ISIJ International, Vo[. 36 (1996), No. 7, pp, 855-861

Effect of Thermo-mechanicalof High-nitrogen Containing

Treatment

Cr-Mn-Nion Mechanical Properties

Austenitic Stainless Steels

Yuji IKEGAMIand Rikio NEMOTO1)

Technical Research Center. Research and DevelopmentDivision. Nippon Yakin KogyoCo., Ltd., Kojima-cho,

1) Products and DevelopmentDepartment, Research and DevelopmentDivision.Kawasaki, 21 OJapan.KogyoCo., Ltd., Kyobashi. Chuo-ku, Tokyo, I04 Japan.

(Received on September29. 1995, accepted in final form on January 18. 1996)

Kawasaki-ku.Nippon Yakin

To develop a high strength nonmagnetic stainiess steel with excel]ent toughness, thermo-mechanicalcontrol process (TMCP)wasapplied to hot-forged products of Cr-Mn-Ni austenitic stainless steels with

various levels of Cand N. The yield strength in the as-hot-forged condition increases with increasing

nitrogen contents. Carbon addition is detrimental to toughness because it promotes carbide precipitation

at the grain boundary during hot forging, causing intergranular fracture at 293 K. In a low carbon and high

nitrogen steel, 0.2•/• yield strength increases and toughness slightly decreases with lowering finish-forging

temperature, It was found that a steel with a chemical composition of Fe-0.06C-20Cr-1 5Mn-4Ni-2Mo-0,64N can be strengthened by TMCPto a level of more than I OOOMPain 0,20/• yield strength with

maintaining its ductility and toughness. This excellent strength-toughness balance can be explained byaustenite grain refinement and substructures generated by TMCPas well as strong solution hardening andgrain size hardening by nitrogen.

KEYWORDS:thermo-mechanical control process; austenitic stain]ess steel; high nitrogen steel; high

manganesesteel; strength; toughness; non-magnetic; hot deformation; recrystallization; grain refinement.

1. Introduction

Austenitic stainless steels have been widely used in avariety of industrial fields becauseof their superior cor-rosion resistance and excellent mechanical properties.

Besides these characteristics, austenitic stainless steels

showsparamagnetism, thus being used as nonmagneticstructural materials in the power-generation industry,

railways and nuclear industries. Relatively low yield

strength of austenitic stainless steels, however, is anobstacle to the applications.

In raising the strength of austenitic stainless steels,

nitrogen alloying has been gaining much attention

because N is a strong solid solution hardener, and hashigher solubility in austenlte than C, and improvescorrosion resistance. Nitrogen solubility in austenite

increases with increasing manganeseand chromiumcontentsl'2); therefore, nitrogen alloying can be applied

effectively to Cr-Mn or Cr-Mn-Ni austenitic stainless

steels. In addition, nitrogen increases grain size harden-ing.3'4) Therefore, its grain size should be as small aspossible to develop high strength austenitic stainless

steels.

It has been reported that thermo-mechanical control

process (TMCP)can give austenitic stainless steels grain

refinement and substructure hardening which raise

strength without much reduction in ductility andtoughness.5 ~ Io) However, there has been little study on

855

the application of TMCPto a high nitrogen alloyed steel

with a nitrogen level of more than O.3 masso/o.

In this study, TMCPwasapplied to hot forging using

high-nitrogen containing corrosion resistance austenitic

stainless steels with nitrogen concentration up to 0.6

mass"/o. The effects of alloying elements of Cand Naswell as forging condition on the mechanical properties

of the TMCPsteels were investigated in order to develop

an ultra-high strength nonmagnetic stainless steel.

2. Experimental Procedures

Chemical compositions of the materials used are given

in Table 1. Steel Nos. I to 7were melted by a 10 kg high

frequency induction melting furnace to examine the

effects of CandNcontents on the mechanical properties

of the steels in the as-forged condition. Nickel contents

were varied to obtain fully austenite microstructures.

The lOkg ingot was forged into a 40mmsquare billet.

The billet was reheated at 1423Kand then forged with-

out further reheating to produce a 25mmdiameter rod.

Finish-forging temperature was varied from I 073 to

1273K. The finish-forging temperature meansthe final

tapping temperature from a 23mmsquare rod to a25mmround rod. Tensile and Charpy impact tests were carried

out at 293K using specimens taken in the longitudinal

direction from the 25mmround rods.

To investigate the microstructural evolution during

@1996 ISIJ

ISIJ International, Vol. 36 (1996), No. 7

Table 1. Chemical compositions of steels tested. (masso/o)

Steel C Si Mn P S Ni Cr Mo V Nl

3456

AB

O.060,17

0,260.37

0.26

0,260,27

0.060.07

o,44

0.46

0.48

0.49

o.40

o.40

0.44

0.320.73

l5. 115.l

I5.0

14.8

15.2

14.7

15.2

15.3

14.5

0.0220.023

0.0230.023

0.0220.0220.0230.0120.029

0.007o.0070,0070,0080.0080.008

0,0070.0040.004

4. 152,02

2.02

2.08

4. 12l ,98

2, IO4.0 l4.24

20.2

20.3

20.4

20.420.2

20.6

20.6

20,016.5

1.99

l ,98

2.05

2.01

l .83

2.08

2.02

l .98

0.05

0.26

0.27

o.25

0.23

0.23

0.240.22

0.240.35

0.6550.605

0.6250.5860.4340.4860.5990.6400.330

1423KX600s deformation temperature(1 073 - 1373K)

cooling rate

=20Kls

/microstrucure

hardness

(a) Test condition for hot work hardening

1423KX600s

2nd deformation

cooling rate=20K/s

/

C:S

O*:~

J:O)C'

~CD.~~ao

Qdd

1200

11oO

1oOo

900

800

700

600

o

N: 0.59-0.660/0

-e- 0.2%yield strength

- {)- - Charpy impact energy

e

finish temperature = 1200K

e

b_

*o

Fig.

4

3

2

1

o

C~JE

\1':~

>c,)

(D,:e)

~CLE>Q(Q,:O

(b) Test conditon for measurementof softening ratio

l. Schematic diagrams of compression test conditions.

hot deformation and its effect on room-temperaturehardness, hot compression testing was performed in

vacuumusing cylindrical specimens with a diameter of8mmanda height of 12mm.Thespecimenwasmachinedfrom an 85mmround billet of Steel A. Its productionprocess will be described later. Figure l(a) shows theschematic diagram of compression test conditions. Afterbeing reheated at 1423K for 600s, the specimen waskept at a deformation temperature from I 023 to 1373Kfor 20 s, and deformed at a strain rate of I s~1 with atrue strain ranging from O.25 to I .2 (hereafter strain).

Figure 1(b) shows the schematic diagram of a two-stagecompression test which was carried out to examine theeffects of interpass time or holding time on micro-structures and a softening ratio. Thesoftening ratio wasevaluated by the following equation:

S ((T~ cr )/(a~-ayO)

where, S: softening ratio,

(T~: flow stress at a strain of O.2 in the first

deformation,cryo : flow stress at a strain of O.05 in the first

deformation, and(Ty: flow stress at a strain of O.05 in the second

C 1996 ISIJ 856

Fig. 2.

0.0 0.1 0,2 0.3 0,4

Ccontent (mass'/.)

Effect of Ccontents on 0.2 "/, yield strength and impact

energy at 293K for as-forged Cr-Mn-Ni steels,

deformation.The specimen was kept at the deformation tempera-

ture for 20 sanddeformedwith a strain of 0.2 at a strain

rate of I s~ 1, followed by the second deformation witha strain of 0.2. A11 the specimens were cooled using Arimmediately after deformation with a cooling rate of20 K/s.

To examine the effect of austenite grain size onstrength, 2mmthick sheets wereprepared by cold-rolling

5mm-thick hot-rolled sheets of Steel A and Steel B.Annealing temperature wasvaried from 1273 to 1423Kwith a holding time ranging from 30 to 120 s. Thegrainsize obtained through this process was ranging frorn 7to 35 ~m.

The microstructures were observed using an opticalmicroscope etched with an aqueous solution of 10010oxalic acid. The substructures were also observed using

a transmission electron microscope.

3. Results and Discussion

3.1. Effect of Non Strength and ToughnessComparedto C

Figure 2showsthe effect of Ccontents on 0.2 o/o yieldstrength andCharpyimpact energy at 293Kof the 25mmround rods in the as-forged condition with a finish-

forging temperature of 1200K. With increasing Ccontents, the yield strength slightly decreases, while theimpact energy drastically decreases. A rapid decrease in

ISIJ International, Vol. 36 (1996), No. 7

CUCL~J~a)Ca)

u)

lOa)'>1~~ao

C\J

O

1Ooo

900

800

700

600

500

Fig. 3.

O Charpy impact energy C= O26•/-

e 0.2•/• yield strength

finish temperature = 1200K

e

o___ ----O--- ~-------o~~~

4

3

2

i

o

c\sE

~i,E>o)q)

coocT:

CLE>c~cax:O

0.6 0.7O,4 O,5

Ncontent (mass'/.)

Effect of Ncontents on 0.2 "/, yield strength and impact

energy at 293K for as-forged CrMn-Ni steels.

Steel No. 3: 0.26C-20Cr-15Mn-2Ni-2Mo-0.63N

1173KX300s 1223KX300s

Steel N0.1 (C=0.060/.)

(a),(b)

Fig. 4.

(a)

Steel N0.3 (C=0.260/0)

(b)

Fig. 5. Microstructural changewith reheating for Steel No. 3.

1400

~ 1200

=i:~)a)(o

IOOO1:

.~)80

c\1

d 800

600

- ~)- - Charpy impact energy

H•- 0.2%yield strength

o'

o

• ---~~~ ~ ~ ' ~b ~~~ ~o

, ee

0.06C-20Cr-15Mn-4Ni-2Mo-0.66N

e

4

(d)

crosS Section (C),(d) : tractUred surface

Cross sections and fracture surfaces of impact test

specimens.

c\'E

'~3 1,~>C"

~2 o

~co

~E~CL

1 *,O

!::

O

toughness occurs whenCexceeds 0.2masso/o. On the

other hand, as shown in Fig. 3, the yield strength in-

creases and the impact energy remalns unchangedwithincreasing Ncontents.

Figure 4showsthe cross sections and fracture surfaces

of the impact test specimens of Steel No. I (C =0.06

masso/o) and No. 3 (C=0.26masso/o). Dimple patterns

were observed on the fracture surface of Steel No, I andthe fractured region was heavily deformed, indicating

that Steel No. I is highly ductile in the as-forged

condition. On the other hand, Steel No. 3 shows nodimple pattern on the fracture surface and intergranular

fracture Is predominant. Both carbide precipitation at

the grain boundaries and lamellar carbide precipitation

were observed In Steel No. 3.

Figure 5showsmicrostructures of Steel No. 3reheated

at I 173 and 1223K for 300s after solution treatment of

l 423K for I.8 ks. Graln boundary and lamellar carbide

857

o1050 11OO 1150 1200 1250 1300

Finish-forging temperature (K)

Fig. 6. Effect of finish-forging temperature on 0,2"/o yield

strength and impact energy at 293K in the as-forged

condition for Steel No, l.

precipitation wasobserved. It wasconfirmed that carbide

precipitation at the grain boundaries started within 30 sat 1223 K; thus, it is difficult to prevent carbides fromprecipitating during hot forging if the steel has a high

level of C.

With regard to the effects of alloying elements onthe mechanical properties of the as-forged steel, Nis significantly useful in increasing the yield strength.

However, Chas little effect on the yield strength and is

considerably detrimental to its toughness. Therefore,

reducing C contents and increasing N contents are

important to obtain a high strength steel without a loss

of toughness through TMCP.

3.2. Effect of Finish-forging Temperature on Strength

and ToughnessFigure 6showsthe effect of finish-forging temperature

on 0.20/0 yield strength and Charpy impact energy at

293Kin the as-forged condition for Steel No. I that has

a low carbon content of 0.06masso/o and a nitrogen

content of 0.66masso/o. By lowering the finish-forging

temperature, the yield strength increases and Charpyimpact energy decreases while the impact energy remainsrelatively high, considering a yield strength level of morethan I OOOMPain the case of a finish-temperature of

l 073 K.

C 1996 ISIJ

ISIJ International, Vol. 36 (1996). No. 7

(~i.

* ~~~,~;:s.

~~~h~~\~

"'e!~:s

'-r-~

~'::

~~~;~_ ~~' JT~ i~:

~._'~~~~~~_'~~'

'~;

}4~~~~:r_.~j• ~~ •

~ i~_ ~l~;~1'

''~~ i '~

/"r~:"-

Fig. 7. Optical micrograph of Steel No.condition.

ll40um1 in the as-forged

(a) SOlution-treated (b) aS COntrol-forged(a) Solution treatment :1323K XI .8ks water quenching 111(b) Finish-forging temperature : 1073K 1um

Fig. 8. Transrnission electron micrographs ofthe steel in the

solution-treated and as-control-forged condition.

500

1073K 1173K

.:~(*,* .

_;~

~.:

,

*:i~~l- ~

,~;

);'~'

!~~:~ *(

_,~f_*~'

\

+* ~;~* :,,--\~ ,:.~~'~~

~;1_~.

.

i~~~,'j~-

+~•t; ;~~

1373K ST: 1423KX600se;i.'

F j~~ '

'~.:\,

:~ ~t:~~~*, *

: +f.

:~~

.

,'i:

r,

~:

~~: ~~;.

~

:F ~~

l' :~.).~

!;:=:,/ :8=0.3, ~=1s~1 III

80umST: Solution treatment before deformation

Fig. lO. Microstructural change with deformation tempera-ture for Steel A (Fe0.06C-20Cr-15Mn~}Ni-2Mo-0.64N).

1,o

>Iu;

~co

I

0.8

450

400

350

300

250

200

eOAA

l073Kl 173K1273K1373K

O06C-20Cr-15Mn-4Ni-2Mo-O64N

8 = Is-1

cooling rate = 20K/s

Fig. 9.

OO O2 0.4 0.6 0.8 1.O I .2 1.4

True strain

Effect of hot compression temperature and strain onhardness at 293K for Steel A.

Theoptical and transmission electron micrographs ofSteel No. I with a finish-temperature of I 073Kare shownin Figs. 7and 8, respectively. It consists of slightly elon-gated fine austenite grains with an average grain size ofless than lOkamwith high dislocation density.

3.3. Hot CompressionTest

In hot forging, deformation temperature decreases asforgihg proceeds. In order to obtain fine microstructures,it is important to understand the effect of deformationtemperature and strains on microstructural changeandhardness. To this end, hot compression tests were per-formed. Figure 9 shows the effect of hot deformation

o_~(rf* 0.6

o).~::(D

'tto 0.4(D

0.2

0.0

Fig. Il.

0.06C-20Cr-15Mn-4Ni-2Mo-0.64N

1st

2nd.press : E= 0.2

press : g = 0.2

~= IS-1

holding time = 60s

C 1996

1150 1200 1250 1300 1350 1400

Deformation temperature (K)

Relationship between deformation temperature andsoftening ratio in the two-stage compression test.

ISIJ 858

temperature and strain on hardness at 293K for Steel

A. Hardness increases with lowering deformation tem-perature and increasing strain. Figure 10 showsmicro-structural changewith deformation temperature. Nore-crysta]lization wasobserved below a deformatlon temper-ature of I 173K. At 1373 K, fine recrystallized grains

were observed along the prior large grain boundarles.The relationship between deformation temperature

and the softening ratio measuredby the two-stage com-pression test is shownin Fig. Il, and the dependenceofthe softening ratio on deformation temperature and in-

terpass holding time is shownin Fig. 12. Below 1250Kwith a softening ratio of less than 0.2, recrystallization

barely occurs as shown in Fig. 13. At 1323 K, full

recrystallization and grain growth take place with aholding time of 60 s. Figure 14 shows microstructuralchange with holdlng time between the Ist and the 2nddeformation in the two-stage compression test. At

ISIJ International, Vol. 36 (1996), No. 7

1.o

0.8

,o

~ 06~'

c(D't:o 0.4a)

0.2

0.06C-20Cr-15Mn-4Ni-2Mo-O.64N

Ist. press 1323Kg = 0.2

2nd. press8 = 0.2

~= Is-1 1273K

o o

e

1223K

,T;

O-

~J~O)a)

1)(1)

.~~~ao

C\S

O

900

800

700

600

500

e steel A: N=0.64•/Q

L Steel B: N=0.33•/-

0.20laYS = 428 + 33.8d-1/2

AA

O.2010YS= 333 + 21 .3d-1!2

A

Fig. 12.

0.0

1O IOO IOOO

Holding time (s)

Dependenceof softening ratio on deformation tem-

perature and holding time.

ilholding time = 60s80um

Fig. 13. Microstructural change with deformation tempera-ture for Steel A,

1Os 30s\. I t J~,

~.~~ll$.~:_=_-

~~~*~~•,i~~~si"'

'~J,..1-~1 : *.~)~~

,~~_.1:: *~~,:.1 ,,~~

~i'Er,

~~~~~~~~~F~~~

*:s~,

1lrl~:5,

.

* *- ';~~~i,

= ~~;-.~;

_-.=*;~~L,~

~T'•.,~~:~:, t~~

II120s 80umdeformation temperature

= 1273K

-• ~f,, :11~

r."

Fig. 14. Microstructural changewith holding time for Steel A.

859

4004 6 iO 12 148

Grain size : d-1/2(mm-1/2)

Fig. 15. Hall-Petch relation forCrMn-Niaustenitic stainless

steels, Steel Aand Steel B.

1273K, the fraction of recrystallization increases withincreasing holding time, Ieading to a microstructure offine austenite grains.

In actual hydraulic press forging, interpass time is

approximately from 10 to 120 s, depending on the shapeof forgings and the type of press equipment. Based onthe above results, it can be said that forging at atemperature between 1273 and 1373Kcontributes tothe refinement of austenite grain size, and forging belowl 250K increases a strength level by work hardening.

3.4. Grain Size Hardening

Theeffect of nitrogen on strength is attributed mainlyto solution hardening andgrain size hardening. Theyield

strength of nitrogen alloyed austenitic stainless steels

can be described by the HallPetch relation, that is,

cry = (TO+ky d~ 1/2. It has been reported that the hardeningcoefficient ky becomeslarger with higher Ncontents.3'4)

As shown in Fig. 7, the steel produced by TMCPhasfine austenite grains. Therefore, to understand the

strengthening mechanismby TMCP,it is necessary to

evaluate howmuchgrain size hardening contributes to

the strength of the steel. Figure 15 showsthe HallPetchrelation of O.2 o/o yield strength at 293Kfor two nitrogenalloyed steel sheets of Steel A and Steel B in the

solution-heat-treated condition. Steel Awith a higher Ncontent has larger ky and (TO than Steel B does. Therelationship between austenite grain size and tensile

strength and hardness also is given in Table 2. As withyield strength, the hardening coefficients of Steel Aarehigher than those of Steel B. It is clear that the dependenceof the hardening coefficient of yield strength on grainsize is muchgreater than that of tensile strength.

Figure 16 shows the relationship between austenite

grain size and 0.2 o/o yield strength of the solution-treated

andcontrol-forged Steel A. In the case of a finish-forging

temperature above 1223K, the yield strength and the

hardening coefficient of the TMCPsteel is slightly higher

than the solution-treated steel. Grain size hardeninggreatly contributes to strengthening in this case. Onthe

other hand, Iowering a finish temperature, the yield

strength and the hardening coefficient of the TMCPsteel

C 1996 ISIJ

ISIJ International, Vol. 36 (1 996), No. 7

Table 2. Relationship between austenite grain size and mechanical properties.

Steel Ncontents (masso/o) Grain size range (d: mm) HallPetch relation

A 0.64 0,010O.035 0.2"/.YS (MPa)=428+33.8d - 1/2

TS(MPa)=808+22.0d - 1!2

HV= 185+ l0.8d~ l/2

(R2=0.99)

(R2=0.98)

(R2=0.97)

B 0.33 0,007-0,035 0.2~/.YS (MPa)=333+21.3d~ l/2

TS(MPa)=670+16.4d ~ 1!2

HV=161+7.8d-1/2

(R2 =0.97)

(R2 =0.97)(R2 =0.98)

YS: Yield Strength, TS: Tensile Strength, HV: Vickers hardness, R: correlation coefficient

Table 3. Typical mechanical properties of a control-forged 85mmround billet of Steel A.

0.20/.YS

(MPa)TS

(MPa)Elongation

(o/.)

Hardness(HRC)

Impact energy(MJ/m2) Permeability

TMCPST

l 180

540l 295

9702251

44 0.543.6

l .004

l .004

Required properties > I069 > I 172 > 10 >40 >O. l I.02

ST:YS:

Solution Treatment (1 323Kx I ,8 ks water quench)Yield Strength, TS: Tensile Strength, HRC:Rockwell Chardness

,TS

CL~:

!:C,,[a)

l:,

,D'>~~ao

C\l

C;

1500

1250

i oOO

750

500

0.06C-20Cr-15Mn-4Ni-2Mo-0.64N

Le

finished below I173Kfinished above 1223Ksolution-treated I

at 293K

l

~

I.

-Ar

' I

_A1

10 1284 6Grain size : d-1/2(mm-1/2)

Relationship between austenite grain size and 0.20/0

yield strength in the solution-treated and control-

forged conditions for Steel A.

>IQ;

~'o

I

Fig. 17.

500

450

400

Fig. 16.

is significantly higher than the solution-treated steel. Thisdifference in strength can be explained by work harden-ing or high dislocation density of the TMCPsteel.

The results obtained so far led us to conclude thatultra high strength of nitrogen alloyed Cr-Mn-Ni aus-tenitic steel processed by TMCPcan be explained by thestrengthening mechanismsof solid-solution hardening,grain size hardening and work hardening, all of whichare enhancedby higher Nconcentration.

3.5. Production of 85mmRound Billet by ControlForging

Ahigh-strength nonmagnetic steel with high ductility

and toughness is required for the material used in

connecting sleeves for large coaxial power transmissioncables. The required properties are listed in Table 3. Thespecification of the mechanical properties are nearly

350

Steel A : 0.06C-20Cr-15Mn-4Ni-2Mo-0.64N

Billet diameter : 85mm

C 1996 ISIJ 860

O IO 20 30 5040Distance from center (mm)

Hardness distribution of an 85mmround billet

of Steel A produced by thermo-mechanical control

forgoing process.

equal to 15-5PHstainless steel in H925condition speci-fied in ASTMA705. To meet this demand,the thermo-mechanical control forging wasapplied to a high nitro-

gen containing Cr-Mn-Ni steel (Steel A). The steel wasmelted by a 1500kg high frequency induction furnace.

A 500kg ingot wasmadeand hot forged to a 140mmsquare billet using a 1000t hydraulic press by the con-ventional forging process. The billet was reheated at

l 453K and subsequently forged into an 85mmroundbillet by a thermo-mechanical control forging process.Forging from 140mmsquare to 80mmsquare in crosssection was conducted between I 123 and 1373K toobtain recrystallized fine austenite grains. Start-forging

temperature from 80mmsquare to 85mmround in crosssection wascontrolled below 1123 K. The final tappingtemperature or finish-forging temperature was betweenl 023and I 073Kto obtain the required strength. Typical

ISIJ International. Vol. 36 (1996), No. 7

3

CWE

.+1':~

2>a)CD

oo,~'

QE.- 1>1c~cl'

cO

o

eee

e

JLee

e

e

e eee eee

600 800 1OOO 1200 14000.2'/. yield strength (MPa)

Fig. 18. Relationship between0.2"/* yield strength and impact

energy at 293K for Steel Aproduced by TMCP.

mechanical properties of the 85mmround billet are givenin Table 3and its hardness distribution is shownin Fig.

17. Thesteel obtained by the thermo-mechanical control

process exhibits ultra-high strength with good toughnessand ductility. Figure 18 shows the relationship between0.20/0 yield strength and Charpy impact energy at 293Kfor Steel Aproduced by TMCP.It clearly demonstratesthat TMCPcan impart Steel Aboth high strength andexcellent toughness.

4. Conclusions

Thermo-mechanical control process was applied to

hot-forged products of high-nitrogen alloyed austenitic

stainless steel to develop a high strength nonmagneticstainless steel. Thefollowing results were obtained in this

study.(1) Increasing N contents in Cr-Mn-Ni austenitic

stainless steel is useful in strengthening the TMCPsteel

without muchreduction in toughness.(2) AdditionofChas little effect on the yield strength

and is significantly detrimental to the toughness of thesteel because carbide precipitation occurs at the grain

boundary during hot forging, resulting in intergranularfracture in the impact test at 293K.

(3) 0.06C20Crl5Mn-4Ni-2Mo-0.64Nsteel can bestrengthened byTMCPto a level of morethan I OOOMPain O.20/0 yield strength with maintaining ductility andtoughness.

(4) Strengthening mechanismsby TMCPis explained

by grain size hardening, substructure hardening~ workhardening and solid solution hardening, all of which areenhancedby higher Nconcentration.

i)

2)

3)

4)

5)

6)

7)

8)

9)

1O)

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861 C 1996 ISIJ