Highly ductile steel sheet and method of manufacturing the same

Abstract

A steel sheet has a C content in the range of 0.04 to 0.25% by mass, a Si content in the range of 0.05 to 3% by mass, a Mn content in the range of 0.1 to 3% by mass, an Al content in the range of 0.01 to 2% by mass, a P content of 0.02% by mass or below and higher than 0% by mass, and a S content in the range of 0 to 0.005% by mass. The number of CaO.Al 2 O 3 inclusions containing CaO, Al 2 O 3 , SiO 2 and MgO, having a CaO content of 5% or above, a SiO 2 content of 0.1% or above and an Al 2 O 3 content of 60% or above, respectively, having major axes of 30 μm or above, and contained in 1 kg of the steel sheet is seventy or below. The steel sheet is highly ductile and has high bore-expandability. A steel slab manufacturing method manufactures a steel slab for forming the steel sheet.

1 . A steel sheet having a C content in the range of 0.04 to 0.25% by mass, a Si content in the range of 0.05 to 3% by mass, a Mn content in the range of 0.1 to 3% by mass, an Al content in the range of 0.01 to 2% by mass, a P content of 0.02% by mass or below and higher than 0% by mass, and a S content in the range of 0 to 0.005% by mass; 
 wherein the number of CaO.Al 2 O 3  inclusions containing CaO, Al 2 O 3 , SiO 2  and MgO, contain 5% or above of CaO, 0.1% or above of SiO 2  and 60% or above of Al 2 O 3  in 100% of oxides including CaO, Al 2 O 3 , SiO 2  and MgO and having major axes of 30 μm or above, and contained in 1 kg of the steel sheet is seventy or below.    
   
   
       2 . The steel sheet according to  claim 1 , wherein the S content is in the range of 0 to 0.003% by mass.  
   
   
       3 . A method of manufacturing a steel slab for forming the steel sheet according to  claim 1 , said method comprising the steps of: 
 producing CaO slag containing CaO, Al 2 O 3 , SiO 2 , MgO, MnO and total Fe, and having a CaO content of 45% or above, a total Fe content of 5% or below, and a MnO content of 3% or below on a molten steel decarburized in a decarburizing furnace and contained in a ladle by adding a CaO flux to the molten steel;    desulfurizing the molten steel to reduce the S content of the molten steel to 0.005% or below by stirring the molten steel and the CaO slag by a gas stirring method in the ladle;    circulating the desulfurized molten steel contained in the ladle in a vacuum circulation degassing apparatus for 10 min or longer; and    forming a steel slabby casting the molten steel circulated in the vacuum circulation degassing apparatus.    
   
   
       4 . The method according to  claim 3 , the CaO slag has a CaO content of 50% by mass or above.  
   
   
       5 . The method according to  claim 3 , wherein the CaO slag has a MnO content of 2% by mass or blow and a total Fe content of 3% by mass or below.