Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
  • C22B9/16—Remelting metals
  • C22B9/18—Electroslag remelting

Definitions

• the present invention relates to a method for manufacturing a stainless martensitic steel comprising a slag remelting step of an ingot of this steel and a cooling step of this ingot
• a stainless martensitic steel is a steel whose chromium content is greater than 10.5%, and whose structure is essentially martensitic.
• ESR Electro Slag Refusion
• 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.
• 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 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.
• the ingot from the slag remelting is, before the skin temperature of this ingot is less than the martensitic transformation temperature Ms of the steel, placed in an oven whose initial temperature T 0 is then greater than the end of pearlitic transformation temperature in Arl cooling of said steel, this ingot being subjected in this oven to a homogenization treatment for at least one holding time t after the temperature of the coldest point of the ingot has reached a homogenization temperature T, this holding time t being equal to at least one hour, and the homogenization temperature T varying between about 900 ° C and the burn temperature of the steel.
• FIG. 1 compares fatigue life curves for a steel according to the invention and a steel according to the prior art
• FIG. 2 shows a fatigue stress curve
• FIG. 3 is a diagram illustrating dendrites and interdendritic regions
• FIG. 4 is a photograph taken under the electron microscope of a fracture surface after fatigue, showing the gas phase having initiated this fracture.
• FIG. 5 schematically shows cooling curves on a time-temperature diagram for a region richer in alphagenic elements and less rich in gamma-ray elements
• FIG. 6 schematically shows cooling curves on a time-temperature diagram for a region less rich in alpha-gene elements and richer in gammagenic elements.
• the dendrites 10 corresponding to the first solidified grains, are by definition richer in elements.
• alphagenes while the interdendritic regions 20 are richer in gamma-like elements (application of the known rule segments on the phase diagram).
• An alphagene element is one that promotes a ferritic type structure (more stable structures 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.
• FIG. 5 is a temperature (T) -time (t) diagram known for a region richer in alphagenes and less rich in gamma elements, such as dendrites 10.
• Curves D and F mark the beginning and the end of the transformation of austenite (region A) into a ferrito-pearlitic structure (FP region). This transformation takes place, partially or fully, when the cooling curve that follows the ingot passes respectively in the region between the D and F curves or in the FP region. It does not occur when the cooling curve is entirely in region A.
• FIG. 6 is an equivalent diagram for a region richer in gammagenic elements and less rich in alphagenic elements, such as the interdendritic regions 20. It will be noted that with respect to FIG. 5, the curves D and F are shifted to the right, that is to say, it will cool more slowly the ingot to obtain a ferritoperlitic structure.
• FIG. 5 and 6 shows three cooling curves from an austenitic temperature, corresponding to three cooling rates; fast (curve C1), average (curve C2), slow (curve C3).
• fast curve C1
• average curve C2
• slow curve C3
• the temperature begins to decrease from an austenitic temperature.
• the cooling rates of the surface and the heart of the ingot are very close. The only difference is that the surface temperature is lower than that of the core because the surface was the first to cool relative to the core.
• the dendrites 10 first turn into ferritic structures during cooling (by crossing the curves D and F of FIG. 5). While the interdendritic regions 20 either do not change (in the case of rapid cooling according to the curve C1) or change later, in whole or in part (in the case of average cooling according to the curve C2 or slow according to the curve C3), to temperatures inferior (see Figure 6).
• the interdendritic regions thus retain a longer austenitic structure.
• the lighter elements are able to diffuse ferritic structure dendrites towards the interdendritic regions 20 of austenitic or all-part structure and to concentrate during the period of coexistence 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.
• the austenite of interdendritic regions tends to locally transform into martensite when the temperature of the steel falls below the martensitic transformation temperature Ms, which is slightly above the temperature. ambient ( Figures 5 and 6).
• martensite has a threshold of solubility in light elements even lower than other metallurgical structures and that austenite. There is therefore more microscopic gaseous phase within the steel during this martensitic transformation.
• 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, immediately after the ESR step, a specific homogenization treatment is carried out on the ingot taken out of the ESR crucible, the formation is reduced. of gaseous phases of light elements.
• the homogenization treatment also leads to a homogenization of the martensitic transformation temperature Ms.
• the diffusion of the alloying elements is far from negligible. Moreover, if the temperature gradient makes it possible to have a warmer surface That the center of the ingot, which the conditions of recovery proposed by the inventors allow, the light elements diffuse towards the surface, which reduces their overall content in the steel.
• the inventors have found that satisfactory results are obtained when the ingot is subjected in this oven to a homogenization treatment during a holding time t after the temperature of the most The cold of this ingot has reached a homogenization temperature T, this time t being equal to at least one hour, and the homogenization temperature T varying between a temperature T min and the burn temperature of this steel.
• the temperature T m j n is approximately equal to 900 ° C.
• the burning temperature of a steel is defined as the temperature in the raw state of solidification at which the grain boundaries in the steel are transformed (or even liquefied), and is greater than T min . This time t of maintaining the steel in the furnace therefore varies inversely with this homogenization temperature T.
• the homogenization temperature T is 950 ° C., and the corresponding holding time t is equal to 70 hours.
• the homogenization temperature T is 1250 ° C. which is slightly lower than the burn temperature, then the corresponding holding time t is equal to 10 hours.
• the homogenization temperature T is selected from a range selected from the group consisting of the following ranges: 950 ° C to 1270 ° C, 980 ° C to 1250 ° C, 1000 ° C to 1200 ° C.
• the minimum hold time t is selected from a range selected from the group consisting of the following ranges: 1 hour to 70 hours, 10 hours to 30 hours, 30 hours to 150 hours.
• the inventors have found that satisfactory results are obtained when the ingot at the outlet of the ESR crucible is placed in an oven whose initial temperature T 0 is greater than the end of pearlitic transformation temperature in cooling Arl of this steel. and when the skin temperature of this ingot remains higher than the martensitic transformation temperature Ms of this steel.
• the initial temperature T 0 of the oven is lower than the homogenization temperature T
• the temperature of the oven is, after the ingot has been placed in this oven, increased to a temperature at least equal to the temperature homogenization.
• the temperature in the center of the ingot therefore remains lower than the skin temperature of the ingot throughout the rise in temperature. This allows a global degassing and more effective ingot.
• the initial temperature T 0 of the oven may be greater than the homogenization temperature, in which case the oven temperature is simply maintained above this homogenization temperature.
• the inventors have found that the homogenization treatment is especially necessary when:
• the maximum dimension of the ingot is less than approximately 910 mm, and the H content of the ingot before slag remelting is greater than 10 ppm, and
• the maximum dimension of the ingot is greater than about 910 mm and the minimum dimension of the ingot is less than about 1500 mm, and the H content of the ingot before slag remelting is greater than 3 ppm, and
• the minimum dimension of the ingot is greater than 1500 mm and the H content of the ingot before slag remelting is greater than 10
• the maximum dimension of the ingot is that of the measurements in its most massive part, and the minimum dimension of the ingot is that of the measures in its least massive part;
• the concentrations of light elements may be greater (greater than 10 ppm) when the minimum dimension of the ingot or of the deformed ingot is greater than a large size threshold (in this case 1500 mm).
• a high threshold (1500 mm) for the minimum dimension of the ingot is as follows: when the minimum dimension of the ingot is greater than this threshold, we approach the case of slow cooling (curve C3) in which there is almost no structural difference between dendrites and interdendritic regions during cooling.
• the cooling rate is sufficiently low so that the temperature is substantially homogeneous between the core of the skin of the ingot, so that the diffusion of light elements to the surface is facilitated, which allows a greater degassing.
• the core of the ingot is, during the cooling, much hotter than its surface, which favors a diffusion of the light elements towards the core and slows down the degassing.
• the slag is previously dehydrated before use in the ESR crucible, because it minimizes the amount of hydrogen present in the slag, and thus minimizes the amount of hydrogen that could pass the slag ingot during the ESR process.
• the inventors have carried out tests on Z12CNDV12 steels produced with the process according to the invention, that is to say with a homogenization carried out immediately after the ingress of the ingot of the ESR crucible according to the following parameters:
• Test No. 1 skin temperature of the ingot at 250 ° C., put in the oven at 400 ° C., rise of the oven at the homogenization temperature of 1250 ° C., metallurgical maintenance (as soon as the coldest temperature of the ingot reaches the homogenization temperature) of 75h, cooling to room temperature.
• Test No. 2 Skin temperature of the ingot at 600 ° C., put in the oven at 450 ° C., rise of the furnace at the homogenization temperature of 1000% metallurgical maintenance (as soon as the coldest temperature of the ingot reaches the temperature homogenization) of 120h, cooling to room temperature. The results of these tests are presented below,
• composition of the Z12CNDV12 steels is the following (standard DMD0242-20 index E):
• the martensitic transformation temperature Ms measured is 220 ° C.
• the amount of Hydrogen measured on the ingots before slag remelting varies from 3.5 to 8.5 ppm.
• 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 bias is shown schematically in FIG. 2.
• the period T represents a cycle.
• the constraint evolves between a maximum value C ma x and a minimum value C min .
• the first curve 15 (in fine lines) 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 situated respectively at a distance of +3 ⁇ s ⁇ and -3 at t from the first curve 15, where ⁇ * is the standard deviation of the distribution of the experimental points obtained during these fatigue tests. and ⁇ 3 ⁇ statistically corresponds to a confidence interval of 99.7%.
• the distance between these two curves 14 and 16 in dashed line is therefore a measure of the dispersion of results.
• Curve 14 is the limiting factor for dimensioning a part.
• 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 load according to FIG. CN average curve is surrounded by two curves 26 and 24 dashed thick line, respectively located at a distance of +3 ⁇ 2 and -3 ⁇ 2 of the second curve 25, ⁇ 2 being the standard deviation of the distribution of points experimental results obtained during these fatigue tests.
• Curve 24 is the limiting factor for dimensioning a part.
• 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.
• 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.
• 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).
• 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 the prior art.
• 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%.
• 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.

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