Institut für Eisenhüttenkunde der RWTH Aachen

New Microalloyed Forging Steels

CBMM, 13th July London

Prof. Dr.-Ing. Wolfgang Bleck

2

Introduction

Strip Products

• Continuous casting of slabs or thin slabs

• Thermomechanical rolling on HSM or CSM

• Cold rolling and batch or continuous annealing

→ in general: well defined process routes

Forging Products

• Semi-product: long products via numerous different

process routes

• Forging: numerous different product forms and

different temperature and deformation cycles

→ difficult process control

→ many constraints: tooling, forces

→ limited control of cooling rates

Niobium in Forging Steels

• High C contents; Martensitic of Bainitic microstructure

→ Transformation Kinetics; robust processes

→ Grain refinement of austenite/martensite?/bainite?

→ Enhanced mechanical properties

3

Projects

Current Nb related Industry Projects at IEHK

• Case-Hardening Steels (Al-reduced)

DFG-Project, Cluster of Excellence WP 5200 Demon

• Precipitation Hardening Ferritic/ Pearlitic Steels

AViF-Project, A 228

• High-Strength Ductile Bainitic Steels

BMWI-Project, IGF 260 ZN

• TRIP-Forging Steels

BMWI-Project, IGF 374 ZN

• Forging Simulation of Nb-Microalloyed Steels

BMWI-Project, IGF 17246 N

• Damage Tolerant Microstructures of Highly Stressed

Components for Mechanical Engineering

DFG-AiF-Project, HiPerComp

4

Microalloyed Gear Steels

Project 1: Case hardening steel for

high temperature carburising processes

Project 2: Al-reduced case hardening steel

for high temperature carburising processes

5

High temperature carburizing

decreases the production

duration and costs of gear

components.

Grain size control of austenite is

needed.

Grain growth can be prevented by

microalloying of case

hardening steels.

Prediction and optimization of

particle size and amount is

performed by numerical

simulation of particle evolution.

Microalloyed Gear Steels

6

Microalloyed Gear Steels Experimental Procedure

Material: 25CrMo4 + Nb/Ti

ST

EM

C Si Mn Cr Mo Ni V Al N Ti Nb

0.24 0.22 0.89 0.92 0.43 0.18 0.008 0.023 0.016 0.009 0.034

ST

EM

ST

EM

ST

EM

ST

EM

Ro

llin

g

Fo

rgin

g

FP-Annealing HT-Case hardening

8

EFTEM / during Processing

N Nb

Al Ti

NbC

100 nm

BG-I, cold formed,

930 °C – 75 min

• Different particles can be found:

AlN, NbC and Ti(C,N)

• Shape factor for AlN >2 and

NbC - Ti(C,N) ~ 1

AlN Ti(C,N)

9

Simulation Results

• Faster ripening of Al-nitrides in comparison to (Ti,Nb)-carbonitrides

• Higher pinning force for (Ti,Nb)-carbonitrides in comparision to Al-nitrides

11

Simulation Results

• Prediction of pinning force

using calculation of particle

evolution along different

process chains

• Small improvement of pinning

force via shortening of

austenitization time

• Decrease of pinning force by

increase of austenitization

temperature up to 1200 °C

equal to increasing the case

hardening temperature to

1100 °C

13

Microalloyed Gear Steels Grain Size Control

Austenite initial grain size ←

start microstructure,

heating conditions,

particle state

Pinning force ←

particle size, amount and

distribution

Description of pinning force

due to Zener force

Increase of particle amount

and decrease of particle

size needed

cmradiusparticler

amountparticlef

cmJenergysurface

cmJforceZenerZ

r

fZ

ZZZ

NCNbTiAlN

NCNbTiAlN

,

/ ,

/ ,

2

3

2

3

),)(,/(

),)(,(

15

Al reduced Case Hardening Steel

• Al reduction is requested for cleanliness improvement.

• The same pinning force at 1050 °C in Nb modified 25CrMo4 steel can be

obtained by an increase of Nb content to 850 ppm.

• Additionally, an increase in solution temperature is needed.

• Conclusion: a combined development of alloy and process parameters is

needed in order to realize this new steel concept.

16

Experimental results of grain size

distribution

• High grain stability of Al-reduced grade for 1050 °C and 1100 °C

• No abnormal growth at 1100 °C

Al Nb Ti N

Ref 227 ppm 337 ppm 89 ppm 166 ppm

Mod 87 ppm 850 ppm 16 ppm 160 ppm

17

Materials Design of

High-Strength Forging Steels

• Project 1: Microalloyed precipitation hardening

ferritic/pearlitic steels (PHFP-M)

• Project 2: High-strength ductile bainitic steels (HDB)

• Project 3: TRIP-Forging Steels

18

Materials Design of Forging Steels Motivation

Commonly used alloy concepts for forging components:

• Quenched and Tempered (Q&T) forging steels (e.g. 42CrMo4)

Need of additional heat treatment, danger of distortion

• Ferritic / Pearlitic precipitation hardening forging steels (e.g. 38MnVS6)

Limited in strength and especially toughness

Necessity for advanced mechanical properties and easy processing

19

Material Design of Forging Steels Alloy Design

C Si Mn S Cr Mo B Nb Ti V N

PHFP 1 0.38 0.60 1.40 0.05 0.04 0.03 <0.0005 <0,001 <0,001 0.10 0.010

PHFP-M 1 0.36 0.68 1.44 0.03 0.15 0.03 <0.0005 0.029 0.022 0.19 0.021

PHFP-M 2 0.30 0.62 1.44 0.03 0.29 0.04 <0.0005 0.049 0.020 0.19 0.012

HDB 1 0.30 1.56 1.52 0.02 1.23 0.08 0.0025 0.029 0.023 <0,001 0.012

HDB 2 0.22 1.47 1.50 0.01 1.31 0.09 0.0025 0.035 0.026 <0,001 0.011

Effect of Alloying Elements:

Nb, V: increase strength by precipitation hardening

Cr: decreases vcrit and increases tensile strength

B, Ti, N: B decreases vcrit in solute state, therefore Ti is added to

form TiN (refinement austenite grain) instead of BN

Si: Si > 1% suppresses the formation of cementite

chemical composition in wt.%

20

Materials Design of Forging Steels PHFP-M

C Si Mn P S Cr Mo Ni Cu N Al Nb Ti V

low Nb 0,35 0,64 1,40 0,008 0,030 0,16 0,06 0,16 0,01 0,0154 0,021 <0,001 0,02 0,11

medium Nb 0,35 0,62 1,41 0,008 0,029 0,17 0,06 0,16 0,01 0,0144 0,023 0,03 0,02 0,10

high Nb 0,35 0,65 1,41 0,009 0,030 0,17 0,06 0,17 0,01 0,0141 0,026 0,06 0,02 0,10

chemical composition in wt.%

Nb(C,N)

MnS

AlN

Increasing Nb content leads to increasing strength, especially yield strength

Amount of mass fraction of

microalloying precipitates: Strength properties:

Precipitation hardening ferritc/pearlitic steels:

21

Correlation Heat Treatment / Microstructure /

Mechanical Properties in HDB Steels

Blocky

M/A

Lath

wid

th in

mm

Str

en

gth

in

MP

a

To

tal

elo

ng

ati

on

in

%

To

un

gh

ne

ss

at

RT

in

J

Transformation temperature in °C

bainitic ferrite

elongated

retained

austenite

blocky

M/A

Mechanical Properties HDB 1 steel:

22

Toughness dependent on Yield Strength

+ MAE

bainitic

microstructure

amount retained austenite after

continuous cooling: ~7,8%

(measured by EBSD analysis)

Common Rail

(HDB 2)

23

Materials Design of Forging Steels Mechanical Properties

PHFP-M

pearlite lamellae spacing l

Ti, V, Nb (C,N)

ferrite fraction

HDB

bainitic ferrite FB + retained austenite R

+ Nb (C,N)

24

Effect of Nb and Mn on the transformation

behaviour of steel 18CrNiMo7

• The variation of small amounts of Mn (+0,5 wt.-%) and Nb

(+0,025 wt.-%) results in a change of transformation behavior

• Therefore the mechanical properties change dramatically especially at very

slow cooling rates

25

Forging of Microalloyed Steels

Simulation the Flow-Behaviour and

Microstructure Evolution

during Multi-Hit Forging of Microalloyed Steels

26

Hot Forging: Process and Materials Simulation

Modeling the Flow-Behaviour and

Microstructure Evolution

Hot-Deformation Flow Stress

σ=f(ε,T,ε) .

Microstructure

Dynamic/ Static

Recovery and Recrystallization

Precipitation of

Microalloying Elements Ti,V,Nb

Grain Growth

σ=f(ε,T,S) .

ρ=g(ε,T,ρ)

Dislocation Density

Empirical Approaches

Internal Variable

27

ρcrit

Critical

Dislocation Density

ρcrit

FV;R

Interconnection of the Sub-Modules

Dislocation Density Evolution/

Static Recrystallization

fconv kdyn,c8,c1,n

Precipitation Model

Temperature,

Strain Rate,

Grain Size,

Chem.Comp.

Dm ρ σ

Dm ρ X

Y/N

CC

CN

CV

kstat

Hot Forging: Process and Materials Simulation

Dislocation Density Evolution/

Dynamic Recrystallization

Temperature,

Strain Rate,

Grain Size,

Chem.Comp.

Temperature,

Strain Rate,

Grain Size,

Chem.Comp.

Temperature,

Strain Rate,

Grain Size,

Chem.Comp.

34

FEM-Simulation in FORGE

Courtesy GmbH

Hot Forging: Process and Materials Simulation

50%

45%

40%

35%

30%

25%

20%

15%

10%

5%

0

0.30

0.27

0.24

0.21

0.18

0.15

0.12

0.09

0.06

0.03

0

High Grade

of Deformation

High Volume

Fraction of DRX

XDXN /-- PHI /--

35

FEM-Simulation in FORGE Austenite Grain Size

Hot Forging: Process and Materials Simulation

l=32

l=27

l=32

l=42 l=47

l=29

DM /µm

55.0

50.0

45.0

40.0

35.0

30.0

25.0

20.0

15.0

10.0

5.5

l PAG in µm

Courtesy GmbH Courtesy GmbH

36

Model Implementation in eesy-2-form

Courtesy GmbH

Simulation of the Flow Stress as

Function of the Dislocation Density

Translation of the Model in

Material Flow Simulation

Meshed geometry of an uniaxial

forging sample

Local distribution of flow stress

(status after 0.35s process time)

Flow stress of the middle element

Hot Forging: Process and Materials Simulation

37

Niobium in New Forging Steels

Multi-Tasking

New use

of Nb in a great

variety of steels

Grain Refinement

Crack Resistance

Increase of:

Strength

Toughness

Ductility

Applications

Short Processing

Fast Precipitation

Kinetics

Multifunctional

Precipitation

Hardening

Al,V Nb

Alloy Design

Economic/ Efficient

Processing

Wide Fields of

Applications

Improved Materials

Properties and

Design

Institut für Eisenhüttenkunde der RWTH Aachen

New Microalloyed Forging Steels

CBMM, 13th July London

Prof. Dr.-Ing. Wolfgang Bleck