Steel sheet, member, and methods for manufacturing same

Abstract

A steel sheet contains, in mass %, C: 0.15 to 0.45%, Si: 1.5% or less, Mn: 1.7% or less, P: 0.03% or less, S: less than 0.0020%, sol. Al: 0.20% or less, N: 0.005% or less, B: 0.0015 to 0.0100%, and at least one of Nb and Ti in a total amount of 0.005 to 0.080%, with the balance being Fe and incidental impurities. The area fraction of martensite with respect to the total area of the microstructure is 95 to 100%, and the diameter of prior γ grains is less than 11.0 μm. The number density A of precipitates having an equivalent circular diameter of 500 nm or more satisfies the formula: A (particles/mm2)≤8.5×105×[B].

1 - 10 . (canceled) 
     
     
         11 . A steel sheet having a chemical composition comprising, in mass %,
 C: 0.15% or more and 0.45% or less,   Si: 1.5% or less,   Mn: 1.7% or less,   P: 0.03% or less,   S: less than 0.0020%,   sol. Al: 0.20% or less,   N: 0.005% or less,   B: 0.0015% or more and 0.0100% or less, and   at least one of Nb and Ti in a total amount of 0.005% or more and 0.080% or less,   with the balance being Fe and incidental impurities,   wherein the steel sheet has a microstructure in which the area fraction of martensite with respect to the total area of the microstructure is 95% or more and 100% or less,   wherein prior austenite grains have an average grain diameter of less than 11.0 μm, and   wherein the number density A of precipitates having an equivalent circular diameter of 500 nm or more satisfies formula (1) below:   
       
         
           
             
               
 
               
                 
                   
                     
                       
                         A 
                         ⁢ 
                             
                         
                           
                             ( 
                             
                               particles 
                               / 
                               
                                 mm 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                       ≤ 
                       
                         8.5 
                         × 
                         
                           10 
                           
                             
                                 
                                 
                             
                             5 
                           
                         
                         × 
                         
                           [ 
                           B 
                           ] 
                         
                       
                     
                   
                   
                     
                       formula 
                       ⁢ 
                           
                       
                         ( 
                         1 
                         ) 
                       
                     
                   
                 
               
             
           
         
         where [B] represents the content of B (% by mass). 
       
     
     
         12 . The steel sheet according to  claim 11 , wherein the chemical composition further comprises, in mass %, one or two or more selected from the following groups A to D:
 group A:   one or two selected from Cu: 1.0% or less and Ni: 1.0% or less,   group B:   one or two or more selected from Cr: 1.0% or less, Mo: less than 0.3%, V: 0.5% or less, Zr: 0.2% or less, and W: 0.2% or less,   group C:   one or two or more selected from Ca: 0.0030% or less, Ce: 0.0030% or less, La: 0.0030% or less, REMs (excluding Ce and La): 0.0030% or less, and Mg: 0.0030% or less,   group D:   one or two selected from Sb: 0.1% or less and Sn: 0.1% or less.   
     
     
         13 . The steel sheet according to  claim 11 , wherein the steel sheet comprises a coated layer on a surface thereof. 
     
     
         14 . The steel sheet according to  claim 12 , wherein the steel sheet comprises a coated layer on a surface thereof. 
     
     
         15 . A member formed using the steel sheet according to  claim 11 . 
     
     
         16 . A member formed using the steel sheet according to  claim 12 . 
     
     
         17 . A member formed using the steel sheet according to  claim 13 . 
     
     
         18 . A member formed using the steel sheet according to  claim 14 . 
     
     
         19 . A method for manufacturing a steel sheet, the method comprising:
 heating a steel slab having the chemical composition according to  claim 11  such that the surface temperature of the slab is increased from 1000° C. to a heat holding temperature of 1250° C. or higher at an average heating rate of 10° C./minute or less and then holding the steel slab at the heat holding temperature for 30 minutes or longer;   then setting a residence time at 900 to 1000° C. to 20 seconds or longer and 150 seconds or shorter and performing hot finish rolling under the condition of a finish rolling temperature of 850° C. or higher;   performing cooling at an average cooling rate of 40° C./second or more in a range from the finish rolling temperature to 650° C.;   then performing coiling at a coiling temperature of 650° C. or lower to obtain a hot rolled steel sheet;   subjecting the hot rolled steel sheet to cold rolling at a rolling reduction of 40% or more to obtain a cold rolled steel sheet; and   subjecting the cold rolled steel sheet to continuous annealing including   heating the cold rolled steel sheet from 400° C. to an annealing temperature of 830 to 950° C. at an average heating rate of 1.0° C./second or more,   holding the cold rolled steel sheet at the annealing temperature for 600 seconds or shorter,   cooling the cold rolled steel sheet from a cooling start temperature of 680° C. or higher to a finish cooling temperature of 260° C. or lower at an average cooling rate of 70° C./second or more, and   then holding the cold rolled steel sheet at a holding temperature of 150 to 260° C. for 20 to 1500 seconds to thereby manufacture a steel sheet.   
     
     
         20 . A method for manufacturing a steel sheet, the method comprising:
 heating a steel slab having the chemical composition according to  claim 12  such that the surface temperature of the slab is increased from 1000° C. to a heat holding temperature of 1250° C. or higher at an average heating rate of 10° C./minute or less and then holding the steel slab at the heat holding temperature for 30 minutes or longer;   then setting a residence time at 900 to 1000° C. to 20 seconds or longer and 150 seconds or shorter and performing hot finish rolling under the condition of a finish rolling temperature of 850° C. or higher;   performing cooling at an average cooling rate of 40° C./second or more in a range from the finish rolling temperature to 650° C.;   then performing coiling at a coiling temperature of 650° C. or lower to obtain a hot rolled steel sheet;   subjecting the hot rolled steel sheet to cold rolling at a rolling reduction of 40% or more to obtain a cold rolled steel sheet; and   subjecting the cold rolled steel sheet to continuous annealing including   heating the cold rolled steel sheet from 400° C. to an annealing temperature of 830 to 950° C. at an average heating rate of 1.0° C./second or more,   holding the cold rolled steel sheet at the annealing temperature for 600 seconds or shorter,   cooling the cold rolled steel sheet from a cooling start temperature of 680° C. or higher to a finish cooling temperature of 260° C. or lower at an average cooling rate of 70° C./second or more, and   then holding the cold rolled steel sheet at a holding temperature of 150 to 260° C. for 20 to 1500 seconds to thereby manufacture a steel sheet.   
     
     
         21 . The method for manufacturing a steel sheet according to  claim 19 , the method further comprising, after the continuous annealing, subjecting a surface of the steel sheet to coating treatment. 
     
     
         22 . The method for manufacturing a steel sheet according to  claim 20 , the method further comprising, after the continuous annealing, subjecting a surface of the steel sheet to coating treatment. 
     
     
         23 . A method for manufacturing a member, the method comprising the step of subjecting the steel sheet according to  claim 11  to at least one of forming and joining to thereby obtain a member. 
     
     
         24 . A method for manufacturing a member, the method comprising the step of subjecting the steel sheet according to  claim 12  to at least one of forming and joining to thereby obtain a member. 
     
     
         25 . A method for manufacturing a member, the method comprising the step of subjecting the steel sheet according to  claim 13  to at least one of forming and joining to thereby obtain a member. 
     
     
         26 . A method for manufacturing a member, the method comprising the step of subjecting the steel sheet according to  claim 14  to at least one of forming and joining to thereby obtain a member.