Strengthening of steels was felt needed (20s-30s). Pearlite was seen to exert large effect on properties when

present in small amount. Advantage of grain refinement was demonstrated by

Arrowsmith in 1924. Its influence on the properties of steel was conclusively

shown in 1935. Effect of carbon or pearlite, on strength, ductility and

notch toughness was recorded in 1937. Pearlite continued to be the main for strengthening agent

of rolled steel till 1960s.

Solid solution hardening of ferrite. Ferrite grain size. Amount of Pearlite. Interlamellar spacing of the Pearlite. Age hardening effect in the ferrite.

The original Austenite grain size which, in general, controls the number of ferrite nuclei.

The transformation temperature, itself controlled by alloy composition and the rate of cooling.

Second phase particles deliberately added either to restrain Austenite grain growth or nucleate ferrite.

Robertson discovered bainite (an aggregate of ferrite and carbide) in 1929.

Davenport and Bain introduced Bainite in isothermal transformation of steel in 1930.

Utility of TTT diagram was established. Upper and lower bainite was introduced by Mehl in

1939. During 1930s understanding of hardening of steel by

quenching was gained. Heat treatment principles were evolved (30-40).

In 1940 two types of martensites (now called lath and twinned) were observed in Fe-Ni alloys.

Averbach, Cohen and D.P.Antia defined three stage of tempering:

1. Precipitation of ε-carbide and low carbon martensite(upto2009C).

2. Decomposition of retained austenite(200-3009C).3. Cementite forms and grows(above 3009C). Negative influence of Pearlite on weldability was

established. Multiple welding started after world II.

HAZ properties were poor in ferrite Pearlite steels. In sheet steel Pearlite was again found to be

detrimental in respect of ductility and formability. In late 40s fourth stage of tempering and secondary

hardening was demonstrated. In Cr steel, Cr7C3 gradually replaces Fe3C at

4509C/5009C(Cohen). Lamont showed that Fe3C changes to Cr7C3 through a

transitional structure. Roch demonstrated that Fe3C dissolves and Cr7C3

forms. Graphitisation in steel was noted.

If hyper eutectoid steels with Cr and containing Al, graphitisation is easier.

Microstructures of steels were described by ferrite, bainite and martensite.

After World War II prevention of brittle fracture and enhancement notch toughness was felt

Barr and Tipper demonstrated the effect of increase in ferrite grain size in increasing ductile to brittle transition temperature in 1947.

Upto 60s the situation in hot rolled steel did not improve.

Pearlite was found to have adverse effect on resistance to brittle fracture and negative influence on FATT even if the grain refinement was significant.

1950-609% Cr steel Does not show secondary hardening, Cr7C3 forms by

separate nucleation. Vanadium remains in solution and gives finer

dispersion and secondary hardening. Sudden drop in hardness is explained by loss in

coherency. In V steel V4C3 give secondary hardening.

Importance of crystal structure of carbides were shown.

V added Cr steel indicates that V goes insolution in Cr7C3 and no V4C4 forms.

Effect of addition of Ta and Nb1. Delays peak.2. Rises peak hardness.3. Delays Softening.

Carbides are more finally dispersed in minor added steels.

Expansion of lattice of V4C3(0.25-0.5%) to mean that minor elements have gone in solution in V4C3 separate carbides were also notced.

Mo steels Mo2C precipitates and give similar results, acicular

Mo2C and massive Mo6C also Ta rich carbide forms. Mo2C/V4C3 initially coherent. Ta/Nb slows down diffusion, retard growth(going in

solution).

Separate dispersion of Ta/Nb carbide reduces overageing.

In low temperature tempering of 9Cr,4V steel and ε carbides donot form if C is less than 0.25% carbon.

Imperfect Structure of cementite established at 1709C. Hence imperfect to perfect Fe3C is a gradual process taking place by diffusion.

Pearlite free /pearite reduced steel was intended for improving weldabilty/toughness by reducing C and increasing Nb/V.

C-Mn-Nb fine grained steel commenced on 1958. In 1959 Beiser demonstrated that Nb increases

strength. in 1960 as hot rolled Nb steel was developed and

effect of Nb was demonstrated. Controlled rolling opened up since 1960 BISRA

research.

Nucleation of γ takes place immediately at pearlite colony boundaries and growth is controlled by carbon diffusion. It is also nucleated at α/ Fe3C boundary with in pearlite. Dissolution of Fe3C then takes place. In alloyed pearlite (α+M3C) the growth of γ takes place in lamellar form as carbide dissolution lags behind transformation. The habit plane is fixed carbide lamellae still persisting.

Compettitive austenite Peraequilibrium austenite.

Austenite nucleates in the Fe-17Cr-0.5C at ferrite grain boundary with a specific orientation relationship (Kardjumov-Sach) with atleast one adjacent ferrite grain. Close packed planes are parallel {111}γ//{110}α. So a single variant austenite at grain boundary. But growth takes place both by semicoherent and incoherent boundary migration and is controlled by diffusion. No orientation is needed. Δ ferrite how ever does not show such habit.

1. Grain refinement.2. Precipitation hardening.3. Solid solution hardening.4. Transformation hardening.During 1970s three varieties of steels weredeveloped1. Acicular ferrite.2. Dual phase(α+M).3. Ferrite+pearlite.

1. Quantitative relation between microstructure and property.

2. Innovation in hot rolling technology.3. Optimisation strength/ toughness balance.4. Improvement in strength/formability relation.

By quantitative metallography and regression analysis mathematical relations are developed.

Ti/Nb/V have their individual advantages and disadvantages of additions.

Directionality is removed by inclusion shape control.

Oil Embargo in 1970s demanded high strength/weight ratio in automobiles.

α+P replaced by α+M. α+M contains ≈15V%M,results in YS decrease, UTS

increase El% stretch formability increase. This is due to:

1. Nature and behaviour of martensite, its ductility and cohesiveness with ferrite.

2. Low carbon in ferrite.3. Transformation of retained γ.

Pearlite was detrimental, but martensite is found to be useful.

Accelerated cooling can give low temperature transformation product, improving properties.

Flow stress during rolling below recrystallisation is due to combined effect of grain refinement solid solution strengthening, retained work hardening and strain induced precipitation.

Structure refinement. Improvement in strength/toughness.Process:I. Stage- 10009CII.Stage- 1000-9009CIII.Stage- warm worked γ in non crystalline region.Two types of γα transformation At γ grain boundary above recrystallisation

temperature Fine grain γ >ASTM6=> ferrite+pearlite if

normalised

Deformation in non recrystallised γ α nucleates at grain interiors itragranularly.Deformation in γ+α(α substructure)Controlled rolling parameters1. Selection of composition.2. Slag reheating temperature.3. γ recrystallisation.4. Deformation in nonrecrystallisation.5. Deformation in γ-α two phase region.6. Controlled cooling.

Retardation of recrystallisation1. Strain induced precipitation of Nb(C,N).2. Solute Nb In controlled rolling strain energy stored during deformation acceleratesγ to α transformation i.e. a Strain induced transformation.

Restoration have two types:1. Static.2. Dynamic.Three static restoration processes:a) Recovery.b) Recrystallisation.c) Metadynamic recrystallisation.d -1 (dynamic recrystallisation) α log z

Z=εexp(Q diff/RT)

Dynamic recrystallisation is not achievable in Nb steel to refine γ. Also structure obtained through dynamic restoration is thermodynamically unstable. Hence staticrecrystallisation needed.

Nucleation at grain boundary corners. Migration of recrystallisation front into deformed

matrix. It does not occur at all conditions.Conditions: Resistance for completion of recrystallisation is small

in plain carbon steel but large in Nb steel due to precipitation.

Reduction in recrystallisation region- fine uniform grains of γ.

Within partial recrytallisation region mixed structure recrystallised, recovered γ-grains.

Low temperature pearlite(80s) Steel 1.50% C, 0.4%Mn, 0.3%Si after isothermal

treatment at various temperature shows pearlite and bainite.

C curve with a single maximum is recognised to be complicated by the superposition of two three maxima

A higher amount of Cu may be retained in solid solution if the finish rolling temperature is kept high. in that case the ageing response of the experimental steels is improved.

It is further concluded that the addition of boron in Cu bearing HSLA steels delays the precipitation of Cu during post TMCP ageing.

The slower air cooling produces considerable precipitation of Cu and segrigation of boron at the grain boundaries.

The TMCP alloys exhibit a higher amount of αq and higher fraction of αB in their microstructure.

In aged condition, the precipitates of Cu are formed. Less dislocated massive ferrite contains less nucleation sites for Cu precipitation.

It is concluded that the precipitation of Cu in the present series of steel is mainly controlled by the diffusion of Cu in iron.

Boron lowers the activation energy for precipitation reaction and envisages the heterogeneous nucleation of Cu in such steels.

There have been several attempts to predict mechanical properties of a steel. A few examples: Properties of microalloyed steel (Dumortier et al

Mat.Sci. Forum,1998) Strength variation in thin steel sheets (Myllykoski,

J.Mater.Process.Technology,1998) Structure- property relationship of C-Mn steel (Liu

Etal., j.Mater.Process.Technology,1996) Tensile properties of mechanically alloyed

iron(Bhadeshia et. al., Mat.Sci.Tech.1998)

Hot strength of steels (kong et. Al., ISIJ Int.,1999)

Estimation of hot torsion stress strain curve in iron alloys (Bhadeshia et. al.,ISIJ Int.,1999)

A relationship has to be chosen before analysis. The relationship chosen tends to be linear, or with

nonlinear terms added together to form a pseudo-linear equation.

The regression equation, once derived, applies across the entire span of the input space.

A non linear regression technique. Layers input hidden and output. Use of tanh as transfer function. Learning from training. Prdiction of unknown output.

Martensite - start temperatureVermeulen et. al. (Ironmaking steelmaking, 1996) demonstrated that neural network model can estimate Ms much more effectively than numerous regression equation used for many decays. CCT diagramContinuous cooling transformation of austenite has been modelled with ANN method using, chemical composition, austenitisation temp. and cooling rate as

inputs by Vermeulen et.al.(steel recearch,1997)

Later the process was further improved by P.J.Vander worlk et.al.(Intelligent processing of high performance materials,Res. And Tech.org., France, 1998)and J.Wang et.al.ISIJ(ISIL Int.,1999).

Austenite formation: Bhadeshia et.al. created a ANN model in which Ac1

and Ac3 temperatures of steels are estimated as a function of chemical composition and the heating rate(Material science and technology, 19996)

DSIT is essentially a process of spontaneous dislocation induced discontinous decomposition of metastable austenite under deformation to nano sized carbides and ultra fine ferrite.

Hudgson et. al.,(1999) postulated three critical factors for the formation of UFF grains during SIT:

1. A high shear strain.2. A heavy under cooling.3. An appropriate deformation temperature.

In DSIT austenite to ferrite transformation is given to take place during the coarse of deformation.

The idea of DSIT centers on the creation of high density of intragranular(IG) nucleation sites within highly strained metastable austenite so as to produce UFF grains in the final microstructure.

Various process have been carried out on various alloys of low C steels to attain UFF grains by heating samples to austenitisation temperature and deform them by various methods at various cooling temperatures.

DIFT is a nucleation controlling process i.e. the transformation is mainly accomplished through the continuous nucleation process, whereas TMCP is grain growth and coarsening dominant process.

The growth of ferrite grains is inhibited to a great extent during DIFT due to the rapid and repeated nucleation of ferrite grains at γ/α interface, which results in a final finer grain size than that obtained by TMCP.

The grain number increase during DIFT, but it decrease during TMCP.

In recent years several studies on grain refinement of ferrite have been conducted by various methods like:

1. Equal Channel Angular Pressing(ECAP).2. High pressure torsion(HPT).3. Accumulative Roll Bonding(ARB). In order to optimize the relationship between the

mechanical properties and microstructure of steels ECAP has demonstrated the capability to produce a great refinement of the steel microstructure.

The structural and phase transformations which take place in low carbon steels during deformation by high pressure torsion(HPT) and subsequent heating gives refined grain structure has been studied using transmission electron microscopy and X-ray structural analysis methods.

Brittle-ductile transition(BDT) behaviour was investigated in low carbon steels deformed by Accumulative Roll Bonding(ARB) process.