Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2000-07-07
2002-03-19
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S619000, C148S620000
Reexamination Certificate
active
06358338
ABSTRACT:
The invention relates to the manufacture of strip made of ferrous alloys. More particularly it relates to the manufacture of strip made of an iron-carbon-manganese alloy by direct casting in the form of thin strip.
Hadfield steels, comprising Fe—Mn(11 to 14%)-C (1.1 to 1.4%), which may be termed “high manganese steels”, have been known for a long time. They have the feature of being very strong and able to undergo ageing under the effect of repeatedly applied friction forces or impacts. Also known are austenitic steels of the Fe—Mn(15 to 35%)-Al (0 to 10%)-Cr (0 to 20%)-C (0 to 1.5%) type which derive simultaneously from Hadfield steels and from Fe—Cr—Ni austenitic stainless steels in which the nickel is progressively replaced by manganese and the chromium progressively replaced by aluminium. These high manganese steels are characterized by a high work-hardenability which allows them to combine a high strength level with excellent ductility. Thus, they can be advantageously used for the manufacture of reinforcing elements manufactured for the motor-vehicle industry by drawing or stamping. These steels owe their high work-hardenability to mechanical twinning, possibly enhanced by the &ggr;→&egr; martensitic transformation. By propagating, the twins facilitate plastic deformation but, where mutually impeding one another, they also contribute to increasing the yield stress.
Various documents discuss the composition and the manufacture of such very-high manganese steels, for example WO 93/13233, WO 95/26423, WO 97/24467. These steels have always, until now, been manufactured by the conventional process of continuous casting of thick slabs approximately 200 mm in thickness/hot rolling/cold rolling/annealing/pickling/skin-pass. This process essentially has three drawbacks. Firstly its cost, due to the use of a strip mill which is a plant requiring a very high investment and consuming a great deal of energy, since it is needed to greatly reheat the slabs before they are rolled. Secondly, there is a risk of the strip hot-cracking during this reheat, during which a thick layer of scale also forms, this being unfavourable both to the surface quality of the product and to the metallurgical efficiency of the manufacturing process. Thirdly, overall, it is a long manufacturing process not always making it possible to react promptly to a pressing demand on the part of a customer.
The object of the invention is to propose a method of manufacturing strip made of ferrous alloys having a high manganese content more rapidly and less expensively than the known conventional method and making it possible to obtain products at least as good in quality as those by that previous method.
For this purpose, the subject of the invention is a process for producing strip made of an iron-carbon-manganese alloy, in which:
a thin strip, having a thickness of 1.5 to 10 mm, is cast directly on a casting machine from a liquid metal of composition, in percentages by weight: C ranging between 0.001 and 1.6%; Mn ranging between 6 and 30%; Ni≦10% and with (Mn+Ni) ranging between 16 and 30%; Si≦2.5%; Al≦6%; Cr≦10%; (P+Sn+Sb+As)≦0.2%; (S+Se+Te)≦0.5%; (V+Ti+Nb+B+Zr+rare earths)≦3%; (Mo+W)≦0.5%; N≦0.3%; Cu≦5%; the balance being iron and impurities resulting from the smelting;
the said strip is cold rolled with a reduction ratio ranging between 10 and 90% in one or more steps; and
the said strip undergoes recrystallization annealing.
The invention also relates to a strip that can be produced by this process.
As will have been understood, the invention relies firstly on the use of a process for casting liquid metal directly in the form of a thin strip. The latter may possibly undergo in-line hot rolling by means of a plant of small size, the manufacturing and running cost of which is very much less than that of a strip mill. In addition, the omission of the hot rolling on a strip mill eliminates the risks of hot cracking during the reheat of which mention was made. Following thereafter are cold-rolling, annealing and possibly skin-pass operations, the execution of which, according to the embodiments which will be specified, allows the desired product properties to be obtained.
The invention will be more clearly understood on reading the description which follows.
The process of directly casting thin steel strip from 1.5 to 10 mm thickness is well known at the present time, especially in its form called “twin-roll casting”. The liquid steel solidifies against the side walls of two closely spaced horizontal rolls, which are internally cooled and rotating in opposite directions, and emerges beneath the rolls in the form of a solidified strip. The latter may be coiled directly and then sent to the cold-processing plants, or may undergo in-line hot rolling before being coiled. According to the invention, the use of such a process makes it possible to shorten the process for manufacturing strip made of high manganese steel by eliminating the pass through the strip mill, whereas this pass is necessary in the conventional process which begins by casting slabs. This elimination is all the more advantageous when the high manganese austenitic steels are characterized by the absence of a phase transformation while they are being cooled. This is because one of the conventional functions of the hot rolling of ferritic, carbon or stainless steels is the refinement of the microstructure just before the phase transformation occurs. However, high manganese steels, which offer the best strength/ductility compromise at the forming temperature are completely austenitic, at least before deformation, from their point of solidification to the end of their cooling. Therefore there is no significant metallurgical advantage in hot rolling high manganese austenitic steels. Its function is limited to a simple thickness reduction of the product in order to obtain a strip capable of being cold rolled. In such cases, there is therefore no drawback in obtaining, by thin strip casting, a strip having a thickness relatively close to its final thickness, as long as the said strip is free of any central porosity after it has been cast. Light in-line hot rolling, as described above, is sufficient to close up any such porosity.
The invention applies to the manufacture of high manganese steels which have the following composition, the percentages being percentages by weight:
their carbon content ranges between 0.001 and 1.6%, preferably between 0.2 and 0.8%; a content of less than 0.2% requires the pool of liquid steel to be decarburized, which can be expensive to carry out, particularly when manganese is already present in a significant quantity; moreover, this minimum amount of 0,2% allows to obtain an interaction between carbon and dislocations:carbon, by locking the dislocations, allows a further hardening compared to twinning, and allows to improve the tensile strength by 50 to 100 Mpa; an amount greater than 0.8% makes it more difficult to optimize the contents of the other alloying elements for the purpose of obtaining the most favourable mechanical properties;
their manganese content ranges between 6 and 30%, bearing in mind that the total of their manganese and nickel contents ranges between 16 and 30% and that their nickel content may range up to 10%;
their silicon content may range up to 2.5%, bearing in mind that this element is only optional;
their aluminium content is less than 6%, bearing in mind that this element is only optional;
if chromium is present, the chromium content is at most 10%;
their phosphorus content may range up to 0.2%, it being known that tin, antimony and arsenic, which may possibly be present, are, from this standpoint, similar to phosphorus and compatibilized with it in the composition of the steel; above this level, there is a risk of obtaining defects in the segregated zones of the strip; these defects are caused by delays in the solidification at the point where segregation occurs; if the product is hot rolled while metal in the
Faral Michel
Faral Odile
Guelton Nicolas
Combs-Morillo Janelle
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Usinor
Wyszomierski George
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