EP2488670B1 - Degassing of martensitic stainless steel before remelting beneath a layer of slag - Google Patents

Description

The present invention relates to a method of manufacturing a stainless martensitic steel comprising a slag remelting step of an ingot of this steel and a cooling step of this ingot.

In the present invention, the percentages of composition are percentages by weight unless otherwise specified.

A stainless martensitic steel is a steel whose chromium content is greater than 10.5%, and whose structure is essentially martensitic.

It is important that the fatigue strength of such a steel is the highest possible, so that the life of parts made from this steel is maximum.

For this, it seeks to increase the inclusion cleanliness of the steel, that is to say, to reduce the amount of undesirable inclusions (certain alloy phases, oxides, carbides, intermetallic compounds) present in the steel. Indeed, these inclusions act as crack initiation sites which lead, under cyclic stress, to premature failure of the steel. Experimentally, a considerable dispersion of the results of fatigue tests is observed on test specimens of this steel, that is to say that for each level of fatigue strain imposed strain, the service life (corresponding to the number of cycles leading to the rupture of a fatigue test piece in this steel) varies over a wide range. Inclusions are responsible for the minimum statistical values of steel fatigue life (low values in the range).

To reduce this dispersion of the fatigue strength, that is to say up those low values, and also to increase its average value in resistance to fatigue, it is necessary to increase the inclusion cleanliness of the steel. We know the technique of slag remelting, or ESR (Electro Slag Refusion). For example, EP0577997A1 discloses such a method. In this technique, the steel ingot is placed in a crucible into which a slag (mineral mixture, for example lime, fluoride, magnesia, alumina, spath) has been poured so that the lower end of the ingot quenches in the slag. . Then we do pass an electric current into the ingot, which serves as an electrode. This current is high enough to heat and liquefy the slag and to heat the lower end of the steel electrode. The lower end of this electrode being in contact with the slag, melts and passes through the slag in the form of fine droplets, to solidify below the layer of supernatant slag, into a new ingot that grows gradually. The slag acts, inter alia, as a filter which extracts the inclusions from the steel droplets, so that the steel of this new ingot located below the slag layer contains fewer inclusions than the initial ingot (electrode). . This operation is carried out at atmospheric pressure and air.

Although the ESR technique makes it possible to reduce the dispersion of the fatigue strength in the case of stainless martensitic steels by elimination of inclusions, this dispersion in terms of lifetime of the parts still remains too important.

Non-destructive ultrasonic testing, performed by the inventors, showed that these steels practically had no known hydrogen defects (flakes).

The dispersion of the fatigue strength results, specifically the low values of the range of results, is therefore due to another undesirable mechanism of premature initiation of cracks in the steel, which leads to its premature failure in fatigue.

The present invention aims to provide a manufacturing method that allows to raise these low values, and thus reduce the dispersion of the fatigue strength of stainless martensitic steels, and also to increase its average value in resistance to fatigue.

This object is achieved by virtue of the fact that the ingot, before the slag remelting step, undergoes degassing under vacuum for a time sufficient to reach a hydrogen content in the ingot of less than 3 ppm.

Thanks to these provisions, it reduces the formation of microscopically sized gas phases (not detectable by industrial non-destructive testing means) and consist of light elements within the steel, and thus avoids the premature crack initiation from these microscopic phases which leads to the premature failure of the steel in fatigue.

• la figure 1 compare des courbes de durée de vie en fatigue pour un acier selon l'invention et un acier selon l'art antérieur,
• la figure 2 montre une courbe de sollicitation en fatigue,
• la figure 3 est un schéma illustrant les dendrites et les régions interdendritiques,
• la figure 4 est une photographie prise au microscope électronique d'une surface de fracture après fatigue, montrant la phase gazeuse ayant initié cette fracture.

The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings in which:

• the figure 1 compares fatigue life curves for a steel according to the invention and a steel according to the prior art,
• the figure 2 shows a fatigue stress curve,
• the figure 3 is a diagram illustrating dendrites and interdendritic regions,
• the figure 4 is a photograph taken under an electron microscope of a fracture surface after fatigue, showing the gas phase that initiated this fracture.

During the ESR process, the steel that has been filtered by the slag cools and gradually solidifies to form an ingot. This solidification takes place during cooling and is carried out by growth of dendrites, as illustrated in FIG. figure 3 . In agreement with the phase diagram of the stainless martensitic steels, the dendrites 10, corresponding to the first solidified grains are by definition richer in alphagenes elements while the interdendritic regions 20 are richer in gamma elements (application of the known rule of the segments on the phase diagram). An alphagene element is an element that favors a ferritic type structure (structures that are more stable at low temperature: bainite, ferrite-pearlite, martensite). A gamma element is an element that promotes an austenitic structure (stable structure at high temperature). There is therefore segregation between dendrites 10 and interdendritic regions 20.

This local segregation of chemical composition is then preserved throughout the manufacturing process, even during subsequent hot forming operations. This segregation is therefore found on both the solid ingot of solidification and on the subsequently deformed ingot.

Indeed, once the material has solidified, the dendrites 10 first turn into ferritic structures during cooling, while the interdendritic regions 20 are subsequently converted, in whole or in part, to lower temperatures, and therefore retain longer a austenitic structure.

During this cooling in the solid state, locally, there is a structural heterogeneity with coexistence of austenitic microstructure and ferritic type. Under these conditions, the light elements (H, N, O) are more soluble in the austenite than in the ferritic structures, so tend to concentrate in the interdendritic regions 20. This concentration is increased by the higher content of elements. gammagens in interdendritic regions. At temperatures below 300 ° C, the light elements only diffuse at extremely low speeds and remain trapped in their region. After the interdentitic zones have been converted into a total or partial ferritic structure, the solubility limit of these gaseous phases is reached under certain concentration conditions and these gaseous phases form pockets of gas (or a substance in a physical state allowing great malleability and incompressibility).

During the cooling phase, the more the ingot at the outlet of the ESR (or the subsequently deformed ingot) has a large diameter (or, more generally, the larger the dimension of the ingot is large) or the cooling rate of the ingot is low. , plus the light elements are able to diffuse dendrites towards the interdendritic regions and to concentrate there during the period of cohabitation of the ferritic and austenitic structures. The risk that the solubility of these light elements is exceeded locally in the interdendritic regions is accentuated. When the concentration in light elements exceeds this solubility, it appears then in the steel microscopic gas pockets containing these light elements.

In addition, during the end of cooling, the austenite of the interdendritic regions tends to locally transform into martensite when the temperature of the steel falls below the Martensitic transformation temperature Ms, which is above room temperature. . However, martensite has a lower solubility threshold in light elements than austenite. There is therefore more microscopic gaseous phase within the steel during this martensitic transformation.

During subsequent deformations that the steel undergoes during hot forming (eg forging), these phases flatten in sheet form.

Under fatigue stress, these leaves act as stress concentration sites, which are responsible for premature crack initiation by reducing the energy required to initiate cracks. There is thus a premature failure of the steel, which corresponds to the low values of the fatigue resistance results.

These conclusions are corroborated by the observations of the inventors, such as the electron micrograph of the figure 4 the watch.

In this photograph of a fracture surface of a stainless steel martensitic, there is a substantially globular zone P from which radiates fissures F. This zone P is the imprint of the gaseous phase consisting of the light elements, and which is at the origin of the formation of these fissures F which, by propagating and agglomerating, created a zone of macroscopic fracture.

The inventors have carried out tests on stainless martensitic steels, and have found that when, before the slag remelting, such a steel in the liquid state undergoes a vacuum degassing operation for a time sufficient to reach a desired in H (hydrogen) in this ingot less than 3 ppm by weight, then on the one hand this content of H (hydrogen) is insufficient for a recombination between H and O (oxygen) and N (Nitrogen) in the gaseous phases that may form after the slag remelting of this steel.

On the other hand, this reduced content of gaseous elements remains lower than that which would lead to a solubility exceeding of these gaseous phases even in martensite after concentration in the austenitic structures coexisting with the ferritic structures. This makes it possible to keep the concentration of gamma-elements in the interdendritic regions and the concentration of alpha-gene elements in the dendrites substantially constant. The risk of unwanted gaseous phases in the steel is reduced.

Preferably, the slag is dehydrated before use in the ESR crucible. Indeed, it is possible that the H concentration in the steel ingot from ESR slag remelting is greater than the H concentration in this ingot before its slag remelting. In this case, hydrogen can pass from slag to ingot during the ESR process. By dehydrating the slag beforehand, the amount of hydrogen present in the slag is minimized, and thus the amount of hydrogen that could pass from slag to ingot during the ESR process is minimized.

Preferably, the ESR liquid metal ingot is degassed under vacuum for a time sufficient to reach a hydrogen content in the ingot after the slag remelting step of less than 3 ppm.

The vacuum degassing process of an alloy is known, the description below is therefore brief. It consists in placing the still liquid ingot in an enclosure in which at least the primary vacuum is made. Alternatively, such a degassing under vacuum can be performed by dipping into the liquid steel, which is contained in a container, a duct connected to a pocket in which one has evacuated. The steel is sucked into this pocket by the vacuum that prevails and then falls into the container through the conduit. The bag may also include an inlet pipe and an outlet pipe which are both immersed in the liquid steel, in which case the steel flows through the pocket by entering through the inlet pipe and out of the pipe. outlet duct.

Upstream of the vacuum degassing process, the steel generally undergoes refining at ambient atmosphere. This refining makes it possible to obtain a fine chemical concentration and to reduce as much as possible in the desired range the content of sulfur and carbon. In the case of martensitic stainless steels, the most economical industrial plant used is Argon Oxygen Decarburization (AOD), which is carried out in an ambient atmosphere. The assembly consisting of this AOD process followed by vacuum degassing as described above, constitutes a process which has the advantage of being cheaper and faster to perform than processes for extracting impurities that take place. in a vacuum chamber, such as VOD (Vacuum-Oxygen-Decarburization).

The inventors have carried out tests on Z12CNDV12 steels produced with the process according to the invention, that is to say with degassing of the ingot carried out according to the above parameters before the ESR, and the results of these tests are presented below.

The composition of the Z12CNDV12 steels is as follows: (DMD0242-20 standard E: C index (0.10 to 0.17%) - Si (<0.30%) - Mn (0.5 to 0.9%) - Cr (11 to 12.5%) - Ni (2 to 3%) - Mo (1.50 to 2.00%) - V (0.25 to 0.40%) - N ₂ (0.010 to 0.050%) - Cu (<0.5%) - S (<0.015%) - P (<0.025%) and satisfying the criterion 4.5 ≤ (Cr - 40.C - 2.Mn - 4.Ni + 6.Si + 4.Mo + 11.V - 30.N) <9.

The figure 1 qualitatively shows the improvements made by the method according to the invention. The value of the number N of rupture cycles necessary to break a steel specimen subjected to a cyclic stress in tension as a function of the pseudo-alternating stress C is obtained experimentally (this is the stress experienced by the test specimen under imposed deformation , according to Sncma DMC0401 standard used for these tests).

Such a cyclic solicitation is schematically represented in figure 2 . The period T represents a cycle. The constraint changes between a maximum value C _max and a minimum value C _min .

By fatigue testing a statistically sufficient number of test pieces, the inventors obtained points N = f (C) from which they drew a mean statistical curve C-N (stress C as a function of the number N of fatigue cycles). The standard deviations on the stresses are then calculated for a given number of cycles.

On the figure 1 the first curve 15 (in fine line) is (schematically) the average curve obtained for a steel produced according to the prior art. This first average curve CN is surrounded by two curves 16 and 14 in dashed fine lines. These curves 16 and 14 are located respectively at a distance of +3 σ ₁ and -3 σ ₁ from the first curve 15, where σ ₁ is the standard deviation of the distribution of the experimental points obtained during these fatigue tests, and ± 3σ ₁ corresponds statistically to a confidence interval of 99.7%. The distance between these two dashed lines 14 and 16 is therefore a measure of the dispersion of the results. Curve 14 is the limiting factor for dimensioning a part.

On the figure 1 the second curve 25 (in thick line) is (schematically) the average curve obtained from the results of fatigue tests carried out on a steel produced according to the invention under a solicitation according to the figure 2 . This second average CN curve is surrounded by two curves 26 and 24 in dashed thick lines, located respectively at a distance of +3 σ ₂ and -3 σ ₂ from the second curve 25, σ ₂ being the standard deviation of the distribution of the experimental points obtained during these fatigue tests. Curve 24 is the limiting factor for dimensioning a part.

It is noted that the second curve 25 is located above the first curve 15, which means that under fatigue stress at a stress level C, the steel test pieces produced according to the invention break on average to a number N of cycles higher than that where the steel test pieces according to the prior art are broken.

In addition, the distance between the two curves 26 and 24 in thick dashed line is smaller than the distance between the two curves 16 and 14 in dashed fine lines, which means that the dispersion in fatigue resistance of the developed steel according to the invention is lower than that of a steel according to the prior art.

The figure 1 illustrates the experimental results summarized in Table 1 below.

Table 1 gives the results for a fatigue load of oligocyclic fatigue according to the figure 2 with a stress C _{min of} zero, at a temperature of 250 ° C, at N = 20,000 cycles, and N = 50,000 cycles. Oligocyclic fatigue means that the bias frequency is of the order of 1 Hz (the frequency being defined as the number of periods T per second). <i> Table 1 </ i> Test conditions in oligocyclic fatigue Steel according to the prior art Steel made according to the invention NOT Temperature C _min Dispersion C _min Dispersion 2 · 10 ⁵ 200 ° C 100% = M 120% M 130% M 44% M 5 · 10 ⁴ 400 ° C 100% = M 143% M 130% M 90% M

It should be noted that for a given value of the number N of cycles, the minimum value of fatigue stress required to break a steel according to the invention is greater than the minimum fatigue stress value M (set at 100%) necessary to break a steel according to art prior. The dispersion (= 6 σ) of the results at this number N of cycles for a steel according to the invention is less than the dispersion of the results for a steel according to the prior art (dispersions expressed as a percentage of the minimum value M).

Advantageously, the carbon content of the stainless martensitic steel is lower than the carbon content below which the steel is hypoeutectoid, for example a content of 0.49%. Indeed, such a low carbon content allows a better diffusion of the alloying elements and a lowering of the temperatures of solution of the primary or noble carbides, which leads to a better homogenization.

For example, martensitic steel, before its slag remelting, was made in the air.

Claims (5)

1. A method of fabricating a stainless martensitic steel, comprising a step of electroslag remelting of an ingot of said steel then a step of cooling said ingot, characterized in that before the electroslag remelting step, the ingot undergoes vacuum degassing in the liquid metal state for a time sufficient to obtain a hydrogen content in said ingot of less than 3 ppm.
2. A method of fabricating a stainless martensitic steel according to claim 1, characterized in that the slag used in said remelting step has been dehydrated in advance.
3. A method of fabricating a stainless martensitic steel according to claim 1 or claim 2, characterized in that said ingot undergoes said vacuum degassing for a time that is sufficient to obtain a hydrogen content in said ingot after said electroslag remelting step of less than 3 ppm.
4. A method of fabricating a stainless martensitic steel according to any one of claims 1 to 3, characterized in that before said vacuum degassing, said ingot undergoes refining at ambient atmosphere.
5. A method of fabricating a stainless martensitic steel according to any one of claims 1 to 4, characterized in that the carbon content of said steel is less than the carbon content below which the steel is hypoeutectoid.

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