High-strength steel sheets with excellent formability and method for manufacturing the same

Abstract

Disclosed is a high-strength ferrite-martensite steel sheet with excellent formability comprising: 0.05-0.15 weight % of C; 0.15 weight % or less of Si; 0.5-2.7 weight % of Mn; 0.1-0.7 weight % of Al; 0.005-0.03 weight % of P; 0.01-0.3 weight % of Sb; 0.002-0.02 weight % of S; at least one element selected from the group consisting of 0.01-0.6 weight % of Mo and 0.0005-0.0035 weight % of B; and the balance Fe and incidental impurities.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of Korean Patent Application No. 10-2006-0106092 filed with the Korean Intellectual Property Office on Oct. 31, 2006, the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a high-strength steel sheet and a method for manufacturing the same, and more particularly, to high-strength steel sheets with excellent formability and galvanizability, which can be used as a steel material for an automotive application and a method for manufacturing the same.

[0004] 2. Background

[0005] Numerous researchers in the automotive industries have intensively studied to reduce automotive body weight to meet the standards required by environmental regulations and to cope with resource exhaustion problems.

[0006] For example, great efforts have been made to provide high-strength steel sheets to suppress exhaust gas and improve fuel efficiency as well as to reduce automotive body weight.

[0007] While higher strength steel materials are preferred for the purpose of vehicle users safety, they have a lower formability. For the products with complicated form, the formability is much lowered.

[0008] This can be readily inferred from the fact that the higher the strength of the steel sheet is, the more the yield stress is increased and the more the drawing formability represented by r-value falls sharply.

[0009] To overcome such problems, various attempts have been made to achieve both high-strength and high ductility of these steel materials. For example, numerous studies have provided multiphase steels with high-strength and high ductility, including TRIP steel using a transformation induced plasticity phenomenon, dual phase (DP) steel with martensite-ferrite matrix, and TWIP steel using a twin phenomenon.

[0010] Among them, the DP steel has been considered as a preferred material in terms of the press formability. Especially, the DP steel fabricated through continuous annealing followed by gas jet cooling line (GCL) shows low yield stress, high ductility and excellent bake hardening (BH).

[0011] However, while the DP steels have good formability, they have the drawback that their r-values are relatively low, meaning that their deep drawing properties are not excellent.

[0012] Currently, high strength steels are required to have both high ductility and superior contraction (i.e., high r-value). This is true even for high-strength steel sheets having a tensile strength of 440 MPa, 490 Mpa, 590 Mpa or more.

[0013] Various attempts have been made to increase the r-values of DP steels and improve the contraction property. For example, Japanese Patent Publication Nos. 1991-097812, 1991-097813 and 1993-209228 disclose methods for increasing r-values of cold rolled steel sheets for deep drawing, which employs two cold rolling processes and two annealing processes.

[0014] However, the methods disclosed in the above patents are not suitable for the high-strength cold rolled steel sheets having a tensile strength of more than 440 Mpa. Moreover, due to the cold rolling and annealing processes, each of which is required to be performed twice, the methods also exhibit low productivity and requires increased material cost. Accordingly, such methods are not suitable for mass production.

[0015] The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.

SUMMARY OF THE INVENTION

[0016] In one aspect, the present invention provides a high-strength DP steel sheet that can have excellent formability and contraction property even with one cycle of cold rolling and annealing process and can be fabricated through a hot dip galvanization process. This advantage can be achieved by using effective amounts of certain elements including Sb.

[0017] In a preferred embodiment, the present invention provides a high-strength ferrite-martensite steel sheet with excellent formability and contraction property, comprising: 0.05-0.15 weight % C; 0.15 weight % or less Si; 0.5-2.7 weight % Mn; 0.1-0.7 weight % Al; 0.005-0.03 weight % P; 0.01-0.3 weight % Sb; 0.002-0.02 weight % S; at least one element selected from the group consisting of 0.01-0.6 weight % Mo and 0.0005-0.0035 weight % B; and the balance Fe and incidental impurities.

[0018] Preferably, such steel sheet may further comprise 0.15 weight % or less of at least one element selected from the group consisting of Ti, Nb and V in accordance with a target tensile strength.

[0019] Suitably, .gamma.-Fiber orientation ((111)//RD orientation) of the ferrite matrix may be 0.35 volume % or more and cube orientation ((100)<001>) may be 0.2 volume % or less.

[0020] In another aspect, the present invention provides a method for manufacturing a high-strength steel sheet of a dual phase structure that has excellent formability and contraction property even with one cycle of cold rolling and annealing process, comprising the step of adding effective amount of Sb so as to suppress the development of cube orientation and rotated cube orientation ((100)<011>) and increase the development of (111)//RD orientation.

[0021] In such a method, the amount of Sb is preferably 0.3 weight % or less, and more preferably, 0.01-0.3 weight %.

[0022] In still another aspect, the present invention provides a method for manufacturing a high-strength steel sheet with excellent formability and superior contraction property, comprising the steps of: (a) providing a slab comprising: 0.05-0.15 weight % C; 0.15 weight % or less Si; 0.5-2.7 weight % Mn; 0.1-0.7 weight % Al; 0.005-0.03 weight % P; 0.01-0.3 weight % Sb; 0.002-0.02 weight % S; at least one element selected from the group consisting of 0.01-0.6 weight % Mo and 0.0005-0.0035 weight % B; and the balance Fe and incidental impurities; (b) reheating the slab at 1,050-1,250.degree. C.; (c) hot rolling the slab to adjust a reduction ratio at the last (final) stand to be 10% or less; (d) winding the hot rolled steel sheet at 600-750.degree. C. and then cold rolling the wound steel sheet; and (e) recrystallization-annealing the cold rolled steel sheet and then quenching the recrystallization-annealed steel sheet.

[0023] Preferably, recrystallization annealing may be performed at 750-850.degree. C. for 30-180 seconds. Also preferably, quenching may be carried out at a rate of 15-2,000.degree. C./s.

[0024] Suitably, 0.15 weight % or less of at least one element selected from the group consisting of Ti, Nb and V in accordance with a target tensile strength may be added to provide a slap.

[0025] In a further aspect, motor vehicles are provided that comprise a described high-strength steel sheet.

[0026] It is understood that the term "vehicle" or "vehicular" or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like. The present high-strength steel sheets will be particularly useful with a wide variety of motor vehicles.

[0027] Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be described with reference to certain exemplary embodiments thereof illustrated in the attached drawings in which:

[0029] FIG. 1 is a graph showing volume fractions of .gamma.-Fiber orientation in accordance with added amount of antimony in Examples and Comparative Examples;

[0030] FIG. 2 is a graph showing volume fractions of cube orientation in accordance with added amount of antimony in Examples and Comparative Examples; and

[0031] FIG. 3 is a graph showing r-values in accordance with added amount of antimony in Examples and Comparative Examples.

DETAILED DESCRIPTION

[0032] Hereinafter, preferred embodiments of the present invention will now be described in detail with reference to the attached drawings.

[0033] As discussed above, in one aspect, the present invention provides a high-strength DP steel sheet that has excellent formability and contraction property.

[0034] A preferred example of the steel sheet of the present invention is a high-strength ferrite-martensite steel sheet with excellent formability and superior contraction property, which comprises: 0.05-0.15 weight % C; 0.15 weight % or less Si; 0.5-2.7 weight % Mn; 0.1-0.7 weight % Al; 0.005-0.03 weight % P; 0.01-0.3 weight % Sb; 0.002-0.02 weight % S; at least one element selected from the group consisting of 0.01-0.6 weight % Mo and 0.0005-0.0035 weight % B; and the balance Fe and incidental impurities. Such steel sheet is composed of a dual phase structure of ferrite and martensite.

[0035] The inventor of the present invention has examined the development of microstructure, recrystallization and development of phase transformation according to the kinds of alloying elements in cold rolled steel sheets and the conditions of manufacturing process in order to solve the problems associated with the conventional art. While conducting such researches, the present inventor confirmed that the development of .gamma.-Fiber orientation ((111//RD) that has a direct effect on the r-value is influenced primarily by the amount of solid solution carbon C among the alloying components.

[0036] A certain amount of carbon (C) is required for formation of the martensite necessary for obtaining a target tensile strength. Accordingly, the amount of solid solution carbon can be reduced with the generation of carbides by utilizing titanium (Ti), vanadium (V), niobium (Nb), etc. against the solid solution carbons not necessary for formation of such martensite.

[0037] It is possible to develop recrystallization texture of final product obtained after recrystallization-annealing process be a method known in the art. However, the method still has limitations in controlling the texture due to the reduction of the solid solution carbon amount.

[0038] Accordingly, in order to overcome these restrictive limitations, the present inventor has conducted researches on an optimum design of alloying components and a control of texture with the addition of components, which requires no additional process.

[0039] Through his intensive studies, the present inventor has discovered that a high r-value can be obtained when 0.3 weight % or less of antimony (Sb) is used. This limited amount of Sb can suppress the developments of .alpha.-Fiber orientation ((110)H/RD) and cube orientation ((100)<001>) and increase the development of (111)//RD orientation.

[0040] Moreover, it has been well known that, when antimony (Sb) is added to the steel material in which 10 ppm or less of sulfur (S) exists as an impurity, the cube orientation develops, thereby improving physical properties of the steel sheet.

[0041] Based on these facts, the present inventor has discovered that the developments of cube orientation and rotated cube orientation ((100)<011>) can be suppressed and the development of (111)//RD orientation can increase, if 10 ppm or more of sulfur (S) and 0.3 weight % or less of antimony (Sb) are contained in the steel material.

[0042] Based on this principle, in another aspect, the present invention provides a method for manufacturing a high-strength steel sheet of a dual phase structure that has excellent formability and superior contraction property even with one cycle of cold rolling and annealing process, comprising the step of adding effective amount of Sb so as to suppress the development of cube orientation and rotated cube orientation ((100)<011>) and increase the development of (111)//RD orientation.

[0043] In a preferred embodiment, accordingly, the amount of S may be 10 ppm or more and the amount of Sb may be 0.3 weight % or less. More preferably, the amount of S may be 0.002-0.02 weight % and the amount of Sb may be 0.01-0.3 weight %.

[0044] In particular, a slab comprising the alloying elements as described above is subjected to a hot rolling process having a reduction ratio in the final stand of a hot strip finishing mill adjusted to of 10% or less. The hot rolled sheet is subjected to a box annealing process at 600-750.degree. C. The resulting annealed sheet is then subjected to a winding process.

[0045] The thus-obtained hot rolled steel sheet is subjected to a pickling process and a cold rolling process of 65-75%. Subsequently, the cold rolled steel sheet is subjected to a degreasing process, a recrystallization annealing at 750-850.degree. C. for 30-180 seconds and a quenching process at a rate of 15-2,000.degree. C./s based on the characteristics of the quenching line.

[0046] The basic phase of the DP cold rolled steel sheet prepared in the above-described method is composed of ferrite matrix, and bainite and martensite structures co-exist in a ratio over a certain level in accordance with a target tensile strength and a cooling rate of the annealing line.

[0047] In a more preferred embodiment, a high-strength steel sheet cab be fabricated in the following manner. A slab comprising: 0.05-0.15 weight % C; 0.15 weight % or less Si; 0.5-2.7 weight % Mn; 0.1-0.7 weight % Al; 0.005-0.03 weight % P; 0.01-0.3 weight % Sb; 0.002-0.02 weight % S; at least one element selected from the group consisting of 0.01-0.6 weight % Mo, 0.0005-0.0035 weight % B; and, optionally, 0.15 weight % Ti, Nb or V; and the balance Fe and incidental impurities is subjected to a reheating process at 1,050-1,250.degree. C. and a hot rolling process at a reduction ratio of 10% or less in the last stand of the hot strip finishing mill in consideration of the strip flatness and crown control. The hot rolled steel sheet is wound up at 600-750.degree. C. The resultant hot rolled steel sheet is subjected to a cold rolling process at a reduction ratio of 65-75% in thickness of 1.4 mm. The cold rolled steel sheet is subjected to a recrystallization annealing process at 750-850.degree. C. for 30-180 seconds. Finally, the recrystallization annealed steel sheet is quenched at a rate of 15-2,000.degree. C./s.

[0048] In the reheating process of the slab, heating at less than 1,050.degree. C. can increase the roll force caused by the rolling resistance due to the low temperature, deteriorating the hot rolling workability by By contrast, heating at more than 1,250.degree. C. can make it difficult to control the microstructures, volume fractions and texture controls of the end product due to micro-precipitations contained in quantities by the increased recreation of solid solution.

[0049] The hot rolling process includes a front rolling method in that the front stand of the hot strip finishing mill executes the rolling process and a rear rolling method in that the rear stand of the hot strip finishing mill executes the rolling process, and both methods affect the development of the texture of the end product.

[0050] In both rolling methods, the reduction ratio in the last (final) stand commonly influences the strip flatness of hot rolled steel sheets, which has a close relation with the productivity. Accordingly, the reduction ratio in the last hot rolling step is limited to 10% or less.

[0051] Furthermore, as described above, the hot rolled steel sheet is wound up (coiled) at 600-750.degree. C. A temperature below 600.degree. C. can cause a problem in that the transformation structure generated during the hot rolling process is not completely recrystallized. In contrast, a temperature over 750.degree. C. can cause a problem in pickling due to an oxide film of a dense structure formed on the surface of hot rolled steel sheet.

[0052] In addition, it is desirable that the recrystallization annealing process be carried out at 750-850.degree. C., since it is difficult to control the ratio of martensite phase required for a certain level of tensile strength, if the recrystallization annealing process is performed outside the above range.

[0053] Moreover, it is desirable that the annealing time be limited to 30-180 seconds. An annealing for less than 30 seconds can prohibit the cold rolled structure from being completely annealed and can affect the phase transformation temperature. On the other hand, annealing for more than 180 seconds can reduce overall productivity although it may reach an ideal temperature.

[0054] Furthermore, the cooling process is carried out at a rate of 15-2,000.degree. C./s after the recrystallization annealing process. A cooling rate lower than the above range can make it difficult to ensure the driving energy required for the martensite transformation. By contrast, a cooling rate higher than the above range can make it impossible to achieve a commercially and practically available result.

[0055] The steel sheet fabricated through the above processes has a matrix structure composed of ferrite and martensite and has characteristics in that .gamma.-Fiber orientation ((111//RD) of ferrite matrix is controlled to 0.35 volume % or more and cube orientation ((100)<001>) is suppressed to 0.2 volume % or less.

[0056] Next, the reasons for limiting the content of the respective alloying elements to a specific range will be described.

[0057] C: 0.05-0.15 Weight %

[0058] Carbon can affect the formability of steel. Excessive amount of carbon deteriorates the formability.

[0059] Moreover, it moves from the inside of ferrite to the austenite phase in the temperature range of phase transformation, thus stabilizing the austenite phase.

[0060] However, since an amount exceeding 0.15 weight % affects the formation of the dual phase and deteriorates the weldability, it is desirable that the carbon content be 0.15 weight % or less.

[0061] On the other hand, an amount less than 0.05 weight % cannot ensure the necessary strength.

[0062] Si: 0.15 Weight % or Less

[0063] Silicon is a ferrite stabilizing element that suppresses the precipitation of cementite and increases the ferrite fraction to improve the elongation and increases the strength by strengthening solid solution.

[0064] Excessive amount of Si causes a defective surface texture and deteriorate the weldability. It also lowers the paintability due to the generation of oxides.

[0065] Accordingly, 0.15 weight % or less is preferred.

[0066] Mn: 0.5-2.7 Weight %

[0067] Manganese is an austenite stabilizing element that increases the strength with the generation of low temperature phase transformation such as acicular ferrite, bainite or martensite.

[0068] Moreover, despite the phase transformation by the diffusional transformation, it causes a texture memory phenomenon to occur by decreasing the ferrite region.

[0069] If the amount added is less than 0.5 weight %, it is difficult to control the cooling rate for suppressing the formation of ferrite, whereas, if it is more than 2.7 weight %, it causes the formation of segmentation on the sheet during the hot rolling process, the deterioration of weldability and the hydrogen induced embrittlement.

[0070] Accordingly, it is desirable that the manganese content be maintained at 0.5-2.7 weight %.

[0071] Al: 0.1-0.7 Weight %

[0072] Aluminum, like silicon, suppresses the precipitation of cementite to delay the transformation process.

[0073] It increases the ferrite fraction to improve the elongation and does not deteriorate the chemical conversion coating property and the hot dip galvanizability; however, it provides a cause of the embrittlement.

[0074] Moreover, it lowers the actual yield by the solution strengthening along with silicon added during the cold rolling process.

[0075] Accordingly, it is desirable that the aluminum content be limited to the range of 0.1-0.7 weight %.

[0076] P: 0.005-0.03 Weight %

[0077] Phosphorus increases the strength of steel sheet by the solid strengthening and plays a role of stabilizing the remaining austenite by the increase of solid solution carbon caused by the suppression of oxide formation.

[0078] If added excessively, it causes the deterioration of weldability and local ductility due to the grain boundary segregation of P, promotes the development of cube orientation and inhibits the development of (111)//RD orientation.

[0079] Accordingly, it is desirable that the phosphorus content be limited to the range of 0.005-0.03 weight %.

[0080] Sb: 0.01-0.3 Weight %

[0081] Antimony promotes the formation of (111)//RD orientation that is an advantageous textile for r-value and inhibits the formation of cube orientations and rotated cube orientations. These effects are increased if silicon (S) is contained in an appropriate range.

[0082] If added excessively, it affects the steel manufacture and the hot rolling. Accordingly, it is desirable that the antinomy content be limited to the range of 0.01-0.3 weight %.

[0083] S: 0.002-0.02 Weight %

[0084] If sulfur is added in an appropriate range, its effect is increased. However, if added excessively, it forms rough MnS on the hot rolled steel sheet, which results in cracks. Accordingly, it is desirable that sulfur content be limited to the range of 0.002-0.02 weight %.

[0085] Mo: 0.01-0.6 Weight %

[0086] Molybdenum, like Mn, is an element that stabilizes the low temperature phase transformation. If added excessively, it deteriorates the formability. Accordingly, it is desirable that molybdenum content be limited to the range of 0.01-0.6 weight %.

[0087] B: 0.0005-0.0035 Weight %

[0088] Boron, like Mn, is an element that stabilizes the low temperature phase transformation. Outside the range of 0.0005-0.0003 weight %, the above effect cannot be obtained. In particular, if added excessively, it has a bad effect on the plating adhesion. Accordingly, it is desirable that boron content be limited to the range of 0.0005-0.0035 Weight %.

[0089] Ti, Nb and V: 0.15 Weight % or Less

[0090] Titanium (Ti), niobium (Nb) and vanadium (V) are grain refining elements. If added excessively, each of them decreases the overall productivity. Accordingly, it is desirable that their contents be limited to 0.15 weight % or less.

[0091] Subsequently, the present invention will be described in more detail with reference to Examples and Comparative Examples. The following examples are presented to illustrate further various aspects of the present invention, but are not intended to limit the scope of the invention in any aspect.

EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLES 1 TO 3

[0092] Steel slabs having the compositions listed in the following Table 1 were heated at 1,200.degree. C. and then subjected to hot rolling, winding (coiling), pickling, cold rolling processes in turn, thus fabricating steel sheets of 1.0 mm in thickness. Subsequently, a recrystallization annealing process was carried out for the steel sheets under the conditions as listed in Table 2.

[0093] The reduction ratio in the last hot rolling step was set at 10%, the coiling temperature was set at 650.degree. C., the reduction ratio in the cold rolling step was set at 65% and the cooling rate after the recrystallization annealing process was set at 20.degree. C./s.

[0094] Tensile strengths, yield strengths and r-values were measured for the steel sheets fabricated through the recrystallization annealing process in Examples 1 to 6 and Comparative Examples 1 to 3. In addition, (111)//RD orientations and (100)<001>orientations by volume weight were calculated using an EBSD technique. The data are shown in the following Table 2.

TABLE-US-00001 TABLE 1 Composition (weight %) Class C Si Mn P Al S Sb Mo B Ti Nb V Example 1 Invented 0.055 0.15 2.1 0.012 0.42 0.003 0.035 0.15 0.002 -- -- -- Steel Example 2 Invented 0.068 0.13 1.69 0.011 0.95 0.0045 0.045 -- 0.002 -- -- -- Steel Example 3 Invented 0.076 0.04 1.90 0.023 1.2 0.008 0.04 0.08 -- -- 0.01 -- Steel Example 4 Invented 0.08 0.07 2.0 0.057 1.7 0.007 0.06 0.18 -- 0.03 -- -- Steel Example 5 Invented 0.08 0.11 1.34 0.01 1.0 0.005 0.05 0.25 0.002 -- -- -- Steel Example 6 Invented 0.09 0.12 1.56 0.056 0.7 0.003 0.08 0.25 0.002 -- -- -- Steel Comparative Compared 0.03 0.18 1.42 0.041 0.88 0.001 0.055 0.11 0.001 -- 0.01 -- Example 1 Steel Comparative Compared 0.06 0.18 1.69 0.011 0.94 0.0045 -- -- 0.002 -- -- -- Example 2 Steel Comparative Compared 0.15 0.1 1.54 0.013 1.16 0.009 0.65 0.9 -- -- 0.01 0.05 Example 3 Steel

TABLE-US-00002 TABLE 2 Tensile Annealing Annealing Strength Elongation (111)//RD (100)<001> Class Temp (.quadrature.) Time (s) (MPa) (%) (vol %) (vol %) R-value Example 1 Invented 790 60 586 31 0.45 0.1 1.45 Steel Example 2 Invented 790 60 635 29.4 0.38 0.13 1.33 Steel Example 3 Invented 790 60 623 30 0.375 0.12 1.31 Steel Example 4 Invented 820 60 652 28 0.36 0.16 1.3 Steel Example 5 Invented 820 60 671 27 0.35 0.16 1.3 Steel Example 6 Invented 820 60 734 26 0.35 0.19 1.28 Steel Comparative Compared 790 60 682 23 0.29 0.23 1.1 Example 1 Steel Comparative Compared 790 60 630 28 0.34 0.22 1.05 Example 2 Steel Comparative Compared 850 60 950 18 0.22 0.3 0.75 Example 3 Steel

[0095] As shown in Table 2, it was possible to manufacture high-strength steel sheets with high r-values and excellent formability by adding antimony (Sb) in an optimum amount.

[0096] Moreover, FIGS. 1 and 2 depict volume fractions (volume %) of (111)//RD orientation and (100)<001>orientation in accordance with the addition of antimony (Sb), and FIG. 3 depicts r-values in accordance with the addition of antimony (Sb).

[0097] The steel sheets according to the present invention, as shown in FIG. 1, showed the (111)//RD orientation as an orientation distribution of 0.035%. Referring to FIG. 2, the orientation density was lowered. In contrast, the orientation density is increased for comparative examples.

[0098] Moreover, referring to FIG. 3, high r-values were obtained if the added amount of antimony is satisfied with the condition of the present invention.

[0099] As described above, the present invention can provide a dual phase high-strength steel sheet for a hop dip galvanization that has a high revalue and excellent formability and contraction, manufactured by adding antimony (Sb) in an appropriate range to alloying components and carrying out hot rolling, coiling, cold rolling and recrystallization annealing processes under optimum process conditions.

[0100] Particularly, it is possible to manufacture a dual phase high-strength steel sheet with high tensile strength with one cycle of cold rolling and annealing process, thereby increasing productivity, reducing manufacturing cost and facilitating mass production.

[0101] The present invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A high-strength ferrite-martensite steel sheet with excellent formability and superior contraction property, comprising: 0.05-0.15 weight % of C; 0.15 weight % or less of Si; 0.5-2.7 weight % of Mn; 0.1-0.7 weight % of Al; 0.005-0.03 weight % of P; 0.01-0.3 weight % of Sb; 0.002-0.02 weight % of S; at least one element selected from the group consisting of 0.01-0.6 weight % of Mo and 0.0005-0.0035 weight % of B; and the balance Fe and incidental impurities.

2. The high-strength steel sheet as recited in claim 1 further comprising: 0.15 weight % or less of at least one element selected from the group consisting of Ti, Nb and V in accordance with a target tensile strength.

3. The high-strength steel sheet as recited in claim 1, wherein .gamma.-Fiber orientation ((111)H/RD orientation) of the ferrite matrix is 0.35 volume % or more and cube orientation ((100)<001>) of the ferrite matrix is 0.2 volume % or less.

4. A method for manufacturing a high-strength steel sheet with excellent formability and superior contraction property comprising the steps of: providing a slab comprising: 0.05-0.15 weight % of C; 0.15 weight % or less of Si; 0.5-2.7 weight % of Mn; 0.1-0.7 weight % of Al; 0.005-0.03 weight % of P; 0.01-0.3 weight % of Sb; 0.002-0.02 weight % of S; at least one element selected from the group consisting of 0.01-0.6 weight % of Mo and 0.0005-0.0035 weight % of B; and the balance Fe and incidental impurities, reheating the slab at 1,050-1,250.degree. C.; hot rolling the slab to adjust a reduction ratio at the last stand to be 10% or less; winding the hot rolled steel sheet at 600-750.degree. C. and then cold rolling the wound steel sheet; and recrystallization-annealing the cold rolled steel sheet and then quenching the recrystallization-annealed steel sheet.

5. The method as recited in claim 4, wherein the recrystallization annealing is performed at 750-850.degree. C. for 30-180 seconds and the quenching is carried out at a rate of 15-2,000.degree. C./s.

6. The method as recited in claim 4, wherein 0.15 weight % or less of at least one element selected from the group consisting of Ti, Nb and V in accordance with a target tensile strength is added to provide a slap.

7. A method for manufacturing a high-strength steel sheet of a dual phase structure that has excellent formability and superior contraction property even with one cycle of cold rolling and annealing process, comprising the step of adding an effective amount of Sb so as to suppress the development of cube orientation and rotated cube orientation ((100)<011>) and increase the development of (111)//RD orientation.

8. The method as recited in claim 7, wherein the amount of Sb is 0.3 weight % or less.

9. The method as recited in claim 8, wherein the amount of the amount of Sb is 0.01-0.3 weight %.

10. A motor vehicle comprising the steel sheet of claim 1.