JP5312230B2 - Soft high carbon steel sheet with small punching and manufacturing method thereof - Google Patents

Description

  The present invention relates to a soft high carbon steel sheet having a small punching and a manufacturing method thereof.

  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 has been disclosed, no attempt has been made to reduce punching in the high carbon steel sheet.

  A high carbon steel plate is high carbon, so it originally has the required hardness and is excellent in punchability, but if it is softened by carbide spheroidizing annealing, the punchability will deteriorate and it will occur by punching. Kaeri grows big.

  In particular, in a high carbon steel plate containing 0.65% by mass or more of C, it is difficult to achieve both softening of the material and improvement of punchability, particularly suppression of punching.

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 0.65% by mass or more of C, it is difficult to achieve both softening of the material and improvement of punchability (particularly, suppression of punching). In a high carbon steel sheet containing 0.65% by mass or more and 0.85% by mass or less of C, the object is to soften the material and improve punchability (particularly, suppression of punching burrs), and solve the problem It aims at providing a high carbon steel plate and its manufacturing method.

The inventors of the present invention have intensively studied a method for solving the above-described problems. As a result, with the combination of annealing conditions and cooling conditions, when the area of carbides of 0.5 μm ² or less is suppressed to 15% or less of the total area of carbides in the thickness cross section of the structure before final cold rolling, It has been found that cold rolling introduces voids in the steel structure, and as a result, punching remarks are remarkably reduced, and punching performance is remarkably improved.

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

(1) By mass%, C: 0.65-0.85%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.005-0.03% , 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,
(I) the hardness is 170 HV or less, and
(Ii) After continuous casting, hot rolling, pickling and annealing, in the plate thickness section of the structure before final cold rolling when cold rolling and annealing are performed once or twice, 0.5 μm ² or less of carbide area of state, and are within 15% of the total area of the carbide, and,
(Iii) the final cold rolling reduction ratio is Ru der than 30% to 15%
A soft high-carbon steel sheet with small punching characteristics.

  (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 with small punching as described in (1) above, containing 1.0% or one or more of 1.0%.

  (3) In mass%, Nb: 0.01 to 0.5%, V: 0.01 to 0.5%, Ta: 0.01 to 0.5%, and W: 0.01 to The soft high-carbon steel sheet with small punching as described in (1) or (2) above, containing 0.5% of one type or two or more types.

  (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 sheet with small punching as described in any one of (1) to (3) above.

(5) The soft high carbon with a small punching defect according to any one of the above (1) to (4), wherein 100 or more voids per 1 mm ² of the observed structure are present in the thickness cross section of the structure steel sheet.

(6) A 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 at 1300 ° C. or less for 90 minutes or less. The finish rolling is finished at 800 to 940 ° C., 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. or less until winding up to 400 to 400 ° C. taken Maki below 650 ° C., after pickling, after performing softened box annealing, cold rolling and annealing method for manufacturing a 1 Kai施 be high carbon steel sheet, after softening box annealing and softening box annealing A method for producing a soft high carbon steel sheet with small punching, characterized in that the cold rolling is performed under the following conditions.
(I) softening box annealing, after heating from room temperature up to a temperature of Ac ₁ ~Ac ₁ + 100 ℃, and held for 3 hours or more, then, in the course of cooling to Ar ₁ or less, the temperature of the heating and holding temperature to Ar ₁ In the range, cooling at 20 ° C./hr or less and holding for 0.5 hr or more after cooling are repeated once or more.
(Ii) The rolling reduction of cold rolling after softening box annealing is 15% or more and 30% or less.

(7) A continuous cast slab satisfying the component composition according to any one of (1) to (4) is subjected to hot rolling directly after casting or at 1300 ° C. or less for 90 minutes or less. The finish rolling is finished at 800 to 940 ° C., 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. or less until winding up to 400 to 400 ° C. It is a manufacturing method of a high carbon steel sheet which is subjected to cold rolling and annealing twice after scraping at less than 650 ° C., pickling, softening box annealing, and after softening box annealing and softening box annealing. A method for producing a soft high-carbon steel sheet with small punching, characterized in that cold rolling and processing after cold rolling are performed under the following conditions.
(I) Softening box annealing is heated for 3 hours or more after heating from room temperature to a temperature of 680 ° C. or higher and Ac ₁ or lower.
(Ii) The rolling reduction of cold rolling after softening box annealing shall be 40% or more.
(Iii) The treatment after cold rolling is performed again at a temperature of 680 ° C. or more and Ac ₁ or less so that the ferrite particle diameter dα satisfies the following formula (1), and then the rolling reduction is 15% or more and 30% or less. subjected to cold rolling, then, subjected to a box grilled blunt.
dα> 2.5-1.7ln (C mass%) (1)

  According to the present invention, in the high carbon steel sheet of C: 0.65 to 0.85%, voids are introduced into the steel sheet structure, and punching is significantly reduced. Therefore, the soft high carbon steel sheet having excellent punchability and A manufacturing method thereof can be provided.

It is a figure which shows the aspect in which 0.5 micrometer < ² > or less carbide | carbonized_material exists in 15% or less of the total area of a carbide | carbonized_material in a plate | board thickness cross-sectional structure. It is a figure which shows the aspect in which a void (white arrow) exists in a plate | board thickness cross-sectional structure | tissue. 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 grade of punching, and hardness (HV).

The soft high carbon steel sheet of the present invention is in mass%, C: 0.65-0.85%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.00. 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 being Fe and inevitable In steel plates made of mechanical impurities,
(I) the hardness is 170 HV or less, and
(Ii) After continuous casting, hot rolling, pickling and annealing, in the plate thickness section of the structure before final cold rolling when cold rolling and annealing are performed once or twice , 0.5 μm ² or less of carbide The area is within 15% of the total area of the carbide,
It is characterized by that.

  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: 0.65-0.85%
C is an important element for securing the strength of the steel sheet, and is added by 0.65% or more to ensure the required strength. If it is less than 0.65%, the hardenability deteriorates and the strength as a high-strength steel sheet for machine structures cannot be obtained, so the lower limit is made 0.65%. If it exceeds 0.85%, it takes a long time for heat treatment to ensure toughness and workability, so the upper limit is made 0.85%. Preferably, it is 0.70 to 0.80%.

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 0.8 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 strengthen the mechanical properties of the steel sheet of the present invention, a necessary amount of one or more of Nb, V, Ta, and W may be added.

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%.

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. In the steel sheet of the present invention, Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0.003 to 0. 0.03% of one kind or two kinds or more are 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 Ti, Zn, Zr are mixed as unavoidable impurities, but in the steel sheet of the present invention, the above does not impair the characteristics of the steel sheet of the present invention. Allow mixing of elements. It should be noted that elements other than Ti, 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
(I) the hardness is 170 HV or less, and
(Ii) After continuous casting, hot rolling, pickling and annealing, in the plate thickness section of the structure before final cold rolling when cold rolling and annealing are performed once or twice , 0.5 μm ² or less of carbide The area is within 15% of the total area of the carbide,
It is characterized by that.

  In order to ensure the softness of the steel sheet, the hardness is limited to 170 HV or less. If the hardness exceeds 170 HV, the workability decreases.

In the steel sheet of the present invention, it is important that the area of carbide of 0.5 μm ² or less is within 15% of the total area of carbide in the thickness cross section of the structure before final cold rolling.

If fine carbides of 0.5 μm ² or less are present in excess of 15% of the total area of the carbides, void formation is suppressed in the cold rolling of the next step. In the plate thickness cross section of the previous structure, the area of carbides of 0.5 μm ² or less was limited to 15% of the total area of carbides.

That is, the steel sheet of the present invention is a combination of annealing conditions and cooling conditions, and the carbide area of 0.5 μm ² or less is suppressed within 15% of the total carbide area in the sheet thickness cross-sectional structure before final cold rolling. It is.

FIG. 1 shows an embodiment in which carbides of 0.5 μm ² or less exist in 15% or less of the total area of carbides in the plate thickness cross-sectional structure.

After continuous casting, hot rolling, pickling, and annealing, in the sheet thickness cross-sectional structure before final cold rolling when cold rolling and annealing are performed once or twice , the carbide area of 0.5 μm ² or less It is a novel finding that the present inventors have found that when the steel sheet is controlled to be within 15% of the total area, the burrs of the steel sheet generated when the steel sheet is punched can be remarkably reduced. An annealing condition and a cooling condition for suppressing the area of carbide of 0.5 μm ² or less within 15% of the total area of the carbide in the plate thickness cross-sectional structure will be described later.

In the steel sheet of the present invention, it is preferable that 100 or more voids exist per 1 mm ² of the observation structure in the thickness cross section of the steel sheet structure. The presence of 100 or more voids per 1 mm ² of the observed structure in the structure of the plate thickness cross section significantly improves punchability, and the burrs generated after punching become as small as possible.

It is a new 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 observation structure in the structure of the plate thickness cross section. 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 one or more exist. 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. 2 shows an embodiment in which voids exist in the steel structure subjected to box annealing after cold rolling. It can be seen that the void is formed adjacent to the carbide. 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 fracture surfaces is promoted by the effect of void connection, and punchability is improved. improves.

  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. When heating is performed at a temperature exceeding 1300 ° C. or when the heating is performed for a long time exceeding 90 minutes, de-C is remarkable in the surface layer portion of the slab in the heating process, and the hardenability of the steel sheet surface is deteriorated. Therefore, the heating temperature is 1300 ° C. or less and the heating time is 90 minutes or less. From the viewpoint of suppressing de-C, the heating temperature is preferably 1200 ° C. or less, and the heating time is preferably 60 minutes or less.

  In addition, although a continuous cast slab is directly or reheated and used for hot rolling, there is almost no difference in steel plate characteristics between direct rolling and rolling after reheating.

  Hot rolling may be either normal hot rolling or continuous hot rolling in which slabs are joined in finish rolling. The end temperature of hot rolling (hot end temperature) is set by adding the viewpoint of surface defects to the viewpoint of productivity, sheet thickness accuracy, and anisotropy improvement. If the hot rolling end temperature is less than 800 ° C., defects due to baking occur frequently, whereas if it exceeds 940 ° C., the occurrence frequency of scale defects 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 650. Slowly cool to below ℃.

  After hot rolling, the hot-rolled steel sheet is cooled to 650 ° C. at a cooling rate of 30 ° C./second or more. If the cooling rate is less than 30 ° C./second, a pearlite band accompanying segregation is generated, This is because coarse carbides are easily present even after annealing, leading to deterioration of workability. From the viewpoint of suppressing the deterioration of workability, the hot-rolled steel sheet is cooled at 30 ° C./second or more.

  The reason why the hot-rolled steel sheet is slowly cooled to a coiling temperature of 400 to 650 ° C. at a cooling rate of 20 ° C./second or less is to promote uniform pearlite transformation and bainite transformation of the pearlite structure. When cooled rapidly to the above temperature range, the winding shape of the coil is disturbed due to the supercooled austenite, so that wrinkles are generated and the yield is lowered.

  The reason for winding a hot-rolled steel sheet at a coiling temperature of 400 to 650 ° C. is that when it is less than 400 ° C., partly martensitic transformation occurs, the steel sheet strength increases, handling becomes difficult, or during cold rolling. This is because the yield is lowered, such as causing gauge hunting due to the uneven structure. On the other hand, when the coiling temperature exceeds 650 ° C., the scale of the hot-rolled steel sheet becomes thick and not only the pickling property is lowered, but also the oxidation of the surface layer part and the grain boundary oxidation progress. Therefore, the winding temperature is 400 to 650 ° C.

After pickling the steel plate and cleaning the surface, the steel plate is subjected to softening box annealing. In the manufacturing method of the present invention, the steel sheet is subjected to softening box annealing, and the area of carbides of 0.5 μm ² or less is suppressed to 15% or less of the total area of carbides, thereby softening the material and improving punchability. At the same time.

  Here, in FIG. 3, the aspect of the heat processing in this invention manufacturing method is shown.

As shown in FIG. 3, the softening box annealing is performed by heating a steel plate (hot rolled plate, cold rolled plate) from room temperature to Ac ₁ to Ac ₁ + 100 ° C. and holding for 3 hours or more. By holding for 3 hours or more, lamellar carbide is dissolved in austenite.

After the above holding for 3 hours or more, the steel sheet is cooled to Ar ₁ or lower as shown in FIG. 3. In this cooling process, cooling at 20 ° C./hr or lower in the temperature range of the heating holding temperature to Ar ₁ ( (A) in FIG. 3) and holding for 0.5 hr or more after cooling ((b) in FIG. 3) are repeated once or more ((c) in FIG. 3).

The step cooling repeating cooling and holding one or more times, the thickness cross section of the final cold rolling before the tissue, an area of 0.5 [mu] m ² or less of carbides can be suppressed within 15% of the total area of the carbide .

When the cooling rate exceeds 20 ° C./hr or the retention after cooling is less than 0.5 hr, the desired area ratio related to carbides of 0.5 μm ² or less cannot be obtained when Ar ₁ or less is reached.

Another embodiment of heat treatment in the production method of the present invention will be described .

After performing the above softening box annealing, the above steel plate was cooled to Ar ₁ below, subjected to rolling reduction of 30% or less of the cold rolling, then subjected to box annealing. The annealing temperature is preferably Ac ₁ or less.

By subjecting the steel sheet subjected to softening box annealing to cold rolling with a rolling reduction of 15% or more and 30% or less, voids can be efficiently introduced into the steel simultaneously with softening.

Further, softening box annealing, the hot rolled sheet was heated from room temperature to Ac ₁ temperature below performs holds more than 3 hours, then the hot-rolled sheet, reduction ratio of 40% or more cold rolling alms, then ferrite grain size (alpha particle diameter) d _alpha is, so as to satisfy the following formula (1), again, subjected to annealing at Ac ₁ or lower.
d _α> 2.5-1.7ln (C mass%) (1)

  By subjecting the steel sheet subjected to the above-mentioned softening box annealing to cold rolling with a rolling reduction of 40% or more, the recrystallization of ferrite can be used as a driving force, and the carbide can be coarsened.

Moreover, Ac in the steel plate subjected ₁ the following annealing, subjected to the following cold rolled 30% reduction ratio of 15% or more, then, subjected to box annealing. The annealing temperature is preferably Ac ₁ or less.

By subjecting the steel sheet annealed with Ac ₁ or less to cold rolling at a reduction rate of 15% or more and 30% or less, voids are generated on the coarse carbide. When the steel sheet is cold-rolled with a reduction ratio of more than 30%, ferrite recrystallizes and voids disappear during the recrystallization, so the reduction ratio is preferably 30% or less. In Table 4, the lower limit of the rolling reduction is preferably 15% based on the minimum value of 15% of the cold rolling rate of the invention examples.

Next, the steel sheet after cold rolling is subjected to box annealing in a temperature range of Ac ₁ or less to promote ferrite grain growth while leaving voids in the matrix. When box annealing is performed in a temperature range exceeding Ac ₁ , voids disappear during phase transformation from ferrite to austenite. Therefore, the box annealing temperature is preferably a temperature range of Ac ₁ or lower.

  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, which has a small punching resistance and excellent punchability, is subjected to a hardening treatment after punching. 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 Table 3, and the hardness (HV), carbide, voids, and ferrite grain size were investigated for the steel sheet structure before the final cold rolling. . Table 2 summarizes the annealing conditions and the state of carbides of the steel sheet after hot-rolled sheet annealing and before cold rolling, and the numbers indicate the component composition corresponding to No in Table 1; The alphabet indicates that various hot-rolled sheet annealing was applied to the hot-rolled steel sheet having each component composition. Table 3 summarizes the state of the carbide before the second cold rolling of the steel sheet that has been cold rolled and annealed after the hot rolled sheet annealing, and the numbers are component compositions corresponding to No in Table 1. The alphabet following the number indicates that various hot-rolled sheet annealing, cold-rolling and cold-rolling sheet annealing were performed on the hot-rolled steel sheets having the respective component compositions. Further, the steel sheet was punched to evaluate the degree of punching. The results are shown in Table 4. Table 4 is a steel sheet that has been cold-rolled and annealed once or twice after the hot-rolled sheet annealing, and arranged the carbide state before the final cold rolling, the hardness after the final cold rolling and the final annealing, and the punchability. However, after the numbers of No and the alphabet following the numbers are subjected to hot-rolled sheet annealing shown in Table 2 or hot-rolled sheet annealing, cold-rolled and annealed as shown in Table 3, further cold-rolled and annealed in Table 4 It shows that it was manufactured. The degree of punching was evaluated according to the criteria shown in Table 5.

FIG. 4 shows the relationship between the evaluation of the degree of punching and the hardness (HV). In the figure, ◯ indicates an invention example, and ◇ indicates a comparative example. The invention example is located in the upper left in the figure, and it can be seen that, compared with the comparative example, the punching is remarkably small and the punching property is excellent.

  As described above, according to the present invention, in a high carbon steel sheet of C: 0.65 to 0.85%, voids are introduced into the steel sheet structure, and punching is significantly reduced, so that the punchability is excellent. A soft high carbon steel sheet and a method for producing the same can be provided. 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 (7)

In mass%, C: 0.65-0.85%, Si: 0.05-0.4%, Mn: 0.5-2.0%, P: 0.005-0.03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01%, with the balance consisting of Fe and inevitable impurities,
(I) the hardness is 170 HV or less, and
(Ii) After continuous casting, hot rolling, pickling and annealing, in the plate thickness section of the structure before final cold rolling when cold rolling and annealing are performed once or twice, 0.5 μm ² or less of carbide area of state, and are within 15% of the total area of the carbide, and,
(Iii) the final cold rolling reduction ratio is Ru der than 30% to 15%
A soft high-carbon steel sheet with small punching characteristics.   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 a small punching resistance according to claim 1, characterized in that it contains at least one kind or two or more kinds.   Further, Nb: 0.01 to 0.5%, V: 0.01 to 0.5%, Ta: 0.01 to 0.5%, and W: 0.01 to 0.5 The soft high-carbon steel sheet having a small punching resistance according to claim 1 or 2, characterized by containing at least one kind or two or more kinds.   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 sheet with small punching according to any one of claims 1 to 3.   5. The soft high carbon steel plate with small punching according to any one of claims 1 to 4, wherein there are 100 or more voids per 1 mm <2> of the observed structure in the thickness cross section of the structure. A continuous cast slab satisfying the component composition according to any one of claims 1 to 4 is subjected to hot rolling after casting, directly or at 1300 ° C or lower for 90 minutes or less, and 800 to 940. Finish rolling at ℃, then strongly cool the hot-rolled steel sheet to 650 ° C at 30 ° C / s or more, and then slowly cool to 20 ° C / s or less until winding, at 400 to 650 ° C or less. taken Maki, after pickling, after performing softened box annealing, cold rolling and annealing method for manufacturing a 1 Kai施 to high-carbon steel, the cold-rolled after softening box annealing and softening box annealing A method for producing a soft high carbon steel sheet with small punching, characterized in that it is carried out under the following conditions.
(I) softening box annealing, after heating from room temperature up to a temperature of Ac ₁ ~Ac ₁ + 100 ℃, and held for 3 hours or more, then, in the course of cooling to Ar ₁ or less, the temperature of the heating and holding temperature to Ar ₁ In the range, cooling at 20 ° C./hr or less and holding for 0.5 hr or more after cooling are repeated once or more.
(Ii) The rolling reduction of cold rolling after softening box annealing is 15% or more and 30% or less. A continuous cast slab satisfying the component composition according to any one of claims 1 to 4 is subjected to hot rolling after casting, directly or at 1300 ° C or lower for 90 minutes or less, and 800 to 940. Finish rolling at ℃, then strongly cool the hot-rolled steel sheet to 650 ° C at 30 ° C / s or more, and then slowly cool to 20 ° C / s or less until winding, at 400 to 650 ° C or less. A method of manufacturing a high carbon steel sheet which is subjected to cold rolling and annealing twice after performing softening box annealing after scraping, pickling, and softening box annealing, cold rolling after softening box annealing, and A method for producing a soft high carbon steel sheet with small punching, characterized in that the treatment after cold rolling is performed under the following conditions.
(I) Softening box annealing is heated for 3 hours or more after heating from room temperature to a temperature of 680 ° C. or higher and Ac ₁ or lower.
(Ii) The rolling reduction of cold rolling after softening box annealing shall be 40% or more.
(Iii) The treatment after cold rolling is performed again at a temperature of 680 ° C. or more and Ac ₁ or less so that the ferrite particle diameter dα satisfies the following formula (1), and then the rolling reduction is 15% or more and 30% or less. subjected to cold rolling, then, subjected to a box grilled blunt.
dα> 2.5-1.7ln (C mass%) (1)