Sumitomo Metal Industries, Inc.

Toshirou Tomida

Kaori Kawano

Mitsuru Yoshida

Masayuki Wakita

Norio Imai

Tamotsu Toki

Masanori Yasuyama

Hitomi Nishibata

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof are provided. The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and said average crystal grain diameter D (&#x3bc;m) satisfy the relation defined by the formula (3) given below:1.2&#x2266;&#x2266;7&#x2003;&#x2003;formula (1)&#x2266;2.7+5000/(5+350&#xb7;C+40&#xb7;Mn)&#x2003;&#x2003;formula (2)&#x2266;0.1&#x2003;&#x2003;formula (3)wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.

Method of producing cold-rolled steel sheet

A cold-rolled steel sheet of carbon steel or low-alloy steel comprising C: 0.01 to 0.25% by mass with a ferrite main phase is has an average ferrite crystal grain diameter D (&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfying formulas (5,6) and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and D (&#x3bc;m) satisfying formula (3):1.2&#x2266;D&#x2266;9.3&#x2003;&#x2003;(5)&#x2266;5.0&#x2212;2.0&#xb7;Cr+5000/(5+350&#xb7;C+40&#xb7;Mn)2&#x2003;&#x2003;(6)&#x2266;0.1&#x2003;&#x2003;(3)and, at said &#xbc; inch depth, the area percentage of ferrite crystal grains the crystal grain diameter d (&#x3bc;m) of which satisfying formula (4) is at least 80%:/3&#x2266;&#x2266;3&#x2003;&#x2003;(4)wherein C, Cr and Mn represent mass % of the respective elements in the steel.

Cold-rolled steel sheet

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances, machine structures, and of constructing buildings, and a manufacturing method thereof is provided. The steel sheet is a hot-rolled steel sheet of a carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D(&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by formulas (1) and (2) given below and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and said average crystal grain diameter D(&#x3bc;m) satisfy the relation defined by the formula (3) given below:1.2&#x2266;D&#x2266;7&#x2003;&#x2003;formula (1)&#x2266;2.7+5000/(5+350&#xb7;C+40&#xb7;Mn)&#x2003;&#x2003;formula (2)&#x2266;0.1&#x2003;&#x2003;formula (3)wherein C and Mn represent the content (in % by mass) of the respective elements in the steel.

Method of producing hot-rolled steel sheet

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof are provided. The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and said average crystal diameter D (&#x3bc;m) satisfy the relation defined by the formula (3) given below:1.2&#x2266;D&#x2266;7&#x2003;&#x2003;formula (1)&#x2266;2.7+5000/(5+350&#xb7;C+40&#xb7;Mn)&#x2003;&#x2003;formula (2)&#x2266;0.1&#x2003;&#x2003;formula (3)wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.

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Hyogo, Japan

Toshirou Tomida