Determining the Conditions for Dynamic Recrystallization in Hot Deformation of C Mn V Steels and the Effects of Cr and Mo Additions

Transcription

1 , pp Determining the Condition for Dynamic Recrytallization in Hot Deformation of C Mn V Steel and the Effect of Cr and Mo Addition Charle de Abreu MARINS, 1) Evgueni POLIAK, 2) * Leonardo Barboa GODEFROID 3) and Nina FONSEIN 2) 1) ArcelorMittal ubarão, Vitória, Epírito Santo, Brazil. 2) ArcelorMittal Global R&D, Eat Chicago, IN, USA. 3) Univeridade Federal de Ouro Preto, Ouro Preto, Mina Gerai, Brazil. (Received on July 19, 2013; accepted on Augut 26, 2013) he refinement of microtructure and it homogeneity during controlled hot trip rolling i primarily achieved by controlling the autenite recrytallization before it tranformation during accelerated cooling. he paper decribe the methodology to determine the deformation condition favorable for dynamic recrytallization (DRX). Uing thi methodology it become poible to delineate the condition for potdeformation tatic and metadynamic recrytallization a well. he work i baed on vicoplatic power law formalim applied to teady tate flow within wide range of deformation temperature and train rate. wo euation of the ame form but with different coefficient can be ued depending on whether the teady tate flow i controlled by dynamic recovery (DRV) or DRX. he tranition from DRV- to DRX at the correponding value of Zener-Hollomon parameter Z t can be viewed a the demarcation between tatic and metadynamic recrytallization occurring after deformation. he approach i illutrated uing low carbon Mn V teel. Alloying with Cr and epecially with Mo uppree DRX and MDRX a manifeted by increaing Z t. KEY WORDS: low alloy teel; hot deformation; dynamic recrytallization; dynamic recovery; power law. 1. Introduction Controlled hot trip rolling of low carbon low alloyed teel aim at refining the microtructure to enhance mechanical propertie of both a hot rolled trip and after further proceing. he refinement of microtructure i primarily achieved by controlling the recrytallization of autenite before it tranformation during accelerated run-out table cooling. 1 3) he recrytallization behavior of autenite during and after hot deformation operation i thu one of the key factor determining the hot rolling trategy and proceing parameter. In developing the trategy for controlled rolling it i important to keep in mind that partial recrytallization of hot rolled autenite prior to the beginning of it tranformation i highly detrimental for the propertie of teel product even after downtream proceing. he type and kinetic of phae tranformation in cooling of deformed autenite differ dramatically from thoe of recrytallized autenite. 4) When autenite i only partially recrytallized before tranformation, the deformed and recrytallized portion tranform into different phae and at different rate which reult in highly variable microtructure and hence propertie after cooling. hi i further exacerbated by temperature, train and train rate variability within a bar and between bar typical for * Correponding author: evgueni.poliak@arcelormittal.com DOI: indutrial hot rolling. Becaue of thee variation in proceing parameter the partial recrytallization of autenite i practically uncontrollable. he induced variability of microtructure and hot band mechanical propertie cannot be eliminated in cold rolling and i inevitably carried over into the annealing. herefore, minimization of downtream nonuniformity of microtructure and propertie to improve the final uality of teel heet product reuire obtaining more uniform and well controlled hot band microtructure, particularly by avoiding partial recrytallization of autenite during hot trip mill proceing. In hot rolling of autenite, the recrytallization of the following type i known to occur. Firt to mention i the dynamic recrytallization (DRX) during deformation in the roll bite. After deformation (i.e., between the rolling pae and after exiting the lat finihing pa) the conventional tatic recrytallization (SRX) can take place if the train in the preceding rolling pae ha not been high enough to initiate DRX. 5) When train i ufficient to ignite DRX, the metadynamic recrytallization (MDRX) occur after deformation. 6) If DRX i incomplete then after deformation both SRX and MDRX can operate imultaneouly: dynamically recrytallized portion of material continue to recrytallize metadynamically while the remaining portion can undergo SRX. 6) here i an important practical ditinction between MDRX and SRX: the former tart immediately after deformation wherea certain time ha to elape before the onet ISIJ

2 of SRX. In other word, SRX reuire the o-called incubation time for initiation, while for MDRX the incubation time i zero. 7) Becaue of that, controlling the autenite recrytallization to prevent mixed non-uniform grain tructure in hot rolled teel implie two important apect. Firt, it i neceary either to avoid DRX during deformation (which eliminate MDRX between pae and after deformation) or, on the contrary, to enure the completion of DRX. Second, the initiation of SRX can be uppreed by making the incubation time longer than the characteritic time period of rolling proce (interpa time, gap between the lat pa exit and the beginning of tranformation in accelerated cooling, etc.). Converely, the completion of SRX can be achieved by the extending the time available for SRX after deformation. hu, SRX control can be accomplihed by adjuting the rolling peed o a to prevent SRX or, alternatively, to enure it full completion before the onet of tranformation. It i MDRX that i more harmful in term of generating nonhomogeneou grain tructure in autenite, which i aggravated by lack of control over MDRX initiation becaue of the abence of the incubation time. For thee reaon, the firt tep in autenite recrytallization control i to define the proceing condition favorable for DRX and hence for MDRX. hi would enable deigning the hot rolling proce a to avoid or facilitate the recrytallization of thee type. he paper decribe the methodology for determining the range of deformation condition under which DRX can occur. Steel chemitry ha a profound impact on autenite recrytallization. Alloying with the element known to low down the initiation and progre of any kind of recrytallization can render better control over microtructure evolution in hot rolling. Another objective of the preent work i to tudy the effect of chemical compoition on the DRX and MDRX range of proceing parameter for a number of C Mn V teel. able 1. Alloy content, ma% γ α tranformation temperature, C Chemical compoition and autenite-to-ferrite tranformation temperature of the tudied teel. Steel 1 Steel 2 Steel 3 Steel 4 Steel 5 Steel 6 C Mn P S Si Al Mo Cr V N ortho A e3 para A e3 (5) A r3 (2) A r A e3 ortho, A e3 para calculated, hermocalc A r3 (5), A r3 (2) meaured by dilatometry after autenitization at C for 30 ec and ubeuent cooling at 5 and 2 C/ec, repectively. 2. Experimental he preent work employed C Mn V teel with variable Cr or Mo content a hown in able 1. Steel were laboratory melt and cat in 20 kg ingot. After cooling, the ingot were reheated at C and hot rolled with the laboratory hot rolling mill to 20 mm thick plate. Portion of the plate were machined into tandard cylindrical Ø 10 mm 15 mm pecimen for teting in uniaxial compreion uing Gleeble 3500 thermomechanical imulator. Prior to teting the pecimen were olution treated for 30 min at C to diolve poible precipitate in hot rolled plate and thu to imulate the condition of the autenite after lab reheating in indutrial hot rolling. During the tet, the pecimen were reheated at 9 C/ec to 1100 C and oaked at thi temperature for 30 ec to enure full autenitization but at the ame time to prevent ignificant grain growth. hi wa followed by cooling of the pecimen at 5 C/ec to tet temperature of C varied in 50 C increment and oaking for 60 ec before deformation. Since the euilibrium A e3 tranformation temperature for the tudied teel are relatively high (cf. able 1) the dilatometry meaurement of A r3 temperature after reheating at C and cooling at variou rate were performed uing MMC dilatometer to make ure that autenite tranformation did not take place before and during deformation in Gleeble. A hown in able 1, the A r3 temperature pertinent to the decribed thermal profile of the compreion tet are ignificantly lower than the tet temperature. After oaking at tet temperature, the pecimen were iothermally compreed in one hit up to true train of about 0.8 (55% height reduction) at contant train rate of 0.1, 1.0, and 10.0 ec 1. he load troke machine data were converted into the true tre true train flow curve uing contant volume and uniform deformation aumption. 3. Reult and Dicuion 3.1. Stre Strain Curve he tre - train curve generated in Gleeble compreion tet are exemplified in Fig. 1. hey are typical for low carbon low alloyed teel. he full et of curve i hown only for the leat alloyed Steel 1 (Fig. 1(a)). For Steel 4 with the highet Cr content (Fig. 1(b)) and Steel 6 with the highet Mo content (Fig. 1(c)) only the curve for one train rate are diplayed to better illutrate the derivation that follow. he flow curve for other tudied teel are ualitatively imilar to thoe in Fig. 1. At low tet temperature the tre - train curve do not reveal any ign either of deformation in autenite + ferrite range or a dynamic γ α-tranformation, both of which would manifet themelve in flow tre drop and continuou oftening after tre peak. 8,9) he abence of uch manifetation may be regarded a additional confirmation that the pecimen were indeed deformed in ingle phae he detailed reult of dilatometry tudie intended for development of the hot trip mill run-out table cooling trategie are to be publihed eparately ISIJ 228

3 autenite. Baed on their hape the flow curve can be claified into the following three categorie. Firt, there are curve with typical DRX hape exhibiting pronounced tre peak and teady tate flow. hee DRX-type curve correpond to higher tet temperature and lower train rate. In contrat, the tre-train curve obtained at lower temperature and higher train rate how no tre peak but only the teady tate flow. Such curve are traditionally aociated with dynamic recovery (DRV) only and can therefore be referred to a of the DRV-type *. Finally, at intermediate temperature, everal curve can be identified in Fig. 1 having what can be called a tranient -type hape with poorly pronounced very broad low tre peak barely ditinguihable from the teady tate. he occurrence of MDRX can be anticipated after deformation under the condition at which the DRX-type flow curve are oberved. In cae of DRV-type flow curve, SRX i the only expectable pot-deformation recrytallization. Steel chemitry within the tudied range of compoition doe not affect the hape of the flow curve. For all teel, the above three type of tre-train curve were detected. It hould be noted that with Cr addition of up to 1 wt.% the flow tre lightly increae. Alloying with Mo raie the overall level of flow tre more appreciably. Fig. 1. Example of tre train flow curve. et temperature vary in 50 C increment from top (750 C) to bottom (1 050 C) curve: (a) Steel 1, (b) Steel 4, (c) Steel Effect of Strain Rate and emperature on Steady State Stre In thi Section, the approach to analyzing the flow tre i outlined with the objective to determining the deformation condition favorable for DRX and hence for MDRX. he detail are illutrated uing the tet data for Steel 1. he reult for other teel and the effect of alloying are dicued in the next Section. he preent analyi employ the teady tate tree σ only. In agreement with numerou experimental obervation of iothermal contant train rate deformation, the high temperature teady tate flow tre and microtructure are determined only by the applied train rate and temperature. 11) hu, the conideration baed on teady tate tree allow avoiding dicrepancie temming from poible effect of microtructure prior to deformation and particularly thoe of the initial autenite grain ize a, for example, in the cae of the peak tree. 11) Alo it i worth recalling that in hot deformation of autenite the tre peak are oberved only when DRX take place. Since DRX i known to begin before the tre peak i reached 11 16) the microtructure at tre peak i in fact a mixture of recrytallized and nonrecrytallized autenite. Such mixed microtructure lead to highly untable hardening behavior: light increae in train beyond the peak induce rapid and dratic change in flow tre and train hardening rate. here i no phyical analogy to uch behavior in cae of DRV. In the teady tate deformation, on the other hand, the microtructure i table and independent of the initial microtructure. he flow tre i determined olely by the controlling oftening mechanim and i invariant with increaing train a long a the temperature and train rate are maintained contant and the deformation remain uniform. From thi viewpoint, a unified analyi of DRX and DRV controlled flow i only viable for the teady tate. he vicoplatic power law for the teady tate flow i now aumed to hold for the entire range of tudied deformation condition (abolute temperature and train rate ε ). he effect of and ε on the flow tre can then be evaluated by plotting the experimentally obtained value of lnσ againt reciprocal abolute temperature for each train rate. hee plot for Steel 1 are diplayed in Fig. 2. A expected, the teady tate tre increae with decreaing tet temperature and increaing train rate. * Stre-train curve with the hape imilar to that of the DRV-type are alo oberved in cae of continuou DRX (CDRX) predominantly in high tacking fault energy material, e.g., Al. 10) However, the initiation of CDRX reuire high train (ε ~ 2 4) 10) that far exceed the train range applied in compreion tet of thi work. herefore, CRDX i excluded from the preent conideration ISIJ

4 Fig. 2. emperature and train rate dependence of teady tate tre, Steel 1. he leat uare fitting reveal that each lnσ 1/ plot i bet approximated by two linear egment (Fig. 2) indicating two ditinct teady tate behavior. wo familie of linear egment are readily identifiable in Fig. 2: at higher temperature (lower 1/), lnσ follow traight line with teeper lope while at higher 1/ the lope are eentially lower. Within each family the lnσ 1/ plot are not parallel, their lope decreae a the train rate i increaed. With reference to Fig. 1(a), it can be noticed that for each train rate the two linear egment in Fig. 2 correlate with the hape of the flow curve. he lnσ low 1/ linear egment (higher lope) correpond to the DRX-type flow curve, i.e. thee egment pertain to the temperature and train rate of the occurrence of DRX. he higher 1/ egment correpond to the DRV-type curve, i.e., to the range of deformation condition under which the teady tate flow i controlled by DRV, and not by DRX. At given train rate, the flow curve with the tranienttype hape are oberved at temperature at which the lope of linear egment change harply. In thi temperature range, lnσ can be eually well attributed to either of the two linear egment. Noteworthy that the value of lnσ within the temperature range of the tranition exhibit weak train rate dependence. Naturally, each linear egment in Fig. 2 can be decribed by the euation ln σ... (1) = +aσ, where i the lope of the given egment (at given train rate) and a σ i the intercept. he value of and a σ for the two familie of linear egment are plotted in Fig. 3 a the function of ln ε. hee plot are alo fairly linear o that thee coefficient can be preented a =... (2) 0 + a ln ε a... (3) σ = aσ0 + bln ε where 0 and a σ 0 are the value of and a σ at ε = 1 ec 1, repectively. Combination of E. (1) (3) yield the euation 1 lnσ = ( + a ln ε) + aσ + bln 0 0 ε.... (4) Fig. 3. which i eual to Strain rate dependence of coefficient (a) and a σ (b) in E. (1). Partial differentiation of E. (4) with repect to ln ε allow determining the train rate enitivity of the teady tate tre, m = lnσ, lnε 1 m = a + b... (5) if 0, a, a σ 0 and b are independent of train rate. On the other hand, partial differentiation of E. (4) with repect to 1/ give the temperature enitivity of the teady tate tre: lnσ 1 = ( ) From E. (2) and (4) the temperature enitivity of tre i = 0 + a ln ε.... (6) which i identical to E. (2) if 0, a, a σ 0 and b are independent of temperature. A uggeted by the experimental data in Fig. 2 and 3, for each family of linear egment the coefficient 0, a and b can be indeed conidered contant. hen the train rate enitivity of tre, E. (5), appear to be only the function of temperature and the temperature enitivity of tre, E. (6), i a function of train rate only. By denoting a = ln A, σ 0 E. (4) can be rewritten in two euivalent formulation of the power-law vicoplatic contitutive relationhip: 16) ε b σ ( ε, )= A ε exp 0 + a ln ε... (7-1) 2014 ISIJ 230

5 or b a σ ε A ( + ) 0 (, )= ε exp...(7-2) where A i a caling contant and either of the term Aexp ( 0 + a ln ε /) or Aexp( 0/) i a vicoity-like parameter preented in the form of the Arrheniu law. Both of E. (7) apply to the two familie of linear egment in Fig. 2. With numerical value of the empirical coefficient 0, a and b for Steel 1 computed from Fig. 2, the train rate and temperature enitivitie of the teady tate tre, E. (5) and (6), can be repectively expreed a DRX DRV m = ; m = DRX DRV = ln ε ; = ln ε... (8) where the ubcript DRX and DRV refer to the low and high 1/ range in Fig. 2, repectively, or, euivalently, to the DRX- and DRV type flow curve in Fig. 1(a), repectively. A follow from E. (8), at given train rate the temperature enitivity of teady tate tre i higher in cae of DRX, DRX > DRV. Alo, at given temperature the train rate enitivity of tre i higher for DRX, m DRX > m DRV. he tranition from DRV controlled to DRX controlled teady tate flow i accompanied by increae in both train rate and temperature enitivitie of the teady tate tre. At the DRV DRX tranition σ DRV DRX = σ. With E. (4) and uing the linear combination of the coefficient in E. (1) (3) it become poible to determine the train rate ε t, at which tranition take place at the given temperature, or, alternatively, to determine the temperature t, at which the tranition occur at given train rate. When < t, the flow tre never become high enough to initiate DRX at the given train rate. In other word, at < t, DRX can never take place at given train rate regardle of the applied train. A follow from Fig. 2, the tranition temperature t i higher the higher i the train rate. On the other hand, when ε > ε t the flow tre i alo never high enough to ignite DRX at given temperature and o DRX can never be expected to occur, again irrepective of the applied train. he data in Fig. 2 indicate that the tranition train rate ε t i higher the higher i the deformation temperature. hu, both ε t ( = cont) and t ( ε = cont) delineate the range of the occurrence of DRX. Under favorable condition, i.e., at > t and/or ε < ε t, DRX can only tart when the critical train for the onet of DRX i reached. When train rate or temperature do not favor DRX ( < t, ε > ε t ) the DRX critical train can never be reached during deformation. he preented experimental evidence of coupling between the DRX DRV tranition temperature t and train rate ε t point at the poibility to decribe thi tranition uing the Zener-Hollomon parameter (temperature compenated train rate): Z ε exp Q R... (9) = ( def ) where R i the univeral ga contant and Q def i the apparent activation energy of deformation. In term of Z the power law euation ha the form 17,18) n σ = AZ.... (10) hi formulation implie that the coefficient A, n and Q def remain contant with reaonable accuracy. Putting A = A and uing E. (4), the relationhip between σ and Z can be rewritten a 1 nln Z = lnσ A a...(11) b ln = ( + ln ε) + ln 0 ε A hown above, the coefficient A, 0, a and b remain contant a long a the teady tate flow i controlled either by DRV or DRX, i.e. for a ingle family of linear egment in Fig. 2. he DRX DRV tranition i decribed by dicontinuou change in A, 0, a and b at the temperature and/or train rate of tranition or, euivalently, by change in train rate and temperature enitivitie of the teady tate tre. Coneuently, at the DRX DRV tranition the coefficient A, n and Q def mut change dicontinuouly a well. hat i, the coefficient in E. (10) hould be different depending on whether the teady tate flow i controlled by DRV or by DRX. Accordingly, the parameter Z hould be defined eparately for DRX and DRV domain. By definition, n = lnσ = m lnε and R lnσ R Q m m def = ( 1 ) = ε Within the framework of the preent approach the train rate enitivity of teady tate tre m and temperature enitivity are not contant even for a ingle family of linear egment in Fig. 2 but vary with temperature and train rate, repectively, cf. E. (5) and (6). hen n and Q def hould not be in fact contant for a family of linear egment. hi, however, doe not mean that the power law in form of E. (10) cannot be applied. 16) Following traditional treatment, the coefficient n can be taken eual to the average train rate enitivity of tre over the temperature range at which a ingle dynamic oftening mechanim operate, 17) n = m. he apparent activation energy Q def can be expreed through the temperature enitivity of tre averaged over the range of ln ε. In thi tudy ln ε = 0, o that Q def can be defined a R Q... (12) m def = 0 For the two familie of linear egment in Fig. 2, m DRX = n DRX = 0.16, m DRV = n DRV = 0.08, Q DRX def = 355 kj/mol, Q DRV def = 275 kj/mol. hee value are conitent with the general view that DRV and DRX are aociated with different apparent activation energie. When the teady tate flow i controlled by DRV the activation energy Q def i comparable with the activation energy for iron elf-diffuion in autenite (284 kj/mol 19) ) while in cae of DRX the activation energy Q def i ignificantly higher. 20,21) Figure 4 how the dependence of teady tate flow ISIJ

6 Fig. 4. Dependence of experimental and calculated teady tate flow tree on Zener-Hollomon parameter with Z computed eparately for DRV and DRX uing the repective value of Q def. Fig. 5. Effect of temperature, train rate and teel compoition on the teady tate flow tree. tree, both obtained experimentally and calculated with E. (10), on Zener-Hollomon parameter. he latter wa computed eparately for DRV and DRX uing the repective value of Q def. he flow tree were then calculated with the correponding value of n. he two calculated ln σ log Z plot interect at Z t. At Z < Z t the experimental data align DRX uite well with ln σ computed uing Q def and n DRX. On the other hand, at Z > Z t the experimental ln σ match the DRV calculation that employ Q def and n DRV. Change in controlling mechanim i manifeted by abrupt change in the lope of experimental ln σ log Z plot at Z t. hat i, Fig. 4 how that the dominant oftening mechanim i determined by the lowet level of the teady tate tre. Although the change in flow controlling mechanim can be attributed to certain tranition value of Zener-Hollomon parameter Z t, the actual DRX DRV tranition pan over ome range of Z t, Fig. 4. Mot likely thi range pertain to the deformation condition under which the DRX and the DRV teady tate tree are cloe to each other. Within the range of Z t the flow curve of the tranient type are oberved. he range of Z t alo explain light variation in tranition tre with train rate (or temperature) mentioned earlier with reference to Fig. 2. It i alo worth noting that in part the catter in Z t can be contributed by purely mechanical factor, i.e., by low accuracy of maintaining contant train rate of 10 ec 1. At thi train rate the total time of deformation i about 0.08 ec, which i comparable with the time for ram acceleration and deceleration in Gleeble 3500 machine o that in uch tet the train rate i not in fact contant. Nonethele, Z t can be viewed a the demarcation between DRX and DRV and hence a lower bound for potdeformation MDRX, imilarly to t and ε t Effect of Alloying he procedure and analyi decribed in the previou Section and illutrated for Steel 1 were applied to all teel tudied in thi work. he related effect of teel chemitry are now addreed. A already noted, alloying with up to 1% Cr doe not ignificantly impact the overall level of flow tre and the teady tate tre in particular (Fig. 5) although the latter tend to grow lightly with increaing Cr content due to olid olution trengthening of autenite. Alloying with Mo induce more appreciable trengthening epecially at lower train rate and higher temperature, Fig. 5. he plot in Fig. 5 alo do not indicate the occurrence of phae tranformation in the entire temperature range employed here a no tre decreae with decreaing temperature i oberved, a hould have been had the tranformation taken place. 9) emperature and train rate dependence of the teady tate flow tree for Steel 2 6 are diplayed in Fig. 6 in the form of lnσ 1/ plot. hee are imilar to the plot hown in Fig. 2 for Steel 1. In all cae, two linear egment can be identified correponding to the DRX-type and DRVtype flow curve. he DRV DRX tranition temperature t determined from the plot in Fig. 6 are diplayed in Fig. 7 a function of train rate and teel chemitry. A wa oberved in cae of Steel 1, for all other teel t increae a the train rate i increaed. Addition of 0.25 to 0.75% Cr raie the tranition temperature by ome C. he effect of alloying with Cr i more pronounced at higher train rate. However, with increaing of Cr content up to 1% the tranition temperature drop to the ame level a for low Cr teel (0.25% Cr), thi behavior i oberved at all train rate. hee variation indicate that mall addition of Cr tend to lightly hinder DRX but at higher Cr content thi effect diappear which can indicate the change in the way chromium influence the deformation behavior of autenite. At mall Cr concentration, a weak olid olution trengthening may determine the deformation behavior of autenite while at higher concentration (around 1 ma%) the decreae in the tacking fault energy of autenite due to preence of Cr may tart to dominate thu facilitating recrytallization. Alloying with Mo trongly inhibit DRX a manifeted by ignificantly higher tranition temperature than in cae of alloying with Cr, Fig. 7. Even low Mo concentration i uite efficient in uppreing DRX and thi effect become tronger with higher Mo content irrepective of the train rate, Fig. 8. Suppreing of autenite recrytallization by molybdenum detected in thi tudy i conitent with finding of other reearcher. 5,22,23) 2014 ISIJ 232

7 Fig. 6. Linearized temperature and train rate dependence of ln σ for Steel Practical Implication he reult preented above can be directly applied to controlled hot rolling proce deign aimed to maximize the homogeneity of autenite grain tructure. o avoid DRX in a given rolling pa (o that to avoid immediate MRDX after the pa), the bar temperature mut be below the tranition temperature t. In particular, if DRX ha to be avoided in the entire finihing rolling euence the finihing mill entry temperature mut be lower than t. In typical hot trip mill the finihing entry temperature are about C and the firt pa train rate are cloe to 10 ec 1. A follow from Fig. 7 and 8, under uch condition all of the teel tudied here are prone to DRX which indicate that DRX will take place in the firt finihing pa if the reduction in thi pa exceed the DRX critical train. In thi cae MDRX will tart immediately upon the exit from the roll bite. A the reult, highly non-homogeneou autenite grain tructure will emerge that can be further carried over through the entire finihing mill. he likelihood of uch an unfortunate cenario i much le for Steel 6 with 0.5 wt.% Mo ince thi compoition ha the highet tability againt DRX and MDRX a manifeted by the highet tranition temperature at 10 ec 1 (Fig. 7 and 8). he poible countermeaure to prevent generation of mixed autenite grain microtructure by DRX and MDRX can be een in lowering the finihing mill entry temperature and/or increaing the entry peed and hence the train rate. hi way both proceing parameter can be brought outide the DRX domain. It i alo poible to lower the reduction in the firt pa making it below DRX critical train. All of thee meaure, however, may not be viable and readily applicable. Lowering the entry temperature and/or increaing the rolling peed lead to higher flow tree and hence to higher roll eparation force. Redrafting the mill a to reduce the firt pa reduction may alo be harmful ince in thi cae more draft hould be taken in downtream pae ISIJ

8 Fig. 7. Effect of teel chemitry and train rate on DRV DRX tranition temperature. train rate, the tranition from DRV- to DRX controlled platic flow occur at certain temperature t; imilarly, at contant temperature the tranition from DRV- to DRX flow occur at certain train rate ε t. More generally, thi tranition can be decribed by the correponding value of Zener- Hollomon parameter Z t. t i higher the higher i the train rate; ε t i higher the higher the deformation temperature. (3) he tranition from DRV- to DRX controlled flow when deformation temperature and/or train rate are varied can be viewed a the demarcation between tatic and metadynamic recrytallization occurring after deformation. (4) Alloying of 0.12% C 1.7% Mn teel with up to 0.75% Cr hinder DRX a manifeted by higher DRX DRV tranition temperature. However, t decreae with further addition of chromium. Alloying with up to 0.5% Mo efficiently uppree DRX; the effect of molybdenum i far tronger than that of chromium. Acknowledgement he permiion for publication of thi paper from ArcelorMittal i acknowledged with deep gratitude. he author are indebted to Dr. D. Bhattacharya of ArcelorMittal Global R&D for hi encouragement and upport of thi work. hey are alo thankful to Prof. F. Boratto of Pontifícia Univeridade Católica de Mina Gerai for fruitful and timulating dicuion. REFERENCES Fig. 8. Schematic map of teady tate flow controlling mechanim for the tudied teel compoition. where the roll force can alo increae becaue of higher reduction and higher flow tree due to lower temperature of the trip. An alternative trategy can imply raiing the finihing mill entry temperature above t, reducing the finihing mill entry peed and/or applying heavy reduction in the firt pa to enure the highet poible extent of the progre of DRX. In any cae, however, the hot rolling trategy hould be carefully elected with account for product dimenion, configuration and capabilitie of the particular hot trip mill. 4. Concluion (1) Hot compreion tet of low carbon low alloy teel at temperature of C and train rate of ec 1 indicate that the platic flow i controlled either by DRV or DRX depending on the deformation temperature and train rate. he vicoplatic power law formalim can be applied to teady tate flow within the above range of deformation condition. DRV- and DRX controlled teady tate tree can be decribed by euation of the ame form but with different coefficient. (2) A the deformation temperature i increaed at fixed 1) J. J. Jona: ISIJ Int., 40 (2000), ) J. J. Jona: Proc. of 2nd Int. Conf. hermomechanical Proceing MP Liege, Belgium, Verlag Stahleien GmbH, Dueldorf, (2004), 35. 3) M. Venkatraman and. Venugopalan: Proc. of 2nd Int. Conf. hermomechanical Proceing MP Liege, Belgium, Verlag Stahleien GmbH, Dueldorf, (2004), 99. 4) R. H. Larn and J. R. Yang: Mater. Sci. Eng. A, A278 (2000), ) C. Roucoule, P. D. Hodgon, S. Yue and J. J. Jona: Metall. Mater. ran., 25A (1994), ) J. H. Bianchi and L. P. Karjalainen: J. Mater. Proce. echnol., 160 (2005), ) P. D. Hodgon: Proc. of Int. Conf. HERMEC-97, ed. by. Chandra and. Sakai, Mineral, Metal and Material Society, Warrendale, PA, (1997), ) J. K. Choi, D.-H. Seo, J. S. Lee and W. Y. Choo: ISIJ Int., 43 (2003), ) N. K. Park, A. Shibata, D. erada and N. uji: Acta Mater., 61 (2013), ) S. Gourdet and F. Montheillet: Mater. Sci. Eng. A, A283 (2000), ). Sakai and J. J. Jona: Acta Metall., 32 (1984), ) N. D. Ryan and H. J. McQueen: Can. Metall. Q., 29 (1990), ) E. I. Poliak and J. J. Jona: Acta Metall. Mater., 44 (1996), ) S. B. Davenport, N. J. Silk, C. N. Spark and C. M. Sellar: Mater. Sci. echnol., 16 (2000), ) A. I. Fernandez, P. Uranga, B. Lopez and J. M. Rodriguez-Ibabe: Mater. Sci. Eng., A361 (2003), ) F. Montheillet and J. J. Jona: Metall. Mater. ran. A, 27A (1996), ) H. J. McQueen and N. D. Ryan: Mater. Sci. Eng., A 322 (2002), ) H. Mirzadeh, J. M. Cabrera and A. Najafizadeh: Acta Mater., 59 (2011), ) Smithell Metal Reference Book, 7th ed., Butterworth-Heinemann Ltd., Oxford, UK, (1992). 20) J. J. Jona, C. M. Sellar and W. J. McG. egart: Int. Metall. Rev., 14 (1969), 1. 21) N. D. Ryan and H. J. McQueen: J. Mech. Working echnol., 12 (1986), ) H. L. Andrade, M. G. Akben and J. J. Jona: Metall. ran., 14A (1983), ) S. F. Medina and J. E. Mancilla: ISIJ Int., 36 (1996), ISIJ 234