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Abstract: Cold-rolled high strength steel sheets for automobiles contain more Mn and Si based alloy elements, which, under the reducing atmosphere in the annealing furnace, will get enriched on the strip surface. Since the furnace rollers and the strips contact frequently, alloy elements on the roller surface will form hard protuberance. We call this protuberance "pick-up". In addition, with the increase of the amount of dust in the furnace atmosphere, the occurrence of furnace roller pick-up will also increase. Through chemical reaction and physical accumulation analysis combined with practical experience, the causes of pick-up in different furnace sections were analyzed. Methods of preventing furnace roller pick-up in the high strength steel sheet processing line were discussed and solutions to improving the strip surface quality in the continous annealing furnace were provided.
Key words: high strength steel; continuous annealing furnace; roller pick-up; furnace atmosphere cleanliness
1 Introduction
With the development of the automotive industry, the demand for automotive steel sheets,especially for thin-gauge sheets,is growing. These sheets have to meet the increasingly strict requirements on safety, energy saving,low emission,beauty,corrosion resistance,etc. In order to reduce vehicle weight, lower fuel consumption,reduce emissions and improve security, the development of high-strength automotive steel sheets has become an inevitable trend.
For example, TWIP steel is a Fe and Mn-based austenitic steel. By controlling the Mn (17% -24% ) and C (0. 5% - 0. 7% ) content, the best mechanical properties can be acheived. Compared with DDQ grade sheet steel, TWIP steel has a strength 2-5 times higher,but it can also be stamped into automotive parts with a variety of complex shapes. As high-strength automotive steel sheets have a high content of alloying elements such as Mn and Si, these alloying elements will concentrate on the strip surface under the reducing atmosphere in the annealing furnace. Then with the the frequent contact between furnace rollers and strips, the alloying elements will form hard protuberance on the surface of rollers. We call this hard protuberance "pick-up". It will seriously affect the surface quality of automotive steel sheets.
2 Characteristics of pick-up and morphology of strip surface defects
An annealing furnace,as shown in Fig. 1,generally consists of the following components : the preheating furnace (PHF),heating furnace (HF),soaking furnace ( SF ), slow cooling...