KR20000043777A - Mid-carbon steel excellent in quench and fine blanking workability and method of manufacturing the same - Google Patents

Abstract

PURPOSE: A method of manufacturing medium-carbon steel is provided to have fine blanking characteristic of low-carbon steel and to obtain quenching of high-carbon steel at the same time. CONSTITUTION: Steel consists of, by weight %: 0.15-0.35% of C, 0.1-0.5% of Si, 0.5-1.5% of Mn, 0.1-1.0% of Cr, not more than 0.005% of S, not more than 0.02% of P, 0.0005-0.005% of B, 2-7% of Ti/N, Fe and incidental impurities. The steel is hot-rolled at a temperature of 1240°C for 250 minutes or shorter and coiled at a temperature of 550-630°C to manufacture a hot-rolled steel plate. Finally, the manufactured steel plate is annealed.

Description

Medium carbon steel with excellent hardening heat treatment and fine blanking processability and its manufacturing method

The present invention relates to a medium carbon steel for manufacturing a medium carbon steel sheet used in automobile parts and mechanical parts, and more particularly, to a method for manufacturing a medium carbon steel sheet having high hardening heat treatment property and excellent fine blanking workability. will be.

In recent years, much of mechanical parts and automobile parts are manufactured by blanking. Blanking processing refers to shearing a steel sheet between a punch and a die and pressing it with a punch. The blanking type is generally divided into general blanking and fine blanking. In general blanking, the parts are simply machined by placing the steel plate on the die and punching it down. It is a technology that processes parts while simultaneously grabbing the material with a punch and pulling it downward. Parts manufactured by general blanking have a shear surface of 30%, a fracture surface of 70%, a poor blanking cross section, an inaccurate component, and slight deformation during processing. On the other hand, the parts processed by fine blanking have a very beautiful blanking cross section because the shear surface is 100%, the precision of the parts is the level of the machined parts, and no deformation occurs at all. In addition, since fine blanking does not require secondary processing compared to general blanking, the production process can be omitted, and the production of precision parts similar to the machining can be omitted, but the productivity is much higher than machining. .

Fine blanking differs from general blanking in that it is processed differently, so the characteristics of the material required for fine blanking are also different. The conditions for the fine blanking material should be uniform without anisotropy and large ductility.

There are various kinds of methods for producing fine blanking steel sheet, from low carbon steel to high carbon steel. Japanese Laid-Open Patent Publications (JP 58-104160 A) and Japanese Patent Publications (JP 5-14784 B2) provide methods for manufacturing fine blanking steel sheets using low carbon steels with relatively low carbon contents, but low-carbon steels have low hardness and ductility. The fine blanking property may be excellent, but the hard quenching heat treatment is poor, so it is difficult to obtain high hardness and high strength from the mechanical parts to the required parts, and there is a problem in that thick parts cannot be manufactured.

Japanese Patent Publication (JP 62-2008 B2) discloses a method for manufacturing high carbon fine blanking steel with high carbon content in order to improve the quenching heat treatment and to obtain high strength after heat inference. Even after spheroidizing annealing, the hardness and the ductility are low, so that the fine blanking property of the part is not only degraded, but the wear of the fine blanking mold is severe, which causes frequent mold replacement.

Accordingly, the present inventors have repeatedly conducted studies and experiments to obtain a steel having both high quenching heat treatment of high carbon steel and excellent fine blanking property of low carbon steel, and proposed the present invention based on the results. The medium carbon steel having the middle range of carbon steel is used to control the inclusions and microstructures so that they have excellent fine blanking characteristics at low carbon steel levels, and the addition of trace amounts of boron and alloying elements such as Mn and Cr It is an object of the present invention to provide a method for producing a medium carbon steel strip having both quench heat treatment and fine blanking properties by having quench heat treatment.

In order to achieve the above object, the present invention provides, by weight%, C: 0.15-0.35%, Si: 0.1-0.5%, Mn: 0.5-1.5%, Cr: 0.1-1.0%, S: 0.005% or less, and P: 0.02% or less, B: 0.0005 -0.005%, Ti / N: 2-7, and the steel material consisting of the remaining Fe and unavoidable impurities, and the steel first, the heating time is 250 minutes or less at a heating temperature of 1240 ℃ or less After hot rolling, a hot rolled steel sheet is manufactured at a coiling temperature of 550 ° C. to 630 ° C., and a method for producing a medium carbon steel sheet having excellent hardening heat treatment and fine blanking workability is performed by spheroidizing annealing the prepared hot rolled steel at a transformation temperature below A _c1. It characterized in that to provide.

In addition, the present invention is in weight%, C: 0.15-0.35%, Si: 0.1-0.5%, Mn: 0.4-1.5%, Cr: 0.1-1.0%, S: 0.010% or less, P: 0.02% or less, B : 0.0005-0.005%, Ti / N: 2-7, Ca: 0.001 -0.01%, and the steel material consisting of the balance Fe and unavoidable impurities, and the commercial steel, first, the heating time is 250 minutes at a heating temperature of 1240 ℃ or less After hot rolling, a hot-rolled steel sheet is manufactured at a coiling temperature of 550 ° C. to 630 ° C., and the annealed heat treatment and fine blanking workability are performed in which the prepared hot rolled steel sheet is spheroidized and annealed at a transformation temperature below A _c1. It provides a manufacturing method.

Hereinafter, the present invention will be described in more detail.

The present inventors have studied the working conditions of carbon, boron, Mn, Cr, S, Ca, and hot rolling conditions and spheroidizing annealing conditions in steel that affect the hardening heat treatment and fine blanking properties of medium carbon steel, It was found that steel having excellent quench heat treatment and fine blanking properties can be produced from steels containing boron, Mn, Cr, S, and Ca properly.

The present invention, starting from the above point of view, in a weight ratio of C: 0.15-0.35%, Si: 0.1-0.5%, Mn: 0.5-1.5%, Cr: 0.1-1.0%, S: 0.005% or less, P: 0.02% or less, B: 0.0005-0.005%, Ti / N: Medium carbon steel containing 2-7, consisting of remainder Fe and impurities,

Or by weight ratio: C: 0.15-0.35%, Si: 0.1-0.5%, Mn: 0.4-1.5%, Cr: 0.1-1.0%, S: 0.010% or less, P: 0.02% or less, B: 0.0005-0.005% , Ti / N: 2 to 7, Ca: 0.001-0.01%, and relates to a heavy carbon steel composed of balance Fe and impurities,

In addition, the present invention hot-rolled steel sheet of the composition as described above hot-rolled at a heating temperature of 1240 ℃ or less at a heating time of 250 minutes or less and under the usual hot rolling conditions to produce a hot rolled steel sheet with a temperature of 550 ℃ ~ 630 ℃ Thereafter, the present invention relates to a method for producing a medium carbon steel sheet in which spheroidizing annealing of a manufactured hot rolled steel sheet is performed at an _Acl transformation temperature or less.

Hereinafter, the reason for limiting the composition range of the inventive steel will be described.

C is 0.15-0.35% by weight (hereinafter referred to simply as "%") as the most important element for increasing the hardness of steel. In order to obtain the hardness (HRC: 30) required for the mechanical parts by hardening and tempering heat treatment, it is necessary to contain C at least 0.15%. However, if the amount of c is too high, the hardness after spheroidizing annealing becomes high and the ductility decreases, so the fine blanking property is greatly reduced.

Si is contained 0.1-0.5% as a very important element as a deoxidizer. If the amount of Si is too small, since there is little deoxidation effect, it contains 0.1% or more. However, if the amount of Si is too high, decarburization becomes severe during reheating, and red scale occurs, so that pickling is difficult, so that surface defects easily occur, and hardness is high due to solid solution strengthening and ductility is lowered, so it is limited to 0.5% or less.

Mn is an important element for improving the hardenability, and should be contained 0.5% or more to obtain the effect. However, when the content is increased, the structure in which the ferrite and ferrite are distributed in layers (hereinafter referred to as a band structure) is likely to occur, and the upper limit is limited to 1.5% because it suppresses spheroidization of cementite, increases hardness, and decreases ductility. .

Cr is an effective element that has less solid solution strengthening effect than Mn and improves hardenability without promoting the development of band structure. In the present invention, Cr was added mainly to suppress band structure development and to improve hardenability, and the lower limit thereof was 0.1%. However, when Cr is added too much, the hardness becomes high and the ductility falls, so the upper limit was made 1.0%.

S forms an emulsion and is elongated in the rolling direction during hot rolling to increase the anisotropy of the steel sheet, thereby greatly reducing the fine blanking property. The smaller the amount of S, the better. However, the upper limit was set at 0.005%, which is a range in which the fine blanking property is not significantly reduced.

The p was set to an upper limit of 0.02% because stabilization of cementite prevents spheroidization and is present together with Mn to segregate to promote band structure.

The B is an important element that segregates at the austenite grain boundary and suppresses the production of pearlite during the hardening heat treatment, thereby improving the hardenability, and the hardenability is increased when the B content is contained at least 0.0005%. However, when B is contained in an amount of 0.005% or more, the effect of improving the hardenability is small, and boron nitride or boron carbide is formed in the steel, thereby lowering the ductility.

The Ti is added in advance to prevent B from forming an oxide or nitride to lower the effect of improving hardenability. The ratio of Ti / N must be at least 2 to have the effect of hardenability. However, if the Ti / N ratio is too high, titanium carbonitride is formed, which increases the hardness of the matrix and decreases the ductility. The upper limit was set to seven.

The Ca was added to spheroidize the emulsion to reduce the anisotropy of the material, and at least 0.001% of Ca should be contained in order to obtain the Ca addition effect. However, if the amount of Ca is too high, the amount of Ca is limited to 0.1% or less because oxide-based nonmetallic inclusions increase to decrease ductility. On the other hand, even when Ca is added, the smaller the amount of S, the better. However, when Ca is present, S is precipitated as CaS rather than MnS, so the effect of S on material anisotropy is reduced. Therefore, when Ca is added, the upper limit of the amount of S is limited to 0.01%.

Hereinafter, the method of manufacturing a steel strip using the steel materials of the above components is demonstrated.

In the process of manufacturing a steel strip using the medium carbon steel of the above composition, in the hot rolling process, it is important to prevent surface decarburization and to suppress the band structure by miniaturizing the structure.

The surface decarburization reduces the hardenability of the high carbon steel to inhibit wear resistance and durability, as well as cause deformation during heat treatment. To prevent this, the reheat temperature should be lowered and the time saved. Surface decarburization of about 1.0% or less of the thickness of the hot-rolled steel sheet does not significantly affect the hardness and deformation of the final heat treatment material. For this purpose, the heating temperature is limited to 1240 ° C. or less, and the heating time is 250 minutes or less.

Band tissue suppression through the tissue micronization is important for improving spheroidizing annealing, fine blanking, ductility, heat treatment heat treatment. That is, the finer the structure and the more the band structure is suppressed, the more spheroidization of cementite during spheroidization annealing is achieved, thereby improving the fine blanking property and lowering the heat-treatment heat treatment. In the hot rolled steel, the tissue micronization is most dependent on the coiling temperature. The higher the coiling temperature, the more coarse the tissue and the more difficult the spheroidization. However, if the coiling temperature is too low, the structure is fine, but bainite is formed, the strength is too high, and even after spheroidizing annealing, the hardness of the fine blanking mold is greatly caused. Therefore, the coiling temperature was limited to 550 ℃ ~ 630 ℃, because the coarse structure and the band structure is developed above 630 ℃, the bainite structure below 550 ℃ increases the strength and hardness after spheroidization annealing.

In the present invention, the hot rolled steel sheet obtained through the hot rolling process as described above is subjected to spheroidizing annealing at or below A _cl transformation temperature, and then subjected to fine blanking using the obtained Kusao-annealed steel sheet, and the transformation temperature at which austenite transformation occurs. The product can be obtained by maintaining the normal quenching heat treatment temperature higher than 30 ° C. above the phosphorus A _c3 transformation temperature, then obtaining martensite structure through oil cooling and tempering to the hardness required for the final product.

Wherein the spheroidizing annealing in geongjeong the size of the cementite spheroidization annealing performed in the above A _cl transformation temperature is coarsened or a coarse pearlite is regenerated fine blanking property may be degraded, or heat treatment quenching property is deteriorated. Therefore, the nodular annealing temperature is below the A _cl transformation temperature.

The temperature and time of the quenching heat treatment can be applied to the usual conditions, in order to obtain the hardness required for the machine parts, the cementite is kept sufficiently dissolved at a temperature of 30 ℃ or more of the _A3 transformation temperature and then quenched in oil and the final product It is preferable to perform tempering to obtain a hardness of.

The present invention will be described in more detail with reference to the following examples.

Example 1

Steels having the components shown in Table 1 were prepared. Steel grade No. 1-5 is a steel within the chemical composition specified in the present invention, and steel grade No. 6-13 is a steel melted for comparison in the present invention.

In order to derive the reheating condition to suppress the surface decarburization of the hot rolled steel sheet, using the invention steel 1 of Table 1, the heating temperature was constant at 1240 ° C. and the heating time was measured at 220 minutes, 250 minutes, and 280 minutes. Is shown in Table 2. As shown in Table 2, it can be seen that the depth of the decarburized layer increases as the heating time increases, and the heating conditions for obtaining the decarburized layer within 1.0% of the thickness of the hot-rolled steel sheet targeted by the present invention are the heating temperature. It can be seen that less than 250 minutes at 1240 ℃ or less.

Example 2

In this embodiment, in order to derive the coiling condition after hot rolling, the heating temperature was constant at 1240 ° C. using the inventive steel 1 shown in Table 1, and the heating time was heated at 230 minutes. After finishing-rolling to 5.0 mm, it wound up at 500 degreeC, 600 degreeC, and 650 degreeC, respectively, and produced the hot rolled steel plate of thickness 5.0mm. The hot rolled steel sheet was pickled and spheroidized annealing was performed in a non-oxidizing atmosphere to prepare a spheroidized annealing steel sheet. Spherical annealing was carried out for 10 hours at 710 ° C. lower than A _cl transformation temperature, which is the normal condition. The hardness was measured for the hot rolled steel sheet thus prepared, and the hardness and the fine blanking property were measured for the spheroidized steel sheet. The method of measuring fine blanking property was manufactured by directly manufacturing automotive door parts with a thickness of 5.0mm by using a fine blanking press.If the shear surface is 90% or more from the manufactured product, it is good (marked ◎), and if the shear surface is 90% or less, ○) and the number of measurement parts was set to 50 pieces each. Table 3 shows the results. As shown in Table 3, when the coiling temperature is 500 ° C, the hot rolled steel sheet has a bainite structure, and after the spheroidizing annealing, carbides are finely distributed and excellent in fine blanking, but the hardness is too high, resulting in a shorter mold life. . When the coiling temperature of the invention condition is 600 ℃, ferrite and pearlite are finely distributed in the hot-rolled steel sheet, after spheroidizing annealing, the cementite is unevenly distributed, so that the hardness is low but the fine blanking property is not excellent. Therefore, it can be seen that the winding temperature range of the mold life aimed at the present invention and excellent fine blanking property is in the range of 550 ~ 630 ℃.

Example 3

Next, the steels of Table 1 were heated at a heating temperature of 1240 ° C. for 230 minutes, followed by finish rolling to a thickness of 5.0 mm at 850 ° C. under normal hot rolling conditions, and then wound at 600 ° C. The wound hot rolled sheet was pickled and spheroidized annealing was performed in a non-oxidizing atmosphere. Spherical annealing was performed at 710 degreeC which is A _c1 temperature or less for 10 hours. The fine blanking process was performed using a spheroidized annealed steel sheet, followed by heating at 880 ° C. for 30 minutes, followed by ignition of 60 ° C. temperature. Hardness and fine blanking properties were measured for spherical annealed steel sheets, and hardenability of the hardened steel sheets was measured to evaluate hardenability. The fine blanking evaluation method is the same as the method mentioned in Example 2.

The results measured as described above are shown in Table 4 and Table 7 to distinguish the characteristics of each alloy element for convenience. At this time, the hardness after spheroidizing annealing of automotive parts and mechanical parts targeted by the present invention is Hv 160 or less, and the fine blanking property should be excellent in shear surface of 90% or more, and when tempered at 400 ℃ for 1 hour after hardening Becomes Hv 300 or more. For this purpose, the hardening hardness of the core must be Hv 500 or more.

Table 4 looks at the impact of carbon. As shown in Table 4, when the carbon content is 0.1%, the fine blanking property is good, but the particle size is too low than the target value, and when the carbon content is 0.45%, the particle size is satisfied, but the hardness is too high in the spheroidized annealing state. It can be seen that the castle is not excellent. Therefore, it can be seen that the carbon range of 0.15-0.35% is appropriate for simultaneously obtaining the target fine blanking properties and hardness.

Table 5 looks at the effect of Cr. As shown in Table 5, when the Cr content is 0.7%, the hardness increase after the spheroidal annealing is not very high and the fine blanking property is good. However, when the Cr amount is 1.5%, the Cr suppresses the spheroidization of the cementite, the hardness of the spheroidizing annealing is very high and the pin is increased. It can be seen that the ranking property is also reduced.

Table 6 looks at the effects of the amount of B and Ti. As shown in Table 6, B and Ti do not have a significant effect on spheroidizing annealing or fine blanking properties, but when B is not added, the particle size decreases considerably, since B is greatly improved even when B is added in a small amount. And when B is added but not Ti, B is mainly precipitated as BN, and it can be seen that it does not contribute significantly to the improvement of quenchability. In other words, the purpose of Ti addition is that Ti has greater affinity for nitrogen (N) than B, so that before forming N in the steel, B forms TiN so that B exists in the solute state, thereby improving the hardenability.

Table 7 looks at the effects of S amount and Ca amount. As shown in Table 7, when Ca is not added, fine blanking property is good when S is 0.004%, but fine blanking property is decreased when S is 0.01%. This is because S combines with Mn to form MnS inclusions, because MnS inclusions are ductile and elongate easily during hot rolling. On the other hand, when Ca is added, the fine blanking property is improved even if the amount of S is slightly higher. However, when S is too large, the amount of emulsion that is stretched even when Ca is added increases the fine blanking property. Therefore, in order to secure the fine blanking property, it is necessary to lower S to 0.005% or less, but when S cannot be lowered to 0.005% or less, the fine blanking property can be improved by spheroidizing inclusions with Ca. However, even when Ca is added, too much S deteriorates the fine blanking property, so S must be 0.01% or less even in Ca-added steel.

Chemical composition (wt%) Remarks C Si Mn Cr S P B Ti N Ca Invention steel One 0.2 0.3 1.0 0.2 0.004 0.015 0.002 0.02 0.005 - basic 2 0.3 0.28 0.95 0.21 0.003 0.014 0.002 0.018 0.008 - C effect 3 0.2 0.25 0.95 0.7 0.003 0.018 0.002 0.019 0.005 - Cr effect 4 0.2 0.27 1.2 0.2 0.003 0.013 0.004 0.015 0.004 - B effect 5 0.2 0.3 1.0 0.22 0.008 0.014 0.002 0.018 0.004 0.004 Ca effect Comparative steel 6 0.1 0.2 0.9 0.2 0.003 0.015 0.002 0.015 0.004 - C effect 7 0.48 0.21 1.0 0.2 0.004 0.015 0.002 0.016 0.005 - 8 0.2 0.25 2.0 0.2 0.004 0.016 0.002 0.018 0.004 - Mn effect 9 0.2 0.2 1.0 1.5 0.004 0.015 0.002 0.017 0.004 - Cr effect 10 0.2 0.23 1.0 0.2 0.01 0.015 0.002 0.016 0.004 - S effect 11 0.2 0.2 1.0 0.2 0.015 0.015 0.002 0.018 0.005 0.004 Ca effect 12 0.2 0.25 1.0 0.2 0.004 0.015 0.002 0 0.005 - Ti effect 13 0.2 0.23 1.0 0.2 0.004 0.015 0 0.02 0.004 - B effect

Decarburization layer depth in hot rolled steel sheet according to reheating condition of steel grade No.1 (mild steel sheet thickness: 5.0㎜) Reheating conditions Decarburized Layer Depth (㎛) Remarks Condition 1 1240 ℃ × 220 minutes 45 Invention Condition Condition 2 1240 ° C × 250 minutes 51 Comparative condition Condition 3 1240 ° C × 280 minutes 58 Comparative condition goal 50 1.0% of steel plate thickness

Table 3 shows the structure and hardness of the hot rolled steel sheet according to the coiling temperature of steel grade No. 1

Hardness and fine blanking property (steel plate thickness 5.0mm) of a spheroidized annealed steel sheet are shown.

Winding temperature (℃) Hot Rolled Sheet Hardness (Hv) Hot Rolled Sheet Microstructure Spherical hardened plate hardness (Hv) Spherical annealed plate fine blanking Remarks 500 230 Bainite 170 ◎ (★) Comparative condition 600 200 Ferrite-Perlite 150 ◎ Invention Condition 650 175 Band structure 135 ○ Comparative condition

◎: Shear surface 90% or more ○: Shear surface 90% or less

★: High hardness, shortened life span

Effect of Carbon Content on Spherical Annealing Hardness, Fine Blanking, Hardening Hardness and Tempering Hardness (Steel Thickness: 5.0㎜) C amount (wt%) Hardness after spheroidization (Hv) Fine Blanking Hardness Hardness (Hv) Tempering Hardness (Hv) Steel grade Inventive Example 1 0.2 150 ◎ 510 340 Inventive Steel 1 Inventive Example 2 0.3 156 ◎ 550 370 Inventive Steel 2 Comparative Example 6 0.1 140 ◎ 400 250 Comparative Steel 6 Comparative Example 7 0.45 170 ○ 600 400 Comparative Steel 7

◎: Fine blanking property is good, ○: Normal

Table 5 shows the effect (chromium thickness: 5.0 mm) on the amount of chromium on spheroidizing annealing, fine blanking, quenching and tempering.

Cr amount (wt%) Hardness after spheroidization (Hv) Fine Blanking Hardness Hardness (Hv) Tempering Hardness (Hv) Steel grade Inventive Example 1 0.2 150 ◎ 510 340 Inventive Steel 1 Inventive Example 3 0.7 155 ◎ 530 360 Invention Steel 3 Comparative Example 9 1.5 170 ○ 560 390 Comparative Steel 9

◎: Fine blanking property is good, ○: Normal

Effect of Boron and Titanium on Spherical Annealing Hardness, Fine Blanking, Hardening Hardness and Tempering Hardness (Steel Thickness: 5.0 ㎜) B amount (wt%) Ti amount (wt%) Hardness after spheroidization (Hv) Fine Blanking Hardness Hardness (Hv) Tempering Hardness (Hv) Steel grade Inventive Example 1 0.004 - 150 ◎ 510 340 Inventive Steel 1 Comparative Example 10 0.01 - 152 ○ 515 350 Inventive Steel 4 Inventive Example 5 0.008 0.004 153 ◎ 505 210 Comparative Steel 13 Comparative Example 11 0.015 0.004 155 ○ 511 240 Comparative Steel 12

◎: Fine blanking property is good, ○: Normal

Effect of S and Ca content on tempering hardness on spheroidizing annealing, fine blanking, quenching hardness and tempering hardness (steel thickness: 5.0 ㎜) S amount (wt%) Ca amount (wt%) Hardness after spheroidization (Hv) Fine Blanking Hardness Hardness (Hv) Tempering Hardness (Hv) Steel grade Inventive Example 1 0.004 - 150 ◎ 510 340 Inventive Steel 1 Comparative Example 10 0.01 - 152 ○ 515 342 Comparative Steel 10 Inventive Example 5 0.008 0.004 153 ◎ 505 345 Inventive Steel 5 Comparative Example 11 0.0015 0.004 155 ○ 511 343 Comparative Steel 11

◎: Fine blanking property is good, ○: Normal

As described above, according to the present invention, it has an intermediate carbon content between low carbon steel and high carbon steel, and has excellent fine blanking at low carbon steel level through control of inclusions and microstructures, and also adds trace boron and alloys. By adding the element to secure the hardening heat treatment of high carbon steel at the same time, it is possible to obtain the safety of mechanical parts and automobile parts, and to provide the effect of increasing the demand for medium carbon steel strip and coping with various demands of the demand.

Claims (4)

By weight%, C: 0.15-0.35%, Si: 0.1-0.5%, Mn: 0.5-1.5%, Cr: 0.1-1.0%, S: 0.005% or less, P: 0.02% or less, B: 0.0005-0.005% , Ti / N: A medium-carbon steel with excellent heat treatment heat treatment and fine blanking workability, characterized in that it comprises 2-7, the balance Fe and unavoidable impurities. The steel material having the composition of claim 1 is first hot-rolled at a heating temperature of 1240 ° C. or less with a heating time of 250 minutes or less, and then a hot rolled steel sheet is prepared at a winding temperature of 550 ° C. to 630 ° C. _A method for producing a medium carbon steel sheet having excellent heat treatment heat treatment and fine blanking workability, characterized by performing spheroidizing annealing at a transformation temperature below _c1 . By weight%, C: 0.15-0.35%, Si: 0.1-0.5%, Mn: 0.4-1.5%, Cr: 0.1-1.0%, S: 0.010% or less, P: 0.02% or less, B: 0.0005-0.005% , Ti / N: 2-7, Ca: 0.001-0.01%, heavy carbon steel with excellent heat treatment heat treatment and fine blanking workability, characterized in that consisting of the remaining Fe and unavoidable impurities. The steel material having the composition of claim 3 is first hot-rolled at a heating temperature of 1240 ° C. or less with a heating time of 250 minutes or less, and then a hot rolled steel sheet is prepared with a winding temperature of 550 ° C. to 630 ° C. _A method for producing a medium carbon steel sheet having excellent heat treatment heat treatment and fine blanking workability, characterized by performing spheroidizing annealing at a transformation temperature below _c1 .