A New Approach to EAF Melted Steels

Transcription

1 A New Approach to EAF Melted Steels Author: Mauricio Torres Melt Department Manager Co-Authors: Ohannes Mangoyan - Vice President of Manufacturing Charles Scherrer Plant Metallurgist

2 McConway & Torley, LLC McConway & Torley, LLC (M&T) is a green sand steel foundry located in Pittsburgh, PA. For over 145 years, M&T has been producing railroad castings. As the leading manufacturer of couplers and other railroad castings, M&T produces over 30,000 tons of finished castings per year using an automated green sand Kunkel Wagner system and two 20-ton electric arc furnaces (EAF). M&T boasts full cleaning room and heat treating capabilities. We pride ourselves for recycling scrap steel and converting it to functional castings. The Need for Change The current deoxidation practice at M&T consists of adding SiMn to the EAF after blowdown. A considerable percentage of SiMn is sacrificed to kill the steel while the balance of the SiMn acts as an alloying addition. As a result of this practice we incur the following: Excessive and inefficient SiMn usage Non-homogeneous bath temperature Non-homogeneous bath chemistry Excessive slag generation / disposal Inconsistent alloy recovery Unacceptably high back and leg injuries rates A continuous improvement team led by Hovig Mangoyan took the initiative to attack this problem and explore various options. The agreed upon direction was to efficiently deoxidize the steel in the EAF and perform the alloying step in the ladle during the tapping process. Improvement Team Findings 1. Determination of the of the SiMn (Silicon Manganese) loss in the furnace: Substantial data was collected and analyzed. To our surprise, the actual SiMn loss was much higher than initially anticipated. Over 30% of the SiMn was lost as a deoxidizer. Inconsistent and unpredictable deoxidation is the prime source for inconsistent alloy recovery. 2. Non homogeneous bath chemistry and temperature: While EAF s are recognized to be efficient scrap melters, they are also known for non-homogeneous chemistry and temperature due to the lack of stirring action in the EAF. SFSA Technical and Operating Conference, December 2016 Page 2 of 14

3 3. Method of handling alloy additions: The current practice consists of shoveling SiMn through the slag door. Due the fact that this method is labor intensive, the alloy addition to the EAF by means of shovels is unreliable and inconsistent. 4. Safety findings: Our safety records indicate high incident rate related to heat exposure, back pain, and leg / knee injuries. CURRENT PRACTICE BLOW DOWN AND ALLOY ADDITIONS Current Practice Loss of SiMn Our current practice calls for adding SiMn to the furnace after the blow down (refining) of the heat in order to reduce the active oxygen in the steel and bring the Manganese to the desired level in the steel. After blow down of the heat, the oxygen levels in the bath are very high, averaging 163 ppm of oxygen, resulting in an oxygen level before tap of 30.1 ppm after addition of SiMn. Bringing the oxygen level in the steel to equilibrium with the SiMn results in a loss of the alloy and the formation of SiO 2 and MnO which will float and become part of slag. SFSA Technical and Operating Conference, December 2016 Page 3 of 14

4 Current Practice Manganese Recovery % M n C o n t e n t % 66% 67% 69% 62% 63% 900 lbs 960 lbs 840 lbs 880 lbs 920 lbs 1040 lbs Silico- Manganese Added (lbs.) %Final Mn %Mn Added % Mn Recovery 66% Avg. Recovery Improving the Process It is clear that the excessive free oxygen after blowdown is the root cause of the low SiMn recovery. Naturally, we had to search for a substitute deoxidizer with higher affinity for oxygen than Manganese. The Ellingham Diagram offered the answer to our question. We determined that aluminum was the most economical and practical deoxidizer to be used. This solution however presented a new challenge: due to the low specic gravity and melting temperature of aluminum, it would be challenging to add aluminum through the slag and into the steel. We concluded that a more dense form of aluminum had to be explored. As another option, an alternate method of introducing aluminum in the bath had to be found. SFSA Technical and Operating Conference, December 2016 Page 4 of 14

5 Changes To The Melting Steps CURRENT PRACTICE: Furnace Charge EAF Melt Refining O 2 Blowdown Alloy Addition Ladle Tapping / Deoxidation NEW PRACTICE: Furnace Charge EAF Melt Ladle Refining O 2 Blowdown Deoxidation Tapping / Alloying Ferro-Aluminum (composition: 30% Aluminum 70% Iron) Initially, we opted to experiment with the addition of Ferro-Aluminum pigs due to the higher specic gravity and melting temperature. Test results were very encouraging in terms of final oxygen levels, reduced SiMn addition levels, and considerably higher SiMn recoveries. In comparison to our current practice, it was easy for the operator to add the FeAl to the furnace. Oxygen levels as low as 3.1ppm were achieved prior to tapping. With this practice it is highly recommended to stir argon gas into the bath to achieve a homogeneous chemistry and temperature. Despite the favorable results, this option was not economical due to the high cost of FeAl compared to that of aluminum. SFSA Technical and Operating Conference, December 2016 Page 5 of 14

6 Ferro-Aluminum Addition and Manganese Recovery Ferro-Aluminum % 85% 80% 76% % 80% % Theoretical Mn 0.4 % Final Mn % Mn Recovery Heat #1 Heat #2 Heat #3 Heat #4 Heat #5 Heat #6 740 lbs 740 lbs 740 lbs 700 lbs 740 lbs 700 lbs Silico-Manganese Added (lbs.) 80.83% Avg. Recovery Manganese Recovery Using Ferro-Aluminum HEAT #1 HEAT #2 HEAT #3 HEAT #4 HEAT #5 HEAT #6 Total lbs FeAl Deoxidation lbs. FeAl Al lbs Prelim Mn SiMn lbs ElMn lbs Theoretical Mn Final Mn Recovery % Carbon(C) Manganese (Mn) Silicon (Si) Chrome (Cr) Nickle (Ni) Moly (Mo) Aluminum (Al) Phosphorus (P) Sulfur (S) SFSA Technical and Operating Conference, December 2016 Page 6 of 14

7 Aluminum Plunge (composition: 100% 10 lb. Notch Bar Aluminum) In our second experiment, we plunged aluminum bars thru the slag into the steel bath. The test data showed similar encouraging low oxygen levels and good Mn recoveries. In this trial, we plunged two-30 lb. aluminum bars. We also ran argon to stir the bath during the addition of the aluminum; as a result we had a very uniform chemistry with a tremendously lower level of oxygen (3ppm). This practice was equally successful and more economical; however, this solution was deemed impractical due to the fact that it is dependent on employee skill besides the fact that it is physically demanding. PLUNGE DEVICE SFSA Technical and Operating Conference, December 2016 Page 7 of 14

8 Aluminum Bar Plunging vs. Recovery of Manganese Aluminum Bar Addition vs. Manganese Recovery % 83% 83% 78% 78% 84% Heat #1 Heat #2 Heat #3 Heat #4 Heat #5 Heat #6 Addition of 690 lbs. of Silico-Manganese % Theoretical Mn % Final Mn % Mn Recovery 80.83% Avg. Recovery Manganese Recovery Plunging Aluminum Bars HEAT #1 HEAT #2 HEAT #3 HEAT #4 HEAT #5 HEAT #6 Total lbs. Al Al lbs Deoxidation Al Ladle lbs Prelim Mn SiMn lbs ElMn lbs Theoretical Mn Final Mn Recovery % Carbon (C) Manganese (Mn) Silicon (Si) Chrome (Cr) Nickle (Ni) Moly (Mo) Aluminum (Al) Phosphorus (P) Sulfur (S) SFSA Technical and Operating Conference, December 2016 Page 8 of 14

9 Aluminum Wire Injection (composition: 100% Aluminum) Our final experiment, consisted of introducing aluminum into the bath by feeding 100% aluminum wire into the bath using a standard wire feed machine. Simultaneously, we stirred the bath with an argon lance. In this case, the recovery of Mn was the highest with minimal employee input. Aluminum Wire Addition vs. Manganese Recovery Aluminum Wire Injection % M n C o n t e n t % 89% 83% 83% 84% 86% HEAT #1 HEAT #2 HEAT #3 HEAT #4 HEAT #5 HEAT #6 % Theoretical Mn % Final Mn % Recovery 85.16% Avg. Recovery 690 lbs. 690 lbs. 690 lbs. 690 Silico-Manganese Added lbs. (lbs.) 670 lbs. 650 lbs. SFSA Technical and Operating Conference, December 2016 Page 9 of 14

10 Manganese Recovery Using Aluminum Wire HEAT #1 HEAT #2 HEAT #3 HEAT #4 HEAT #5 HEAT #6 Total lbs. Al Wire Al lbs Prelim # Mn SiMn lbs Theoretical Mn Final Mn Recovery % Carbon (C) Manganese (Mn) Silicon (Si) Chrome (Cr) Nickle (Ni) Moly (Mo) Aluminum (Al) Phosphorus (P) Sulfur (S) Calcium (Ca) Is Aluminum Wire Injection The Way To Go? As in any new proposed process, we had to validate the process. We compared the following properties: Chemistry Mechanical Properties Charpy Microstructure SFSA Technical and Operating Conference, December 2016 Page 10 of 14

11 Average Heat Chemistry for Current Practice vs. New Practice C % Mn % Si % Cr % Ni % Mo % Al % P % S % AIM.24/.30.90/ /.65.40/.70.40/.70.12/ Min.030 Max.030 Max CURRENT PRACTICE NEW PRACTICE Average Physical Proprieties of Aluminum Wire Deoxidized Heats AAR Min. for E-Grade Steel CURRENT PRACTICE NEW PRACTICE UTS (psi) YS (psi) % EL % RA CV -40 F 120, , , , , , Standard Deviation Comparisons of Current Practice vs. New Practice UTS (psi) YS (psi) % EL % RA CURRENT PRACTICE 6,658 2,482 2,482 6,591 NEW PRACTICE 5,093 1,635 1,635 3,729 Microstructure Comparison Magnification: um um Current Practice New Practice SFSA Technical and Operating Conference, December 2016 Page 11 of 14

12 Discussion and Conclusions Once an efficient and economically feasible furnace deoxidation has been established, the ladle alloying became possible while offering significant savings due to reduced alloy additions. This practice translates into a 14.5 lbs. /ton of SiMn savings. Two additional pieces of equipment had to be purchased to fully implement the new process. AVERAGE ANNUAL SAVINGS Silico-Manganese $352,795 Savings Aluminum Cone/Wire $ 35,167 + Additional Costs Total Savings per Year $317,628 1) Robot This piece of equipment will allow us to wire feed aluminum into the furnace and simultaneously bubble argon into the steel to stir the bath. This robot will also measure and record furnace temperatures, perform the oxygen blow down and take chemistry samples for chemical analysis from the bath. With this piece of equipment we not only improve the deoxidation practice, but we also reduce the heat exposure of the furnace operator. The furnace operator will no longer be required to blow down the heats and take steel samples. The purchase of this equipment will also reduce the amount of man-hours needed to produce a ton of steel. Because argon is needed to stir the bath when aluminum wire is injected; the argon stir will improve the homogeneity of the steel before tap to help achieve a more uniform temperature and chemistry. SFSA Technical and Operating Conference, December 2016 Page 12 of 14

13 Preliminary Robot Design 2) Alloy Feeder An Automatic Alloy Feeder is the second piece of equipment required. This machine will allow us to automatically feed the alloy additions to the ladle during tapping. Load cells are incorporated in the machine to weigh the appropriate amount of alloy needed in each heat and also record the amount of usage for inventory purposes. The alloy feeder will eliminate the handling of one ton of alloy per heat by the operators. This will reduce on safety incidents by eliminating the weighing, staging and shoveling a half ton of alloy into the furnace multiple times a day. This strenuous physical activity was the root cause of many injuries in the last several years. SFSA Technical and Operating Conference, December 2016 Page 13 of 14

14 Final Alloy Feeder Design SFSA Technical and Operating Conference, December 2016 Page 14 of 14