JP2011012316A - Soft high-carbon steel sheet superior in punchability and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To soften the material and enhance the punchability of a high-carbon steel sheet which contains 0.70-0.95 mass% C.SOLUTION: The soft high-carbon steel sheet superior in punchability includes, by mass%, 0.70-0.95% C, 0.05-0.4% Si, 0.5-2.0% Mn, 0.005-0.03% P, 0.0001-0.006% S, 0.005-0.10% Al, 0.001-0.01% N and the balance Fe with unavoidable impurities, and has a structure which includes 100 pieces or more of voids per 1 mmof the observed structure.

Description

  The present invention relates to a soft high carbon steel sheet excellent in punchability and a method for producing the same.

  High carbon steel plates are widely used as materials for chains, gears, clutches, saws, blades and the like. When a product is produced from a high carbon steel plate, it is usually cured after forming by heat treatment such as quenching and tempering. Therefore, high carbon steel sheets are required to have workability that can withstand complicated and severe processing.

  Usually, in order to impart required workability to a high-carbon steel sheet, annealing to spheroidize carbide is employed (see, for example, Patent Documents 1 to 5). As a result, punching properties are also required, and it is the actual situation that carbide spheroidizing annealing for softening the material cannot cope with the requirements.

  For example, in Patent Document 1, C: 0.50 to 0.70% by mass, Si: 0.5% by mass or less, Mn: 1.0 to 2.0% by mass, P: 0.02% by mass or less, S: 0.02% by mass, Al: 0.001 to 0.10% by mass, V: 0.05 to 0.50% by mass, Ti: 0.02 to 0.20%, Nb: 0.01 1 to 2 or more of 0.50 mass%, the balance is Fe and inevitable impurities, the spheroidization rate of the carbide is 95% or more, the carbide having a maximum particle size of 2.5 μm or less dispersed Although a high carbon steel sheet excellent in hardenability, fatigue characteristics, and toughness is disclosed, the punchability of the high carbon steel sheet is not improved.

  A high carbon steel plate is high carbon, and thus has a high hardness and excellent punchability. However, the punchability is lowered due to softening.

  In particular, in a high carbon steel plate containing 0.70% by mass or more of C, it is difficult to achieve both softening of the material and improvement of punchability.

JP 2009-024233 A JP 2008-303415 A JP 2008-156712 A JP 2008-069452 A JP 2007-291495 A

  As described above, in a high carbon steel sheet containing 0.70% by mass or more of C, it is difficult to achieve both softening of the material and improvement of punchability. An object of the present invention is to provide a high carbon steel sheet and a method for producing the same, which are intended to improve the softness of the material and improve the punchability in a high carbon steel sheet containing .95% by mass or less.

  The inventors of the present invention have intensively studied a method for solving the above-described problems. As a result, it has been found that the punchability is remarkably improved when voids are introduced into the steel structure by a combination of annealing conditions and cooling conditions.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) By mass%, C: 0.70 to 0.95%, Si: 0.05 to 0.4%, Mn: 0.5 to 2.0%, P: 0.005 to 0.03% , S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01%, with the balance being Fe and inevitable impurities, and A soft high-carbon steel sheet having excellent punchability, wherein the structure has 100 or more voids per 1 mm ^{2 of} the observed structure.

  (2) In mass%, Cr: 0.05 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.05 to 0.5%, and Mo: 0.01 to The soft high-carbon steel sheet having excellent punchability according to (1) above, containing 1.0% or one or more of 1.0%.

  (3) By mass%, Nb: 0.01-0.5%, V: 0.01-0.5%, Ta: 0.01-0.5%, B: 0.001-0. (1) or (2) characterized by containing one or more of 01%, Ti: 0.005 to 0.2%, and W: 0.01 to 0.5% A soft high carbon steel plate with excellent punchability as described.

  (4) By mass%, Sn: 0.003-0.03%, Sb: 0.003-0.03%, and As: 0.003-0.03%, or one or more The soft high carbon steel plate excellent in the punchability according to any one of the above (1) to (3).

  (5) The continuous cast slab satisfying the component composition according to any one of (1) to (4) is subjected to hot rolling directly after casting or heated to 1300 ° C. or less and 90 minutes or less, Finish rolling at 800 to 940 ° C., and then hot-rolled steel sheet is strongly cooled to 650 ° C. at 30 ° C./s or more, and then slowly cooled to 20 ° C. or less until winding up to 400 to 650 A method for producing a soft high-carbon steel sheet having excellent punchability, characterized by scoring at a temperature lower than 0 ° C., pickling, and softening box annealing.

(6) the softening box annealing, carried and held at room temperature from Ac ₁ to Ac ₁ + After heating to 50 ° C., 5 hours or more, then, after cooling below 100 ° C. / hr, the temperature of the Ar ₁ ~Ae ₁ Hold for 3 hours or more in the range, and then slowly cool down to Ar ₁ or less at 10 ° C./hr or less to form a pearlite block having a lamellar spacing of 0.5 μm or more in the structure with an area ratio of 10% or more. The manufacturing method of the soft high carbon steel plate excellent in the punchability as described in said (5) characterized by these.

(7) The steel sheet subjected to the softening box annealing is subjected to cold rolling with a reduction ratio of 30% or less, and then subjected to box annealing for 3 hours or more in a temperature range of Ac ₁ or less (5 ) Or (6), the manufacturing method of the soft high carbon steel plate excellent in the punchability.

(8) The steel sheet subjected to box annealing is subjected to cold rolling with a reduction rate of 30% or less and box annealing that is held for 3 hours or more in a temperature range of Ac ₁ or less once or more ( The manufacturing method of the soft high carbon steel plate excellent in the punchability as described in 7).

  (9) The box annealing is performed in an atmosphere of 95% or more of hydrogen, a dew point of less than −20 ° C. up to 400 ° C., and a dew point of more than 400 ° C. of less than −40 ° C. (7 ) Or (8), a method for producing a soft high carbon steel sheet having excellent punchability.

  ADVANTAGE OF THE INVENTION According to this invention, the soft high carbon steel plate excellent in the punchability which introduce | transduced the void into steel structure, and its manufacturing method can be provided.

It is a figure which shows the aspect in which a void exists in the steel structure which gave the cold rolling after annealing a hot-rolled sheet. It is a figure which shows the aspect in which a void (white arrow) exists in the steel structure after annealing a cold-rolled sheet. It is a figure which shows the aspect of the heat processing in this invention manufacturing method. It is a figure which shows the relationship between the hardness of this invention steel, and comparative steel, and punchability.

The soft high carbon steel sheet of the present invention is in mass%, C: 0.70 to 0.95%, Si: 0.05 to 0.4%, Mn: 0.5 to 2.0%, P: 0.00. 005 to 0.03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.0010 to 0.01%, the balance being Fe and inevitable In a steel plate made of mechanical impurities, the structure has 100 or more voids per 1 mm ^{2 of} the observed structure.

  First, the reason for limitation relating to the component composition of the soft high carbon steel sheet of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”) will be described. Hereinafter, “%” means “mass%”.

C: 0.70 to 0.95%
C is an important element for ensuring the strength of the steel sheet, and is added at 0.70% or more to ensure the required strength. If it is less than 0.70%, the hardenability is lowered and the strength as a high-strength steel sheet for machine structures cannot be obtained, so the lower limit is made 0.70%. If it exceeds 0.95%, it takes a long time for heat treatment to ensure toughness and workability, so the upper limit is made 0.95%. Preferably, it is 0.75 to 0.85%.

Si: 0.05-0.4%
Si acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.05%, the effect of addition cannot be obtained, so the lower limit is made 0.05%. If it exceeds 0.4%, the surface properties are deteriorated due to scale wrinkling during hot rolling, so the upper limit is made 0.4%. Preferably, it is 0.10 to 0.3%.

Mn: 0.5 to 2.0%
Mn acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.5%, the effect of addition cannot be obtained, so the lower limit is made 0.5%. If it exceeds 2.0%, impact characteristics after quenching and tempering are promoted, so the upper limit is made 2.0%. Preferably, it is 0.5 to 1.5%.

P: 0.005 to 0.03%
P is a solid solution strengthening element and is an element effective for the strength of the steel sheet. Since excessive inclusion will inhibit toughness, the upper limit is made 0.03%. Reduction to less than 0.005% causes an increase in refining cost, so the lower limit is made 0.005%. Preferably, it is 0.007 to 0.02%.

S: 0.0001 to 0.006%
S forms a non-metallic inclusion and causes a deterioration in workability and toughness after heat treatment, so the upper limit is made 0.006%. Reducing to less than 0.0001% causes a significant increase in refining costs, so the lower limit is made 0.0001%. Preferably, it is 0.001 to 0.004%.

Al: 0.005-0.10%
Al acts as a deoxidizing agent and is an element effective for fixing N. If it is less than 0.005%, the effect of addition cannot be sufficiently obtained, so the lower limit is made 0.005%. If it exceeds 0.10%, the effect of addition is saturated and surface flaws are likely to occur, so the upper limit is made 0.10%. Preferably, it is 0.01 to 0.05%.

N: 0.001 to 0.01%
N is an element that forms a nitride. If nitride precipitates during slab bending correction in curved continuous casting, the slab may crack, so the upper limit is made 0.01%. A smaller amount is preferable, but a reduction to less than 0.001% leads to an increase in refining costs, so the lower limit is made 0.001%. Preferably, it is 0.004 to 0.007%.

  In order to enhance the mechanical properties of the steel sheet of the present invention, a required amount of one or more of Cr, Ni, Cu, and Mo may be added.

Cr: 0.05-1.0%
Cr is an element effective for improving hardenability. If it is less than 0.05%, there is no effect of addition, so the lower limit is made 0.05%. If it exceeds 1.0%, the effect of addition is saturated, so the upper limit is made 1.0%. Preferably, it is 0.07 to 0.7%.

Ni: 0.01 to 1.0%
Ni is an element effective for improving toughness and hardenability. If it is less than 0.01%, there is no effect of addition, so the lower limit is made 0.01%. If it exceeds 1.0%, the effect of addition is saturated and the cost is increased, so the upper limit is made 1.0%. Preferably, it is 0.05 to 0.5%.

Cu: 0.05 to 0.5%
Cu is an element effective for ensuring hardenability. If it is less than 0.05%, the effect of addition is insufficient, so the lower limit is made 0.05%. If it exceeds 0.5%, it becomes too hard and the cold workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.08 to 0.2%.

Mo: 0.01 to 1.0%
Mo is an element effective for improving hardenability and improving resistance to temper softening. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 1.0%, the effect of addition is saturated, so the upper limit is made 1.0%. Preferably, it is 0.05 to 0.5%.

  In order to further enhance the mechanical properties of the steel sheet of the present invention, one or more of Nb, V, Ta, B, and W may be added in a required amount.

Nb: 0.01 to 0.5%
Nb is an element that forms carbonitride and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so the lower limit is made 0.01%. If it exceeds 0.5%, the effect of addition is saturated, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

V: 0.01 to 0.5%
V, like Nb, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

Ta: 0.01 to 0.5%
Ta, like Nb and V, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

B: 0.001 to 0.01%
B is an element effective for enhancing the hardenability by adding a small amount. If it is less than 0.001%, there is no effect of addition, so the lower limit is made 0.001%. If it exceeds 0.01%, the castability deteriorates, and a B-based compound is generated to reduce toughness. Therefore, the upper limit is made 0.01%. Preferably, it is 0.003 to 0.007%.

Ti: 0.005 to 0.2%
Ti acts as a deoxidizer and is an element effective for fixing N. From the relationship with the amount of N, addition of 0.005% or more is necessary. Even if Ti is added in excess of 0.2%, the effect of addition is saturated and the cost is not only increased, but also Ti-based precipitation due to promotion of nitrogen absorption during the production process, reduction of effective carbon amount due to carbide formation, etc. This increases the amount of material, inhibits the growth of austenite grains during quenching heat treatment, and deteriorates the hardenability, so the upper limit is made 0.2%. Preferably, it is 0.01 to 0.4%.

W: 0.01-0.5%
W is an element effective for strengthening a steel sheet. If it is less than 0.01%, the effect of addition does not appear, so the lower limit is made 0.01%. If it exceeds 0.5%, the workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.04 to 0.2%.

  When scrap is used as a raw material for the steel sheet of the present invention, one or more of Sn, Sb, and As are inevitably mixed in by 0.003% or more, and any of them may be 0.03% or less. For example, since the punchability and hardenability of the steel sheet of the present invention are not hindered, Sn: 0.003-0.03%, Sb: 0.003-0.03%, and As: 0.0. Inclusion of one or more of 003 to 0.03% is allowed.

  In the steel sheet of the present invention, the amount of O is not specified, but when the oxide aggregates and coarsens, the ductility decreases, so O is preferably 0.0025% or less. A smaller amount of O is preferable, but since it is technically difficult to reduce it to less than 0.0001%, a content of 0.0001% or more is allowed.

  When scrap is used as a melting raw material of the steel sheet of the present invention, elements such as Zn and Zr are mixed as unavoidable impurities. Allow mixing. In addition, elements other than Zn, Zr and the like are allowed to be mixed as long as the characteristics of the steel sheet of the present invention are not impaired.

As described above, the steel sheet of the present invention is characterized in that, in addition to the component composition, the structure has 100 or more voids per 1 mm ^{2 of} the observed structure.

The steel sheet of the present invention is a combination of annealing conditions and cooling conditions, in which 100 or more voids are introduced in the structure adjacent to the carbide and 1 mm ^{2 of} the observed structure.

It is a novel finding that the present inventors have found that the punchability of the steel sheet is remarkably improved by the presence of 100 or more voids per 1 mm ² of the observed structure in the structure. An annealing condition and a cooling condition for introducing a void into the structure will be described later.

The observation of the tissue is preferably performed with a scanning electron microscope. Count the number of voids per 1 mm ^{2 of the} observed tissue and confirm that there are at least 100 voids. If the number of voids is less than 100 per 1 mm ^{2 of} the observed tissue, it becomes difficult to ensure the required punchability.

  FIG. 1 shows an embodiment in which voids are present in a steel structure subjected to cold rolling after annealing a hot-rolled sheet. The structure before introducing the void is preferably a structure in which the area ratio of the pearlite block having a lamellar spacing of 0.5 μm or more is 10% or more.

  Voids are generated on acicular carbides rather than spheroidized carbides. When the lamellar interval is less than 0.5 μm, the interval becomes too fine, and the carbide is finely divided in the subsequent cold working, which becomes a factor for inhibiting softening. If the lamellar spacing is 0.5 μm or more, the carbide after the division is also relatively large, so that it does not become a factor that hinders softening.

  The area ratio of a pearlite block having a lamellar spacing of 0.5 μm or more is preferably 10% or more. If it is less than 10%, the formation frequency of voids on the lamellar decreases, and the formation frequency of voids becomes insufficient. The area ratio of the pearlite block is more preferably 20% or more.

  FIG. 2 shows a mode in which voids exist in a steel structure obtained by annealing a cold-rolled sheet. It can be seen that the void is formed adjacent to the carbide (see white arrow in the figure). Since voids can usually be the starting point of fatigue fracture in steel structures, they should be avoided in structural materials, but at the time of punching, the formation of fractured surfaces is promoted by the effect of void connection, and punchability is improved. Will improve.

  Next, a manufacturing method of the steel sheet of the present invention (hereinafter referred to as “the manufacturing method of the present invention”) will be described.

  A continuously cast slab (cold slab) to be subjected to hot rolling is heated to 1300 ° C. or less and 90 minutes or less. If the heating temperature exceeds 1300 ° C. or the heating time exceeds 90 minutes, de-C in the surface portion of the slab becomes prominent in the heating step, and the hardenability of the steel sheet surface deteriorates. Therefore, the heating temperature is 1300 ° C. or less. The time is 90 minutes or less. From the viewpoint of suppressing de-C, the heating time is preferably 1200 ° C. or less, and the heating time is preferably 60 minutes or less.

  In addition, the continuous cast slab is directly or reheated and subjected to hot rolling, but when directly subjected to hot rolling and after reheating and subjected to hot rolling, the steel sheet characteristics are improved. There is almost no difference.

  Hot rolling may be either normal hot rolling or continuous hot rolling in which slabs are joined in finish rolling. The lower limit of the hot rolling end temperature (hot rolling end temperature) is set to 800 ° C. in terms of surface defects in addition to productivity, sheet thickness accuracy, and anisotropy improvement. If the hot rolling end temperature is lower than 800 ° C., wrinkles due to baking frequently occur. On the other hand, when the hot rolling end temperature exceeds 940 ° C., the occurrence frequency of wrinkles due to scale increases, the product yield decreases, and the cost increases. Therefore, the hot rolling end temperature is set to 800 to 940 ° C.

  The steel sheet after hot rolling is cooled to 650 ° C. at a cooling rate of 30 ° C./second or more after the finish rolling, and subsequently, at a cooling rate of 20 ° C./second or less, at a coiling temperature of 400 to 600 ° C. Allow to cool slowly.

  The reason why the cooling rate to 650 ° C. is 30 ° C./second or more after hot rolling is that if the cooling rate is less than 30 ° C./second, a pearlite band is generated due to segregation, and coarse carbides are present even after annealing. This is because it tends to exist and leads to deterioration of workability. In order to suppress deterioration of workability, the hot-rolled steel sheet is cooled at a cooling rate of 30 ° C./second or more after finish rolling.

  The reason why the cooling rate at a coiling temperature of 400 to 650 ° C. is set to 20 ° C./second or less is to promote uniform pearlite transformation and bainite transformation of the pearlite structure. When the hot-rolled steel sheet is rapidly cooled to less than 400 to 650 ° C., supercooled austenite is generated, the coiled shape is disturbed due to the supercooled austenite, and wrinkles are generated on the steel sheet surface. When wrinkles occur on the surface of the steel sheet, the yield greatly decreases.

  The reason why the hot-rolled steel sheet is wound at a coiling temperature of less than 400 to 650 ° C. is that when it is less than 400 ° C., the martensitic transformation occurs in part and the strength of the steel sheet increases, making handling difficult. This is because, when cold rolling is performed, gauge hunting occurs due to non-uniform structure, resulting in a decrease in yield.

  On the other hand, when it winds at the temperature of 650 degreeC or more, the scale of a hot-rolled steel plate will become thick and pickling property will fall, and the oxidation progress of a surface layer part and grain boundary oxidation will advance. Therefore, the hot-rolled steel sheet is wound at a winding temperature of 400 to less than 650 ° C.

  After pickling the steel plate and cleaning the surface, the steel plate is subjected to softening box annealing. In the production method of the present invention, the steel plate is subjected to softening box annealing to form coarse lamellar carbide. Here, in FIG. 3, the aspect of the heat processing in this invention manufacturing method is shown.

As shown in (1) in FIG. 3, the softening box annealing is performed by heating the steel sheet from room temperature to Ac ₁ to Ac ₁ + 50 ° C. and holding it for 5 hours or more. The holding for 5 hours or more promotes the spheroidization of the carbide and dissolves the fine lamellar in the austenite.

After holding for 5 hours or longer, the steel sheet is preferably cooled at 100 ° C./hr or less, as shown in (2) in FIG. 3, and then, Ar ₁ to Ar as shown in (3) in FIG. 3. Hold at Ae ₁ temperature range for 3 hours or more. After this holding, the steel plate is cooled (see (4) and (5) in FIG. 3). If it is possible to cool the steel plate with high accuracy while keeping the temperature distribution in the coil uniform, the cooling rate May exceed 100 ° C./hr.

The steel sheet after cooling is held in the temperature range of Ar _{1 to} Ae ₁ for 3 hours or more to form a lamellar structure with a coarse interval. If the lamellar spacing is coarse, the amount of voids introduced increases in the next cold rolling process. Immediately after holding in the above temperature range for 3 hours or more, when the steel sheet is cooled at a cooling rate of more than 10 ° C./hr, pearlite with a narrow lamellar spacing is generated. Therefore, the cooling rate immediately after holding is preferably 10 ° C./hr or less. (See (4) in FIG. 3).

  If the area ratio of the pearlite block is less than 10%, void generation during cold rolling is not promoted, so the area ratio of the pearlite block is preferably 10% or more.

  The steel sheet subjected to softening box annealing is subjected to cold rolling with a reduction rate of 30% or less to form a large number of voids on the needle-like θ (see (6) in FIG. 3). When cold rolling with a rolling reduction of more than 30% is applied to the steel sheet, ferrite recrystallizes and voids disappear during the recrystallization. Therefore, the rolling reduction of cold rolling is preferably 30% or less.

Next, the steel sheet after cold rolling is subjected to box annealing for 3 hours or more in a temperature range of Ac ₁ or less (see (7) in FIG. 3). This box annealing promotes ferrite grain growth while leaving voids in the matrix. If the box annealing is less than 3 hours, the softening does not proceed sufficiently.

If the annealing temperature is greater than Ac ₁ , voids disappear during the phase transformation from ferrite to austenite, so the box annealing temperature is preferably Ac ₁ or less.

It is preferable to perform cold rolling with a rolling reduction of 30% or less and box annealing that is held for 3 hours or more in a temperature range of Ac ₁ or less at least once. By repeating the cold rolling and box annealing, void formation and ferrite grain growth can be effectively promoted.

  The box annealing is preferably performed in an atmosphere of 95% or more of hydrogen, a dew point up to 400 ° C. of less than −20 ° C., and a dew point of over 400 ° C. of less than −40 ° C.

  In addition to uniformizing the temperature distribution in the coil, annealing is performed in an atmosphere of 95% or more of hydrogen in order to suppress a decrease in hardenability due to nitrogen penetration. In order to suppress decarburization during annealing, the dew point up to 400 ° C is set to less than -20 ° C, and the dew point above 400 ° C is set to less than -40 ° C.

  The steel sheet of the present invention having excellent punchability is subjected to a solidification treatment after punching, and is subjected to a quenching treatment. By this quenching treatment, the steel sheet of the present invention after punching has a required strength.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
The steel sheet having the component composition shown in Table 1 was annealed under the annealing conditions shown in Table 2, and the number of voids in the structure and product characteristics (hardness and punchability) were investigated. The results are also shown in Table 2. In Table 2, the annealing conditions (1) to (7) correspond to (1) to (7) in FIG.

  In Table 2, “burr generation height” was evaluated according to the criteria shown in Table 3.

  In Table 2, No. 1A (using the steel plate No. 1 in Table 1) is a comparative example in which the amount of carbide is not sufficient and void formation during cold rolling is small. No. 2A is a comparative example in which the amount of carbide is not sufficient and the formation of voids during cold rolling is small.

  No. 3B is a comparative example in which the temperature is insufficient in step (1), acicular carbides remain, the hardness is high, and fine (α + θ) is generated by strong processing. No. 4B is a comparative example in which a pearlite having a low lamellar spacing was generated in step (3), and it was difficult to soften.

  No. 5A is a process (1) to (4) in which the spheroidization of the carbide progresses too much, and in step (7), the two-phase annealing is performed, voids introduced by cold rolling disappear, and the punchability is reduced. It is a comparative example. No. 6A is a comparative example in which pearlite having a high cooling rate and a fine lamellar spacing was newly generated in step (4), and void introduction was reduced.

  No. 7A is a comparative example in which the temperature is insufficient in step (1), acicular carbides remain, the hardness is high, and fine (α + θ) is generated by strong processing. No. 7C ″ is a comparative example in which fine (α + θ) is generated by the strong processing of the second cold rolling while forming the structure of the present invention by the first cold rolling annealing.

  No. 9B is a comparative example in which two-phase annealing was performed in step (7), voids introduced by cold rolling disappeared, and punchability was reduced.

  No. 10A is a comparative example in which the temperature is insufficient in step (1), acicular carbides remain, the hardness is high, and fine (α + θ) is generated by strong processing. No. 10B is a comparative example in which fine (α + θ) is generated by strong processing. No. 10C ″ is the first cold-rolled sheet annealing to form the structure of the present invention, but the second cold rolling is a strong work, and fine (α + θ) is generated, and the second annealing is a two-phase annealing. Thus, this is a comparative example in which voids introduced by cold rolling disappear and ridge removal performance is reduced.

  No. 11A is a comparative example in which the carbide is highly stable due to its high Mn and is not easily softened industrially. No. 12A is a comparative example in which two-phase annealing was performed in step (7), voids introduced by cold rolling disappeared, and punchability was reduced. No. 12B is a comparison in which, while forming the structure of the present invention by the first cold-rolled sheet annealing, the second annealing became a two-phase annealing, voids introduced by cold rolling disappeared, and the ridge removal performance decreased. It is an example.

  N0.15A is a comparative example in which the amount of carbide is large and voids are easily generated, but it is difficult to soften. In contrast, no. 3A, no. 4A, no. 6B, no. 6B ', no. 6C, no. 6C ', no. 7B, no. 7C, no. 7C ', no. 8A, no. 9A, no. 10C, no. 10C ', no. 12B, no. 12B ', no. 12C, no. 13A and No. 14A is an invention example.

  FIG. 4 shows the relationship between the burr height evaluation (1 to 6) and the hardness (HV). In the figure, ◯ and ● marks are invention examples, and ◇ and ♦ marks are comparative examples. The invention example is located in the upper left in the figure, and it can be seen that the punchability is remarkably superior to the comparative example.

  As described above, according to the present invention, it is possible to provide a soft high carbon steel sheet excellent in punchability in which voids are introduced into the steel structure and a method for producing the same. Therefore, since this invention expands the use of a high carbon steel plate greatly, its applicability is high in the steel product manufacturing industry.

Claims (9)

In mass%, C: 0.70 to 0.95%, Si: 0.05 to 0.4%, Mn: 0.5 to 2.0%, P: 0.005 to 0.03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01%, the balance is made of Fe and inevitable impurities, and the structure is A soft high-carbon steel sheet having excellent punchability, characterized by having 100 or more voids per 1 mm ^{2 of} the observation structure.   Further, Cr: 0.05-1.0%, Ni: 0.01-1.0%, Cu: 0.05-0.5%, and Mo: 0.01-1.0 The soft high-carbon steel sheet having excellent punchability according to claim 1, comprising:   % By mass, Nb: 0.01 to 0.5%, V: 0.01 to 0.5%, Ta: 0.01 to 0.5%, B: 0.001 to 0.01%, It contains one or more of Ti: 0.005 to 0.2% and W: 0.01 to 0.5%, and is excellent in punchability according to claim 1 or 2. Soft high carbon steel plate.   In addition, it contains one or more of Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0.003 to 0.03%. The soft high carbon steel plate excellent in punching property according to any one of claims 1 to 3.   The continuous cast slab satisfying the component composition according to any one of claims 1 to 4 is subjected to hot rolling directly after casting, or 1300 ° C or less and 90 minutes or less, and 800 to 940 ° C. Then, finish rolling is finished, and then the hot-rolled steel sheet is strongly cooled to 650 ° C. at 30 ° C./s or more, and then slowly cooled to 20 ° C./s or less until winding, and then rolled at 400 to less than 650 ° C. The manufacturing method of the soft high carbon steel plate excellent in the punchability characterized by performing softening box annealing after taking and pickling. The softening box annealing, after heating from room temperature to Ac ₁ ~Ac ₁ + 50 ℃, carried and held over 5 hours, then, after cooling below 100 ° C. / hr, 3 in a temperature range of Ar ₁ ~Ae ₁ It is held for a period of time or more, and then slowly cooled to 10 ° C./hr or less to Ar ₁ or less, and a pearlite block having a lamellar spacing of 0.5 μm or more is formed in the structure by 10% or more by area ratio. The manufacturing method of the soft high carbon steel plate excellent in the punchability of Claim 5. The steel sheet subjected to the softening box annealing is subjected to cold rolling with a reduction rate of 30% or less, and then subjected to box annealing for 3 hours or more in a temperature range of Ac ₁ or less. The manufacturing method of the soft high carbon steel plate excellent in the punchability of description. The steel sheet subjected to box annealing is subjected to cold rolling with a reduction rate of 30% or less and box annealing that is held for 3 hours or more in a temperature range of Ac ₁ or less once or more. A method for producing a soft high carbon steel sheet having excellent punchability.   The box annealing is performed in an atmosphere of 95% or more of hydrogen, a dew point up to 400 ° C of less than -20 ° C, and a dew point of over 400 ° C of less than -40 ° C. The manufacturing method of the soft high carbon steel plate excellent in the punchability of description.