JPH0556465B2 - - Google Patents

Description

[Detailed description of the invention]

(1) Industrial Application Field The present invention relates to an oxygen probe that measures oxygen activity in molten metal using a zirconia solid electrolyte. (2) Prior Art In general, in metal smelting, controlling the amount of dissolved oxygen has a significant effect on the quality of the material, so the oxygen activity in the molten metal is measured during the smelting process. In recent years, oxygen probes using zirconia-based solid electrolytes have been used to measure oxygen activity values, and have achieved considerable success in measuring oxygen activity in molten steel. This oxygen probe utilizes the principle of an oxygen concentration battery to measure the difference between the oxygen potential of the reference electrode material and the oxygen potential in the molten metal using electromotive force.
As shown in FIG. 6, a zirconia-based solid electrolyte 3 containing zirconia as a main component such as ZrO ₂ -MgO and a molten metal side electrode 4 are embedded in the end face of the support body housing 2 provided with a heat-resistant exterior material 1. Set up
The hollow tip of the former is filled with a reference electrode material 5, and the base side is filled with stable heat-resistant powder 6, and the oxygen activity in the molten metal is measured using a lead wire connected to the reference electrode material 5. 7 and a lead wire 7a connected to the molten metal side electrode 4 are led to a recorder. A thermocouple 8 is usually provided in the housing 2 so that the molten metal temperature can be measured at the same time, and a metal cap 9 is usually provided at the end of the probe body to protect the sensor part from slag during immersion measurement. ing. (3) Problems to be Solved by the Invention Incidentally, in the case of this type of oxygen probe, it is absolutely necessary that the measured values be highly reliable, but in addition, there is a problem when it is immersed in high-temperature molten metal. Since there is a limit to the immersion time, it is also necessary to have a short response time so that the electromotive force can be stabilized. However, in the case of conventional oxygen probes, when the zirconia-based solid electrolyte 3 (hereinafter simply referred to as solid electrolyte 3) protruding from the end surface of the supporting body housing 2 is immersed in molten metal in an unprotected state, the solid electrolyte 3 There was a problem that measurements were impossible due to spalling and destruction. This spalling is thought to occur in the following process. When the oxygen probe is immersed in molten metal, the tip of the protrusion of the solid electrolyte is rapidly heated to the temperature of the molten metal, while the root embedded in the support body housing 2 is embedded in refractory cement. Therefore, it is hardly heated. Therefore, a large temperature gradient occurs at the boundary between the protrusion and the root, and the difference in the degree of volumetric expansion at the boundary increases, resulting in large strain, which causes spalling at the boundary and destroys the solid electrolyte 3. It is. Conventional oxygen probes that have prevented such destruction of the solid electrolyte 3 include those in which the entire solid electrolyte 3 is covered with a closed tube cap at one end, and those in which the entire solid electrolyte 3 is coated with a refractory coating agent (Utility Model 55).
-172851) are known. These oxygen probes use caps and coatings to reduce thermal shock when immersed in molten metal, but since the part filled with reference electrode material 5 is also covered, the inside of the probe is exposed to the temperature of the molten metal. It took a long time to reach this point, the response time was long, and the responsiveness was poor. For this reason, a stable waveform could not be obtained within the allowable immersion time, and oxygen activity could not be accurately measured in some cases. In particular, in the case of a coating coated with a coating agent, there is a risk that the oxide forming the coating film will contaminate the molten metal surrounding the solid electrolyte 3, and if the coating film becomes thick to a certain extent, the diffusion potential of the film itself cannot be ignored. It was not possible to determine whether the measured electromotive force was due to the true oxygen potential difference between the reference electrode and the molten metal, and the reliability of the measured values was lacking. (4) Means for Solving the Problems The present inventors investigated the behavior of the solid electrolyte 3 in order to develop an oxygen probe that does not impair responsiveness and reliability, and that does not destroy the solid electrolyte 3 when immersed in molten metal. As a result of various studies, the following findings were obtained. (a) The fixed electrolyte 3 has a protruding length of 10 mm or more from the end surface of the support body housing 2, preferably 10 to 60 mm.
The protrusion is made to have a diameter of mm, but the breakage due to thermal shock during measurement is due to spalling that occurs at the boundary between the root and the protrusion, as described above. However, when examining the state after destruction, spalling occurred only at the protruding base of the solid electrolyte 3 and did not extend to the tip portion filled with the reference electrode material 5. (b) When examining the part of the solid electrolyte 3 that actually functions as an electrolyte in an oxygen concentration battery, only the part filled with the reference electrode material 5 is found, and the other parts do not function as an electrolyte. Based on these findings, the present inventors coated the solid electrolyte 3 from the end surface of the supporting body housing 2 to the vicinity near where the reference electrode material 5 is filled with a refractory coating agent. When the thermal shock to the protruding base is alleviated, the solid electrolyte 3 is not destroyed, and the part filled with the reference electrode material 5 is exposed, so that responsiveness and reliability are not impaired. found. Thus, the present inventors have developed an oxygen probe having a structure in which a zirconia-based closed-end tube solid electrolyte filled with a reference electrode material in the length direction is protruded from the end surface of the support main body housing, and the protruding base on the outer surface of the solid electrolyte. We have developed an oxygen probe characterized in that a portion of the reference electrode material from the housing end surface to at least 2 mm before the housing side end is coated with a refractory coating agent over a distance of 10 mm or more from the housing end surface. In the present invention, the area coated with the refractory coating agent is at least 10 mm or more from the end face of the housing. If it is less than 10 mm, a large thermal gradient will occur at the boundary between the solid electrolyte root and the protrusion, and the thermal shock mitigation effect will be small. It is from. In addition, the object of the covering part is at least 2 points from the housing side end of the reference electrode material 5.
The reason why it is set to 1 mm before the reference electrode material 5 is because if the coating part reaches the reference electrode material 5, it will prevent the temperature of the reference electrode material 5 from increasing, and the response will decrease, and the measured electromotive force will be influenced by the coating agent, which will reduce the reliability of the measured value. This is because there will be no more. The refractory coating agent that covers the solid electrolyte 3 may be of any composition as long as it can alleviate thermal shock to the solid electrolyte 3. For example, commercially available Al ₂ O ₃ type or ZrO ₂ type refractory coating agent may be used. It can be anything. This refractory coating agent may be applied to the solid electrolyte 3 by applying it to the solid electrolyte 3 in advance and then embedding it in the support body housing 2, but it is easier to apply it using a brush or the like after embedding it. A coating thickness of approximately 50 μm after drying is sufficient for thermal shock mitigation. Note that there is no problem even if the coating thickness is uneven. (5) Embodiment Figures 1 and 2 show an embodiment of the oxygen probe of the present invention, and each member of the oxygen probe is similar to the conventional oxygen probe shown in Figures 5 and 6. Only the solid electrolyte 3 is coated with a refractory coating agent 10 for 10 mm or more from the end surface of the support body housing 2 toward the tip side. Table 1 shows that in this oxygen probe, the length l of the refractory coating from the end face of the support body housing 2 was varied, and low carbon aluminum killed steel and low carbon rimmed steel (molten steel temperature was
The figure shows the spalling occurrence rate when immersed at 1600±20°C. The spalling incidence was determined by manufacturing 50 refractory coatings with different coating lengths l for each coating length.
Spalling did not occur when the coating length was 10 mm or more.

[Table] Also, FIGS. 3 and 4 show similar oxygen probes in which the distance L between the tip of the refractory coating agent 10 that coats the solid electrolyte 3 and the housing side end of the reference electrode material 5 is varied, and the same molten steel as described above is shown. The response time and electromotive force were measured by immersing the solid electrolyte 3 in the solid electrolyte 3 and compared with those of the uncoated solid electrolyte 3. It was slower than the uncoated product with electrolyte 3, and the electromotive force was also erroneous with respect to the true oxygen potential difference between the reference electrode and the molten steel. In particular, when the part filled with the reference electrode material 5 was covered, the response time was slow and the electromotive force had a large error of 40 mV or more. Moreover, this error differed depending on the type of refractory coating agent 10, and highly reliable measured values could not be obtained. On the other hand, when the distance L was 2 mm or more, both the response time and the electromotive force were the same as the uncoated product. (6) Effects As described above, in the oxygen probe of the present invention, the protruding base side of the solid electrolyte is coated with a refractory coating agent, so the thermal shock applied to the protruding base is alleviated, and destruction of the solid electrolyte can be prevented. . In addition, the outer surface of the solid electrolyte in the part filled with the reference electrode material is not coated with a refractory coating agent, so it is not affected by the refractory coating agent.
Excellent responsiveness and reliability.

[Brief explanation of drawings]

FIGS. 1 and 2 show one embodiment of the oxygen probe of the present invention, with FIG. 1 being a sectional view and FIG. 2 being a bottom view. Figures 3 and 4 are similar to Figure 1;
3 shows the response time and electromotive force when the distance between the tip of the refractory coating coating the solid electrolyte and the housing side end of the reference electrode material was varied in the oxygen probe shown in FIG. 2. FIGS. 5 and 6 are a cross-sectional view and a bottom view, respectively, of a conventional oxygen probe. 2... Support body housing, 3... Zirconia solid electrolyte, 5... Reference electrode material, 10... Refractory coating agent, l... Refractory coating agent coating length, L... Tip of refractory coating agent and the housing edge of the reference electrode material.

Claims (1)

[Claims] 1. In an oxygen probe having a structure in which a zirconia-based one-end closed-tube solid electrolyte whose part in the length direction is filled with a reference electrode material protrudes from the end face of the housing of the support body, the protruding base on the outer surface of the solid electrolyte extends from the end face of the housing. An oxygen probe characterized in that a portion of the reference electrode material up to at least 2 mm before the housing side end is coated with a refractory coating agent over a distance of 10 mm or more from the housing end face.