Carlos García de Andrés

Acicular ferrite formation in a medium carbon steel with a two stage continuous cooling

I. Madariaga*, I. Gutie ́rrez*, C. Garcı ́a-de Andre ́s** and C. Capdevila** * Centro de Estudios e Investigaciones Te ́cnicas de Guipu ́zcoa (CEIT) P Manuel Lardizabal, 15, 20009 San Sebastia ́n, Basque Country, Spain ** Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalu ́rgicas (CENIM), Consejo Superior de Investigaciones Cientı ́ficas (CSIC), Avda. Gregorio del Amo, 8. 28040 Madrid, Spain

Industrialised nanocrystalline bainitic steels. Design approach

Abstract Nanostructured microstructures consisting of a mixture of very thin plates of bainitic ferrite separated by C-enriched austenite are the main characteristics of the so called NANOBAIN steel family. This paper shows the theoretical approach followed in the design of a new, industrially viable, generation of NANOBAIN steels, a process in which industrial demands such as simpler chemical compositions, faster transformation kinetics and sufficient hardenability have also been considered. The microstructural and mechanical characterization of the bainitic microstructures obtained by isothermal transformation at different temperatures, come to confirm and to validate the theoretical approach used.

Influence of Second Phase Particles on Recrystallisation of Cold-Rolled Low Carbon Microalloyed Steels during Isothermal Annealing

The recrystallisation behaviour of two cold-rolled low carbon microalloyed steels with different Ti content was investigated by thermoelectric power (TEP) measurements and metallography. It was shown that recrystallisation was more sluggish in the higher Ti grade steel as compare with traditional ELC and LC steels. The existence of Ti in solid solution together with the pinning effect of Nb-, V- and Ti-rich particles are likely to be responsible for such behaviour. Metallographic etching with picral revealed that cementite act as preferential nucleation site for recrystallisation.

Evaluation of the Austenitic Grain Growth by Thermoelectric Power Measurements

The influence of the deformation grade on the recrystallised grain size has been studied in the AISI 304 stainless steel. Therefore, cold rolled samples of this material with reductions varying between 30% and 80% were annealed at different temperatures and subsequently quenched. The mean austenitic grain sizes were measured and compared. Moreover, the correlation between the variation of the thermoelectric power and the grain growth was investigated for each degree of prior deformation.

Discussion on the Rate Controlling Process of Coarsening of Niobium Carbonitrides in a Niobium Microalloyed Steel

Austenite grain growth in microalloyed steels is governed by the coarsening of fine precipitates present at grain boundaries below the grain coarsening temperature. Zener model is widely used in metals to describe the pinning effect of second phase particles precipitated in the matrix. In this work it has been discussed whether grain boundary or volume diffusion is the rate controlling process for the coarsening of the niobium carbonitrides. Calculations on austenite grain growth kinetics, obtained coupling Zener theory and both rate controlling processes of precipitate coarsening, have been compared against experimental austenite grain size results under nonisothermal heating conditions. In this sense, it has been concluded that the coarsening of niobium carbonitrides is mainly controlled by volume diffusion of Nb in austenite.

Modelling the Influence of Cementite on Static Recrystallisation in Cold-Rolled Low-Carbon Steels

In this work the prominent influence of Particle Stimulated Nucleation (PSN) on the overall process of nucleation and subsequent grain growth is discussed and a global recrystallisation kinetics model is proposed. This model accounts for the effect of the most relevant industrial parameters, such as cold reduction and annealing temperature. Moreover, not only the role of the cementite content, which is function of the nominal carbon content, is included in the model, but also the morphology of the second phase particles. Experimental evidence is given to ascertain the accuracy of the theoretical predictions.

Proposition of Two Parameters for a Good Characterisation of the Austenitising Condition of Microalloyed Steels

The characterisation of the compositional condition (particle dissolution/precipitation level),and the microstructural condition (austenite grain size and deformation state) are of vital importance for the subsequent phase transformations which occur during continuous cooling after thermomechanical processes of microalloyed steels. Thermoelectric power (TEP) measurement has been revealed as powerful method to analyse dissolution/precipitation processes together with recovery and recrystallisation phenomena. In this sense, this paper deals with characterising the austenitising condition by two fundamental parameters such as the prior austenite grain size (PAGS) and TEP. Likewise, to validate this hypothesis, the austenitising condition after a forging process in a medium carbon microalloyed steel has been simulated by a simple heat treatment which reproduce the same PAGS and TEP values. A comparison between the continuous cooling transformations occurred from both conditions, allow us to confirm that these parameters well characterise the autenite state of a microalloyed steel.

Effect of the Microalloying Elements on Nucleation and Growth Kinetics of Allotriomorphic Ferrite in Medium Carbon-Manganese Steels

The influence of alloying elements on nucleation and growth kinetics of allotriomorphic ferrite has been studied using dilatometric techniques and microstructural analysis in four medium carbon-manganese steels (0.3%C - 1.4% Mn). A careful comparison between a C-Mn steel and microalloyed steels containing V, Ti and Mo subjected to isothermal transformation confirmed that all these elements delay the allotriomorphic ferrite transformation. More significant effect has been found by adding Mo, compared with the effect of the V and Ti.

Influence of Plastic Deformation on Recrystallized Microstructure of Fe-Base ODS Alloy

The ferritic oxide dispersion strengthened alloys (ODS) are manufactured using the mechanical alloying process. The development of a coarse grained microstructure during the recrystallization has been noted and discussed by a number of authors but, the mechanism of grain control remains uncertain. Recent work has emphasized the large influence of non-uniformities on the development of the recrystallized microstructure. The purpose of the present work was to study the effect of non-uniform plastic strain on recrystallization of Fe-base ODS alloy named MA 957.