Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2000-07-21
2001-12-11
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S602000
Reexamination Certificate
active
06328826
ABSTRACT:
The invention relates to the continuous casting of steel in the form of thin strip. It relates more particularly to the fabrication of “TRIP” steel in the form of thin strip directly from liquid metal.
TRansformation Induced Plasticity (TRIP) steel combines high strength and high ductility, which makes it particularly suitable for shaping. These properties are obtained by virtue of its particular microscopic structure. It has, within a ferritic matrix, a hard bainite and/or martensite phase and residual austenite representing from 5 to 20% of the structure. TRIP steel plate is usually obtained either by continuous casting of slabs followed by hot rolling (the quickest method, and therefore the most economic, but which produces relatively thick products) or by continuous casting of slabs followed by hot rolling, cold rolling and annealing (the method used for thin products). The bainite stabilizes the austenite.
The following problem makes it difficult to make high-quality TRIP steel plate from strip obtained by the conventional continuous casting and hot rolling method. After hot rolling the initial slab, the austenite has to be stabilized during the bainitic transformation that occurs during coiling of the strip, which takes place at a temperature of 400° C. (±50° C.). The hot-rolled strip is cooled to its coiling temperature by sprinkling it with water. This cooling occurs in a range of temperatures in which a phenomenon referred to as “rewetting” can occur. This phenomenon is caused by the instability of steam in contact with the strip caused by Leidenfrost's phenomenon, some of the steam returning to the liquid state. The resulting localized contact between the strip and liquid water, rather than steam, leads to inconsistent quenching of the strip. Inconsistent cooling leads to significant inconsistencies in the microstructure of the strip, degrading its mechanical properties.
The object of the invention is to make it possible to produce high-quality TRIP steel strip reliably on a short production line, i.e. one that does not include any cold rolling or annealing step.
To this end, the invention provides a method of fabricating “TRIP” steel in the form of thin strip, wherein:
a strip from 1.5 to 10 mm thick, preferably from 1 to 5 mm thick, is cast directly from liquid steel having the composition (in weight percent) C% from 0.05 to 0.25, (Mn+Cu+Ni) % from 0.5 to 3, (Si+Al) % from 0.1 to 4, (P+Sn+As+Sb) % not greater than 0.1, (Ti+Nb+V+Zr+rare earths) % less than 0.3, Cr % less than 1, Mo % less than 1, V % less than 1, the remainder being iron and manufacturing impurities;
said strip is hot rolled on-line in one or more passes at a temperature higher than the Ar
3
temperature of said steel and with a reduction ratio from 25 to 70%;
first forced cooling of said strip is carried out at a cooling rate from 5 to 100° C./s;
the strip remains at temperatures from 550 to 400° C. for the time needed for bainitic transformation to occur therein with a residual austenite content greater than 5%, whilst preventing the formation of perlite, after which the transformation is interrupted by second forced cooling of said strip to a temperature below 400° C.; and
said strip is coiled at a temperature below 350° C.
The invention also provides thin “TRIP” steel strip obtained by the above method.
Clearly a first essential aspect of the invention is the continuous casting of the steel to form thin strip directly from the liquid metal, instead of the standard process of casting slabs to be hot rolled on a strip mill. The strip produced in this way is hot rolled on-line and then cooled to the range of temperatures in which the bainitic transformation occurs. It is only when that transformation has occurred and the required microstructure typical of TRIP steel has been obtained that further cooling is applied, which interrupts the transformation and cools the strip towards its coiling temperature. That temperature is lower than that of hot-rolled strip produced by the conventional process because the bainitic transformation has already occurred and because maintaining the coiled strip in the range of temperatures at which the transformation occurred would entail the risk of undesirable changes to the microstructure.
REFERENCES:
patent: 5470539 (1995-11-01), Nomura et al.
patent: 5567250 (1996-10-01), Akamatsu et al.
patent: 6027587 (2000-02-01), Hodgson et al.
patent: 6210496 (2001-04-01), Takagi et al.
patent: 0 666 332 (1995-08-01), None
patent: 0881 306 (1998-12-01), None
patent: 60-043425 A (1995-03-01), None
patent: WO 98/20180 (1998-05-01), None
Babbit Michel
Faral Michel
Faral Odile
Issartel Christophe
Iung Thierry
Combs-Morillo Janelle
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Usinor
Wyszomierski George
LandOfFree
Method of fabricating “TRIP” steel in the form... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of fabricating “TRIP” steel in the form..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of fabricating “TRIP” steel in the form... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2586792