MXPA99008446A - Manufacturing method for stainless steel thin strip - Google Patents

Abstract

Manufacturing thin strip of stainless steel by direct solidification of liquid steel, with rolling mill with cylinders with specified diameter, the strip temperature and the rate of reduction of the strip thickness are specified Manufacturing a thin strip of stainless steel comprises direct solidification of liquid steel in a casting installation comprising two cooled moving walls;solidification is finished when the strip leaves the walls;and hot laminating this strip using a rolling mill with cylinders having a diameter of 400-900 mm, the strip temperature after leaving the rolling mill is 800-1100 degrees C;the rate of reduction of the strip thickness during hot lamination is 15-50%.

Description

PROCEDURE FOR THE MANUFACTURE OF A STAINLESS STEEL BAND FIELD OF THE INVENTION The invention relates to the manufacture of thin strips of stainless steel, directly from liquid metal, by solidification inside an ingot mold constituted by two cooled walls moving at the same speed as the solidified strip, such as the walls external of two cylinders in rotation with horizontal axes.

BACKGROUND OF THE INVENTION In the casting process which is under industrialization under the name of "casting between cylinders", one of the main problems related to the quality of the band is the possible presence of porosities in the center of the band. When these porosities are found in the products at the exit of the further transformations suffered by the band (such as pickling, annealing, cold rolling and other processing operations), they limit the REF .: 31310 field of application of the products because of the alteration of the mechanical properties that procure them. The causes of appearance of these porosities in the center of the bands cast between cylinders, can be similar to those that provoke (in a higher dimensional scale) the reinsurances in the ingots and the central porosities in the products of classic continuous casting, namely the closing by the solid metal of bags that enclose the metal also liquid during the solidification of the product (which, normally, is substantially finished when the strip removes the walls of the ingot mold, that is to say, that the center of the band is not found more than totally in the liquid state) is not done in a completely regular manner. The cooling and solidification of the liquid metal that these bags enclose are accompanied by a contraction of this metal, which makes an empty space appear. This can not be filled before the completion of the solidification, because this closed bag is fed only by the new liquid metal. These porosities must be distinguished from the spherical faults called "vents" which are due to an exit of dissolved gas and which occurs most of the time in the vicinity of the surface of the products.

EP 0 396 862 proposes a method that focuses on suppressing the central porosities, and also other internal and surface faults, during the casting of the steel bands between two cylinders. According to this method, the casting cylinders comprise on the surfaces of precisely dimensioned circumferential grooves, and arranged in an offset manner on the two cylinders. It is thus intended to avoid the peeling of the solidified metal coatings on the surfaces of the rolls, which carry irregularities in the solidification of the strip. However, it seems that the only prevention of the detachments of these is insufficient to completely avoid the appearance of central porosities. JP 8252653 provides a method according to which a hot rolling of the strip is practiced in line with the casting under conditions that comply with the following inequality: r > (2.74 x 10"x 10" 2T + 43.55) (to / Wo) r: reduction rate of hot rolling; T: temperature of the hot rolled in ° C; t0: diameter of the porosity in the direction of the thickness of the band; - W0: diameter of the porosity in the direction of the length of the band. It is needed, according to the procedure, that the hot rolling is carried out with a sufficient reduction rate so that the porosities are closed during this lamination, and these minimum rates depend on the temperature of the laminate (i.e. which is the band on its entry into the influence of the cylinders) and the shape and orientation of the porosities. However, it has been found that these lamination conditions are still insufficient to obtain a certain closure of all the porosities, and above all that they do not always prevent the closed porosities from opening again during the transformation of the band or the implementation of the products that are issued, which causes their rupture.

DESCRIPTION OF THE INVENTION The aim of the invention is to propose a process that guarantees a definitive closure of the central porosities that appeared in the center of the band after its complete solidification. To this end, the subject of the invention is a method of manufacturing a thin strip of stainless steel, by direct solidification of the liquid steel in the form of a strip of thickness less than or equal to 8 mm in a casting installation comprising two walls cooled in motion and by hot rolling of the strip whose solidification is substantially finished when it removes the walls, characterized in that hot rolling is carried out in a rolling mill of which the working rolls have a diameter comprised between 400 and 900 mm, because the temperature of the strip at its exit from the laminator is between 800 and 1100 ° C, and because the rate of reduction of the thickness of the strip during hot rolling is between 15 and 50%. Preferably, the hot rolling is carried out in line with the casting of the strip. The casting installation can be of the "cast between cylinders" type. As in the aura or field comprised, the objective of the invention is to achieve, by the combination of demands that carry on the diameter of the working rolls of the hot rolling mill, the temperature of the band at its exit from the cylinders and the rate of reduction of the thickness of the band during the hot rolling. The invention is applied to the casting of stainless steels of all kinds, which classically have carbon amounts less than or equal to 1%, amounts of silicon less than or equal to 1%, amounts in manganese less than or equal to 15%, amounts in chrome comprised between 10 and 30%, amounts in copper less than or equal to 5% and amounts in nitrogen less than or equal to 0.5% (these quantities are expressed in percentages by weight). These steels can also contain significant amounts of nickel (up to 40%) or molybdenum (up to 8%). On the other hand, as is commonly the case, other elements are present in the metal, either as impurities, or as alloying elements, in particular sulfur, phosphorus, titanium, niobium, zirconium. Your total must not exceed 2% by weight. As has been said, a thin band of stainless steel cast between cylinders is strongly susceptible to develop porosities in its center, during its solidification, when a liquid bag is closed by the solid metal. This phenomenon occurs at the end of the solidification of the pasty zone, also called «equi je zone», located between the two solidified layers at the contact of the cylinders, also called «columnar zones». The area equieje is very difficult to matrix, and its thickness can vary depending on the speed of solidification of the columnar zones. Thus, the equieje zone can be closed locally in advance, at the levels where the crossing of the colonized areas has been faster than normal. Below the closing point of the equieje zone, the liquid bags can not be more correctly realized in liquid metal, and the porosities are formed by contraction of the metal during the solidification of these liquid bags. However, this case also remains rare, and in fact, generally, the isolation of a liquid bag is produced by the regrouping in the liquid of glass crystals that become a plug that obstructs the equieje zone. The porosities that form in the area equieje are constituted by the sets of channels and cavities free of gas, whose maximum dimension according to the thickness of the sheet or palastro corresponds to the thickness of the area equieje (or 100 to 400 μm), and which can reach a length of 1 to 2 mm in the other directions. As it has been said, it is not a spherical blow or vein that would have as origin a gas outlet, or an internal fault that ends at the surface of the strip.

The idea of the basis of the invention is to create, during the hot rolling of the solidified strip, the conditions such that they lead not only to a closing of the central porosities as is already known, but also to a true welding of the opposite walls of the porosities, that the laminate has allowed to diminish. In this way, it is ensured that the porosities do not risk opening during the subsequent laying of the band or during the use of the products thus made. During the hot rolling of the band, two stages happened. First, the internal walls of failure progressively approached as the thickness of the band was reduced, until its contact. Then, once this contact was made, a welding of the walls takes place by diffusion of the constituent elements of the steel through the interface. But an effective welding of the walls must already have been obtained from before the exit of the band from the influence of the rollers of the rolling mill, otherwise the relaxation of the compression of the band which occurs at the exit of the cylinders leads to a partial peeling of the walls. The efficiency of the welding depends essentially on two parameters: the duration of the forced contact of the walls in the mill and the temperature at which this contact is made. This forced contact should take place as soon as possible after the belt has entered the laminator, and its duration depends mainly on a given rolling speed (which is, in the case of an in-line rolling, in large quantities). part conditioned by the thickness of the band before its rolling), the diameter of the working cylinders of the rolling mill and the reduction rates of their thickness that they impose on the band. When more the diameter of the cylinders and the reduction rate are high, then the forced contact of the walls of the porosities is rapid and prolonged. However, it can not be satisfied with affirming that it suffices, in order to solve the problem satisfactorily, to laminate the band with a reduction rate and a diameter of cylinders as high as possible. In fact, a very high reduction rate, which would go below the hot deformation capacities of the band, leads to the appearance of superficial cracks in the band called "cracks", which are absolutely to avoid. On the other hand, the temperature at which the forced contacting of the porosity walls takes place depends not only on the inlet temperature of the band in the laminator, but also on the duration of the contact between the band and the cylinders, because this contact causes a cooling of the band. If, for a given temperature of the band, the cylinders have a very high diameter, the cooling of the band causes the risk of driving it at an insufficient temperature so that the welding of the porosity walls can be complete. To this effect, the value of the temperature of the band at its exit from the cylinders will provide a good indication of the real possibility that the walls have had to weld together in the influence of the cylinders. The temperature of the band at the exit of the cylinders must be sufficient to allow the welding of the porosities, but it must not be too high, in order to avoid an excessive thermal load on the cylinders. This would lead to a degradation of its surface which leads to a deterioration of the appearance of the surface of the strip in the form of excessive roughness. The object of the invention can not be achieved without obtaining annoying side effects for the overall quality of the band which, if combined with the diameter of the rolls, the reduction rate and the temperature of the strip at the exit of the rolling mill. properly. To determine how these parameters should be combined, series of tests or tests have been carried out, during which, for a given type of stainless steel, the diameter of the working rolls of the rolling mill, the rate of reduction of the thickness of the band and the temperature of the band are varied at the exit of the laminator. The laminator is arranged in line with the casting installation. Each test has made the object of a characterization that allows to determine if the welding of the porosities has been effective or not. This characterization consisted of breaking a tensile specimen and examining the breaking sides. If the sides have porosities that have been opened during the tensile test, it follows that the welding has not been satisfactory. If the sides do not exhibit any apparent porosity, the weld was judged to have been satisfactory. Table 1 lists the steel compositions in which the tests have been carried out, the results of which are given in Table 2. The amounts of the different elements are given in percentage by weight. Table 1 also provides the thicknesses at the outlet of the casting rolls of the bands in which the tests have been carried out, as well as the corresponding casting speeds, measured between the casting rolls and the hot rolling mill.

Table 1: Composition of test casting steels, cast strip thickness and casting speed The compositions of type A and Ar castings correspond to those of classic austenitic stainless steels of type AISI 304. Type B castings correspond to ferritic stainless steels of type AISI 430. Type C castings correspond to ferritic stainless steels of type A and Ar. AISI 409 type stabilized with titanium. Table 2 shows the results of the tests carried out in the bands left from these castings, with the corresponding test conditions.

Table 2: Results of tests carried out in types A, A ', B, C castings The result of these tests is that an effective welding of the porosities is obtained without the appearance of cricketers and without excessive roughness on the surface of the band. when the three following conditions are met: - a diameter of the working rolls of the rolling mill comprises between 400 and 900 mm; - a rate of reduction of the thickness of the band during the lamination comprised between 15 and 50%; - Nail temperature at the exit of the laminator of at least 800 ° C and at most 1100 ° C. On the other hand, there was no influence, under the test conditions, on the combination of the thickness of the casting velocity-band: the results of type A castings are identical to those of the castings of type A, for casting parameters on the other hand identical. These tests, as has been said, have been carried out with a hot rolling mill arranged in line with the casting installation and which precedes the winding installation of the band. In the spirit of the invention, this feature is indispensable, and the hot rolling can be carried out in a separate installation of the casting and winding installation, thus after unwinding and overheating of the pure or unmixed casting band. However, an online laminate is recommended for several reasons. Mainly, this solution presents the economic advantages linked to the continuous nature of the operations. In the first place, the manufacturing process of the band is abbreviated or reduced. In addition, the coil is economical, and also the reheating facility of relatively high power, since the cast strip can be sufficiently heated to obtain the appropriate rolling temperatures, possibly with the help of a cape that stops the band radiation between its exit from the casting cylinders and its entry into the laminator. However, if a superheat of the band is found necessary, it can be done with the help of an induction furnace of reduced power, sufficient to raise the temperature of the defoliating band of a few hundred degrees. On the other hand, an in-line lamination, eliminating the need for a winding of the raw casting band, at the same time eliminates the risks of deterioration of the strip during this winding that will take place in a relatively thick band having a non-recrystallized structure. Finally, the suppression of the reheating of the band from the ambient temperature to the hot rolling temperature suppresses the reoxidations of the band surface which are usual during this operation. These reoxidations would form the calamine, which would require embedding both in the band and in the rolls of the laminator and thus lead to a degradation of the appearance of the surface of the product after pickling or deoxidation. The invention applies not only to casting installations between cylinders, but to any other type of casting installation of thin strips of stainless steel between two surfaces cooled in motion, such as the deflected strips.

It is noted that in relation to this date the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, the content of the following is claimed as property

Claims (3)

1. A method of manufacturing a thin strip of stainless steel by direct solidification of the liquid steel in the form of a strip of thickness less than or equal to 8 mm in a casting installation comprising two walls cooled in motion and by hot rolling said band whose solidification is substantially finished when it removes the walls, characterized in that the hot rolling takes place in a rolling mill whose working cylinders have a diameter comprised between 400 and 900 mm, because the temperature of the strip at its exit from the rolling mill is between 800 and 1100 ° C, and because the rate of reduction of the thickness of the strip during hot rolling is between 15 and 50%.

2. The method according to claim 1, characterized in that the hot rolling is carried out on an installation placed in line with the casting installation.

3. The process according to claim 1 or 2, characterized in that the cooled walls of the casting installation are constituted by the surfaces of two cylinders in rotation to horizontal axes.