KR101115842B1 - Bake hardening cold rolled steel sheet having superior workability and high strength, and process for producing the same - Google Patents

Abstract

The present invention relates to a hardening hardening type high strength cold rolled steel sheet used as a material for automobiles, home appliances, and the like. This cold rolled steel sheet is C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, Mo: 0.01% to 0.2% At least one selected from the group consisting of P: 0.03-0.2%, Si: 0.1-0.8% and Cr: 0.2-1.2%, and is composed of the remaining Fe and other unavoidable impurities and has an average size of MnS precipitates. It consists of 0.2 micrometer or less. Also provided is a method for producing this cold rolled steel sheet.

In the cold rolled steel sheet of the present invention, by controlling the content of carbon in the steel in which fine MnS is precipitated, the baking hardening property is secured, and the addition of Mo results in a high plastic anisotropy index and a low in-plane anisotropy index.

Cold Rolled Steel, Hardening Hardening, Mo, Plastic Anisotropy Index, MnS

Description

BAKE HARDENING COLD ROLLED STEEL SHEET HAVING SUPERIOR WORKABILITY AND 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 hardening hardening type high strength cold rolled steel sheet used as a material for automobiles, home appliances, etc. More specifically, by controlling the content of the fine MnS precipitates and carbon to secure the hardening hardening properties while excellent processability by the addition of Mo It relates to a high strength cold rolled steel sheet and a method of manufacturing the same.

In order to improve the dent resistance, exterior hardening type cold rolled steel sheets are frequently used for exterior materials such as automobiles. The hardened hardened cold rolled steel is a steel in which a yield of solid solution carbon is retained in the steel sheet and the dislocation generated during press molding is fixed by using solid heat of 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 solid carbon remains, and it has a hardening hardening capacity of about 10 to 20 Mpa after the quenching 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.

In the case of IF steel, Ti and Nb are added to completely precipitate the carbon or nitrogen dissolved in the steel to improve moldability. The hardening hardening characteristic is given to the IF steel by hardening. 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. In the case of small hardening IF steel, in order to employ an appropriate amount of carbon, not only the amount of carbon added, but also the amount of Ti or Nb added, as well as the amount of sulfur and nitrogen that react with Ti and Nb to form precipitates are controlled within a very narrow range. Because of this, it is difficult to secure stable quality and costs a lot of production.

The present invention provides a cold rolled steel sheet and a method for manufacturing the same, which improve workability while securing the hardening hardening characteristic in a high strength cold rolled steel sheet without adding Ti and Nb.

Cold rolled steel sheet of the present invention for achieving the above object, by weight% C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, Mo: 0.01% to 0.2%, and a component for increasing 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, and the remaining Fe and other unavoidable impurities It is composed of, 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, Mo: 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 hardening characteristics without adding Ti and Nb as a high strength cold rolled steel sheet.

If the carbon content is adjusted to 0.003-0.005% on a cold rolled steel sheet without adding Ti and Nb, and finely distributes the precipitates of MnS, the yield strength after baking is increased. In addition, when Mo is added, the plastic anisotropy index is increased and the in-plane anisotropy index is lowered, thereby significantly improving workability.

As shown in FIG. 1, the finer the precipitate of MnS, the smaller the amount of solid solution carbon in the grains. If the carbon content is 0.003 ~ 0.005% in the finely distributed steel of MnS precipitates, it will have the hardening characteristic. This suggests that when the MnS precipitates are minutely distributed, carbon precipitates around the MnS precipitates, which does not cause aging at room temperature, but dissolves the carbon in the coating sub-heat treatment to significantly increase the yield strength.                     

For this purpose, the content of carbon should be adjusted to 0.003 ~ 0.005%, the size of the fine precipitate of MnS was determined to be less than 0.2㎛.

Paying attention to these new facts, we have studied how to finely distribute MnS. 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 0.004% C-0.4% Si-0.15% Mn-0.008% P-0.015% S-0.03% Al-0.0012% N-0.05% Mo, the composition 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), the Mn and S component ratio (0.58 * Mn / S) when the cooling rate is adjusted for the case of less than 10 confirms that the precipitate size of MnS can satisfy 0.2㎛ or less Can be.

Based on this aspect, the cold rolled steel sheet of the present invention and a method of manufacturing the same will be described in detail below.

[Cold rolled steel sheet of the present invention]

The content of carbon (C) is preferably 0.003% to 0.005%.                     

If the content of carbon is less than 0.003%, the amount of hardening of baking in steel is small, and if the content of carbon is more than 0.005%, the moldability decreases rapidly.

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, and carbon is precipitated around the MnS precipitate, and the precipitated carbon is dissolved in the coating baking treatment to give baking hardening capacity. It is preferable to make content of manganese into 0.05 to 0.2%. 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 to produce a coarse MnS precipitate is poor in hardening properties.

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 precipitates but also the size of precipitated MnS is very coarse, which is not good for baking hardening characteristics. 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.

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

Aluminum is an element added as a deoxidizer, but in the present invention, nitrogen is added in order to prevent formability deterioration due to solid nitrogen by precipitating nitrogen in steel. If the aluminum content is less than 0.01%, the amount of solid solution is high because the amount of solid solution decreases, and when the aluminum content is more than 0.1%, the amount of aluminum in the solid solution state is high and the ductility is lowered.

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

Nitrogen is an element inevitably added during steelmaking, and in the case of more than 0.004%, the moldability is lowered, so 0.004% or less is preferable.

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

Molybdenum is added to ensure the processability, the content is more than 0.01% to improve the processability, when the content exceeds 0.2% there is a fear that the workability is no longer improved and cause hot brittleness.

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 value, which guarantees high strength in steels controlling MnS precipitates according to the present invention. When the phosphorus content is 0.03% or more to secure 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 as MnS, which affects the hardening hardening ability, yield strength, and in-plane anisotropy index, depending on the amount of manganese and sulfur added. 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, MnS precipitates are coarse, and the hardening hardening property is lowered, yield strength, in-plane Anisotropy index is 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 hardening hardening, yield strength and in-plane anisotropy index. Especially, when the average size of MnS is more than 0.2㎛, the hardening hardening characteristic drops sharply and the in-plane anisotropic index is reduced. Also 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 carried out at a finish rolling temperature above the Ar ₃ transformation temperature. This is because when the finish rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is 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). , 0.0038% C-0.4% Si-0.43% Mn-0.011% P-0.009% S-0.035% Al-0.0043% N-0.05% Mo). 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 exceeds 700 ℃ MnS precipitates grow too coarse, so the hardening hardening characteristics are not good.                     

[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

In Table 1, the steel slabs of Samples 1-8 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 steel sheet was 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 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. The specimen was measured for yield strength, tensile strength, elongation, plastic anisotropy index (r _m value), and in-plane anisotropy index (Δr) 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. The quench hardening properties are shown in Table 2 as the yield strength measured after quenching after 20% heat treatment at 200 ° C. after adding 2% of the strain to the specimen.

sample Chemical composition (% by weight) 0.58 *
Mn / S C Mn P S Al N Mo Ti Scope of the Invention 0.003-0.005 0.05-0.2 0.03-0.2 0.005-0.03 0.01-0.1 ≤0.004 0.01-0.2 ≤10 One 0.0044 0.1 0.05 0.008 0.034 0.0018 0.025 7.25 2 0.0035 0.09 0.053 0.01 0.024 0.0022 0.17 5.22 3 0.0046 0.11 0.11 0.012 0.035 0.0023 0.059 5.32 4 0.0033 0.08 0.15 0.011 0.023 0.0028 0.11 4.22 5 0.0045 0.1 0.146 0.01 0.033 0.0015 0.27 5.8 6 0.0035 0.08 0.052 0.006 0.04 0.0015 0 7.73 7 0.0042 0.10 0.102 0.010 0.05 0.0026 0 5.8 8 0.0039 0.08 0.151 0.012 0.035 0.0018 0 3.87 9 0.0028 0.4 0.07 0.01 0.04 0.0016 - 0.022 11.6

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) Sofufu
Yield strength
(MPa) Secondary processing
Brittle
(DBTT- ℃) One 259 361 44 1.95 0.31 321 -80 0.12 Invention steel 2 245 355 46 1.89 0.3 315 -70 0.11 Invention steel 3 309 410 38 1.78 0.32 351 -50 0.11 Invention steel 4 370 456 33 1.69 0.31 410 -40 0.09 Invention steel 5 380 466 31 1.38 0.24 424 -40 0.1 Comparative steel 6 252 362 43 1.65 0.25 305 -70 0.13 Comparative steel 7 305 411 36 1.52 0.29 346 -50 0.12 Comparative steel 8 377 460 32 1.46 0.27 414 -40 0.09 Comparative steel 9 222 358 44 1.68 0.45 258 0 - Conventional Steel

As shown in Tables 1 and 2, Sample Nos. 1 to 4 are inventive steels satisfying the present invention, having high plastic anisotropy index, excellent secondary work brittleness, and a hardening hardening characteristic.

In the case of Sample No. 5, an excessive amount of Mo was added, resulting in a low plastic anisotropy index. In the case of Sample No. 6-8, Mo was not added and workability was not good. Sample No. 9 was Ti-added steel, which had poor secondary work brittleness and high in-plane anisotropy index.

[Example 2]

The steel slabs of Table 3 were reheated at 1200 ° C., finished hot rolled, cooled at 200 ° C./min and wound up at 650 ° C. The wound hot rolled steel sheet was cold rolled at a reduction ratio of 75% and continuously annealed. 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 mechanical properties were measured in the same manner as in Example 1.

sample Chemical composition (% by weight) 0.58 *
Mn / S C Mn Si P S Al N Mo Scope of the Invention 0.003-0.005 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 One 0.0046 0.1 0.25 0.01 0.008 0.035 0.0032 0.028
- 7.25 2 0.0044 0.08 0.25 0.011 0.01 0.033 0.0012 0.16
- 4.64 3 0.0035 0.09 0.45 0.011 0.011 0.023 0.002 0.073
- 4.75 4 0.0038 0.1 0.62 0.009 0.01 0.03 0.0032 0.11
- 5.8 5 0.0045 0.11 0.65 0.01 0.011 0.043 0.0022 0.29 5.8 6 0.0039 0.09 0.24 0.009 0.006 0.05 0.0022 - 8.7 7 0.0042 0.09 0.43 0.013 0.012 0.03 0.0026 - 4.35 8 0.0035 0.1 0.62 0.011 0.009 0.035 0.0025 - 6.4

Sample Number Mechanical properties Precipitate
Average size
(Μm) Remarks Yield strength
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Firing
Anisotropy
Indices
(r _m ) Inside
Anisotropy
Indices
(△ r) Sofufu
Yield Steel
Degree (MPa) Secondary processing
Brittle
(DBTT- ℃) One 255 355 46 1.98 0.32 302 -80 0.1 Invention steel 2 255 359 45 1.89 0.28 310 -80 0.09 Invention steel 3 310 415 42 1.78 0.25 338 -60 0.11 Invention steel 4 360 454 36 1.62 0.27 410 -50 0.13 Invention steel 5 366 460 34 1.32 0.20 419 -50 0.09 Comparative steel 6 250 360 45 1.64 0.25 312 -80 0.09 Comparative steel 7 315 421 40 1.52 0.22 348 -60 0.11 Comparative steel 8 366 460 35 1.46 0.29 414 -50 0.11 Comparative steel

As shown in Tables 3 and 4, Sample Nos. 1 to 4 are inventive steels satisfying the present invention, have high plastic anisotropy index, excellent secondary work brittleness, and have hardening hardening properties.

In the case of Sample No. 5, an excessive amount of Mo was added, resulting in low plastic anisotropy index. In the case of Sample No. 6-8, Mo was not added and workability was not good.

Example 3

The steel slabs of Table 5 were reheated at 1200 ° C., finished hot rolled, cooled at 200 ° C./min and wound up at 650 ° C. The wound hot rolled steel sheet was cold rolled and continuously annealed 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. The mechanical properties were measured in the same manner as in Example 1.

sample Chemical composition (% by weight) 0.58 *
Mn / S C Mn Cr P S Al N Mo Scope of the Invention 0.003-0.005 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 _One 0.0034 0.11 0.33 0.011 0.009 0.034 0.0012 0.019 7.09 ₂ 0.0047 0.09 0.3 0.01 0.011 0.029 0.0023 0.16 4.75 ₃ 0.0042 0.09 0.68 0.012 0.01 0.042 0.0022 0.069 5.22 ₄ 0.0041 0.1 0.81 0.009 0.011 0.025 0.0015 0.1 5.27 ₅ 0.0034 0.12 0.85 0.012 0.01 0.024 0.0022 0.26 5.27 ₆ 0.0044 0.1 0.35 0.01 0.007 0.04 0.0024 0 8.29 ₇ 0.0032 0.09 0.65 0.01 0.012 0.04 0.0032 0 4.35 ₈ 0.0038 0.11 0.82 0.012 0.017 0.05 0.0018 0 3.75

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) Sofufu
Yield strength
(MPa) Secondary processing
Brittle
(DBTT- ℃) One 235 359 46 1.95 0.29 295 -80 0.09 Invention steel 2 245 364 43 1.90 0.23 307 -80 0.12 Invention steel 3 310 415 39 1.70 0.28 360 -60 0.13 Invention steel 4 354 460 34 1.63 0.27 415 -50 0.12 Invention steel 5 350 455 36 1.34 0.22 420 -50 0.11 Comparative steel 6 239 360 44 1.62 0.20 293 -80 0.09 Comparative steel 7 306 420 38 1.44 0.22 359 -60 0.10 Comparative steel 8 350 462 33 1.40 0.21 428 -50 0.09 Comparative steel

As shown in Tables 5 and 6, Sample Nos. 1 to 4 are inventive steels satisfying the present invention, have high plastic anisotropy index, excellent secondary work brittleness, and have hardening characteristics.

In the case of Sample No. 5, an excessive amount of Mo was added, resulting in low plastic anisotropy index. In the case of Sample No. 6-8, Mo was not added and workability was not good.

Example 4

The steel slabs in Table 7 were reheated at 1200 ° C., finished hot rolled, cooled at 200 ° C./min and wound up at 650 ° C. The wound hot rolled steel sheet was cold rolled and continuously annealed 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. The mechanical properties were measured in the same manner as in Example 1.

sample Chemical composition (% by weight) 0.58 *
Mn / S C Mn P Si Cr S Al N Mo Scope of the Invention 0.003-0.005 0.05-
0.2 0.03-
0.2 or ≤0.015 0.1-
0.6 0.2-
1.2 0.005-
0.03 0.01-0.1 ≤0.004 0.01-
0.2 ≤10 One 0.0041 0.11 0.065 0.48 - 0.01 0.03 0.0022 0.08 6.38 2 0.0038 0.1 0.075 - 0.35 0.012 0.042 0.0035 0.06 4.83 3 0.0043 0.1 0.012 0.2 0.32 0.015 0.029 0.0018 0.04 3.87 4 0.0034 0.09 0.05 0.2 0.32 0.012 0.04 0.0022 0.1 4.35

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) Sofufu
Yield strength
(MPa) Secondary processing
Brittle
(DBTT- ℃) One 351 474 36 1.59 0.17 406 -60 0.1 Invention steel 2 335 462 35 1.55 0.15 390 -60 0.11 Invention steel 3 328 419 39 1.67 0.19 358 -70 0.09 Invention steel 4 365 481 34 1.55 0.15 426 -60 0.1 Invention steel

As shown in Tables 7 and 8, Sample Nos. 1 to 4 are inventive steels satisfying the present invention, have high plastic anisotropy index, excellent secondary work brittleness, and have hardening characteristics.

 As described above, the cold rolled steel sheet provided in accordance with the present invention has a small hardening property, a high plastic anisotropy index, a low in-plane anisotropy index, and thus less wrinkles during processing, and has less useful effects after processing.

Claims (5)

By weight% C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, Mo: 0.01-0.2%, P: 0.03-0.2% Wherein 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, and the average size of the MnS precipitate is less than 0.2 μm, Hardened hardened high strength cold rolled steel with excellent workability with a Brittle Transition Temperature (DBTT) of -40 ℃ or less. C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, P: 0.015% or less, Mo: 0.01-0.2%, by weight% Si: 0.1-0.8%, the weight ratio of Mn and S satisfies the following conditions 0.58 * Mn / S ≤ 10, is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate is less than 0.2㎛ Hardening-type hardened cold rolled steel sheet having excellent workability, which is composed of a ductile-brittle transition temperature (DBTT) of -40 ° C. or less. C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, P: 0.015% or less, Mo: 0.01-0.2%, by weight% Cr: 0.2-1.2%, 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 is less than 0.2㎛ Hardening-type hardened cold rolled steel sheet having excellent workability, which is composed of a ductile-brittle transition temperature (DBTT) of -40 ° C. or less. C: 0.003-0.005% by weight, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, Mo: 0.01-0.2%, here P: 0.03- 2 or 3 selected from the group of 0.2%, Si: 0.1-0.8%, Cr: 0.2-1.2%, and the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, It is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate is less than 0.2㎛, and the hard-hardened high strength cold rolled steel sheet having excellent workability with a ductile-brittle transition temperature (DBTT) of -40 ℃ or less. delete

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