Benefits of thermodynamic and microstructure simulations towards reliable continuous casting of steels

Transcription

1 M. Apel 1), B. Böttger 1), D.G. Eskin 3), B. Santillana 2) 1) Access, Aachen, Germany 2) Tata Steel RD&T, POBox 10000, 1970CA, IJmuiden, The Netherlands 3) BCAST, Brunel University, Uxbridge UB8 3PH, U.K. Benefits of thermodynamic and microstructure simulations towards reliable continuous casting of steels

2 Outline Background and industrial application Microstructure differences MICRESS results Unexpected peritectic solidification and Cp 2

3 Breakouts!!!! Solidified shell bursts open Lost Production (24 hours) Machine damage Loss of over 200 K-euro's Safety 3

4 The iceberg Breakouts A few Cracks Few more Solidification issues Plenty 4

5 Solidification issues ZDT, ZST, LIT, DCP Critical stresses & strains Thermal Combined with high heat extraction Mechanical Chemical Micro/macr osegregatio n,peritectic solidificatio n 5

6 Problem Alloy compositions: LCAK HSLA LR-HSLA Steel grade (in 0,001 %) min aim max C Mn N (ppm) 50 cracking! Al soluble Nb 5 Ti 10 V 5 Ca (ppm) Steel grade (in 0,001%) min aim max C Mn N (ppm) not cracking! Al soluble Nb Ti 10 V Ca (ppm) Steel grade (in 0,001%) min aim max C Mn N (ppm) cracking! Al soluble Nb Ti 10 V 40 Ca (ppm) Why 6

7 Macrostructure (same heat transfer conditions) HSLA LCAK Macroetching for the two steel grades considered in this study. 7

8 17 mm 18 mm HSLA LCAK Macroetching for the two steel grades considered in this study. 8

9 Measured PDAS (um) Primary Dendrite Arm Spacing HSLA LCAK Distance from slab surface (mm) 9

10 Sumitomo hot tensile test ZDT ZST ZDT ZST FV85 Hot Tensile Tests 1300 C 1320 C 1340 C 1360 C 1380 C 1400 C 1420 C 1440 C 1460 C 1480 C Brittle temperature range (ΔT B ) HSLA Brittle temperature range (ΔT B ) LCAK 10

11 Aims of this project Perform Phase Field Simulations of microstructure formation of the first solid shell under Continuous casting conditions Quantify simulated solidification microstructure Find out whether the observed differences of the cracking risk can be understood via the chemical composition 11

12 Comparison LCAK and HSLA (same heat transfer conditions) LCAK (Fe-C-Mn), r 0 =0.065 µm, σ=10 %*r 0, N=10 8 HSLA (Fe-C-Mn-N-Nb-V), r 0 =0.065 µm, σ=10 %*r 0, N=10 8 HSLA is finer and more equiaxed! 12

13 Comparison LCAK and HSLA: Carbon composition distribution at 0.4s LCAK (Fe-C-Mn), r 0 =0.065 µm, σ=10 %*r 0, N=10 7 HSLA (Fe-C-Mn-N-Nb-V), r 0 =0.065 µm, σ=10 %*r 0, N=10 7 HSLA is finer, more equiaxed and has a wider solidification range 13

14 LCAK vs HSLA at same heat transfer conditions The difference between LCAK and the HSLA steels is the as-cast structure LCAK has a coarser structure and tends to form more columnar dendrites than the HSLA Finer grains, more equiaxed & thinner dendrites exhibit more isotropic and uniform mechanical properties =>less tendency for hot tearing formation and decreased macrosegregation. Coarser structure also exhibits a wider BTR (also shown in the SMI hot tensile tests) 14

15 Thermo-Calc C: 0.060% Mn: 0.875% Nb: 0.016% N: 0.015% V: 0.141% Ti: 0.010% C: 0.060% Mn: 0.875% Nb: 0.016% N: 0.010% V: 0.040% Ti: 0.010% A: FV85 B: FV83 Figure 1. Scheil calculations for FV83 and FV85 considering the maximum values for the A (for HSLA steel grade), the phases listed are as follows: compositions. 1: LIQUID: liquid steel 2: LIQUID BCC_A2: liquid and the δ ferrite (L+ δ) 3: LIQUID BCC_A2 FCC_A1#2: liquid, δ ferrite (L+ δ) and TiN 4: LIQUID FCC_A1#1 FCC_A1#2: liquid, γ austenite (L+ γ) and TiN Consequently, in B (for LR-HSLA steel grade), the phases listed are as follows: 1: LIQUID: liquid steel 2: LIQUID BCC_A2: liquid and the δ ferrite (L+ δ) 3: LIQUID BCC_A2 FCC_A1#2: liquid, δ ferrite (L+ δ) and TiN 4: LIQUID BCC_A2 FCC_A1#1 FCC_A1#2: liquid, δ ferrite and γ austenite (L +δ+γ) and TiN 5: LIQUID FCC_A1#1 FCC_A1#2: liquid, γ austenite (L+ γ) and TiN 15

16 Temperature (ºC) Carbon potential-ferrite potential δ δ P δ+γ δ 1 δ+l γ 1 γ L γ 2 L P γ+l L 2 Reaction: L Transformations: L L 1370 C δ C γ C L The actual [7] peritectic covers the range Cd to CL, but regarding peritectic grades prone to defects the peritectic range is: C δ < C p < C γ Composition (wt% C) Figure 1. Schematic Fe-C phase diagram showing the peritectic reaction and transformation with the proximity to C δ (left hand /low C extremity) being considered particularly important. 16

17 Carbon potential-ferrite potential The changes in the Fe C phase diagram through chemistry can be described using an effective carbon content Cp expression of the form: C p = [%C] + X Mn [%Mn] + X Si [%Si] + with X x as a coefficient. This approach works on the principle whereby the equivalent concentrations of austenite forming elements are added to the Cp value, and the equivalent concentrations of ferrite forming elements are subtracted. However, from the +/- signs of various coefficients, the Cp approach may no be very straightforward for determining the selectivity between Austenite and Ferrite. The TRICO formula is calculated from a statistical analysis done at TRICO steel Decatur, Alabama, USA (now Nucor Decatur). The SMS formula is based on plant experience. Reference Mn Si Al(tot) N P S Nb V Ti Cu Sn Cr Ni Mo B Ca Wolf Trico BSSTC SMS Howe

18 Cp Cp Cp Cp BS-STC & Howe formulae BS STC No BO's FV83 No BO's FV85 NO BO's F12L No BO's FN 80 No BO's FN 81 BO's upper limit lower limit LR-HSLA BS STC No BO's FV83 No BO's FV85 BO's upper limit lower limit HSLA Ladle Ladle A: Cp for all the ladles from 1 st January No BO's 2009 FV83 B: Ladles corresponding to FV83 and FV85 up to 31 st Howe Howe No BO's FV83 No BO's FV85 December NO BO's F12L No BO's FV85 No BO's FN Figure 1. Carbon potential calculated No with BO's FN 81 BS STC formula. BO's 0.12 BO's upper limit 0.12 upper limit lower limit lower limit Ladle Ladle A: Cp for all the ladles from 1 st January 2009 B: Ladles corresponding to FV83 and FV85 up to 31 st December Figure 1. Carbon potential calculated with A. Howe formula. 18

19 Peritectic range (Cp) Cp Cp Wolf formula & Blazek limits Wolf No BO's FV83 No BO's FV85 NO BO's F12L No BO's FN 80 No BO's FN 81 BO's upper limit lower limit Wolf No BO's FV83 No BO's FV85 BO's upper limit lower limit Ladle Ladle A: Cp for all the ladles from 1 st January 2009 B: Ladles corresponding to FV83 and FV85 up to 31 st 0.2 December Figure 1. Carbon potential calculated with Wolf formula LCAK Cδ LCAK Cγ MA Cδ MA Cγ MA2 Cδ MA2 Cγ LR-HSLA Cδ LR-HSLA Cγ HSLA Cδ HSLA Cγ LCAK Cδ LCAKCγ MA Cδ MA Cγ MA2 Cδ MA2 Cγ LR-HSLA Cδ LR-HSLA Cγ HSLA Cδ HSLA Cγ 0 19

20 CONCLUSIONS LCAK is more sensitive to cracking because the microstructure is coarse LR-HSLA may have cracking issues because is almost a peritectic grade so may have late peritectic solidification Next step: DSC-DTA Aims of the project achieved! Perform Phase Field Simulations of microstructure formation of the first solid shell under Continuous casting conditions Quantify simulated solidification microstructure Find out whether the observed differences of the cracking risk can be understood via the chemical composition Next step: other steel grades to simulate 20

21 Thank you for your attention Questions