The huge advantages of continuous casting technology in production efficiency and production cost are the driving force of continuous casting of stainless steel production. At the same time, the “double high” characteristics of stainless steel (high alloy content and high surface quality requirements) have made the pace of continuous casting of stainless steel production always lag behind the overall development of continuous casting technology. In recent years, the progress of continuous casting equipment and process level has made the characteristics of high surface quality requirements of stainless steel products and high control accuracy of continuous casting fit together, making continuous casting the mainstream process in stainless steel production.
Baosteel Stainless Steel Branch invested in two stainless steel production lines with an annual output of 720,000 tons each in April 2004 and May 2005, each production line including a slab continuous casting machine and corresponding supporting equipment. Since the solidification characteristics and metallurgical characteristics of stainless steel are well considered in the design of continuous casting equipment, and a relatively reasonable process route is compiled for the surface and internal quality requirements of stainless steel slabs, the quality of stainless steel products basically meets the requirements of the post-process and achieves the goal of process design.
1 Process design for improving the surface quality of stainless steel continuous casting slabs
Improving the surface quality of stainless steel slabs is mainly reflected in reducing the grinding rate of slabs, including lower surface and subcutaneous slag inclusion detection rates, basically no surface longitudinal cracks and transverse cracks, regular and shallow vibration marks and other factors.
The surface quality of stainless steel slabs mainly depends on the flow field distribution of molten steel in the crystallizer during the continuous casting process, the type and parameters of crystallizer vibration, liquid level control technology, the melting behaviour of protective slag and the reaction of slag, and the reasonable setting of the first and second cooling parameters.
1) Design of the flow field distribution of molten steel in the copper mould tube
The flow field distribution of molten steel in the CCM tube determines the turbulence amplitude of the liquid surface and the floating law of inclusions. At the same time, the different behaviours of molten steel streams also have a certain influence on the melting characteristics of protective slag and the growth law of slab shells. For stainless steel continuous casting, the surface quality of the slab determines the grinding loss and the surface finish level that the final product can achieve, which is a key factor reflecting the competitiveness of stainless steel products. Therefore, in the technical reserve work before hot commissioning, the flow field distribution of the crystallizer is proposed as a key issue. The calculation software package based on the C+ + Builder language is used to simulate the flow field in the slab crystallizer. The research content includes the fluctuation of the crystallizer liquid level, the impact position and impact strength of the narrow surface flow stream of the crystallizer, the distribution of the reflux area, etc., and according to the solidification characteristics and high-temperature mechanical properties of different types of stainless steel, the SEN geometry, tapping hole angle, insertion depth and other process parameters are adjusted. The orthogonal method is used to screen out a reasonable combination of process parameters and apply them in actual production.
2) Determination of copper mould tube vibration mode
The type and vibration mode of the crystallizer vibration device will determine the depth and regularity of the slab vibration mark. The trough of irregular and deep vibration marks is often the origin of the slab transverse cracks and the gathering place of slag particles and bubbles. If the slab grinding amount is insufficient, then due to the characteristics of stainless steel that it is not easy to oxidize, these problems will be brought to the cold-rolled products and form scale defects. At the root of such defects, slag inclusions containing Na+, K+ and other protective slag-specific elements can often be detected.
The slab continuous casting machine of the stainless steel branch company has selected the leaf spring hydraulic vibration mode, and the hydraulic cylinder is controlled by a proportional valve closed-loop system. At the equipment level, it has the ability to realize the sinusoidal and non-sinusoidal vibration modes of the mould tube and can switch between different amplitudes and frequencies during the casting process.
The selection of vibration process parameters takes into account the reduction of vibration mark depth as much as possible and the improvement of slab surface quality under the premise of ensuring reasonable consumption of protective slag. During the hot commissioning of stainless steel slab continuous casting, a series of tests on the relationship between process parameters such as amplitude, vibration frequency, pulling speed, negative slip time and vibration mark depth were carried out, and the vibration process parameters were determined based on this.
According to the data, under normal conditions, stainless steel slabs will produce 0.2~0.3mm iron oxide scale in the hot rolling heating furnace. The surface defects of the slabs within this range can be removed simultaneously with the iron oxide scale. If the vibration mark depth is within this range, the slab does not need to be ground. Therefore, based on the results of the above research work, reasonable vibration process parameters and the pulling speed range required to ensure the surface quality of the slab are determined.
3) Continuous casting tube liquid level control technology
In addition to the optimized design of the steel liquid flow field, the crystallizer liquid level automatic control system also plays a vital role in the control of the crystallizer liquid level. In the process design of the continuous casting machine of the stainless steel branch, the built-in eddy current crystallizer liquid level detection device (VUZ system) is selected to detect the liquid level of the molten steel in the crystallizer and feedback control the position of the stopper rod. The liquid level control accuracy is ±3mm.
In actual use, when the casting speed changes or other operations cause the crystallizer liquid level to fluctuate, the system responds in time and controls the liquid level fluctuation value within a relatively small range.
4) Stainless steel protective powder selection
The behaviour of protective slag during the casting process directly affects the surface quality of stainless steel slabs. The literature points out that the thickness distribution of the three-layer structure between the crystallizer copper plate and the slab shell, namely the liquid phase, glass phase, and crystal phase, is closely related to the corresponding slab surface quality. In the selection of protective slag for stainless steel continuous casting slabs, the different requirements of high-temperature mechanical properties and solidification characteristics of different types of stainless steel on the performance of protective slag are mainly considered.
For example, according to the composition design range and typical composition points of AISI304 stainless steel, the phase composition and distribution range during the solidification process were calculated using the Schaeffler formula.
The calculation results show that within the current composition design range, the microstructure of 304 steel is more than 90% of the γ phase and 4%~9% of the α phase. Therefore, the solidification shrinkage rate of the shell in the crystallizer will be relatively large, and the shell growth will be uneven. The direct consequence is the formation of longitudinal strip-shaped depression defects on the surface of the slab, and cracks can be detected in some depressions. Therefore, the selection of protective slag first considers higher basicity and crystallization temperature to achieve a uniform and gentle heat extraction in the crystallizer and minimize the uneven growth of the shell in the crystallizer.
5) Cold strength setting
The setting of the cooling water volume of the crystallizer takes into account the thermodynamic properties of the primary shell of stainless steel and the optimal working temperature of the protective slag film to avoid crack defects and steel leakage accidents out of the crystallizer. The addition of Cr and Ni greatly influences the high-temperature strength, thermal conductivity, and solidification shrinkage of stainless steel. Therefore, the cold strength strategy is also different. For 400 series stainless steel, the high-temperature strength of the billet shell is relatively low (the temperature range of the billet shell in the crystallizer), and a small strain may form a billet shell crack. Uniform and gentle crystallizer cooling is conducive to reducing the cracks formed on the billet shell surface. Therefore, the crystallizer cooling strength of 400-series stainless steel is weaker than that of 300-series stainless steel.
6) Secondary cooling strength strategy
The secondary cooling of the slab continuous casting machine of the stainless steel branch adopts steam-water mixed cooling. The flow control of cooling water in each circuit is calculated and determined by the second-level computer according to the casting speed, steel type and temperature correction factor. The key points of the specific secondary cooling strength design of stainless steel slab continuous casting are:
(1) The specific water volume setting takes into account the lower thermal conductivity of stainless steel to meet the metallurgical length restriction conditions; (2) Considering the thermodynamic properties of stainless steel, the cooling strength is weaker than that of carbon steel, and the cooling uniformity is emphasized; (3) Considering the crack sensitivity of 300 series stainless steel and the low high temperature strength and easy high temperature creep of 400 series stainless steel.
When setting the water flow rate of the water inlet circuit during the casting process, the actual water inlet temperature measured by the circuit is compared and corrected with the target temperature. When the water inlet temperature is ≤25℃ or ≥35℃, the secondary cooling water automatic control system will adjust the cooling water flow rate.
7) Casting speed control
Under the current vibration parameter setting, the increase in casting speed reduces the negative slip rate, reduces the vibration mark depth, and improves the surface quality of the slab. However, with the decrease in negative slip rate, the consumption of protective slag decreases and the leakage rate increases, so the casting speed level of 1.2~1.3m/min is more in line with the current production situation of the stainless steel branch.
2 Analysis of continuous casting process effect
In production practice, after hot commissioning, most process designs have been proven to guarantee the quality of the product effectively. For stainless steel continuous casting, the grinding rate and grinding loss directly reflect the quality of the slab surface and are important indicators for measuring slab quality.
Actual grinding rate index of 304 steel from January to August 2006 (grinding rate = number of slabs ground in one continuous casting / total number of slabs cast in one continuous casting). At present, the grinding rate of 304 steel slabs of the stainless steel branch has reached the level of the model plant, especially since March 2006, when the grinding rate of 304 steel has been below 20%. Considering the requirement that the head and tail slabs in the continuous casting furnace must be ground, the grinding rate of normal slabs has been below 15%. Although there is an impact of the difference in grinding standards caused by different final uses, it also reflects the good surface quality level of stainless steel slabs to a certain extent.
3 Conclusion
In the process design and technology reserve stage, the slab continuous casting project of Baosteel Co., Ltd. Stainless Steel Branch has carried out a lot of research work on the characteristics of stainless steel. Numerical simulation, theoretical calculation, field tests and other means have been applied to the design of the stainless steel continuous casting process. Most of the process measures have been proven to effectively guarantee the surface quality of stainless steel slabs in production practice. The slab grinding rate is basically stable below 30%, reaching the level of the process model plant.
