KR100478723B1 - Method for manufacturing cold-rolled steel sheet having improved dent-resistance and processability - Google Patents

Abstract

The present invention relates to a method for manufacturing a cold rolled steel sheet having excellent dent resistance and workability, and more specifically, yield strength of 18 to 22 kgf / mm ² , tensile strength of 32 to 35 kgf / mm ^2, and rm value (average value of vertical anisotropy) of 1.8. It relates to a method for producing a cold rolled steel sheet as described above.

The production method of the tensile strength 32kgf / mm grade ² cold rolled steel sheet excellent in the dent resistance and workability of the present invention, by weight% C: 0.0050% or less, Mn: 0.3 to 0.5%, P: 0.02 to 0.04%, Si: to Extremely low carbon steel containing 0.03%, N: 0.0030% or less, S: 0.015% or less, Ti: 0.045 to 0.065%, and other unavoidably contained elements, and reheat cracked slab in a conventional manner and hot rolled Hot-rolled coils are wound in the range of 600 to 700 ° C. with the hot finish rolling temperature at or above Ar3 transformation point, prepared by pickling in the usual manner, and cold-rolled in the range of 70 to 85%, and then in the range of 800 to 830 ° C. Annealing at, cooled to -10 ~ -30 ℃ / sec and overaging treatment, characterized in that the cold rolled steel sheet is produced by applying an elongation in the range of 0.6 to 1.2% during temper rolling work.

Description

METHOD FOR MANUFACTURING COLD-ROLLED STEEL SHEET HAVING IMPROVED DENT-RESISTANCE AND PROCESSABILITY}

The present invention relates to a cold rolled steel sheet manufacturing method of the tensile strength 32Kgf / mm ² grade excellent in DENT-RESTSTANCE and workability, more specifically, manganese which is a solid solution strengthening element in ultra-low carbon Al-killed steel [Mn], the proper addition of [P], and, [C], [Ti] to adjust the component to the yield strength 18~22kgf / mm ^2, tensile Yangtze FIG 32~35kgf / mm ^2, and the value rm (perpendicular anisotropy average ) Cold Rolled Steel Sheets with 1.7 or more are manufactured to improve yield strength by ² ~ 3kgf / mm ² or more compared to existing ultra low carbon steels with sufficient processability. The present invention relates to a method for manufacturing a cold rolled steel sheet that can be applied to an outer sheet material, and can be used as a steel sheet that can reduce the thickness of a material due to an increase in strength, thereby contributing to environmental regulations and improving fuel efficiency of automobile companies.

In the case of the cold rolled steel sheet for automobiles related to the present invention, conventionally, a low carbon steel-based box (BAF) annealing method is produced, or an ultra-low carbon steel based CAL annealing method is used to produce cold rolled steel sheet having excellent workability. have.

Cold-rolled steel sheet by box annealing is 0.02 ~ 0.04% [C] low carbon base, heated to an annealing temperature of 650 ~ 700 ℃, then gradually cooled to precipitate sufficient carbonitride to secure workability. In addition, the surface quality is inferior and the productivity is very low. Recently, most of them are producing cold rolled products by CAL annealing method using ultra low carbon steel. In the case of CAL annealing, IF steel with [Ti] and [Nb] added to [C]: 0.005% or less, [P]: 0.015% and [S]: 0.015% or less Added to precipitate carbon and nitrogen in steel) and cold-rolled steel sheet by using high temperature annealing method of 800 ℃ or higher, so drawing performance and ductility can be maximized, but yield strength and tensile strength are very low. This is relatively disadvantageous. Dent resistance withstands local deformation such as indentation. As a method of measuring this, a method of measuring a depth after applying a predetermined load to a steel plate with a test ingot is used.

Recently, in order to overcome this problem of dent resistance, the ultra-low carbon steel containing [C]: 0.002 ~ 0.004% is strictly controlled, so that the hardening amount after baking is more than 3kgf / mm ² Hardened cold rolled steel sheets have been developed. However, these hardened hardened steels are very difficult to be supplied for overseas automobiles because they have very strict compositional control during steel making, which makes the steel making work very poor. It is true.

The present invention is to solve the problems of the prior art as described above, by using a continuous annealing method of producing a cold rolled steel sheet having a tensile strength of 32kg or more, rm value (average value of vertical anisotropy) 1.8 or more excellent in dent resistance The purpose is to provide.

In order to achieve the above object, the method for producing a tensile strength of 32 kgf / mm grade ² cold rolled steel sheet having excellent dent resistance of the present invention is [C]: 0.005% or less, [Mn]: 0.3 to 0.5%, [P] by weight% ]: 0.015%, [S]: 0.015% or less, [N]: 0.003% or less, [Ti] 0.45-0.065% of composition and the aluminum-kilted steel containing the element which is inevitable in the manufacture of steel, 1050-1200 After homogenizing at ℃, hot rolling finish temperature is set to 900 ~ 940 ℃ of Ar3 transformation point office workers, hot rolling is carried out in the temperature range of 600 ~ 700 ℃, pickling treatment in the usual manner, then cold pressure of 70 to 80% Cold rolling is carried out at a rate. In case of continuous annealing, high temperature work in 800 ~ 830 ℃ temperature range, and cooling rate is -10 ~ -30 ℃ / sec after cooling and overaging treatment, and cold rolling is applied by elongation of 0.6 ~ 1.2% in temper rolling work. To prepare a steel sheet.

The flame-retardant steel sheet manufactured as described above exhibits yield strength of 18 to 22 kgf / mm ² and tensile strength of 32 kgf / mm ² or more, and has an excellent average workability of 1.8 or more in terms of vertical anisotropy. Thus, automotive materials, especially roofs, doors, hoods, etc. It can be advantageously used as a base steel sheet such as an outer plate material that requires excellent workability and dent resistance.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments.

In the present invention, the composition of the steel sheet is% by weight [C]: 0.005% or less, [Mn]: 0.3-0.5%, [P]: 0.2-0.4%, [S]: 0.015% or less, [N]: 0.003 % Or less, [Ti] 0.45 to 0.065%, the remaining amount of Fe and other inevitable elements, the reason for the content limitation of each component is as follows.

Carbon [C] is an element that inhibits workability, and if it is more than 0.005%, solid carbon existing in solid solution in the hot-rolled steel sheet may remain. Since such dissolved carbon inhibits the aggregate structure which is advantageous for the workability of the cold rolled steel sheet, the amount of the dissolved carbon should be reduced as much as possible. Generally, solid solution of carbon is removed by adding titanium [Ti]. However, if the amount of carbon is large, the content of [Ti] should also increase, resulting in deterioration of steel workability due to this, and the occurrence of surface defects in cold rolled steel sheet. The upper limit is limited to 0.005%.

Manganese [Mn] should contain at least 10 times of the [S] content to prevent red brittleness caused by sulfur [S] elements, and 0.3% is set as the lower limit to increase the strength by solid solution strengthening. Above 0.5%, the grain size was reduced so much that the workability decreased, so the upper limit was limited to 0.5%.

In the case of phosphorus [P] element, the addition of small amount is excellent in solid solution strengthening effect and FeTiP precipitation strengthening effect, but when it exceeds 0.5%, secondary processing brittleness due to grain boundary segregation is appeared. Limited to%.

In the case of nitrogen [N], it precipitates in combination with [Ti] in steel. Nitrogen is generally precipitated before [C], so if the amount of nitrogen increases, the consumption of [Ti] by nitrogen increases. Therefore, the upper limit of nitrogen is increased because the amount of [Ti] to remove solid solution carbon increases. It was set as 0.003%.

Aluminum [Al] is added to the steel for deoxidation. When the amount of aluminum is less than 0.02%, oxygen remains in the steel, forming oxides with [Mn] and [Ti] during steelmaking. If it is more than 0.05%, aluminum is added more than necessary to increase the manufacturing cost and generate a large amount of surface defects of the steel sheet, so the upper limit of [Al] is limited to 0.05%.

In the case of titanium [Ti], 0.045% or more is added to improve the workability of cold rolled steel sheets due to the penetration of solid solution elements such as carbon and nitrogen that impair workability, and when 0.065% or more is added, nozzle clogging occurs in the steelmaking process. This results in poor workability and increased surface defects caused by oxidative inclusions. Therefore, the content of [Ti] was limited to 0.045 to 0.065%.

The steel melt | dissolved with the said composition is homogenized at about 1050-1200 degreeC. When the homogenization treatment temperature is more than 1200 ℃ mainly because the fine precipitates precipitate due to poor workability, the homogenization treatment temperature was limited to 1050 ~ 1200 ℃. The hot rolling finish temperature was performed at 900 to 940 ° C, directly above the Ar3 transformation point, and hot rolled at 600 to 700 ° C to prepare a hot rolled coil. After pickling the hot rolled coil, cold rolling was performed at a cold reduction rate of 70 to 85%. The reduction ratio was limited to more than 70% because it affects the drawing workability, and the upper limit was limited to 85% in consideration of the rolling mill load during cold rolling. In the annealing operation after cold rolling, the annealing temperature was in the range of 800 to 830 ° C. If the temperature is less than 800 ° C., recrystallization and sufficient grain growth do not occur, so that the workability is degraded. In the case of more than 830 ° C., the strength is degraded, so the annealing temperature is limited to 800 to 830 ° C. Temper rolling is carried out for shape correction, roughening, and YP-EL, but less than 0.6% yields lack of yield strength, and when applied above 1.2%, work load and material deterioration occurs. Therefore, the temper rolling elongation was limited to 0.6 ~ 1.2%.

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

EXAMPLE

Table 1 shows the composition performance, hot rolling temperature and material performance of the tested steel.The steel melted in the high-frequency vacuum induction furnace was used to cast ingots of 60mm (thickness) × 170mm (width) × 200nn (length) to 1200 After homogenizing at 1 ℃ for 1 hour, hot rolling was performed at a pilot mill of 3.2mm in thickness, hot rolling finish temperature was 900 ℃, and after water spray cooling for 1 hour at 600 ℃ and 700 ℃ Hot rolled winding simulation (SIMULATION). The hot rolled material was cold rolled at a cold reduction rate of 70 to 80% after pickling, and the annealing heat treatment was performed by changing the annealing temperature to 800, 820, and 840 ° C using an infrared reflective heating furnace. After cooling to -25 ° C / sec and overaging. The chemical composition, the coiling temperature, the annealing temperature and the material performance values of the inventive and comparative materials thus produced are shown in Table 1 and Table 2. In the field production, the temper rolling was carried out, but in the laboratory manufacturing, the yield strength was lower than the standard because the temper was not applied.

As shown in Table 1, the yield and tensile strengths increased with increasing carbon content, lower coiling temperature, and lower annealing temperature. In the case of Ti + Nb composite steels of Comparative 1, 2, and 3, tensile strength of 32kgf / mm ² or more can be secured, but the material dependence on hot coiling temperature is very large, and the material deviation according to the difference in cooling history by coil length direction after hot rolling There is a possibility. However, in the case of invention steel, TS 32kgf / mm ² or more, rm 1.8 or more can be secured, and the material deviation is small according to the coiling temperature and annealing temperature. I think it will be easy.

Table 3 shows the results of the actual application of the invention steel, which is similar to the material of laboratory materials, but yield strength of 0.6 ~ 1.2% is applied to temper rolling, so it is possible to secure YP 18kgf / mm ² or more and yield strength compared to existing steel. ² ~ 3kgf / mm ^{2 is} high, it shows that it can be applied to the steel sheet of automobile exterior materials having excellent dent resistance. Table 4 shows the results of measuring the dent resistance of the invention steel applied to automobiles.

Table 1

Chemical Composition and Performance of Comparative and Invented Steels division Chemical composition (%) CT (℃) Material performance C Mn P S S0L-AL Ti Nb YP TS EL R Comparison 1 Comparison 2 0.0023 0.34 0.25 0.007 0.039 0.027 0.009 700600 12.812.9 32.033.0 44.541.3 2.01.73 Comparison 3 Comparison 4 0.0035 0.34 0.25 0.007 0.051 0.032 0.009 700600 12.913.4 32.533.8 44.041.0 2.011.70 Comparison 5 Comparison 6 0.0048 0.36 0.24 0.007 0.053 0.036 0.010 700600 13.015.0 33.235.8 43.538.5 1.971.60 Invention 1 Invention 2 0.0029 0.36 0.24 0.007 0.045 0.045 - 700600 12.813.1 32.933.4 44.943.5 1.831.80

[Table 2]

Performance of material according to hot rolling and annealing temperature of invention steel and comparative steel division Hot rolled winding temperature (℃) CAL annealing temperature (℃) Material performance YP TS EL R Comparative 3 Comparative 3-1 700 800820840 12.912.811.9 32.532.432.0 44.044.045.0 2.012.082.10 Comparison 4 Comparison 4-1 600 800820840 13.413.212.0 33.833.733.0 41.041.342.0 1.701.721.77 Invention 1 Invention 1-1 700 800820840 12.812.512.2 32.932.532.3 44.945.645.6 1.832.032.10 Invention 2 invention 2-1 600 800820840 13.113.012.3 33.433.232.6 43.544.544.8 1.801.821.85

Table 3

Material Performance of In-situ Steel (Gwangyang 1 Cold Rolled Steel) and Existing Ultra Low IF Cold Rolled Steel division Chemical composition (%) Material performance C Mn P S0L-AL Ti YP TS EL R Existing Material <= 0.005 <= 0.15 <= 0.015 0.02 to 0.06 0.035-0.055 15-16 28-31 46-48 1.8 to 2.0 Invention steel <= 0.005 0.3-0.05 0.02 to 0.04 0.02 to 0.05 0.045-0.065 18-20 32 to 34 44 to 45 1.8 to 1.9

Table 4

Comparison of dent resistance between invention steel and existing feedstock division R / H L / H ① ② ③ ④ ⑤ ⑥ ⑦ ⑧ Existing Material 4.65 8.85 4.50 8.05 4.55 7.00 3.80 6.55 NSC 4.65 8.10 4.35 6.65 4.75 9.15 4.00 6.90 Triumphal 3.80 5.90 3.95 6.10 3.95 6.95 3.50 6.10

Accordingly, the present invention is characterized by being able to effectively produce and provide a cold rolled steel sheet having both excellent dent resistance and workability suitable as an outer panel material for automobiles by the method described above.

Claims (1)

% By weight [C]: 0.005% or less, [Mn]: 0.3 to 0.5%, [P]: 0.2 to 0.4%, [S]: 0.015% or less, [N]: 0.003% or less, [Al] 0.02 to Hot rolled finish temperature is 900 ~ 940 ℃, which is directly above Ar3 transformation point after homogenizing the aluminum-kilted steel with the composition of 0.05% and [Ti] 0.45 ~ 0.065% and inevitably contained in the manufacture of steel at 1050 ~ 1200 ℃. After hot rolling is carried out at a temperature in the range of 600 to 700 ° C., pickling is carried out by a conventional method, followed by cold rolling at a cold rolling rate of 70 to 85%, and then annealing continuously at a temperature in the range of 800 to 830 ° C. After cooling at -10 ~ -30 ℃ / sec, overaging treatment, yield strength of 18 ~ 22kgf / mm ² , tensile strength 32 ~ 35kgf / mm ² and vertical by applying elongation 0.6 ~ 1.2% in temper rolling Cold rolled steel sheet manufacturing method excellent in dent resistance and workability, characterized by securing an anisotropic average value of 1.8 or more.