Process for the continuous casting of steel

Abstract: In the continuous casting of steel, at the same time and in a plurality of zones of the mould different mould powders are utilised, if necessary continuously adapted to the specific instantaneous casting conditions present in said plurality of mould zones; products thus obtained are substantially free from surface defects.

Description: Present invention refers to a process for the continuous casting production of products free from surface defects and to the products thus produced; more specifically, the present invention refers to a process for the production of steel products substantially free form surface defects, through optimisation of the lubrication and thermal transfer conditions in the mould, utilising at the same time and in a plurality of mould zones different mould powders whose composition can be modified, during the casting operations, according to specific working conditions, particularly according to the temperature in said various mould zones. STATE OF THE ART The continuous casting represents a very important improvement in the production of rolled steel products, as it permitted to improve productivity, yields and quality. However, the productivity of the continuous casting machines badly fits with the treatment speed of subsequent roughing and hot rolling plant. Moreover, the continuous casting can induce on the produced slabs surface defects which are easily transferred into the end product, which is not acceptable for various steel types, such as stainless steels which usually require a perfect surface finish. Due to the above, and according to the desired end products, the continuously cast slabs are stocked, cooled down, inspected, possibly ground to eliminate the defects, reheated and then roughened and hot rolled in some millimetre thick strips; then, if necessary, the hot rolled strips can be also cold rolled. Such a working scheme is obviously rather costly, for instance depending on the necessity to have storage areas, manpower for inspection and grinding, power and furnaces for heating. Due to the necessity to minimise the costs in an industrial field since long considered ripe, a new thin bodies continuous casting technique was recently introduced, which is divided into two main fields: the continuous casting of thin slabs, from about 40 to about 80 mm thick, and the casting of strips, less than 10 mm, and in many cases about 2 to 5 mm, thick. Such techniques utilise widely different apparatuses, the thin slab ones being at the moment of a generically traditional type, i.e. with oscillating parallelepiped or shaped moulds, while in the ones for strip casting the molten steel is cast between two cooled, counter-rotating rolls, whose distance gives the strip thickness. Up to now, the thin slabs casting technique is much more industrially utilised, and the moulds are mostly of the "funnel" type, with the upper part having a section widening in the central zone, to accommodate a nozzle bringing molten steel to the mould from an above placed tundish. This section widening gradually reduces towards the mould exit, at which the mould has a perfectly rectangular transversal section. The above technique is known as CSP (Compact Strip Production). The diffusion of the thin slabs casting technique permitted more flexible steel plants, eliminating the slow points proper of the traditional process, and permitting considerable savings in the plant costs, in that roughing mills and, in some cases, slab re-heating furnaces are no longer necessary. The so called "minimills" developed, which with simple and relatively cheap plants, can produce hundred thousands tons of steel per year for each production line, typically a million tons per year. However, also this new technique has some inconveniences, particularly relating to the surface defectosity of the products. For many types of steels, particularly the carbon ones, this usually does not represent a major problem, provided a strip of lower quality can be accepted, both in that such steels are less prone to develop surface defects during the casting, and because the surface finish of relevant end product is not of paramount importance. On the contrary, for other steel types, specifically for the stainless ones, and for any casting method, this problem is very important, for three main reasons: first because such steels can easily form surface defects during the continuous casting, then because a mirror finish of the cold rolled strip is usually required, due to relevant end uses, and finally because the process characteristics do not permit a proper inspection and grinding of the slabs. Most of the defects found at the surface of continuously cast slabs can be derived from phenomena occurring during the steel passage through the casting mould, and particularly (i) to the steel fluodynamic conditions as it enters the mould and during its passage through it; (ii) to the thermal transfer conditions between steel and mould; (iii) to the protection conditions of the liquid bath into the mould; and (iv) to the lubrication conditions at the interface between the already solidified and the still solidifying steel and the mould internal walls. In the traditional continuous casting, such conditions are influenced by introducing onto the free steel surface in the mould some specific powders (mould powders) which protect the liquid steel from oxidation and/or from other effects of the environment, and then melt and are drawn to the interface between steel and mould where they provide a beneficial effect on lubrication and thermal transfer. The presence of a proper amount of liquid slag onto the steel surface is obtained through a continuous addition, at a proper speed, of mould powders into the mould, which powders must have a proper melting speed. Moreover, the liquid slag must have a viscosity matching the specific casting conditions, in order to penetrate into the gap between the solidifying steel surface and the inner mould walls, and then perform its specific functions. The friction between steel and mould is linked to the molten powders viscosity and to the casting speed, according to the formula F l = µ (V l- V c )A/d l In which F l is the liquid friction; µ is the viscosity; V l is the mould speed; V c is the casting speed; A is the contact surface and d l is the thickness of the liquid layer. Other factors influencing the lubrication are the slag structure, whether amorphous or crystalline, surely solid in proximity of the mould walls, and its crystallisation temperature. Still another important factor is the liquid slag surface tension, particularly for the high speed casting in which the slag must have a low surface tension to ensure a lesser adhesion between solidifying steel and liquid slag, thus diminishing the tendency to incorporate slag into the steel. It would thus be rather convenient to realise constant conditions in the relationships of thermal transfer, fluodynamic, lubrication and so on, which develop in the mould. But the conditions within the mould change, even sharply, during the transients, (for instance while changing ladle); to take such modifications into account, in the traditional continuous casting attempts are made to control the casting conditions and, if any important transition occurs, the type of powder is changed according to the circumstances. However, it must be noted that, even in the continuous casting of thick slabs, a sudden modification of the powder type will in any case cause further defects in the slabs. It is thus clear that a problem of surface quality exists in the continuously cast bodies and that, notwithstanding the interesting advantages connected to the production of products having an improved surface quality, this problem was not yet sufficiently studied. The present invention aims to obviate such inconveniences, proposing a process for the continuous casting of steel bodies practically free from surface defects. DESCRIPTION OF THE INVENTION The present invention is based on the hypothesis that the defects in the continuous casting essentially stem from the different thermal and fluodynamic conditions present along the mould perimeter, as a function of mould geometry, of the nozzle bringing liquid metal into the mould, of the casting speed and of the lubricating powder. Such conditions are particularly important in the thin slab continuous casting, due to the large dimensions of thenozzle, which occupies a much more important space within the mould than in the traditional casting, exerting a larger influence on the thermal and fluodynamic conditions within the mould. The onset of surface defects rises, in any type of casting, during the transients, such as the casting start and end, the ladle changes during the sequence casting, and the variation in steel speed and level. Moreover, in the thin slabs continuous casting the differences naturally occurring between the central zone, in which the nozzle in immersed and in which there are lower temperatures and lower liquid steel recirculating speed, and hence a lesser thermal exchange, and the peripheral zones towards the lesser mould walls, in which the steel motion is much more active and the temperatures higher. Those conditions are particularly important in the casting of the austenitic steels, in that during their solidification a phase transition occurs from the ferritic phase, stable at high temperature, to the austenitic one, stable at lower temperatures and having a higher shrinkage coefficient; as a consequence, in the mould central zone, in which steel temperature and speed are lower, the steel solidifies earlier and earlier becomes detached from the mould than in the lateral zones, thus promoting the formation of longitudinal grooves or striations. At the lesser sides of the mould, where steel temperature and speed are higher, it frequently happens that the upwardly directed steel currents catch slag particles which are embedded into the solidifying still soft steel skin, thus forming inclusions. The latter, during subsequent hot rolling, or even during the cold rolling, break the thin steel layer covering them, thus forming cracks and other surface defects, such as slivers. Thus, it is clear that, for the austenitic steels it is very important to adjust the casting conditions to level as far as possible the thermal transfer conditions present around the nozzle to the ones at the mould zones near the lesser sides thereof. For the steels which are ferritic at room temperature, an important problem is the low solidification speed, which can entail an insufficient thickness of the solidified steel in the zone around the nozzle, and then slab bulging or, worse, breakouts. Moreover, it is dear that such a variety of thermal and fluodynamic conditions around the mould perimeter can cause different infiltration of slag between mould and steel, and then lubrication alterations which can decay up to causing local welding of the steel skin to the mould walls and consequently further surface defects. The above analysis shows how the temperature differences between different mould zones play a particularly important role in the formation of surface defects in the thus produced bodies. It is therefor essential to form in any case, between the solidifying skin and the mould walls a proper slag layer, apt to equalise the thermal transfer and the lubrication conditions between steel and mould. The technical problem solved by present invention is, thus, to dynamically control and adjust the thermal transfer and lubrication conditions in the mould between the latter and the cast body; a dynamic optimisation of the casting conditions into the mould can be obtained, conveniently utilising the characteristics of various mould powders, added one by one or in mixture into the mould. The process according to present invention refers to the continuous casting of steels, in which molten steel is cast into a generically known mould in which different thermal transfer conditions are formed between cast steel and various mould zones, the cast steel being covered with a layer of mould powder which melts and slips between solidifying steel and mould inner walls, and is characterised in that said casting powder layer ha a composition gradually changed along the inner perimeter of the mould, according to the thermal transfer conditions established between the steel and the various mould zones. The casting powders layer is composed by at least two different conveniently mixed powders. Said mixture of at least two different casting powders is created according to the specific thermal transfer conditions formed in each of said mould zones, each mixture being added into the mould at the location of the mould zone for which it was created. The physical and chemical characteristics of the casting powders taken into account in present invention are: (i) the melting speed, in mg/s; (ii) the end of melting temperature, in °C; (iii) the viscosity, in dPa .s; (iv) the temperature of vitrification start, in °C; (v) the crystallisation span, in °C, and (vi) the slag basicity index. Generally speaking, it must be kept in mind that the lower the steel temperature and speed, the lower must be the end of melting temperature and the viscosity of the casting powder, while the higher must be its melting speed. As long as the steels more difficult to cast are concerned, the austenitic steels, for instance, require, compared to the ferritic ones, powders having higher melting speed and basicity index, and lower temperature of melting end and viscosity. The powders characteristics must specifically be adjusted to the time and space conditions of the process (process transients, thermal profiles along the mould perimeter, and so on). Depending on the above, on the thermal transfer measures and on the casting parameters, the powder mixtures more apt to each of the casting conditions specific to a given mould zone are identified, each mixture being then fed to relevant zone. In this decision the characteristics gradients are taken into account occurring from the spontaneous mixing of the different powders once fed into the mould. It was verified that such a spontaneous mixing does not give troubles, in that it generally follows the conditions change between adjacent zones. Obviously, iterating both the thermal transfer measures as well as the ones relating to casting parameters ensure that the different powder mixture utilised levels the solidifying conditions, that this happens in any case after only a few iterations and that such evenness lasts in time. For example, the austenitic steels optimal values of the mould powder characteristics are: Melting speed (mg/s): 70 - 140, preferably 90 - 120; Melting end temperature (°C): 1050 - 1250, preferably 1100 - 1130; Viscosity (at 1300 °C, dPa.s): 0.1 - 1, preferably 0.2 - 0.8; Temperature of vitrification start (°C): 1200 -1050, preferably 1150 - 1100; Crystallisation range (°C): 1050 - 800, preferably 950 - 800; Basicity index: 1 - 1.3. For the ferritic steels, the values of the above characteristics are: Melting speed (mg/s): 25 - 70, preferably 35 - 60; Melting end temperature (°C): 1150 - 1270, preferably 1160 - 1200; Viscosity (at 1300 °C, dPa.s): 0.5 - 1.2, preferably 0.7 - 1.0; Temperature of vitrification start (°C): 1150 - 900, preferably 1100 - 1000; Crystallisation range (°C): 1000 - 700, but preferably must not crystallise; Basicity index: 0.8 - 1. The process according to present invention is then characterised by the combination in co-operation relationship of the following steps: to provide at least two types of mould powders having different chemical and physical characteristics; to measure the thermal transfer conditions in a plurality of zones of the mould; to determine the casting conditions; to provide a plurality of powder admission points into the mould, for each of such points mixing said at least two types of powders according to the casting general conditions and to the specific thermal transfer conditions of the corresponding mould zones of said plurality of zones; to feed each of said powder mixtures at the relevant admission point for which it has been calculated; to measure again the thermal transfer conditions in said plurality of zones of the mould and repeat the above steps up to obtain a substantial uniformity of said conditions. As far as the thermal transfer conditions are concerned, the mould temperatures ant relevant heat fluxes are measured in a plurality of zones near the steel meniscus. More specifically, such measures are made in at least a series of points duly horizontally spaced from each other, each series being horizontally placed along the perimeter of a mould transversal section. The casting conditions include the transfer rate of steel from tundish to mould and the different speeds of the steel within the mould, both on the horizontal and on the vertical plane. According to present invention four powders were tested, the characteristics of which are reported in Table 1. POWDER IB 2 (CaO/SiO 2 ) Viscosity at 1300°C (dPa s) T vitrification start °C T crystallization start °C T crystallization end °C Melting rate (mg/s) A 1.00 1.0 1100 970 750 34 B 0.90 1.2 1100 870 700 40 C 1.20 1.1 1120 1010 830 58 D 1.35 0.5 1045 970 900 100 To obtain specific information on the mixtures obtainable from the above powders, a number of mixtures were prepared, whose compositions and characteristics are reported in Table 2. MIXTURES (%bw) IB 2 (CaO/SiO 2 ) Viscosity at 1300 °C (dPa s) T vitrification start. °C T crystallization start °C T crystallization end °C Melting rate (mg/s) E 30%A-70%B 0.93 1.0 1075 900 700 40 F 50%A-50%B 0.95 0.9 1100 950 720 37 G 70%A-30%B 0.97 1.0 1090 970 700 35 H 30%C-70%D 1.30 0.4 1100 950 830 81 I 50%C-50%D 1.27 0.05 1050 1070 930 82 J 70%C-30%D 1.24 0.7 1080 950 860 81 The following will expose, just as non limiting Examples of the objects and scope of present invention, the casting specifications and the results obtainable for two types of stainless steels. EXAMPLE 1 An AISI 400 (ferritic) stainless steel having a known per se composition was cast in slabs 215 mm thick and 1300 mm wide, using a casting speed of 1 m/min. The mould had an 800 mm height, with variable oscillation amplitude and frequency. The mould was equipped with 54 thermocouples arranged in superimposed horizontal rows. For this steel powders A and B were utilised. At the beginning of casting operations was utilised only the A powder. The stabilised thermal flux at the meniscus corresponding to practically complete absence of surface defects, measured through the thermocouples, was at around 1400 kW/m 2 . During the casting operations, a reduction in thermal flux at the meniscus was detected, from 1415 to 1060 kW/m 2 . Such a situation was purposely maintained for a period of time. In the meantime, the information available on utilised powders and their mixtures permitted to predict that a mixture of A and B powders similar to the E one in Table 2 would be suitable. In fact, at a composition of 37%A-63%B (bw) the thermal flux was back to around 1400 kW/m 2 , at which value the general cooling conditions were again stabilised. The final rolled product did show, in correspondence to the thermal flux reduction, a marked increase of the surface defects, which sharply decreased up to practically complete absence of surface defects within a few tens of metres from the correction intervention, such showing how this intervention was timely and effective. EXAMPLE 2 An AISI 304 (austenitic) stainless steel having a known per se composition was cast in slabs 215 mm thick and 1300 mm wide; casting speed was 1,25 m/min. The mould was 800 mm high, and had variable oscillation frequency and amplitude. The mould was equipped with 54 thermocouples, arranged in superimposed horizontal rows. Three intervention zones were selected; (i) central, around the nozzle; (ii) terminal, at the short walls of the mould; and (iii) central, between the other two zones. C and D powders were chosen for this steel. At the beginning of casting operations, only powder C was utilised. The stabilised thermal flux at the meniscus, measured through the thermocouples, stabilised at around 1420 kW/m 2 in zone (i), at around 1800 kW/m 2 in zone (ii) and around 1630 kW/m 2 in zone (iii). After casting 100 m of slab, the thermal flux in zone (i) was uniformed with the one in the other two zones by adding powder D to the initial one, to reach a composition similar to the J one in Table 2; the thermal flux in the (i) zone was brought at around 1650 kW/m 2 . Due to the mixing effect of the two powders in zone (i), also in zone (iii) the thermal flux changed to around 1680 kW/m 2 . A check of the rolled product obtained from said slab did show, in correspondence to the initial part of the slab, a diffuse distribution of defects, sharply decreased at values of less than 1% in correspondence to the second part of the slab, cast after obtaining the new thermal flux conditions. Also in this case the intervention was timely and effective. Similar results were obtained both in experimental trials of thin bodies casting and with steels (stainless and common) different from the ones here reported.

Claims: Process for the continuous casting production of substantially surface defects free bodies, in which the molten steel is cast, with known and controlled casting conditions, into a generically known mould, in which the thermal transfer conditions established between the steel and a plurality of mould zones are continuously monitored, the cast steel being covered with a layer of mould powders, having known characteristics, which melt and slip between the solidifying steel and the mould internal walls, characterised in that the composition of said powder layer is continuously controlled and changed along the inner perimeter of the mould, according to the thermal transfer conditions establishing between the steel and said plurality of mould zones. Process for the continuous casting production of substantially surface defects free bodies according to claim 1, in which the layer of mould powders is formed by at least a mixture of at least two different mould powders. Process for the continuous casting production of substantially surface defects free bodies according to claim 2, in which said mixture of at least two mould powders is created according to the specific thermal transfer conditions established in each mould zone, each mixture being added into the mould zone for which it was created. Process for the continuous casting production of substantially surface defects free bodies according to any one of the preceding claims, in which in order to find said thermal transfer conditions, the mould temperature and relevant thermal flows are continuously measured in a plurality of zones near the steel meniscus. Process for the continuous casting production of substantially surface defects free bodies according to claim 4, in which said temperature measures are taken in at least a series of points horizontally spaced from each other, each series being horizontally distributed along the perimeter of a transversal section of the mould. Process for the continuous casting production of substantially surface defects free bodies according to any one of the preceding claims, in which to determine the casting conditions the transfer rate of steel from tundish to mould and the steel speeds into the mould, both on the horizontal and on the vertical plane, are utilised. Process for the continuous casting production of substantially surface defects free bodies according to any one of the above claims, in which mould powders are used having the following characteristics: Melting speed (mg/s): 70 - 140; Melting end temperature (°C): 1050 - 1250; Viscosity (at 1300 °C, dPa.s): 0.1 - 1; Temperature of vitrification start (°C): 1200 - 1050; Crystallisation range (°C): 1050 - 800; Basicity index: 1 - 1.3. Process for the continuous casting production of substantially surface defects free bodies according to claim 7, in which mould powders are used having the following characteristics: Melting speed (mg/s): 80 - 120; Melting end temperature (°C): 1100 - 1130; Viscosity (at 1300 °C, dPa .s): 0.2 - 0.8; Temperature of vitrification start (°C): 1150 - 1100; Crystallisation range (°C): 950 - 800. Process for the continuous casting production of substantially surface defects. free bodies according to any one of the above claims, in which mould powders are used having the following characteristics: Melting speed (mg/s): 25 - 70; Melting end temperature (°C): 1150 - 1270; Viscosity (at 1300 °C, dPa s): 0.5 - 1.2; Temperature of vitrification start (°C): 1150 - 900; Crystallisation range (°C): 1000 - 700; Basicity index: 0.8 1. Process for the continuous casting production of substantially surface defects free bodies according to any one of the above claims, in which mould powders are used having the following characteristics: Melting speed (mg/s):35 - 60; Melting end temperature (°C): 1160 - 1200; Viscosity (at 1300 °C, dPa.s): 0.7 - 1.0; Temperature of vitrification start (°C): 1100 - 1000; Crystallisation range (°C): does not crystallise; Basicity index: 0.8 - 1.

Inventors: CAPOTOSTI ROMEO, CIMARELLI TOSCA, SPACCAROTELLA ANTONIO, CAPOTOSTI, ROMEO, CIMARELLI, TOSCA, SPACCAROTELLA, ANTONIO
Applicants: THYSSENKRUPP ACCIAI SPECIALI, THYSSENKRUPP ACCIAI SPECIALI TERNI S.P.A.
Application year: 1999
Main classification: B22D11
IPC codes: B22D11