FeSi 75 vs FeSi 45: Which One for Steelmaking, Which One for Foundry (A 5-Step Method)

2026-09-14 10:52:37

To put the same 100 tonnes of silicon into a furnace, FeSi75 means shipping 133 t and FeSi45 means shipping 222 t. Those extra 89 tonnes have to be paid for as freight, given warehouse space, and heated to molten-metal temperature — or, in an electric arc furnace, they become iron you were going to buy anyway.

Here is the answer up front: there is no better grade, only a better calculation. Do not ask which is better; ask what you are measuring against. The order is fixed: work out how much silicon you actually need → look at which process step you add it in → check the impurity budget → compare price per tonne of silicon rather than per tonne of alloy → then add freight, temperature loss and storage. Five steps, and the answer comes out on its own — and it regularly comes out differently for different plants.

The most common mistake in this business is treating the two grades as high-spec and low-spec versions of the same product. They are not. Their impurity profiles differ, the process steps they suit differ, and their sensitivity to addition method differs.

1. The short version: this is a question of basis, not of quality

If you only want one line you can use today:

Ask yourself: am I buying silicon, or do I also want the iron that comes with it?

If you want the silicon itself— the steel is already made and you are just trimming silicon or deoxidising — then the 55% iron in FeSi45 is a liability: more freight, more heat to absorb, more of your impurity budget consumed.
If you also want the iron— melting scrap in an EAF, or melting in a cupola or induction furnace — then that 55% is raw material, and FeSi45 is more economical than it looks on paper.

That one question settles most cases. For the rest, run the five steps.

One caveat before we start: every price in this article is an illustrative number used to demonstrate the arithmetic, not a market quotation. Ferrosilicon moves, and the gap between FeSi75 and FeSi45 on a per-silicon basis narrows, widens and occasionally inverts. Re-run the numbers every time you ask for a quote.

2. What actually differs between the two grades

Chinese ferrosilicon is graded under GB/T 2272 “Ferrosilicon”; FeSi75 and FeSi45 are two widely used designations, nominal 75% and 45% silicon. Check the current version in force and your own specification for the exact grade boundaries and impurity sub-grades.

Comparison	FeSi75	FeSi45	What it means for selection
Nominal silicon	~75%	~45%	Basis of the grade name
Physical tonnage for the same silicon units	1.00 t (baseline)	1.67 t	67% more to buy, ship and store
Non-silicon share of each tonne	~25% (mostly iron)	~55% (mostly iron)	Over half of FeSi45 by weight is iron
Density trend	Lower	Higher	More silicon means lower density → FeSi75 lingers at the slag line more easily
Sensitivity to addition method	High (surface additions oxidise)	Moderate (sinks more readily)	FeSi75 needs stream addition or plunging
Impurity input at equal silicon units	1.0× (baseline)	~1.67×	Matters when the impurity budget is tight
Shipping and storage tonnage	1.00 t (baseline)	1.67 t	Freight is billed on gross tonnes — a straight ×1.67
Typical uses	Steelmaking deoxidation and alloying, low-Al/low-Ti grades, base for inoculants and nodularizers, reducing agent	Bulk silicon adjustment in EAF and foundry melting, applications with generous impurity headroom	Different applications, not high-spec vs low-spec
Correct basis for comparison	price per tonne ÷ 0.75	price per tonne ÷ 0.45	Comparing price per tonne on its own is wrong

3. Step 1: work out how much silicon you actually need

Every pricing mistake traces back to this step being skipped. You are not buying tonnes of alloy, you are buying kilograms of silicon.

Silicon to add = (target Si% − Si% before addition) × metal weight

Alloy to charge = silicon to add ÷ (alloy Si content × recovery rate)

Example: 60 t of steel, target 0.35% Si, currently 0.05%, recovery assumed at 80%.

Silicon to add = (0.35% − 0.05%) × 60,000 kg = 180 kg Si
With FeSi75: 180 ÷ (0.75 × 0.80) = 300 kg
With FeSi45: 180 ÷ (0.45 × 0.80) = 500 kg

Same 180 kg of silicon, 200 kg more material — a difference of 67%. That 67% runs through every step that follows: freight, temperature loss, storage, impurities.

One warning about recovery rate. It is not a constant. It varies with addition method and with grade. FeSi75 has lower density, so if you throw it onto the slag surface it tends to sit at the slag-metal interface and oxidise, and recovery drops visibly; added into the tapping stream or plunged, it does much better. FeSi45 is denser and less fussy. The two grades should not share one recovery figure — if they do, step 1 is already skewed.

4. Step 2: look at where you add it — feedstock or cold charge

This step decides whether that extra 55% iron in FeSi45 is an asset or a liability, and it is the step most often overlooked.

The same grade can be economical in an EAF and uneconomical in a ladle.

Addition point	What happens to the extra 55% iron from FeSi45	Verdict on FeSi45
EAF melting	Counted as charge — you are melting scrap and adding iron anyway	Positive, economics better than they look
Cupola / induction furnace (foundry Si trim)	Counted as hot metal — you need that iron anyway	Positive
BOF tapping stream deoxidation	Cold charge, draws on tapping temperature	Negative, eats your temperature margin
LF trimming	Cold charge, but reheating is available	Slightly negative (reheating costs power and time)
Ladle / in-stream inoculation	Inoculants are high-silicon based; comparison does not apply	Not applicable — see §8 case 5

Why this matters: a lot of purchasing happens at a desk, with no view of what happens at the furnace. The same FeSi45 quotation is a saving in an EAF shop and may be opposed by the melter in a BOF shop because of temperature loss — and temperature loss does not appear on a quotation.

5. Step 3: check the impurity budget — aluminium and phosphorus are the hard limits

The arithmetic here is simple: at equal silicon units, FeSi45 brings in roughly 1.67× the impurities of FeSi75. The question is whether you can carry it.

Start with aluminium. Aluminium means almost opposite things on the two process routes:

Steelmaking: aluminium is itself a deoxidiser, so a little of it in ferrosilicon is usually harmless, sometimes useful.
Low-aluminium grades (electrical/silicon steel, deep-drawing steel, bearing steel): aluminium is a hard limit. These grades demand low Al, low Ti and low C, and FeSi45 is off the table here— not because it is inferior, but because its impurity headroom is simply insufficient.
Foundry: aluminium promotes hydrogen pickup in the iron, and in green-sand practice it is associated with pinholing. So the aluminium content of inoculation-grade ferrosilicon has to be controlled. The association is broadly accepted, but the threshold varies with process — verify it against your own practice and write the limit into your specificationrather than copying someone else’s number.

Then phosphorus, sulphur and carbon. At small addition rates (1–5 kg/t is typical in steelmaking) the input from these is usually negligible. But foundry silicon trim is much larger — targeting 1.8–3.2% Si works out at roughly 25–45 kg of FeSi75 per tonne of iron — and at that scale a 1.67× difference becomes worth calculating properly.

One-line test: work out how much of each impurity you are allowed to introduce per tonne of steel or iron, then see how much of that allowance each grade consumes. Whichever grade consumes more of the allowance is the one that costs you more to use — regardless of its price per tonne.

6. Step 4: compare price per tonne of silicon, not per tonne of alloy

This is the one formula worth memorising:

Price per tonne of silicon = price per tonne of alloy ÷ silicon content

Example (figures for illustration):

Grade	Price per tonne (example)	Si content	Price per tonne of Si
FeSi75	$1,300	0.75	$1,733 / t Si
FeSi45	$700	0.45	$1,556 / t Si

Compare the headline prices and FeSi45 looks 46% cheaper. Measured on silicon, it is 10% cheaper.

That 10% is the whole of FeSi45’s price advantage. Step 7 asks whether it covers the extra costs.

A counter-intuitive note: the per-silicon gap moves with the market. There have been periods when the two grades came close on a per-silicon basis, or inverted — and at those moments FeSi45’s price advantage disappears while its extra freight and temperature loss remain. So the conclusion is not “FeSi45 is cheaper”. It is “recalculate every time”.

7. Step 5: add the costs that ride along

Four of them, roughly in order of impact:

① Freight and landed cost (the one most often missed) Ocean freight is billed on gross tonnes, not on silicon units. FeSi45’s extra 67% of physical tonnage means 1.67× the freight. The same applies to inland haulage from the discharge port.

② Temperature loss and reheating See §4. In BOF tapping deoxidation, the extra alloy draws on tapping temperature. An LF can put the heat back, but that costs power and time.

③ Storage and handling Meeting a year’s silicon demand with FeSi45 means holding about 67% more physical material — warehouse space, number of jumbo bags, handling hours, all up. Where warehouse space is tight or inland haulage is long, this item can outweigh the freight.

④ Exposure to dusting Ferrosilicon disintegrates under damp conditions and, with moisture present, can release gases including phosphine — a documented handling risk in ferroalloy storage and transport. The larger the stored tonnage and the longer the storage period, the larger the exposure — and FeSi45 requires about 67% more of it.

Combined example (100 t of silicon, figures illustrative):

Cost item	FeSi75 (133.3 t)	FeSi45 (222.2 t)
Goods	$173,333	$155,556
Ocean freight (example $40/t)	$5,333	$8,889
Subtotal	$178,667	$164,444

On these two lines FeSi45 saves $14,222, about 8.0% — note that the 10% visible on unit price has already shrunk to 8%.

That is where the decision line sits: if reheating, storage, impurity handling and dusting risk together exceed 8% of the silicon cost, FeSi75 is the right answer; below 8%, FeSi45 pays. The exact figure differs from plant to plant and has to come from your own furnace data.

8. Decision tree: which grade in seven situations

If you would rather not run the numbers, use this table:

#	Your situation	Choose	Why
1	Small addition (< 5 kg/t) and a tight impurity budget	FeSi75	The absolute price difference is small; headroom and temperature matter more
2	EAF melting, iron units welcome	FeSi45	The iron carried in counts as charge you were buying anyway
3	Large foundry silicon trim (target Si ≥ 1.5%)	Calculate the per-silicon price first; usually FeSi45	The price gap is magnified at volume — but check the aluminium limit first
4	Low-Al / low-Ti grades (electrical, deep-drawing, bearing)	Low-Al FeSi75	Not enough impurity headroom; FeSi45 is out
5	Base material for inoculants / nodularizers	FeSi75 based	A high-silicon carrier gives more controllable chemistry and sizing
6	Use as a reducing agent (e.g. magnesium production)	FeSi75	Silicon content governs reducing power; cost is secondary
7	High discharge-port freight / tight storage / long inland haul	FeSi75	About 40% less physical tonnage for the same silicon

One-line test (if you cannot remember the table): am I buying silicon, or do I also want the iron?

Just the silicon → lean towards FeSi75
The iron as well → lean towards FeSi45, but run the impurity budget first

9. Four common traps

Trap one: comparing price per tonne. Covered above, but it bears repeating — the visual shock of $1,300 against $700 makes people think they are halving the cost, when the real figure is 10%.

Trap two: forgetting that freight and storage scale with physical tonnage. Negotiating 2% off the unit price while paying 5% more in freight is a net loss. Those two items belong on the same sheet.

Trap three: tipping FeSi75 onto the slag surface. Higher silicon means lower density, so FeSi75 sits at the slag-metal interface and oxidises, and recovery falls. Add it into the tapping stream or plunge it, and match the sizing to the addition method (sizing and recovery are covered in B14). FeSi45 is comparatively forgiving — which is exactly why using one recovery figure for both grades gives the wrong answer.

Trap four: treating FeSi45 as “budget FeSi75”. It is not. The impurity profiles differ, the process steps they suit differ, and their sensitivity to addition method differs. Using FeSi45 as a cheap substitute on a low-aluminium grade is a classic route to an incident; insisting on FeSi75 in an EAF without costing the iron units is money left on the table.

2. What we do · Frequently asked questions

Henan Longchuang Metallurgical Materials Co., Ltd. supplies ferrosilicon (FeSi75 / FeSi72 / FeSi65 / FeSi45), rare-earth magnesium ferrosilicon (nodularizers), inoculants, cored wire, and manganese and chromium series alloys — over 20 product lines.

Send us your target composition, monthly tonnage and destination port — quotation and available tonnage within 【12 hours】

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