BAIXAR OUTRO Austenite Grain Coarsening in Microalloyed Steels

7/27/2019 BAIXAR OUTRO Austenite Grain Coarsening in Microalloyed Steels

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Au steni te Gra in Coarsen ing in M icroa l loye d Stee ls

L . J . C U D D Y a n d J . C . R A L E Y

A uniform, f ine-grain s tructure is essent ia l in s teels , par t icular ly for s tr ip and pla te , that are to meetdem ands f o r h igh s t r eng th and toughness . To pr oduce such m ic r os tr uc tur es , eve r y s tep o f the h igh-

tem per a ture p r oces s ing o f the s tee l m us t be ca r e f u l ly con t r o l l ed , beg inn ing wi th g r a in coa r sen ing tha toccurs dur ing reheat ing for s lab rol l ing. Extremely coarse or nonuniform grain s tructures in the

r ehea ted s lab a r e d i f f i cu l t to r e f ine by subsequent ho t wor k ing . Accor d ing ly , the g r a in - coa r sen ing

behavior o f l abor a to r y hea ts o f C - Mn- S i base s tee l s and of such s tee l s wi th add i t ions o f A1, V, T i , o rNb was exam ined to u nder s tand the p r inc ip le s gover n ing the behav ior o f th i s c la ss o f s t ee l s . Thegr a incoa r sen ing tem per a ture ( the t em per a tur e a t which abnor m al o r d i s con t inuous g r ow th occur s )

va r ie s w i th the typ e and concen t r a t ion of the m ic r oa l loy add i t ion ; an appr ox im a te r e la t ion i s p r esen ted .

Gen eral ly the grain-coarsening temperature increases with, but is lower than, the tempe rature requiredf or com ple te d i s so lu t ion of the m ic r oa l loy ca r b ide o r n i t r ide p r esen t . Thus , s t ee l s con ta in ing theve r y inso lub le T iN coa r sen a t m uch h ighe r t em per a tur es than s tee l s con ta in ing the m or e so lub le

VCN. These r e su l t s agr ee qua l i t a t ive ly wi th p r ed ic t ions o f m ode ls o f g r a in - boundar y p inn ing byprecipi ta te par t ic les .

I . I N T R O D U C T I O N

THE grain-coarsening behavior of s teels dur ing reheat ing

for hot rol l ing is an impor tan t factor in the des ign o f thermo-mechanical process ing to achieve f ine-grained product . Inthe cour se o f s tud ies o f the r m om echanica l t r ea tm ent o fm ic r oa l loyed s tee l s , a body of g r a in - coa r sen ing da ta has

been accumulated for C-Mn-Si base s teels and for s teels ofthis base composit ion with addit ions of A1, V, Ti , or Nb.These observat ions are summar ized, and the pat terns that

evolve are interpreted in terms of mo dels of pinning o f grainboundar ies by precipi ta te par t ic les .

Mo st mo dels o f par t ic le pinning of grain boun dar ies 1-~ are

m odi f ica t ions o f the Zen er concep t and the r e f ore r educe toa s imilar form ulat ion. Z ene r ' s m odel 1 balances the force( 27 /R) tha t a r is e s f r om the dec r ease in g r a in - boundar y a r ea

pe r vo lum e ( thus ene r gy) dur ing the g r owth o f an i so la ted

grain of radius R (assu med equ al to the radius of curvature)and gr a in - boundar y ene r gy 7 , aga ins t the p inn ing f o r ce

(TrrnsT) cau sed b y a d ens ity ns of par t ic les of radius r in the

boundar y . F or p inn ing , n s r = E l / R , wher e s = 2 /7 r f o r theZener f o r m ula t ion , bu t va r ie s wi th the de ta i l s o f the m ode l .I f the r e i s no segr ega t ion to boundar ie s o r sweeping up ofpa r ti c le s by bo undar ie s , the boun dar y s im ply sam ples the

volum e ave r age , n v, of par tic les that l ie within + r of the

p lane o f the boundar y . Thus , the p inn ing condi t ion i s

n v R r 2 = ~' [1]

where ~ ' = 1 7r in the Zener mo del . In terms of the volum efract ion of spher ical par t ic les , iv = 4/ 3 7rr3nv, the re la t ion

becom esd

D = s ' -- [ la ]i v

where the diameters d = 2r for par tic les and D = 2R forgrains . In Zener ' s model ~ :" = 2/3.

G l a d m a n ' s m o d i f ic a t io n2 cons ide r s the abno r m al g r owthof a large grain into a matr ix of smaller , pinned grains . In

L . J . C U D D Y , A s s o c i a t e R e s e a r c h C o n s u l t a n t , an d J . C . R A L E Y , R e -

s e a r c h M e t a l l u r g ic a l A n a l y s t , a r e w i t h U . S . S t e e l C o r p o r a t i o n , R e s e a r c hL a b o r a t o r y , 1 2 5 J a m i s o n L a n e , M o n r o e v i l l e , P A 1 5 1 4 6 .

M a n u s c r i p t s u b m i t te d O c t o b e r 1 , 1 9 8 2 .

this case ~:" = 7r /6 [ (3/2 - 2/Z )] whe re Z is the ra t io of

d iam ete r s o f the g r owing and p inned gr a ins . F or the nor -

m a l ly obse r ved r ange o f Z = ~ /2 to 2 , ~ " = 0 . 05 to 0 . 26 .He l lm an and Hi l l e r t3 cons ide r ed the e f f ec ts o f the r a t io o f

boundary curvature radius to par t ic le radius , the re la t ionbetween curvature and grain-s ize dis tr ibut ion, and par t ic le-

s ize dis tr ibut ion to ar r ive a t approximate re la t ions for nor -

m a l and abnor m al g r a in g r owth . F or sm a l l vo lum e f r ac t ionsof par t ic les , ~:" = 4/ 9 for normal growth , and ~:" < 4 /3 for

abnor m al g r owth .F igur e 1 shows the s ize D o f the g r ains p inned by a g iven

va lue o f d / i v acco rding to these three mo dels . 1 '2 '3 The

pr ed ic ted s ize va r ie s by o ve r an o r de r o f m agni tude , depend-

ing upon the a s sum pt ions o f the m od e l . I n a l l the m ode ls ,however , the same trend exis ts . To maintain a f ine grain

s ize requires a large volume f ract ion of very f ine precipi-

t a te s . These two pa r am ete r s , f v and d, are f ixed ini t ia l lyby the l eve l o f m ic r oa l loy add i t ions and the p r io r the r m o-m echanica l t r ea tm ent o f the s tee l . However , changes tha t

occur dur ing subsequent r ehea t ing f o r ho t r o l l ing , f o r ex-a m p l e , d e p e n d e n t i r e l y o n t h e r a t e o f d i s s o l u t i o n o fpa r ti c les , w hich i s f ixed by the type o f m ic r oa l loy add i t ion .

Par t ic le dissolut ion has tw o ef fects : a direct ef fect in reduc-ingfv and an indirect ef fect of increas ing the ra te of par t ic le

r ipening ( increas ing d) by providing the higher solute level

103 f10

d / f v , / z m

F i g . 1 - - R e l a t i o n b e t w e e n g r a i n s i z e , D, p a r t i c l e s i z e , d , a n d v o l u m e

f r a c t i o n , f v , p r e d i c t e d b y 3 m o d e l s .

M E T A L L U R G IC A L T R A N S A C T IO N S A V O L U M E 1 4 A , O C T O B E R 1 9 8 3 - 1 9 8 9

mperatura descimento de grãomentosoligantes

ueio dopelasculas

elo der

man

He

H



necessary for greater mass transport. Such a pinning model

predicts, for example, that boundaries pinned by highly

insoluble TiN particles would resist growth to higher

temperatures than boundaries pinned by a similar initial

distribution of the more soluble VCN.

In the absence of detailed measurements of the relation

between particle size and density, we attempted to derive a

phenomenological relation between steel composition and

coarsening behavior. In particular, a relation was sought

between the grain-coarsening temperature and the type and

concentration of microalloying elements, the factors that

regulate particle dissolution.

I I . E X P E R I M E N T A L

15 minutes for temperatures between 1150 and 1250 °C.

For higher temperatures or longer times the specimens

were heated in capsules back-filled with argon. Prior exami-

nation had shown that these procedures limited the surface

layer that was damaged by decarburization and oxygen dif-

fusion to less than 0.5 mm. The heat treated specimens

were quenched in iced brine and surface ground to remove

2 to 3 mm of material.

For metallographic examination, specimens were pol-

ished and etched by swabbing in a saturated aqueous

picral solution heated to 80 °C. If the resulting structure

was not clear, specimens were tempered for 17 hours at

-500 °C and re-etched. Grain sizes were determined by

linear intercept or, for large grains, from grain-size charts.

Grain coarsening in microalloyed steels was studied by

annealing samples at temperatures between 900 and

1300 °C. These steels were prepared from 135 or 225 kg

vacuum-melted heats that were cast as 125 mm or 175 mm

slabs. The base steels were nominally 1.4Mn, 0.01P, 0.01S,

and 0.25Si. To these were added various levels of C, N, A1,

V, Ti, or Nb, as shown in Table I.

The ingots were reheated to 1250 °C and rolled to 25 mm

plate, finishing at or above 1100 °C, and air cool ed.Samples 25 by 25 by 6 mm were cut from the as-rolled plate

for grain coarsening heat treatments. They were heated in air

for 30 minutes at temperatures up through 1150 °C, and for

I I I . R E S U L T S

The grain coarsening behavior as a function of tem-

perature in three types of steels is shown in Figure 2. With

increasing temperature, grain coarsening in the C-Mn base

steels is continuous, producing a gradual increase in size of

relatively uniform equiaxed grains (Figure 3(a)). Grain

growth in the microalloyed steels is, however, discon-

tinuous. Growth is suppressed at the lower temperatures,presumably because o f pinning of grain boundaries by alloy

carbides or nitrides. At higher temperatures, isolated grains

begin to grow rapidly at the expense of the fine surrounding

T a b le I . T e m p e r a t u r e s f o r C o m p l e t e D i s s o l u t i o n o f M i c r o a l l o y C a r b i d e s o r N i t r id e s , T ~,a n d C o r r e s p o n d i n g G r a i n - C o a r s e n i n g T e m p e r a t u r e s , T o, i n t h e E x p e r i m e n t a l S t e e l s

Steel C, Wt Pct N, Wt Pct Microalloy, Wt Pct Prec ipitate Ts, °C Observed To, °C

A 0.065 0.007 . . . .B 0.09 0.002 . . . .

C 0.065 0.005 0.068 A1 A1 N (1) 1230 1090D 0.08 0.006 0.10 A1 A1N ~) 1320 1050E 0.068 0.006 0.035 A1 A1N~" 1165 1000F 0.076 0.019 0.043 A1 A1N l) 1370 1100G 0.067 0.017 0.090 A1 A1N ) 1490 1200H 0.070 0.031 0.057 A1 A1N > 1515 1200

I 0.078 0.006 0.098 V VN~2) 995 980J 0.08 0.013 0.11 V VN~2) 1070 1050K 0.07 0.017 0.075 V VN~z) 1060 1000L 0.074 0.018 0.25 V VN~2) 1180 1050M 0.063 0.017 0.198 V VN~2~ 1150 1050

N 0.069 0.007 0.059 Ti Ti C ~3) 1090 1090O 0.053 0.001 0.225 Ti Ti C (3) 1225 1150

P 0.069 0.008 0.019 Ti TiN ~2> 1690 1250Q 0.068 0.009 0.020 Ti TiN~2) 1710 1250R 0.073 0.005 0.020 Ti TiN<z) 1645 1200S 0.079 0.002 0.016 Ti TiN~2) 1535 1100

T 0.077 0.007 0.057 Nb NbCN°) 1205 1090U 0.08 0.011 0.05 Nb NbCN 3) 1200 1150V 0.055 0.015 0.023 Nb NbCN°~ 1065 1100W 0.058 0.014 0.044 Nb NbCN°~ 1150 1150X 0.054 0.014 0.066 Nb NbCN~3) 1195 1150Y 0.071 0.004 0.011 Nb NbCN~3) 995 1000Z 0.075 0.003 0.046 Nb NbCN~3> 1165 1100

AA 0.064 0.017 0.048 Nb NbCN~3~ 1175 1140BB 0.06 0.015 0.11 Nb Nb CN (3) 1285 1200

(1) Reference 5(2) Reference 6(3) Reference 7

1990- VOLUME14A,OCTOBER1983 METALLURGICALTRANSACTIONSA

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Argô

Ata

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Miccredessus

btem

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duaI-

a

_z<re

ZwV-

<

>

300 I m I

C-Mn /Z~

2OO

O STEELA

Z~ STEELB100

0

2OO

• STEEL

100 rl STEELK

Nb

200 j

100

STEELAA

• STEELW0 I I900 1000 1100 1200 1300

SOAKTEMPERATURE, C

Fig . 2 --Gra in coarsen ing behavior in 3 types of s tee l s (30-min soaks) .Steels are identified in Table I.

grains. This prod uces a duplex structure (Figure 3(b)) repre-

sented by the cross-hatching in Figure 2 at - 1000 °C for theV steels and at 1150 °C for the Nb steels.

Different samples of the same steel exhibited quite re-producible grain-coarsening behavior. How ever, for a given

family of steels, the temperature at wh ich discontinuousgrowth occurs varies with the concentration and distribution

of precipitate particles. The thermomech anical histories of

all specimens used were standardized to minim ize the vari-

ation in initial particle distribution. No direct observationsof the initial particle distributions were made. However,

the observation that upon reheating to lower temperatures(900 to 1000 °C) the specimens all contained grains of about

the same size (15 to 20 ~m) suggests that the initial par-ticle population densities were comparable in all the steels.

Thus, the variation in the coarsening behavior in any familyof microalloyed steels is believed to be due primarily tothe range of microalloy concentration. For example, Fig-

ure 4 illustrates an increase of about 200 °C in the grain-coarsening temperature of Nb steels as the Nb content is

increased from 0.01 to 0.11 wt pct.

The con centration effect may be illustrated by plotting thegrain-coarsening temperature vs the microalloy concen-

tration for four types of steels (Figure 5). Such a co rrelationfails to recognize that the thermal stab ility of the precipitateis dependent upon the concentration of C or N with which

the microalloy interacts. Specifically, the equilibrium tem-

perature at which a p recipitate completely dissolves depen dsupon the product of the concentrations of the interactingspecies, assuming equality of concentration and activity.

For the dissolution of a simple carbide MC to its soluble

components, MC ~ [M] + [C], the product of the solublecomponents (the solubility product) can be expressed by

100/Jm

(a) (b)Fig. 3 - (a) Continuous grain grow th in base steel heated 15 rain at 1000 °C. (b) Discontinuous grain growth in AI steel heated 15 rain at 1000 °C.

METALLURGICALTRANSACTION S A voL UM E 14A, OCTOBE R 1983 m 1991



200

0 0.011 Nb

O 0.048 Nb

O 0.11 Nb

= -uJ

<

z<~¢z:

<~ruJ><

100

0.011Nb

O0.048Nb

0.11Nb

I o !1

0 I

9 0 0 1 1 0 0 1 3 0 0

SOAKING TEMPERATURE, °C

F i g. 4 - - E f f e c t o f N b c o n t e n t o n t h e g r a i n - c o a rs e n i n g te m p e r a tu r e .

H a t c h e d b a r s i n d i c a t e t h e r a n g e o f d u p l e x g r a i n s p r o d u c e d a t t h e c o a r s e n -

i n g t e m p e r a t u r e s .

t I I I

I I I

1300 TiN

~: 1200

0 A IN

v,

u~ 1000

< CN

8z - I I / / O A , N

o o o / -

8 0 0 r~ T i N

0.00 0.04 0.08 0.12 0.16 0.20 024 0.28

M IC R O ALLO Y C O N C EN TR ATIO N , a tom %

F i g . 5 - - I n c r e a s e i n th e g r a i n - c o a r s e n i n g t e m p e r a t u r e o f 4 t y p e s o f m i c r o -

a l l o y e d s t e e l s w i t h i n c r e a s e i n t h e m i c r o a l l o y c o n c e n t r a t i o n .

l o g [ M ] [ C ] = F - Q/ T wher e Q a nd F a r e cons tan t sknow n f r om publ i shed so lub i l i ty da ta , and T i s the abso lu tetemperature.5'6'7 Thus, f o r a s t eel wi th know n conten t s o f M

and C , the t em per a ture T , a t which these a r e com ple te ly in

solut ion is

QL = [21

F - log(M • C)

The co ncentra t ions of the interact ing species , the precipi ta tef or m ed , and the t em per a tur e r equ i r ed f o r com ple te d i s -solut ion of that quanti ty of precipi ta te are l is ted for each

steel in Table I . Also l is ted is the observe d grain-coa rsening

temperature for each s teel , (To) . The gra in-coarsen ing tem-peratures are plot ted agains t the temperatures for complete

precipi ta te solut ion in F igure 6.Over the r ange o f com pos i t ions exam in ed the r e i s a l inea r

r e la tion be tween the g r a in coa r sen ing tem per a tur e and the

so lu t ion t em per a tur e o f the m ic r oa l loy pr ec ip i ta te in each

t y p e o f m i c r o a l l o y e d s t e e l . T h e g r a i n - c o a r s e n i n g t e m -perature general ly l ies below the solut ion temperature , the

d i f f e rence inc r eas ing wi th inc r eas ing so lu t ion t em per a tur e ,

that is , with increas ing thermal s tabi l i ty of the precipi ta te .How ever , in no case does the g r a in - coa rsen ing tem per a tur e

r ise above about 1250 °C for 30-minute soak t imes .The e lem ent m os t e f f ec t ive in inh ib i t ing g r a in g r owth i s

T i . Whe n T i s t ee l s a re r ap id ly coo led f r om the m e l t , a s wer e

these heats , a dispers ion of very f ine , near ly insoluble , TiNpar t ic les forms. When the s teels are reheated, this ar ray ofpa r ti c le s p ins the g r a ins a t a s i ze o f abou t 20 / ~m d iam ete r.

This g r a in s t r uc tur e r em a ins s tab le to qu i te h igh tem -per a tu r es ; f o r exam ple , in 15- m inute annea l s the 20 /zmgr a ins wer e s tab le to t em per a tur es be tween 1200 and

1250 °C (Figure 7(a)) ; in iso therma l anneals a t 1200 °C the

s t ruc ture r e s is ted abnor m al g r owth f o r be tween 90 and 120m i n u t e s ( F i g u r e 7 ( b )) . P r e l i m i n a r y s t u d i e s o f c a rb o n -

extract ion repl icas 8 reveal that the pinnin g is cau sed b y TiNpar t i c le s whose ave r age s ize i s 100 A and whose num berdensi ty is approx imately 1 × 1015 cm -3. The Zen er mo del(Eq. [1] ) predicts that this ar ray would produce pinned

gr a ins appr ox im a te ly 13 gm in d iam ete r which agr ees rea -sonab ly wi th the obse r ved s ize o f 20 /xm . Dur ing gr a incoa r sen ing , the pa r t i c le s g r ew to an ave r age d iam ete r o f

160 A, but the dis tr ibut ion is less uniform.

I V . D I S C U S S I O N

The resul ts of this s tudy cor roborate in a qual i ta t ive waythr ee o f the p r ed ic t ions o f m o de ls f o r the p inn ing of g r a inboundar ie s by pa r t i c le s . ( 1 ) The leas t so lub le pa r t i c le s

( TI N) im pede bou ndar y m ot ion to the h ighes t t em per a tur e .( 2) The h igh e r the p r oduc t o f the concen t r a t ions o f the in te r-

ac t ing e lem ents , the h ighe r i s the g r a in - coa r sen ing tem -perature . (3) The grain-coarsening temperature l ies below

the pa r t i c le so lu t ion t em per a tur e because boundar y un-pinning requires that the par t ic les dissolve and grow only tothe extent that the pinn ing force fa l ls below a cr i t ical value.

, , f '

rl TiN /

; i N / N + , A ,,o1o .++° / / t , /

+ + "o . / P + o / /0 Ti N

1000

++ /

800 1000 1200 1400 1600

SOLUTIONTEMPERATURE,C

F i g . 6 - - R e l a t i o n b e t w e e n o b s e r v e d g r a in - c o a r s e n i n g t e m p e r a t u r e o f a u s -

t e n it e a n d c o m p u t e d t e m p e r a t u re f o r c o m p l e t e d i s s o l u t i o n o f t h e m i c r o a ll o y

c a r b i d e s / n i t r i d e s i n t h e a u s t e n i t e ,

1992- -VO LUM E 14A, OCTOBER 1983 METALLURGICAL TRANSACTIONS A

Relaen

res

tem

cregerest

de T

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temaumo au

Titânio

BAIXAR OUTRO Austenite Grain Coarsening in Microalloyed Steels

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