Micro-alloyed steels

Question 1

Characteristics

Answer 1

> ~0.1wt% carbon, 1-2wt% Mn, small amount of alloys, low cost
ferrite matrix with fine carbides and high dislocation density, very fine ferrite grains
high strength from fine ferrite grains, solid solution hardening

Question 2

Properties

Answer 2

> excellent weldability and toughness, low DBTT< low cost

> typical comp: C: 0.12 wt%, Mn: 1.35 wt%, Nb: 0.02 wt%, V: 0.04 wt%

Question 3

Rolling sequence

Answer 3

1. steel is austenitized to form small, uniform and equiaxed austenite grains
2. high temp roughing passes done to refine grain size by sequential recrystallization
3. hold steel to cool it to lower temp for static recrystallization
4. low temp finishing passes to get high dislocation density
5. controlled coolign for fine ferrite grains

Question 4

Role of micro-alloying elements

Answer 4

> carbides/nitrides of Nb, V, Ti, etc. almost completely dissolve at highest temp during cold rolling
as temp ^, solubility of carbides/nitrides v, gives precipitation which retards grain growth
rate of coarsening of precipitates is very low so they’re fine
carbides and lower temp increase strain of recrystallization of austenite
rate of grain growth decreases as precipitates are on GB

Question 5

Microstructural evolution during controlled rolilng

Answer 5

> highest rolling temp: a few residual carbides present, restricting grain growth of austenite
as temp decreases during roughing passes, carbides precipitate on GB and severely restrict grain growth
steel recrystallizes many times during roughing passes, grains become smaller and more precipitation of carbides
with more precipitation, recrystallization requires more deformation
during delay period, static recrystallization occurs, precipitation of carbides continues, equiaxed fine grain austenite obtained
during finishing passes, recrystallizaiton doesn’t occur due to lack of high strain; high dislocation density
during air cooling, deformed austenite –> ultra fine ferrite with finely dispersed carbide/nitrides

Question 6

Strengthening mechanisms

Answer 6

> fine ferrite = higher yield stress
fine carbides = higher yield stress (depends on interparticle distance, avg particle size, vol fraction precipitate)
alloys give solid soln strengthening effect
high dislocation density = high strength (depends on finish rolling temp)
amount of deformation during controllled rolling must be at least 30% to get increase in strength)

Question 7

Effect of process parameters

Answer 7

a. completing rolling at temp right when transf is complete give high yield stress due to increase in dislocation density in ferrite, fine structure of ferrite, and finer grains
b. when finishing temp is above transformation temp, the faster cooling rate changes ferrite from equized to Widmanstaten with a higher dislocation density and higher yield stress
c. higher austenitization temp gives higher yield stress as more carbides are dissolved giving higher vol fraction of fine carbides during precipitatio. soaking temp shouldn’t be higher than temp at which all precipitates are dissolved