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