Steel sheet for hardening, hardened member, and method for manufacturing steel sheet for hardening

Abstract

An aspect of the present invention is a steel sheet for quench hardening, satisfying a prescribed composition and having a Mn concentration satisfying the formula (1): S1+S2<−10×[Mn]+44 (1), where [Mn] is Mn concentration in a steel sheet analyzed by inductively coupled plasma emission spectrography (% by mass); S1 is an area % of the region where the Mn concentration analyzed by an electron beam microprobe analyzer in the structure at a position of ¼ of the steel sheet thickness is two times or more the [Mn]; and S2 is an area % of the region where the Mn concentration analyzed by an electron beam microprobe analyzer in the structure at a position of ¼ of the steel sheet thickness is 0.5 times or less the [Mn].

1 : A steel sheet comprising a composition comprising, in terms of % by mass,
 C: more than 0.2% and 0.4% or less,   Si: 0.8% or more and 1.4% or less,   Mn: 1% or more and 3% or less,   P: more than 0% and 0.02% or less,   S: more than 0% and 0.002% or less,   sol.Al: 0.02% or more and 0.06% or less,   N: more than 0% and 0.01% or less,   O: more than 0% and 0.01% or less,   B: 0.0005% or more and 0.005% or less,   Ti: 0.005% or more and 0.1% or less, and   iron,   wherein a Mn concentration satisfies the following formula (1):
     S 1+ S 2<−10×[Mn]+44  (1)
 
   wherein   [Mn] is Mn concentration in a steel sheet analyzed by inductively coupled plasma emission spectrography (% by mass),   S1 is an area % of the region where the Mn concentration analyzed by an electron beam microprobe analyzer in structure at a position of ¼ of the steel sheet thickness is two times or more the [Mn], and   S2 is an area % of the region where the Mn concentration analyzed by an electron beam microprobe analyzer in structure at a position of ¼ of the steel sheet thickness is 0.5 times or less the [Mn].   
     
     
         2 : The steel sheet for quench hardening according to  claim 1 , wherein an area ratio of ferrite at a position of ¼ of the steel sheet thickness is 0% or more and 50% or less. 
     
     
         3 : The steel sheet for quench hardening according to  claim 1 , wherein the composition satisfies
 B: 0.001% or more and 0.005% or less, in terms of % by mass.   
     
     
         4 : The steel sheet for quench hardening according to  claim 1 , wherein the composition further comprises at least another element selected from the group consisting of
 Cr: more than 0% and 3% or less,   Mo: more than 0% and 3% or less,   Nb: more than 0% and 0.1% or less, and   V: more than 0% and 0.1% or less, in terms of % by mass.   
     
     
         5 : A quench hardened member manufactured using the steel sheet for quench hardening according to  claim 1 , the quench hardened member having hardness of 515 HV or more and marked T direction bendability. 
     
     
         6 : A method for manufacturing steel sheet for quench hardening according to  claim 1 , the method comprising:
 finish rolling in an austenite region to carry out a process satisfying the following formula (2):
   6.0<2×10 4 ×(ln[ R ]+10)/((ln[ t ]+70)×[ T ])  (2)
 
   wherein   [R] is average cooling rate from “finish rolling temperature” to “coiling temperature” (° C./s),   [t] is retention time (h) from “coiling temperature” to “coiling temperature −50° C.”, and   [T] is “coiling temperature” (° C.).   
     
     
         7 : The method for manufacturing steel sheet for quench hardening according to  claim 6 , wherein the average cooling rate is 10° C./s or more and 200° C./s or less. 
     
     
         8 : The method for manufacturing steel sheet for quench hardening according to  claim 6 , wherein the retention time is 0.25 hours or more and 15 hours or less. 
     
     
         9 : The method for manufacturing steel sheet for quench hardening according to  claim 6 , wherein the coiling temperature is 320° C. or higher and 650° C. or lower.