KR20050116347A - Cold rolled steel sheet having excellent formability and aging resistance, and process for producing the same - Google Patents

Abstract

Provided are a cold rolled steel sheet used as a material for automobiles and home appliances, and a manufacturing method thereof. Cold rolled steel sheet of the present invention, by weight% C: 0.0005 ~ 0.003%, Mn: 0.03 ~ 0.2%, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, P: 0.015% or less, Cu: 0.005 to 0.2%, the Mn, Cu, S satisfy the conditions Mn + Cu ≤ 0.3, 0.5 * (Mn + Cu) / S: 2 ~ 20, and is composed of the remaining Fe and other unavoidable impurities The average size of the precipitates is less than 0.2㎛. In the cold rolled steel sheet of the present invention, the amount of carbon dissolved in the crystal grains is reduced by fine precipitates, thereby improving workability while improving aging resistance.

Description

 COLD ROLLED STEEL SHEET HAVING EXCELLENT FORMABILITY AND AGING RESISTANCE, AND PROCESS FOR PRODUCING THE SAME}

The present invention relates to a cold rolled steel sheet used as a material for automobiles, home appliances, and the like, and more particularly, to a cold rolled steel sheet improved in aging characteristics and workability by securing a solid amount of carbon in crystal grains by fine precipitates. It is about.

Cold rolled steel sheets used in automobiles and home appliances require strength and formability as well as anti-aging properties. Aging is a kind of strain aging phenomenon in which hardening occurs as the solid elements (C, N) adhere to dislocations, causing a defect called a stretcher strain.

The aging resistance of the cold rolled steel sheet can be secured by the normal annealing of the aluminum-kilted steel, but the annealing has a long annealing time, which leads to low productivity and severe material deviations for each part. Therefore, IF steel (Interstitial Free Steel) which is continuously annealed by adding strong carbon and nitride forming elements such as Ti and Nb is mainly used.

In order to manufacture IF steel, strong carbon and nitride forming elements such as Ti and Nb are added. Since Ti and Nb raise the recrystallization temperature, they must be annealed at a high temperature. This leads to lower productivity and higher costs using more energy. In addition, annealing at a high temperature has a disadvantage in that various defects such as fine defects and shape defects are likely to occur. In addition, since Ti and Nb have strong oxidizing properties, many nonmetallic inclusions are generated during steelmaking, causing surface defects of the steel sheet. In addition, IF steel has a disadvantage in that the so-called secondary processing brittleness is generated, which is brittle after processing due to weak grain boundaries. To prevent this, efforts have been made to prevent secondary processing brittleness by adding elements such as B. In particular, IF steel has a disadvantage of generating a lot of defects in the surface treated products such as plating and painting.

In order to solve such a problem, Ti and Nb non-added steels without adding Ti or Nb have been proposed. For example, Japanese Unexamined Patent Application Publication Nos. Hei 6-093376, 6-093377, and 6-212354 do not add Ti and Nb but instead strictly control the C: 0.0001 to 0.0015% in steel with 0.0001 to 0.003% of B. It is a technology that improves Hyosung. However, in this prior art, aging resistance is not sufficient, and quenching after annealing is recommended to ensure aging resistance. Therefore, since the pickling treatment is required to remove the oxide film generated during water cooling, the surface is not good and additional costs are required. In addition, these steels have a disadvantage of low strength, high in-plane anisotropy, wrinkles, and high ear (ear).

On the other hand, the present inventors have proposed a method for producing a cold rolled steel sheet having excellent elongation processing characteristics by improving the ductility without adding Ti, Nb in the Republic of Korea Patent Publication No. 2000-0039137. The manufacturing method of this cold rolled steel sheet is C: 0.0005-0.002% or less, Mn: 0.05-0.3%, S: 0.015% or less, P: 0.015% or less, Al: 0.01-0.08%, N: 0.001-0.005 by weight% %, The C + N + S + P is less than 0.025% and hot-rolled steel slab containing the remaining Fe and other inevitable elements with a finish rolling temperature of Ar ₃ transformation point or more, and then below 750 ℃ After winding at a temperature, it is cold rolled at a reduction ratio of 50 to 90% and continuously annealed for 10 seconds or more at a temperature in the range of 650 to 850 ° C. The cold rolled steel sheet thus obtained is excellent in aging resistance and ductility while maintaining the plastic anisotropy index at a predetermined level or more. Korean Laid-Open Patent Publication No. 2000-0039137 should strengthen the desulfurization and dephosphorization capability in order to strictly control C + N + S + P to 0.025%. In addition, the yield strength is too low in terms of material, there is a problem to use a thicker material. In processing, there is a problem that the in-plane anisotropy index (Δr value) is too high to cause many wrinkles and break.

In addition, the present inventor has proposed a method of manufacturing a cold rolled steel sheet that can improve the yield strength in high strength steel of tensile strength 35kgf / mm ² without adding Ti, Nb in Republic of Korea Patent Publication No. 2002-0049667. The method for producing this cold rolled steel sheet is C: 0.0005 to 0.003%, Mn: 0.1% or less, S: 0.003 to 0.02%, P: 0.03 to 0.07%, Al: 0.01 to 0.1%, and N: 0.005% or less by weight. , Cu: 0.05% to 0.3%, Cu / S atomic ratio 2 to 10 hot rolled to more than Ar ₃ transformation point, cold rolled to 50 ~ 90% reduction rate, 10 at a temperature of 700 ~ 880 ℃ range It is continuous annealing for 5 ~ 5 minutes.

The cold rolled steel sheet thus obtained has a yield strength of 240 MPa in 340 MPa class high strength steel. However, since the aging index is greater than 30 MPa, the aging resistance cannot be guaranteed. In addition, the plastic anisotropy index (r _m ) is 1.8, and the workability needs to be improved. In particular, the in-plane anisotropy index is higher than 0.5 so that many wrinkles are generated. There is a problem that is broken.

The present invention is to provide a cold rolled steel sheet and a method for producing the same having excellent aging properties without adding Ti and Nb, and further having high plastic anisotropy index and low in-plane anisotropy index.

Cold rolled steel sheet of the present invention for achieving the above object, by weight% C: 0.0005 ~ 0.003%, Mn: 0.03 ~ 0.2%, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, P: 0.015% or less, Cu: 0.005% to 0.2%, Mn, Cu, and S satisfy the conditions Mn + Cu ≦ 0.3, 0.5 * (Mn + Cu) / S: 2 to 20, and the remaining Fe and It is composed of other unavoidable impurities, and the average size of the precipitate is less than 0.2㎛.

In the cold rolled steel sheet of the present invention, the C may be greater than 0.002% and less than 0.003%. In addition, the precipitates are MnS, CuS, (Mn, Cu) S, and the number of precipitates is preferably ³ × 10 ⁶ or more. More preferably, it is said 0.5 * (Mn + Cu) / S: 2-7, and the said precipitate number is 2 * ^{10 <8>} or more. The cold rolled steel sheet of the present invention may additionally contain 0.01% to 2% of Mo and / or 0.01% to 2% of V. In the case where V is added, it is most preferable that V and C satisfy 0.25 * V / C of 1-20.

In the method for manufacturing a cold rolled steel sheet of the present invention, after reheating a steel slab satisfying the above-mentioned emphasis of the present invention to a temperature of 1100 ° C. or more, hot rolling is performed at a finish rolling temperature of Ar ₃ or higher and at a speed of 300 ° C./min or more. It is made by cooling the apparatus, winding at a temperature of 700 ° C. or lower, cold rolling, and continuous annealing.

Hereinafter, the present invention will be described in detail.

The present inventors have discovered the following new facts in the course of research for improving the characteristics of aging resistance without adding Ti and Nb. That is, the precipitates of MnS, CuS, and (Mn, Cu) S affect the plastic anisotropy index and the in-plane anisotropy index along with the aging characteristics.

As shown in Figure 1, the finer the distribution of the precipitates of MnS, CuS, (Mn, Cu) S is to reduce the amount of solid solution carbon in the grains to improve the aging characteristics. In order to secure the anti-aging characteristics, the amount of dissolved carbon in the grains should be about 20 ppm or less. For this, it can be stably secured when the average size of precipitates of MnS, CuS, and (Mn, Cu) S is 0.2 μm or less.

Paying attention to these new facts, we have studied how to finely distribute MnS, CuS, and (Mn, Cu) S. As a result, (1) the content of Mn, Cu, and S with the following conditions Mn + Cu ≦ 0.3, while the content of Mn is 0.03 to 0.2%, the content of S is 0.003 to 0.025%, and the content of Cu is 0.005 to 0.2%. 0.5 * (Mn + Cu) / S: It should be adjusted to satisfy 2 ~ 20. (2) After finishing rolling, if the cooling rate is over 300 ℃ / min, MnS, CuS, (Mn, The average size of the precipitates of Cu) S is fine to 0.2 탆 or less.

Figure 2 (a) is composed of 0.0025% C-0.13% Mn-0.009% P-0.015% S-0.04% Al-0.0029% N-0.04% Cu and a steel of 0.5 * (Mn + Cu) /S:5.67. The graph shows the size of the precipitates according to the cooling rate after hot rolling. Referring to the graph of FIG. 2 (a), as the cooling rate increases for the case of satisfying 0.5 * (Mn + Cu) / S ≦ 20, precipitates of MnS, CuS, and (Mn, Cu) S have finer average sizes. You can see it done.

According to the present invention, by weight% C: 0.0005 to 0.003%, Mn: 0.03 to 0.2%, S: 0.003 to 0.025%, Al: 0.01 to 0.08%, N: 0.004% or less, P: 0.015% or less, Cu: 0.005 0.2%, when Mn, Cu and S satisfy the following conditions Mn + Cu ≦ 0.3, 0.5 * (Mn + Cu) / S: 2 to 20, the aging resistance when the average size of precipitates is 0.2 μm or less While the basic anisotropy index (r _m ) is high while the in-plane anisotropy index (Δr) is low.

Furthermore, the present inventors found that the more the precipitates of MnS, CuS, (Mn, Cu) S in the distribution of MnS and CuS than the complex precipitates of Mn and Cu, the fine precipitates were uniformly distributed and thus the plastic anisotropy index increased. It was confirmed that the in-plane anisotropy index was lowered and the workability was greatly improved. That is, when the ratio of 0.5 * (Mn + Cu) / S is in the range of 2-7, the number of single precipitates of MnS and CuS is larger than that of (Mn, Cu) S composite precipitates, so that the distribution number of precipitates is increased to improve workability. Confirmed.

Furthermore, in the present invention, when Mo is further included, the processing characteristics are remarkably improved. In addition, when V is further added, V precipitates solid solution C, and the aging index becomes OMPa, thereby securing non-aging characteristics.

The cold rolled steel sheet of the present invention and a manufacturing method thereof will be described in detail below.

[Cold rolled steel sheet of the present invention]

The content of carbon (C) is preferably 0.0005 to 0.003%.

If the content of carbon (C) is less than 0.0005%, the grains of the hot rolled sheet are coarse to lower the strength and to increase the in-plane anisotropy. In addition, when the carbon content is more than 0.003%, the amount of solid carbon in the steel is high, making it difficult to secure aging resistance and the crystal grains of the annealing plate become fine, resulting in significantly lower ductility. In the present invention, by using a fine precipitate, the carbon content can be made less than 0.002% to 0.003% or less in the steelmaking process.

The content of manganese (Mn) is preferably 0.03 to 0.2%.

Manganese is known as an element that precipitates solid sulfur in steel as MnS to prevent hot shortness caused by solid sulfur. In the present invention, when the content of manganese and sulfur is appropriate, very fine MnS is precipitated to ensure the resistance to aging, while improving the yield strength and in-plane anisotropy, based on the results of the study, the content of manganese is preferably 0.03 to 0.2%. Do. When the content of manganese is 0.03% or more, it is possible to secure fine MnS precipitates, and when the content of manganese is more than 0.2%, manganese has a high content of coarse MnS, resulting in poor aging resistance.

The content of sulfur (S) is preferably 0.003 to 0.025%.

When the content of sulfur (S) is less than 0.003%, the amount of precipitates of MnS, CuS, and (Mn, Cu) S is small, and the precipitates are very coarse in size, and thus the aging resistance is not good. If the content of sulfur is more than 0.025%, the content of the solid solution of sulfur is so high that the ductility and moldability is greatly lowered, there is a fear of red brittle brittleness.

The content of aluminum (Al) is preferably 0.01 to 0.08%.

Aluminum is an element added as a deoxidizer, but in the present invention, nitrogen is added to prevent precipitation of solid nitrogen in aging. If the content of aluminum is less than 0.01%, the amount of nitrogen is high so that the aging phenomenon can not be prevented, and if the content of aluminum is more than 0.08%, the amount of aluminum in the solid solution is high, the ductility is reduced.

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

Nitrogen is an element inevitably added during steelmaking, and if it is more than 0.004%, the aging index is increased and moldability is deteriorated, so 0.004% or less is preferable.

The content of phosphorus (P) is preferably 0.015% or less.

When the content of phosphorus is more than 0.015%, ductility and moldability are lowered, so it is preferable to be 0.015% or less.

The content of copper (Cu) is preferably 0.005 to 0.2%.

Copper (Cu) improves aging characteristics, in-plane anisotropy, and plastic anisotropy by reducing precipitated carbon in grains by forming precipitates of less than 0.2 µm when the ratio of S to S and the cooling rate before winding in the hot rolling process are appropriate. Based on the addition of 0.005 ~ 0.2%. When the content of copper is 0.005% or more, it can be finely precipitated, and when it exceeds 0.2%, it is coarse precipitated and the aging resistance is not good.

The sum of the Mn and Cu is preferably 0.3% or less. This is because when the sum of Mn and Cu exceeds 0.3%, the size of the precipitate increases, making it difficult to secure aging characteristics.

It is preferable that the weight ratio of said Mn, Cu, and S satisfy | fills 0.5 * (Mn + Cu) / S: 2-20. S is combined with Mn and Cu to precipitate as MnS, CuS and (Mn, Cu) S. The precipitates are affected by the addition amount of Mn, Cu and S, affecting the aging index, plastic anisotropy index and in-plane anisotropy index. Crazy According to the present invention, an effective precipitate is obtained when the addition ratio of Mn, Cu and S [0.5 * (Mn + Cu) / S (wherein the content of Mn, Cu, S is in weight%)] is 2 or more, In the case of more than 20, the precipitates are coarse to increase the aging index, and the characteristics of plastic anisotropy index and in-plane anisotropy index are not good. In the range of 0.5 * (Mn + Cu) / S in the range of 2-20, the average size of the precipitates is less than 0.2㎛. The type and distribution of the precipitates starting from 7 is 0.5 * (Mn + Cu) / S. The number varies significantly. That is, if the ratio of 0.5 * (Mn + Cu) / S is 7 or less, much more uniformly distributed single precipitates of MnS and CuS are finer than the composite precipitates of (Mn, Cu) S. If the ratio of 0.5 * (Mn + Cu) / S is greater than 7, the distribution number decreases despite the small difference in the size of the precipitates, because the amount of the composite precipitates of (Mn, Cu) S increases.

In the component system of the present invention, the average size of the precipitate is preferably 0.2 µm or less.

According to the results of the present invention, the size of MnS, CuS, and (Mn, Cu) S precipitates directly affect the aging index, plastic anisotropy index, and in-plane anisotropy index. When the average size of these precipitates is 0.2 µm or less, the aging index is satisfied, and in order to improve both the plastic anisotropy index and the in-plane anisotropy index, 0.2 µm or less is preferable.

Further, when the distribution number of precipitates of 0.2 μm or less in the component system of the present invention is 3 × 10 ⁶ or more, the plastic anisotropy index is increased and the in-plane anisotropy index is lowered, thereby greatly improving workability. 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. . Further, in the present invention 0.5 * (Mn + Cu) / S ratio of time of 2-7 days, the number of distribution of the precipitate of less than 0.2㎛ is greatly enhanced by more than 2X10 ⁸ workability becomes better.

At least one of molybdenum (Mo) and vanadium (V) may be further added to the steel formed as described above.

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

Molybdenum is added as an element to increase the plastic anisotropy index, the plastic anisotropy index is increased only when the content is more than 0.01%, the plastic anisotropy index does not increase any more it may cause hot brittleness.

The content of vanadium (V) is preferably 0.01 to 0.2%.

Vanadium is added to precipitate the solid solution C to secure the non-aging characteristics, the content of which is more than 0.01% to obtain the non-aging characteristics, when exceeding 0.2%, the plastic anisotropy index is lowered.

The weight ratio of the V and C (0.25 * V / C, V, C content of the weight%) is more preferably satisfying 1 to 20. If the weight ratio of V and C is less than 1, the precipitation effect of solid solution C is not large. If it exceeds 20, the plastic anisotropy index is lowered.

[Manufacturing method of cold rolled steel sheet]

The present invention is characterized by satisfying the average size of MnS, CuS, (Mn, Cu) S precipitates in a cold rolled plate through hot rolling and cold rolling to satisfy the above-described stress. The average size of these precipitates in the cold rolled plate is affected by the conditions of addition, reheating temperature, winding temperature, etc., but is directly affected 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. If the reheating temperature is less than 1100 ℃, the reheating temperature is low, the coarse CuS produced during the continuous casting is not completely dissolved, the coarse precipitates remain even after hot rolling.

Hot rolling is preferably performed at a finish rolling temperature above 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 degraded due to the formation of the rolled grain but also the ductility is greatly reduced.

After hot rolling, the cooling rate before winding is preferably 300 ° C / min or more. According to the present invention, even when the temperature is 2 ≦ 0.5 * (Mn + Cu) / S ≦ 20, if the cooling rate is less than 300 ° C./min, the average size of the precipitate exceeds 0.2 μm. In other words, as the cooling rate increases, a large number of nuclei are generated and the precipitate becomes fine. If 0.5 * (Mn + Cu) / S is over 20, the coarse precipitates undissolved in the reheating process, and even if the cooling rate is fast, the number of new nuclei is generated and the precipitates do not become fine (Fig. 2b, 0.0025% C). -0.4% Mn-0.01% P-0.01% S-0.05% Al-0.0016% N-0.15% Cu). Referring to the graph of Figure 2, the faster the cooling rate is the size of the precipitate becomes fine, so there is no need to limit the upper limit of the cooling rate, even if the cooling rate is more than 1000 ℃ / min cooling rate is no longer increased 300-1000 degreeC / min 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 is higher than 700 ° C, precipitates grow too coarsely to lower aging resistance.

[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 500-900 degreeC. If the continuous annealing temperature is less than 500 ° C., recrystallization is not completed and the target ductility value cannot be secured. If the annealing temperature is higher 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.

Mechanical properties in the examples were measured by processing the cold rolled steel sheet to a standard specimen according to the ASTM (ASTM E-8 standard). Yield strength, tensile strength, elongation, plastic anisotropy index (r _m value), in-plane anisotropy index (Δr value) and aging index (AI, Aging Index) were measured using a tensile tester (INSTRON, Model 6025). In Examples, the plastic anisotropy index (r _m ) and the in-plane anisotropy index (Δr) were obtained by the following equation. r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2

In addition, the average size of the precipitate in the specimen and the number of distribution of the precipitate is a measure of the size and distribution of all the precipitates present in the substrate.

Example 1

The steel slabs of Table 1 were reheated at 1200 ° C., hot rolled to finish, cooled at a rate of 600 ° C./min, and wound up at 650 ° C. The wound hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio of 75%. However, in Table 1, sample 10 was reheated at 1050 ° C., hot rolled to finish, cooled at 50 ° C./min, and wound up at 750 ° C. At this time, the finish rolling temperature was 910 ° C. above Ar ₃ transformation point, and continuous annealing was performed at 10 ° C. Heating was carried out at 750 ° C. for 40 seconds at a rate of ° C./sec.

Psalm Number Chemical composition (% by weight) Mn + Cu≤0.3 0.5 * (Mn + Cu) / S = 2-20 C Mn P S Al Cu N 0.0005 ~ 0.003 0.03-0.2 ≤0.015 0.003-0.025 0.01 ~ 0.08 0.005-0.2 ≤0.004 A1 0.0021 0.08 0.012 0.005 0.04 0.082 0.0023 0.16 16.2 A2 0.0018 0.11 0.009 0.009 0.04 0.04 0.0019 0.15 8.33 A3 0.0022 0.09 0.012 0.011 0.05 0.05 0.0024 0.14 6.36 A4 0.0024 0.15 0.008 0.021 0.05 0.04 0.0018 0.19 4.52 A5 0.0022 0.05 0.008 0.018 0.04 0.035 0.0024 0.09 2.36 A6 0.0024 0.4 0.011 0.012 0.05 0.023 0.0038 0.4 17.6 A7 0.0028 0.05 0.012 0.018 0.04 0.012 0.0023 0.06 1.72 A8 0.0025 0.25 0.01 0.008 0.03 0.18 0.0015 0.4 26.9 A9 0.0022 0.15 0.013 0.005 0.03 0.12 0.0026 0.27 27 A10 0.0025 0.1 0.010 0.010 0.03 0.042 0.0014 0.14 7.1

Psalm Number Mechanical properties Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (△ r) Aging Index (AI- (MPa) A1 198 292 51 2.32 0.38 17 0.09 4.5 X 10 ⁶ Invention steel A2 208 309 52 2.35 0.35 16 0.08 9.4 X 10 ⁶ Invention steel A3 221 314 55 2.51 0.26 21 0.06 2.2 X ^{10 8} Invention steel A4 218 310 56 2.55 0.28 18 0.05 3.5 X ^{10 8} Invention steel A5 205 300 58 2.68 0.31 23 0.05 4.1X10 ⁸ Invention steel A6 175 282 58 2.83 0.93 35 0.38 8.5 X 10 ⁴ Comparative steel A7 163 270 60 2.78 1.12 36 0.48 4.3X10 ⁴ Comparative steel A8 169 278 52 2.23 0.93 44 0.53 4.5 X 10 ⁴ Comparative steel A9 189 286 51 1.93 0.79 42 0.33 6.3 X 10 ⁴ Comparative steel A10 181 291 55 2.45 0.88 35 0.38 7.1 X 10 ⁴ Comparative steel

In Tables 1 and 2, the average size of precipitates is 0.2 µm or less, and the aging index can be guaranteed to be 30 MPa or less, while the plastic anisotropy index is 2.3 or more and the in-plane anisotropy index is 0.4 or less, which is extremely excellent in workability. . In particular, in the case of A3 to A5 having a ratio of 0.5 * (Mn + Cu) / S of 7 or less, the strength and formability were very excellent, and it was confirmed that very fine precipitates precipitated by MnS or CuS alone were uniformly distributed. Due to the distribution characteristics of these precipitates, the plastic anisotropy index is high, and the in-plane anisotropy index is low, which shows very excellent processing characteristics. When the ratio of 0.5 * (Mn + Cu) / S was 7 or more, the smaller the number of precipitates was, the smaller the number of precipitates was, indicating that a large amount of (Mn, Cu) S complex precipitates were present.

A6 (comparative steel), the ratio of 0.5 * (Mn + Cu) / S satisfies the scope of the present invention, but the Mn + Cu content is out of the scope of the present invention is low in strength, poor formability and high aging index.

A7 (comparative steel) has a low strength, poor formability, and high aging index due to the low ratio of 0.5 * (Mn + Cu) / S and Mn + Cu content.

A8 (comparative steel) and 9 (comparative steel) have a ratio of 0.5 * (Mn + Cu) / S higher than the range of the present invention, which is low in strength, poor in formability and high in aging index.

A10 (comparative steel) had a poor cooling resistance due to the slow cooling rate and the coarse precipitates.

Example 2

The steel slabs of Table 3 were reheated at 1200 ° C., hot rolled to finish, cooled at a rate of 600 ° C./min, and wound up at 650 ° C. The wound hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio 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 750 ℃ to 10 ℃ / second.

Psalm Number Chemical composition (% by weight) 0.5 * (Mn + Cu) / S C Mn P S Al Cu N Mo 0.0005 ~ 0.003 0.03-0.2 ≤0.015 0.003-0.025 0.01 ~ 0.08 0.005-0.2 ≤0.004 0.01 ~ 0.2 B1 0.0023 0.11 0.01 0.011 0.024 0.08 0.0033 0.018 8.64 B2 0.0015 0.08 0.008 0.012 0.046 0.054 0.0026 0.073 5.58 B3 0.0023 0.1 0.011 0.008 0.035 0.065 0.0018 0.16 10.3 B4 0.0027 0.12 0.008 0.01 0.024 0.046 0.0028 0.25 8.3

Psalm Number Mechanical property Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (△ r) Aging Index (AI- (MPa) B1 209 302 50 2.83 0.45 25 0.09 3.5 X 10 ⁶ Invention steel B2 192 290 53 2.88 0.42 15 0.08 4.4 X 10 ⁶ Invention steel B3 211 304 54 2.79 0.44 25 0.07 2.2 X ^{10 8} Invention steel B4 220 315 49 2.05 0.37 18 0.05 5.5X10 ⁸ Comparative steel

As shown in Tables 3 and 4, in the case of B1 to 3, it was found that workability is improved by addition of Mo. In the case of B4, when Mo was excessively added, workability was rather poor.

Example 3

The steel slabs of Table 5 were reheated at 1200 ° C, hot rolled to finish, cooled at a rate of 600 ° C / min, and wound up at 650 ° C. The wound hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio 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 750 ℃ to 10 ℃ / second.

Psalter Chemical composition (% by weight) 0.5 * (Mn + Cu) / S 0.25 * V / C C Mn P S Al Cu N V 0.0005 ~ 0.003 0.03-0.2 ≤0.015 0.003-0.025 0.01 ~ 0.08 0.005-0.2 ≤0.004 0.01 ~ 0.2 2 ~ 20 1-20 C1 0.0023 0.12 0.011 0.009 0.033 0.082 0.0023 0.021 11.2 2.28 C2 0.0019 0.09 0.009 0.011 0.032 0.064 0.0018 0.082 7 10.8 C3 0.0025 0.11 0.012 0.012 0.045 0.054 0.0024 0.17 6.83 17 C4 0.0022 0.11 0.01 0.011 0.031 0.043 0.0016 0.26 6.95 29.5

Sample Number Mechanical property Average size of precipitate (㎛) Number of precipitates (pcs / mm ² ) Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (△ r) Aging Index (AI- (MPa) C1 162 291 51 2.21 0.29 0 0.08 4.2 X 10 ⁶ Invention steel C2 158 289 53 2.32 0.31 0 0.07 6.1X10 ⁶ Invention steel C3 174 296 52 2.29 0.3 0 0.07 2.2 X ^{10 8} Invention steel C4 202 323 46 1.96 0.29 0 0.05 7.5X10 ⁸ Comparative steel

As shown in Tables 5 and 6, the non-aging characteristics were obtained by the addition of V. In the case of C4, when V was added in excess, it was found that the workability was severely deteriorated.

Example 4

The steel slabs of Table 7 were reheated at 1200 ° C., hot rolled to finish, cooled at a rate of 600 ° C./min, and wound up at 650 ° C. The wound hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio 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 750 ℃ to 10 ℃ / second.

Specimen number Chemical composition (% by weight) 0.5 * (Mn + Cu) / S 0.25 * V / C C Mn P S Al Cu N Mo V 0.0005 ~ 0.003 0.03-0.2 ≤0.015 0.003-0.025 0.01 ~ 0.08 0.005-0.2 ≤0.004 0.01 ~ 0.2 0.01 ~ 0.2 2 ~ 20 1-20 D1 0.0017 0.1 0.01 0.009 0.036 0.042 0.0032 0.019 0.023 7.89 3.38 D2 0.0022 0.12 0.013 0.01 0.022 0.054 0.0024 0.075 0.085 8.7 9.66 D3 0.0025 0.09 0.011 0.011 0.035 0.035 0.0034 0.15 0.16 5.68 16 D4 0.0022 0.12 0.012 0.01 0.035 0.074 0.0035 0.35 0.22 6.5 25

Psalm Number Mechanical property Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (△ r) Aging Index (AI- (MPa) D1 159 298 53 2.52 0.39 0 0.09 3.2 X 10 ⁶ Invention steel D2 162 302 52 2.63 0.31 0 0.08 4.3X10 ⁶ Invention steel D3 166 306 51 2.69 0.42 0 0.07 4.6 X ^{10 8} Invention steel D4 212 332 45 2.11 0.29 0 0.06 6.5X10 ⁸ Comparative steel

As shown in Tables 7 and 8, it was found that workability is also improved and non-aging is obtained by the complex addition of Mo and V. In the case of D4, too much Mo and V were added, resulting in poor workability.

As described above, the cold rolled steel sheet provided in accordance with the present invention has excellent plasticity anisotropy index and low in-plane anisotropy index while ensuring the aging resistance.

1 is a graph showing the change in the amount of solid solution carbon in the grain according to the size of the precipitate.

2 is a graph showing the size of the precipitate according to the cooling rate,

2A is the case of 0.5 * (Mn + Cu) / S ≦ 20,

2B shows the case of 0.5 * (Mn + Cu) / S ≦ 20.

Claims (22)

C: 0.0005 to 0.003%, Mn: 0.03 to 0.2%, S: 0.003 to 0.025%, Al: 0.01 to 0.08%, N: 0.004% or less, P: 0.015% or less, Cu: 0.005 to 0.2% Mn, Cu, and S satisfy the conditions Mn + Cu ≦ 0.3, 0.5 * (Mn + Cu) / S: 2 to 20, and include Fe and other unavoidable impurities, and have an average size of precipitates. Aging cold rolled steel sheet excellent in workability which is 0.2 micrometer or less. According to claim 1, wherein C is more than 0.002% ~ 0.003% or less aging cold rolled steel sheet excellent in workability. According to claim 1, wherein the precipitate is MnS, CuS, (Mn, Cu) S excellent aging cold rolled steel sheet, characterized in that the workability. The aging cold rolled steel sheet of claim 1, wherein the number of precipitates is 3 × 10 ⁶ or more. The aging cold-rolled steel sheet having excellent workability according to claim 1, wherein 0.5 * (Mn + Cu) / S: 2 to 7. The aging cold-rolled steel sheet having excellent workability according to claim 5, wherein the number of precipitates is 2 × ^{10 8} or more. The aging cold rolled steel sheet having excellent workability according to any one of claims 1 to 6, wherein Mo is contained in an amount of 0.01 to 2%. The aging cold rolled steel sheet excellent in workability according to any one of claims 1 to 6, wherein V is further contained 0.01 to 2%. The aging cold-rolled steel sheet having excellent workability according to claim 8, wherein V and C satisfy 0.25 * V / C of 1 to 20. 8. The aging cold rolled steel sheet having excellent workability according to claim 7, wherein V is further added in an amount of 0.01 to 2%. 11. The aging cold rolled steel sheet having excellent workability according to claim 10, wherein V and C satisfy 0.25 * V / C of 1 to 20. By weight%, C: 0.0005 ~ 0.003%, Mn: 0.03 ~ 0.2%, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, P: 0.015% or less, Cu: 0.005 ~ 0.2%, Mn, Cu and S satisfy the following conditions Mn + Cu ≤ 0.3, 0.5 * (Mn + Cu) / S: 2 ~ 20, the steel is formed by containing the remaining Fe and other unavoidable impurities temperature of 1100 ℃ or more After reheating, hot rolling with finishing rolling temperature above Ar ₃ transformation point, cooling at 300 ℃ / min or higher, winding at temperature below 700 ℃, cold rolling, continuous annealed cold rolled steel sheet Manufacturing method. The method of claim 12, wherein the C is greater than 0.002% and less than 0.003%. The method of claim 12, wherein the precipitate is MnS, CuS, (Mn, Cu) S. The method of claim 12, wherein the precipitate water is 3 × 10 ⁶ or more. 13. The method of claim 12, wherein the 0.5 * (Mn + Cu) / S: 2 to 7 is excellent in workability. 17. The method of claim 16, wherein the precipitate water is 2 × ^{10 8} or more. 18. The method for producing an aging cold rolled steel sheet having excellent workability according to any one of claims 12 to 17, further comprising 0.01 to 2% of Mo. 18. The method according to any one of claims 12 to 17, wherein V is further added in an amount of 0.01 to 2%. 20. The method of claim 19, wherein V and C are 0.25 * V / C satisfying 1-20. 19. The method according to claim 18, wherein V is further added in an amount of 0.01 to 2%. 22. The method of claim 21, wherein V and C are 0.25 * V / C satisfying 1-20.