What those grade names really mean

There's more to selecting a grade of steel than simply using the short form such as "250" or "Grade 250", there are important detail differences between the different steels covered by these general headings.

Each steel grade has a number of explicit attributes associated with the particular standard that it complies with, whereas a general term can cover more than one grade or standard. When steel is purchased to meet a particular standard, the purchaser is assured that it will meet certain chemistry and/or mechanical property requirements.

However, other attributes are less obvious, but can have a significant effect on the performance of the steel. Many of these attributes are learned through experience. When users do not have the experience or background in using particular steel grades, they may not be aware of precisely which grade offers the most suitable attributes for their purpose. This article highlights some of the different characteristics inherent in specific types of steels, and also what some of the common terms used in the industry are.

"Grade 250"

Probably the most common grade of steel used is often referred to as "Grade 250" or just "250". But "oils ain't oils", as the saying goes, and there are two significantly different types of "250". Grade 250 can be produced on either the Plate Mill in flat plate form (AS/NZS 3678 - 250 XLERPLATE®*), or the Hot Strip Mill in coil form (AS/NZS 1594 - HA250 XLERCOIL®). The different production routes can lead to differences in the properties and performance of the two grades, and these differences can be important in certain applications.

The Hot Strip Mill produces coil plate with thicknesses up to a maximum of 12mm, while the Plate Mill can produce plate to a minimum thickness of 5mm. Hence there is an overlap in the thickness range between 5 and 12mm for plate supplied from the two mills.

Which is better" That depends on the intended end use of the material, and it is more a question of which is more suitable, rather than which is "better". In some cases there will be no particular issues with the end use when selecting between AS/NZS 3678-250 XLERPLATE® from the Plate Mill and AS/NZS 1594-HA250 XLER® coil plate. In these cases the choice will come down to choosing a size (width) that will allow for better nesting of components and hence less yield loss.

But there are times when the differences will make a significant difference to the end result. The two types of plate can differ in such characteristics as surface scale, flatness, silicon content, grain structure and even dimensional tolerances. Where any of these characteristics is critical to the application, it becomes important to know which plate is which.

Surface Scale:

XLER® coil plate (HA250) has less surface scale than plate from the Plate Mill (3678-250 XLERPLATE®). Consequently it tends to be better for laser cutting. However, for thicknesses greater than 8mm, the flatness of the plate from the Plate Mill tends to be better than that of coil plate and may be preferred if flatness was critical.

Silicon Content:

Australian steels generally have low silicon levels. Coil plate will generally, but not always, have lower amounts of silicon than plate from the plate mill. Hence coil plate is generally better for galvanizing, especially where appearance is important. Lower levels of silicon produce a galvanized finish that is brighter and more aesthetically pleasing. For further information, Contact Us or the Galvanizers Association of Australia on 03 9654 1266.

Flatness:

Where flatness is critical, AS/NZS 3678-250 XLERPLATE® from the plate mill may be preferred to the HA250 XLER® coil plate for thicker gauges. This is due to the uncoiling of the thick coil to cut into plate lengths. In such applications it is also advisable to Contact Us to discuss what levels of flatness can be achieved and what options may be available in producing product to meet requirements.

Grain structure:

The different rolling processes also lead to differences in the grain structure of steel produced on the two mills. The 5 and 6mm thick 3678-250 XLERPLATE® will tend to be significantly stronger than the 5 and 6mm thick HA250 XLER® coil plate. As a result, where a tight bend radius is required the HA250 XLER® coil plate grade will be preferable. However, the difference between a transverse bend (across the plate) and a longitudinal bend (along the plate) is greater in the HA250 XLER® coil plate than in the 3678-250 XLERPLATE® grade. This is because the plate mill process tends to involve more cross rolling than the hot strip mill. Consequently the 3678-250 XLERPLATE® has a more uniform grain structure and more uniform properties than HA250 XLER® coil plate.

Standard Variations

In addition ot these practical variations, the two standards AS/NZS 3678 and AS/NZS 1594 include several important differences in specification between the two types of "Grade 250". These issues include dimensional tolerances, surface quality and strength, and similar issues also apply to AS/NZS 3678 - 350 XLERPLATE® and AS/NZS 1594 - HA350 XLERCOIL® grades.

The two standards have different tolerances for thickness, width, length and flatness. Where any of these attributes are critical, standard AS/NZS 1365 should be consulted. The standards also have different requirements for surface quality and structural integrity, which again are detailed in the respective standards.

Even within the AS/NZS 3678 standard there are different strength requirements for 250 grade. As thickness increases, the minimum yield strength required for 3678-250 XLERPLATE® decreases. At 5mm thickness the minimum yield strength required by the standard is 280 Mpa, while for thicknesses greater than 80mm the minimum yield strength is 230 Mpa.

By contrast, there is no such variation for coil plate. For the AS/NZS 1594-HA250 XLER® coil plate specification the minimum yield strength of 250 megapascals applies to the full thickness range covered by the standard.

Mild Steel

Mild steel is another common term used in the industry, but it too is widely misunderstood. For a start, it is not actually a grade - this "grade" comes with limited specifications or guarantees with respect to properties or performance. The term is sometimes used interchangeably with 250 grade, but the two are not the same, and purchasers of "mild" should be aware that such steel might not meet the requirements of 250 grade.

In practical terms, "mild steel" generally refers to any low strength, carbon steel with a carbon level less than 0.2 percent. This may give rise to problems because mild steel has no specified (hence guaranteed) strength levels, or other mechanical properties, and it is not covered by any specific restrictions on its chemistry. Neither is it covered in terms of tolerances or quality by any standard, which clearly makes it unsuitable where any of the specifications or characteristics may be critical.

When purchasing steel for any purpose, it is important to ensure that the correct grade of steel is purchased for the desired application. If unsure of what grade should be specified, Contact Us

By John Dryden