KR101115703B1 - Non aging cold rolled steel sheet having high strength, and process for producing the same - Google Patents

Abstract

The present invention relates to a high strength cold rolled steel sheet used as a material for automobiles, home appliances, and the like. This cold rolled steel sheet is C: 0.003% or less, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, V: 0.01-0.2% by weight, P: 0.03-0.2%, Si: 0.1-0.8%, Cr: 0.2-1.2%, including one or more selected from the group, wherein the weight ratio of Mn and S is 0.58 * Mn / S≤ It satisfies 10, is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate is less than 0.2㎛. In addition, a method for producing a cold rolled steel sheet of the present invention is also provided.

Cold rolled steel, non-aging, high strength, in-plane anisotropy index, MnS

Description

 Non-aging high strength cold rolled steel sheet and its manufacturing method {NON AGING COLD ROLLED STEEL SHEET HAVING HIGH STRENGTH, AND PROCESS FOR PRODUCING THE SAME}

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

2 is a graph showing the size of MnS precipitates according to the cooling rate.

The present invention relates to a high-strength cold rolled steel sheet used as a material for automobiles, home appliances, and more particularly, to a high-strength cold rolled steel sheet having fine MnS precipitates and having improved non-aging characteristics by the addition of V and a method of manufacturing the same. will be.

Cold rolled steel sheets used in automobiles and home appliances require strength and formability as well as anti-aging properties. Aging is a type of strain aging that causes a defect called stretcher strain as hardening occurs as the invasive solid-solution elements C and N adhere to dislocations over time.

The aging resistance 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 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 lowers productivity and uses more energy to raise costs. 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 for securing 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. The surface is not good and there is additional cost. 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 the steel slab containing the remaining Fe and other inevitable elements. This cold rolled steel sheet has good ductility and age resistance while maintaining plastic anisotropy index above a certain level. Since this cold rolled steel sheet controls C + N + S + P to 0.025%, the desulfurization and dephosphorization ability should be enhanced. In addition, the yield strength is too low in terms of material, there is a problem to use a thicker material, has not secured the non-aging characteristics.

On the other hand, as a non-aging cold rolled steel sheet having a high yield strength, a cold rolled steel sheet having a high content of Mn and P in ultra low carbon steel and Ti is added. This cold rolled steel sheet has a poor secondary brittle resistance to ductile-brittle transition temperature of 0-30 ℃ so that breakage occurs at impact even at room temperature.

The present invention provides a high strength cold rolled steel sheet having a non-aging characteristic and excellent secondary processing brittleness without adding Ti and Nb, and a method of manufacturing the same.

Cold rolled steel sheet of the present invention for achieving the above object, by weight% C: 0.003% or less, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, V : 0.01% to 0.2%, and a component that increases strength by solid solution strengthening is appropriately added, and the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, with remaining Fe and other unavoidable impurities It is formed, and the average size of the MnS precipitate is less than 0.2㎛.

In the present invention, the components for improving the strength by solid solution strengthening are representative of P, Si, Cr and the like, and their contents are preferably P: 0.03-0.2%, Si: 0.1-0.8%, Cr: 0.2-1.2%. Do. Therefore, one or two or more components selected from the group of P, Si and Cr may be added, and the content of P when one or two of Si, Cr is added is preferably 0.015% or less.

In addition, the cold rolled steel sheet manufacturing method of the present invention, by weight% C: 0.003% or less, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, V: 0.01 ˜0.2%, P: 0.03-0.2%, Si: 0.1-0.8%, Cr: 0.2-1.2%, including one or two or more selected from the group, wherein the weight ratio of Mn and S is satisfy 0.58 * Mn / S≤10, and a balance of Fe and other unavoidable impurities, the steel after the finish-rolling temperature of Ar ₃ transformation point in the hot rolling and at least 200 ℃ / min speed over reheated to a temperature above 1100 ℃ composition is in Cooling, winding at a temperature of 700 ° C. or less, followed by cold rolling, and continuous annealing.

Hereinafter, the present invention will be described in detail.

The present inventors have discovered the following new facts in the course of research for improving the secondary workability with non-aging characteristics without adding Ti and Nb as a high strength cold rolled steel sheet.

The distribution of fine MnS precipitates decreases the amount of dissolved carbon in the grains, thereby improving the aging characteristics. As shown in FIG. 1, the finer the distribution of MnS precipitates in the cold rolled steel sheet, the smaller the amount of dissolved carbon in the grains, thereby improving the aging resistance characteristics. In order to secure the anti-aging properties, the amount of dissolved carbon in the grains should be about 15 ppm or less, and the size of the fine precipitates of MnS was determined to be 0.2 μm or less. Accordingly, in the present invention, there is an advantage in that the content of C can be expanded to 0.003% of the load less in the steelmaking process.

By paying attention to these new facts, we have studied how to finely distribute MnS in cold rolled steel. As a result, (1) it is necessary to adjust the content ratio (0.58 * Mn / S) of 10 or less while (2) Mn content of 0.05 to 0.2% and S content of 0.005 to 0.03%. In addition, when the cooling rate is 200 ℃ / min or more after the end of the rolling rolling, it is possible to obtain a fine MnS precipitate of 0.2 ㎛ or less.

That is, Figure 2 (a) is a steel of 0.002% C-0.4% Si-0.10% Mn-0.01% P-0.010% S-0.03% Al-0.0024% N-0.05% V the ratio of Mn and S (0.58 * It is a graph that investigates the size of precipitates according to the cooling rate after hot rolling steel of composition with Mn / S) of 5.8. Looking at the graph of Figure 2 (a), it is confirmed that the precipitate size of MnS can satisfy 0.2㎛ or less when the cooling rate is adjusted for the case where the component ratio (0.58 * Mn / S) of Mn and S satisfies 10 or less. Can be.

According to the present invention, when the fine MnS precipitates are distributed, the amount of solid solution carbon in the grains is reduced, and the remaining carbon may be precipitated by the addition of V to secure non-aging characteristics. In the present invention, the fine MnS precipitates contribute to the improvement of yield strength and in-plane anisotropy index.

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 0.003% or less.

If the carbon content is more than 0.003%, the amount of solid carbon in steel is high, making it difficult to secure inaging properties, and the crystalline ductility of the annealing plate becomes fine, which greatly reduces the ductility. Therefore, the content of carbon (C) is preferably 0.003% or less, more preferably, the content of carbon (C) is 0.0005 to 0.003%. This is because when the content of carbon (C) is less than 0.0005%, the grains of the hot rolled sheet are coarse to lower the strength and increase the in-plane anisotropy. In the present invention, the amount of carbon in the grains can be lowered by the MnS precipitate, so that the carbon content can be increased to 0.003%, so that the decarburization treatment can be omitted to lower the carbon content as much as possible. The range is less than 0.003%.

The content of manganese (Mn) is preferably 0.05-0.2%.

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 inferiority of the base, while improving the yield strength and in-plane anisotropy, based on the results of the study, the manganese content is preferably 0.05 to 0.2%. Do. Manganese content of more than 0.05% can achieve the above effects, when the content of manganese is more than 0.2% high manganese content is generated coarse MnS precipitate is poor inaging age.

The content of sulfur (S) is preferably 0.005-0.03%.

When the content of sulfur (S) is less than 0.005%, not only the amount of MnS precipitated is small but also the size of the precipitated MnS is very coarse, which results in poor aging. If the content of sulfur is more than 0.03%, the content of solute is high so that the ductility and moldability is greatly lowered, there is a fear of red brittleness. When the content of sulfur is in the range of 0.005 ~ 0.03%, it becomes easy to adjust the precipitate size of MnS to the desired range. More preferable content of S is 0.016 to 0.03%.

The content of aluminum (Al) is preferably 0.01-0.1%.

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. When the aluminum content is less than 0.01%, the amount of solid solution is not high enough to prevent the aging phenomenon completely, and when the aluminum content is more than 0.1%, the amount of aluminum present in the solid solution state is too high 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.

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

Vanadium is added in order to precipitate solid solution C to secure the non-aging characteristics. The content of the vanadium is 0.01% or more to obtain the non-aging characteristics. When the content exceeds 0.2%, the plastic anisotropy index is lowered.

The weight ratio (0.25 * V / C) of V and C preferably satisfies 1-20.

When the weight ratio of V and C is less than 1, the precipitation effect of the solid solution C is not large, and if it exceeds 20, the plastic anisotropy index is lowered.

One or two or more of P, Si, Cr as solid solution strengthening elements

The content of phosphorus (P) is preferably 0.03 to 0.2%.

Phosphorus is an element having a high solid solution strengthening effect and a small decrease in r (rankford) value, which guarantees high strength in steels controlling MnS precipitates according to the present invention. When the content of phosphorus is more than 0.03% to ensure the strength, in the case of more than 0.2% ductility is lowered to limit the upper limit to 0.2%. In the present invention, when securing high strength with only one or two of Si and Cr, the content of phosphorus (P) is preferably 0.015% or less.

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

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

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

Chromium is an element that lowers the secondary brittleness temperature and decreases the aging index by Cr carbide while having a high solids strengthening effect. The chromium guarantees high strength in steels controlling MnS precipitates and lowers the 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 weight ratio of Mn and S preferably satisfies 0.58 * Mn / S ≦ 10.

Manganese and sulfur combine to precipitate MnS, which affects the aging index, yield strength, and in-plane anisotropy index due to the addition of manganese and sulfur. According to the present invention, when the addition ratio of manganese and sulfur (0.58 * Mn / S, where the content of Mn, S in weight%) is more than 10, the MnS precipitate is coarse to increase the aging index, yield strength, in-plane anisotropy The characteristics of the exponent are not good.

The average size of the MnS precipitates is preferably 0.2 μm or less.

According to the results of the present invention, the size of MnS precipitates directly affects the aging index, yield strength, and in-plane anisotropy index. Especially, when the average size of MnS exceeds 0.2 µm, the aging index is rapidly increased and the in-plane anisotropy index is also high. Increases. Therefore, the average size of the MnS precipitates is preferably 0.2 μm or less.

[Manufacturing method of cold rolled steel sheet]

The present invention is characterized in that an average size of MnS precipitates in a cold rolled sheet is hot and cold rolled to satisfy the above-described stress. The size of MnS precipitates in the cold rolled plate is affected by the ratio of Mn / S and the manufacturing process, but in particular 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 lower than 1100 ℃, the reheating temperature is low, the coarse MnS produced during the continuous casting is not completely dissolved, the coarse MnS remains 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 less 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 200 ° C / min or more. According to the present invention, even if the component ratio (0.58 * Mn / S) of Mn and S is 10 or less, the precipitate size of MnS exceeds 0.2 µm when the cooling rate is less than 200 ° C / min. In other words, as the cooling rate increases, a large number of nuclei are generated, thereby minimizing MnS precipitates. In the case where the composition ratio of Mn and S (0.58 * Mn / S) is more than 10, there are many coarse MnS precipitates unresolved in the reheating process, and even if the cooling rate is fast, new nuclei are generated less and the precipitates do not become fine (Fig. 2b). , 0.024% C-0.4% Si-0.43% Mn-0.01% P-0.009% S-0.035% Al-0.0043% N-0.05% V). Referring to the graph of Figure 2, the faster the cooling rate is the size of the MnS precipitate becomes finer, 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 200-1000 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, MnS precipitates grow too coarsely, lowering the aging resistance.

[Cold rolling condition]

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

[Continuous Annealing]

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 500-900 degreeC. If the continuous annealing temperature is less than 500 ° C., the recrystallized grain is too fine to obtain a target ductility value. If the annealing temperature is higher than 900 ° C., the strength decreases due to coarsening of the recrystallized grain. The continuous annealing time keeps the recrystallization complete. If it is about 10 seconds or more, the recrystallization is completed.

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

Example 1

The steel slabs of Table 1 were reheated at 1200 ° C, hot rolled to finish, cooled at a rate of 200 ° C / min, and wound up at 650 ° C. The wound hot rolled sheet was subjected to cold rolling and continuous annealing at 75% reduction rate. 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. The obtained annealing plate was processed into a standard specimen according to ASTM E-8 standard to investigate the mechanical properties. Yield strength, tensile strength, elongation, plastic anisotropy index (r _m value), in-plane anisotropy index (△ r) and aging index (AI, Aging Index) were measured using a tensile tester (INSTRON, Model 6025). . Where r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 and Δr = (r ₀ −2r ₄₅ + r ₉₀ ) / 2.

sample Chemical composition (% by weight) 0.58 *
Mn / S 0.25 * V / C C Mn P S Al V N Ti Scope of the Invention ≤0.003 0.05-0.2 0.03-0.2 0.005-0.03 0.01-0.1 0.01-0.2 ≤0.004 - ≤10 1-20 One 0.0018 0.11 0.05 0.011 0.03 0.02 0.002 5.8 2.78 2 0.0022 0.08 0.048 0.01 0.024 0.15 0.0024 4.64 17 3 0.0028 0.1 0.113 0.012 0.033 0.062 0.002 5.8 5.5 4 0.0018 0.09 0.152 0.008 0.02 0.1 0.0025 6.53 13.9 5 0.0018 0.12 0.148 0.011 0.035 0.25 0.0019 6.33 34.7 6 0.0023 0.08 0.052 0.006 0.04 0 0.0015 7.73 0 7 0.0018 0.10 0.102 0.010 0.05 0 0.0026 5.8 0 8 0.0025 0.08 0.151 0.012 0.035 0 0.0018 3.87 0 9 0.0024 0.4 0.07 0.01 0.04 - 0.0016 0.05 11.6 0

Sample Number Mechanical properties Precipitation
water
Average
size
(Μm) Remarks Yield strength
(MPa) The tensile strength
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) Aging Index
(AI- (MPa) Secondary processing
Brittle
(DBTT- ℃) One 203 355 44 1.76 0.23 0 -70 0.12 Invention steel 2 205 360 43 1.79 0.25 0 -70 0.13 Invention steel 3 262 411 41 1.58 0.19 0 -60 0.14 Invention steel 4 303 459 38 1.48 0.17 0 -50 0.1 Invention steel 5 315 467 36 1.36 0.28 0 -40 0.13 Comparative steel 6 241 356 47 1.83 0.32 28 -70 0.11 Comparative steel 7 299 402 42 1.65 0.25 23 -50 0.09 Comparative steel 8 352 456 35 1.53 0.23 27 -40 0.14 Comparative steel 9 210 353 40 0.73 0.58 0 0 Conventional Steel

As shown in Tables 1 and 2, Sample Nos. 1 to 4 are invention steels satisfying the present invention. The aging index is 0 Mpa, which has non-aging characteristics, has a plastic anisotropy index of a certain level or higher, and the in-plane anisotropy index is low. It is excellent in workability and excellent in secondary work brittleness.

Sample No. 5 was steel with an excessive amount of V added, and the workability was not good. Sample No. 6-8 was a V-free steel, which did not secure non-aging characteristics. Sample No. 9 was Ti-added steel, and the secondary workability resistance was not good.

 [Example 2]

The steel slabs of Table 3 were manufactured under the conditions of Example 1. Mechanical properties were also measured in the same manner as in Example 1.                     

Sample Number Chemical composition (% by weight) 0.58 *
Mn / S 0.25
* V / C C Mn Si P S Al N V Scope of invention ≤
0.003 0.05-0.2 0.1
-0.8 ≤
0.015 0.005
-0.03 0.01
-0.1 ≤
0.004 0.01
-0.2 ≤10 1-20 One 0.0022 0.11 0.25 0.01 0.009 0.034 0.0022 0.021 7.08 2.39 2 0.0025 0.08 0.26 0.009 0.01 0.03 0.002 0.16 4.64 16 3 0.0026 0.11 0.43 0.01 0.012 0.025 0.0026 0.067 5.32 6.44 4 0.0018 0.09 0.63 0.009 0.01 0.033 0.0031 0.1 5.22 13.9 5 0.0021 0.15 0.63 0.011 0.012 0.036 0.0033 0.25 7.25 29.8 6 0.0019 0.11 0.22 0.01 0.008 0.04 0.0012 0 7.78 0 7 0.0025 0.09 0.43 0.015 0.011 0.04 0.0016 0 4.75 0 8 0.0018 0.1 0.62 0.011 0.009 0.035 0.0025 0 6.4 0

Sample Number Mechanical properties Precipitation
water
Average
size
(Μm) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) Aging Index
(AI- (MPa) Secondary processing
Brittle
(DBTT- ℃) One 198 360 47 1.77 0.32 0 -70 0.13 Invention steel 2 202 366 47 1.69 0.28 0 -80 0.14 Invention steel 3 259 410 44 1.58 0.21 0 -60 0.14 Invention steel 4 310 459 41 1.44 0.18 0 -50 0.12 Invention steel 5 315 466 37 1.35 0.17 0 -50 0.14 Comparative steel 6 241 356 50 1.75 0.36 24 -80 0.11 Comparative steel 7 299 402 44 1.61 0.29 28 -60 0.09 Comparative steel 8 352 456 38 1.47 0.31 22 -50 0.14 Comparative steel

As shown in Tables 3 and 4, Sample Nos. 1 to 4 are invention steels satisfying the present invention. The aging index is 0 Mpa, which has non-aging characteristics, has a plastic anisotropy index of a certain level or higher, and the in-plane anisotropy index is low. It is excellent in workability and excellent in secondary work brittleness.

Sample No. 5 was steel with an excessive amount of V added, and the workability was not good. Sample No. 6-8 was a V-free steel, which did not secure non-aging characteristics.

Example 3

The steel slabs of Table 5 were manufactured under the conditions of Example 1. Mechanical properties were also measured in the same manner as in Example 1.                     

sample Chemical composition (% by weight) 0.58 *
Mn / S 0.25
* V / C C Mn Cr P S Al N V Scope of invention ≤
0.003 0.05-0.2 0.2
-1.2 ≤
0.015 0.005
-0.03 0.01
-0.1 ≤
0.004 0.01
-0.2 ≤10 1-20 One 0.0015 0.11 0.33 0.01 0.01 0.023 0.0022 0.02 6.38 3.33 2 0.0022 0.09 0.3 0.009 0.008 0.033 0.002 0.16 6.53 18.2 3 0.0017 0.12 0.65 0.01 0.012 0.04 0.0031 0.061 5.8 8.97 4 0.0024 0.1 0.93 0.012 0.009 0.023 0.0022 0.1 6.44 10.4 5 0.002 0.11 0.9 0.015 0.012 0.035 0.0025 0.27 5.32 33.7 6 0.0024 0.09 0.32 0.01 0.007 0.05 0.0012 0 7.46 0 7 0.0022 0.11 0.63 0.015 0.012 0.04 0.0028 0 5.31 0 8 0.0018 0.11 0.95 0.011 0.015 0.03 0.0022 0 4.25 0

city
Ryo
time
number Mechanical properties Precipitate
Average size
(Μm) 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- ℃) One 197 352 47 1.65 0.28 0 -80 0.11 Invention steel 2 202 360 48 1.72 0.31 0 -80 0.12 Invention steel 3 263 412 44 1.51 0.19 0 -60 0.09 Invention steel 4 312 460 36 1.36 0.15 0 -50 0.13 Invention steel 5 320 471 32 1.24 0.16 0 -50 0.12 Comparative steel 6 235 352 47 1.70 0.32 21 -80 0.08 Comparative steel 7 299 418 44 1.51 0.25 18 -60 0.07 Comparative steel 8 349 459 36 1.42 0.19 16 -50 0.11 Comparative steel

As shown in Tables 5 and 6, Sample Nos. 1 to 4 are invention steels satisfying the present invention. The aging index is 0 Mpa, which has non-aging characteristics, has a plastic anisotropy index of a certain level or higher, and the in-plane anisotropy index is low. It is excellent in workability and excellent in secondary work brittleness.

Sample No. 5 was steel with an excessive amount of V added, and the workability was not good. Sample No. 6-8 was a V-free steel, which did not secure non-aging characteristics.

Example 4

The steel slabs of Table 7 were manufactured under the conditions of Example 1. Mechanical properties were also measured in the same manner as in Example 1.

sample Chemical composition (% by weight) 0.58 *
Mn / S 0.25
*
V / C C Mn P Si Cr S Al N V Scope of the Invention ≤
0.003 0.05-
0.2 0.03
-0.2
or
≤
0.015 0.1
-0.8 0.2
-1.2 0.005
-0.03 0.01
-0.1 ≤
0.004 0.01
-0.2 ≤10 1-20 One 0.0014 0.11 0.052 0.49 - 0.011 0.035 0.0018 0.07 5.8 12.5 2 0.0026 0.09 0.068 - 0.35 0.01 0.04 0.0030 0.065 5.22 6.25 3 0.0027 0.1 0.012 0.2 0.32 0.015 0.029 0.0018 0.048 3.87 4.44 4 0.0018 0.12 0.048 0.28 0.26 0.009 0.042 0.0022 0.12 7.73 16.7

Sample Number Mechanical properties Precipitate
Average size
(Μm) 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
(DBTT- ℃) One 325 482 34 1.59 0.19 0 -60 0.12 Invention steel 2 316 462 35 1.62 0.15 0 -60 0.09 Invention steel 3 297 419 39 1.67 0.19 0 -70 0.1 Invention steel 4 324 481 34 1.55 0.15 0 -60 0.09 Invention steel

As shown in Tables 7 and 8, Sample Nos. 1 to 4 are invented steels satisfying the present invention, and have non-aging characteristics as high strength steels excellent in workability and secondary workability.

As described above, the present invention provides a high-strength cold rolled steel sheet having a plastic anisotropy index of a predetermined level or more, having low in-plane anisotropy, excellent workability, excellent secondary work brittleness characteristics, and non-aging characteristics.

Claims (7)

By weight% C: 0.003% or less (excluding 0), Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less (excluding 0), V: 0.01 ~ 0.2%, P: 0.03-0.2%, the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S≤10, is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate A non-aging high strength cold rolled steel sheet having a thickness of 0.2 μm or less and a ductile-brittle transition temperature (DBTT) of −50 ° C. or less. By weight% C: 0.003% or less (excluding 0), Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less (excluding 0), V: 0.01 ~ 0.2%, P: 0.015% or less (excluding 0), Si: 0.1-0.8%, and the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and the remaining Fe and other unavoidable A non-aging high strength cold rolled steel sheet composed of impurities and having an average size of MnS precipitates of 0.2 µm or less, and having a ductile-brittle transition temperature (DBTT) of -50 ° C or less. By weight% C: 0.003% or less (excluding 0), Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less (excluding 0), V: 0.01 ~ 0.2%, P: 0.015% or less (excluding 0), Cr: 0.2-1.2%, and the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and the remaining Fe and other unavoidable A non-aging high strength cold rolled steel sheet composed of impurities and having an average size of MnS precipitates of 0.2 µm or less, and having a ductile-brittle transition temperature (DBTT) of -50 ° C or less. By weight% C: 0.003% or less (excluding 0), Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less (excluding 0), V: 0.01 ~ 0.2%, including two or three selected from the group: P: 0.03-0.2%, Si: 0.1-0.8%, Cr: 0.2-1.2%, wherein the weight ratio of Mn and S is 0.58 * Non-aging high strength cold rolled steel sheet that satisfies Mn / S ≦ 10, is composed of remaining Fe and other unavoidable impurities, has an average size of MnS precipitates of 0.2 μm or less, and has a ductile-brittle transition temperature (DBTT) of -50 ° C. or less . The non-aging high strength cold rolled steel sheet according to any one of claims 1 to 4, wherein a weight ratio (0.25 * V / C) of V and C is 1-20. delete delete

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