AU2003266431A1 - Process for manufacturing a thin stainless steel strip - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 DIVISIONAL APPLICATION NAME OF APPLICANT(S): UGINE SA ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street Melbourne, 3000.

INVENTION TITLE: "Process for Manufacturing a thin stainless steel strip" The following statement is a full description of this invention, including the best method of performing it known to me: la The invention relates to the manufacture of thin stainless steel strip, directly from liquid metal, by solidification within a mould consisting of two cooled walls moving at the same speed as the solidified strip, such as the external walls of two rotating rolls having horizontal axes.

In this casting process, which is under industrial-scale development and is referred to as "twin-roll casting", one of the major problems associated with the quality of the strip is the possible presence of porosity in the core of the strip.

When this porosity is found on products coming from the subsequent conversions undergone by the strip (such as pickling, annealing, cold rolling and other conversion operations), it limits the field of application of the products because of the degradation in the mechanical properties that result from this porosity.

The causes for the appearance of this porosity in the core of twin-roll cast strip may be similar to those which result (on a larger dimensional scale) in shrinkage cavities in the ingots and the central porosity in conventional continuous casting products, namely closure by solid metal of pockets containing still-liquid metal when the solidification of the product (which normally is substantially completed by the time the strip leaves the walls of the mould, that is to say the core of the strip is not totally in the liquid state) does not take place completely uniformly.

The cooling and solidification of the liquid metal that these pockets contain are accompanied by contraction of this metal, which causes a void to appear. This cannot be filled before the end of solidification since this closed pocket is no longer fed with fresh liquid metal.

These pores must be distinguished from spherical defects called "blowholes" which are due to a release of dissolved gas and usually occur near the surface of the products.

-2 Document EP 0,396,862 proposes a process aimed at eliminating the central porosity, and also any internal and surface defects, during the twin-roll casting of steel strip. According to this process, the casting rolls have on their surfaces precisely dimensioned circumferential grooves arranged in an offset manner on both rolls. The aim is thus to prevent the solidified metal shells on the .surfaces of the rolls separating, which would result in irregularities in the solidification of the strip. However, it seems that the sole prevention of such separation is insufficient to completely prevent the appearance of central porosity.

Document JP 8252653 proposes a process in which, in line with the casting operation, the strip undergoes hot rolling under conditions satisfying the following inequality: r (2.74x0- 5

T

2 -6.88xl0-2T+43.55) (t 0 /w 0 with: r: hot-rolling reduction ratio; T: hot-rolling temperature in °C; to: pore diameter in the thickness direction of thestrip; w 0 pore diameter in the width direction of the strip.

According to this process, it is therefore necessary for the hot rolling to be carried out with a reduction ratio sufficient for the pores to be closed up during this rolling, and this minimum ratio depends on the rolling temperature (that is to say the temperature at which the strip enters the roll nip) and on the shape and the orientation of the pores. However, it has been found that these rolling conditions are still insufficient for reliably closing up all the pores, and above all that they do -not always prevent the closed pores from opening up again during conversion of the strip or use of the products which stem therefrom, causing them to break.

r 3 The invention advantageously proposes a process guaranteeing that the central pores appearing in the core of the strip after it has fully solidified are definitely closed.

For this purpose, the subject of the invention is a process for manufacturing a thin stainless steel strip 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 plant comprising two cooled moving walls and by hot rolling the said strip, the solidification of which is substantially complete by the time it leaves the said walls, characterized in that the hot rolling is carried out on a rolling mill, the work rolls of which have a diameter of between 400 and 900 mm, in that the temperature of the strip as it leaves the rolling mill is between 800 and 1100 0 C and in that the thickness reduction ratio of the strip during hot rolling is between 15 and Preferably, the hot rolling is carried out in line with the casting of the strip. The casting plant may be of the "twin-roll casting" type.

As will have been understood, the advantages of the invention may be achieved by the combination of requirements with regard to the diameter of the work rolls of the hot-rolling mill, the temperature of the strip on leaving the rolls and the thickness reduction ratio of the strip during hot rolling.

The invention applies to the casting of stainless steels of any class, which conventionally have carbon contents less than or equal to silicon contents less than or equal to manganese contents less than or equal to 15%, chromium contents between and 30%, copper contents less than or equal to 5% and nitrogen contents less than or equal to 0.5% (these contents are expressed as percentages by weight). These steels may also contain large amounts of nickel (up to or of molybdenum (up to Moreover, as is usually the case, other elements are present in the metal, either as impurities or as alloying elements, 4 particularly sulphur, phosphorus, titanium, niobium and zirconium. Their total content must not exceed 2% by weight.

As mentioned, a thin twin-roll cast stainless steel strip is highly likely to develop porosity in its core, during its solidification, when a liquid pocket is closed up by solid metal. This phenomenon occurs at the end of solidification of the pasty region, also called the "equiaxed region", located between the two solidified shells in contact with the rolls, which shells are also called the "columnar regions". The equiaxed region is very difficult to control, and its thickness may vary depending on the rate of solidification of the columnar regions. Thus, the equiaxed region may locally close up in the anticipated manner at the points where the growth of the columnar regions has been more rapid than normal. Downstream from the point of closure of the equiaxed region, the liquid pockets can no longer be correctly refed with liquid metal, and pores form by contraction of the metal as these liquid pockets solidify. Nevertheless, this case remains quite rare and in fact, in general, the isolation of a liquid pocket occurs by the clustering of equiaxed crystals in the liquid, which form a plug obstructing the equiaxed region. The pores which form in the equiaxed region consist of groups of channels and cavities containing no gas, the maximum dimension of which, in the thickness direction of the sheet, corresponds to the thickness of the equiaxed region 100 to 400 pum), which channels and cavities may be as long as 1 to 2 mm in the other directions. As mentioned, these are not spherical blowholes, which would arise by the release of gas, or internal defects emerging on the surface of the strip.

The basic idea of the invention is to create, while the solidified strip is being hot rolled, conditions such that they lead not only to closure of the central pores, as is already known, but also to veritable welding of the opposed pore walls, that the 5 rolling has allowed to come together. In this way, it may be ensured that there will be no risk of the pores opening during subsequent forming of the strip or during use of the products thus produced. As the strip is being hot rolled, two steps occur. Firstly, the internal walls of the defect gradually come together as the thickness of the strip is reduced, until they come into contact with each other. Then, once this contact has been made, welding of the walls occurs by diffusion of the constituent elements of the steel across the interface. However, the walls must have already been effectively welded immediately before the strip leaves the roll nip of the rolling mill, otherwise the release of the compression of the strip which occurs on leaving the rolls would cause partial separation of the walls.

The effectiveness of the welding depends essentially on two parameters: the duration of the forced contacting of the walls in the rolling mill and the temperature at which this contacting takes place.

This forced contact therefore must take place as soon as possible after the strip enters the rolling mill, and its duration mainly depends, for a given rolling speed (which, in the case of in-line rolling, is largely determined by the thickness of the strip before it is rolled), on the diameter of the work rolls of the rolling mill and its thickness reduction ratio that they impose on the strip. The larger the diameter of the rolls and the higher the reduction ratio, the more rapid and prolonged the forced contacting of the pore walls. However, it may not be sufficient to state that, in order for the problem posed to be solved satisfactorily, all that is required is to roll the strip with a reduction ratio and a roll diameter which are as high as possible. This is because too high a reduction ratio, which would exceed the hot deformability of the strip, leads to the appearance of surface cracks, on the strip which are absolutely to be avoided. Moreover, the temperature at which the forced contacting of the pore walls takes -6 place depends not only on the entry temperature of the strip in the rolling mill but also on the duration of the contact between the strip and the rolls, since this contact causes the strip to cool. If, for a given entry temperature of the strip, the rolls have a very large diameter, there is a risk that the cooling of the strip which they cause would take the latter to a temperature not high enough for the pore walls to be able to be fully welded. In this regard, the value of the temperature of the strip as it leaves the rolls gives a good indication of the actual possibility that the pore walls have had of being welded to each other in the roll nip.

The temperature of the strip on leaving the rolls must therefore be high enough to allow welding of the pores, but it must not be too high either, so as to avoid an excessive thermal load on the rolls. This would result in degradation of their surface, leading to a deterioration in the surface appearance of the strip in the form of excessive roughness. The objective of the invention can therefore be achieved without obtaining secondary effects adversely affecting the general quality of the strip only if the diameter of the rolls, the reduction ratio and the strip temperature on leaving the rolling mill are combined in a suitable manner.

In order to determine how these parameters must be combined, series of trials were carried out, during which, for a given type of stainless steel, the diameter of the work rolls of the rolling mill, the thickness reduction ratio of the strip and the temperature of the strip on leaving the rolling mill were varied. The rolling mill was placed in line with the casting plant. Each trial involved a characterization making it possible to determine whether or not the welding of the pores had been effective. This characterization consisted in breaking a tensile test piece and examining the fracture surface. If the fracture surface exhibits pores which 7 were open during the tensile test, it may be concluded that the welding had not been satisfactory. If the fracture surface exhibits no apparent porosity, the welding is judged to have been satisfactory.

Table 1 gives the compositions of the steels on which the trials were carried out, the results of the trials being given in Table 2. The contents of the various elements are given in percentages by weight.

Table 1 also gives the thicknesses of the strips on leaving the casting rolls, on which strips the trials were carried out, as well as the corresponding casting speeds measured between the casting rolls and the hotrolling mill.

Table 1: Composition of the steels of the trial casting runs, the thickness of the cast strip and the casting rates Casting Casting Casting Casting runs A runs A' runs B runs C C 0.05 0.05 0.04 0.01 Mn 1.5 1.5 0.4 0.2 P 0.04 0.04 0.04 0.03 S 0.01 0.01 0.01 0.01 Si 0.3 0.3 0.3 Ni 8.6 8.6 0.3 0.1 Cr 18 18 16.5 11.5 Cu 0.5 0.5 0.2 0.2 Mo 0.5 0.5 0.1 0.1 Ti 0.01 0.01 0.01 0.15 N 0.05 0.05 0.04 0.01 Strip thickness 4 mm 2 mm 3 mm 3 mm Casting speed 25 m/min. 100 m/min. 60 m/min. 60 m/min.

The compositions of the type A and A' casting runs correspond to those of conventional austenitic stainless steels of the AISI 304 type. The type B casting runs correspond to ferritic stainless steels of the AISI 430 type. The type C casting runs correspond to ferritic stainless steels of the titanium-stabilized AISI 409 type.

-8- Table 2 gives the results of trials carried out on the strip resulting from these casting runs, with the corresponding trial conditions.

Table 2: Results of the trials carried out on the type A, B and C casting runs According to the invention Casting run type Diameter of the rolling mill rolls (mm) Reduction ratio Temperature of the strip leaving the rolling mill o Pore welding Other defects no A 300 50 1100 no none no A 400 10 1100 no none no A 400 15 750 no none yes A,A' ,B,C 400 15 800 yes none yes A,A',B,C 400 15 1100 yes none no A 400 15 1150 yes excessive roughness no A 400 50 750 no none yes A,A',B,C 400 50 800 yes none yes A,A',B,C 400 50 1100 yes none no A 400 50 1150 yes excessive roughness no A 400 60 1100 yes crazes no A 900 10 1100 no none no A 900 15 750 no none yes A,A',B,C 900 15 800 yes none yes A,A',B,C 900 15 1100 yes none no A 900 15 1150 yes excessive roughness no A 900 50 750 no none yes A,A',B,C 900 50 800 yes none yes A,A',B,C 900 50 1100 yes none no A 900 50 1150 yes excessive roughness no A 900 60 1100 yes crazes no A 1000 50 750 no none It is apparent from these trials that effective pore welding is obtained without the appearance of surface cracks and without excessive roughness on the surface of the strip when the following three conditions are combined: a diameter of the rolling-mill work rolls of between 400 and 900 mm; a thickness reduction ratio of the strip during rolling of between 15 and 9 a temperature of the strip on leaving the rolling mill of at least 800 0 C and at most 11000C.

On the other hand, no influence was noted, under the trial conditions, of the strip thickness/casting speed combination: the results of the type A' casting runs are identical to those of the type A casting runs for casting parameters which are, moreover, identical.

As mentioned, these trials were carried out with a hot-rolling mill placed in line with the casting plant and before the strip coiling plant. Within the context of the invention, this characteristic is not absolutely essential and the hot rolling may be carried out on a plant separate from the casting and coiling plant, and therefore after the as-cast strip has been uncoiled and reheated. However, in-line rolling is recommended for various reasons. First of all, this solution has economic advantages associated with the continuous nature of the operations. In the first instance, the strip manufacturing process is shortened.

In addition, a saving may be made by not having a coiler and also a relatively high-power reheat plant since the cast strip may be hot enough to obtain the suitable rolling temperatures, possibly with the aid of a cover stopping the radiation from the strip between its exit from the casting rolls and its entry into the rolling mill. However, if it proves to be necessary to reheat the strip, this may be carried out using a lowpower induction furnace, the power being sufficient to raise the temperature of the running strip a few hundred degrees. Moreover, in-line rolling, dispensing with the need to coil the as-cast strip, eliminates by the same stroke the risk of damage to the strip during this coiling which would take place on a relatively thick strip having a non-recrystallized structure.

Finally, by not having to reheat the strip from room temperature up to the hot rolling temperature, surface reoxidation of the strip which normally occurs during this operation is eliminated. Such reoxidation would 10 form scale which would run the risk of being encrusted both into the strip and into the rolling-mill rolls, and thus of causing a degradation of the surface appearance of the product after pickling.

The invention applies not only to twin-roll casting plants but to any type of plant for casting thin stainless steel strip between two cooled moving surfaces, such as running strip.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Claims (4)

1. Process for manufacturing a thin stainless steel strip by direct solidification of the liquid steel in the form of a strip of thickness less than or equal to 8mm in a casting plant comprising two cooled moving walls and by hot rolling the said strip, the solidification of which is substantially complete by the time it leaves the said walls, wherein the hot rolling is carried out on a rolling mill, the work rolls of which have a diameter of between 400 and 900mm, the temperature of the strip as it leaves the rolling mill is between 800 and 1100°C and the thickness reduction ratio of the strip during hot rolling is between 15 and

2. Process according to Claim 1, wherein the hot rolling is carried out on a plant placed in line with the casting plant.

3. Process according to Claim 1 or 2, wherein the cooled walls of the casting plant consist of the surfaces of two rotating rolls having horizontal axes.

4. Process for manufacturing stainless steel strip substantially as hereinbefore described with reference to the Examples. Stainless steel strip when manufactured by a process according to any one of claims 1 to 4. DATED: 11 October, 2002 by DAVIES COLLISON CAVE Patent Attorneys for the Applicant(s): UGINE SA