Method for influencing the elongation of cast iron

The invention relates to a method of influencing the properties of cast iron by adding magnesium to a cast iron melt.

[0002] The content of magnesium unbound in cast iron melts is generally recognized as a decisive factor for the formation of spheroidal or vermicular graphite in cast iron treated with magnesium. Modern practice in the regulation of ductile iron production is based on determining the total magnesium content, i.e. unbound and bound magnesium, using spectrographic samples. However, this method offers an incomplete image because the content of unbound magnesium is unknown, and the measurement does not provide information about oxygen activity. Oxygen activity, which is in equilibrium with unbound magnesium, however, is a factor determining the formation of the graphite form. The so-called. malleable cast iron is ordinary gray cast iron which has been treated with the additive that forms the ball, therefore, the main part of graphite carbon in the cast iron is the so-called spheroidal graphite or ball-type graphite. Spheroidal graphite in cast iron needs to be analyzed in terms of shape, size and amount of particles, because these parameters affect the mechanical properties of cast iron. Visual analysis is complex or subjective, even for semi-automated analyzes. The measurements related to this matter are known, for example, from US 5 675 097. In DE 199 28 456A1, measurements concerning the spatial structure of graphite in cast iron based on the determination of oxygen content and free from defects in visual methods are described. Thanks to this, it is possible to react more quickly, and the targeted influencing of production increases productivity or reduces the amount of foundry shortage. The quality of cast iron is well regulated.

[0003] Further examples of oxygen measurement for controlling the composition of cast iron treated with the addition of Mg are known from JP2006063396, US6,544,359 B1 and WO 99/45156.

[0004] The efficiency of the treatment of iron with the addition of magnesium can be verified, for example, by means of metallographic or spectrographic analyzes of samples clotted in white or also by thermal analyzes.

[0005] Pure magnesium or magnesium alloy is generally used to promote the formation of a ball of cast iron. Some of the added magnesium takes away oxygen and sulfur from iron, the remaining part is called unbound Magnesium content that regulates oxygen activity. The share of unbound magnesium in the melt is a factor determining the spheroidality of cast iron. The proportion of unbound magnesium in the melt decreases with time, while oxygen activity increases. This affects the structure and mechanical properties of cast iron.

[0006] Sensors for determining oxygen activity in a metal melt are known, e.g., from DE 103 10 387 B3. A solid electrolyte tube is disclosed thereon, which on its external surface has a coating of a mixture of calcium zirconate and fluoride, for which, for example, the measurement of the concentration of sulfur, silicon or coal in an iron melt can be carried out.

The object of the present invention is to indicate a process control method by means of which the existing technique will be improved, while the mechanical properties of the cast iron should be influenced in a directed manner already in the liquid phase.

[0008] This task has been solved thanks to the features of the independent claim. Preferred embodiments of the invention are set forth in the dependent claim. The method according to the invention is characterized in particular by the fact that the oxygen content in the cast iron melt is measured and that magnesium is added to the cast iron so long as the oxygen content in the cast iron melt at 1420 ° C as a reference temperature of 00.8 to 0. 1 ppm. Since the measurement of oxygen is more accurate than the possible measurement of magnesium (magnesium is present in the melt as unbound magnesia and as bound magnesium, and therefore an accurate measurement is not possible), it is more accurate to determine the mechanical properties of cast iron.

[0009] This allows the correlation with mechanical properties to be indicated, as already described in US 5,675,097, e.g. in terms of tensile strength, elongation and resistance to deformation. In the case of cast iron, it has been surprisingly found that it exhibits a maximum elongation when the addition of magnesium takes place until the oxygen content is less than 0.1 ppm, according to the invention between 0.08 and 0.1 ppm. In the case of lower or higher content, the elongation of the cast iron decreases again. It is advantageous to add about 200 to 750 ppm magnesium to the cast iron to obtain the desired oxygen content.

[0010] A sensor for measuring the oxygen content of a cast melt, equipped with an electrochemical measuring cell that includes a solid electrolyte tube, is characterized in that a layer of zirconium dioxide is applied to the outwardly facing surface of the solid electrolyte tube. In particular, the zirconium oxide layer may be stabilized with calcium oxide, yttrium oxide and / or magnesium oxide. Preferably the layer is stabilized with up to 30% by weight. calcium oxide, up to 25 wt.% magnesium oxide and / or up to 52 wt.% yttrium oxide. The particularly preferred layer is stabilized with approx. 4 to 6% by weight. calcium oxide. More preferably, the sensor layer is plasma sprayed. It preferably has a thickness of about 30 to 50 μm, in particular about 40 μm. The tube with the solid electrolyte on which the layer is located is preferably a zirconium oxide tube, which can be stabilized with approx. 2% by weight magnesium oxide.

[0011] In the following, an embodiment of the invention will be described based on the drawing. The illustration shows:

Fig. 1 dependence between the amount of graphite particles and oxygen content (oxygen activity aO),

Fig. 2 the relationship between relative expansion and oxygen content,

Fig 3. cross-section through the sensor head and

4 is a partial section through a further version of the sensor.

[0012] In Figure 1, it can be seen that the number of graphite particles increases with increasing oxygen content (oxygen activity aO). By controlling the oxygen content by adding magnesium, it is possible to set the number of graphite particles. Thanks to this, it is possible to influence the properties of cast iron in a way already targeted in the melt. Maximum spheroidality occurs in the case of oxygen activity between approx. 0.10 and 0.12 ppm (valid at 1420 ° C). If the oxygen activity drops below 0.10 ppm, the spheroidality decreases. This corresponds to the well-known experience of foundry practice that too high a share of magnesium has a negative impact on spheroidality.

[0013] Fig. 2 shows the relationship between the relative expansion of iron and the oxygen content. The maximum elongation can be observed at approx. 0.08 ppm. In the case of lower oxygen activity, the elongation is slightly smaller, presumably due to the lower spheroidality. If the oxygen activity exceeds the optimal value, the elongation decreases permanently. The graphic shows that it is possible to influence the amount of oxygen in the cast melt by adding magnesium to the relative elongation of the cast iron.

[0014] Fig. 3 shows a sensor. In the metal tube 1 there are electric wires 2 (Cu / CuNi / conductor) in the sand fill 3. Using the connector 4, the electric wires are connected to the lance or to another holder and further to the analyzing unit. The other end of the wires 2 is connected to the thermocouple 5 and to the electrochemical measuring cell 6. The electrochemical measuring cell 6 has a solid electrolyte tube (ZrO2 cell) with a steel impact shield as the outer coating. The ZrO2 cell has on its outer surface a layer of zirconium dioxide, which is stabilized with 5% by weight. calcium oxide. This layer has a thickness of approx. 40 μm. It is not shown separately in the drawing, since the solid electrolyte tubes are generally known.

[0015] The thermocouple 5 is fixed in the sealing cement of the thermocouple. The measuring cell 6 is also fixed in the cement 8, its end located inside the sensor is closed by means of a sealing plug 9 through which electrical contacts are led out. Both sensing elements 5; 6 are connected to each other by means of a plastic clip 10. Through the thermally insulating part 11, the conduits are guided through the interior of the metal tube 1. In the immersion of the sensor, on the outer side of the metal tube 1, the sand body 12 is placed for protection.

[0016] Fig. 4 shows a similar arrangement in which the sensor contacts in the carrier tube are shown 13. The carrier tube 13 is made of cardboard and, with its front facing the sand body 12, the side is surrounded by a pipe for splash guard 14 made of molding sand or cement. Sensor elements 5 themselves; 6 are for protection during transport and during dipping in the melt first surrounded by a metal cap 15, which during or after immersion of the sensor in the metal melt melts and releases the sensor elements 5; 6.

[0017] Sensor elements 5 themselves; 6 are for protection during transport and during dipping in the melt first surrounded by a metal cap 15, which during or after immersion of the sensor in the metal melt melts and releases the sensor elements 5; 6.

Heraeus Electro-Nite International NV Plenipotentiary:

PL-PAT-2012-876

EP 2 228 643 B1