high tensile valve spring steel in kobe steel · improving fatigue strength and sag resistance....
TRANSCRIPT
High Tensile Valve Spring Steel
in Kobe Steel
Tomokazu Masuda
Kobe Steel, Ltd.
8th international congress of spring industry
High power ⇒Higher Rev.
Fuel Efficiency
Engine
Weight reduction
Reduce spring load
Thinner wire
Strengthening
Valve Spring
★Benefit of strengthening
Reducing 1g/spring enhances fuel efficiency by 0.2% → Can increase rev. by 100~200rpm
(1)Fatigue Resistance:No fracture by fatigue until 100,000km
(2)Sag Resistance:Few loss of spring load (free length)
+(3)Answer for higher performance engines (Thinner wire)
Increase in natural vibration
Reduce friction at cam
Strengthening Valve Spring Steel
Need to strengthen the
material
Required Properties
Steel
Trend of High Strength Steel Grades for Valve Spring in KOBE STEEL
・Adopting turbo-charger, higher power, higher rev., and higher engine temperature.
・Downsizing engines for fuel efficiency, and computer-controlled engines.
High Strength
10%up
Fatigue
str
ength
(M
Pa)
1975 1985 1995 2005
SAE9254
KHV7
KHV7
+Nitriding
KHV10N
+Nitriding
KHV12N
+Nitriding
(New generation steel)
Year
Steel grade C Si Mn Ni Cr V
SAE9254 Si-Cr steel 0.55 1.40 0.65 - 0.65 -
KHV7 Si-Cr-V steel 0.62 1.45 0.60 - 0.60 0.12
KHV10N High_Si-Cr-V steel 0.58 2.00 0.85 0.30 0.95 0.10
KHV12N High_Si-High_Cr-V steel 0.60 2.15 0.45 0.20 1.75 0.27
20%up
Table. Chemical compositions of wire rod for valve spring (mass%)
Desired Properties and Methods for High Tensile Valve Spring
Desired Property
Fatigue
High Temp. Sag Resistance
Productivity
Withstand 1×108 cycles
Key Technology Method
Design composition for high temp. temper softening resistance
Design composition for higher strength and toughness
Design composition for acquiring nitriding effect
Reduce harmful inclusions Establish inclusion
refining technology
High Si add.
High V add.
High Cr add.
Concept and mechanical properties of KHV10N and KHV12N will be presented.
High Tensile Strength
1000
800
600
400
200
0 Change in tensile
str
ength
MP
a
0 0.5 1.0 1.5 2.0
C
Cr Si
V
Alloying Element Content mass%
Relationship between alloying element contents and change in tensile strength after
having been tempered at 450℃ for 1 hour
Increasing/adding C, Cr, Si, V is effective way to improve strength and toughness.
They can contribute to solid solution and precipitation strengthening, and refinement
of grain size.
Relationship between Si content and residual shear strain
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0
Resid
ual shear
str
ain
γ×
10
-4
1.5 2.0 2.5
Si content (mass%)
High Sag Resistance
Si has an effect to resist cementite to break/gather up by heat and strain. It
also resist C diffusion.
Si is effective against sag resistance for spring steel.
Approx. over 2%Si maximizes sag resistance property.
Influence of grain size at initiation point of FATT
High toughness
50
0
-50
-100
-150
1.0 2.0 3.0 4.0 5.0 6.0
D-1/2 (mm-1/2)
FA
TT
(℃
)
Grain refinement is useful for increasing toughness.
Segregation of impurities at grain boundary is reduced by increasing grain boundary
area.
In addition to strengthening grain refinement, loss of toughness with
increasing tensile strength can be prevented.
Quenching temperature (℃)
13
12
11
10
9
8
7
6
5
750 800 850 900 950 1000 1050
Auste
nite
gra
in s
ize G
.S.
No.
SUP7
V alloyed steel
Grain refinement by V
Relationship between quenching temperature and austenite grain size for V-alloyed
steel and SUP7
Fine grain size can be acquired at austenitizing temperature even below
quenched at 900℃ since V carbides/nitrides are remaining.
Quenching at above 950℃, V carbides/nitrides decompose, thus, grain growth
cannot be suppressed.
Grain refinement by Cr
0 0.5 1.0 1.5 2.0
Cr content (mass%)
16
15
14
13
12
11
10
9
8 Auste
nite g
rain
siz
e n
um
be
r G
S#
Relationship between austenite grain size and Cr content for spring steel
To suppress grain growth, improving thermal stability of cementite by increasing
%Cr is effective.
By stabilizing cementite, austenitic grain growth is suppressed.
Effect of Cr on grain growth is maximized by adding more than 1.5%Cr.
950 900 850 800 750 700 650 600
Fatigue
str
ength
(M
Pa)
1500 1600 1700 1800 1900 2000 2100 2200
High Fatigue Strength
Effect of tensile strength of steel wire for valve spring on fatigue strength
Tensile strength (MPa)
Fatigue strength increases with increasing tensile strength,
but results are unstable due to decrease in toughness.
Inclusion control
Toughness improvement Possible to reach target fatigue strength
corresponding to tensile strength.
Steel Hardness after Nitriding
Cr
V
Si
Al
0 1 2 3 4 5
Alloy contents (mass%)
900 800 700 600 500 400 300 200
Hard
ness
(BH
N)
Effect of alloy contents of steel on
nitriding hardness
0 0.1 0.2 0.3 0.4 0.5 center
Distance from steel surface (mm)
800
600
400
200
Hard
ness
(mH
V)
Base
V
V+Al
V+Cr
V+Cr+Al
Hardness distribution of V, Cr, Al added
steels after nitriding
Easily nitride compound forming elements, Al, Cr, and V, are
effective for nitriding hardness
Combination of Al, Cr, and V can enhance hardness after nitriding.
Developed steel increased nitriding hardness with Cr and V
Steel grade C Si Mn Ni Cr V
KHV12N Ultra High Tensile 0.60 2.15 0.45 0.20 1.75 0.27
KHV10N Super High Tensile 0.58 2.00 0.85 0.30 0.95 0.10
SAE9254 Regular 0.55 1.40 0.65 - 0.65 -
Chemical compositions of wire rod for valve spring (mass%)
Billet Hot-rolling Shaving Patenting Drawing Oil-tempering
Eddy-current testing Cold coiling Stress relieving Coil-end grinding
Nitriding Shot peening Strain aging Setting
Manufacturing process of valve spring
Aim is to improve strength and toughness by increasing C, Si, Cr and adding V
Example
In order to maximize developed steels’ performance of fatigue property, nitriding
and shot peening are also applied.
Result
Relationship between annealing temperature and mechanical properties in
developed steels
2300
2200
2100
2000
1900
1800
1700
1600
1500
1400
Tensile
str
ength
(M
Pa) KHV12N
KHV10N
SAE9254
KHV12N
KHV10N
SAE9254
300 350 400 450 500 550
Tempering temperature (℃)
70
60
50
40
30
20
10
0
300 350 400 450 500 550
Tempering temperature (℃) R
eduction o
f are
a
(%)
KHV10N shows higher temper softening resistance and decrease in tensile strength
is less compared to SAE9254.
KHV12N obtains even higher temper softening resistance than KHV10N. Also,
secondary hardening was shown around 400℃.
Hardness after Nitriding of Developed Steels
700
600
500
400
300
200
100
0
Vic
kers
Hard
ness H
V
SAE9254 KHV10N KHV12N
Steel grade
KHV10N shows higher temper softening resistance and decrease in inner hardness
after nitriding is less than SAE9254 because KHV10N contains higher Si.
KHV12N obtains even better nitriding property due to addition of Cr and V.
Developed steel can improve both fatigue and sag resistance by
increasing residual stress on the surface while nitriding and
increasing inner hardness.
160
150
140
130
120
110
100
90 Fatigue
str
ength
(S
AE
9254=
100%
)
SAE9254 KHV10N KHV12N
Steel grade
High Fatigue Strength of Developed Steels
KHV10N can improve fatigue strength by 40% compared to SAE9254.
KHV12N can improve fatigue strength by 10% compared to KHV10N.
SAE9254 KHV10N KHV12N
Steel grade
120
100
80
60
40
20
0
Resid
ual shear
str
ain
(SA
E9254=
100%
)
High Sag Resistance of Developed Steels
KHV10N can decrease residual shear strain by 40% than SAE9254.
KHV12N can decrease residual shear strain by 30% than KHV10N.
Weight and height of valve spring can be reduced with our developed steels.
These can contribute to lighter automobiles and higher performance valve train.
Conclusions
The followings were acquired by investigating the effect of C, Si, Cr and V on
improving fatigue strength and sag resistance.
(1) Tensile strength was enhanced by increasing C content.
(2) Addition of over 2% Si allowed to improve sag resistance.
(3) Toughness was improved due to grain size refinement by increasing Cr
content and adding V.
(4) KHV10N and KHV12N, containing above mentioned elements, were
able to improve fatigue strength and sag resistance compared to SAE9254
Future works
・Improving fatigue strength and sag resistance of KHV12N
・Improving toughness of KHV10N, KHV12N
Addition to considering chemical composition and productivity,
relationship between toughness and microstructure will be investigated.
We kindly ask for your cooperation on developing new steels for lighter
automobiles and better valve train performance.
Process change on valve spring
quality (super clean) wire rod
Reforming the Structure of the Steel Business
Kobe
Works
Kakogawa
Works
Crude Steel Capacity : 1.4 million
tons/year
Products : Wire rods, Bars
Kobe Works Kakogawa Works
Crude Steel Capacity: 6.8 million
tons/year
Products : Wire rods, Sheets, Plates
Production approval has been required.
At the present, engine valve spring (super clean) material has been
manufactured at Kobe Works only.
Rebuilding the Business Foundation for Stability and Growth.
Shut down the upstream equipment including blast furnace at Kobe Works and transfer the upstream operation to Kakogawa Works in 2017.
Kobe Steel confirmed that there is no quality difference especially
cleanliness of steel between Kobe and Kakogawa Works.
Currently, our customers (wire makers and spring makers) have been
evaluating the properties to obtain approval from final customers.
Current status