JPH07119754B2 - Trace oxygen analysis method for steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for analyzing a trace amount of oxygen in steel, wherein oxygen in the steel is increased by thoroughly removing oxygen adhering to the surface of an analysis sample. Related to the technology to measure with precision.

[Prior Art] With the improvement in quality of steel, an extremely small amount of oxygen in steel has come to be regarded as important, and the accuracy of analysis in an extremely small amount region has become an important issue. For example, in Al deoxidized steel, the amount of oxygen in the steel is also a measure of the amount of inclusions in the steel, and in high-grade high-carbon steels used as bearing materials, the amount of inclusions is severely limited, and the amount of oxygen in the steel is 10 ppm. The following are required:

A commonly used analytical method for such trace oxygen is to heat a sample of about 1 g in a rod shape to around 2000 ° C in a graphite crucible in an oxygen-free environment to melt the sample and simultaneously remove oxygen in the sample. It is a method of extracting as CO. But,
In this case, CO may contain oxygen that was truly present in the steel and oxygen that was present as a surface oxide on the sample surface in a mixed manner.

In this way, oxygen existing on the surface and unrelated to inclusions in the steel is removed from the measurement target.Therefore, the surface oxygen was removed by grinding the sample surface layer with a grinder as a pre-analysis treatment. It was However, when the minute amount of oxygen is discussed, surface oxidation during grinder polishing becomes a problem.

Various polishing methods that do not leave oxygen on the surface have been studied, and as a result, in recent years, the oxide layer on the surface has been finally removed by electrolytic polishing using a liquid with little surface contamination. For example, iron and steel, vol.73, No.4,1981, S41
According to 1), acetylacetone-based or complex acid-based electrolytic polishing is recommended as the polishing method with the least surface contamination. Then, these electrolytic solutions are removed by washing with an organic solvent such as acetone, and the organic solvent is volatilized to purify the sample surface.

[Problems to be Solved by the Invention] However, the cleaning effect of the organic solvent of the electropolishing liquid containing oxygen differs depending on the condition of the surface layer of the sample, and the problem that the electropolishing liquid is not sufficiently removed remains.

The present invention was made to solve this problem.
The purpose is to accurately measure the amount of trace oxygen in iron and steel by strictly removing the electrolytic solution from the sample surface.

[Means for Solving the Problem] A means for achieving this object is a method of electrolytically polishing a sample as a pretreatment when a trace amount of oxygen in steel is extracted by heating and measured. 1200 ℃ or more 1400 ℃
This is a microanalysis method for steel in which preheating is performed at the following temperature, and then oxygen is extracted from steel to measure carbon by main heating that is usually performed.

[Operation] A chemical used for electrolytic polishing contains oxygen. For this reason,
Strictly removing the polishing liquid after electropolishing is an important matter, but when cleaning it off by liquid cleaning,
The cleaning effect depends on the shape of the surface to be cleaned. For example,
Even in the case of steel samples, the state of surface irregularities after electropolishing varies depending on the sample, and it is difficult to remove the sample with severe irregularities and the polishing liquid may remain. In particular, those having a high carbon content tend to remain. Figure 1 shows such a sample
It is the result of measuring oxygen extracted with heating at 1700 ° C. In the figure, the vertical axis represents oxygen signal intensity and the horizontal axis represents time. Two peaks can be seen in the extraction curve. The first peak begins to appear after heating for several tens of seconds, but the peak is low and disappears, and the peaks later are large. The appearance of the first peak differs depending on the sample, and it hardly appears in the sample with a smooth surface, but it appears more prominently in the sample with more unevenness. This first peak is due to oxygen generated from the polishing liquid left on the surface.

If you preheat even a sample with such unevenness,
The first mountain above disappears. Examining the relationship between this effect and the preheating temperature, if the temperature is low, the effect cannot be obtained, and if it is too high, oxygen in the steel may partly be extracted. That is, it is considered that the electrolytic polishing is decomposed by preheating or volatilizes even if it is not decomposed, and the preheating temperature may be the volatilization temperature of the polishing agent, and may be a temperature at which oxygen in the steel is not extracted. . Actually, 1200 ° C or higher and 1400 ° C or lower is appropriate.
If the temperature is less than 00 ° C, sufficient effect may not be obtained unless the preheating time is extended. Fig. 2 shows that the preheating temperature is 1400
The oxygen signal intensity curve in the case of ° C and the case of 1600 ° C are shown. In the figure, curve A shows the case of 1400 ° C., which is the case where the temperature is raised to 2000 ° C. and then the main heating is performed following preheating, and curve B shows the case of 1600 ° C. In A, after the first mountain has completely descended, the next large mountain appears due to the main heating and separates the two mountains, but in B, the next large mountain appears and cannot be separated before the first mountain has completely descended. That is, at 1600 ° C, surface oxygen is mixed with oxygen in steel to be extracted. As described above, when the preheating temperature exceeds 400 ° C., there is a possibility that part of oxygen in the steel will be extracted.

[Examples] Rod-shaped samples A and B having different diameters of 5 mm and produced by sucking up molten steel were analyzed under different preheating conditions and the surface oxygen content and the oxygen content in steel were measured. Regarding preheating, conditions outside the scope of the present invention were also performed as comparative examples, and the extracted oxygen amount was also compared as the oxygen amount in steel for the conventional example in which preheating was not performed.

For the sample, grind it with 5% hydrochloric acid
Ethanol solution (hereinafter referred to as hydrochloric acid system) or 10% acetylacetone / 1% trimethylammonium chloride /
Electrolytic polishing was performed at room temperature using a methanol solution (hereinafter referred to as acetylacetone system) at a current density of 5 mA / cm ² for 5 minutes, and then washed with acetone, and then subjected to analysis.
The analysis was carried out by the melting heating infrared absorption method. First, the preheating temperature was maintained for a predetermined time to extract surface oxygen, and then the temperature was raised to 2000 ° C. to measure the oxygen content in the steel. That is, the sample is put in a graphite crucible and heated by induction heating while flowing Ar gas, and oxygen is captured as CO,
It was measured by the amount of infrared absorption. Table 1 shows the preheating conditions and the analysis results.

In the examples, the oxygen extracted by the preheating had a large variation from 5 ppm to 9 ppm, but regarding the oxygen in the steel, the average oxygen amount in the sample type A was 11.0 ppm, the inter-sample variation was 0.9 ppm, and the average value in the sample type B. The variation between samples was as small as 0.2 ppm at 6.5 ppm. On the other hand, in the comparative example, the preheating temperature is too low in the sample No. 9, the oxygen content in the steel is considerably larger than that in the example,
The surface oxygen content was low. This is because surface oxygen was not sufficiently decomposed by preheating. In addition, Sample No. 10 and No. 11, which have too high preheating temperature, apparently have a large amount of surface oxygen and a small amount of oxygen in the steel.
At 1, a mountain weight was also seen in the oxygen signal intensity curve. In the conventional example, the amount of oxygen in steel was large, and it was almost equal to the total of the amount of surface oxygen and the amount of oxygen in steel of the same sample type in the example and the comparative example.

[Advantages of the Invention] As described above, according to the present invention, the oxygen in the steel is preliminarily heated at a specified temperature before being extracted by heating and measured, so that oxygen on the surface due to the remaining electrolytic polishing liquid is removed. It is possible to measure only oxygen in steel. For this reason,
The present invention is highly effective in providing accurate information about oxygen in steel, which is an analytical method with excellent accuracy and plays an important role in improving the quality of steel.

[Brief description of drawings]

FIG. 1 is a signal intensity graph of extracted oxygen for explaining the action, and FIG. 2 is a signal intensity graph of extracted oxygen for each preheating temperature.

Claims (1)

[Claims] 1. A method for electrolytically polishing a sample as a pretreatment when heating and extracting a trace amount of oxygen in steel for measurement.
A method for microanalysis of steel, which comprises preheating at a temperature of 1200 ° C or more and 1400 ° C or less before measuring an oxygen content.