The smelting process of Z3CN20-09M steel for nuclear power pump casings has been improved. The molten steel and alloy are melted separately, and after blending, the ingredients are simply fine-tuned and the VOD operation is performed. It can ensure the quality and greatly shorten the smelting time.
Keywords: stainless steel; duplex method; VOD; Z3CN20-09M
In the past, two processes were usually used to smelt ultra-low carbon stainless steel: one is to provide rough molten steel in an electric furnace, adjust the nickel and molybdenum composition in the electric furnace, adjust chromium, manganese, and silicon in the refining furnace, and then pour it into another refining package for VOD operation; The other method is to adjust chromium, manganese and silicon alloys in addition to nickel and molybdenum when smelting rough molten steel in an electric furnace, continue to adjust the chromium composition in the refining furnace, and finally pour into another refining package for VOD operation. The above two methods have shortcomings such as cumbersome process and long smelting time. We have made a major process improvement in smelting Z3CN20-09M steel, that is, two electric furnaces are used to smelt low-phosphorus molten steel and alloy respectively. After blending, the ingredients are simply fine-tuned and the VOD operation is performed.
Steelmaking equipment
A company’s newly renovated VOD equipment was designed and modified by itself based on the original 130 t VD ladle refining furnace. The vacuum cover of the original VD furnace is equipped with a Laval tube water-cooled oxygen lance, a furnace gas temperature indicator and a micro-oxygen content analyzer to provide guarantee for VOD end-point carbon control. The vacuum cover is also equipped with an electronic video monitoring system to observe the reaction progress and splashing in the furnace in real time. There are two 500 kg vacuum feeding bins, which can add slag-making materials and deoxidized alloys after the VOD is completed. The vacuum pumping equipment is a 500 kg steam jet pump, which is divided into four stages, and the vacuum degree can reach below 133 Pa.
The electric arc furnaces that provide rough molten steel are 40t EBT and 30t EAF electric arc furnaces.
Smelting process
The test steel type is Z3CN20-09M, and the molten steel smelting capacity is 70 t. The composition of the finished product and the requirements for rough molten steel are shown in Table 1.
Process flow
A 40t eccentric bottom electric furnace is used to provide the VOD refining furnace with pure low-phosphorus oxidized molten steel without adjusting any alloying elements. At the same time, a 30t electric furnace is used to specifically melt the alloy. The process flow is shown in Figure 1.
Table 1 The main chemical composition of Z3CN20-09M steel(mass fraction,%)
| C | Si | Mn | P | S | Cr | Ni | ||
| Standard requirements | ≤0.04 | ≤1.50 | ≤1.50 | ≤0.030 | ≤0.015 | 19.00～21.00 | 8.00～11.00 | |
| Electric furnace roughing | 40 t(EBT) | ≤0.03 | _ | _ | ≤0.003 | _ | _ | _ |
30 t(EAF) | 0.70～0.90 | _ | _ | _ | _ | 50.00～53.00 | 22.00～24.00 | |
| VOD refinement | Before dumping | 0.30～0.40 | ≤0.50 | ≤0.30 | ≤0.020 | ≤0.020 | 19.50～20.00 | 8.00～8.50 |
Figure 1 The flow diagram of testing process for Z3CN20-09M steel
In order to recover the chromium in the slag as much as possible, the alloy from the 30t electric furnace is directly turned into the LF refining ladle with slag. In order to minimize the recovery of P during the refining period, the low-phosphorus oxidized molten steel smelted in the 40t electric furnace is mixed into the refining package using the slag sticking method, and the amount of slag is strictly controlled. After fine-tuning the composition, the molten steel is poured into another refining package slag, and then the VOD operation is performed. After VOD, fine-tuning the composition and temperature control are performed, and finally pouring is carried out.
Key points of 30t EAF electric furnace smelting process
(1)Ingredients
The task of the 30t electric furnace is to melt the required alloy. Since carbon cannot be oxidized during alloy melting, medium carbon and
Low carbon chromium alloy. The ingredient structure is shown in Table 2.
Table 2 The mix structure of 30 t electric furnace
| Low chromium/t | Medium chromium/t | Nickel plate/t | Total ingredients/t | Amount of C (mass fraction, %) |
| 6 | 16 | 5.8 | 27.8 | 0.56 |
(2)Operation points
During the alloy melting process, Cr and Ni are adjusted according to the finished product specifications. Since there is no standard steel during the entire smelting process, only the carbon content in the steel can be analyzed. The slagging amount is 5%, and the ratio of lime and fluorite is 4:1. To avoid adding carbon during melting, oxygen can be blown appropriately. After melting, add FeSi powder in small quantities in batches to reduce the chromium in the slag. The chemical composition after melting and before tapping is shown in Table 3.
Table 3 The chemical composition of 30 t electric furnace after melting down and before pouring(mass fraction,%)
| C | Si | Mn | P | S | Cr | Ni | |
| After melting | 0.79 | 0.53 | 0.42 | 0.016 | 0.005 | 50.13 | 21.93 |
| Before tapping | 0.79 | 0.72 | 0.43 | 0.015 | 0.005 | 49.68 | 21.08 |
Key points of smelting process of 40 t EBT electric furnace
(1)Ingredients
The ingredient structure is shown in Table 4.
Table 4 The mix structure of 40 t electric furnace
| Pig iron/t | High-quality scrap steel/t | Total batching amount/t | Amount of blending C (mass fraction, %) |
| 7.2 | 40.8 | 48 | 0.60 |
(2)Operation points
The 40 tEBT electric furnace provides low-phosphorus crude molten steel for VOD refining. The key is to create oxidized slag. Ensure the alkalinity of the slag and the temperature of the molten steel to achieve the purpose of rapid phosphorus removal. Carbon and phosphorus are required to be as low as possible, and a certain amount of decarburization must be ensured.
When P ≤ 0.003%, C ≤ 0.03% in the molten steel, and the temperature of the molten steel ≥ 1660°C, steel can be tapped. During the tapping process, the chartered electronic scale and the required amount of molten steel are used to strictly check the slag, and the slag is also stuck when mixing steel. The composition before tapping is shown in Table 5.
Table 5 The chemical composition of 40 t electricfurnace before pouring(mass fraction,%)
| C | P | Mo | Cr | Ni |
| 0.03 | 0.002 | 0.33 | 0.04 | 0.02 |
Key points of refining furnace smelting process
(1) Operation before repacking
At the refining furnace station, two refining packages must be prepared. One of them is built with magnesia carbon bricks for LF operation and the other is built with magnesia chrome bricks for VOD operation. The baking temperature of the refining package is required to be greater than 800℃. Turn in the alloy from the 30 t rough smelting furnace, mix the stuck slag into the 40 t rough molten steel in the rough smelting furnace, and then regenerate the slag and use electricity to heat it up. Sampling and fine-tuning the ingredients to meet the requirements in Table 1, raising the temperature to above 1680°C, and then pouring the bag. Strictly block the slag when pouring the bag. After repacking, enter the VOD station for VOD blowing. The ingredients after blending are shown in Table 6.
Table 6 The chemical composition after mixing(mass fraction,%)
C | Si | Mn | P | S | Cr | Ni | Mo | N | Cu |
0.32 | 0.08 | 0.20 | 0.014 | 0.014 | 18.25 | 7.15 | 0.21 | 0.0349 | 0.05 |
(2)VOD operation
The VOD blowing process flow is shown in Figure 2.
Figure 2 The flow diagram of VOD air refining process
As can be seen from Figure 2, after VOD oxygen blowing, the slag can be adjusted under vacuum, and then the vacuum degree is raised to within 133 Pa and maintained for 15 to 20 minutes for VD operation. After removing the vacuum, fine-tune the alloy and tap the steel at the appropriate temperature.
The main process parameters of the VOD process are shown in Table 7. The chemical composition before and after VOD operation is shown in Table 8.
Table 7 The process parameters of VOD course
| Process parameters | Parameter value |
| Refining container capacity/t | 130 |
| Liquid steel volume/t | 70 |
| Free space height inside bag/mm | 1380 |
| Oxygen gun stroke/m | 1.2 |
| Oxygen pressure/MPa | 0.8 |
| Nitrogen flow/L·min-1 | 70～80 |
| Oxygen blowing time/min | 32 |
| Maximum instantaneous flow rate of oxygen/m³·h-1 | 990 |
| Total oxygen consumption/m³ | 589.2 |
| Vacuum degree/Pa during VOD oxygen blowing | 1333.2～1999.8 |
Table 8 The chemical composition beforeand after VOD process(mass fraction,%)
| C | Si | Mn | P | S | Cr | Ni | Mo | |
| Before VOD | 0.32 | 0.08 | 0.22 | 0.015 | 0.011 | 19.30 | 8.52 | 0.24 |
| After VOD | 0.017 | 0.16 | 0.19 | 0.014 | 0.010 | 19.02 | 8.50 | 0.24 |
Discussion
The successful trial of Z3CN20-09M steel using the duplex method has opened up new ideas for the company in the process of smelting stainless steel. Compared with the traditional VOD stainless steel smelting process, the duplex method + VOD method has the following characteristics:
(1) Shorten the smelting time
Summarizing the VOD production over the past few years, the electric furnace smelting cycle is relatively long. Since the rough smelting of molten steel in the traditional process requires preconditioning of part of the chromium and dephosphorization and reduction in the electric furnace, the smelting process in the electric furnace is relatively prolonged. It usually takes 4 to 5 hours to smelt one furnace. The amount of alloy added to the refining furnace is large, and the smelting time is also greatly extended.
In the new process, the electric furnace divides labor and cooperates. The alloyed electric furnace is loaded with alloys according to the calculated quantity. There is no need to analyze the chemical composition and dephosphorization. When the temperature is suitable, steel can be tapped. The general smelting time is 2h~2.5h. The 40tEBT electric furnace is responsible for providing oxidized molten steel. There is no need to mix alloys, as long as C, P and temperature meet the process requirements. Its smelting time is generally 2.5 h~3h. After rough smelting of molten steel and mixing, the main components and temperature basically meet the process requirements, and the amount of alloy added is very small, which greatly shortens the smelting time, generally 3h~4h. The smelting time of traditional process is 5h~7h.
(2) Improved chromium recovery rate
In the traditional process, because part of the chromium is added in the electric furnace during rough steelmaking, the recovery rate is low, generally around 90%. After adopting the new process, since the molten steel from the electric furnace that melts the alloy is fully loaded with slag and is turned into the refining bag, the chromium in the slag can be reduced and recycled as much as possible, and the recovery rate is generally 97% to 99%.
(3) The process can be further improved
When formulating the test plan, we used reverse packaging for the sake of safety. In fact, after the crude steel is mixed, the composition and temperature have already met the VOD operating conditions. Therefore, the repacking process can be eliminated in the future, further simplifying the process, shortening the smelting time, and reducing the smelting cost.
Conclusion
Through the trial of Z3CN20-09M steel, it was proved that it is completely feasible to melt the alloy in the entire furnace of crude steel and use the duplex method + VOD method to smelt stainless steel. Not only can it fully guarantee the quality, but it also speeds up the smelting speed, shortens the smelting time, and achieves the goal of increasing production and efficiency.