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Vol.30, No.4, 401 ~ 409, 1992

Title

Effects of Chemical Composition and Coiling Temperature on the Bake Hardening Characteristics of Low - Carbon Hot Rolled Strips

김교성Gyo Sung Kim, 권오준Oh Joon Kwon

Abstract

Laboratory hot rolling experiments were performed to develop bake hardenable hot-rolled strips. The effects of steel chemistry as well as coiling temperature(CT) were studied to find out the optimum processing condition. It was found that the ferrite grain size was decreased as increasing the carbon content up to about 120ppm. The carbon content more than 120ppm did not affect the ferrite grain size significantly. When the carbon content was high matrix cementites were observed at the CT`s ranging between 400 and 500℃, but few matrix cementites were observed at the CT`s lower or higher than the range. Matrix cementite was not formed either when the carbon content was low. This can be analyzed by the degree of supersaturation and diffusivity of carbon. It was observed that the major factor which determined the amount of bake hardening (BH) was the content of soluble C and N. When CT was 200℃, BH was increased with the carbon content. When CT was 700℃, BH showed a maximum point at C=50-100ppm. When CT was 500℃, BH was low and did not show a carbon content dependency. This was attributed to the fact that precipitation of matrix cementites lowered the solute carbon content. BH in AI-containing steels was very low when CT was 700℃. This was thought to be due to the formation of AlN which not only decreased the solute nitrogen content but also accelerated the cementite precipitation. It was also found that fine grain size showed higher BH at a given solute (C+N) content.

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