Investigation on the behaviour of medium carbon and vanadium microalloyed steels by hot forging test

Effect of Cooling Rate and Vanadium Content on the Microstructure and Hardness of Medium Carbon Forging Steel

Laboratory experiments of a high-strength Ti-Nb microalloyed steel were conducted with two cooling modes, i.e., a large (35 °C s−1) cooling rate in the initial stage followed by slow (8.5 °C s−1) cooling rate (termed as FFC) and a slow (8.5 °C s−1) cooling rate in the initial stage followed by large (35 °C s−1) cooling rate (LFC) during cooling process. Based on the results of laboratory experiments, an industrial trial was performed with similar steel on a continuous hot strip production mill. The grain size in LFC sample (2.83 μm) is smaller than that in FFC sample (3.80 μm), and the volume fraction of precipitates in LFC sample (1.04%) is more than that in FFC sample (0.81%). Both results of laboratory experiments and industrial tests confirm that the strengthening effect of the LFC mode is much better than that of the FFC mode from the viewpoints of both fine-grain strengthening and precipitation strengthening. The present study provides a new approach to improve the property of microalloyed steels produced by continuous hot rolling technology.

The Effects of Cooling Mode on Precipitation and Mechanical Properties of a Ti-Nb Microalloyed Steel

The effect of different microstructures on the fatigue behaviour of a medium carbon vanadium microalloyed steel has been studied. Specimens were subjected to a controlled closed die forging followed by cooling in sand, air or oil, respectively. The hardness and fatigue properties of the microalloyed steel are determined and compared with those of ferrite-pearlite and martensite microstructures obtained by cooling with different mediums after forging. Relatively fine ferrite and pearlite increase the fatigue strength of the steel, while the martensite structure reduces the fatigue strength. Characteristics of fatigue fracture surface morphology are summarized and related to fatigue crack initiation and propagation mechanisms in the forged medium carbon microalloyed steel. The cooling rate has a remarkable effect on the microstructure, hardness, and fatigue behaviour at room temperature.

Microstructural Effects on Fatigue Behaviour of a Forged Medium Carbon Microalloyed Steel

La construction electrique reposant en grande partie sur l’utilisation des proprietes electromagnetiques du fer et de ses alliages, les concepteurs ont besoin d’une connaissance precise de ces proprietes. Les caracteristiques des materiaux magnetiques doux, typiquement celles de l’acier des roues polaires d'une machine a griffes, peuvent etre notablement impactees par les procedes de fabrication. Celles-ci doivent alors etre caracterisees directement sur la piece finale. Dans ce travail, on propose une methode de mesure non destructive combinant une demarche operatoire et la simulation par elements finis afin de caracteriser la conductivite electrique locale des roues polaires. Afin d’adapter les parametres experimentaux a la geometrie de la piece consideree, et a l’aide d’un diagramme 5M (ou diagramme d'Ishikawa), une demarche d’optimisation de la methode est entreprise. Puis l’incertitude de mesure est determinee a l’aide de deux demarches differentes. Cette methode est ensuite exploitee pour mener une etude sur une population de roues polaires extraites de la chaine de production. Dans un second temps, la methode est etendue pour la mesure de permeabilite incrementale. Des essais sur toles epaisses ont permis de valider l’approche proposee a l'aide du meme type de capteur. Par ailleurs, une etude complementaire par elements finis 3D a permis de mettre en evidence les limites de l'approche analytique, notamment lorsque la permeabilite de la plaque et son epaisseur deviennent trop importantes. Finalement, cette methode est appliquee au cas des roues polaire, dans le cadre d’une approche qualitative, en vue d’une application de type controle qualite.

Caractérisation des propriétés électromagnétiques des matériaux magnétiquement doux : application aux roues polaires de machine à griffes

Two low carbon steel grades were used in the present investigation. One of them was microalloyed with Ti, V, and Nb. Both steel grades were subjected to a controlled hot forging followed by either cooling in air or quenching water. The microstructures of all TMT conditions are dominated by ferrite phase with different morphologies and grain sizes according to both chemical composition and cooling rate. Polygonal ferrite is considered to be a dominated phase of air cooled microstructures for both steel grades that is responsible for decreasing the hardness, yield, and tensile strength with the attendant increase in ductility. Water quenching leads to a formation of relatively fine polygonal ferrite in low carbon steel or transformation into acicular ferrite in low carbon microalloyed steel. Relatively fine polygonal ferrite and acicular ferrite increase strength but decrease ductility. The cooling rate has a negligible effect on the impact toughness at room temperature.

S.A. El-Rady

Papers

Effect of cooling rate after controlled forging on properties of low carbon multi-microalloyed steels