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Fundamentals of Automotive Steel Sheets - Characteristics of Steel Sheets
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Characteristics of Steel Sheets
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Steel sheets are of two types; hot rolled or cold rolled: |
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Hot rolled steel sheet is rolled above re-crystallization temperature (normally 1,472-1,652 °F (800-900 °C)). | |
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Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature). | |
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In automotive body components, hot rolled steel sheets are used for frames and other parts of vehicles that carry heavy loads such as trucks. Cold rolled steel sheets are mostly used for unibodies of passenger vehicles. Because corrosion resistance is required for such parts as outer panels, side frames and wheelhouses, zinc plated cold rolled steel sheets are widely used: |
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During the hot rolling process, slabs must be rolled while they are still hot. As they are cooled while being rolled, they cannot be rolled thin flat. Cold rolled steel sheets can be rolled at normal temperature, making it possible to manufacture thin steel sheets. | |
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As hot rolled steel sheets are rolled at a high temperature, black oxidation, or mill scale, appears on the sheet’s surface. Sheets that have been washed with oxide (pickling washer) have gray surfaces. The surface of a cold rolling steel sheet is smooth and glossy, often called polished sheet. It is subjected to the picking process, cold rolling, annealing, and temper rolling process after the hot rolling process. | |
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Most of the body component parts are press formed. The workability on presswork is determined by technical specifications of materials. Those technical specifications are, in turn, determined by material contents as well as the extent of temper rolling.
When the workability compared in terms of elongation rate obtained by an elongation test, hot rolled steel has about 35% while a cold rolled steel has about 43%, indicating that a cold rolled steel sheet has better workability. | | | 
