Controlling converter (BOF) slag discharge is not only a crucial process technology for improving molten steel quality but also a key measure for reducing steelmaking costs. This article discusses the detrimental effects of converter slag discharge on smelting costs and molten steel quality, and compares the discharge rates of various slag-blocking methods. Using Benxi Steel as an example, this article focuses on the principles of the slag-blocking method using a sliding plate for upper and lower slag detection, along with the problems encountered during its use and their solutions, ultimately achieving the desired results.

In recent years, with shifts in global steel consumption patterns, the demand for high-quality, high-value-added steel has increased. At the same time, my country’s conventional steel production is in oversupply, making the development of efficient and low-cost clean steel processing platforms a key research focus for major steel mills. Reducing converter slag discharge is an effective way to improve molten steel cleanliness, enhance converter steel product quality, and reduce steelmaking costs. Effective slag-blocking during converter tapping not only improves molten steel quality and alloy yield, but also provides favorable conditions for refining operations. Comparative studies have shown that the sliding plate slag blocker effectively and stably controls slag discharge.

1.Steelmaking Plant Production Process Flow

The steelmaking production process flow is shown in Figure 1.

(1) Hot metal desulfurization:Spraying magnesium powder + lime powder to desulfurize hot metal. Hot metal is 100% desulfurized.

(2) Converter smelting: 180t top and bottom double-blown converter. During the tapping process, deoxidation and alloying are carried out according to the requirements of the steel grade. Slag stopper is used in the later stage.

(3) Refining: 180t LF refining furnace. The composition of the steel is adjusted to the target range according to the requirements of the steel grade.

2.The harm of converter slag volume to smelting cost and molten steel quality

2.1 Reduce alloy yield

During the steel tapping and alloying process, the FeO content of about 15% in the converter final slag will react with the Mn in the alloy to form MnO, which reacts with the Al in the deoxidized alloy to form Al2O3 inclusions. The reaction equations are as follows (1) and (2):

Mn＋(FeO)＝Fe＋(MnO)(1)
2Al＋3(FeO)＝3Fe＋(Al2O3)(2)

2.2 Harmful elements enter the molten steel

Most steel grades have strict control requirements on silicon and phosphorus. Low silicon steel grades require a silicon content of no more than 0. Low phosphorus steel grades require a phosphorus content of no more than 0. During the refining and desulfurization process, desulfurization requires a reducing atmosphere. Using aluminum to make slag reduces the FeO content in the ladle top slag, which is beneficial to desulfurization. However, at the same time, Al reacts with P2O5 and SiO2 in the top slag to generate elemental P and Si that enter the molten steel. The reaction equations are as follows (3) and (4), resulting in the steel grade P and Si mass fraction exceeding the standard.

10Al＋3(P2O5)＝6P＋5(Al2O3)(3)
4Al＋3(SiO2)＝3Si＋2(Al2O3)(4)

2.3***Carbon steel grade inclusions and “aluminum burning”

***Carbon steel requires RH vacuum treatment. No deoxidation operation is performed during the steelmaking and tapping process. Instead, oxygen in the molten steel is used for decarburization during the refining vacuum treatment process to make the carbon mass fraction meet the steel grade requirements. After RH decarburization, aluminum balls are used to remove oxygen from the steel. Some of the added aluminum combines with oxygen in the steel to form the deoxidation product Al2O3 (most of which floats to the surface before casting), another portion is consumed by the FeO in the ladle top slag to form component aluminum, and the remainder is consumed by the FeO in the ladle top slag. Experiments have shown that the aluminum “recovery rate” (i.e., the ratio of the sum of the aluminum consumed by combining with oxygen in the steel to form the deoxidation product Al2O3 and the aluminum consumed to form component aluminum in the steel) after adding aluminum balls fluctuates significantly, ranging from as high as 60% to as low as 30%. Analysis shows that the aluminum “recovery rate” is directly related to the FeO mass fraction in the ladle top slag: the higher the FeO mass fraction, the lower the aluminum “recovery rate.”

After RH refining and deoxidation, the [O] mass fraction in the steel is extremely low, reaching 3×10⁻⁶ to 5×10⁻⁶. This makes it impossible to consume the Al in the steel. Instead, the FeO in the ladle slag continuously provides oxygen to the steel, enabling the 3[O] + 2Al (Al₂O₃) reaction to proceed. Over time, the amount of Al₂O₃ inclusions in the steel gradually increases, and most of them do not have time to float up, entering the billet with the steel flow. Large Al₂O₃ particles directly affect the surface quality of automotive sheet metal.

3.Selection and Research of Slag Stopping Technology

3.1 Selection of Slag Stopping Technology

Converter slag control generally uses slag stoppers, slag stoppers, slag stoppers, and slag stopper markers. With technological advancements, pneumatic slag stoppers, slide plate slag stoppers, and infrared slag detection systems have emerged. The slag removal rates of various slag stopper methods are shown in Table 1. The slide plate slag stopper combined with slag detection achieves a slag removal rate of 2–4 kg per ton of steel, making it the most stable and reliable slag removal method.

3.2 Slag Discharge Model During Converter Tapping

Of the slag removed from the converter to the ladle, the initial slag accounts for approximately 30%, the slag carried down from the molten steel surface by the vortex effect accounts for approximately 30%, and the final slag accounts for approximately 40%.

3.3 Principle of Automatic Control of Slide Plate Slag Stopping for Tapping

During converter smelting, the slide plate is open. At the end of converter smelting, the hydraulic station pump is manually activated, and the converter tilting begins. When the converter reaches the 35° position, a signal is issued to close the slide plate, causing it to automatically close. When the converter tilts to 75°-80°, the slag has completely risen, and a slide-open command signal is issued, causing the slides to open and steel tapping to begin. At the end of tapping, when the infrared slag detection system detects slag, a slide-close command signal is issued, causing the slides to automatically close. After the converter reverses its tilt to the vertical position, a slide-open command signal is issued, causing the slides to open.

4.Problems with Slide Plate Slag Stops and Solutions Developed

4.1 The distance between the lower edge of the taphole and the upper edge of the ladle is small, resulting in insufficient space for the slide plate mechanism.

The No. 6 converter at Bengang Plate Steel Plant previously used a standard slag stop. As long as the converter’s maximum rotation radius met the requirement to avoid contact with other equipment, the taphole’s peak point should be no more than 400 mm from the ladle. Therefore, the original design had a maximum taphole distance of 500 mm from the ladle. With the introduction of slide plates for slag stoppers, the slide plate mechanism was installed outside the taphole, increasing the taphole length by 530 mm. This increased the converter’s rotation radius after installation, and the safety distance between the lower edge of the taphole and the upper edge of the ladle was insufficient. To address this issue, the taphole length was innovatively shortened by 300 mm. A horizontal slide plate technology was also employed to ensure the safe and stable operation of the slide plate slag stop mechanism.

4.2 Reduced Taphole Life

Due to the implementation of the new slide plate slag stop technology, the taphole life was low during the commissioning phase, reaching 89 taps. This failure to meet process requirements significantly impacted production rhythm control. On-site monitoring revealed that vibration from the assembly and disassembly equipment during bowl brick replacement significantly impacted the taphole. By improving the quality of the fireclay and the service life of the bowl bricks, and reducing the frequency of bowl brick replacements, the taphole lifespan reached a maximum of 213 times and an average of 185 times, exceeding the original design requirement of 150 times and meeting production requirements.

4.3 Study on the Timing of Slide Closure

The slide slag barrier utilizes infrared to detect the slag ratio in the steel stream to determine whether to close the slide. If the slag ratio is set too low, slag curl during tapping can cause the slag ratio in the steel stream to reach the shutdown alarm value. The slag detection triggers a slide closure signal, preventing a large amount of molten steel from being fully tapped, resulting in secondary tapping or residual steel in the furnace. If the slag ratio is set too high, a large amount of converter final slag enters the ladle, defeating the intended slag barrier effect. In order to solve this problem, the tapping time is generally 5.5-9 minutes, and the converter tilting angle is 100°~110° at the end of tapping. It is set that when the tapping time is less than 4 or the converter tilting angle is less than 97° during the tapping process, the infrared slag detection only detects the slag ratio and provides detection data, but does not provide a slide plate closing signal. Under abnormal conditions, it can be closed manually. When the tapping time is ≥4 minutes and the converter tilting angle is ≥97°, the slide plate is immediately closed when the slag ratio reaches the alarm signal, and the tapping operation is ended. Through experimental calibration, when the slag ratio reaches 30% (30 frames per second, if the slag ratio in the steel flow is greater than 30% for 5 consecutive frames, a slide plate closing signal is issued), it can effectively reduce the residual steel caused by the wrong closing of the slide plate and effectively control the slag amount (see Table 2 test data).

5.Conclusion

(1) The slide plate slag blocking process can more effectively control the slag amount than the slag blocking standard. (2) Shortening the taphole length effectively addresses the limited space problem without affecting the slide plate’s slag blocking. The shortened taphole life and modified slide plate’s slag blocking achieved the desired results, providing a basis for other manufacturers’ slide plate slag blocking modifications.
(3) When the slag ratio reaches 30%, closing the slide plate effectively reduces residual steel caused by mistakenly closing the slide plate and effectively controls the amount of slag released.