FR2635534A1 - METHOD FOR OBTAINING SPHEROIDAL GRAPHITE BRIDGES - Google Patents

Abstract

1degre (s) The invention relates to a process for producing spherodal or vermicular graphite casting iron from a base iron. It consists of a late treatment of said base cast iron by means of a single additive, acting as nodulizer and inoculant, having the following composition, expressed in% by weight: Silicon 41 to 65 Magnesium 2 to 30 Bismuth 0.1 to 4 Rare earths 0.5 to 4 Ca, Ba, Sr <4 (for each element) Aluminum <1.5 Iron balance 2degre (s) The late treatment of the base iron is carried out either by introducing the additive divided into the casting jet of the cast iron at the entrance of the mold or by the technique called "in-mold".

Description

METHOD FOR OBTAINING SPHEROIDAL GRAPHITE BRIDGES

DOKAINE TRCENIOE OF THE INVNTION

  The present invention relates to a process for obtaining spheroidal graphite cast irons by late treatment of the cast iron by means of a single nodulising additive to prevent inoculation.

STATE OF THE TBCMIUA

  Spheroidal graphite cast irons are normally obtained by treatment of a base cast iron with a first additive called unodulizer which provides the formation of nodular or spheroidal graphite, and which is most often magnesium, for example in the form of a fersilicon-magnesium alloy, with or without rare earth, then with an inoculant which modifies the germination of graphite and avoids the formation of cementite which would weaken the melting; the inoculant is most often based on silicon,

  for example an iron-silicon alloy.

  It is also known that the effect of nodulisatioh treatment is short-lived (a few minutes) due to fainting

  the effect of magnesium at the temperature of molten iron.

  It is therefore necessary to cast the melt in the molds very shortly after introduction of the additives. This requirement is not very compatible with the operating conditions of modern large-capacity and largely automated smelters because it is very difficult to keep for a long time (half an hour or an hour, for example) a reserve of treated cast iron, in pockets or large capacity automatic casting furnaces, this would be desirable. The solution is then to constitute a pool of untreated molten iron and to carry out a late treatment ", by nodulisation then

  inoculation, in the few moments before the introduc-

  cast iron in the impression of the pieces inside the molds. These "late" treatments are carried out essentially by two types of processes - the in-mold processes, in which the crushed or molded additives are placed in dwellings in the iron feed channel, - the processes. in which the additive (s), more or less finely pulverized (s) are introduced into the cast iron stream at the outlet of the ladle or automatic casting furnace, during the pouring into the mold. device of the latter type have been described in our French patent FR 2588571 (= US 4723763) .They make it possible to perform these additions in a continuous and controlled manner, under conditions which ensure the elimination of dirt that could

cause this operation.

  The present invention can be implemented both in the in-mold processes and in the jet processes and, in general, in all the processes of late processing,

  that is to say in the few moments that precede the introduc-

  casting in the indentations of the pieces inside

mussels.

  OBJECT OF THE INVENTION The object of the present invention is a process for the production of spheroidal or vermicular cast iron, characterized in that the base cast is treated later.

  by means of a single additive, simultaneously

  inoculation, this additive being a ferro-silico-magnesium having an addition of bismuth and at least one metal of

the family of Rare Earths.

  It has already been advocated, previously, the use of bismuth in inoculants. For example, the French patent FR 2511044 (= US 4432793), in the name of the Applicant describes an inoculant, known under the trademark "Sphérix", which is a ferro-silicon with 70-75% silicon, comprising from 0.005 to 3% of at least one of the bismuth, lead or antimony metals, and 0.005 to 3% of at least one rare earth metal. But this treatment is complementary to the obligatory nodulisation by magnesium,

performed in a first stage.

  In the Japanese patent application KOKAI / JA-59/43823, in the name of KUSAKA, it is recommended the introduction of bismuth and cerium into the nodulizer, but the subsequent inoculation with ferro-silicon remains necessary to obtain the optimum characteristics.

of spheroidal cast iron.

  The present invention makes it possible to avoid the inoculation stage and to obtain directly an apheroidal cast iron having a high number of graphite nodules, and a total absence of carbides even in the case of very thin pieces.

millimeters.

  The additive making it possible to obtain this result has a composition within the following limits (expressed in percent by weight): Silicon 41 to 65 Magnesium: 2 to 30 Bismuth: 0.1 to 4 Rare earths: 0.5 to 4 Ca , Ba, Sr: <to 4 (for each element) Al: <to 1.5 Iron: difference i 100 In what follows, the term "rare earths" will be designated at least one metal of the group of lanthanides, in particular the cerium, possibly associated with other metals of the same family (such as lanthanum) in a variable proportion depending on the origin

  ore and the treatments he has undergone.

  Compared with the prior art, the method which is the subject of the invention is essentially distinguished by the fact that it combines nodulisation and inoculation in a single operation which is performed late, which leads to a simplification of procedures and equipment. used, therefore a gain in cost price and in production.

of the foundry workshop.

EXAMPLE OF IMPLEMENTATION

  The invention has been implemented with the device which is the subject of our application FR 2588571, often called 'hourglass' because of its particular shape and which allows the direct introduction of the finely powdered additive into the casting jet. in contra-controlled atmosphere. Thus, according to the invention, a spheroidal cast iron having the following final composition (% by weight) was prepared: C: 3.7 Si: 2.5 Mn: 0.15 P: 0.043 Mg: 0.039 For this reason, carried out a 'late' treatment of the base cast with a single, nodulising-inoculating additive having the following composition (l by weight): Si: 48.2 Mg: 6.1 TR: 0.56 Bi : 0.33 Ca: 0.44 Al: 0.71 Fe: balance. TR: essentially cerium, plus other metals

  of the group, usually accompanying cerium.

  A comparative test, according to the prior art, was carried out on the same cast iron with a conventional nodulizer having the following composition (l by weight) Si: 44.5 Ng: 5.6 TR: 0.6 Ca: 0, 83 Al: 1.21 Mn: 0.37

  Iron: balance. Bismuth content: zero. TR: as above.

  In both cases, the nodulizer was introduced at the dose of

1 t of the weight of the cast iron.

  In the comparative trial, nodulisation was followed by inoculation

  classical treatment with a silicon-based ferro-silicon inoculant doped with alkaline-earth additions,

  the dose of 0.2% relative to the weight of cast iron.

  In both cases, the cast iron was cast, at a temperature of 1400 C, in the form of bars having diameters of 30, 20, 12 and 6 millimeters. The casts were made in sand Beta Set. On these bars, a micrographic count of the graphite nodules, per millimeter-square cross-section, and the detection of the presence of carbide (cementite) Fe3C were carried out. The results were as follows diameter cast iron bars: nodules / mm2 and presence 30 20 12 6 carbides (0 / N) Comparative test, 550 750 1100 prior art N N N O According to the invention 660 900 2000 3400

N N N N

  This table shows that: on the one hand the number of graphite nodules is substantially higher in the case of the treatment according to the invention, on the other hand, the obtaining of parts of a thickness of 6 mm without carbide, very difficult to obtain according to the prior art, is carried out in very good conditions according to the invention, which allows the use of this process for all cast iron parts & reduced thickness at

less than 6 mm.

  The quantity of additive to be introduced into the base cast, for the implementation of the invention, is determined by those skilled in the art according to the final content referred to, in the cast iron, of the two magnesium elements and bismuth. Average contents of 0.025 to 0.060% by weight of magnesium and 0.005 to 0.02% of bismuth are generally considered satisfactory. They can therefore be obtained, taking into account the volatility of the two elements, with a quantity of additive of the order of 1% compared to

cast iron weight to be treated.

REVMENDICATIONS

  1- Process for producing spheroidal graphite cast iron

  dal or vermicular from a base cast iron, characterized in that a late treatment of said base cast is carried out by means of a single additive, acting as a nodulizer and inoculant, having the following composition, expressed in percent by weight Silicon ............... 41 to 65 Magnesium .............. 2 to 30 Bismuth ................ 0.1 to 4 Rare earths ..... ...... 0.5 to 4 Ca, Ba, Sr .............. <4 (for each element) Aluminum ............ .. <1.5 Iron .................... balance 2 Method according to claim 1, characterized in that the

  late treatment of the basic cast is carried out by introducing

  of the additive divided in the casting stream of the

cast iron at the entrance of the mold.

  3 - Process according to claim 1, characterized in that the late treatment of the base cast is carried out according to

the so-called 'in-moldw technique.