KR100742951B1 - Baking hardening type cold rolled steel sheet having good formability and process for producing the same - Google Patents

Abstract

Provided is a cold rolled steel sheet and a method of manufacturing the same, which show improved yield strength and in-plane anisotropy by fine (Mn, Cu) S precipitates in IF steel.

The cold rolled steel sheet is, in weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, Mn: 0.01-0.3%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004% or less, P : 0.2% or less, B: 0.0001-0.002%, Nb: 0.001 ~ 0.05%, remaining Fe and other inevitable impurities,

Cu, Mn, S, C, Nb is Cu + Mn: 0.08-0.4%, (Mn / 55 + Cu / 63.5) / (S / 32): 1-30,

Cs (solute carbon): Meets 5-30

Where Cs = (C-Nbx12 / 93) x10000)

The average size of the (Mn, Cu) S precipitates is 0.2 탆 or less.

In the present invention, unlike the technique of adding Cu for corrosion resistance in IF steel or the technique of using Cu as the precipitated phase of ε-Cu, MnS precipitates are used together with Cu as fine CuS precipitates.

Stiffening hardening type, automotive material, (Mn, Cu) S precipitate, in-plane anisotropy, yield strength

Description

BAKING HARDENING TYPE COLD ROLLED STEEL SHEET HAVING GOOD FORMABILITY AND PROCESS FOR PRODUCING THE SAME}

Japanese Unexamined Patent Publication No. 1994-240365

Japanese Unexamined Patent Publication No. 1995-216460

The present invention relates to a hardened hardened cold rolled steel sheet used as a material for automobiles, home appliances, and more particularly, cold rolled steel sheet having improved yield strength and in-plane anisotropy by fine (Mn, Cu) S precipitates in IF steel. It relates to a manufacturing method.

In addition, the hardening type cold rolled steel sheet is used for exterior materials such as automobiles in order to increase dent resistance. The hardened hardened cold rolled steel is a steel in which an appropriate amount of solid solution carbon remains in the steel sheet and the potential generated during press molding is fixed to solid solution carbon using heat from the coating furnace to increase the yield point.

There are Al-Killed steel and IF steel (Interstitial Free Steel).

In the case of Al-Killed steel, which is an ordinary annealing material, a small amount of dissolved carbon remains, and it has a hardening hardening capacity of about 10-20 MPa after the calcination treatment while securing aging characteristics. In the case of the annealing material, the yield strength rising after the baking treatment is low, and there is a disadvantage in that the productivity is low because it is annealed for a long time.

IF steel is a steel grade which adds Ti and Nb to improve the formability by completely depositing carbon or nitrogen dissolved in steel, and it is the hardening hardening IF steel that gives the hardening hardening characteristic to the IF steel. The baking hardening type IF steel controls the adding amount of Ti or Nb and the adding amount of carbon so that an appropriate amount of carbon remains in the steel to give the baking hardening characteristic. As related technologies, there are Japanese Patent Application Laid-open Nos. 1994-240365 and 1995-216460.

Japanese Laid-Open Patent Publication No. 1994-240365 combines Cu and P in order to secure corrosion resistance in a hardened type IF steel.

Japanese Laid-Open Patent Publication No. 1995-216340 discloses hot rolling while adding a small amount of Ti to the extent that the solid solution C cannot be fixed to TiC in a small hardening type IF steel including a combination of Cu and P and Nb as necessary to secure corrosion resistance. It is proposed that excellent control of depth can be obtained by controlling the conditions.

The above prior arts are steels in which Cu is added in an amount of 0.05-1.0% but actually 0.2% or more, in order to secure corrosion resistance in the hardening type IF steel. They do not have a review of in-plane anisotropy. When the in-plane anisotropy is low, wrinkles are less generated during processing, and there is less advantage of generating ears after processing. In addition, the yield ratio (yield strength / tensile strength) is not very high in the prior arts. If the yield strength is higher than the same strength, the thickness of the steel sheet can be reduced, thereby reducing the weight. Steel grades with high yield ratios in the prior arts are component systems designed with high P content and Cu content. If the content of P is high, the plating property is not good. If the content of Cu is high, the production cost is high.

It is an object of the present invention to provide a cold rolled steel sheet and a method of manufacturing the same, which can lower in-plane anisotropy while enhancing yield strength by (Mn, Cu) S precipitates in a small hardening type IF steel.

Cold rolled steel sheet of the present invention for achieving the above object,

By weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, Mn: 0.01-0.3%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.001-0.05%, remaining Fe and other inevitable impurities,

Cu, Mn, S, C, Nb is Cu + Mn: 0.08-0.4%, (Mn / 55 + Cu / 63.5) / (S / 32): 1-30,

Cs (solute carbon): Meets 5-30

Where Cs = (C-Nbx12 / 93) x10000)

The average size of the (Mn, Cu) S precipitates is 0.2 탆 or less.

In the present invention, the precipitate is preferably ¹ × 10 ⁶ / mm ² or more, more preferably 1 × 10 ⁷ / mm ² or more.

The cold rolled steel sheet of the present invention has the characteristics of a soft cold rolled steel sheet of 280MPa grade and high strength cold rolled steel sheet of 340MPa or more according to the component design.

When the content of P in the above component system is 0.015% or less, a soft cold rolled steel sheet of 280 MPa grade is obtained. The cold rolled steel sheet further contains one or two of the solid solution strengthening elements Si and Cr, or when the P content is 0.015 to 0.2%, high strength characteristics of 340 MPa or more are secured. In the case of high strength steel containing P alone, the content of P is preferably 0.03 to 0.2%. 0.1-0.8% for Si and 0.2-1.2% for Cr are preferred. In the case of containing at least one of Si and Cr, the content of P may be variously designed in the range of 0.2% or less.

If you want to improve the workability in the cold rolled steel sheet of the present invention may further comprise Mo 0.01 ~ 0.2%.

In the method for producing a cold rolled steel sheet, the slab that satisfies the component system of the present invention is reheated to a temperature of 1100 ° C. or higher, and then hot-rolled at a finish rolling temperature of Ar ₃ or higher and cooled at a speed of 300 ° C./min or higher, and 700 ° C. After winding up at the following temperature, it cold-rolls and continuously anneales.

Hereinafter, the present invention will be described in detail.

In the present invention, (Mn, Cu) S precipitate is an expression including MnS precipitate, CuS precipitate or a composite precipitate of Mn and Cu.

The present invention is completed on the basis of the results of the study that when the (Mn, Cu) S precipitates are secured in the small hardening type IF steel, the grains become fine and the yield strength is enhanced and the in-plane anisotropy index is lowered to improve workability. Cu is mainly used as an element to improve corrosion resistance in IF steel, and the amount of addition is actually 0.2% or more. Or the technique which uses Cu as a precipitated phase of (epsilon) -Cu is known. However, there is no known technique for securing fine CuS precipitates or MnS precipitates in IF steel to achieve grain refinement.

In the present invention, it is useful to utilize S managed as an impurity in the IF steel to secure fine (Mn, Cu) S. S is preferentially precipitated with Ti and Zr, and the remaining amount of S is precipitated as (Mn, Cu) S. Since the IF steel of the present invention is an Nb-only IF steel to which no sulfide forming elements such as Ti and Zr are added, S is precipitated as (Mn, Cu) S according to the addition conditions of Mn, Cu, and S.

The (Mn, Cu) S precipitate makes the grains fine. On the other hand, the solid solution carbon not precipitated by Nb is more present in the grain boundary than in the grains to ensure the room temperature non-aging characteristics to improve the baking hardening characteristics. The dissolved carbon remaining in the grains is relatively free to move, which affects the aging characteristics in combination with the operating potential. On the other hand, solid solution carbon segregating at a more stable position, such as grain boundaries or precipitates, is activated at high temperatures such as coating baking treatment, thereby affecting the baking hardening characteristics. As such, the decrease in the amount of solid solution carbon in the grains means that carbon exists in a more stable position, that is, around grain boundaries or fine precipitates, thereby affecting the hardening characteristic.

The finely distributed (Mn, Cu) S precipitates according to the present invention have a positive effect on the increase in yield strength due to precipitation strengthening, improvement of strength-ductility balance characteristics, and in-plane anisotropy index. For this purpose, if the (Mn, Cu) S precipitates should be finely distributed, the content of Mn, Cu and S, their component ratio conditions, and the manufacturing conditions, in particular, the cooling rate after the hot rolling is affected.

First, C, Cu, Mn, S, Al, P, N, B, and Nb serving as basic components will be described.

The content of carbon (C) is preferably 0.001-0.01%.

If the content of carbon (C) is less than 0.001%, the amount of hardening of baking is small, and if it is more than 0.01%, moldability is lowered. The higher the carbon content, the larger the hardened portion. Considering this, the carbon (C) content is more preferably 0.003-0.01%, or 0.005-0.01%.

The content of copper (Cu) is preferably 0.01 to 0.2%.

Cu forms fine CuS precipitates to finer grains, lowering the in-plane anisotropy index and enhancing yield strength by precipitation strengthening. For this purpose, the Cu content must be more than 0.01% to precipitate finely, and when it exceeds 0.2%, it is coarsened. Preferable Cu content is 0.03-0.2%.

The content of manganese (Mn) is preferably 0.01-0.3%.

Manganese is known as MnS to prevent hot shortness caused by solid sulfur by precipitating sulfur in solid state in steel. From this technical point of view, it is common to add a high content of manganese. However, in the present invention, when the sulfur content is appropriate while lowering the content of manganese, MnS precipitates very finely, thereby improving the properties of plastic anisotropy and in-plane anisotropy by grain refinement and improving the yield strength characteristics by precipitation strengthening. Based on the results of the study, the content of manganese should be less than 0.2%. In order to secure these characteristics, the content of manganese should be 0.01% or more. If the content is less than 0.01%, red brittleness may occur due to the large amount of sulfur remaining in the solid state, and the content of manganese exceeds 0.3%. In the case of, the content of manganese is high and coarse MnS precipitates are formed, making it difficult to secure strength. More preferably, the content of Mn is made 0.2% or less.

The content of sulfur (S) is preferably 0.005-0.08%.

Sulfur (S) reacts with Cu to form fine (Mn, Cu) S precipitates. When the content of S is less than 0.005%, not only the amount of precipitates precipitated is small but also the number of precipitates precipitated is very small. If the content of sulfur is more than 0.08%, the content of the solid solution of sulfur is so high that the ductility and formability is greatly lowered, there is a fear of red brittleness.

The content of aluminum (Al) is preferably 0.1% or less.

Aluminum is an element added as a deoxidizer, but it is added to precipitate nitrogen in the steel to completely prevent aging by solid nitrogen. If the aluminum content is more than 0.1%, the amount of aluminum present in the solid solution state is high, thereby reducing the ductility. Preferred Al content is 0.01-0.1%.

The content of nitrogen (N) is preferably 0.004% or less.

Nitrogen is an element that is inevitably added during steelmaking, and is preferably managed at 0.004% or less.

The content of phosphorus (P) is preferably 0.2% or less.

Phosphorus is an element having a high solid solution strengthening effect and a small decrease in r value, and guarantees high strength in steels for controlling precipitates according to the present invention. In steel grades requiring strength of 280 Mpa, the content of P should be less than 0.015%. For high strength steel of 340Mpa or higher, it is recommended to set it as 0.016 ~ 0.2%. If the content of P is more than 0.2%, it is preferable that the ductility is lowered so as to limit the upper limit to 0.2%. When Si and Cr are added in the present invention, the P content can be designed in various strengths while keeping the content of P or less within 0.2%.

The content of boron (B) is preferably 0.0001 to 0.002%.

Boron is added to prevent secondary processing brittleness. For this purpose, the boron content is preferably 0.0001% or more. If the boron content exceeds 0.002%, deep drawing may be greatly degraded.

The content of niobium (Nb) is preferably 0.001 to 0.05%.

Nb is added for the purpose of ensuring inaging and improving moldability. Nb is a strong carbide generating element that is added to steel to precipitate NbC precipitates. In addition, NbC precipitates have an effect of greatly improving the processing ability of the soil by developing the aggregated structure during annealing. When the amount of Nb added is less than 0.001%, the amount of precipitation of NbC precipitates is too small, so that there is little effect of improving the processability of Omrim because of less development of texture. When Nb exceeds 0.05%, the amount of NbC precipitates is too high, resulting in low processability and elongation, which may significantly reduce moldability.

In the present invention, (Mn, Cu) S precipitates are affected by the component ratios of Mn, Cu, and S. Namely, the amount of Mn + Cu is 0.4% or less, preferably 0.08-0.4%.

Moreover, it is preferable to satisfy (Mn / 55 + Cu / 63.5) / (S / 32): 1-30. Here, Cu, Mn and S are weight%.

When the value of (Mn / 55 + Cu / 63.5) / (S / 32) is 1 or more, the effective (Mn, Cu) S precipitates are precipitated. .

In the present invention, Nb and C can be a component design in terms of non-aging and baking properties. When Nb precipitates C as NbC, the component must be designed to secure solid carbon which can secure the baking hardening characteristics. That is, Cs (solute carbon) should satisfy 5-30. Cs is determined by the following relationship.

Cs = (C-Nbx12 / 93) x10000

Here, C and Nb are by weight, and the value calculated as Cs, that is, the content unit of solid solution carbon is ppm.

If the Cs value is 5ppm or more, it is possible to secure the hardening of the baking. If it exceeds 30ppm, it is difficult to secure the non-aging due to the high content of solid solution carbon.

The finer the precipitates in the component system of the present invention, the more advantageous it is. Preferably, the average size of the (Mn, Cu) S precipitates is 0.2 μm or less. According to the results of the present invention, especially when the average size of the precipitate is more than 0.2㎛, the strength is low, the in-plane anisotropy index is not good.

Furthermore, although the precipitate of 0.2 micrometer or less is distributed in a large amount in the component system of this invention, the distribution number is not specifically limited. Preferably, the number of distribution of the precipitates is at least 1 × 10 ⁶ per mm ² , more preferably at least 1 × 10 ⁷ / mm ² . The larger the distribution of precipitates, the higher the plastic anisotropy index and the lower in-plane anisotropy index are. In general, when the plastic anisotropy index increases, the in-plane anisotropy index rises and there is a limit to increasing the plastic anisotropy index in terms of processability. .

In the present invention, when applied to a high-strength steel sheet of 340 MPa grade or more, one or two or more solid solution strengthening elements such as P, that is, P, Si, and Cr may be added. As described above with respect to P, redundant descriptions are omitted.

The content of silicon (Si) is preferably 0.1-0.8%.

Si is an element having a high solid solution strengthening effect and a low drop in elongation, which ensures high strength in steels for controlling precipitates according to the present invention. When the content of Si is more than 0.1% to secure the strength, in the case of more than 0.8% ductility is reduced.

The content of chromium (Cr) is preferably 0.2 to 1.2%.

Cr is an element that lowers the secondary brittleness temperature and decreases the aging index by Cr carbide while having a high solid-solution strengthening effect, and assures high strength in steels for controlling precipitates according to the present invention and also lowers in-plane anisotropy index. The Cr content is more than 0.2% to secure the strength, in the case of more than 1.2% ductility is reduced.

In the cold rolled steel sheet of the present invention, molybdenum (Mo) may be further added.

The content of molybdenum (Mo) is preferably 0.01 to 0.2%.

Mo is added as an element to increase the plastic anisotropy index, the content of the plastic anisotropy index is increased when the content is more than 0.01%, if the content exceeds 0.2%, the plastic anisotropy index is no longer increased and there is a risk of causing hot brittleness.

[Manufacturing method of cold rolled steel sheet]

The present invention is characterized by satisfying the mean size of (Mn, Cu) S precipitates in a cold rolled sheet through hot rolling and cold rolling to satisfy the above-described stress. The average size of (Mn, Cu) S precipitates in the cold rolled plate is influenced by the component design and manufacturing processes such as reheating temperature and winding temperature, but especially by the cooling rate after hot rolling.

[Hot Rolling Condition]

In the present invention, the steel that satisfies the above-mentioned emphasis is reheated and hot rolled. The reheating temperature is preferably 1100 ° C or more. This is because when the reheating temperature is lower than 1100 ° C., the coarse precipitates generated during continuous casting remain completely insoluble due to the low reheating temperature, so that many coarse precipitates remain even after hot rolling.

Hot rolling is preferably carried out under the conditions of the finish rolling temperature higher than the Ar ₃ transformation temperature. This is because when the finish rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is reduced by the formation of the rolled grain but also the strength is lowered.

It is preferable that the cooling rate before winding after hot rolling shall be 300 degreeC / min or more. Even if the component ratio is controlled to obtain a fine precipitate according to the present invention, if the cooling rate is less than 300 ° C / min, the average size of the precipitate may exceed 0.2 ㎛. In other words, as the cooling rate increases, a large number of nuclei are generated and the precipitate becomes fine. The faster the cooling rate, the finer the precipitate is, so it is not necessary to limit the upper limit of the cooling rate.However, even if the cooling rate is faster than 1000 ° C / min, the finer effect of the precipitate is no longer increased, so the cooling rate is 300 to 1000 ° C / min. This is more preferable.

[Coiling condition]

Winding is performed after hot rolling as above, but the winding temperature is preferably 700 ° C or lower. If the coiling temperature exceeds 700 ℃, precipitates grow too coarse, making it difficult to secure strength.

[Cold rolling condition]

Cold rolling is preferably performed at a reduction ratio of 50 to 90%. If the cold reduction rate is less than 50%, the amount of nucleation of the annealing recrystallization is small, so that grains grow too large during annealing, resulting in a decrease in strength and formability due to coarsening of the annealing recrystallization grains. If the cold reduction ratio is more than 90%, the moldability is improved, but the nucleation amount is too high, so the annealing recrystallized grain is too fine to decrease the ductility.

[Continuous Annealing]

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 700-900 degreeC. If the continuous annealing temperature is less than 700 ° C., recrystallization is not completed and the target ductility value cannot be secured. If the annealing temperature is more than 900 ° C., the strength decreases due to coarsening of the recrystallized grains. The continuous annealing time keeps the recrystallization complete. If it is about 10 seconds or more, the recrystallization is completed. Preferably the continuous annealing time is in the range of 10 seconds to 30 minutes,

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

The steel slabs of Table 1 were reheated, hot rolled to finish, cooled to 400 ° C./min, wound up at 650 ° C., and then cold rolled and continuously annealed at a reduction rate of 75%. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 830 ℃ to 10 ℃ / second.

The obtained annealing plate was processed into a standard specimen according to ASTM E-8 standard to investigate the mechanical properties. The specimen was measured for yield strength, tensile strength, elongation, plastic anisotropy index (r _m value), in-plane anisotropy index (Δr value) and aging evaluation index using a tensile tester (INSTRON, Model 6025). Where r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, △ r = (r ₀ -2r ₄₅ + r ₉₀ ) / 2, and the aging evaluation index is 100 ° C for 1.0% skin pass-rolled specimen after annealing X 2hr. Yield point elongation rate measured after heat treatment.

The quench hardening properties were obtained by adding 2% strain to the specimen and measuring the yield strength after heat treatment at 170 ° C. for 20 minutes, and subtracting the yield strength value before heat treatment as the BH value.

division Chemical composition (% by weight) C Mn P S Al Cu Nb B N Etc A1 0.0018 0.1 0.008 0.009 0.035 0.082 0.003 0.0007 0.0013 - A2 0.0022 0.12 0.026 0.011 0.042 0.1 0.004 0.0005 0.0022 - A3 0.0018 0.07 0.044 0.012 0.029 0.068 0.003 0.0008 0.0027 Si: 0.05 A4 0.0028 0.11 0.082 0.013 0.043 0.084 0.006 0.0006 0.0023 Si: 0.26 A5 0.0039 0.07 0.12 0.011 0.035 0.11 0.008 0.0008 0.0023 Si: 0.39 A6 0.0028 0.09 0.083 0.012 0.033 0.15 0.006 0.0005 0.0021 Si: 0.27 Mo: 0.085 A7 0.0037 0.08 0.073 0.01 0.052 0.13 0.008 0.0009 0.0015 Si: 0.33 Cr: 0.28 A8 0.0025 0.42 0.055 0.009 0.043 0 0.038 0.005 0.0024 - A9 0.0039 0.07 0.115 0.011 0.036 0.11 0.003 0 0.0027 Si: 0.1

division Cu + Mn (Mn / 55 + Cu / 63.5) / (S / 32) Cs Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) A1 0.18 11.1 14.129 0.06 1.1 X 10 ⁷ A2 0.22 10.9 16.839 0.06 1.8 X 10 ⁷ A3 0.14 6.25 14.129 0.06 1.5X10 ⁷ A4 0.19 8.18 20.258 0.05 2.5 X 10 ⁷ A5 0.18 8.74 28.677 0.05 3.2 X 10 ⁷ A6 0.24 10.7 20.258 0.05 2.5 X 10 ⁷ A7 0.21 11.2 26.677 0.04 4.4 X 10 ⁷ A8 0.42 27.2 -24.03 0.25 6.5X10 ⁴ A9 0.18 8.74 35.129 0.06 5.3 X 10 ⁶ Cs = (C-Nbx12 / 93) x10000

Sample Number Mechanical properties Remarks Yield strength (MPa) Tensile strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (Δr) Aging Index (%) BH value (MPa) 2nd processing brittleness (DBTT- ℃) A1 190 308 48 1.91 0.31 0 35 -50 Invention steel A2 209 329 46 1.85 0.27 0 41 -50 Invention steel A3 227 347 46 1.82 0.22 0 36 -70 Invention steel A4 295 396 39 1.73 0.21 0 45 -60 Invention steel A5 341 463 33 1.53 0.14 0 58 -50 Invention steel A6 331 446 35 1.71 0.22 0 51 -50 Invention steel A7 355 457 34 1.59 0.18 0 46 -50 Invention steel A8 196 350 40 1.85 0.29 0 0 -50 Comparative steel A9 369 451 32 1.29 0.198 2.5 95 -30 Comparative steel

In the present invention, the above embodiment is only one example, and the present invention is not limited thereto. Anything that has substantially the same configuration as the technical idea described in the claims of the present invention and achieves the same operation and effect is included in the technical scope of the present invention.

As described above, the present invention refines the grains by distributing fine (Mn, Cu) S precipitates in the hardening type IF steel, thereby lowering the in-plane anisotropy index and yielding by strengthening (Mn, Cu) S precipitation strengthening. It is to increase the strength.

Claims (8)

delete By weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, Mn: 0.01-0.3%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.001-0.05%, remaining Fe and other inevitable impurities, Cu, Mn, S, C, Nb is Cu + Mn: 0.08-0.4%, (Mn / 55 + Cu / 63.5) / (S / 32): 1-30, Cs (solute carbon): Meets 5-30 Where Cs = (C-Nbx12 / 93) x10000) The average size of (Mn, Cu) S precipitates is 0.2 µm or less, The precipitated water hardening type cold rolled steel sheet excellent in workability, characterized in that 1X10 ⁷ / mm ² or more. [Claim 3] The bake hardened cold rolled steel sheet having excellent workability according to claim 2, further comprising one or two of Si: 0.1 to 0.8% and Cr: 0.2 to 1.2%. The hardened hardened cold rolled steel sheet according to claim 2 or 3, wherein Mo is contained in an amount of 0.01 to 0.2%. delete By weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, Mn: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.001-0.05%, remaining Fe and other inevitable impurities, Cu, Mn, S, C, Nb is Cu + Mn: 0.08-0.4%, (Mn / 55 + Cu / 63.5) / (S / 32): 1-20, Cs (solute carbon): Meets 5-30 Where Cs = (C-Nbx12 / 93) x10000) One and then re-heating the slabs that meet at a temperature above 1100 ℃ by a finish rolling temperature above the Ar ₃ transformation point, hot-rolled and cooled by more than 300 ℃ / min speed and take-up at a temperature below 700 ℃ the next, 50 to 90% Cold rolling at a rolling reduction rate, and continuous annealing for 10 seconds to 30 minutes in the temperature range of 700 ~ 900 ℃, the workability of (Mn, Cu) S precipitates with an average size of less than 0.2㎛ 1X10 ⁷ / mm ² or more distributed Excellent method for hardening hardened cold rolled steel sheet. The method for producing a hardened hardened cold rolled steel sheet having excellent workability according to claim 6, further comprising one or two of Si: 0.1 to 0.8% and Cr: 0.2 to 1.2%. The method for producing a hardened hardened cold rolled steel sheet having excellent workability according to claim 6 or 7, wherein Mo is contained in an amount of 0.01 to 0.2%.