BOF Magnesia Carbon Brick — Zonal Converter Lining Grades

One Vessel, Several Different Failure Modes

A BOF is not lined with one refractory. It is lined with five or six zones that fail by different mechanisms, at different rates, and almost never at the same time. The cone takes scrap impact and mechanical abrasion. The charge pad sees the same plus thermal shock from cold scrap. The slag line is dissolved by FeO-rich slag. The bottom carries bottom-stirring erosion and steel penetration. The taphole assembly has to hold dimensional stability while molten steel flows through it at every tap.

If all of those zones get the same brick, one of them becomes the campaign-limiting zone and the rest of the vessel is thrown away while it still has life in it. This is why XZK supplies a zonal grade matrix rather than a single "BOF grade": seventeen mapped grades from 4% to 19% carbon, each assigned to a location where its particular balance of slag resistance, hot strength and oxidation life is the right one.

Reading the Grade Matrix

Carbon does three things in an MgO-C brick. It is not wetted by basic slag, so it slows penetration. It carries heat away from the hot face, which keeps the brick below the temperature where it softens. And it gives the lining enough compliance to absorb thermal cycling without spalling.

What carbon costs you is hot strength and oxidation life. That is the trade the matrix is built around:

• 19% C, 72% MgO — maximum slag resistance, lowest apparent porosity (≤ 2.0%), for slag line and metal zone service where dissolution dominates.
• 15–17% C, 74–77% MgO — the working compromise for sidewall and bottom, where mechanical load and thermal cycling matter as much as slag.
• 13% C, 79% MgO — taphole seating and sleeve blocks, where dimensional stability and oxidation resistance outrank slag resistance.
• 4% C, 89% MgO — the safety lining. Deliberately low carbon, because a conductive high-carbon backup raises shell temperature and makes the working lining run hotter.

Why Antioxidants Are Not "More Is Better"

Aluminium, silicon, silicon carbide and boron carbide all slow decarburisation, but they work in different temperature bands and they leave different residues. Overdose a metallic antioxidant and you gain oxidation life at the cost of low-temperature strength and, in some cases, of the very hot strength you were protecting. Underdose it and the decarburised layer thickens until slag penetrates and the structure spalls.

We select the package by position and by oxygen potential rather than applying a fixed recipe across the vessel. If you tell us your slag FeO and your tap-to-tap time, we can usually predict which formulation will hold and which will run out early.

Manufacture and Batch Consistency

XZK basic refractories are pressed on 630-ton to 2,000-ton class CNC hydraulic presses from automatically weighed batches, then tempered under controlled atmosphere. Every batch is chemically assayed and tested for bulk density, apparent porosity, cold crushing strength and hot modulus of rupture, and ships with a Certificate of Analysis.

On shapes that have to close a ring — taphole seating and sleeve blocks especially — we diamond grind the seating faces and dry trial-fit the assembly before it leaves the works. Field cutting is where most relining schedules slip, and it is almost always cheaper to remove that step in the factory than on the furnace floor.

What We Need to Quote Properly

Vessel capacity, the zone-by-zone drawing or a worn-lining survey, campaign life by zone, tap-to-tap time, slag chemistry if you have it, and target heats. With that we return a zone-by-zone proposal with grades and quantities in about 48 hours.

If you do not have a drawing, send us the campaign record and a description of where the lining comes down first. That alone is often enough to identify which zone is limiting and what grade change would move it.

Where This Grade Sits in the Lining System

Carbon-bonded basic grades are never specified in isolation. Carbon level, antioxidant package and bulk density are all set by the zone, and the zone map is set by the vessel — which is why a grade that performs well in one position can fail early in the next.

XZK supplies bof magnesia carbon brick as one zone of a zoned package rather than as a standalone item: the grades normally zoned alongside it — among them magnesia carbon brick — are documented on the magnesia-carbon and basic refractories page, and the vessel-level architecture — where each zone starts and ends, and which mechanism actually limits the campaign — is set out on the BOF converter page.

The ArcelorMittal Kryvyi Rih 300 t BOF lining programme reference documents how this class of material performed in full service, with the measured campaign figures rather than datasheet values.