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Level 2 Model as a Metallurgical System


Metallurgical nature of the flow stress

The metallurgical impacts on the flow stress exists at least in the two areas. Besides the retained strain that affects the strain thus the flow stress, the grain size change accompanied by the metallurgical process also changes the flow stress.

During rolling there is dynamic recrystallization, and in the inter-pass period, there are static and metadynamic recrystallization. As long as about 95% of the metal is recrystallized, the grains start to grow. With the change of the grain size, the flow stress changes. If a portion of the strain is not removed by the recrystallization, it will be carried forward to the next pass as the retained strain and thus cause the errors of the strain and thus flow stress in the next pass. In the case there is phase transformation, two materials are actually involved and theoretically, two sets of the flow stress coefficients should be used.

As showed in the Table 1, the Material in the flow stress section (left) is associated with both the Phase and the Grain size (right), and the Strain is affected by the Retained strain. In the high-speed rolling, the portion of the Strain rate contribution to the flow stress is affected by the Temperature due to the significant heat generation. In addition, the Phase transformation involves heat release or heat absorb as well as the change of the material, and so it has a great impact to the flow stress. The metallurgical process would get much more complicated when the precipitation, etc., exists during the processes such as hold.

Table 1 Flow stress and metallurgical parameters

Flow Stress

Metallurgical parameter

- Material
- Strain
- Strain rate
- Temperature

- Phase
- Grain size
- Retained strain
- Temperature

Property variations

During a study to improve the performance of the Level 2 model and prior studies, the microstructure and the properties of the steel were investigated to understand the source of errors in the prediction of force and torque. Clear and persistent variations occurred along the length, width and thickness of the steel. This would be expected from the observed variations in processing and the known metallurgical reactions that occurred from the processing. The variations help to explain some of the inconsistent force and torque and the difficulty in reaching the target mechanical properties for an order.

The microstructure through the thickness of the steel showed that the grains at the center were larger and less flat than the grains on the surface. This would be expected from the metallurgical slip lines of flow during plastic deformation. Depending on the roll diameter and the draft during rolling, the resulting grain nucleation and growth are predictable. Even the variation in the retained strain at the center after the delays between passes could be predicted. None of these processes were included in typical Level 2 models, although these metallurgical processes have a large influence on the mechanical properties of the steel, particularly the impact properties critical in X grades.

There were also observed differences in the microstructure and the temperatures of the steel across the width. Near the edges of the steel, the temperatures were about 100C colder during rolling and quenching. The Level 2 models do not track this difference or the effects of the temperature on the flow stress and the percentage of plastic deformation near the edge. Most models do track the variations in the forces across the work rolls, but do not model the metallurgical results of these differences, so it is not surprising that the Level 2 did not predict the total force to roll the steel through the gap in the finishing passes

Down the length of the steel there were also many differences. Even short plates had changes in temperatures, changes in flow stress, and subsequent changes in microstructure along the length, particularly for X grades. When rolling steel from coil furnaces, the temperature differences were over 100C and the metallurgical effects were significant. Most Level 2 models track the temperatures at the ends of the steel but do not taken into account the pattern of variations from end to end, that affects grain size, phases present and retained strain in the steel. These details are critical to predicting the properties of the steel and to maximizing the utilization of the rolling mill.



[1] B. Li and J. Nauman, Metallurgical, Modeling and Software Engineering Issues in the Further Development of the Steel Mill Level 2 Models, AIST Annual Conference 2008, May 5 8 2008.

[2] I. Tamura, et al., Thermomechanical Processing of High-strength Low-alloy Steels. Butterworths & Co. 1988. ISBN 0-408-11034-1.

[3] A. Hensel & T. Spittel, Kraft- und Arbeitsbedarf bildsameer Formgebungsverfahren. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, Germany. 1977.

[4] Y. Saito, et al., The mathemarical model of hot deformation resisitance with reference to microstructural changes during rolling in plate mill. Transaction ISIJ, 1985, 25(11).

[5] Suzuki, et al, Studies on the Flow Stress of Metal and Alloys. Univ. of Tokyo. 1968.

[6] B. Li and J. Nauman, Metal Pass 108 Mill-Related Projects. Online at www.metalpass.com/consulting.

[7] B. Li, D. Cyr and P. Bothma, Level 2 Model Improvements at Evraz Oregon Steel Mills. To be published in AISTech 2009.


Related Resources

Metal Pass Work List on Level 2 and Mill Modeling

Metal Pass Research Reports

Metal Pass Recent Publications

Technical papers published in the February and March of 2008. Those publications are primarily on Level 2, Level 2 model and process automation.


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