KR100723180B1 - Cold rolled steel sheet having good formability 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, in weight%, C: 0.005% or less, Mn: 0.2% or less, Cu: 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%, Ti: 0.005 ~ 0.15%, remaining Fe and other inevitable impurities,

Cu + Mn: 0.05-0.4, 0.003≤S ^* ≤0.015, 1≤ (Mn / 55 + Cu / 63.5) / (S ^* / 32) ≤30, 0.8≤ (Ti ^* / 48) / (C / 12) ≤5.0

Where S ^* = S-0.8 × (Ti-0.8 × N (48/14)) × (32/48), Ti ^* = Ti-0.8 × S (48/32) -0.8 × N (48/14) , Satisfies and the average size of (Mn, Cu) S precipitates is 0.2 탆 or less.

In the cold rolled steel sheet of the present invention, as the (Mn, Cu) S precipitates of the nanosize are distributed, crystal grains become fine, thereby decreasing the in-plane anisotropy index, and the yield strength is enhanced by the (Mn, Cu) S precipitates. Therefore, the content of P and Mn added to secure high strength can be greatly reduced, thereby improving weldability and plating characteristics.

Cold rolled steel, non-aging, high strength, secondary processing brittleness, in-plane anisotropy index, precipitate

Description

Cold rolled steel sheet with excellent workability and manufacturing method {COLD ROLLED STEEL SHEET HAVING GOOD FORMABILITY AND PROCESS FOR PRODUCING THE SAME}

Japanese Laid-Open Patent Publication No. 57-0413349,

Korean Unexamined Patent Publication No. 2003-0052248,

Japanese Unexamined Patent Publication No. Hei 6-057336

Japanese Patent Laid-Open No. 9-227951

Japanese Patent Laid-Open No. 10-265900

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

Cold rolled steel sheets used in automobiles and home appliances require strength and workability, as well as non-aging characteristics. Aging is a kind of strain aging that causes hardening as the solid elements (C, N) adhere to dislocations, leading to a defect called stretcher strain.

The non-aging property of the cold rolled steel sheet can be secured by the annealing of the aluminum-kilted steel, but the annealing has the disadvantage that the annealing time is long and the productivity is low and the material deviation is severe for each part.

Therefore, IF steel (Interstitial Free Steel) which adds strong carbon and nitride forming elements such as Ti and Nb and performs continuous annealing is mainly used. IF steels have a non-aging effect by removing some or all of the carbon and nitrogen employed.

The high-strength method of IF steel is the employment strengthening technology by P. Japanese Laid-Open Patent Publication No. 57-0413349 secures strength by adding 0.04 to 0.12% of P in Ti-added IF steel. In the IF steel of the ultra low carbon component system, P causes the problem of secondary processing brittleness due to grain boundary segregation.

In addition, Korean Laid-Open Patent Publication No. 2003-0052248 improves processability and secondary processing brittleness by adding 0.5-2.0% of Mn, Al, and B together instead of P addition. However, Mn used in the ultra low carbon steel is of high quality, so that the manufacturing cost increases due to the large amount of Mn added, and in particular, when the Mn content is increased, the plating properties are not good.

In JP-A-6-057336, 0.5-2.5% of Cu is added to IF steel to form an ε-Cu precipitated phase to secure strength. High strength is ensured by the precipitated phase of ε-Cu, but the processing characteristics are not good.

In contrast, techniques for improving the processability or surface defects by carbides using Cu as precipitation nuclei of carbides are disclosed in Japanese Patent Laid-Open Nos. 9-227951 and 10-265900.

Japanese Patent Application Laid-Open No. 9-227951 discloses a slab of IF steel containing 0.01-0.05% of Cu after rough rolling at 1100-950 ° C. for at least 70 seconds to precipitate CuS and hot roll to obtain a precipitate of Cu-Ti-CS. The hot rolled plate is obtained and cold rolled to secure workability. Cold rolling of a hot rolled sheet having a precipitate of Cu-Ti-CS increases the nuclei of crystal grains which form {111} parallel to the plate surface around these precipitates during recrystallization, resulting in improved workability as a result. .

Japanese Patent Laid-Open No. 10-265900 discloses surface defects by reducing the solid solution C in the steel sheet by precipitating CuS in IF steel to which 0.005-0.1% Cu is added and making the CuS precipitate effectively function as precipitation nuclei of carbides. It is a technique to improve.

In Japanese Patent Application Laid-Open Nos. 9-227951 and 10-265900, the coarse CuS precipitates are used in the intermediate process, and finally, the cold rolled steel sheet secures carbides. It is adding more than S amount by ratio. That is, since the amount of Ti added is larger than the amount of S in atomic ratio so that S reacts most with Ti, the precipitate of CuS alone is not utilized.

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

Cold rolled steel sheet of the present invention for achieving the above object, in weight%, C: 0.005% or less, Mn: 0.2% or less, Cu: 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%, Ti: 0.005 ~ 0.15%, and the remaining Fe and other unavoidable impurities,

Cu + Mn: 0.05-0.4, 0.003≤S ^* ≤0.015, 1≤ (Mn / 55 + Cu / 63.5) / (S ^* / 32) ≤30, 0.8≤ (Ti ^* / 48) / (C / 12) ≤5.0

Where S ^* = S-0.8 × (Ti-0.8 × N (48/14)) × (32/48), Ti ^* = Ti-0.8 × S (48/32) -0.8 × N (48/14) Satisfies and the average size of the (Mn, Cu) S precipitate is less than 0.2㎛.

In the present invention, the fine (Mn, Cu) S precipitate is preferably ¹ × 10 ⁶ / mm ² or more.

In the present invention, the content of C is preferably 0.0005-0.003%. As for said Cu, 0.03-2.0% is preferable.

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. When the cold rolled steel sheet further contains one or two of P, Si and Cr as solid solution strengthening elements, high strength characteristics of 340 MPa or more are secured. When P is contained 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, the (Mn, Cu) S precipitate is an expression including Mn alone emulsion, Cu alone emulsion, and a complex emulsion of Mn and Cu.

The present invention is completed on the basis of the research results that when the (Mn, Cu) S precipitate is secured in the cold rolled steel sheet, the grains become fine to improve yield strength and lower in-plane anisotropy index to improve workability.

In order to secure the fine CuS precipitate on the cold rolled steel sheet, it is necessary to secure S precipitated as (Mn, Cu) S. Since Ti forms precipitates with C, N, and S, it is necessary to control their relationship so that S can be precipitated as (Mn, Cu) S. For this purpose, S, Ti, Mn, Cu, N, C It is important to manage the relationship.

According to the present invention, when the fine (Mn, Cu) S precipitate is secured, the grains are refined and the dissolved carbon is more present in the grain boundary than in the grains, thereby ensuring room temperature non-aging characteristics. The dissolved carbon remaining in the grain is relatively free to move, so it does not affect the room temperature aging characteristics in combination with the operating potential. In addition, 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, the (Mn, Cu) S precipitates must be finely distributed, and the content of Ti, Mn, Cu, C, N, and S, their composition ratio, and the cooling rate after the hot rolling are affected.

Cold rolled steel sheet of the present invention has a high yield strength can reduce the thickness of the steel sheet has a light weight effect. In addition, the in-plane anisotropy has the advantage of less wrinkles generated during processing and less ear generation after processing. The cold rolled steel sheet of the present invention and a manufacturing method thereof will be described in detail below.

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

The content of carbon (C) is preferably 0.005% or less.

If the content of carbon (C) exceeds 0.005%, a large amount of expensive Ti should be added to remove the solid solution carbon which greatly degrades aging and plastic anisotropy. In this case, the manufacturing cost rises and the recrystallization temperature increases. Therefore, the annealing temperature should be increased, otherwise the crystal grains of the annealing plate become fine and the ductility is greatly lowered, and the plating property is also lowered during plating. More preferable content of carbon (C) is 0.003% or less.

Preferably, the lower limit of the content of carbon (C) is made 0.0005%. 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.

The content of manganese (Mn) is less than 0.2%, preferably in the range of 0.01-0.2%.

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 is more preferably 0.01% or more. When the content of manganese is more than 0.2%, the content of manganese is high, making coarse MnS precipitates, making it difficult to secure strength.

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 0.01% or more to precipitate finely, and when it exceeds 0.2%, it precipitates coarsely. Preferable Cu content is 0.03-0.2%.

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

Sulfur (S) reacts with Mn and 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 sulfur content is more than 0.08%, there is a high content of solute sulfur so that ductility and processability are greatly lowered, and there is a fear of red brittleness. More preferably, the content of sulfur (S) is preferably 0.005-0.02%.

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 to limit the upper limit to 0.2%. In the present invention, when Si and Cr are added, various strength designs are possible while the content of P is in the range of 0.2% or less.

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 titanium is 0.005 to 0.15%, preferably 0.005-0.1%.

Titanium is added for the purpose of securing ineffectiveness and improving workability. Titanium is a strong carbide-generating element, added to steel to precipitate TiC precipitates, thereby securing ineffective aging. If the addition amount of titanium is less than 0.005%, the precipitation amount of TiC precipitate is too small, and there is little effect of improving the processability of the soil because of less development of the texture. If Ti exceeds 0.15%, the TiC precipitates are too large to reduce the grain refining efficiency, resulting in an increase in in-plane anisotropy, lowering the yield strength and greatly degrading the plating properties.

The present invention is characterized in controlling the component ratios of Ti, S, C, N and Cu from the viewpoint of securing fine (Mn, Cu) S in the cold rolled steel sheet. In IF steel, Ti is known to precipitate precipitates such as TiC, TiN, TiS, Ti (C, N), Ti ₄ C ₂ S ₂ , and in order to utilize these precipitates, Ti, S, C, N, Cu, The component relationship of Mn is controlled. However, in the present invention, in order to secure (Mn, Cu) S precipitates, the relationship between Ti, S, C, N, Mn, and Cu is managed.

[Relationship 1]

Cu + Mn: 0.05-0.4

[Relationship 2]

0.003≤S ^* ≤0.015,

Where S ^* = S-0.8 × (Ti-0.8 × N (48/14)) × (32/48)

 [Relationship 3]

1≤ (Mn / 55 + Cu / 63.5) / (S ^* / 32) ≤30

 [Relationship 4]

0.8≤ (Ti ^* /48)(C/12)≤5.0

Where Ti ^* = Ti-0.8 × S (48/32) -0.8 × N (48/14),

In relations 1, 2, 3 and 4, the contents of Ti, S, C, N and Cu are by weight.

If the amount of Mn + Cu is 0.05% or more, it is possible to secure (Mn, Cu) S precipitates, and in the case of more than 0.4%, it is difficult to secure aging characteristics due to the large size of precipitates. Preferably, the amount of Mn + Cu is made 0.3% or less.

In relation 2, S ^* represents the content of S reacting with Mn and Cu in the content of total S in the cold rolled steel sheet. Sufficient (Mn, Cu) S precipitates can be obtained when the content of S ^* is 0.003-0.015% by weight. If the content of S ^* is less than 0.003%, the amount of precipitation of (Mn, Cu) S precipitates is too small, and if the content of S ^* is more than 0.015%, there is a possibility that the ductility and workability will be greatly reduced due to the high content of solid solution S. .

Equation 3 is to manage the relationship between Mn, Cu and S to secure a fine (Mn, Cu) S precipitate, and the effective (Mn, Cu) S precipitates will be precipitated when the value of relation 2 is 1 or more, In the case of more than 30, (Mn, Cu) S precipitates are coarse, resulting in poor workability and yield strength.

Equation 4 is to allow the remaining Ti reacts with C in the total Ti content, and the remaining Ti reacts with C. If the value is less than 0.8, it is difficult to guarantee room temperature aging because of the high content of C present in solid solution. In the case of the TiC precipitate, the TiC precipitate is coarsened so that the yield strength is lowered and the plating property is not good.

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, when the distribution number of precipitates of 0.2 μm or less in the component system of the present invention is 1 × 10 ⁶ or more per mm ^2, 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. .

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. P has already been mentioned, so duplicate descriptions are omitted.

The content of silicon (Si) is 0.8% or less, preferably in the range of 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. More preferably, it is 0.5% or less of range.

The content of chromium (Cr) is 1.2% or less, 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. More preferably, it is 0.5% or less.

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

The content of molybdenum (Mo) is 0.2% or less, preferably in the range of 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 affected by the conditions of addition, reheating temperature, winding temperature, etc., but it 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. 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%, since the annealing recrystallization nucleation amount is small, the grains grow too large during annealing, resulting in a decrease in strength and workability due to coarsening of the annealing recrystallization grains. If the cold reduction rate exceeds 90%, the workability is improved, but the nucleation amount is too large, 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.

division Chemical composition (% by weight) C Mn Cu S Al N P B Ti Etc A1 0.0007 0.11 0.09 0.02 0.035 0.0008 0.043 0.0007 0.029 Si: 0.08 A2 0.0012 0.08 0.12 0.032 0.039 0.0021 0.08 0.0009 0.049 Si: 0.17 A3 0.0028 0.11 0.16 0.041 0.025 0.0019 0.11 0.0005 0.064 Si: 0.3 A4 0.0013 0.09 0.11 0.035 0.043 0.0023 0.082 0.0011 0.057 Si: 0.26 Mo: 0.1 A5 0.0015 0.1 0.09 0.05 0.025 0.001 0.075 0.0012 0.069 Si: 0.32 Cr: 0.21 A6 0.0035 0.45 0.14 0.009 0.033 0.0024 0.048 0.005 0 A7 0.0031 0.13 0.03 0.012 0.038 0.0021 0.118 0 0.15 Si: 0.33

division Cu + Mn S ^* (Mn / 55 + Cu / 63.5) / (S ^* / 32) (Ti ^* / 48) / (C / 12) Average size of (Mn, Cu) S precipitates (㎛) Number of (Mn, Cu) S precipitates (pcs / mm2) A1 0.2 0.0057 19.173 One 0.04 4.5 X 10 ⁶ A2 0.2 0.0089 11.972 1.01 0.04 5.2 X 10 ⁶ A3 0.27 0.0096 14.994 0.86 0.03 6.3 X 10 ⁶ A4 0.2 0.008 13.535 1.67 0.04 7.3 X 10 ⁶ A5 0.19 0.0147 7.0611 1.04 0.04 8.9X10 ⁶ A6 0.59 0.0125 26.566 -1.2 0.25 1.5X10 ⁴ A7 0.16 -0.065 -1.398 10.5 0.16 4.3X10 ⁴ S ^* = S-0.8 × (Ti-0.8 × N (48/14)) × (32/48) Ti ^* = Ti-0.8 × S (48/32) -0.8 × N (48/14)

 division Mechanical properties Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (Δr) Aging Evaluation Index (%) 2nd processing brittleness (DBTT- ℃) A1 222 352 46 2.04 0.39 0 -70 Invention steel A2 288 402 39 1.87 0.32 0 -60 Invention steel A3 338 454 35 1.68 0.29 0 -50 Invention steel A4 329 449 34 1.88 0.28 0 -50 Invention steel A5 383 452 35 1.64 0.29 0 -50 Invention steel A6 238 342 43 1.21 0.59 1.73 -60 Comparative steel A7 302 433 30 1.65 0.48 0 -50 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 is different from the technique of using Cu as the precipitated phase of ε-Cu in IF steel or the technique of using Cu as the precipitate nucleus of carbide by depositing Cu as CuS, and the precipitate of nanosized (Mn, Cu) S. Is distributed on the cold rolled steel sheet to refine the grains, thereby lowering the in-plane anisotropy index and increasing the yield strength by strengthening (Mn, Cu) S precipitation.

Claims (16)

By weight%, C: 0.005% or less, Mn: 0.2% or less, Cu: 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%, Ti: 0.005-0.15%, the remaining Fe and other inevitable impurities, Cu + Mn: 0.05-0.4, 0.003≤S ^* ≤0.015, 1≤ (Mn / 55 + Cu / 63.5) / (S ^* / 32) ≤30, 0.8≤ (Ti ^* / 48) / (C / 12) ≤5.0 Where S ^* = S-0.8 × (Ti-0.8 × N (48/14)) × (32/48), Ti ^* = Ti-0.8 × S (48/32) -0.8 × N (48/14) The cold rolled steel sheet excellent in workability, which satisfies and has an average size of 0.2 μm or less of (Mn, Cu) S precipitates. The cold rolled steel sheet having excellent workability according to claim 1, wherein the number of precipitates of (Mn, Cu) S is 1 × 10 ⁶ / mm ² or more. The cold rolled steel sheet having excellent workability according to claim 1, wherein the C content is 0.0005-0.003%. The cold rolled steel sheet having excellent workability according to claim 1, wherein the Cu is 0.03-0.2%. The cold rolled steel sheet according to any one of claims 1 to 4, wherein the content of P is 0.015% or less. The cold rolled steel sheet according to any one of claims 1 to 4, wherein the content of P is 0.03 to 0.2%. The cold rolled steel sheet according to any one of claims 1 to 4, wherein the cold rolled steel sheet further includes one or two kinds of Si: 0.8% or less and Cr: 1.2% or less. The cold rolled steel sheet according to any one of claims 1 to 4, wherein the cold rolled steel sheet further contains 0.2% or less of Mo. By weight%, C: 0.005% or less, Mn: 0.2% or less, Cu: 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%, Ti: 0.005-0.15%, the remaining Fe and other inevitable impurities, Cu + Mn: 0.05-0.4, 0.003≤S ^* ≤0.015, 1≤ (Mn / 55 + Cu / 63.5) / (S ^* / 32) ≤30, 0.8≤ (Ti ^* / 48) / (C / 12) ≤5.0 Where S ^* = S-0.8 × (Ti-0.8 × N (48/14)) × (32/48), Ti ^* = Ti-0.8 × S (48/32) -0.8 × N (48/14) After reheating the slab satisfying the temperature above 1100 ℃ and hot rolling with the finish rolling temperature above the Ar ₃ transformation point, cooling it at a speed above 300 ℃ / min, winding it at a temperature below 700 ℃, and then 50-90% Cold rolling at a reduction ratio, continuous annealing at a temperature of 700-900 ℃ to produce a cold rolled steel sheet excellent in workability in which (Mn, Cu) S precipitates having an average size of 0.2㎛ or less is distributed. 10. The method of claim 9, wherein the number of precipitates of the (Mn, Cu) S is 1 × 10 ⁶ / mm ² or more. 10. The method of claim 9, wherein the C content is 0.0005-0.003%. 10. The method of claim 9, wherein the Cu is 0.03-0.2%. The method of manufacturing a cold rolled steel sheet having excellent workability according to any one of claims 9 to 12, wherein the content of P is 0.015% or less. The method for manufacturing a cold rolled steel sheet having excellent workability according to any one of claims 9 to 12, wherein the content of P is 0.03 to 0.2%. The method for manufacturing a cold rolled steel sheet having excellent workability according to any one of claims 9 to 12, wherein the cold rolled steel sheet further includes one or two kinds of Si: 0.8% or less and Cr: 1.2% or less. . The method for manufacturing a cold rolled steel sheet having excellent workability according to any one of claims 9 to 12, wherein the cold rolled steel sheet further contains Mo or less 0.2%.