IFMASS10

295

ferrite and related microstructures, due to poor toughness of bainitic/martensitic micro-

structures /15, 16/. Since bainites and acicular ferrite nucleates in the same temperature

interval, the main difference between them is related to nucleation places. While bainites

nucleate on austenitic grain boundaries, acicular ferrite nucleates on the particles within

the grains. Therefore, it is suggested that particles are necessary for nucleation of acicular

ferrite. The extent of nucleation depends, among others, on composition, crystal structure,

number, size and interparticle distance. Second condition is grain size on annealing tem-

perature, i.e. grain should have some optimal size, rather larger than smaller, because

larger grains will decrease temperature of austenite decomposition, to the temperature

range in which acicular ferrite is a dominant structure. Hardenability has similar role.

Second phase particles are usually oxides and nitrides or sulphides; MnS particles served

as preferential places for precipitation of VN, which has great potential for nucleation of

intragranular proeutectoid ferrite (intragranularly nucleated ferrite – INI), which in turn

serves as nucleation site for acicular ferrite /15, 24/. Therefore, even considerably high

content of sulphur (130 ppm) did not damaged toughness, i.e. nucleation of acicular

ferrite would be not possible without MnS particles. This conclusion is very important for

further practical design of materials /25/.

Service life with present crack, introduced in fracture mechanics has been adopted in

materials science also, i.e. in spite of the presence of particles in steels, their control allow

the application of materials.. On the other hand, the requirement for clean steel is

necessary, like in use of seamless tubes /26/. The benefit of carbides in wear resistant and

tool steels/27/, and influence of impurities on deformation and fracture behaviour in

aluminium 7000 series alloys /28/, although important are not discussed in this

presentation.

5. CONCLUSIONS

Development of material that can meet all requirements together with reasonably low

costs is the eternal question and challenge.

Based on knowledge of fracture mechanics, consideration of particles in steel is not

only in direction of lower content of impurities, but more to CONTROL of shape and

distribution of second phase particles. It is much easier and reasonably cheaper to control

the shape of inclusions, than to produce super clean steel. If the shape of second phase is

modified into sphere, than low stress concentration will occur. Also, by its influence on

grain refinement and transformation behaviour, the presence of particles turned to be

beneficial and unavoidable.

REFERENCES

R.W.Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, J.Wiley and

Sons, New York (1996)

J.F.Knott, Fundamentals of Fracture Mechanics, Butterworths, London (1973)

Dj.Drobnjak, Lecture Notes, Faculty of Technology and Metallurgy, Belgrade (1996)

Kiessling, R., Nordberg, H.,

Influence of Inclusions on Mechanical Properties of Steel,

Production and Application of Clean Steels, Iron and Steel Institute, London (1972) p.179

Brooksbank.D, Andrews, K.W,

Stress Field Around Inclusions and Their Relation to

Mechanical Properties

, Production and Application of Clean Steels, Iron and Stel INstitute,

London (1972) p.186.