KR20060115646A - Baking hardening type cold rolled steel sheet with high yield ratio and process for producing the same - Google Patents

Abstract

A non-aging type cold rolled steel sheet capable of enhancing yield strength and lowering in-plane anisotropy by fine AlN precipitates and a method for manufacturing the same are provided. A non-aging type cold rolled steel sheet with high yield ratio comprises, by weight percent, 0.005% or less of C, 0.08% or less of S, 0.1% or less of Al, exceeding 0.004% to 0.02% or less of N, 0.2% or less of P, 0.0001 to 0.002% of B and 0.002 to 0.04% of Nb with the balance being Fe and other inevitable impurities, wherein the Al, N, Nb and C, by weight percent, satisfy (Al/27)/(N/14)=1 to 10 and (Nb/93)/(C/14)=0.8 to 5.0, and wherein AlN precipitates have an average size of 0.2 mum or less.

Description

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

Japanese Unexamined Patent Publication No. 1982-0413349

Japanese Unexamined Patent Publication No. 1998-158783

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 IF steel, and a manufacturing method thereof. .

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 phenomenon in which hardening occurs as the solid elements (C, N) adhere to dislocations, causing a defect called a 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.

Japanese Patent Application Laid-open No. Hei 10-158783 secures press forming and surface characteristics by controlling hot rolling conditions in a Ti or Ti-Nb composite system to manage microstructure. In this prior art, the strength is secured by Si, Mn, and P. However, Mn is high quality, so that the production cost is increased by the addition of a large amount of Mn. In particular, when the content of Mn is increased, the plating properties are not good. In addition, there is no mention of the characteristics of in-plane anisotropy and yield strength. Considering the aspect of the component system that secures strength as a solid solution element, 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 less than 0.01% 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, by weight, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004% or more-0.02% or less, P: 0.2% or less , B: 0.0001-0.002%, Nb: 0.002 ~ 0.04%, including the remaining Fe and other unavoidable impurities,

Al, N, Nb, C is the weight percent,

(Al / 27) / (N / 14): 1-10,

(Nb / 93) / (C / 14): We satisfy 0.8-5.0,

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

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

1X10 ⁶ pcs / mm ² or more is good. In the present invention, according to the conditions of the Nb, C has a non-aging characteristics and baking hardening characteristics. In order to secure the non-aging characteristics, C must be precipitated as NbC, and the condition is that Nb and C satisfy the following conditions (Nb / 93) / (C / 14): 0.8 to 5.0. In the present invention, Al and N satisfy the following conditions, (Al / 27) / (N / 14): 1-10, more preferably (Al / 27) / (N / 14): 1-5 It is.

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 soft cold rolled steel sheet of 280 MPa grade is obtained. When the cold rolled steel sheet further contains one or two of P, Si and Cr as solid solution strengthening elements, high strength characteristics of 340 MPa or more are secured. When P is contained 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 when the AlN precipitate is secured in the IF steel, the grains become fine to improve the yield strength and lower the in-plane anisotropy index to improve workability.

In the IF steel, N is managed as an impurity. In Japanese Laid-Open Patent Publication No. 1998-158783, N is 0.01% or less, but this is managed as an impurity, and it is not actively using fine AlN precipitates. Since the prior art is a Ti-based IF steel, N cannot be finely precipitated into AlN because N is precipitated as TiN.

In the present invention is to secure a fine AlN in the IF steel. S preferentially reacts with Ti and Zr to precipitate most of them. Since the IF steel of the present invention is an Nb-only IF steel to which no sulfide forming elements such as Ti and Zr are added, N is precipitated as AlN according to the addition conditions of Al and N.

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 and N, their component ratio conditions, 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 Nb as basic components will be described.

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

If the content of carbon (C) exceeds 0.005%, a large amount of expensive Nb should 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.

Al forms fine AlN precipitates with N, which enhances the yield strength by grain refinement and precipitation strengthening. To this end, add up to 0.1%. When the Al content is more than 0.1%, there is a fear that the ductility decreases because the Al content is high in solid solution.

The content of nitrogen (N) is preferably more than 0.004% -0.02% or less. More preferably, it is 0.005-0.02%.

If the N content is less than 0.004%, the number of precipitated AlNs is small, and the effect of grain refinement and precipitation strengthening is small. If the N content exceeds 0.02%, the aging guarantee due to solid nitrogen is difficult, so the upper limit is set to 0.02%.

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%. For high strength steel of 340Mpa or higher, it is recommended to set it as 0.016 ~ 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 niobium (Nb) is preferably 0.002 to 0.04%.

Nb is added for the purpose of ensuring inaging and improving moldability. Nb is a strong carbide-generating element, added to steel to precipitate NbC precipitates, thereby securing ineffective age by depositing solid carbon. In addition, NbC precipitates have an effect of greatly improving the processing ability of the soil by developing the aggregated structure during annealing. If the amount of Nb added is less than 0.002%, the amount of precipitation of NbC precipitates is so small that there is little development of aggregates, and thus there is little effect of improving Omrim processability. When Nb exceeds 0.04%, the amount of NbC precipitates is too high, resulting in low rimability and low elongation, which may significantly reduce moldability.

In order to secure a fine AlN precipitate in the present invention, Al and N must satisfy the following relationship, (Al / 27) / (N / 14): 1-10. More preferably, it satisfies 1-5. If the addition ratio of Al and N (0 (Al / 27) / (N / 14)) is less than 1, aging by solid solution N may occur.In the case of more than 10, the effect of strengthening strength is enhanced by precipitation of coarse AlN precipitates. Few.

In the present invention, Nb and C can be a component design in terms of non-aging and baking properties. Nb / C satisfies the following relationship, (Nb / 93) / (C / 14): 0.8 to 5.0, in order to secure non-aging characteristics. Nb combines with C to precipitate NbC precipitates to remove C in the solid solution state in the steel, thereby securing ineffectiveness and developing aggregated structure during annealing to improve the processability of Orim forest (Nb / 93) / (C / 14) If the value is less than 0.8, it is difficult to secure the non-aging property, and the processability of soiling is lowered. If the value of (Nb / 93) / (C / 14) exceeds 5.0, the amount of Nb remaining in the solid solution state in the steel is large and the ductility is greatly reduced.

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, when the distribution number of precipitates of 0.2 μm or less in the component system of the present invention is 1 × 10 ⁶ or more per mm ^2, the plastic anisotropy index is increased, and the in-plane anisotropy index is lowered, thereby greatly improving workability. 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. P has already been mentioned, so duplicate 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.

[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 affected by the conditions of addition amount, manufacturing process such as reheating temperature, winding temperature, etc., 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 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 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, as the cooling rate increases, a large number of nuclei are generated and the precipitate becomes fine. The faster the cooling rate, the finer the precipitate is, so there is no need to limit the upper limit of the cooling rate. 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.

division Chemical composition (% by weight) C S Al N P B Nb Etc A1 0.0014 0.006 0.053 0.0072 0.051 0.0005 0.015 A2 0.0023 0.01 0.062 0.0082 0.082 0.0004 0.021 Si: 0.12 A3 0.0018 0.011 0.055 0.0127 0.118 0.0004 0.037 Si: 0.25 A4 0.0024 0.008 0.049 0.0074 0.029 0.0007 0.028 Si: 0.2 Mo: 0.06 A5 0.0032 0.013 0.053 0.0088 0.109 0.0009 0.035 Si: 0.18 Cr: 0.13 A6 0.0018 0.013 0.052 0.0018 0.052 0.005 0 A7 0.0035 0.009 0.008 0.023 0.125 0.0005 0.062

division (Al / 27) / (N / 14) (Nb / 93) / (C / 14) Average size of precipitates (㎛) Number of precipitates (pcs / mm ² ) A1 3.82 1.38 0.06 5.3 X 10 ⁵ A2 3.92 1.18 0.05 5.6 X 10 ⁵ A3 2.25 2.65 0.05 6.8X10 ⁶ A4 3.43 1.51 0.05 5.5 X 10 ⁶ A5 3.12 1.41 0.04 6.3 X 10 ⁶ A6 15 0 0.11 4.5 X 10 ⁴ A7 0.18 2.29 0.08 8.4 X 10 ⁴

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 (%) One 209 349 44 2.03 0.25 -60 0 Invention steel 2 282 399 37 1.72 0.24 -50 0 Invention steel 3 339 457 34 1.73 0.27 -50 0 Invention steel 4 219 360 42 2.21 0.29 -50 0 Invention steel 5 354 449 33 1.73 0.21 -60 0 Invention steel 6 189 348 45 1.32 0.43 -40 0.94 Comparative steel 7 335 457 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. Any thing that has substantially the same structure and the same effect as the technical idea described in the claim of the present invention is included in the technical scope of this invention.

As described above, the present invention is to refine the grain size by dispersing the nanosize AlN precipitates on the cold rolled steel sheet, thereby lowering the in-plane anisotropy index, and further enhancing yield strength by precipitation strengthening.

Claims (10)

By weight%, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004% or more-0.02% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04 Composed of the remaining Fe and other unavoidable impurities, including% Al, N, Nb, C is the weight percent, (Al / 27) / (N / 14): 1-10, (Nb / 93) / (C / 14): We satisfy 0.8-5.0, A high yielding ratio, non-aging cold rolled steel sheet having an average size of AlN precipitates of 0.2 µm or less. The high yielding ratio of non-aging cold rolled steel sheet according to claim 1, wherein the precipitate is 1 × 10 ⁶ particles / mm ² or more. The non-aging cold rolled steel sheet according to claim 1, wherein the value of (Al / 27) / (N / 14) satisfies 1-5. The high yield ratio non-aging cold rolled steel sheet according to claim 1, 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 1 or 4, wherein Mo is contained in an amount of 0.01 to 0.2%. By weight%, C: 0.005% or less, S: 0.08% or less, Al: 0.1% or less, N: 0.004% or more-0.02% or less, P: 0.2% or less, B: 0.0001-0.002%, Nb: 0.002-0.04 Composed of the remaining Fe and other unavoidable impurities, including% Al, N, Nb, C is the weight percent, (Al / 27) / (N / 14): 1-10, (Nb / 93) / (C / 14): satisfies 0.8 ~ 5.0, and then re-heating the slab to a temperature of at least 1100 ℃ by a finish rolling temperature above the Ar ₃ transformation point, hot-rolled and cooled by more than 300 ℃ / min speed, and A method for producing a high yielding ratio, non-aging cold rolled steel sheet in which AlN precipitates having an average size of 0.2 μm or less are distributed after winding at a temperature of 700 ° C. or lower, followed by cold rolling. The method of claim 6, wherein the precipitate is 1 × 10 ⁶ particles / mm ² or more. The method of manufacturing a non-aging cold rolled steel sheet having a high yield ratio according to claim 6, wherein the value of (Al / 27) / (N / 14) satisfies 1-5. 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%. 10. The method of manufacturing a high yielding ratio non-aging cold rolled steel sheet according to claim 6 or 9, wherein Mo is contained in an amount of 0.01 to 0.2%.