WO2008072873A1

WO2008072873A1 - Formable high strength cold-rolled steel sheet with excellent weather resistance and method manufacturing the same

Info

Publication number: WO2008072873A1
Application number: PCT/KR2007/006426
Authority: WO
Inventors: Jai Ik Kim; Kee Jo Jeong; Jong Hwa Kim; Soo Hee Lee
Original assignee: Posco
Priority date: 2006-12-12
Filing date: 2007-12-11
Publication date: 2008-06-19
Also published as: CN101553586B; JP5101628B2; CN101553586A; JP2010512461A; KR100815709B1

Abstract

There are provided a formable high strength cold-rolled steel sheet with excellent weather resistance and a tensile strength of 80 or more kgf/mm, a method for manufacturing the same. The formable cold-rolled steel sheet includes, by weigh: carbon (C): 0.10 toϑ.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% and cobalt (Co): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities, wherein a low-temperature transformation structure is present in a volume of 10 to 30% and the balance is a ferrite structure.

Description

FORMABLE HIGH STRENGTH COLD-ROLLED STEEL SHEET WITH EXCELLENT WEATHER RESISTANCE AND METHOD

MANUFACTURING THE SAME

Technical Field

[1] The present invention relates to a formable high strength cold-rolled steel sheet with excellent weather resistance that is used for construction, railway vehicles, containers and the like, and a method for manufacturing the same, and more particularly, to a weather-resistant cold-rolled steel sheet with high strength characteristics as well as excellent workability, the steel sheet being able to ensure a low-temperature transformation structure in components of the Cu-Co system, and a method for manufacturing the same.

[2]

Background Art

[3] Materials such as stainless steel or aluminum have been used to manufacture lightweight containers or railway vehicles and lengthen their span of life in the art. Physical properties required for such products include bending workability, weldability, durability and the like, as well as strength characteristics.

[4]

[5] For the transport structures, there has been an urgent demand for lightweight containers to increase the weight of cargo that may be transported once. 20 or 40 feet containers prepared according to the ISO standards were widely used in the past, but 45 to 53 feet-long containers have been increasingly used with these trends, and these long containers have their own weight of greater than 3 tons. Therefore, one point that may be first considered to manufacture a lightweight container is to reduce a thickness of a steel sheet through the manufacture of high-strength steel. For example, assume that 1 TEU (twenty-foot equivalent unit as a unit for a container, referred to as one 20-feet container) of container cargo is shipped and transported across the Atlantic Ocean, then if the container may be reduced in weight by about 10%, it is possible to reduce the cost of used raw materials. Also, it is possible to reduce an amount of CO generated in the use of mineral resources, there by to stop global warming.

[6]

[7] For this purpose, a high strength steel plate having a tensile strength of 80 kgf/mm or more is preferably used as one of the container materials. In particular, the containers have been basically manufactured with steels having excellent weather resistance since the containers should be resistant to various climatic conditions on the ground or on the sea, depending on the transportation conditions.

[8]

[9] For example, a weather-resistant cold-rolled steel sheet, SPA-C, (see Korean standard KS D3542 and JIS G3125) has been widely used in the art, but these steels have a low tensile strength of about 50 kg/mm . The transportation cost is increased due to the increased steel weight when a larger volume of products are made of the steels with a low tensile strength. Also, a high-strength cold-rolled steel sheet having a tensile strength of 60 to 80 kg/mm has been used in structural members of automobiles, but the resulting high-strength cold-rolled steel sheets do not have a desired weather resistance since they are manufactured on the basis of the strength characteristics.

[10]

[11] Recently, larger containers whose weight is considerably reduced have been manufactured in the field of the container industry to reduce the manufacturing cost and cope with the environmental problems. Therefore, there have been attempts to highly improve transportation efficiency. In particular, there has been a demand for a steel plate having weather resistance and high strength, and there have been also proposed techniques about methods for manufacturing these steel plates.

[12]

[13] For example, Japanese Patent Laid-open Publication No. Hei7-207408 proposes a method for manufacturing a hot-rolled steel strip including: heating steel at 1100 to 1300⁰C, the steel comprising, by weight: C: 0.008% or less, Si: 0.5 to 2.5%, Mn: 0.1 to 3.5%, P: 0.03 to 0.20%, S: 0.01% or less, Cu: 0.05 to 2.0%, Al: 0.005 to 0.1%, N: 0.008% or less, Cr: 0.05 to 6.0%, Ni: 0.05 to 2.0%, Mo: 0.05 to 3.0%, and B: 0.0003 to 0.002%; stopping the rolling of the steel at 800 to 95O⁰C, and coiling the steel at 400 to 700⁰C. For the patent, it is, however, shown that it is difficult to obtain a steel strip having a tensile strength of 80kg/mm since the steel strips has a tensile strength of 60 to 70 kg/mm only in a small minority of embodiments, but has a tensile strength of tensile strength of 50 kg/mm in the case of most embodiments. Among the above composition, the components such as Cr, Mo and the like should be added in a large content to improve hardenability of the steel, which leads to the deteriorated weldability and the increased manufacturing cost.

[14]

[15] Also, Japanese Patent Laid-open Publication No. Heil 1-21622 proposes a method including: heating steel at 1050 to 1300⁰C, the steel comprising, by weight: C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%, P: 0.03 to 0.15%, S: 0.02% or less, Cu: 0.4% or less, Al: 0.01 to 0.1%, Cr: 0.1% or less, Ni: 0.4 to 4.0%, and Mo: 0.1 to 1.5%; hot- rolling 40% or more of the steel at 95O⁰C or above; stopping the rolling of the steel at 900 to 750⁰C, and cooling the hot-rolled steel with the air. For the above patent, it was shown that most of the steel sheets have a tensile strength of 50 kg/mm , and a small minority of the steel sheets have a tensile strength of 60 kg/mm . Therefore, this technique may generally apply only to a steel plate having a tensile strength of 50kg/mm . Also, it is disclosed that P is added in a large content of 0.03 to 0.15% to improve corrosion resistance of the steel in a seawater environment, but the addition of a large content of P results in the center segregation of the cold-rolled sheet, and therefore the workability of the steel plate may be deteriorated abruptly.

[16]

[17] Furthermore, Japanese Patent Laid-open Publication No. Hei6- 104858 discloses a technique including: adjusting a content of titanium in steel to 12. IX (%)/Mn (%)> ti.eff

1.0, the steel comprising, by weight: 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 0.05%, Cr: 0.3 to 1.25%, N : 0.006% or less, and Ti: 0.06 to 0.20%; re-heating the steel at 1180⁰C or above; hot-rolling the steel at 880 to 950⁰C, and coiling the steel at 650⁰C or below. For this patent, it was shown that a content of Ti is added in connection with an amount of the added Mn as an element that controls precipitates. In the case of the patent, it was, however, revealed that the steel sheet has a tensile strength of 60 kg/mm that is lower than the tensile strength (80 kg/mm ) of a steel sheet required for the present invention. [18]

Disclosure of Invention

Technical Problem

[19] An aspect of the present invention provides a formable high strength cold-rolled steel sheet that has excellent weather resistance and workability as well as a tensile strength of 80 kg/mm or more.

[20]

Technical Solution

[21] According to an aspect of the present invention, there is provided a formable high strength cold-rolled steel sheet with excellent weather resistance, comprising, 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% and cobalt (Co): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities, wherein a low-temperature transformation structure is present in a volume of 10 to 30% and the balance is a ferrite structure.

[22] [23] The low-temperature transformation structure may include at least one selected from the group consisting of martensite and bainite. The low-temperature transformation structure preferably includes a majority of martensite and a minority of bainite.

[24]

[25] According to another aspect of the present invention, there is provided a method for manufacturing a formable high strength cold-rolled steel sheet with excellent weather resistance, the method including: reheating steel at 1150 to 1300⁰C, the steel comprising, 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% and cobalt (Co): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities; hot-rolling the steel at a finish rolling temperature of 750 to 93O⁰C; coiling the hot-rolled steel at 400 to 65O⁰C; cold-rolling the coiled steel; and continuously annealing the cold-rolled steel at a temperature conditions of (A transformation point+30°C ) to (A transformation point or below) to prepare 10 to 30% of a low-temperature transformation structure and the balance of a ferrite structure.

[26]

Advantageous Effects

[27] According to present invention, the formable high strength cold-rolled steel sheet may be useful to ensure excellent workability, as well as weather resistance and mechanical characteristics at the same time. Therefore, there may be provided a steel plate having high added values including, for example, the use of in the stretching process, etc.

[28]

Best Mode for Carrying Out the Invention

[29] Hereinafter, the present invention will be described in detail.

[30] The present inventors have made ardent attempts to develop a cold-rolled steel sheet that meets weather resistance as well as various physical properties and ensure a high tensile strength, for example a tensile strength of 80kg/mm or more, and therefore the present invention was completed on the basis of the above facts. The present invention is characterized in that a low-temperature structure is prepared by optimizing contents of the added components such as Mn and Cr in the weather- resistant component system of Cu-Co, thereby to give weather resistance and workability as well as high strength characteristics. The components in the cold-rolled steel sheet according to the present invention will be described in more detail. [31]

[32] Carbon (C) is preferably in a range of 0.10 to 0.20 % by weight (hereinafter, simply referred to as percentage (%)).

[33] Carbon (C) is an element that is added to improve strength of a steel plate. In this case, the tensile strength and yield strength of a steel sheet are enhanced with an increasing content of the carbon (C), but its upper limit is preferably 0.20% since the addition of the excessive carbon (C) causes the deterioration of workability. Meanwhile, when the content of the carbon (C) is less than 0.10%, it is impossible to ensure a sufficient precipitation- strengthening effect, and transverse cracks also frequently occurs in the continuous casting process when the carbon (C) is present in the content range of around 0.09%.

[34]

[35] Silicon (Si) is preferably in a range of 0.05 to 0.25%.

[36] Silicon (Si) is an element that serves to give deoxidation of molten steel and a solid solution strengthening effect, and also to improve corrosion resistance of a steel sheet by forming a compact Fe SiO oxide layer with Fe in a surface layer of the steel sheet at a high temperature. In this case, the silicon (Si) is preferably added in a content of at least 0.05% to meet the above-mentioned effects. As a result, the silicon (Si) should be added to improve weather resistance of a steel sheet, but weldability and coating property of the steel sheet may be deteriorated when the silicon (Si) is added in an excessive content. Therefore, the silicon (Si) is preferably added in the content of 0.25% or less.

[37]

[38] Manganese (Mn) is preferably in a range of 1.0 to 2.5%.

[39] Manganese (Mn) is an element that is effective to strengthen steel by means of a solid solution and plays important roles in increasing strength of the steel and improving hot workability of the steel. However, the manganese (Mn) functions to inhibit ductility and workability of the steel through the formation of the MnS. When a content of the manganese (Mn) is low, the workability of the steel is desirable, but hardenability of the steel is deteriorated, which makes it difficult to ensure a desired strength of the steel. Therefore, the manganese (Mn) is preferably added in a content of 1.0% or more to ensure a desired strength of the steel. On the contrary, the addition of the excessive manganese (Mn) is adverse to its economical efficiency and weldability caused by the excessive use of the expensive alloy element, and therefore the upper limit of the manganese (Mn) is preferably 2.5%.

[40]

[41] Phosphorus (P) is preferably in a range of 0.02% or less.

[42] Phosphorus (P) is preferably added in a large amount with respect to the corrosion resistance since the phosphorus (P) functions to improve corrosion resistance of steel. However, since the phosphorus (P) causes the most serious center segregation during a molding process, the addition of a large amount of the phosphorus (P) results in the deterioration of weldability and toughness. Therefore, the content of the phosphorus (P) is preferably limited to a content range of 0.02% or less.

[43]

[44] Sulfur (S) is preferably in a range of 0.01% or less.

[45] Sulfur (S) has been known to be an element that is effective to improve corrosion resistance of steel, but a content of the sulfur (S) is preferably as low as possible since the sulfur (S) binds to Mn in the steel to form a non-metallic inclusion that functions as a corrosion starting point. Therefore, the content of the sulfur (S) is preferably set to a range of 0.01% or less, and more preferably 0.005% or less.

[46]

[47] Aluminum (Al) is preferably in a range of 0.02 to 0.07%.

[48] Aluminum (Al) is an element that is generally effective to improve deoxidation of molten steel and corrosion resistance of steel. When the aluminum (Al) is added an excessive amount, an inclusion is increasingly formed in the steel to deteriorate workability of the steel, and therefore the content of the aluminum (Al) is preferably set to a range of 0.02 to 0.07%.

[49]

[50] Niobium (Nb) is preferably in a range of 0.02 to 0.08%.

[51] Niobium (Nb) is an element that has effects to enhance strength of a steel plate by binding and extracting C, N , etc. from steel, as well as to delay recrystallization of ferrite. In this case, the niobium (Nb) is preferably added in a content of 0.02% or more so as to ensure a desired strength of the steel. On the contrary, the increased manufacturing cost and the deteriorated hot rolling workability may be caused when the content of the added niobium (Nb) exceeds 0.08%.

[52]

[53] Nickel (Ni) is preferably in a range of 0.05 to 0.3%.

[54] Nickel (Ni) is an element that functions to improve corrosion resistance of steel, as well as to prevent casting strains that are generally caused in the molding of Cu-added steel. In this case, the nickel (Ni) is added in a content of 0.05% or more so as to show the above effects. However, when the content of the nickel (Ni) exceeds 0.3%, the corrosion resistance may be deteriorated on the contrary to the effects, and the manufacturing cost may be increased by the excessive use of the expensive alloy element.

[55]

[56] Copper (Cu) is preferably in a range of 0.1 to 0.5%.

[57] Copper (Cu) is an element that improves corrosion resistance of steel by forming a stable rust layer under corrosive environments. In this case, the copper (Cu) is added in a content of 0.1% or more so as to ensure a desired corrosion resistance. However, when the amount of the added copper (Cu) exceeds 0.5%, grain boundary cracks may be caused in the casting of steel, and a surface of a hot-rolled steel plate may be also made coarse.

[58]

[59] Chromium (Cr) is preferably in a range of 0.8 to 1.5%.

[60] Chromium (Cr) is an element that functions to form a stable rust layer as in the copper (Cu). In this case, the chromium (Cr) is preferably added in a content of 0.8% or more so as to ensure corrosion resistance and give strength. Also, when the amount of the added chromium (Cr) exceeds 1.5%, crevice corrosion may be caused on the contrary to the effects, and the manufacturing cost may also be increased sharply.

[61]

[62] Cobalt (Co) is preferably in a range of 0.01 to 0.1%.

[63] Cobalt (Co) is an element that reacts with Cu and Cr to facilitate formation of products that inhibits the corrosion in a surface layer of steel, the Cu and Cr being added to ensure corrosion resistance in the steel. In this case, the cobalt (Co) is preferably added in a content of 0.01% or more so as to obtain the above effects. However, when the amount of the added cobalt (Co) exceeds 0.10%, the manufacturing cost may be increased on the contrary to the contribution to the effect to improve corrosion resistance.

[64]

[65] The composition of the cold-rolled steel sheet includes the above-mentioned components, and the balance of Fe and other inevitable impurities. Alloy elements may be added to improve physical properties of weather-resistant steel, when necessary. Here, although the alloy elements that are not described in the exemplary embodiments of the present invention are added to the composition of the cold-rolled steel sheet, it is not construed that the alloy elements are excluded in the scope and sprite of the present invention.

[66]

[67] A microstructure in the cold-rolled steel sheet of the present invention includes 10 to 30 vol.% (hereinafter, simply referred to as percentage(%)) of a low-temperature transformation structure, and the balance of a ferrite structure. The low-temperature transformation structure is present in a volume of 10% or more to reduce yield strength and increase strain hardening exponent, and therefore it is possible to ensure excellent workability. When the volume of the low-temperature transformation structure is less than 10%, it is difficult to ensure a desired workability since a discontinuous yield behavior appears when solid solution elements remain in the steel, which leads to the processing defects in the process. When the volume of the low-temperature transformation structure exceeds 30%, the workability is not good. According to the present invention, the low-temperature transformation structure includes martensite and bainite, and they may be used alone or in combination thereof. The low-temperature transformation structure the most preferably include martensite as a major component and bainite as a minor component.

[68]

[69] Hereinafter, the method for manufacturing a cold-rolled steel sheet according to the present invention will be described in detail.

[70] A high-strength cold-rolled steel sheet having excellent weather resistance and workability and a tensile strength of 80kg/mm or more may be manufactured by rolling the steel having the above-mentioned chemical composition at a reheating temperature of 1150 to 1300⁰C and a finish rolling temperature of 750 to 93O⁰C in a hot rolling operation, coiling the steel at 400 to 65O⁰C, and heat-treating the coiled steel at a temperature of (A transformation point+30°C) to (A transformation point or below) in cold-rolling and continuous annealing processes, wherein the resulting steel has a low-temperature transformation structure.

[71]

[72] When the reheating temperature is less than 115O⁰C, a solidification grain structure formed in the molding process is destroyed insufficiently, which leads to the serious progress of center segregation. Therefore, the finally formed grains are mixed to significantly deteriorate workability and impact toughness of steel. Also, when the reheating temperature exceeds 1300⁰C, scale formation by oxidation reaction is facilitated to significantly decrease a thickness of a slab, and coarse grains are formed in the reheating operation, which leads to the deteriorated impact toughness. Accordingly, the reheating temperature is preferably in a range of 1150 to 1300⁰C since the increase in the heating unit results in the high economic loss.

[73]

[74] When the finish hot-rolling temperature exceeds 93O⁰C, the steel is not hot-rolled uniformly over its entire thickness to form micro grains insufficiently, and therefore impact toughness of the steel is deteriorated due to the presence of the coarse grains. On the contrary, when the finish hot-rolling temperature is less than 75O⁰C, the admixture of the grains is made abruptly since the steel is hot-rolled in a low temperature period, which leads to the deteriorated corrosion resistance and workability. Therefore, the finish hot rolling temperature is preferably limited to a temperature range of 750 to 93O⁰C.

[75]

[76] The coiling process is carried out after the finish hot rolling process. In this case, when the coiling temperature exceeds 65O⁰C, the strength of the steel sheet is low due to the insufficient extraction, and therefore it is impossible to securely obtain a desired strength of 80 kg/mm . On the contrary, when the coiling temperature is less 400⁰C, a hard phase may be formed when the steel is cooled and maintained at low temperature. As a result, it is difficult to ensure rolling properties since a roll force of a rolling mill is increased in the cold rolling process, and therefore it is preferred to limit the coiling temperature to a temperature range of 400 to 65O⁰C.

[77]

[78] Meanwhile, when the above-mentioned finish rolling process is completed, the steel is preferably cooled in a run-out table (ROT) prior to the coiling process. According to the exemplary embodiments of the present invention, a cooling rate of the steel is preferably in a range of 20 to 4O⁰C per second, but the present invention is not particularly limited thereto. When the cooling rate is less than 2O⁰C per second, growth of the grains is accelerated to form relatively coarse grains, thereby to reduce strength of the steel. When the cooling rate exceeds 4O⁰C per second, a hard second phase steel such as bainite may be formed to deteriorate cold rolling properties of the steel.

[79]

[80] The hot-rolled steel is cold-rolled under cold-rolling conditions, followed by undergoing a continuous annealing process. In this case, the annealing temperature is preferably set to a temperature range between (A transformation point+30°C ) and (A transformation point or below) so as to ensure desired physical properties of the steel. After the annealing process, the resulting steel sheet preferably includes 10 to 30% of a low-temperature transformation structure and the balance of a ferrite structure.

[81]

[82] When the annealing temperature is less than (A transformation point+30°C ) in the continuous annealing process, ductility may be seriously deteriorated due to the presence of the deformed grains in the cold rolling process, and it is difficult to obtain a desired volume fraction of the second phase steel in the cooling process after the annealing of the steel, which leads to the deteriorated workability. On the contrary, when the annealing temperature exceeds the A transformation point, a surface of the steel may be increasingly damaged by the high-temperature annealing. For the continuous annealing process according to the present invention, a cooling process may be used if the cooling process may be carried out under cooling conditions that are required for the present invention. A water cooling system may be preferred, and it may also be preferred to cool the steel with a cooling gas.

[83]

[84] According to the exemplary embodiment of the present invention, the cold-rolled steel sheet of the present invention has weather resistance and high strength characteristics as well as meets various machining properties such as bendability and elongation. Also, the cold-rolled steel sheet of the present invention may ensure physical properties such as weldability, coating properties and the like through the control of the components such as Si, etc.

[85]

Mode for the Invention

[86] Hereinafter, the exemplary embodiments of the present invention will be described in more detail.

[87] [88] [Embodiment 1] [89] The steels having the compositions as listed in the following Table 1 were measured for standardized corrosion resistance index (CI) and weather resistance. The results are listed in the following Table 2.

[90] As the weather resistance test, a salt spray test (SST) for the steels was carried out at 3O⁰C for 480 hours in 5% saline (NaCl solution). The results are listed in the following Table 2.

[91] [92] The corrosion resistance index is an index obtained by evaluating weather resistance of steel according to the ASTM GlOl standard. It has been known that the weather resistance of the steel grows better as the corrosion resistance index increases. Here, the corrosion resistance index is mainly calculated on the basis of the alloy element, and represented by the following equation.

[93] 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

[94] [95] Table 1

[96] [97] Table 2

[98] [99] As listed in the Table 2, it was revealed that the comparative steels 1 to 3 are difficult to be used in the aspect of the weather resistance since the comparative steels have a low corrosion resistance index value and their weight losses by the corrosion are high. Also, it was seen that the comparative steel 4 and the inventive steels 1 and 2 have excellent weather resistance with respect to the weight loss and corrosion resistance index.

[100] [101] [Embodiment 2] [102] Each of cold-rolled steel sheets was prepared from the inventive steels 1 and 2 and the comparative steels 1 to 4 as listed in the Table 1 of Embodiment 1 under the conditions as listed in the following Table 3, and the cold-rolled steel sheets were measured for mechanical properties and machining characteristics. The results are listed in the following Table 4.

[103] [104] Table 3

[105] [106] Table 4

[107] [108] As listed in the Table 4, it was revealed that the inventive compositions 1 to 4, where the content ranges of the chemical components in the inventive steels and the manufacturing conditions of the inventive steels meet the requirements of the method according to the present invention, have a tensile strength of 80 kgf/mm or more, and 10% or more of a volume fraction of a second phase including a martensite phase after the annealing process. Therefore, the manufacture of the cold-rolled steel sheets having weather resistance, high strength and high workability is possible since cracks do not appear during the banding and stretching processes.

[109] [HO] On the contrary, the comparative compositions 1 to 5, where the content ranges of the chemical components in the inventive steels meet the range of the present invention but the manufacturing conditions of the inventive steels are out of the range of the present invention, fail to show desired workability. That is, ductility of the steel is less than 10% since the annealing temperature condition did not satisfy the annealing temperature range of the present invention that is greater than (A transformation point+30°C), and cracks appeared mainly in an annealing plate during the stretching process since a volume fraction of the second phase including a martensite phase is greater than 10%.

[111] [112] Meanwhile, when the contents of C and Cr are out of the content range of the composition according to the present invention, the comparative compositions 6 and 7 prepared at the manufacturing conditions according to the present invention have a relatively good workability when compared to the comparative steel 1 whose weather resistance condition does not meet the desired level, but the comparative compositions 6 and 7 whose tensile strength is considered to be a level of 80 kgf/mm did not ensure a tensile strength of 80 kgf/mm or more.

[113] [114] Also, it was revealed that the comparative steel 2, where the contents of Si and P are low and the contents of Mn and Co is high in the chemical composition when compared to the compositional range of the steels according to the present invention, and the comparative steel 3, where the contents of Mn and Cr are low when compared to the compositional range of the steels according to the present invention, did not ensure weather resistance, and the comparative steels 2 and 3 did not ensure a desired volume fraction of a second phase after the annealing process even when they were prepared under the manufacturing conditions according to the present invention (Comparative compositions 8 and 9). Therefore, it was revealed that the comparative steels 2 and 3 did not ensure workability as listed in Table 4.

[115]

[116] Meanwhile, when the comparative steel 4 whose weather resistance was relatively good was annealed at a lower temperature than the annealing temperature range of the present invention (Comparative example 10), the comparative steel 4 met the desired tensile strength condition, but cracks appeared in the comparative steel 4 when the second phase is processed in a volume fraction of 10% or less at a low annealing temperature. On the contrary, the comparative steel 4 did not ensure the desired high strength characteristics when the comparative steel 4 was annealed at the increased annealing temperature (Comparative example 11).

[117]

[118] While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] A formable high strength cold-rolled steel sheet with excellent weather resistance, comprising, 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% and cobalt (Co): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities, wherein a low-temperature transformation structure is present in a volume of 10 to 30% and the balance is a ferrite structure.

[2] The formable high strength cold-rolled steel sheet of claim 1, wherein the low- temperature transformation structure comprises at least one selected from the group consisting of martensite and bainite.

[3] A method for manufacturing a formable high strength cold-rolled steel sheet with excellent weather resistance, the method comprising: reheating steel at 1150 to 1300⁰C, the steel comprising, 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% and cobalt (Co): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities; hot-rolling the steel at a finish rolling temperature of 750 to 93O⁰C; coiling the hot-rolled steel at 400 to 65O⁰C; cold-rolling the coiled steel; and continuously annealing the cold-rolled steel at a temperature conditions of (A transformation point+30°C) to (A transformation point or below) to prepare 10 to 30% of a low-temperature transformation structure and the balance of a ferrite structure.