How to Choose and Use CPC for Steelmaking

Introduction

        Carbon adjustment seems simple. You need a certain carbon level in the steel, so you add a recarburizer. But not all carbon is the same. If you have ever seen carbon recovery bounce between 60% and 90% from one heat to the next, you know there is more to it.

        Calcined petroleum coke (CPC) is one of the most common recarburizers used in steelmaking. It is made from raw petroleum coke that has been heated to drive off volatile matter, leaving behind a dense, high-purity carbon material. When chosen and fed correctly, CPC gives you consistent carbon recovery without introducing unwanted impurities.

        At JINLI GROUP, we supply CPC to steel mills and foundries around the world. This article is not a chemistry lecture. It is a practical guide on what to look for in a CPC product and how to use it without wasting carbon or damaging your steel.

The Two Main Grades: CPC and GPC

        Before going further, let me clear up a common confusion. You will hear two terms: CPC (calcined petroleum coke) and GPC (graphitized petroleum coke). Both are used as recarburizers, but they are different.

        CPC is calcined at around 1300°C. It has a crystalline structure but is not fully graphitized. GPC is heated above 2500°C, which aligns the carbon structure into graphite. GPC has lower sulfur and nitrogen and it dissolves faster. It is also more expensive.

        For most carbon steel, low-alloy steel and even many specialty grades, CPC works perfectly well. The key is to match the grade to your steel’s impurity limits. If you need very low sulfur or nitrogen, you may need GPC. But for everyday production, CPC offers the best value.

What to Check on a CPC Specification Sheet

        When a supplier sends you a CPC analysis, do not just look at the fixed carbon number. Here is what matters most.

        Fixed carbon should be 98.5% or higher. Lower than that means more ash, which turns into slag and wastes your money.

        Sulfur content is critical. For steels that require low sulfur, use CPC with less than 0.5% sulfur. Some CPC grades have sulfur as high as 3%, those are only suitable for cast iron or very dirty steel practices. Know your steel specification before ordering.

        Nitrogen content matters for aluminum-killed steels that are sensitive to nitrogen embrittlement. High nitrogen CPC can cause pinholes and reduced ductility. If you make drawing quality or pipe grades, ask for CPC with nitrogen below 300 ppm.

        Moisture seems minor, but wet CPC causes hydrogen pickup and poor recovery. Good CPC has less than 0.5% moisture. Store it in dry conditions.

        Particle size affects dissolution rate and recovery. Fine particles dissolve quickly but can be lost to the slag or sucked into the dust collector. Coarse particles dissolve slowly and may not fully dissolve before casting. A well-graded CPC with a range of 1 to 5 mm is a good starting point.

How CPC Dissolves in Steel —— The Practical Side

        Carbon dissolution is not instant. The CPC particles must be wetted by the steel, then the carbon diffuses into the melt. Several factors influence how fast and completely this happens.

        Firstly, temperature. Hotter steel dissolves carbon faster. If you add CPC at 1550°C, it will take longer than at 1600°C. That means you need to add it early enough in the heat, not at the very end when the steel is already heading to the caster.

        Secondly, stirring. Good argon stirring brings the CPC particles into contact with fresh steel. Without stirring, the carbon can float on the surface and react with slag instead of dissolving. Always add CPC under strong stirring.

        Thirdly, particle size distribution. If the CPC contains too many fines (below 0.5 mm), those tiny particles will oxidize in the air or float on the slag. They never reach the steel. If the CPC has too many coarse lumps (above 8 mm), they may sink to the bottom and dissolve too slowly. A consistent size range gives the most reliable recovery.

Where to Add CPC

        Different shops add CPC at different points. Each has its own advantages.

        In the EAF during melting, CPC can be added with the scrap or through the roof. The high temperature and long residence time ensure complete dissolution. Recovery is typically 90% to 95%. However, some carbon will be lost to the slag and to oxidation from the oxygen lance. Also, adding too much CPC in the EAF can increase your slag volume.

        In the ladle after tapping, CPC is added under argon stirring. This gives you precise control because you can adjust after knowing the tap carbon. Recovery is usually 70% to 85% depending on how well the ladle is stirred and the slag condition. The downside is that you have limited time before casting.

        In the furnace after meltdown but before refining, many mills add a portion of CPC to make up the carbon target, then fine-tune in the ladle. This hybrid approach is common and works well.

        Here is a tip. If you add CPC to the ladle, do not just dump it on top of the slag. Use a small pipe or a tube to inject the powder below the slag surface or mix the CPC with some steel scrap to help it sink. Otherwise, most of the carbon will float on the slag and slowly oxidize.

Why Recovery Varies and What to Do

        Recovery means how much of the added carbon actually stays in the steel. If you add 100 kg of CPC with 98% fixed carbon, theoretically you add 98 kg of carbon. If the steel gains only 70 kg, recovery is about 71%.

        Low recovery comes from several sources.

        Oxidation is the biggest one. Carbon reacts with oxygen from the air, from the slag or from dissolved oxygen in the steel. That is why you should always add CPC after deoxidation? Actually no, carbon is itself a deoxidizer. But if you add CPC to steel with high oxygen activity, some carbon will burn away as CO gas. The solution is to add CPC early, before oxygen gets too high or to add it together with a deoxidizer like aluminum.

        Slag absorption is another loss. If the slag is foamy or thick, CPC particles can get trapped and never reach the steel. Keep the slag thin and basic. If you have to add CPC into a deep slag, use a lance to inject it below the slag line.

        Particle loss happens with very fine CPC. The fines can be pulled into the dust collection system or blown away by the argon bubbles. Using a product with controlled particle size helps.

        A good target recovery for CPC added in the ladle is 80% to 85%. If you are consistently below 75%, check your addition method and your stirring practice.