Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Patent
1998-04-15
2000-04-25
Ip, Sikyin
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
148546, 148601, 148602, C21D 109
Patent
active
060539968
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method for the manufacture of a strip of formable steel.
DESCRIPTION OF THE PRIOR ART
EP-A-370575 describes a method for making formable steel strip in which liquid steel is formed in a continuous casting machine into a thin slab with a thickness smaller than 100 mm, and, with use of the casting heat, the steel slab is rolled in the austenitic range into an intermediate slab. The intermediate slab is cooled to a temperature below Ar.sub.3 and, at a temperature below T.sub.t, at which 75% of the material is converted into a ferrite, and above 200.degree. C. is rolled into the strip. A drawback of this method is that, to use it for manufacturing a steel strip with good forming properties, it requires a complicated plant, not least because of the proposed large reduction in the ferritic range and the recrystallisation furnaces needed for obtaining a desired structure. Related methods, less relevant to the present discussion are disclosed in EP-A-306076 and EP-A-504999.
SUMMARY OF THE INVENTION
One object of the invention is to provide a method which can be carried out continuously and with a simple plant, and by which a steel strip with good forming properties can be obtained.
In one aspect the invention provides a method for the manufacture of a strip of formable steel, comprising the steps of, in the following order: thickness of not more than 100 mm, austenitic region, into an intermediate slab having a thickness in the range 5 to 20 mm, Ar.sub.3 temperature of the steel, homogenisation thereof, pass applying a thickness reduction of more than 50%, the intermediate slab being below a temperature T.sub.t at which 75% of the steel is converted into ferrite and above 200.degree. C.,
This method requires a smaller number of process stages. By this method good forming properties may be achieved without the steel strip requiring recrystallisation annealing. The finishing train by which the intermediate slab is rolled into the strip may be of simple construction because only a relatively small reduction is made. Another advantage is that, because the mean temperature during the entire process is on average higher, the rolling forces are on average lower. The plant for carrying out the method may then be built lighter and with a lower installed capacity.
Another advantage is that storage from the homogenisation can allow sufficient time for precipitation to TiC in the case of IF steel.
Preferably the steel strip is coiled at a temperature above 600.degree. C. So-called self-annealing then occurs in the coiled coil as a consequence of the heat content of the steel strip.
Another advantage of the relatively thin intermediate slab is that the thickness reduction in the ferritic region is relatively small and that the relationship between exit speed and entry speed is thus relatively low. The exit speed may be set at around a conventional value of 600 m/min, for which technology is available. Because the intermediate slab is relatively thin the entry speed is still high. The advantage of this is that the time that the intermediate slab is exposed to the surrounding atmosphere, thus allowing oxide to form on its surface, is brief. Therefore, with the method it is possible to make a strip with little oxide space. The entry speed is preferably >0.8 m/s.
Improved deformation properties of the steel strip are obtained because the intermediate slab undergoes at least one pass having at least 50% reduction in the ferritic region. Such deformation is quite adequate for introducing recrystallisation. In addition the advantage is achieved that, with such deformation, the temperature drop of the steel as a consequence of heat loss to the surroundings and to the mill rolls may be considerably compensated for by the deformation energy introduced into the steel during rolling. By applying this reduction, virtually no heat loss occurs in the rolling train, so that the intermediate slab can be rolled in the first mill stands at relatively low temperatures and less oxide wil
REFERENCES:
patent: 4885041 (1989-12-01), den Hartog et al.
patent: 5042564 (1991-08-01), Van Perlstein et al.
Cornelissen Marcus Cornelis Maria
Den Hartog Huibert Willem
Pronk Cornelis
Hoogovens Staal BV
Ip Sikyin
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