A mold simulator for continuous casting of steel: Part II. The formation of oscillation marks during the continuous cast
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I. INTRODUCTION
OSCILLATION marks are defects formed on the surfaces of continuously cast steel products. These marks form in an orientation perpendicular to the direction of casting, and are normally parallel to each other. While the physical surface defects of the oscillation marks themselves do not pose a significant problem at this time, some other defects have been found to be regularly associated with oscillation marks. These defects include entrapped argon bubbles, inclusions, and segregation. The oscillation marks may also act as nucleation sites for surface cracking. It is commonly believed that the main casting parameter affecting oscillation mark formation is the negative strip time. Negative strip time is defined as the period during which the mold is moving downward faster than the strand, while the remaining duration of the oscillation cycle is called the positive strip period. For sinusoidal oscillation, negative strip time is quantified by the following equation:[1] tn
vc 1 arccos a b pf psf
[1]
where tn is the negative strip period, f is the frequency of oscillation (Hz), vc is the casting speed, and s is the stroke. An increase in the duration of the negative strip period will result in an increase in the severity of oscillation marks. Therefore,
A. BADRI is with Shell Oil, Malaysia. T.T. NATARAJAN, Senior Research Engineer, C.C. SNYDER, Senior Technician, and K.D. POWERS, Project Analyst, are with the U. S. Steel Research and Technology Center, Monroeville, PA 15140. F.J. MANNION, General Manager, is with U.S. Steel, Slovakia. M. BYRNE, Director of Research, is with ISG Technical Center, Bethlehem, PA 18016. A.W. CRAMB is with the Department of Metallurgical and Materials Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted February 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
one solution to alleviate the effect of oscillation marks would be to decrease the negative strip period. However, negative strip time is also understood to be the period of time during which the liquid mold flux infiltrates between the shell and the steel, where it acts as a lubricant. Therefore, it may not be possible to eliminate the negative strip time without producing disastrous consequences such as sticker breakouts. From Eq. [1], the operating parameters that affect negative strip time are casting speed, oscillation stroke, and oscillation frequency. Although the effects of these parameters on the negative strip period are not independent, the following discussion will consider them separately. Since casting speed is directly related to productivity, the ideal solution would be to minimize the negative strip time while maximizing the casting speed. An upper bound on the casting speed is reached when the casting speed is equal to the maximum downward velocity of the mold (sf ), since at this point the negative strip time is zero. Any velocity greater than this value would result in casting without negative strip and an increased likelihood
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