In the steel industry, there is a lot of talk referencing cold rolled steel and hot rolled steel, which can be confusing. After all, steel is steel right? Well, not exactly. So what is the difference between cold rolled steel and hot rolled steel; and, how is cold rolled steel made?
To start out, cold rolled steel is a carbon flat rolled steel product that is manufactured by rolling a hot rolled steel coil between two rollers, thereby compressing the steel from it’s original thickness. Cold rolled steel is rolled below its re-crystallization temperature at room temperature which is about 1,000 degrees lower than what the original hot rolled steel is rolled at.
The cold rolling process increases the strength, ductility, and hardness of the steel. This hardness of the steel, also known as temper, is controlled by how much original strain is introduced by the rollers on the original steel during the cold rolling process.
In addition, the cold rolling process creates a smoother, more aesthetic and tactile surface finish on the steel than a traditional hot rolled product. Among others, this is one of the primary reasons why cold rolled steel is used in many consumer products and durable goods. Finally, cold rolled steel has more precise dimensions than hot rolled steel and is able to hold tighter tolerances.
Cold rolled steel sheet, blanks, slit coil and coil is produced in a handful of conditions including quarter hard, half hard, full hard, and the most common “skin pass” /skin rolled.
Skin pass cold rolled steel is created when the metal is only reduced by about a 0.5% to 1% thickness during the cold rolling process. As a result, skin pass material is more ductile for bending and forming into final products.
Quarter hard, half hard, and full hard cold rolled steel sheet, blanks, coil and slit coil are all reduced further during the cold rolling process than skin passed in corresponding order. All of these applications have their own uses in the manufacturing and fabricating process.
Because the steel is compressed thinner in quarter hard, half hard, and full hard applications than in skin pass, the steel’s yield is increased while its ductility decreases. For example, full hard cold rolled steel can only be bent at a 45* angle without fracturing while skin pass cold rolled steel can be bent at 90*.
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