RU2297900C2 - Steel strip producing method and thin steel strip produced by such method - Google Patents

Abstract

FIELD: foundry, namely casting of thin steel strip in two-roll casting machine.

SUBSTANCE: inclusions occurred at solidifying and killing are in liquid state at temperature of initial solidification of steel. Method provides content of oxygen in casting bath of melt steel no less than 100 ppm and content of free oxygen 30 - 50 ppm in order to receive metallic particles with content of oxide inclusions corresponding to total content of oxygen in melt steel and to initiate nucleation and to create intensive heat flux during initial stage of steel solidification on surfaces of casting rolls. Thin steel strip with thickness less than 5 mm of solidified steel contains solidified oxide inclusions distributed in such a way that in surface zones of strip with depth 2 micrometers density of inclusions per surface area unit is equal at least 120 inclusions/mm². Combination of high content of oxygen of melt steel and short time period of melt steel presence in casting bath provides possibility for producing thin steel strip with enhanced ductile properties.

EFFECT: possibility for producing steel strip with high content of oxygen distributed in oxygen inclusions, improved ductile properties of thin steel strip.

19 cl, 8 dwg, 1 ex, 2

Description

This invention relates to a method for manufacturing a steel strip, in particular it is applicable to the continuous casting of thin steel strip in a twin roll casting machine.

When casting in a two-roll casting machine, liquid metal is fed between a pair of horizontal casting rolls rotating in opposite directions, which are cooled, as a result of which the metal particles solidify on the moving surfaces of the rolls and are pressed together in the gap between the rolls to produce hardened steel product fed down from the said gap between the rolls. The term "gap" is used here to indicate the area in which the rolls are located closest to each other. Liquid metal can be poured from the ladle into a smaller container, from which it enters through a pouring cup located above the gap so as to direct the liquid metal into the gap between the rolls, as a result of which a molten bath of molten metal is formed, supported by the surfaces of the rolls directly above the gap and extending along its entire length. This casting bath is usually limited by side plates or supports pressed against the end surfaces of the rolls so that they can rotate freely to prevent the casting bath from flowing out at both outflow ends, although other means, such as electromagnetic barriers, can also be installed.

When casting a thin steel strip in a two-roll casting machine, liquid steel in the casting bath will generally be at a temperature of the order of 1500 ° C and higher and, therefore, it is necessary to create very high cooling rates on the casting surfaces of the rolls. It is especially important to create a high heat flux and extensive nucleation during the initial solidification of steel on casting surfaces to form metal particles. US Pat. No. 5, 720, 336 describes how to increase the heat flux during initial solidification by adjusting the chemical composition of the steel melt so that a substantial portion of the metal oxides formed as deoxidation products are in a liquid state at the initial solidification temperature to create practically a liquid layer at the boundary between the molten metal and each of the casting surfaces. As described in US Pat. Nos. 5, 934, 359 and 6, 059, 014, as well as in International Application AU 99/00641, nucleation in steel upon initial solidification can be influenced by using the texture of the casting surface. In particular, International Application AU 99/00641 describes that a disordered texture of ridges and grooves can stimulate initial solidification by creating potential nucleation centers distributed over the casting surfaces. The authors of the present invention determined that the formation of nuclei also depends on the presence of oxide inclusions in the steel melt and that it is disadvantageous to use “pure” steel when casting strips in a two-roll casting machine, in which the number of inclusions formed during deoxidation is minimized, and this last by surprise.

Steel intended for continuous casting is deoxidized in the ladle before casting. In the case of casting in a two-roll casting machine, steel is generally subjected to oxidation in a ladle using silicon and manganese, although aluminum deoxidation with the addition of calcium can be used to control the formation of Al ₂ O ₃ solid inclusions, which can lead to tightening of the holes for the metal to flow into a metal feed system through which molten metal is fed into a casting bath. Until now, it was believed that it was desirable to ensure optimum steel purity by processing in a ladle in order to minimize the total oxygen content in liquid steel. However, the authors of the present invention have determined that lowering the level of oxygen in the steel decreases the volume of inclusions and, if the total oxygen content in the steel decreases below a certain level, the nature of the initial contact between the steel and the surfaces of the rolls may deteriorate to such an extent that there will be a level of nucleation insufficient to provide fast initial solidification and high heat flux. Liquid steel is treated by deoxidation in a ladle so that the total oxygen content is in a range that ensures satisfactory solidification on the casting rolls and the manufacture of a strip product of satisfactory quality. The oxide inclusions contained in the liquid steel (typically MnO, CaO, SiO ₂ and / or Al ₂ O ₃ ) are sufficient to provide the necessary density of nucleation centers on the surfaces of the rolls for initial solidification, and the resulting strip product exhibits a characteristic distribution of solidified inclusions.

A method of manufacturing a steel strip by continuous casting, comprising the steps of:

a) assembling a pair of cooled casting rolls with the creation of a gap between them and the limiting parts adjacent to the ends of the said gap;

b) supplying a liquid low carbon steel having a total oxygen content of at least 100 ppm and a free oxygen content of 30 to 50 ppm between a pair of casting rolls to create a casting bath between said rolls;

c) rotating the casting rolls in opposite directions and solidifying the molten steel to create metal particles on the surface of said rolls having an oxide content corresponding to the total oxygen content of the molten steel to produce a thin steel strip; and

d) manufacturing a hardened thin steel strip in the gap between the casting rolls of said hardened particles.

The total oxygen content in the molten steel in the casting bath may range from 100 to 250 ppm. In particular, it may be approximately 200 ppm. Mild steel may have a carbon content in the range of 0.001-0.1 wt.%, A manganese content in the range of 0.1-2.0 wt.% And a silicon content in the range of 0.01-10 wt.%. The steel may have an aluminum content of the order of 0.01 mass% or less. For example, aluminum may contain as little as 0.008 wt.% Or less. Liquid steel may be silicon / manganese deoxidized steel.

Oxide inclusions can be inclusions formed during solidification and inclusions formed during deoxidation, the former are formed during cooling and solidification of the steel during casting, and the latter are formed during the deoxidation of molten steel before the casting process. The hardened steel may contain oxide inclusions, usually consisting of one or more of the following oxides: MnO, SiO ₂ and Al ₂ O ₃ distributed in steel with a density in the range of 2-4 g / cm ³ .

Before being fed into the gap between the casting rolls to form a casting bath, molten steel can be refined in a ladle by heating steel and slag-forming material, as a result of which molten steel is coated with slag containing silicon, manganese and calcium oxides. Liquid steel is mixed by blowing inert gas, for example for desulfurization, and in the case of steel deoxidized with silicon / manganese, oxygen can be supplied to produce steel having a desired total oxygen content of at least 100 ppm and usually less than 250 parts per million. Desulfurization can reduce the sulfur content in molten steel to less than 0.01 wt.%.

A thin steel strip made by continuous casting in a twin roll casting machine, as described above, has a thickness of less than 5 mm and is made of hardened steel containing hardened oxide inclusions. The distribution of inclusions can be such that in two surface regions of the said strip 2 microns deep from the outer surfaces, the content of hardened inclusions per unit area is at least 120 inclusions / mm ² .

The hardened steel may be silicon / manganese deoxidized steel, and the oxide inclusions may contain one or more of the following oxides: MnO, SiO ₂ and Al ₂ O ₃ . Inclusions typically may have a size in the range of 2-12 microns, with at least a majority of the inclusions having a size in this range.

Using the method described above, a unique steel with a high oxygen content distributed in oxygen inclusions is made. What is especially important, the combination of a high oxygen content in liquid steel and a short residence time of liquid steel in a casting bath leads to the creation of a thin steel strip with improved plastic properties.

For a more detailed disclosure of the present invention, the following are some specific examples with reference to the accompanying drawings, in which:

Figure 1 shows the influence of the melting points of inclusions on the heat flux obtained during casting tests in a two-roll machine using steels deoxidized with silicon / manganese;

figure 2 shows the distribution of manganese obtained using energy dispersive spectroscopy, which shows a strip of small inclusions formed during solidification in a hardened steel strip;

Figure 3 is a graph showing the effect of changing the ratio of the content of manganese to silicon on the liquidus temperature of inclusions;

Figure 4 shows the relationship between the alumina content (measured for inclusions contained in the strip) and deoxidation efficiency;

Figure 5 is a state diagram of a ternary system for MnO-SiO ₂ -Al ₂ O ₃ ;

6 shows the relationship between the presence of inclusions containing alumina and liquidus temperature;

7 shows the effect of oxygen contained in molten steel on surface tension; and

Fig. 8 is a graph for calculation results regarding the number of inclusions suitable for nucleation at various degrees of steel purity.

The inventors of the present invention conducted extensive casting experiments in a twin roll casting machine, the type of which is described in detail in US Pat. Nos. 5,184,668 and 5,277,243, with the aim of manufacturing a steel strip having a thickness of the order of 1 mm or less. Such casting tests using steel deoxidized by silicon and manganese showed that the melting temperature of oxide inclusions contained in liquid steel affects the heat fluxes that occur during solidification of the steel, as shown in Figure 1. Oxides with a 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 bath, causing a higher heat transfer rate. Liquid inclusions do not occur if said melting point is higher than the temperature of the steel in the casting bath. Thus, a significant decrease in the intensity of heat transfer occurs if the melting point of the inclusions exceeds approximately 1600 ° C.

Foundry tests using steel deoxidized by aluminum showed that in order to prevent the formation of inclusions of aluminum oxide with a high melting point (2050 ° C), it is necessary to carry out calcium treatment to create liquid inclusions of CaO · Al ₂ O ₃ .

Oxide inclusions arising in solidified metal particles and, therefore, in a thin steel strip contain inclusions formed during cooling and solidification of steel, and inclusions formed during deoxidation and refining in the ladle.

The level of free oxygen in the steel decreases significantly during cooling in the meniscus, which leads to the formation of inclusions near the surface of the strip. These inclusions are formed mainly from MnO · SiO ₂ arising from the following reaction:

Mn + Si + 3O = MnO · SiO ₂

The image of inclusions arising during cooling and solidification on the surface of the strip, obtained using energy dispersive spectroscopy, is shown in Figure 2. You can see that these inclusions are extremely small (typically less than 2-3 microns) and are located in the surface strip with a depth of 10-20 microns. A typical distribution of the size of inclusions in the strip is shown in Figure 3 of the article entitled "Recent Developments of Project M, a joint development of the low carbon steel strip casting process carried out by BHP and IHI" presented at METEC Congress 99, Dusseldorf, Germany, June 13-15 1999.

The relative content of inclusions arising during cooling and solidification is determined primarily by the levels of Mn and Si in steel. Figure 3 shows that the ratio of Mn to Si has a significant effect on the liquidus temperature of inclusions. In steel deoxidized with manganese and silicon, having a carbon content in the range of 0.001-0.1 wt.%, Manganese content in the range of 0.1-2.0 wt.%, Silicon content in the range of 0.1-10 wt.% and an aluminum content of the order of 0.01 mass% or less, such oxide inclusions may occur during cooling of the steel in the upper regions of the casting bath. In particular, steel may have the following composition, designated M06, wt.%:

Carbon 0.06 Manganese 0.06% Silicon 0.28% Aluminum 0.002%

Other inclusions occur during the deoxidation of molten steel in the ladle using Al, Si and Mn. Therefore, the oxide inclusions formed during deoxidation are mainly based on MnO · SiO ₂ · Al ₂ O ₃ . These inclusions are randomly distributed in the strip and larger than the inclusions formed during cooling and solidification, located near the surface of the strip.

The content of aluminum oxide in the inclusions has a strong effect on the level of free oxygen in steel. Figure 4 shows that with an increase in the content of alumina, the content of free oxygen in the steel decreases. When aluminum oxide is introduced, MnO · SiO ₂ inclusions dissolve, which leads to a decrease in their activity, which in turn reduces the level of free oxygen, as can be seen from the following reaction:

Mn + Si + 3O + Al ₂ O ₃ ⇔ (Al ₂ O ₃ ) · MnO · SiO ₂ .

For inclusions based on MnO-SiO ₂ -Al ₂ O _{3, the} influence of the composition of inclusions on the liquidus temperature can be determined from the state diagram of the ternary system shown in FIG. 5. Analysis of oxide inclusions contained in a thin steel strip showed that the ratio of MnO / SiO ₂ is typically in the range of 0.6-0.8, and for such a ratio it was found that the content of aluminum oxide in oxide inclusions has the strongest effect on temperature melting inclusions (liquidus temperature), as shown in Fig.6.

The inventors of the present invention have determined that it is important for the casting method of the present invention that the inclusions formed upon cooling, solidification and deoxidation are in a liquid state at the initial solidification temperature of the steel, and that the liquid steel in the casting bath contains oxygen at a level of at least at least 100 ppm to produce metal particles with oxide inclusions corresponding to the total oxygen content in the molten steel to stimulate ducation embryos and creation of high heat flux during the initial solidification of the steel on the casting surfaces of the rolls. Both inclusions formed during cooling and solidification, and inclusions formed during deoxidation, are oxide inclusions and are nucleation centers, and also significantly affect the formation of nuclei during the metal solidification process, but ultimately control the speed inclusions formed during deoxidation, as their concentration may vary. The inclusions formed during deoxidation are much larger in size and typically exceed 4 microns, while the inclusions formed during cooling and solidification are generally smaller than 2 microns in size and their base is MnO · SiO ₂ , while Al ₂ O _{3 is} absent, while inclusions formed during deoxidation contain this oxide.

In casting tests carried out using the aforementioned M06 grade steel, deoxidized with silicon / manganese, it was found that if the total oxygen content in the steel during refining in the ladle decreases to low levels of less than 100 ppm, the heat fluxes decrease and the quality casting is impaired, while good results can be achieved if the total oxygen content exceeds at least 100 parts per million and typically amounts to about 200 parts per million n Such levels of oxygen in the bucket result in total oxygen levels in the intermediate tundish of at least 70 ppm, and free oxygen levels of 20-60 ppm, which in turn provides the same or slightly lower levels of oxygen in the foundry bath. The total oxygen content can be measured using the "LECO" measuring device and depends on the degree of "flushing" during processing in the bucket, that is, the amount of argon passed through the porous insert or top lance during the bubbling of steel in the bucket, as well as on the processing time . The total oxygen content was measured using a LECO TC-436 nitrogen / oxygen meter using the usual procedures described in the TC-436 Nitrogen / Oxygen Meter User Guide offered by LECO (Form No. 200-403, last revised in April 1996, chap. 7, pp. 7-1 to 7-4).

In order to determine whether the increased heat fluxes obtained at higher levels of total oxygen content are due to the presence of oxide inclusions as nucleation centers, casting tests were carried out using steels for which deoxidation in the ladle was carried out using silicocalcium (Ca-Si), and the results were compared to casting mild silicon deoxidized steel known as M06 grade. The results are shown in the following table.

Table 1
Differences in heat flux for M06 and Ca-Si steels Casting No. Mark Casting speed, m / min Bath height mm Total heat dissipation, MW M 33 M06 64 171 3,55 M 34 M06 62 169 3,58 About 50 Ca-Si 60 176 2.54 About 51 Ca-Si 66 175 2,56

Although the levels of Mn and Si were close to their levels in ordinary grades deoxidized with silicon, the level of free oxygen in Ca-Si melts was lower, and oxide inclusions contained more CaO. Heat fluxes in Ca-Si melts were lower, despite the lower melting temperature of inclusions (see table 2).

table 2
Calcium / Silicon Deoxidation Formulations Mark Free oxygen, ppm The composition of the slag, wt.% Inclusion melting temperature, ° С SiO ₂ MnO Al ₂ O ₃ Cao Ca-Si 23 32,5 9.8 32.1 22.1 1399

The levels of free oxygen in Ca-Si grades were lower, typically 20-30 ppm compared to 40-50 ppm for M06 grades. Oxygen is a surface-active element, and therefore, it is assumed that a decrease in oxygen content decreases wetting between molten steel and casting rolls and causes a decrease in heat transfer intensity. However, it can be seen from FIG. 7 that a decrease in oxygen content from 40 to 20 ppm may not be sufficient to increase surface tension to levels that explain the observed decrease in heat flux.

It can be concluded that a decrease in the content of free oxygen and the total oxygen content in the steel reduces the volume of inclusions and, thus, reduces the number of oxide inclusions for the initial nucleation. This may adversely affect the nature of the initial contact between the steel and the surface of the rolls. Submersible tests have shown that in order to create sufficient heat flux during initial solidification in the upper region of the casting bath or the meniscus region, a density of nucleation centers per unit area of approximately 120 / mm ² is required. An immersion test involves lowering a chilled billet into a molten steel bath at such a rate that it simulates the conditions created on the casting surfaces of a two-roll casting machine with a high degree of approximation. Steel hardens on a cooled billet as it moves through the liquid bath, creating a layer of hardened steel on the surface of this billet. The thickness of this layer can be measured at various points throughout its area to reflect changes in the speed of solidification and, consequently, the actual intensity of heat transfer in different places. Thus, it is possible to measure the final solidification rate, as well as the total heat flux. In addition, it is possible to study the microstructure of the strip surface in order to establish a relationship between changes in the microstructure during solidification and changes in the observed solidification rates and heat transfer values, and investigate the structures associated with the formation of nuclei on a cooled surface during initial solidification. A device for conducting immersion tests is described in more detail in US patent No. 5, 720, 336.

The relationship between the oxygen content in liquid steel during initial nucleation and heat transfer was investigated using the model described in Appendix 1. This model assumes that all oxygen inclusions are spherical and uniformly distributed in steel. It is assumed that the surface layer has a thickness of 2 μm, and that only inclusions in this surface layer can participate in the nucleation process during the initial solidification of the steel. The initial data for the model were the total oxygen content in the steel, the diameter of the inclusions, the strip thickness, casting speed and the thickness of the surface layer. The output was the percentage of inclusions of their total amount in steel, required to provide the necessary density of nucleation centers per unit area equal to 120 / mm ² .

Fig. 8 is a graph of the percentage of oxide inclusions in the surface layer required to participate in the process of nucleation in order to achieve the necessary density of nucleation centers per unit area on the degree of purity of steel, which is expressed by the total oxygen content, it is assumed that the thickness the strip is 1.6 mm and the casting speed is 80 m / min. This graph shows that with an inclusion size of 2 μm and a total oxygen content of 200 ppm, to achieve the required density of nucleation centers per unit area of 120 / mm ² , 20% of the total number of oxygen inclusions in the surface layer is required. However, with a total oxygen content of 80 ppm, approximately 50% of inclusions are required to reach a threshold nucleation rate, and with a total oxygen content of 40 ppm, the level of oxide inclusions will be insufficient to provide the necessary density of nucleation centers per unit area. Accordingly, when steel is refined by deoxidation in a ladle, the oxygen content in the steel can be controlled to obtain a total oxygen content in the range of 100-250 ppm, and typically about 200 ppm. This will lead to the fact that the layers of two micron thickness adjacent to the casting rolls during initial hardening will contain oxide inclusions having a density per unit area of at least 120 / mm ² . These inclusions will be present in the surface layers of the final hardened strip product and can be detected by appropriate studies, for example, by energy dispersive spectroscopy.

An example embodiment of the invention

Input data

The threshold density of nucleation centers per unit area, number / mm ² (necessary to achieve sufficient heat transfer intensity) 120 This value was obtained during experimental immersion tests. Width of rolls, m one Strip thickness mm 1,6 The weight of steel in the bucket, t 120 The density of steel, kg / m ³ 7800 The total oxygen content, parts per million 75 The density of inclusions, kg / m ³ 3000

Output

The mass of inclusions, kg 21,42857 Diameter of inclusion, m 2 × 10 ^-6 The volume of inclusions, m ³ 0,0 Total number of inclusions 170609645131938 1.5 The thickness of the surface layer, microns (one side) 2 The total number of inclusions, only on the surface 4265241128298.4536 These inclusions can participate in the initial embryo formation Casting speed, m / min 80 Strip length, m 9615.38462 The surface area of the strip, m ² 19230,76923 The total number of centers required for the formation of nuclei 2307692,30760 % of the available inclusions required to participate in the process of embryo formation 54.10462

Annex 1

Character List

w = roll width, m

t = strip thickness, mm

m _s = weight of steel in the bucket, tons

_s = steel density, kg / m ³

ρ _i = density of inclusions, kg / m ³

ρ _t = total oxygen content in steel, parts per million

d = diameter of inclusions, m

v _i = the volume of one inclusion, m ³

m _i = mass of inclusions, kg

N _t = total number of inclusions

t _s = thickness of the surface layer, microns

N _s = total number of inclusions present in the surface layer (which can participate in the process of nucleation)

u = casting speed, m / min

L _s = strip length, m

A _s = strip surface area, m ²

N _req = total number of inclusions required to provide the necessary density of nucleation centers

NC _t = required density of nucleation centers per unit area, number / mm ² (obtained from immersion tests)

N _av = percentage of total inclusions in liquid steel at the surface of casting rolls suitable for the initial nucleation process

Equations

Note: for steel deoxidized Mn / Si, to create 1 kg of inclusions with a composition of 30% MnO, 40% SiO ₂ and 30% Al ₂ O ₃ requires 0.42 kg of oxygen;

for steel deoxidized Al (with the introduction of Ca), to create 1 kg of inclusions with a composition of 50% Al ₂ O ₃ and 50% CaO requires 0.38 kg of oxygen

Using Equation 1, the mass of inclusions in steel is calculated.

Using Equation 2, the volume of one inclusion is calculated, and it is assumed that the inclusions are spherical in shape.

Using Equation 3, the total number of inclusions present in the steel is calculated.

Using Equation 4, the total number of inclusions present in the surface layer is calculated (it is assumed to be 2 μm on each side). Note that only these inclusions can participate in the initial formation of nuclei.

Equation 5 and Equation 6 are used to calculate the total surface area of the strip.

Using Equation 7, the number of inclusions needed on the surface is calculated to achieve the necessary nucleation rate.

Equation 8 is used to calculate the percentage of the total number of inclusions present on the surface that should be involved in the process of nucleation. Note that if this number is more than 100%, then the number of inclusions on the surface is insufficient to provide the necessary rate of nucleation.

Claims (19)

1. A method of manufacturing a steel strip by continuous casting, comprising the steps a) assembling a pair of cooled casting rolls with the creation of a gap between them and limiting parts adjacent to the ends of the said gap; b) supplying a liquid low carbon steel having a total oxygen content of at least 100 ppm and a free oxygen content in the range of 30-50 ppm between a pair of casting rolls to create a casting bath between said rolls; c) rotating the casting rolls in opposite directions and solidifying the liquid steel to create metal particles on the surface of said rolls having an oxide content level corresponding to the total oxygen content of the liquid steel to produce a thin steel strip; and d) manufacturing a hardened thin steel strip in the gap between the casting rolls of said hardened particles. 2. The method of manufacturing a steel strip according to claim 1, in which the molten steel in the casting bath has a carbon content in the range of 0.001-0.01 wt.%, A manganese content in the range of 0.01-2.0 wt.% And a silicon content of the range of 0.01-10 wt.%. 3. A method of manufacturing a steel strip according to claim 2, in which the molten steel in the casting bath has an aluminum content of about 0.01 wt.% Or less. 4. A method of manufacturing a steel strip according to any one of claims 1 to 3, in which molten steel in the casting bath has an oxygen content in the range of 100-250 ppm. 5. A method of manufacturing a steel strip according to any one of claims 1 to 3, in which molten steel contains oxide inclusions containing one or more of the following oxides: manganese, silicon and aluminum, distributed in steel with a density in the range of 2-4 g / cm ³ . 6. A method of manufacturing a steel strip according to any one of claims 1 to 3, in which the predominant part of the inclusions has a size in the range of 2-12 microns. 7. A method of manufacturing a steel strip according to any one of claims 1 to 3, in which the sulfur content in the liquid steel is less than 0.01 wt.%. 8. A method of manufacturing a steel strip according to claim 1, comprising an additional step: e) refining liquid steel in the ladle before creating a casting bath by heating steel and slag-forming material in the ladle to obtain molten steel coated with slag containing silicon, manganese and calcium oxides; mixing liquid steel in the ladle by blowing inert gas to cause desulfurization, followed by the introduction of oxygen to produce liquid steel having a total oxygen content of more than 100 ppm. 9. The method of manufacturing a steel strip according to claim 8, in which the desulfurization reduces the sulfur content in the liquid steel to a level of less than 0.01 wt.%. 10. A method of manufacturing a steel strip according to claim 8 or 9, in which the hardened steel is steel deoxidized by silicon or manganese, and said inclusions comprise one or more of the following compounds: manganese oxide, silicon oxide and alumina. 11. A method of manufacturing a steel strip according to claim 8 or 9, in which the predominant part of the inclusions has a size in the range of 2-12 microns. 12. A method of manufacturing a steel strip according to claim 8 or 9, in which the hardened steel has a total oxygen content in the range of 100-250 parts per million. 13. A thin steel strip made by casting in a two-roll casting machine, having a thickness of less than 5 mm and consisting of hardened steel containing hardened oxide inclusions distributed in such a way that in the surface regions of the said strip with a depth of 2 μm, their density per unit area is at least 120 inclusions / mm ² . 14. The thin steel strip of claim 13, wherein the steel is deoxidized by silicon or manganese, and said inclusions comprise one or more of the following compounds: manganese oxide, silicon oxide, and alumina. 15. The thin steel strip according to item 13 or 14, in which most of the inclusions have a size in the range of 2-12 microns. 16. The thin steel strip according to item 13 or 14, in which the hardened steel has an oxygen content expressed as a total content in the range of 100-250 ppm, and the molten steel from which the strip is obtained contains free oxygen in the range of 30-50 parts per million. 17. A thin steel strip made by casting in a two-roll casting machine, having a thickness of less than 5 mm and consisting of hardened steel containing oxide inclusions, the distribution of which corresponds to the total oxygen content of this steel, comprising 100-250 parts per million, and the content of free oxygen in liquid steel in the range of 30-50 ppm. 18. The thin steel strip according to claim 17, in which the predominant part of the hardened steel is deoxidized by silicon or manganese, and said inclusions contain one or more of the following compounds: manganese oxide, silicon oxide and alumina. 19. A thin steel strip according to claim 17 or 18, in which most of the inclusions have a size in the range of 2-12 microns.

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