Patent Publication Number: US-2009223603-A1

Title: Method for manufacturing ferritic stainless steel slabs with equiaxed grain structures and the ferritic stainless steel manufactured by it

Description:
BACKGROUND ART 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures and the ferritic stainless steel manufactured by it, and more specifically to a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures and the ferritic stainless steel manufactured by it which control concentration of alumina inclusions in molten steel to maximize an available TiN generation effect serving as a non-uniform nucleating site of ferrite when solidifying it, thereby improving equiaxed crystal ratio. 
     2. Description of Related Art 
     In general, there is a case that Ti of 0.2-0.5% is added to a ferritic stainless steel with Cr concentration of 10-30% in order to improve its corrosion-resistance. In this case, TiN is formed by the following equation from a steel making process to a continuous casting process, wherein when its size and distribution are proper, it serves as a non-uniform nucleating site of ferrite in solidifying molten steel, thereby obtaining equiaxed grain structures. 
       Ti+N=TiN  [Equation 1] 
     In order to manufacture equiaxed grain structures by controlling TiN generating process from a steel making process to a continuous casting process as described above, there has been reported techniques as follows. 
     U.S. Pat. No. 5,868,875 has described that Ti concentration satisfies (% Ti/48)/[% C/120+(% N/14)]&gt;1.5, in making Ti deoxidation for molten steel with Cr: 8˜25%, Mn: 0.1˜1.5% Mn, Si: 1.5% or less, N: 0.05% or less, C: 0.08% or less, and Al: 0.01% or less. 
     Also, European Patent No. 924313 has described that equiaxed crystal ratio in slab of 50% or more can be assured when satisfying [% Ti]×% N]≧0.14×[% Al] among the concentrations of Ti, Al and N. 
     However, in making Ti deoxidation the stainless molten steel as in the above patents, if oxygen concentration in the molten steel is high, most of the injected Ti can be oxidized in the form of Ti oxide. This can serve as a factor impeding TiN generation according to the above equation 1. 
     Also, since the generation of excessive TiN oxide can arise the clogging of a submerged entry nozzle in a continuous casting process and the defect of steel making ability in a slab surface, simply the application of Ti deoxidation has any limitation. 
     European Patent No. 1491646 has described that slabs with equiaxed grain structures can be manufactured by adding Mg of 2˜50 ppm to the molten steel consisting of Cr: 10˜20%, C: 0.001˜0.01%, Si: 0.01˜0.3%, Mn: 0.01˜0.3%, N: 0.001˜0.02%, and Ti: 0.05˜0.3%. 
     In other words, it has been proposed that if the composition and distribution of the inclusions are controlled to satisfy 17.4 (Al 2 O 3 )+3.9 (MgO)+0.3 (MgAl 2 O 3 )+18.7 (CaO)≦500 and (Al 2 O 3 )+(MgO)+(MgAl 2 O 4 )+(CaO)≧95 (each component is mol % base), these inclusions will serve as a non-uniform nucleating site of ferrite when solidifying the molten steel. 
     However, in this case, when it is impossible to simultaneously control the concentration of each oxide, that is, even when the concentration of a specified component such as Al 2 O 3  is very high or the concentration of MgAl 2 O 4  is very low, the above equation can be satisfied, however, the equiaxed grain structures cannot easily be obtained. This is because if the concentration of Al 2 O 3  in the inclusions is excessively high, the difference of lattice mismatched degree between Al 2 O 3 /TiN or Al 2 O 3 /ferrite becomes very large, that is, interface energy becomes large so that these inclusions have a difficulty to serve as the non-uniform nucleating site of ferrite when solidifying the molten steel. 
     Japanese Patent No. 2002-030324 has described that equiaxed crystal ration in slab of 70% or more can be obtained by controlling basicity of CaO—SiO 2 -based slag at 1.2˜2.4, wherein the slag keeps chemical equilibrium with molten steel with Cr: 10˜30%, Si: 0.2˜3.0%, and Ti: 0.05˜0.3%, and controlling to be [% Al]/[% Ti]=0.01˜0.1 in the molten steel, that is, [% Ti]/[% Al]=10˜100. 
     However, when operating with the compositions of high Ti and low Al within the range of the concentration, there is risk of generating a large amount of Ti oxide. This may be a factor causing a nozzle clogging or a defect of a slab surface during a casting process. 
     Japanese Patent No. 2000-1602999 has described that when making vacuum oxygen decarburization refining, equiaxed crystal ratio in slab of 60% or more can be obtained by injecting CaO and Al to make basicity of CaO—Al 2 O 3 -based slag to be the range of 0.7˜2.5 and making molten steel stirring for 5 minutes or more, and adding Ti to allow only TiN nitride to have area ratio of 0.01% or more. 
     However, it is not easy to properly form TiN independently from oxidation-inclusions in the stainless molten steel over which the inclusions in the form of oxide is distributed. This serves the oxidation-inclusions as the non-uniform nucleating site when forming the TiN and conforms to the fact that oxide-Tin composite inclusions, wherein the TiN is crystallized using the oxidation-inclusions as a nucleus when observing the inside of the slabs with substantially equiaxed grain structures, are distributed in large amount. 
     Japanese Patent No. 2004-043838 has described that when refining molten steel with Cr: 9˜30%, equiaxed crystal ratio can be improved by allowing [% Ti]*[% N] to be equal to the range of 0.007˜0.004 in the molten steel and casting the molten steel after injecting proper deoxidizer in it so that the measurement of oxygen activity in the molten steel using an oxygen sensor is log a 0 =−5˜−3. 
     However, it is judged that whenever stainless molten steel is refined, it takes relatively much time to measure the oxygen activity in the molten steel using the oxygen sensor, its accuracy is slightly degraded, and a concrete kind of the deoxidizer is not specified so that it becomes slightly ambiguous in applying it to an actual operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is proposed to solve the problems in a prior art as described above. It is an object of the present invention to provide a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures and the ferritic stainless steel manufactured by it, which control concentration of the alumina inclusions in molten steel using composite deoxidation of Si/M/AlTi in vacuum oxygen decarburization refining process to maximize an available TiN generation effect serving as a non-uniform nucleating site of ferrite when solidifying it, thereby manufacturing ferritic stainless steel slab with high equiaxed crystal ratio and excellent formability, that is, low ridging defect. 
     ADVANTAGEOUS EFFECTS 
     In contrast to a method for manufacturing steel slabs with equiaxed grain structures depending on techniques such as a casting temperature control and an electromagnetic mixing power control and the like made in the prior art, a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures and the ferritic stainless steel manufactured by it according to the present invention as described above precisely controls components of molten steel, such as Si and Mn, etc., and concentration of alumina inclusions to effectively generate available TiN so that it can manufacture ferritic stainless steel slabs with equiaxed grain structures and high equiaxed crystal ratio while improving operating stability, thereby obtaining ferritic stainless steel slabs with equiaxed grain structures having excellent formability, that is, low ridging defect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart illustrating a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures according to a preferred embodiment of the present invention; 
         FIG. 2  is a graph illustrating a change in equiaxed crystal ratio in slabs according to the control of alumina inclusion concentration; 
         FIG. 3  is a graph illustrating the result of  FIG. 2  as distributed data; 
         FIG. 4  is a graph illustrating the decrease of Al 2 O 3  and TiOx inclusions by a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures according to a preferred embodiment of the present invention, compared to the conventional example; and 
         FIG. 5  is an electronmicroscope photograph illustrating the form of oxide-TiN composite inclusions distributed inside slabs with equiaxed grain structures by a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures according to a preferred embodiment of the present invention, compared to the conventional example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to accomplish the object, there is provided a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures comprising the steps of: performing oxygen decarburization reaction by blowing oxygen from the upper part of the molten steel in a vacuum oxygen decarburization ladle; injecting Al in the molten steel to which the oxygen decarburization reaction is made for Cr 2 O 3  reduction; making composite deoxidation by injecting deoxidizer in the molten steel into which the Al is injected for the Cr 2 O 3  reduction; making alloying process by injecting alloying metal in the molten steel; first judging for judging whether Al concentration is in the range of a setting value by analyzing the Al concentration in the molten steel; if the Al concentration satisfies the setting value, stirring it using inert gas and second judging for judging whether alumina inclusion concentration in the final molten steel corresponds to a target value; and if the alumina inclusion concentration satisfies the target value, continuously casting the molten steel. 
     Here, in the composite deoxidating step, the deoxidizer may be Si and Mn and in the allyoing step, the alloying metal may be Ti with 0.2-0.4% by mass. 
     Also, in the continuous casting step, the alumina inclusion concentration in the molten steel satisfies the following condition. 
       [Al] alumina &lt;70 ppm 
     (where, [Al] alumina =[Al] total −[Al] dissolved ) 
     In the continuous casting step, it is preferable that the component of the molten steel satisfies the following condition. 
       [Si]+[Mn]=0.5˜1.0%, 
     however, % is % by mass. 
     Further, it is preferable that the final composition of a refined slag in the vacuum oxygen decarburization refining ladle satisfies the following condition. 
       1.1≦(% CaO)/(% Al 2 O 3 )≦1.4 
       4≦(% TiO 2 )/(% SiO 2 )≦6 
     however, % is % by mass. 
     Also, the molten steel is 80˜85 ton. 
     Preferably, the setting value of Al concentration in the first judging step is 0.05˜0.12% by mass, wherein if the Al concentration is less than 0.05% by mass, it further comprises the step of additionally injecting Al of 30˜40 kg relative to the molten steel of 80˜85 ton and if the Al concentration is 0.12% by mass or more, it further comprises the step of additionally injecting quicklime of 250˜300 kg relative to the molten steel of 80˜85 ton. 
     Further, the target value of the alumina inclusion concentration in the second judging is 70 ppm or less. 
     The ferritic stainless steel slabs with equiaxed grain structure of the present invention are manufactured according to a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures of the present invention, wherein it is characterized in that the alumina inclusion concentration is 70 ppm or less and equiaxed crystal ratio in slabs is 40% or more. 
     Hereinafter, preferred embodiments of the present invention will be described in a more detailed manner with reference to the accompanying drawings. 
       FIG. 1  a flow chart illustrating a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures according to a preferred embodiment of the present invention. 
     A method for manufacturing ferritic stainless steel slabs with equiaxed grain structures according to a preferred embodiment of the present invention comprises the steps of: performing (S 10 ) oxygen decarburization reaction by blowing oxygen from the upper part of the molten steel in a vacuum oxygen decarburization ladle; injecting Al in the molten steel to which the oxygen decarburization reaction is made for Cr 2 O 3  reduction (S 20 ); making composite deoxidating (S 30 ) by injecting deoxidizer in the molten steel into which the Al is injected for the Cr 2 O 3  reduction; making alloying process (S 40 ) by injecting alloying metal in the molten steel; first judging (S 50 ) for judging whether Al concentration is in the range of a setting value by analyzing the Al concentration in the molten steel; if the Al concentration satisfies the setting value, stirring (S 60 ) it using inert gas and second judging (S 70 ) for judging whether the alumina inclusion concentration in the final molten steel corresponds to a target value; and if the alumina inclusion concentration satisfies the target value, continuously casting (S 80 ) the molten steel. 
     Here, the setting value of the concentration of Al in the first judging step (S 50 ) is 0.05˜0.12% by mass (S 51 ), wherein it is preferable that if the Al concentration is less than 0.05% by mass (S 52 ), it further comprises the step of injecting (S 54 ) Al and if the Al concentration is 0.12% by mass or more, injecting quicklime (S 56 ). 
     Also, in the composite deoxidating step (S 30 ), the deoxidizer is Si and Mn and in the allyoing step (S 40 ), the alloying metal is Ti. 
       FIG. 2  is a graph illustrating a change in equiaxed crystal ratio in slabs according to the control of the alumina inclusion concentration and  FIG. 3  is a graph illustrating the result of  FIG. 2  as distributed data. 
     Referring to  FIGS. 2 and 3 , it may be recognized that the equiaxed crystal ration in slabs is increased as the alumina inclusion concentration in the molten steel is decreased. Here, a proper concentration of the alumina inclusions can be set to be less than 70 ppm. When satisfying the condition, the slabs with equiaxed crystal ratio of 40˜100% can be obtained. At this time, if the alumina inclusion concentration exceeds 70 ppm, the creation of available TiN is suppressed so that it is impossible to assure a targeted equiaxed crystal ratio. 
     In order to control the components of the molten steel as above, Cr 2 O 3  in the slag generated from the oxidizer is decreased by completing oxygen decarburization reaction by blowing oxygen from the upper part of the molten steel of 80˜85 ton in a vacuum oxygen decarburization (VOD) ladle and then injecting Al. 
     The step of decreasing Cr 2 O 3  by injecting Al controls a targeted composition by making composite deoxidation by injecting Si and Mn and then injecting Ti. 
     After elapsing about 5 minutes based on a Ti injection, the Al concentration is primarily analyzed. If the primary Al concentration is too low, Al is further injected and if too high, quicklime is further injected. Subsequently, they are stirred using inert gas in the bottom of the ladle so that the alumina inclusion concentration in the final molten steel is controlled to conform to the targeted range to make a continuous casting. 
     In the vacuum oxygen decarburization process, there is oxygen of concentration corresponding to [Cr]/(Cr 2 O 3 ) equilibrium as the following equation 2 in the molten steel after making oxygen blowing for the oxygen decarburization. 
       2[Cr]+3[0]=(Cr 2 O 3 )  [Equation 2] 
     According to the document previously known, it is known that the oxygen concentration in the molten steel of about 20% [Cr] in equilibrium with Cr 2 O 3  at 1650? is 0.06˜0.07% level. As a result, it derives the reaction such as the following equations 3 and 4 by adding Al as effective deoxidation element for the deoxidation in the molten steel and the Cr 2 O 3  reduction in the slag. 
       2[Al]+3[0]=(Al 2 O 3 )  [Equation 3] 
       (Cr 2 O 3 )+2[Al]=(Al 2 O 3 )+2[Cr]  [Equation 4] 
     However, in case of making deoxidation according to the equations 3 and 4 as above, there is a large amount of Al 2 O 3  inclusions in the molten steel. At this time, the alumina inclusions formed are aggregated and grown so that they is floated or removed, however, in case that their size is fine on the order of several μm, they would be remained inside of the molten steel until casting. 
     Also, equiaxed crystal ratio and existence and nonexistence of defect are indicated in the following table 1 by comparing [% Al] alumina  and [% Si]+[Mn] of the prior art with the case of the present invention. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Equiaxed crystal 
               
               
                   
                 [% Al] alumnia   
                 [% Si] + [% Mn] 
                 ratio in slab(%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Conventional 
                 0.0275 
                 0.40 
                 26 
               
               
                 example 1 
               
               
                 Conventional 
                 0.0195 
                 0.25 
                 29 
               
               
                 example 2 
               
               
                 Conventional 
                 0.0124 
                 0.24 
                 23 
               
               
                 example 3 
               
               
                 Conventional 
                 0.0110 
                 0.36 
                 25 
               
               
                 example 4 
               
               
                 Conventional 
                 0.0105 
                 0.42 
                 35 
               
               
                 example 5 
               
               
                 Conventional 
                 0.0080 
                 0.13 
                 23 
               
               
                 example 6 
               
               
                 Conventional 
                 0.0076 
                 0.23 
                 27 
               
               
                 example 7 
               
               
                 Conventional 
                 0.0083 
                 0.42 
                 26 
               
               
                 example 8 
               
               
                 Conventional 
                 0.0120 
                 0.44 
                 25 
               
               
                 example 9 
               
               
                 Conventional 
                 0.0085 
                 0.42 
                 32 
               
               
                 example 10 
               
               
                 Example 1 of the 
                 0.0020 
                 0.89 
                 100 
               
               
                 invention 
               
               
                 Example 2 of the 
                 0.0018 
                 0.67 
                 90 
               
               
                 invention 
               
               
                 Example 3 of the 
                 0.0025 
                 0.78 
                 100 
               
               
                 invention 
               
               
                 Example 4 of the 
                 0.0022 
                 0.57 
                 81 
               
               
                 invention 
               
               
                 Example 5 of the 
                 0.0032 
                 0.96 
                 78 
               
               
                 invention 
               
               
                 Example 6 of the 
                 0.0026 
                 0.88 
                 75 
               
               
                 invention 
               
               
                 Example 7 of the 
                 0.0034 
                 0.66 
                 70 
               
               
                 invention 
               
               
                 Example 8 of the 
                 0.0052 
                 0.79 
                 61 
               
               
                 invention 
               
               
                 Example 9 of the 
                 0.0065 
                 0.69 
                 52 
               
               
                 invention 
               
               
                 Example 10 of the 
                 0.0062 
                 0.75 
                 43 
               
               
                 invention 
               
               
                   
               
            
           
         
       
     
     As indicated in the table 1, the present invention can obtain higher equiaxed crystal ratio of 40% or more, compared with the example of the prior art. 
     Meanwhile, TiN is formed in the molten steel by the reaction of the equation 1; however, a point in time forming it varies depending on the composition and temperature of the molten steel. If TiN is formed in the ladle or a tundish prior to solidifying the molten steel, TiN may be generated through a uniform nucleating and growing by the reaction of Ti atom with N atom, however, it is advantageous that the nucleating is made at the third interface in terms of thermodynamics, for example, at the interface of oxidation-inclusions/molten steel, etc. At this time, as the oxidation-inclusions providing the non-uniform nucleating site of TiN, there are Al 2 O 3 , MgO, TiO x , MgO—Al 2 O 3 , CaO—TiO x , MgO—Al 2 O 3 —TiO x , etc., in some cases. As an indirect index indicating the easy of the non-uniform nucleating of TiN at the surface of these inclusions, there is a lattice mismatched degree defined as the following equation 5. 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                        
                       
                         
                           I 
                           oxide 
                         
                         - 
                         
                           I 
                           TiN 
                         
                       
                        
                     
                     
                       0.5 
                       × 
                       
                         ( 
                         
                           
                             I 
                             oxide 
                           
                           + 
                           
                             I 
                             TiN 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     where l oxide  and l TiN , respectively, means lattice constants of the crystals of the oxidation-inclusions and TiN. It means that it is difficult to serve as the non-uniform nucleating site as δ between two substances becomes large. 
     For example, δ Al203-TiN  between Al 2 O 3  of a hexagonal structure and TiN of a face-centered cubic (FCC) structure is approximately 0.1, whereas δ MgO(or spinel)-TiN  between MgO or MgAl 2 O 3  spinel of the same face-centered cubic and the TiN is approximately 0.0002. As a result, it may be expected that the MgO-based inclusions will easily serves as the non-uniform nucleating site, rather than the Al 2 O 3 -based inclusions. 
     Therefore, in case that there is a large amount of the alumina inclusions in the molten steel, the TiN nucleating is degraded so that upon solidifying, it may be a factor to cause the non-uniform nucleating of ferrite to degrade. 
     Accordingly, in order to manufacture the slabs with equiaxed grain structures, it is necessary to decrease alumina inclusions in the molten steel. At the same time, in order to decrease TiO x -based inclusions, it is necessary to obtain a composite deoxidation effect by injecting Si/Mn. 
       FIG. 4  is a graph illustrating the decrease of Al 2 O 3  and TiOx inclusions by a method for manufacturing ferritic stainless steel slabs with equiaxed grain structures according to a preferred embodiment of the present invention, compared to the conventional example. 
     Referring to  FIG. 4 , when [Si]+[Mn] is 0.1˜0.4% level in an existing molten steel and [Al] is constant at 0.03%, critical [Ti] in which only Ti 3 O 5  inclusions are not formed is approximately 0.45%, whereas [Si]+[Mn] is 0.6˜0.94% and [Al] is 0.03%, it may be appreciated that the generation of Ti 3 O 5  can be suppressed up to about 0.5% [Ti]. Also, when [Si]+[Mn] concentration is low and [Ti] is constant at 0.4%, critical [Al] in which only Ti 3 O 5  inclusions are not formed is approximately 0.027% level, whereas when [Si]+[Mn] concentration is high and [Ti] is 0.4%, it may be appreciated that the Ti 3 O 5  inclusions cannot be formed up to [Al]=0.024% level. 
     As described above, in order to inject [Si]+[Mn] in the molten steel to be conformed to the range of targeted concentration % and in particular, control the alumina inclusion concentration to 70 ppm or less, the [Al] concentration is primarily analyzed in the molten steel at a point in time elapsing about 5 minutes after the injection. 
     Since if the [Al] concentration analyzed is less than 0.05%, a sufficient Cr 2 O 3  reduction is not made and if 0.12% or more, the concentration of the final alumina inclusions exceeds 70 ppm, if the Al concentration analyzed is less than 0.05% and 0.12% or more, Al of 30˜40 kg or quicklime of 250˜300 kg is further injected. Subsequently, they are stirred using inert gas in the bottom of the ladle so that the alumina inclusion concentration in the final molten steel is controlled to be 70 ppm or less to make a continuous casting, thereby facilitating the generation of available TiN. 
     Here, when the Al concentration is less than 0.05%, if Al less than 30 kg is injected, its adding effect is insignificant relative to the molten steel of 80˜85 ton so that effective Cr 2 O 3  reduction cannot be achieved, and if Al exceeding 40 kg is injected, the Al concentration may exceed 0.12%. Accordingly, it is preferable that the amount of Al added is 30˜40 kg. 
     Also, when the Al concentration is 0.12% or more, if quicklime less than 250 kg is injected, the surface defect in the final product may be caused, and if quicklime exceeding 300 kg is injected, the concentration of the final Al inclusions may exceed 70 ppm. Accordingly, it is preferable that the amount of quicklime added is 250˜300 kg. 
     And, it is preferable that the final composition of refined slag in the vacuum oxygen decarburization ladle is 1.1≦(% CaO)/(% Al 2 O 3 )≦1.4 and 4≦(% TiO 2 )/(% SiO 2 )≦6. 
     When (% CaO)/(% Al 2 O 3 ) is less than 1.1, the alumina inclusion concentration may exceed 70 ppm, and when (% CaO)/(% Al 2 O 3 ) exceeds 1.4, the surface defect and the like in the final product may be caused. Accordingly, 1.1≦(% CaO)/(% Al 2 O 3 )≦1.4 is preferable. 
     Further, when (% TiO 2 )/(% SiO 2 ) is less than 4, the effect for suppressing the generation of excess Ti oxide is insignificant and when (% TiO 2 )/(% SiO 2 ) exceeds 6, the generation of TiN as serving the non-uniform nucleating site of ferrite may be degraded due to the excess TiN oxide upon solidifying. Accordingly, 4≦(% TiO 2 )/(% SiO 2 )≦6 is preferable. 
     With this manner, it is possible to manufacture slabs with equiaxed grain structures by assuring equiaxed crystal ratio of 40% or more in the slabs of 200-220 mm in thickness as well as decrease ridging defect generated in forming the final cold rolling product. 
     EXAMPLE 
     The example of the present invention will now be described. 
     In order to have the composition of Fe-17% Cr, scrap iron and ferroalloy are melted in an electric furnace and then subjected to oxygen decarburization process in an AOD refining furnace to be tapped into a ladle at 1780° C. There are molten steel and slag in the ladle. In other to improve vacuum oxygen decarburization efficiency, the slag is removed in a mechanical manner. At this time, the temperature of the molten steel is about 1600° C. The ladle is moved to the vacuum oxygen decarburization refining stand in which a vacuum cover is put on it and oxygen gas is then supplied to it using a lance on the upper part of the molten steel. With this process, after oxygen decarburization reaction is completed, the temperature of the molten steel is raised up to about 1670° C. and the composition of the molten steel is analyzed as in the following table 2. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Component 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 Si 
                 Mn 
                 S 
                 Cr 
                 Ti 
                 Al 
                 N 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Concentration 
                 0.005 
                 0.0 
                 0.3 
                 0.008 
                 17 
                 0.0 
                   
                 0.01 
               
               
                 (% by weight) 
               
               
                   
               
            
           
         
       
     
     After completing oxygen blowing, Al of about 320 kg is injected in order to make Cr 2 O 3  reduction and deoxidation of the molten steel under vacuum atmosphere. At this time, by injecting Si and Mn along with Al, the generation of Ti oxide is suppressed. By injection Ti in the form of sponge, [Ti] in the molten steel is 0.3% level. 
     At a point in time elapsing 5 minutes after injecting the ferroalloy, since [Al] in the molten steel is 0.04% level, Al of 30 kg is further injected as described above and Ar is then supplied from the bottom of the ladle to stir the molten steel for about 20 minutes. After the refining process is completed under vacuum, the temperature of the molten steel is about 1600?. In order to control a casting temperature to 1550° C., coolant and the like is injected under the atmosphere. 
     After the ladle process, the ladle is transferred to a continuous casting process. The component of the molten steel in the final tundish is analyzed as in the following table 3. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Component 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 Si 
                 Mn 
                 S 
                 Cr 
                 Ti 
                 Al alumina   
                 Al alumina   
                 N 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Concen- 
                 0.004 
                 0.3 
                 0.4 
                 0.003 
                 18 
                 0.30 
                 0.02 
                 0.004 
                 0.011 
               
               
                 tration 
               
               
                 (% by 
               
               
                 weight) 
               
               
                   
               
            
           
         
       
     
     In the component of the molten steel in the table 3, [Al alumina ] concentration existing as alumina is 40 ppm. It may be appreciated that the concentration is within the range of the present invention. 
     At this time, N concentration in the molten steel is about 110 ppm and reacts with 0.30% [Ti] to contribute the generation of available TiN as shown in  FIG. 5 , thereby obtain slabs with equiaxed grain structures. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.