High Strength Low Alloy Steel
Large diameter pipeline
High strength low alloy (HSLA) steels have been developed since the 1960s originally for large diameter oil- and gas pipelines. The requirement was high strength as compared to mild carbon steel, combined with improved toughness and good weldability.
HSLA steel typically contains 0.07 to 0.12% carbon, up to 2% manganese and small additions of niobium, vanadium and titanium in (usually max. 0.1%). in various combinations. The material is preferrably produced by a thermomechanical rolling process, which maximizes grain refinement as a basis for improved mechanical properties.
Molybdenum has played an important role in the initial development. The addition of 0.1-0.2% molybdenum produces a fine grain structure of acicular ferrite and substantially enhances the precipitation hardening effects achieved with the other alloying elements.
Consequently, an estimated 2 million tons of Mo containing HSLA steels for pipelines have been produced worldwide during the 1970s, within the following percent range of chemical compositions:
| C | Mn | Nb | V | Mo |
|---|---|---|---|---|
| 0.06 - 0.12 | 1.4 - 1.8 | 0.02 - 0.05 | 0 - 0.06 | 0.2 - 0.35 |
During the following years developments of the rolling and cooling techniques resulted in improvements of the as rolled microstructure to the extent, that API X70- (70ksi yield strength) requirements can largely be met without the addition of molybdenum. However, for oil and gas transmission pipelines through regions with extreme climate conditions substantial quantities of molybdenum continue to be used to meet the low temperature toughness requirements of the steel. Likewise, for applications where the wall thickness exceeds 20 mm the addition of molybdenum is common to obtain a uniform structure with the desired combination of strength and toughnes and good weldability properties.
Presently, there is a strong trend towards increasing the operating pressure of the future long distance gas pipelines. This will take the required steel properties to X80 and higher. Steel producers are making good progress to meet this challenge, and it is not unlikely, that molybdenum will see a comeback in HSLA steels in that the present base formula ( e.g. 0.08 C, Nb, Ti) will be upgraded again with 0.1 to 0.2% Mo.
