Steel sheet and method of production of same

Abstract

A steel sheet excellent in shapeability and bending strength is disclosed. The steel sheet of the present disclosure has a predetermined chemical composition, has a predetermined microstructure, has a difference of within 10.0% between a number density of precipitates in the tempered martensite at a first surface at a front side of the steel sheet and a number density of precipitates in the tempered martensite at a second surface at a back side of the steel sheet, and has a yield strength of 600 MPa or more.

1 . A steel sheet,
 having a chemical composition comprising, by mass %,
 C: 0.10 to 0.30%, 
 Si: 0.60 to 1.20%, 
 Mn: 1.00 to 3.50%, 
 P: 0.0200% or less, 
 S: 0.0200% or less, 
 Al: 0.001 to 1.000%, 
 N: 0.0200% or less, 
 Ti: 0 to 0.500%, 
 Co: 0 to 0.500%, 
 Ni: 0 to 0.500%, 
 Mo: 0 to 0.500%, 
 Cr: 0 to 2.000%, 
 O: 0 to 0.0100%, 
 B: 0 to 0.0100%, 
 Nb: 0 to 0.500%, 
 V: 0 to 0.500%, 
 Cu: 0 to 0.500%, 
 W: 0 to 0.1000%, 
 Ta: 0 to 0.1000%, 
 Sn: 0 to 0.0500%, 
 Sb: 0 to 0.0500%, 
 As: 0 to 0.0500%, 
 Mg: 0 to 0.0500%, 
 Ca: 0 to 0.0500%, 
 Y: 0 to 0.0500%, 
 Zr: 0 to 0.0500%, 
 La: 0 to 0.0500%, 
 Ce: 0 to 0.0500% and 
 a balance of Fe and impurities, 
   having a microstructure comprising, by area ratio,
 a total of ferrite, pearlite and bainite: 0% or more and 30.0% or less, 
 retained austenite: 10.0% or more and 30.0% or less, and 
 a balance of fresh martensite and tempered martensite, 
   having a difference of within 10.0%, the difference being the difference between a number density of precipitates in the tempered martensite at a first surface at a front side of the steel sheet and a number density of precipitates in the tempered martensite at a second surface at a back side of the steel sheet, and   having a yield strength of 600 MPa or more.   
     
     
         2 . The steel sheet according to  claim 1 ,
 having the chemical composition comprising, by mass %, one or more of
 Ti: 0.001 to 0.500%, 
 Co: 0.001 to 0.500%, 
 Ni: 0.001 to 0.500%, 
 Mo: 0.001 to 0.500%, 
 Cr: 0.001 to 2.000% 
 O: 0.0001 to 0.0100% 
 B: 0.0001 to 0.0100%, 
 Nb: 0.001 to 0.500%, 
 V: 0.001 to 0.500%, 
 Cu: 0.001 to 0.500%, 
 W: 0.0001 to 0.1000%, 
 Ta: 0.0001 to 0.1000%, 
 Sn: 0.0001 to 0.0500%, 
 Sb: 0.0001 to 0.0500%, 
 As: 0.0001 to 0.0500%, 
 Mg: 0.0001 to 0.0500%, 
 Ca: 0.0001 to 0.0500%, 
 Y: 0.0001 to 0.0500%, 
 Zr: 0.0001 to 0.0500%, 
 La: 0.0001 to 0.0500%, and 
 Ce: 0.0001 to 0.0500%. 
   
     
     
         3 . The steel sheet according to  claim 1 , wherein
 the microstructure includes the retained austenite in an acicular form.   
     
     
         4 . A method of production of a steel sheet, the method comprising
 hot rolling a steel slab having a chemical composition according to  claim 1  to obtain a hot rolled steel sheet,   coiling the hot rolled steel sheet,   pickling the hot rolled steel sheet,   cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet,   Q-annealing the cold rolled steel sheet,   IA-annealing the Q-annealed cold rolled steel sheet, and   performing aging treatment between the Q-annealing and the IA-annealing, wherein
 at the Q-annealing, heating the cold rolled steel sheet to an austenite single phase and 1000° C. or less and cooling to obtain an area ratio of 90.0% or more of martensite structures, 
 at the IA-annealing, holding the cold rolled steel sheet at a dual phase region of ferrite and austenite, and obtaining retained austenite, and 
 the aging treatment includes:
 aging treatment 1 in which tensile deformation of a bending R of 2.0 m or less is applied to one of a front side and a back side of the cold rolled steel sheet at 0 to 40° C. for 20 hrs or more; and, 
 aging treatment 2 in which tensile deformation of a bending R of 2.0 m or less is applied to the other side between the front side and back side of the cold rolled steel sheet at 0 to 40° C. for 20 hrs or more. 
 
   
     
     
         5 . The method of production according to  claim 4 , the method comprising
 holding the cold rolled steel sheet in the dual phase region of ferrite and austenite in the IA-annealing, then forming coating layers comprising zinc, aluminum, magnesium, or an alloy of these at the front and back surfaces of the cold rolled steel sheet in the process of cooling the cold rolled steel sheet down to room temperature.   
     
     
         6 . The method of production according to  claim 4 , the method comprising
 obtaining the retained austenite in an acicular form by the IA annealing.   
     
     
         7 . The steel sheet according to  claim 2 , wherein
 the microstructure includes the retained austenite in an acicular form.   
     
     
         8 . The method of production according to  claim 5 , the method comprising
 obtaining the retained austenite in an acicular form by the IA annealing.   
     
     
         9 . A method of production of a steel sheet, the method comprising
 hot rolling a steel slab having a chemical composition according to  claim 2  to obtain a hot rolled steel sheet,   coiling the hot rolled steel sheet,   pickling the hot rolled steel sheet,   cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet,   Q-annealing the cold rolled steel sheet,   IA-annealing the Q-annealed cold rolled steel sheet, and   performing aging treatment between the Q-annealing and the IA-annealing, wherein
 at the Q-annealing, heating the cold rolled steel sheet to an austenite single phase and 
 1000° C. or less and cooling to obtain an area ratio of 90.0% or more of martensite structures, at the IA-annealing, holding the cold rolled steel sheet at a dual phase region of ferrite and austenite, and obtaining retained austenite, and 
 the aging treatment includes:
 aging treatment 1 in which tensile deformation of a bending R of 2.0 m or less is applied to one of a front side and a back side of the cold rolled steel sheet at 0 to 40° C. for 20 hrs or more; and, 
 aging treatment 2 in which tensile deformation of a bending R of 2.0 m or less is applied to the other side between the front side and back side of the cold rolled steel sheet at 0 to 40° C. for 20 hrs or more. 
 
   
     
     
         10 . The method of production according to  claim 9 , the method comprising
 holding the cold rolled steel sheet in the dual phase region of ferrite and austenite in the IA-annealing, then forming coating layers comprising zinc, aluminum, magnesium, or an alloy of these at the front and back surfaces of the cold rolled steel sheet in the process of cooling the cold rolled steel sheet down to room temperature.   
     
     
         11 . The method of production according to  claim 9 , the method comprising
 obtaining the retained austenite in an acicular form by the IA annealing.   
     
     
         12 . The method of production according to  claim 10 , the method comprising
 obtaining the retained austenite in an acicular form by the IA annealing.