Metal treatment – Stock – Ferrous
Patent
1994-12-20
1996-10-22
Yee, Deborah
Metal treatment
Stock
Ferrous
148661, 148541, E21D 804
Patent
active
055672507
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an as-cast thin steel sheet having a casting thickness of 0.5 to 5 mm and particularly to a thin steel sheet having an excellent stretch-flange ability and a process for producing the same.
BACKGROUND ART
At the present time, a thin steel sheet having a sheet thickness of 1.4 to 5 mm is produced as a hot-rolled steel sheet by using, as a starting material, a slab having a thickness exceeding 200 mm and subjecting the material to hot rolling. In the above current process, the basis of the technique for formation of an intended structure in the present saturation, that is, the regulation of the structure, is to increase the number of nucleation sites in transformation by causing recrystallization in the material in the step of hot-rolling the material to refine a coarse austenitic structure to increase the intergranular area or by rolling the material in a non-recrystallization region to introduce a deformation zone (a zone where the dislocation density is locally high) or by using other means, thereby enabling the structure of ferrite or the like, produced during cooling, to be refined.
Incidentally, in the conventional process, the grain diameter of the austenite before transformation is not more than 20 .mu.m, and also in the structure obtained by transformation, the grain diameter of the ferrite, for example, is not more than 20 .mu.m.
One of the hot-rolled steel sheets developed in the current process, which is a material required formability after punching (this material being used in, for example, strengthening components (members, wheels, etc.) of automobiles) is a high-strength hot-rolled steel sheet having an excellent stretch-flange ability (enlargeability). Such a steel sheet should have both a high strength as a strengthening member and workability. Up to now, high-strength steel sheets having a strength of up to 60 to 70 kgf/mm.sup.2 have been developed. As disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 61-19733 and 1-162723, the steel sheets have a composite structure comprising a fine ferrite and a fine (in terms of packet size) low-temperature transformation phase (a fine pearlite, bainite or temper martensite). The term "packet" used herein is intended to mean a group of small units of a low-temperature transformation phase comprising a group of similar grain orientations which are identified by etching or the like. It is known that the local ductility, such as stretch-flange ability, is generally lowered when a phase having a hardness much greater than ferrite, such as cementite or martensite of large size, occupies, and attention has been paid particularly to homogenization and refinement (to not more than about 20 .mu.m) of the structure.
On the other hand, advances in casting techniques in recent years have enabling a thin cast strip having a thickness corresponding to that of the hot-rolled steel sheet to be produced by a twin roll casting process or the like. Since hot rolling used in the prior art can be completely omitted, this process has been studied as a cost-effective and energy-saving process mainly for producing a material for a cold-rolled steel sheet subjected to cold rolling/annealing. However, when the thin cast strip, as such, is regarded as a material corresponding to a hot-rolled steel sheet, since the austenite grain diameter is as large as about 1000 .mu.m, the structure mainly composed of ferrite also is generally likely to coarsen significantly. For this reason, the properties of the thin cast strip have hardly been studied.
The present inventors have aimed at the above thin cast strip and made studies with a view to producing a steel sheet having an excellent toughness or strength-ductility balance from the thin cast strip. As a result, they have succeeded in forming a fine bainite or Widmanstatten ferrite structure by cooling the material in an austenite region, i.e., in the temperature range of from 900.degree. to 400.degree. C., at a cooling rate of 1.degree. to 30.degree. C./se
Akamatsu Satoshi
Matsumura Yoshikazu
Nippon Steel Corporation
Yee Deborah
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