STEEL STRIP STRIP
FIELD OF THE INVENTION This invention relates to the casting or molding of steel strips. It has particular application to the continuous casting of thin strips of steel in a double roller casting machine. BACKGROUND OF THE INVENTION In a double roller caster, the molten metal is inserted between a pair of counter-rotating horizontal casting rolls, which are cooled so that the metal covers solidify on the moving surfaces and put together at the point of contact between them to produce a solidified strip product supplied downwardly from the point of contact between rollers. The term "point of contact" is used herein to refer to the general region in which the rollers are closer together. The molten metal can be poured into a pouring cauldron into a smaller container from which it flows to a metal supply nozzle located above the roller contact point so that it directs it towards the point of contact between the rolls for forming a molten pool of molten metal supported on the casting surfaces of the rollers, immediately above the point of contact between rollers and extending along the length of the contact point between rollers. This melt bath is usually confined between side plates or dams held in sliding engagement with the end surfaces of the rolls so that the two ends of the melt are represented against the outflow, although alternatively other means have also been proposed such as electromagnetic barriers. When thin strips of steel are cast in a double roller caster, the steel molded in the melt bath will generally be at a temperature in the order of 1500 ° C and above, and therefore it is necessary to achieve very high cooling rates. on the casting surfaces of the rollers. It is particularly important to achieve a high heat flow and extensive nucleation in the initial solidification of the steel, on the molding surfaces, to form the metal covers. U.S. Patent 5,720,336 describes how the heat flow can be increased over the initial solidification by adjusting the melting chemistry of the steel such that a substantial proportion of the metal oxides formed as deoxidation products are liquid at the initial solidification temperature to form a substantially liquid layer at the interface between the molten metal and each molding or casting surface. As described in U.S. Patents 5,934,359 and 6,059,014 and International Application AU 99/00641, the nucleation of the steel in the initial solidification can be influenced by the texture of the casting or molding surface. In particular, International Application AU 99/00641 discloses that a random texture of the peaks and depressions can improve the solidification sites by providing potential nucleation sites distributed across all the molding surfaces. It has now been determined that nucleation is also dependent on the presence of oxide-inclusions in the steel melt and that surprisingly it is not advantageous in the molding or casting of strips in double rolls for molding with "clean" steel in which the The number of inclusions formed during deoxidation has been minimized in the molten steel before molding or casting. The steel for molding or continuous casting is subjected to the deoxidation treatment in the pouring cauldron before pouring. In molding with double rollers the steel is generally subjected to deoxidation in a casting cauldron with silicon and manganese, although it is possible to use deoxidation with aluminum with the addition of calcium to control the formation of solid Al203 inclusions that can clog fine passages of the metal flow, in the metal supply system, through which the molten metal is supplied to the melting bath. Hitherto it has been thought desirable to aim for an optimum cleaning of the steel by treating with the pouring kettle to minimize the total level of oxygen in the molten metal. Nevertheless, it has now been determined that by decreasing the oxygen level of the steel the volume of inclusions is reduced and if the total oxygen content of the steel is reduced below a certain level, the nature of the initial contact between the steel and the surfaces of the rollers, can be adversely affected to the extent that there is not enough nucleation to generate rapid initial solidification and high heat flow. The molten steel is cut or roughened by deoxidation in the casting kettle such that the total oxygen content falls within a range which ensures satisfactory solidification in the casting or molding rolls and the production of a satisfactory strip product. The molten steel contains a distribution of the oxide inclusions (typically MnO, CaO, Si02 and / or Al203) sufficient to provide adequate density of the nucleation sites on the surfaces of the rollers for initial solidification and the resulting strip product exhibits a characteristic distribution of solidified inclusions.
DESCRIPTION OF THE INVENTION A method for manufacturing a steel strip by continuous casting is provided, comprising the steps of: a. assemble or assemble a pair of cooled molding rolls having a point of contact between them and with adjacent closures adjacent to the ends of the contact point between rolls; b. introduce low-melted carbon steel, having a total oxygen content of at least 100 ppm and a free oxygen content between 30 and 50 ppm, between the pair of cast or cast rolls to form a melting bath between the molding rolls; c. rotating the cast or cast rolls in the opposite direction and solidifying the molten steel to form metal covers on the surface of the cast or cast rolls with levels of oxygen inclusions reflected by the total oxygen content of the molten steel, to promote the formation of the thin steel strip; and d. forming the thin steel strip, solidified through the point of contact between rollers, of the casting rollers from said solidified covers.
The total oxygen content of the molten steel in the melt bath can be between 100 ppm and 250 ppm. More specifically, it can be about 200 ppm. Low carbon steel can have a carbon content in the range of 0.001% to 0.1% by weight, a manganese content in the range of 0.1% to 2.0% by weight and a silicon content in the range of 0.01 % up to 10% by weight. The steel may have an aluminum content of the order of 0.01% or less by weight. Aluminum can for example be as small as 0.008% or less by weight. The molten steel can be a plated steel or deoxidized with silicon / manganese. Oxide inclusions are solidification inclusions and deoxidation inclusions. The solidification inclusions are formed during the cooling and solidification of the steel in the casting, and the deoxidation inclusions are formed during the deoxidation of the molten steel before casting or molding. The solidified steel may contain oxide inclusions usually comprised of any one or more of MnO, SiO2 and Al203 distributed through the steel at a density of inclusions in the range of 2 gm / cm3 and 4 gm / cm3. The molten steel can be refined in a pouring cauldron before introduction between the casting or molding rolls by heating a steel charge and slag-forming material in the casting cauldron, whereby molten steel is formed covered by a slag containing oxides of silicon, manganese and calcium. The molten steel can be agitated by injecting an inert gas into it to cause desulphurisation, and with steels such as a steel that is plated or deoxidized with silicon / manganese, then injecting oxygen, to produce steel having the desired total oxygen content. of at least 100 ppm and usually less than 250 ppm. Desulfurization can reduce the sulfur content of the molten steel to less than 0.01% by weight. The thin steel strip produced by continuous casting with double rolls as described above has a thickness of less than 5 mm and is formed of a solidified steel containing solidified oxide inclusions. The distribution of the inclusions may be such that in the two surface regions of the strip at a thickness of 2 microns from the outer surfaces they contain solidified inclusions at a density per unit area of at least 20 inclusions / mm 2. The solidified steel may be a steel that is plated or deoxidized with silicon / manganese and the oxide inclusions comprise any one or more inclusions of MnO, SiO2 and Al203. The inclusions will typically vary in size between 2 and 12 microns, so that at least a majority of the inclusions are in that size range. The method described above produces a single steel with high oxygen content, distributed in oxide inclusions. Specifically, the combination of the high oxygen content in the molten steel and the short residence time of the molten steel in the melt bath results in a thin steel strip with improved ductility properties. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention can be described in more detail, some specific examples will be given with reference to the accompanying drawings in which: Figure 1 shows the effect of the melting points of the inclusions in the heat fluxes obtained in the tests of molding or casting with double rollers, using steels rested or deoxidized with silicon / manganese. Figure 2 is an energy dispersive spectroscopy (EDS) map of Mn, showing a band of solidified solid inclusions in a solidified steel strip; Figure 3 is a graph showing the effect of the variation of the manganese to silicon contents at the temperatures of the liquid phase of the inclusions;
Figure 4 shows the relationship between the content of alumina (measured from the inclusions of the strip) and the effectiveness of deoxidation; Figure 5 is a ternary phase diagram for MnO. Si02.Al203; Figure 6 shows the relationship between inclusions with alumina content and the temperature of the liquid phase; Figure 7 shows the effect of oxygen in a molten steel on surface tension; and Figure 8 is a diagram of the results of calculations concerning the inclusions available for nucleation at different levels of steel cleaning. DETAILED DESCRIPTION OF THE PREFERRED METHOD Extensive casting tests have been conducted in a double roll caster, of the type fully described in U.S. Patents 5,184,668 and 5,277,243 to produce steel strips in the order of 1 mm thickness or less. Such casting tests using plated or deoxidized steel of silicon and manganese have shown that the melting point of the oxide inclusions in the molten steel have an effect on the heat fluxes obtained during the solidification of the steel as illustrated in Figure 1 The oxides with low melting point improve the heat transfer contact between the molten metal and the surfaces of the casting rolls in the upper regions of the melting bath, generating high heat transfer speeds. Liquid inclusions do not occur when the melting point is higher than the temperature of the steel in the melting bath. Therefore, there is a dramatic reduction in the rate of heat transfer when the melting point of the inclusions is greater than about 1600 ° C. The tests of casting or molding with steels deoxidized with aluminum, have shown that to avoid the formation of inclusions of alumina with high melting point (melting point 2050 ° C), it is necessary to have a treatment with calcium to provide inclusions of C O . Al203 liquid. The oxide inclusions formed in the layers or shells of solidified metal and in turn the thin steel strip comprise inclusions formed during the cooling and solidification of the steel, and the deoxidation inclusions formed during the refining of the pouring cauldron. The level of free oxygen in the steel is dramatically reduced during cooling in the meniscus, resulting in the generation of solidification inclusions near the surface of the strip. These solidification inclusions are formed predominantly from MnO.Si02, for the following reaction:
Mn + Si + 30 = MnO.Si02 The appearance of the solidification inclusions on the surface of the strip, obtained from a Dispersive Energy Spectroscopy (EDS) map, is shown in Figure 2. It can be seen that the inclusions of solidification are extremely fine (typically less than 2 to 3 microns) and are located in a band located within 10 to 20 microns of the surface. A typical size distribution of the inclusions across the strip is shown in Figure 3 of our document titled Recent Developments in Project M the Joint Development of Low Carbon Steel Strip Casting by BHP and IHI (Recent developments in Project M, the joint development of strips of steel with low carbon content through BHP and IHI), presented at the METEC 99 congress, Dusseldorf Germany (June 13-15, 1999). The comparative levels of the solidification inclusions are determined mainly by the levels of Mn and Si in the steel. Figure 3 shows that the ratio of Mn to Si has a significant effect on the temperature of the liquid phase of the inclusions. A deoxidized manganese and silicon steel having a carbon content in the range of 0.001% to 0.1% by weight, a manganese content in the range of 0.1% to 2.0% by weight and a silicon content in the range of 0.1 % to 10% by weight and an aluminum content of the order of 0.01% or less by weight, it can produce such oxide inclusions during the cooling of the steel in the upper regions of the melting bath. In particular, the steel can have the following composition, called M06-. Carbon 0.06% by weight Manganese 0.6% by weight Silicon 0.28% by weight Aluminum 0.002% by weight The deoxidation inclusions are generated during the deoxidation of the molten steel in the casting cauldron with Al, Si and Mn. Then, the composition of the oxide inclusions formed during deoxidation is based mainly on MnO.Si02.Al203. These deoxidation inclusions are located randomly in the strip and are thicker than the solidification inclusions near the surface of the strip. The alumina content of the inclusions has a strong effect on the level of free oxygen in the steel. Figure 4 shows that with the increase in the alumina content, the free oxygen in the steel is reduced. With the introduction of alumina, the inclusions of MnO.Si02 are diluted with a subsequent reduction in their activity, which in turn reduces the level of free oxygen, as observed from the reaction below: Mn + Si + 30 + Al203 <; - > (A1203) .MnO.Si02 For the inclusions based on Mn0-SiO2-Al2O3, the effect of the inclusion composition on the temperature of the liquids can be obtained from the ternary phase diagram shown in figure 5. The analysis of the Oxide inclusions in the thin steel strip, has shown that the Mn0 / Si02 ratio is typically within 0.6 to 0.8 and for this regimen, it was found that the alumina content of the oxide inclusions had the strongest effect at the point of fusion of the inclusions (temperature of the liquid phase), as shown in figure 6. It has been determined that it is important for the molding or casting according to the present invention to have the solidification and deoxidation inclusions in such a way that they are liquid at the initial solidification temperature of the steel that the molten steel in the melt bath has an oxygen content of at least 100 ppm to produce metal covers with levels of inclusions of oxide reflected by the total oxygen content of the molten steel to promote nucleation and high heat flux during the initial solidification of the steel on the surfaces of the casting rolls. Both solidification and deoxidation inclusions are oxide inclusions and provide nucleation sites and contribute significantly to nucleation during the solidification process of the metal, but the deoxidation inclusions are finally controlled in speed or proportion in which their concentration can vary. The deoxidation inclusions are much larger, typically greater than 4 microns, while the solidification inclusions are generally less than 2 microns and are based on MnO.Si02 and do not have Al203 while the deoxidation inclusions also have AI2O3. It has been found in the casting or molding tests using the above grade M06 of the de-oxidized silicon / manganese steel that if the total oxygen content of the steel is reduced in the refining process of the casting kettle to low levels of less than 100 ppm, the heat fluxes are reduced and the molding is affected while good molding or casting results can be achieved if the total oxygen content is at least above 100 ppm and typically in the order of 200 ppm. These oxygen levels in the pouring cauldron result in total oxygen levels of at least 70 ppm and free oxygen levels between 20 and 60 ppm in the refractory tundish, and in turn the same or slightly lower levels of oxygen in the bath of fusion. The total oxygen content can be measured by a "Leco" instrument and controlled by the degree of "lift" during the treatment in the pouring cauldron, ie the amount of argon bubbles through the pouring kettle via a porous plug or upper lance, and the duration of the treatment. The total oxygen content was measured by conventional methods using the LECO TC-436 nitrogen / oxygen determinant described in the NEC nitrogen / oxygen determiner instruction manual available from LECO (Form No. 200-403, Rev. Apr. 96, Section 7 on p. 7 - 1 to 7-4). To determine whether the improved heat fluxes obtained with higher total oxygen contents was due to the availability of oxide inclusions in the nucleation sites, the casting tests were performed with steels in which deoxidation in the pouring cauldron It was carried out with calcium silicide (Ca-Si) and the results compared with casting with steel deoxidized with Si and low carbon content known as M06 grades of steel. The results are established in the following table:
Table 1 Difference of heat flow between grades M06 and Cal
Although the Mn and Si levels were similar to the normal deoxidized Si grades, the free oxygen level in the Ca-Si heats was lower than the oxide inclusions that contained more CaO. The heat fluxes in the Ca-Si heats were lower despite a lower inclusion melting point (see Table 2). Table 2 Compositions of the slag with deoxidation of Ca-Si
The free oxygen levels in the Ca-Si grades were lower, typically 20 to 30 ppm compared to 40 to 50 ppm with the M06 grades. Oxygen is an active element on the surface and then the reduction of the oxygen level is expected to reduce the wetting between the molten steel and the casting rolls and causes a reduction in the heat transfer rate. However, from Figure 7 it appears that the reduction in oxygen from 40 to 20 ppm may not be sufficient to increase the surface tension to levels that explain the observed reduction in heat flow. It can be concluded that decreasing the levels of free and total oxygen in the steel reduces the volume of inclusions and thus reduces the number of oxide inclusions for the initial nucleation. This has the potential to adversely impact the nature of the initial contact between the steel and the roll surface. The immersion or bath test work to deoxidize has shown, that a nucleation density per unit area of approximately 120 / mm2, is required to generate sufficient heat flow in the initial solidification in the upper region or the meniscus of the bath of fusion. Deoxidation bath testing involves advancing a cooled block into a bath of molten steel at such a rate that the conditions on the casting or molding surfaces of a double roller caster are simulated very closely. The steel solidifies on the cooled block as it moves through the molten bath to produce a layer of solidified steel on the surface of the block. The thickness of this layer can be measured in points across its area to form a map of the variations in the solidification rate and hence the effective rate of heat transfer in various locations. It is then possible to produce a total solidification rate as well as measurements of the total heat flow. It is also possible to examine the microstructure of the strip surface to correlate changes in the solidification microstructure with the changes in observed solidification rates and heat transfer values and examine the structures associated with nucleation in the initial solidification on the cooled surface. A bath test apparatus for deoxidizing is more fully described in U.S. Patent No. 5,720,336. The ratio of the oxygen content of the liquid steel in the initial nucleation to the heat transfer has been examined using a model described in Appendix 1. This model assumes that all oxide inclusions are spherical and uniformly distributed through the steel. A surface layer is assumed to be 2 microns and that only inclusions present in the surface layer could participate in the nucleation process in the initial solidification of the steel. The input to the model was the total oxygen content in the steel, the diameter of inclusions, the thickness of the strip, the casting speed and the thickness of the surface layer. The output was the percentage of total inclusions in the steel required to fill or meet a nucleation density per area of 120 / mm2. Figure 8 is a graph of the percentage of oxide inclusions in the surface layer required to participate in the nucleation process to achieve the nucleation density per unit area at different levels of steel cleaning when expressed by the total oxygen content, assuming a strip thickness of 1.6 mm and a casting speed of 80 / min. This shows that for an inclusion size of 2 microns and 200 ppm of total oxygen content, 20% of the total total oxide inclusions available in the surface layer are required to achieve the target nucleation density per unit area of 120 / mm2. However, with a total oxygen content of 80%, about 50% of the inclusions are required to achieve the critical nucleation rate and at a total oxygen level of 40 ppm, there will be an insufficient level of oxygen inclusions to comply with the target nucleation density per unit area. Accordingly, when deburring the steel by deoxidation in the pouring cauldron, the oxygen content of the steel can be controlled to produce a total oxygen content in the range of 100 to 250 ppm and typically approximately 200 ppm. This will result in the two micron depth layers adjacent to the casting rolls in the initial solidification containing oxide inclusions having a density per unit area of at least 120 / mm2. These inclusions will be present in the outer surface layers of the final solidified strip product and can be detected by appropriate examination, for example by energy dispersive spectroscopy (EDS). EXAMPLE
INPUTS Critical nucleation density 120 This value is per unit area No / mm2 obtained
(necessary to reach the baths of sufficient deoxidation transfer rates) experimental test work
Roller width, m 1 Thickness of the strip, mm 1.6 Ton of the pouring cauldron, t 120 Density of the steel, kg / m3 7800 Total oxygen, ppm 75 Inclusion density, kg / m3 3000 OUTPUTS Mass of inclusions, kg 21.42867 Diameter of inclusions, m 2.00E-06 Volume of inclusions, m3 0.0 Total number of inclusions, 1706096451319381.5 Thickness of the surface layer um (one side) 2 Total number of inclusions 4265241128298.4536 These superficial inclusions can participate only in the initial process of nucleation
Casting speed, m / min 80 Strip length, m 9615.38462 Strip surface area, m2 19230.76923 Total number of nucleation sites- 2307692.30760 required% of available inclusions 54.10462 that need to participate in the nucleation process APPENDIX 1 List of symbols w = width of the roller, mt = thickness of the strip, mm plus weight of the steel in the casting cauldron, tons d3 density of the steel, kg / m3 d? = density of inclusions, kg / m3 ot = total oxygen in steel, ppm d = inclusion diameter, mi = volume of an inclusion, m3 mi = mass of inclusions, kg Nt = total number of inclusions ts = thickness of the surface layer, um Ns = total number of inclusions present on the surface (which may participate in the nucleation process) u = casting speed, m / min Ls = strip length, m As = surface area of the strip , m2 Nreq = Total number of inclusions required to meet the target nucleation density NCt = target nucleation density per unit area, number / mm2 (obtained from the deoxidation bath test) Nav =% of the total inclusions available in the molten steel on the surface of the casting rollers for the initial nucleation process.
Equations (1) mi = (0t x ms x O.OOU / 0.42 Mote: for steel deoxidized with Mn-Si, 0.42 kg of oxygen is needed to produce 1 kg of inclusions with a composition of 30% MnO, 40% of Si02 and 30% of Al203 For the steel deoxidized with Al (with Ca injection), 0.38 kg of oxygen is required to produce 1 kg of inclusions of a composition of 50% Al203 and 50% CaO. (2) Vi = 4.19 x (d / 2) 3 (4) Ns = (2.0 ts x 0.001 x Nt / t) (5); Ls = (ms x 1000) / (dß xwxt / 1000 (6) As = 2.0 x La xw (7) Nreg = As X 10S X NCt (8) Nav% = (Nreq / Ns) x 100. 0
The ec. 1 calculates the mass of the inclusions in the steel. The ec. 2 calculates the volume of an inclusion assuming they are spherical.
The ec. 3 calculates the total number of inclusions available in the steel. The ec. 4 calculates the total number of inclusions available in the surface layer (assuming it is 2 um on each side). Note that these inclusions can only participate in the initial nucleation. The ec. 5 and Eq. 6 used to calculate the total surface area of the strip. The ec. 7 calculates the number of inclusions needed on the surface to comply with the target nucleation rate. The ec. 8 used to calculate the percentage of total inclusions available on the surface that must participate in the nucleation process. Note that if this number is greater than 100% then the number of inclusions on the surface is not sufficient to meet the target nucleation rate.