B.G. Thomas, M. Okelman, J. Sengupta, C. Ojeda, and G.G. Lee

National Science Foundation SIRG DMI 05-28668, Continuous Casting Consortium

National Science Foundation Support

DMI 05-28668
10/1/2005 to 9/30/2008
NSF Division: 05-526, "Sensors and Sensor Networks (Sensors)"
NSF Program Director: Abhijit V. Deshmukh, (703) 292-8330
NSF Grants Official: Maria Valerio, (703) 292-8212

Industry Support

Sumitec Seimens (VAI Services): Facilities

Sumitec (VAI Services) of Benton Harbor, Michigan runs a refurbishing and electroplating facility for continuous casting molds. Mike Powers of Sumitec helped us to test our designs using their real-world facility. This helped to fix the problems with scale-up or other differences between lab-scale experiments and full commercial implementation of the new sensor technology we have developed..

Nucor Steel, Decatur, Alabama, (Ron O'Malley)

Nucor Steel, Decatur, Alabama, has agreed to test our new sensor in service once the design is proven in future trials.

Project Summary

Monitoring of mold level and meniscus behavior is important for controlling quality during the continuous casting process.  This project aimed to develop new sensors to measure temperature in the mold very near to the meniscus, initially to use as a new research tool to investigate meniscus behavior to better understand defect formation.  The ultimate goal is to revolutionize online thermal monitoring of industrial continuous casting molds.  A process has been developed to insert sensors manufactured at UW Madison into the mold coating layer.  Tests of sensor integrity was conducted, data collected, and the signals analyzed using computational models.  The meniscus region was further modeled computationally to predict events during an oscillation cycle – including modeling of the sensor itself. This determined the relationship between the sensor signal and the actual meniscus events.  Insights gained enabled optimization of the size and location of the new sensors and interpretation of their signals to gain maximum benefit from their installation into operating molds.

Summary of Activities: Click here for full Activities PDF (182 KB)

Work at the University of Illinois has focused on discovering a new technology for implementing better sensors into the continuous casting mold used at the steel plant. This has involved experimental investigation of 1) the attachment of glass fiber and nickel-based sensors onto the copper mold surface, and 2) ability to embed the sensors into the mold coating during the eletro-plating process, and computational modeling investigations of 1) heat-transfer consequences of imperfect plating to determine standards for success, and 2) mechanisms of initial solidification including heat extraction during oscillation mark formation, in order to extract useful results from the finished sensors in the future. The ultimate goal is to discover a new and better sensor technology for in-mold monitoring of the continuous-casting process.

Experimental Studies: sensor strip attachment and electroplating embedding

Before any research could be carried out to understand the behavior of embedded sensors, it is imperative that a robust method be developed to embed sensors in manufacturing tooling. Therefore, there was an absolute need to refine the embedding process through plating studies, as the sensor strip was attached to the copper mold via the electroplating process used to apply the nickel coating layer. Any attachment method must provide a secure bond between the sensor strip and the copper mold face, have no air gaps, survive the acid pretreatment steps, and allow the sensor strip to be plated successfully before the copper mold can be put into service. It is possible to attempt to plate the sensor strip, which is fabricated encapsulated by nickel, by placing it on the copper mold face and submerging the mold face into the plating tank of the mold manufacturer. Two different trials spanning a range of aspect ratios were performed to evaluate the plating ability to attach a nickel strip to a copper substrate via commercial nickel electroplating.

Modeling Studies: Consequences of Gap Formation and Initial Solidification Phenomena

An incorrect plating procedure can result in an air gap forming between the sensor strip and copper substrate. In service an air gap present in the continuous casting mold can limit heat transfer and cause a localized high temperature region near the sensor. Such a temperature increase can contribute to plating layer failure or lead to the sensor spalling off. Computational methods have been utilized to quantify this behavior. This is needed in order to evaluate the maximum size of gap that would still allow performance of the sensor.

Work to improve understanding of meniscus phenomena has proceeded at the University of Illinois using computational models, metallography, and microscope analysis of plant samples, previous laboratory experiments, and conventional temperature measurements. A new mechanism for the formation of oscillation marks has been developed, which involves freezing of the meniscus.

Discovery of New Sensor Technology

All of the project activities aimed at discovering, developing, and implementing a new technology for monitoring temperature and other phenomena in the continuous casting mold using efficient and accurate new sensors.

Findings Click here for Findings PDF (92 KB)

Experimental Studies: sensor strip attachment and electroplating embedding

• The wireless system was tested in the presence of an electromagnetic field at a commercial steel company and was shown to maintain communication between the thermocouple node and base station transceiver.


• During the initial plating study, nickel adhered well to the top, bottom, and sides of the nickel strip, as well as the copper substrate.


• Often, when electroplating onto a thin strip suspended a small distance from the substrate, a void commonly forms in the gap, due to the starvation of nickel atoms when two sections of growing grains impinge just past the edge of the nickel strip. This also causes the void width to extend wider than the strip width, seen as seams at the edge of the void.


• The rate of plating is greatest in areas where sharp corners exist, due to higher current density.


• More plating reaches under the nickel strip as the sensor width decreases and/or gap thickness increases. More specifically, more plating reaches under the nickel strip as the aspect ratio, defined as the strip width over gap thickness, decreases.


• Complete filling under the nickel strip is predicted to occur when the aspect ratio is less than or equal to one.


• A sensor strip can be attached to a copper substrate via conductive silver paste and successfully plated over without any air gaps. However, the time and skill involved in this process is considerable.


• According to the ultrasonic welding equipment manufacturer, past experiences have indicated that ultrasonic welding near, around, or on top of sensors has lead to irreversible damage.


• Although the ultrasonic welding equipment is able to attach the two dissimilar metals, the machining pattern caused by the weld horn is unavoidable.


• Due to their non-conductivity and geometry, fiber optic sensors can be plated over without an air gap developing
  

Modeling Studies: Consequences of Gap Formation and Initial Solidification Phenomena

• It was found that an air gap in the nickel plating layer can cause stress to increase by 19%.


• A wider gap makes it more difficult for heat to conduct around the gap, increasing the temperature at the hotface: doubling the width of the air gap increases the hotface temperature by 65°C, while doubling the thickness of the air gap increases the hotface temperature by only 5°C.
  
  
• Oscillation marks and hooks which comprise the initial solidification structure form due to meniscus freezing and overflow. This brings heat to the meniscus in a characteristic periodic heat flux, which is usually increasing during the negative strip time. A detailed computational model of this has revealed the fundamentals of this behavior and a detailed mechanism for the phenomena.

Discovery of New Sensor Technology

A method to measure temperature and/or heat flux near the surface of the hot face of continuous casting molds has been designed. It consists of embedding a thin optical fiber with fiber Bragg gratings inside a thin stainless steel tube into the nickel coating layer during electrodeposition onto the surface of the copper molds used to continuous cast steel slabs and/or billets. During casting, this sensor will monitor the thermal condition of the mold. The sensors inside the fiber function using optical-based technology (resonating frequency of light captured in an embedded optical fiber system causes the wavelength of light emitted along the fiber to depend on thermal strain, which varies with the temperature). Embedded sensors have the advantage of real-time monitoring at critical locations as well as immunity to electromagnetic interference and resistance to hostile environments, but cannot be commercial successful without a robust attachment method. An alternative method embeds a rectangular strip which contains micro- and nano-layers deposited using nano-fabrication technology, that functions using conventional thermocouple technology (difference between the voltages that the temperature induces in two different metals) using conductive silver paste. In either case, the key aspects of the new invention (making it better than existing thermocouples) are:

• The small size of the active sensor is much smaller than current thermocouple 'beads', allowing greater sensitivity to temperature variations both spatially and temporally.


• The small size of the entire sensor, allowing accurate knowledge of its position near the surface of the mold.


• The active part of the sensors being embedded into a strip, allowing it to be manufactured in a controlled environment, and handled, prior to attaching to the dirty environment of the mold.
  
  
• The attachment method to the mold, consisting of attaching (via embedding into the coating layer) of the sensor to the conditioned mold surface during the electroplating process. This method allows the sensor to be close to (but not at) the mold surface without drilling a hole through the mold. The distance of less than 1 mm from the surface is more than an order of magnitude closer to the hot face surface than conventional thermocouples.


• Extension of the sensor fiber/strip beyond the top of the mold, allowing easy extraction of the sensor signals to a computer, such as by attachment to a miniature circuit box for wireless transmission to a computer located elsewhere.
  
  
• The new sensor provides crucial information about the temperature and heat flux state of the mold. It solves several problems inherent to conventional thermocouple systems (currently used to monitor mold wall temperature), and conventional mold level sensors (currently used to monitor mold level). If a second optical fiber is embedded, then the sensor can additionally monitor thermal stresses in the mold surface, enabling it to provide feedback to signal crack formation in the coating layer.

Publications:

Li, X., C.W. Wong, D. Dornfeld, and B.G. Thomas, “Research on Subwavelength Microphotonic Sensors for In-situ Monitoring with High Spatial and Temporal Resolution in Manufacturing Environments”, Contacting and Solidification in Casting-by-Design”, Proceedings of 2006 NSF Design, Service, and Manufacturing Grantees and Research Conference, St. Louis, Missouri, July 24-27, 2006, 9p. Click here for a PDF version. (644 KB)

Sengupta, J., B.G. Thomas, H.J. Shin, G.G. Lee, and S.H. Kim, “Mechanism of Hook Formation during
Continuous Casting of Ultra-low Carbon Steel Slabs,” Metallurgical and Materials Transactions A, 37A:5,
1597-1611, May 2006. Click here for a PDF version (1.18 MB)

Ojeda, C., J. Sengupta, B.G. Thomas, J. Barco, and J.L. Arana, “Mathematical Modeling of Thermal-Fluid
Flow in the Meniscus Region During an Oscillation Cycle,” AISTech 2006 Steelmaking Conference
Proceedings, Cleveland, OH, AIST, Warrendale, PA, Vol. 1, 1017-1028, May 1-4, 2006. Click here for a PDF version (607 KB)

Sengupta, J., B.G. Thomas, H.J. Shin, and S.H. Kim, “Mechanism of Hook Formation in Ultra-low
Carbon Steels based on Microscopy Analysis and Thermal-stress Modeling,” AISTech 2006 Steelmaking
Conference Proc., AIST, Warrendale, PA, Vol. 1, 903-914, May 1-4, 2006. Click here for a PDF version (1.45 MB)

Thomas, B.G., J. Sengupta, and C. Ojeda, “Mechanism of Hook and Oscillation Mark Formation In Ultra-
Low Carbon Steel,” Second Baosteel Biennial Conference, Shanghai, PRC Vol. 1, 112-117, May 25-26, 2006. Click here for a PDF version (957 KB)