EP3536815A1 - Method for manufacturing an ingot of low-alloy steel - Google Patents

Abstract

The invention relates to a method for manufacturing a low alloy steel ingot comprising the following steps: a) melting all or part of an electrode by a vacuum arc remelting process, the electrode comprising, before fusion , iron and carbon, the molten portion of the electrode being collected in a crucible and thus forming a molten bath in the crucible, andb) solidification of the molten bath by heat exchange between the molten bath and a cooling fluid, heat exchange achieved to impose a mean solidification rate during step b) less than or equal to 45 microns / s and to obtain a low alloy steel ingot.

Description

Background of the invention

The invention relates to methods for manufacturing low alloy steel ingots and to steel parts obtainable by such methods.

In the case of mechanical parts with high integrity that are stressed alternately under heavy stress, it may be necessary to size according to minimal curves enveloping all the results characterizing the desired properties including fatigue properties. However, the minimum design curves depend not only on the average value but also on the dispersion of results. This is particularly true for parts used in the aeronautical field with which a statistical analysis is generally taken into account. Reducing the dispersion of the results therefore makes it possible to trace the minimum dimensioning curves and, consequently, to improve the performances of the parts, for example by making it possible to lighten the parts, to extend their service life or to to increase the constraints to which they may be exposed. The reduction of the dispersion of the results advantageously makes it possible to confer a competitive technical differentiation as well as an economic gain in the material used.

The service life during oligo-cyclic fatigue stresses may depend, on the one hand, on the energy consumed at the moment of initiation on one of the particles present in the metallic material leading to a microcracking and, on the other hand, the propagation of the crack.

Due to a lack of accommodation, some particles may experience premature cracking, reducing the priming energy and, therefore, the lifetime compared to a single matrix. The nature of the particle, its shape, its individual size, its spatial distribution and its tendency to congregate with other particles can directly influence the reduction of this priming energy. A dispersion in the types of primers can induce a large dispersion of the priming energy reductions and can, therefore, further reduce the curve enveloping the minimum points (lowering of the mean and increase of the standard deviation).

This can particularly be the case for steels and, in particular, for remelted low alloy steels. It is known to manufacture steel grades by remelting a metal in a vacuum electric arc furnace (using a vacuum arc remelting process, in English: "Vacuum arc remelting"). . Such a step makes it possible to improve the inclusion cleanliness by filtering certain particles already present in the metal before this reflow.

In the case of low alloyed steels, the presence of inclusions particles of sulphide and / or oxide type, isolated, agglomerated or aligned can have an influence on the lifetimes of oligo-cyclic fatigue. The pre-reflow operations currently being implemented are aimed at minimizing the likelihood of such particles.

However, there may remain either exogenous particles or particles which due to imperfect solubility can reform during cooling.

In addition, it may be desirable to employ as stable a method of remelting as possible in order to obtain regular flotation of oxide and sulphide rafts on the liquid surface from the center to the edge of the crucible of the crucible. oven. However, each reflow oven has a certain dispersion, inducing a dispersion of the sizes of these primers and, consequently, disparities in the lifetimes of the products obtained.

There is a need to obtain low alloy steel parts with improved service lives.

There is a need to obtain low alloy steel parts with reduced disparities in mechanical properties.

There is a need for low alloy steel fabrication processes to reduce the impact of reflow oven instabilities.

There is still a need for new processes for manufacturing low alloy steel parts.

Object and summary of the invention

1. a) fusion de tout ou partie d'une électrode par un procédé de refusion à l'arc sous vide, l'électrode comportant, avant fusion, du fer et du carbone, la partie fondue de l'électrode étant recueillie dans un creuset et formant ainsi au sein du creuset un bain fondu, et
2. b) solidification du bain fondu par échange thermique entre le bain fondu et un fluide de refroidissement, l'échange thermique réalisé permettant d'imposer une vitesse moyenne de solidification durant l'étape b) inférieure ou égale à 45 µm/s et d'obtenir un lingot d'acier faiblement allié.

For this purpose, the invention proposes, in a first aspect, a method for manufacturing a low alloy steel ingot comprising the following steps:

1. a) melting all or part of an electrode by a vacuum arc remelting process, the electrode comprising, before melting, iron and carbon, the molten portion of the electrode being collected in a crucible and thus forming in the crucible a molten bath, and
2. b) solidification of the melt by heat exchange between the melt and a cooling fluid, the heat exchange achieved to impose a mean solidification rate during step b) less than or equal to 45 microns / sec and obtain a low alloy steel ingot.

"Low-alloy steel" means a steel for which no alloying element is present in a mass content greater than 5.00%. In other words, in a low-alloy steel, each of the chemical elements, other than iron, is present in a mass content less than or equal to 5.00%.

For the purpose of the invention, the "molten bath" comprises the liquid part obtained after melting the electrode and the pasty part situated between the liquid part and the ingot obtained.

By "average speed of solidification during step b)" is meant the ratio (distance traveled by the solidification front during step b)) / (duration of step b)). The solidification front corresponds to the boundary between the ingot obtained and the pasty zone of the molten bath. The distance traveled by the solidification front is equal to the distance, measured along the longitudinal axis of the crucible, which is traversed by the bottom of the molten bath (ie by the point of the molten bath closest to the bottom of the crucible and situated at the contact of the solidification front). The duration of step b) is the time during which solidification of the melt is performed.

The invention advantageously makes it possible to obtain low alloy steel ingots with reduced inclusions of size and alignment.

The invention advantageously makes it possible to obtain low alloy steel ingots having a dispersion of the inclusion population obtained during the reduced production compared to the ingots manufactured by the methods of the state of the art.

The ingots obtained by the process according to the invention advantageously have mechanical properties as well as improved lifetimes compared to ingots made by known processes.

In the invention, a solidification rate of the melt sufficiently low is imposed so that all or part of the inclusions present in the melt "up" faster to the surface of the melt than the solidification front. Thus, in the invention, the average solidification rate is chosen to be less than the flotation rate (i.e. the rate of rise to the surface of the melt) of all or part of the inclusions present in the melt. As a result, the invention advantageously allows the inclusions to float on the surface of the molten bath and to prevent them from being trapped in the ingot obtained.

où K est une constante physique décrivant la constante de l'accélération de la pesanteur et la viscosité dynamique à une température donnée, r est le rayon de l'inclusion et Δ (MV) est la différence de masse volumique entre l'inclusion et le bain fondu.The mechanisms of flotation or decantation of inclusions within the melt can be described by the Stokes equations. For example, the flotation vf velocity of inclusions is given by the equation: $$\mathrm{vF}=\mathrm{K}\mathrm{.}{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">r</figure-callout>}}^{\mathrm{2}}\mathrm{.}\mathrm{\Delta }\left(\mathrm{MV}\right)$$

where K is a physical constant describing the constant of gravitational acceleration and dynamic viscosity at a given temperature, r is the radius of inclusion and Δ (MV) is the density difference between inclusion and melted bath.

Equation 1 shows that small inclusions take longer to rise to the surface than larger inclusions in a radius-to-square ratio. In addition, equation 1 shows that an increase in the density difference increases the flotation rate.

où ΔD est l'accroissement de distance par rapport au fond du creuset, mesuré le long de l'axe longitudinal du creuset, entre la position initiale de l'inclusion et la position où l'inclusion est située à la surface du bain fondu.The flotation time _flotation of an inclusion, corresponding to the time required for an inclusion to rise to the surface of the melt, can be estimated by the following equation: $${\mathrm{t}}_{\mathrm{floatation}}=\mathrm{Dd}/\mathrm{vF}$$

where ΔD is the distance increase from the bottom of the crucible, measured along the longitudinal axis of the crucible, between the initial position of the inclusion and the position where the inclusion is located on the surface of the melt.

Due to the control of the solidification rate implemented in step b), the flotation time of all or part of the inclusions present in the melt is less than the duration of step b).

In an exemplary embodiment, the average rate of solidification imposed during step b) may advantageously be less than the flotation rate of all or part of the nonmetallic inclusions present in the melt.

The average rate of solidification imposed during step b) may advantageously be less than the flotation rate of inclusions present in the melt and able to crystallize in the melt but not in the ingot obtained. In particular, the average rate of solidification imposed during step b) may advantageously be less than the flotation rate of alumina Al ₂ O ₃ and / or the flotation rate of lime aluminates of formula [(Al ₂ O ₃ ) _x (CaO) _y ] present in the melt.

For inclusions of aluminas or lime aluminates that have close densities, the flotation rates and, therefore, the flotation times can be similar. For inclusion rays of 2 μm, the flotation times may, for example, be less than 60 minutes.

Thus, the duration of step b) may, for example, be greater than or equal to 60 minutes, for example 100 minutes.

In an exemplary embodiment, the method according to the invention may further comprise after step b) a step c) of homogenizing the alloying elements present in the ingot obtained. Step c) may, for example, comprise a heat treatment of the ingot obtained by submitting the ingot at a temperature below its melting temperature.

Such a step is advantageous insofar as it makes it possible to diffuse the alloying elements from a zone heavily loaded with alloying elements to a zone weakly loaded with alloying elements.

In an exemplary embodiment, the method according to the invention may, furthermore, comprise, after step c), a step d) of shaping to hot ingot. Step d) may make it possible to obtain from the ingot a semi-finished product, for example in the form of a bar or a sheet.

The average speed of solidification imposed during step b) may preferably be less than or equal to 40 μm / s, preferably 35 μm / s, preferably 30 μm / s, particularly preferably 25 μm / s. .

It is particularly advantageous to impose such average rates of solidification during step b). Indeed, in the process of vacuum arc reflow, the reflow oven can have instabilities which can lead to the return to the bottom of the molten bath of rafts of inclusions. The presence of such instabilities may lead to an increase in the time required for the inclusions to rise to the surface of the melt and remain there. Operating at such average rates of solidification advantageously makes it possible to further increase the difference between the time required to solidify the melt and the time required for an inclusion to rise to the surface. Consequently, the negative impact of the instabilities of the reflow oven is advantageously reduced because the solidification is slower, thus leaving time for the inclusions possibly returned to the bottom of the molten bath to rise to the surface.

Thus, the flotation time of all or part of the inclusions present in the melt may advantageously be less than or equal to two-thirds, or even half, of the duration of step b).

The diameter of the electrode before fusion can, for example, be between 650 mm and 1200 mm.

By "electrode diameter" is meant the largest dimension of the electrode measured perpendicular to the longitudinal axis of the electrode.

Preferably, the electrode may, before melting, be of cylindrical shape.

The use of a cylindrical electrode advantageously makes it possible, after merging, to obtain an upward movement of the inclusions within the molten bath essentially directed along the longitudinal axis of the crucible. This advantageously allows to further limit the amount of inclusions trapped in the ingot obtained after solidification due to a rise of inclusions more direct to the surface of the molten bath.

The invention is not limited to the implementation of a cylindrical electrode before melting. Indeed, the electrode may, alternatively, be of conical or parallelepipedal shape before melting.

In an exemplary embodiment, the diameter of the molten bath may, for example, be between 650 mm and 1200 mm. The diameter of the molten bath can be between 700 mm and 950 mm. The diameter of the molten bath can be between 650 mm and 950 mm. The diameter of the molten bath can be between 700 mm and 1200 mm.

Unless otherwise stated, the diameter of the molten bath corresponds to its largest dimension measured perpendicularly to the longitudinal axis of the crucible. For example, for a crucible having a cylinder shape, the diameter of the molten bath is measured perpendicular to the height of the cylinder. The diameter of the melt is measured without taking into account the thickness of the side wall of the crucible.

Preferably, the average speed of solidification imposed during step b) may be greater than or equal to 9 μm / s and more preferably greater than or equal to 14 μm / s.

The use of such values of solidification rates is advantageous because it makes it possible to obtain particularly little micro-segregations of solidification during step b). This advantageously makes it possible to further improve the mechanical properties of the ingot obtained, such as the toughness or the isotropy of the static mechanical properties.

Moreover, the more the ingot comprises micro-segregations, the longer the duration of the step c) of homogenization can be important.

Consequently, the use of such values of solidification rates also advantageously makes it possible to obtain a better industrial efficiency for the process, for example by avoiding homogenization times greater than 200 hours, or even avoiding even have homogenization times greater than 100 hours.

The use of these solidification rates thus advantageously makes it possible to reduce the cost of the process and to improve productivity.

The person skilled in the art will know, thanks to his general knowledge, adapt the cooling carried out in order to obtain the desired rates of solidification during step b).

The cooling fluid may, for example, be a coolant. In an exemplary embodiment, the combination of a cooling liquid and a cooling gas can be used during step b) to achieve the heat exchange. In this case, the cooling gas may be chosen from: helium, argon or nitrogen.

The cooling liquid may, for example, be selected from: water, a polymer fluid or molten sodium. The water used as coolant may optionally include additives such as anti-scale and / or anti-bacterial additives.

The cooling fluid may, for example, be in motion relative to the crucible during all or part of step b). The circulation velocity of the cooling fluid carrying out the heat exchange may, for example, be greater than or equal to 1000 l / min, preferably between 2000 and 6000 l / min, during all or part of step b).

For example, the cooling fluid may, before the start of the heat exchange, be at a temperature of less than or equal to 80 ° C.

The crucible may, for example, comprise, in particular consist of, a heat-transfer metal. The crucible may, for example, comprise, in particular consist of, copper or brass.

In an exemplary embodiment, the carbon may be present in the electrode before melting in a mass content of between 0.09% and 1.00%.

In an exemplary embodiment, the electrode may further comprise before chromium melting in a mass content of between 0.10% and 5.50%.

In an exemplary embodiment, the electrode may further comprise before molybdenum melting at a mass content less than or equal to 5.00%, for example between 0.05% and 5.00%.

The use of these elements at such mass contents advantageously gives the ingot obtained satisfactory mechanical properties.

• ▪ du carbone en une teneur massique comprise entre 0,09% et 1,00%,
• ▪ du manganèse en une teneur massique inférieure ou égale à 6,00%, par exemple comprise entre 0,010% et 6,00%,
• ▪ du nickel en une teneur massique inférieure ou égale à 5,50%, par exemple comprise entre 0,010% et 5,50%,
• ▪ du silicium en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%,
• ▪ du chrome en une teneur massique comprise entre 0,10% et 5,50%,
• ▪ du molybdène en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,05% et 5,00%,
• ▪ du vanadium en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,005% et 5,00%, et
• ▪ optionnellement un ou plusieurs autres éléments d'alliage, l'ensemble des autres éléments d'alliage étant présent en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%.

In an exemplary embodiment, the electrode may comprise, before melting, iron as well as:

• ▪ carbon in a mass content of between 0.09% and 1.00%,
• ▪ manganese in a mass content of less than or equal to 6.00%, for example between 0.010% and 6.00%,
• Nickel at a mass content of less than or equal to 5.50%, for example between 0.010% and 5.50%,
• Silicon at a mass content of less than or equal to 3.00%, for example between 0.010% and 3.00%,
• ▪ chromium in a mass content of between 0.10% and 5.50%,
• Molybdenum in a mass content of less than or equal to 5.00%, for example between 0.05% and 5.00%,
• Vanadium in a mass content of less than or equal to 5.00%, for example between 0.005% and 5.00%, and
• Optionally one or more other alloying elements, all the other alloying elements being present in a mass content of less than or equal to 3.00%, for example between 0.010% and 3.00%.

• ▪ carbone en une teneur massique comprise entre 0,09% et 1,00%,
• ▪ manganèse en une teneur massique inférieure ou égale à 6,00%, par exemple comprise entre 0,010% et 6,00%,
• ▪ nickel en une teneur massique inférieure ou égale à 5,50%, par exemple comprise entre 0,010% et 5,50%,
• ▪ silicium en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%,
• ▪ chrome en une teneur massique comprise entre 0,10% et 5,50%,
• ▪ molybdène en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,05% et 5,00%,
• ▪ vanadium en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,005% et 5,00%,
• ▪ optionnellement un ou plusieurs autres éléments d'alliage, l'ensemble des autres éléments d'alliage étant présent en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%, et
• ▪ fer en complément à 100,00%.

In an exemplary embodiment, the electrode may have, before melting, the following composition:

• ▪ carbon in a mass content of between 0.09% and 1.00%,
• Manganese in a mass content lower than or equal to 6.00%, for example between 0.010% and 6.00%,
• Nickel in a mass content less than or equal to 5.50%, for example between 0.010% and 5.50%,
• Silicon in a mass content of less than or equal to 3.00%, for example between 0.010% and 3.00%,
• ▪ chromium in a mass content of between 0.10% and 5.50%,
• Molybdenum in a mass content of less than or equal to 5.00%, for example between 0.05% and 5.00%,
• Vanadium in a mass content of less than or equal to 5.00%, for example between 0.005% and 5.00%,
• Optionally one or more other alloying elements, all the other alloying elements being present in one mass content less than or equal to 3.00%, for example between 0.010% and 3.00%, and
• ▪ iron in addition to 100.00%.

• le nombre de champs comportant des inclusions de type D de niveau de sévérité égal à 0,5 est inférieur à 5,
• aucun champ comportant des inclusions de type D de niveau de sévérité égal à 1 n'est obtenu, et
• aucun champ comportant des inclusions de type B de niveau de sévérité égal à 0,5 n'est obtenu.

The present invention also relates to a low-alloy steel part comprising iron and carbon, the part extending along a longitudinal axis, the part being such that, when it is evaluated according to method D of the ASTME standard 45-10, the following results are obtained in analysis along the longitudinal axis:

• the number of fields with type D inclusions with a severity level of 0.5 is less than 5,
• no field with D type inclusions of severity level equal to 1 is obtained, and
• no field with type B inclusions with a severity level of 0.5 is obtained.

Unless otherwise indicated, both thin inclusions ("thin") and thick inclusions ("heavy") are counted.

Such a part according to the invention advantageously has improved fatigue strength compared to the parts of the state of the art. In addition, when a plurality of these parts is analyzed, it is found that the dispersion of the results obtained in terms of lifetime is lower than that presented by a sample of parts produced by known methods.

The part can be obtained by implementing a method as described above. The part may include non-metallic inclusions. The part may correspond to the ingot obtained at the end of step b) or possibly of step c) described above. The part may also correspond to the half-product obtained after implementation of step d) described above.

• le nombre de champs total comportant des inclusions de type D de niveau de sévérité égal à 0,5 est inférieur ou égal à 15, de préférence à 10.

In an exemplary embodiment, when the part is evaluated according to method D of ASTME 45-10, the following result can be obtained by adding the three measurement results obtained along the longitudinal axis of the part and along two axes perpendicular to this longitudinal axis:

• the total number of fields comprising type D inclusions with a severity level equal to 0.5 is less than or equal to 15, preferably greater than 10.

In an exemplary embodiment, the carbon may be present in the part in a mass content of between 0.09% and 1.00%.

In an exemplary embodiment, the part may further comprise chromium in a mass content of between 0.05% and 5.00%.

In an exemplary embodiment, the part may further comprise molybdenum in a mass content less than or equal to 5.00%, for example between 0.05% and 5.00%.

• du carbone en une teneur massique comprise entre 0,09% et 1,00%,
• du manganèse en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,005% et 5,00%,
• du nickel en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,010% et 5,00%,
• du silicium en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%,
• du chrome en une teneur massique comprise entre 0,05% et 5,00%,
• du molybdène en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,05% et 5,00%,
• du vanadium en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,005% et 5,00%, et
• optionnellement un ou plusieurs autres éléments d'alliage, l'ensemble des autres éléments d'alliage étant présent en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%.

In an exemplary embodiment, the part may comprise iron as well as:

• carbon in a mass content of between 0.09% and 1.00%,
• manganese in a mass content less than or equal to 5.00%, for example between 0.005% and 5.00%,
• nickel in a mass content less than or equal to 5.00%, for example between 0.010% and 5.00%,
• silicon in a mass content less than or equal to 3.00%, for example between 0.010% and 3.00%,
• chromium in a mass content of between 0.05% and 5.00%,
• molybdenum in a mass content of less than or equal to 5.00%, for example between 0.05% and 5.00%,
• vanadium in a mass content of less than or equal to 5.00%, for example between 0.005% and 5.00%, and
• optionally one or more other alloying elements, all the other alloying elements being present in a mass content of less than or equal to 3.00%, for example between 0.010% and 3.00%.

• carbone en une teneur massique comprise entre 0,09% et 1,00%,
• manganèse en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,005% et 5,00%,
• nickel en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,010% et 5,00%,
• silicium en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%,
• chrome en une teneur massique comprise entre 0,05% et 5,00%,
• molybdène en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,05% et 5,00%,
• vanadium en une teneur massique inférieure ou égale à 5,00%, par exemple comprise entre 0,005% et 5,00%,
• optionnellement un ou plusieurs autres éléments d'alliage, l'ensemble des autres éléments d'alliage étant présent en une teneur massique inférieure ou égale à 3,00%, par exemple comprise entre 0,010% et 3,00%, et
• fer en complément à 100,00%.

In an exemplary embodiment, the part may have the following composition:

• carbon in a mass content of between 0.09% and 1.00%,
• manganese in a mass content less than or equal to 5.00%, for example between 0.005% and 5.00%,
• nickel in a mass content less than or equal to 5.00%, for example between 0.010% and 5.00%,
• silicon in a mass content less than or equal to 3.00%, for example between 0.010% and 3.00%,
• chromium in a mass content of between 0.05% and 5.00%,
• molybdenum in a mass content of less than or equal to 5.00%, for example between 0.05% and 5.00%,
• vanadium in a mass content of less than or equal to 5.00%, for example between 0.005% and 5.00%,
• optionally one or more other alloying elements, all of the other alloying elements being present in a mass content of less than or equal to 3.00%, for example between 0.010% and 3.00%, and
• iron in addition to 100.00%.

The part according to the invention may, for example, comprise different alloying elements in the proportions indicated in Table 1 given below. <u> Table 1 </ u> % C % Mn %Or %Yes % Cr % Mo % V Room 1 0.13 0.2 3.4 0.2 4.1 4.3 1.2 Room 2 0.15 0.5 3.2 0.3 1.0 0.3 <0.1 Exhibit 3 0.20 0.5 3.2 0.3 1.0 0.3 <0.1 Room 4 0.32 0.7 <0.4 0.3 3.3 2.0 0.3 Exhibit 5 0.35 0.5 3.9 0.3 1.8 0.4 <0.1 Room 6 0.40 <0.5 1.8 <0.5 0.8 0.3 <0.1 Exhibit 7 0.40 0.5 <0.4 0.2 3.2 1.0 0.2 Exhibit 8 0.40 0.3 <0.4 0.9 5.0 1.3 0.5 Exhibit 9 0.41 0.8 1.8 1.7 0.8 0.4 0.08 Exhibit 10 0.81 0.2 <0.4 0.2 4.1 4.3 1.0

Advantageously, the part may have a cylindrical shape. Alternatively, the part may, for example, have a conical or parallelepiped shape.

According to yet another of its aspects, the present invention relates to a low-alloy steel piece comprising iron and carbon and obtainable by implementing a method as described above.

Such a piece may, for example, have the same constituents present at the same mass contents as for the piece described above.

Brief description of the drawings

• les figures 1 et 2 représentent, de manière schématique et partielle la mise en oeuvre d'un procédé selon l'invention.

Other characteristics and advantages of the invention will emerge from the following description of particular embodiments of the invention, given by way of non-limiting examples, with reference to the appended drawings, in which:

• the figures 1 and 2 represent schematically and partially the implementation of a method according to the invention.

Detailed description of embodiments

As illustrated in figure 1 , the electrode 1 intended to be melted is present in the interior volume delimited by the crucible 10. The electrode 1 may, previously, have been developed by any means known as air preparation or elaboration by vacuum induction. The electrode 1 may, as shown, have a cylindrical shape before melting. As explained above, it is not beyond the scope of the present invention if an electrode having another form before fusion is implemented.

The crucible 10 is, for example, copper. The crucible 10 extends along a longitudinal axis X. A generator G imposes a potential difference between the crucible 10 and the electrode 1. A first terminal of the generator G can, as shown, be connected to the electrode 1 and a second terminal of the generator G may, as shown, be connected to the bottom 11 of the crucible 10. The potential difference imposed between the crucible 10 and the electrode 1 by the generator G makes it possible to create electric arcs 3 in the space 2 in which there is a void. These electric arcs 3 make it possible to melt the electrode 1 and to carry out step a).

The molten portion of the electrode 1 is collected in the crucible 10 and thus forms a melt 20. The melt 20 comprises a liquid portion 21 located on the electrode side 1 and a pasty portion 22 located between the liquid portion 21 and the ingot 30. The ingot 30 is obtained by cooling the molten portion of the electrode. The solidification front 34 separates the resulting ingot from the melt 20 and propagates during step b) to the free surface of the melt bath 20. Water circulates around the crucible 10 to continuously cool the crucible 10 as well as the melt 20 and to ensure the solidification of the latter.

In addition, as shown in figure 1 , a cooling channel 13 is present within the side wall 12 and the bottom wall 14 of the crucible 10. A coolant can circulate within the cooling channel 13 to also participate in the solidification of the melt 20 .

As shown, the ingot 30 is present, during step b), between the melt 20 and the bottom 11 of the crucible 10 as well as between the melt 20 and the side wall 12 of the crucible 10. In addition, at least a portion of the peripheral surface 31 of the ingot 30 may not be in contact with the side wall 12 of the crucible 10 and be separated from the latter by a space 33. In some cases, in this space 33, a gas (for example He , Ar, N ₂ ) can be injected to improve cooling.

At the end of step b), the resulting ingot 30 may have a cylindrical shape.

The figure 2 provides a simplified representation of certain details of the method according to the invention. The electrode 1 comprises before fusion inclusions 40. These inclusions 40 may be non-metallic inclusions. As shown, the end 1a of the electrode 1 is, during melting, melted by the energy of the electric arcs 3. Molten electrode drops are produced which will be collected by the crucible 10. The crucible 10 is , as explained above, cooled with water. The melt 20 has a diameter d equal to the internal diameter of the crucible 10.

The melt 20 may, as shown, have during all or part of step b) a hemispherical shape. Such a shape can for example be obtained when using a crucible 10 of cylindrical shape. The melt 20 may take other forms, for example a semi-quasi-ovoid shape. Such a shape can for example be obtained when using a parallelepiped shaped crucible.

The distance e between the free surface of the melt 20 and the electrode 1 is advantageously kept constant during step b). This distance e can be driven either in voltage (V) or in pulses 5. In the illustrated example, the electrode 1 is during step b) moved along the longitudinal axis X of the crucible 10 in order to maintain the distance e constant.

During the melting of the electrode 1, the drops 5 fall and are collected by the crucible 10. The drops 5 may include inclusions 40 which were initially present in the electrode 1. Once introduced into the melt 20, the inclusions 40 may be entrained towards the bottom 26 of the melt 20 (ie the point of the melt 20 closest to the bottom 11 of the crucible and in contact with the solidification front 34).

From the thermal point of view, the melt 20 has an axial portion having a temperature greater than that of its peripheral portion. This leads to a natural convection, corresponding to the engaged forces of buoyancy, which starts from the bottom 26 of the melt 20, joins the free surface 25 of the melt 20 and goes to the edge 27 of the molten bath 20. This convection is shown schematically in FIG. the figure 2 by the arrows 28a and 28b.

During reflow, the inclusions 40 either solid or liquids of lower density than that of the melt 20 will tend to rise to the surface 25 with some velocity by flotation mechanisms as explained above.

On the free surface 25 of the melt 20 are present aggregates 41 formed of agglomerated inclusions 40. These aggregates 41 are driven towards the periphery of the ingot 30 where they will be fixed.

As represented in figure 2 the solidification front 34 propagates from the bottom 11 of the crucible 10 to the free surface 25 of the molten bath. The solidification front 34 propagates during step b) along the longitudinal axis X of the crucible 10 as shown by the arrow 35. As shown, the solidification front 34 may retain its shape during all or part of the step b). The average rate of rise of the solidification front 34 is controlled so as to be less than the rate of rise to the surface of all or part of the inclusions 40 as explained above. We have shown figure 2 the successive positions P ₁ and P ₂ occupied by the bottom 26 of the melt 20. The distance d ₁ traversed by the bottom 26 of the melt is measured along the longitudinal axis X of the crucible 10.

Examples Example 1

An electrode having the following chemical composition: C 0.42% - Mn 0.82% - Ni 1.80% - Si 1.70% - Cr 0.80% - Mo 0.40% - V 0.08% and Fe in addition (the contents are mass) was melted by a process of vacuum arc reflow. The diameter of the electrode before fusion was 920mm.

• tension appliquée : 25 Volts,
• intensité appliquée : 9 kA, et
• impulsion : 250 gouttes coupe-circuit d'électrode fondue produites par minute.

The conditions applied during vacuum arc reflow are as follows:

• applied voltage: 25 volts,
• applied intensity: 9 kA, and
• pulse: 250 drops melted electrode cutoff produced per minute.

These conditions make it possible to obtain a flow rate of molten electrode drops equal to 9.5 kg / minute.

The molten electrode drops are collected in a crucible with a diameter of 975 mm and form a molten bath in the copper crucible.

The melted bath is then solidified by heat exchange between the molten bath and water circulating at 3000 l / minute with a temperature thermostated at 38 ° C. at the inlet and a continuous injection of He at 20mbar.

The heat exchange achieved makes it possible to impose a mean solidification rate during step b) equal to 24 μm / s.

After solidification, a low alloy steel ingot having the following chemical composition is obtained: C 0.41% - Mn 0.80% - Ni 1.80% - Si 1.70% - Cr 0.80% - Mo 0 , 40% - V 0.08% and Fe in addition (the contents are by mass).

The results obtained in terms of inclusion cleanliness according to method D of the ASTME 45-10 standard are given below in number of fields along the longitudinal axis: Level of severity cut AT B C D 0.5 end 0 0 0 3 0.5 thick 0 0 0 1 1 End 0 0 0 0 1 thick 0 0 0 0 1.5 End 0 0 0 0 1.5 thick 0 0 0 0

The sum of the fields with inclusions of type D in the 3 directions is 7.

Example 2 (comparative)

An electrode having the following chemical composition: C 0.42% - Mn 0.83% - Ni 1.81% - Si 1.72% - Cr 0.85% - Mo 0.38% - V 0.09% and Fe in addition (the proportions are mass) was melted by a vacuum arc remelting process.

The diameter of the electrode before fusion was 550mm.

• tension appliquée : 25 Volts,
• intensité appliquée : 11 kA, et
• impulsion : 330 gouttes coupe-circuit d'électrode fondue produites par minute.

The conditions applied during vacuum arc reflow are as follows:

• applied voltage: 25 volts,
• applied intensity: 11 kA, and
• pulse: 330 drops melted electrode cutoff produced per minute.

These conditions make it possible to obtain a flow rate of molten electrode drops greater than or equal to 12 kg / minute +/- 0.6 kg / minute.

The molten electrode drops are collected in a crucible with a diameter of 600 mm and form a molten bath in the copper crucible.

The melted bath is then solidified by heat exchange between the molten bath and water flowing at 1500 l / min with a temperature controlled at 38 ° C. at the inlet and without gas injection.

The heat exchange achieved makes it possible to impose a mean solidification rate during step b) equal to 49 μm / s.

After solidification, a low alloy steel ingot having the following chemical composition is obtained: C 0.41% - Mn 0.81% - Ni 1.82% - Si 1.73% - Cr 0.85% - Mo 0 , 38% - V 0.09% and Fe in addition (the contents are by mass).

The results obtained in terms of inclusion cleanliness according to method D of the ASTME 45-10 standard are given below in number of fields along the longitudinal axis: Level of severity cut AT B C D 0.5 end 0 5 0 28 0.5 thick 0 1 0 15 1 End 0 1 0 2 1 thick 0 0 0 0 1.5 End 0 0 0 0 1.5 thick 0 0 0 0

The sum of the fields comprising inclusions of type B or D in the 3 directions is 87. Such an ingot has mechanical properties significantly lower than those of the ingot according to the invention.

The expression "containing / containing a" must be understood as "containing / containing at least one".

The expression "understood between ... and ..." or "from ... to ..." must be understood as including the boundaries.

Claims (6)

A piece (30) made of low-alloy steel having iron and carbon, the workpiece (30) extending along a longitudinal axis, the workpiece (30) being such that when it is evaluated according to the method D of ASTM 45-10, the following results are obtained in analysis along the longitudinal axis: • the number of fields with type D inclusions with a severity level of 0.5 is less than 5, • no field with inclusions of type D of level of severity equal to 1 is obtained, and • no field with type B inclusions with a severity level of 0.5 is obtained. Part (30) according to claim 1, characterized in that when it is evaluated according to method D of ASTME 45-10, the following result is obtained by adding the three measurement results obtained along the longitudinal axis of the part and along the two axes perpendicular to this longitudinal axis: • the total number of fields with D type inclusions with a severity level of 0.5 is less than or equal to 15. Part (30) according to any one of claims 1 and 2, characterized in that the carbon is present in a mass content of between 0.09% and 1.00%. Part (30) according to any one of claims 1 to 3, characterized in that it further comprises chromium in a mass content of between 0.05% and 5.00%. Part (30) according to any one of claims 1 to 4, characterized in that it further comprises molybdenum in a mass content less than or equal to 5.00%. Part (30) according to any one of claims 1 to 5, characterized in that it comprises iron as well as: • carbon in a mass content of between 0.09% and 1.00%, • manganese in a mass content of less than or equal to 5.00%, • nickel in a mass content of less than or equal to 5.00%, Silicon at a mass content of less than or equal to 3.00%, Chromium in a mass content of between 0.05% and 5.00%, • molybdenum in a mass content of less than or equal to 5.00%, and Vanadium at a mass content of less than or equal to 5.00%.

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