KR20060115637A - Hardened hardened cold rolled steel with excellent in-plane anisotropy and its manufacturing method - Google Patents

Abstract

Provided are a cold rolled steel sheet and a method of manufacturing the same, which show improved yield strength and in-plane anisotropy by fine CuS precipitates and AlN precipitates in a composite IF steel of Ti and Nb.

This cold-rolled steel sheet is, in weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B : 0.0001-0.002%, Ti: 0.005 ~ 0.15%, Nb: 0.002-0.04%, remaining Fe and other inevitable impurities,

Cu, S, Al, N, C, Ti, Nb is

1≤ (Cu / 63.5) / (S ^★ / 32) ≤30, 1≤ (Al / 27) / (N ^★ / 14) ≤ 10, Cs (solute carbon) is 5-30

Satisfying,

(Where S ^★ = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48), Cs = (C-Nbx12 / 93-Ti ^★ x12 / 48) x10000,

^{Ti ★ = Ti-0.8x ((} 48/14) xN + (48/32) xS) However, if Ti ^★ <0 il referred to as Ti ^★ = 0]

The average size of the CuS precipitates and AlN precipitates is less than 0.2 μm.

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, AlN precipitate is used together while using Cu as a fine CuS precipitate.

Description

Baking HARDENING TYPE COLD ROLLED STEEL SHEET HAVING REDUCED PLANE ANISOTROPY 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 CuS precipitates and AlN precipitates in IF steel, and a method of manufacturing the same. It is about.

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.

An object of the present invention is to provide a cold-rolled steel sheet and a method of manufacturing the same, which can lower in-plane anisotropy while improving yield strength by CuS precipitates and AlN 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%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004-0.02% , P: 0.2% or less, B: 0.0001-0.002%, Ti: 0.005 to 0.15%, Nb: 0.002-0.04%, remaining Fe and other unavoidable impurities,

Cu, S, Al, N, C, Ti, Nb is

1≤ (Cu / 63.5) / (S ^★ / 32) ≤30, 1≤ (Al / 27) / (N ^★ / 14) ≤ 10, Cs (solute carbon) is 5-30

Satisfying,

(Where S ^★ = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48), Cs = (C-Nbx12 / 93-Ti ^★ x12 / 48) x10000,

^{Ti ★ = Ti-0.8x ((} 48/14) xN + (48/32) xS) However, if Ti ^★ <0 il referred to as Ti ^★ = 0]

The average size of the CuS precipitates and AlN precipitates is less than 0.2 μm.

In the present invention, Al and N are less than N: 0.004% and less than -0.02%, preferably N: 0.0041-0.02%, more preferably N: 0.005-0.02%, under Al: 0.1% or less. Al / 27) / (N ^* / 14): 1-10 is satisfied so that precipitates of AlN having an average size of 0.2 μm or less are distributed.

In the present invention, the fine CuS precipitates and AlN precipitates are preferably ¹ × 10 ⁶ / 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.

The present invention is completed on the basis of the results of the study that when the fine CuS precipitate and AlN precipitate are secured in the small hardening type IF steel, the grains become fine and the yield strength is improved and the in-plane anisotropy index is lowered to improve the 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. In addition, N is managed from the viewpoint of fixing TiN and AlN by Ti or Al as impurities. Either try to lower N as much as possible, or settle and fix N, but it will be distributed as a coarse precipitate. These techniques lack awareness of the positive effects on the steel when large amounts of fine AlN precipitates are distributed.

As described in the present invention, a technique for achieving grain refinement by securing AlN precipitates together with fine CuS precipitates in the IF steel is not known.

In order to secure CuS precipitates and AlN precipitates in Ti-based IF steels, it is necessary to secure S precipitated with CuS and N precipitated with AlN. In Ti-based IF steel, Ti reacts with C, N, and S to precipitate TiC, TiN, Ti (C, N), Ti ₄ C ₂ S _2, etc., thus allowing S and N to precipitate as CuS and AlN. Management of various ingredients is required to secure proper amount.

According to the present invention, CuS precipitates and AlN precipitates are fine grains. When the grains become fine, more dissolved carbon not precipitated by Ti is present in the grain boundaries than in the grains. Accordingly, while the room temperature non-aging characteristics are secured, the baking hardening characteristics are improved. 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.

According to the present invention, finely distributed CuS precipitates and AlN precipitates have a positive effect on the increase in yield strength and the strength-ductility balance characteristics due to precipitation strengthening, and the in-plane anisotropy index. For this purpose, CuS and AlN precipitates should be finely distributed, which means that Cu and S also contain Al and N, their component ratios, and manufacturing conditions, in particular, the cooling rate after hot rolling.

First, C, Cu, S, Al, P, N, B, Ti, and Nb 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 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 Cu to form fine CuS 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.

Al forms fine AlN precipitates with N to enhance yield strength by grain refinement and precipitation strengthening. To this end, add up to 0.1%. When the Al content is more than 0.1%, there is a fear that the ductility decreases because the Al content is high in solid solution.

The content of nitrogen (N) is preferably more than 0.004% -0.02% or less. More preferably, it is 0.005-0.02%.

If the content of N is less than 0.004%, the number of precipitated AlN is small, so the effect of grain refinement and strengthening of precipitation is small. If it exceeds 0.02%, it is difficult to guarantee aging by solid nitrogen, so it should be less than 0.02%. desirable.

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%. 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 titanium (Ti) is to be 0.005 ~ 0.15%.

Titanium is added for the purpose of securing inaging and improving moldability. Titanium is a strong carbide-generating element and is added to steel to precipitate TiC precipitates to precipitate insoluble solids. If the addition amount of titanium is less than 0.005%, the precipitation amount of TiC precipitate is so small that there is little development of aggregate structure, so there is little effect of improving the processability. 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 content of niobium (Nb) is preferably 0.002 to 0.04%.

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. If the amount of Nb added is less than 0.002%, the amount of precipitation of NbC precipitates is so small that there is little development of aggregates, and thus there is little effect of improving Omrim processability. When Nb exceeds 0.04%, the amount of NbC precipitates is too high, resulting in low rimability and low elongation, which may significantly reduce moldability.

The present invention is characterized in controlling the component ratios of Ti, Nb, S, C, N, Cu, and Al from the viewpoint of securing fine CuS and AlN 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 the composition of Ti, S, C, and N in order to utilize these precipitates. Is controlling. However, the present invention manages the relationship between Ti, Nb, S, C, N, Cu, Al in order to secure an appropriate amount of solid solution carbon while securing CuS and AlN precipitates.

[Relationship 1]

1≤ (Cu / 63.5) / (S ^★ / 32) ≤30

Where S ^★ = S-0.8x (Ti-0.8x (48/14) xN) x (32/48)

 [Relationship 2]

1≤ (Al / 27) / (N ^★ / 14) ≤ 10

Where N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48)

 [Relationship 3]

Cs (solute carbon): 5-30

Where Cs = (C-Nbx12 / 93-Ti ^★ x12 / 48) x10000,

Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS)

If Ti ^* is negative, set it to 0.

Cu, S, Al, N, C, Ti, Nb is the weight percent.

In relation 1, S ^★ represents the content of S reacting with Cu in the total S content in the cold rolled steel sheet, and in order to ensure fine CuS precipitates, the value of relation 1 preferably satisfies 1-30. When the value of relation 1 is greater than or equal to 1, effective CuS precipitates are precipitated, and in the case of more than 30, CuS precipitates are coarse, resulting in poor workability and yield strength.

In relation 2, N ^★ represents the content of N that does not react with Ti in the total content of N. In order to secure fine AlN precipitates, the value of relation 2 preferably satisfies 1-10. When the value of relation 2 is greater than or equal to 1, effective AlN precipitates are precipitated, and in the case of more than 10, AlN precipitates are coarse, resulting in poor workability and yield strength. More preferably, the value of relation 2 satisfies 1-6.

In equation 3 represents the content of N, S and the reaction and the rest of Ti Ti ^★ is in the content of total Ti, was dissolved N and if the Ti ^★ value of the negative, which in the zero Ti ^★ is a negative value It means that Ti is not enough to precipitate S, so that the employment C is not newly created, so the value is 0. The Cs represents the content of solid solution carbon not precipitated with TiC and NbC. That is, Cs (solute carbon) should satisfy 5-30. The unit of content of Cs, ie, the solid solution carbon, calculated in Equation 3 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.

In the component system of the present invention, the finer the distribution, the more advantageous. Preferably, the average size of the precipitate 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 precipitates is at least 1 × 10 ⁶ per 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. The Cr ensures high strength in steels for controlling precipitates and 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 in that the average size of CuS precipitates and AlN precipitates in a cold rolled sheet is hot and cold rolled to satisfy the above-mentioned emphasis. The average size of CuS precipitates and AlN precipitates in cold rolled plates is influenced by the design process, such as reheating temperature and winding temperature, together with the component design, 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 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 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 there is no need to limit the upper limit of the cooling rate. 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.

C P S Al Cu Ti Nb B N Etc A1 0.0015 0.008 0.008 0.052 0.09 0.009 0.004 0.0006 0.0073 A2 0.0017 0.022 0.01 0.038 0.11 0.01 0.004 0.0009 0.011 A3 0.0018 0.045 0.008 0.032 0.12 0.005 0.003 0.0005 0.0075 Si: 0.07 A4 0.0023 0.081 0.011 0.052 0.13 0.011 0.004 0.001 0.0103 Si: 0.14 A5 0.0026 0.118 0.011 0.028 0.16 0.021 0.005 0.0009 0.012 Si: 0.2 A6 0.0021 0.046 0.021 0.052 0.09 0.038 0.004 0.0009 0.0118 Mo: 0.082 A7 0.0015 0.045 0.008 0.067 0.12 0.011 0.003 0.0007 0.0071 Cr: 0.23 A8 0.0022 0.044 0.01 0.028 0 0.021 0.022 0.0009 0.0015 A9 0.0042 0.12 0.009 0.052 0.13 0 0 0.0008 0.0073 Si: 0.15

sample (Cu / 63.5) / (S ^★ / 32) (Al / 27) / (N ^★ / 14) Cs Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) A1 3.27 3.624 15 0.06 2.7 X 10 ⁶ A2 2.67 1.718 17 0.06 3.9 X 10 ⁶ A3 3.71 1.935 18 0.06 3.6 X 10 ⁶ A4 3.24 2.493 23 0.05 6.4 X 10 ⁶ A5 4.65 1.426 26 0.05 8.3X10 ⁶ A6 2.52 3.059 21 0.05 8.5 X 10 ⁶ A7 4.83 5.129 15 0.04 7.9 X 10 ⁶ A8 0 -24.2 -8.5 0.2 3.7 X 10 ⁴ A9 3.33 2.746 42 0.05 5.1 X 10 ⁶ S ^★ = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14 / 48), Cs = (C-Nbx12 / 93-Ti ^★ x12 / 48) x10000, Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS)

Sample number Mechanical properties Remarks Yield strength (MPa) Tensile strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (Δr) Aging Evaluation Index BH value (MPa) 2nd processing brittleness (DBTT- ℃) A1 201 315 48 2.05 0.29 0 38 -40 Invention steel A2 213 347 46 1.96 0.27 0 42 -50 Invention steel A3 212 353 42 1.93 0.27 0 38 -40 Invention steel A4 294 418 35 1.79 0.24 0 53 -50 Invention steel A5 323 451 34 1.69 0.21 0 48 -40 Invention steel A6 254 394 38 1.79 0.28 0 55 -50 Invention steel A7 231 387 37 1.71 0.27 0 35 -40 Invention steel A8 205 348 40 2.03 0.46 0 0 -50 Comparative steel A9 299 452 31 1.21 0.17 4.4 84 -40 Comparative steel

In the present invention, the above embodiment is only one example, and the present invention is not limited thereto. Any thing that has substantially the same structure and the same effect as the technical idea described in the claim of the present invention is included in the technical scope of this invention.

As described above, the present invention is to finely grains by distributing fine CuS precipitates and AlN precipitates in the small hardening IF steel, thereby lowering the in-plane anisotropy index, and further enhancing yield strength by precipitation strengthening.

Claims (10)

By weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002% , Ti: 0.005-0.15%, Nb: 0.002-0.04%, remaining Fe and other inevitable impurities, Cu, S, Al, N, C, Ti, Nb is 1≤ (Cu / 63.5) / (S ^★ / 32) ≤30, 1≤ (Al / 27) / (N ^★ / 14) ≤ 10, Cs (solute carbon) is 5-30 Satisfying, (Where S ^★ = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48), Cs = (C-Nbx12 / 93-Ti ^★ x12 / 48) x10000, ^{Ti ★ = Ti-0.8x ((} 48/14) xN + (48/32) xS) However, if Ti ^★ <0 il referred to as Ti ^★ = 0] A hardened hardened cold rolled steel sheet having excellent in-plane anisotropy, in which the average size of CuS precipitates and AlN precipitates is 0.2 µm or less. The cold hardened steel sheet having excellent in-plane anisotropy according to claim 1, wherein the number of precipitates is 1 × 10 ⁶ / mm ² or more. According to claim 1, wherein (Al / 27) / (N ^★ / 14) is a small hardened type cold-rolled steel sheet having excellent in-plane anisotropy, characterized in that 1-6. The small hardening type cold rolled steel sheet having excellent in-plane anisotropy according to claim 1, further comprising one or two of Si: 0.1 to 0.8% and Cr: 0.2 to 1.2%. 5. The hardened hardened cold rolled steel sheet having excellent in-plane anisotropy according to claim 1 or 4, further comprising 0.01 to 0.2% of Mo. By weight%, C: 0.001-0.01%, Cu: 0.01-0.2%, S: 0.005-0.08%, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002% , Ti: 0.005-0.15%, Nb: 0.002-0.04%, remaining Fe and other inevitable impurities, Cu, S, Al, N, C, Ti, Nb is 1≤ (Cu / 63.5) / (S ^★ / 32) ≤30, 1≤ (Al / 27) / (N ^★ / 14) ≤ 10, Cs (solute carbon) is 5-30 To meet the slabs, (Where S ^★ = S-0.8x (Ti-0.8x (48/14) xN) x (32/48), N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48), Cs = (C-Nbx12 / 93-Ti ^★ x12 / 48) x10000, ^{Ti ★ = Ti-0.8x ((} 48/14) xN + (48/32) xS) However, if Ti ^★ <0 il referred to as Ti ^★ = 0] After reheating to 1100 ℃ or higher, hot rolling with the finish rolling temperature above Ar ₃ transformation point, cooling at 300 ℃ / min or higher, winding at temperature below 700 ℃, cold rolling, continuous annealing and average size A method for producing a hardened hardened cold rolled steel sheet having excellent in-plane anisotropy in which CuS precipitates and AlN precipitates of 0.2 µm or less are distributed. The method for producing a hardened hardened cold rolled steel sheet having excellent in-plane anisotropy according to claim 6, wherein the precipitate water is 1 × 10 ⁶ particles / mm ² or more. The method of claim 6, wherein (Al / 27) / (N ^★ / 14) is 1-6. The method for producing a small hardened type cold rolled steel sheet having excellent in-plane anisotropy 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 in-plane anisotropy according to claim 6 or 9, wherein Mo is contained in an amount of 0.01 to 0.2%.