Method for Producing Hot Rolled Strip with a Multiphase Microstructure

Abstract

For the production of hot strip referred to as TRIP steel (transformation induced plasticity), with a multiphase structure and with outstandingly good deformation properties along with high strengths, from the hot-rolled state, the invention proposes a method which is carried out with a predetermined chemical composition of the steel grade used within the limits 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements and with a combined rolling and cooling strategy in such a way that a structure comprising 40-70% ferrite, 15-45% bainite and 5-20% residual austenite is obtained, wherein the finish rolling of the hot strip ( 7 ) is performed to set a very fine austenite grain (d<8 μm) in the final forming operation (6′) at temperatures between 770 and 830° C. just above Ar3 in the region of the metastable austenite, and a controlled two-stage cooling ( 10, 11, 12 ) is carried out after the last rolling stand ( 6′ ) of the hot strip ( 7 ) to a strip temperature in the range of bainite formation of 320-480° C., with a holding time of about 650-730° C., the beginning of which is determined by the entry of the cooling curve into the ferrite region and the duration of which is determined by the transformation of the austenite into at least 40% ferrite.

1 . A method for producing hot-rolled strip that consists of TRIP ( tr ansformation- i nduced  p lasticity) steel with both high strength values and outstanding deformation properties, a refinement of dual-phase steels with a predetermined chemical composition of the steel grade that is used within the following limits: 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements, and a multiphase microstructure which consists of 40-70% ferrite, 15-45% bainite, and 5-20% retained austenite, from the hot-rolled state in a thin-slab continuous casting and rolling plant (CSP plant) ( 1 ), wherein the finish rolling of the hot-rolled strip ( 7 ) for adjusting a very fine austenite grain (d<8 μm) during the last deformation is carried out at temperatures of 770-830° 0  C., just above Ar 3  in the range of metastable austenite, and wherein immediately after the last rolling stand ( 6 ′), a controlled two-stage cooling of the hot-rolled strip ( 7 ) to a strip temperature in the range of bainite formation of 320-480° C. is carried out with a holding time at about 650-730° C., whose start is determined by the entry of the cooling curve ( 26 ) into the ferrite range and whose duration is determined by the transformation of the austenite to at least 40% ferrite, wherein the controlled two-stage cooling of the hot-rolled strip ( 7 ) is carried out in a cooling line ( 10 ) that consists of a succession of water cooling zones ( 11   1-7 ,  12 ) that are spaced a certain distance apart, can be automatically controlled, and are equipped with water spray heads ( 13 ), which evenly spray the upper and lower surfaces of the hot-rolled strip  7  with a specific amount of water. 
     
     
         2 . A method in accordance with  claim 1 , wherein the cooling rate is V=30-150 K/s, and preferably V=50-90 K/s, depending on the chemical composition of the steel grade that is used and on the geometry of the hot-rolled strip ( 7 ). 
     
     
         3 . A method in accordance with  claim 1 , wherein the controlled two-stage cooling of the hot-rolled strip ( 7 ) is carried out in a cooling line ( 10 ) that consists of a succession of water cooling zones ( 11   1-7 ,  12 ) spaced a certain distance apart. 
     
     
         4 . A hot-rolled strip ( 7 ) with TRIP steel properties, characterized by a chemical composition within the following limits: 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements; an elastic limit tensile strength ratio R p0.2 /R m  in the range of 0.45-0.75; and a possible level of strength with respect to combinations of tensile strength R m  and elongation after fracture A:
     R   m =600-700 MPa    A> 25%       R   m =700-800 MPa    A> 23%       R   m =800-900 MPa    A >21%       R   m =900-1,000 MPa    A >18%       R   m >1,000 MPa    A> 15%.