EP1676929B1 - Process of manufacturing compacted graphite cast iron - Google Patents

Abstract

Production of cast iron made from compact graphite comprises adding a pre-conditioning agent containing a rare earth metal to a cast iron melt during casting, in which the added amount of rare earth metal is measured depending on the amount of sulfur contained in the amount of cast iron melt.

Description

The invention relates to a method for producing compact graphite-containing cast iron with strengths in the range of 350 to 500 MPa from a molten cast iron.

When classifying the different types of cast iron according to their strength properties, compact graphite cast iron, commonly referred to by the abbreviation "GJV", having strengths in the range of 350 to 500 MPa between cast iron with lamellar graphite, commonly referred to as "GJL", and Having strengths in the range of 150 to 350 MPa, and ductile iron, which is commonly referred to as "GJS" and has strengths of 350 to 1000 MPa. The particular advantage of cast iron with compact graphite consists in a favorable combination of high strength and good thermal conductivity and good damping behavior.

For example, in the article " Piston rings made of cast iron with vermicular graphite "by Wolfgang Knothe and Otto Liesenberg, in foundry technology, Volume 26, Issue 10/1980, pages 297-298 described, technically easy to control way of producing cast iron with compact graphite is to add a rare earth metal, such as cerium in an amount to the cast iron melt in an amount depending on the sulfur content is set. An overdose must be avoided, however, in order to avoid the formation of undesirable microstructure and the so-called "white solidification" ( Foundry Lexicon Edition 2001, 18th ed., Page 573 ).

As in detail from the European Patent EP 1 068 365 B1 Further, in addition to the treatment with cerium or other rare earth metals, moreover, magnesium treatment of the cast iron melt to be cast is usually carried out to produce magnesium silicates in the cast iron. These magnesium silicates have proven to be particularly effective nucleating agents. However, the addition of magnesium to the molten iron also deoxidizes the melt. Since, however, the effectiveness of the magnesium silicates as nucleating agents depends on the oxygen present in the melt, precise control of the oxygen content of the melt is therefore of particular importance.

The information required to accurately determine the oxygen content of a cast iron melt can be obtained, for example, from thermal analyzes, EMF measurements, or other analytical techniques related to nucleation and seed growth events. So is in the EP 1 068 365 B1 It has been proposed, for the purpose of setting an optimum oxygen content in a melt intended for the production of compact graphite cast iron, first to produce a starting melt with a low sulfur content, as is conventional practice in the production of nodular cast iron. The silicon content is set lower than a desired final value, so that based on the C contents saturated SiO ₂ , is near, for example, 1400 ° C. The actual melt temperature "TM" is subsequently set to a value slightly below the temperature "TB" at which bubbles form due to the exit of CO gas from the melt. After a certain time at a specific temperature during which the melt absorbs oxygen from the environment, the now obtained oxygen content of the melt is measured by a standard thermal analysis method. In addition to the oxygen content, this must also provide information regarding the types of crystallization behavior of the melt and its oxide inclusions. When the oxygen content has reached a value of 50-100 ppm, silicon is added until the respectively calculated temperature "TE" is now only about 20 ° C. below the actual temperature "TM" of the melt.

In practical implementation, the known ways of determining the amounts of oxygen and mixed oxides prove to be difficult to handle, which are to be added to an iron melt in order to achieve the desired graphite formation. In addition, they require a high expenditure on equipment, which is not only costly, but is prone to failure under the existing in practice hard operating conditions.

Based on the above-described prior art, therefore, the object of the invention was to provide a method which allows high reliability and low cost, the safe production of cast iron with compact graphite.

mit k = 2, 8 - 3, 5;
M_REM, M_S angegeben in kg pro Tonne Eisenschmelze.This object is achieved by a method for producing compact graphite cast iron having strengths in the range of 350 to 500 MPa from a cast iron melt, wherein the cast iron melt is added during pouring a rare earth-containing preconditioning agent, according to the invention solved in that the added amount M _REM is measured on rare earth metal as a function of the amount M _S of the sulfur contained in the respective cast quantity of cast iron melt, according to the following proviso: $${\mathrm{M}}_{\mathrm{REM}}=\mathrm{k}\times {\mathrm{M}}_{\mathrm{S}};$$

with k = 2, 8 - 3, 5;
M _REM , M _S expressed in kg per ton of molten iron.

The invention provides to add a rare earth metal to the cast iron according to the procedure which is already known per se, with which the formation of compact graphite in the cast iron is specifically supported. The rare earth metal is preferably the cerium which has already been used successfully for this purpose.

Unlike previously provided in the prior art, the added amount of rare earth metal according to the invention measured on the basis of the sulfur content of the cast iron melt, which is present after their melting. This sulfur content is dependent on the sulfur content of the materials from which the molten iron to be cast is melted, and is detected by default as part of the melting.

It has been found that the easily detectable routine sulfur content of the melt in is a direct linear relationship with the amount of rare earth metal required for the preconditioning of the melt to form the compacted graphite in cast iron. Contrary to what has been assumed by experts up to now, the procedure according to the invention thus requires no complex measuring and control processes in order to determine the respective required amount of Mg-containing master alloy which has to be added to the melt in order to form the desired graphite in iron casting.

According to the invention, firstly a base melt is melted, which contains, for example (in% by weight) 3.50-3.90% C, 1.10-2.20% Si, 0.30-0.50% Mn, O, 05-0.07% Cr, 0.005-0.025% S, 0.40-0.90% Cu, 0.09-0.10% Sn, up to 0.01% Ti and the remainder iron and unavoidable impurities , Basically, the melting can be carried out in a cupola with downstream desulfurization. Preferably, however, the molten iron is produced in the electric furnace in order to achieve the lowest possible sulfur contents of the molten iron. These are preferably in the range of 0.005 to 0.020 wt .-%, with optimized properties of the resulting cast iron set when the S content of the molten iron 0.007 to 0.020 wt .-%. The typical C content of molten iron cast to compacted graphite cast iron is in the range of 3.65-3.80 wt%. The Si content of such iron melts is typically from 1.10 to 2.00 wt%.

Immediately before treatment with magnesium, the iron casting melt is then added to the amount of rare earth metal determined in accordance with the invention. To this Purpose can be used in practice already proven commercial treatment agents. Such treating agents typically comprise (in wt%) 47-55% Ce, 24-35% La, 8-15% Nd and 3-8% Pr. The addition of the treatment agent is preferably carried out immediately before the magnesium treatment.

When cerium is used in accordance with the invention as a rare earth element for the preconditioning of the molten iron, the free enthalpies of formation between cerium, oxygen and sulfur are greater than the free Gibbs enthalpies between silicon and oxygen. Therefore, it can be assumed that complete conversion of the cerium to cerium oxysulfides, cerium sulfides and cerium oxides occurs. These particles promote the homogeneous nucleation catalysis, so that the result is a cast iron with an optimum for the desired property spectrum expression of the graphite is obtained.

Practical experiments have shown that optimized properties of the cast iron set when the inventive addition of rare earth metal is made such that the graphite present in the cast iron is present only 5 to 30% in spherical shape.

The addition of rare earth metal to the respective molten iron carried out according to the invention surprisingly leads to the formation of the smallest possible rare earth particles which are well distributed in the melt. Practical experiments have shown that when using Cerhaltigem treatment agent can produce a cast iron with optimized properties by the cerium addition so it is made that the cast iron obtained contains 10 ^-2 to 10 ^-3 atomic% of cerium sulfides.

Depending on the specific composition of the particular treating agent and the added in combination with the respective rare earth metal other nucleation or oxidation effective elements of the method used to determine the quantity M _REM added factor k can be varied between 2.5 and 3.5. Studies have shown that the success of the invention with the use of cerium-containing treatment agents is particularly secure when the factor k is in the range of 3.0 to 3.3, in particular equal to 3.2.

The preconditioning carried out in accordance with the invention can be followed, in a manner known per se, by a magnesium treatment in which a Mg-containing inoculant is added in order to adjust the Mg content in the resulting casting to from 0.008% by weight to 0.014% by weight. is. Due to the higher cooling rate of thinner walled castings, the Mg content of such parts should be in the lower portion of this range, while thicker walled castings should have Mg levels up to the upper limit of that range.

The invention enables a very simple procedure in the preconditioning of a cast iron melt. Thus, the foundry, which is concerned with the treatment of the melt, requires only an indication of the amount of molten iron contained in the respective amount of molten iron melt M _S. Based on this quantity he can then, for example, based a simple diagram determine the amount required in each case on each provided rare earth metal. An example of such a diagram is attached.

hergestellte Zusammenhang beispielhaft für ein Cer-haltiges Behandlungsmittel dargestellt, wobei der Faktor k gleich 3,2 gesetzt ist.In this diagram, the invention is based on the formula $${\mathrm{M}}_{\mathrm{REM}}=\mathrm{k}\times {\mathrm{M}}_{\mathrm{S}}$$

prepared as an example of a cerium-containing treatment agent, wherein the factor k is set equal to 3.2.

Claims (12)

1. Method for manufacturing cast iron comprising compact graphite, with strength values in the range from 350 to 500 MPa, from a cast iron melt, wherein during casting a pre-conditioning agent containing rare earth metal is added to the cast iron melt, characterised in that the quantity added M_REM of rare earth metal, as a function of the quantity M_s of the sulphur contained in the quantity of cast iron melt cast in each case, is measured in accordance with the following scale: $${\mathrm{M}}_{\mathrm{REM}}\mathrm{=}\mathrm{k}\mathrm{\times }{\mathrm{M}}_{\mathrm{S}}\mathrm{;}$$

   

   
   where k = 2.8 - 3.5;
   M_REM, M_s given in kg per tonne of iron melt.
2. Method according to Claim 1, characterised in that the graphite in the cast iron obtained is present in spheroidal formation in the amount of 5 to 30 %.
3. Method according to any one of the preceding claims, characterised in that a cast iron melt is cast with the following composition (as % by weight): C: 3.50 - 3.90 % Si: 1.10 - 2.20 % Mn: 0.30 - 0.50 % Cr: 0.05 - 0.07 % S: 0.005 - 0.025 % Cu: 0.40 - 0.90 % Sn: 0.09 - 0.10 % Ti: ≤ 0.01 % the remainder being iron and unavoidable impurities.
4. Method according to Claim 3, characterised in that the C-content of the iron melt amounts to 3.65 - 3.80 % by weight.
5. Method according to either of Claims 3 and 4, characterised in that the Si content of the iron melt amounts to 1.10 to 2.00 % by weight.
6. Method according to any one of Claims 3 to 5, characterised in that the S content of the iron melt amounts to 0.005 to 0.020 % by weight.
7. Method according to Claim 6, characterised in that the S content of the iron melt amounts to 0.007 to 0.020 % by weight.
8. Method according to any one of the preceding claims, characterised in that the factor k used to determine the quantity M_REM added lies between 3.0 and 3.3.
9. Method according to Claim 8, characterised in that the factor k used to determine the quantity M_REM is equal to 3.2.
10. Method according to any one of the preceding claims, characterised in that an inoculant containing Mg is added to the iron cast melt after pre-conditioning in order to adjust an Mg content in the cast item obtained, which amounts to 0.008 to 0.014 % by weight.
11. Method according to any one of the preceding claims, characterised in that the rare earth metal is Cer.
12. Method according to Claim 11, characterised in that the cast iron obtained contains 10^-2 to 10^-3 atom-% ceroxisulphides.

Images