AS-ROLLED MULTI-PHASE MICROALLOYED STEEL BARS WITH IMPROVED PROPERTIES
VALJANE VEČFAZNE MIKROLEGIRANE JEKLENE PALICE Z IZBOLJŠANIMI LASTNOSTMI
DJORDJE DROBNJAK1, A. KOPRIVICA2
'Faculty of Tech & Met University of Belgrade, Belgrade, YugosIavia 2Institute for Ferrous Metallurgy, Niksic Yugoslavia
Prejem rokopisa - received: 1997-10-01; sprejem za objavo - accepted for publication: 1997-10-21
A series of experimental steels, based on a 0.3 C, 1.5 Mn, 0.1 V composition, with and vvithout 0.01% Ti addition, vvas made by laboratory and full-scale casting, and fabricated into 22 mm dia bars by full-scale hot-rolling. Multi-phase Polygonal Ferrite-Pearlite-Non Polygonal Ferrite (PF-P-NPF) structures, with varying amount of NPF, are obtained in as-rolled bars. Acicular Ferrite (AF) and classical Bainite Sheaves (BS) are found to be dominant NPF morphologies in steels vvith a lovv (<30%) and a high (>40%) fraction of NPF, respectively. In Ti-bearing PF-P-AF steels, the PF-grain and P-colony size control, obtained through fine TiN particles, vvhich also provide preferential sites for intragranular nucleation of AF, a tensile strength of 800/850 MPa and 900/950 MPa in 0.2 and 0.3 C steels vvas obtained, vvhile maintaining the room temperature CVN impact energy at a level of 65/75 J and 40/50 J, respectively. A high fraction of grain boundary nucleated BS is promoted by increasing the content of Cr. Mo or Mn above the base level. The main effect of introducing BS into structure is a drop in impact toughness. Even so, in some BS dominated steels (notably Cr treated) an impact energy of 30/35 J is maintained at a tensile strength level of 1050/1100 MPa. These results have provided a bases for the development of as-rolled medium carbon microalloyed engineering bars. that achieve satisfactory properties in the as-rolled conditions vvithout the need for subsequent heat treatment.
Key vvords: microalloying, bar-rolling, polygonal ferrite, pearlite, non-polygonal ferrite, bainite, acicular ferrite, strength, toughness
Vrsta eksperimentalnih jekel z osnovno sestavo 0.3 C, 1.5 Mn in 0.1 V z ali brez dodatka 0.01% Ti je bila izdelana z laboratorijskim in z industrijskim vlivanjem ter industrijsko izvaljana v 22 mm palice. Dosežene so več fazne poligonalni ferit-perlitne - poligonalni ferit (PF-P-NPF) mikrostrukture z različnim deležem NPF. Acikularni ferit (AF) in klasični bajnitni snopi (BS) so prevladujoče NPF morfologije v jeklih z majhnim (<30%) in velikim (>40%) deležem NPF. V PF-P-AF jeklih z dodatkom Ti, kjer je dosežena kontrola velikosti zrn PF in P kolonij z izločki TiN, ki so tudi prednostna mesta za intragranularno nukleacijo AF, je bila dosežena pri 0.2 in 0.3 C jeklih natezna trdnost 800/850 in 900/950 MPa. Pri tem je ostala CVN udarna energija pri temperaturi ambienta 65/75 oz. 40/50 J. Velik delež BS z nukleacijo na kristalnih mejah se doseže z dodatkom Cr, Mo ali Mn nad osnovno vsebnostjo. Glavna posledica prisotnosti BS v mikrostrukturi je zmanjšanje udarne žilavosti. Vendar je bila tudi v nekaterih jeklih s prevladujočo BS (predvsem z dodatkom Cr) ohranjena udarna energija 30/35 J pri trdnosti 1050/1100 MPa. Ti rezultati so bili osnova za razvoj srednje ogljičnih konstrukcijskih jekel, ki imajo v valjanem stanju dobre lastnosti brez dodatne toplotne obdelave.
Ključne besede: mikrolegiranje, valjane palice, poligonalni ferit, perlit, ne poligonalni ferit, bainit, acikularni ferit, trdnost,
žilavost
1 INTRODUCTION
Medium-carbon microalloyed (MA) forging steels have been extensively studied over the past decade, and the results are reported at several international confer-ences (e.g.1"3). There are many examples to show that MA steels can replace Q&T steels in as-forged and air-cooled conditions, without subsequent heat treatment. Hovvever, in the ferrite-pearlite (FP) version, these steels suffer from inferiors notch toughness. In the last few years, bainite (B) and martensite (M) type grades, have received considerable attention as viable candidates to replace FP steels. Compared to M type (e.g.45) the air-hardened B-type grades need not quench, but the im-provement in toughness is not as-spectacular4613, and in some instances a deterioration in toughness is claimed14" 16. This as vvell as other disadvantage of B-grades, such as a lovv yield ratio13-17 and poor machinability7, may be among the reason why they are scarcely used17.
The strength and toughness of as-hot rolled bars have been less extensively studied in comparison to forgings.
Hovvever, vvhile considerable vvork has been done to improve the toughness of conventionally or controlled rolled bars in FP version (e.g.131819), little effort has been made to improve the toughness of bars by introducing B into structure. Improvements achieved in this vvork are mostly based on recent results vvhich shovv2021 that, vvhile some B-morphologies are beneficial, the other are detrimental to toughness.
2 EXPERIMENTAL DETAILS
2.1 Material
A series of experimental heats, based on 0.3% C, 1.5% Mn, 0.1% V composition (small variations among different heats are included in Table 1) vvith and vvithout 0.01% Ti addition, is used in this vvork. A number of heats vvere modified by either reducing the content of some additions (e.g.: C to 0.2%; N to 15-60 PPM; V, Ni and Cu to traces) or increasing the content of other additions above the base level (e.g.: Mn to 1.55/1.65 or
1.72/1.78%; Cr to 0.37/0.38 or 0.57%; Mo to 0.21%; V to 0.16/19%; N to 160/240 ppm; S to 150/250 ppm).
Table 1: Steel compositions (in wt.%) Tabela 1: Sestava jekel (v ut.%)
C Si	Mn	P	S	Cr Mo Ni Al Cu	N	V
0.27 0.29	1.47	0.006	0.006	0.21 0.03 0.15 0.02 0.18	0.010	0.10
0.32 0.39	1.57	0.010	0.010	0.30 0.06 0.19 0.03 0.31	0.012	0.13
2.2	Casting and hot vvorking of steels
Either laboratory vacuum or full-scale casting was used to produce 60 kg and 2630 kg ingots, labeled L and I-ingots, respectively. L-casting practice proved2" to be effective in producing the fine TiN particles in Ti-bearing steel, capable of imposing a pinning effect on austenite grain boundaries during subsequent reheating and rolling. The ingots were fabricated into 22 mm dia bars on production facilities. They were first fabricated into 120 mm (I-ingots) and 80 mm squares (L-ingots) by hot-roll-ing on a bloom mili or by press-forging, respectively. Then, the squares were hot-rolled to 22 mm bars, on either a continuous (120 mm squares) or a cross-country (80 mm squares) bar mili, using the conventional rolling practice, which involved soaking at 1150°C and finish-rolling above 950°C.
2.3	Testing
Room temperature properties are evaluated from round tensile 0» = 40 mm; do = 8 mm) and standard Charpy V-notch (CVN) longitudinal specimens, which were taken mid-radius from the as-rolled bars. A few im-pact tests were run at -50°C. Conventional metal-lographic techniques were used for revealing the microstructure.
3 RESULTS
3.1 Structure
A multi-phase Polygonal Ferrite-Pearlite-Non Po-lygonal Ferrite (PF-P-NPF) structure, with varying amount of NPF, is developed in the as-hot rolled bars. Acicular Ferrite (AF) and classical Bainite Sheaves (BS) are found to be dominant NPF morphology in steels with a lovv (<30%) and a high (>40%) fraction of NPF (to be referred to as PF-P-AF and PF-P-BS steels), respectively.
Intragranularly nucleated, mostly needlelike plates, vvhich radiate in many directions, referred to as AF, are shovvn in Figure la, and as a part of PF-P-AF structure, in Figure lb. The former austenite grain boundaries are decorated by Grain Boundary Idiomorphs (GBI) in Figure la, but Grain Boundary Alotriomorphs (GBA) or Widmanstatten Sideplates (WSP) are also frequently observed20. Grain boundary nucleated parallel ferrite plates, referred to as BS, are shovvn in Figure 2a, and as a part of a PF-P-BS structure, in Figure 2b.
AF is dominant NPF morphology in steels vvith composition vvithin the limits given in Table 1, vvhile a high fraction of BS is promoted by increasing Mn, Cr and Mo content above the base level. The tvvo morphologies (in-cluding some transient variants) generally coexist in various proportions. In PF-P-AF steels, the addition of Ti increases the AF/BS ratio (Ti addition also refines the PF-grain and P-colony size). In PF-P-BS steels the BS/AF ratio depends upon the alloying addition. Thus, in 0.57% Cr and 0.21% Mo steels, BS coexist vvith a de-tectable fraction of AF (Figure 3a), vvhile in 1.72/1.77% Mn steels BS are the only morphology present (Figure 3b). Ali three steels are L-cast and Ti-treated, but the tvvo latter are virtually free of PF and P.
3.2 Structure-Propert>• Relationship
Strength and Impact Properties (YS = Yield Strength; TS = Tensile Strength; CVN20 = Charpy V-Notch impact energy at 20°C) are, in terms of NPF fraction, shovvn in Figures 4a and 4b, respectively. Each data point repre-sents an average of tvvo (strength) and three to five tests (toughness). The TS remains virtually unchanged vvith increasing fraction of NPF up to about 70% and then
Figure 1: (a) Acicular Ferrite (AF), Grain Boundary Idiomorphs (GBI) and Pearlite(P); (b) Polygonal Ferrite-Pearlite-Acicular Ferrite (PF-P-AF)
Slika 1: (a) Acikularni ferit (AF), idiomorfi (GBI) in perlit (P) po kristalnih mejah; (b) poligonalni ferit perlit - acikularni ferit (PF-A-AF)
Figure 2: (a) Bainite Sheaves (BS); (b) Polygonal Ferrite-Pearlite-Bainite Sheaves (PF-P-BS)
Slika 2: (a) Bainitni snopi (BS); (b) poligonalni ferit perlit - bainitni snopi (PF-B-BS)
Figure 3: (a) Bainite Sheaves (BS) and Acicular Ferrite (AF); (b) Bainite Sheaves (BS)
Slika 3: (a) Bainitni snopi (B) in acikularni ferit (AF); (b) bainitni snopi (BS)
slightly increases, vvhile the YS first decreases and than, at 30-40% of NPF, attain a constant level. These changes are neither consistent with variations in fraction nor in morphology of NPF (AF = open symbols; BS = closed symbols, in Figure 4). A relatively broad scatter band in Figure 4a is presumably a reflection of small variations in composition. For instance, decreasing C from 0.3% to 0.2% produces a drop in YS and TS from 600/650 to 550/600 and from 900/950 to 800/850 MPa, respectively. Moreover Ti, Cr and Mn produce an effect vvhich can be discriminated vvithin the scatter-band itself. Thus, Ti-free PF-P-AF steels (Region la) exhibit a higher strength level then Ti-bearing steels (Region lb), vvhile 0.57% Cr PF-P-BS and 1.72/1.77% Mn B-steels (Region 2a) ex-hibit a higher TS level (1050/1100 and 1150/1200 MPa, respectively) than the other PF-P-BS grades (Region 2b).
Data of Figure 4b shovv that an increase in impact energy vvith increasing fraction of NPF is interrupted by a pronounced drop on passing from AF dominated - Region 1 to BS dominated - Region 2. Within the PF-P-AF Region lc, the energy attains a level of 65/75 J at 20°C (33 J at -50°C), corresponding to YS = 550/600 MPa and TS = 800/850 MPa; and vvithin the Region lb, a level of 40/50 J at 20°C (32 J at -50°C), corresponding to YS =
600/650 MPa and TS = 900/950 MPa, in 0.2% C/10% AF and 0.3% C/30% AF steels, respectively. Both steels are L-cast and Ti-treated. In general, the L-cast/Ti-treated steels exhibit a higher impact energy level (+Ti band in Figure 4b) than the Ti-free steels (-Ti band). The I-cast/ Ti-treated steels take an intermediate position (open squares). Within the PF-P-BS Region 2, the en-ergy attains a level of 30/35 J at 20°C (25/30 J at -20°C), corresponding to YS = 600 MPa and TS = 1050/1100 MPa; and vvithin the Region 2c, a level of 15/25 J at 20°C, corresponding to YS = 550/600 MPa and TS = 1150/1200 MPa, in 0.57% Cr/70% BS and 1.75% Mn/98% BS steels, respectively. In 0.57% Cr steel, in addition to BS, the NPF morphology comprises a detect-able fraction of AF (Figure 3a), vvhile in 1.75% Mn steels, BS is the only NPF morphology present (Figure 3b).
4 DISCUSSION
4.1 Structure
A slight difference in composition of the steels stud-ied in this vvork seems to be decisive in controlling not
535
PF-P-AF PF-P-BS
MS #(L)MiS°
60

a
> o
0 10 20 30 40 50 60 70 80 90 100 Non—Polygonal Ferrite (NPF), %
Figure 4: (a) Yield Strength (YS) and Tensile Strength (TS) as a Function of Non-Polygonal Ferrite (NPF) Fraction; (b) Charpy V-Notch Impact Energy at 20 (CVNzo) as a Function of NPF Fraction (PF = Polygonal Ferrite; P = Pearlite; AF = Acicular Ferrite; BS -Bainite Sheaves; L = L-Cast Ingots; I = I Cast-Ingots; +Ti = Ti-Bearing Steels; -Ti = Ti-Free Steels)
Slika 4: (a) Meja plastičnosti (YS) in natezna trdnost (TS) v odvisnosti od deleža ne poligonalnega ferita (NPF); (b) Charpy V udarna energija pri 20°C (CVN20) v odvisnosti od deleža NPF (PF - poligonalm fent, P - perlit, AF - acikularni ferit, BS - bainitni snopi, L-L - liti ingoti, I-I - liti ingoti, +Ti - jekla s Ti, -Ti - jekla brez Ti)
only the fraction of NPF but, together vvith inclusions, its morphology also.
Thus, the AF, vvhich is assumed to be either intra-granulary nucleated bainite22 or Widmanstatten ferrite23, together vvith various proportions of PF (e.g. Grain Boundary Idiomorphs, GBI) and pearlite, P (Figure 1), is produced in PF-P-AF grades, vvith a relatively low har-denability (<30% NPF). The Grain Boundary Ferrite (GBF) is assumed24 to render austenite grain boundaries inactive in respect to intergranular nucleation of BS, what in turn promotes the intragranular nucleation of AF on TiN or MnS inclusions20. The nucleation potential of inclusions varies vvith composition, crystal structure and dispersion, i.e. vvith their number, size and spacing22-25'29. For example, a low degree of misfit betvveen the ferrite matrix and the substrate crystal lattice is assumed25"31 to increase the nucleation potential of inclusions. There-fore, TiN vvith a misfit ratio of 3.827 should be much more effective than MnS vvith misfit ratio of 8.825. This could account for a higher fraction of AF observed in Ti-
bearing (fine and coarse particles, vvhich are presumably developed in L and I-cast ingots respectively, seems to be equally potent nucleation sites) than in Ti-free (high S) steels. Hovvever, in steels vvith high V and N, VN particles can be precipitated on MnS, before austenite is transformed to ferrite3031. Since the misfit ratio of VN, estimated at 1.3 for (001) plane27, is lovver than that of TiN, the former particles, i.e. MnS coated by VN, should be more potent nucleation sites for AF. While the effect is not quite apparent in the present steel, the fact remains that the fraction of AF is higher in high-S than in lovv-S/Ti-free steels.
The BS are produced in PF-P-BS grades vvith relativen high hardenability (>40% NPF). The nucleation of GBF is considerably suppressed, and an abrupt transition from AF dominated to BS dominated NPF morphology occurs not only in Ti-free but also in Ti-bearing steels. This supports the assumption that removal of GBF frees the boundaries to nucleate BS concurrently vvith intragranular AF.
4.2 Properties
ViflH and Tensi|p .Strpnih (YS. TSV It is vvell knovvn (e.g.6 8) that the YS of Polygonal Ferrite-Pearlite (PF-P) steels increases vvith increasing fraction of P. Additional strengthening is obtained by precipitation of V (C,N) particles in ferrite. Precipitation strengthening effect de-creases vvith decreasing N-content. In Ti-bearing PF-P-AF steels tested in this vvork, Ti forms nitride particles vvhich reduce the N available for VN precipitation and hence, reduce (from Region la to lb in Figure 4a) the strengthening associated vvith this precipitation. Gradual replacement of PF-P structure vvith NPF structure re-duces the YS, as shovvn in Figure 4a, due to suppression of precipitation vvithin the NPF phase (e.g.811). Hovvever, the TS is maintained at the same level (Region 1), and even increases (Region 2a) in some steels. This latter observation can be associated vvith a high strain-harden-ing rate imposed by bainite32.
rVN,„ Impact Tonphness (CVN.n), The present results, together vvith data presented in previous papers20'21, indicate that AF and BS are beneficial and detrimental to toughness, respectively. The brittle fracture of low-C bainites can be related to the cleavage facet size33, vvhich in the present steels can be related to either AF-plate or BS-packet size. AF-plates give rise to a finer facet size, and thus to a higher toughness, vvhile BS, in addition to being coarser itself (individual laths vvithin a sheaf have little effect, since the cleavage crack is deflected at the sheaf and not at the lath boundary33), contain coarse m-terlath carbides (a feature characteristic of upper bainite), vvhich are detrimental to toughness33.
The Ti-bearing PF-P-AF steels (Region lb in Figure 4b) exhibit a considerably higher toughness in compari-son to Ti-free steels (Region la). This is because the PF-grain and P-colony size is finer and the AF fraction is presumably higher in Ti-bearing steels vvhich contain
90
80
70 -
60
150 Z
> 40 O
30 i 20 10 "I 0
CVN +20°C
□ PF-P (Ref. 13,18)
■	PF-P-AF (This vvork)
■	PF-P-BS (This work) CVN J" • PF-P-AF (This vvork) -50°C l @ PF-P-BS (This vvork)
600 700 800 900 1000 1100
Tensile Strength, MPa
1200
Figure S: Tensile Strength (TS) vs. Charpy V-Notch Impact Energy (CVN20 = 20°C; CVN.50 = -50°C) of Conventionally Hot Rolled 0.2-0.5% C Polygonal Ferrite-Pearlite (PF-P), and 0.2-0.3% C Polygonal Ferrite-Pearlite-Acicular Ferrite (PF-P-AF) and Polygonal Ferrite- Pearlite- Bainite Sheaves (PF-P-BS) Bar Steels Slika 5: Natezna trdnost (TS) v odvisnosti od Charpy V udarne energije (CVN20 = 20°C CVN50 = 50°C) pri konvencionalno vroče valjanih 0.2 - 0.5% C poligonalni ferit perlit (PF-P) in 0.2 - 0.3% C poiigonalni ferit perlit - acikularni ferit (PF-P-AF) in poligonalni ferit perlit - bainitni snopi (PF-P-BS) jeklenih palicah
fine TiN particles. The role of particles is tvvofold, first they inhibit the austenite grain grovvth, and second, they provide the nucleation sites for intragranular nucleation of AF. Hovvever, Ti-bearing/I-cast steels exhibit a lovver toughness than Ti-bearing/L-cast steels, in spite of AF is a dominant NPF morphology in both. Coarser TiN particles in I-cast steels are not as effective in pinning austenite grain boundaries as fine particles in L-cast steels and lead to a coarser austenite grain, and conse-quently coarser PF-grain and P-colony size.
5 CONCLUDING REMARKS
The conventionally hot-rolled and air-cooled multi-
phase Polygonal Ferrite-Pearlite-Non Polygonal Ferrite
(PF-P-NPF) microalloyed Ti-bearing steels, tested in this
vvork (closed and shaded symbols in Figure 5), exhibit
improved impact properties in comparison to conven-
tional^ hot rolled PolygonaI Ferrite-Pearlite (PF-P)
steels (open symbols), tested previously (e.g.18-19).
The 0.2% C/10% AF and 0.3% C/30% AF PF-P-AF
steels, based on a 1.5% Mn - 0.1% V - 0.01% Ti compo-
sition, vvith an impact energy level of 65/75 J and 40/50
J at 20°C (33 and 32 J at -50°C), and tensile strength
level of 800/850 MPa and 900/950 MPa respectively, can replace Q&T steels vvhich currently achieve these prop-erty levels.
The use of a higher Cr or Mn contents give rise to
PF-P-BS structure vvith 70-98% BS and increased tensile
strength level up to 1050/1200 MPa at the expense of re-duced toughness.
Acknovvledgment
This vvork vvas supported by Steel Mili, Niksic, Yu-goslavia and permission to publish this paper is acknovv-ledged.
6 REFERENCES
1	Fundamentals of Microalloying Forging Steels, eds. G. Krauss and S. K. Banerji, The Metallurgical Society, VVarrendale, PA 1987
2	Microalloyed Bar and Forging Steels, ed. M. Finn, The Canadian Institute of Mining and Metallurgy, Montreal, QU 1990
3	Fundamentals and Applications of Microalloying Forging Steels, eds. C. J. Van Tyne, G. Krauss and D. K. Matlock, The Minerals, Metals and Materials Society, VVarrendale, PA 1996
4	W. A. Szilva et al., As Ref. 2, p. 227
5	M. Katsumata et al., Phys. Metali, of Direct-Quenched Steels, eds. R. A. Taylor et al., The Metallurgical Society, VVarrendale, PA 1993, p.
247
6	H. Hara and M. Kobayashi, Hot Forged MicroaIloyed Steels in Automobile Components, Vanadium Avvard, The Institute of Metals, 1987
7Vanadium As Forged Steels for Automobile Components, Vanitec Monograph No3, 1987 8 W, E. Heitmann and P. B. Babu, As Ref. 1, p. 55 9G. Tither, T. B. Cameron and D. E. Diesburg, As Ref. 1, p. 269 10P. H. VVright et al., As Ref. 1, p. 541
11	P. A. Khalid and D. V. Edmonds, As Ref. 2, p. 1
12	Y. Koyasu et al., As Ref. 2, p. 202
13	M. Cristinacce and P. F. Reynolds, As Ref. 3, p. 29
14	J. F. Held, Metal Progress, 128 (1985) 12, 17
15	V. Ollilainen, H. Hurmola and H. Pantinen, Journal of Materials En-ergy Systems, 5 (1984) 221
16B. Garbarz and R. Kuziak, Microalloying '95, The Iron and Steel So-ciety, VVarrendale, PA 1995, p. 409
17	H. Tokada and Y. Koyasu, As Ref.3, p. 143
18	P. E. ReynoIds and J. H. Reynolds, As Ref. 3, p. 79
19D. Tostenson, R. Bertolo and B. Glasgal, As Ref. 3, p. 327 20Dj. Drobnjak and A. Koprivica, As Ref. 3, p. 93 21 Dj. Drobnjak and A. Koprivica, Metal 97, Ostrava, Czech Republic 1997, p. 162
22H. K. D. H. Bhadeshia, Bainite in Steels, The Institute of Materials, London, 1992, p. 245
23	R. A. Ricks, P. R. Hovvell and G. S. Barritte, Journal of Materials Science, 17 (1982) 732
24	S. S. Babu and H. K. D. H. Bhadeshia, Materials Science and Tech-nology, 6 (1990) 1005
25	A. R. Mills, G. Thevvlis and J. A. VVhiteman, Ibid., 3 (1987) 12, 1051 26G. Thevvlis, Ibid., 10 (1994) 2, 110
27	Y. Tomita et al., ISIJ International, 34 (1994) 10, 829
28	J. L. Lee, Acta Metallurgica and Materialia, 42 (1994) 10, 3291
29	J. L. Lee and Y. T. Pan, ISIJ International, 35 (1995) 8, 1027 30F. Ishikavva and T. Takahashi, Ibid., 35 (1995) 9, 1128
31	F. Ishikavva, T. Takahashi and T. Ochi, Metallurgical and Materials Transactions /1, 25A (1994) 5, 929
32	K. Irvine and F. B. Pickering, ISI Spec. Rep. 93, London, 1965, 110
33	D. V. Edmonds and R. C. Cohrane, Metallurgical Transaction, 21A (1990) 6, 1527