Patent Publication Number: US-2011073223-A1

Title: Steel sheet for galvanizing with excellent workability, and method for manufacturing the same

Description:
TECHNICAL FIELD 
     The present invention generally relates to a high strength ductile steel sheet for extra deep drawing for interior or exterior plates of automobile bodies and the like, and to a method for manufacturing the same. More particularly, the present invention relates to a steel sheet for galvanizing with excellent workability, which has a tensile strength of 28˜50 kgf/mm 2 , and has excellent properties in terms of formability, secondary work embrittlement resistance, fatigue properties of welded joints and surface quality, 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 Si, Mn, Ti, Al and the like to highly pure steel which is minimized in contents of impurities in the steel. 
     After cold rolling a hot rolled steel sheet, annealing is performed at a temperature of 700° C. or more for recrystallization of work hardened structures. However, since most of the added elements described as above have a higher affinity to oxygen than Fe, the added elements are grown to surface agglomerates, such as MnO, SiO 2 , Al 2 O 3 , TiO and the like, in singular or composite forms during the cold annealing process. 
     As an amount of the surface agglomerates increases, wettability of plating bath is deteriorated, and an alloying reaction in hot plating is further interfered, thereby causing surface defects such as non-plated regions and the like. 
     In addition, coarsening of the surface agglomerates causes fine dents on the surface of the plated steel sheet via attachment of the coarsened surface agglomerates to a hearth roll of a continuous furnace, thereby providing a significantly adverse influence on surface quality of the steel sheet. 
     In order to solve problems regarding plating defects described as above, JP2002-146477, JP2001-64750 and JP2002-155317 disclose techniques aiming to enhance plating properties by adding specific elements such as Cr, Sb and the like to a steel sheet, and JP2001-288550 discloses a technique aiming to suppress formation of the surface agglomerates during cold annealing through pre-oxidation of hot coil before cold rolling. 
     However, since these techniques do not ensure the effect through addition of the specific elements and cannot be realized by currently available equipment for hot rolling, cold rolling and continuous annealing, these techniques are not commercially applied in practice. 
     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 steel sheet for galvanizing with excellent workability, which has a tensile strength of 28˜50 kgf/mm 2 , and has excellent properties in terms of formability, secondary work embrittlement resistance, fatigue properties of welded joints and surface quality. 
     It is another object of the present invention to provide a method for manufacturing the steel sheet for galvanizing with excellent workability. 
     Technical Solution 
     In accordance with one aspect of the invention, the above and other objects can be accomplished by the provision of a steel sheet for galvanizing with excellent workability, comprising, by weight %: C, 0.010% or less, Si: 0.1% 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%, Mo: 0.05% or less, one or both of Sb: 0.005˜0.05% and Sn: 0.005˜0.05%, a total amount of Sb and Sn being in the range of 0.005˜0.1% when both of Sb and Sn are added to the steel sheet, and the balance of Fe and other unavoidable impurities, wherein the steel sheet has surface agglomerates having an average diameter of 1 μm or less, and a tensile strength of 28˜50 kgf/mm 2 . 
     Preferably, the average diameter of the surface agglomerates is suppressed in growth by 10% or less for an increase in annealing temperature of 10° C. 
     In accordance with another aspect of the invention, a method for manufacturing a steel sheet for galvanizing with excellent workability comprises: reheating a steel slab comprising by weight %: C, 0.010% or less, Si: 0.1% 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%, Mo: 0.05% or less, one or both of Sb: 0.005˜0.05% and Sn: 0.005˜0.05%, a total amount of Sb and Sn being in the range of 0.005˜0.1% when both of Sb and Sn are added to the steel sheet, and the balance of Fe and other unavoidable impurities; hot rolling the reheated steel slab with finish rolling at a finish rolling temperature of a single phase austenite region to form a steel sheet, followed by coiling the hot rolled steel sheet; cold rolling the hot rolled steel sheet; and continuously annealing the cold rolled steel sheet at a temperature of 700° C. or more. 
     ADVANTAGEOUS EFFECTS 
     As apparent from the above description, the present invention provides a steel sheet for galvanizing with excellent workability, which has a tensile strength of 28˜50 kgf/mm 2 , and excellent properties in terms of formability, secondary work embrittlement resistance, fatigue properties of welded joints and surface quality. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing and other objects and features of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a graph depicting change in amount of surface agglomerates according to kinds of steel and annealing temperature; 
         FIG. 2  is micrographs showing distribution of surface agglomerates on surfaces of air cooled steel sheets and water cooled steel sheets according to kinds of steel and after hot coiling; and 
         FIG. 3  is micrographs showing distribution of surface agglomerates on steel sheets according to kinds of steel and annealing temperatures. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments will be described in detail with reference to the drawings hereinafter. 
     The steel sheet according to the present invention will be described in terms of composition. 
     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. An excessive content of carbon requires an increase in contents of Ti and Nb which are carbide and nitride formation elements, causing a disadvantage of increased manufacturing costs. Thus, the carbon content is preferably 0.010% or less. 
     Si is an element which causes temper color upon annealing, and non-plated regions upon plating as well as surface scale. Thus, the silicon content is preferably 0.1% or less. 
     Mn is added as a substitutional solid solution strengthening element for ensuring strength of steel. However, if a Mn content exceeds 1.5%, an r-value of the steel is rapidly deteriorated along with elongation, and if the Mn content is less than 0.06%, the steel suffers from embrittlement due to S in the steel. Thus, the Mn content is preferably in the range of 0.06˜1.5%. 
     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 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 0.03˜0.15%. 
     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 making it possible to avoid deterioration in formability due to solid solution of S. 
     Thus, according to the present invention, S content is preferably 0.020% or less, which can deviate from a region causing edge cracks during rolling. 
     For cold-rolled steel products, a 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 at a sufficiently low amount in consideration of manufacturing costs. 
     According to the present invention, however, Sol. Al serves to allow deep drawability to be stably secured at a lower annealing temperature. 
     In other words, 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. If the Sol. Al content exceeds 0.40%, it causes an increase of the manufacturing costs, and deterioration in efficiency of continuous casting operation. Thus, the Sol. Al content is preferably in the range of 0.10˜0.40%. 
     Furthermore, 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 to coarsen the precipitates, it serves as a critical component which provides further enhanced workability of the steel with small added amounts of Ti and Nb in comparison to the conventional IF steel. 
     N deteriorates the workability of the steel if it exists in a solid solution state in the steel. Furthermore, if an excessive content of N resides in the steel, it is necessary to increase added amounts of Ti and Nb for fixing N as precipitates in the steel. Thus, the N content is preferably 0.010% or less. 
     B is a grain boundary strengthening element, and effective to enhance fatigue properties of spot welded joints while preventing grain boundary embrittlement by P. In order to obtain the effect by addition of B, it is necessary to have B content of 0.0002% or more. However, if the B content exceeds 0.0020%, the steel suffers from rapid deterioration of the workability and surface properties of the plated steel sheet. Thus, the B content is preferably 0.0002˜0.0020%. 
     Mo is added to enhance the secondary work embrittlement resistance and the plating properties. If Mo content exceeds 0.05%, the effect of enhancing the secondary work embrittlement resistance and the plating properties is significantly reduced, and it is disadvantageous in terms of the manufacturing costs. Thus, the Mo content is preferably 0.05% or less. 
     Ti and Nb are very important elements in view of the workability. Ti and Nb must be added to the steel in an amount of 0.003˜0.010%, and an amount of 0.003˜0.040%, respectively, in consideration of a minimum and optimum amount for securing the workability (in particular, r-value) enhancing effect. 
     According to the present invention, Sb is a very important element, and exhibits remarkable effects to suppress generation of surface agglomerates such as MnO, SiO 2 , Al 2 O 3 , and the like, and to suppress coarsening of the surface agglomerates by a temperature increase and other variations in hot processing. In order to obtain these effects, the Sb content must be 0.005% or more. However, if Sb is added exceeding a specific content, the above effects cannot be obtained, and thus the Sb content has an upper limit of 0.05%. 
     Sn exhibits similar effects to those of Sb in the steel. In order to obtain these effects, the Sn content must be 0.005% or more, but if Sn is added above a specific content, the above effects cannot be obtained, and thus the Sb content has an upper limit of 0.05%. 
     When adding Sb and Sn to the steel at the same time, a total content of Sb and Sn is preferably in the range of 0.005˜0.1%. 
     A process of manufacturing steel sheet according to the present invention will be described hereinafter. 
     After reheating a steel slab having the composition as described above, the steel slab is subjected to hot rolling with finish rolling at a finish hot rolling temperature of a single phase austenite region, coiling, and cold rolling, thereby providing a cold rolled steel sheet. Then, continuous annealing is performed at a temperature of 700° C. or more. 
     Generally, an increase in temperature results in increases in ductility and r-value of the steel. 
     However, as an annealing temperature increases, the surface agglomerates (singular or composite oxides of Si, Al, Mn and the like) on the steel sheet increase in amount, exhibiting a remarkable tendency of coarsening. As a result, it is likely to generate surface defects such as non-plate regions or surface dents. 
     According to the present invention, growth of the surface agglomerates can be suppressed in the annealing temperature region, thereby ensuring excellent surface quality of the steel sheet. 
     Preferably, the reheating of the steel slab is performed at a temperature in the range of 1,100˜1,300° C., the finish rolling is performed at a temperature in the range of 830˜920° C., and the coiling is performed at a temperature in the range of 500˜700° C. 
     According to the present invention, the hot rolled steel sheet may be subjected to air cooling or water cooling after hot coiling. 
     In addition, a reduction ratio is preferably 65% or more upon the cold rolling in order to obtain a high r-value of 1.9 or more. 
     If the cold rolled steel sheet is annealed at a significantly low temperature, it is difficult to obtain the high r-value of 1.9 or more. On the contrary, if the cold rolled steel sheet is annealed at an excessively high temperature, there is possibility of generating a problem in passing properties of the steel strip due to hot annealing. Thus, the annealing temperature is 700° C. or more, and preferably in the range of 780˜860° C. 
     Preferably, growth of an average diameter of granular surface agglomerates is suppressed by 10% or less for an increase in annealing temperature of 10° C. in the annealing temperature range described as above. 
     In a typical method of manufacturing a steel sheet for extra deep drawing, continuous annealing of the cold rolled steel sheet is performed at a temperature in the range of 880˜930° C. 
     As such, since the continuous annealing temperature of the present invention is lower than the annealing temperature applied to manufacturing of the typical extra deep drawing, the present invention is more economic and has superior operability. 
     According to the invention, it is possible to produce a steel sheet for galvanizing with excellent workability, which has surface agglomerates having an average diameter of 1 μm or less, and a tensile strength of 28˜50 kgf/mm 2 . 
     The surface agglomerates mainly comprise singular or composite oxides of Si, Al, Mn, Ti and the like. 
     The surface agglomerates are preferably suppressed in growth of the average diameter by 10% or less for an increase in annealing temperature of 10° C. 
     MODE FOR THE INVENTION 
     The present invention will be described in detail by reference to examples. 
     Example 
     After reheating steel slabs having the composition as shown in Table 1 to 1,200° C., the steel slabs were subjected to hot rolling with finish rolling at a finish hot rolling temperature of 890° C. and air cooling. Then, the hot rolled steel sheets were subjected to cold rolling at a reduction ratio of 80%, thereby producing cold rolled steel sheets. 
     In addition, for the steel sheets of Steel Kinds 4 and 5, water cooling and cold rolling at the reduction ratio of 80% were performed after coiling. 
     For the steel sheets of Steel Kinds 1˜3 among the steel sheets produced as above, annealing was performed for 86 seconds at a temperature in the range of 780˜830° C. in N 2 -10% H 2  atmosphere. Then, amounts of Mn and Al formed as surface agglomerates on the steel sheets were measured, and results of the measurements are shown in  FIG. 1 . 
     For the steel sheets of Steel Kinds 4 and 5 (subjected to air cooling and water cooling, respectively), annealing was performed for 86 seconds at a temperature in the range of 860° C. in N 2 -10% H 2  atmosphere. Then, shapes of surface agglomerates formed on the steel sheets were investigated, and results of the measurements are shown in  FIG. 1 . 
     For the steel sheets of Steel Kinds 4 and 5 subjected to air cooling after coiling, annealing was performed for 86 seconds at a temperature in the range of 800˜850° C. in N 2 -10% H 2  atmosphere. Then, shapes of surface agglomerates formed on the surface of the steel samples were investigated, and results of the measurements are shown in  FIG. 3 . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Steel 
                 Composition(wt %) 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Kind 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Mo 
                 S—Al 
                 Nb 
                 Ti 
                 B 
                 Sn 
                 Sb 
                 N 
                 Remark 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0.0038 
                 0.01 
                 0.81 
                 0.08 
                 0.01 
                 0.01 
                 0.1 
                 0.005 
                 0.04 
                 0.0002 
                 — 
                 — 
                 0.0019 
                 CS 
               
               
                 2 
                 0.0037 
                 0.01 
                 0.82 
                 0.08 
                 0.01 
                 0.01 
                 0.1 
                 0.005 
                 0.04 
                 0.0002 
                 0.05 
                 — 
                 0.0022 
                 IS 
               
               
                 3 
                 0.0041 
                 0.01 
                 0.81 
                 0.08 
                 0.01 
                 0.01 
                 0.1 
                 0.005 
                 0.04 
                 0.0002 
                 — 
                 0.05 
                 0.0022 
                 IS 
               
               
                 4 
                 0.0019 
                 0.06 
                 0.87 
                 0.08 
                 0.01 
                 0.09 
                 0.115 
                 0.007 
                 0.021 
                 0.0005 
                 — 
                 — 
                 0.0040 
                 CS 
               
               
                 5 
                 0.0023 
                 0.06 
                 0.87 
                 0.08 
                 0.01 
                 0.09 
                 0.15 
                 0.007 
                 0.021 
                 0.0006 
                 — 
                 0.01 
                 0.005 
                 IS 
               
               
                   
               
               
                 IS: Inventive steel, CS: Comparative steel 
               
            
           
         
       
     
     As can be understood from  FIG. 1 , Steel Kinds 2 and 3 satisfying the conditions of the present invention have a lower amount of surface agglomerates of Mn and Al than that of Steel Kind 1 which does not satisfy the conditions of the present invention. 
     In addition, as can be understood from  FIG. 2 , the Steel Kind 5 satisfying the conditions of the present invention has a lower amount and a smaller size of surface agglomerates than those of the Steel Kind 5 which does not satisfy the conditions of the present invention. 
     Particularly, the steel sheet of the Steel Kind 4 subjected to air cooling after hot coiling has surface agglomerates remarkably increased in size compared with the steel sheet subjected to water-cooling after coiling. On the other hand, the steel sheet of the Steel Kind 5 subjected to water-cooling after hot coiling has surface agglomerates which have substantially the same size as that of surface agglomerates on the steel sheet subjected to air cooling after coiling. 
     In addition, as can be understood from  FIG. 3 , for the Steel Kind 4, the surface agglomerates is increased in size according to an increase in annealing temperature. For the Steel Kind 5, the size of the surface agglomerates is not substantially changed irrespective of the increase in annealing temperature. 
     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.