87-87-1-pb

REVISTA DE METALURGIA, 43 (6)NOVIEMBRE-DICIEMBRE, 448-457, 2007

ISSN: 0034-8570

448

11.. IINNTTRROODDUUCCTTIIOONN

The studied steel initially typified as HR multiphasesteel falls into the category of high strength structuralsteels [1] and has been successfully used at industrial

scale in the construction of demolition crane arms. Itsattractiveness resides in the optimum combinationof resistance to plastic deformation, good ductilityand high toughness measured at low temperatures(KCV at 20 C and 40 C ). After cutting the HR

EEvvaalluuaattiioonn ooff tthhee mmeecchhaanniiccaall pprrooppeerrttiieess aafftteerr tthheerrmmaall ttrreeaattmmeennttooff aa ssttrruuccttuurraall hhoott rroolllleedd mmuullttiipphhaassee sstteeeell(())

J. Asensio-Lozano* and J.T. Panta-Mesones**

AAbbssttrraacctt The present paper corresponds to the experimental study conducted on a hot rolled (HR) multiphase (MP) steel,in which hardness, tensile and toughness properties were measured after the application of a series of subcritical andintercritical heat treatments (HT) to the hot rolled stock. The aforementioned values were compared to thecorresponding ones in the as-rolled state and after normalizing. The microstructure in the longitudinal plane of thestrip was analyzed by light optical microscopy in the as-rolled state and in the HT samples. Longitudinal (L) andtransverse (T) tensile and toughness specimens were cut to characterize every condition studied. Toughness propertieswere evaluated by means of Charpy V-notch tests conducted at 20 C, 0 C, 20 C, 40 C, 60 C and 80 C . Itwas observed that the yield stress increased with the increase in the heat treatment temperature in the subcritical range,while the tensile strength decreased slightly over the same range of temperatures. Uniform and total elongation onlyshowed a slight improvement when the treatment was conducted at 620 C and 700 C, while the best toughnessresponse corresponded to the sample treated at 500 C for operating temperatures comprised between 40 C and roomtemperature (RT).

KKeeyywwoorrddss Multiphase steels. Heat treatment. Tempering. Normalizing. Mechanical properties. Hardness. Tension test. Charpyimpact toughness.

EEvvaalluuaacciinn ddee llaass pprrooppiieeddaaddeess mmeeccnniiccaass ttrraass ttrraattaammiieennttoo ttrrmmiiccoo ddeeuunn aacceerroo mmuullttiiffaassee eessttrruuccttuurraall llaammiinnaaddoo eenn ccaalliieennttee

RReessuummeenn El presente estudio corresponde al trabajo experimental desarrollado en un acero multifase laminado en caliente, enel que se evaluaron las propiedades de dureza, traccin y tenacidad a impacto, tras realizar tratamientos trmicossubcrticos e intercrticos al material laminado en caliente. Los valores precedentes se comparan con el material departida laminado en caliente y tras tratamiento de normalizado. Se analiza la microestructura en microscopa ptica dereflexin, en el plano longitudinal tanto en el estado laminado como en las muestras tratadas trmicamente. Seestudiaron los comportamientos longitudinales y transversales en traccin y frente a impacto de todas las condicionesde material. Las propiedades de tenacidad al impacto se evaluaron mediante ensayo Charpy con entalla en V a lastemperaturas de ensayo de 20 C, 0 C, 20 C, 40 C, 60 C y 80 C. Se ha podido observar que el lmite elsticoaumenta con la temperatura de tratamiento en el rango subcrtico de temperaturas, en tanto que la carga de roturadisminuye levemente en ese mismo dominio de temperaturas. Se detect una leve ganancia del alargamiento longitudinaly transversal en muestras tratadas a 620 C y 700 C ; en tanto que la mejor respuesta frente a tenacidad al impacto seobtuvo tras tratamiento a 500 C para temperaturas de operacin entre 40 C y temperatura ambiente.

PPaallaabbrraass CCllaavvee Aceros multifase. Tratamientos trmicos. Revenido. Normalizado. Propiedades mecnicas. Dureza. Traccin. Tenacidadal impacto Charpy.

() Trabajo recibido el da 27 de noviembre de 2006 y aceptado en su forma final el da 26 de febrero de 2007.* Departamento de Ciencia de los Materiales e Ingeniera Metalrgica, Escuela de Minas, Universidad de Oviedo, c/Independencia n 13,E-33004 Oviedo, SPAIN (ESPAA).** Departamento de Ingeniera de Minas, Metalrgica y Materiales, Facultad de Ingeniera, Universidad Nacional de Trujillo, Av. Juan Pabloll s/n, Trujillo-La Libertad, PERU (PER).

EVALUATION OF THE MECHANICAL PROPERTIES AFTER THERMAL TREATMENT OF A STRUCTURAL HOT ROLLED MULTIPHASE STEELEVALUACIN DE LAS PROPIEDADES MECNICAS TRAS TRATAMIENTO TRMICO DE UN ACERO MULTIFASE ESTRUCTURAL LAMINADO EN CALIENTE

REV. METAL. MADRID, 43 (6), NOVIEMBRE-DICIEMBRE, 448-457, 2007, ISSN: 0034-8570 449

strip to the specified plate dimensions, the steel piecesmay be subjected to severe bending operations dueto the uniform elongation that the as-rolled materialpossesses, followed by welding. The weldability ofthis steel falls into the fairpoor category, requiring apost-weld heat treatment (PWHT) of the heataffected zone (HAZ) affecting an entire portion ofthe assembled component. This is due to the presenceof alloying elements leading to a medium-highequivalent carbon content (Ceq) of approximately0.5 % wt. As a result, impact properties in the HAZafter PWHT represent a key issue for safety operation,and often lead to high energy absorbed values beingspecified within the acceptance safety standards. Forinstance, the Charpy values for demolition cranearms are restricted to steels complying with: KCV(20 C ) $ 40 J and KCV (40 C) 27 J. The usualtensile properties of the as-treated plate are given bythe 0.2 percent offset yield stress (0.2 %YS) varyingin the 640-670 MPa range, with a tensile strength(TS) value in excess of 800 MPa, and a totalelongation (AT) equal to or greater than 18 %

[2].These results are typical of ferrite-bainite low carbonsteels[3].

The aforementioned PWHT specificationstimulated the investigation of the mechanicalproperties in the subcritical temperature range,intercritical and supercritical temperatures appliedto the as-rolled material. One of the goals was toexplore the mechanical behavior of the as-rolled stockafter being subjected to HT cycles in which thechosen treatment temperatures were in the subcriticalregion and normalizing. The exploration of the HTresponse after tempering of the as-received material isdue to the interest in quantifying the mechanicaleffect in the tension of alloy carbides, precipitated inthe subcritical region and contributing to secondaryhardening. The increase in strength is oftenaccompanied by the development of ductility and/ortoughness troughs which must be assessed[4]. Acomplementary goal was to investigate the possibilityof applying low temperature PWHT in the subcriticalregion in order to replace the expensive normalizingprocess which is often mandatory to restore anacceptable toughness response in the HAZ, and toverify the potential development of new steel qualities

derived from simple heat treatments applied to theas-rolled strip[5 y 6].

22.. EEXXPPEERRIIMMEENNTTAALL MMAATTEERRIIAALL AANNDDPPRROOCCEEDDUURREE

The as-received stock corresponds to a 7 mm thickstrip processed by controlled rolling followed byaccelerated cooling and subsequent coiling at thehot strip mill facilities that the ARCELOR-MITTALsteelmaking group has at its factory located in Avils,northern Spain. The steel is categorized as a highstrength steel (HSS) microalloyed with Mo-Nb-Tiand B.

The experimental composition of the HR stripexpressed in weight per cent is given in table I. Therecorded values for S and C were analyzed bycombustion in a LECO CS 444 analyzer. Nitrogen wasanalyzed using a LECO TC 136 analyzer. For theremaining elements, chemical analysis was conductedwith an ARL 4460 optical emission spectrometer. Theresults presented in table I are the average of 6determinations.

As part of the experimental procedure, a series ofHT were applied to rectangular samples of 400 30 7in mm cut from the HR strip at 0 (longitudinal) and90 (normal) to the rolling direction (RD).

Several plates were placed in a HERAEUS K-1253model furnace. Those placed immediately above andbelow the sample of interest had a K-type Chromel-Alumel thermocouple inserted to monitor tempera-ture changes during the thermal cycle of the sampleof interest placed in between. Two sets of treatmentswere chosen. An initial series of independenttreatments were carried out at 300, 400, 500, 620and 700 C for 2 h at temperature. The second seriesconsisted of a single normalizing treatment at 920C for 30 min. In all cases, the samples were heatedfrom room temperature (RT) at a heat-up rate of 10C min1 and, on concluding the soaking process,were removed from the furnace and allowed to coolin air.

The hardness test was executed in accordance withthe UNE-EN ISO 6507-1 standard[7], which describesthe Vickers hardness (HV) method. The load employed

TTaabbllee II.. Chemical composition of the steel expressed in weight percent.

Tabla I. Composicin qumica del acero expresado en porcentaje en peso.

CC MMnn SSii SS PP CCuu CCrr NNii AAll NNbb TTii MMoo BB NN

0.085 1.859 0.081 0.004 0.013 0.018 0.020 0.017 0.025 0.056 0.049 0.295 0.001 0.007

J. ASENSIO-LOZANO AND J.T. PANTA-MESONES

450 REV. METAL. MADRID, 43 (6), NOVIEMBRE-DICIEMBRE, 448-457, 2007, ISSN: 0034-8570

was 10 Kg. Hardness specimens of 30 30 7 in mmwere cut from the heads of the tensile test specimens,and the longitudinal and transverse planes weresurface ground and polished to give orthogonal planes.Five tests were conducted for every specimen in boththe longitudinal and transverse plane in the centerof the thickness. An EMCO model M4U hardnesstester was used for the determinations.

On completing the HT schedules, these plateswere then machined to give tensile specimens witha calibrated length of 80 mm, a gauge length of 60mm, 17 mm width and 7 mm thickness, inaccordance to the UNE-EN 10002-1 standard[8]. Thespecimens were tensile tested with an Instron 4488 -H1897 series universal tensile test machine equippedwith a 60 T load cell, at a fixed crosshead speed of2.5 mm min1. The parameters evaluated were: 0.2% offset engineering yield stress (0.2 % YS);engineering tensile strength (TS); uniformengineering elongation (%eu); and total strain atfracture (%et).

Charpy V-notch test were conducted inaccordance with the UNE-EN 10045-1 standard[9]

on longitudinal and transverse specimens at 20 C, 0C, 20 C, 40 C and 80 C to describe theductile-brittle transition curve on an energy versustest temperature plot for every HT studied.

The research study was completed with thecharacterization of the microstructure of the steel bylight optical metallography (LOM). For this purpose,samples were cut from the HR stock, normalizedsamples at 920 C, and 620 C treated samples asrepresentative of the highly tempered steel, and700 C treated samples after mounting them inbakelite. After conventional grinding and polishing,the samples were etched with nital-10 and also withLePera, the latter showing the martensite plusretained austenite in white, the bainite constituentin brown and the ferritic matrix in a dark greybackground[10]. Micrographs were taken with a ZEIS-AXIOVERT 25 LOM connected to a digital cameraand archiving software equipment.

33.. RREESSUULLTTSS AANNDD DDIISSCCUUSSSSIIOONN

The chemical analysis of the steel corresponds to alow carbon steel with high Mn, and the presence ofmicroalloying elements for grain size control such asAl, Nb and Ti, together with elements such as Moand B used to provide the required hardenability inorder to promote the transformation of residualaustenite into a bainitic aggregate and also intomartensite plus retained austenite (M-A) islands.

The as-rolled material observed under LOM shows

fine, elongated ferrite grains (Fig. 1 a)) with thepresence of dispersed constituents.

After LePera etching (Fig. 2 a)), the disperseconstituents are revealed as bainitic aggregates andM-A islands.

With increasing temperatures in subcritical HT,the volume fraction of M-A gradually decreases from8 %-vol. in the as-rolled state to almost 1 %-vol. afterthe 620 C treatment (Figs. 1 b) and 2 b)). Inaddition, the M-A island sizes becomes reduced from2 m to 1 m, corresponding to the as-rolled and620 C treated samples, respectively[5].

The sample treated at 700 C (Figs. 1 c) and 2 c))corresponds to the studied HT, in which theevaluated M-A volume fraction is smallest, around0.3 %-vol., the size of the M-A being estimated tobe very fine, under 1 m, and well dispersed over theferritic matrix[5].

The normalizing HT at 920 C gives rise to aferritic-martensitic structure, as seen in figure 2 d),with the absence of bainite. It is worth noting thatthe volume fraction of M-A reached approximately 9%-v., exceeding that of the as-rolled state, with asomewhat smaller island size (~0.8 m), though welldispersed over the entire thickness. After nital etching,the ferritic grain size presents a more equiaxed habitand is appreciably larger in size than that of the start-out as-rolled material (Figs. 1 a) and 1 d)). Insummary, the microstructure after normalizingcorresponds to that of a DP steel (Fig. 2 d)).

The Vickers hardness values measured in alongitudinal and transverse plane are represented infigure 3. These show the existence of a peak valuedue to precipitation hardening at 620 C, and thedevelopment of two softer conditions, one at 400 Cand the other at 700 C . These are indicative of theredissolution of cementite prior to the secondaryhardening resulting from the dispersion of alloycarbides. The increase in temperature above 620 Cleads to coarsening and solution of precipitates, withthe subsequent loss of their hardening effect[11].

The stress-strain tensile curves corresponding tothe as-received steel (Fig. 4) show a continuous elasticto plastic transition, and the yield stress (612-616MPa) and tensile strength (831-838 MPa) are almostequal in the longitudinal and transverse direction,only showing anisotropic behaviour for the totalelongation (22.4-25.5 %) and uniform elongation(12.6-15 %). Increasing the HT temperature makesthe elasto-plastic transition become morepronounced. At 500 C, the yield point phenomenonbecomes incipient and most pronounced at 620 C inthe subcritical range of tested temperatures. Thetreatments at 700 C and normalizing at 920 Cdefine a set of curves characteristic of dual-phase

EVALUATION OF THE MECHANICAL PROPERTIES AFTER THERMAL TREATMENT OF A STRUCTURAL HOT ROLLED MULTIPHASE STEEL


steels, with a distinctive combination of low yieldstresses together with the total absence of the yieldpoint effect and elevated elongation values (Fig. 5).

The yield stress shows very little anisotropybetween the longitudinal and transverse directions,though it shows values that increased with theincrease in temperature, reaching a maximum stresslevel at 620 C as seen in figure 6. The lowest 0.2%YS value corresponded to the normalized value. Asfor the tensile strength, this is less sensitive to thetreatment temperature than the yield stress.Furthermore, despite the decreasing trend shown bythis parameter with increasing treatment temperaturesin the tempering range represented in figure 6, itshows a peak at 700 C close to that of as-rolled stock,in addition to the lowest value in the normalizedcondition. The anisotropy of the tensile strength isquite small compared to the yield stress.

Both uniform and total elongation for the differenttemperatures studied are represented in figure 7. Asfor uniform engineering ductility, an increase in thetreatment temperature gradually reduces theanisotropy between the longitudinal and transversedirection. Ductility has a trough at 300 C where itreaches a minimum of 12-13 % elongation; and from400 C to 700 C the average gain in uniformelongation can be estimated at 0.63 % per 100 Cincrease in temperature. The normalizing treatmentrenders a material with similar uniform elongationto the as-rolled stock, though completely isotropic.Total elongation shows a degree of anisotropy thatincreases with the increase in temperature in thesubcritical range of temperatures up to 620 C, wherea maximum in total elongation was obtained. At700 C and 920 C, both anisotropy and totalductility are reduced, while the normalizing treatment

FFiigguurree 11.. Light optical micrographs of the longitudinal plane after nital-10 etching of: (a) as-rolledmaterial, and after HT at (b) 620 C; (c) 700 C; and (d) normalized at 920 C.

Figura 1. Micrografas pticas tomadas en el plano longitudinal tras ataque con nital-10 de: (a) aceroen estado bruto de laminacin en caliente, y tras tratamiento trmico a (b) 620 C; (c) 700 C; y (d)normalizadas a 920 C.



achieves a total elongation value of 24 percent. Theratio of yield stress to tensile strength expressed by0.2YS/TS for the corresponding longitudinal andtransverse values at the different HT temperaturesinvestigated is represented in figure 8. This ratio isincluded among the rational criteria for the safeapplication of structural steels, indicating on the onehand that the safest structural application from astress perspective can be obtained after tempering at620 C [12]. However, the most favourable formingconditions could correspond to either the 700 C orto the normalized samples[13].

One of the most critical requirement for structuralapplication is the subzero impact response. This isdue to regular low operating conditions of structuralcomponents in movable structures that often operateunder the risk of severe impacts, such as the arms ofdemolition cranes, etc. For this reason, the absorbed

energy in a Charpy V-notch test at +20 C, 0 C,20 C, -40 C, 60 C and 80 C in thelongitudinal and transverse direction was evaluatedat each HT temperature, the results being representedin figures 9 and 10. In both energy plots, the as-rolledand the normalized curves were taken as referencematerial for comparison, since they respectivelyrepresent the upper and lower bounds of the absorbedenergy versus test temperature curves. Figure 9corresponds to the longitudinal values. It is worthnoting that only the tempering at 500 C improvedthe impact toughness from 40 C up to RT. Theplot corresponding to the steel tempered at 400 Cvery closely follows the upper bound curve given bythe as-rolled material. The tempering at 620 C,however, renders absorbed energies that are lowerthan that corresponding to the normalized materialat test temperatures equal to 40 C and below. The

FFiigguurree 22.. Light optical micrographs of the longitudinal plane after LePera etching of: (a) as-rolledmaterial, and after HT at (b) 620 C; (c) 700 C; and (d) normalized at 920 C.

Figura 2. Micrografas pticas tomadas en el plano longitudinal tras ataque con reactivo LePera de: (a)acero en estado bruto de laminacin en caliente, y tras tratamiento trmico a (b) 620 C; (c) 700 C;y (d) normalizadas a 920 C.



representation of the Charpy toughness values fortransverse specimens at different test temperaturesare shown in figure 10. The transverse values are

lower than the corresponding longitudinal ones; theas-rolled and normalized specimens are alsorepresented as reference materials in this figure. Thebest result corresponds to the steel treated at 620 C,followed by that treated at 500 C . At 400 C and700 C, the treated steel behaves similarly to thereference steels. A summary of the impact behaviourat selected operational temperatures is given in figure11. KCV values at 20 C and 40 C are representedfor all the studied conditions. These show a maximumfor the 500 C tempered steel, which is the only HTcondition that improves that of the start-out as-rolledMP steel. There is a drop in impact toughness at620 C, while for the intercritical HT at 700 C andthe normalizing at 920 C, the values are low andpresent the greatest anisotropy between the twostudied directions. As a result of the standard set forthe Charpy V-notch tests, KCV (20 C ) 40 J andKCV (40 C ) 27 J, only the as-rolled and the steelstempered at 400 C and 500 C satisfy theaforementioned prerequisite in both longitudinal andtransverse directions. Only the steel treated at 620C does not satisfy any of the criteria.

44.. CCOONNCCLLUUSSIIOONNSS

As regards the observed microstructure and theevaluation of steel hardness, it may be concluded thattempering at 620 C of the hot rolled stock providesa peak in hardness values of 265 MN/mm2 (HV 10kg) as a result of the decomposition of bainite andM-A constituents leading to precipitation hardeningstrengthening by Mo and Cr alloy carbide formationin a ferritic matrix. It was observed that higher HTtemperatures lead to lower hardness values, probablydue to precipitate coarsening and further eventualsolution. The observations for the 0.2 % YS coincidewith the above hypothesis, as the 620 C treatedsample also leads to a maximum stress value, varyingbetween 725 and 746 MPa in the longitudinal andtransverse directions, respectively. This represents anincrease of almost 10 % with respect to the as-rolledreference material. The TS, however, registers a slightdecrease of around 2.5 % with respect to the as-rolledsteel. This, however, has little influence, since the0.2 YS/TS quotient still renders a value that confersthe optimum ratio for static structural applicationsamong all the analyzed conditions.

On the other hand, the material that exhibits theoptimum combination of cold ductility withoutexcessive loss of strength is the 700 C HT stock.The values obtained for the engineering uniform andtotal elongation gave average longitudinal andtransverse results of 15 % and 22 %. Due to the low

FFiigguurree 33.. HV10kg Vickers hardness as a functionof the HT temperature. Error bars indicate thedegree of scatter corresponding to the averagevalue plus minus the standard deviation.

Figura 3.Valores de la dureza Vickers con cargade 10kg en funcin de la temperatura detratamiento trmico. Las barras de error expresanel grado de dispersin correspondientes al valormedio mas / menos el valor de la desviacinestndar.

TEMPERATURE (C)

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FFiigguurree 44.. Engineering tensile test curves in thelongitudinal and transverse directions of the as-rolled state. A numerical summary of the mostrelevant tensile properties and a graphicalenlargement of the elasto-plastic transition areincluded in the graph.

Figura 4. Curvas de traccin ingenieriles enprobeta longitudinal y transversal del acero enel estado bruto de laminacin en caliente. Seincluyen dos ventanas, una expresa losresultados numricos mas relevantes y la otraes una ampliacin de la transicin elasto-plstica.

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anisotropy, this heat treatment may be consideredthe most ductile steel condition with a potential forstructural applications among all the studiedconditions.

Finally, low temperature impact toughness is best

achieved by the 500 C HT stock with values inexcess to 90 J and 60 J for the energy absorbed in alongitudinal Charpy V-notch test at 20 C and 40C, respectively, with an average 14 % improvementwith respect to the as-rolled stock.

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FFiigguurree 55.. Engineering tensile test curves in the longitudinal and transverse directions for the differentHT analyzed: a) 300 C, b) 400 C, c) 500 C, d) 620 C, e) 700 C and f) normalized at 920 C . Ithas been included the numerical summary of the most relevant engineering tensile properties togetherwith a graphical enlargement window of the elasto-plastic transition in order to facilitate identificationof the yield point phenomenon.

Figura 5. Curvas de traccin ingenieriles en probetas longitudinales y transversales para los diferentestratamientos estudiados: a) 300 C, b) 400 C, c) 500 C, d) 620 C, e) 700 C y f) normalizada a 920 C.Se incluyen dos ventanas, una expresa los resultados numricos mas relevantes y la otra es unaampliacin de la transicin elasto-plstica que facilita la observacin del fenmeno de fluencia en fro.



AAcckknnoowwlleeddggeemmeennttss

The authors wish to express their gratitude toARCELOR-MITTAL for the granting of access tothe facilities in their HR laboratory. Our specialgratitude goes out to all the factory engineers involvedin the project, in particular to J. Glvez-CarroblesDip. Eng., in charge of the development of HRmultiphase steels during the period comprised between2002-2004, and promoter of a joint research projectbetween the University of Oviedo and ARCELOR-Spain. Our thanks also to Valeriano Barrn-DopazoDip. Eng. and M. Malanda Dip. Eng., both belongingto the HR Quality Department for their invaluablehelp in many technical and non-technical issues. Ourgratitude is made extensive to all the personnel at thehot roll and pickling department involved in thisproject. Lastly, we also wish to express our thanks toOviedo University Vice-Rectorate for Research forproviding the economic support to carry out the proofreading of the manuscript.

RREEFFEERREENNCCEESS

[1] D. LLEWELLYN AND D. HILLIS, IronmakingSteelmaking 23 (1996) 471-477.

FFiigguurree 66.. Effect of the HT temperature on the0.2 percent offset yield stress and tensile strengthvalues in the longitudinal and transversedirections.

Figura 6. Efecto de la temperatura de tratamientotrmico sobre el lmite elstico al 0,2 por cientoy la carga de rotura en direccin longitudinal ytransversal.

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FFiigguurree 77.. Evolution of both uniform and totalengineering elongation as a function of thetreatment temperature.

Figura 7. Evolucin de los alargamientosingenieriles uniforme y total, en funcin de latemperatura de tratamiento trmico.

(%eu) (%et)


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FFiigguurree 88.. Temperature dependence of the0.2YS/TS ratio for the different HT analyzedconditions.

Figura 8. Dependencia del cociente 0.2YS/TScon la temperatura para cada tratamiento trmicoanalizado.

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FFiigguurree 1100.. Temperature dependence of thetransverse Charpy V-notch impact energy for thedifferent steel conditions analyzed. For the sakeof comparison, the graph includes the as-rolledand normalized curves corresponding to: a) 400C, b) 500 C, c) 620 C, and d) 700 C.

Figure 10. Dependencia de la energa absorbidaen el ensayo Charpy con entalla en V en probetatransversal respecto de la temperatura de ensayode los diferentes estados del acero estudiados.Para facilitar su comparacin se incluyen lascurvas correspondientes al estado laminado encaliente y normalizado para: a) 400 C,b) 500 C, c) 620 C, y d) 700 C.

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FFiigguurree 99.. Temperature dependence of thelongitudinal Charpy V-notch impact energy for thedifferent steel conditions analyzed.The curves forthe as-rolled material and after normalizing areincluded in plots a) - d) as upper and lower boundsof the materials response: a) 400 C, b) 500 C,c) 620 C, and d) 700 C treated samples.

Figura 9. Dependencia de la energa absorbidaen el ensayo Charpy con entalla en V en probetalongitudinal respecto de la temperatura de ensayode los diferentes estados del acero estudiados.Las curvas correspondientes al estado laminadoen caliente y normalizado se incluyen como lmitessuperior e inferior aproximados de la energaabsorbida para las muestras tratadas a: a) 400 C,b) 500 C, c) 620 C, y d) 700 C.

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[2] http://www.fcs.arcelor.com/content/email.pl., A22Aceros de muy alto lmite elstico y baja aleacin,2006, pp. 1-6.

[3] J. PERO-SANZ, ACEROS Metalurgia FsicaSeleccin y diseo, 1th ed., CIE Dossat, 2000,Madrid, 2004, pp. 326-331.

[4] R.W.K. HONEYCOMBE AND H.K.D.H.BHADESIA, Steels: Microstructure and Properties,Edward Arnold, London, 1995, p. 135.

[5] J. ASENSIO AND J. PANTA, Proc. 12th Int.Metallographie-Tagung, Fortschritte in derMetallographie, 2006, MAT INFO Werkstoff-informationsgesellschaft, Leoben, 2006, pp.297-304.

[6] J. ASENSIO AND J. PANTA, Mater. Char.,Submitted for publication), August 2006.

[7] UNE EN-ISO 6507-1 (1988).[8] UNE-EN10002-1 (2002).[9] UNE-EN 10045-1 (1990).

[10] F.S. LEPERA, Metallography 12, Elsevier NorthHolland INC, New York, 1979, pp. 263-268.

[11] W.C. LESLIE AND E. HORNBOGEN, PhysicalMetallurgy, 4th ed., R.W. Cahn and P. Haasen,Elsevier Science BV, Cambridge, 1996, pp.1556-1620.

[12] B.L. JONES AND D.L. JONSON, Proc. 150thAnniversary of the discovery of Vanadium, Steelsfor lines pipe and pipeline fittings, 1981, TheMetals Society, Grosvenor House, London,1981, p. 2/5.

[13] J. PERO-SANZ, Ciencia e Ingeniera de Materiales,5th ed., CIE Dossat, 2000, Madrid, 2006, p. 459.

FFiigguurraa 1111.. Summary of the Charpy V-notch testvalues at -20 C and -40 C in the longitudinaldirection for the as-rolled strip and for each HTstudied.

Figura 11. Resumen de los datos del ensayoCharpy a 20 C y a 40 C en probetalongitudinal del acero en estado bruto delaminacin en caliente y para cada tratamientotrmico estudiado.

TEMPERATURE (C)

R.T. 400 500 620 700 9200

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80

100

120

20C40C

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RB

ED

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GY

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