Niobium microalloying in AHSS

PERFORMANCE ENHANCEMENT OF
ADVANCED HIGH STRENGTH STEELS
BY NIOBIUM MICROALLOYING
Matt Enloe, CBMM North America
Hardy Mohrbacher, NiobelCon bvba
Rafael Mesquita, CBMM
1

OUTLINE
• Introduction
• Conventional Role of Nb in HSS
• Precipitation Strengthening
• Grain Refinement
• Nb Metallurgy in AHSS
• Multiphase Steels (DP, MP, CP)
• TRIP-Aided Steels (CFB, Q&P)
• Nb Effects on Delayed Fracture
• Conclusions
2

4
INTRODUCTION – FUNDAMENTALS OF Nb
• Small atoms such as carbon or nitrogen reside on interstitial lattice sites.
• Large atoms such as manganese, silicon, niobium, etc., substitute iron atoms in the
lattice.
• Carbon form carbides with iron and several alloying elements (e.g. niobium, titanium,
vanadium).
(C, N)
Iron matrix
Alloying elements
e.g. Mn, Si, Nb
, e.g., Nb(C,N)

5
Reheating
furnace
Rough rolling Finish rolling Cooling Coil
Austenitizing
(1200-1250°C)
Dissolution of
alloying
elements
Cooling to coiling
temperature
(700-500°C)
γ → α transformation
Thickness
reduction
200 → 25-50 mm
Multiple static
recrystallizations
(1150-1000°C)
Thickness
reduction
25-50 → 5-2 mm
Potentially without
recrystallization
(TMCP)
(1000°C-820°C)
MAJOR PROCESS STEPS IN THE HOT ROLLING MILL (STRIP)
Effects of Nb Present at Each Stage of Process – Precipitation is Strain Dependent
Finishing

6
undissolved
dissolved

7
Precipitation of Nb during steel rolling is nucleation dependent – kinetics matter.

8
CONVENTIONAL ROLE OF Nb IN HSS

9
• Nb carbonitride precipitates strengthen ferrite through traditional mechanisms

10
• Nb as solute or in carbonitride precipitate acts to delay austenite
recrystallization during hot rolling
• Similar effects occur in reheating following cold rolling
• The result is ferritic grain refinement

11
bad
good

12
bad
good
For strength - energy related optimizations in HSS,
grain refinement should be the first mechanism
that is optimized/maximized.
How do these well-proven and understood effects
translate to advanced steels for automotive
applications?

13
NIOBIUM (Nb) MICROALLOYING IN AHSS

14
Rm
Rp0.2
Ag A80
%M (↑)
HVM (↑)
HVF (↑)
dF (↓)
dM (↓)
%B (↑)
%M (↑)
HVM (↑)
HVF (↑)
%B (↑)
dF (↓)
dM (↓)
%M (↑)
%B (↑)
HVM (↑)
HVF (↑) dF (↓)
Strain
Stress
Effects are strongly promoted by Nb microalloying

15
Control of DP microstructure through prior hot band microstructure:
• Fine ferrite grain size
• Fine pearlite distribution
• Achievable by addition of microalloy + controlled rolling
Optimization of DP/MP/CP Properties Will Require Resistance to Strain Partitioning to
Enhance Local Formability with Maintenance of Uniaxial Tensile Properties
This is Achievable Through:
• Structural Refinement
• Reduction in Martensite/Ferrite Hardness Differences
MICROSTRUCTURAL INFLUENCES ON THE MECHANICAL PROPERTIES
OF DP/MP/CP STEEL

16
INFLUENCES ON HOLE-EXPANSION BEHAVIOR
During hole-expanding, micro-cracks
propagate mostly along the phase
interfaces in dual-phase steel in case of
low stretch-flange-formability.
Microcracks tend to propagate through
ferrite or martensite phase in dual-
phase steel in case of high stretch-
flange formability.
The difference in hardness of ferrite and
martensite is the dominant factor of the
stretch-flange formability in dual-phase
steel. In addition, the volume fractions
of phases also influence the formability.
Kohei HASEGAWA, Kenji KAWAMURA, Toshiaki URABE and Yoshihiro HOSOYA
ISIJ International, Vol. 44 (2004), No.3, pp. 603-609
51%F / 49%M 66%F / 34%M
0.12%C – 1.9%Mn
Low
temperature
tempering
250
30
Hardness difference (HVM – HVF)
Holeexpansionratio(%)
300 350 400 450 500
40
50
60
70
80

17
MICROSTRUCTURAL REFINEMENT OF DUAL PHASE STEEL
• Martensite islands with variable
C content.
• Martensite clustering.
• Greater potential for crack
propagation under local straining.
DP 780 DP 590
Ferrite
Martensite
DP780 DP590
+Nb
• Smaller martensite islands
• Reduced martensite clusters.
• Increased YS and TS.
• Improved hole expansion ratio and
bendability.
• Lower C necessary to Achieve UTS

18
Standard DP780Standard DP780 DP780 + NbDP780 + Nb
15 µm 15 µm

19
longitudinal
noNb
withNb
10000
8000
8000
4000
2000
0
0 20 40 60 80 100 120 140
Bending angle (deg)
Bendingforce(N)
DP780+Nb
DP780
Bending radius: 0.2 mm
Sheet gage: 1.2 mm
Optimized Alloy Concepts Will Utilize:
• Grain Refinement
• Minimization of C Content (MP and
DP980 Grades Achievable at Sub-
Peritectic C Levels).
• Continued or Greater Emphasis on
Clean Steel Practices (low S)
Modular Concepts Exist and Have Been
Demonstrated to Achieve Balanced
Global and Local Formability for
DP/MP/CP 590-980 MPa

20
Nb-Added Chemistry Nb-Added Chemistry
HER Half Dome Edge Stretch - DIC

21
NIOBIUM (Nb) MICROALLOYING IN RA AHSS

PRODUCTION OF CLASSIC AND RA MULTIPHASE STEELS

STAGES IN FORMING A RETAINED AUSTENITE MICROSTRUCTURE
Ms
Bs
Fs
time
temperature
1
2
3 4
5
Ms’
g: austenite or retained austenite
abf: bainitic ferrite
abf*: bainitic ferrite with lowered carbon
concentration
am: martensite
am*: martensite with lowered carbon
concentration
am’: carbon-enriched martensite
abf* abf*
g
g
g g
g
g
abf
g
g
g
am*
g
g
g
am*
am’
g
g
g
am
abf
stage 1 stage 2 stage 3 stage 4 stage 5

CARBON PARTITIONING: EFFECT OF GRAIN SIZE

Ref.: A. García-Junceda, C. Capdevila, F.G. Caballero, C. García de
Andrés
Ref.: Hong–Seok Yang and H. K. D. H. Bhadeshia Ref.: Seok-Jae Lee and Young-Kook Lee
• Niobium is very efficient in controlling PAGS (Zener Pinning).
• This can help to enhance robustness of Q&P process.
PAGS EFFECT ON MS TEMPERATURE

EFFECT OF PAGS ON MARTENSITE CHARACTER
Error of volume fractions is ±0.02
Tγ, °C 900 1000 1100
PAGS, µm 14±1 24±1 67±1
fM1 0.62 0.73 0.81
fRA 0.16 0.15 0.14
fM2 0.22 0.12 0.05
Mechanical stabilization of austenite phase
due to a reduction of its grain size causes a
reduction in Ms – C kinetics reduce at same
fraction.
Lower fM1 implies lower carbon content
available for diffusion into the adjacent
austenite during partitioning.
This leads to more fM2 resulting in higher
strength for a given austenite character.
M1 is carbon depleted martensite (softer).
M2 is carbon enriched martensite (harder).

0
2
4
6
8
10
300 350 400 450 500 550
fg0
(vol%)
TA
(o
C)
EFFECT OF OVER-AGING TEMPERATURE AND ALLOYING CONCEPT ON RETAINED
AUSTENITE IN RA (CARBIDE FREE BAINITIC) STEEL
0.0
0.5
1.0
1.5
300 350 400 450 500 550
Cg0
(mass%)
T
A
(
o
C)
BF+M+gR BF+gRMs
0.5Si-1.0Al-
0.05Nb-0.2Mo
0.5Si-1.0Al
1.5Si

0
5
10
15
20
25
30
300 350 400 450 500 550
TEl(%)
T
A
(
o
C)
(a)
EFFECT OF Al, Nb AND Mo ADDITIONS TO 0.2% C-1.5% Mn CFB STEEL
0
10
20
30
40
50
60
70
80
300 350 400 450 500 550
(%)
T
A
(
o
C)
(b)

PRECIPITATION BEHAVIOR OF NB AND/OR MO BEARING 0.2C-1.5SI-1.5MN STEELS
0
0.05
0.1
0.15
0.2
0.25
0.3
400 600 800 1000 1200 1400
f
NbC
,f
NbMoC
(vol%)
T (
o
C)
0.05Nb-0.20Mo
0.02Nb-0.10Mo
0.05Nb
0.02Nb
200nm
NbC
NbMoC

0
200
400
600
800
1000
1200
1400
900 1200 1500 1800 2100
TS (MPa)
DFL(MPa)
DELAYED FRACTURE LIMIT (DFL) OF CFB STEELS
0.2C-(0.2-1.5)Si-1.5Mn
1.5Si (base
steel)
0.2Si
Martensitic
steel
1.0Al-0.5Si
1.0Al-0.05Nb-
0.2Mo-0.5Si
0
200
400
600
800
1000
1200
1000 1200 1400 1600 1800 2000
DFL(MPa)
TS (MPa)
TS=DFL TS=DFL
0.4C-(0.2-1.5)Si-1.5Mn
Martensitic steel
Bainitic
steel
CFB steel
No forming
After forming
Sugimoto, et al., Materials Science and Technology, 2009, Vol 25, No 9

HYDROGEN EMBRITTLEMENT RESISTANCE IN PHS STEELS
Zhang et al., Materials Science & Engineering A, 2015, Vol 626
0 Nb 0.05 Nb

CONCLUSIONS
• Nb strengthens conventional low carbon ferritic high strength steel by
conventional mechanisms of grain size refinement and precipitation hardening.
• Grain size refinement may be considered the “basis” of strengthening
mechanisms due to its positive influence on fracture toughness (resistance to
crack propagation) – others require trade-off
• Nb additions in AHSS are considered for optimization of the primary
strengthening mechanisms, namely the multiphase structure created. The
demonstrated benefits of Nb in this regard include:
• Refinement and Homogeneity of Microstructure for Local Formability in MP
• Precipitation Hardening of Ferritic and Bainitic Constituents by NbC
• Modification of Martensite / Austenite Characters in RA Steels for Better
Property Balance of Process Robustness
• Precipitates Act as H-trapping Sites

THANK YOU!
MATTHEW.ENLOE@CBMM.COM

Niobium microalloying in AHSS

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