KR101125930B1 - Non-aging cold rolled steel sheet having excellent resistance to second work embrittleness and high strength, 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, etc. The 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, P: 0.015% or less, Si: 0.1-0.8%, the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and the remaining Fe and other unavoidable It is composed of impurities, and the average size of MnS precipitate is less than 0.2㎛. In addition, a method for producing this cold rolled steel sheet is also provided.

Cold rolled steel sheet of the present invention can be used in the extreme region with an aging index of 30MPa or less and a ductile-brittle transition temperature (DBTT) of -50 ℃ or less can ensure the non-aging. In addition, yield strength is more than 240Mpa and yield strength-ductility balance is more than 12000 can reduce the product thickness, there is a light weight effect. In addition, the plastic anisotropy index (r _m ) is 1.4 or more, the in-plane anisotropy index (△ r) is 0.4 or less, less wrinkles during processing, and less ear generation after processing.

Cold rolled steel, non-aging, high strength, secondary processing brittleness, in-plane anisotropy, MnS precipitate

Description

 High-strength, non-aging cold rolled steel with excellent secondary processing brittleness and its manufacturing method

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 the like, and more particularly, due to the fine MnS precipitate in Si-added steel, the yield strength and the strength-ductility balance are excellent and the secondary machining characteristics are excellent. It relates to an excellent high strength non-aging cold rolled steel sheet and a method of manufacturing the same.

Cold rolled steel sheets used in automobiles and home appliances require strength and formability as well as non-aging characteristics. 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 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 increase the recrystallization temperature, so they must be annealed at high 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 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, this prior art does not have sufficient aging, and quenching after annealing is recommended to secure aging. In this case, since most of the water is cooled, 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 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, cold rolling is carried out at a reduction ratio of 50 to 90%, followed by continuous annealing for 10 seconds or more at a temperature in the range of 650-850 ° C. The cold rolled steel sheet thus obtained is excellent in aging and ductility while maintaining the plastic anisotropy index above a certain level. However, the cold rolled steel sheet obtained in Korean Unexamined Patent Publication No. 2000-0039137 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 to control the carbon content to 0.002% or less. In addition, in order to control C + N + S + P to 0.025%, desulfurization and dephosphorization capacity must be strengthened, which is very disadvantageous in terms of productivity and cost. In addition, the yield strength is too low on the material side, there is a problem to use a thicker material, and when processing, there is a problem in that the in-plane anisotropy index (△ r) is too high wrinkles are generated a lot.

On the other hand, cold rolled steel sheet having a high yield strength and non-aging cold rolled steel sheet is known to increase the content of Mn and P of 0.3 to 0.7% in ultra low carbon steel and to add Ti. The cold-rolled steel sheet has a poor second brittle resistance to ductile-brittle transition temperature of 0 ~ 30 ℃ so that breakage occurs at impact even at room temperature.

According to the present invention, a cold rolled steel sheet having excellent inferior brittle resistance and excellent yield strength, strength-ductility balance characteristics, and small in-plane anisotropy without having Ti and Nb added thereto and having substantial non-aging characteristics, and a manufacturing method thereof To provide, for that purpose.

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, Si: 0.1-0.8%, 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 0.2 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, P: 0.015 % Or less, Si: 0.1 ~ 0.8%, the weight ratio of Mn and S satisfy the following conditions 0.58 * Mn / S ≤ 10, and after reheating the steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ℃ or more Hot rolling with the finish rolling temperature above Ar ₃ transformation point, cooled at a rate of 200 ° C / min or more, wound at a temperature below 700 ° C, cold rolled at a rolling reduction rate of 50-90%, and a temperature range of 500-900 ° C. Continuous annealing at.

Hereinafter, the present invention will be described in detail.

The present inventors have discovered the following new facts in the course of research for securing the high strength properties together with the secondary processing characteristics without the aging characteristics without adding Ti and Nb to the Si-added cold rolled steel sheet. In other words, for the high strength, the precipitate of MnS in the Si-added steel has an effect on the aging characteristics, the yield strength and the strength-ductility balance characteristics due to the precipitation strengthening, and the secondary processing characteristics.

As shown in Figure 1, the finer distribution of the precipitate of MnS in the Si-added high strength cold-rolled steel sheet is to reduce the amount of solid solution carbon in the grains to improve the aging 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%, which is less load in the steelmaking process.

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 0.002% C-0.4% Si -0.10% Mn-0.01% P-0.010% S-0.03% Al-0.0024% N, the composition 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.

According to the present invention, Si is 0.1 to 0.8 in the basic component system of C: 0.003% or less, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004%, and P: 0.015% or less. If the weight ratio of Mn and S meets the following conditions 0.58 * Mn / S≤10 with the addition of% and the average size of the MnS precipitate is less than 0.2㎛, the secondary processing brittleness is basically obtained at high strength and inferior aging. Also becomes excellent. As the precipitate becomes finer, the appropriate amount of carbon remains in the grain boundary to strengthen the grain boundary, thereby preventing brittle fracture which may occur due to the weak grain boundary after processing.

 The steel of the present invention satisfies the yield strength of 240 MPa or more, the yield strength-ductility balance of 12000 or more, and the in-plane anisotropy index Δr satisfies 0.4 or less. If the yield strength is high, the thickness of the steel sheet can be reduced, thereby reducing the weight, and also having low in-plane anisotropy, less wrinkles during processing, and less ears after processing.

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 and the inferiority is basically secured while improving the secondary processing brittleness, yield strength, and in-plane anisotropy, based on the results of the study. It is preferable to set it as%. If the content of manganese is less than 0.05%, red brittleness may occur due to the large amount 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. Hyosung becomes worse.

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.

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

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

Silicon is an element having high solid solution strengthening effect and low drop in elongation, which ensures high strength in steels controlling MnS precipitates according to the present invention. If the content of silicon is more than 0.1% to secure the strength, in the case of more than 0.8% 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, secondary processing brittleness, 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, the MnS precipitate is coarse to increase the aging index, secondary brittleness, Yield strength and in-plane anisotropy index 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. Secondary brittle brittleness is also poor. 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. 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). , 0.0024C-0.4Si -0.43Mn-0.01P-0.009S-0.035Al-0.0043N). 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 inaging.

[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]

The ingots in Table 1 were reheated, hot rolled to finish, cooled, then wound up, and then cold rolled and continuous 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. 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

Reheating Temperature (℃) Cooling rate
(℃ / min.) Winding
Temperature (℃) C Mn Si P S

Al N Ti One 0.0019 0.11 0.22 0.01 0.008 0.04 0.0012 - 7.78 1200 200 650 2 0.0025 0.09 0.43 0.015 0.011 0.04 0.0016 - 4.75 1200 200 650 3 0.0018 0.1 0.62 0.011 0.009 0.035 0.0025 - 6.4 1200 200 650 4 0.0026 0.42 0.25 0.01 0.01 0.03 0.0028 - 24.4 1200 200 650 5 0.0017 0.4 0.41 0.015 0.011 0.04 0.0032 - 21.1 1200 200 650 6 0.0022 0.43 0.62 0.01 0.008 0.05 0.0039 - 31.2 1200 200 650 7 0.015 0.12 - 0.0081 0.0033 0.03 0.0021 - 21.0 1250 50 650 8 0.0011 0.23 - 0.0083 0.0033 0.04 0.0028 - 24.7 1250 50 650 9 0.0008 0.23 - 0.0073 0.0093 0.05 0.0019 - 14.3 1250 50 650 10 0.0010 0.22 - 0.0059 0.0122 0.03 0.0035 - 10.4 1250 50 650 11 0.0024 0.4 0.01 0.07 0.01 0.04 0.0016 0.05 11.6 1200 200 650 12 0.0028 0.4 0.012 0.121 0.012 0.05 0.0021 0.05 19.3 1200 200 650 13 0.0018 0.4 0.01 0.167 0.01 0.05 0.0019
0.05 8.7 1200 200 650 Sample Nos. 7, 8, 9, and 10 are conventional soft cold rolled steel sheets, which are the invention steels 1, 2, 3, and 4 of Korean Patent Publication No. 2000-039137.
Sample numbers 11, 12 and 13 are conventional high strength cold rolled steel sheets

Sample Number Mechanical properties Average size of precipitate
(Μm) Remarks Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index
(△ r) Aging Index
(AI- (MPa) 2nd processing brittleness
(DBTT- ℃) One 241 356 50 1.75 0.28 24 -80 0.11 Invention steel 2 299 402 44 1.61 0.26 28 -60 0.09 Invention steel 3 352 456 38 1.47 0.31 22 -50 0.14 Invention steel 4 231 346 45 1.72 0.58 42 -70 0.49 Comparative steel 5 289 394 39 1.63 0.48 35 -60 0.35 Comparative steel 6 343 445 35 1.42 0.42 40  -40 0.62 Comparative steel 7 142 261 62.1 1.84 1.21 13 -50 0.52 Conventional Steel 8 145 264 60.2 1.8 0.98 12 -50 0.43 Conventional Steel 9 150 272 58.3 1.75 0.82 9 -50 0.46 Conventional Steel 10 160 279 56.3 1.72 0.68 12 -50 0.39 Conventional Steel 11 210 353 40 1.73 0.58 0 + 0 0.21 Conventional Steel 12 265 395 35 1.65 0.35 0 + 20 0.25 Conventional Steel 13 308 451 37 1.54 0.41 0 + 30 0.35 Conventional Steel

As shown in Table 2, Samples 1 to 3 (inventive steel) contained the chemical composition and the manufacturing conditions in the range suggested by the present invention, the yield strength was 240 MPa or more, and the yield strength-ductility balance was 12000 with the elongation of 38% or more. That's it. In addition, the plastic anisotropy index is 1.4 or more, the in-plane anisotropy index is 0.4 or less, and has excellent moldability, and the aging index is 30 MPa or less. In addition, it has excellent secondary processing brittleness with a ductile-brittle transition temperature of -50 ° C or less. Such material characteristics can be exhibited by controlling the size of the precipitate to be 0.2 µm or less.

On the other hand, samples 4-6 (comparative steel) have a 0.58 * Mn / S ratio of 20 or more and a large precipitate, which makes it difficult to guarantee aging for more than one year.

In addition, samples 7 to 10 (conventional steel) are disclosed in Korean Laid-Open Patent Publication No. 2000-039137 with a ratio of 0.58 * Mn / S of 10 or more, which is outside the scope of the present invention, and the cooling rate after hot rolling is too low to obtain a very coarse precipitate. The precipitation strength did not appear at all, so the yield strength was less than 160MPa, resulting in a maximum strength-ductility balance of 9008. The steel sheet of such material is too low in strength to increase the thickness by more than 20% in order to obtain the same strength in the same product. In addition, in the case of workability, the in-plane anisotropy index, which causes wrinkles, is very high as 0.7 or more.

In addition, samples 11, 12, and 13 (conventional steel) are conventional high-strength cold-rolled steel sheets, but have an excellent aging index but have a high ductility-brittle transition temperature.

As described above, the cold rolled steel sheet provided in accordance with the present invention can guarantee aging at an aging index of 30 Mpa or less, while having a ductile-brittle transition temperature of -50 ° C. or less, and having excellent secondary work brittleness, and yield strength of 240 MPa. Yield strength-ductility balance is more than 12000 and in-plane anisotropy index is less than 0.4, which 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, Si: 0.1-0.8%, said Mn and S weight ratio satisfies the following conditions 0.58 * Mn / S ≤ 10, is composed of the remaining Fe and other unavoidable impurities, high strength ratio excellent in secondary processing brittleness with an average size of MnS precipitates of 0.2 ㎛ or less Aging cold rolled steel sheet. [Claim 2] The high strength, non-aging cold rolled steel sheet of claim 1, wherein C is greater than 0.002% and less than 0.003%. The high strength non-aging cold rolled steel sheet of claim 1, wherein S is 0.016 to 0.03%. The cold rolled steel sheet has a yield strength of 240 Mpa or more, a yield strength-ductility balance of 12000 or more, a ductility-brittle transition temperature of -50 ° C. or less, and an in-plane anisotropy index of 0.4 or less. High strength non-aging cold rolled steel with excellent secondary work brittleness. 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, Si: 0.1-0.8%, said The weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S≤10, and reheats the steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ° C or higher, followed by hot rolling with a finish rolling temperature of at least Ar ₃ transformation point. Rolling, cooling at a rate of 200 ° C./min or more, winding at a temperature of 700 ° C. or less, cold rolling at a reduction ratio of 50 to 90%, and secondary annealing including continuous annealing at a temperature range of 500-900 ° C. Process for producing high strength non-aging cold rolled steel sheet with excellent workability. The method of claim 5, wherein the C is 0.002% or more and 0.003% or less. The method of claim 5, wherein the S is 0.016 to 0.03%. The cold rolled steel sheet has a yield strength of 240 Mpa or more, a yield strength-ductility balance of 12000 or more, a ductile-brittle transition temperature of -50 ° C. or less, and an in-plane anisotropy index of 0.4 or less. Method for producing high strength non-aging cold rolled steel sheet with excellent secondary processing brittleness.

Images