中国微合金化技术与含铌钢发展30周-of Evolution(8)

中国微合金化技术与含铌钢发展30周-of Evolution

High Strength Microalloyed Linepipe: Half a Century of Evolution 27

During evolution of linepipe steels over the period depicted in Figure 5, several essential events, chronicled below, can be credited with changing of specifications or with stimulating metallurgical developments in both linepipe and plate steel.

These factors are presented in Table 3 below:

中国微合金化技术与含铌钢发展30周-of Evolution

28

中国含铌钢技术发展30周年国际研讨会论文集

Table 3: Technology development incentives 1943 - Present

The trend toward higher strength levels (up to 120 ksi yield strength and above) has been led by developments in DSAW linepipe where the benefits translate into lighter wall thicknesses for high pressure, long distance onshore pipelines. Similar metallurgical approaches are used for HFERW linepipe, which is made from hot-rolled coiled skelp, but yield strength levels have not yet exceeded 80 ksi. In offshore pipelines the useful yield strength is limited to 65-70 ksi due to the need for negative buoyancy and to guard against plastic collapse. In the most extreme cases wall thicknesses in the range 1.125-1.875 inch are being installed in water depths >8000 ft. as detailed later. In the case of seamless linepipe, quenching and tempering is used to achieve the desired yield strength level which is now approaching 100 ksi in offshore riser applications. Examples of the chemical compositions and microalloying element contents used for each grade, product type, and application will be presented later.

It should be noted that the early lower strength steels presented in Figure 5 had very poor toughness from both a fracture energy absorption and impact transition temperature viewpoint. The low absorbed energies in the Charpy test were related to high carbon and sulfur contents and poor steel cleanness, especially in the case of semi-killed steels, whilst the high Charpy and Battelle Drop Weight Tear Test transition temperatures were caused by the coarse ferrite grain sizes and high carbon contents. As a consequence, the early high strength steels often relied on normalizing to achieve adequate grain refinement. In the mid to late 1960’s the benefits of low finish rolling temperatures were discovered and thermomechanical processing was quickly introduced on a broad scale. Simultaneously, lower carbon contents were