Description

HIGH STRENGTH COLD ROLLED STEEL SHEET HAVING

EXCELLENT FORMABILITY AND COATING PROPERTY,

ZINC-BASED METAL PLATED STEEL SHEET MADE OF IT

AND THE METHOD FOR MANUFACTURING THEREOF

Technical Field

[1] The present invention relates to a steel sheet which is primarily used as structural members and reinforcement of the car body, a zinc-based metal plated steel sheet(zinc-coated steel sheet) made of it and a method for manufacturing the same. More specifically, the present invention relates to a high-strength, cold rolled steel sheet having tensile strength of more than 490 MPa and excellent coating properties, a zinc -based metal plated steel sheet(zinc-coated steel sheet) made of it and a method for manufacturing the same.

[2]

Background Art

[3] Recently, with strengthening and expansion of regulations for motor vehicle passenger safety, a great deal of research and study has been actively undertaken to achieve weight reduction and strengthening of the car body, in order to improve impact resistance of the car body. To cope with such a trend, high-tensile strength steel sheets having tensile strength of more than 490 MPa are actively employed to simultaneously meet weight reduction and strengthening of the car body.

[4] Further, since most of automotive steel sheets are shaped by press finishing, they are required to have excellent press formability. High ductility is also essentially required to secure excellent press formability. That is, the automotive steel sheets are high-tensile strength steel sheets, and the most important thing that should be considered in the automotive steel sheets is high ductility.

[5] Strengthening of the automotive steel sheet results in significant deterioration of the formability and coating properties of the steel sheet, thus suffering from various difficulties in practical applications thereof.

[6] Further, the automotive steel sheet also requires high corrosion resistance, and therefore hot-dip galvanized steel sheets having excellent corrosion resistance have been conventionally used as the automotive steel sheet. That is, such steel sheets can be manufactured with excellent corrosion resistance at low production costs, since they are manufactured through a continuous hot-dip galvanizing line where recrystallization annealing and coating are carried out in the same line.

[7] Further, alloyed hot-dip galvanized steel sheets, which were subjected again to heat treatment after hot-dip galvanizing, are widely used in terms of excellent weldability and formability, in addition to excellent corrosion resistance.

[8] That is, in order to achieve further weight-reduction and strengthening of the car body, there is a need in the art for the development of high-tensile strength cold rolled steel sheets having excellent formability, and high-tensile strength hot-dip galvanized steel sheets having excellent corrosion resistance via the continuous hot-dip galvanizing line.

[9] As a representative conventional art relating to the high-tensile strength hot-dip galvanized steel sheets having excellent formability, mention may be made of Korean Patent Publication Laid-open No. 2002-0073564. This patent relates to a steel sheet having a composite structure of soft ferrite and hard martensite, and discloses a method of manufacturing a hot-dip galvanized steel sheet having improved elongation ratio and r-value (Lankford value). However, the above-mentioned conventional art suffers from a difficulty to secure excellent coating quality due to the addition of large amounts of silicon (Si), and problems associated with increased productions costs due to the addition of large amounts of titanium (Ti) and molybdenum (Mo).

[10] Further, Japanese Unexamined Patent Publication No. 2004-292891 suggests a method of manufacturing a high tensile strength steel sheet. This conventional art is directed to a steel sheet composed of a composite structure containing ferrite as a primary phase, the retained austenite as a secondary phase, and bainite and martensite as low temperature transformation phases, and suggests a method of manufacturing a steel sheet having improved ductility and stretch flangeability.

[11] However, this conventional art suffers from a difficulty to achieve desired coating quality due to the addition of large amounts of silicon (Si) and aluminum (Al), and a difficulty to secure desired surface quality upon steel-making and continuous casting. Further, this art also suffers from a difficulty to secure flatness of the steel sheet due to the risk of partial deformation of the steel sheet upon cooling, since cooling of the steel sheet should be carried out at a rate of more than 100°C/sec in order to obtain high strength.

[12] Further, as a conventional art of alleviating the problems of coating properties suffered by the high-tensile strength steel sheets, there is Japanese Unexamined Patent Publication No. 2002-088447 which relates to a steel sheet composed of a composite structure including ferrite as a primary phase, and discloses a method of obtaining good workability and coating properties. However, this patent suffers from difficulties in practical application thereof, due to increased production costs resulting from one or more heat treatment processes prior to coating, in order to obtain good workability.

[13]

Disclosure of Invention

Technical Problem

[14] Therefore, the present invention has been made in view of the above problems, and the present invention provides advantages capable of achieving excellent coating properties and high tensile strength of more than 490 MPa, by the addition of antimony (Sb) to a steel material. Further, the present invention provides advantages of securing desired formability of a steel sheet. In addition, the present invention provides advantages of securing bake hardenability after coating of the steel sheet.

[15]

Technical Solution

[16] In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a cold rolled steel sheet, comprising 0.01-0.2 wt% of carbon (C), 0.01-2.0 wt% of silicon (Si), 0.5-4.0 wt% of manganese (Mn), less than 0.1 wt% of phosphorous (P), less than 0.03 wt% of sulfur (S), less than 1.0 wt% of soluble aluminum (SoLAl), 0.001-0.1 wt% of nitrogen (N), 0.005-1.0 wt% of antimony (Sb), and the balance of iron (Fe) with inevitable impurities.

[17] In one embodiment of the present invention, the cold rolled steel sheet may satisfy the following inequality of (Si/28+Al/27)/(N/14)>10, for Si, Al and N. In this case, contents of the soluble aluminum (SoLAl) and nitrogen (N) in the cold rolled steel sheet are preferably in a range of 0.01 to 1.0%, and in a range of 0.001 to 0.03%, respectively.

[18] In one embodiment of the present invention, the cold rolled steel sheet may satisfy the following inequality of N*=(N/14)/(Al/27+Ti/48+Nb/93+V/51+B/l l)>0.2, for N, Al, Ti, Nb, V and B. In this case, contents of the soluble aluminum (SoLAl) and nitrogen (N) in the cold rolled steel sheet are preferably in a range of less than 0.2%, and in a range of 0.01 to 0.1%, respectively.

[19] In one embodiment of the present invention, the cold rolled steel sheet may further comprise at least one selected from the group consisting of:

[20] a) 0.001-0.1% of at least one of titanium (Ti), niobium (Nb) and vanadium (V);

[21] b) 0.01-2.0% of chromium (Cr) and 0.001-1.0% of molybdenum (Mo); and

[22] c) less than 0.01% of boron (B).

[23]

[24] In one embodiment of the present invention, a steel structure of the cold rolled steel sheet may have ferrite as a primary phase and a martensite fraction of 2 to 70% as a secondary phase.

[25] In one embodiment of the present invention, a zinc-coated steel sheet includes the above cold rolled steel sheet as a base steel sheet and has a zinc-coating layer on at

least one surface of top and bottom surfaces of the base steel sheet.

[26] The zinc-coated steel sheet may have one coating layer of zinc coating and alloying hot-dip zinc coating, without being limited thereto.

[27] In one embodiment of the present invention, there is provided a method for manufacturing a cold rolled steel sheet, comprising re -heating a steel slab satisfying the steel composition of the present invention at a temperature of 1100 ⁰ C to 1300 ⁰ C; subjecting the steel slab to hot finish rolling at a temperature ranging from the Ar transformation point to 1000 ⁰ C; winding the steel slab at a temperature of 45O ⁰ C to 75O ⁰ C; pickling and cold-rolling the steel slab; continuously annealing the steel slab at a temperature of 75O ⁰ C to 900 ⁰ C for 10 to 1000 sec; cooling the steel slab to 600 ⁰ C to 72O ⁰ C at a rate of 1 to 10°C/sec (primary cooling); and cooling the steel slab to 100 ⁰ C to 400 ⁰ C at a rate of 1 to 100°C/sec (secondary cooling).

[28] When it is desired to make the zinc-coated steel sheet, the cold rolled steel sheet is subjected to hot-dip galvanizing at a temperature of 45O ⁰ C to 500 ⁰ C for less than 10 sec.

[29]

Advantageous Effects

[30] According to the present invention, a steel sheet secures high tensile strength of more than 490 MPa, in conjunction with improvement of coating properties. In addition, excellent formability of the steel sheet is also secured. Further, bake hard- enability after coating of the steel sheet is enhanced. Therefore, the steel sheet of the present invention can be applied as structural members and reinforcement of au- tomotives.

[31]

Brief Description of the Drawings

[32] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[33] FIG. 1 is a graph showing a relationship between (Si/28+Al/27)/(N/14) and TS*E1 in the present invention;

[34] FIG. 2 is a graph showing a relationship between N* and TS*E1 and between N* and BH in the present invention; and

[35] FIG. 3 is a photograph showing surface-enriching properties with addition of Sb in the present invention.

[36]

Best Mode for Carrying Out the Invention

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

[38] As a result of a variety of extensive and intensive studies and experiments to solve the problems associated with surface defects resulting from the addition of large amounts of silicon (Si) and manganese (Mn), the inventors of the present invention discovered that it is possible to inhibit enriching and coarsening of oxides on a steel sheet surface by the addition of a proper amount of antimony (Sb). That is, appropriate addition of antimony (Sb) interferes with migration of the oxides to grain boundaries, thereby resulting in significant reduction of the probability of surface defects due to Si and Mn. Therefore, it is possible to secure excellent coating properties even when large amounts of Si and Mn are added.

[39] Further, when it is desired to further improve the formability in conjunction with the above-mentioned coating properties, in accordance with one embodiment of the present invention, it is preferred to control a content of nitrogen (N) so as to satisfy either or both of the following inequalities:

[40]

[41] (Si/28+Al/27)/(N/14)≥10 (l)

[42] N*=(N/14)/(Al/27+Ti/48+Nb/93+V/51+B/l l)>0.2 (2)

[43]

[44] Further, in one embodiment of the present invention, the bake hardenability is also increased due to solute nitrogen (N) after coating of the steel sheet, when the solute nitrogen is secured.

[45] Hereinafter, the composition components of the steel according to the present invention will be described.

[46]

[47] Steel Composition

[48]

[49] Carbon (C): 0.01-0.2%

[50] Carbon (C) is a very important component to increase strength of a steel sheet and to secure a composite structure composed of ferrite and martensite. Where the carbon content is lower than 0.01%, it is impossible to obtain the steel strength which is desired by the present invention. On the other hand, where the carbon content is higher than 0.2%, the steel may be highly susceptible to deterioration of toughness and weldability. Therefore, the carbon content is preferably limited to the range of 0.01-0.2%.

[51]

[52] Silicon (Si): 0.01-2.0%

[53] Silicon (Si) is a useful element which is capable of securing desired strength of a steel sheet while not causing deterioration of ductility of the steel sheet. In addition, silicon is an element promoting the formation of martensite by promoting the

formation of ferrite and facilitating the enrichment of carbon into the untransformed austenite. Where the silicon content is lower than 0.01%, it is difficult to secure the above-mentioned effects. On the other hand, where the silicon content is higher than 2.0%, surface properties and weldability may be deteriorated. Therefore, the silicon content is preferably limited to the range of 0.01 to 2.0%.

[54]

[55] Manganese (Mn): 0.5-4.0%

[56] Manganese (Mn) is an element having significant solid-solution strengthening effects, simultaneously with promotion of formation of a composite structure composed of ferrite and martensite. Where the manganese content is lower than 0.5%, it is difficult to secure the steel strength which is desired by the present invention. On the other hand, where the manganese content is higher than 4.0%, this may result in high susceptibility to problems associated with the weldability and hot-rolling performance. Therefore, the manganese content is preferably limited to the range of 0.5 to 4.0%.

[57]

[58] Phosphorus (P): Less than 0.1%

[59] Phosphorus (P) serves to strengthen a steel sheet, but an excessive amount thereof may result in degradation of the press formability. Therefore, the phosphorus content is preferably limited to a range of less than 0.1%.

[60]

[61] Sulfur (S): Less than 0.03%

[62] Sulfur (S) is an impurity element present in the steel, and is likely to inhibit the ductility and weldability of a steel sheet. Therefore, the sulfur content is preferably limited to a range of less than 0.03%.

[63]

[64] Soluble aluminum (Sol. Al): Less than 1.0%

[65] Soluble aluminum (Sol.Al) is an element which combines with oxygen in the steel to thereby exert deoxidation effects, and, in conjunction with silicon (Si), is effective to improve the martensite hardenability by distribution of carbon in ferrite into austenite. If the content of Sol.Al exceeds 1.0%, the deoxidation effects and improvement of the martensite hardenability are saturated and production costs are increased. Therefore, the content of Sol.Al is limited to the range of less than 1.0%. The content of Sol.Al is preferably in the range of 0.01 to 1.0%, more preferably less than 0.2%.

[66]

[67] Nitrogen (N): 0.001-0.1%

[68] Nitrogen (N) is a component effective to stabilize austenite. Where the nitrogen content is lower than 0.001%, it is difficult to achieve such stabilizing effects. On the

other hand, where the nitrogen content is higher than 0.1%, there is no significant increase in stabilizing effects of austenite, in conjunction with problems associated with the weldability and increased production costs. Therefore, the nitrogen content is limited to the range of 0.001 to 0.1%.

[69] Preferably, the nitrogen content is in the range of 0.01 to 0.1%. Nitrogen combines with titanium (Ti), niobium (Nb) and aluminum (Al) to form nitrides, thereby increasing yield strength of steel. In the present invention, sufficient amounts of nitrogen are added so as to increase the yield strength of steel after coating. Nitrogen serves as a primary cause for a sharp increase of the yield strength of steel, in a manner that nitrogen remains in the form of solute N within crystal grains prior to coating, and then interferes with the dislocation movement after coating to thereby elevate a yield point. If the nitrogen content is lower than 0.01%, it is difficult to achieve such effects. On the other hand, if the nitrogen content exceeds 0.1%, there is no significant increase in the yield strength-improving effects, in conjunction with problems associated with the weldability and increased production costs.

[70] According to one embodiment of the present invention, upon considering the weldability and production costs, the nitrogen content is preferably in the range of 0.001 to 0.03%, even though it may not secure sufficient strength by solute nitrogen.

[71]

[72] Antimony (Sb): 0.005-1.0%

[73] Antimony (Sb) is a very important element in the present invention, and is an essential component added to secure excellent coating properties. Sb, as shown in FIG. 2, reduces surface defects by inhibiting surface enrichment of oxides such as MnO, SiO and Al O , and exhibits excellent effects on inhibition of coarsening of surface- enriched materials resulting from elevation of temperatures and changes of the hot- rolling process. Where the antimony content is lower than 0.005%, it is difficult to secure the above effects. On the other hand, a continuing increase of the antimony content does not lead to further significant increase of such effects and may also present problems associated with production costs and degradation of the workability. Therefore, the antimony content is preferably limited to the range of 0.005 to 1.0%.

[74]

[75] In addition to the above-mentioned composition components, one or more elements selected from titanium (Ti), niobium (Nb) and vanadium (V), and chromium (Cr), molybdenum (Mo) and boron (B) may be further added to the steel.

[76]

[77] One or more elements selected from titanium (Ti), niobium (Nb) and vanadium (V):

0.001-0.1%

[78] Titanium (Ti), niobium (Nb) and vanadium (V) are elements effective to increase

the strength of the steel sheet and to achieve grain refinement. Where the content of Ti, Nb and V is lower than 0.001%, it is difficult to achieve desired effects. On the other hand, where the content of Ti, Nb and V exceeds 0.1%, this may result in increased production costs, and decreased ductility of ferrite due to excessive amounts of precipitates. Therefore, the Ti, Nb and V content is preferably limited to the range of 0.001 to 0.1%.

[79]

[80] Chromium (Cr): 0.01-2.0%

[81] Chromium (Cr) is a component added to improve the hardenability of the steel and to secure high strength. Where the chromium content is lower than 0.01%, it is difficult to secure such effects. On the other hand, where the chromium content exceeds 2.0%, this may result in saturation of such effects and deterioration of the ductility. Therefore, the chromium content is preferably limited to the range of 0.01 to 2.0%.

[82]

[83] Molybdenum (Mo): 0.001-1.0%

[84] Molybdenum (Mo) is a component added to retard transformation of austenite into pearlite and simultaneously to achieve ferrite refinement and improve the strength. Where the molybdenum content is lower than 0.001%, it is difficult to obtain such effects. On the other hand, where the molybdenum content exceeds 1.0%, this may result in saturation of such effects and deterioration of the ductility. Therefore, the molybdenum content is preferably limited to the range of 0.001 to 1.0%.

[85]

[86] Boron (B): Less than 0.01%

[87] Boron (B) is a component which retards transformation of austenite into pearlite upon cooling of the steel during an annealing process. Where the boron content exceeds 0.01%, this may result in degradation of the coating adhesion, due to excessive enrichment of boron on the steel sheet surface. Therefore, the boron content is preferably limited to the range of less than 0.01%.

[88]

[89] In one embodiment of the present invention, it is preferred that Si, Al and N satisfy the inequality 1 of (Si/28+Al/27)/(N/14)>10. In this case, contents of the soluble aluminum (Sol. Al) and nitrogen (N) in the steel sheet are preferably in a range of 0.01 to 1.0%, and in a range of 0.001 to 0.03%, respectively.

[90] Inequality 1 is a very important equation in terms of the formability of the present invention. As shown in FIG. 1, where the inequality value is smaller than 10, it is difficult to secure excellent TS * El balance. On the other hand, where the inequality value is 10 or higher, it is possible to secure TS * El balance of more than 15,000.

[91] That is, ferrite-formation promoting elements Si and Al are appropriately added to

actively induce formation of ferrite, thereby facilitating enrichment of carbon into austenite and improving the hardenability to promote martensitic transformation. The ratio of Al and N is controlled to appropriately form AlN precipitates, thereby preventing pearlite band formation during the hot rolling process to induce refining and dispersion of pearlite, consequently achieving fine dispersion of martensite in the final annealing process. As a result, it is possible to secure high strength and high ductility of the steel.

[92] In one embodiment of the present invention, when one or more components of Al,

Ti, Nb, V and B are added, it is preferred to satisfy the following inequality of N*=(N/14)/(Al/27+Ti/48+Nb/93+V/51+B/l l)>0.2. In this case, contents of the soluble aluminum (Sol.Al) and nitrogen (N) in the steel sheet are preferably in the range of less than 0.2%, and in a range of 0.01 to 0.1%, respectively.

[93] N* means a content of nitrogen remained after the formation of nitrides by combination of nitrogen with Al, Ti and Nb etc., and plays an important role in the present invention. As shown in FIG. 2, where the value of N* is lower than 0.2, it is difficult to secure excellent TS * El balance and a BH value. On the other hand, where the value of N* is 0.2 or higher, it is possible to secure TS * El balance of more than 15,000 and a BH value of more than 80 MPa. That is, N*, referring to the remaining nitrogen after formation of nitrides, serves as an austenite-stabilizing element, similar to carbon, and therefore promotes martensitic transformation during a cooling process. In addition, nitrogen enriched within the martensite leads to increased strength of the steel. As a result, an improved elongation ratio can be obtained at the same strength. Further, the bake hardenability is also improved by the solute nitrogen (N) after coating of the steel sheet.

[94] In a preferred embodiment, the steel of the present invention is composed of the above-mentioned components and the balance of iron (Fe) with inevitable impurities. If necessary, other alloying elements may also be added. Therefore, it should be understood that the steel of the present invention does not exclude steels with addition of other alloying elements, although not mentioned in embodiments of the present invention.

[95] In accordance with the present invention, there is provided a cold rolled steel sheet as composed above. In addition, there is provided a zinc-coated steel sheet having a zinc coating layer on at least one surface of top and bottom surfaces of the cold rolled steel sheet.

[96] Hereinafter, final structure of the cold rolled steel sheet and zinc-coated steel sheet after heat treatment thereof will be described in more detail.

[97] In one embodiment of the present invention, the steel composed as above may be subjected to heat treatment suited for the cold rolled steel sheet and hot-dip galvanized

steel sheet to control a microstructure of the steel, thereby imparting desired physical properties. The steel sheet in the present invention is made to have ferrite as a primary phase and a martensite fraction of 2 to 70% as a secondary phase. Where the martensite fraction is lower than 2%, it is difficult to obtain high tensile strength which is desired by the present invention. On the other hand, where the martensite fraction is higher than 70%, this may result in a sharp decrease of an elongation ratio. Therefore, the martensite fraction is preferably limited to the range of 2 to 70%. Further, in the present invention, it is also possible to secure physical properties which are desired by the present invention, when bainite as the secondary phase is contained in a content of less than 5%, in addition to martensite.

[98]

[99] Hereinafter, a method for manufacturing a cold rolled steel sheet having the above- specified steel composition and structure will be described in more detail.

[100]

[101] Hot rolling

[102] First, a steel slab, as composed above, is re-heated at a temperature of 1100 ⁰ C to

1300 ⁰ C. Where the re-heating temperature is lower than 1100 ⁰ C, structural homogeneity and re-dissolution of Ti and Nb are not sufficiently achieved. On the other hand, where the re-heating temperature is higher than 1300 ⁰ C, this may result in high susceptibility to coarsening of the steel sheet structure and the occurrence of problems associated with manufacturing processes. Therefore, the re-heating temperature is preferably limited to the range of 1100 ⁰ C to 1300 ⁰ C.

[103] Then, the steel slab is subjected to hot finish rolling at a temperature ranging from the Ar transformation point to 1000 ⁰ C. Where the hot finish rolling temperature is lower than the Ar transformation point, this may lead to high possibility of a dramatic increase in the hot deformation resistance and problems associated with manufacturing processes. On the other hand, where the hot finish rolling temperature is higher than 1000 ⁰ C, this may probably result in high risk of excessively thick oxide scales and coarsening of the steel sheet structure. Therefore, the hot finish rolling temperature is preferably limited to a temperature ranging from the Ar transformation point to 1000 ⁰ C.

[104] After the hot finish rolling was complete, the thus-rolled steel slab was wound at a temperature of 45O ⁰ C to 75O ⁰ C. Where the winding temperature is lower than 45O ⁰ C, excessive formation of martensite or bainite leads to excessively increased strength of the hot rolled steel sheet, thereby resulting in problems associated with manufacturing processes such as imperfect shapes due to heavy load upon cold rolling. On the other hand, where the winding temperature is higher than 75O ⁰ C, this may result in severe surface enrichment by elements such as Si, Mn and B, which lower the wettability of

hot-dip galvanizing. Therefore, the winding temperature is preferably limited to a range of 45O ⁰ C to 75O ⁰ C.

[105] The hot rolled steel sheet may be processed into a cold rolled steel sheet by cold rolling, if necessary.

[106]

[107] Cold rolling

[108] The thus-wound hot rolled steel sheet was subjected to pickling and cold rolling. In one embodiment of the present invention, the cold rolling is preferably carried out at a reduction ratio of 30 to 80%. Where the cold-rolling reduction ratio is lower than 30%, it is difficult to achieve a desired thickness of the steel sheet and it is also difficult to achieve shape correction of the steel sheet. On the other hand, where the cold-rolling reduction ratio is higher than 80%, this may result in high susceptibility to the occurrence of cracks in edges of the steel sheet, and increased cold rolling load.

[109]

[110] The thus-cold rolled steel sheet may be subjected to annealing treatment, if necessary.

[I l l]

[112] Annealing

[113] Next, the cold rolled steel sheet may be subjected to continuous annealing at a temperature of 75O ⁰ C to 900 ⁰ C for 10 to 1000 sec. The continuous annealing is intended to perform recrystallization, simultaneously with formation of ferrite and austenite and distribution of carbon. Where the continuous annealing temperature is lower than 75O ⁰ C, it is difficult to achieve sufficient recrystallization and sufficient formation of austenite, and it is therefore difficult to obtain the steel strength which is desired by the present invention. On the other hand, where the continuous annealing temperature exceeds 900 ⁰ C, this may result in decreased productivity and excessive formation of austenite, thereby decreasing the ductility. Therefore, the continuous annealing temperature is preferably limited to the range of 75O ⁰ C to 900 ⁰ C.

[114] Further, where the continuous annealing time is shorter than 10 sec, it is difficult to form sufficient amounts of austenite. On the other hand, where the continuous annealing time is longer than 1000 sec, this may result in decreased productivity and excessive formation of austenite. Therefore, the continuous annealing time is preferably limited to the range of 10 to 1000 sec.

[115] Thereafter, the thus-continuously annealed steel sheet is cooled to 600 ⁰ C to 72O ⁰ C at a rate of 1 to 10°C/sec (primary cooling). The primary cooling step is intended to increase the ductility and strength of the steel sheet by securing an equilibrium carbon concentration of ferrite and austenite. Where the primary cooling termination temperature is lower than 600 ⁰ C or higher than 72O ⁰ C, it is difficult to obtain the steel

ductility and strength which are desired by the present invention. Therefore, the primary cooling termination temperature is preferably limited to the range of 600 ⁰ C to 72O ⁰ C.

[116] Further, where the primary cooling rate is lower than l°C/sec, this may result in susceptibility to the formation of pearlite during the cooling process. On the other hand, where the primary cooling rate is higher than 10°C/sec, it is difficult to achieve the equilibrium carbon concentration, thus making it difficult to obtain desired ductility and strength of the steel sheet. Therefore, the primary cooling rate is preferably limited to the range of 1 to 10°C/sec.

[117] After primary cooling, the steel sheet is cooled to 100 ⁰ C to 400 ⁰ C at a rate of 1 to

100°C/sec (secondary cooling), and then held at that temperature for 10 to 1000 sec to form a composite structure composed of ferrite and martensite. Where the secondary cooling rate is lower than l°C/sec, this results in formation of largely pearlite or bainite as the secondary phase, thus making it difficult to secure the desired ductility and strength. On the other hand, where the secondary cooling rate is higher than 100°C/sec, there is necessary excessive facility investment. Therefore, the secondary cooling rate is preferably limited to the range of 1 to 100°C/sec.

[118] Further, where the secondary cooling termination temperature is lower than 100 ⁰ C, it is difficult to stably secure a composite structure composed of ferrite and martensite. On the other hand, where the secondary cooling termination temperature is higher than 400 ⁰ C, pearlite and bainite are largely formed as the secondary phase, thus making it difficult to secure the desired ductility and strength. Therefore, the secondary cooling termination temperature is preferably limited to the range of 100 ⁰ C to 400 ⁰ C.

[119] In addition, where the hold time after secondary cooling is shorter than 10 sec, it is difficult to stably secure a composite structure steel. On the other hand, where the hold time is longer than 1000 sec, this may result in decreased productivity and a difficulty to desired steel strength. Therefore, the hold time is preferably limited to the range of 10 to 1000 sec. Thereafter, the steel sheet is cooled to room temperature to manufacture a cold-rolled annealed steel sheet.

[120]

[121] coating

[122] Where appropriate, the hot-rolled steel sheet, the cold-rolled steel sheet and the annealed cold-rolled steel sheet (hereinafter, referred to simply as "steel sheet") may be coated. In one embodiment of the present invention, zinc coating or alloyed zinc coating may be applied to coating of the steel sheet. There is no particular limit to coating methods and for example, mention may be made of hot-dip coating, electrolytic coating, evaporation deposition coating and cladding. From a productivity point of view, hot-dip coating is preferred. Even though the coating method will be

described according to the most preferred embodiment, the present invention is not limited thereto.

[123]

[124] Hot - dip galvanizing

[125] Hot-dip galvanizing of a steel sheet is preferably carried out at a coating temperature of 45O ⁰ C to 500 ⁰ C for less than 10 sec. Where the coating temperature is lower than 45O ⁰ C, zinc coating is not sufficiently achieved. On the other hand, where the coating temperature is higher than 500 ⁰ C, this may result in excessive zinc coating. Therefore, the coating temperature is preferably limited to the range of 45O ⁰ C to 500 ⁰ C.

[126] Further, where the hot-dip galvanizing time is longer than 10 sec, this may result in excessive zinc coating. Therefore, the coating time is preferably limited to the range of less than 10 sec. After the hot-dip galvanizing was complete, the steel sheet is cooled to room temperature.

[127] After the hot-dip galvanizing, the steel sheet may be cooled to room temperature to manufacture a galvanized steel sheet, or otherwise the steel sheet may be subjected to alloy coating treatment to manufacture an alloyed galvanized steel sheet. The alloyed galvanized steel sheet may be subjected to alloying heat treatment at a temperature of 44O ⁰ C to 58O ⁰ C for less than 30 sec. Where the alloying heat treatment temperature is lower than 44O ⁰ C or higher than 58O ⁰ C, this may result in unstable alloying. Further, where the alloying heat treatment time exceeds 30 sec, this may result in excessive alloying.

[128]

Mode for the Invention

[129] EXAMPLES

[130] Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only for illustrating the present invention and should not be construed as limiting the scope and spirit of the present invention.

[131]

[132] Example 1

[133] Steel slabs having a steel composition as set forth in Table 1 below were subjected to vacuum melting, and heating in a heating furnace at a re -heating temperature of 115O ⁰ C to 125O ⁰ C for 1 hour, followed by hot rolling and winding. The hot rolling was terminated at a temperature of 85O ⁰ C to 95O ⁰ C. The winding temperature was set to 65O ⁰ C. Then, the hot rolled steel sheets were subjected to pickling and cold rolling at a cold-rolling reduction ratio of 50 to 70%. The cold rolled steel sheets were subjected to continuous annealing, and primary and secondary cooling, under conditions given in

Table 2. For the tensile test, JIS 5 test pieces were taken from the continuously- annealed cold rolled steel sheets and examined for the quality of material thereof.

[134] Further, in order to observe coating properties, the continuously-annealed steel sheets manufactured as above were heated to 46O ⁰ C, subjected to hot-dip galvanizing for 5 sec and alloying treatment at 500 ⁰ C for 10 sec, and cooled to room temperature, followed by observation of whether the steel sheets were coated or not by naked eyes.

[135] Mechanical properties and coating properties of Inventive and Comparative steels are given in Table 3 below.

[136] [137] Table 1

[138] [139] Table 2

Table 3

[141] [142] As shown in Tables 1 to 3, Inventive materials (1 to 10), satisfying the steel composition and manufacturing method specified in the present invention, exhibited a value of Inequality 1 of more than 10. That is, as shown in FIG. 1, Inventive materials could secure the formability which is desired by the present invention, as evidenced by TS x El balance of more than 15,000 at a value of Inequality 1 of more than 10. Further, Inventive materials exhibited high tensile strength of more than 490 MPa, and exhibited, as shown in FIG. 3, excellent coating properties with the addition of antimony (Sb). Therefore, steel materials of the present invention can be used as automotive structural members and reinforcement having high tensile strength of more than 490 MPa, high ductility and excellent coating properties.

[143] On the other hand, Comparative materials (11 to 14), manufactured using Comparative steels (K to N) that do not meet the composition range of the present invention, exhibited a value of Inequality 1 of less than 10. That is, as shown in FIG. 1, Comparative materials could not secure the formability which is desired by the present invention, as evidenced by TS x El balance of less than 15,000. Further, Comparative steels are steels with no addition of Sb, and Comparative material 12 with a low content of Si and Mn exhibited excellent coating properties, whereas Comparative materials 13 and 14 exhibited poor coating properties due to addition of large amounts of Si and Mn.

[144] [145] Example 2 [146] Steel slabs having a steel composition as set forth in Table 4 below were subjected to vacuum melting, and heating in a heating furnace at a re -heating temperature of 115O ⁰ C to 125O ⁰ C for 1 hour, followed by hot rolling and winding. The hot rolling was

terminated at a temperature of 85O ⁰ C to 95O ⁰ C. The winding temperature was set to 65O ⁰ C. Then, the hot rolled steel sheet was subjected to pickling and cold rolling at a cold-rolling reduction ratio of 50 to 70%. The cold rolled steel sheet was subjected to continuous annealing, and primary and secondary cooling, under conditions given in Table 5.

[147] For the tensile test, JIS 5 test pieces were taken from the annealed cold rolled steel sheet and examined for the quality of material thereof. Further, in order to simulate the quality of the material after coating in automotive parts, 2% strain was applied to the thus-prepared JIS 5 test pieces which were then boiled in oil at 17O ⁰ C for 20 min, followed by the tensile test. A BH value was calculated from the following equation:

[148] BH = YS (after coating) - strength (after application of 2% strain) [149] [150] Further, in order to observe coating properties, the continuously-annealed steel sheets manufactured as above were heated to 46O ⁰ C, subjected to hot-dip galvanizing for 5 sec and alloying treatment at 500 ⁰ C for 10 sec, and cooled to room temperature, followed by observation of whether the steel sheets were coated or not by naked eyes.

[151] Mechanical properties and coating properties of Inventive and Comparative steels are given in Table 6 below.

[152] [153] Table 4

[154] [155] Table 5

Table 6

[157] [158] As shown in Tables 4 to 6, Inventive materials (1 to 8), manufactured according to the manufacturing method of the present invention using Inventive steels (A to H)

satisfying steel composition specified in the present invention, exhibited an N* value of more than 0.2. That is, as shown in FIG. 2, Inventive materials could secure the formability and bake hardenability which are desired by the present invention, as evidenced by TS x El balance of more than 15,000 and a BH value of more than 80 MPa, at an N* value of more than 0.2. Further, Inventive materials exhibited high tensile strength of more than 490 MPa, and could secure, as shown in FIG. 3, automotive steel sheets having excellent coating properties with the addition of antimony (Sb). Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.