From Scrap to Spec: EAF Steel Grade Selection and the Chemistry of Final Adjustments
The EAF makes steel. The downstream equipment - ladle furnace, degasser, continuous caster - defines what kind of steel. The EAF job is to deliver liquid steel at the right temperature and the right base chemistry. The ladle metallurgy job is to hit the tight specifications. A modern EAF-ladle-caster sequence can produce rebar, structurals, plate, wire rod, special bar quality, and some sheet grades. Each grade has different requirements, and the path through the EAF and the ladle is different for each.
Here is how grade selection works and where the chemistry tightens up.
Start with the order book and the available scrap.
A 60-ton EAF heat costs $35 to $60 per ton in raw materials and energy. The steel sells for $50 to $150 per ton above the raw material cost (the "metallurgical margin"). The margin is highest for the tightest specs - special bar quality (SBQ) and plate grades - and lowest for the loosest - rebar and merchant bar quality. A melt shop that wants high margin runs more SBQ and plate. A shop that wants volume runs more rebar.
Scrap selection drives the chemistry. EAF scrap is heterogeneous. A bundle of shredded autobody steel has different residual content (Cu, Ni, Cr, Mo, Sn) than a bundle of structural cuttings. The operator's job is to blend scrap mixes to hit a residual envelope that suits the grade being made.
Rebar grade allows high residuals - 0.30 to 0.40 percent copper, 0.20 percent nickel, 0.10 percent chromium are typical limits. Wire rod grade is tighter. Plate grade is tighter still. SBQ is the tightest. Each step down requires a cleaner scrap mix, which costs more.
The scrap management system is where this gets controlled.
A modern EAF scrap yard has scrap classification areas. Heavy melting scrap (HMS) goes in one area. Shredded steel in another. Bundles in another. Pig iron and HBI/DRI in another. Each area has a known chemistry range, tested regularly by sampling. The scrap crane operator pulls from different areas to build a charge mix with a specific residual target.
For rebar, the operator might pull 80 percent HMS plus 20 percent shredded. For plate, the mix shifts to 50 percent HMS, 30 percent pig iron (for its low residual content), and 20 percent DRI. For SBQ, the mix is 40 to 60 percent pig iron and DRI plus low-residual HMS, and the operator is highly selective about which HMS lots are used.
The EAF's job at tap is to deliver:
A specific tap weight (within 1 to 2 percent of target)
2. A specific tap temperature (typically 1620 to 1680 degrees C depending on grade)
3. A specific base chemistry for C, Mn, Si, P, S
4. Acceptable residual levels
5. Clean steel (low N, low H, low total oxygen)
The tap temperature target depends on the downstream equipment and the casting format. A thin slab caster needs lower tap temperature (1580 to 1620 degrees C) because the tundish and the caster are sensitive to high superheat. A thick slab or bloom caster can accept higher tap temperature. The EAF operator sets the temperature to hit the target, knowing that the ladle furnace and the ladle treatment will add or remove 20 to 50 degrees C of thermal energy through heating, alloying, and natural losses.
Ladle furnace (LF) treatment is where the steel grade is finalized.
The LF heats the steel with graphite electrodes (or, in some designs, with a channel inductor). It also stirs the steel with argon through a porous plug, homogenizes the chemistry, and provides a quiet vessel for alloy additions and slag conditioning.
The alloy trim happens in the LF. The base chemistry at EAF tap is approximate - say 0.05 percent C, 0.10 percent Mn, 0.05 percent Si, 0.020 percent P, 0.020 percent S, with some residual Cu, Ni, Cr from the scrap. The LF operator adds alloys to hit the final spec.
For a rebar grade (say ASTM A615 Grade 60), the target is 0.30 to 0.40 percent C, 1.0 to 1.5 percent Mn, 0.20 to 0.40 percent Si, P under 0.040 percent, S under 0.040 percent, plus optional microalloying with V or Nb. The alloy trim typically adds 1.0 to 1.5 percent Mn as FeMn, plus SiMn for Si and Mn, plus carbon as petroleum coke or graphite. Total alloy cost: $3 to $6 per ton.
For a plate grade (say ASTM A36), the target is tighter on residuals and tighter on P and S. The alloy trim uses cleaner FeMn (low-P grade), low-C FeCr if any chrome is needed, and careful carbon trim to hit 0.15 to 0.25 percent. Total alloy cost: $5 to $10 per ton.
For SBQ (say AISI 4140), the spec is tight on C (0.38 to 0.43 percent), Mn (0.75 to 1.00 percent), Cr (0.80 to 1.10 percent), Mo (0.15 to 0.25 percent), plus tight limits on residuals and inclusions. The LF work is more complex - the steel needs vacuum degassing for hydrogen, inclusion modification with calcium-silicon, and tight temperature control. Total alloy cost: $15 to $30 per ton. The selling price premium justifies it.
The desulfurization work happens in the LF too. A reducing slag cover (typically CaO-Al2O3-CaF2 with carbon or aluminum) pulls sulfur from the steel. With good slag practice and bottom stirring, the LF can drop sulfur from 0.025 percent to 0.005 percent. For each 0.005 percent of sulfur removed, the steel gains about $5 to $10 per ton in value for plate and SBQ grades.
Inclusion control is a different topic that is worth mentioning.
Non-metallic inclusions - alumina, silicates, sulfides - are the silent killers in steel. They cause fatigue failures in service, particularly in dynamic loaded parts like crankshafts, axle beams, and rail. The LF and the ladle treatment remove inclusions through three mechanisms: floating (argon bubbles carry inclusions to the slag), dissolution (the slag absorbs certain inclusion types), and modification (calcium-silicon converts solid alumina inclusions into liquid calcium aluminates, which are less harmful and float more easily).
Inclusion control is what separates a commodity rebar producer from a premium SBQ producer. The investment in argon stirring intensity, slag chemistry, calcium treatment, and ladle shrouding (to prevent reoxidation during tap) is substantial. But the price premium for clean steel pays for it several times over.
Vacuum degassing is another step that only some grades need.
A vacuum tank degasser (VTD) or a RH circulation degasser drops the pressure over the steel to below 10 mbar. At that pressure, dissolved gases - hydrogen and nitrogen - come out of solution. Hydrogen drops from 4 to 7 ppm at atmospheric pressure to below 1.5 ppm after vacuum treatment. Nitrogen drops from 50 to 80 ppm to 20 to 30 ppm. The hydrogen removal is critical for heavy plate and forging applications where hydrogen-induced cracking (HIC) and flaking are risks. Nitrogen removal is a bonus for high-quality sheet and tire cord applications.
Continuous casting is the final step, and the steel must arrive at the caster tundish at the right temperature with the right chemistry. Superheat - the temperature above the liquidus - is the critical variable. Too much superheat and the caster runs slow, the grains are coarse, and the breakout risk increases. Too little and the tundish nozzles freeze. The target superheat is 20 to 40 degrees C for most bloom and billet casters, 15 to 30 degrees C for thin slab casters.
The tundish is the last vessel before solidification, and the steel is in the tundish for 5 to 15 minutes depending on the sequence. During that time, inclusions continue to float into the tundish slag, and the temperature drops by 10 to 30 degrees C. The tundish flux and the tundish cover (typically an insulating ladle cover plus a basic tundish flux) protect the steel from reoxidation and heat loss.
Author: MONTE INTELLIGENCE EAF and ladle metallurgy team. For grade selection consulting and process audits, contact helenxu@cnlymonte.com.
