This publication explains how steelmakers can use small levels of niobium, up to 0.10%, in linepipe steels providing pipe manufacturers with a product that meets the desired quality at a competitive cost.

It is important for steel producers to work together with the pipe manufacturers to agree the properties of the steel used to produce the pipes. This ensures that the properties of the pipes can then meet the specific pipeline project requirements.

In essence, the pipe manufacturers require steelmakers to provide:

• Competitive steel prices;

• Delivery time to meet the specified schedule and volume;

• Reliable products that consistently meet the required quality standards, ensuring a smooth and efficient pipe manufacturing process.

The key to achieving comptetitive costs, production capacity and product reliability is through robust processes, using an appropriate alloy design:

Niobium increases the T_nr (non-recrystallization temperature), hence retarding recrystallization. This allows hot rolling at higher temperatures without the risk of recrystallization and growth of the deformed grains. The result is a fine, homogenous austenitic grain size and an austenite with more strain accumulation at the end of the hot rolling process. This finer and deformed austenite transforms, to a desired acicular ferrite microstructure compatible with API requirements. The plates or coils produced by this process, with this refined and homogeneous uniform microstructure, will guarantee that the best possible product will be supplied to pipe manufacturers. It enables them to produce their products to meet the required standards, without any kind of defect that results in a downgrade or material cut away.

Not only does adding up to 0.10% niobium improve the steel properties, it also brings cost savings and increased productivity to the rolling mill. The figure below summarises how this alloying approach brings advantages through enabling rolling at a higher temperature.

The increase of the Tnr temperature, enabling the material to be rolled at temperatures up to 100°C higher than conventional TMCP processing, results in the following advantages:

• Roughing can be done with less passes;

• Crop shears are exposed to a higher temperature steel that has higher plasticity, resulting in better cutting and less wear of the shear components;

• There is less wear on the rolling mill equipment and the electric motors use less energy due to the reduced rolling forces at the higher temperatures;

• The final thickness of the steel is defined by rolling at temperatures around 60°C to 80°C above the temperature at which austenite transforms into ferrite. This makes it easier to select and apply the appropriate cooling rates to the material after it leaves the final finishing stand of the rolling mill. This allows transformation directly into acicular ferrite and bainite, avoiding the strained pancake austenite transforming into ferrite plus perlite, which gives the material the best guarantee of excellent toughness

This approach has been successfully applied by steelmakers, transferring considerable benefits to pipe manufacturers. Steels with up to 0.10% niobium have been used extensively in large scale pipeline projects around the world, including the Second and the Third West-East Pipeline in China, the best examples of this concept being used successfully.