High Carbon Hot Rolled Steel Sheet and method for manufacturing same

Abstract

A high carbon hot-rolled steel sheet, as a hot-rolled spheroidizing annealed material, having both excellent stretch-flange formability and excellent homogeneity of hardness in the sheet thickness direction is provided by a manufacturing method having the steps of: hot-rolling a steel containing 0.2 to 0.7% C by mass at finishing temperatures of (A r3 transformation point−20° C.) or above to prepare a hot-rolled sheet; cooling the hot-rolled sheet to temperatures of 650° C. or below at cooling rates from 60° C./s or larger to smaller than 120° C./s; coiling the hot-rolled sheet after cooling at coiling temperatures of 600° C. or below; and annealing the coiled hot-rolled sheet at annealing temperatures from 640° C. or larger to A c1 transformation point or lower.

1 . A method for manufacturing high carbon hot-rolled steel sheet, comprising the steps of: hot-rolling a steel containing 0.2 to 0.7% C by mass at a finishing temperature of (A r3  transformation point−20° C.) or above to prepare a hot-rolled sheet;
 cooling the hot-rolled sheet to a temperature of 650° C. or below at a cooling rate ranging from 60° C./s or larger to smaller than 120° C./s;   coiling the hot-rolled sheet after cooling at a coiling temperature of 600° C. or below; and   annealing the coiled hot-rolled sheet at an annealing temperature ranging from 640° C. or larger to A c1  transformation point or lower.   
   
   
       2 . The method for manufacturing high carbon hot-rolled steel sheet according to  claim 1 , wherein the step of cooling conducts cooling the hot-rolled sheet to a temperature of 600° C. or below at a cooling rate ranging from 80° C./s or larger to smaller than 120° C./s, and the step of coiling conducts coiling the sheet at a temperature of 550° C. or below. 
   
   
       3 . A high carbon hot-rolled steel sheet which is a hot-rolled spheroidizing annealed material, comprising 0.2 to 0.7% C, 2% or less Si, 2% or less Mn, 0.03% or less P, 0.03% or less S, 0.08% or less Sol.Al., and 0.01% or less N, by mass, the content of carbide having smaller than 0.5 μm of particle size being 15% or less by volume to the total amount of carbide, and the difference between the maximum hardness H V max  and the minimum hardness H V min , ΔHv (=H V max −H V min ), in the sheet thickness direction being 10 or smaller. 
   
   
       4 . The high carbon hot-rolled steel sheet according to  claim 3 , wherein the content of carbide having smaller than 0.5 m of particle size is 10% or less by volume to the total amount of carbide, and the difference between the maximum hardness Hv max and the minimum hardness Hv min, ΔHv (=H V max −H V min ), in the sheet thickness direction is 8 or smaller. 
   
   
       5 . The high carbon hot-rolled steel sheet according to  claim 3 , further comprising at least one element of: about 0.005% or less B, about 3.5% or less Cr, about 3.5% or less Ni, about 0.7% or less Mo, about 0.1% or less Cu, about 0.1% or less Ti, about 0.1% or less Nb, and about 0.1% or less of the total of W, V, and Zr, by mass. 
   
   
       6 . The high carbon hot-rolled steel sheet according to  claim 4 , further comprising at least one element of: about 0.005% or less B, about 3.5% or less Cr, about 3.5% or less Ni, about 0.7% or less Mo, about 0.1% or less Cu, about 0.1% or less Ti, about 0.1% or less Nb, and about 0.1% or less of the total of W, V, and Zr, by mass.