Influence of Carbon Content on the Properties of Low-Carbon Magnesia-Carbon Bricks

 1. Introduction

With the continuous development of the steel industry, the proportion of ultra-low carbon steel in steelmaking is steadily increasing. Traditional magnesia-carbon bricks, which contain relatively high carbon content, are associated with problems such as excessive heat loss, carbon pickup in molten steel, and increased environmental burden. As a result, low-carbon magnesia-carbon bricks have gradually become a major research focus.

However, reducing carbon content inevitably leads to deterioration in high-temperature performance, particularly thermal shock resistance and slag corrosion resistance. It is well known that lowering the carbon content reduces thermal conductivity while increasing elastic modulus, thereby worsening thermal shock stability. In addition, reduced carbon content increases the wettability of the brick by slag and molten steel, resulting in lower slag resistance and higher permeability.

In recent years, Japan has achieved significant progress in the development of low-carbon magnesia-carbon bricks through nanotechnology. This study systematically investigates the influence of different carbon contents on the physical and high-temperature properties of magnesia-carbon bricks. The objective is to identify an optimal carbon range suitable for smelting different steel grades, reduce environmental impact, and improve resource utilization efficiency.

2. Experimental Procedure

2.1 Raw Materials

The experiment used the following raw materials:

• 97% MgO fused magnesia

• Flake graphite (<196 μm)

• Aluminum powder

• Liquid thermosetting phenolic resin as binder

The compositions of the samples are listed in Table 1.

2.2 Sample Preparation

Aggregates and resin were first wet-mixed for 1–2 minutes, after which fine powders were added and mixed for approximately 40 minutes. The mixture was sealed and aged for 12 hours. Specimens were then molded under a pressure of 0.2 MPa per unit area into:

• Cylindrical samples: Ø50 mm × 50 mm

• Strip samples: 25 mm × 40 mm × 130 mm

All specimens were dried at 200 ℃ for 24 hours before testing.

Table 1 Composition of Magnesia-Carbon Brick Samples (wt.%)

3. Analysis of Experimental Results

3.1 Physical Properties

As carbon content increased, the bulk density of the samples generally decreased. The maximum bulk density of 3.13 g·cm⁻³ was observed at 4% carbon content. Between 6% and 10% carbon, bulk density showed little variation; however, above 10%, a significant decline occurred.

The apparent porosity exhibited the opposite trend, reaching a minimum at 4% carbon content. When carbon content exceeded 10%, porosity increased rapidly.

3.2 Mechanical Properties

The room-temperature compressive strength decreased with increasing carbon content. Carbon-free samples showed the highest strength (112.1 MPa). When carbon content was between 2% and 6%, compressive strength remained relatively stable; beyond 6%, it declined sharply.

High-temperature flexural strength is a key indicator of resistance to thermal stress, impact, abrasion, and slag attack. With increasing carbon content, the flexural strength exhibited a zigzag trend. The highest value (8.90 MPa) was observed at 2% carbon content, while the lowest (7.77 MPa) occurred at 4%. Between 6% and 18% carbon, flexural strength first decreased and then increased.

3.3 Thermal Shock Resistance

Thermal shock stability was evaluated by measuring the flexural strength loss rate after three water-cooling cycles at 1100 ℃.

As carbon content increased, the loss rate initially decreased and then increased. Sample #1 showed the highest strength loss, while Sample #5 exhibited the lowest. Samples containing 2–6% carbon showed minimal spalling after thermal shock. When carbon content exceeded 14%, severe surface oxidation caused significant peeling and spalling.

Lower carbon content resulted in greater thermal stress due to mismatch in thermal expansion, leading to higher flexural strength loss.

3.4 Oxidation Resistance

At 1600 ℃ for 3 hours, oxidation resistance varied with carbon content. Samples with excessive or insufficient carbon showed severe oxidation. Samples with 4–10% carbon formed relatively dense decarburized layers, improving oxidation resistance. Overall, carbon content had a limited effect on oxidation resistance, but both extremes were detrimental.

3.5 Slag Resistance

Slag erosion tests were conducted at 1600 ℃ for 3 hours using slag with the composition shown in Table 2.

Table 2 Slag Composition (wt.%)

Slag penetration occurred mainly along periclase grain boundaries. Sample #4 exhibited the best slag resistance, with minimal adhesion and no obvious decarburized layer. Periclase grain growth and silicate phase formation effectively blocked further slag penetration.

Samples with higher carbon content showed increased graphite oxidation, leading to porous decarburized layers and reduced slag resistance. Although high-carbon bricks generally resist slag erosion, surface decarburization ultimately accelerates structural damage.

4. Conclusions

1. Increasing carbon content leads to a reduction in bulk density and compressive strength of magnesia-carbon bricks.

2. Optimal overall performance—high-temperature flexural strength, thermal shock resistance, slag resistance, and oxidation resistance—is achieved when carbon content is 6–8%.

3. Magnesia-carbon bricks with 6–8% carbon content are recommended for the smelting of low-carbon and ultra-low carbon steels.

How to Control Blowing Lance in Converter Steelmaking?

 Five-Stage Lance Position Control (Low–High–Low–High–Low)

The five-stage lance position method is not a rigid rule but a guiding principle for BOF/LD blowing control, aimed at coordinating stirring, slag formation, temperature rise, FeO control, and carbon–oxygen reaction.

① Low (Initial strong stirring)
Lower the lance 100–200 mm below normal at blow start to intensify stirring, accelerate scrap melting, eliminate dead zones, and build early bath temperature. Use higher oxygen pressure (~0.9 MPa) for 60–90 s, then add slag materials in small, frequent batches. Early dolomite addition (MgO) promotes slag melting and early dephosphorization.

② High (Slag formation & heating)
Raise the lance to normal or slightly higher to promote slag melting and uniform heating. Maintain FeO at ~20–25% to ensure lime dissolution. This stage may be longer in low iron-consumption practice. Slag must be fully melted before adding the second batch; excessive lance height leads to low bath temperature, poor reactions, and high-FeO splashing.

③ Low (Suppress FeO & splashing)
As temperature and FeO rise, conditions for splashing emerge. Lowering the lance strengthens the carbon–oxygen reaction, consumes FeO, suppresses splashing, accelerates decarburization, and stabilizes bath stirring.

④ High (Intense reaction control)
This is the most difficult stage. FeO is rapidly consumed, and improper lance adjustment can cause dry slag, violent splashing, phosphorus reversion, and equipment damage. Continuous flame observation is essential; adjust the lance before the flame turns vertical to maintain fluid slag without splashing.

⑤ Low (End-point control)
Gradually lower the lance to carbon-blowing position for ≥40 s to homogenize temperature and composition and reduce slag FeO. Maintain a “soft” flame to avoid lance damage. Once endpoint carbon and temperature meet requirements, raise the lance and finish blowing.

Key Misconceptions & Operational Points

Defects are not simply due to insufficient slag amount

• Unmelted lime = ineffective lime → poor dephosphorization/desulfurization.

• Late melting of oversized or abnormal scrap introduces harmful elements at the end.

Typical 12-minute blow time allocation (reference only)

• Phase 1: 1.5–2 min

• Phase 2: 2–4 min

• Phase 3: 2–3 min

• Phase 4: ~3 min

• Phase 5: ~2 min
  Actual timing must be adjusted by flame and furnace response.

Correct handling of splashing

• High-temperature splashing: add material briefly, then promptly lower the lance.

• Low-temperature splashing: do not raise the lance or add blindly; instead reduce oxygen pressure, add small batches, and stabilize the carbon–oxygen reaction

Core principle
The blow is short but highly dynamic. Operators must understand flame–reaction relationships, make proactive adjustments, and use lance position to control the flame, not chase it. The five-stage method reflects a trend, not a rigid formula—skillful flexibility is the key to stable, high-quality steelmaking.

LMM GROUP ONE STOP SOLUTION FOR STEELMILL