EP1228254B1 - Method for making carbon steel bands, in particular packaging steel bands - Google Patents

Abstract

The invention relates to the iron and steel industry. More specifically, the invention describes the manufacture of steel strip intended to be converted into thin packaging, such as for drinks and preserved food.

Description

The invention relates to the steel industry. More specifically, it relates to manufacturing steel strips intended to be transformed into thin packaging, such as canned food and drink boxes.

• coulée continue de brames d'acier au carbone ;
• laminage à chaud de ces brames sur un train à bandes avec une température de fin de laminage supérieure à la température Ar₃ de la nuance considérée ;
• laminage à froid de la bande à chaud ainsi obtenue, ce laminage à froid pouvant être effectué en une étape unique, ou en deux étapes pouvant être séparées par un traitement thermique, selon l'épaisseur finale désirée pour la bande ;
• recuit de la bande à froid ainsi obtenue, par recuit base ou recuit continu.

The conventional method of manufacturing steel strips intended to then be transformed into thin packaging, in particular for beverages and food products, comprises the following steps:

• continuous casting of carbon steel slabs;
• hot rolling of these slabs on a band train with an end of rolling temperature higher than the temperature Ar ₃ of the grade considered;
• cold rolling of the hot strip thus obtained, this cold rolling can be carried out in a single stage, or in two stages which can be separated by a heat treatment, according to the final thickness desired for the strip;
• annealing of the cold strip thus obtained, by base annealing or continuous annealing.

In practice, the thicknesses of the final strips after cold rolling and annealing are of the order of 0.09 to 0.40 mm. These strips are then cut into sheets and / or blanks, which are stamped to form the desired packaging.

This manufacturing process is long and costly in energy, because it requires the use of separate facilities. In particular, the rolling of the slabs on the band train is expensive, in particular because these slabs must first be reheated to high temperature. On the other hand, the band train is a tool requiring a high investment.

This drawback can be overcome by replacing the continuous casting-reheating furnace-strip train assembly with a direct casting installation for thin strips having a thickness of less than 10 mm. This solution was proposed in the document JP 09-001207, which teaches to flow directly from liquid metal, on a casting installation between two internally cooled counter-rotating cylinders, strips whose composition corresponds to a conventional grade of steels for packaging (C% ≤ 0.15; Mn% ≤ 0.6; P% ≤ 0.025; S% ≤ 0.025; Al% ≤ 0.12%; N% ≤ 0.01; O _total % ≤ 0.007%, all these contents being expressed as weight percentages). The strip thus cast then undergoes pickling, a first cold rolling, a recrystallization annealing and a second cold rolling. The total reduction rate undergone by the strip during cold rolling is between 85 and 95% if we want to obtain satisfactory results on the rate of the drawing horns, the anisotropy coefficient r and the planar anisotropy Δr. The casting between rolls can be followed by a light hot rolling with a reduction rate of 20 to 50%, or even more. The manufacture of the hot strip which must then undergo cold rolling and the associated treatments is thus faster and more economical. However, the need for subsequent cold rolling in two stages separated by annealing tempers these advantages.

Document JP 07118735 discloses the production of a steel strip by casting a thin strip no more than 10 mm thick followed by hot rolling in the austenitic region and forced cooling. The strip is then maintained at a temperature between Ar ₁ at 600 ° C for at least 10 s and then wound.

The object of the invention is to propose a more economical process than known processes for obtaining cold rolled steel strips usable for the manufacture of packaging, in particular for food packaging such as drink cans.

• on coule sous forme d'une bande mince de 0,7 à 10 mm d'épaisseur, directement à partir de métal liquide, un acier ayant une composition adaptée à une utilisation comme acier pour emballage ;
• on effectue une opération de laminage à chaud en ligne de ladite bande, se terminant dans le domaine austénitique dudit acier pour obtenir une bande d'épaisseur inférieure à 3 mm ;
• on effectue un refroidissement forcé de ladite bande à une vitesse de 80 à 400°C/s se terminant dans le domaine ferritique dudit acier ;
• on effectue un laminage à froid de ladite bande à un taux de réduction de 85% au moins en une seule étape ;
• et on effectue un recuit de ladite bande.

To this end, the subject of the invention is a process for manufacturing strips of carbon steel, in particular steel for packaging, according to which:

• a steel having a composition suitable for use as steel for packaging is poured in the form of a thin strip of 0.7 to 10 mm thick, directly from liquid metal;
• performing an in-line hot rolling operation of said strip, ending in the austenitic range of said steel to obtain a strip of thickness less than 3 mm;
• a forced cooling of said strip is carried out at a speed of 80 to 400 ° C / s ending in the ferritic range of said steel;
• cold rolling of said strip is carried out at a reduction rate of 85% at least in a single step;
• and annealing said strip.

The invention also relates to the use of a steel strip obtained according to this process for the manufacture of packaging.

As will be understood, the invention is based on the use of a method of casting between rolls followed by at least one in-line hot rolling step and a particular cooling of the strip. A hot strip is thus obtained which tolerates not then undergo only one cold rolling step (apart from the conventional final passage to the skin-pass) to be given the properties that make it suitable for manufacturing steels for packaging.

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

The process according to the invention begins with casting in the form of strips thin 0.7 to 10 mm thick (preferably 1 to 4 mm) of a semi-finished product low or ultra-low carbon content of a steel that can be used for the packaging of classic composition. This composition, for the main elements present, meets the main criteria (the percentages are expressed in weight percentages): 0% ≤ C ≤ 0.15%; 0% ≤ Mn ≤ 0.6%; 0% ≤ P ≤ 0.025%; ≤ 0% ≤ S ≤ 0.05%; 0% ≤ Al ≤ 0.12%; 0% ≤ N ≤ 0.04%. This steel also contains the usual impurities resulting from the development, and possibly of elements of alloy in small quantity which will not affect not adversely affect the properties of the products during their shaping or use as steels for packaging (it is thus known, in certain steels for packaging, introduce a few thousandths of% boron), the rest being iron. The alloying elements, generally absent, may possibly be present in contents of up to 1%; These elements are notably Si, Cr, Ni, Mo, Cu. For for regulatory reasons, certain alloying elements must be excluded when the steel is intended for packaging; these elements are for example lead, cadmium and arsenic.

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. The most commonly used technique 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 the “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. This technique is particularly recommended for the invention because that it gives access to strip thicknesses of a few mm, and we will refer to this in the continuation of the description. But other methods of direct tape casting can be used thin, such as the casting between two scrolling bands which allows casting of products a little thicker than the casting between cylinders. However, one of the advantages of casting between cylinders is the possibility of obtaining, if necessary, thickness profiles of the extremely flat crosswise band, thanks to the excellent control of the convex cylinders that allow the most advanced methods of practicing this process (see, for example, document EP 0 736 350).

At its exit from the cylinders, the strip preferably crosses an area such as an enclosure inerted by gas blowing, where it is subjected to a non-oxidizing environment (a neutral atmosphere of nitrogen or argon, or even a atmosphere containing a certain proportion of hydrogen to make it reductive), in order to avoid or limit the formation of scale on its surface. At the exit from this inerting zone, it is also possible to place a device for descaling the strip by spraying shot or solid CO ₂ onto its surface or by brushing, in order to eliminate the scale which could have formed despite the precautions taken. . It is also possible to choose to allow the scale to form naturally without seeking to inert the atmosphere surrounding the strip, then to eliminate this scale by a device as just described. The presence of scale on the strip is generally not desired, because of the risks of encrustation of this scale in the surface of the strip during subsequent rolling. Such inlays lead to a poor surface condition of the products. In addition, the scale increases the rolling forces to be applied, and degrades the surface condition of the rolls of the rolling mill.

• une épaisseur inférieure à 3 mm (typiquement 0,9 mm) qui, en liaison avec les taux de réduction qui seront pratiqués lors du laminage à froid qui suivra, permettra d'obtenir des bandes finales ayant l'épaisseur souhaitée ;
• une structure métallurgique qui, toujours en liaison avec les traitements ultérieurement subis par la bande, permet d'obtenir sur la bande les propriétés mécaniques requises pour l'utilisation future du métal, par exemple comme acier pour emballages ;
• un profil travers plus plat que ceux obtenus avec les procédés conventionnels.

As soon as possible immediately after leaving the strip from the inerting or descaling installation, if there is one, a hot rolling operation of the strip takes place, followed by strong cooling. The purpose of this treatment is to obtain a strip having:

• a thickness less than 3 mm (typically 0.9 mm) which, in conjunction with the reduction rates which will be applied during the cold rolling which will follow, will make it possible to obtain final strips having the desired thickness;
• a metallurgical structure which, still in conjunction with the treatments subsequently undergone by the strip, makes it possible to obtain on the strip the mechanical properties required for the future use of the metal, for example as steel for packaging;
• a cross profile flatter than those obtained with conventional methods.

To achieve this result, two variants of manufacturing schemes are proposed.

• de refermer les porosités qui peuvent être présentes au coeur de la bande après sa coulée ;
• de « casser » la microstructure de solidification ;
• et d'améliorer l'état de surface de la bande en écrasant les reliefs qui peuvent être présents à la surface de la bande, en particulier lorsqu'on a utilisé lors de la coulée des cylindres présentant une relativement forte rugosité qui peut être avantageuse pour l'optimisation des transferts thermiques entre les cylindres et les peaux solidifiées.

According to the first variant, a single step of hot rolling of the strip is carried out, ending at a temperature higher than the temperature Ar ₃ of the cast steel, in other words in the austenitic field. This hot rolling is carried out with a minimum reduction rate of 20%, and preferably this rate is greater than 50%. The functions of this hot rolling are:

• to close the porosities which may be present at the heart of the strip after its casting;
• to "break" the solidification microstructure;
• and to improve the surface condition of the strip by crushing the reliefs which may be present on the surface of the strip, in particular when cylinders having a relatively high roughness have been used during casting, which may be advantageous for optimization of heat transfers between cylinders and solidified skins.

This single stage of hot rolling can be carried out by means of the passage strip in a single rolling stand. It can also be done more progressive by passing the strip through two or more rolling stands. The first one cage can, for example, apply to the tape a reduction rate only sufficient to close the porosities, and the second cage then ensures most of the reduction thick to fulfill the other two functions of hot rolling. The bottom line is that the overall reduction rate caused by this or these passages in the or the successive cages and the temperature of the strip after it has passed through the last cage are within the prescribed ranges of values.

According to the second of these variants, hot rolling takes place in two stages, separated by reheating, and optionally by descaling. The first of these steps is carried out either in the austenitic domain, or in the ferritic domain of the cast strip, with a reduction rate of 20 to 70%. It has functions identical to those of the single hot rolling step of the first variant, and can also be carried out by passing the strip through one or more successive rolling mill stands. Preferably, this first rolling step takes place in the ferritic area when it is desired to obtain a final thickness of the strip that is small, because less effort is required to deform the strip evenly over its entire width than when the strip is in the austenitic domain. When this first hot rolling step is carried out by distributing it over several cages, it is however possible to begin this first step in the austenitic field, for example by a relatively light rolling which would mainly aim at closing the porosities, and finish in the ferritic area where the rest of the thickness reduction would be carried out. After this first stage of hot rolling, the strip is allowed to cool down to the ferritic region if it is not already there (if necessary using a slight forced cooling), then a treatment is applied to it. thermal reheating which brings it back into the austenitic domain, therefore above the temperature Ar ₃ . This causes an additional phase change in the strip, which results in an even further refinement of the grains of the metallurgical structure. Then the second stage of hot rolling, in the austenitic field, is carried out with a reduction rate of 10 to 30%. This second hot rolling has the essential function of correcting the geometric defects (poor flatness, saber ...) that the first hot rolling could have caused. Intermediate heating can be achieved by means of an inductor through which the strip passes. For a strip 0.75 mm thick and 850 mm wide running at a speed of 200 m / min, a power of 1.04 MW is necessary if a temperature rise of 100 ° C is sought. Consequently, if a solenoid inductor in longitudinal flux operating at 500 kHz is used, the efficiency of which is usually around 45%, an inductor length of approximately 2 m (including 1.5 m of useful area) is suitable for this purpose. If the strip has a smaller thickness, one can use the technology of induction heating under transverse flow, described in particular in the document "High flux induction for the fast heating of steel semi-product in line with rolling" (Proceedings of the XIII International Congress on Electricity Applications, Birmingham, June 1996). However, in general, other more conventional technologies, such as a muffle furnace under controlled atmosphere, or radiant tubes, can be used to ensure this reheating.

The two variants which have just been described therefore have the common point of ending with a rolling carried out on the strip in the austenitic phase, which therefore ends above the temperature Ar ₃ . In both cases, the method according to the invention continues with cooling of the strip which includes a forced cooling step at a speed of 80 to 400 ° C / s, preferably 100 to 300 ° C / s. This cooling ends in the ferritic field of cast steel, and in general brings the strip to a temperature close to its winding temperature. Its purpose is to avoid excessive growth in the size of the grains before winding and during the stay of the strip in the form of a reel. This winding temperature is typically less than 750 ° C. For the aluminum calmed grades, the winding temperature can be chosen around 550 ° C or 600 ° C or 700 ° C in order to more or less favor the precipitation of aluminum nitrides.

It is important for the reliability of obtaining the desired properties for the strip that this forced cooling takes place homogeneously over the entire width of the strip. The maximum desirable amplitude of the temperature differences from one point to another of the width of the strip at a given time can be calculated at 10 ° C. This homogeneity is more difficult to guarantee if the cooling rate is high, which motivates the recommendation of a maximum speed of 400 ° C / s. Conversely, a minimum speed of 80 ° C / s ensures that the cooling will have the desired metallurgical efficiency. Such cooling rates can be obtained, in particular, by spraying water by means of high-pressure nozzles, or by spraying a water-air mixture or the like (atomization). This forced cooling can start immediately after the austenitic rolling of the strip, but it is advisable to start it only after having allowed the strip to cool at low speed (about 10 ° C / s, which is accessible by a simple exposure in the open air) and pass into the ferritic domain, therefore below Ar ₃ . In this way, one takes full advantage of the refinement of the grain linked to the change of austenite-ferrite phase, while rapid cooling which would begin in the austenitic domain would substantially interfere with the homogeneity of the microstructure. It should be noted, however, that the accelerated cooling should preferably not start at a temperature below Ar ₃ - 10 ° C.

In general, the use of rapid cooling before winding avoids the presence of large grains in the skin of the strip, which are particularly undesirable on steels for packaging. Indeed, these must have, after rolling when cold, very great homogeneity of their final characteristics.

The strip wound and then unwound then undergoes cold rolling at a rate of reduction of at least 85%, preferably more than 90%. This cold rolling can perfectly be executed by simple reduction, i.e. in a single step, and not imperatively in two stages with intermediate annealing as was the case in the document JP 09-001207 already cited (double reduction cold rolling). We get stamping skills comparable to those obtained by known methods, and access to strip thicknesses smaller than 0.09 mm of known methods without however, it is necessary to resort to cold rolling with double reduction. If we don't want get thinner strips than usual, we can get the classic thicknesses with lower reduction rates during cold rolling, which is more economical. he is, of course, possible to carry out a cold rolling of the strip in double reduction if one wishes to obtain an even smaller thickness or mechanical characteristics higher.

As an indication, table 1 can be presented which gives examples of final thicknesses of the strip as a function of its initial thickness after casting and of the rolling rates applied during the hot rolling steps (in one or two steps depending on the chosen variant) and cold rolling. Thickness of the strips obtained as a function of the various casting parameters and
rolling Thickness of the
casting strip (mm) Rolling rate at
hot (%) Strip thickness
hot (mm) Rolling rate
Cold (%) Final thickness of
the strip (mm) 3 65 1.05 85 to 92 0.158 to 0.084 3 70 0.9 85 to 92 0.135 to 0.072 2 60 0.8 85 to 92 0.12 to 0.064 1.5 50 0.75 85 to 92 0.113 to 0.060

After cold rolling, the strip undergoes the usual annealing (basic or continuous) intended to give it its mechanical properties. This annealing can be followed, as usually by stripping, coating and / or passing the skin-pass.

The exit speeds of the strip from the hot rolling mill being of the order of 250 m / min or less, these speeds are compatible with placing on a single line of this rolling mill (therefore the entire casting line) and one or more of the cold rolling, annealing and cold treatment of steels for packaging, whose metal flow is compatible with that of the hot rolling mill. We can cite as examples of such operations, in addition to pickling and the skin-pass which can follow annealing, a lacquering, varnishing, polymer deposition, for example by coextrusion, deposition under vacuum by plasma or electronic bombardment, a metallic coating by electrodeposition. If the cold rolling operation takes place in line with the casting and hot rolling, this involves removing the winding step from the bandaged.

If the invention finds a preferred field of application in the manufacture of steel strips intended to be stamped to form packaging for beverages or canned food, it goes without saying that it can be applied to the manufacture of steel strips intended for other uses for which similar qualities would be required for tapes produced.

Claims (14)

1. Method for manufacturing strips of carbon steel, especially of steel for packaging, in which:

   a steel having a composition suitable for use as packaging steel is cast in the form of a thin strip from 0.7 to 10 mm in thickness, directly from liquid metal;

   an in-line hot rolling operation is carried out on said strip, at the end of which said steel is in the austenitic range, so as to obtain a strip having a thickness of less than 3mm;

   said strip undergoes forced cooling at a rate of 80 to 400°C/s, at the end of which said steel is in the ferritic range;

   said strip undergoes a cold rolling operation with a reduction ratio of at least 85% in at least a single step; and

   said strip undergoes an annealing operation.
2. Method according to Claim 1, characterized in that said strip is cast between two internally cooled horizontal rolls rotating in opposite directions.
3. Method according to Claim 1 or 2, characterized in that said hot rolling operation is carried out in a single step with a reduction ratio of at least 20%.
4. Method according to Claim 3, characterized in that said hot rolling operation is carried out in a single step with a reduction ratio of at least 50%.
5. Method according to Claim 1 or 2, characterized in that said hot rolling operation is carried out in two steps, in that the first of these steps is carried out with a reduction ratio of 20 to 70%, in that, after this first step, the strip is reheated so as to make said steel pass from the ferritic range into the austenitic range, and in that the second rolling step is then carried out with a reduction ratio of 10 to 30%, at the end of which second step said steel is in the austenitic range.
6. Method according to Claim 5, characterized in that said first step is carried out entirely in the ferritic range of said steel.
7. Method according to Claim 5, characterized in that said first step is carried out partly in the austenitic range and partly in the ferritic range of said steel.
8. Method according to Claims 1 to 7, characterized in that, after the strip has been cast, it is made to pass through a region in which it is subjected to a nonoxidizing environment.
9. Method according to one of Claims 1 to 8, characterized in that the strip is subjected to a descaling operation before and/or during the hot rolling.
10. Method according to one of Claims 1 to 9, characterized in that said forced cooling is carried out at a rate of 100 to 300 °C/s.
11. Method according to one of Claims 1 to 10, characterized in that said forced cooling starts when the strip is in the ferritic range of said steel.
12. Method according to one of Claims 1 to 11, characterized in that the strip is coiled at a temperature below 750°C between the forced cooling operation and the cold rolling operation.
13. Method according to one of Claims 1 to 12, characterized in that the reduction ratio of the cold rolling is at least 85%.
14. Use of a steel strip which can be obtained by the method according to any one of Claims 1 to 13, for the manufacture of packaging.