Q390C steel plate is a type of high-strength low-alloy structural steel defined in the Chinese national standard GB/T 1591-2018 "High Strength Low Alloy Structural Steels," known for its excellent strength and good weldability. The "C" in the grade typically indicates the quality level of the material, which relates to the impact test temperature; grade C usually corresponds to an impact test at -20°C.

Q390ARC steel plate, where "A" denotes the delivery condition as hot-rolled, "R" indicates normalizing treatment, and "C" similarly represents the quality level. Normalizing is a heat treatment process that involves heating and slow cooling to improve the mechanical properties and workability of the material.

The main differences between the two lie in their delivery conditions and possible heat treatment processes:

1. Delivery Condition: Q390C steel plate may be supplied in various delivery conditions, such as hot-rolled, normalized, or thermomechanically controlled processed (TMCP), whereas Q390ARC steel plate specifically indicates normalizing treatment.

2. Heat Treatment: Q390C steel plate may not undergo normalizing or may receive other types of heat treatment as needed. Q390ARC steel plate, however, is explicitly normalized, which helps refine the grain structure and enhance the material's toughness and plasticity.

3. Performance: Due to the normalizing treatment, Q390ARC steel plate may exhibit better impact toughness and weldability, especially in low-temperature applications.

4. Application: Although both steel plates have similar chemical compositions, differences in heat treatment may make them suitable for different engineering applications. For example, Q390ARC steel plate may be more appropriate for applications requiring good low-temperature toughness.

5. Cost: Normalizing treatment increases production costs, so Q390ARC steel plate may be slightly more expensive than Q390C steel plate.

6. Supply Conditions: When procuring, users need to select the appropriate type of steel plate based on their specific requirements, including delivery conditions and intended application environment.

In practical applications, engineers must choose the suitable material based on specific engineering requirements and expected usage conditions, while considering factors such as performance, cost, and supply conditions.