Grain oriented steel strip with high magnetic characteristics, and manufacturing process of the same

Abstract

A method for the production of hot-rolled steel strip comprising the following steps: providing a steel slab comprising, in weight percentages: Si: 2.5 to 3.5%, C: 0.05 to 0.1%, Mn: 0:05 to 0.1%, Als: 0.015 to 0.026%, N: 0.0050 to 0.0100%, and further comprising S and/or Se so that S+ (32/79) Se is in an amount of 0.018 to 0.030%, and optionally comprising one or more elements chosen among Sb in an amount of 0.015 to 0.035%, Cu in an amount of 0.08% to 0.25%, Sn in an amount of 0.06% to 0.15%, P in an amount of 0.005% to 0.015%, the balance being iron and unavoidable impurities, reheating said slab to a temperature between 1300° C. and 1430° C., roughing hot-rolling said slab to produce a blank having a thickness below 50 mm, finishing hot-rolling of said blank to produce a hot rolled strip in three rolling passes or more, the temperature of said blank during the first pass being above 1150° C. and at least one rolling pass being performed with a reduction of 40% or more and being immediately followed by a holding of more than 20 sec, the average interpass holding temperature Tav being settled between 1000 and 1200° C., the total finishing hot rolling time t being controlled so that the value of Tav for any portion of said strip further respects the under mentioned equation: T av >T 1 +α 1 (t−78) [eq. 1] with T 1 =992.2+1493(Als) and α 1 =1.204+24.9(Als), T av and being expressed in ° C., t in seconds and Al s in weight %, cooling of said hot-rolled strip from the finish rolling temperature to a temperature below 600° C. in less than 10 sec and coiling of said hot-rolled strip.

1 . A method for the production of hot-rolled steel strip comprising the successive following steps, in this order:
 providing a steel slab comprising, in weight percentages:
 Si: 2.5 to 3.5%, 
 C: 0.05 to 0.1%, 
 Mn: 0.05 to 0.1%, 
 Als: 0.015 to 0.026%, 
 N: 0.0050 to 0.0100%, 
   and further comprising S and/or Se so that S+ (32/79) Se is in an amount of 0.018 to 0.030%,   optionally comprising one or more elements chosen among Sb in an amount of 0.015 to 0.035%, Cu in an amount of 0.08% to 0.25%, Sn in an amount of 0.06% to 0.15%, P in an amount of 0.005% to 0.015%,   the balance being iron and unavoidable impurities,   reheating said slab to a temperature between 1300° C. and 1430° C.,   roughing hot-rolling said slab to produce a blank having a thickness below 50 mm,   finishing hot-rolling of said blank to produce a hot rolled strip in three rolling passes or more, the temperature of said blank during the first pass being above 1150° C. and at least one rolling pass being performed with a reduction of 40% or more and being immediately followed by a holding of more than 20 sec,   the average interpass holding temperature Tav being settled between 1000 and 1200° C., the total finishing hot rolling time t being controlled so that the value of Tav for any portion of said strip further respects the under mentioned equation:
     T   av   >T   1 +α 1 ( t− 78)  [eq. 1]
 
   with T 1 =992.2+1493(Als) and α 1 =1.204+24.9(Als),   T av  and T 1  being expressed in ° C., t in seconds and Al, in weight %, and   cooling of said hot-rolled strip from the finish rolling temperature to a temperature below 600° C. in less than 10 sec, and   coiling of said hot-rolled strip.   
     
     
         2 . A method according to  claim 1 , wherein the roughing and finishing hot rolling is performed using a reversible hot rolling mill. 
     
     
         3 . A method according to  claim 1 , wherein the end hot rolling temperature T end  is higher than ([Ti+α 1 (t−78)]−60° C.) 
     
     
         4 . A method according to  claim 1 , wherein said reheating of the slab is performed at a temperature between 1350° C. and 1430° C. 
     
     
         5 . A hot-rolled steel strip, to comprising, in weight percentages:
 Si: 2.5 to 3.5%,   C: 0.05 to 0.1%,   Mn: 0.05 to 0.1%,   Als: 0.015 to 0.026%,   N: 0.0050 to 0.0100%,   and further comprising S and/or Se so that S+ (32/79) Se is in an amount of 0.018 to 0.030%,   optionally comprising one or more elements chosen among Sb in an amount of 0.015 to 0.035%, Cu in an amount of 0.08% to 0.25%, Sn in an amount of 0.06% to 0.15%, P in an amount of 0.005% to 0.015%, the balance being iron and unavoidable impurities, and   comprising less than 0.0025% of nitrogen linked to aluminium under the form of AlN and presenting five different layers across the strip thickness composed of grains populations with different characteristics.   
     
     
         6 . A hot-rolled strip according to  claim 5 , further comprising outer layers 1 and 5, the zones extending from the surface to ⅙ of strip thickness having a ferritic microstructure composed of more than 80% of equi-axial recrystallized grains with an average grain size d αo  lower than 50 μm and preferably lower than 30 μm, and optionally, atop of at least one of the outer surface of said outer layers, coarse decarburised grains with an average grain size of at least 2d αo . 
     
     
         7 . A hot-rolled strip according to  claim 5 , further comprising intermediate layers 2 and 4, the zones extending from ⅙ of the strip thickness to 2/6 of strip thickness, as measured from the surface, having a ferritic microstructure composed of more than 80% of recrystallized grains with an average grain size d αi  lower than 80 μm and preferably lower than 50 μm, d αi  being such that d αi >d αo . 
     
     
         8 . A hot-rolled strip according to  claim 5 , further comprising a central layer, the central zone of the strip equal to ⅓ of strip thickness, having an α-fiber texture with less than 60% and preferably less than 50% of the area percentage of the layer with grains having a disorientation less than 20° from the α-fiber (<110> crystallographic direction parallel to rolling direction) 
     
     
         9 . A method for the production of cold-rolled grain oriented magnetic strip comprising the successive following steps, in this order:
 providing a hot-rolled strip obtained by the method according to  claim 1 ,   optional cold rolling of said hot-rolled strip,   performing a first annealing of said strip during 40 to 300 sec, in one or more steps, the first step being performed at a temperature between 1050° C. and 1170° C. and optional further steps being performed at lower temperatures,   cooling down said strip to a quenching start temperature between 750 and 940° C.,   quenching the strip to a temperature below 300° C.,   cold rolling said annealed strip, the reduction ratio of the last cold rolling down to final thickness being between 80% and 95%,   performing a primary recrystallization annealing of the strip including a decarburization treatment so as to reach a carbon amount below 0.0030%,   applying an annealing separator onto the strip surface, and   performing a secondary recrystallization annealing of the strip.   
     
     
         10 . A method according to  claim 9 , wherein said first annealing of the strip is performed in two steps, the strip being first held at a temperature between 1050° C. and 1170° C. during 10 to 60 s, and then cooled and held at a temperature between 800° C. and 950° C. during 40 to 240 sec. 
     
     
         11 . A method according to  claim 9 , wherein during said cold rolling of said annealed strip, said strip under rolling is held at a temperature between 170° C. and 300° C. in at least one interpass step after the first cold rolling pass. 
     
     
         12 . A method according to  claim 9 , wherein the primary recrystallization annealing of the strip comprises a holding at a temperature between 780° C. and 900° C., during 60 to 300 sec, in an atmosphere consisting of N 2 , H 2  and H 2 O, the ratio between partial pressure of H 2 O and partial pressure of H 2  being between 0.40 and 0.70. 
     
     
         13 . A method according to  claim 12 , wherein the heating rate of the strip to reach said holding temperature between 780 and 900° C. is at least 150° C./sec in the range between 200° C. and 700° C. 
     
     
         14 . A method according to  claim 9 , wherein the secondary recrystallization annealing comprises a heating of the strip to a temperature between 1000° C. and 1250° C. with a heating rate between 5° C./h and 40° C./h in an atmosphere consisting of N 2  and H 2 , and then a holding of said strip during 5 to 30 h at this temperature in an atmosphere consisting of H 2 .