DEVELOPMENT OF V-BASED MICROALLOYED HIGH STRENGTH HOT ROLLED STRIP. Branislav Škandera a Andrej Samoilov b Hans Peter Degischer a Peter Stiaszny b

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1 METAL 24 Hradec nad Moravicí DEVELOPMENT OF V-BASED MICROALLOYED HIGH STRENGTH HOT ROLLED STRIP Branislav Škandera a Andrej Saoilov b Hans Peter Degischer a Peter Stiaszny b a Technische Universität Wien, Karlsplatz 13, A-114 Wien, Austria, E-ail: bskander@ail.zserv.tuwien.ac.at b voestalpine Stahl GbH, Voest-Alpine Strasse 3, A-42 Linz, Austria, E-ail: andre.saoilov@voestalpine.co Abstract The objective o the project is to develop a new V-based icroalloyed HSLA hot strip grade oering a reduction o production costs by decreasing the rolling orces and the heat treatent eorts. The key requireents are the echanical properties as well as the suitability or welding and noralising treatent or which the produced Nb-based S355 (Nb) steel type serves as benchark. The cheical copositions are calculated with respect to precipitation o V(C,N) by Thero-Calc sotware. The echanical properties and rolling orces are odelled by the voestalpine Stahl CAQC-Syste. Rolling trials are carried out in order to conir these odelling results. The rolling orces decrease by about 11% in coparison with the conventially rolled Nb-types. The reheating teperature reduced by 3 C or a slightly corrected cheical coposition still yielding a rolling orces reduction o about 5% or roughing rolling and about 8% or inish rolling. The low stresses or the V-alloyed slabs and the echanical properties are copared or the dierent rolling trials with respect to the conventional Nb types. 1. Introduction The basic idea o strengthening o structural steels by icroalloying and by theroechanical processing is to exploit the eects o (i) grain reineent and (ii) icroalloy precipitation. Grain reineent is unique in that it provides both strength and toughness whereas precipitates increase strength at the expense o soe loss in toughness, but assist as well in the grain reineent. Weldability is norally aintained in both cases but it is substantially iproved, when grain reineent can be retained during welding. This is the coon basis or all steels icroalloyed with Nb, V or Ti or their cobinations. However, regarding optiised coposition and processing route account ust be taken to the dierences between these various eleents. The ost signiicant properties o V as copared to Nb and Ti are: (i) The solubility o V(C,N) is uch larger; which is o particular interest or reducing the solution teperature or alternatively, to dissolve larger aounts o V at a given teperature. (ii) As opposed to Nb but siilar to Ti, the solubility product o VN is about two orders o agnitude lower than that o VC. This iplies that N as a icroalloying eleent has a decisive role in V-icroalloyed steels, especially or the enhanceent o precipitation strengthening [1]. 1

2 2. Experiental procedure and results The copositions o the investigated elts are given in Table 1. Precipitation was calculated by.25 using Thero-Calc in order to Base+V FRT Base depict the precipitation potential o.2 Base+V the icroalloyed steels described Base+Ti below (Fig.1). Fro the Fig.1 is.15 Base+Ti clearly seen, that while the precipi- Base+N.1 tation o the Nb-icroalloyed steel St52 Nb is nearly coplete at inish rolling Base (313) teperature (FRT), essential part o the V(C,N) precipitation is preci-.5 S355 Nb pitated only at teperatures below FRT. The precipitation o Nb(C,N) Teperature [ C] starts at the reheating teperature o the slab o about 125 C inluencing Fig.1: Calculated precipitation o carbonitrides the rolling orces. However, the precipitation o V(C,N) o the Base (313) steel starts at the teperature o about 98 C, what gives roo to decrease both the reheating teperature and the rolling orces. Rolling was carried out in the hot rolling ill o voestalpine Stahl Linz GbH in three consecutive steps: (A) The technical rolling was carried out to veriy the theoretical predictions on cheically coposition and echanical properties, (B) The irst trial heat was executed to deterine the rolling orces and to optiise the coiling teperature, (C) The second trial heat was carried out to optiise the rolling process, especially the reheating teperature in conjunction with a slight correction o the cheical coposition to achieve the required echanical properties. The echanical properties were easured by tensile testing and by ipact toughness testing or all the coiling teperatures. The saples were taken ro the head, the iddle and the end o the strip length, as well as ro the edge, ¼ and ½ o the strip width. The echanical properties were easured parallel (L-Direction), as well as transversely (T- Direction) to the rolling direction. The saples or ipact toughness were taken ro the iddle o the strip and were tested at 5 C with the crack preparating in the transverse plane (TS). The tensile and the ipact toughness tests at -5 C ater noralising treatent were carried out or the saples taken ro the iddle o the strip length as well The technical rolling Mole raction o precip. carbonitrides The steels were anuactured according to Table 1 in a laboratory unit capable o producing 5 kg ingots. Rolling was carried out in the hot ill at voestalpine by iplanting, i.e. each cast ingot was sliced into two halves and tested or deects. Hal-size ingots o 4 steel copositions were welded together and welded into the two slabs o the steel grade S355Nb with the thickness o 211 (Fig.2). 2

3 Table 1: Cheical copositions o the elts or technical rolling (A) Steel C Mn Al Cr Ni Mo V Nb Ti N Base+ N 312,155 1,52,33,24,19,5,51,2,2,96 Si =,2 Base 313,159 1,5,32,2,19,5,51,2,2,53 P =,1 Base+ V314,163 1,51,23,2,19,5,1,2,2,54 S =,6 Base+ Ti 315,16 1,49,3,2,19,1,51,2,13,66 Cu =, Head Rolling direction Fig. 2: Position o hal-size ingots in the other slab Both slabs were heated to 12 C in a urnace, rolled to 6, in 5 roughing and 7 inishing passes and coiled with two dierent coiling teperatures o 6 C and 68 C, respectively. Finish rolling teperature or both slabs was 88-9 C. The results o tensile testing are shown in Table 2. All easured values are better than required or S355, except the ipact toughness o the steel type Base+V (314) or the coiling teperature o 6 C and R, R p,2 o the steel type Base+Ti (315) or the coiling teperature o 68 C, which are below the required level. Tab. 2: Mechanical properties o technical rolled steels CT Coiling teperature CT= 6 C R R p,2 A [%] IT [J] Ater noralising R R p,2 A [%] Base (313) Base+N(312) Base+V(314) Base+Ti(315) S355 Nb CT= 68 C R R p,2 A [%] IT [J] IT [J] Ater noralising R R p,2 A [%] Base (313) Base+N(312) Base+V(314) Base+Ti(315) S355 Nb Goals >355 >2 >27 IT [J] The echanical properties ater noralizing treatent are in good agreeent with the required liits, the ipact toughness is considerably increased. The values R and R p,2 or both coiling teperatures increase with the increase o Vand N content in steel, whereas the elongation decreases slightly and the ipact toughness decreases considerably. In case o the increased Ti content in steel the results show an opposite trend. 3

4 2.2. The irst trial heat Four slabs were cast with the cheical coposition according to the Base (313) steel as shown in the Table 3. The slabs were rolled under the sae conditions as the steel type S355 with Nb at coiling teperatures o 58 C, 62 C, 64 C and 67 C, respectively. Table 3: The cheical coposition o the trial heats B and C based on Base (313) o Table 1 Steel C Si Mn P S Al Cr Ni Mo V Nb Cu Ti N 1. Trial,6,16,2 1,47,13,42,45,25,18,7,48,3,28,1 heat (B) 4 2. Trial,16,2 1,49,1,3,47,2,1,1,35,2,2,1,6 heat (C) The echanical properties as presented in Fig.3 and 4 satisy the required liits except the tensile (TS) and yield (YS) strength values or the coiling teperature o 67 C. Alost all o the easured values are the sae or both trials, except the ipact toughness value at 5 C o the coiling teperature o 6 C. Tensile (TS) and yield (YS) strength values o the coiling teperature o 68 C show soe dierence between the conventional (trial heat) and the technical (iplant) rolled steels. YS,TS IT, A [J, %] Coiling Teperature [ C] R Rp.2 A [%] IT [J] Fig. 3: The echanical properties First trial heat (B) YS,TS IT, A [J, %] Coiling Teperature [ C] Fig. 4: First trial heat-ater noralising A Base313 V [wt%] V-prec[%] CT 6 C,92,51 CT 68 C,143 B V [wt%] V-prec[%] CT 58 C,68 CT 62 C,11,48 CT 64 C,127 CT 67 C,14 C V [wt%] V-prec[%] CT 6 C,54,35 CT 62 C,51 Tab. 4: Cheical analysis o extracted precipitates, A-Technical rolling, B-1.trial heat, C-2.trial heat The echanical properties ater noralising treatent (Fig. 4) atch the requireents or all coiling teperatures. The ipact toughness values at 5 C exceed the required level. A cheical analysis o precipitated icroalloying eleents (Tab. 4) was ade or extracted residui ro each o the coiling teperatures. The aount o V-precipitated correlates with the coposition and generally increases with increasing coiling teperature. 4

5 Optical icrographs o the industrially rolled strips (Fig. 5) show a ore pronounced banded structure. It is caused by the coarser dendrite structure in the industrially cast slabs in coparison to laboratory cast ingot o the iplant rolled strips. All o the icrographs presented or the trial heats are taken ro the iddle o the strip in width, length and thickness and copared with the icrographs o the technically rolled Base (313) steel. a c d b e g Fig. 5: Optical icrographs o cross sections o the rolled steels a-technical rolling o Base (313) CT 6 C; b- like (a) CT 68 C; c-first trial heat CT 58 C; d- like (c) CT 62 C; e- like (c) CT 64 C; - like (c) CT 67 C; g- Second trial heat CT 6 C; h- like (g) CT 62 C h 2.3. The second trial heat The cheical coposition o the slabs was slightly corrected with respect to the irst trial heat by the reduction o the V-content ro the level o,5 % to,35% (Tab.3), with the ai to reduce production costs and to iprove the ipact toughness. A decrease o the reheating teperature ro 12 C to 117 C was tested exploiting the larger solubility o V(C,N) to reduce the production costs. Two slabs were rolled under the sae conditions as the steel type S355 with Nb but with 3 C lower reheating teperature and at coiling teperatures o 6 C and 62 C. Tab. 5: Coputed teperatures in iddle o the slabs while in the reheating urnace Core Teperature [ C] Average Teperature [ C} 1. Trial heat (B) Trial heat (C)

6 YS,TS YS, TS a) as rolled A, IT [%,J] b) as noralised A, IT [J] Coiling Teperature [ C] TS YS A [%] IT [J] Fig. 6: Mechanical properties o the second trial Heat (C) The results o the tensile and ipact toughness tests or all the coiling teperatures satisy the required liits and are in the sae range or both trials. The ipact toughness values at 5 C or the second trial heat saples are considerably iproved and show less scatter in coparison with saples o the irst heat The rolling orces TS The values o rolling orces, low stresses, reductions and strain rates or both steel types S355+Nb and S355+V were deterined by using CAQC-Syste. Alost all o the easured values o teperatures, reductions, strain rates and rolling orces dier between both steel variants. The rolling orce F is deterined by a) low stress actor k characteristic or the coposition, b) teperature T, c) deoration?, d) deoration rate??, e) geoetry actors (thickness, reduction, radius) Our task is to separate the steel property k ro other paraeters by excluding at irst the geoetrical actors using Sis approxiation: A [%] 6 62 Coiling Teperature [ C] YS IT 5 [J] d F? w? l? 2? k? Q [N] (1) P where w strip width; l d contact length; actor. k - ean low stress; Q P geoetrical The value o k is calculated by the CAQC syste or every segent o the strip. To derive dependencies on b) and d), the Hensel-Spittel approxiation is used: B1T? B2 B3? B4????? k? e?????? e k (2) Statistic_ it 6

7 ? k? Statistic_ it describes the ean low stress obtained by statistic itting. The real low stress (k ) Experiental is a little bit higher that that o the statistic it or the steel with Nb, but saller or the steel with V. Coeicients B1, B4 were calculated by regression analysis or a suicient aount o strips. A k_nor was calculated? k?? Experient al B2 B3? B4???? k _ nor (3)???? e The k_nor contains dependencies on coposition a) and teperature b) explicitly, the other dependencies are included in the iperectness o the approxiations. The dependence on the coposition is plotted in the Figure 7; k_nor describes the pure inluence o the coposition and is calculated by:? k? al?? Experient k Statistic_ Fit k _ nor? (4) k_nor 1,15 1,5 1,15 1,1 1,1 1,5 Mean Plot (k_s355_out_oil_21_phi_select.sta 56v*384c) k_nor 1,15 1,15 1,1 1,1 1,5 1,5 Mean Plot (k_vp52_5_6mm_oil_phi_select.sta 66v*2657c) S355+Nb S355+V 1, k,9,85 1,,95,95,9,85 k 1, 1,,95,95,9,9,85,85,8,75,8,75 V1 V2 V3 V4 V5 F F1 F2 F3 F4 F5 F6 V1 V2 V3 V4 V5 F F1 F2 F3 F4 F5 F6 Rolling stand Rolling stand,8,8,75,75 V1 V2 V3 V4 V5 F F1 F2 F3 F4 F5 F6 V1 V2 V3 V4 V5 F F1 F2 F3 F4 F5 F6 Rolling stand Rolling stand a) the irst trial heat (B) strip thickness 6 b) the second trial heat (C) strip thickness 5,6 Fig. 7: Coparison o the k-values according to (4) or both trial heats B and C V-V5 roughing rolling stands F-F6 inish rolling stands A statistical analysis o the rolling loads showed the decrease about 12% o low stress during the irst trial roughing rolling o the V-alloyed slabs in coparison with the conventional Nb-alloyed ones, see Fig. 7a, regarding rolling stands V1-V5. The decrease o the low stress o the irst trial inishing rolling is about 9% in avour o the Vanadiu steel. The Vanadiu icroalloyed steel o the second trial heat showed the decrease o low stresses o about 5% o the roughing rolling and o about 8 % o inish rolling in coparison with the Niobiu-steel, except or the rolling stand F2, that showed opposite results. In this case, the V-alloyed slabs were rolled at reduced reheating teperature than usually Nballoyed ones. 7

8 3. Suary and conclusions The new HSLA steel type was developed by replacing Nb by an appropriate aount o Vanadiu and by adjusting the nitrogen concentration. The ain advantage lies in the reduced hot rolling eort (orce, teperature) while the echanical properties o the developed HSLA steel are aintained on the requested level. The reheating teperature can be decreased by about 3 C, the coiling teperature can be held in the range o 6-62 C. The kinetic o the V(C, N) precipitation will be studied in order to urther iprove the rolling process and the reproducibility o speciied properties o the V-icroalloyed steel. 4. Reerences 1. R. LAGNEBORG, T. SIWECKI, S. ZAJAC, B. HUTCHINSON: The role o vanadiu in icroalloyed steels, Scandinavian Journal o Metallurgy 1999; 28: S. ZAJAC, T. SIWECKI, B. HUTCHINSON, M. ATTLEGARD: Recrystallised controlled rolling and accelerate cooling or high strength and toughness in V-Ti-N steels. Metal Trans A 1991: 22A: AT DAVENPORT, LC BROSSARD, RE MINER: Precipitation in icroalloyed HSLA steels, J Metals 1975:27: AL DE ARDO: Modern theroechanical processing o icroalloyed steel: Con. Microalloying 95, Pittsburgh June 1995, Iron and Steel Soc D. RAMAKRISHNA, S.P. GUPTA: Coarsening kinetics o grain boundary VC in a HSLA steel; Metallurgica, Vol.2, pp ,