中国微合金化技术与含铌钢发展30周-of Evolution(2)

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 21

raw productivity, plus all subsequent processing steps are metallurgically integrated since they have an influence on final mechanical properties of the pipe.

Emergence of HSLA (Microalloyed) Steels

The effect of vanadium in increasing strength of normalized steels is reported in German literature, circa 1945, whereas the effect of niobium in increasing the strength of hot-rolled carbon-manganese steels is reported in the patent literature as early as 1938-1939 [2-3]. The exact strengthening mechanism was not known but it was surmised that the benefit was predominantly due to beneficial grain refining effects of niobium and vanadium carbides and nitrides.

Other reports at the time [4] indicated that the new steels had poor notch toughness due to the formation of cementite networks on ferrite grain boundaries, or the formation of Widmanstatten ferrite during normal air cooling [5]. Later it was discovered that hardening or strengthening by the precipitation of vanadium or niobium carbonitrides could also harm toughness [6].

In time it was discovered that these problems could be eliminated by increasing manganese content [7] and refining the austenite grain size during hot rolling. In 1967 the effect of niobium in retarding austenite recrystallization was discovered [8-9] which later became the foundation for the large scale introduction of controlled rolling and other thermomechanical processing methods of austenite [10]. Other microalloying elements, plus molybdenum and aluminum, retard austenite recrystallization to some degree but niobium has been found to be most effective Figure 1 [11], and termed indispensable by Kosazu, et al [12].

Figure 1: Retardation of recrystallization by microalloying elements

In the ensuing years (1968-1975) the strengthening mechanisms operating in microalloyed steels became better understood and quantified [13-14] and optimized compositional and