EP2197608B1 - Method of manufacturing stainless steels comprising fine carbonitrides, and product obtained from this method - Google Patents

Description

The invention relates to a method for manufacturing stabilized stainless steels for economically obtaining a very fine dispersion of carbonitrides after solidification, with a minimized risk of nozzle clogging during casting.

The invention also relates to stabilized, continuously cast stainless steels having a very fine dispersion of homogeneously distributed carbonitrides. In order to stabilize these stainless steels, additions of stabilizing elements are made in the bag. It is known that a possible precipitation of chromium carbides at the grain boundaries can lead to a local depletion of chromium and thus to an awareness of intergranular corrosion. Elements such as titanium, zirconium, niobium and vanadium which form carbides, nitrides or carbonitrides which are more stable than chromium carbides, are therefore used as stabilizing elements for fixing carbon and nitrogen.

• compte tenu du délai s'écoulant entre les additions et la solidification en lingotière, une partie des précipités a le temps de coalescer et de s'agglomérer au sein du métal liquide, conduisant à un accroissement de la taille moyenne des précipités et à la présence de certains précipités de taille plus importante. Ceci a une influence néfaste sur les propriétés mécaniques car l'amorçage de l'endommagement intervient en premier lieu sur les précipités de plus grande taille. De plus, certains agglomérats de précipités peuvent se retrouver sur la peau des demi-produits après coulée et entraîner des défauts de surface qui doivent être éliminés par des traitements mécaniques coûteux.
• Par ailleurs, une oxydation partielle des éléments stabilisants peut intervenir et un certain nombre de précipités a le temps de décanter, ce qui diminue notablement le rendement des additions de ces éléments.

The additions of titanium or ferro-titanium pocket are for example in the form of cored wire or sponge. However, there are disadvantages to these early additions, that is to say at the stage of the pocket:

• given the time between additions and solidification in the mold, part of the precipitates has the time to coalesce and agglomerate within the liquid metal, leading to an increase in the average size of the precipitates and the presence some larger precipitates. This has a detrimental influence on the mechanical properties because the initiation of the damage intervenes primarily on the larger precipitates. In addition, some agglomerates of precipitates can be found on the skin of the semi-finished products after casting and cause surface defects that must be eliminated by expensive mechanical treatments.
• In addition, a partial oxidation of the stabilizing elements can occur and a number of precipitates have time to settle, which significantly reduces the yield of additions of these elements.

It is envisaged to stabilize stainless steels at the stage of continuous casting. Continuous casting of steel is a well known process: it consists of pouring from a pocket, a liquid metal in a tundish for regulating the flow and then, from the latter, to make a casting in the upper part of a water-cooled bottomless copper mold with vertical reciprocating motion. The semi-solidified product from the lower part of the mold is extracted by means of rollers.

The liquid steel is introduced into the mold by means of a tubular conduit called nozzle arranged between the tundish and the mold.

It has thus been proposed a casting device allowing additions to the stage of the mold, described in the patent EP269180 of the Metallurgical Research Center: the liquid metal is poured on the top of a refractory material dome of a distributor body. The shape of this dome causes a flow of the metal towards its periphery, the flow being deflected towards the inner wall of the nozzle or an intermediate vertical tubular member. Thus, in the central part of the nozzle under the distributor member, a volume without liquid metal is created in which it is possible to carry out additions via an injection channel. The device thus described is called a hollow jet nozzle or "Hollow Jet Nozzle".

Using this device, the patent BE1014063 discloses a method of adding metal powders to form oxides during solidification. For this purpose, a steel having a dissolved oxygen level (O ₂ ) given from the distributor to the mold is cast, an addition (M) of metal powder is carried out, the M / O ₂ ratio is controlled and the mixture is mixed. the powder to the liquid metal so as to form metal oxides.

Even though the formation of these oxides may play a favorable role by increasing the equiaxed zone fraction on the solidified half-product, this process does not make it possible to provide a response to stabilization. stainless steels since it does not concern the sequestration of carbon and nitrogen. The application of such a method to stainless steels is also not mentioned in this patent.

The patent WO2006096942 discloses an addition of technical ceramic nanoparticles in a hollow jet nozzle. These ceramic particles may be oxides, nitrides, carbides, borides or silicides. These particles are characterized by a high thermal stability, so that no reaction occurs substantially between them and the liquid metal. However, this process is difficult to implement due to agglomeration of the nanoparticles which tend to form larger particles possibly causing the aforementioned defects. Again, the application of such a technique to stainless steels is not mentioned in the patent.

The object of the invention is to provide a process for the manufacture of stabilized stainless steels having a fine and regular dispersion of nitrides and / or carbonitrides. In particular, it seeks to obtain a large number of fine precipitates, less than 2.5 microns in size, while limiting the number of coarse precipitates larger than 10 microns.

Another object of the invention is to propose a process having a better efficiency with regard to the yield of the additions of stabilizing elements, compared with the methods of addition in the bag.

Another object of the invention is to provide a method for minimizing the risk of plugging nozzles in continuous casting of stainless steels.

Another object of the invention is to provide stainless steel semi-finished products having an equiaxed solidification structure at the end of the continuous casting, even without the use of electromagnetic stirring techniques.

Another object of the invention is to provide stainless steel semi-finished products having a good homogeneity on a cross section relative to the direction of continuous casting.

The subject of the invention is thus a process for producing a stabilized stainless steel semi-finished product comprising a casting step using a hollow jet nozzle disposed between a tundish and a continuous casting mold, the nozzle comprising in its upper part a distributor member for deflecting the liquid metal arriving at the inlet of the nozzle, thus defining an interior volume without liquid metal. The method is characterized by supplying, in the form of liquid metal in the tundish, an unstabilized stainless steel containing no precipitates of nitrides, carbides and carbonitrides, and then pouring the liquid metal by means of the nozzle simultaneously performing an addition of metal powder in the interior volume of the hollow jet, the metal powder containing at least one element for stabilizing the stainless steel, the addition being carried out at a temperature of the liquid steel between T _liquidus + 10 ° C and T _liquidus + 40 ° C. The liquid metal is solidified, the solidification starting less than 2 seconds from the addition, to obtain the semi-product

The subject of the invention is also a process according to one of the above modes, characterized in that the stabilizing element is chosen from one or more of the following elements: titanium, niobium, zirconium, vanadium.

Preferably, the stabilizing element is titanium, the titanium, carbon and nitrogen contents of the stainless steel, expressed in mass percentage, satisfying: Ti≥0,15 + 4 (C + N)

In a particular embodiment, the steel is a ferritic stainless steel, or austenitic stainless, or martensitic stainless or austenitic ferritic stainless.

The invention also relates to a stainless steel product made from a semi-finished product produced by a method according to one of the above modes, characterized in that the stabilizing element is titanium and that the number of titanium nitride or carbonitride precipitates less than 2.5 microns in size is greater than 15000 / cm ² .

The number of titanium nitrides or carbonitrides greater than 10 microns in size is preferably less than 50 / cm ² .

In a preferred embodiment, the average interprecipitated distance is less than 15 micrometers.

Other features and advantages of the invention will become apparent from the following description given by way of example and with reference to the figure 1 attached which shows schematically an example of a device for implementing the method according to the invention.

Other features and advantages of the invention will become apparent from the following description given by way of example.

The invention to be described is directed to a wide range of stainless steels capable of being stabilized by additions of titanium, niobium or zirconium, vanadium or other stabilizing elements, these elements being used alone or in combination.

In particular, the invention is advantageously used in the manufacture of ferritic stainless steels of the X 3CrTi17 type, of composition according to NF.EN 10.088-1 and 2: C <0.050, Si <1.00%, Mn < 1.00%, P <0.040%, S <0.015%, Cr: 16.00-18.00%, N <0.045%, 0.15 + 4 (C + N) <Ti <0.080%, the contents being expressed as a percentage by mass.

• On élabore au moyen d'un procédé connu en soi, un métal liquide destiné à la fabrication d'acier inoxydable ferritique, ou inoxydable austénitique, ou inoxydable martensitique ou inoxydable austéno-ferritique. Au stade de la poche, avant coulée, l'acier liquide peut faire l'objet de différentes opérations métallurgiques :
• additions complémentaires pour mise à la nuance de l'acier
• désoxydation du métal liquide
• brassage du bain par un gaz neutre de façon à assurer l'homogénéisation thermique avant coulée

The process according to the invention is as follows:

• A liquid metal for producing ferritic stainless steel, or austenitic stainless steel or martensitic stainless or austenitic ferritic stainless steel is produced by means of a process known per se. At the ladle stage, before casting, the liquid steel can undergo various metallurgical operations:
• additional additions for shading of steel
• deoxidation of the liquid metal
• bath mixing by a neutral gas so as to ensure thermal homogenization before casting

At this stage, even if the liquid metal may possibly contain a small amount of element allowing the stabilization of the stainless steel, no precipitation of this element is involved. The main addition of stabilizing element and its precipitation occur later, as described below.

A liquid metal containing a content of nitrogen N and carbon C present in the form of dissolved elements is poured from the ladle into the distribution basket: the composition and the temperature of the liquid metal are such that it does not exist precipitates of nitrides, carbides, carbonitrides, under these conditions. The carbon and nitrogen contents make it possible to adjust the amounts of stabilizing elements which will be added later.

The ladle is poured into a tundish 1 comprising a bottom with a closure device 2 whose more or less complete closure makes it possible to regulate the flow towards a casting nozzle 3. At this stage, the temperature of the liquid steel must not be too important. It will be seen later that the additions made in the hollow jet nozzle must be carried out at a temperature having a limited distance from the liquidus temperature (referred to as T _liquidus ) of the steel.

By means of his general knowledge and the specificities of the casting device which condition the temperature loss between the tundish and the nozzle, the skilled person will be able to adjust the casting temperature according to the characteristics of the invention set out below. .

As has been explained, the process according to the invention requires the use of a hollow jet nozzle. This nozzle comprises a distribution dome 4 made of refractory material pierced with one or more injection channels which open into the central lower part of the dome in the form of injection tubes 5. It is thus possible to add a driven metal powder. by a vector gas. The injected powder 6 mixes with the liquid metal which has been deflected by the upper part of the dome towards the walls of the nozzle or of an intermediate tubular member between the nozzle proper and the tundish.

The powder supply is carried out by one or more tubes 7 themselves connected to one or more tanks 8. The upper part 9 of these powder reservoirs is pressurized with a carrier neutral gas such as argon, which helps protect the powder from oxidation. A suitable gas flow forces the powder to flow to the hollow jet nozzle with a flow rate corresponding to the amount that it is desired to add. The flow of the powder can also be facilitated by a mechanical device such as a worm. The particle size of the powder must be chosen so as to ensure easy flow between the tanks and the nozzle and a near-immediate melting in the liquid metal. A spherical particle size of between 100 and 200 micrometers is well adapted to these requirements.

• le titane, qui peut être utilisé pur ou sous forme de ferro-titane pour des raisons de coût. Ces additions sont destinées à former des nitrures de titane TiN d'une grande stabilité ou des carbonitrures Ti(C,N)
• le zirconium, formant lui aussi des nitrures et des carbonitrures très stables.
• le niobium, destiné essentiellement à former des carbonitrures Nb(C,N)
• le vanadium, formant également des carbonitrures

This powder contains one or more metallic elements intended to ensure the stabilization of the stainless steel, thus:

• titanium, which can be used pure or in the form of ferro-titanium for cost reasons. These additions are intended to form titanium nitrides TiN of high stability or carbonitrides Ti (C, N)
• zirconium, also forming nitrides and carbonitrides very stable.
• niobium, essentially intended to form carbonitrides Nb (C, N)
• vanadium, also forming carbonitrides

Powders of these metal elements can be naturally mixed so as to achieve a particular combination such as, for example, titanium-niobium bi-stabilization. It is also possible to mix the above powders with ferroalloys or iron powder in order to reduce the overheating temperature at the outlet of the hollow jet nozzle so as to increase the equiaxed zone fraction of the semi-finished product after solidification.

• d'obtenir une précipitation fine et intense de nitrures et de carbonitrures
• de favoriser la solidification sous une forme équiaxe.

Simultaneously with the casting, the addition of the powder comprising the stabilizing element or elements in a liquid metal is carried out at a temperature of between T _liquidus + 10 ° C. and T _liquidus + 40 ° C. This particular range of addition temperature allows all at once:

• to obtain a fine and intense precipitation of nitrides and carbonitrides
• to promote solidification in an equiaxial form.

When the addition temperature is too high compared to the liquidus, the time between the formation of nitrides or carbonitrides and the end of solidification increases, which leads to an increase in size, unwanted phenomenon .

On the other hand, when the addition temperature is too low compared to the liquidus, the process becomes more sensitive to an untimely variation of the manufacturing parameters, there is a risk of clogging of the nozzle.

Upon addition to the hollow jet nozzle, the stabilizing element is melted by contacting the liquid metal within a few tenths of a second. Since the powder is protected from oxidation by the neutral gas until it comes into contact with the liquid metal, the yield of the addition is high.

Sufficient stabilizing elements are added so that nitrogen and carbon are completely precipitated and the solubility product corresponding to the formation of these precipitates is reached or exceeded at the temperature at which the addition is made. The nitrides and / or carbonitrides then immediately precipitate in a very fine form.

After addition, the solidification of the liquid metal is begun in less than 2 seconds, the latter starting on the walls of the mold 10. This very limited hold time of the precipitates in the liquid metal makes it possible to avoid an increase in their size. Those skilled in the art will be able to adapt the various parameters at their disposal such as: height of the injection device with respect to the mold, injection flow rate, more or less significant implementation of the heat exchangers, extraction speed of the half -product, overheating temperature, complementary injection of ferroalloy powder to accelerate the solidification, so that the delay between the addition and the beginning of the solidification is less than 2 seconds.

A preferred embodiment relies on the use of titanium for the purpose of forming a precipitation of fine and dispersed nitrides and / or carbonitrides. According to the invention, the titanium, carbon and nitrogen contents of stainless steel, expressed as a percentage by weight, are such that: Ti≥0.15 +4 (C + N). Under these conditions, the amount of titanium added allows total stabilization of the steel.

A particularity of the stainless steels obtained according to the invention lies in the great homogeneity of the dispersion of the nitrides and carbonitrides with a smaller inter-precipitate mean distance, so that a possible sensitization due to a locally impoverished zone is reduced. .

According to another preferred embodiment of the invention, the above parameters, and in particular the powder injection rate and the superheating temperature, are adapted so as to obtain a completely equiaxed semi-finished solidification structure. This last term designates for example a slab (thickness of the order of 200mm), a slab (thickness of the order of 50-80mm), a thin strip (thickness of the order of 1-3 mm), a slab billet, not yet mechanically deformed hot. Such an equiaxed structure is particularly advantageous in the field of ferritic stainless steels to minimize the ragging defect. It is known that this defect is manifested by the formation of surface irregularities after stamping parallel to the rolling direction. It is due to the presence of heterogeneous structures before cold rolling and annealing, themselves resulting from columnar solidification structures.

The addition of powder proves to be advantageous for obtaining a totally equiaxed structure because the precipitates act as germination sites, thus preventing the formation of a less favorable columnar or basaltic type solidification. The invention thus makes it possible to avoid possibly implementing electromagnetic stirring techniques which are usually used for this purpose.

After manufacture of the semi-finished product, it can be hot-rolled or cold-rolled, pickled, annealed, according to conventional methods, to thereby obtain a product which can take various forms such as hot strip, thin sheet, or long product of various forms.

In the absence of a redissolution treatment, the characteristics of the precipitation are practically identical on the semi-finished products and the products obtained from these semi-finished products. The advantages conferred by the invention on the semi-finished products are thus found on the products obtained. By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example:

Two titanium stabilized ferritic stainless steel castings have been developed whose compositions, expressed in weight percent, are shown in Table 1. Steel A was produced according to the invention under conditions to be exposed, steel B being was manufactured using a conventional continuous casting technique. Table 1 Compositions of steels VS mn Yes Cr Cu Or S Ti V NOT AT 0.016 0.34 0.38 16.27 0.05 0.10 0.006 0.30 0.12 0,015 B 0.02 0.34 0.38 16.16 0.04 0.16 0.006 0.45 0.08 0.012 A = Made according to the invention
B = Made according to a conventional technique

In grade B, the addition of titanium was carried out in pocket, in the form of a titanium sponge.

In the preparation of the grade A according to the invention, the liquid metal in the tundish does not contain titanium. This element was added in a hollow jet nozzle in the form of ferro-titanium powder (titanium 70% -fer 30%) with a particle size of between 100 and 200 microns. The addition temperature of the powder is T _liquidus + 35 ° C. The solidification of the metal begins less than two seconds after addition to the walls of the mold. Different slab-shaped castings have been made according to the invention without encountering a problem of nozzle plugging. This is a consequence of the characteristic late precipitation of the process, the low retention time of the precipitates within the liquid metal and an advantage over conventional addition processes.

After hot rolling of the slabs to obtain strips 3 mm thick, the presence of titanium nitride precipitates was noted on polished sections. The size distribution of these precipitates is measured by image analysis according to the procedure defined in ASTM E1245. The density of the precipitates is expressed in number of precipitates per cm ² .

The mean inter-precipitate distance was also measured. The results of these measurements are as follows: Table 2: Precipitation Distribution Characteristics Number of TiNs less than 2.5μm (N / cm ² ) Number of TiNs larger than 10μm (N / cm ² ) Mean inter-precipitate distance (micrometers) Steel A (invention) 17560 30 14.2 Steel B (reference) 9320 110 24.6 Underlined values: not in accordance with the invention

A density of fine precipitates (<2.5 μm) greater than 15000 / cm ² guarantees a very homogeneous distribution of titanium nitrides. In this way, the trapping of carbon and nitrogen is ensured in a very complete and uniform manner.

A density of coarse precipitates (> 10 μm) less than 50 / cm ² makes it possible to ensure that failure initiation does not occur prematurely during mechanical stressing.

These two characteristics are observed for the steel manufactured according to the method of the invention. Compared to a conventional method, the invention makes it possible to multiply by a factor of about 2 the number of fine precipitates and to divide the number of coarse precipitates by a factor of about 3.

Observations were made on a cross-section with respect to the casting direction on a strip 1m wide and 3mm thick made according to the invention. Measurements made in the center, 1/3 width, 2/3 width and at the edge of the band reveal that the precipitation is very uniform. In particular, the average inter-precipitate distance is virtually identical between the center and the bank of the strip. The semi-finished products or products manufactured according to the invention thus have a high homogeneity of structures and properties.

In addition, the solidification structure examined on polished and etched transverse cuts of slabs is totally equiaxed. The absence of columnar areas is favorable to avoid the lack of crumpling. The yield of the addition of titanium (ratio of the titanium present in the final product to the titanium added in the form of a powder) is 95 to 100% in the process according to the invention. This yield is therefore much higher than that of the conventional process, of the order of 60%.

The method according to the invention thus makes it possible to economically and reliably produce stabilized stainless steel grades having a very fine dispersion of nitrides or carbonitrides.

Claims (4)

1. Process for manufacturing a semi-finished product made of stabilized stainless steel, which includes a casting step by means of a hollow jet nozzle placed between a tundish (1) and a continuous casting mould (10), said nozzle comprising, in its upper part, a distributing member (4) for deflecting the liquid metal arriving at the inlet of said nozzle, thus defining an internal volume with no liquid metal, characterized in that:

   - a non-stabilized stainless steel containing no nitride, carbide and carbonitride precipitates, is delivered in liquid metal form into said tundish; then

   - said liquid metal is cast by means of said nozzle, simultaneously carrying out an addition of metal powder (6) into said internal volume of said hollow jet, said metal powder containing at least one element for stabilizing said stainless steel, said at least one element being chosen from titanium, zirconium, niobium and vanadium, said addition being carried out at a liquid steel temperature between T_liquidus+10°C and T_liquidus+40°C, and in a sufficient amount for the nitrogen and carbon to be fully precipitated and for the solubility product corresponding to the formation of these precipitates to be reached or exceeded at the temperature at which the addition is carried out; then

   - said liquid metal is solidified, the solidification of said liquid metal starting less than 2 seconds after said addition, in order to obtain said semi-finished product.
2. Process according to Claim 1, characterized in that said stabilizing element is titanium, the titanium, carbon and nitrogen contents of said stainless steel satisfying, the contents being expressed in percentages by weight: $$\mathrm{Ti}\ge 0.15+4\left(\mathrm{C}+\mathrm{N}\right).$$

   
3. Process according to either one of Claims 1 and 2, characterized in that said steel is a ferritic stainless steel or an austenitic stainless steel or a martensitic stainless steel or an austeno-ferritic stainless steel.
4. Stainless steel product manufactured from a semi-finished product produced by a process according to any one of Claims 1 to 3, characterized in that it has a fully-equiaxed solidification structure, in that the stabilizing element is titanium and in that the number of titanium nitrides or carbonitrides of size smaller than 2.5 microns is greater than 15 000/cm², in that the mean inter-precipitate distance is less than 15 microns, and in that the number of titanium nitrides or carbonitrides of size greater than 10 microns is less than 50/cm².

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