KR20060018301A - Non-aging cold rolled steel sheet and process for producing the same - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet used as a material for automobiles, home appliances, and the like. The cold rolled steel sheet is C: 0.003% or less, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01 by weight. -0.1%, N: 0.004% or less, P: 0.015% or less, V: 0.01 to 0.2%, and are composed of the remaining Fe and other unavoidable impurities. In the present invention, the weight ratio of V and C (0.25 * V / C) satisfies 1 to 20, and the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and the average size of MnS precipitates It may be made less than 0.2㎛. The present invention also provides a method for producing this cold rolled steel sheet.

Cold rolled steel, unaging, vanadium (V), plastic anisotropy, MnS

Description

 Non-aging cold rolled steel sheet and its manufacturing method {NON-AGING COLD ROLLED STEEL SHEET 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 the MnS precipitates according to the cooling rate.

The present invention relates to a cold rolled steel sheet used as a material for automobiles, home appliances, and the like, and more particularly, to a cold rolled steel sheet and a method of manufacturing the non-aging characteristics improved by the addition of V. Furthermore, in the cold rolled steel sheet of the present invention, the mechanical properties are further improved by the fine MnS precipitate.

Cold rolled steel sheets used in automobiles and home appliances require strength and formability as well as non-aging characteristics. 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 non-aging property of the cold rolled steel sheet can be secured by the annealing of the aluminum-kilted steel, but the annealing has the disadvantage that the annealing time is long and the productivity is low and the material deviation is severe for each part. Therefore, IF steel (Interstitial Free Steel) which 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 Publication Nos. Hei 6-093376, 6-093377, and 6-212354 show that they are strictly managed at C: 0.0001 to 0.0015% in steel with 0.0001 to 0.003% of B added instead of Ti and Nb. It is a technology that improves Hyosung. However, the prior art does not have sufficient aging, and quenching after annealing is recommended to secure the aging. In this case, most of the quenching is performed in order to remove the oxide film generated during the quenching process. 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 excellent aging resistance and ductility while maintaining plastic anisotropy index above a certain level. However, in order to control the carbon content below 0.002%, the cold rolled steel sheet has a disadvantage in that it is costly and very low in productivity because a strong decarburization treatment must be performed in the steelmaking process. Also, in order to control C + N + S + P to 0.025%, desulfurization and dephosphorization ability must be enhanced, which is very disadvantageous in terms of productivity and cost. In addition, it does not guarantee complete non-aging characteristics.

The present invention provides a cold rolled steel sheet having a completely unaging property and excellent workability without adding Ti and Nb, and an object thereof.

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, P : 0.015% or less, V: 0.01% to 0.2%, and is composed of the remaining Fe and other unavoidable impurities.

Furthermore, the manufacturing method of the cold rolled steel sheet of this invention 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, P: 0.015 by weight%. % Or less, V: 0.01 ~ 0.2%, re-heated steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ℃ or more and hot-rolled with the finish rolling temperature above Ar ₃ transformation point and more than 200 ℃ / min It comprises cooling at a rate and winding at a temperature of 700 ° C. or lower, followed by cold rolling and continuous annealing.

In the present invention, the weight ratio (0.25 * V / C) of the V and C satisfies 1 to 20, and the weight ratio of the Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10 to produce the MnS precipitate. More preferably, the average size is 0.2 탆 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 workability without adding Ti and Nb.

In the case of adding V to an unaging cold rolled steel sheet without adding Ti and Nb, V precipitates solid solution C, thereby securing non-aging characteristics. Further, workability and the like are further improved by the fine precipitate of MnS.                     

As shown in FIG. 1, the finer the distribution of MnS precipitates, the smaller the amount of dissolved carbon in the grains, thereby improving the aging resistance. When the size of the fine precipitate of MnS is less than 0.2 µm, the amount of solid solution carbon in the grains may be more advantageous to the aging characteristics. In the present invention, the amount of solid solution carbon is effectively controlled by V, and the fine MnS precipitate exhibits a greater effect on the improvement of the plastic anisotropy index. As such, when V is added together to have a fine MnS precipitate, not only the aging characteristics but also the processability are greatly improved. In the present invention, since the solid solution carbon can be effectively controlled by the MnS precipitates as well as V, there is an advantage that the content of C can be expanded to 0.003% with a low load in the steelmaking process.

By paying attention to these new facts, we have studied how to finely distribute MnS in V-added 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, Fig. 2 (a) is a steel having 0.0018% C-0.15% Mn-0.008% P-0.015% S-0.03% Al-0.0012% N-0.05% V and the ratio of Mn and S (0.58 * Mn / S) It is a graph that investigates the size of precipitates according to the cooling rate after hot rolling steel with composition 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.

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 improve the plastic anisotropy index, and the content of manganese is preferably 0.05 to 0.2%. If the content of manganese is less than 0.05%, red brittleness may occur due to the high content of sulfur remaining in solid solution. If the content of manganese is more than 0.2%, coarse MnS precipitates are formed due to high content of manganese. Anisotropic index is lowered.

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 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 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 (0.25 * 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.

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 plastic anisotropy index because the precipitated state varies 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 to lower the plastic anisotropy index.

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 precipitate directly affects the aging index, yield strength, and plastic anisotropy index. In particular, when the average size of MnS exceeds 0.2 µm, the plastic anisotropy index is lowered. 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 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. If the composition ratio of Mn and S (0.58 * Mn / S) is more than 10, there are many coarse MnS precipitates undissolved 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-0.43% Mn-0.011% 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 coarse to deteriorate the inaging properties.

[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 rate is more than 90%, the moldability is improved, but the nucleation amount is too large, so the annealing recrystallized grain is too fine to decrease the ductility.

[Continuous Annealing]

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 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

The ingots of Table 1 were reheated, rolled up after finishing hot rolling, and then cold-rolled 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. 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 Number Chemical composition (% by weight) 0.58 * Mn / S 0.25 * (0.25 * V / C) C Mn P S Al V N ≤0.003 0.05-0.2 ≤0.015 0.005-0.03 0.01-0.1 0.01-0.2 ≤0.004 One 0.0015 0.1 0.011 0.009 0.033 0.023 0.0025 6.44 3.83 2 0.0024 0.08 0.01 0.01 0.035 0.051 0.0012 4.64 5.13 3 0.0025 0.12 0.008 0.012 0.023 0.08 0.0015 5.8 8 4 0.0015 0.11 0.01 0.02 0.032 0.11 0.002 3.19 18.3 5 0.0027 0.08 0.008 0.01 0.033 0.154 0.0011 4.64 14.3 6 0.0025 0.25 0.011 0.01 0.035 0.07 0.0021 14.5 7 7 0.002 0.4 0.01 0.013 0.022 0.325 0.0013 17.8 30 8 0.0023 0.08 0.01 0.005 0.04 0 0.0015 9.28 0 9 0.0018 0.10 0.011 0.012 0.05 0 0.0026 4.83 0 10 0.0018 0.15 0.008 0.015 0.03 0 0.0012 5.8 0 11 0.0027 0.09 0.012 0.025 0.035 0 0.0018 2.09 0

number Reheating Temperature (℃) Cooling rate (℃ / min) Winding temperature (℃) Remarks One 1200 200 650 Invention steel 2 1200 200 650 Invention steel 3 1200 200 650 Invention steel 4 1200 200 650 Invention steel 5 1200 200 650 Invention steel 6 1200 200 650 Comparative steel 7 1200 200 650 Comparative steel 8 1200 200 650 Comparative steel 9 1200 200 650 Comparative steel 10 1200 200 650 Comparative steel 11 1200 200 650 Comparative steel

Sample Number Mechanical properties Average size of precipitates (㎛) Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) Aging Index (AI- (MPa) One 175 295 50 1.82 0 0.06 Invention steel 2 163 301 53 1.86 0 0.11 Invention steel 3 158 284 49 1.9 0 0.12 Invention steel 4 148 278 49 1.77 0 0.13 Invention steel 5 175 302 49 1.74 0 0.14 Invention steel 6 185 292 50 1.61 18 0.39 Comparative steel 7 182 308 47 1.52 0 0.54 Comparative steel 8 211 309 49 1.83 23 0.05 Comparative steel 9 209 311 52 1.93 22 0.12 Comparative steel 10 201 295 54 1.94 21 0.15 Comparative steel 11 223 319 48 1.88 27 0.14 Comparative steel

As shown in Table 2, Sample No. 1 to 5 is an invention steel that satisfies the present invention can secure non-aging characteristics. In addition, in the case of Sample No. 1-5, the size of MnS precipitate was 0.2 μm or less, and the plastic anisotropy index was high, indicating that the workability was more excellent.

On the other hand, sample No. 6, which is comparative steel, decreased the aging index due to the addition of V, but the precipitate size was too large, the aging index was 18MPa, which did not have perfect non-aging characteristics, and the plastic anisotropy index was also low, resulting in low workability. In the case of Sample No. 7, too much V was added and the plastic anisotropy index was very low. For Sample Nos. 8 to 11, the steel without V added cannot guarantee complete non-aging characteristics.

As described above, the cold rolled steel sheet provided in accordance with the present invention can guarantee complete non-aging characteristics and is excellent in workability.

Claims (8)

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, P: 0.015% or less, V: 0.01-0.2% And, unaging cold rolled steel sheet composed of the remaining Fe and other unavoidable impurities. The non-aging cold rolled steel sheet according to claim 1, wherein a weight ratio (0.25 * V / C) of V and C satisfies 1 to 20. The non-aging cold rolled steel sheet according to claim 1, wherein the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and the average size of MnS precipitate is 0.2 μm or less. The non-aging cold rolled steel sheet according to claim 1, wherein the C is greater than 0.002% and less than 0.003%. 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, P: 0.015% or less, V: 0.01-0.2% After reheating the steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ° C or higher, hot rolling with the finish rolling temperature above the Ar ₃ transformation point, cooling at a speed of 200 ° C / min or more, and winding at a temperature of 700 ° C or less. And then cold rolling, followed by continuous annealing. The method according to claim 5, wherein the weight ratio (0.25 * V / C) of V and C satisfies 1 to 20. The method for producing an unaging cold rolled steel sheet according to claim 5, wherein the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and the average size of MnS precipitates is 0.2 µm or less. 8. The method of claim 5, wherein C is more than 0.002% and less than 0.003%.

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