ES2210887T3 - PROCEDURE FOR PRODUCTION IN CONTINUOUS STEEL COLADA. - Google Patents

Abstract

IN THE CONTINUOUS STEEL COLADA, AT THE SAME TIME AND IN A PLURALITY OF AREAS OF THE MOLD, DIFFERENT MOLDING POWDERS ARE USED AND IN CASE NECESSARY ADAPTED CONTINUOUSLY TO THE SPECIFIC CONDITIONS OF INSTANTANEOUS COLADA PRESENT IN THE PLURALITY OF ZONE. THE PRODUCTS OBTAINED DO NOT HAVE SUBSTANTIALLY DEFECTS ON THE SURFACE.

Description

Procedure for casting production Continuous steel.

The present invention relates to a Procedure for continuous product production of products no surface defects and the products produced from this mode; more specifically, the present invention relates to a procedure for the production of steel products practically without superficial defects, through the optimization of lubrication and heat exchange conditions in the mold, using at the same time and in a plurality of mold areas different cover powders whose composition can be modified, during casting operations, according to conditions of specific operation, particularly according to the temperature in the various areas of the mold mentioned.

State of the art

Continuous casting is a very good improvement. important in the production of rolled steel products, since It allows to improve productivity, yields and quality. However, the productivity of continuous casting machines does not fits well with the speed of treatment of the posterior Roughing and hot rolling plant. In addition, the laundry continuous can produce superficial defects in the plates produced that are easily transferred to the final product, which is not acceptable for various types of steel, such as steels stainless that usually require a perfect finish of the surface.

Due to the above, and according to the products desired finishes, the plates obtained by continuous casting they get rough, they get cold, they are inspected, possibly polish to eliminate defects, overheat and then rough and hot rolled in bands of a few millimeters of thickness; then, if necessary, the laminated bands in Hot can also be cold rolled. Obviously such a scheme operating is quite expensive, for example, depending on the need to have storage areas, personal for the inspection and polishing, energy and furnaces for heating.

Due to the need to minimize costs in an industrial field that is considered mature for a long time, it recently introduced a new continuous casting technique of thin bodies, which is divided into two main fields: the continuous casting of thin plates, between 40 and 80 mm thickness approximately, and the casting of bands, less than 10 mm, and in many cases about 2 to 5 mm thick.

Such techniques use very diverse devices, and in currently those of thin plates are of the generic type traditional, that is, with oscillating parallelepipedic molds or with a concrete form, while in the band casting cast steel sneaks between two cooled rollers that rotate in opposite directions, whose distance gives the thickness of the band.

So far, the plate casting technique thin were used much more industrially, and the molds are mostly of type "funnel", whose upper part has a section that widens in the central area, to accommodate a diver carrying molten steel to the mold from a trough located above. This widening of the section is reduced gradually towards the exit of the mold, in which the mold has a perfectly rectangular cross section. This technique is known as CSP (Compact Configuration to Produce Bands, of English "Compact Strip Production").

The diffusion of the plate casting technique thin allowed more flexible steel plants, eliminating slow points typical of the traditional procedure, and allowing considerable savings in plant costs, since the roughing trains and, in some cases, furnaces plate overheating is no longer necessary. The called "mini-plants", which with simple plants and relatively cheap, they can produce hundreds of thousands of tons of steel per year for each production line, typically one million tons per year.

However, this new technique also has some inconveniences, particularly related to the product surface malfunction. For many types of steel, particularly carbon steels, this is not usually pose a serious problem, as long as a band can be accepted of inferior quality, because both steels are less prone to surface defects during laundry, such as that the surface finish of the relevant final product is not of utmost importance.

On the contrary, for other types of steel, specifically for the stainless, and for any laundry procedure, this problem is very important, for three main reasons: first because in such steels you can form surface defects easily during laundry continue, then because a finishing of mirror of the cold rolled strip, due to the end uses relevant, and finally because the characteristics of the procedure does not allow correct inspection and polishing of the plates.

Most of the defects found in The surface of continuous casting plates can have their origin in the phenomena that occur during the passage of steel to through the mold of the laundry, and particularly (i) at fluid dynamic conditions of the steel when entering the mold and during its passage through it; (ii) to the conditions of heat transfer between steel and mold; (iii) at protection conditions of the liquid bath inside the mold; and (iv) to the lubrication conditions in the interfacial zone between the solidified steel and the one that is still solidifying and the internal walls of the mold.

In traditional continuous laundry, such conditions are influenced by the introduction on the surface free of the steel that is in the mold of specific powders (cover powders) that protect liquid steel from oxidation or other effects of the environment, and then melt and take to the interfacial surface between the steel and the mold, where provide a beneficial effect on lubrication and thermal transfer

The presence of an appropriate amount of slag liquid on the surface of the steel is obtained through a continuous addition, at an appropriate rate, of cover powders in the mold, which must have a melting speed appropriate. In addition, the liquid slag must have a viscosity that Conforms to the specific conditions of the laundry, in order to penetrate the gap between the surface of the steel being solidifying and the inner walls of the mold, and then Perform your specific functions. The friction between the steel and the mold is related to the viscosity of molten powders and with the casting speed, according to the formula

F_ {l} = \ mu (V_ {l-} V_ {c}) A / d_ {l}

Where F_ is the friction of the liquid; µ is the viscosity; V_ {l} is the molding speed; V_ {c} is the casting speed; A is the contact surface and d_ {l} It is the thickness of the liquid layer.

Other factors that influence lubrication are the slag structure, whether it is amorphous or crystalline, solid surely near the walls of the mold, and its temperature of crystallization.

And another important factor is tension surface of liquid slag, particularly for laundry high speed in which the slag must have a low tension surface to ensure lower adhesion between the steel that is solidifying and liquid slag, thus decreasing the tendency to incorporate slag in steel.

In this way, it would be very convenient to create constant conditions in thermal transfer ratios, dynamic fluid, lubrication, etc., which are produced in the mold. But the conditions inside the mold change, even sharply, during transient states, (for example while the pouring spoon is changed); to take into account such modifications, in the traditional continuous casting one tries control the laundry conditions and, if there is any transition important, the type of dust is changed according to the circumstances. However, it should be noted that, even in the continuous casting of thick plates, a sudden modification of the type of dust it will cause in any case more defects in the plates.

In this way it is evident that in the bodies obtained by continuous casting there is a quality problem of the surface and that, despite the interesting advantages related to the production of products that have quality improved surface, this problem had not yet been studied enough.

The present invention aims to obviate such inconveniences, proposing a procedure for laundry Continuous steel bodies virtually flawless superficial.

Description of the invention

The present invention is based on the hypothesis of that defects in continuous casting are essentially the result of the different thermal and fluid dynamic conditions present along the perimeter of the mold., as a function of the geometry of the mold, of the diver that carries the liquid metal to the mold, pouring speed and lubricating powder. Such conditions are particularly important in continuous casting of thin plates, due to the large size of the diver, which occupies a much more important space inside the mold than in traditional laundry, and exerts a greater influence on the thermal and fluid dynamic conditions inside the mold.

The appearance of surface defects arises, in any type of laundry, during transient states, such as the beginning and the end of the laundry, the changes of pouring spoon during sequential casting, and variation in the  speed and level of steel.

In addition, in continuous plate casting thin differences that occur naturally between the area central, in which the diver is immersed and in which there is lower temperatures and a lower recirculation speed of liquid steel, and hence less heat exchange, and areas peripheral to the smaller walls of the mold, in which the steel movement is much more active and temperatures are greater.

Those conditions are particularly important. in the casting of austenitic steels, since during its solidification there is a phase transition from the phase ferritic, stable at high temperature, austenitic, stable at lower temperatures and that have a contraction coefficient higher; consequently, in the central area of the mold, in which the temperature and speed of steel are lower, steel is solidifies sooner and separates from the mold sooner than in the lateral areas, thus promoting the formation of grooves or stretch marks Longitudinal

On the smaller sides of the mold, where the steel temperature and speed are higher, occurs with frequency upstream currents trap slag particles that are embedded in the still soft crust of the steel that is solidifying, thus forming inclusions. These, during subsequent hot rolling, or even during cold rolling, break the thin layer of steel that covers them, thus forming cracks and other surface defects, such as fringes

In this way, it is clear that for austenitic steels it is very important to adjust the conditions of the laundry to level as much as possible the conditions of thermal transfer present around the diver with those of the areas of the mold near the smaller sides of it.

For steels that are ferritic at temperature environment, a major problem is the low speed of solidification, which may be insufficient steel thickness solidified in the area around the diver, and then the buckling of the iron or, worse, breaks of the laundry crust.

In addition, it is clear that such a variety of thermal and fluid dynamic conditions around the perimeter of the mold can cause various infiltration of slag between the mold and steel, and then, alterations in the lubrication that it can decompose to cause local welding of the crust of the steel to the walls of the mold and, consequently, more defects superficial.

The above analysis shows how the differences of temperature between the different areas of the mold play a particularly important role in defect formation superficial in the bodies produced in this way.

Therefore it is essential to train in any case a suitable slag layer between the crust that is being solidifying and mold walls, appropriate to match the thermal transfer and lubrication conditions between the Steel and mold.

The technical problem solved hereby In this way, the invention is to dynamically control and adjust the thermal transfer conditions and lubrication in the mold between it and the molded body; you can get an optimization dynamics of casting conditions inside the mold, using conveniently the characteristics of various powders of cover, added one by one or mixed into the mold.

The process according to the present invention is refers to the continuous casting of steels, in which molten steel it sneaks into a generically known mold in which they form different thermal transfer conditions between steel casting and various areas of the mold, and the cast steel is covered by a layer of cover dusts that melts and slips between the steel that is solidifying and the interior walls of the mold, and is characterized in that said layer of covering powders it has a gradually modified composition along the inner perimeter of the mold, according to the transfer conditions  thermal established between the steel and the various areas of the mold.

The cover powder layer is composed of at least two different powders mixed conveniently.

Said mixture of at least two cover powders Different is created according to the transfer conditions thermal forming in each of said areas of the mold, adding each mixture inside the mold at the location of the area of the mold for which it was created.

The physical and chemical characteristics of cover powders that are taken into account here invention are: (i) the melting rate, in mg / s; (ii) the final melting temperature, in ° C; (iii) viscosity, in dPa s; (iv) the start temperature of vitrification, in ° C; (v) the crystallization interval, in ° C, and (vi) the basicity index of the scum.

In general, you have to keep in mind that how much the lower the temperature and the speed of the steel, the lower they will have to be the final melting temperature and viscosity of coverage powders, while their melting speed will have to be older With regard to the most difficult steels of molding, austenitic steels, for example, require, in comparison with ferritics, powders that have a velocity of higher melt and basicity index, and a final temperature of lower melting and viscosity.

The characteristics of the powders must specifically adjust to the time and space conditions of the procedure (transient states of the procedure, profiles thermal along the perimeter of the mold, etc.).

Depending on the above, the measures of thermal transfer and molding parameters are identified the most appropriate powder mixtures for each of the specific molding conditions for a given mold area, then introducing each mixture in the relevant area. In this decision the characteristics gradients are taken into account that occur from the spontaneous mixing of the different powders a Once introduced into the mold. It was found that such a spontaneous mixture It does not give problems, since it usually follows the change of conditions between adjacent areas.

Obviously, the repetition of the measures of thermal transfer as well as those referring to the parameters molding ensures that the mixture of different powders used levels solidification conditions, that this happens in any case after only a few repetitions and how regular Stay over time.

For example, the optimal values of the characteristics of the powders for steels Austenitic are:

Melting Rate (mg / s): 70 to 140, preferably 90 to 120;

Final melting temperature (° C): 1050 to 1250, preferably 1100 to 1130;

Viscosity (at 1300 ° C, dPa.s): 0.1 to 1, preferably 0.2 to 0.8;

Start temperature of vitrification (° C): 1200 to 1050, preferably 1150 to 1100;

Crystallization range (° C): 1050 to 800, preferably 950 to 800;

Basicity index: 1 to 1.3.

For ferritic steels, the values of Previous features are:

Fusion rate (mg / s): 25 to 70, preferably 35 to 60;

Final melting temperature (° C): 1150 to 1270, preferably 1160 to 1200;

Viscosity (at 1300 2C, dPa.s): 0.5 to 1.2, preferably 0.7 to 1.0;

Start temperature of vitrification (° C): 1150 to 900, preferably 1100 to 1000;

Crystallization range (° C): 1000 to 700, but preferably it should not crystallize;

Basicity index: 0.8 to 1.

The process according to the present invention is characterized then by the combination of the following stages in A cooperative relationship:

provide at least two types of powders coverage that have different chemical characteristics and physical;

measure thermal transfer conditions in a plurality of areas of the mold;

determine the molding conditions;

provide a plurality of admission points inside the mold, mixing for each of those points said to less two types of powders according to the general molding conditions and the specific thermal transfer conditions of the corresponding mold zones of said plurality of zones;

enter each of the powder mixtures in the relevant admission point for which it was calculated;

re-measure transfer conditions thermal in said plurality of mold areas and repeat the previous stages until obtaining a considerable uniformity of these conditions.

Regarding the conditions of thermal transfer, mold temperatures and flows Relevant heat are measured in a plurality of areas near the meniscus.

More specifically, such measures are performed in at least one series of points duly spaced from each other horizontally, and each series is placed horizontally at along the perimeter of a cross section of the mold.

Molding conditions include speed of transfer of the steel from the trough to the mold and the different speeds of the steel inside the mold, both in the horizontal plane as in the vertical.

According to the present invention four were tested powders, whose characteristics are reported in table 1.

TABLE 1

Powder lB_ {2} (CaO / SiO_ {2}) Viscosity to T_ {beginning} T_ {beginning} T_ {final} Speed from 1300 ° C (dPa s) vitrification ° C _{crystallization} ° C crystallization ° C Fusion (mg / s) TO 1.00 1.0 1100 970 750 3. 4 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

For specific information on mixtures that can be obtained from the preceding powders, several mixtures of whose compositions and characteristics are reported in table 2.

TABLE 2

Mixes IB_ {2} (CaO / SiO_ {2}) Viscosity to T_ {beginning} T_ {beginning} T_ {final} Speed  from (% in weigh) 1300 ° C (dPa s) vitrification ºC _{crystallization} ºC C crystallization Fusion (mg / s) AND 30% A- 0.93 1.0 1075 900 700 40 70% B F 50% A- 0.95 0.9 1100 950 720 37 50% B G 70% A- 0.97 1.0 1090 970 700 35 30% B H 30% C- 1.30 0.4 1100 950 830 81 70% D I 50% C- 1.27 0.05 1050 1070 930 82 50% D J 70% C- 1.24 0.7 1080 950 860 81 30% D

They will be presented below, only as examples Non-limiting objects and the scope of this invention, the characteristics of the molding and the results that They can be obtained for two types of stainless steel.

Example 1

A stainless steel AISI 400 (ferritic) that it has a known composition of itself molded into plates of 215 mm thick and 1300 mm wide, using a speed of 1 m / min laundry The mold had a height of 800 mm, with variable oscillation amplitude and frequency.

The mold was provided with 54 thermocouples arranged in overlapping horizontal rows.

Powders A and B were used for this steel.

At the beginning of the casting operations used only powder A. The thermal flux stabilized in the meniscus, corresponding to the almost total absence of defects Superficial, measured through thermocouples, was 1400 kW / m2 approximately.

During the casting operations a reduction of the thermal flow in the meniscus, from 1415 to 1060 kW / m2. Such a situation was expressly maintained during a time frame. Meanwhile, the information available on used powders and mixtures allowed to predict that a mixture of powders A and B similar to mixture E of table 2 would be suitable. From in fact, at a composition of 37% A-63% B (by weight) the thermal flux was approximately 1400 kW / m2 again, value to which the general conditions of cooling.

The final rolled product showed, in correspondence with the reduction of thermal flux, a remarkable increase in surface defects, which decreased sharply until the almost total absence of surface defects in some tens of meters since the correction intervention, thus showing That this intervention was timely and effective.

Example 2

A stainless steel AISI 304 (austenitic) that it has a known composition of itself molded into plates of 215 mm thick and 1300 mm wide; casting speed was 1.25 m / min The mold had a height of 800 mm, with amplitude of variable oscillation and frequency.

The mold was provided with 54 thermocouples arranged in overlapping horizontal rows.

Three intervention zones were chosen; (i) central, around the diver; (ii) terminal, on short walls of the mold; and (iii) central, between the other two zones.

For this steel, powders C and D.

At the beginning of the casting operations used only powder C. The thermal flux stabilized in the Meniscus measured through thermocouples, stabilized at 1420 kW / m2 approximately in zone (i), at 1800 kW / m2 approximately in zone (ii) and at 1630 kW / m2 approximately in the zone (iii).

After casting 100 m of iron, the thermal flux in zone (i) became consistent with that of the other two areas by adding powder D to the initial powder, to reach a composition similar to composition J of table 2; the flow Thermal in zone (i) was approximately 1650 kW / m2. Due to the mixing effect of the two powders in zone (i), the flow in zone (iii) also changed to 1660 kW / m2 approximately.

The verification of the rolled product obtained at starting from said plate showed, in correspondence with the part initial plate, a diffuse distribution of defects, which sharply decreased to values below 1% in correspondence with the second part of the iron, molded after having obtained the new conditions of the thermal flow.

Also in this case the intervention was timely and effective.

Similar results were obtained both in experimental trials of casting of thin bodies as in steels (stainless or common) different from those on which report in this document.

Claims (10)

1. Procedure for continuous casting of bodies with virtually no surface defects, in which molten steel is molded, with known and controlled molding conditions, within a generically known mold, in which transfer conditions are continuously controlled thermally established between the steel and a plurality of mold areas, and the cast steel is covered with a layer of cover powders, which have known characteristics, which melt and slide between the steel being solidified and the inner walls of the mold, characterized in that the composition of said powder layer is continuously controlled and changed along the internal perimeter of the mold, according to the thermal transfer conditions established between the steel and said plurality of mold areas. 2. Procedure for casting production Continuous bodies with virtually no surface defects according to claim 1, wherein the cover powder layer is formed by at least a mixture of at least two cover powders different. 3. Procedure for casting production Continuous bodies with virtually no surface defects according to claim 2, wherein said mixture of at least two powders Different coverage is created according to specific conditions of the thermal transfer established in each area of the mold, adding each mixture in the area of the mold for which it was created. 4. Procedure for casting production Continuous bodies with virtually no surface defects according to any one of the preceding claims, wherein, for find said thermal transfer conditions, the temperature of the mold and the relevant thermal fluxes are continuously measured in a plurality of areas close to the meniscus of steel. 5. Procedure for casting production Continuous bodies with virtually no surface defects according to claim 4, wherein said temperature measurements are take at least a series of horizontally spaced points each other, each series being distributed horizontally along of the perimeter of a cross section of the mold. 6. Procedure for casting production Continuous bodies with virtually no surface defects according to any one of the preceding claims, wherein determine the casting conditions, the speed of transfer of steel from trough to mold and speeds of the steel in the mold, both horizontally and in the vertical. 7. Procedure for casting production Continuous bodies with virtually no surface defects according to any one of the preceding claims, wherein use cover powders that have the following features: Melting rate (mg / s): 70 to 140; Final melting temperature (2C): 1050 a 1250; Viscosity (at 1300 ° C, dPa.s): 0.1 to 1; Start temperature of vitrification (° C): 1200 to 1050; Crystallization range (° C): 1050 to 800; Basicity index: 1 to 1.3. 8. Procedure for casting production Continuous bodies with virtually no surface defects according to claim 7, wherein cover powders are used which They have the following characteristics: Melting rate (mg / s): 80 to 120; Final melting temperature (° C): 1100 a 1130; Viscosity (at 1300 ° C, dPa.s): 0.2 to 0.8; Start temperature of vitrification (° C): 1150 to 1100; Crystallization range (° C): 950 to 800; 9. Procedure for casting production Continuous bodies with virtually no surface defects according to any one of the preceding claims, wherein use cover powders that have the following features: Melting rate (mg / s): 25 to 70; Final melting temperature (° C): 1150 a 1270; Viscosity (at 1300 ° C, dPa.s): 0.5 to 1.2; Start temperature of vitrification (° C): 1150 to 900; Crystallization range (° C): 1000 to 700; Basicity index: 0.8 to 1. 10. Procedure for casting production Continuous bodies with virtually no surface defects according to any one of the preceding claims, wherein use cover powders that have the following features: Melting rate (mg / s): 35 to 50; Final melting temperature (° C): 1160 a 1200; Viscosity (at 1300 ° C, dPa.s): 0.7 to 1.0; Start temperature of vitrification (° C): 1100 to 1000; Crystallization interval (° C): no crystallizes; Basicity index: 0.8 to 1.