KR20000039137A - Method for producing non-aged cold rolled steel sheets with excellent ductility - Google Patents

Abstract

PURPOSE: A method for producing cold rolled steel sheets is provided to have excellent stretching property while having non-aged characteristic by improving ductility. CONSTITUTION: A steel slab is composed of 0.0005-0.002wt% of C, 0.05-0.3wt% of Mn, under 0.015wt% of S, under 0.015wt% of P, 0.01-0.08wt% of Al, 0.001-0.005wt% of N, under 0.025wt% of C+N+S+P, and the remnant wt% of Fe and impurities. Then, the steel slab is hot rolled at a temperature of over the deformation temperature of Ar3 for being wound up at the temperature of under 750°C. The steel slab is cold rolled at a reduction ratio of 50-90% for being annealed at the temperature of 650-850°C for 10 seconds. Thus, non-aged cold rolled steel sheets are produced.

Description

Manufacturing method of non-aging cold rolled steel sheet with excellent ductility

The present invention relates to a method for manufacturing a cold rolled steel sheet for automobiles, and more particularly, to a method for manufacturing a cold rolled steel sheet having high ductility and excellent stretchability by continuous annealing, which can secure non-aging properties.

Recently, in order to improve fuel efficiency and reduce the weight of the vehicle body due to the weight reduction of automobiles, there is an increasing demand for increasing the strength and formability of cold rolled steel sheets for automobiles. Formability, which is considered to be an important characteristic in cold rolled steel sheets for automobiles, is evaluated as plastic anisotropy and ductility as processing characteristics of sheet materials. If the plastic anisotropy index is large, it is evaluated to have excellent rimability and high elongation. In addition, non-aging is considered to be an important characteristic in automotive cold rolled steel sheet, which is a kind of strain aging phenomenon in which hardening occurs as a solid element fixes an electric potential.

BACKGROUND ART Conventional techniques for securing the inaging properties of cold rolled steel sheets for automobiles include a method of continuously annealing by adding a strong carbon and nitride forming element such as titanium or niobium to ultra low carbon steel. However, such carbonitride-forming elements hinder the growth of recrystallized grains during annealing and thus lower ductility, which is disadvantageous for parts requiring long processability. In particular, titanium has a strong oxidizing property, so that many nonmetallic inclusions are produced during steelmaking, causing surface defects in the steel sheet, and niobium has a drawback in that ductility is greatly reduced by greatly increasing the recrystallization temperature. These special element-added steels have a disadvantage of high cost because of high recrystallization temperature and low productivity and high temperature annealing. As another conventional technique, there has been a method of producing an annealing of aluminum-kilted steel, but this method has a long annealing time, and thus has low productivity and severe material variation for each part. Therefore, at present, there is a demand for the production of cold rolled steel sheet having excellent non-aging properties and excellent ductility while being manufactured by a continuous annealing method.

The present invention provides a method for producing a cold rolled steel sheet having excellent elongation processing characteristics by improving ductility, which is a disadvantage of conventional continuous annealed steel, while having substantial non-aging characteristics.

The present invention for achieving the above object, by weight% C: 0.0005-0.002%, 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% and the C + N + S + P is less than 0.025%, hot-rolled steel slab containing the remaining Fe and other unavoidable elements with a rolling finish temperature above the Ar ₃ transformation point and then below 750 ℃ After winding at a temperature of 50-90%, cold rolling at a rate of reduction, and then continuous annealing for more than 10 seconds at a temperature in the range of 650-850 ℃.

Hereinafter, the present invention will be described in detail.

If the carbon (C) is added more than 0.002%, the amount of solid carbon in steel is difficult to secure inaging age, it is difficult to secure the aging, the crystal grains of the annealing plate is fine, the ductility is greatly lowered, so limit the upper limit of carbon to 0.002%. On the other hand, when the carbon content is less than 0.0005%, the grain size of the hot rolled sheet is too coarse, so the ductility is rather reduced, so the lower limit is limited to 0.0005%.

Manganese (Mn) is added to prevent hot shortness due to precipitation of solid solution sulfur in the steel as manganese sulfide, and in the case of the present invention, the steel content of sulfur is limited to 0.015% or less to prevent the heat brittleness. The minimum possible amount of addition is 0.05% as the lower limit. If the manganese content is more than 0.3%, the amount of solid solution is high, so the ductility is reduced, so the upper limit is 0.3%.

In the case of sulfur (S) and phosphorus (P), when the addition amount is large, the ductility and moldability are lowered, so the upper limit is set to 0.015%, respectively.

Aluminum (Al) is an element added as a deoxidizer, but in the present invention, nitrogen is added in the steel to completely prevent aging by solid solution nitrogen. When aluminum is added at 0.01% or more, the aging phenomenon by nitrogen can be completely prevented, whereas when aluminum is added at 0.01% or less, the amount of aluminum nitride in steel is small and the grains of the hot rolled sheet are too coarse, so that after cold rolling and annealing Since the formation of {111} aggregate structure favorable for moldability was small and the r value of plastic anisotropy index was very low, the lower limit of addition of aluminum is made into 0.01%. When the amount of aluminum added was 0.08% or more, the amount of aluminum present in the solid solution state was high, so the ductility was lowered, so the upper limit was made 0.08%.

Nitrogen (N) is an element that is inevitably added during steelmaking, but in the present invention, when the nitrogen content is less than 0.001%, the amount of aluminum nitride is too small, so that the grain size of the hot rolled sheet grows so large that the value of plastic anisotropy index r decreases. Limit to 0.001%. In addition, when the content of nitrogen is 0.005% or more, it is difficult to secure inaging, so the upper limit is made 0.005%.

The carbon, nitrogen, phosphorus and sulfur are elements deteriorating ductility and formability, even if limited to the above range, when the sum of these elements, that is, C + N + P + S is 0.025% or more, the ductility is greatly reduced and the present invention Because the target ductility cannot be secured, the upper limit of C + N + P + S is limited to 0.025%.

The steel slab formed as described above is reheated and hot rolled. At this time, the finish rolling temperature is preferably limited to a temperature of at least Ar ₃ transformation. This is because when hot rolling at a temperature below the transformation temperature, not only the workability deteriorates due to the formation of the rolled grain but also the ductility greatly decreases.

Hot rolling as described above, and then winding, if the coiling temperature at this time is too high, it is preferable to limit to 750 ℃ or less because the crystal grains of the hot rolled sheet is too coarse and the moldability is reduced.

The wound hot rolled sheet is pickled and then cold rolled, and the cold reduction rate is preferably 50-90%. If the cold cold rolling rate is too low, the amount of nucleation of annealing material crystals is small, so the grains grow too large during annealing, and the ductility and formability are deteriorated due to the coarsening of the annealing recrystallized grains. Let 50% of a reduction ratio be a lower limit. In addition, when the cold reduction ratio is 90% or more, the moldability is improved, but the amount of nucleation is so large that the annealing recrystallized grain is rather fine and the ductility is lowered.

Subsequent annealing, the annealing temperature at this time plays an important role in determining the material of the product. In the case of the present invention, the lower limit is 650 ° C. because the recrystallized grain is too fine at 650 ° C. or less, so that a target ductility value cannot be obtained. . Further, above 850 ° C, the annealing temperature is so high that ductility decreases due to coarsening of the recrystallized grains, so the upper limit temperature is set to 850 ° C. Since the recrystallization is completed in 10 seconds or more, the annealing time is 10 seconds.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

Table 1 below shows the chemical components of the inventive steels and the comparative steels and their manufacturing conditions. That is, the steel ingots satisfying the chemical components of Table 1 were maintained in a 1250 ° C. heating furnace for 1 hour, and then hot rolled. At this time, the finish rolling temperature was 900 ℃, the coiling temperature was 650 ℃ and 780 ℃, respectively, the final thickness was 3.2mm. The hot rolled specimen was subjected to pickling to remove the oxide film on the surface and then cold rolled. At this time, the cold rolling rate was 40-70%, and the cold rolled specimens were processed into tensile specimens to investigate mechanical properties and then subjected to continuous annealing. Tensile specimens were processed into standard specimens according to ASTM E-8 standard. Continuous annealing was performed for 30 seconds at annealing temperatures of 750 ° C and 880 ° C. After the annealing was completed, the tensile tester (INSTRON, Model 6025) was used to measure yield strength, tensile strength, elongation, aging index and plastic anisotropy index. The results are shown in Table 2 below.

Sample Number Chemical composition (% by weight) C + N + S + P Hot rolled winding temperature (℃) Cold rolling reduction (%) Annealing Temperature (℃) Remarks C Mn P S Al N One 0.0015 0.12 0.0081 0.0033 0.038 0.0021 0.0150 650 70 750 Invention steel 2 0.0011 0.23 0.0083 0.0054 0.045 0.0028 0.0176 650 40 750 Invention steel 3 0.0008 0.23 0.0073 0.0093 0.052 0.0019 0.0193 650 40 750 Invention steel 4 0.0010 0.22 0.0059 0.0122 0.038 0.0035 0.0226 650 40 750 Invention steel 5 0.0042 0.22 0.0092 0.0066 0.051 0.0030 0.0230 650 40 750 Comparative steel 6 0.0012 0.25 0.0093 0.0074 0.005 0.0025 0.0204 650 40 750 Comparative steel 7 0.0011 0.26 0.0132 0.0185 0.052 0.0023 0.0351 650 40 750 Comparative steel 8 0.0009 0.13 0.0090 0.0080 0.042 0.0005 0.0184 650 40 750 Comparative steel 9 0.0010 0.22 0.0052 0.0120 0.038 0.0035 0.0217 650 40 750 Comparative steel 10 0.0010 0.22 0.0052 0.0120 0.038 0.0035 0.0217 650 70 880 Comparative steel 11 0.0010 0.22 0.0052 0.0120 0.038 0.0035 0.0217 780 70 750 Comparative steel

Sample Number Mechanical property Remarks Yield strength (kgf / mm ² ) Tensile strength (kgf / mm ² ) Elongation (%) Plastic Anisotropy Index Aging Index (kgf / mm ² ) One 14.5 26.6 62.1 1.84 1.32 Invention steel 2 14.8 26.9 60.2 1.80 1.20 Invention steel 3 15.3 27.8 58.3 1.75 0.92 Invention steel 4 16.3 28.5 56.3 1.72 1.20 Invention steel 5 19.3 29.3 50.2 1.45 4.36 Comparative steel 6 17.3 27.3 50.1 1.53 3.83 Comparative steel 7 17.2 29.7 45.2 1.39 1.26 Comparative steel 8 13.2 24.9 45.8 1.75 0.20 Comparative steel 9 12.8 23.9 45.8 1.53 1.30 Comparative steel 10 13.2 24.5 46.9 1.86 1.18 Comparative steel 11 13.8 26.8 49.3 1.58 1.32 Comparative steel

As can be seen in Table 2, in the case of the sample No. 1-4 belonging to the scope of the present invention, it is possible to secure a substantial inferiority of 3 kg / mm ² or less of the aging index, and a high DDQ (plastic drawing anisotropy index of 1.7 or higher). It is not only possible to secure the processability of rims of more than) grade but also applied to parts requiring severe elongation processability with elongation rate of 55% or more and can be easily molded.

On the contrary, in the case of sample No. 5, the carbon content is higher than the range of the present invention, and thus, it is difficult to secure substantial inferiority with an aging index of 4.36 kg / mm ² , and the elongation and plastic anisotropy index are also low. In the case of sample No. 6, the aluminum content is lower than the range of the present invention, so that the aging index is 3.83 kg / mm ² , which does not secure a practical inscription, and the plastic anisotropy index and ductility are also very low. In Sample No. 7, the aging index is 1.26, which can secure non-aging, but the value of C + N + P + S is out of the range of the present invention, and the elongation is greatly reduced. Sample number 8 has a very low elongation because the nitrogen content is out of the range of the present invention, and sample number 9 has a very low elongation out of the range of the cold reduction rate. Sample No. 10 had a high annealing temperature, so the plastic anisotropy index was high, but the recrystallized grain was too coarse, so the ductility was greatly reduced.

As described above, the present invention has an effect of providing a method for producing a cold rolled steel sheet excellent in aging and ductility while maintaining a plastic anisotropy index to a predetermined level or more.

Claims (1)

C: 0.0005-0.002% by weight, 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% and the above C + N + S + The steel slab containing P or less than 0.025% and the remaining Fe and other inevitably contained elements were hot rolled at a finish rolling temperature of more than Ar ₃ transformation point, and then wound at a temperature of 750 ° C. or lower, and then 50-90 A method for producing a non-aging cold rolled steel sheet having excellent ductility, which is cold rolled at a% reduction ratio and continuously annealed for 10 seconds or more at a temperature in the range of 650-850 ° C.