KR101143116B1 - High strength cold rolled steel sheet having excellent resistance to second work embrittleness and aging resistance, and process for producing the same - Google Patents

Abstract

Provided are a high strength cold rolled steel sheet used as a material for automobiles, home appliances, and the like and a method of manufacturing the same. Cold rolled steel sheet of the present invention, by weight% C: 0.0005 ~ 0.003%, Mn: 0.03 ~ 0.2%, P: 0.015% or less, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, Cu: 0.005 to 0.2%, Cr: 0.2 to 1.2%, and Mn, Cu, and S satisfy the conditions Mn + Cu ≦ 0.3%, 0.5 * (Mn + Cu) / S: 2 to 20, and the rest. It is composed of Fe and other unavoidable impurities, and the average size of the precipitate is less than 0.2㎛. According to the present invention, the ductile-brittle transition temperature (DBTT) is low, so that it can be used in extreme temperatures and age-resistant.

Cold rolled steel, aging resistance, high strength, secondary workability, in-plane anisotropy index, (Mn, Cu) S

Description

High strength aging resistant cold rolled steel sheet with excellent secondary processing brittleness and manufacturing method thereof

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.

The present invention relates to a high strength cold rolled steel sheet used as a material for automobiles, home appliances and the like and a method of manufacturing the same. More specifically, the present invention relates to a cold rolled steel sheet having a secondary precipitate resistance with age resistance and securing a solid solution of carbon in a grain by fine precipitates and a method of manufacturing the same.

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 that causes hardening as the solid elements (C, N) adhere to dislocations, leading to a defect called 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. These elements have to be re-annealed at high temperature because they increase the recrystallization temperature. This leads to lower productivity, higher energy costs, and more pollution. 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, which is brittle after processing due to a weak grain boundary, is generated, and thus, efforts to prevent secondary processing brittleness by adding elements such as B are performed. 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, the aging resistance is not sufficient, and quenching after annealing is recommended to ensure aging resistance. In this case, since most of the water cooling is performed, pickling is performed again to remove the oxide film generated during water cooling. This is not good and costs extra. 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 has excellent aging resistance and ductility while maintaining the plastic anisotropy index above a certain level. Since this cold rolled steel is strictly controlled to less than 0.025% C + N + S + P, the decarburization, desulfurization and dephosphorization capacity should be strengthened in the steelmaking process. In addition, the yield strength is too low on the material side, there is a problem that a thicker material should be used, and when processing, there is a problem in that the in-plane anisotropy index (△ r value) is too high to cause a lot of wrinkles and fracture.

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 340MPa class without adding Ti, Nb in the 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. Promote to level. However, since the aging index is greater than 30 MPa, the aging resistance cannot be guaranteed. In particular, the in-plane anisotropy index is higher than 0.5, which causes a lot of wrinkles and causes fracture.

The present invention ensures anti-aging properties without adding Ti and Nb, and further has excellent secondary processing brittleness, and maintains a plastic anisotropy index of a certain level or more and has a low in-plane anisotropy index, and thus has excellent workability. To provide a manufacturing method, the object is.

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%, P: 0.015% or less, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, Cu: 0.005 to 0.2%, Cr: 0.2 to 1.2%, and Mn, Cu, and S are conditions Mn + Cu ≦ 0.3%, 0.5 * (Mn + Cu) / S: 2 to 20 is satisfied, and the remaining Fe and other inevitable impurities are formed, and the average size of the precipitate is 0.2 탆 or less.

Cold rolled steel sheet of the present invention may be less than 0.002% to 0.003% C. The precipitate includes at least one of MnS, CuS, and (Mn, Cu) S. The number of precipitates is preferably 2.2 × 10 ⁶ or more. 0.5 * (Mn + Cu) / S is 2-7, and the number of precipitates is 4.4x10 <^8> or more is preferable. Cold rolled steel sheet of the present invention may further include at least one of Mo: 0.01% to 0.2%, V: 0.01% to 0.2%. As for V and C, it is most preferable that 0.25 * V / C satisfy | fills 1-20.

In addition, the cold rolled steel sheet manufacturing method of the present invention, after reheating the steel slab that satisfies the above-described emphasis at a temperature of 1100 ℃ or more hot-rolled at a finish rolling temperature of more than Ar ₃ transformation point and cooled at a rate of 300 ℃ / min or more It is configured to include a cold rolled, continuous annealing, and then wound at a temperature of 700 ℃ or less.

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 and secondary workability of brittleness without adding Ti and Nb to Cr-added high strength cold rolled steel sheets.

The precipitates of MnS, CuS, and (Mn, Cu) S in cold rolled steel sheets affect the plastic anisotropy index and in-plane anisotropy index along with aging characteristics.

As shown in FIG. 1, the finer the distribution of precipitates (MnS, CuS, (Mn, Cu) S) in the cold rolled steel sheet, the amount of solid carbon in the grains is reduced, thereby improving aging resistance 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 was set under the following condition Mn + Cu ≦ 0.3%, while the content of Mn was 0.03 to 0.2%, the content of S was 0.003 to 0.025%, and the content of Cu was 0.005 to 0.2%. , 0.5 * (Mn + Cu) / S: It needs to be adjusted to satisfy 2 ~ 20, and (2) MnS, CuS, (Mn, The average size of the precipitate of Cu) S is less than 0.2 μm.

That is, Figure 2 (a) is 0.0018% C-0.12% Mn-0.008% P-0.012% S-0.04% Al-0.0021% N-0.04% Cu-0.42% Cr (0.5 * (Mn + Cu) / S: 6.67) is a graph showing the size of precipitates according to the cooling rate after hot rolling. Referring to the graph of FIG. 2 (a), when the cooling rate is adjusted for the case of satisfying 0.5 * (Mn + Cu) / S ≦ 20, the precipitates of MnS, CuS, and (Mn, Cu) S have an average size of 0.2 μm. It can be confirmed that the following can be satisfied.

When the precipitate becomes fine according to the present invention, an appropriate amount of carbon remains in the grain boundary so that the grain boundary is strengthened, so that brittle fracture caused by weak grain boundary after processing is prevented.

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.

In addition, in the cold rolled steel sheet of the present invention, the processing properties are significantly improved when Mo is added in an amount of 0.01 to 0.2%, and when V is added in an amount of 0.01 to 0.2%, the remaining solid solution C is precipitated to secure 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 solution 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. Therefore, the content of carbon (C) is preferably 0.0005 to 0.003% or less. In the present invention, while increasing the content of C to 0.002 ~ 0.003% to ensure the aging resistance can be omitted in the decarburization treatment to reduce the content of carbon 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. The above effects can be obtained when the content of manganese is 0.03% or more, and when the content of manganese is more than 0.2%, the content of manganese is high and coarse MnS precipitates are generated, resulting in poor aging resistance.

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

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

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, it is added to precipitate nitrogen in the steel to completely prevent aging by solid nitrogen. If the aluminum content is less than 0.01%, the amount of solid solution is too large to prevent the aging phenomenon, and if the aluminum content is more than 0.08%, the amount of aluminum present in the solid solution is too large to reduce the ductility.

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 copper (Cu) is preferably 0.005 to 0.2%.

Copper forms precipitates of less than 0.2 µm when the ratio of Cu and S content and the cooling rate before winding is appropriate in the hot rolling process to reduce the dissolved carbon in the grains, thereby aging resistance, secondary processing brittleness, in-plane anisotropy and plastic anisotropy Add 0.005-0.2% based on studies to improve. If the content of copper is 0.005% or more, it can be finely precipitated. If it exceeds 0.2%, it is coarse precipitated and the aging resistance is poor.

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

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

The sum of the Mn and Cu is preferably 0.3% or less. This is because if the sum of Mn and Cu is more than 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 be precipitated as MnS, CuS, (Mn, Cu) S, and the precipitates are different depending on the amount of Mn, Cu and S, and thus the aging index, secondary processing brittleness, plastic anisotropy index, Affects in-plane anisotropy index. 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, 20 In the case of more than that, the precipitate is coarse and the aging index is large, and the characteristics of the plastic anisotropy index and the in-plane anisotropy index are not good, and the average size of the precipitate is 0.2㎛ in the range of 0.5 * (Mn + Cu) / S. The ratio of 0.5 * (Mn + Cu) / S is significantly different from the starting point of 7. The type of precipitate and the number of distributions are significantly different, that is, when the ratio of 0.5 * (Mn + Cu) / S is 7 or less, (Mn, Evenly distributed MnS and CuS homogeneous precipitates are much finer than the composite precipitates of Cu) S. When the ratio of 0.5 * (Mn + Cu) / S is greater than 7, the number of distribution decreases despite the small difference in size of precipitates. This is because the amount of the composite precipitate of (MN, Cu) S increases.

The cold rolled steel sheet of the present invention may further include Mo or / and V.

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.

As for the weight ratio (V / C) of said V and C, it is more preferable to satisfy | fill 1-20. If the weight ratio of V and C is less than 1, the precipitation effect of solid solution C is not large.

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, secondary processing brittleness, plastic anisotropy index, and in-plane anisotropy index, and the average size of these precipitates In the case of more than 0.2 µm, the aging index is particularly high, and the secondary work brittleness, plastic anisotropy index, and in-plane anisotropy index are not good.

Furthermore, when the distribution number of precipitates of 0.2 μm or less in the component system of the present invention is 2.2 × 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. . In addition, in the present invention, when the ratio of 0.5 * (Mn + Cu) / S is 2-7, the distribution number of precipitates of 0.2 μm or less is greatly increased to 4.4 × ^{10 8} or more, and the workability is further improved.

[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 the amount of 0.5 * (Mn + Cu) / S is over 20, the coarse precipitates undissolved during the reheating process may cause a large number of new nuclei to be generated even if the cooling rate is increased, and thus the precipitates do not become fine (Fig. 2b, 0.0018% C). -0.13% Mn- 0.012% P-0.006% S-0.05% Al-0.0027% N-0.15% Cu-0.62% Cr, 0.5 * (Mn + Cu) /S=23.3). 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 lower than 500 ° C, recrystallization is not completed and the target ductility value cannot be obtained. 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 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) Mn + Cu 0.5 *
(Mn + Cu) / S C Mn Cr P S Al N Cu Ti 0.0005 ~ 0.003 0.03-0.2 0.2 ~
1.2 ≤0.015 0.003-0.25 0.01 ~
0.08 ≤0.004 0.005-0.2 ≤0.3 2 ~ 20 A1 0.0024 0.05 0.30 0.01 0.016 0.04 0.0022 0.04 - 0.09 2.81 A2 0.0018 0.12 0.32 0.009 0.012 0.05 0.0019 0.03 - 0.15 6.25 A3 0.0024 0.12 0.6 0.01 0.015 0.04 0.0025 0.05 - 0.17 5.67 A4 0.0027 0.1 0.63 0.01 0.018 0.04 0.0025 0.04 - 0.14 3.89 A5 0.0026 0.18 0.95 0.009 0.008 0.05 0.0022 0.08 - 0.26 16.3 A6 0.0017 0.05 0.32 0.01 0.02 0.04 0.0022 0.01 - 0.06 1.5 A7 0.0023 0.15 0.62 0.01 0.005 0.05 0.0023 0.12 - 0.27 27 A8 0.0025 0.25 0.93 0.012 0.015 0.04 0.0024 0.29 - 0.54 18 A9 0.0024 0.4 0.01 0.07 0.01 0.04 0.0016 - 0.05 - A10 0.0028 0.4 0.012 0.121 0.012 0.05 0.0021 - 0.05 - A11 0.0018 0.4 0.01 0.167 0.01 0.05 0.0019 - 0.05 -

Psalter
number Mechanical properties Precipitate
Average
size
(Μm) Number of precipitates (pcs / mm ² ) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) prescription
Indices
(MPa) Secondary
Processing
Brittle
(℃) A1 265 356 52 1.93 0.22 23 -80 0.06 5.9 X ^{10 8} Invention steel A2 258 352 54 1.95 0.29 27 -70 0.07 4.4 X ^{10 8} Invention steel A3 298 395 45 1.62 0.22 22 -60 0.05 6.2X10 ⁸ Invention steel A4 308 405 46 1.58 0.20 23 -60 0.05 6.1X10 ⁸ Invention steel A5 348 455 38 1.55 0.22 21 -50 0.06 2.2 X 10 ⁶ Invention steel A6 237 342 45 1.65 0.52 43 -70 0.35 4.2 X 10 ⁴ Comparative steel A7 275 390 41 1.54 0.42 42 -60 0.55 7.3 X 10 ⁴ Comparative steel A8 335 440 32 1.38 0.25 38 -40 0.42 5.7 X 10 ⁴ Comparative steel A9 210 353 40 1.73 0.58 0 + 0 0.21 3.1 X 10 ³ Conventional Steel A10 265 395 35 1.65 0.35 0 + 20 0.25 2.5 X 10 ³ Conventional Steel A11 308 451 37 1.54 0.41 0 + 30 0.35 3.5 X 10 ³ Conventional Steel

In Tables 1 and 2, A1 to A5 (Inventive Steel) have an average size of precipitates of 0.2 µm or less, and the aging index can be guaranteed to be 30 Mpa or less, while the plastic anisotropy index is 1.5 or higher and the in-plane anisotropy index is lower than 0.3. High strength is secured. In particular, in the case of A1 to A4 having a ratio of 0.5 * (Mn + Cu) / S of 7 or less, the strength and formability are 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. In the case where the ratio of 0.5 * (Mn + Cu) / S was 7 or more, the smaller number of the precipitates was found to have a large amount of (Mn, Cu) S complex precipitates.

A6 (comparative steel) has a low ratio of 0.5 * (Mn + Cu) / S, low strength, poor formability, and high aging index.                     

A7 (comparative steel) has a ratio of 0.5 * (Mn + Cu) / S higher than the range of the present invention, which results in poor moldability and high aging index.

A8 (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 not good moldability and high aging index.

Nos. 9 ~ A11 (Prior to Steel) are P-added high strength IF (Ti-based) cold rolled steel sheets with excellent aging index but high ductility and brittle transition temperature, which may cause breakage at impact even at room temperature.

[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.

Specimen Number Chemical composition (% by weight) Mn +
Cu 0.5 *
(Mn + Cu) / S C Mn Cr P S Al N Cu Mo 0.0005 ~
0.003 0.03-0.2 0.2-1.2 ≤
0.015 0.003 ~
0.025 0.01 ~ 0.08 ≤0.004 0.005-0.2 0.01
~ 0.2 ≤0.3 2 ~ 20 B1 0.0017 0.11 0.34 0.011 0.013 0.034 0.0029 0.043 0.018 0.15 5.88 B2 0.0022 0.08 0.62 0.009 0.009 0.054 0.0031 0.053 0.092 0.13 7.38 B3 0.0026 0.15 0.92 0.012 0.012 0.038 0.0021 0.086 0.17 0.24 9.83 B4 0.0017 0.09 0.32 0.01 0.013 0.018 0.0032 0.045 0.252 0.14 5.19

Psalter
number Mechanical property Precipitate
Average
size
(Μm) Number of precipitates
(Pcs / mm ² ) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) prescription
Indices
(MPa) Secondary
Processing
Brittle
(℃) B1 258 359 51 2.38 2.7 19 -80 0.07 6.9X10 ⁸ Invention steel B2 303 411 47 1.81 1.9 26 -60 0.06 5.5X10 ⁸ Invention steel B3 351 462 39 1.72 1.8 27 -50 0.07 7.2 X 10 ⁶ Invention steel B4 262 362 49 1.82 2.4 16 -80 0.05 4.9X10 ⁸ Comparative steel

As shown in Tables 3 and 4, it was found that in the case of B1 to B3, workability was 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.

Specimen Number Chemical composition (% by weight) Mn +
Cu 0.5 *
(Mn +
Cu)
/ S 0.25
* V / C C Mn Cr P S Al N Cu V 0.0005 ~ 0.003 0.03-0.2 0.2-1.2 ≤0.015 0.003-0.025 0.01 ~ 0.08 ≤0.004 0.005-0.2 0.01 ~ 0.2 ≤0.3 2 ~ 20 1-20 C1 0.0016 0.09 0.32 0.01 0.012 0.036 0.0022 0.038 0.016 0.13 5.33 2.5 C2 0.0022 0.11 0.64 0.009 0.011 0.043 0.0023 0.054 0.08 0.16 7.45 9.09 C3 0.0027 0.13 0.95 0.013 0.009 0.025 0.0032 0.077 0.16 0.21 11.5 14.8 C4 0.0022 0.11 0.34 0.011 0.008 0.028 0.0019 0.042 0.24 0.15 9.5 27.3

Psalter
number Mechanical properties Precipitation
water
Average
size
(Μm) Number of precipitates (pcs / mm ² ) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) prescription
Indices
(AI-
(MPa)) Secondary
Processing
Brittle
(DBTT- ℃) C1 210 352 52 1.9 0.28 0 -70 0.07 5.6 X ^{10 8} Invention steel C2 255 405 44 1.69 0.22 0 -60 0.07 6.2X10 ⁸ Invention steel C3 313 458 38 1.48 0.2 0 -50 0.06 7.9 X 10 ⁶ Invention steel C4 245 375 45 1.75 0.26 0 -60 0.07 6.7 X ^{10 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.

Chemical composition (% by weight) Specimen Number C Mn Cr P S Al N Cu Mo V Mn +
Cu 0.5 * (Mn + Cu) / S 0.25 * V / C 0.0005 ~ 0.003 0.03-0.2 0.2-1.2 ≤
0.015 0.003-0.025 0.01 ~ 0.08 ≤0.004 0.005-0.2 0.01 ~ 0.2 0.01 ~ 0.2 ≤0.3 2 ~ 20 1-20 D1 0.0018 0.1 0.34 0.012 0.01 0.026 0.0025 0.043 0.022 0.016 0.14 7.15 2.22 D2 0.0024 0.12 0.66 0.011 0.012 0.045 0.0034 0.061 0.08 0.08 0.18 7.54 8.33 D3 0.0027 0.11 0.93 0.011 0.01 0.034 0.0027 0.057 0.16 0.17 0.17 8.55 15.7 D4 0.0018 0.1 0.35 0.009 0.008 0.018 0.0034 0.045 0.22 0.25 0.15 9.06 34.7 D5 0.0027 0.08 0.35 0.011 0.011 0.041 0.0024 0.055 0.38 0.075 0.135 6.14 6.94 D6 0.0025 0.1 0.38 0.01 0.008 0.028 0.0019 0.071 0.075 0.34 0.171 10.7 34

Specimen Number Mechanical property Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) prescription
Indices
(AI-
(MPa)) Secondary
Processing
Brittle
(DBTT- ℃) Precipitation
water
Average
size
(Μm) Number of precipitates
(Pcs / mm ² ) D1 204 349 52 2.21 0.28 0 -70 0.08 4.2 X ^{10 8} Invention steel D2 250 413 44 1.92 0.22 0 -60 0.07 5.4 X ^{10 8} Invention steel D3 317 462 36 1.66 0.18 0 -50 0.07 6.2 X 10 ⁶ Invention steel D4 238 369 47 1.83 0.24 0 -20 0.08 3.7 X ^{10 8} Comparative steel D5 251 362 45 1.85 0.27 0 -70 0.09 1.2X10 ⁸ Comparative steel D6 201 352 48 2.15 0.35 0 -10 0.13 2.8X10 ⁷ 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 a aging index of 30 Mpa or less and can guarantee the aging while having a low ductility-brittle transition temperature, thereby excellent in secondary work brittleness, high anisotropy index, and high in-plane anisotropy index. Is low, and workability is extremely excellent.

Claims (22)

By weight%, C: 0.0005 ~ 0.003%, Mn: 0.03 ~ 0.2%, P: 0.015% or less, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, Cu: 0.005 ~ 0.2%, Cr: 0.2-1.2%, Mn, Cu, S satisfy the following conditions Mn + Cu≤0.3%, 0.5 * (Mn + Cu) / S: 2-20, remaining Fe and other unavoidable impurities It is composed of AlN precipitates and MnS, CuS, (Mn, Cu) S precipitates, the number of precipitates is more than 2.2X10 ⁶ / mm ² , the average of the MnS, CuS, (Mn, Cu) S precipitates A high strength, aging resistant cold rolled steel sheet with excellent secondary processing brittleness of 0.2 μm or less in size. According to claim 1, wherein C is high strength aging cold-rolled steel sheet excellent in secondary processing brittleness, characterized in that less than 0.002% ~ 0.003%. delete delete According to claim 1, wherein 0.5 * (Mn + Cu) / S is high strength aging cold-rolled steel sheet excellent secondary secondary brittleness, characterized in that 2 to 7. The high-strength age cold-rolled steel sheet having excellent secondary processing brittleness according to claim 5, wherein the precipitate water is 4.4 X ^{10 8} / mm ² or more. The high strength aging cold rolled steel sheet having excellent secondary work brittleness according to any one of claims 1, 2, 5, and 6, further comprising Mo in an amount of 0.01 to 0.2%. The high strength aging cold rolled steel sheet excellent in secondary processing brittleness according to any one of claims 1, 2, 5, and 6, wherein V is contained in an amount of 0.01 to 0.2%. According to claim 8, wherein V and C is high strength aging cold-rolled steel sheet excellent secondary secondary brittleness, characterized in that 0.25 * V / C satisfy 1 ~ 20. 8. The high strength, aging resistant cold rolled steel sheet having excellent secondary processing brittleness according to claim 7, wherein V is further included in an amount of 0.01 to 0.2%. The high strength, aging resistant cold rolled steel sheet having excellent secondary work brittleness 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%, P: 0.015% or less, S: 0.003 ~ 0.025%, Al: 0.01 ~ 0.08%, N: 0.004% or less, Cu: 0.005 ~ 0.2%, Cr: 0.2-1.2%, Mn, Cu, S satisfy the following conditions Mn + Cu ≦ 0.3%, 0.5 * (Mn + Cu) / S: 2-20, remaining Fe and other unavoidable impurities After reheating the steel to a temperature of 1100 ℃ or more and hot rolling with a finish rolling temperature of more than Ar ₃ transformation point, cooled at a rate of 300 ℃ / min or more, wound at a temperature of 650 ~ 700 ℃, then cold rolled, Continuous annealing to form AlN precipitates and MnS, CuS, (Mn, Cu) S precipitates, the number of precipitates being at least 2.2 × 10 ⁶ / mm ² , and the MnS, CuS, (Mn, Cu) S precipitates A method for producing a high strength cold-resistant steel sheet excellent in secondary processing brittleness to make the average size of less than 0.2㎛. The method of claim 12, wherein C is 0.002% or more and 0.003% or less. delete delete The method according to claim 12, wherein 0.5 * (Mn + Cu) / S is 2 to 7, and a method for manufacturing high strength aging cold rolled steel sheet having excellent secondary work brittleness. The method according to claim 16, wherein the precipitate water is 4.4X10 ⁸ / mm ² or more. The method according to any one of claims 12, 13, 16, and 17, wherein Mo is further added in an amount of 0.01 to 0.2%. . The method according to any one of claims 12, 13, 16, and 17, wherein V is added in an amount of 0.01 to 0.2%. . 20. The method according to claim 19, wherein V and C are 0.25 * V / C satisfying 1 to 20. 19. The method according to claim 18, wherein V is added in an amount of 0.01 to 0.2%. 22. The method according to claim 21, wherein V and C are 0.25 * V / C satisfying 1-20.

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