An idea of thermodynamics for casting hyper-peritectic steel grades for cotinuous casting cotinuous casting Begoña SantillanaDémian Ruvalcaba Jimenez

From raw materials to steel…

Iron ore

Coal Coke

Blast

furnaceBOS

Converters

Hot metal & scrap

Hot metal Hot metal “liquid”“liquid”

Steel “liquid”Steel “liquid”

2

Ladle

treatment

Continuous

castersLadle tapping

Continuous caster

Ladles

Turret

3

Tundish

Mould

Secondary cooling

Oxy cutters / pendulum shear

Background

• Steel producers are looking for new products with high added value

• Some of these products have compositions in the hyper peritectic range

• Traditionally, steels with a composition around the peritectic point are difficult to cast ….

• The idea is to cast “alloying out of the peritectic”.

• A medium carbon grade is actually cast in IJmuiden:

4

• A medium carbon grade is actually cast in IJmuiden:

• 0.23-0.25%C• 1.5% Mn• 0.3% SI

• But what about the hyper peritectic range? a “new steel” with a mix of the maximum alloying we cast (on different grades):

• 0.2% C• 2 %Mn• 1 % Si (could be more but it will be a so-called electrical steel)

Casting a high alloyed steel

• Peritectic, hyper-peritectic or what??

• Solidification range acceptable for continuous casting?

• Segregation?

• Casting parameters to consider:

• Heat extraction

5

• Heat extraction

• superheat

• casting speed

• etc…

Classification

Hypo-peritectic Hyper-peritectic

6

Peritectic

Classification for steelmaker’s….

Hypo-peritectic 0.06-0.75

Hyper-peritectic 0.18 up to who

knows…

7

Peritectic 0.75-0.18

•This classification is based on sensitivity to formation of depressions and surface cracking in the solidifying shell•Relates to choice of mould fluxes to optimise heat transfer and lubrication in the continuous casting mould•Rule of thumb based on "classical" steel compositions

Binary vs. multi

LIQUID

LIQUID+δ

LIQUID+δ+γ

δ δ+γ

γ

LIQUID+γ

LIQUID

LIQUID+δ

LIQUID+δ+γ

δ

δ+γ LIQUID+γ

0.55 %C

1491ºC

0.16 %C

1491ºC

8

γ+(Mn,Fe)S_Q

γ+(Mn,Fe)S_Q+CNb

γ

Peritectic associated defects

1350

1370

1390

1410

1430

1450

1470

1490

1510

1530

1550

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Tem

pera

ture

(ºC

) δδ+γ

δ+L

γ γ+L

LδP

LP

L2γ2

Cδ

δ1

CLCγ

γ1

1350

1370

1390

1410

1430

1450

1470

1490

1510

1530

1550

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Tem

pera

ture

(ºC

) δδ+γ

δ+L

γ γ+L

LδP

LP

L2γ2

Cδ

δ1

CLCγ

γ1

Schematic Fe-C

diagramγ1δ1

9

Composition (wt% C)Composition (wt% C)

Shell Structure γ δ γ δ γ δ γ δ L γ δ γ δ γ δ γ δ L γ γ γ γ L

Narrow mushy zone but only a Narrow mushy zone and Extended mushy zone narrow strong austenite layer thick strong austenite layer with narrow fully-solid austenite layer

%C

Type B Sticking

Type A Depressions

Defect susceptibility

Carbon potential-Ferrite potential

• The changes in the Fe–C phase diagram through chemistry can be approximated using an effective carbon content Cp expression of the form [2]:

• Cp = [%C] + XMn[%Mn] + XSi[%Si] + …

• Since the key factor to assess the peritectic reaction is the primary ferrite fraction during solidification, a “ferrite Potential” Fp can be

10

ferrite fraction during solidification, a “ferrite Potential” Fp can be defined by the following relationship:

• Fp = 2.5(0.5-%Cp)

Si, Ti, Cr, Mo C, N, Ni, Mn

Lower Cp Higher CpCp

Cp is ‘shifted’ by chemical composition

Carbon potential

gradeA Medium

carbonHyper-

peritecticLCAK

C 0.18-0.22 0.265 0.3 0.045Mn 1.8-2.2 1.3 0 0.22Si 0.8-1.2 0.25 0 0.01

Wolf 1991 0.16 0.19 0.30 0.05

11

Wolf 1991 0.16 0.19 0.30 0.05BS STC 0.18 0.22 0.30 0.06

SMS 0.21 0.24 0.30 0.05Howe 0.17 0.21 0.30 0.06NSC 0.16 0.19 0.30 0.05

Kobe 0.33 0.30 0.17 0.17

Cp

QSP

grade High alloyed

steel

C wt% 0.22

Mn wt% 2.1

Si wt% 1

N wt% 0.0007

Ep 0.32 Peritectic equivalent. <1, starts as ferrite. >1, austenite.

A

12

Ep 0.32 Peritectic equivalent. <1, starts as ferrite. >1, austenite.

Peri Hit 0.7618 Fraction solid (ferrite) at which the Peritectic is encountered

Delta-P 18.34 Temperature range between loss of ferrite (P-end) and solidus

FF@ sol. 28.5 Fraction ferrite at solidus (i.e. if still on peritectic "plateau")

Liq 1498.8 Liquidus, C

P-start 1461.12 Start of peritectic, C

P-end 1451.57 End of peritectic "plateau", i.e. loss of ferrite, C

Sol 1433.23 Solidus, C

Solidification range

fS

Solidus (fS=1)

ZST (fS=0.8)

ZDT (fS=0.99)LIT (fS=0.90)

RT (0.80≤≤≤≤ fS≤≤≤≤ 0.90) Liquidus (fS=0)

fS

Solidus (fS=1)

ZST (fS=0.8)

ZDT (fS=0.99)LIT (fS=0.90)

RT (0.80≤≤≤≤ fS≤≤≤≤ 0.90) Liquidus (fS=0)

13

StrengthDuctility

Temperature

Stage 1Stage 2aStage 2bStage 3

StrengthDuctility

Temperature

Stage 1Stage 2aStage 2bStage 3

Solidification range

Solidification range= Tliq-Tsol

Steel “A” ~63 degrees

LCAK~18 degrees

Thickness mushy zone (CON1D)

A LCAK

Liquidus T(ºC) 1500.3 1532.2Solidus T(ºC) 1435.3 1513.5

Peritectic T(ºC) 1472

Mould exitCastig speed high low high low@ Liquidus (mm) 11.7 10.6 11.8 10.6@ Soliidus (mm) 8.9 7.7 10.7 9.6Thickness "mushy zone" 2.9 2.6 1.1 1

A LCAK

12

14

14

0

2

4

6

8

10

0 20 40 60 80 100

Distance from meniscus (% of mould length)

She

ll th

ickn

ess

(mm

)

Liquidus 5m/min

Solidus 5m/min

Liquidus 4m/min

Solidus 4m/min

Liquidus 5m/min LCAK

Solidus 5m/min LCAK

Liquidus 4m/min LCAK

Solidus 4m/min LCAK

Scheil (Thermo-Calc)

“A”

Hyper-peritectic

15

Medium Carbon

Phase transition (FactSage)Hyper-peritectic

“A”

16

Medium Carbon

Some conclusions

• The “High alloyed steel” is a hyper-peritectic grade, quite early during solidification will pass through the peritectic transformation, how big is that region is unknown.

• “Equilibrium” solidification range is around 60 degrees, thus even if peritectic surface cracking may not occur, the larger the sol-range the more susceptible to other casting defects such as: hot tearing and sticking.

• Non-uniform heat flux in the caster’s could lead to local segregation

17

• Non-uniform heat flux in the caster’s could lead to local segregation changing the solidification kinetics.

• The strength of the solid–mush region needs to be checked for this steel.

• Non equilibrium behaviour during actual casting conditions needs to be taken into account: HOW?

Thank you for your Thank you for your attention

Proposal for the Thermo -Proposal for the Thermo -Calc & MICRESS

meeting…

A triple point is characteristic for MICRESS…

• Obvious for this sort of meeting should be a “triple point” …

• IGUAZU… Argentina, Brasil, Paraguay

20

Why?

• Is a triple-point… & has tropical weather (sunny the whole year round!!!)

• It has plenty of iron & other elements in a form of precious metals

21

Why?

• It has a very inspiring environment …

• the widest waterfall in the world (sorry Zimbabwe but it’s true!)

22

Iguazu falls

23

Why?

• We could find big topics to discuss… as big as the trees growing there…

24

Why?

• We can find a PARROT (not the TC-module!)

• And many “sources of inspiration”

• so maybe “TUCAN” will be the next TC-module???

25