Thin cast steel strip with reduced microcracking

Abstract

A thin cast steel strip and method of making thereof with improved resistance to microcracking, where the steel strip is produced by continuous casting and contains between about 0.003% and about 0.008% sulfur by weight and about 0.010% and about 0.065% carbon by weight. The sulfur content may be between about 0.003% and 0.006%.

1 . A thin cast steel strip produced by continuous casting by steps comprising: 
 a. assembling a pair of internally cooled casting rolls having a nip there between and with confining closures adjacent the ends of the nip;    b. introducing molten carbon steel having a sulfur content of between about 0.003% and about 0.008% by weight and a carbon content of between about 0.010% and about 0.065% by weight between the pair of casting rolls to form a casting pool supported on the casting surfaces of the casting rolls;    c. counter rotating the casting rolls to form solidified metal shells on the casting surfaces of the casting rolls; and    d. forming from said solidified shells thin steel strip downwardly through the nip between the casting rolls.    
   
   
       2 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten steel is a low carbon steel.  
   
   
       3 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten steel has a carbon content between about 0.025% and about 0.060% by weight.  
   
   
       4 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten steel has a carbon content between about 0.032% and about 0.045% by weight.  
   
   
       5 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the sulfur content of the molten steel is between about 0.004% and about 0.006% by weight.  
   
   
       6 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten carbon steel in the casting pool has a total oxygen content of at least 70 ppm and a free oxygen content between 20 and 60 ppm.  
   
   
       7 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the steel strip contains, by weight, less than 0.06% aluminum, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium.  
   
   
       8 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the steel strip contains fine oxide particles of silicon and iron distributed through the microstructure having an average precipitate size less than 50 nanometers.  
   
   
       9 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the steel strip contains fine oxide particles distributed through the microstructure capable of producing an average austenite grain size of less than 50 microns up to at least 1000° C. for a holding time of at least 20 minutes.  
   
   
       10 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the steel strip contains fine oxide particles capable of restricting ferrite recrystallization for strain levels up to 10% and temperatures up 750° C. with hold times up to 20 minutes.  
   
   
       11 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten metal in the casting pool contains a nitrogen content below about 120 ppm and a free hydrogen content below about 6.9 ppm and the sum of the partial pressures of the hydrogen and nitrogen is less than 1.15 atmospheres.  
   
   
       12 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten steel includes nitrogen content below about 100 ppm and free hydrogen content below about 6.9 ppm and the sum of the partial pressures of the hydrogen and nitrogen is less than 1.15 atmospheres.  
   
   
       13 . The thin steel strip produced by continuous casting as claimed in  claim 1 , wherein the molten steel includes free nitrogen content below about 85 ppm and free hydrogen content below about 6.9 ppm and the sum of the partial pressures of the hydrogen and nitrogen is less than 1.15 atmospheres.  
   
   
       14 . A method of casting thin steel strip comprising: 
 a. introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a sulfur content between about 0.003% and about 0.008% by weight and a carbon content between about 0.01% and about 0.065% by weight; and    b. solidifying the molten steel to form metal shells on the casting rolls and form thin.    
   
   
       15 . A thin steel strip produced by: 
 casting steel strip from molten steel having a sulfur content between about 0.003% and about 0.008% by weight and a carbon content between about 0.010% and about 0.065% by weight.    
   
   
       16 . The thin steel strip as recited in  claim 15 , wherein the steel strip is less than 5 mm in thickness.  
   
   
       17 . The thin steel strip as recited in  claim 15 , wherein the steel strip is less than 2.5 mm in thickness.  
   
   
       18 . The thin steel strip as recited in  claim 15 , wherein the steel strip is formed of a solidified steel containing solidified oxide inclusions distributed in the surface regions of the strip to a depth of 2 microns from the surface contain such inclusions to a per unit area density of at least 120 inclusions/mm 2 .  
   
   
       19 . The thin steel strip as recited in  claim 15 , wherein the steel strip is formed of a solidified steel containing oxide inclusions distributed to reflect a total oxygen content in the range 70 ppm to 250 ppm and free oxygen content between 20 and 60 ppm in the made steel from which the strip is made.  
   
   
       20 . A method of casting thin steel strip comprising: 
 a. assembling a pair of cooled casting rolls having a nip there between and confining end closures adjacent to ends of the casting rolls;    b. introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting rolls confined by the end closures, with the molten steel having a sulfur content between about 0.003% and about 0.008% by weight and a carbon content between about 0.010% and about 0.065% by weight;    c. counter-rotating the casting rolls to form solidified metal shells on casting surfaces of the casting rolls; and    d. forming from the solidified metal shells downwardly through the nip between the casting rolls thin steel strip.    
   
   
       21 . A steel composition comprising: 
 a. a carbon content of between about 0.010% and about 0.065% by weight, a silicon content of 2.5% by weight or less, a manganese content of 2.0% by weight or less, and an aluminum content of 1.0% by weight or less; and    b. a component to reduce the formation of surface microcracks in the cast thin strip, the component comprising a sulfur content between about 0.003% and about 0.008% by weight.    
   
   
       22 . The steel composition as recited in  claim 20  wherein the silicon content is between about 0.01% and about 2.5% by weight, the manganese content is between about 0.01% and about 2.0% by weight, and the aluminum content is 0.01% by weight or less.