Steel sheet, member, and methods for manufacturing them

Abstract

A steel sheet; a related member; and methods for manufacturing them are disclosed. The steel sheet has a chemical composition including specific amounts of C, Si, Mn, P, S, sol. Al, and N in mass %. The steel sheet has a specific ratio of the total of polygonal ferrite, upper bainite, retained γ, fresh martensite, tempered martensite, and lower bainite, and a specific ratio of a remaining microstructure. The steel sheet has a specific ratio of the number of fresh martensite grains and retained γ grains having an equivalent circular diameter of less than 0.8 μm, and has a specific ratio of the number of fresh martensite grains and retained γ grains having an aspect ratio of 2.0 or more and an equivalent circular diameter of 0.8 μm or more.

1 - 6 . (canceled) 
     
     
         7 . A steel sheet having a chemical composition comprising, in mass %:
 C: 0.10 to 0.24%,   Si: 0.4% or more and less than 1.60%,   Mn: 2.0 to 3.6%,   P: 0.02% or less,   S: 0.01% or less,   sol. Al: less than 1.0%, and   N: less than 0.015%,   the chemical composition satisfying formula (1) below,   the balance being Fe and incidental impurities,
 the steel sheet comprising a microstructure in which: 
 the area fraction of polygonal ferrite is 5% or more and 25% or less, 
 the area fraction of upper bainite is 5% or more and 50% or less, 
 the volume fraction of retained austenite is 3% or more and 20% or less, 
 the area fraction of fresh martensite is 12% or less (including 0%), 
 the total of the area fractions of tempered martensite and lower bainite is 10% or more and 50% or less, and 
 the area fraction of a remaining microstructure is 5% or less, 
 the microstructure being such that: 
 the ratio of the total number of fresh martensite grains and retained austenite grains having an equivalent circular diameter of less than 0.8 μm is 50% or more relative to the number of all fresh martensite grains and all retained austenite grains, and 
 the ratio of fresh martensite grains and retained austenite grains having an aspect ratio of 2.0 or more and an equivalent circular diameter of 0.8 μm or more is 30% or more relative to the number of fresh martensite grains and retained austenite grains having an equivalent circular diameter of 0.8 μm or more, 
   
       
         
           
             
               
                 
                   
                       
                     
                       
                         Si 
                         / 
                         Mn 
                       
                       < 
                       
                         
                           0 
                           . 
                           5 
                         
                         ⁢ 
                         0 
                       
                     
                   
                 
                 
                   
                     Formula 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         
           wherein in formula (1), Si and Mn indicate the Si content (mass %) and the Mn content (mass %), respectively. 
         
       
     
     
         8 . The steel sheet according to  claim 7 , wherein
 the chemical composition further comprises, in mass %, one or two selected from the following group A and group B:   group A:   one, or two or more selected from:   Nb: 0.2% or less,   Ti: 0.2% or less,   V: 0.2% or less,   B: 0.01% or less,   Cu: 0.2% or less,   Ni: 0.2% or less,   Cr: 0.4% or less, and   Mo: 0.15% or less,   group B:   one, or two or more selected from:   Mg: 0.0050% or less,   Ca: 0.0050% or less,   Sn: 0.10% or less,   Sb: 0.10% or less, and   REM: 0.0050% or less.   
     
     
         9 . A member obtained using the steel sheet described in  claim 7 . 
     
     
         10 . A member obtained using the steel sheet described in  claim 8 . 
     
     
         11 . A method for manufacturing a steel sheet, comprising, after hot rolling and pickling are performed on a steel slab having the chemical composition described  claim 7 , a cold rolling step of performing a cold rolling treatment on the hot rolled steel sheet to produce a cold rolled steel sheet, and
 an annealing step of performing an annealing treatment on the cold rolled steel sheet to produce a steel sheet,   the cold rolling step being such that the cold rolled steel sheet is obtained by performing the cold rolling treatment in such a manner that:   the cumulative cold rolling reduction ratio is 30 to 85%, and   the rolling reduction ratio in a first pass is 5% or more and less than 25%, thereby controlling the area fraction of the total of microstructures having {111} <0-11> orientation, {111} <11-2> orientation, {211} <0-11> orientation, and {100} <011> orientation to 35% or more and 75% or less relative to all bcc phase microstructures,   the annealing step being such that the annealing treatment comprises:   heating the cold rolled steel sheet at an average heating rate of 0.5 to 15° C./sec in a range of temperatures of 500° C. or above and Ac1 or below, to an annealing temperature T being 840° C. or below and satisfying 0.6≤(T−Ac1)/(Ac3−Ac1)<1.0;   after the heating, soaking and holding the steel sheet at the annealing temperature T in a furnace atmosphere having a dew point Td of −50° C. or above and −30° C. or below, thereby producing a steel sheet having a number density of acicular austenite microstructures of 5 microstructures/1000 μm 2  or more;   subsequently performing first cooling of cooling the steel sheet at an average cooling rate of 6.0° C./sec or more in a range of temperatures of 750 to 550° C., to a first cooling stop temperature Tc1 of 550° C. or below and 400° C. or above;   after the first cooling, subjecting the steel sheet to first holding at the first cooling stop temperature Tc1 for 25 seconds or more;   after the first holding, performing second cooling of cooling the steel sheet at an average cooling rate of 3.0 to 80° C./s in a range of temperatures of 350° C. or below and 200° C. or above, to a second cooling stop temperature Tc2 of 320° C. or below and 150° C. or above;   subjecting the steel sheet to second holding at the second cooling stop temperature Tc2 for 2 to 20 seconds;   after the second holding, subjecting the steel sheet to over-aging and holding in a range of temperatures of 350 to 500° C. for 20 to 3000 seconds; and after the over-aging and holding, performing third cooling of cooling the steel sheet.   
     
     
         12 . A method for manufacturing a steel sheet, comprising, after hot rolling and pickling are performed on a steel slab having the chemical composition described in  claim 8 , a cold rolling step of performing a cold rolling treatment on the hot rolled steel sheet to produce a cold rolled steel sheet, and
 an annealing step of performing an annealing treatment on the cold rolled steel sheet to produce a steel sheet,   the cold rolling step being such that the cold rolled steel sheet is obtained by performing the cold rolling treatment in such a manner that:   the cumulative cold rolling reduction ratio is 30 to 85%, and   the rolling reduction ratio in a first pass is 5% or more and less than 25%, thereby controlling the area fraction of the total of microstructures having {111} <0-11> orientation, {111} <11-2> orientation, {211} <0-11> orientation, and {100} <011> orientation to 35% or more and 75% or less relative to all bcc phase microstructures,   the annealing step being such that the annealing treatment comprises:   heating the cold rolled steel sheet at an average heating rate of 0.5 to 15° C./sec in a range of temperatures of 500° C. or above and Ac1 or below, to an annealing temperature T being 840° C. or below and satisfying 0.6≤(T−Ac1)/(Ac3−Ac1)<1.0;   after the heating, soaking and holding the steel sheet at the annealing temperature T in a furnace atmosphere having a dew point Td of −50° C. or above and −30° C. or below, thereby producing a steel sheet having a number density of acicular austenite microstructures of 5 microstructures/1000 μm 2  or more;   subsequently performing first cooling of cooling the steel sheet at an average cooling rate of 6.0° C./sec or more in a range of temperatures of 750 to 550° C., to a first cooling stop temperature Tc1 of 550° C. or below and 400° C. or above;   after the first cooling, subjecting the steel sheet to first holding at the first cooling stop temperature Tc1 for 25 seconds or more;   after the first holding, performing second cooling of cooling the steel sheet at an average cooling rate of 3.0 to 80° C./s in a range of temperatures of 350° C. or below and 200° C. or above, to a second cooling stop temperature Tc2 of 320° C. or below and 150° C. or above;   subjecting the steel sheet to second holding at the second cooling stop temperature Tc2 for 2 to 20 seconds;   after the second holding, subjecting the steel sheet to over-aging and holding in a range of temperatures of 350 to 500° C. for 20 to 3000 seconds; and after the over-aging and holding, performing third cooling of cooling the steel sheet.   
     
     
         13 . A method for manufacturing a member, comprising a step of subjecting the steel sheet described in  claim 7  to at least one working of forming and joining to produce a member. 
     
     
         14 . A method for manufacturing a member, comprising a step of subjecting the steel sheet described in  claim 8  to at least one working of forming and joining to produce a member.