Why Vacuum Degassing is Needed
Dissolved gases — hydrogen, nitrogen, and oxygen — are soluble in liquid steel in quantities that significantly exceed the solubility limits in the solid. During solidification, excess dissolved hydrogen diffuses to defect sites and grain boundaries, causing hydrogen-induced cracking (HIC) in heavy plate, forgings, and rail steel. Nitrogen above approximately 80 ppm causes strain ageing in low-carbon steel strip, making it unsuitable for deep-drawing applications. Carbon removal below approximately 30 ppm — essential for interstitial-free (IF) steel used in automotive body panels — cannot be achieved by BOF or EAF chemistry alone and requires vacuum treatment. Vacuum degassing resolves all three by exposing the steel to a very low partial pressure of the offending gas, which drives it out of solution according to Sievert's Law.
RH Degasser (Ruhrstahl-Heraeus)
The RH degasser is the dominant vacuum treatment process for large-scale flat-rolled production. Two refractory-lined snorkels project downward from the base of the vacuum vessel: the up-snorkel is immersed in the steel ladle and argon is injected through it, entraining steel upward into the vacuum vessel at circulation rates of 60–150 t/min. Steel overflows from the up-snorkel side to the down-snorkel and returns to the ladle, creating a continuous circulation loop. Inside the vessel, at pressures of 0.5–2.0 mbar, dissolved CO bubbles nucleate on the circulating steel droplets and are pumped away. Alloying additions can be made via a gravity hopper or injection lance into the circulating steel stream, enabling chemistry control under vacuum. A full decarburisation treatment to <30 ppm C takes approximately 12–20 minutes of circulation under vacuum.
RH Oxygen Blowing (RH-OB / RH-KTB)
For IF steel production, oxygen can be blown into the circulating steel via a top lance (RH-OB, or the Japanese KTB variant) to promote decarburisation at higher initial carbon levels — arriving carbon from the BOF is typically 0.03–0.06% and must be reduced to <0.003% for IF grades. The injected oxygen reacts with dissolved carbon: C + O → CO (g), accelerating the vacuum decarburisation reaction and reducing treatment time. After decarburisation is complete, aluminium additions for deoxidation are made, followed by titanium or niobium additions to tie up any residual interstitial carbon and nitrogen. This sequence — BOF → RH-OB decarburisation → Al deoxidation → Ti/Nb addition → continuous caster — defines the production route for virtually all automotive exposed-quality panels worldwide.
Tank Degasser (VD) and VOD
The vacuum tank degasser (VD) is a simpler alternative: the ladle is placed on a cover and sealed, and vacuum is applied via steam ejectors or mechanical pumps. Argon is stirred through the bottom porous plugs throughout the treatment. The VD is widely used for hydrogen removal in heavy plate (linepipe, offshore, pressure vessel) and forging grades, where the target is <1.5 ppm H to prevent hydrogen-induced cracking. The vacuum oxygen decarburisation (VOD) vessel is a tank degasser with an oxygen lance, used primarily for stainless steel refining — blowing oxygen under vacuum allows deep decarburisation (to <0.03% C) without excessive chromium oxidation, which would occur at atmospheric pressure.
Hydrogen Removal and Heavy Plate Quality
As-tapped steel from the BOF or EAF contains 4–7 ppm of dissolved hydrogen, depending on scrap moisture, lime quality, and atmospheric humidity. For thick-section products — plates >50 mm, large forgings, railway axles, rail — hydrogen above approximately 1.5–2.0 ppm causes hydrogen-induced cracking (hairline cracking, flaking), which can appear hours to days after rolling as the steel cools and hydrogen diffuses to microstructural traps. Vacuum degassing to <1.5 ppm H, followed by controlled slow cooling of the product, eliminates this failure mode. For thin-gauge flat-rolled products, hydrogen diffuses out rapidly during rolling and coiling and vacuum degassing for H removal is not required.
RH is Essential for Automotive IF Steel
Without RH treatment, it is impossible to produce interstitial-free (IF) steel — the ultra-low carbon (<30 ppm C, <30 ppm N) grade used for automotive exposed panels, deep-drawn door inners, and outer skins. IF steel's exceptional formability — elongation >40%, r-bar >1.6 — is entirely dependent on the removal of interstitial carbon and nitrogen that would otherwise pin dislocations and cause Lüders band stretcher-strain markings on formed panels. Every integrated flat-rolled plant supplying automotive OEMs operates at least one RH degasser.