Steel Sheet for Deep Drawing Having Excellent Secondary Work Embrittlement Resistance, Fatigue Properties and Plating Properties, and Method for Manufacturing the Same

Info

Publication number: 20080196799
Type: Application
Filed: Jul 7, 2006
Publication Date: Aug 21, 2008
Applicant: POSCO (Nam-ku, Pohang)
Inventors: Hee-Jae Kang (Cheonlanam-do), Kwang-Keun Chin (Cheonlanam-do), Sang-Ho Han (Cheonlanam-do), Chang-Sik Choi (Cheonkanam-do)
Application Number: 11/994,630

Abstract

A steel sheet for deep drawing used for automobiles, and a method for manufacturing the same are disclosed. The steel sheet comprises, by weight %, C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.10˜0.40%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0<(Ti[%]+Al[%]/16+6B[%])/3.43N[%]<4.1, and wherein the composition of Nb, Al, and C satisfies the relationship; 0.7<(Nb[%]+Al[%]/20)/7.75C[%]<3.5. The steel sheet exhibits excellent secondary work embrittlement, fatigue properties of welded joints, and an appealing plated surface as well as excellent formability.

Description

Description

TECHNICAL FIELD

The present invention is based on, and claims priority from, Korean Application Number 2005-61691, filed Jul. 8, 2005, the disclosure of which is incorporated by reference herein in its entirety.

The present invention relates to steel sheets for deep drawing mainly used for interior or exterior plates of automobile bodies, and the like. More particularly, the present invention relates to steel sheets for deep drawing, which have a tensile strength of 28˜50 f/ while exhibiting excellent secondary work embrittlement resistance, fatigue properties of welded joints, and plating properties as well as excellent formability, and to a method for manufacturing the same.

BACKGROUND ART

In recent years, as components of automobile body have had a tendency of becoming more complicated in shapes and integrated as a single component, steel sheets for the automobile body have been required to have further enhanced formability. In addition, the steel sheets for the automobile body also have been required to have excellent secondary work embrittlement and fatigue properties of welded joints in terms of using conditions of the automobiles, and to have an appealing plated surface.

Generally, steel sheets having enhanced formability and strength are produced in such a way of adding formability enhancing elements, that is, carbide and nitride formation elements such as Ti, Nb and the like, and strength enhancing elements, that is, solid solution strengthening elements such as Mn, P, Si and the like to a highly pure steel which is minimized in contents of impurities in the steel. Due to inherent restrictions in properties of the steel, however, it is difficult to enhance the formability and the strength at the same time.

In particular, since the steel sheet for extra deep drawing is produced using the highly pure steel, it commonly suffers from embrittlement of grain boundaries, which results in significant deterioration of secondary work embrittlement resistance and fatigue properties of welded joints.

In order to manufacture products which overcome such problems described above, Japanese furnace makers have made various investigations, and developed techniques for manufacturing steel sheets for deep drawing as follows.

Generally, the steel sheets for deep drawing are produced using, so called, ultra-low carbon interstitial free (IF) steel, which is produced by adding the carbide and nitride formation elements such as Ti, Nb, and the like as a single component or a combination thereof to ultra-low carbon steel while lowering an amount of interstitial solid solution elements such as C or N to 50 ppm or less during a steel making process in order to ensure good formability. As common features of the conventional techniques which produce the steel sheet for deep drawing by use of the IF steel, although the carbide and nitride formation elements such as Ti, Nb, and the like are added in an amount of 0.01˜0.07% to the ultra-low carbon steel in order to ensure workability, the steel lacks in the interstitial solid solution strengthening elements which serve to strengthen the grain boundaries, causing the secondary work embrittlement while deteriorating the fatigue properties at the spot welded joints.

This problem becomes serious in high strength steel for deep drawing which contains the solid solution strengthening elements such as P, Mn, and the like. In this regard, techniques disclosed in Japanese Patent Laid-open publication Nos. (H) 6-57373 and (H) 7-179946 suggest addition of grain boundary strengthening elements such as B and the like, and techniques disclosed in Japanese Patent Laid-open publication Nos. 2000-303144 and 2001-131695 suggest limitation of carbon content in the steel to 60 ppm or less. However, these techniques also suffer from the problems such as deterioration in workability, plating properties of GA products using the steel sheets produced by the techniques, and the like.

In addition, inventors of the present invention invented a high strength steel sheet for extra deep drawing useful for automobiles and the like, and a method for manufacturing the same disclosed in Korean Patent Laid-open Publication No. 2004-0002768, which comprises, by weight %, C: 0.010% or less, Si: 0.02% or less, Mn: 1.5% or less, P: 0.03˜0.15%, S: 0.02% or less, Sol. Al: 0.03˜0.40%, N: 0.004% or less, Ti: 0.005˜0.040%, Nb: 0.002˜0.020%, and at least one of B: 0.0001˜0.0020% and Mo: 0.005˜0.02%, thereby enhancing the workability of Ti—Nb added steel. However, although this method can enhance the workability by controlling Ti and Nb in combination, it fails to ensure the secondary work embrittlement and the fatigue properties which have been required for the steel plate of the automobile in recent years.

DISCLOSURE OF INVENTION Technical Problem

Therefore, the present invention has been made to solve the above problems, and it is an object of the present invention to provide a high strength steel sheet for deep drawing, which is controlled in contents of Ti, Al, B and N, and in contents of Nb, Al and C combinationally, while increasing the content of Al, which is advantageous in terms of formability and plating properties, and reducing the content of Ti, which is disadvantageous in terms of the plating properties, and the like, thereby providing excellent properties in terms of secondary work embrittlement resistance, and fatigue properties of welded joints as well as formability while exhibiting an appealing surface quality.

Technical Solution

In accordance with one aspect of the invention, the above and other objects can be accomplished by the provision of a high strength steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties, comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.10˜0.40%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1, and wherein the composition of Nb, Al, and C satisfies the relationship: 0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5.

In accordance with another aspect of the invention, a method for manufacturing a high strength steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties comprises: reheating a steel slab at a temperature of 1,100˜1,250° C., the steel slab comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.10˜0.40%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1, and wherein the composition of Nb, Al, and C satisfies the relationship: 0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5; rough rolling the reheated steel slab; finish rolling the rough rolled steel slab at a finish rolling temperature of 880° C. or more, followed by coiling the hot rolled steel sheet; cold rolling the coiled steel sheet at a reduction ratio of 65% or more; and continuously annealing the cold rolled steel sheet at a temperature of 780˜860° C.

Advantageous Effects

As apparent from the above description, the steel sheets for deep drawing according to the present invention exhibit excellent secondary work embrittlement, fatigue properties of welded joints, and an appealing plated surface as well as excellent formability compared with the conventional high strength steel sheets for deep drawing.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments will be described in detail hereinafter.

A high strength steel sheet according to the present invention has characteristics in that it is controlled in contents of Ti, Al, B and N, and in contents of Nb, Al and C combinationally, while increasing the content of Al, which is advantageous in terms of formability and plating properties, and reducing the content of Ti, which is disadvantageous in terms of the plating properties, and the like, thereby exhibiting excellent properties in terms of secondary work embrittlement resistance, fatigue properties of welded joints and plating properties as well as formability.

The steel sheet according to the present invention will be described in terms of composition and manufacturing method hereinafter.

[Composition]

C: 0.010 wt % or less (hereinafter,%)

C acts as an interstitial solid solution element in steel, and obstructs formation of {111} texture, which is advantageous in terms of workability in the course of forming the texture in a steel sheet upon cold rolling and annealing. If carbon content exceeds 0.010%, it is necessary to increase the contents of Ti and Nb which are carbide and nitride formation elements, causing a disadvantage in terms of manufacturing costs. Thus, the carbon content is preferably 0.010% or less.

Si: 0.02% or less

Si is an element which causes a defect of surface scale. If silicon content exceeds 0.02%, there arise problems such as temper color upon annealing, and non-plated parts upon plating. Thus, the silicon content is preferably 0.02% or less.

Mn: 0.06˜1.5%

Mn is a substitutional solid solution strengthening element for ensuring strength. If Mn content is less than 0.06%, the steel suffers from embrittlement due to S in the steel, whereas, if the Mn content exceeds 1.5%, an r-value of the steel is rapidly deteriorated along with elongation. Thus, the Mn content is preferably in the range of 0.06˜1.5%.

P: 0.15% or less

P is also a representative solid solution strengthening element which is added to the steel along with Mn for increasing the strength. When P is added to Ti—Nb added steel as in the steel of the present invention, it results in growth of the {111} texture, advantageous in terms of the r-value, through grain refinement, grain boundary segregation, and the like. However, if P content exceeds 0.15%, the steel suffers from rapid reduction in elongation along with significant increase in brittleness. Thus, the P content is preferably in the range of 0.15% or less.

S: 0.020% or less

When producing steel for deep drawing, S content in the steel is generally restricted to a low degree of 0.005% or less. According to the present invention, however, since the steel contains Mn, all amounts of S in the steel are precipitated as MnS, thereby enabling deterioration of formability due to solid solution S to be avoided. Thus, S content is preferably 0.020% or less, which deviates from a region causing edge cracks during rolling.

Sol. Al: 0.10—0.40%

For cold-rolled steel products, Sol. Al content of the steel is generally controlled to be in the range of 0.02˜0.07% while dissolved oxygen in the steel is maintained in a sufficiently low state in consideration of manufacturing costs. According to the present invention, Sol. Al serves to allow deep drawability to be stably secured at a lower annealing temperature. In addition, Sol. Al diffuses to the surface of the steel along the grain boundaries, and makes a plated layer dense, thereby enhancing powdering resistance of the steel.

According to the present invention, if the Sol. Al content is 0.10% or more in the steel, it coarsens the precipitates in the steel, remarkably obstructs effect of suppressing recrystallization by P, thereby activating the recrystallization, and aids in development of the {111} texture and enhancement of the powdering resistance. If the Sol. Al content exceeds 0.40%, it causes an increase of the costs, and deterioration in efficiency of continuous casting operation.

Thus, the Sol. Al content is preferably in the range of 0.10˜0.40%. In the steel sheet of the present invention, since the Sol. Al content influences formation of Ti or Nb-based precipitates as the carbide and nitride such that the precipitates become coarsened, it serves as a critical component, which provides further enhanced formability of the steel with small added amounts of Ti and Nb in comparison to the conventional IF steel.

N: 0.010% or less

N generally exists in a solid solution state, and deteriorates the formability of the steel. If N content exceeds 0.010%, it is necessary to increase added amounts of Ti and Nb for fixing N as precipitates. Thus, the N content is preferably 0.010% or less.

Ti: 0.003˜0.010%

Ti is a very important element in terms of the formability. In order to provide effect of enhancing the formability (in particular, r-value), Ti must be added to the steel in an amount of 0.003% or more. However, if Ti content exceeds 0.010%, it is disadvantageous in terms of manufacturing costs and plating properties in galvannealing. Thus, the Ti content is preferably in the range of 0.003˜0.010%.

Nb: 0.003˜0.040%

Nb is also a very important element in terms of the formability like Ti. In order to provide the effect of enhancing the formability (in particular, r-value), Nb must be added to the steel in an amount of 0.003% or more. However, if Nb content exceeds 0.040%, it is disadvantageous in terms of the manufacturing costs and the plating properties. Thus, the Nb content is preferably in the range of 0.003˜0.040%.

B: 0.0002˜0.0020%

B is a grain boundary strengthening element, and effective to enhance fatigue properties of spot welded joints while preventing grain boundary embrittlement by P. If B content is less than 0.0002%, the steel fails to achieve the effect described above, whereas, if the B content exceeds 0.0020%, there arise problems of rapid reduction in the formability, and deterioration in surface properties of plated steel sheet. Thus, the B content is preferably 0.0002˜0.0020%.

According to the present invention, the steel sheet comprises the balance of Fe and other unavoidable impurities in addition to the above components. Additionally, the steel sheet of the present invention may further comprise Mo in order to further enhance the secondary work embrittlement resistance and the plating properties. At this time, Mo content is preferably 0.05% or less. The reason is that, if the Mo content exceeds 0.05%, the effect of enhancing the secondary work embrittlement resistance and the plating properties by the Mo content is significantly reduced, and it is disadvantageous in terms of the manufacturing costs.

According to the present invention, in order to simultaneously secure the formability, plating properties, secondary work embrittlement resistance and fatigue properties of the steel which has the composition with a low Ti content and a high Al content as described above, it is necessary to control the contents of Al, B and N in combination to addition of Ti as in the following Expression 1. Specifically, according to the present invention, since the Ti content is low in comparison to the conventional steel sheet, there is a high possibility of deterioration of formability. In this regard, in order to avoid the deterioration of formability due to the low content of Ti while ensuring the secondary work embrittlement resistance, the fatigue properties, and the plating properties at the same time, the present invention suggests the following Expression 1:

1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1 1

In other words, according to the present invention, it is necessary to satisfy the relationship of 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1 due to the following reason.

In the steel, Ti, Al and B react with N, forming nitrides. Thus, if the contents of these elements in the steel are significantly low, solid solution N causes an aging phenomenon while deteriorating drawability. On the other hand, if the contents of these elements in the steel increases above predetermined amounts, the steel suffers from deterioration of the plating properties and the stretching properties upon machining.

In other words, if a calculated value of the expression is less than 1.0, the steel suffers from not only the aging phenomenon and the deterioration in drawability, but also failure in ensuring the secondary work embrittlement resistance and the fatigue properties. On the other hand, if the calculated value exceeds 4.1, the steel suffers from deterioration in the plating properties and the stretching properties. Accordingly, it is preferably to control the content of Ti, Al, B and N so as to satisfy the relationship of 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1

In addition, according to the present invention, in order to ensure the deep drawability and the stretching properties more stably, it is necessary to control the contents of the components so as to satisfy the following Expression 2. Specifically, due to the low Ti content of the steel according to the present invention, it is necessary to further ensure the deep drawability and the stretching properties. To this end, the present invention controls the contents of Nb, Al and C in combination according to the following Expression 2:

0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5 2

If a calculated value is less than 0.7, the drawability can be deteriorated due to instable scavenging of C in the steel. On the other hand, if the calculated value exceeds 3.5, there is a problem of deterioration in stretching properties due to an increase in an amount of solid solution Nb in the steel.

In the steel sheet of the present invention, Nb—Ti—Al—N—C based composite precipitates are formed. At this point, if an average size of Nb—Ti—Al—N—C based composite precipitates is controlled to be 40□ or more, it is more preferable since it can further enhance the formability of the steel sheet. In addition, according to the present invention, the formability and the plating properties can be further enhanced by restricting a fraction of Ti₄C₂S₂to be 50% or more and a fraction of TiC to be below 5% among the Nb—Ti—Al—N—C based precipitates. Since Ti₄C₂S₂is a precipitate advantageous in terms of the formability and the plating properties desired to be obtained by the present invention, if the fraction of Ti₄C₂S₂is controlled to be 50% or more, it is possible to secure further enhanced formability and plating properties.

Meanwhile, since TiC is a precipitates disadvantageous in terms of the plating properties, if the fraction of TiC is restricted to be below 5%, it is possible to secure further enhanced plating properties. The control of the composite precipitates as described above is closely related to a ratio of a reduction amount of rough rolling to a reduction amount of finish rolling (also hereinafter referred to as a reduction amount ratio) in hot rolling when manufacturing of the steel sheet according to the present invention, which will be described below.

According to the present invention, the steel sheet can be produced to have a desired tensile strength by controlling the components to satisfy the above composition and the following Expression 3:

28≦27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%]>50 3

According to the present invention, it is possible to control the contents of the components such that a calculated value of 27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%] is in the range of 28˜50. This expression is a regression expression of tensile strength according to the present invention, which expresses an influential degree of each component to the tensile strength as a coefficient based on experience. When satisfying the above relation, it is possible to easily secure good properties of commercially available steel sheets for deep drawing with tensile strength of 28, 35, 40 and 45 f/ levels.

A process of manufacturing steel sheet for deep drawing according to the present invention will be described hereinafter.

[Manufacturing Process]

First, a steel slab having the composition as described above is reheated to a temperature of 1,100˜1,250° C. If the reheating temperature is less than 1,100° C., it is difficult to perform hot rolling, whereas, if the reheating temperature exceeds 1,250° C., surface defects can be created.

Then, the reheated steel slab is subjected to hot-rolling(comprising rough rolling and finish rolling) and coiling. At this point, when performing the hot rolling, a finish rolling temperature is preferably controlled to be 880° C. or more. The reason is that, if the finish rolling temperature is less than 880° C., mixed grains are created, causing negative properties of products. In addition, according to the present invention, in order to improve an r-value of the products, it is desirable that a ratio of a reduction amount of rough rolling to a reduction amount of finish rolling, that is, a reduction amount ratio is suitably controlled during the hot rolling.

Specifically, the reduction amount ratio is preferably controlled in the range of 1.0˜3.5. The reason is that, if the reduction amount ratio is less than 1.0, the reduction amount of the finish rolling is significantly increased, causing an increase of load while making it difficult to control the fraction of Ti₄C₂S₂among the precipitates to be 50% or more and to control the fraction of TiC to be below 5%. On the other hand, if the reduction amount ratio exceeds 3.5, the effect of improving the r-value is negligible. Controlling of the reduction amount ratio will be described in detail hereinafter.

In the steel of the present invention, Ti, Nb and the like react with impurity solid solution elements, and form precipitates, size and distribution of which significantly influence the formability of the final cold rolled products. In other words, if the precipitates mainly having a size of several hundreds of or more are uniformly distributed instead of ultra fine precipitates having a size of several dozens of or less in a state wherein all the impurity elements such as C, N, S and the like in the hot rolled steel sheet are fixed as the precipitates, the r-value of the cold rolled steel sheet as the final product is remarkably improved.

Meanwhile, since a temperature range of allowing these precipitates to be actively formed in the steel is equal to the temperature range of hot rolling, the size and distribution of the precipitates in the ultra-low carbon steel significantly depend on a hot rolling temperature and the reduction amount. Since formation of the precipitates is promoted via dynamic precipitation during the rolling process, an increase of the reduction amount in the temperature region of most actively enabling the precipitation results in easy formation of the precipitates.

Accordingly, as the reduction amount of the finish rolling is increased, it is advantageous in formation of the precipitates. In this case, since the formation of the precipitates is based on the dynamic precipitation, the precipitates mainly have the size of several hundreds of or more so that an average size of Nb—Ti—Al—N—C based composite precipitates in the steel becomes 40 or more. In addition, the increase in reduction amount of the finish rolling can cause an increase in fraction of Ti₄C₂S₂which is advantageous in terms of the formability and the plating properties, and a decrease in fraction of TiC, which is disadvantageous in terms of the plating properties.

In other words, according to the present invention, the reduction amount ratio is restricted due to the following reasons: increasing the reduction amount of the finish rolling serves not only to allow the precipitates mainly having the size of several hundreds of or more to be distributed in the steel sheet without forming the solid solution elements therein, but also to increase the fraction of the precipitate, which is advantageous in terms of the formability and the plating properties, while decreasing the fraction of the precipitate, which is disadvantageous in terms of the plating properties, thereby improving the r-value and the plating properties of the final product.

Then, the coil hot-rolled steel sheet is subjected to cold rolling and continuous annealing. At this time, a reduction ratio of the cold rolling is preferably restricted to be 65% or more since the reduction ratio below 65% makes it difficult to obtain a high r-value of 1.9 or more. In addition, the continuous annealing is preferably performed at a temperature of 780˜860° C.

The reason is that an annealing temperature less than 780° C. makes it difficult to obtain a high r-value of 1.9 or more, and an annealing temperature above 860° C. provides a high possibility of causing problems to threading of strips during the operation due to high temperature annealing. Since the continuous annealing temperature of the present invention is significantly lower than a temperature region (880˜930° C.) used by the conventional method for manufacturing the steel sheet for deep drawing, it is advantageous in manufacturing cost, and provides excellent producibility.

The cold rolled steel sheet produced as above can be subjected to a typical plating process, if necessary. The plating process may be, for example, galvanizing, galvannealing, and the like.

The present invention will be described in detail with reference to examples. It should be noted that these examples are provided for the illustrative purpose, and thus do not restrict the scope of the invention.

MODE FOR THE INVENTION EXAMPLE 1

After reheating steel slabs having the composition as shown in Table 1 to 1,180° C., the steel slabs were subjected to hot rolling with finish rolling at a temperature of 910° C., and coiling at a temperature of 650° C. The coiled steel sheets were subjected to cold rolling and continuous annealing under conditions shown in Table 2. Then, mechanical properties of the cold rolled steel sheets were evaluated, results of which are shown in Table 2. At this point, the secondary work embrittlement of each steel sheet was evaluated by use of ductile brittle transition temperature (DBTT) obtained in such a way of dropping a plumb bob to a cup formed at a process ratio of 1.9 after laying the cup in a lateral direction. The fatigue properties were evaluated under a condition wherein, when applying a load repetitiously a total of ten million times to point welded samples with a cycle of 60 Hz, the samples did not fail. The powdering resistance was evaluated according to a detached ratio of a plated layer due to cupping, which was calculated in terms of a weight ratio.

TABLE 1 Steel Components No. C Si Mn P S Sol.Al N Ti Nb B Mo Exp. 1 Exp. 2 Exp. 3 Note IS 1 0.0018 0.01 0.08 0.001 0.01 0.12 0.0026 0.009 0.01 0.001 2.52 1.15 29.6 28 IS 2 0.0025 0.01 0.12 0.008 0.007 0.21 0.0051 0.008 0.015 0.0004 1.34 1.32 31 Kgf/mm² IS 3 0.0035 0.01 0.07 0.005 0.006 0.11 0.0032 0.006 0.021 0.0005 1.45 0.98 31.3 level IS 4 0.0012 0.01 0.09 0.005 0.008 0.31 0.0026 0.008 0.014 0.0003 3.27 3.17 30.4 CS 1 0.0016 0.01 0.08 0.006 0.007 0.13 0.0061 0.003 0.018 0.0003 0.62 1.98 31 CS 2 0.002 0.01 0.12 0.009 0.009 0.041 0.0021 0.021 0.031 3.27 2.13 33.1 CS 3 0.0026 0.01 0.08 0.008 0.012 0.038 0.0024 0.02 0.019 2.72 1.04 31.2 IS 5 0.0031 0.01 0.52 0.04 0.007 0.15 0.0018 0.006 0.015 0.0005 0.02 2.98 0.94 36.4 35 IS 6 0.0036 0.01 0.58 0.038 0.01 0.21 0.0031 0.007 0.024 0.0007 0.02 2.29 1.24 37.7 Kgf/mm² IS 7 0.0031 0.01 0.61 0.043 0.011 0.14 0.0017 0.006 0.021 0.0003 0.02 2.84 1.17 37.9 level IS 8 0.0021 0.01 0.54 0.042 0.008 0.25 0.0024 0.008 0.009 0.0011 0.02 3.67 1.32 36 CS 4 0.0035 0.01 0.51 0.044 0.008 0.14 0.0056 0.003 0.012 0.0005 0.02 0.77 0.7 36.3 CS 5 0.0033 0.01 0.48 0.061 0.007 0.03 0.0021 0.045 0.0008 7.17 0.06 35.6 CS 6 0.0031 0.01 0.38 0.058 0.012 0.04 0.0029 0.048 0.0005 5.38 0.08 34.8 IS 9 0.0021 0.01 0.86 0.087 0.009 0.14 0.0026 0.008 0.017 0.0008 0.03 2.42 1.47 42.9 40 IS 10 0.0031 0.01 0.76 0.084 0.007 0.24 0.0021 0.007 0.028 0.0011 0.03 3.97 1.66 43.7 Kgf/mm² IS 11 0.0027 0.01 0.81 0.085 0.012 0.17 0.0028 0.009 0.016 0.0012 0.03 2.79 1.17 42.4 level IS 12 0.0034 0.01 0.84 0.091 0.008 0.34 0.0031 0.006 0.01 0.0007 0.03 2.96 1.02 42.2 CS 7 0.0044 0.01 0.85 0.096 0.008 0.14 0.0072 0.003 0.013 0.0002 0.03 0.52 0.59 43 CS 8 0.0039 0.01 0.8 0.091 0.007 0.04 0.0025 0.052 0.0006 6.78 0.07 39.8 CS 9 0.0032 0.01 0.78 0.094 0.01 0.05 0.0029 0.049 0.0009 5.78 0.1 40 IS 13 0.0017 0.01 0.88 0.11 0.009 0.26 0.0025 0.007 0.032 0.0009 0.03 3.34 3.42 47.1 45 IS 14 0.0021 0.01 1.12 0.091 0.007 0.14 0.0023 0.006 0.028 0.0011 0.03 2.71 2.15 46 Kgf/mm² IS 15 0.0031 0.01 0.83 0.102 0.007 0.17 0.002 0.009 0.033 0.0008 0.03 3.56 1.73 46.2 level IS 16 0.0025 0.01 1.15 0.087 0.012 0.31 0.0026 0.008 0.026 0.0012 0.03 3.88 2.14 45.6 CS 10 0.0064 0.01 1.11 0.089 0.01 0.11 0.0056 0.003 0.026 0.0012 0.03 0.89 0.64 45.6 CS 11 0.0038 0.01 0.83 0.095 0.009 0.04 0.0028 0.043 0.0007 5.17 0.07 40.3 CS 12 0.0033 0.01 0.95 0.105 0.008 0.03 0.0022 0.049 0.0005 7.14 0.06 41.8 IS: Inventive steel, CS: Comparative steel, Exp: Expression

TABLE 2 Cold Average reduction CA size of Steel ratio Temp. TS Elongation r- DBTT FS Powdering precipitates No. (%) (° C.) (kgf/mm²) (%) value (° C.) (kgf) resistance (nm) IS 1 78 835 28.9 49.8 2.32 −70 85 10% 54 IS 2 78 830 30.1 47.9 2.24 −70 80 14% 56 IS 3 78 830 30.4 47.6 2.08 −80 85 6% 49 IS 4 78 835 29.7 50.4 2.19 −80 85 8% 51 CS 1 78 830 31.3 45.8 1.78 −50 75 15% 14 CS 2 78 830 29.9 47.9 2.22 −40 75 12% 12 CS 3 78 830 28.7 48.7 1.98 −50 70 19% 24 IS 5 75 825 35.2 43.2 2.34 −70 130 12% 68 IS 6 75 830 35.9 44.1 2.41 −70 140 6% 62 IS 7 75 815 36.1 45.0 2.28 −70 130 10% 60 IS 8 73 810 36.8 44.3 2.45 −60 140 5% 70 CS 4 75 830 37.2 41.2 1.74 −50 125 15% 11 CS 5 75 830 35.8 45.2 1.89 −50 120 18% 28 CS 6 75 830 35.4 45.3 1.85 −60 120 14% 23 IS 9 75 815 42.3 35.9 2.21 −50 150 8% 55 IS 10 78 830 41.8 36.2 2.18 −50 150 9% 52 IS 11 75 798 41.6 37.0 2.26 −40 160 4% 60 IS 12 78 825 42.1 36.7 2.41 −50 150 3% 69 CS 7 75 830 43.1 34.2 1.67 −40 140 12% 10 CS 8 75 830 41.4 37.2 1.82 −40 140 9% 18 CS 9 73 830 40.9 36.8 1.79 −40 150 19% 20 IS 13 68 793 45.5 33.9 2.18 −40 170 11% 52 IS 14 68 812 46.3 33.2 2.13 −40 160 13% 55 IS 15 70 820 46.6 34.0 2.26 −40 170 6% 63 IS 16 70 828 47.1 33.7 2.34 −40 170 4% 70 CS 10 70 830 47.4 30.1 1.57 −30 160 13% 8 CS 11 68 840 45.2 34.2 1.78 −30 150 12% 21 CS 12 70 830 45.9 33.8 1.75 −40 160 20% 18 IS: Inventive steel, CS: Comparative steel, CA: Continuous annealing, TS: Tensile strength, FS: Fatigue strength, Powdering resistance: Reduction in weight of plated layer

As can be appreciated from Table 2, Inventive steels 1˜16 satisfying the conditions of the present invention exhibit excellent properties in terms of secondary work embrittlement resistance, fatigue properties, and plating properties (powdering resistance) as well as formability.

However, Comparative steels 1˜12 not satisfying the conditions of the present invention in terms of composition and relations between the components exhibit deteriorated properties in terms of secondary work embrittlement resistance, fatigue properties, and plating properties (powdering resistance) as well as formability compared with the inventive steels. In particular, for Comparative steels 1, 4, 7 and 10 satisfying the composition according to the present invention while not satisfying the relations between the components, elongation, r-value, secondary work embrittlement resistance and fatigue properties are lower than the inventive steels.

EXAMPLE 2

After reheating steel slabs having the compositions of Inventive Steels 1 and 5 in Table 1 to 1,180° C., the steel slabs were subjected to hot rolling with finish rolling at a temperature of 910° C., and coiling at a temperature of 650° C. Here, when performing the hot rolling, a ratio of a reduction amount of rough rolling to a reduction amount of finish rolling was based on the condition as shown in Table 3. The coiled steel sheets were subjected to cold rolling and continuous annealing under the conditions (the conditions of Inventive Steels 1 and 5) shown in Table 2.

Then, mechanical properties and distribution of precipitates of samples were evaluated, results of which are shown in Table 3.

TABLE 3 Average size Reduction of Ti₄C₂S₂ Example Steel amount Powdering precipitates Fraction TiC Fraction No. No. ratio r-value resistance (nm) (%) (%) IE 1 IS 1 0.8 1.95 15 16 43 12 IE 2 IS 1 2.1 2.32 10 56 62 1 IE 3 IS 1 3.7 1.82 19 32 38 9 IE 4 IS 5 0.7 1.93 18 28 40 8 IE 5 IS 5 2.2 2.34 12 68 65 2 IE 6 IS 5 3.9 1.92 15 15 42 10 IE: Inventive example, IS: Inventive steel, Powdering Resistance: Reduction in weight of plated layer

As can be appreciated from Table 3, Inventive Examples 2 and 5 produced according to a reduction amount ratio of 1.0˜3.5 exhibit excellent r-values and plating properties compared with Inventive Examples 1, 3, 4 and 6 produced without satisfying the reduction amount ratio of 1.0˜3.5. In addition, it was revealed that these results were obtained due to an increased average size of precipitates, an increased fraction of Ti₄C₂S₂which is advantageous in terms of the formability and the plating properties, and a decreased fraction of TiC, which is disadvantageous in terms of the plating properties.

It should be understood that the embodiments and the accompanying drawings have been described for illustrative purposes, and the present invention is limited only by the following claims. Further, those skilled in the art will appreciate that various modifications, additions and substitutions are allowed without departing from the scope and spirit of the invention according to the accompanying claims.

Claims

1. A steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties, comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.010˜0.040%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities,

wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1, and

wherein the composition of Nb, Al, and C satisfies the relationship: 0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5.

2. The steel sheet according to claim 1, further comprising: 0.05% or less of Mo.

3. The steel sheet according to claim 1, wherein an average size of Nb—Ti—Al—N—C based composite precipitates is 40 nm or more, the composite precipitates comprising Ti4C2S2 in a fraction of 50% or more, and TiC in a fraction less than 5%.

4. The steel sheet according to claim 1, wherein the contents of Mn, P, Nb, Mo and B are controlled to provide a tensile strength in the range of 28˜50 kgf/mm2 calculated according to a tensile strength calculation expression TS: TS=27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%].

5. A method for manufacturing a steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties, comprising:

reheating a steel slab at a temperature of 1,100˜1,250° C., the steel slab comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06˜1.5%, P: 0.15% or less, S: 0.020% or less, Sol. Al: 0.10˜0.40%, N: 0.010% or less, Ti: 0.003˜0.010%, Nb: 0.003˜0.040%, B: 0.0002˜0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0≦(Ti[%]+Al[%]/16+6B[%])/3.43N[%]≦4.1, and wherein the composition of Nb, Al, and C satisfies the relationship: 0.7≦(Nb[%]+Al[%]/20)/7.75C[%]≦3.5;

rough rolling the reheated steel slab;

finish rolling the rough rolled steel slab at a finish rolling temperature of 880° C. or more, followed by coiling the hot rolled steel sheet;

cold rolling the coiled steel sheet at a reduction ration of 65% or more; and continuously annealing the cold rolled steel sheet at a temperature of 780˜860° C.

6. The method according to claim 5, wherein the steel slab further comprises 0.05% or less of Mo.

7. The method according to claim 5, wherein an average size of Nb—Ti—Al—N—C based composite precipitates is 40 nm or more, the composite precipitates comprising Ti4C2S2 in a fraction of 50% or more, and TiC in a fraction less than 5%.

8. The method according to claim 5, wherein the contents of Mn, P, Nb, Mo and B are controlled to provide a tensile strength in the range of 28˜50 kgf/mm2 calculated according to a tensile strength calculation expression TS: TS=27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%].