Year

1991

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

A series of experiments using heat-treatment, carbon replication extraction, single-pass rolling in the stable γ region and in the (γ+ α) two-phase region have been carried out to investigate the effect of Ti and Ti-Nb additions on the austenite grain coarsening behaviour, the alloy carbonitride particle size distribution, γ-> α transformation and the restoration characteristics of deformed y and α in a series of commercial slab cast 0.13%C-1.45%Mn base steels. The microalloyed steels consisted of 0.016%Ti, 0.076%Ti, 0.025Nb and 0.019%Ti plus 0.024%Nb. The Ti: N ratios in the three Ti-bearing steels were ~2.66, 10.1 and 2.1, respectively.

Experimental results showed that the effective austenite grain boundary pinning particles are TiN and TiNbN, and a Ti: N ratio near to the stoichiometric ratio (3.42) significantly increases the austenite grain coarsening temperature (GCT), due to the substantially complete combination of Ti (or Ti and Nb ) with N to form fine dispersed TiN (or TiNbN) particles. A high Ti level with Ti: N ratio in excess of the stoichiometric ratio resulted in a lower GCT due to the formation of coarse TiN and Ti4C2 S2 particles. Although Nb alone does not strongly influence the GCT due to the low solution temperature of Nb(CN). Ti-Nb additions resulted in a higher GCT , which was still lower than that for a single Ti addition, because Nb decreases the stability of the Ti-rich nitride particles. A s expected, the G C T was a function of holding time at remperature and decreased with increasing holding time.

Solution treatment and hot rolling after slab casting resulted in a decrease of volume fraction of fine nitride particles, and a increase of mean particle size due to the solution of fine particles, thereby lowering GCT . Re-precipitation of fine particles during rolling and air cooling only partly compensated for the loss of fine particles during reheating.

Ti and Ti-Nb additions retarded recrystallization of deformed γ after rolling in the stable γ region ( > Ar3 ), particularly at temperatures1100°C), the effect of Ti and Ti-Nb additions on retarding γ recrystallization was not significant, due mainly to the solution of grain boundary pinning particles of TiN and NbTi(CN). In the temperature range ~950-1100°C, the retarding effect of Ti and Ti -Nb on γ recrystallization was remarkable, due to the re-precipitation of TiN or NbTi(CN) particles during or after rolling. When rolling at low temperatures (<~950), Ti and Ti-Nb additions substantially retarded γ recrystallization, by retarding the incubation time by 1-2 orders of magnitude and reducing the recrystallization rate, due to re-precipitated TiN in the Ti steel and re-precipitated and strain-induced NbTi(CN) particles in the Ti-Nb steel.

For each steel there was a critical temperature for γ recrystallization (Tc), below which recrystallization of deformed γ did not occur within holding times < 1800 sec, and the deformed γ grains retained a "pancaked" shape. Ti and Ti-Nb additions raised Tc to ~850°C and 900°C, respectively, whereas Tc for the C-Mn steel was ~800°C.

During and/or after rolling in the stable γ region, γ grains can be refined by recrystallization. Further refinement can be achieved by decreasing the reheating temperature before rolling, lowering the rolling temperature or increasing the rolling reduction. However, there is a limiting extent of γ grain refinement possible by recrystallization.

When rolling was performed at temperatures near the critical temperature (Tc), deformation bands and deformed twin boundaries were produced in the γ grains. During the subsequent cooling, these boundaries acted as nucleation sites for a grains and contributed significantly to refinement.

For the same reheating and rolling conditions in the stable γ region, Ti and Ti- Nb additions to the base steel led to finer initial γ grains after reheating; retardation of the recrystallization of deformed γ; and suppression of the growth of recrystallized γ grains. These effects resulted in fine recrystallized γ grains, or high volume fractions of unrecrystallized γ, which transformed into fine α grains. These effects are due to undissolved and re-precipitated TiN particles in the Ti steel; and to solute Ti and Nb atoms, and re-precipitated and strain induced NbTi(CN) particles in the Ti-Nb steel.

The effect of Ti alone on retarding γ recrystallization was weaker than the effect of Ti-Nb additions in the two low Ti steels, due to the smaller amount of Ti than Ti plus N b , the smaller volume fraction of TiN compared with NbTi(CN), and the finer initial γ grain size in the Ti steel compared with the Ti-Nb steel. However, Ti can exert a stronger effect than Ti-Nb additions on suppressing γ grain growth due to the higher stability of TiN than NbTi(CN).

Rolling in the (γ + α) two-phase region accelerated the γ-> α transformation, and the presence of Ti or Ti plus N b enhanced this accelerative effect.

In undeformed and lightly deformed samples, α grains nucleated mainly at γ grain boundaries during and/or after rolling. In samples rolled with reductions higher than the critical value, α grains nucleated at γ grain boundaries as well as interiors, mainly at the boundaries of deformation bands and deformed recrystallization twins. Rolling in the (γ + α ) region is a 'hot-deformation' of the ferrite, and restoration of deformed α occurred during and/or after rolling, by static and/or dynamic recovery, and static recrystallization. Deformed α grains changed into cell and/or subgrains or equiaxed grains, depending on the degree of restoration, which in turn depended on rolling reduction, isothermal holding time and the effect of the Ti and Ti-Nb additions.

Recovery and recrystallization of deformed α proceeded rapidly in the C-Mn steel, but was sluggish in the Ti and Ti-Nb steels. The incubation time for recrystallization of deformed α was retarded by 1-2 orders of magnitude in the Ti and Ti-Nb steels compared with the base steel, resulting in higher volume fractions of cell and/or subgrain structures in the deformed α, and correspondingly higher a hardness in the Ti and Ti-Nb steels than in C - M n steel, for similar rolling and holding conditions.

In the 0.016%Ti steel, retardation of recovery and recrystallization of deformed α is due to the pinning effect of undissolved and re-precipitated TiN particles on dislocations, α grain and subgrain boundaries. In the Ti-Nb steel retardation of recovery and recrystallization of deformed α is due to solute-drag by N b and Ti atoms in solution, and the pinning effect of re-precipitated and/or straininduced NbTi(CN) particles on dislocations and α grain and subgrain boundaries.

Dual Ti and N b additions exerted a stronger retarding effect on restoration of deformed α than Ti alone, because of the higher volume fraction of NbTi(CN) in the Ti-Nb steel than TiN in the Ti steel. However, the effect of Ti on stabilizing straininduced substructures in deformed γ and accelerating nucleation of new α in deformed γ was stronger than that of Ti-Nb additions, especially in samples rolled with medium reductions (15-30%). This observation is a result of the higher stability ofTiN than NbTi(CN).

Ti and Ti-Nb additions produced finer initial γ and α grains after reheating before rolling, and stabilized the strain induced substructures during and after rolling. These substructures resulted in more nucleation sites for new α grains. Ti and Ti-Nb additions also retarded recovery and recrystallization processes in deformed α, resulting in higher volume fractions of cells and α subgrains. In addition, Ti and Ti-Nb suppressed the growth of recrystallized α grains. All of these effects led to higher hardness (and strength) in the microalloyed steels than in the CM n steel after rolling in the (γ + α) two-phase region.

Rolling in the (γ + α) two-phase region is effectively 'cold-working' of austenite and during and after rolling, recrystallization did not occur in the deformed γ. Restoration of deformed γ proceeded, in effect, by the strain-induced γ -> α transformation.

Therefore, the microalloying additions of Nb and particularly Ti to a commercial 0.13%C-1.4%Mn base steel had a marked grain refining effect on both γ and α which is a result of grain growth inhibition and retardation of restoration of both γ and α, during the hot rolling process.

The present work has clarified the role of micro-additions of Ti and Ti-Nb on the grain coarsening and hot rolling characteristics of a C-Mn base steel and makes original contributions in terms of both the linking of particle size distribution data to the grain coarsening behaviour during reheating, and the analysis of the effect of microalloy additions on the structure and properties developed during two phase rolling.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.