Jfe Steel Corporation

Takuya Yamazaki

Chikara Kami

Akio Tosaka

Shinjiro Kaneko

Kaneharu Okuda

Kazunori Osawa

Takashi Ishikawa

An object of the present invention is to provide a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet in which tensile strength is effectively increased by press forming and heat treatment while maintaining excellent deep drawability in press forming. Specifically, a steel composition contains less than 0.01% of C, 0.005 to 1.0% of Si, 0.01 to 1.0% of Mn, 0.005 to 0.050% of Nb, 0.005 to 0.030% of Al, 0.005 to 0.040% of N, 0.0005 to 0.0015% of B, 0.05% or less of P, and 0.01% or less of S, the balance substantially composed of Fe, in which the following equations (1) and (2) are satisfied:N %&#x2267;0.0015+14/93&#xb7;Nb %+14/27&#xb7;Al %+14/11&#xb7;B %&#x2003;&#x2003;(1)C %&#x2266;(12/93)&#xb7;Nb %.&#x2003;&#x2003;(2)

Cold rolled steel sheet and galvanized steel sheet having strain age hardenability and method of producing the same

An object of the present invention is to provide a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet in which tensile strength is effectively increased by press forming and heat treatment while maintaining excellent deep drawability in press forming. Specifically, a steel composition contains less than 0.01% of C, 0.005 to 1.0% of Si, 0.01 to 1.0% of Mn, 0.005 to 0.050% of Nb, 0.005 to 0.030% of Al, 0.005 to 0.040% of N, 0.0005 to 0.0015% of B, 0.05% or less of P, and 0.01% or less of S, the balance substantially composed of Fe, in which the following equations (1) and (2) are satisfied:N%&#x2267;0.0015+14/93.Nb%+14/27.Al%+14/11.B%&#x2003;&#x2003;(1)C%&#x2267;(12/93).Nb%&#x2003;&#x2003;(2)

The present invention presents a high tensile strength cold rolled steel sheet having excellent formability, impact resistance and strain age hardening characteristics, and the production thereof. As a specific means, a slab having a composition which contains, by mass %, 0.15% or less of C, 0.02% or less of Al, and 0.0050 to 0.0250% of N at N/Al of 0.3 or higher, and has N in a solid solution state at 0.0010% or more, is first hot rolled at the finish rolling delivery-side temperature of 800&#xb0; C. or above, and is subsequently coiled at the coiling temperature of 750&#xb0; C. or below to prepare a hot rolled plate. Then, after cold rolling, the hot rolled plate is continuously cooled at a temperature from the recrystallization temperature to 900&#xb0; C. at a holding time of 10 to 120 seconds, and is cooled by primary cooling in which the hot rolled plate is cooled to 500&#xb0; C. or below at a cooling rate of 10 to 300&#xb0; C./s, and furthermore if necessary, by secondary cooling in which a residence time is 300 seconds or less in a temperature range of the primary cooling stopping temperature or below and 350&#xb0; C. or higher. Provided is a steel sheet containing a ferritic phase having an average crystal grain size of 10 &#x3bc;m or less at an area ratio of 50% or more, and if necessary, a martensitic phase at an area ratio of 3% or more as a second phase.

High tensile strength cold rolled steel sheet having excellent strain age hardening characteristics and the production thereof

The present invention presents a high tensile strength cold rolled steel sheet having excellent formability, impact resistance and strain age hardening characteristics, and the production thereof. As a specific means, a slab having a composition which contains, by mass %, 0.15% or less of C, 0.02% or less of Al, and 0.0050 to 0.0250% of N at N/Al of 0.3 or higher, and has N in a solid solution state at 0.0010% or more, is first hot rolled at the finish rolling delivery-side temperature of 800&deg; C. or above, and is subsequently coiled at the coiling temperature of 750&deg; C. or below to prepare a hot rolled plate. Then, after cold rolling, the hot rolled plate is continuously cooled at a temperature from the recrystallization temperature to 900&deg; C. at a holding time of 10 to 120 seconds, and is cooled by primary cooling in which the hot rolled plate is cooled to 500&deg; C. or below at a cooling rate of 10 to 300&deg; C./s, and furthermore if necessary, by secondary cooling in which a residence time is 300 seconds or less in a temperature range of the primary cooling stopping temperature or below and 350&deg; C. or higher. Provided is a steel sheet containing a ferritic phase having an average crystal grain size of 10 &mgr;m or less at an area ratio of 50% or more, and if necessary, a martensitic phase at an area ratio of 3% or more as a second phase.

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Chiba, Japan

Takuya Yamazaki