Plate Supply Condition

01 May 2009

Hot Rolling

Various plate standards, domestic and international, specify or provide a range of alternatives for the supply condition of plate. Typically these supply conditions are As Rolled, Normalized, Normalized Rolled and Thermo-mechanical Controlled Rolled. This article explains the various Supply Conditions and the implications for the customer in choosing which supply condition to specify when ordering plate. This article will discuss hot rolling processes only and will not discuss quench and tempered plate.

Prior to discussing plate supply conditions it is necessary to have a basic understanding of steel metallurgy as it relates to the plate rolling process. How steel is rolled affects the mechanical properties of the final plate. This is shown schematically below. When a steel slab is reheated in a reheat furnace (~1200-1250°C) the steel forms a structure referred to as austenite. When the slab exits the furnace and is rolled it begins to cool. However, in the early stages of rolling the temperature remains high and the deformation of the slab leads to new grains of austenite to form and grow within the austenite structure. This occurs by a process known as recrystallisation and grain growth. The structure at this stage is one of coarse austenite. When this plate cools to room temperature it results in a microstructure of fairly coarse grained ferrite and pearlite which gives average strength and toughness. This process is generally known as hot rolling.

If the steel has microalloying elements such as niobium, titanium, vanadium or aluminium added, there is a point during rolling where the austenite no longer recrystallises but remains in a deformed condition. This is known as the non-recrystallisation temperature. If rolling is carried on below this temperature but usually above the Ar3 (the temperature where the austenite starts to transform into ferrite), the structure at the end of rolling consists of fine "pancaked" grains of deformed austenite. Some rolling may take place just below the Ar3 temperature to give optimal strength and toughness. On cooling to room temperature, the large surface area of the pancaked grains allows for a lot of ferrite nucleation sites and as such gives a fine grained ferrite/pearlite microstructure which is both strong and tough. This process is known as thermo-mechanical controlled rolling (TMCR). Some steel manufacturers employ water spray cooling after rolling in a process known as accelerated cooling, alternatively known as thermo-mechanical controlled processing (TMCP). This usually results in very fine ferrite/pearlite microstructure giving excellent strength and toughness.

How does this relate to plate supply condition? In short the four plate supply conditions depend on the properties required and the fabrication processes in producing the final product.

With "As Rolled" plate there is only a moderate strength requirement and usually no or only moderate toughness requirements for the steel. There is no stipulation on the finish rolling temperature (the temperature at which the final pass in the rolling mill is carried out), but generally the finish rolling temperature of an "As Rolled" plate will be above the Ar3 temperature. Examples of "As Rolled" plate are AS/NZS 3678-250 and AS 1548-PT460NR

With plate supplied in the Normalized condition the plate rolling process is similar to "As Rolled" plate. However, following rolling the plate is subjected to an additional heat treatment (Normalizing). Normalising is usually specified where there is some high temperature forming process during fabrication. It may also be used where the mechanical properties specified are not achievable by rolling alone. In "Normalizing" the plate is heated up to a temperature just above the A3 (~900°C for most plate steel grades) and held for a period of time. During this heating period the ferrite and pearlite structure transforms back into austenite. Because the temperature is held close to the A3 a uniform fine-grained austenite structure develops. After heating, the plate is cooled in still air. As the plate cools, the fine-grained austenite structure transforms again into a fine-grained ferrite/ pearlite mixture. The result is a fine-grained structure with improved strength and toughness compared to "As Rolled". Examples of grades are AS 1548-PT490NL40.

The next common Supply Condition is "Thermo-mechanical Controlled Rolling" (TMCR). This may also be referred to as "Thermo-mechanical Controlled Processing" (TMCP) in some standards. Note the terminology of "Controlled Rolling" to describe the TMCR process is not strictly true as any process that controls the temperature at which final deformation is carried out is "Controlled Rolled". Hence, by definition the "Normalized Rolled" process described below is a "Controlled Rolling" process. In the TMCR process the final rolling passes occur at or just below the A3 temperature. As a consequence of the low finish rolling temperatures the recrystallisation of the austenite structure is inhibited and the subsequent transformation of austenite leads to a fine-grained ferrite and pearlite structure. The fine-grained structure produces plate with good strength and toughness. However, because rolling is completed at a temperature below the A3 there is some deformation (or cold work) of the ferrite formed as a result of the transformation from the austenite. As a result if a TMCR steel is heated up to above the A3 (as in hot forming of dished ends) the cold worked ferrite will transform back into austenite and the plate will lose strength. As a consequence hot forming of TMCR grades is limited to ~620°C to prevent any significant loss of strength or toughness. One of the advantages to TMCR is that it allows equivalent strength and toughness as normalised steels at lower carbon equivalents, without the extra cost of heat treatment. The disadvantage is that it cannot be fully utilised in thicker (usually greater than 50mm) plate. Examples of TMCR steels are AS/NZS 3678-400 and AS 1548-PT490TL50.

The final supply conditon, "Normalized Rolling", is a rolling process whereby the finish rolling passes of the plate are tightly controlled such that they occur in a narrow range of temperatures just above the Ar3. In doing so the mechanical properties of the plate are equivalent to that obtained by carrying out a "Normalizing" heat treatment. Furthermore, if the "Normalized Rolled" plate were subsequently given a "Normalizing" heat treatment the specified properties of the plate would be met. Hence, "Normalized Rolled" plate offers a less expensive option then "Normalizing" as it eliminates the need for a subsequent heat treatment process. "Normalized Rolling" is not effective in heavier gauge plate as it is difficult to get the required reduction through the thickness of the plate at the correct temperature (similar to TMCR). Hence to achieve the required properties in heavier gauge plate it may be necessary to carry out a Normalizing heat treatment process. Currently there are no examples of "Normalised Rolled" supply condition in Australian Standards (the 2008 revision of AS1548 will introduced it). Examples of international grades that can be supplied are EN 10025-2-S355J2 and EN 10028-3-P355NL.

Thus it can be seen that although, the various supply conditions can seem confusing at first, each when specified correctly leads to the supply of steel with the correct mechanical properties for the lowest cost.

By John Dryden