Optimizing Steel Strength and Toughness with Ferrosilicon

Ferrosilicon affects steel strength and toughness by supplying silicon for deoxidation and alloying. Silicon dissolved in the ferrite matrix can increase yield strength, tensile strength and hardness through solid-solution strengthening. However, its effect on impact toughness and ductility is not always positive or negative. The actual result depends on the steel grade, silicon concentration, microstructure, heat treatment and other alloying elements. To achieve the required mechanical properties, you need to balance deoxidation efficiency with the final silicon content specified for your steel.

When selecting ferrosilicon for steelmaking, check the actual silicon content, impurity limits, particle size and batch Certificate of Analysis (COA). You can review our ferrosilicon product specifications and supply options to compare available grades.

In steelmaking, ferrosilicon is more than a source of silicon. Its addition affects both the refining process and the chemical composition of the finished steel. The silicon consumed during deoxidation contributes to oxygen control, while the silicon retained in the steel influences its final mechanical properties.

For steelmakers, the practical challenge is to obtain the required deoxidation result without exceeding the silicon content permitted by the product specification. Understanding the relationship between silicon, strength and toughness helps you select the appropriate ferrosilicon grade and establish a more consistent addition practice.

Ferrosilicon is an iron-silicon alloy commonly used as a deoxidizer and alloying addition in steelmaking. Its influence on steel properties comes mainly from the silicon transferred into the molten steel.

During refining, silicon reacts with dissolved oxygen and forms silicon oxide. The reaction can be represented in simplified form as:

[Si] + 2[O] → (SiO₂)

The resulting oxide products must be managed through slag-metal reactions, flotation and subsequent refining practice. Their removal depends on the oxygen level, slag composition, temperature, stirring and inclusion-control conditions.

After the deoxidation reactions, the silicon retained in the steel becomes part of its chemical composition. It can dissolve in the iron matrix and contribute to solid-solution strengthening.

Therefore, ferrosilicon influences steel production through two related but different functions:

• Deoxidation: Silicon reacts with dissolved oxygen during refining and helps control oxide formation.
• Alloying: Residual silicon contributes to the mechanical and physical properties of the finished steel.

These functions should not be confused. FeSi75 refers to the silicon content of the supplied ferroalloy, not the silicon content of the finished steel. The final steel composition depends on the addition quantity, actual batch chemistry and silicon recovery.

For a more detailed explanation of the refining mechanism, see our guide to ferrosilicon as a deoxidizer in steelmaking.

2.1 Solid-Solution Strengthening

One of the main mechanisms through which silicon increases steel strength is solid-solution strengthening.

When silicon dissolves in ferrite, silicon atoms occupy substitutional positions in the iron crystal lattice. The difference between silicon and iron atomic characteristics creates local lattice strain fields. These strain fields interact with dislocations and make their movement more difficult.

Because plastic deformation in steel is closely related to dislocation movement, restricting this movement generally increases the stress required to deform the material.

The resulting strengthening effect can contribute to:

• Higher yield strength: More stress is required before permanent plastic deformation begins.
• Higher tensile strength: The steel can withstand greater tensile stress before failure.
• Increased hardness: Resistance to localized indentation may increase.

However, the actual strengthening contribution depends on the silicon concentration, steel microstructure and other strengthening mechanisms, including grain refinement, precipitation and dislocation strengthening.

2.2 Effects on Yield Strength and Tensile Strength

Yield strength and tensile strength are two important indicators used to evaluate the mechanical performance of steel.

Yield strength describes the stress at which permanent deformation begins, while tensile strength represents the maximum engineering stress reached during a tensile test.

Silicon can increase both properties through solid-solution strengthening. However, the magnitude of the increase cannot be determined from the ferrosilicon grade alone. You also need to consider the final steel chemistry and processing conditions.

2.3 Experimental Evidence: Silicon Content and Steel Strength

A 2023 study published in Nuclear Materials and Energy investigated the effect of silicon content on the microstructure and mechanical properties of 9Cr ferritic/martensitic steel.

The researchers prepared experimental steel samples with different silicon concentrations ranging from 0 to 1.0 wt.% and evaluated their mechanical properties after controlled processing and heat treatment.

The reported results are summarized below:

What does this study tell us?

Under the specific experimental conditions, increasing silicon from 0 to 1.0 wt.% increased yield strength, tensile strength and hardness, while total elongation remained nearly unchanged. The researchers identified solid-solution strengthening as the main explanation for the strength increase.

This is useful evidence that higher silicon content does not automatically cause a proportional reduction in ductility.

Important limitation: These results apply to the investigated 9Cr ferritic/martensitic steel and its specific processing conditions. They should not be used as universal performance values for carbon steel, structural steel or other alloy systems.

Research source: Effect of Si Content on the Microstructure and Mechanical Properties of 9Cr-Ferritic/Martensitic Steels, Nuclear Materials and Energy, 2023.

Increasing steel strength is only part of the objective. You also need to maintain sufficient toughness and ductility for the intended service conditions.

Silicon does not have one universal effect on these properties. Its influence depends on the steel composition, phase distribution, grain structure, heat treatment and testing temperature.

3.1 Silicon and Impact Toughness

Impact toughness describes the ability of steel to absorb energy during rapid loading. It is commonly evaluated through the Charpy V-notch impact test.

In some ferritic/martensitic steels, silicon strengthening can increase strength while reducing impact toughness under particular microstructural conditions.

For example, research on HT-9 ferritic/martensitic steel found that the effects of silicon and tempering temperature were closely related to solid-solution strengthening, delta-ferrite formation and carbide precipitation.

The study showed that these mechanisms can influence strength, hardness, ductility and impact toughness in different directions. The combined result was not a simple linear relationship between silicon content and mechanical performance.

This means you should not assume that a higher silicon concentration will always improve or always reduce impact toughness.

3.2 Silicon and Ductility

Ductility describes how much plastic deformation a material can undergo before fracture. Total elongation and reduction of area are commonly used indicators.

Silicon can affect ductility through its influence on matrix strength and microstructure. However, the final result also depends on carbon content, alloying elements, grain size, inclusions and heat treatment.

When a steel grade requires both high strength and good formability, you should evaluate the combined effects rather than selecting the silicon target independently.

3.3 Factors That Influence the Strength-Toughness Balance

For steelmakers, the practical objective is to maintain the required silicon level while controlling the other elements that influence the final microstructure and mechanical performance.

You can also read our guide on how excessive impurities in ferrosilicon affect steel quality to understand why impurity control matters during alloy procurement.

The role of silicon changes according to the type of steel being produced. A silicon level suitable for one application may not be appropriate for another.

When you select a ferrosilicon grade, start with the final steel specification and identify the mechanical and processing requirements of the product.

Electrical steel is a particularly important example. Silicon is used to increase electrical resistivity and reduce eddy-current losses in electrical applications. In this case, the silicon target is closely associated with magnetic performance rather than simply maximizing tensile strength.

For other steel grades, the correct silicon concentration must be determined from the relevant product standard and production requirements.

One of the most important tasks in steelmaking is controlling the amount of silicon consumed during deoxidation and the amount retained in the finished steel.

5.1 Deoxidation Requirements

When ferrosilicon enters molten steel, part of its silicon reacts with dissolved oxygen. The amount consumed depends on the initial oxygen content, the refining route, the slag condition and the use of other deoxidizers.

Adding more ferrosilicon does not automatically produce better steel. Excessive addition may increase the final silicon content beyond the permitted range, while insufficient addition may fail to achieve the required deoxidation and composition targets.

For this reason, you should evaluate the required deoxidation effect and the required final silicon content together.

5.2 Controlling the Final Silicon Content

Before calculating an addition, confirm the following information:

• Initial silicon content of the molten steel.
• Target silicon content in the finished steel.
• Actual silicon content of the ferrosilicon batch.
• Expected silicon recovery under the actual operating conditions.
• Silicon consumption associated with the deoxidation practice.
• Contributions from other silicon-bearing additions.

The batch COA is important because the actual silicon content may differ from the nominal grade designation. Using the measured value improves the accuracy of the material balance.

5.3 Ferrosilicon Addition Calculation

For a preliminary estimate of the amount required to increase the silicon content, you can use the following mass-balance formula:

Ferrosilicon Addition (kg) =
Steel Weight (kg) × Required Si Increase (mass fraction)
÷ (Actual Si Fraction × Silicon Recovery Fraction)

Calculation example:

Assume you are treating a 100-ton steel heat and need to increase its silicon content by 0.10 percentage points. The supplied ferrosilicon contains 75% Si, and the assumed silicon recovery is 90%.

This example estimates the material required for the specified silicon increase. It does not independently calculate the silicon consumed by deoxidation. You must account for that consumption and other process losses separately when establishing the actual furnace addition.

For a complete worked example and additional calculation guidance, see how much Ferrosilicon 75 to add in steelmaking.

FeSi65, FeSi72 and FeSi75 are common commercial ferrosilicon grades. The grade number describes the silicon level of the alloy, not the silicon concentration required in the finished steel.

When comparing grades, consider the silicon contribution per tonne of alloy, the impurity limits, the addition method and the total purchasing cost.

Note: The figures above describe commercial grade designations. Confirm the guaranteed chemical composition, impurity limits and applicable delivery standard in the purchase specification and batch COA.

6.1 When Should You Choose FeSi75?

FeSi75 provides a higher silicon concentration than FeSi72 or FeSi65. For the same theoretical silicon input, a higher-silicon grade requires less alloy mass, assuming comparable recovery and other process conditions.

However, FeSi75 is not automatically the best choice for every steel grade. You should compare the actual silicon requirement, aluminum limit, phosphorus and sulfur limits, available particle size and total cost.

For a detailed comparison, read our guides to Ferrosilicon 72 and 75 for steelmaking and Ferrosilicon 75 applications in steelmaking.

6.2 Check the Complete Chemical Specification

Silicon content is only one part of the purchasing specification. Aluminum, calcium, carbon, phosphorus and sulfur may also need to be controlled according to your steel grade.

If your production route has strict aluminum requirements, compare standard and low-aluminum material before confirming an order. Our guide to low-aluminum ferrosilicon for steelmaking explains why aluminum control matters.

You can also review our ferrosilicon grade selection guide to compare common commercial grades and their applications.

Consistent steel properties require consistent control of both the incoming ferroalloy and the finished steel. A ferrosilicon COA confirms the chemical composition of the supplied material, while mechanical testing determines whether the finished steel meets its specified performance requirements.

7.1 What to Check in a Ferrosilicon COA

Before accepting a ferrosilicon shipment, compare the reported batch composition with the purchase specification.

For batch-level inspection, read our Ferrosilicon 75 COA verification guide. It explains which values and supporting information you should check before accepting a shipment.

7.2 How to Verify the Mechanical Properties of Finished Steel

A COA for ferrosilicon cannot prove the tensile strength or impact toughness of the finished steel. These properties must be evaluated through mechanical testing of representative steel samples.

For reliable comparisons, keep the steel grade, sampling location, specimen orientation, heat-treatment condition and testing temperature consistent. A tensile result and a Charpy impact result measure different properties and should not be used interchangeably.

7.3 Why Batch Consistency Matters

Even when two ferrosilicon batches meet the same nominal grade, their actual chemical compositions may differ. Variations in silicon content and impurities can change the amount of each element introduced into the steel.

We recommend checking the actual COA values, agreed impurity limits and batch identification before shipment. This gives you a more reliable basis for calculating additions and tracing any production variation.

Q1: Does ferrosilicon increase steel strength?

Ferrosilicon supplies silicon, which can increase steel yield strength and tensile strength through solid-solution strengthening. The actual effect depends on the final silicon concentration, steel composition, microstructure and processing conditions.

Q2: Does higher silicon content always improve steel toughness?

No. Silicon can increase strength, but its effect on impact toughness depends on the steel grade and microstructure. In some ferritic/martensitic steels, solid-solution strengthening may reduce impact toughness under particular conditions. You should evaluate strength and toughness together.

Q3: Can silicon improve both strength and ductility?

It can under certain conditions. Experimental research on 9Cr ferritic/martensitic steel found that increasing silicon content improved strength while total elongation remained nearly unchanged. However, this result should not be generalized to every steel grade.

Q4: Which ferrosilicon grade is suitable for high-strength steel?

FeSi65, FeSi72 and FeSi75 may all be considered depending on the target silicon content, impurity limits and steelmaking route. The correct selection should be based on the final steel specification and actual batch chemistry, not simply on choosing the highest silicon grade.

Q5: How do you calculate the ferrosilicon addition for steelmaking?

Calculate the required silicon increase using the steel weight, initial and target silicon contents, actual silicon concentration of the ferroalloy and expected recovery rate. You must also account for silicon consumed during deoxidation and contributions from other alloy additions.

Q6: How can you test whether silicon improves steel strength?

Use tensile testing to determine yield strength, tensile strength and elongation. Charpy V-notch testing evaluates impact energy, while hardness testing measures resistance to indentation. Compare samples with controlled chemistry and processing conditions.

Q7: What impurities should you check when buying ferrosilicon?

Commonly controlled elements include aluminum, calcium, carbon, phosphorus and sulfur. Their acceptable limits depend on the steel grade and the purchase specification. You should verify the actual batch values on the COA.

Q8: Is FeSi75 better than FeSi72 for steelmaking?

Not in every situation. FeSi75 provides a higher silicon concentration per unit mass, but FeSi72 may be more economical when the required silicon input and final chemistry permit its use. Compare actual Si content, impurity limits, recovery and delivered cost.

Q9: Can a ferrosilicon COA guarantee the mechanical properties of steel?

No. A ferrosilicon COA verifies the chemical composition of the alloy. The mechanical properties of finished steel must be confirmed through appropriate tensile, impact, hardness and other specified tests.

The following research papers and testing standards provide additional technical information about silicon strengthening and the evaluation of steel mechanical properties.

1. Effect of Si Content on the Microstructure and Mechanical Properties of 9Cr-Ferritic/Martensitic Steels (2023).
   Read the research paper.
2. Effects of Silicon Content and Tempering Temperature on the Microstructural Evolution and Mechanical Properties of HT-9 Steels.
   Read the full research paper.
3. ASTM E8/E8M - Standard Test Methods for Tension Testing of Metallic Materials.
   View ASTM standard information.
4. ISO 6892-1 - Metallic Materials: Tensile Testing at Room Temperature.
   View ISO standard information.
5. ISO 148-1 - Metallic Materials: Charpy Pendulum Impact Test.
   View ISO standard information.
6. ASTM E18 - Standard Test Methods for Rockwell Hardness of Metallic Materials.
   View ASTM standard information.

We supply ferrosilicon grades including FeSi45, FeSi65, FeSi72 and FeSi75 for steelmaking and other metallurgical applications.

When you source ferrosilicon from us, you can discuss the following requirements before confirming your order:

• Chemical composition: Silicon content and agreed limits for Al, Ca, C, P, S and other specified elements.
• Particle size: 10–50 mm, 10–100 mm and other agreed size ranges.
• Quality documentation: Batch COA and agreed inspection requirements.
• Packaging: Jumbo bags or other export packaging arrangements.
• Shipment: Export arrangements and loading schedules based on the order and destination port.

Our production and supply system integrates smelting, crushing, screening and packaging. We focus on matching the supplied grade and batch specifications with your actual production requirements.

If you are selecting ferrosilicon for steelmaking, send us your target silicon content, impurity limits, particle size and required quantity. We can review your specification and provide a suitable product option.

ZHEN AN INTERNATIONAL CO., LIMITED - Your partner for ferrosilicon and metallurgical raw materials.