Jfe Steel Corporation

Hidekazu Ookubo

Takeshi Fujita

Yoshimasa Funakawa

Nobusuke Kariya

Norio Kanamoto

Tokunori Kanemura

Masato Shigemi

Yoshiharu Kusumoto

Taro Kizu

Shoji Kasai

Noriaki Kosaka

Yusuke Yasufuku

Shinji Yamazaki

The high strength steel sheet has a chemical composition including 0.08% to 0.20% of C, 0.3% or less of Si, 0.1% to 3.0% of Mn, 0.10% or less of P, 0.030% or less of S, 0.10% or less of Al, 0.010% or less of N, 0.20% to 0.80% of V, and the remainder composed of Fe and incidental impurities on a percent by mass basis, and a microstructure which includes 95% or more of ferrite phase on an area percentage basis, in which fine precipitates are dispersed having a distribution in such a way that the number density of precipitates having a particle size of less than 10 nm is 1.0&#xd7;10/&#x3bc;mor more and the standard deviation of natural logarithm values of precipitate particle sizes with respect to precipitates having a particle size of less than 10 nm is 1.5 or less.

High strength steel sheet

A semi-manufactured steel material has a chemical composition including, by mass %, C: 0.055% to 0.15%, Si: not more than 0.2%, Mn: not more than 1.3%, P: not more than 0.03%, S: not more than 0.007%, Al: not more than 0.1%, N: not more than 0.01%, and Ti: 0.14% to 0.30%, the balance comprising Fe and inevitable impurities. In the composition, 1.0&#x2266;([C]/12)/([Ti*]/48) is satisfied ([C], [S], [N] and [Ti]: contents (mass %) of the respective elements, and [Ti*]=[Ti]&#x2212;3.4&#xd7;[N]&#x2212;1.5&#xd7;[S]), and the contents of niobium and boron as impurities are limited to Nb: less than 0.03% and B: less than 0.0005%.

Hot rolled steel sheet and method for manufacturing the same

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.

Method of manufacturing high carbon cold-rolled steel sheet

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Chiba, Japan

Hidekazu Ookubo