KR100742930B1 - Non-aging type cold rolled steel sheet with high yield ratio and process for producing the same - Google Patents

Abstract

Provided are a cold rolled steel sheet and a method of manufacturing the same, which yield strength and in-plane anisotropy by fine AlN precipitates in a Ti-based IF steel.

The cold rolled steel sheet is, by weight, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002%, Ti: 0.005 Contains -0.15% and is composed of the remaining Fe and other unavoidable impurities,

The Ti, N. C satisfies the following relationship, 1≤ (Al / 27) / (N ^★ / 14) ≤10, 0.8≤ (Ti ^★ / 48) / (C / 12) ≤5.0,

Where N ^★ = N-0.8x (Ti-0.8x (48/32) xS) x (14/48), Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS )]

The average size of the AlN precipitates is less than 0.2 μm.

In the present invention, the fine AlN precipitates are distributed in the IF steel to increase the yield ratio while lowering the in-plane anisotropy.

Automobile material, AlN precipitate, in-plane anisotropy, yield strength, room temperature non-aging

Description

Non-aging cold rolled steel sheet with high yield ratio and its manufacturing method {NON-AGING TYPE COLD ROLLED STEEL SHEET WITH HIGH YIELD RATIO AND PROCESS FOR PRODUCING THE SAME}

Japanese Laid-Open Patent Publication No. 57-0413349

Japanese Unexamined Patent Publication No. 10-158783

The present invention relates to a non-aging cold rolled steel sheet used as a material for automobiles, home appliances, and more particularly, to a cold rolled steel sheet improved in yield strength and in-plane anisotropy by fine AlN precipitates in a Ti-based IF steel. It is about.

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 adds strong carbon and nitride forming elements such as Ti and Nb and performs continuous annealing is mainly used. IF steels have a non-aging effect by removing some or all of the carbon and nitrogen employed.

The high-strength method of IF steel is the employment strengthening technology by P. Japanese Laid-Open Patent Publication No. 57-0413349 secures strength by adding 0.04 to 0.12% of P in Ti-added IF steel.

In Japanese Unexamined Patent Publication No. Hei 10-158783, while lowering P, high strength is secured by using a solid solution strengthening element of Mn and Si together. This prior art secures press formability and surface characteristics by controlling hot rolling conditions while securing strength by using Mn as a solid solution strengthening element in Ti or Ti-Nb composite systems. This prior art uses a solid solution strengthening element up to 0.5%, Mn is of high quality, the production cost is increased by the addition of a large amount of Mn, in particular, if the content of Mn increases, the plating properties are not good. This prior art makes no mention of the characteristics of in-plane anisotropy and yield strength. Considering the use of Mn as an element for strengthening employment, the characteristics of high yield ratio cannot be obtained. In the prior art, Al is added 0.1% as a deoxidizer, and in the case of N it is managed to 0.01% or less as impurities. Therefore, it is not a component system which uses AlN precipitate.

It is an object of the present invention to provide a cold rolled steel sheet and a method of manufacturing the same, which can lower in-plane anisotropy while enhancing yield strength by fine AlN precipitates.

Cold rolled steel sheet of the present invention for achieving the above object, in weight%, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002%, Ti: 0.005-0.15%, and is composed of the remaining Fe and other unavoidable impurities,

The Ti, N. C satisfies the following relationship, 1≤ (Al / 27) / (N ^★ / 14) ≤10, 0.8≤ (Ti ^★ / 48) / (C / 12) ≤5.0,

Where N ^★ = N-0.8x (Ti-0.8x (48/32) xS) x (14/48), Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS )]

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

In the present invention, the fine AlN precipitate is preferably ¹ × 10 ⁵ / mm ² or more, more preferably 1 × 10 ⁶ / mm ² or more.

The cold rolled steel sheet of the present invention has the characteristics of a soft cold rolled steel sheet of 280MPa grade and high strength cold rolled steel sheet of 340MPa or more according to the component design.

If the content of P in the above component system is 0.015% or less, a 280 MPa grade soft cold rolled steel sheet is obtained. The cold rolled steel sheet further contains one or two of the solid solution strengthening elements Si and Cr, or when the P content is 0.015 to 0.2%, high strength characteristics of 340 MPa or more are secured. In the case of high strength steel containing P alone, the content of P is preferably 0.03 to 0.2%. 0.1-0.8% for Si and 0.2-1.2% for Cr are preferred. In the case of containing at least one of Si and Cr, the content of P may be variously designed in the range of 0.2% or less.

If you want to improve the workability in the cold rolled steel sheet of the present invention may further comprise Mo 0.01 ~ 0.2%.

In the method for producing a cold rolled steel sheet, the slab that satisfies the component system of the present invention is reheated to a temperature of 1100 ° C. or higher, and then hot-rolled at a finish rolling temperature of Ar ₃ or higher and cooled at a speed of 300 ° C./min or higher, and 700 ° C. After winding up at the following temperature, it cold-rolls and continuously anneales.

Hereinafter, the present invention will be described in detail.

The present invention is completed on the basis of the research results that the fine grains of AlN precipitated in the Ti-based IF steel, the grain becomes fine, the yield strength is enhanced and the in-plane anisotropy index is lowered to improve the workability.

In the IF steel, N is managed as an impurity. In Japanese Unexamined Patent Application Publication No. Hei 10-158783, N is 0.01% or less, but this is managed as an impurity and does not actively use fine AlN precipitates. Prior art is a Ti-based IF steel, since N is precipitated as TiN, it is necessary to manage all the components in order to precipitate N into AlN, but N is managed only as impurities.

In the present invention is to secure a fine AlN in the IF steel. N preferentially reacts with Ti and Zr to precipitate most of them. Since the IF steel of the present invention is Ti-added IF steel, Ti reacts with C, N, and S. Therefore, it is necessary to manage the general components so that N precipitates into AlN.

According to the present invention, when the grain is refined by the fine AlN precipitate, the solid solution carbon is more present in the grain boundary than in the grain, thereby ensuring room temperature non-aging characteristics. The dissolved carbon remaining in the grain is relatively free to move, so it does not affect the room temperature aging characteristics in combination with the operating potential. In addition, the finely distributed AlN precipitates according to the present invention have a positive effect on the increase in yield strength and the improvement of the strength-ductility balance characteristics, and the in-plane anisotropy and plastic anisotropy. To this end, AlN precipitates should be finely distributed, which affects the content of Al, N, Ti, C and their component ratios, and the cooling rate after hot rolling.

Cold rolled steel sheet of the present invention has a high yield strength can reduce the thickness of the steel sheet has a light weight effect. In addition, the in-plane anisotropy has the advantage of less wrinkles generated during processing and less ear generation after processing. The cold rolled steel sheet of the present invention and a manufacturing method thereof will be described in detail below.

First, C, S, Al, P, N, B, and Ti, which are basic components, will be described.

The content of carbon (C) is preferably 0.005% or less.

When the carbon content exceeds 0.005%, a large amount of expensive Ti must be added to remove the solid solution carbon which greatly degrades aging and plastic anisotropy. In this case, the manufacturing cost rises and the recrystallization temperature increases. Therefore, the annealing temperature should be increased, otherwise the crystal grains of the annealing plate become fine and the ductility is greatly lowered, and the plating property is also lowered during plating. More preferable content of carbon (C) is 0.003% or less. Preferably, the lower limit of the content of carbon (C) is made 0.0005%. If the content of carbon (C) is less than 0.0005%, the grains of the hot rolled sheet are coarse to lower the strength and to increase the in-plane anisotropy.

The content of sulfur (S) is preferably 0.08% or less.

When the content of sulfur (S) is more than 0.08%, the content of solute is high, so the ductility and formability are greatly lowered, and there is a fear of red brittleness.

The content of aluminum (Al) is preferably 0.1% or less.

Aluminum reacts with N to form fine AlN precipitates to increase yield strength by grain refinement and precipitation strengthening. More preferred Al content is 0.01-0.1%.

The content of nitrogen (N) is preferably 0.004-0.02%.

If the content of N is less than 0.004%, the number of precipitated AlN is small, so the effect of grain refinement and precipitation strengthening is small. When the nitrogen content exceeds 0.02%, aging guarantee by solid nitrogen is difficult.

The content of phosphorus (P) is preferably 0.2% or less.

Phosphorus is an element having a high solid solution strengthening effect and a small decrease in r value, and guarantees high strength in steels for controlling precipitates according to the present invention. In steel grades requiring strength of 280 Mpa, the content of P should be less than 0.015%. In the high strength steel of 340Mpa grade or more, it is good to set it as 0.015 ~ 0.2%. If the content of P is more than 0.2%, it is preferable that the ductility is lowered to limit the upper limit to 0.2%. In the present invention, when Si and Cr are added, various strength designs are possible while the content of P is in the range of 0.2% or less.

The content of boron (B) is preferably 0.0001 to 0.002%.

Boron is added to prevent secondary processing brittleness. For this purpose, the boron content is preferably 0.0001% or more. If the boron content exceeds 0.002%, deep drawing may be greatly degraded.

The content of titanium (Ti) is to be 0.005 ~ 0.15%.

Titanium is added for the purpose of securing inaging and improving moldability. Titanium is a strong carbide-generating element and is added to steel to precipitate TiC precipitates to precipitate insoluble solids. If the addition amount of titanium is less than 0.005%, the precipitation amount of TiC precipitate is too small, and there is little effect of improving the processability of the soil because of less development of the texture. If Ti exceeds 0.15%, the TiC precipitates are too large to reduce the grain refining efficiency, resulting in an increase in in-plane anisotropy, lowering the yield strength and greatly degrading the plating properties.

In the present invention, the content of Ti, Al, N, C in order to secure AlN precipitates are managed as follows. In the following relationship, each component is used in weight%.

[Relationship 1]

1≤ (Al / 27) / (N ^★ / 14) ≤10

[Relationship 2]

N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48)

Relationship 1 is to secure a fine AlN precipitate. In Equation 1, N ^* is the content of N remaining in the total N content without reacting with Ti and remaining with Al. N ^* is determined by relation 2. In order to secure a fine AlN precipitate, it is preferable that the value of relation 1 satisfies 1-10. When the value of relation 1 is greater than or equal to 1, effective AlN precipitates are precipitated, and in the case of more than 10, AlN precipitates are coarse, so that the characteristics of workability and yield strength are not good.

In the present invention, carbon is precipitated as TiC. Therefore, the room temperature aging characteristics are affected by the conditions of the solid solution carbon not precipitated by TiC. In consideration of this, it is most preferable that Ti and C satisfy the following conditions.

[Relationship 3]

0.8≤ (Ti ^★ / 48) / (C / 12) ≤5.0,

[Relationship 4]

Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS)

Equation 3 is to secure room temperature non-aging characteristics by precipitating TiC to remove carbon in solid solution. In relation 3, Ti ^* is the content of Ti reacting with N and S in the total Ti content and remaining with C. Ti ^* is determined by the relation 4

If the value of the relation 3 is less than 0.8, it is difficult to secure the room temperature non-aging characteristics, and if the value of the relation 3 exceeds 5, the amount of Ti remaining in the solid solution state in the steel is high, the ductility is lowered.

In the component system of the present invention, the finer the distribution, the more advantageous. Preferably, the average size of the AlN precipitate is 0.2 μm or less. According to the results of the present invention, especially when the average size of the precipitate is more than 0.2㎛, the strength is low, the in-plane anisotropy index is not good.

Furthermore, although the precipitate of 0.2 micrometer or less is distributed in a large amount in the component system of this invention, the distribution number is not specifically limited. Preferably, the number of distribution of precipitates is at least 1 × 10 ⁵ , more preferably at least 1 × 10 ⁶ per mm ² . The larger the distribution number of precipitates, the higher the plastic anisotropy index and the lower in-plane anisotropy index are. In general, when the plastic anisotropy index increases, the in-plane anisotropy index rises and there is a limit to increasing the plastic anisotropy index in terms of processability. .

In the present invention, when applied to a high-strength steel sheet of 340 MPa grade or more, one or two or more solid solution strengthening elements such as P, that is, P, Si, and Cr may be added. As described above with respect to P, redundant descriptions are omitted.

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

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

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

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

In the cold rolled steel sheet of the present invention, molybdenum (Mo) may be further added.

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

Mo is added as an element to increase the plastic anisotropy index, the content of the plastic anisotropy index is increased when the content is more than 0.01%, if the content exceeds 0.2%, the plastic anisotropy index is no longer increased and there is a risk of causing hot brittleness.

The cold rolled steel sheet obtained according to the present invention satisfies 58% or more of yield ratio (yield strength / tensile strength) while sufficient workability (ductility, plastic anisotropy index).

[Manufacturing method of cold rolled steel sheet]

The present invention is characterized in that an average size of AlN precipitates in a cold rolled sheet is hot and cold rolled to satisfy the above-described stress. The average size of AlN precipitates in the cold rolled plate is influenced by the component design and the manufacturing process such as reheating temperature and winding temperature, but especially 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. This is because when the reheating temperature is lower than 1100 ° C., the coarse precipitates generated during continuous casting remain completely insoluble due to the low reheating temperature, so that many coarse precipitates remain even after hot rolling.

Hot rolling is preferably carried out under the conditions of the finish rolling temperature higher than 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 reduced by the formation of the rolled grain but also the strength is lowered.

It is preferable that the cooling rate before winding after hot rolling shall be 300 degreeC / min or more. Even if the component ratio is controlled to obtain a fine precipitate according to the present invention, if the cooling rate is less than 300 ° C / min, the average size of the precipitate may exceed 0.2 ㎛. In other words, the faster the cooling rate, the greater the number of nuclei generated and the finer the precipitate. The faster the cooling rate, the finer the precipitate is, so it is not necessary to limit the upper limit of the cooling rate.However, even if the cooling rate is faster than 1000 ° C / min, the finer effect of the precipitate is no longer increased, so the cooling rate is 300 to 1000 ° C / min. This 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 ℃, precipitates grow too coarse, making it difficult to secure strength.

[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 700-900 degreeC. If the continuous annealing temperature is less than 700 ° C., recrystallization is not completed and the target ductility value cannot be secured. If the annealing temperature is more than 900 ° C., the strength decreases due to coarsening of the recrystallized grains. The continuous annealing time keeps the recrystallization complete. If it is about 10 seconds or more, the recrystallization is completed. Preferably the continuous annealing time is in the range of 10 seconds to 30 minutes,

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

Example 1

The steel slabs of Table 1 were reheated, hot rolled to finish, cooled to 400 ° C./min, wound up at 650 ° C., and then 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 830 ℃ 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), in-plane anisotropy index (Δr value) and aging evaluation index using a tensile tester (INSTRON, Model 6025). Where r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, △ r = (r ₀ -2r ₄₅ + r ₉₀ ) / 2, and the aging evaluation index is 100 ° C for 1.0% skin pass-rolled specimen after annealing X 2hr. Yield point elongation rate measured after heat treatment.

sample Chemical composition (% by weight) C P S Al Ti B N Etc A1 0.0008 0.008 0.023 0.042 0.059 0.0007 0.0103 A2 0.0017 0.035 0.025 0.044 0.074 0.001 0.0135 Si: 0.13 A3 0.0025 0.061 0.034 0.039 0.095 0.0009 0.015 Si: 0.24 A4 0.0012 0.085 0.025 0.024 0.066 0.0007 0.0117 Si: .11 Mo: 0.06 A5 0.0006 0.12 0.023 0.038 0.061 0.0008 0.0112 Cr: 0.13 A6 0.0038 0.042 0.013 0.032 0 0.0005 0.0012 A7 0.0014 0.12 0.009 0.055 0.14 0.0005 0.012 Si: 0.13

sample (Ti ^★ / 48) / (C / 12) N ^★ (Al / 27) / (N ^★ / 14) Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) A1 0.67 0.003 7.32 0.05 6.3 X 10 ⁵ A2 1.03 0.0032 7.06 0.05 6.3 X 10 ⁵ A3 1.31 0.0024 8.59 0.05 8.4X10 ⁶ A4 0.81 0.0033 3.77 0.05 7.3 X 10 ⁶ A5 1.12 0.0034 5.78 0.05 6.2 X 10 ⁶ A6 -1.2 0.0048 3.43 0.05 4.5 X 10 ⁵ A7 17.2 -0.018 -1.6 0.28 3.5 X 10 ³ Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS) N ^★ = N-0.8x (Ti-0.8x (48/32) xS)) x (14/48)

Sample Number Mechanical properties Remarks Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (Δr) 2nd processing brittleness (DBTT- ℃) Aging Evaluation Index (%) A1 209 349 44 2.03 0.25 -60 0 Invention steel A2 282 399 37 1.72 0.24 -50 0 Invention steel A3 339 457 34 1.73 0.27 -50 0 Invention steel A4 219 360 42 2.21 0.29 -50 0 Invention steel A5 354 449 33 1.73 0.21 -60 0 Invention steel A6 189 348 45 1.32 0.43 -40 0.94 Comparative steel A7 335 457 26 1.53 0.24 -40 0 Comparative steel

In the present invention, the above embodiment is only one example, and the present invention is not limited thereto. Anything that has substantially the same configuration as the technical idea described in the claims of the present invention and achieves the same operation and effect is included in the technical scope of the present invention.

As described above, the present invention is intended to refine the grains by distributing fine AlN precipitates in the IF steel, thereby lowering the in-plane anisotropy index and increasing yield strength by strengthening precipitation by AlN.

Claims (8)

delete By weight percent, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002%, Ti: 0.005-0.15% And remaining Fe and other inevitable impurities, The Ti, N. C satisfies the following relationship, 1≤ (Al / 27) / (N ^★ / 14) ≤10, 0.8≤ (Ti ^★ / 48) / (C / 12) ≤5.0, Where N ^★ = N-0.8x (Ti-0.8x (48/32) xS) x (14/48), Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS) ] The average size of AlN precipitates is less than 0.2㎛ The precipitate water is 1 X 10 ⁶ / mm ² or more, characterized in that the non-aging cold rolled steel sheet. The non-aging cold rolled steel sheet according to claim 2, further comprising one or two of Si: 0.1 to 0.8% and Cr: 0.2 to 1.2%. The non-aging cold rolled steel sheet according to claim 2 or 3, wherein Mo is further added in an amount of 0.01 to 0.2%. delete By weight percent, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004-0.02%, P: 0.2% or less, B: 0.0001-0.002%, Ti: 0.005-0.15% And remaining Fe and other inevitable impurities, A slab in which Ti and N.C satisfy the following relationship, 1≤ (Al / 27) / (N ^★ / 14) ≤10, 0.8≤ (Ti ^★ / 48) / (C / 12) ≤5.0, Where N ^★ = N-0.8x (Ti-0.8x (48/32) xS) x (14/48), Ti ^★ = Ti-0.8x ((48/14) xN + (48/32) xS) ] After reheating to 1100 ℃ or higher, hot rolling with finishing rolling temperature above Ar ₃ transformation point, cooling at 300 ℃ / min or higher, winding at temperature below 700 ℃, and cold rolling at 50 ~ 90% reduction rate And annealing continuously for 10 seconds to 30 minutes in a temperature range of 700 to 900 ° C., in which the AlN precipitate having an average size of 0.2 μm or less is distributed 1 × 10 ⁶ / mm ² or more. The method for producing a high yielding ratio non-aging cold rolled steel sheet according to claim 6, further comprising one or two of Si: 0.1 to 0.8% and Cr: 0.2 to 1.2%. The method for producing a high yielding ratio non-aging cold rolled steel sheet according to claim 6 or 7, further comprising 0.01% to 0.2% of Mo.