When it comes to the quality of continuous casting, steelmakers know that mold oscillation plays a vital role. One important concept that’s often discussed in regards to mold oscillation and continuous casting is negative strip, which is usually expressed as negative strip time.
This article will define what theoretical and actual negative strip time is, why it’s important to the quality of your continuous casting process, and how you can measure this time to optimize quality and yield.
What is Negative Strip, or Negative Strip Time?
When you consider the function of a mold oscillator, you know that as you are casting, the mold oscillator is moving up and down, generally in a sinusoidal pattern. When the mold is moving upward relative to the strand, that period of time is known as positive strip. When the mold is moving downward relative to the strand, that is known as negative strip.
Negative strip time refers to the amount of time during the downward movement of the mold when the mold speed is higher than casting speed.
Negative strip can also be expressed as a percentage, which refers to the average mold speed during the downstroke relative to the casting speed, or as a ratio, which refers to the ratio of the negative strip and the total cycle time.
How Does It Affect Your Continuous Casting Process?
It’s important to understand the negative strip time of your continuous casting process, as it can affect the quality of your steel in a few ways.
- In general, longer times lead to the formation of deeper oscillation marks.
- Longer time does also have some benefits, specifically in improving the mold-lubricating behavior in a way that has proven useful in improving surface quality and preventing breakouts.
For operators, understanding and keeping a close eye on the negative strip of your continuous casting system is essential to ensure ideal casting conditions. Identifying the ideal negative strip parameters for your casting process can help ensure the optimal balance between surface quality and mold lubrication.
How is Negative Strip Time Calculated?
This is calculated using the following formula:
where:
f = frequency (cycles per minute)
h = stroke (mm)
Vc = casting speed (metres per min)
What Does This Calculation Tell You?
This calculation tells you the theoretical negative strip time of your casting process based on your nominal parameters, however by measuring actual oscillation systems both in cold and hot conditions the actual strip time can be found. Often this does not match the calculated time due to deviations in the oscillation cycle, which can upset the lubrication condition inside the mold and can lead to surface quality problems and breakouts.
Negative strip time is just one calculation of many that operators use to evaluate and improve the quality of their continuous casting process. If you have questions, or you’re looking for overall greater insight into the performance of your system’s mold oscillator, the KISS Technologies team has the expertise and the products to support you. Give our team a call at 616-394-1401 or contact us online today for more information.