EP1061139A1 - Method of manufacturing deep drawing steel sheets by direct casting of thin strips and sheets obtained by this method - Google Patents

Abstract

Stampable steel sheets are produced by: direct casting of a steel strip; first hot rolling in one or more steps at 950 degrees C to the Ar3 temperature, with total reduction ≥ 10%; second hot rolling in one or more steps in ferritic phase at 850 degrees C, with total reduction ≥ 50% using a lubricant to obtain a sheet of thickness of ≤ 2 mm; and complete recrystallization throughout the thickness at 700-800 degrees C. The composition of the molten steel used to produce the steel strip comprises (in weight %): carbon ≤ 0.1%, manganese 0.03-2, silicon 0-0.5, phosphorus 0-0.1%, boron 0-0.002%, titanium 0-0.15, and iron and inevitable impurities the remainder. Most preferably, the cast steel strip has carbon content less than 0.007%, the sum of carbon and nitrogen less than 0.007%, the sum of Ti and Nb less than 0.04%, manganese content 0.3-2. Casting of the strip is performed between two cooled horizontal rolls rotating in opposite directions. Between casting and the first hot rolling the strip is passed through a non-oxidizing atmosphere and/or is subjected to descaling. Forced cooling of the strip and can be carried out between the first and second hot rolling stages. Between the first hot rolling and the second hot rolling the strip is passed through a non-oxidizing atmosphere and/or is subjected to descaling. An Independent claim is given for a sheet produced by the above process.

Description

The invention relates to the field of the manufacture of steel sheets of low thickness intended to be stamped. More specifically, it concerns steel sheets ordinary low and very low carbon.

• coulée continue de brames d'épaisseur 200 mm environ ;
• laminage à chaud desdites brames jusqu'à obtenir des bandes d'une épaisseur de l'ordre de 4 mm ;
• laminage à froid, recuit (base ou continu) et passage dans un laminoir écrouisseur (skin-pass) desdites bandes (opérations que l'on désigne par le terme « traitements à froid », même si pour certains,tels que le recuit, un réchauffage est nécessaire), que l'on découpe ensuite pour obtenir des tôles.

Conventionally, thin sheets (0.5 to 1.5 mm) made of carbon steel intended to be stamped and used, for example in the automotive industry, are obtained by the following manufacturing process:

• continuous casting of slabs with a thickness of around 200 mm;
• hot rolling said slabs until strips of a thickness of the order of 4 mm are obtained;
• cold rolling, annealing (basic or continuous) and passage through a cold-rolling mill (skin-pass) of the said strips (operations which are designated by the term "cold treatments", even if for some, such as annealing, a reheating is necessary), which is then cut to obtain sheets.

The composition of these sheets can be summarized as follows (the percentages are weight percentages).

For so-called “low carbon” sheets, there must be a carbon content less than 0.1%, preferably less than 0.03%, with even more preferably a sum of the carbon and nitrogen contents of less than 0.03%, a manganese content between 0.03 and 0.3%, a silicon content between 0.05 and 0.3%, a phosphorus content of between 0.01 and 0.1%. When you want sheets having a particularly high resistance, a carbon content is imposed less than 0.03% and a manganese content of between 0.3 and 2%. Additions of boron (up to 0.008%) and titanium (from 0.005 to 0.06%) in low carbon sheets are also possible.

For so-called “ultra-low carbon” sheets, there must be a carbon content less than 0.007%, and preferably a nitrogen content also very low, does not not exceeding a few tens of ppm. The contents of the other elements are the same than for low carbon sheets, with optional titanium microadditions (from 0.005 to 0.06%) and / or niobium (0.001 to 0.2%).

A process which can replace the previous one consists in casting the steel in slabs thin at their exit from the mold for continuous casting (for example 40 to 100 mm) and then carry out their hot rolling on rolling mill stands in line with the casting installation, this rolling can include various stages during which the steel is in the ferritic or austenitic state (see document WO 97/46332). In this process, at least one reheating of the slab, preceding the first hot rolling, is necessary, as well as subsequent cooling and reheating to allow carry out the desired metallurgical transformations of the product. We can thus realize various types of products, especially high formability sheets for the automotive industry.

In the foregoing methods, using conventional hot rolling mills to obtaining the final thickness of the strip before cold rolling, the speed of running of the strip at the outlet of the hot rolling installation is of the order of 600 at 950 m / min depending, in particular, on the thickness of the product. These speeds are relatively high, in particular in relation to the usual running speeds of the products in the installations ensuring the “cold” treatments of the strips obtained in the rest of the manufacturing process, for example in compact annealing lines, hardened or plating. This induces productivity differences between these various installations, which oblige to store the products in their intermediate states in the form of coils, pending “cold” treatments. This leads to management product flow which is not optimal, even in the most favorable case where all casting, rolling and "cold" treatment facilities would be grouped together the same industrial site.

The object of the invention is to provide a method of manufacturing sheet metal with high stampability with better productivity than traditional methods, by a shortening of manufacturing chains.

• on coule directement à partir de métal liquide une bande d'acier d'épaisseur 1,5 à 10 mm ayant la composition en pourcentages pondéraux : carbone moins de 0,1%, manganèse de 0,03 à 2%, silicium de 0 à 0,5%, phosphore de 0 à 0,1%, bore de 0 à 0,002%, titane de 0 à 0,15%, le reste étant du fer et des impuretés résultant de l'élaboration ;
• on procède à un premier laminage à chaud en une ou plusieurs étapes de ladite bande, en phase austénitique, à une température comprise entre 950°C et la température Ar₃ de ladite bande avec un taux de réduction global d'au moins 10% ;
• on procède à un second laminage à chaud en une ou plusieurs étapes de ladite bande, en phase ferritique, à une température inférieure à 850°C, avec un taux de réduction global de 50% au moins en présence d'un lubrifiant, afin d'obtenir une tôle laminée à chaud d'épaisseur inférieure ou égale à 2 mm ;
• et on procède à une recristallisation complète sur toute l'épaisseur de ladite bande par un séjour entre 700 et 800°C.

To this end, the subject of the invention is a method of manufacturing steel sheets suitable for stamping obtained from a strip, characterized in that:

• a steel strip 1.5 to 10 mm thick is poured directly from liquid metal, having the composition in weight percentages: carbon less than 0.1%, manganese from 0.03 to 2%, silicon from 0 to 0.5%, phosphorus from 0 to 0.1%, boron from 0 to 0.002%, titanium from 0 to 0.15%, the rest being iron and impurities resulting from the production;
• a first hot rolling is carried out in one or more stages of said strip, in the austenitic phase, at a temperature between 950 ° C. and the temperature Ar ₃ of said strip with an overall reduction rate of at least 10%;
• a second hot rolling is carried out in one or more stages of the said strip, in the ferritic phase, at a temperature below 850 ° C., with an overall reduction rate of 50% at least in the presence of a lubricant, in order to '' obtain a hot rolled sheet of thickness less than or equal to 2 mm;
• and a complete recrystallization is carried out over the entire thickness of said strip by a stay between 700 and 800 ° C.

The strip can then undergo cold treatments, or be cut to form sheets which will be shaped directly.

The subject of the invention is also a sheet which can be obtained from tapes produced by the above process.

As will be understood, the invention is first based on the use of a process for the direct casting of thin strips from liquid metal, known per se. The process of casting the strip between two internally cooled horizontal cylinders and rotated in opposite directions is well suited for this purpose. The strip coming out of the cylinders is then subjected to thermal and thermomechanical treatments which make it suitable for undergo the usual cold treatment operations that are applied to the laminated strips hot obtained by traditional methods. The respective productivities usual installations for direct casting of thin strips and installations for cold treatment of said strips being very comparable, the management of the production of sheet metal suitable for stamping is very simplified. It is even sometimes possible to completely pass from the cold rolling step, necessary in conventional dies, this which makes the manufacture of sheets and the resulting products faster and more economic.

The use of an installation for casting thin strips between cylinders to getting carbon steel strips 1.5 to 10 mm thick is fine today known. In some cases (for example in document WO 95/26840), it is practiced on these strips hot rolling online. It is also known to practice on these bands various heat treatments by heating and / or cooling cycles online, in order to influence the metallurgical structure of the strips, for example to refine the grain size by phase changes α → γ → α (see documents JP 61-189846 and JP 63-115654).

The method according to the invention is particularly suitable for the manufacture of sheets with high drawability, low steel (less than 0.1%, preferably less than 0.05%) and ultra-low (less than 0.007%) carbon content. Their manganese content can vary from 0.03 to 2%, the highest grades (from 0.3%) corresponding to steels for which a particularly high resistance is required. Their silicon content can range from 0 to 0.5%. their phosphorus content ranges from 0 to 0.1%. In the general case, Additions of boron (up to 0.002%) and titanium (up to 0.15%) are possible. Preferably, these steels have a low nitrogen content. For low carbon steels, the sum of the carbon and nitrogen contents does not optimally exceed 0.03%. For the ultra-low carbon steels, the sum of the carbon and nitrogen contents must not optimally not exceed 0.007%. These ultra-low carbon steels can also contain small amounts of elements such as titanium and niobium (with Ti + Nb not exceeding 0.04%) whose function is to trap carbon and nitrogen under form of carbonitrides. Other chemical elements resulting from the production of metal may be present as impurities which do not radically modify the properties of sheets obtained thanks to the compositions which have just been described. Ultra-low contents in carbon and nitrogen are preferred because these elements will be in solid solution during the deformation, taking into account the method of manufacturing sheets according to the invention; their presence can create dynamic aging problems during deformation, and therefore increase the rolling forces to be applied in the ferritic field.

In accordance with the process according to the invention, the casting is carried out directly at from liquid metal with a thin strip of 1.5 to 10 mm thick, usually 1.5 to 4 mm thick. The casting of this strip between two cylinders, as we said, is well suited to this process and to the thicknesses most commonly cast, and it is this nonlimiting example which will be considered in the following description.

The solidified strip leaving the casting space delimited by the cylinders then optimally crosses an area in which measures are taken to avoid or at least strongly limit the formation of scale on its surface, such as an enclosure inerted by a non-oxidizing atmosphere, ie a neutral atmosphere (nitrogen, argon) or reducing atmosphere (atmosphere containing hydrogen), in which the oxygen content is lowered as much as possible. One can also choose to let the scale form naturally over a certain distance, then remove the formed scale, for example using brushes or a device projecting shot or solid CO ₂ on the surface of the strip. Such a device can also be installed at the outlet of an inerting zone to remove the small amount of scale which may have formed there.

Immediately after the inerting or descaling zone (where they exist), the strip undergoes a first in-line hot rolling. It is in particular because of this rolling that the presence of scale on the surface of the strip must optimally be avoided, since the presence of scale requires greater rolling forces than in its absence. In addition, the scale can become embedded in the surface of the strip during rolling, and a poor surface finish of the final product is thus obtained, which can make this final product unsuitable for the most demanding uses from this point of view. . This first rolling takes place in the temperature range between 950 ° C. and the temperature Ar ₃ of the cast grade, that is to say in the lower zone of the austenitic range. Its role is multiple. On the one hand, it makes it possible to close the central porosities which may possibly have formed at the heart of the strip during its solidification. On the other hand, it "breaks" the microstructure resulting from solidification and makes it possible to form ferrite grains from a hardened austenite. Finally, it has a beneficial influence on the surface condition of the strip by reducing its roughness. To achieve these objectives, a minimum reduction rate of 10% is to be expected, and a rate of around 20% is typically practiced. It is generally obtained by passing the strip through a single rolling stand comprising, in a known manner, a pair of working rolls (and possible supporting rolls), but it can be carried out progressively by passing of the strip in several such successive cages.

After this first hot rolling in the austenitic phase, the strip to cool and pass into the ferritic area, where it will undergo a second rolling hot. This cooling can take place naturally, by simple radiation tape in the open air, or can be obtained by forced projection of air or water on the surface of the strip, which shortens the path traveled by the strip between the two rolling stages. Forced cooling can take place, at the option of the operator, before, during, or after the ferritic transformation of the strip, or with several of these stages. The exact methods of forced cooling depend on the parameters casting operations, such as the thickness of the strip, its running speed, the distance between the two rolling mills, etc. The bottom line is that by the time the tape undergoes its second hot rolling, it is in the ferritic area at a temperature below 850 ° C, preferably below 750 ° C to have a work hardened structure and avoid recrystallization.

This second hot rolling takes place with a reduction rate of at least 50%, preferably at least 70%, obtained by passing the strip through a single cage or in several successive cages. Its objective is to develop the textures of the product which will then be favorable to the drawing properties. Deformation rates high will favor the development of {111} crystal orientation in the future recrystallization. This lamination must be carried out in the presence of a lubricant, in order to to homogenize the textures in the thickness of the sheet avoiding the development of shear textures at a quarter of the strip thickness. It also reduces the forces to be exerted on the strip during the ferritic deformation.

If it appears necessary, we can provide between the first and the second rolling hot inerting and / or descaling means of the strip similar to those previously described, in order to prevent the second rolling from taking place on a strip slightly burned. Because of the high reduction rates applied during the second hot rolling, avoid scale encrustation at this stage if you want to obtain an excellent surface condition of the strip.

After the second rolling, the strip in the ferritic state must be recrystallized. For this purpose, it can be wound at high temperature, between 700 and 800 ° C (typically 750 ° C), so that its recrystallization is complete over its entire thickness and that one is assured of obtaining an optimal texture. If after the second rolling the strip temperature is lower than 700 ° C, the strip must be reheated to bring it back to the desired temperature range. This reheating will, in most cases, of the order of a hundred degrees and can be obtained by passing the strip through an induction furnace. The advantage of an induction oven compared to an oven equipped, for example, with gas burners, is that it makes it possible to obtain rapid heating of the product, and above all uniform over the entire thickness of the band. In fact, recrystallization can then take place at least in large part during this reheating. The reheating rates of the strip which can usually be obtained by induction furnaces with the usual configuration and power (from 0.5 to 1.5 MW / mm ² of strip) allow reheating of approximately 100 ° C of a 0.75 mm thick strip in an oven with a length of around 2 m. The installation of such an oven between the second rolling installation and the winding installation is therefore entirely possible on a conventional thin strip casting installation without excessively lengthening it. For reheating the thinnest strips, it is possible to use advantageously an inductor of the transverse flow type, the power of which reaches from 1 to 3MW / mm ² of strip, as described in the document “High flux induction for the fast heating of steel semi-product in line with rolling ”by G. PROST, J. HELLEGOUARC'H, JC. BOURHIS and G. GRIFFAY, Proceedings of the XIII International Congress On Electricity Applications, Birmingham, June 1996.

The hot sheets obtained by the process according to the invention have a thickness less than or equal to 2 mm, preferably less than or equal to 1 mm, depending on the thickness of the initial strip and the rolling rates applied to it. According to intended uses, it is possible to use them directly, especially if their thickness is particularly weak, for example less than 0.7 mm (while hot sheets obtained by traditional processes are too thick for direct use), or then subject them to the usual "cold" treatment operations: cold, annealed (continuous or base annealed), skin-pass, especially if you want to obtain end of very thin sheets. To these operations can be added surface treatments usual (descaling, pickling ...) which accompany them in the classic processes of manufacture of sheet metal for stamping.

Finally, as the speed of travel of the strips leaving the second rolling mill at hot is generally less than 250 m / min, it is compatible with an execution in line of at least the first of said “cold” transformation operations. In particular, if we were able to obtain by reheating a complete execution of the recrystallization following the second rolling, it is possible to envisage dispensing with winding, and to introduce the strip (after having possibly cooled it or allowed it to cool to a adequate temperature) directly in one or more successive "cold" treatment: cold rolling mill, continuous annealing, skin-pass, coating line.

Claims (18)

Process for the manufacture of steel sheets suitable for stamping obtained from a strip, characterized in that: a steel strip 1.5 to 10 mm thick is poured directly from liquid metal, having the composition in weight percentages: carbon less than 0.1%, manganese from 0.03 to 2%, silicon from 0 to 0.5%, phosphorus from 0 to 0.1%, boron from 0 to 0.002%, titanium from 0 to 0.15%, the rest being iron and impurities resulting from the production; a first hot rolling is carried out in one or more stages of said strip, in the austenitic phase, at a temperature between 950 ° C. and the temperature Ar ₃ of said strip with an overall reduction rate of at least 10%; a second hot rolling is carried out in one or more stages of the said strip, in the ferritic phase, at a temperature below 850 ° C., with an overall reduction rate of 50% at least in the presence of a lubricant, in order to '' obtain a hot rolled sheet of thickness less than or equal to 2 mm; and a complete recrystallization is carried out over the entire thickness of said strip by a stay between 700 and 800 ° C. Process according to claim 1, characterized in that the carbon content of the cast steel strip is less than 0.05%. Method according to claim 2, characterized in that the sum of the contents in carbon and nitrogen of the cast steel strip does not exceed 0.03%. Process according to claim 1, characterized in that the carbon content of the cast steel strip is less than 0.007%. Process according to claim 4, characterized in that the sum of the contents of carbon and nitrogen content of the cast steel strip does not exceed 0.007%. Method according to claim 4 or 5, characterized in that the cast steel contains titanium and / or niobium with Ti + Nb not exceeding 0.04%. Method according to one of claims 1 to 6, characterized in that the content of manganese of the cast steel strip is between 0.3 and 2%. Method according to one of claims 1 to 7, characterized in that the casting of said strip takes place by pouring liquid metal between two cooled horizontal cylinders internally rotated in opposite directions. Method according to one of claims 1 to 8, characterized in that between the casting and the first rolling, the strip passes through a zone with a non-oxidizing atmosphere and / or is subjected to a descaling operation. Method according to one of claims 1 to 9, characterized in that one proceeds to forced cooling of the strip between the first and the second hot rolling. Method according to one of claims 1 to 10, characterized in that between the first and second rolling, the strip passes through a zone with a non-oxidizing atmosphere and / or is subjected to a descaling operation. Method according to one of claims 1 to 11, characterized in that the second lamination is carried out with an overall reduction rate of at least 70%. Method according to one of claims 1 to 12, characterized in that the second rolling is carried out at a temperature below 750 ° C. Method according to one of claims 1 to 13, characterized in that said recrystallization is obtained by winding the strip between 700 and 800 ° C. Method according to one of claims 1 to 13, characterized in that said recrystallization is obtained at least in part by reheating the strip carrying it between 700 and 800 ° C throughout its thickness. Method according to one of claims 1 to 15, characterized in that one subjects then the strip with one or more “cold” treatments, such as rolling, annealing, skin-pass, dip coating, electrodeposition coating. Method according to one of claims 1 to 13, 15, 16, characterized in that at less the first of said “cold” treatments is carried out in line with the manufacture of the hot rolled and recrystallized strip. Sheet suitable for stamping, characterized in that it is capable of being obtained from strips produced by the method according to one of claims 1 to 17.