Comportement au revenu d'un acier du type Z80CDSV8.2.1.1 pour cylindres de laminoirs à chaud

Tempering behaviour of a Z80CDSV.2.1.1 steel for hot rollers.

Laboratoire de Structure des Matériaux Métalliques - Université Paris Sud - Centre d'Orsay

Abstract

Transformations in Cr-Mo-Si steels with a specific addition of Vanadium have been characterized precisely as well as their effect on the hardness of the materials. A preliminary treatment in the austenitic field leads to a limited enrichment of the matrix as well as temperature is less than 1050°C : only M₃C and M₂₃C₆ carbides may then be dissolved. On the contrary, the dissolution of M₇C₃ and MC carbides is observed over 1100°C but is only partial even after treatment at 1150°C. After quenching from three distinct temperatures (1050-1100 and 1150°C), the microstructure consists of martensite, residual austenite and primary carbides (mainly M₇C₃ and MC type) ; different tempering treatments have been performed (isochronal, isothermal or on continuous heating) using various physical methods (dilatometry, DTA and magnetic measurements, TEM observations and hot hardness testing). According to the maximum θR temperature reached, the following structural evolutions were observed :

• •

  ε carbide precipitates at θR < 250° C in the a’ matrix, then M₃C carbide forms between 250 and 350°C which transforms in M₇C₃ carbide from 450 to 500°C. At the same time (300- 500°C) a secondary hardening occurs linked to the formation of very fine V₄C₃ carbides : a progressive increase in hot hardness is observed while holding isothermally at 300-450°C.

• •

  From 450 to 600°C, the residual austenite is destabilized owing to the precipitation of small carbides in α'/γ interfaces and may be transformed on cooling either in bainite or secondary martensite. On the other hand that impoverished austenite may be transformed in (α + carbides) between 600-700°C. Besides in the same temperature range, M₆C and M₂₃C₆ may be formed at the expense of fine M₇C₃ carbides previously formed.