ES2392736T3 - New additive comprising lead and / or a lead alloy intended to treat liquid steel baths - Google Patents

Abstract

Additive comprising metallic lead and / or one or a few lead alloys, intended to treat liquid steel baths, said additive in the form of a lined wire composed of a metal sheath and a finely divided filling material, the latter being constituted by a metal lead and / or lead alloy powder and a powder containing a material capable of releasing a gas, inert with respect to liquid steel, at the temperature of the liquid steel bath, said additive being characterized in that said powder of metallic lead and / or lead alloy consists of a granulometric fraction GR between 200 µm and 500 µm, and because said granulometric fraction GR has the following characteristics: - in the 200 µm sieve: GR 5%; - in the 300 µm sieve: 90% GR 10%; - in the 400 µm sieve: 40% GR 100%; - in the 500 µm sieve: 100% GR 90%.

Description

New additive comprising lead and / or a lead alloy intended to treat liquid steel baths.

The present invention relates to an additive in the form of a lined wire for the treatment of liquid steel baths to obtain steels with a high lead content.

Lead is well known because it improves the machinability of steels because, insoluble in steel, it forms exogenous lead inclusions (nodules) that act as a lubricant, and favor the division of chips during the machining of rolled steels. It suffers, on the contrary, from important inconveniences of use, due to its toxicity, its strong density (higher than that of liquid steel) and its low melting point. It is introduced into the baths of liquid steel by injection of balls or shotguns through a submerged lance or in the form of lined wire, the latter technique being known to be generally more flexible and more reliable.

The addition yields currently obtained with the classic lined wires do not allow significant additions of lead, under penalty of generating significant amounts of harmful fumes and consequently considerable discomfort with respect to hygiene and safety of personnel.

EP 0 316 921 describes an additive containing lead for steel baths, in the form of a lined wire composed of a metal sheath and a finely divided filler material, containing the latter metallic lead and / or lead alloys and a lime-containing material that releases carbon dioxide (CO2) at the temperature of the liquid steel bath. The release of CO2 in the steel bath produces, around the lined wire, an intense turbulence that has the effect of emulsifying the lead particles in the liquid steel and dragging them in motion inside the bath, thus improving their distribution in the bath of liquid steel On the other hand, the use of this additive has allowed to limit the emissions of harmful fumes and to better control the addition process, while increasing the addition performance with respect to those previously known.

However, it has been found that the use of this type of additive does not allow a uniform distribution of lead inclusions during casting in the liquid steel container. On the other hand, the final steel product obtained thanks to the use of this additive does not present a homogeneous distribution of lead inclusions. In addition, the yield of the addition of lead in the liquid steel bath is still below 70%.

The present invention aims to alleviate these drawbacks by providing a new additive comprising a metal lead and / or lead alloy powder whose size and granulometric distribution are very specific, said powder being associated with a compound capable of ensuring a homogeneous distribution of the lead in the bath of liquid steel.

To this end, the invention relates, according to a first aspect, to an additive comprising metallic lead and / or one or a few lead alloys, intended to treat liquid steel baths, said additive being presented in the form of a lined wire composed of a metal sheath and a finely divided filler material, the latter being constituted by a metal lead and / or lead alloy powder and a powder of a compound suitable for releasing a gas, inert with respect to liquid steel, to the temperature of the liquid steel bath, said additive being characterized in that said metal lead and / or lead alloy powder consists of a granulometric fraction GR comprised between 200 µm and 500 µm, and because said granulometric fraction GR has the following characteristics:

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in a 200 m sieve: GR � 5%;

-

in a 300 µm sieve: 90% � GR � 10%;

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in a 400 µm sieve: 40% � GR � 100%;

-

in a 500 µm sieve: 100% � GR � 90%;

According to a second aspect, the invention relates to a method of treating liquid steel baths by means of an additive comprising metallic lead and / or one or a few lead alloys, which comprises a step of adding to said steel baths. of an additive that is presented in the form of a lined wire composed of a metal sheath and a finely divided filler material, the latter being constituted by a metal lead and / or lead alloy powder and a compound powder suitable for releasing a gas, inert with respect to liquid steel, at the temperature of the liquid steel bath, said metallic lead and / or lead alloy powder consisting of a granulometric fraction GR that is comprised between 200 µm and 500 µm , and which has the following characteristics:

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in a 200 µm sieve: GR � 5%;

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in a 300 µm sieve: 90% � GR � 10%;

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in a 400 µm sieve: 40% � GR � 100%;

-

in a 500 µm sieve: 100% � GR � 90%;

According to a third aspect, the object of the invention is the use of the additive comprising metallic lead and / or

one or more lead alloys, described above, to treat liquid steel baths.

The invention also relates, according to a fourth aspect, to any rolled steel product with a high lead content obtained by the above procedure, characterized in that the lead nodules are smaller than 100 µm in size and, for the most part, at a height minus 80%, distributed in rolled steel according to a random distribution type as shown in Figure 3a. This distribution gives the rolled steel optimum machining characteristics.

The use of this new additive makes it possible to substantially improve the lead addition yields and therefore the possibility of making more important additions in satisfactory hygiene and safety conditions. It also allows to obtain a better distribution of the final lead nodules in solid steel while reducing the phenomena of remanence and contamination of the refractories of the containers used for the treatment of these steels. The production costs of these steels are thus improved.

Other features and advantages of the invention will become more clear from the reading of the detailed description and the following embodiments, as well as the attached figures, in which:

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Figure 1 represents the characteristics of sizes and granulometric distributions of the granulometric fraction GR;

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Figure 2 shows the variation in lead yield as a function of the amount of additive added to the steel bath expressed in length of lined wire added per ton of liquid steel;

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Figure 3 schematically represents the different types of distribution of lead nodules in the final solid steel product;

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Figure 4 represents a diagram that allows the calculation of the minimum distance with the nearest lead nodule.

The present invention relates to a new additive comprising metallic lead and / or one or more lead alloys, intended for the treatment of liquid steel baths to obtain steels with high lead content. In a known manner, this additive is presented in the form of a lined wire composed of a metal sheath and a finely divided filler material, the latter being constituted by a metal lead and / or lead alloy powder and a powder of a compound capable of releasing a gas, inert with respect to liquid steel, at the bath temperature of liquid steel.

Advantageously, said metal lead and / or lead alloy powder consists of a granulometric fraction GR between 200 µm and 500 µm. This granulometric fraction is preferably in the form of small granules or very fine balls.

Typically, said granulometric fraction GR has the following characteristics:

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in a 200 µm sieve: GR 5%;

-

in a 300 µm sieve: 90% GR 10%;

-

in a 400 µm sieve: 40% GR 100%;

-

in a 500 µm sieve: 100% GR 90%;

These granulometric characteristics are schematically represented in the attached figure 1.

This granulometric distribution (contained in the area materialized in Figure 1) gives the lined wire optimal filling properties that lead to a metallurgical treatment with good performance of liquid steel baths. The selection of said granulometric distribution guarantees a level of residual porosity much lower than those observed in lined wires made from a classic lead powder. The porosity is thus between 5% and 20% maximum, while for a classical wire, the value is generally between 15 and 40%.

The metal sheath surrounding the additive is made of a material capable of dissolving in the steel bath with a sufficiently high speed to allow the release of said additive and without introducing into it unwanted components. Preferably, the metal sheath is made of non-alloy mild steel. Its thickness is between 0.1 and 1 mm, preferably between 0.2 and 0.6 mm.

On the other hand, the diameter of the lined wire according to the invention is between 5 and 20 mm, preferably between 9 and 15 mm.

The additive according to the invention is in the form of a lined wire containing 100 to 1000 g of lead per

wire meter

In the case of the powder of the compound capable of spontaneously releasing a gas, inert with respect to liquid steel, at the temperature of the liquid steel bath (between approximately 1550 and 1650 ° C), it is also presented in finely divided form, with a particle size less than 1 mm, preferably less than 0.5 mm.

Advantageously, the release of the gas bubbles in the liquid steel bath creates an updraft that leads to a very random distribution of the lead inclusions formed from the granulometric fraction according to the invention, thus unifying their distribution in the bath of liquid steel

In a particular embodiment, the compound capable of spontaneously releasing a gas, inert with respect to liquid steel, is a mineral compound such as limestone (calcium carbonate) or uncooked dolomite, and said inert gas with respect to liquid steel is carbon dioxide. In this case, the mineral compound is used in an amount of 3 to 30% by weight with respect to the weight of the metallic lead and / or of the alloy or of the lead alloys used (s).

According to a second aspect, the invention relates to a method of treating liquid steel baths by means of an additive comprising metallic lead and / or one or a few lead alloys, which comprises a step of adding to said baths a additive that is presented in the form of a lined wire, described above.

A lined wire containing said powder allows to obtain a lead yield in the liquid steel superior to that obtained with a classic wire or also with that described in EP 0 316 921.

Figure 2 presents some industrial results that refer to the lead yield as a function of the amounts added (the variation of the lead yield as a function of the amount of additive added to the steel bath expressed in wire length lined per ton of steel) .

Lead yield is defined by the following equation:

Ypb = (CF - CI) / CA

In this equation:

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CI is the initial lead content in the liquid steel container;

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CF is the final lead content obtained in the liquid steel container;

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CA is the target lead content in the liquid steel container;

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Ypb is the performance of lead addition.

The lined wire containing the lead dust and / or a lead alloy whose granulometry respects the specificities described in the present invention, allows to obtain a higher lead yield than with a conventional powder. It also allows to obtain very regular and constant yields whatever the length of the wire injected in the bath of liquid steel. Thus, the smaller dispersion makes it possible to substantially increase the chances of success of obtaining the lead forecast in the final steel.

Thanks to higher performance, hygiene and safety conditions during the treatment of liquid steel containers are also substantially improved. There is less release of harmful fumes above the container. The phenomena of sedimentation of lead at the bottom of the container and contamination of the refractory walls of the containers are also very small.

The addition of the additive according to the invention in the liquid steel container takes place before casting. Depending on the final lead content desired, an amount between 0.1 and 10 kg of additive is introduced in the form of wire lined per ton of liquid steel to be treated. The lined wire is wound in the steel bath at a speed between 50 and 200 m / min, preferably between 100 and 150 m / min.

The following two examples illustrate the high lead yield values obtained by using the new additive according to the invention.

Example 1

13.6 mm outer diameter lined wire

Strap thickness between 0.35 mm and 0.40 mm

Amount of calcium carbonate in the mixture: 6.3% by weight

Metric wire weight: 970 g / m

Injection speed: 120 m / min

Weight of liquid steel in the container: 95 t

Expected lead content: 0.260%

Lead content obtained after treatment: 0.248%

The yield of the added lead in coated wire obtained is: 71.8%

Example 2

13.6 mm outer diameter lined wire

Strap thickness between 0.35 mm and 0.40 mm

Amount of calcium carbonate in the mixture: 5.8% by weight

Injected wire length: 334 m

Wire injection speed: 150 m / min

Liquid steel weight in the container: 115 t

Expected lead content: 0.200%

Initial lead content in the liquid steel bath: 0.009%

Lead content obtained after treatment: 0.191%

The yield of the added lead in lined wire obtained is: 72.0%

Since the granulometry of the lead particles contained in the lined wire has been chosen in a very small range of 200 to 500 µm, lead inclusions, insoluble in liquid steel, are distributed evenly throughout the container.

Advantageously, the reduced size of the lead inclusions greatly reduces its sedimentation at the bottom of the container. This leads to a constant lead content in the liquid steel bath from the beginning of the emptying of the container until the end of the laundry. The solidified product is therefore more homogeneous in lead content, whatever the amount of liquid steel that remains to melt. This is illustrated in the following example.

Example 3

13.6 mm outer diameter lined wire

Strap thickness between 0.35 mm and 0.40 mm

Amount of calcium carbonate in the mixture: 6.5% by weight

Amount of powder injected by lined wire: 297 kg

Injection speed: 120 m / min

Container weight: 95 t

Expected lead content: 0.260%

Lead content obtained on the first cast levers: 0.252%

Lead content on longliners in the middle of the container: 0.245%

Lead content on longliners at the end of emptying the container: 0.249%

The term "longliner" designates a solidified steel unit (round, rectangular or polygonal section steel ingot).

On the other hand, the use of the lined wire containing a finely divided lead powder according to the invention makes it possible to reduce the washing operations of the containers used for the production of liquid steels with a high lead content. Refractory vessels are less contaminated by significant lead infiltration. The steelmaker notes less lead residues in the opening / closing systems of the pouring hole as well as in the joints between the refractory bricks.

The percentage of wastes from rolled products (bars) of steel with a high lead content is greatly reduced thanks to the use of the additive containing a lead and / or lead alloy powder whose granulometry is described in the present invention. The bars are rejected if the size and distribution of lead nodules do not meet the specifications imposed by the steelmaker's client. Thanks to the additive according to the invention, 100% of the bars are suitable, while the use of a lined wire containing a classic lead powder can lead to up to 30% of waste.

In addition to the smaller sizes, the lead nodules are more adequately distributed in the laminated product that thus favors the machining properties. There is no standard or international method to characterize the distribution of lead nodules in rolled products; Therefore, a qualification criterion for the distribution of the population of lead nodules in laminated products has been specifically developed.

Thus, the applicant has specifically developed a qualification criterion for the distribution of the population of lead nodules in rolled products. It therefore defines distribution rates and the associated criteria and finally, the conditions of experimental measurements.

Thanks to an in-depth study by digital simulation that models the different distributions considered in Figure 3, the applicant has demonstrated relevant indices that allow qualifying these distributions. It has also determined the limits associated with each of these indices. In this way, it has been shown that an IR distribution index greater than 1.4% allows a random distribution to be differentiated with a tolerance interval of 99% from the other types of distribution.

The IR index is defined in this way:

10 IR: distribution rate

D: diagonal of the analysis area say: minimum distance between lead nodules (closest ones - figure 4) NI: number of lead nodules with an associated minimum distance.

15 Lead nodules are distributed randomly (thus favoring mechanization), when IR is greater than 1.4%.

This IR index is relevant only on the condition of taking into account a large number of lead nodules. This number has been set at 500. A specific analysis method has been developed.

The characterization of the lead nodules is carried out on the surface of a sample extracted in the middle of a radius of a rolled steel bar with a diameter greater than 1 40 mm, and observed in the direction of the laminate. The surface of the extracted sample is polished to a 1 µm sheet.

25 Lead nodules are identified and characterized by observing the surface of the sample with a scanning electron microscope equipped with a retro-diffuse electron detector (MEB.FEG) coupled to an image analyzer. With this observation mode and thanks to the chemical contrast, the lead nodules appear with a medium gray level clearly superior to that of the steel matrix and the inclusions of others

30 natural (such as sulphides, oxides, nitrides, etc.), which allows them to be easily distinguished and isolated.

The measurement method consists of observing a surface of a minimum of 25 mm2, in a square shaped area centered at half the radius of the bar. All lead nodules with a small diameter of Feret greater than 2 microns are taken into account and measured. More than 500 lead nodules must be taken into account. For

Each of these nodes, the position parameters (X and Y coordinates in the reference of the area examined) and the main morphological parameters (surface of the nodule, diameter of Feret, etc.) are stored in a result file.

The distribution parameters are then calculated in order to demonstrate the distribution of lead nodules.

40 that allow to optimize the mechanization properties of the product. Several types of distributions are taken into account, represented in the attached figure 3: random (figure 3a), in heaps (figure 3b), in bands (figure 3c) or in a network (figure 3d). In order to ensure a sufficient difference in mechanization aptitude with respect to that known for laminated steels with high lead content treated with classic lined wires, it has been determined that the proportion of nodules distributed according to the random type should be maximum and preferably

45 at least equal to 80%.

The use of an additive that is presented in the form of a lined wire containing a lead powder, whose granulometry is as defined in Figure 1, allows to obtain a homogeneous rolled steel product that contains lead nodules of very small size , less than 100 µm, and distributed according to a distribution

50 very mostly random. This distribution gives the steel improved machining properties with respect to those obtained on steels treated with classic lined wires or those described in EP 0 316 921.

The use of the new additive comprising metallic lead and / or a lead alloy, according to the invention, for

The treatment of liquid steel containers to obtain steels with a high lead content has numerous advantages and leads in particular to:

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the improvement of the performance of the addition of lead in the liquid steel bath;

60 - the improvement of the processing and treatment conditions of steels with a high lead content: in fact, the increase in the addition performance allows reducing the emission of harmful fumes and therefore an improvement of the hygiene and of security for steelworks personnel;

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a strong improvement in the distribution and fineness of lead inclusions in liquid steel that allows

keep the lead content of the steel from the beginning of the laundry constant until the container is completely emptied;

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a sharp decrease in the sedimentation of lead at the bottom of the vessel and thus an almost disappearance

5 of the lead residues in the opening / closing systems of the casting holes as well as in the joints of the refractory bricks;

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the improvement of the final machining properties of rolled steel products thanks to a better distribution of lead nodules in rolled steel according to a distribution mostly

10 of random type, characterized by an innovative method specially developed to alleviate the absence of international standard method;

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the sharp reduction in the percentages of waste products laminated by internal defects

related to the presence of undesirable large lead nodules and / or poorly distributed in the 15 rolled steel products.

Claims (11)

1. Additive comprising metallic lead and / or one or a few lead alloys, intended to treat liquid steel baths, said additive in the form of a lined wire composed of a metal sheath and a 5 finely divided filling material, the latter being constituted by a metal lead and / or lead alloy powder and a powder containing a material suitable for releasing a gas, inert with respect to liquid steel, at bath temperature of liquid steel, said additive being characterized in that said metal lead and / or lead alloy powder consists of a granulometric fraction GR comprised between 200 µm and 500 µm, and because said granulometric fraction GR has the following characteristics:

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in the 200 µm sieve: GR 5%;

-

in the 300 µm sieve: 90% GR 10%;

-

in the 400 µm sieve: 40% GR 100%;

-

in the 500 µm sieve: 100% GR 90%; fifteen

2.

Additive according to claim 1, wherein the metal sheath is made of non-alloy mild steel.

3.

Additive according to any one of claims 1 or 2, wherein the metal sheath has a thickness of 0.1 to 1 mm,

preferably 0.2 to 0.5 mm. twenty 4. Additive according to any of claims 1 to 3, wherein the lined wire has a diameter of 5 to 20 mm, preferably 9 to 15 mm. 5. Additive according to any of claims 1 to 4, wherein the filler material has a size of 25 particles not exceeding 1 mm. 6. Additive according to any one of claims 1 to 5, wherein the lined wire contains 100 to 1000 g of lead per meter. 7. Additive according to any of claims 1 to 6, wherein the material suitable for releasing a gas, inert with respect to liquid steel, is a mineral compound consisting of limestone (calcium carbonate) or uncooked dolomite, being the gas released then carbon dioxide. 8. Additive according to claim 7, wherein the mineral material is present in an amount of 3 to 30% by weight with respect to the weight of lead used or the alloy (or alloys) of lead used (s). 9. Process for treating liquid steel baths by means of an additive comprising metallic lead and / or one or a few lead alloys, comprising a step of adding to said container an additive which is presented in the form of a lined wire composed of a metal sheath and a filler material 40 finely divided, the latter being constituted by a metal lead and / or lead alloy powder and by a powder of material capable of releasing a gas, inert with respect to liquid steel, at the temperature of the liquid steel bath, consisting of said metallic lead and / or lead alloy in a granulometric fraction GR comprised between 200 µm and 500 µm, and having the following characteristics: 45 -in the 200 µm sieve: GR 5%;

-

in the 300 µm sieve: 90% GR 10%;

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in the 400 µm sieve: 40% GR 100%;

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in the 500 µm sieve: 100% GR 90%;

Method according to claim 9, in which 0.1 to 10 kg of wire lined per ton of liquid steel to be treated is introduced. 11. Method according to one of claims 10 or 11, wherein the wire wrapped in the liquid steel container at a speed of 50 to 200 m / min, preferably 100 to 150 m / min. 55

12.

Use of an additive comprising metallic lead and / or one or some lead alloys according to any one of claims 1 to 8 for treating liquid steel baths.

13.

Product rolled in steel with high lead content, whose lead nodules are smaller than

60 100 µm, characterized in that it has been obtained by the method according to one of claims 9 to 11, the distribution of the nodules being defined according to the following formula: in which: IR: distribution index D: diagonal of the analysis area di: minimum distance between the nearest lead nodules NI: number of lead nodules with an associated minimum distance, IR is greater than 1.4%.