Cast iron properties influence method

The invention concerns a method for affecting the properties of cast iron by adding magnesium to the molten iron. Further, the invention relates to a sensor for measuring the oxygen content of the iron melt with an electrochemical measuring cell having a solid electrolyte tube.

On the whole, free magnesium content of the molten iron is considered as a specific factor for the formation of spheroidal or vermicular graphite in magnesium treated iron. The current practice for regulating the production of ductile cast iron consists of determining the total magnesium content, that is, free and bound magnesium, with the help of the samples spectrographically. True, this method gives an incomplete picture, since the content of free magnesium is not known, and measurements do not give any information about the activity of oxygen. However, the activity of oxygen, which is in equilibrium with free magnesium is the determining factor in the formation of graphite shape. The so-called ductile cast iron is the normal gray (foundry) cast iron, which is processed by spheroidizing additive, so that most of the iron is in the form of graphite carbon in the so-called globular graphite (nodular graphite iron) or spherical graphite. Globular graphite cast iron must be analyzed with respect to the shape, size and number of particles, as these parameters affect the mechanical properties of cast iron. Visual analysis is complex and subjective, even if partially automated analysis. Measurements for this purpose are known for example from document US 5675097. US in DE 19,928,456 A1 describes measurements to determine the spatial structure of the graphite in the iron, which are based on the determination of oxygen and characterized by impaired visual methods. This allows you to react quickly and direction affect the production, increasing output or reducing marriage during casting. The quality of iron becomes a well-regulated.

Result treatment with magnesium in the pig iron can be determined, for example by spectrographic analysis or metallographic crystallized from a metastable structure samples (weiβerstarrung) or also by thermal analysis.

In general, pure magnesium or magnesium alloy is used to promote the spherical shape iron. Some of the added magnesium recovered in the iron oxygen and sulfur, the remaining part is the so-called free fractions of magnesium, which regulates the activity of oxygen. The content of free magnesium in the melt is the determining factor for globular cast iron. The proportion of free magnesium in the melt decreases with time, while the oxygen activity increases. This influences the structure and mechanical properties of cast iron.

Sensors for determining the oxygen activity in the molten metal are known, for example from German Patent DE 10310387 B3. There is described a solid electrolyte tube which at its outer surface has a coating of a mixture of lead zirconate and calcium fluoride so that, for example, iron in the melt can be conducted to measure the concentration of sulfur, silicon or carbon.

The prior art is also known to influence the properties of cast iron by adding magnesium to the molten iron according to US 6544359, B1, 08.04.2003.

The objective of this invention is to provide a method and sensor for controlling the manner in which existing equipment is improved, and a targeted effect on the mechanical properties of cast iron must be provided in the liquid phase.

The problem is solved by the features of the independent claims. Preferred embodiments are given in the dependent claims. In particular, the method according to the invention is characterized in that the measured oxygen content in the molten iron and molten iron that magnesium is added as long as the oxygen content in the molten iron at a temperature of about 1420 ° C as the reference temperature is about 0.005-0.2 ppm (1 / m). Since oxygen measurement is more accurate than hitherto possible measurement of magnesium (magnesium is in the melt as free magnesium and magnesium as linked so that the accurate accounting is not possible), the analysis of the mechanical properties of the cast iron becomes more accurate. The skilled person may install and use the correlation between the presence of smaller particles of graphite at the oxygen reducing on one side and the smaller particles of graphite at an elevated oxygen content, on the other hand.

Thus, the mechanical properties can be correlated, as described in U.S. Patent 5675097, for example in respect of tensile strength, elongation and resistance to deformation. For iron unexpectedly it turned out that the maximum elongation is achieved if the additive is magnesium occurs as long as the oxygen content becomes less than 0.1 ppm, preferably between 0.08 and 0.1 ppm. At lower or higher oxygen content extension iron decreases again. Preferably, if the iron is added to the melt about 200-750 ppm magnesium to achieve the desired oxygen content.

The sensor according to the invention is characterized in that on the outwardly facing surface of the solid electrolyte tube coated with a layer of zirconia. In particular, the zirconia may be stabilized by a layer of calcium oxide, yttrium oxide and / or magnesium oxide. Preferably, the layer is stabilized up to 30 wt.% Calcium oxide, up to 25 wt.% Of magnesium oxide and / or up to 52 wt.%

yttrium oxide. It is particularly preferred that the layer is stabilized about 4-6 wt.% Of calcium oxide. Preferably, the sensor layer is applied by spraying the plasma. It preferably has a thickness of about 30-50 microns, particularly about 40 microns. Tube of a solid electrolyte on which a layer is preferably a tube of zirconia which may be stabilized with about 2 wt.% Of magnesium oxide.

Further embodiments of the invention describe using images. They show:

1 - the relationship between the number of particles of graphite and oxygen content (oxygen activity aO);

2 - the relationship between elongation and oxygen content;

Figure 3 - a cross-sectional view of the sensor head according to the invention;

4 - a partial section of a further embodiment of the sensor.

Figure 1 shows that the number of graphite particles with a high oxygen content (oxygen activity aO) increases. Therefore, by adjusting the oxygen content of magnesium additive can set the number of graphite particles. Thereby deliberately influence the properties in the cast iron melt. Maximum observed for globular oxygen activity of about 0.10 to 0.12 ppm (1 / mn) (applies to 1420 ° C). If the oxygen activity drops below 0,10 ppm, decreases globular.

This corresponds to a well-known experience of foundry practice that an increased proportion of magnesium has a negative impact on the globular.

Figure 2 shows the relationship between the elongation of iron and oxygen content. Maximum extension (deflection), the difference at about 0,08 ppm. At lower oxygen activity slightly less elongation, presumably due to the decreased globular. If the oxygen activity exceeds the optimum value, this leads to a continuous reduction in elongation. The graph shows that it is possible by setting the oxygen content in molten pig iron by adding magnesium influence on elongation of cast iron.

3 shows a sensor according to the invention. The metal tube 1 are disposed two electric wires (Cu / CuNi / wire) with standard excipient 3. A connecting element 4 electrically connected to the measuring wire rod or other support, and further with a computing unit. The other end of the wires 2 is connected with a thermocouple 5 and 6. The electrochemical measurement cell Electrochemical measuring cell 6 has a solid electrolyte tube (cell ZrO2) with a steel protective screen as an outer shell. Cell ZrO2 on its outer surface has a layer of zirconia, which is stable 5 wt.% Of calcium oxide. This layer has a thickness of 40 microns. In the drawing it is not presented separately because of the solid electrolyte tubes known in principle.

Thermocouple 5 is fixed in sealing cement 7. thermocouple measuring cell 6 is also fixed in cement 8, it locks it from the inside end of the sensor located with a sealing plug 9 derived by means of electrical contacts. Both sensor element 5, 6 are connected with each other through plastic clip 10. The thermally insulated portion of the wire 11 is conducted through the interior of the metal tube 1. For immersion sensor on the outside of the metal tube 1 is provided sand body to protect the sensor.

Figure 4 shows a similar device for which is shown contacting the sensor in the tube carrier 13. The tube carrier 13 is formed from cardboard and with its front, close to the sandy side of the body is surrounded by a protective tube 14 with coating, which is formed of foundry sand or cement .

Sami sensor elements 5, 6 for protection during transport and immersion in a melt initially closed metal lid 15 which during or after the immersion sensor into the molten metal melts and releases the sensor elements 5, 6.