Hot rolled steel sheet and method of manufacturing same

Abstract

A hot rolled steel sheet comprises: a predetermined chemical composition; a microstructure in which a total area ratio of a tempered bainite phase and a tempered martensite phase is 70% or more, a total area ratio of a coarse pearlite phase, a martensite phase, and a retained austenite phase is 10% or less, the tempered bainite phase and the tempered martensite phase have laths with an average width of 1.0 μm or less as a substructure, a proportion of Fe-based carbides with an aspect ratio of 5 or less in Fe-based carbides precipitated inside and at boundaries of the laths is 80% or more, and MC-type carbides with an average particle size of 20 nm or less are dispersed and precipitated inside and at the boundaries of the laths; and an average dislocation density of 1.0×10 14 m −2 or more and 5.0×10 15 m −2 or less.

1 . A hot rolled steel sheet comprising:
 a composition containing, in mass %,   C: 0.03% or more and 0.20% or less,   Si: 0.4% or less,   Mn: 0.5% or more and 2.0% or less,   P: 0.03% or less,   S: 0.03% or less,   Al: 0.1% or less,   N: 0.01% or less, and   Ti: 0.03% or more and 0.15% or less,   with a balance being Fe and incidental impurities;   a microstructure in which a total area ratio of a tempered bainite phase and a tempered martensite phase is 70% or more, a total area ratio of a coarse pearlite phase, a martensite phase, and a retained austenite phase is 10% or less, the tempered bainite phase and the tempered martensite phase have laths with an average width of 1.0 μm or less as a substructure, a proportion of Fe-based carbides with an aspect ratio of 5 or less in Fe-based carbides precipitated inside and at boundaries of the laths is 80% or more, and MC-type carbides with an average particle size of 20 nm or less are dispersed and precipitated inside and at the boundaries of the laths; and   an average dislocation density of 1.0×10 14  m −2  or more and 5.0×10 15  m −2  or less.   
     
     
         2 - 7 . (canceled) 
     
     
         8 . The hot rolled steel sheet according to  claim 1 , wherein the composition further contains, in mass %, at least one selected from groups (a) to (c):
 (a) one or more of V: 0.01% or more and 0.3% or less, Nb: 0.01% or more and 0.1% or less, and Mo: 0.01% or more and 0.3% or less;   (b) B: 0.0002% or more and 0.010% or less; and   (c) one or more of REM, Zr, As, Cu, Ni, Sn, Pb, Ta, W, Cr, Sb, Mg, Ca, Co, Se, Zn, and Cs: 1.0% or less in total.   
     
     
         9 . The hot rolled steel sheet according to  claim 1 , comprising a coated or plated layer on a surface thereof. 
     
     
         10 . The hot rolled steel sheet according to  claim 8 , comprising a coated or plated layer on a surface thereof. 
     
     
         11 . A method of manufacturing a hot rolled steel sheet, comprising:
 hot rolling a steel raw material having the composition according to  claim 1 , whereby the steel raw material is heated to an austenite single phase region and subjected to rough rolling and finish rolling to obtain a steel sheet, and the steel sheet is cooled and coiled after the finish rolling;   pickling the steel sheet after the hot rolling; and   then continuous annealing the steel sheet,   wherein in the hot rolling, a finisher delivery temperature is 850° C. or more and 1000° C. or less, an average cooling rate to 500° C. after the finish rolling is 30° C./s or more, and a coiling temperature is 500° C. or less, and   in the continuous annealing, a maximum heating temperature of the steel sheet is 700° C. or more and (A 3  point+A 1  point)/2 or less, a time during which a temperature of the steel sheet is 600° C. or more and 700° C. or less in heating the steel sheet to the maximum heating temperature is 20 s or more and 1000 s or less, a time during which the temperature of the steel sheet is more than 700° C. is 200 s or less, an average cooling rate to 530° C. when cooling the steel sheet from the maximum heating temperature is 8° C./s or more and 25° C./s or less, and a time of holding the steel sheet in a temperature range of 470° C. or more and 530° C. or less after the cooling stops is 10 s or more.   
     
     
         12 . The method of manufacturing a hot rolled steel sheet according to  claim 11 , further comprising
 performing a coating or plating treatment on the steel sheet, after the continuous annealing.   
     
     
         13 . A method of manufacturing a hot rolled steel sheet, comprising:
 hot rolling a steel raw material having the composition according to  claim 8 , whereby the steel raw material is heated to an austenite single phase region and subjected to rough rolling and finish rolling to obtain a steel sheet, and the steel sheet is cooled and coiled after the finish rolling;   pickling the steel sheet after the hot rolling; and   then continuous annealing the steel sheet,   wherein in the hot rolling, a finisher delivery temperature is 850° C. or more and 1000° C. or less, an average cooling rate to 500° C. after the finish rolling is 30° C./s or more, and a coiling temperature is 500° C. or less, and   in the continuous annealing, a maximum heating temperature of the steel sheet is 700° C. or more and (A 3  point+A 1  point)/2 or less, a time during which a temperature of the steel sheet is 600° C. or more and 700° C. or less in heating the steel sheet to the maximum heating temperature is 20 s or more and 1000 s or less, a time during which the temperature of the steel sheet is more than 700° C. is 200 s or less, an average cooling rate to 530° C. when cooling the steel sheet from the maximum heating temperature is 8° C./s or more and 25° C./s or less, and a time of holding the steel sheet in a temperature range of 470° C. or more and 530° C. or less after the cooling stops is 10 s or more.   
     
     
         14 . The method of manufacturing a hot rolled steel sheet according to  claim 13 , further comprising
 performing a coating or plating treatment on the steel sheet, after the continuous annealing.