EP1072689A1 - Process of manufacturing thin "TRIP" type steel strips and strips obtained thereby - Google Patents

Abstract

To make the TRIP (TRansformation Induced Plasticity) band, the band is continuously cast, 1.5-10 mm thick, preferably 1-5 mm thick. Composition is C 0.5-0.25, (Mn+Cu+Ni) is 0.5-3%, (Si+Al) is 0.1-4%, (P+Sn+As+Sb) is up to 0.1%, (Ti+Nb+V+Zr+rare earths) is up to 0.3%, Cr is below 1%, as are Mo, V; the balance being iron and impurities. The band is hot-laminated above the steel Ar3 temperature, with a reduction of 25-70% in one or more passes. The first forced cooling takes place at 5-100 degrees C/s. The band is held at 550-400 degrees C sufficiently long for the bainitic transformation with a proportion of residual austenite exceeding 5%, whilst avoiding perlite formation. The transformation is interrupted by a second forced cooling to 400 degrees C. The band is finally wound up, below 350 degrees C. An Independent claim is included for the band of steel so produced.

Description

The invention relates to the continuous casting of thin steel strips. More in particular, it relates to the manufacture of thin steel strips of the “TRIP” type directly from liquid metal.

Steels usually designated by the term TRIP (this term meaning "Transformation Induced Plasticity") are steels which simultaneously have a very high strength and high ductility, which makes them particularly suitable for laying form. These properties are obtained thanks to their particular microscopic structure. They have, in fact, within a ferritic matrix, a hard phase of bainite and / or martensite, as well as residual austenite representing 5 to 20% of the structure. The TRIP steel sheets are usually obtained by either continuous casting from slabs - hot rolling (the shortest route, therefore the most economical, but which provides relatively thick products), either by continuous casting slabs - hot rolling - cold rolling - annealing (used for products of low thickness). The bainite stabilizes the austenite.

Production of high-quality TRIP steel sheets from strips obtained by the conventional continuous casting die - hot rolling is however made difficult by the following problem. After the hot rolling of the initial slab, we look for a stabilization of the austenite during the bainitic transformation which occurs during the winding of the strip, this taking place at a temperature of 400 ° C (± 50 ° C). For bring the hot rolled strip to its winding temperature, cooling by water sprinkling is necessary. Now this cooling occurs in a domain of temperatures where a phenomenon known as "rewetting" is likely to occur. This rewetting is due to instability of the water vapor formed by heat-forming on contact with the strip, the vapor partly returning to the liquid state. So there are locally water (liquid) - strip contacts instead of water (vapor) - strip contact, and this leads to heterogeneities in the strip quenching phenomenon. These heterogeneities in the cooling results in notable heterogeneities in the microstructure of the strip, whose mechanical properties they impair.

The object of the invention is to make possible the reliable production of steel strips High quality TRIP through a short manufacturing chain, i.e. not including cold rolling and annealing step.

• on coule directement à partir d'acier liquide une bande d'épaisseur comprise entre 1,5 et 10 mm, preférentiellement 1 à 5 mm, ledit acier ayant la composition (en pourcentages pondéraux) C% compris entre 0,05 et 0,25, (Mn + Cu + Ni)% compris entre 0,5 et 3, (Si + Al)% compris entre 0,1 et 4, (P + Sn + As + Sb)% inférieur ou égal à 0,1 , (Ti + Nb + V + Zr + terres rares)% inférieur à 0,3, Cr% inférieur à 1, Mo% inférieur à 1, V% inférieur à 1, le reste étant du fer et des impuretés résultant de l'élaboration ;
• on effectue un laminage à chaud en ligne de ladite bande à une température supérieure à la température Ar₃ dudit acier avec un taux de réduction compris entre 25 et 70%, en une ou plusieurs passes ;
• on effectue un premier refroidissement forcé de ladite bande à une vitesse comprise entre 5 et 100°C/s ;
• on laisse séjourner la bande à des températures comprises entre 550 et 400°C pendant le temps nécessaire pour que s'y produise une transformation bainitique avec une proportion d'austénite résiduelle supérieure à 5%, tout en évitant la formation de perlite, puis on interrompt cette transformation par un second refroidissement forcé de ladite bande qui l'amène à une température inférieure à 400°C ;
• on effectue un bobinage de ladite bande à une température inférieure à 350°C.

To this end, the subject of the invention is a process for manufacturing thin steel strips of the "TRIP" type, according to which:

• a strip of thickness between 1.5 and 10 mm, preferably 1 to 5 mm, is poured directly from liquid steel, said steel having the composition (in weight percentages) C% between 0.05 and 0.25 , (Mn + Cu + Ni)% between 0.5 and 3, (Si + Al)% between 0.1 and 4, (P + Sn + As + Sb)% less than or equal to 0.1, ( Ti + Nb + V + Zr + rare earths)% less than 0.3, Cr% less than 1, Mo% less than 1, V% less than 1, the rest being iron and impurities resulting from the production;
• hot rolling is carried out in line of said strip at a temperature higher than the temperature Ar _{3 of} said steel with a reduction rate of between 25 and 70%, in one or more passes;
• a first forced cooling of said strip is carried out at a speed of between 5 and 100 ° C / s;
• the strip is left to stay at temperatures between 550 and 400 ° C. for the time necessary for a bainitic transformation to occur there with a proportion of residual austenite greater than 5%, while avoiding the formation of perlite, then interrupts this transformation by a second forced cooling of said strip which brings it to a temperature below 400 ° C;
• said strip is wound at a temperature below 350 ° C.

The subject of the invention is also a thin steel strip of the “TRIP” type, likely to be obtained by the above process.

As will be understood, a first essential aspect of the invention is the use of a continuous casting process of steel in thin strips directly to from liquid metal, instead of a conventional slab casting process intended to be hot rolled on a strip train. The strip thus produced is subjected to hot rolling online and then cooling which brings it into the temperature range where the bainitic transformation occurs. It’s only after this transformation occurred and the desired microstructure, typical of TRIP steels, was obtained that a second cooling takes place which interrupts the transformation, and brings the strip its winding temperature. This is located at a lower value than that of hot rolled strips produced by the conventional process, since the transformation bainitique has already taken place, and that an extended stay of the wound strip in the range of temperatures where this transformation took place could lead to an evolution undesirable microstructure.

The invention will be better understood on reading the description which follows.

The continuous casting of thin strips directly from liquid metal is a technique which has been tested for several years for the casting of carbon steels, stainless steels and other ferrous alloys, but has never been used before for the production of TRIP type steels. The most commonly used technique in casting thin strips of ferrous alloys, which is reaching the stage industrial, is the technique called "casting between cylinders", according to which we introduce liquid metal between two close cylinders with horizontal axes, rotated in direction reverse and internally cooled. The pouring space is closed laterally by refractory plates applied against the flat side faces of the cylinders. Of “Skins” of solidified metal are formed on each of the cylinders, and meet at the level the neck (the area where the gap between the cylindrical side surfaces of the cylinders is greatest low and corresponds substantially to the desired thickness for the strip) to form a solidified strip. Before being wound, the strip can then undergo various treatments thermal and / or thermomechanical such as one or more hot rolling, cooling, reheating ... It is a particular set of such treatments which constitutes one of the essential elements of the invention.

According to the invention, a steel is cast, the composition of which is defined as follows (all percentages are weight percentages).

Its carbon content is between 0.05 and 0.25%. The lower limit is required by the stabilization of the residual austenite, which takes place during cooling of the strip by rejection of carbon from the ferritic phase in the phase austenitic. Above 0.25%, it is considered that the strip will no longer have a weldability sufficient for the usual applications of TRIP steels.

Its manganese content is between 0.5 and 3%. Manganese has for functions to stabilize austenite (it is a gamma element) and to harden steel. In below 0.5%, these effects are not sufficiently marked. Above 3%, the effect gammagen becomes too important to guarantee the formation of a ferritic matrix, and more manganese segregates exaggeratedly, which degrades the properties tape mechanics. Manganese can be partially substituted by copper and / or nickel which also have gamma effects.

On the other hand, one can optionally impose a copper content between 0.5 and 2% (remaining within the framework Mn + Cu + Ni between 0.5 and 3%). The copper added specifically allows hardening by precipitation. In addition, the copper being insoluble in cementite, it allows like silicon and aluminum to obtain a beneficial effect for the residual austenite. On the other hand, the conditions of rapid cooling imposed by the casting of thin strips makes it possible to avoid problems of degradation of the surface condition of the product which dissuade from making this addition of copper in TRIP steels produced by conventional processes.

The total of its silicon and aluminum contents is between 0.1 and 4%. These elements prevent precipitation of cementite in austenite and promote formation high temperature ferrite. Compared to the silicon contents usually encountered on TRIP steels (from 0.2 to 1.5%), it will be noted that the process according to the invention may authorize higher contents, for reasons and under conditions which will be seen later.

The cumulative content of phosphorus, tin, arsenic, antimony must not exceed 0.3%, to limit the fragility of the products, and preferably the phosphorus content does not not exceed 0.05%.

It is also possible to add titanium, niobium, vanadium, zirconium or rare earths, in contents the sum of which does not exceed 0.3%. These elements form carbides, nitrides or carbonitrides that block the growth of grains at high temperature and increase the resistance by the precipitation effect.

Finally, it is necessary to avoid an excessive presence of elements which would slow down the bainitic transformation. This is the case for chromium, molybdenum and vanadium. In all However, the contents of each of these elements must not exceed 1%. Optimally, the total of their contents should not exceed 0.3%, and even more preferably 0.05%.

The other elements present in the steel are those which one expects usually to be found as impurities resulting from processing, in proportions which do not have a significant influence on the properties sought for steels TRIP.

Liquid metal whose composition meets the criteria set out above is cast on a casting installation between cylinders, so as to continuously form a solidified strip whose thickness can range from 0.5 to 10 mm and more conventionally range from 1 at 5 mm. At its exit from the cylinders, the strip preferably crosses an inerting zone, such as that a sealed enclosure, inside which is maintained in the vicinity of the strip a non-oxidizing atmosphere for the metal, thanks to an insufflation of a neutral gas (nitrogen or argon) lowering the oxygen content to a very low level. We can also consider give this atmosphere reducing properties by introducing hydrogen into it.

The purpose of this blanketing is to avoid, or at least significantly limit, the scale formation on the surface of the strip, the presence of which during the step of hot rolling which will follow, would lead to the appearance of defects such as scale inlays on the surface of the strip. The inerting device can be replaced or supplemented by a device ensuring the removal of the scale formed, by example a set of rotating brushes. An advantage of using such a device of inerting and / or descaling before hot rolling is that it increases the tolerable silicon metal content. Indeed, in the conventional process of manufacturing TRIP steels by slab casting - hot rolling, we prefer to avoid, most often, to impose a silicon content greater than 0.25%, because otherwise the conditions of formation calamine are generally such that there is a significant appearance of fayalite (iron and silicon oxide), very difficult to remove before hot rolling. In conventional installations where the casting of slabs and their cooling are carried out in the open air, the slabs which have already been heavily calcined remain at the room temperature, and should be reheated in a large oven (so difficult to inert) located off the casting line before being sent to the band train. To limit the formation of scale heavily loaded with fayalite and thus obtain a state correct surface area of the strip, it is therefore preferable in the usual manufacturing process hot-rolled TRIP steels, to limit the silicon content of the metal to the value previously cited, whereas, as we said, higher contents would present significant metallurgical advantages. The use of a casting between cylinders provided from an online hot rolling mill has, from this point of view, the advantage that it is much more easy to prevent or limit the formation of fayalite over the short distance between casting and rolling (or removing the fayalite that may have formed) only in a facility classic.

After it has been poured, and after having passed through any inerting zone, the strip is then hot-rolled in line, in known manner, to give it a thickness generally between 1 and 3 mm. This rolling must be carried out in the austenitic domain, therefore at a temperature higher than the temperature Ar ₃ of the cast grade. It is carried out with a total reduction rate of between 25 and 70%. The role of this inline hot rolling is twofold. It must first close the porosities which may have formed at the heart of the strip during its solidification. Above all, it must "break" the microstructure resulting from solidification, so as to refine it and make it possible to obtain the desired final microstructure. This hot rolling can take place in one or more passes, that is to say by passing the strip through a single rolling stand, or by passing the strip through several successive stands, the first ensuring a slight reduction aimed at closing the porosities, and the one or the following ensuring the obtaining of the final thickness. As an example, the following triplets can be proposed (casting thickness / reduction ratio in hot rolling / final thickness): examples of triplets (casting thickness / reduction rate in hot rolling / final thickness) Initial strip thickness (mm) Hot rolling rate (%) Final strip thickness (mm) 4 25 3 4 50 2 2 40 1.2 1.5 40 0.9 1 60 0.6

After this hot rolling, a first forced cooling of the tape, for example by means of a sprinkling of water. This cooling aims to form the within the band a ferritic structure, while avoiding the appearance of perlite. To this end, it must be carried out at a speed between 5 and 100 ° C / s, preferably between 25 and 80 ° C / s, which is perfectly compatible with conventional technologies of cooling of strips having the thicknesses considered. Cooling speed too low would cause perlite to appear, making transformation impossible bainitique which constitutes one of the essential characteristics of the invention. A speed too high cooling risk of not allowing the ferritic structure to be obtained as sought for the matrix, because one would pass directly in the field bainitic, even in the martensitic domain. The range of cooling speeds preferential allows to better ensure the obtaining of an optimal result.

This first cooling must be such, in speed and duration, that it brings about the strip in a thermal state which allows the strip to remain in the air in the area of temperatures 550-400 ° C, preferably 530-470 ° C (in order to obtain the austenite rate sought for reasonable retention times, while ensuring that no training will be no perlite) for the time necessary for a transformation to occur bainitique stabilizing the proportion of austenite remaining at more than 5%, while avoiding the perlite formation. Once this result is obtained, the band undergoes a second forced cooling, for example by spraying water, so as to bring the strip out from the previous temperature range (therefore less than 400 ° C), preferably until its winding temperature, which must be less than 350 ° C. This temperature range of winding is chosen to avoid any major change in the structure of the wound strip, such as a precipitation of carbides which would destabilize the austenite.

The length of stay of the strip in air without forced cooling necessary for obtaining the bainitic transformation as desired varies according to the casting parameters precise, i.e. the composition of the tape and its speed of movement in the the corresponding installation. This duration must be determined experimentally, in using the classic transformation curves of the steel grades considered, and function of the precise residual austenite level that one wishes to obtain. Austenite level high improves ductility, but conversely, an austenite level of less than 5% at the end of bainitic transformation will provide insufficient martensite formation to obtain the TRIP effect. For example, on a grade of 0.2% carbon, 1.5% manganese and 1.5% of silicon, an austenite content of 6% is obtained for maintaining the strip of 10 s at 470 ° C or 20s at 530 ° C. In practice, the duration of this stay may be generally between 5 and 30 s.

If we assume that the casting strip has an initial thickness of 3 mm and a speed of 60 m / min at its exit from the rolls (which is common in a casting installation between rolls), the running speed of the strip hot rolled in the bainitic transformation zone varies according to the hot rolling rate applied to it. Table 2 shows examples of strip running speeds in the bainitic transformation zone as a function of the hot rolling rate, taking into account the previous hypotheses. strip running speeds in the bainitic transformation zone as a function of the hot rolling rate (casting thickness 3 mm, casting speed 60 m / min) Hot rolling rate (%) Belt speed (m / s) 25 1.3 40 1.7 60 2.5 70 3.3

Under these conditions, if it is decided to impose an end temperature on the strip rolling of 900 ° C, a cooling rate in the first spray zone of 50 ° C / s, a stay of 10 s at 500 ° C in the bainitic transformation zone and a speed cooling in the second spray zone of 50 ° C / s to bring the strip to less than 250 ° C, the strip will take 20 to 25 s to reach the rolling stand up to the winder. If these two organs are about 40 m apart, which is reasonable on a usual installation between rolls, the speed of the web after its rolling must therefore be approximately 2 m / s, which is perfectly compatible with conclusions drawn from Table 2. Technologically, the implementation of the process according to the invention therefore does not pose any major problem. To get the desired result, one can also act on the length of the cooling zones and on the flow of the liquid of cooling in each of these areas. For this purpose, if the cooling zones are consist of a succession of water spray bars, you can choose to use a variable number of ramps to flexibly adjust the lengths of these zones.

It will be understood that the essential step of the method according to the invention is the stay of the strip in the bainitic transformation field after its hot rolling, to which the second cooling imposes a short duration, as well as the realization of the winding of the strip in a range of temperatures where the bainitic transformation has already happened. This prevents the course of the bainitic transformation from being affected by the rewetting phenomenon, and improves the reliability of obtaining a homogeneous microstructure within the band. The fact of making the strip by casting between cylinders (or, so general, by direct casting of thin strips from 1.5 to 10 mm and in particular from 1 to 5 mm thick) and hot rolling in line is almost a prerequisite for the economic viability of carrying out the bainitic transformation in these conditions. Indeed, it would be possible to carry out this bainitic transformation by stay at 550-400 ° C for one to a few seconds of a strip leaving a train at classic bands. However, taking into account the usual tape running speeds at the exit of a band train which are significantly higher than the running speeds at the outlet of a rolling mill in line between rolls, this would require a distance oversized (of the order of 500 m) between the output of the strip train and the winder. That would remove any economic interest in this solution. In addition, by carrying out rolling with hot and bainitic transformation in line with casting, we don't need a intermediate heating, costly in energy. Finally, metallurgical transformations brought into play by the method according to the invention, where the temperature of the strip only decrease between its casting and its winding, are not likely to be hindered by structures which would have been obtained after a first cooling of the product to room temperature and would remain at least residual after the reheating before hot rolling. This could be the case if the supply chain manufacturing between the casting of the initial semi-finished product and the winding of the final strip had to be discontinuous.

After their winding, the strips obtained by the process according to the invention are ready to be used in the same way as TRIP steel strips as well composition obtained by the conventional continuous casting slab die - rolling to hot.

Claims (13)

Process for manufacturing thin steel strips of the “TRIP” type, according to which: a strip having a thickness of between 1.5 and 10 mm, preferably 1 to 5 mm, is poured directly from liquid steel, said steel having the composition (in weight percentages) C% of between 0.05 and 0.25, (Mn + Cu + Ni)% between 0.5 and 3, (Si + Al)% between 0.1 and 4, (P + Sn + As + Sb)% less than or equal to 0.1, (Ti + Nb + V + Zr + rare earths)% less than 0.3, Cr% less than 1, Mo% less than 1, V% less than 1, the rest being iron and impurities resulting from production; hot rolling is carried out in line of said strip at a temperature higher than the temperature Ar _{3 of} said steel with a reduction rate of between 25 and 70%, in one or more passes; a first forced cooling of said strip is carried out at a speed of between 5 and 100 ° C / s; the strip is left to stay at temperatures between 550 and 400 ° C. for the time necessary for a bainitic transformation to occur there with a proportion of residual austenite greater than 5%, while avoiding the formation of perlite, then interrupts this transformation by a second forced cooling of said strip which brings it to a temperature below 400 ° C; said strip is wound at a temperature below 350 ° C. Process according to claim 1, characterized in that the phosphorus content steel is less than or equal to 0.05% Method according to claim 1 or 2, characterized in that the total of the contents in chromium, molybdenum and vanadium does not exceed 0.3%. Process according to claim 3, characterized in that the total of the contents of chromium, molybdenum and vanadium does not exceed 0.05%. Method according to one of claims 1 to 4, characterized in that the content of copper is between 0.5 and 2%. Method according to one of claims 1 to 5, characterized in that the speed of the first cooling is between 25 and 80 ° C / s. Method according to one of claims 1 to 6, characterized in that after the first cooling, the strip is left to stay between 530 and 470 ° C. for the time necessary for a bainitic transformation to occur with a proportion residual austenite greater than 5%, while avoiding the formation of perlite. Method according to one of claims 1 to 7, characterized in that the duration of the stay of said strip in the area where the bainitic transformation occurs is between 5 and 30 s. Method according to one of claims 1 to 8, characterized in that said second cooling brings said strip to its winding temperature. Method according to one of claims 1 to 9, characterized in that between its casting and its hot rolling, said strip passes through an area where it is maintained at near its surface a non-oxidizing atmosphere for the metal. Method according to one of claims 1 to 10, characterized in that before the hot rolling, a descaling operation is carried out on the surface of said strip. Method according to one of claims 1 to 11, characterized in that said strip is cast between two close cylinders with horizontal axes rotated in reverse and internally cooled. “TRIP” type thin steel strip, capable of being obtained by method according to one of claims 1 to 12.