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Comparison of mechanical properties of HSLA steel deformed by hot rolling and by flat compression under simulated conditions
Journal of Mechanical Working Technology, 5 (1981) 45--68
45
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
COMPARISON OF MECHANICAL PROPERTIES OF HSLA STEEL DEFORMED BY HOT ROLLING AND BY FLAT COMPRESSION UNDER SIMULATED CONDITIONS
O. PAWELSKI
Max-Planck-Institut flit Eisenforschung GmbH, Diisseldorf (West Germany) and V. GOPINATHAN*
Indian Institute of Technology, Madras-36 (India) (Received April 21, 1980; accepted October 2, 1980)
Industrial Summary Steels required for the transportation of oil pipes must have a very low ductile-brittle transition temperature apart from good mechanical properties because of the severe atmospheric conditions to which they are subjected. To achieve this, either alloying or optimization of the rolling mill schedule with micro-alloy additions is employed. Because of the economic advantages the latter method is becomingpopular, but the main difficulty of this method is that the optimum rolling schedules are not available for all steel compositions. To achieve this, several laboratory simulation tests have been tried; one such test is fiat compression. In this work, the validity of flat compression as a laboratory test to simulate hot rolling conditions is investigated. To this end, rolling and fiat compression tests were carried out under similar deformation conditions at different temperatures between 850 and 1100°C using Nb micro-alloyed steel NV4-4. From the rolled and the fiat compression test specimens, tensile, hardness, impact and structural test-pieces were taken and tested, and the results obtained then compared for the two processes. The results obtained show that the hardness distribution, the impact strength, all the tensile properties and the austenite grain shape, size and distribution, agree very well for rolling and the fiat compression test, under similar deformation conditions. Based on these results it is concluded that the flat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the optimum rolling schedule.
1. Introduction Steels used f o r the t r a n s p o r t a t i o n o f oil pipes must n o t onl y be strong t o withstand th e oil pressure but t h e y must also n o t fail under the severe atmospheric conditions -- in m a n y occasions below --60°C -- t o which t h e y are subjected. Because o f these severe constraints the steels selected for the m a n u f a c t u r e o f oil pipes must have a very low ductile-brittle transition *Present address: Central Metal Forming Institute, HMT Limited, Hyderabad-500 854, India.
0378-3804/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
46 temperature, apart from having good other mechanical properties. In the last decade, several developed countries have been engaged in finding a suitable material for oil pipes. In general, there are t w o possible methods b y which the steel can be given the required properties for oil transporation purposes. These are: (i) b y the addition of alloying elements like Ni in amounts more than 5 to 10%, (ii) by the optimization of rolling mill schedules with micro-alloy additions to the steel. The main difficulty of the first method, which was widely used until recently, is that it is costly and the alloying elements are scarce. In the second method, elements such as Nb, Ti, Mo, V are added to low carbon steel in amounts varying from 0.01% to about 0.5%, either individually or in combination. When this steel is roiled under the so-called optimum roiling conditions it is then possible to obtain the required properties. The obvious advantage of this technique is that it is highly economical and the costly alloying elements can be spared for other purposes. The main drawback of this technique, however, is the establishment of the correct optimum rolling schedule. Even slight changes in any one of the rolling parameters -- such as rolling temperature, number of passes, deformation in each pass, strain rate in each pass, cooling rate between passes and at the end of roiling, austenitizing temperature, etc. - - c a n alter the mechanical properties. The influence of these parameters on the structure and properties of HSLA steels has been studied b y many authors [1--16], b u t despite this the o p t i m u m rolling schedule is n o t available for all steel compositions. Several research institutions and industries are currently devoting their efforts and resources to establishing this o p t i m u m rolling schedule for different steel grades. As mentioned earlier, the parameters involved are so numerous that for establishing the optimum rolling schedule several thousand experiments have to be carried out. To do this in industry, under actual roiling conditions, is n o t only time consuming b u t is also costly and measurements are difficult. Hence, several laboratory simulation tests have been developed to tackle this problem, the four c o m m o n l y used being: cylindrical compression, tension, torsion and flat compression. Each test has its own merits and drawbacks, which are described elsewhere in the literature [ 1 7 - - 1 8 ] . The details of individual tests have been described by several authors [19--23]. The requirements of a good simulation test for rolling have been described by Pawelski et al. [ 2 4 ] . A study of these different tests clearly shows that the flat compression test meets most of the requirements of a rolling simulation test, and several research programmes are currently being carried o u t using this method. Naturally the question arises, h o w well do the results obtained in a flat compression test compare with those obtained under actual rolling conditions? Little information is available in the literature to answer this question. Work has thus been carried out to compare the mechanical properties of steel deformed b y h o t rolling with those of steel deformed b y the flat corn-
47
pression test, at different forming temperatures, the results of which investigation are reported herein. Further w o r k carried out to compare the material flow and the deformation resistance will be reported at a later date
[25]. 2. Experimental procedure Experiments were carried out using Nb micro-alloyed steel NV4-4, the composition of which is given in Table 1. The mechanical properties of the steel in the as-received condition are shown in Table 2. All the rolling experiments were carried o u t using a 2-high rolling mill with a roll diameter of 180.3 mm and a barrel length of 200 mm. Before rolling, the specimens were austenitized either at 1226 or 1250°C for 1 h in an argon atmosphere and then cooled in still air until the required rolling temperature was reached. When rolling in several passes, immediately after the first pass the mill was reversed and the t o p roll adjusted to the new height, rolling then being continued. After rolling, the specimens were either cooled in still air or quenched in water or cooled in a furnace held at 600°C, according to the requirements. The specimen temperature was controlled during and after rolling by inserting a chromel-alumel 2 mm outer
3. Results and discussions 3.1. Hardness Figure 3 shows the hardness distribution obtained along the width and thickness directions of specimens rolled at different temperatures and in the as-received condition. The hardness variation curves across the thickness show that the hardness at the centre is very low, and that the hardness then increases with increasing distance from the centre, reaching a maximum value at a b o u t 2 to 3 mm from the surface, thereafter decreasing as the surface is approached. This trend is observed for all of the specimens tested. In the asreceived condition, however, such marked variation in hardness across the specimen thickness is n o t observed. Similarly, along the width direction the hardness remains generally constant for all rolling temperatures, except near the edges, where the hardness decreases. *Abbreviation for the german word "Warmumformsimulator".
0.14
C
0.22
Si 1.43
Mn 0.021
P 0.001
S
Chemical c o m p o s i t i o n ( w e i g h t %)
0.04
Nh 0.015
N
NV44
Steel t y p e
o"U
8L=25 mm
407 410 423
542 542 542
26.4 28.4 29.4
( N / r a m : ) ( N / r a m 2) (%)
ey
Tensile p r o p e r t i e s
~
70 72 71
(%)
°'Z
1229
( N / m m 2)
130.8
+19°(3
128.3
--10°C
Impact strength a K ( J / c m 2)
102.4
--40°C
21.4
+19°C
21.2
--10°C
17.3
--40°C
Lateral e x t e n s i o n ( b m a x - - bo)/bo (%)
a y = yield or 0.2% p r o o f s t r e n g t h , o u = u l t i m a t e tensile s t r e n g t h , 5 = f r a c t u r e e l o n g a t i o n , ~ = r e d u c t i o n in area o f cross-section, o z = true f r a c t u r e s t r e n g t h , aK = C h a r p y i m p a c t s t r e n g t h u s i n g D V M K s p e c i m e n as p e r D I N 50 115, bma x = m a x i m u m w i d t h o f t h e i m p a c t s p e c i m e n , b0 = initial w i d t h o f t h e i m p a c t s p e c i m e n .
Mechanical p r o p e r t i e s o f t h e steel tested, in t h e as-received c o n d i t i o n
TABLE 2
NV4-4
Steel t y p e
Chemical c o m p o s i t i o n o f t h e steel t e s t e d
TABLE 1
QO
49 -250 --
I I.
I i
roiling direction
-10~--
4
(~) Hardness specimen
I
z,5.2 o x---/ i Lz,-z0.6 ,4 (~) Impact specrmen rL
l n--
'
I
L
-
-
~
-
,
u~ M
1
.
8
(]) P-8 ~'
120
60
"t .4
(~) Tensile specimen
LI
Fig. 1. Location and dimensions of different test-pieces from rolling experiment specimens.
.
7.
r=0.25-0.025 ,, ~
.
~i
@ Impc]ct specimen
@
Tensile s p e c l m e n
i
,
@ Hordness and structural specimen
i'
f
Aft dimensions
in m m
Initial length =11,0
Fig. 2. Location and dimensions of different test-pieces from fiat compression test specimens.
In order to study this variation in a more detailed manner, hardness measurements were m a d e at 1 m m intervals across the thickness and at 2 or 4 m m intervals across the width over the width X thickness area of the rolled specimens. Figure 4 shows the hardness profile obtained in one half of the specimen in the as-received condition. It can be seen that the specimen is harder at the centre and that the hardness decreases with distance from the middle. Figures 5--7 s h o w similar hardness distributions after rolling at three
0
I
o "1-
-8
'- 350
"r375
>
o400
425
3
B
o
0
0
i
-4
~,,, I"J
0",
u'l
0
c/I 0
z,
f
!
I\
o
?
-4 uq
'
0
350
-8 Distance
o
4O0
425
o
oq
-4 from
0 the
/~/ ~'~
-:~
"4
0 0
I
~'1
0
-8
350
~00
8 mm
,.ozbJ i
~00
~,25
NV10
4 8 -8 -4 0 4 c e n t r e a | o n g the t h i c k n e s s ,
,I
L
J
~
o
Hardness
/
0
-
-4
0
~li ~
U'I
/
\
/,,
\
>
Ln
0
8-10
-6
125' t
150
175
200
o o
cI
-
D..
2.
-2
0
2
L.
6
>.. r"'
7
>
01 0
51
different temperatures and water quenching after rolling. Here, an inverted trend can be observed, viz. the hardest region is not at the centre but is about 2 to 3 m m below the surface on both sides o £ t h e specimen. Such a trend is
width :100ram HVl0 160-165
ho:20.3 mm
155-160 [~
150-155
i~5-15o
Fig. 4. Hardness distribution in a typical test specimen in the as-received condition.
width = 100ram
II Q
HV 10
B
~oo -~5
"~
direction of rolling
1226°C/1 h
/
[ ~
~,~o c water quenching
3 7 0 - 390 '(370
ho =20.3ram
6t =32.75%
ht : 13.65mm
~l : 0397
\
deformolion t
Fig. 5. Hardness distribution in a specimen rolled at 1100°C and water quenched after rolling.
Fig. 3. Hardness distribution along the width and thickness directions of specimens rolled at different temperatures and in the as-received condition.
52
• :::?~!.::}~-< ~; " :::::::::::::::::::::::::::: ...... --:~r.~ --I-~i.;?::: ................. ~ ; ~ ; : , ' ~ : t ~
i
~ldth : 100ram = HVlO
(~ direction of roiling
~.oo /45
1250°Cnh
380 - 400
/'
// "x.~,,,..~deformotion
\,~
//
L_2 36o- sso
,/
L ~ 5]
/
_
h° :20.3ram
8:26.1%
h, :15.0rnm
~ :0.303
'
Fig. 6. Hardness distribution in a specimen rolled at 900°C and water quenched after rolling.
width = ]UUmm HV10
[:::3 360 380 /~ 360 deformot ion
Q direction of roiling l~ I
1250°C/1h
/
r~s°~°~s~3~-'l"\
ho =20.3ram h :lS,7mm i
61 =22.7% ~ =0.257
~-weter quenching
\ t
Fig. 7. H ~ d n ~ rolling.
d ~ t r i b u t i o n in a s l ~ e i m e n rolled at 8 5 0 ° 0 and w a t e r q u e n c h e d after
observed not only for water cooled specimens but also for air cooled specimens, even though the absolute hardness value is less in the latter case. Such a distribution of hardness can be due to any of the following 3 reasons: (i) variation in cooling rate across the specimen thickness; (ii) variation in strain (deformation due to rolling) across the specimen thickness; and
53 1250OC/lh n
•ol I "
•
water
• .7. •
-
o
' --
"a "'al.%~ °
~)ebe
"
""
20
....
/
\
/
o
o o •
o.%° '
30
•
o
~, ~
~
• *
0,%0 • !-e
• o
•
~
h o = 20"3 m m h, = 1 4 . 5 m m
/
V
~1 = 2 8 . 6 %
~4 =0a37
~
40
quenching]
g50°C
r
• screw l e f t oscrew right
go .cmo
o a
°~o°
o o
rolling
•
~---
direction
2O
t o~ 10 ¢-.
4O ++ : omPOo o •o ~oo i
m30 ID r" 2g
•
" ~.
•
•
Op
D ii Ii I
oo., uOeOo , o°
_
9
IV: ....
o
o•
2 20
F-
10
Z.O
o,
== o
30
•
• "o
o
• i~
o D
•
• u
o
'e
direction~
,~-" I I
° oC
,%
o. . . . "
o •
o
~ o o
: o -"
before after
rolling/ rolling
20 0
2
4 Thickness,
mm
Fig. 8. Strain variation acro~ the thickness o f a rolled specimen at different locations. "screw left" and "screw right" refer to the left-hand-side and the right-hand-side of the screw, respectively.
54 (iii) variation in chemical composition across the specimen thickness. Based on the cooling curves calculated using the finite difference equation given by Schmidt [27] it was found that the m a x i m u m variation in temperature between the centre and surface is of the order of 20 to 30°C, which is too small to explain the marked difference in hardness, reported earlier. The measured variation in strain -- obtained using the screw technique [25] - - a t different locations of the rolled specimen is shown in Fig. 8. These results show that within the measurement scatter the distribution of thickness strain across the thickness is constant over the entire width of the specimen, indicating that the hardness variation reported earlier is also n o t due to the strain distribution. A measurement of the chemical composition variation across the specimen thickness showed a drastic reduction in C content (from 0.14% to 0.04%) and nitrogen c o n t e n t (from 0.017% to 0.014%) near the surface. The variation of these elements across the thickness is comparable with the hardness variation reported earlier, indicating that variation in chemical composition is the main cause for the observed hardness distribution. The hardness distributions obtained in the fiat compression test for the three deformation temperatures are shown in Figs. 9--11. Here, again, similar variations to those for rolling are observed. A study of these figures clearly indicates that the hardness distributions in rolling and in the flat compression test are closely comparable under similar deformation conditions. Such good agreement is observed for the air-cooled specimens as well as for those rolled or deformed in several passes or strokes.
width ~ 100 mm I
HVIO /.00- 415
1~ t
1226°C/lh
390- 400
. t , deformation
ho=20.3 mm - • h I =13.5 mm
81
- 33.w% ~1 = 0.408
\
t
Fig. 9. Hardness distribution in a fiat compression test specimen deformed at 1100°C and
water-quenched after deformation (transverse section).
55
"9" I
HVIO
I
~oo-~3o
E~
3~o-~oo
E~
360-380
t250°C/lh
quenching
t/
<360
V
"o
h~ = 15.1 mm
F_, = 25.6 %
deformation
~. t
Fig. 10. I-lardneu distribution in a fiat compression test specimen deformed at 900°C and water-quenched after deformation (transverse section).
"
"
-~=
",,
, ,
\ " ..... \ "\ " ' "
"'
: ',, ? ' i ~ ,\ •
,.:17 ] '. , ,:.
wmdth = 100 mm 7250°C/lh
E~
3ao-~oo 360-380 / ho=20.3mm /-.3~0
'%-,-,,,,---- deformotion
h =15.gmm
C,=21.7°Io ~=0.2~A,
X~xuenchlng ~.
t"
Fig. 11. Hardness distribution in a fiat compression test specimen deformed at 850°C and water-quenched after deformation (transverse section).
3.2. Impact strength Figures 1 2 - - 1 4 s h o w the variation o f i m p a c t strength values as a f u n c t i o n o f testing temperature u n d e r different d e f o r m a t i o n c o n d i t i o n s for b o t h rolling and fiat c o m p r e s s i o n . It can be seen that for a l m o s t all t h e d e f o r m a t i o n c o n ditions t h e i m p a c t strength variations in rolling and in the fiat c o m p r e s s i o n test are comparable. O n l y in the case o f furnace-cooled specimens after
56 100 ~ !
1226°Cllh
I
I
80
~t =32.75%
~ =0.397
rE.t =26.6% ~=0.309
60
/
40
/
1 // /
20 u 0
-40
-20
0
c"
/
+20 -40 -20 Testing temperoture, °C o Rolling
0
test
~I00 '~. 12'26°C/'lh
/
L-
~ 8o t 4-
40
+20
S
/
E BO
t
/
_./ /
~ 1226oC/lh
20
1=32"75°/° 62 0
(c)
-40
-20
I
t
!
;(d) I ,2o -4o -2b Testing tempereture, °C
~t°~: ~7"3°/° , ,
5
\
*"
.2'o
Fig. 12. Comparison of impact strengths as a function o f test temperature for similar conditions in rolling and in the fiat compression test.
rolling (Figs. 14(c) and (d)) is there some difference between the rolling and the fiat compression test results, the fiat compression test specimens giving a higher value than the rolled specimens; for all other conditions, the values are almost identical.
57
/--~~L
~,',12so"c/1,
100
•
"
'
80 tot : 26.1%
Jl |m 4-
60
/
/
Z.(3
I
/I 0
-20
/
_11Z
{al
0
/
(b) 0
-40 -20 0 ÷20 Testing temperature .°C
crl L.
//
÷2
o Rol ring . Fief compression test 100
÷/
0
Q,.
60 ~0 20
//
/ -40
//
S
/
7c"'\ II
, \ Bsooc ~o,~
1=22"2
IIIIII
{c)
-20 0 .20 TeslJng temperature, °C
Fig. 13. Comparison of impact strengths as a function o f test temperature for similar conditions in rolling and in the fiat compression test.
3.3. Tensile properties Tables 3--6 compare all of the tensile results obtained in the rolling and in the fiat compression tests, for different deformation conditions. For 1100°C deformation temperature the results of six rolling specimens are shown (Table 3), displaying good consistency. Hence, for all the subsequent conditions
58 120
?,"
100
80
40
o
/
t~ /
/
/
,.,i'
F_tot =49.3%
0
-4o
2b
6
(b)
t'
-~o
+2~
Testing temperature,°C
14() 120
100
80
2o
+20
o l~Ot Llng +Flat compression test
{c)
E
"q~CJor.oc, F"t°t =31"04%
~.~,~ furnote \600°C/21h ~1=32.75% 52=24.5% "X~ai
y
cn r-
I
/ ~oir
20
£
f
/
60
I
/
i:
/
(d)
i/7
+ ,#
/
i
/
Z
/
/
¢C11h
60
/
g,ooV
/
40
r E~tot = 63.7%
-4b
-2'0
t
\g 2o°c
£tot
545%
~
0 ' +20 0 ' -2b Testing t e m p e r a t u r e , ° C
T
'
6
'+Zo
Fig. 14. C o m p a r i s o n o f impact strengths as a f u n c t i o n o f test temperature for similar conditions in rolling and in the fiat c o m p r e s s i o n test.
three tests for rolling and two for the fiat compression test were carried out. A study of the results given in Tables 3--6 shows that there is, in general, good agreement for all the tensile properties of the rolled and the flat compression test specimens.
....~_~ cooling
condition
'~:0.309
~t=23.2%
"~t;0.263
[ h°"20"3mm h , - - l~So i. r6 ~ mcooling ~
~" 1226':'C/lh
~ :26.6%
/ho=20.3mmhtfl/-..g mm
~~_gso~
4~' 1226°C/lh ~ .
Deformation
t
in
T"
598 598 598
591 593 606
382 383 409
397 399 399
% N/mm2 598 596 591 604 593 606
~ N/mm 2 373 372 367 378 372 379
27.2 27.6 27.6
26.8 26.8 27.6
(~..=25mm % 30.0 27.6 27.6 258 26~ 27.6
Roiling
74 70 71
1389 1245 1309
387 394
392 39/,
601 603
601 606
1250 1201 1309
%
69 69 69
O~y
N/mm2 N/mm21N/mm2 1218 382 591 1309 382 596 1309 1282 1212 1279
% 69 71 71 69 70 70
~Z
test
27.2 24.8
28.4 30.4
% 24.8 29.2
73 75
71 73
% b..... 73 70
~L=25m m ~
Fiat compression
5=fracture elongation =yield or 0.2o/,proof strength, ~u =ultimate tensite strength, deformation CYz=true fracture strength = reduction in a r e a of c r o s s s e c t i o n ,
No.
SI.
Comparison of tensile properties obtained in rolling and in the fiat compression test for different deformation conditions
TABLE 3
~z
1469 1507
1309 1356
N/mm2 1394 1279
r,D
el
.e,
mm~
~i
ii i 2 :!4~ .75°~omm %:
1226PC/1h
~
~1=23.2
h o =20.3
1250°C/lh
g
i
950°C
=0.263
~sso°C
125 O°C/lh
~
DeformGtion condition
ng
T~ 412 412 407
407
423 392
611 608 603
601 601 606
26.4 29.2 28.0
27.6 28.4 24.8
N/ram2 N/mm2 % 433 611 27.2 402 616 24.8
Ro[ting
71 72 72
65 69 69
% 64 66
1361 1413 1359
1127 1218 1282
359 377
397 397
489 486
596 591
N/ram~ N/mm21N/mm2 1132 382 591 1165 387 598
33.2 33.6
22.8 20.0
°/.o., 27.2 26.0
70 74
65 75
% 69 71
Fiat compression test
CYy=yield or o.2O/oproofstrength, cYu=ultimate tensile strength, 6--fracture etongation ~F = reduction in oreo of cross section, d"z:true fracture strength ~deformation
No.
SL
Comparison of tensile properties obtained in rolling and in the fiat compression test for different deformation conditions
TABLE 4
1010 1194
1160 1487
N/ram2 1218 1309
O
--"~790°C
~tot=O,303
Etot = 49"3%
'=-'+'o'"
"++2 -: -: '",". .=+' / .
cooling
.. °ling
"~tol :0.678
"
m
fur nac • cooling
~°tot = 0.493
/ -' - ~ ~ . ~ c" ' " ~ - /
v&I 1226~11h
~fnt =38~9%
61=223% 62=21%
\800oc t hi=IS.Tram h 2 = ~ i r
ir
,~ 11250°C/lh
etot=26.1%
.
condition
' h 1=15.6mm h 2 ~ 61 =23.2% £2=3.9%
I
"~' ~1226oC/lh
Deformation
t
i..
t_-_ 443 /+18
/*28 /*38 428
/.1/* /*07 382
517 519
621 621 621
608 606 591
N/mm 2 N/mm 2
29.2 27.6
2B.8 26.4 27.2
29.2 26+8 26.8
%
Rolling
73 71
65 66 65
73 71 70
%
1149 1082
1201 1165 1186
1375 1309 1263
394 392
/.18 /.18
397 399
585 586
596 591
588 588
N/mm2 N/mm 2 N/mm 2
30.8 28.8
27.6 24./*
27.6 30.8
%
72 76
65 70
75 69
%
Flat compression t e s t
8:fracture elongation cYy=yield or o.2°/oproof strength, d~u=ultimate tensile s t r e n g t h , CYz=true f r a c t u r e s t r e n g t h ~ ....... deformation y : reduction in area of cross section,
No.
St.
Comparison of tensile properties obtained in rolling and in the flat compression test for different deformation conditions
TABLE 5
1268 1529
t127 1162
1487 1185
N/mm 2
F
~tot :31.0/.%
~ : 0tot . 3 7 ~ ~'2
q-,-
furnace
"
cooling
_
E.tot =6/..5%
t2tot :1.04
"~
/
furnace~
-~
-"'4,6o~/!,~/
__~"~. 920oC
~1010°C
,,
t~3:~s./. ~ :2o.,,,
.
~' 1250°C/lh
/~ _ ~ . , _ ~ c ° ~ ' n ~ 7
/~X'X X /go0~c ~ . .
~1 1250°C/lh
r
/ ~--23.2% M~o.3"/.
"~
---~oc/orooce coo,og
condition
/h1:lS-6mm h 2 : 1 4 m m ~ a l r
/
---~8sg~c
@i 1226%/lh
Deformation
L12 412
3tl 316 316
Z.07 405
545 509
514 51L 509
532 532
N/m m 2 N/ram 2
30.0 32.4
3L.4 340 33.6
29.6 28.8
%
Roiling
75 74
76 73 75
71 71
%
1283 1265
1306 1170 1281
1117 1134
3L6 383
341 377
378 368
504 506
489 489
504 501
N/mm 2 N/ram 2 N/ram 2
36.0 35.2
32.4 32.8
31.2 32.4
%
80 81
74 71
76 77
%
FLat compression test
CYy=yieldor0.2°/oproofs!.rength, o"u=u[timate tensite s t r e n g t h , ~ =fracture e l o n g a t i o n "y" : r e d u c t i o n in area of cross section, d"z=true f r a c t u r e s t r e n g t h ~deformation
12
11
10
No.
St.
Comparison of tensile properties obtained in rolling and in the fiat compression test for different deformation conditions
TABLE 6
1L32 1500
1115 1030
1274 1305
N/mm 2
63
The averages of the tensile properties shown in Tables 3--6 are compared in Figs. 15 and 16 for the two processes. The equation of the best-fit straight line, along with the correlation coefficient obtained using regression analysis, is also given in these figures. It can be seen that all the tensile properties of rolled and fiat compression test specimens agree within a maximum error of about 5%. 3.4. M icrostructure In order to study bow the processes of rolling and fiat compression influence the austenite grain shape, size and distribution, specimens after rolling /.60
i
i
i
i
O'yR=13.6 .1.00 O'YFcT / ¢-
k=O.5Z. * /~// .+ o / /
/.20
C
~E
/
// o one pass or stroke + two or four posses or strokes
2"•g 300
, , , I I 340 3l~0 420 /.60 500 Yield or o.2"/,proof strength in f l a t compression test JF~"cT'N/mm2
I
300
i O"UR=113"2* 0"82 L O"UFcT+
620
k =o.g7
.,
o ~"
~E
//
Z
,J'/ soo/
=o c
"- & 6 0 / 460
/
k= correlation coefficient
IIitll
500 5/.0 580 620 Ultimate tensile strength
in flQt compression test
660
O'UFcT , N/ram 2
Fig. 15. Comparison of average yield and ultimate tensile strength for rolling and the fiat compression test.
64 40
~36
o one pass or stroke / ÷ two or four passes or strokes,,/
/.
E~ C
/
/"
y/"
-~ 32 r
/•R
= 8.19 +0 202
~FCT
C
k=0.754
~t - 28
*o
O
0 LL.
28 32 36 40 Fracture elongation in f l a t compression test SFCT ,°/o
2~
I 80
/
~ 76 .c_ 72
= -23.99.1.31 "~FCT
// =
o-64/
Iltll
I 64
68
72
I
7
Itll
Reduction in area in flat compression test ~rFcm,%
Fig. 16. Comparison of average fracture elongation and reduction in area of cross section for rolling and the fiat compression test.
and fiat compression were water-quenched after deformation at different temperatures. These specimens were stress-relieved at 300°C for 1 h and prepared for austenite grain study. The results obtained are shown in Figs. 17 and 18, where it can be seen that for all the deformation temperatures tested there is a very good resemblance in austenite grain shape, size and distribution of the rolled and the fiat compression test specimens. Similar microstructures obtained after holding to different times in air after deformation at 900 and 950°C showed a very good resemblance between the two processes.
65
Rolling
Flat compression test
)P
\
t--
1226~11h
ho= 20,3mm 15t = 2 6 . 1 %
air----/ \
\',
t z-
t
1250°C/Ih
~ ~ . - w a t er / ho=2?,3mm \ 300oC_li.h { h4 =15.1mm ~ F-----~/ ~,~ =25.6 %
\+
+.. 7". i , .
+ +
\
i;g"~+~': ~'¢: .. . . . . . .
.+
.+
,1250oC/lh -+~ o. ,++
2-",, ho=2Oo3mm
"+-'N /water y alr---.7
h1 -t5 7 mm
~300eCllh
-e
/ t
Fig. 17. C o m p a r i s o n of austenite grains o b t a i n e d u n d e r the same test c o n d i t i o ~ in rolling and in the fiat compression test. E t c h a n t : Picric acid + CuC12 + Agepon.
66
Rol ling
Flat compression test
1.1~ls 2.1xzu \ 113mm E,~=3235 17.2%
h 1 = 13.65
i =20,3ram F..= .=/,/,.3%\
/
\~:--~-
tpQss 2toss
~water
/ g~ 32.75 30./. %
300~"/lh t
II. _':;.",~ :? _ Y=" " _
t
F~I=25.6 % F~2=23"2% 1~3=20.2% F./.=13.1% ho =20.3ram hi. =8 mm e,tot = 61%
r - - ~ ai t ~ _ --~ t
Fig. 18. C o m p a r i s o n of a u s t e n i t e grains o b t a i n e d u n d e r t h e s a m e t e s t c o n d i t i o n s in rolling a n d in t h e fiat c o m p r e s s i o n test. E t c h a n t : Picric acid + CuCI 2 + A g e p o n .
67 4. Conclusions The results of these investigations may be summarized as follows: (1) In spite of the inert (argon) atmosphere employed for austenitizing the specimens, there is a marked amount of decarburization and reduction in total nitrogen content at the surface of the specimens which can be ascertained by examination of microstructure and chemical composition. (2) The variation of the hardness distribution shows that the highest hardness does not occur at the centre, but occurs at about 2 to 3 mm from both surfaces. A similar type of hardness is obtained in rolling as in the fiat compression test, irrespective of the deformation temperature and the cooling conditions after deformation. (3) The above type of hardness distribution is attributed mainly to the variation in chemical composition across the specimen thickness. (4) Under steady~state conditions there exists a generally uniform strain across the thickness and width of the rolled specimen. (5) The impact strengths in rolled and fiat compression test specimens agree very well, for all impact test temperatures explored. (6) There is a linear relationship between all tensile properties of rolled and fiat compression test specimens, under similar deformation conditions. Based on regression analysis the mathematical relationship has been evaluated and the maximum deviation between the two results found to be about 5%. (7) Under identical deformation conditions the austenite grain size, shape and distribution in rolling and in the fiat compression test are very similar. (8) The simulated fiat compression test is comparable to rolling from the point of view of hardness distribution, impact strength, tensile properties and austenite grain size, shape and distribution, under similar deformation conditions. (9) The fiat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the optimum rolling schedule.
References 1 G. Robiller, L. Meyer and S.R. Datta, Thyssen Technische Bericht, 7(1) (1975) 14-25. 2 R.A.P. Djaic and J,J. Jonas, J. Iron & Steel Inst., 210(4) (1972) 256--261. 3 R. Priestner, C.C. Earley and J.H. Rendall, J. Iron & Steel Inst., 206(12) (1968) 1252--1261. 4 D.J. Walker and R.W.K. Honeycombe, Metal Science, 12(9) (1978) 445--452. 5 I. Weiss and J.J. Jonas, AIME symposium on "Precipitation effects in HSLA steels", Denver, Colo., USA, Feb. 1978. 6 A. Jones and B. Walker, Metal Science, 8(1974) 397--406. 7 N.V. Thikhiy, Ya.I. Spektor, M.I. Sinelnikov, R.I. Entin and R.V. Yatsenko, Fiv. Met. Metalloved, 38(6)(1974) 1250--1255. 8 H.D. Bartholot, H.J. Engell, W. Esche and K. Kaup, Stahl u. Eisen, 91(4) (1971) 204--220.
68 9 K.J. Irvine, T. Gladman, J. Orr and F.B. Pickering, J. Iron & Steel Inst., 208(8) (1970) 717--726. 10 M. Fukuda, T. Hashimoto and K. Kunishige, The Sumitomo Search, 9(9) (1973) 8--23. 11 T. Gladman, I.D. Mc.Iror and D. Dulieu, Micro-alloying '75 Conference, Washington, U.S.A., Oct. 1975. 12 R.A. Petkovic, M.J. Luton and J.J. Jonas, International symposium on "Hot forming of steel", ~trbsk~ Pleso, C.S.S.R., Sept. 1974. 13 R. Roussev, Trans. ISIJ, 13 (1973) 105--110. 14 H. Matsubara, T. Osuka, I. Kozasu and K. Tsukada, Trans. ISIJ, 12 (1972) 435--443. 15 I. Kozasu, T. Shimizu and K. Tsukada, Trans. ISIJ, 12 (1972) 305--313. 16 T. Brnlin, Annals CIRP, 25(1) (1976) 185--189. 17 O. Pawelski, Z. Metailkunde, 68(2) (1977) 79--89. 18 D. Bauer, Metall, 32(8) (1978) 776--784. 19 E. Neuschiitz and H. Rohloff, Stahl u. Eisen, 96(25--26) (1976) 1303--1307. 20 H. Weiss, D.H. Skinner and J.H. Everitt, J. Phys. E (Scientific Inst.), (6) (1973) 709. 21 S. Fulop, K.C. Cadien, M.J. Luton and H.J. McQueen, J. Testing and Evaluation, 5(6) (1977) 419. 22 W. Vanovsek and H. Trenkler, Berg. u. Hiittenwes., (9) (1977) 397. 23 R. Kaspar and O. Pawelski, Proc. 19th, Int. Mach. Tool Des. Res. Conf., Manchester, Sept. 1978, Macmillan, London, 1979, pp. 247--253. 24 O. Pawelski, U. Riidiger and R. Kaspar, Stahl u. Eisen, 98(5) (1978) 181--189. 25 O. Pawelski, V. Gopinathan, J. Mech. Work. Tech., 5 (1981). 26 V. Gopinathan, Research report on "A comparative study of hot rolling with fiat compression test", Dept. Metal Forming, MPI, Diisseldorf, May 1980. 27 E. Schmidt, August-F~ppl. festschrift, Berlin 1924, pp. 179--189.
45
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
COMPARISON OF MECHANICAL PROPERTIES OF HSLA STEEL DEFORMED BY HOT ROLLING AND BY FLAT COMPRESSION UNDER SIMULATED CONDITIONS
O. PAWELSKI
Max-Planck-Institut flit Eisenforschung GmbH, Diisseldorf (West Germany) and V. GOPINATHAN*
Indian Institute of Technology, Madras-36 (India) (Received April 21, 1980; accepted October 2, 1980)
Industrial Summary Steels required for the transportation of oil pipes must have a very low ductile-brittle transition temperature apart from good mechanical properties because of the severe atmospheric conditions to which they are subjected. To achieve this, either alloying or optimization of the rolling mill schedule with micro-alloy additions is employed. Because of the economic advantages the latter method is becomingpopular, but the main difficulty of this method is that the optimum rolling schedules are not available for all steel compositions. To achieve this, several laboratory simulation tests have been tried; one such test is fiat compression. In this work, the validity of flat compression as a laboratory test to simulate hot rolling conditions is investigated. To this end, rolling and fiat compression tests were carried out under similar deformation conditions at different temperatures between 850 and 1100°C using Nb micro-alloyed steel NV4-4. From the rolled and the fiat compression test specimens, tensile, hardness, impact and structural test-pieces were taken and tested, and the results obtained then compared for the two processes. The results obtained show that the hardness distribution, the impact strength, all the tensile properties and the austenite grain shape, size and distribution, agree very well for rolling and the fiat compression test, under similar deformation conditions. Based on these results it is concluded that the flat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the optimum rolling schedule.
1. Introduction Steels used f o r the t r a n s p o r t a t i o n o f oil pipes must n o t onl y be strong t o withstand th e oil pressure but t h e y must also n o t fail under the severe atmospheric conditions -- in m a n y occasions below --60°C -- t o which t h e y are subjected. Because o f these severe constraints the steels selected for the m a n u f a c t u r e o f oil pipes must have a very low ductile-brittle transition *Present address: Central Metal Forming Institute, HMT Limited, Hyderabad-500 854, India.
0378-3804/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
46 temperature, apart from having good other mechanical properties. In the last decade, several developed countries have been engaged in finding a suitable material for oil pipes. In general, there are t w o possible methods b y which the steel can be given the required properties for oil transporation purposes. These are: (i) b y the addition of alloying elements like Ni in amounts more than 5 to 10%, (ii) by the optimization of rolling mill schedules with micro-alloy additions to the steel. The main difficulty of the first method, which was widely used until recently, is that it is costly and the alloying elements are scarce. In the second method, elements such as Nb, Ti, Mo, V are added to low carbon steel in amounts varying from 0.01% to about 0.5%, either individually or in combination. When this steel is roiled under the so-called optimum roiling conditions it is then possible to obtain the required properties. The obvious advantage of this technique is that it is highly economical and the costly alloying elements can be spared for other purposes. The main drawback of this technique, however, is the establishment of the correct optimum rolling schedule. Even slight changes in any one of the rolling parameters -- such as rolling temperature, number of passes, deformation in each pass, strain rate in each pass, cooling rate between passes and at the end of roiling, austenitizing temperature, etc. - - c a n alter the mechanical properties. The influence of these parameters on the structure and properties of HSLA steels has been studied b y many authors [1--16], b u t despite this the o p t i m u m rolling schedule is n o t available for all steel compositions. Several research institutions and industries are currently devoting their efforts and resources to establishing this o p t i m u m rolling schedule for different steel grades. As mentioned earlier, the parameters involved are so numerous that for establishing the optimum rolling schedule several thousand experiments have to be carried out. To do this in industry, under actual roiling conditions, is n o t only time consuming b u t is also costly and measurements are difficult. Hence, several laboratory simulation tests have been developed to tackle this problem, the four c o m m o n l y used being: cylindrical compression, tension, torsion and flat compression. Each test has its own merits and drawbacks, which are described elsewhere in the literature [ 1 7 - - 1 8 ] . The details of individual tests have been described by several authors [19--23]. The requirements of a good simulation test for rolling have been described by Pawelski et al. [ 2 4 ] . A study of these different tests clearly shows that the flat compression test meets most of the requirements of a rolling simulation test, and several research programmes are currently being carried o u t using this method. Naturally the question arises, h o w well do the results obtained in a flat compression test compare with those obtained under actual rolling conditions? Little information is available in the literature to answer this question. Work has thus been carried out to compare the mechanical properties of steel deformed b y h o t rolling with those of steel deformed b y the flat corn-
47
pression test, at different forming temperatures, the results of which investigation are reported herein. Further w o r k carried out to compare the material flow and the deformation resistance will be reported at a later date
[25]. 2. Experimental procedure Experiments were carried out using Nb micro-alloyed steel NV4-4, the composition of which is given in Table 1. The mechanical properties of the steel in the as-received condition are shown in Table 2. All the rolling experiments were carried o u t using a 2-high rolling mill with a roll diameter of 180.3 mm and a barrel length of 200 mm. Before rolling, the specimens were austenitized either at 1226 or 1250°C for 1 h in an argon atmosphere and then cooled in still air until the required rolling temperature was reached. When rolling in several passes, immediately after the first pass the mill was reversed and the t o p roll adjusted to the new height, rolling then being continued. After rolling, the specimens were either cooled in still air or quenched in water or cooled in a furnace held at 600°C, according to the requirements. The specimen temperature was controlled during and after rolling by inserting a chromel-alumel 2 mm outer
3. Results and discussions 3.1. Hardness Figure 3 shows the hardness distribution obtained along the width and thickness directions of specimens rolled at different temperatures and in the as-received condition. The hardness variation curves across the thickness show that the hardness at the centre is very low, and that the hardness then increases with increasing distance from the centre, reaching a maximum value at a b o u t 2 to 3 mm from the surface, thereafter decreasing as the surface is approached. This trend is observed for all of the specimens tested. In the asreceived condition, however, such marked variation in hardness across the specimen thickness is n o t observed. Similarly, along the width direction the hardness remains generally constant for all rolling temperatures, except near the edges, where the hardness decreases. *Abbreviation for the german word "Warmumformsimulator".
0.14
C
0.22
Si 1.43
Mn 0.021
P 0.001
S
Chemical c o m p o s i t i o n ( w e i g h t %)
0.04
Nh 0.015
N
NV44
Steel t y p e
o"U
8L=25 mm
407 410 423
542 542 542
26.4 28.4 29.4
( N / r a m : ) ( N / r a m 2) (%)
ey
Tensile p r o p e r t i e s
~
70 72 71
(%)
°'Z
1229
( N / m m 2)
130.8
+19°(3
128.3
--10°C
Impact strength a K ( J / c m 2)
102.4
--40°C
21.4
+19°C
21.2
--10°C
17.3
--40°C
Lateral e x t e n s i o n ( b m a x - - bo)/bo (%)
a y = yield or 0.2% p r o o f s t r e n g t h , o u = u l t i m a t e tensile s t r e n g t h , 5 = f r a c t u r e e l o n g a t i o n , ~ = r e d u c t i o n in area o f cross-section, o z = true f r a c t u r e s t r e n g t h , aK = C h a r p y i m p a c t s t r e n g t h u s i n g D V M K s p e c i m e n as p e r D I N 50 115, bma x = m a x i m u m w i d t h o f t h e i m p a c t s p e c i m e n , b0 = initial w i d t h o f t h e i m p a c t s p e c i m e n .
Mechanical p r o p e r t i e s o f t h e steel tested, in t h e as-received c o n d i t i o n
TABLE 2
NV4-4
Steel t y p e
Chemical c o m p o s i t i o n o f t h e steel t e s t e d
TABLE 1
QO
49 -250 --
I I.
I i
roiling direction
-10~--
4
(~) Hardness specimen
I
z,5.2 o x---/ i Lz,-z0.6 ,4 (~) Impact specrmen rL
l n--
'
I
L
-
-
~
-
,
u~ M
1
.
8
(]) P-8 ~'
120
60
"t .4
(~) Tensile specimen
LI
Fig. 1. Location and dimensions of different test-pieces from rolling experiment specimens.
.
7.
r=0.25-0.025 ,, ~
.
~i
@ Impc]ct specimen
@
Tensile s p e c l m e n
i
,
@ Hordness and structural specimen
i'
f
Aft dimensions
in m m
Initial length =11,0
Fig. 2. Location and dimensions of different test-pieces from fiat compression test specimens.
In order to study this variation in a more detailed manner, hardness measurements were m a d e at 1 m m intervals across the thickness and at 2 or 4 m m intervals across the width over the width X thickness area of the rolled specimens. Figure 4 shows the hardness profile obtained in one half of the specimen in the as-received condition. It can be seen that the specimen is harder at the centre and that the hardness decreases with distance from the middle. Figures 5--7 s h o w similar hardness distributions after rolling at three
0
I
o "1-
-8
'- 350
"r375
>
o400
425
3
B
o
0
0
i
-4
~,,, I"J
0",
u'l
0
c/I 0
z,
f
!
I\
o
?
-4 uq
'
0
350
-8 Distance
o
4O0
425
o
oq
-4 from
0 the
/~/ ~'~
-:~
"4
0 0
I
~'1
0
-8
350
~00
8 mm
,.ozbJ i
~00
~,25
NV10
4 8 -8 -4 0 4 c e n t r e a | o n g the t h i c k n e s s ,
,I
L
J
~
o
Hardness
/
0
-
-4
0
~li ~
U'I
/
\
/,,
\
>
Ln
0
8-10
-6
125' t
150
175
200
o o
cI
-
D..
2.
-2
0
2
L.
6
>.. r"'
7
>
01 0
51
different temperatures and water quenching after rolling. Here, an inverted trend can be observed, viz. the hardest region is not at the centre but is about 2 to 3 m m below the surface on both sides o £ t h e specimen. Such a trend is
width :100ram HVl0 160-165
ho:20.3 mm
155-160 [~
150-155
i~5-15o
Fig. 4. Hardness distribution in a typical test specimen in the as-received condition.
width = 100ram
II Q
HV 10
B
~oo -~5
"~
direction of rolling
1226°C/1 h
/
[ ~
~,~o c water quenching
3 7 0 - 390 '(370
ho =20.3ram
6t =32.75%
ht : 13.65mm
~l : 0397
\
deformolion t
Fig. 5. Hardness distribution in a specimen rolled at 1100°C and water quenched after rolling.
Fig. 3. Hardness distribution along the width and thickness directions of specimens rolled at different temperatures and in the as-received condition.
52
• :::?~!.::}~-< ~; " :::::::::::::::::::::::::::: ...... --:~r.~ --I-~i.;?::: ................. ~ ; ~ ; : , ' ~ : t ~
i
~ldth : 100ram = HVlO
(~ direction of roiling
~.oo /45
1250°Cnh
380 - 400
/'
// "x.~,,,..~deformotion
\,~
//
L_2 36o- sso
,/
L ~ 5]
/
_
h° :20.3ram
8:26.1%
h, :15.0rnm
~ :0.303
'
Fig. 6. Hardness distribution in a specimen rolled at 900°C and water quenched after rolling.
width = ]UUmm HV10
[:::3 360 380 /~ 360 deformot ion
Q direction of roiling l~ I
1250°C/1h
/
r~s°~°~s~3~-'l"\
ho =20.3ram h :lS,7mm i
61 =22.7% ~ =0.257
~-weter quenching
\ t
Fig. 7. H ~ d n ~ rolling.
d ~ t r i b u t i o n in a s l ~ e i m e n rolled at 8 5 0 ° 0 and w a t e r q u e n c h e d after
observed not only for water cooled specimens but also for air cooled specimens, even though the absolute hardness value is less in the latter case. Such a distribution of hardness can be due to any of the following 3 reasons: (i) variation in cooling rate across the specimen thickness; (ii) variation in strain (deformation due to rolling) across the specimen thickness; and
53 1250OC/lh n
•ol I "
•
water
• .7. •
-
o
' --
"a "'al.%~ °
~)ebe
"
""
20
....
/
\
/
o
o o •
o.%° '
30
•
o
~, ~
~
• *
0,%0 • !-e
• o
•
~
h o = 20"3 m m h, = 1 4 . 5 m m
/
V
~1 = 2 8 . 6 %
~4 =0a37
~
40
quenching]
g50°C
r
• screw l e f t oscrew right
go .cmo
o a
°~o°
o o
rolling
•
~---
direction
2O
t o~ 10 ¢-.
4O ++ : omPOo o •o ~oo i
m30 ID r" 2g
•
" ~.
•
•
Op
D ii Ii I
oo., uOeOo , o°
_
9
IV: ....
o
o•
2 20
F-
10
Z.O
o,
== o
30
•
• "o
o
• i~
o D
•
• u
o
'e
direction~
,~-" I I
° oC
,%
o. . . . "
o •
o
~ o o
: o -"
before after
rolling/ rolling
20 0
2
4 Thickness,
mm
Fig. 8. Strain variation acro~ the thickness o f a rolled specimen at different locations. "screw left" and "screw right" refer to the left-hand-side and the right-hand-side of the screw, respectively.
54 (iii) variation in chemical composition across the specimen thickness. Based on the cooling curves calculated using the finite difference equation given by Schmidt [27] it was found that the m a x i m u m variation in temperature between the centre and surface is of the order of 20 to 30°C, which is too small to explain the marked difference in hardness, reported earlier. The measured variation in strain -- obtained using the screw technique [25] - - a t different locations of the rolled specimen is shown in Fig. 8. These results show that within the measurement scatter the distribution of thickness strain across the thickness is constant over the entire width of the specimen, indicating that the hardness variation reported earlier is also n o t due to the strain distribution. A measurement of the chemical composition variation across the specimen thickness showed a drastic reduction in C content (from 0.14% to 0.04%) and nitrogen c o n t e n t (from 0.017% to 0.014%) near the surface. The variation of these elements across the thickness is comparable with the hardness variation reported earlier, indicating that variation in chemical composition is the main cause for the observed hardness distribution. The hardness distributions obtained in the fiat compression test for the three deformation temperatures are shown in Figs. 9--11. Here, again, similar variations to those for rolling are observed. A study of these figures clearly indicates that the hardness distributions in rolling and in the flat compression test are closely comparable under similar deformation conditions. Such good agreement is observed for the air-cooled specimens as well as for those rolled or deformed in several passes or strokes.
width ~ 100 mm I
HVIO /.00- 415
1~ t
1226°C/lh
390- 400
. t , deformation
ho=20.3 mm - • h I =13.5 mm
81
- 33.w% ~1 = 0.408
\
t
Fig. 9. Hardness distribution in a fiat compression test specimen deformed at 1100°C and
water-quenched after deformation (transverse section).
55
"9" I
HVIO
I
~oo-~3o
E~
3~o-~oo
E~
360-380
t250°C/lh
quenching
t/
<360
V
"o
h~ = 15.1 mm
F_, = 25.6 %
deformation
~. t
Fig. 10. I-lardneu distribution in a fiat compression test specimen deformed at 900°C and water-quenched after deformation (transverse section).
"
"
-~=
",,
, ,
\ " ..... \ "\ " ' "
"'
: ',, ? ' i ~ ,\ •
,.:17 ] '. , ,:.
wmdth = 100 mm 7250°C/lh
E~
3ao-~oo 360-380 / ho=20.3mm /-.3~0
'%-,-,,,,---- deformotion
h =15.gmm
C,=21.7°Io ~=0.2~A,
X~xuenchlng ~.
t"
Fig. 11. Hardness distribution in a fiat compression test specimen deformed at 850°C and water-quenched after deformation (transverse section).
3.2. Impact strength Figures 1 2 - - 1 4 s h o w the variation o f i m p a c t strength values as a f u n c t i o n o f testing temperature u n d e r different d e f o r m a t i o n c o n d i t i o n s for b o t h rolling and fiat c o m p r e s s i o n . It can be seen that for a l m o s t all t h e d e f o r m a t i o n c o n ditions t h e i m p a c t strength variations in rolling and in the fiat c o m p r e s s i o n test are comparable. O n l y in the case o f furnace-cooled specimens after
56 100 ~ !
1226°Cllh
I
I
80
~t =32.75%
~ =0.397
rE.t =26.6% ~=0.309
60
/
40
/
1 // /
20 u 0
-40
-20
0
c"
/
+20 -40 -20 Testing temperoture, °C o Rolling
0
test
~I00 '~. 12'26°C/'lh
/
L-
~ 8o t 4-
40
+20
S
/
E BO
t
/
_./ /
~ 1226oC/lh
20
1=32"75°/° 62 0
(c)
-40
-20
I
t
!
;(d) I ,2o -4o -2b Testing tempereture, °C
~t°~: ~7"3°/° , ,
5
\
*"
.2'o
Fig. 12. Comparison of impact strengths as a function o f test temperature for similar conditions in rolling and in the fiat compression test.
rolling (Figs. 14(c) and (d)) is there some difference between the rolling and the fiat compression test results, the fiat compression test specimens giving a higher value than the rolled specimens; for all other conditions, the values are almost identical.
57
/--~~L
~,',12so"c/1,
100
•
"
'
80 tot : 26.1%
Jl |m 4-
60
/
/
Z.(3
I
/I 0
-20
/
_11Z
{al
0
/
(b) 0
-40 -20 0 ÷20 Testing temperature .°C
crl L.
//
÷2
o Rol ring . Fief compression test 100
÷/
0
Q,.
60 ~0 20
//
/ -40
//
S
/
7c"'\ II
, \ Bsooc ~o,~
1=22"2
IIIIII
{c)
-20 0 .20 TeslJng temperature, °C
Fig. 13. Comparison of impact strengths as a function o f test temperature for similar conditions in rolling and in the fiat compression test.
3.3. Tensile properties Tables 3--6 compare all of the tensile results obtained in the rolling and in the fiat compression tests, for different deformation conditions. For 1100°C deformation temperature the results of six rolling specimens are shown (Table 3), displaying good consistency. Hence, for all the subsequent conditions
58 120
?,"
100
80
40
o
/
t~ /
/
/
,.,i'
F_tot =49.3%
0
-4o
2b
6
(b)
t'
-~o
+2~
Testing temperature,°C
14() 120
100
80
2o
+20
o l~Ot Llng +Flat compression test
{c)
E
"q~CJor.oc, F"t°t =31"04%
~.~,~ furnote \600°C/21h ~1=32.75% 52=24.5% "X~ai
y
cn r-
I
/ ~oir
20
£
f
/
60
I
/
i:
/
(d)
i/7
+ ,#
/
i
/
Z
/
/
¢C11h
60
/
g,ooV
/
40
r E~tot = 63.7%
-4b
-2'0
t
\g 2o°c
£tot
545%
~
0 ' +20 0 ' -2b Testing t e m p e r a t u r e , ° C
T
'
6
'+Zo
Fig. 14. C o m p a r i s o n o f impact strengths as a f u n c t i o n o f test temperature for similar conditions in rolling and in the fiat c o m p r e s s i o n test.
three tests for rolling and two for the fiat compression test were carried out. A study of the results given in Tables 3--6 shows that there is, in general, good agreement for all the tensile properties of the rolled and the flat compression test specimens.
....~_~ cooling
condition
'~:0.309
~t=23.2%
"~t;0.263
[ h°"20"3mm h , - - l~So i. r6 ~ mcooling ~
~" 1226':'C/lh
~ :26.6%
/ho=20.3mmhtfl/-..g mm
~~_gso~
4~' 1226°C/lh ~ .
Deformation
t
in
T"
598 598 598
591 593 606
382 383 409
397 399 399
% N/mm2 598 596 591 604 593 606
~ N/mm 2 373 372 367 378 372 379
27.2 27.6 27.6
26.8 26.8 27.6
(~..=25mm % 30.0 27.6 27.6 258 26~ 27.6
Roiling
74 70 71
1389 1245 1309
387 394
392 39/,
601 603
601 606
1250 1201 1309
%
69 69 69
O~y
N/mm2 N/mm21N/mm2 1218 382 591 1309 382 596 1309 1282 1212 1279
% 69 71 71 69 70 70
~Z
test
27.2 24.8
28.4 30.4
% 24.8 29.2
73 75
71 73
% b..... 73 70
~L=25m m ~
Fiat compression
5=fracture elongation =yield or 0.2o/,proof strength, ~u =ultimate tensite strength, deformation CYz=true fracture strength = reduction in a r e a of c r o s s s e c t i o n ,
No.
SI.
Comparison of tensile properties obtained in rolling and in the fiat compression test for different deformation conditions
TABLE 3
~z
1469 1507
1309 1356
N/mm2 1394 1279
r,D
el
.e,
mm~
~i
ii i 2 :!4~ .75°~omm %:
1226PC/1h
~
~1=23.2
h o =20.3
1250°C/lh
g
i
950°C
=0.263
~sso°C
125 O°C/lh
~
DeformGtion condition
ng
T~ 412 412 407
407
423 392
611 608 603
601 601 606
26.4 29.2 28.0
27.6 28.4 24.8
N/ram2 N/mm2 % 433 611 27.2 402 616 24.8
Ro[ting
71 72 72
65 69 69
% 64 66
1361 1413 1359
1127 1218 1282
359 377
397 397
489 486
596 591
N/ram~ N/mm21N/mm2 1132 382 591 1165 387 598
33.2 33.6
22.8 20.0
°/.o., 27.2 26.0
70 74
65 75
% 69 71
Fiat compression test
CYy=yield or o.2O/oproofstrength, cYu=ultimate tensile strength, 6--fracture etongation ~F = reduction in oreo of cross section, d"z:true fracture strength ~deformation
No.
SL
Comparison of tensile properties obtained in rolling and in the fiat compression test for different deformation conditions
TABLE 4
1010 1194
1160 1487
N/ram2 1218 1309
O
--"~790°C
~tot=O,303
Etot = 49"3%
'=-'+'o'"
"++2 -: -: '",". .=+' / .
cooling
.. °ling
"~tol :0.678
"
m
fur nac • cooling
~°tot = 0.493
/ -' - ~ ~ . ~ c" ' " ~ - /
v&I 1226~11h
~fnt =38~9%
61=223% 62=21%
\800oc t hi=IS.Tram h 2 = ~ i r
ir
,~ 11250°C/lh
etot=26.1%
.
condition
' h 1=15.6mm h 2 ~ 61 =23.2% £2=3.9%
I
"~' ~1226oC/lh
Deformation
t
i..
t_-_ 443 /+18
/*28 /*38 428
/.1/* /*07 382
517 519
621 621 621
608 606 591
N/mm 2 N/mm 2
29.2 27.6
2B.8 26.4 27.2
29.2 26+8 26.8
%
Rolling
73 71
65 66 65
73 71 70
%
1149 1082
1201 1165 1186
1375 1309 1263
394 392
/.18 /.18
397 399
585 586
596 591
588 588
N/mm2 N/mm 2 N/mm 2
30.8 28.8
27.6 24./*
27.6 30.8
%
72 76
65 70
75 69
%
Flat compression t e s t
8:fracture elongation cYy=yield or o.2°/oproof strength, d~u=ultimate tensile s t r e n g t h , CYz=true f r a c t u r e s t r e n g t h ~ ....... deformation y : reduction in area of cross section,
No.
St.
Comparison of tensile properties obtained in rolling and in the flat compression test for different deformation conditions
TABLE 5
1268 1529
t127 1162
1487 1185
N/mm 2
F
~tot :31.0/.%
~ : 0tot . 3 7 ~ ~'2
q-,-
furnace
"
cooling
_
E.tot =6/..5%
t2tot :1.04
"~
/
furnace~
-~
-"'4,6o~/!,~/
__~"~. 920oC
~1010°C
,,
t~3:~s./. ~ :2o.,,,
.
~' 1250°C/lh
/~ _ ~ . , _ ~ c ° ~ ' n ~ 7
/~X'X X /go0~c ~ . .
~1 1250°C/lh
r
/ ~--23.2% M~o.3"/.
"~
---~oc/orooce coo,og
condition
/h1:lS-6mm h 2 : 1 4 m m ~ a l r
/
---~8sg~c
@i 1226%/lh
Deformation
L12 412
3tl 316 316
Z.07 405
545 509
514 51L 509
532 532
N/m m 2 N/ram 2
30.0 32.4
3L.4 340 33.6
29.6 28.8
%
Roiling
75 74
76 73 75
71 71
%
1283 1265
1306 1170 1281
1117 1134
3L6 383
341 377
378 368
504 506
489 489
504 501
N/mm 2 N/ram 2 N/ram 2
36.0 35.2
32.4 32.8
31.2 32.4
%
80 81
74 71
76 77
%
FLat compression test
CYy=yieldor0.2°/oproofs!.rength, o"u=u[timate tensite s t r e n g t h , ~ =fracture e l o n g a t i o n "y" : r e d u c t i o n in area of cross section, d"z=true f r a c t u r e s t r e n g t h ~deformation
12
11
10
No.
St.
Comparison of tensile properties obtained in rolling and in the fiat compression test for different deformation conditions
TABLE 6
1L32 1500
1115 1030
1274 1305
N/mm 2
63
The averages of the tensile properties shown in Tables 3--6 are compared in Figs. 15 and 16 for the two processes. The equation of the best-fit straight line, along with the correlation coefficient obtained using regression analysis, is also given in these figures. It can be seen that all the tensile properties of rolled and fiat compression test specimens agree within a maximum error of about 5%. 3.4. M icrostructure In order to study bow the processes of rolling and fiat compression influence the austenite grain shape, size and distribution, specimens after rolling /.60
i
i
i
i
O'yR=13.6 .1.00 O'YFcT / ¢-
k=O.5Z. * /~// .+ o / /
/.20
C
~E
/
// o one pass or stroke + two or four posses or strokes
2"•g 300
, , , I I 340 3l~0 420 /.60 500 Yield or o.2"/,proof strength in f l a t compression test JF~"cT'N/mm2
I
300
i O"UR=113"2* 0"82 L O"UFcT+
620
k =o.g7
.,
o ~"
~E
//
Z
,J'/ soo/
=o c
"- & 6 0 / 460
/
k= correlation coefficient
IIitll
500 5/.0 580 620 Ultimate tensile strength
in flQt compression test
660
O'UFcT , N/ram 2
Fig. 15. Comparison of average yield and ultimate tensile strength for rolling and the fiat compression test.
64 40
~36
o one pass or stroke / ÷ two or four passes or strokes,,/
/.
E~ C
/
/"
y/"
-~ 32 r
/•R
= 8.19 +0 202
~FCT
C
k=0.754
~t - 28
*o
O
0 LL.
28 32 36 40 Fracture elongation in f l a t compression test SFCT ,°/o
2~
I 80
/
~ 76 .c_ 72
= -23.99.1.31 "~FCT
// =
o-64/
Iltll
I 64
68
72
I
7
Itll
Reduction in area in flat compression test ~rFcm,%
Fig. 16. Comparison of average fracture elongation and reduction in area of cross section for rolling and the fiat compression test.
and fiat compression were water-quenched after deformation at different temperatures. These specimens were stress-relieved at 300°C for 1 h and prepared for austenite grain study. The results obtained are shown in Figs. 17 and 18, where it can be seen that for all the deformation temperatures tested there is a very good resemblance in austenite grain shape, size and distribution of the rolled and the fiat compression test specimens. Similar microstructures obtained after holding to different times in air after deformation at 900 and 950°C showed a very good resemblance between the two processes.
65
Rolling
Flat compression test
)P
\
t--
1226~11h
ho= 20,3mm 15t = 2 6 . 1 %
air----/ \
\',
t z-
t
1250°C/Ih
~ ~ . - w a t er / ho=2?,3mm \ 300oC_li.h { h4 =15.1mm ~ F-----~/ ~,~ =25.6 %
\+
+.. 7". i , .
+ +
\
i;g"~+~': ~'¢: .. . . . . . .
.+
.+
,1250oC/lh -+~ o. ,++
2-",, ho=2Oo3mm
"+-'N /water y alr---.7
h1 -t5 7 mm
~300eCllh
-e
/ t
Fig. 17. C o m p a r i s o n of austenite grains o b t a i n e d u n d e r the same test c o n d i t i o ~ in rolling and in the fiat compression test. E t c h a n t : Picric acid + CuC12 + Agepon.
66
Rol ling
Flat compression test
1.1~ls 2.1xzu \ 113mm E,~=3235 17.2%
h 1 = 13.65
i =20,3ram F..= .=/,/,.3%\
/
\~:--~-
tpQss 2toss
~water
/ g~ 32.75 30./. %
300~"/lh t
II. _':;.",~ :? _ Y=" " _
t
F~I=25.6 % F~2=23"2% 1~3=20.2% F./.=13.1% ho =20.3ram hi. =8 mm e,tot = 61%
r - - ~ ai t ~ _ --~ t
Fig. 18. C o m p a r i s o n of a u s t e n i t e grains o b t a i n e d u n d e r t h e s a m e t e s t c o n d i t i o n s in rolling a n d in t h e fiat c o m p r e s s i o n test. E t c h a n t : Picric acid + CuCI 2 + A g e p o n .
67 4. Conclusions The results of these investigations may be summarized as follows: (1) In spite of the inert (argon) atmosphere employed for austenitizing the specimens, there is a marked amount of decarburization and reduction in total nitrogen content at the surface of the specimens which can be ascertained by examination of microstructure and chemical composition. (2) The variation of the hardness distribution shows that the highest hardness does not occur at the centre, but occurs at about 2 to 3 mm from both surfaces. A similar type of hardness is obtained in rolling as in the fiat compression test, irrespective of the deformation temperature and the cooling conditions after deformation. (3) The above type of hardness distribution is attributed mainly to the variation in chemical composition across the specimen thickness. (4) Under steady~state conditions there exists a generally uniform strain across the thickness and width of the rolled specimen. (5) The impact strengths in rolled and fiat compression test specimens agree very well, for all impact test temperatures explored. (6) There is a linear relationship between all tensile properties of rolled and fiat compression test specimens, under similar deformation conditions. Based on regression analysis the mathematical relationship has been evaluated and the maximum deviation between the two results found to be about 5%. (7) Under identical deformation conditions the austenite grain size, shape and distribution in rolling and in the fiat compression test are very similar. (8) The simulated fiat compression test is comparable to rolling from the point of view of hardness distribution, impact strength, tensile properties and austenite grain size, shape and distribution, under similar deformation conditions. (9) The fiat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the optimum rolling schedule.
References 1 G. Robiller, L. Meyer and S.R. Datta, Thyssen Technische Bericht, 7(1) (1975) 14-25. 2 R.A.P. Djaic and J,J. Jonas, J. Iron & Steel Inst., 210(4) (1972) 256--261. 3 R. Priestner, C.C. Earley and J.H. Rendall, J. Iron & Steel Inst., 206(12) (1968) 1252--1261. 4 D.J. Walker and R.W.K. Honeycombe, Metal Science, 12(9) (1978) 445--452. 5 I. Weiss and J.J. Jonas, AIME symposium on "Precipitation effects in HSLA steels", Denver, Colo., USA, Feb. 1978. 6 A. Jones and B. Walker, Metal Science, 8(1974) 397--406. 7 N.V. Thikhiy, Ya.I. Spektor, M.I. Sinelnikov, R.I. Entin and R.V. Yatsenko, Fiv. Met. Metalloved, 38(6)(1974) 1250--1255. 8 H.D. Bartholot, H.J. Engell, W. Esche and K. Kaup, Stahl u. Eisen, 91(4) (1971) 204--220.
68 9 K.J. Irvine, T. Gladman, J. Orr and F.B. Pickering, J. Iron & Steel Inst., 208(8) (1970) 717--726. 10 M. Fukuda, T. Hashimoto and K. Kunishige, The Sumitomo Search, 9(9) (1973) 8--23. 11 T. Gladman, I.D. Mc.Iror and D. Dulieu, Micro-alloying '75 Conference, Washington, U.S.A., Oct. 1975. 12 R.A. Petkovic, M.J. Luton and J.J. Jonas, International symposium on "Hot forming of steel", ~trbsk~ Pleso, C.S.S.R., Sept. 1974. 13 R. Roussev, Trans. ISIJ, 13 (1973) 105--110. 14 H. Matsubara, T. Osuka, I. Kozasu and K. Tsukada, Trans. ISIJ, 12 (1972) 435--443. 15 I. Kozasu, T. Shimizu and K. Tsukada, Trans. ISIJ, 12 (1972) 305--313. 16 T. Brnlin, Annals CIRP, 25(1) (1976) 185--189. 17 O. Pawelski, Z. Metailkunde, 68(2) (1977) 79--89. 18 D. Bauer, Metall, 32(8) (1978) 776--784. 19 E. Neuschiitz and H. Rohloff, Stahl u. Eisen, 96(25--26) (1976) 1303--1307. 20 H. Weiss, D.H. Skinner and J.H. Everitt, J. Phys. E (Scientific Inst.), (6) (1973) 709. 21 S. Fulop, K.C. Cadien, M.J. Luton and H.J. McQueen, J. Testing and Evaluation, 5(6) (1977) 419. 22 W. Vanovsek and H. Trenkler, Berg. u. Hiittenwes., (9) (1977) 397. 23 R. Kaspar and O. Pawelski, Proc. 19th, Int. Mach. Tool Des. Res. Conf., Manchester, Sept. 1978, Macmillan, London, 1979, pp. 247--253. 24 O. Pawelski, U. Riidiger and R. Kaspar, Stahl u. Eisen, 98(5) (1978) 181--189. 25 O. Pawelski, V. Gopinathan, J. Mech. Work. Tech., 5 (1981). 26 V. Gopinathan, Research report on "A comparative study of hot rolling with fiat compression test", Dept. Metal Forming, MPI, Diisseldorf, May 1980. 27 E. Schmidt, August-F~ppl. festschrift, Berlin 1924, pp. 179--189.