Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1997-06-25
2000-02-22
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148648, C21D 800
Patent
active
060275871
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the production and processing of steels to achieve ultrafine microstructures. For example, in a ferrite containing steel, ultrafine microstructures are considered to be those having a significant proportion of grains of a size less than 5 microns in a plain carbon steel, or less than 3 microns in a microalloyed steel.
2. Related Art
One of the principal aims of modem steel processing methods is to refine ferrite grain size. A small ferrite grain size is desirable as this results in a steel with improved strength and toughness.
In recent years, there have been several reports in the scientific literature of different techniques for producing low carbon microalloyed steels with ultra fine ferrite grains. One type of approach has relied upon the expectation that dynamic recrystallisation at temperatures only a little above the austenite to ferrite transformation temperature (Ar.sub.3) will produce a small grain size. Controlled rolling schedules have thus been devised, using laboratory simulation by torsion or compression testing, which exploit dynamic recrystallisation after strain accumulation.
In one case, Kaspar et al reported production of austenite grains down to 1 to 4 micron in a compression tested Nb-V microalloyed steel which transformed on cooling to ferrite with a mean grain size less than 5 micron ["Thermec 88" Proc.Int.Conf. on Physical Metallurgy of Thermomechanical Processing of Steels and Other Metals, I.S.I.J. 1988, 2, 713]. Samuel et al reported that torsion testing of niobium microalloyed steels produced austenite and ferrite grain sizes of 5 and 3.7 micron, respectively, in deformation schedules where strain accumulation from successive passes led to dynamic recrystallisation [I.S.I.J. Int., 1990, 30, 216].
U.S. Pat. No. 4,466,842 to Yada et al describes a hot-rolled ferritic steel composed of 70% or more of equiaxed ferrite grains having an ultra-fine grain size of 4 .mu.m or less. This steel is produced by hot working at approximately the Ar.sub.3 point and by one or more passes of hot working having a minimum required total reduction ratio of at least 75%. Due to hot working, dynamic transformation of austenite and/or dynamic recrystallisation of ferrite takes place.
For plain carbon steels, Matsumura and Yada [I.S.I.J. 1987, 27, 492 and "Thermec 88" I.S.I.J. 1988, 1, 200] disclosed hot working schedules using laboratory compression and rolling tests to produce ferrite grain sizes below 3 micron. By imposing large strains just above the Ar.sub.3, they induced transformation during the deformation (despite the increase in temperature from the heat of deformation) and then continued to work the ferrite sufficiently for it to recrystallise dynamically. Rapid quenching after the deformation, while preventing coarsening of the ferrite grains, led to some martensite formation. By imposing strains up to 4, microstructures with 70-80% ferrite as fine as 1 to 2 micron were produced. Reducing the amount of intercritical deformation tended to reduce the volume fraction of ferrite and to increase the mean grain size.
Other techniques to produce ultrafine grains have been more involved. Ameyama et al. ["Thermec 88", I.S.I.J. 1988, 2, 848] disclosed low temperature deformation and brief austenitising cycles, combined with the addition of 3% Mn and 1% Mo to enhance austenite nucleation on reheating, to produce austenite grain sizes down to 1 micron in diameter. Kurzydlowski et al. [Z. Metallkunde, 1989, 80, 469] also disclosed repeated cold deformation and anneal cycles, together with boron additions, to produce austenitic stainless steels with grain sizes down to 1 micron diameter. Although these methods are of considerable scientific interest, they are a relatively expensive means of producing ultrafine grains.
More recently, Beynon et al have reported [Materials Forum 1992, 16, 37] the production of ultrafine Nb microalloyed ferrite, with an average grain size of approximately 1 micron, using laboratory hot torsion tests. Th
REFERENCES:
patent: 4466842 (1984-08-01), Yada et al.
patent: 5200005 (1993-04-01), Najah-Zadeh et al.
Gibbs Russell Keith
Hickson Mark Richard
Hodgson Peter Damian
The Broken Hill Proprietary Company Limited
Yee Deborah
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