Jfe Steel Corporation

Yoshiharu Kusumoto

Takeshi Fujita

Nobusuke Kariya

Hidekazu Ookubo

Norio Kanamoto

A high carbon hot-rolled steel sheet which is a hot-rolled spheroidizing annealed material, including 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, which contains carbide having a particle size of smaller than 0.5 &#x3bc;m in a content of 15% or less by volume to the total amount of carbide, and the difference between the maximum hardness Hand the minimum hardness H, &#x394;H(=H&#x2212;H), in the sheet thickness direction being 10 or smaller.

High carbon hot-rolled steel sheet

A high carbon cold-rolled steel sheet 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 (Atransformation point &#x2212;20&#xb0; C.) or above to prepare a hot-rolled sheet; cooling the hot-rolled sheet to temperatures of 650&#xb0; C. or below at cooling rates from 60&#xb0; C./s or larger to smaller than 120&#xb0; C./s; coiling the hot-rolled sheet after cooling at coiling temperatures of 600&#xb0; C. or below; cold-rolling the coiled hot-rolled sheet at rolling reductions of 30% or more to prepare a cold-rolled sheet; and annealing the cold-rolled sheet at annealing temperatures from 600&#xb0; C. or larger to Atransformation point or lower.

Growing community of inventors

Chiba, Japan

Yoshiharu Kusumoto