JP5618065B2 - Bi-based inoculant for spheroidal graphite cast iron and method for producing spheroidal graphite cast iron using the same - Google Patents

Description

  The present invention relates to a Bi-based inoculant for spheroidal graphite cast iron and a method for producing spheroidal graphite cast iron using the same.

As described in Non-Patent Document 1, in the production of spheroidal graphite cast iron, a graphite spheronizing agent (hereinafter also simply referred to as a spheronizing agent) and an inoculum are added to molten cast iron (hereinafter also simply referred to as a molten metal). Is done.
As spheroidizing agents, Mg, Mg-based alloys, Ca-based alloys, REM (rare earth element) -based alloys, and Si-based alloys are practically used, and among these, Mg and Mg-based alloys are most frequently used.

Inoculation is performed for the purpose of increasing the number of graphite grains and improving mechanical properties, preventing chilling, and preventing abnormal graphite. Ferrosilicon (Fe-Si (containing Ca)) is mainly used as the inoculum, and the amount added is 0.4% or less in terms of Si.
On the other hand, Bi is known to have an effect as an inoculum (Patent Documents 1 to 3).

In Patent Document 1, as a reason for adding B: 0.0005 to 0.05% to the molten metal composition of the ductile cast iron used as the inner layer of the centrifugal cast composite roll, a small amount of Bi has the effect of finely crystallizing graphite. Although the number of grains is increased, it is stated that the addition of an excessive amount reduces the amount of graphite and causes a tendency to cause embrittlement of cast iron ([0017]).
In Patent Document 2, as a reason for adding Bi: 0.001 to 0.05% to the limiting requirement of the inner layer composition of the composite roll for centrifugal casting rolling, Bi, which is positioned as a harmful element, is dispersed as fine particles in the molten metal. Experiments focusing on the formation of a number of new interfaces between these and the liquid phase of Bi, and if the amount is too small, the effect of uniform fine dispersion of graphite is small, and if too much, the material becomes brittle due to the formation of Bi-containing carbides ([0012]).

  Patent Document 3 describes the following points. In other words, the effect of adding Bi is remarkably limited to small castings, and the effect of fine crystallization of graphite is hardly observed in large castings such as rolling rolls ([0014]). Only with the addition of bismuth, the time for Bi to last until solidification after casting (until the crystallization of graphite) is extremely short. In large-sized castings, it takes a lot of solidification time after casting, so that effect cannot be achieved. On the other hand, in the invention described in this document, a small amount of Bi (Bi: 0.0005 to 0.05%) and an appropriate amount of Sn or Sn + Cu (Sn: 0.01 to 0.2%, Cu: 0.1% to 2.0%), the synergistic effect of these components enables a remarkable effect of fine crystallization of graphite even in large castings such as rolling rolls, and a composite roll for rolling with a particularly slow solidification rate. Abnormal black at the upper shaft end of the inner layer The generation of lead (chunky graphite) can be prevented and the toughness of the inner layer can be significantly increased ([0015]).

Japanese Patent No. 3002392 JP 2002-317237 A JP-A-2005-270991

Edited by the Japan Iron and Steel Institute, “Third Edition Steel Handbook Vol. V, Casting / Forging / Powder Metallurgy” October 1, 1982, published by Maruzen, p.76-92

  However, with the conventional inoculation technique using only Bi, the yield of Bi is low and unstable, and the effect of increasing the number of graphite grains is unstable. Particularly, the effect is difficult to appear in a thick cast (roll shaft material). There was a problem such as. Such a problem that the inoculation effect of Bi is unstable is not sufficiently solved even with the technique of Patent Document 3.

Inventors have made repeated experiments and studies in order to solve the above problems, as a Bi-based inoculant, a Bi was mixed with Ni and Cu, or preferably alloyed by using a thing The present inventors have found that the yield of Bi can be improved, the number of graphite grains can be increased, and that the Bi inoculation effect can be sufficiently exerted even in thick castings.
That is, the present invention is as follows.
(1) A Bi-based inoculant for inoculating a cast iron melt of spheroidal graphite cast iron, Bi : 0.1% by mass or more and less than 40% by mass, Mg: 0-30% by mass, Si: 0-50% by mass, Cu: containing from 1 to 50% by weight, and wherein the Rukoto a balance being Ni and unavoidable impurities, Bi-based inoculant for spheroidal graphite cast iron.
(2) the in Bi-based inoculant, Mg in mass%: Bi inoculant for spheroidal graphite cast iron according to is contained 0.5% or more and 30% or less formed by the (1).
( 3 ) In the method for producing spheroidal graphite cast iron, the Bi-based inoculant for spheroidal graphite cast iron described in (1) or (2) above is used as at least part of the inoculant added to the molten cast iron. Method for producing spheroidal graphite cast iron.

In the above ( 3 ), the inoculum to be used is placed in a pan together with the graphite spheroidizing agent, and the cast iron melt is poured into the pan, or at the time of addition or addition of the inoculum to the cast iron melt Immediately after that, the cast iron melt may be stirred.

  According to the present invention, the yield of Bi can be stably improved, the number of graphite grains can be increased, and the Bi inoculation effect can be sufficiently exerted even in a thick casting.

Predicted diagram of the mechanism of increasing the number of graphite grains by Bi inoculation Prediction of the effect of Bi-based inoculants containing Ni Schematic diagram showing the outline of the experiment in the example

The present inventors predicted the mechanism by which Bi inoculation of molten metal (meaning of Bi-based inoculant addition, hereinafter the same) increases the number of graphite grains as shown in FIG. That is, Bi is a unique element that does not dissolve in the molten metal 2 but separates into two phases as liquid Bi particles 1 and floats in the molten metal 2, and it is considered that the liquid Bi particles 1 act as a nucleation substance of the graphite 3 .
Bi melt in Bi concentration after inoculation as low as several 10ppm or less, but is still more information is known about the dispersion status, the present inventors have found that among the dispersed particles of Bi was filled with Ni and Cu form (a Ni -Cu-Bi alloy, and it may contain Mg or Si et) mixture or preferably an alloy having a, by using as the Bi-based inoculant, more liquid Bi particles in the melt It can be finely and stably dispersed, and furthermore, the amount of Bi added can be reduced without reducing the nucleation sites in the molten metal compared to the conventional case, so that quality problems such as deterioration of the shape of graphite and Zaku defects are also solved. I found out.

  The effect of using the Bi-based inoculant is expected as follows. For example, when a Bi-based inoculant containing Ni is melted in a molten metal, Bi and Ni are melted together to form a Ni-Bi melt (alloy). Next, this Ni-Bi melt is dispersed into the melt mainly composed of Fe. At that time, Ni elutes from the Ni-Bi melt 4 as shown in FIG. With the interfacial reaction, the interfacial tension between Ni-Bi melt 4 and molten metal 2 decreases significantly. Since the Ni-Bi melt in the melt can be regarded as a dispersed droplet in the quasi-viscous region, the droplet diameter is inversely proportional to the 1/2 power of the energy dissipation density of the turbulent vortex and is proportional to the interfacial tension. The Ni-Bi melt 4 is more finely dispersed due to a significant decrease in interfacial tension. Further, as shown in FIG. 2 (b), since Ni is eluted from the dispersed fine Ni-Bi melt 4 thereafter, the volume is reduced and finally smaller liquid Bi particles 1 are generated. In addition, since a Ni concentrated layer is formed around the Ni-Bi melt, the temperature for generating graphite nuclei increases, and as a result, the nucleation ability of graphite improves. The same effect can be expected when Cu is used as an element combined with Bi instead of or in addition to Ni.

However, Bi content in Bi-based inoculant (mass%) shall be the less than 0.1% to 40%. If this is less than 0.1%, the amount of Ni or Cu added will increase, making it difficult to apply the present technology. On the other hand, if it is 40% or more, fine dispersion of Bi will be difficult.
Further, it is preferable to add Mg: 0.5% to 30% by mass% in the Bi-based inoculant because the Ni-Bi melt is more finely dispersed by Mg explosive reaction.

As described above, the Bi-based inoculant of the present invention is preferably an alloy rather than a mixture from the viewpoint of further stabilizing the Bi-inoculation effect. A preferred composition (component content; mass%) of such an alloy is as follows.
· Ni -Cu-Bi alloy: Bi: less than 0.1% or more 40%, Mg: 0 (more preferably 0.5) ~30%, Si: 0~50 %, Cu: 1~50%, balance Ni and incidental Impurities In the production method of the present invention, the Bi-based inoculant of the present invention is used as at least a part of the inoculant added to the molten cast iron. The Bi-based inoculant of the present invention can be used alone, but can also be used in combination with other inoculants such as ferrosilicon which has been mainly used conventionally. The molten metal composition may be a normal one.

  As a preferable addition method to the molten metal, there is a pouring method (see Non-Patent Document 1, p. 81). That is, the inoculum to be used is installed in a pan (ladder) together with a spheroidizing agent, and molten cast iron is poured into the pan. The spheroidizing agent may be one that has been conventionally used. In the infusion method, the molten metal in the pan is stirred by the explosive reaction of the spheroidizing agent, so that the reaction between the additive substance and the molten metal is promoted. Therefore, no separate stirring means for stirring the molten metal is required. Although methods other than the infusion method can be employed, in that case, it is necessary to promote the reaction between the additive substance and the molten metal by stirring the molten metal with another stirring means at the time of addition to the molten metal or immediately after the addition, Such other stirring means include mechanical stirring, gas bubbling, or addition to the pouring stream. However, in the aforementioned Mg-containing inoculant, stirring can be omitted.

  Moreover, it is good to determine the addition amount of Bi type inoculant to a molten metal so that Bi content (mass%) in the casting product cast from this molten metal may be 0.0005-0.0025%.

  The following casting experiment was conducted as an example. In this experiment, as shown in FIG. 3, molten metal 2 (molten metal weight 21 kg) having a composition within a normal cast iron molten metal composition range melted in a high-frequency melting furnace 5 (rated molten amount 30 kg) was converted into an Mg treatment pan 6 After spheroidizing treatment and inoculation by pouring, a Y block casting 11 having dimensions shown in FIG. 3 was cast into a Y block mold 7. The casting temperature was 1330-1380 ° C.

The infusion method was performed at the following levels.
(Level 1) Prior to pouring, Mg alloy as spheroidizing agent 8 and ferrosilicon as inoculating agent 9 are installed in the pocket provided on the bottom of the pan 6 for Mg treatment, and the cover material 10 is placed thereon. Cover with. Spheroidizing agent 8 and inoculant 9 have a content (mass%) in the molten metal of 1.6% (equivalent to Mg 0.08%) for spheroidizing agent Mg alloy and 0.27% (equivalent to 0.2% of Si) for inoculant ferrosilicon It was installed only in the amount.
(Level 2) In Level 1, further, metal Bi was installed between the spheroidizing agent 8 and the inoculant 9 as Bi-based inoculant 9A in such an amount that the content (mass%) in the molten metal was 0.005%. . Otherwise, it was the same as Level 1.
(Level 3) In Level 2, Ni-Bi alloy (Ni-1.0Bi (the numerical unit is mass%)) is used as the Bi-based inoculant 9A instead of metal Bi, and the content (mass%) in the molten metal is 0.5%. It was installed in an amount that would be equivalent to Bi 0.005%. Otherwise, it was the same as Level 1.
(Level 4) In Level 2, instead of metal Bi as Bi-based inoculant 9A, Cu-Bi alloy (Cu-1.0Bi (the numerical unit is mass%)) is 0.5% in content (mass%) in the molten metal. It was installed in an amount that would be equivalent to Bi 0.005%. Otherwise, it was the same as Level 1.
(Level 5) In level 2, Ni-Cu-Bi alloy (Ni-40Cu-1.0Bi (the numerical unit is mass%)) instead of metal Bi as Bi-based inoculant 9A is contained in the molten metal (mass%). ) Is 0.5% (equivalent to Bi 0.005%). Otherwise, it was the same as Level 1.
(Level 6) In level 2, Ni-Mg-Bi alloy (Ni-5.0Mg-1.0Bi (numerical unit is mass%)) instead of metal Bi as Bi-based inoculant 9A is contained in the molten metal (mass %) Is 0.5% (equivalent to Bi 0.005%). Otherwise, it was the same as Level 1.
(Level 7) In Level 2, as a Bi-based inoculant 9A, instead of metal Bi, a mixture of metal Bi and metal Ni (Bi mass / Ni mass = 1/99) is contained in the molten metal (mass%). Installed only in an amount of 0.5% (equivalent to 0.005% Bi). Otherwise, it was the same as Level 1.

The following investigation was performed about the obtained Y block casting.
(A) The composition was examined by chemical analysis. The results are shown in Table 1.

Comparing the Bi content in the casting of Table 1 with the Bi content in the molten metal (Bi content when the Bi yield is 100%), the present invention example has a higher Bi yield than the comparative example. Yes.
(B) A test piece is taken from a portion having a height of 65 mm from the bottom of the Y block casting, and the number density of graphite particles is measured by observing the polished surface (no etching) with an optical microscope, and a tensile test in the longitudinal direction of the Y block ( The tensile strength TS and elongation EL were measured using a test piece diameter of 10 mm. The results are also shown in Table 1.

As shown in Table 1, in this example, at level 1 (comparative example) in which no Bi-based inoculant was added, chunky graphite was generated, the graphite particle number density was extremely low, and the elongation was low. . In Level 2 (comparative example) where Bi metal was used as an inoculum, chunky graphite was not generated, but the graphite particle number density was as low as 98 particles / mm ² , indicating a balance between strength and ductility. × EL was about 7000 (MPa ·%), and Bi yield was as low as 18%.

On the other hand, in the present invention embodiment with no occurrence of background Yanki graphite, graphite grain number density is made 140 pieces / mm ² or more and graphite fine dispersion. Further, the Bi yield is markedly increased to 40 to 50%, which is considered to indicate the stability of the inoculation effect according to the present invention. Further, TS × EL indicating the balance between strength and ductility is 8500 (MPa ·%) or more, and the mechanical property improving effect according to the present invention is clearly shown.

  In the above-described embodiment of the present invention, the effect of performing the casting experiment with the mold shown in FIG. 3 was shown, but the present invention is not limited to the mold size or the amount of molten metal, and is smaller. Alternatively, the effect is sufficiently exhibited even in a large casting such as a hot rolling roll.

1 Liquid Bi particles
2 Molten metal
3 Graphite
4 Ni-Bi melt
5 induction melting furnace
6 Pan (Mg processing pan)
7 Y block mold
8 Spheroidizing agent
9 Inoculum
9A Bi-based inoculum
10 Cover material
11 Y block casting

Claims (3)

A Bi-based inoculant for inoculating a cast iron melt of spheroidal graphite cast iron, Bi : 0.1 mass% or more and less than 40 mass%, Mg: 0-30 mass%, Si: 0-50 mass%, Cu : 1 containing 50 mass%, and wherein the Rukoto a balance being Ni and unavoidable impurities, Bi-based inoculant for spheroidal graphite cast iron. Wherein the Bi-based inoculant, Mg in mass%: Bi inoculant for spheroidal graphite cast iron according to claim 1 formed by incorporating 0.5% to 30% or less. In the manufacturing method of spheroidal graphite cast iron, the Bi type | system | group inoculation agent for spheroidal graphite cast iron of Claim 1 or 2 is used as at least one part of the inoculum added to cast iron molten metal, The manufacturing method of spheroidal graphite cast iron characterized by the above-mentioned .

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