Determination of hydrogen content in molten steel

Hydrogen is the main cause of many defects and failures in the steelmaking process. The main function of the VD furnace is vacuum degassing. The vacuum chamber is evacuated by a vacuum pump, and the molten steel is degassed under vacuum to reduce the hydrogen and nitrogen content in the molten steel. At the same time, the steel slag interface reaction is used for desulfurization. Accurately measuring the hydrogen content in molten steel is a main technical indicator to measure the degassing effect of the VD furnace. In order to make special steel, the content of [H] in molten steel after VD treatment is required to be controlled to be less than or equal to 0.12ml. per hundred grams of steel. Hangzhou Steel Changxing Electric Furnace uses the immersion direct-reading hydrogen analyzer (Multi-lab Hydris System) of Heraeus Company to measure the hydrogen content in the molten steel before and after VD treatment.
1 Measuring principle of Hydris hydrogen determination system

2 Advantages of Hydris hydrogen determination system

2.1 The thermal conductivity detector (TCD) is used to monitor the thermal conductivity rate of the nitrogen-carrying gas leaving the heating wire, and the data calculation is completed by the built-in microprocessor. The success rate of measurement is high. System accuracy: within the measuring range of 0-6ppm, the error is <0.15ppm; within the measuring range of 6-10ppm, the error is <0.20ppm.

2.2 Corresponding to different steel grades, the hydrogen activity coefficient fH is also different. Therefore, it is necessary to adopt different K/f coefficients for steel plants with large changes in steel grade and smelting temperature. The Hydris hydrogen determination system provides an RS-232 serial port for the communication between the hydrogen meter and the secondary computer. Before the measurement, the operator manually enters the furnace number, and the steel number information is taken from the laboratory through the secondary computer network. The meter automatically adjusts the K/f coefficient according to the obtained steel grade, steel temperature, and measurement data through the calculation of the microprocessor, and finally obtains the correct hydrogen content. This helps steel workers to take corresponding measures in time according to the hydrogen content required by the number of steelmaking.

2.3 The instrument provides two RS-232 ports for data communication, and a parallel port for printer connection.

2.4 The measurement range is between 0.4~13ppm, and the measurement time is between 28 seconds and 70 seconds.

2.5 The storage capacity of measurement records is 120.

3 Measurement and operation sequence

3.1  Check the nitrogen inlet pressure of the pneumatic cabinet and the use of the loop filter, and confirm that the blinking cursor on the lower right foot of the meter screen indicates the ready state (READY).

3.2  Press the start button on the pneumatic cabinet or the F2 key on the instrument, the system starts to work, and the fixed hydrogen muzzle blows out. The blowing pressure on the meter screen is constantly rising. About 10 seconds, the yellow light on the pneumatic cabinet will light up and the red light will stay on.

3.3  Install the probe carefully, tighten it firmly, but do not hit it with force. After 4~5 seconds, the green light is on, the red light is off, and the yellow light remains on. At this time, the blowing pressure on the screen of the meter is continuously rising, to about 4500Pa, the yellow light is off, only the green light is on, and the system enters the state of preparation for measurement.

3.4  Argon blowing must be stopped before measurement, and the molten steel is calm for 1~2 minutes. At the same time, avoid measuring during alloy addition and avoid measuring immediately after alloy addition.

3.5 Carefully lift the fixed hydrogen gun and insert it into the molten steel. Keep the probe as vertical as possible and insert the molten steel to a depth of 300~400mm. Hold it, hold it, and keep it in balance. Don’t shake it left and right. Before the end of the measurement, the fixed hydrogen gun must not be lifted until the red light indicating the end of the measurement lights up (at the same time the buzzer in the pneumatic cabinet sounds).

3.6” After the measurement, take out the probe carefully in time and check whether the contact block is clean. Put the fixed hydrogen gun, do not leave the probe after the test on the fixed hydrogen gun, so as not to damage the fixed hydrogen gun.

3.7 Through the calculation of the built-in microprocessor of the meter, the hydrogen content is displayed on the meter screen.

3.8  After the system enters the ready-to-measure state (the green light on the pneumatic cabinet is on), there is no measurement for some reason, and the system will automatically return to the start state after waiting for a period of time. If you need to measure, just do the startup procedure again. Press CLR+F2 at any time to interrupt the measurement process.

4  Several factors affecting the determination of hydrogen content

The online hydrogen determination system is a high-precision detection instrument. Improper operation can easily cause inaccurate measurement data and may also cause damage to the equipment.

4.1 In order to ensure that the system can work normally, an air simulation test is necessary before measurement. The hydrogen content is required to be between 1.2ppm and 1.6ppm, and the straw pressure Psu (Pvac) is required to be greater than 730 hPa.

4.2 The correct setting of instrument parameters is an important link to ensure the accuracy of hydrogen content measurement.

4.3 The loop filter is a very important part of the fixed hydrogen system. Filters can remove water, oil and dust from the pipeline because they will contaminate the thermal conductivity detector (TCD) and block the movement of pumps and valves. Therefore, it is extremely important to check the life of the filter during use. Generally speaking, the service life of the filter is 35 times.

4.4 Once the system leaks, the measurement process will not be able to proceed. You must first perform the leak detection procedure, check the relevant links, find the leaking parts, and take sealing measures. Only after the leak detection result is OK, can the hydrogen measurement be performed.

4.5 The suction pipe pressure (Psu) is too high or too low will cause the measurement process to be unable to proceed. The upper limit setting value of the suction pipe pressure (Psu) is 1150hpa, and the lower limit setting value is 400 hpa. Such failures are caused by major hardware failures, such as clogged straws and system gas leakage.

4.6 The blowpipe pressure (Pbl) exceeds the upper limit of the pump (the standard setting value of the upper limit is 2500hpa), indicating that the blowpipe is blocked, and the probe may be damaged. Check the measuring gun.

4.7 In order to ensure measurement accuracy, the instrument needs to be calibrated every quarter.

5 Conclusion

The online hydrogen determination system is a high-precision detection instrument. As a consumable part, the cost of the fixed hydrogen probe is relatively high. In order to minimize the consumption of the fixed hydrogen probe and to ensure the success rate and accuracy of the measurement, the relevant operating procedures must be strictly followed in the field measurement. If there is a problem in the measurement process, you can analyze the problem according to the curve record on the meter screen to find the problem. At the same time, the maintenance and regular maintenance of the instrument system are also very important.