Nippon Steel Corporation

Yoshikazu Matsumura

Satoshi Akamatsu

Naoki Yoshinaga

Osamu Akisue

Koe Nakajima

Kohsaku Ushioda

Kunio Nishimura

Shigeru Ogawa

Masafumi Miyazaki

Hidekuni Murakami

Toshiaki Mizoguchi

Kazuo Koyama

Yoshiyuki Ueshima

Hideki Oka

Hiroshi Katoh

Hiroyuki Nakashima

Yasumitsu Onoe

Hidemaro Takeuchi

Shigenori Tanaka

Hiroshi Yada

Kiyomi Shio

Yoshimori Fukuda

Yoshio Morimoto

Shozo Sekino

Twin-roll continuous casting method

The present invention provides a cold-rolled steel sheet and a hot-dip galvanized steel sheet having good bake hardenability, non-aging properties at room temperature and good formability and a process for producing the same. An extra low carbon steel or an extra low carbon steel containing at least one element selected from Ti and Nb is annealed at a temperature of not lower than the AC.sub.3 transformation point to bring the structure after annealing to a structure of low-temperature transformation products. This makes it possible to provide a steel sheet that has a combination of high paint-bake hardenability and non-aging properties at room temperature and is excellent also in formability in respect of average r value (deep drawability) and elongation (punch stretchability). In particular, with respect to paint-bake hardenability, a BH property on a high level up to about 10 kgf/mm.sup.2 can be imparted according to need, and it is possible to provide a cold-rolled steel sheet and a hot-dip galvanized cold-rolled steel sheet that also have a non-aging property at room temperature.

Cold-rolled steel sheet and hot-dip galvanized cold-rolled steel sheet

Process for producing thin cast strip

Thin steel sheet having excellent stretch-flange ability and process for

Non-aging at room temperature ferritic single-phase cold-rolled steel

A novel hot-rolled ferritic steel composed of 70% or more of equiaxed ferrite grains having an ultra-fine grain size of 4 .mu.m or less. This steel is produced by a hot working at approximately the Ar.sub.3 point and by one or more passes of the hot working having a total reduction ratio of at least 75%. When a plurality of passes is carried out, the time between passes is less than one second. Due to hot working, dynamic transformation of austenite and/or dynamic recrystallization of ferrite takes place. The total reduction ratio may be at least 35% for a high purity steel.

Ferritic steel having ultra-fine grains and a method for producing the

A process for producing a steel having excellent strength and toughness, which comprises continuously casting a molten steel containing 0.01 - 0.10% Al and 0.002 - 0.009% N with an average cooling rate of not lower than 3.degree. C/min. down to 500.degree. C to obtain a steel slab heating the steel slab to a temperature ranging from 900.degree. to 1150.degree. C and rolling the steel slab.

Process for producing a steel having excellent strength and toughness

Growing community of inventors

Futtsu, Japan

Yoshikazu Matsumura