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.