2010 August Issue
Full Table of Contents

Continuous Casting Technologies

A surface investigation of high-strength steel rougher work roll necks was conducted to determine the effect of microstructure on wear performance. The relationship between wear performance and core material chemistry was also examined in this study.

This paper describes a method to measure the intrinsic lubricity in all lubrication regimes of importance to steel cold rolling. The application of the Stribeck curve proves useful in the differentiation and development of advanced cold rolling lubricants.

This paper presents the development of the UNI plus coiler and explains the necessary reinforcement and optimization of the mechanical, hydraulic and electrical design. A modernization concept of a recent project is discussed along with operational results.

Thermal spray coating has helped to reduce dross buildup, wear and corrosion of the rolls used in hot-dip galvanizing. A new MoB-based thermal spray coating has given better results with non-wetting of the roll surface and higher roll life compared with coating based on tungsten carbide.

The new Pomini automatic loader, designed to lift rolls with chocks up to 25 tons, has a patented lifting beam and a control system that minimizes the center of mass excursions.  The system allows a faster setup of the grinding phase, which leads to reduced loading/unloading time.

Spot welding of advanced high-strength steels (AHSS) poses significant challenges because the high-carbon equivalent of AHSS can result in welds that are perceived to be of lower quality based on conventional weld quality criteria. Specifically, welds in AHSS are more prone to interfacial fracture, whereas button pullout failure is the preferred fracture mode because it indicates the weld is stronger than the base material. However, some evidence suggests that welds in AHSS that exhibit interfacial fracture have comparable strengths to welds that exhibit button pullout failure in prior-generation steels. Therefore, parameters that can indicate weld integrity should be investigated more thoroughly for the new generation of high-strength, high-ductility steels. In this paper, processing-microstructure-mechanical behavior relationships are presented for TRIP780 and HSLA-50, which fall in the class of AHSS and prior-generation steel, respectively. If a weld is defect-free, the parameter that most strongly controls mechanical properties, such as peak load and energy absorbed to failure, is the weld diameter in both the TRIP and HSLA steel. There could be multiple factors that correlate with weld diameter, such as hardness and stress profiles across a weld, that in turn affect weld mechanical properties. However, these factors are shown to vary minimally with weld diameter, and therefore the load-carrying capacity of the weld versus the load-carrying capacity of the base material controls mechanical behavior. For welds that exhibit interfacial fracture, the crack initiation fracture toughness can be calculated, which is normalized by the weld geometry and allows for the integrity of welds of different diameters or different sheet thicknesses to be compared. The fracture toughness of welds in TRIP780 and HSLA-50 is very similar because it is most strongly influenced by the fracture toughness of martensite, which is comparable for the two steel compositions.

The addition of expensive alloying elements is often needed to form the desired microstructures of DP and TRIP grades in hot rolled steel flat products. These alloying elements effectively modify the rate at which diffusional phase transformations occur, making it possible to produce the required microstructure within the constraints of a hot line’s rolling speed, runout table length, and the heat extraction capabilities of its existing laminar cooling systems.
This paper presents Compact Cooling systems as an alternative to this approach, whereby processing enhancements may enable new alloy designs and product strategies. Compact Cooling systems provide heat transfer rates that exceed those of conventional laminar cooling systems; hence they can bring about the required phase transformations with shorter runout table length requirements, reduce the levels of “hardenability” additions needed in the production of hot rolled DP and TRIP grades, and enable the implementation of new microstructure/product concepts.