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BOC experts offer means to improve the quality of annealed steel products

BOC Gases : 14 January, 2002  (Company News)
A team of BOC metals specialists have developed an approach for improving the quality of annealed steel by reducing carbon deposition on the surface of the cold rolled steel strip. Surface quality is extremely important because annealed steel is usually painted or galvanized, and any surface imperfection, such as carbon deposits, iron fines, or oxides will affect the quality of the final product.
'A 100-percent hydrogen atmosphere is the best atmosphere for the production of annealed steel,' says Mircea Stefan Stanescu, Ph.D. senior customer support specialist, BOC. 'But even in this atmosphere, carbonaceous deposition is occasionally encountered, and even a thin film of carbon on the surface of annealed steel strip interferes with the subsequent coating process.'

In a paper presented in late 2001 at the 21st American Society for Metals International, Heat Treating Society Conference in Indianapolis, Ind., Stanescu and BOC co-authors Paul Stratton and Arthur Klassen reviewed the causes of carbon deposition on annealed steel. They also made recommendations on ways to reduce or eliminate carbonaceous deposition in 100-percent hydrogen annealing of steel strip coils.

Annealing, which involves heating steel to a predetermined temperature, holding it for a certain time, and then cooling to room temperature, is done to improve ductility and reduce brittleness. Annealing is carried out intermittently during the working of cold rolled steel to restore ductility lost through processing.

According to Stanescu, the most common causes of carbonaceous deposition on annealed steel are the presence of rolling lubricants and tramp oils on the cold rolled strip before annealing and, during annealing, methanation and carbon back deposition, also related to the existence of rolling lubricants.

To address this problem, the BOC experts recommend examining the lubricants used by the rolling mills through which the steel passes during processing, and obtaining base-line data on the current annealing practices. This information is used to determine the causes of carbonaceous deposition and, consequently, the recommendations necessary to optimize the annealing cycles for the best surface quality and minimum hydrogen consumption per ton of annealed product.

'In our study,' Stanescu says, 'we found that the highest quality annealed steel strip, wire or rod is produced using the optimum hydrogen flow rate profile and annealing cycle to give the most uniform temperature within the load, and the most effective evaporation and evacuation of the rolling lubricant.'
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