KR100815709B1 - Formable high strength cold-rolled steel sheet with excellent weather resistance and method manufacturing the same - Google Patents

Abstract

A high strength cold-rolled steel sheet having weatherability, formability, and at least 80 kgf/mm^2 of tensile strength is provided, and a method for manufacturing the same is provided. A high strength cold-rolled steel sheet with excellent weatherability and formability has a composition comprising, by weight percent, 0.10 to 0.20% of carbon(C), 0.05 to 0.25% of silicon(Si), 1.0 to 2.5% of manganese(Mn), 0.02% or less of phosphorous(P), 0.01% or less of sulfur(S), 0.02 to 0.07% of aluminum(Al), 0.02 to 0.08% of niobium(Nb), 0.05 to 0.30% of nickel(Ni), 0.1 to 0.5% of copper(Cu), 0.8 to 1.5% of chromium(Cr), and 0.01 to 0.10% of cobalt(Co) with the balance of Fe and other inevitable impurities, and is comprised of 10 to 30% of a low temperature transformed structure, and the balance of ferrite. The low temperature transformed structure comprises at least one of martensite and bainite.

Description

Formable High Strength Cold-Rolled Steel Sheet With Excellent Weather Resistance And Method Manufacturing The Same}

Japanese Laid-Open Patent Publication No. 7-207408

Japanese Patent Application Laid-Open No. 11-21622

Japanese Patent Application Laid-open No. Hei 6-104858

     The present invention relates to a high strength weather resistant cold rolled steel sheet used in construction, railroad cars, containers, and the like, and more particularly, to a low temperature transformation structure in a Cu-Co-based component system, thereby having a high workability with excellent workability. It relates to a cold rolled steel sheet and a method of manufacturing the same.

     Conventionally, materials such as stainless steel or aluminum have been used for the purpose of reducing the weight and extending the service life of a container or a railway vehicle. Characteristics required for such products include bending properties, bendability, weldability, durability, and the like.

In addition, in the case of a transport structure to increase the weight of the cargo that can be transported at one time, the demand for lightweight containers has been actively made. In the past, ISO standardized 20 or 40 foot containers were mainly used, but the trend is that the utilization of long containers of 45 to 53 feet is increasing. For these long containers, the container itself weighs only 3 tons. It is over. Therefore, the first consideration to reduce the weight of the container is to reduce the material thickness through the high strength of the applied material. For example, if one TEU (Twenty-foot Equivalent Units) container cargo is transited across the Atlantic after being shipped to the vessel, the weight of the container is reduced by about 10%. If you can, you can save more than 10% of the raw materials used. In addition, the amount of CO ₂ generated when using ore raw materials can be suppressed, thereby coping with global warming.

To this end, it is preferable to apply a high strength steel sheet of 80kgf / mm ² or more as a container material. In particular, since containers have to withstand various climatic conditions in the sea or on land, depending on the transport conditions, the use of steel with excellent weather resistance is required.

For example, conventionally, weather-resistant cold-rolled steel SPA-C material (see industrial standards KS D3542 and JIS G3125) has been mainly used, but these steels have a low tensile strength of 50 kg / mm ² , thereby making larger products. Increasing transportation costs due to its own weight has become a limiting factor. In addition, although there are high-strength cold-rolled steel having a tensile strength of 60 ~ 80kg / mm ² grade for the structural member of the vehicle, even in the case of these materials, there is a problem in that it does not exhibit the desired weather resistance as the emphasis on strength characteristics.

Recently, in the container industry, in order to cope with cost reduction and environmental problems, a large container is manufactured by greatly reducing the weight of the container itself. Accordingly, attempts have been made to greatly increase the efficiency of transportation, and in particular, there is a demand for steel sheets having weather resistance and high strength and techniques for manufacturing these materials.

For example, in Japanese Patent Laid-Open No. 7-207408, C 0.008% or less, Si 0.5-2.5%, Mn 0.1-3.5%, P 0.03-0.20%, S 0.01% or less, Cu 0.05-2.0%, Al 0.005- Steels containing 0.1% and N 0.008% or less, Cr 0.05-6.0%, Ni 0.05-2.0% and Mo 0.05-3.0%, B 0.0003-0.002% are heated at 1100-1300 ° C and the rolling ends at 800-950 ° C. A method for producing a hot rolled steel strip, characterized in that the winding to 400 ~ 700 ℃. However, only a few embodiments of this technique show a tensile strength of 60 to 70 kg / mm ^2, and in most cases, a tensile strength of 50 kg / mm ² indicates that 80 kg / mm ² can not be obtained. appear. In addition, there is a problem in that weldability is deteriorated and manufacturing cost is increased by adding a large amount of hardenability improving elements such as Cr and Mo among the component components.

As another example, Japanese Patent Laid-Open No. 11-21622 discloses C 0.15% or less, Si 0.7% or less, Mn 0.2-1.5%, P 0.03-0.15%, S 0.02% or less, Cu 0.4% or less, Al 0.01- or less. Steel containing 0.1% and less than 0.1% of Cr, 0.4 to 4.0% of Ni, and 0.1 to 1.5% of Mo is heated to 1050 to 1300 ° C, hot rolled at 40% or more at 950 ° C or more, and then finished rolling at 900 to 750 ° C. And air cooling is proposed. However, even at this time, most of the tensile strength is 50kg / mm ² grade, only a few showed the tensile characteristics of 60kg / mm ² grade, this technique can be said to be mainly applied to 50kg / mm grade ² steel sheet. In addition, although the effect of improving the corrosion resistance in the seawater atmosphere by adding a large amount of P to 0.03 ~ 0.15% is mentioned, the addition of a large amount of P causes a central segregation of the cold rolled material, such that the workability of the steel sheet is sharply degraded there was.

In addition, Japanese Patent Laid-Open No. 6-104858 discloses C 0.02 to 0.12%, Si 0.5% or less, Mn 0.1 to 2.0%, P 0.07 to 0.15%, S 0.02% or less, Cu 0.25 to 0.55% or less, Al 0.01 to Steels containing 0.05% and Cr 0.3-1.25%, N ₂ 0.006% or less, Ti 0.06-0.20% are controlled in the range 12.1X _{ti, eff} (%) / Mn (%)> 1.0 and reheated at 1180 ° C. or higher, After hot rolling at 880 ~ 950 ℃, it provides the technology of winding below 650 ℃. This technique adds Ti content in conjunction with the amount of Mn added as a precipitate control element. However, the embodiment of this technique also has a tensile strength of 60kg / mm ² class lower than the 80kg / mm ² class aimed at the present invention.

Thus, in the present invention to solve the above problems, it is to ensure weather resistance and workability while having a high strength of more than 80kg / mm ² tensile strength.

In order to achieve the above object, the present invention, by weight% of carbon (C) 0.10 ~ 0.20%, silicon (Si) 0.05 ~ 0.25%, manganese (Mn) 1.0 ~ 2.5%, phosphorus (P) 0.02% or less, sulfur ( S) 0.01% or less, aluminum (Al) 0.02-0.07%, niobium (Nb) 0.02-0.08%, nickel (Ni) 0.05-0.30%, copper (Cu) 0.1-0.5%, chromium (Cr) 0.8-1.5% , Cobalt (Co) containing 0.01 ~ 0.10%, is composed of the remaining Fe and other unavoidable impurities, it is composed of 10-30% low-temperature transformation structure and the remaining ferrite.

In the present invention, the low temperature transformation tissue is one containing at least one or more of martensite and bainite. Preferably, the low temperature metamorphic tissue mainly contains martensite and some bainite.

The method for producing a cold rolled steel sheet according to the present invention is 0.1% to 0.20% of carbon (C), 0.05 to 0.25% of silicon (Si), 1.0 to 2.5% of manganese (Mn), 0.02% or less of phosphorus (P), sulfur ( S) 0.01% or less, aluminum (Al) 0.02-0.07%, niobium (Nb) 0.02-0.08%, nickel (Ni) 0.05-0.30%, copper (Cu) 0.1-0.5%, chromium (Cr) 0.8-1.5% , Cobalt (Co) 0.01 ~ 0.10%, the steel consisting of the remaining Fe and other unavoidable impurities reheated at 1150 ~ 1300 ℃ hot-rolled under the finish rolling conditions of 750 ~ 930 ℃ and wound at 400 ~ 650 ℃ Next, after cold rolling, continuous annealing at a temperature of (A ₁ transformation point + 30 ° C.) to (A ₃ transformation point or less) is performed to obtain 10-30% of the low temperature transformation structure and the structure of the remaining ferrite.

Hereinafter, the present invention will be described in detail.

The present inventors have completed the present invention by repeating research and experiments to secure high strength of 80kg / mm ² or more while satisfying weather resistance along with various processing characteristics. The present invention is characterized by optimizing the addition amount of Mn, Cr, etc. in the weathering component system of Cu-Co and securing a low temperature structure, weather resistance, workability with high strength characteristics. The components of the cold rolled steel sheet of the present invention will be described in detail.

Carbon (C) is preferably 0.10 to 0.20% by weight (hereinafter simply referred to as%).

C is an element added to improve the strength of the steel sheet, the tensile strength and yield strength increases as the amount added, but the workability worsens when added excessively, the upper limit is preferably 0.20%. On the other hand, if the amount of C is less than 0.10%, not only does not obtain sufficient precipitation strengthening effect, but also has a problem in that a crack occurs frequently at around 0.09%.

As for silicon (Si), 0.05 to 0.25% is preferable.

Si not only provides molten steel deoxidation and solid solution strengthening effect, but also forms a dense oxide of Fe ₂ SiO ₄ together with Fe in the surface layer of the steel at high temperature to improve corrosion resistance. Addition is preferred. Therefore, Si should be added to improve weather resistance, but if excessively added, there is a problem in that weldability and plating property deteriorate.

As for manganese (Mn), 1.0 to 2.5% is preferable.

Mn is an element that is effective for strengthening by solid solution and is an important element for increasing the strength of steel and improving hot workability, but is also an element that inhibits the ductility and workability of a material due to MnS formation. If the content of Mn is small, it is advantageous for workability but poor in hardenability, so that it is difficult to secure strength, so that 1.0% or more is preferable to secure the target strength. On the other hand, when Mn is excessively added, the upper limit is preferably 2.5% because there is a problem of deterioration in economics and weldability due to the addition of a large amount of expensive alloying elements.

The phosphorus (P) is preferably 0.02% or less.

Since P plays a role of improving the corrosion resistance of the steel, it is preferable to add a large amount in terms of corrosion resistance. However, since P is the element causing the largest segregation of the core during casting, a large amount of P decreases weldability and toughness. Therefore, the content is preferably limited to 0.02% or less.

Sulfur (S) is preferably 0.01% or less.

S is known as an effective element for improving corrosion resistance, but it is desirable to reduce the content as much as possible because it forms a non-metallic inclusion which serves as a starting point of corrosion in combination with Mn in the steel. Therefore, the amount of S is preferably 0.01% or less, and more preferably 0.005% or less.

As for aluminum (Al), 0.02 to 0.07% is preferable.

Al is generally an element that is effective in improving deoxidation and corrosion resistance of molten steel, but when it is added excessively, there is a problem of decreasing the workability by increasing the amount of inclusions in the steel, so the content is preferably set to 0.02 to 0.07%.

Niobium (Nb) is preferably 0.02% to 0.08%.

Nb is not only an effect of delaying the recrystallization of ferrite but also an element exhibiting the effect of increasing the strength of the steel sheet by being combined with C and N _{2 in} steel to precipitate, so that addition of 0.02% or more is preferable for securing the target strength. On the other hand, when the addition amount of Nb exceeds 0.08%, it becomes a factor that increases the manufacturing cost and the hot rolling workability.

As for nickel (Ni), 0.05 to 0.3% is preferable.

Ni is generally an element that improves corrosion resistance as well as preventing casting cracks generated during casting in Cu-added steel. In order to exert such effects, addition of 0.05% or more is preferable. However, if the content of Ni exceeds 0.3%, rather than worsening the corrosion resistance, there is a cost problem due to the excessive use of expensive alloying elements.

As for copper (Cu), 0.1 to 0.5% is preferable.

Cu is an element which forms a stable rust layer in a corrosive atmosphere and improves corrosion resistance, and is preferably added at least 0.1% in order to ensure target corrosion resistance. However, when the addition amount of Cu exceeds 0.5%, not only does it cause grain boundary cracking during playing, but also there is a problem of roughening the surface state of the hot rolled steel sheet.

Chromium (Cr) is preferably 0.8 to 1.5%.

Cr is an element that plays a role of forming a stable rust layer, such as Cu, is preferably 0.8% or more in order to secure corrosion resistance and obtain strength. In addition, when the addition amount of Cr exceeds 1.5%, rather than acting as a factor causing the hole corrosion, but also sharply increases the manufacturing cost.

Cobalt (Co) is preferably 0.01 to 0.1%.

Co is an element that reacts with Cu, Cr, etc., added to secure corrosion resistance in steel, and promotes formation of a surface layer corrosion inhibiting product, and in order to obtain such an effect, 0.01% or more is preferable. However, when the amount of Co added exceeds 0.10%, it contributes to an increase in manufacturing cost rather than a contribution of improving the corrosion resistance.

Including the above-mentioned components, it becomes an unavoidable impurity and remainder Fe. If necessary, an alloying element may be added to the weathering steel to add a property improvement, and an alloying element which is not disclosed in the embodiment of the present invention is added and is not interpreted as being excluded from the scope of the present invention.

In the cold-rolled steel sheet of the present invention, the microstructure includes 10-30 vol.% (Simply, expressed in%) of the low temperature transformation structure and the remaining ferrite. The low temperature transformation structure is 10% or more, which lowers the yield strength and increases the work hardening index, thereby ensuring excellent workability. If the low-temperature transformation structure is less than 10%, discontinuous yielding behavior is exhibited according to the remaining solid solution in the steel, and machining defects occur in the machining process, making it difficult to secure the target workability. If the low temperature transformation tissue exceeds 30%, the workability is poor. In the present invention, the low-temperature metamorphic tissue may include martensite, bainite, and the like, and one or two or more of them are included. The cold metamorphic tissue is mainly martensite and most preferably some bainite is present.

Hereinafter, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.

The steel having the above chemical composition was rolled at 400 ~ 650 ℃ after rolling at reheating temperature of 1150 ~ 1300 ℃ and finishing rolling temperature of 750 ~ 930 ℃ in hot rolling step, by cold rolling and continuous annealing (A ₁ transformation point + 30 ℃). By heat treatment at a temperature of ~ (A ₃ transformation point or less) to include a low temperature transformation structure can be produced a high strength cold rolled steel sheet having excellent weather resistance and workability having a tensile strength of 80kg / mm ² or more.

If the reheating temperature is less than 1150 DEG C, since the solidification structure formed during casting is insufficient and the center segregation is well developed, mixing of the finally formed crystal grains occurs and workability and impact toughness are significantly reduced. In addition, if the reheating temperature exceeds 1300 ℃, the formation of scale by oxidation is promoted, and the slab thickness decreases and the impact toughness decreases due to the coarsening of grains when reheating, and the economic loss due to the increase of the heating unit is large. As for the management range of reheating temperature, 1150-1300 degreeC is preferable.

If the finishing hot rolling temperature is higher than 930 ° C, uniform hot rolling is not performed throughout the thickness, resulting in insufficient grain refinement, resulting in a drop in impact toughness due to grain coarsening. On the contrary, when the hot rolling is finished at a temperature of less than the final hot rolling temperature of 750 ° C., as the hot rolling is finished in the low temperature region, it is preferable to limit the finish hot rolling temperature to 750 to 930 ° C. because the hybridization of the crystal grains proceeds rapidly.

Winding after finishing hot rolling, when the coiling temperature exceeds 650 ℃ it is difficult to secure a stable strength of the target strength of 80kg / mm ² as a sufficient precipitation effect is not obtained. On the other hand, at winding temperatures of less than 400 ° C., hard phases are generated during cooling and holding, and as the roll force of the rolling mill rises in the cold rolling process, rolling properties cannot be secured. It is preferable to limit to 400-650 degreeC.

On the other hand, it is preferable to cool in a run-out-table (ROT) before winding up after said finishing hot rolling. According to a preferred embodiment of the present invention, the cooling rate at this time is preferably 20 ~ 40 ℃ per second, but is not necessarily limited thereto. If the cooling rate is less than 20 ° C per second, the coarse grains are formed by the promotion of grain growth, which may be a cause of strength degradation. If the cooling rate exceeds 40 ° C per second, a hard second phase such as bainite may be formed, resulting in poor cold rolling.

The hot rolled material is subjected to rolling under normal cold rolling conditions and subjected to a continuous annealing process. At this time, in order to secure the target material properties, it is preferable to manage the annealing temperature (A ₁ transformation point + 30 ° C or more) to A ₃ transformation point or less. After annealing, it is desirable to include the low temperature metamorphic tissue in the range of 10-30% and become the tissue of the remaining ferrite.

If the annealing temperature is lower than (A ₁ transformation point + 30 ℃) in the continuous annealing process, the ductility drops rapidly due to the deformation grain during cold rolling, and the volume fraction of the second phase targeted in the cooling process after annealing cannot be obtained. Therefore, there is a problem that the workability is lowered. On the other hand, when the A ₃ transformation point is exceeded, there is a problem in that the surface defects of the steel are increased by high temperature annealing. Cooling in the continuous annealing of the present invention may be any cooling condition to obtain the low temperature transformation structure required by the present invention. Preferably it can be water cooled and it is also possible to cool using cooling gas.

According to an embodiment of the present invention, the cold rolled steel sheet of the present invention has weather resistance and high strength while satisfying various processing characteristics, that is, bending property and stretchability. In addition, the cold rolled steel sheet of the present invention also ensures the properties such as weldability, plating property, etc. by control of components such as Si.

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

Example 1

Table 2 shows the results of evaluation of the standardized corrosion resistance index (CI) and weather resistance test for steels as shown in Table 1 below.

The weather resistance test was conducted for a salt spray test (SST, ST) for 5 hours in a 5% salt solution (NaCl solution) at 30 ° C. and the results are shown in Table 2.

Corrosion resistance index is a weather resistance evaluation index specified in ASTM G101. The higher this value is, the better the weather resistance of steel is. It is mainly calculated based on alloying elements and is defined as follows.

Corrosion Resistance Index (CI) = 26.01 (% Cu) + 3.88 (% Ni) + 1.2 (% Cr) + 1.49 (% Si) + 17.28 (% P)-7.29 (% Cu) (% Ni)-9.10 (% Ni) (% P)-33.39 (% Cu) ²

Steel grade Chemical composition (% by weight) C Si Mn P S Al Nb Co Ni Cu Cr Inventive Steel 1 0.15 0.08 1.54 0.009 0.003 0.042 0.051 0.05 0.19 0.35 0.96 Inventive Steel 2 0.16 0.15 1.98 0.008 0.003 0.037 0.047 0.06 0.14 0.36 1.02 Comparative Steel 1 0.05 0.15 1.54 0.012 0.006 0.033 0.024 0.04 0.21 0.25 0.36 Comparative Steel 2 0.16 0.65 0.54 0.094 0.005 0.042 - - 0.46 0.75 1.05 Comparative Steel 3 0.13 0.05 0.56 0.011 0.004 0.034 0.066 - 0.15 0.36 - Comparative Steel 4 0.12 0.20 1.34 0.009 0.024 0.051 - - 0.12 0.31 0.76

River bell Corrosion Resistance Index (CI) Corrosion degree (weight loss, gr / cm ² ) Weatherability Assessment Inventive Steel 1 6.676799 0.0264 Good Inventive Steel 2 6.787588 0.0256 Good Comparative Steel 1 5.687628 0.0312 Defective Comparative Steel 2 3.454711 0.0518 Defective Comparative Steel 3 5.474161 0.0398 Defective Comparative Steel 4 6.404425 0.0296 Good

As shown in Table 2, the comparative steels (1) to comparative steels (3) had a low corrosion resistance index value and a large weight loss due to corrosion, making it difficult to apply in terms of weather resistance. In case of (2), it was found that weather resistance was excellent in terms of weight loss and corrosion resistance index value.

Example 2

In Table 1 of Example 1, using the invention steel (1), (2) and comparative steels (1) to (4), by working under the same conditions as Table 3 to produce a cold rolled steel sheet for each material mechanical Table 4 shows the results of evaluating the properties and processing characteristics.

division Used steel grade Reheating Temperature (℃) Hot rolled finish temperature (℃) Cooling rate (℃ / s) Winding temperature (℃) Annealing Temperature (℃) Invention 1 Inventive Steel 1 1200 800 25 620 800 Invention 2 1200 910 25 620 850 Invention 3 Inventive Steel 2 1200 860 30 500 830 Invention 4 1250 880 30 500 880 Comparative Material 1 Inventive Steel 1 1080 700 25 450 750 Comparative Material 2 1200 870 25 600 650 Comparative Material 3 1220 870 30 600 700 Comparative Material 4 Inventive Steel 2 1250 850 20 600 700 Comparative Material 5 1250 850 20 600 750 Comparative Material 6 Comparative Steel 1 1250 830 30 560 800 Comparative Material7 1250 830 25 560 850 Comparative Material 8 Comparative Steel 2 1200 860 30 620 830 Comparative Material 9 Comparative Steel 3 1190 860 25 600 830 Comparative Material 10 Comparative Steel 4 1200 870 20 620 700 Comparative Material 11 1200 870 25 620 850

division Yield strength (kgf / mm ² ) Tensile Strength (kgf / mm ² ) Elongation (%) Second phase fraction Machinability Volume fraction (%) Martensite fraction (%) Bainite fraction (%) Invention 1 42.1 88.7 16.9 14.2 13.3 0.9 Good Invention 2 46.9 91.2 17.1 15.8 14.2 1.6 Good Invention 3 58.2 93.1 18.2 15.4 13.2 2.2 Good Invention 4 62.5 96.2 18.7 17.4 16.8 0.6 Good Comparative Material 1 76.4 99.8 6.2 6.1 4.9 1.2 Crack occurrence Comparative Material 2 72.5 81.4 9.1 1.4 0.0 1.4 Crack occurrence Comparative Material 3 82.8 91.4 7.8 0.0 0.0 0.0 Crack occurrence Comparative Material 4 90.4 91.9 6.0 0.0 0.0 0.0 Crack occurrence Comparative Material 5 77.1 92.2 8.4 0.0 0.0 0.0 Crack occurrence Comparative Material 6 45.7 62.8 19.0 3.2 3.2 0.0 Good Comparative Material7 40.1 60.4 17.5 3.8 3.6 0.2 Good Comparative Material 8 76.5 82.4 5.2 0.0 0.0 0.0 Crack occurrence Comparative Material 9 62.4 76.1 12.4 0.0 0.0 0.0 Good Comparative Material 10 84.6 85.3 2 0.0 0.0 0.0 Crack occurrence Comparative Material 11 46.5 72.5 19.4 5.4 5.4 0.0 Good

As shown in Table 4, in the case of the inventive materials (1) to (4) in which the chemical composition and the production conditions satisfy the scope of the method of the present invention, the tensile strength is 80 kgf / mm ² As mentioned above, the volume fraction of the second phase including the martensite phase after annealing was secured at 10% or more, and no cracking occurred during bending and stretching, thus producing cold-rolled steel sheets having weather resistance, high strength, and high workability. It was possible.

On the other hand, the comparative materials (1) to (5) which satisfy the chemical composition range of the inventive steel but whose manufacturing conditions are out of the range of the present invention failed to obtain the target workability. That is, since the annealing temperature does not satisfy the annealing range of (A ₁ transformation point + 30 ° C.) or more of the present invention, the ductility is not less than 10%, and the volume fraction of the second phase including the martensite phase in the annealing plate is also 10. As it did not exceed%, most processing cracks occurred during stretching.

On the other hand, in the case of the comparative material (6) and the comparative material (7) manufactured in the manufacturing conditions range of the present invention for the comparative steel (1) that C and Cr, etc. were not able to secure the weatherability target as the deviation of the present invention composition Although relatively good processability, tensile strength was not possible to secure a tensile strength of at least 80kgf / mm ² who aims the ^second level 80kgf / mm.

In addition, even in the case of comparative steel (2) having a high chemical composition, Si, P, and low Mn, Co and the like compared to the composition range of the present invention, and the comparative steel (3) having a low Mn, Cr and the like, weather resistance could not be ensured. Even when these materials were worked in the production range of the present invention (Comparative Materials 8 and 9), it is not possible to secure the target volume fraction of the second phase after annealing. there was.

On the other hand, when the annealing was performed at a temperature lower than the present annealing range for the comparative steel (4), which had relatively good weather resistance (Comparative Material 10), the tensile strength satisfied the target level but the volume fraction of the second phase was 10% as the annealing temperature was low. There was a problem that processing cracks occur during processing to less than. On the other hand, in the case of raising the annealing temperature (Comparative Material 11), there was a problem that the target high strength properties could not be secured.

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, according to the present invention, it is possible to provide a steel sheet with high added value, such as applications requiring stretching, as well as securing weatherability and mechanical properties at the same time and obtaining excellent workability.

Claims (3)

By weight%, carbon (C) 0.10 to 0.20%, silicon (Si) 0.05 to 0.25%, manganese (Mn) 1.0 to 2.5%, phosphorus (P) 0.02% or less, sulfur (S) 0.01% or less, aluminum (Al) 0.02 to 0.07%, niobium (Nb) 0.02 to 0.08%, nickel (Ni) 0.05 to 0.30%, copper (Cu) 0.1 to 0.5%, chromium (Cr) 0.8 to 1.5%, cobalt (Co) 0.01 to 0.10% A high strength cold rolled steel sheet having excellent weatherability and workability, which is composed of remaining Fe and other unavoidable impurities, and composed of 10 to 30% low temperature transformation structure and remaining ferrite. The high strength cold rolled steel sheet having excellent weatherability and workability according to claim 1, wherein the low temperature transformation structure includes at least one of martensite and bainite. By weight%, carbon (C) 0.10 ~ 0.20%, silicon (Si) 0.05 ~ 0.25%, manganese (Mn) 1.0 ~ 2.5%, phosphorus (P) 0.02% or less, sulfur (S) 0.01% or less, aluminum (Al) 0.02 to 0.07%, niobium (Nb) 0.02 to 0.08%, nickel (Ni) 0.05 to 0.30%, copper (Cu) 0.1 to 0.5%, chromium (Cr) 0.8 to 1.5%, cobalt (Co) 0.01 to 0.10% Re-heat the steel, which is composed of the remaining Fe and other unavoidable impurities, at 1150 to 1300 ° C., hot rolled to a finish rolling condition of 750 to 930 ° C., and wound at 400 to 650 ° C., followed by cold rolling, (A ₁ transformation point A method for producing a high strength cold rolled steel sheet having excellent weather resistance and workability, comprising continuously annealing at a temperature of + 30 ° C) to (A ₃ transformation point or less) to obtain a 10-30% low-temperature transformation structure and a structure of the remaining ferrite.