Key Control Points for Protective Pouring in the Continuous Casting Process

Core Objectives of Protective Pouring

Various Types of Tundish Injection Protection

a—Submerged Nozzle; b—Sealing Tube; c—Protective Cover; d—Liquid Nitrogen

1—Tundish; 2—Mould Powder; 3—Mold; 4—Liquid Nitrogen Ring

Protective pouring, the lifeline of continuous casting quality control, aims to isolate the steel from air throughout the entire process, blocking the path of secondary oxidation, reducing inclusion formation, and improving castability. During the transfer of molten steel from the ladle to the mold, there are four key oxidation risk zones: ladle inflow, tundish steel level, tundish inflow, and mold steel level. Key control measures must cover both physical isolation and atmospheric protection in these four areas. Practice has shown that standardized implementation of protective pouring can reduce the subsurface inclusion defect rate in ingots by over 60% and increase the aluminum recovery rate in aluminum-containing steel to over 95%.

Precision Control Technology for Key Equipment

(I) Control of Protective Equipment from Ladle to Tundish

1. Shroud System Optimization

Al-C shrouds are used for physical isolation. During installation, they must be inserted 100mm ± 5mm below the tundish steel surface and have a concentricity deviation of ≤3mm with the ladle outlet to prevent steel splashing and oxidation due to misalignment. Argon gas must be introduced to seal the joints, with the pressure controlled at 0.2-0.4MPa to ensure the free oxygen content in the protective atmosphere is less than 1%. Otherwise, the protective effect will be lost. Experience at one steel plant showed that when the shroud installation deviation exceeds 5mm, the total oxygen content in the steel increases by more than 0.003%.

2. Tundish Cover System Configuration

A double-layer slag cover (a bottom layer of insulating slag and an upper layer of adsorbent slag) is used, with a thickness of 50-80mm. This reduces heat loss and absorbs floating inclusions in the molten steel. The cover must be baked to a moisture content of less than 0.5% to prevent gas release at high temperatures that can cause molten steel surging.

(II) Equipment Control from the Tundish to the Mold

1. Precise Selection and Installation of Submerged Entry Nozzles (SEN)

Select the sprue nozzle structure based on the cross-section of the ingot: a single-hole, straight-through type is used for small billets, and a double-side-hole type is used for large blooms and slabs. The side hole inclination angle should be controlled between 15° and 35°, and the outlet diameter should be matched to the casting speed (e.g., a φ20mm outlet should be used for a casting speed of 1.2 m/min). During installation, the mold must be aligned with a deviation of ≤2 mm. The insertion depth should be controlled between 120-180 mm for slabs and 80-120 mm for billets. Too deep an insertion depth can easily lead to inclusion accumulation, while too shallow an insertion depth can exacerbate liquid level fluctuations.

2. Mold Sealing Structure Design

A flexible sealing chamber surrounds the tundish for injection, continuously introducing argon to form a gas curtain. The flow rate is adjusted to 10-20 L/min based on the casting speed, providing dual protection with the mold slag.

Optimization of Process Medium Parameters

(I) Refined Control of Shielding Gas

1. Dynamic Adjustment of Argon Parameters

The ladle shroud sealing argon gas adopts a “low-pressure constant-flow” mode, maintaining a stable flow rate of 5-10 L/min to prevent sudden pressure changes from causing gas entrainment in the molten steel. The mold argon gas flow must be linked to the casting speed: for every 0.2 m/min increase in casting speed, the flow rate increases by 2 L/min, but not exceeding a maximum of 25 L/min, to prevent excessive stirring that could cause liquid level fluctuations greater than ±3 mm.

2. Atmosphere Monitoring and Feedback

An online oxygen content monitor is installed in the sealing chamber. When O2 exceeds 1%, an automatic alarm is triggered and the argon flow rate is increased to ensure a stable shielding atmosphere.

(II) Full-Process Control of Mold Slag

1. Control of Key Mold Slag Performance Indicators

Select the slag system based on the characteristics of the steel grade: For aluminum-containing steel, use an acidic slag with a low Al₂O₃ content (≤8%) and a basicity of 0.85-1.10; for high-speed continuous casting (casting speed >4 m/min), use a slag with a low viscosity (0.5-0.8 Pa・s) and a fast melting rate. The core control standards are as follows:

Melting Temperature: 1100-1250°C (matched to the molten steel temperature)

Liquid Slag Layer Thickness: 10-15mm (maintained through frequent and small additions)

Consumption: 0.3-0.5kg/t steel (slab), 0.5-0.7kg/t steel (billet)

2. Operating Procedures

Use a multi-point, even addition method, adding slag every 2-3 minutes, with each addition not exceeding 0.5kg/m², to avoid localized crusting. During the steel pouring process, continuously monitor the slag layer and immediately adjust the slag addition frequency if the sintered layer thickness exceeds 20mm. One steel mill reduced the longitudinal crack defect rate of ingots by 42% by optimizing the mold slag addition process.

Abnormal Handling