EP0988901B1 - Manufacturing method for stainless steel thin strip - Google Patents

Abstract

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

The invention relates to the manufacture of thin strips of stainless steel, directly from liquid metal, by solidification inside an ingot mold consisting of two cooled walls moving at the same speed as the solidified strip, such as the outer walls of two rotating cylinders with horizontal axes.

In this casting process which is being industrialized under the name of "Casting between cylinders", one of the major problems related to the quality of the strip is the possible presence of porosities in the core of the strip. When these porosities found on the products at the end of the subsequent transformations undergone by the strip (such as pickling, annealing, cold rolling and other operations of processing), they limit the scope of the products due to spoilage mechanical properties they provide.

The causes of the appearance of these porosities at the heart of the bands cast between cylinders can be similar to those that cause (on a dimensional scale upper) the shrinkage in the ingots and the central porosities in the products of classic continuous casting, namely the closure of pockets containing solid metal still liquid metal when the solidification of the product (which normally is substantially completed when the strip leaves the walls of the ingot mold, i.e. the core of the strip is no longer completely in the liquid state) is not carried out completely regularly. The cooling and solidification of the liquid metal that enclose these pockets are accompanied by a contraction of this metal, which causes a empty space. This can not be filled before the end of solidification, because this pocket closed is no longer fed by new liquid metal. These porosities must be distinguished from spherical faults called "blowing" which are due to a release of gas dissolved and most often occur near the surface of products.

Document EP 0 396 862 proposes a process aimed at eliminating the porosities central, and also other internal and superficial defects, when casting strips of steel between two cylinders. According to this process, the casting rolls have on their surfaces of the circumferential grooves precisely dimensioned, and arranged in offset on the two cylinders. We thus aim to avoid peeling of the skins solidified metal on the surfaces of the cylinders, which would cause irregularities in solidifying the strip. However, it appears that the only prevention of these detachments is insufficient to completely avoid the appearance of central porosities.

• r : taux de réduction du laminage à chaud ;
• T : température de laminage à chaud en °C ;
• t₀: diamètre de la porosité dans le sens de l'épaisseur de la bande ;
• w₀ :diamètre de la porosité dans le sens de la largeur de la bande.

Document JP 8252653 proposes a process according to which a hot rolling of the strip is carried out in line with the casting under conditions respecting the following inequality: r≥ (2.74 x 10 -5 T 2 - 6.88 x 10 -2 T + 43.55) (t 0 / w 0 ) with:

• r: reduction rate of hot rolling;
• T: hot rolling temperature in ° C;
• t ₀ : diameter of the porosity in the direction of the thickness of the strip;
• w ₀ : diameter of the porosity in the direction of the width of the strip.

According to this process, it is therefore necessary that the hot rolling takes place with a rate of sufficient reduction for the porosities to be closed during this rolling, and this rate minimum depends on the rolling temperature (i.e. the temperature at which finds the strip as it enters the grip of the cylinders) and the shape and orientation porosities. However, it has been found that these rolling conditions are still insufficient to ensure that all the pores are closed, and above all that they do not always prevent the closed porosities from reopening during the transformation of the strip or the implementation of the products which result therefrom, which causes their rupture.

The object of the invention is to propose a method guaranteeing closure final of the central porosities appeared in the core of the strip after its solidification complete.

To this end, the subject of the invention is a method for manufacturing a thin strip. 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 cooled walls in movement and by hot rolling of said strip, the solidification is substantially completed when it leaves said walls, characterized in what hot rolling is done on a rolling mill whose working rolls have a diameter between 400 and 900 mm, in that the temperature of the strip on leaving the rolling mill is between 800 and 1100 ° C, and in that the rate of thickness reduction 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 bandaged. The casting installation can be of the “casting between cylinders” type.

As will be understood, the object of the invention is achieved by the combination requirements relating to the diameter of the working rolls of the hot rolling mill, the temperature of the strip at its exit from the cylinders and the rate of reduction of the thickness of the strip during hot rolling.

The invention applies to the casting of stainless steels of all classes, which have conventionally carbon contents less than or equal to 1%, silicon contents less than or equal to 1%, manganese contents less than or equal to 15%, chromium contents between 10 and 30%, copper contents less than or equal to 5% and nitrogen contents less than or equal to 0.5% (these contents are expressed in weight percentages). These steels can also contain significant quantities nickel (up to 40%) or molybdenum (up to 8%). By the way, as it is usually the case, other elements are present in the metal, impurities, either as alloying elements, in particular sulfur, phosphorus, titanium, niobium, zirconium. Their total must not exceed 2% by weight.

As mentioned, a thin strip of stainless steel poured between cylinders is highly likely to develop porosities in its heart, during its solidification, when a liquid pocket is closed with solid metal. This phenomenon occurs at the end of solidification of the pasty zone, also called "equiaxial zone", located between the two skins solidified in contact with the cylinders, also called "columnar areas". The area equiaxial is very difficult to control, and its thickness can vary depending on the speed of solidification of columnar areas. Thus, the equiaxial zone can close locally from anticipated, at levels where the growth of columnar areas has been faster than the normal. Downstream of the closing point of the equiaxial zone, the liquid bags do not can no longer be properly replenished with liquid metal, and porosities form by contraction of the metal during the solidification of these liquid pockets. This case remains nevertheless quite rare, and in fact, generally, the isolation of a liquid pocket occurs by the grouping in the liquid of equiaxial crystals which come to form a plug obstructing the equiaxial zone. The porosities which form in the equiaxial zone are consisting of sets of gas-free channels and cavities, the dimension of which maximum according to the thickness of the sheet corresponds to the thickness of the equiaxial zone (i.e. 100 at 400 µm), and which can reach a length of 1 to 2 mm in the other directions. As we said, it is not a spherical blow which would originate from a evolution of gas, or an internal fault opening on the surface of the strip.

The idea behind the invention is to create, during the hot rolling of the strip solidified, conditions such that they lead not only to a closure of central porosities as it is already known, but also a real welding of opposite walls of the porosities, which the rolling made it possible to bring together. In this way, we ensures that the porosities are not likely to open during subsequent shaping of the strip or when using the products thus produced. When hot rolling the strip, two stages follow one another. First, the internal walls of the defect are gradually approach as the thickness of the strip decreases, until brought into contact. Then, once this contact has been made, the walls are welded produced by diffusion of the constituent elements of steel through the interface. But one efficient welding of the walls must already have been obtained before the exit of the strip the grip of the rolls of the rolling mill, because otherwise the relaxation of the compression of the strip which occurs at the outlet of the cylinders leads to a partial detachment of the walls.

The efficiency of welding essentially depends on two parameters: the duration of the forced contact of the walls in the rolling mill and the temperature at which it takes place this contact. This forced contact must therefore take place as soon as possible after entry of the strip in the rolling mill, and its duration depends mainly, for a speed of given rolling (which is, in the case of in-line rolling, largely conditioned by the thickness of the strip before it is rolled), the diameter of the working rolls of the rolling mill and the reduction rate of its thickness which they impose on the strip. The longer the cylinder diameter and reduction rate, the higher the forced contact of the walls of the porosities is rapid and prolonged. We cannot, however, be satisfied to assert that it suffices, in order to solve the problem posed satisfactorily, to laminate the strip with a reduction ratio and a cylinder diameter as high as possible. Indeed, a reduction rate too high, which would go beyond the deformation capacities to hot of the strip, leads to the appearance of surface cracks on the strip called "Coves", which are absolutely to be avoided. On the other hand, the temperature at which takes place the forced contacting of the walls of the poros depends not only on the temperature of entry of the strip into the rolling mill, but also of the duration of contact between the strip and the cylinders, because this contact causes the strip to cool. If, for a temperature entry of the given strip, the cylinders have a very large diameter, the cooling of the strip which they cause risks leading it to a temperature insufficient for that the welding of the walls of the porosities can be complete. For this purpose, the value of the temperature of the strip as it leaves the cylinders provides a good indication of the actual possibility that the walls of the porosities had to weld to each other in the grip of the cylinders.

The temperature of the strip at the outlet of the cylinders must therefore be sufficient to allow the welding of porosities, but it must not be too high either, so avoid excessive thermal load on the cylinders. This would lead to a degradation of their surface resulting in a deterioration of the surface appearance of the strip in the form of excessive roughness. The objective of the invention cannot therefore be achieved without getting annoying side effects for the overall quality of the tape that if we combine the diameter of the cylinders, the reduction rate and the temperature of the strip leaving the rolling mill adequately.

To determine how these parameters should be combined, we carried out series of tests, 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 strip and the temperature of the strip leaving the rolling mill. The rolling mill was arranged in line with the casting facility. Each test was subject to a characterization allowing determine whether the porosity welding has been effective or not. This characterization consisted to break a tensile test piece and to examine the facies of rupture. If the facies present porosities which opened during the tensile test, it is deduced that the welding was not satisfactory. If the facies have no apparent porosity, the welding is found to have been satisfactory.

Table 1 groups together the compositions of the steels on which the tests were carried out, the results of which are given in Table 2. The contents of the various elements are given in percentages by weight. Table 1 also gives the thicknesses at the outlet of the casting rolls of the strips on which the tests were carried out, as well as the corresponding casting speeds, measured between the casting rolls and the hot rolling mill.

The compositions of types A and A 'flows correspond to those of steels classic austenitic stainless steels type AISI 304. Type B castings correspond to AISI 430 type ferritic stainless steels. Type C castings correspond to AISI 409 ferritic stainless steels stabilized with titanium.

Table 2 shows the results of tests carried out on the strips from these castings, with the corresponding test conditions. Results of tests carried out on types A, A ', B, C Field of the invention Type of casting Diameter of rolling mill rolls (mm) Reduction rate (%) Temperature of the strip leaving the rolling mill (° C) Welding of porosities Other faults no AT 300 50 1100 no nothingness no AT 400 10 1100 no nothingness no AT 400 15 750 no nothingness Yes A, A ', B, C 400 15 800 Yes nothingness Yes A, A ', B, C 400 15 1100 Yes nothingness no AT 400 15 1150 Yes too strong roughness no AT 400 50 750 no nothingness Yes A, A ', B, C 400 50 800 Yes nothingness Yes A, A ', B, C 400 50 1100 Yes nothingness no AT 400 50 1150 Yes too strong roughness no AT 400 60 1100 Yes critics no AT 900 10 1100 no nothingness no AT 900 15 750 no nothingness Yes A, A ', B, C 900 15 800 Yes nothingness Yes A, A ', B, C 900 15 1100 Yes nothingness no AT 900 15 1150 Yes too strong roughness no AT 900 50 750 no nothingness Yes A, A ', B, C 900 50 800 Yes nothingness Yes A, A ', B, C 900 50 1100 Yes nothingness no AT 900 50 1150 Yes too strong roughness no AT 900 60 1100 Yes critics no AT 1000 50 750 no nothingness

• un diamètre des cylindres de travail du laminoir compris entre 400 et 900 mm ;
• un taux de réduction de l'épaisseur de la bande lors du laminage compris entre 15 et 50%;
• une température de la bande en sortie du laminoir d'au moins 800°C et d'au plus 1100°C.

It emerges from these tests that an effective welding of the porosities is obtained without the appearance of cracks and without excessive roughness on the surface of the strip when the following three conditions are met:

• a diameter of the working rolls of the rolling mill between 400 and 900 mm;
• a reduction rate of the strip thickness during rolling between 15 and 50%;
• a temperature of the strip leaving the rolling mill of at least 800 ° C. and at most 1100 ° C.

On the other hand, no influence was noted, under the test conditions, of the Strip thickness-casting speed combination: results of type A 'castings are identical to those of type A castings, for other casting parameters identical.

These tests, as we said, were carried out with a hot rolling mill arranged in line with the casting installation and preceding the tape winding installation. In the spirit of the invention, this characteristic is not essential, and rolling to hot can be carried out on a separate installation from the casting installation and winding, so after unwinding and reheating the raw casting strip. However, inline lamination is advised for a variety of reasons. First of all, this solution has economic advantages linked to the continuous nature of operations. In first, the tape manufacturing process is shortened. In addition, we do the economy of a winder, and also of a reheating installation of relatively high power since the casting strip can be hot enough to obtain the adequate rolling temperatures, possibly using a hood to stop the radiation of the strip between its exit from the casting cylinders and its entry into the rolling mill. If a reheating of the strip is however necessary, it can be carried out at using an induction furnace of reduced power, sufficient to raise the temperature of the strip running a few hundred degrees. On the other hand, in-line rolling, in eliminating the need for a winding of the raw casting strip, also eliminates suddenly the risks of deterioration of the strip during this winding which would take place on a relatively thick strip with an unrecrystallized structure. Finally, the removal reheating the strip from room temperature to the temperature of hot rolling removes the usual reoxidation of the strip surface during this operation. These re-oxidations would form scale, which could cause become embedded both in the strip and in the rolls of the rolling mill and thus lead to degradation of the surface appearance of the product after pickling.

The invention applies not only to installations for casting between rolls, but to any other type of casting installation of thin strips of stainless steel between two cooled surfaces in motion, such as moving bands.

Claims (3)

1. Method of manufacturing a thin stainless steel strip by direct solidification of liquid steel in the form of a strip of a thickness less than or equal to 8mm in a casting apparatus comprising two cooled walls in motion, and by hot rolling of the said strip the solidification of which is substantially achieved when it leaves the said walls, characterised in that hot rolling is carried out on a rolling mill the working cylinders of which have a diameter of between 400 and 900 mm, in that the temperature of the strip when it comes out of the rolling mill is between 800 and 1100°C, and in that the rate of reduction in the thickness of the strip when it is hot rolled is between 15 and 50%.
2. Method according to claim 1, characterised in that hot rolling is carried out on apparatus arranged in line with the casting apparatus.
3. Method according to claim 1 or 2, characterised in that the cooled walls of the casting apparatus are formed by the surfaces of the two cylinders in rotation on horizontal axes.