Engineering of forged products of microalloyed constructional steels

Transcription

1 f Achievements in Materials and Manufacturing Engineering OLUME 15 ISSUE 1-2 March-April 2006 Engineering f frged prducts f micrallyed cnstructinal steels J. Adamczyk, M. Opiela* Divisin f Cnstructin and Special Materials Engineering, Institute f Engineering Materials and Bimaterials, Silesian University f Technlgy, ul. Knarskieg 18a, Gliwice, Pland * Crrespnding authr: address: marek.piela@plsl..pl Received ; accepted in revised frm Manufacturing and prcessing ABSTRACT Purpse: Effect f the therm-mechanical treatment cnditins n the structure and mechanical prperties f the frged elements f cnstructinal C-Mn steels with Ti,, B and N micradditins. Design/methdlgy/apprach: Metallgraphy, electrn micrscpy, tensile test, hardness measurements, hardenability calculatins, Charpy- tests have been used. Findings: The therm-mechanical treatment allws t btain the fine-grained austenite structure during ht plastic defrmatin, and gives frged elements btaining: yield pint R p0,2 ver 690, UTS ver 770, hardness 220 up t 250 HB and breaking energy K ver 180J after high tempering. Research limitatins/implicatins: It is predicted TEM investigatins n structure f the frged elements after therm-mechanical treatment. Practical implicatins: Investigatins carried ut shwed full usability f micr-allyed steels fr prducing frged machine parts with the high strength and cracking resistance, using the energy-saving therm-mechanical treatment methd. Originality/value: Prductin cnditins f energy-saving therm-mechanical treatment f frged elements f HSLA cnstructinal steels with the diversified hardenability, were presented. Keywrds: Therm-mechanical treatment; Micrallyed steels; Frged elements; Mechanical prperties; Cracking resistance 1. Intrductin Die frging is generally used as a manufacturing methd f machine parts frm nn-ally and ally steels. Frged elements frm nn-ally steels are usually nrmalized t imprve their prperties thanks t grain refinement. Frged elements f ally steels have t be sft annealed r tempered in high temperatures, befre their mechanical treatment and in ready state quenched and tempered in high temperatures with dimensin crrectin by machining - especially grinding and finishing. Pssibility f cst reductin during prducing frged elements because f eliminatin r reductin heat treatment, t tempering r artificial ageing nly have appeared with develping micrallyed steels HSLA type. This kind f steels cnsists up t 0,5 % f C and up t 2% f Mn and Nb, Ti and in quantity up t 0,1% and smetimes als Zr and small cntent f N and up t 0,005% B which increase the hardenability f these steels. Ht wrking f these steels in well chsen cnditins allws t create finegrained structure with dispersive particles f MX interstitial phases (M Nb, Ti and ; X N and C), precipitated in plastically defrmed austenite. Thse particles are limiting a grain grwth f recrystallized γ phase. Fine-grained structure (as a prduct f transfrmined austenite) which ccures in adjustable cling cnditins and precipitatin hardening (fllws frm presence f MX phase particles) are deciding abut cnsiderable increasing f strength prperties with preserving a gd cracking resistance f manufactured prducts. That prblems, widely described in many publicatins and als in prceedings f many scientific cnferences, are als shwn in papers [1-4]. Cpyright by Internatinal OCSCO Wrld Press. All rights reserved Research paper 153

2 Jurnal f Achievements in Materials and Manufacturing Engineering lume 15 Issue 1-2 March-April 2006 Metallic micr-allys intrduced t the steel and als B have a strng affinity t xygen and nitrgen. Besides that Ti and Zr have als affinity t sulfur. This means that the liquid steel, just befre intrducing micr-allys, shuld be well dexidided and desulphurized. Because f that steel heated in xygen-blwn cnverter r in electric arc furnace have t be treated in the drawing f a ladle furnace in cnditins which guarantee energetic curse f chemical reactin with liquid base slag (which is n the tp f liquid metal) and pwder substance blwn t the liquid metal (which are needed fr dexidizing, desulphurizatin and mdificatin f nn-metallic inclusins) and allying elements and micr-additives (withut misture and crystalic water thanks t theirs rasting). Gases used t blw inside these additives (argn r nitrgen) making intensive mixing f bath and hmgenizing f chemical cmpsitin f the steel. The final stage f ladle treatment is vacuum dexidizing f the bath and cntinuus casting in argn atmsphere, prtecting steel frm secndary xidizing and nitriding. Ingt intersectin is chsen taking t accunt intersectin f frge charge with required plastic prcessing f ready frging. Prcessing f frge elements with high mechanical prperties frm micr-allyed HSLA steels require well chsen plastic treatment cnditins accmmdated t kind and kinetics f disslutin (precipitatin) MX interstitial phases (frm intrduced t the steel micrallys) in austenite described by equatin: lg[m][x] = B A/T, (1) where: [M] and [X] - weight cncentratin f metallic and metallid micr-additives disslved in steel austenite in T temperature, A and B - cnstants. alue f cnstants A and B in equatin (1) fr chsen interstitial phases and BN phases are presented in table 1. Table 1. alues f the cnstants A and B in the equatin (1) fr selected carbides and nitrides [1, 2] MX cns. AlN C N TiC TiN NbC NbN BN A B 1,79 3,02 6,72 5,33 0,32 3,04 2,8 5,24 In case f using brn, it is needed t intrduce Ti t the bath in amunt necessary t bind a nitrgen frm the steel t TiN. This peratin prevents pssibilities f fixing the nitrgen t stable nitride BN type. Plastic defrmatin f HSLA steels especially the final stage is prvided in a range f temperatures related with precipitatin prcesses f MX phases in austenite. The imprtant think is that in the temperature f the end f plastic defrmatin all micr-additives intrduced t the steel have t be chemically bunded t MX phases. In these cnditins prcess f the plastic defrmatin is cntrlled by dynamic recvery. Technical - ecnmical aspects are respnsible fr this, that cntemprary predminant part f die frged elements fr needs f: mtrizatin industry, maining and agriculture machines and thers are dne frm micr-allys machine-steels with ferriticpearlitic structure. These steels cntain up t 0,5 % f C, up t 0,8% f Si and micr-additives f Nb, Ti, and N (table 2). The main cnditin t btain desirable mechanical prperties f frged elements is gd chise f prcessing parameters (charge heating temperature and plastic prcessing temperature defrmatin distributin and the strain speed are very difficult t be cntrlled) [3]. Charge heating temperature shuld nt ttally disslved MX phases in the slid slutin because this culd cause superfluus grw f grain size. High speed defrmatin and shrt brakes between cnsecutive defrmatin stages during frging, limiting the effective run f static recrystallizatin - respnsible fr refining austenite grain size. Althugh transfrmatin γ α f plastically defrmed austenite (bth carse-and fine-grained) is starting at the grain bundary and strip defrmatin, but in case f carse - grained γ phase this nt assure t btain sufficient refine structure and expected mechanical prperties f frged elements. Frging elements prepared in these cnditins, cled dwn frm end f plastic prcessing temperature n a fresh air have high strength (thanks t strng precipitatin prcesses, and t high vlume f pearlite) but small cracking resistance [5]. Frm data putted in table 2 it is bvius that elements frm micr-allys steels frged in adjust cnditins have cnsiderably less cracking resistance in cmparisn with ne made frm steels with allying elements fr tughening. Because f this smetimes theirs applicatin are limited due t standard requirement and allw regulatins. Hwever, they have higher fatigue resistance, mainly gd machinability and they d nt need a heat treatment. Higher strength prperties (especially cracking resistance) in cmparisn with frging elements with ferritic-pearlitic structure have elements, frged using therm-mechanical treatment frm lw-ally steels fr tughening, with micradditins f Ti, Nb and and als N and B. This methd is based n steel plastic defrmatin in adjusted frging cnditins with usual r isthermal quenching frged elements directly frm end frging temperature advantageusly after t 0,5 time pass, which is needed t btain 50% vlume f statically recrystallized austenite. Direct cmmn quenching after passing t 0,5 time eliminate heat treatment f frged elements nly t high tempering. Hwever, isthermal quenching eliminates cmpletely expensive tughening (fig. 1). Temperature A c3 A c1 M s M f charge heating temperature t 0,5 quenching plastic defrmatin isthermal quenching Time tempering Fig. 1. The scheme f prductin f frge element frm HSLA steels using therm-mechanical treatment [3] 154 Research paper J. Adamczyk, M. Opiela

3 Manufacturing and prcessing Table 2. Chemical cmpsitin and mechanical prperties f chsen prducts f micrallys steels fr frged elements [5-7] Steel type Cmpnent cntents, % C Mn Si P S Nb+ Nb ther R e Mechanical prperties Metasafe D800 1) 0,15 0,25 1,3 2,0 0,1 0,4 <0,030 <0,015 0,10 0, Metasafe D900 1) 0,25 0,30 1,3 2,0 0,1 0,4 <0,030 <0,015 0,10 0, Metasafe D1000 1) 0,35 0,50 1,3 2,0 0,1 0,4 <0,030 <0,015 0,10 0, MnS3 2) 0,44 0,50 0,70 1,0 0,50 0,035 27MnSiS6 2) 0,25 0,30 1,30 1,6 0, ,035 38MnSiS5 2) 0,35 0,40 1,20 1,5 0, ,035 0,065 0,050 0,065 UTS DM J (Ti) <0,050 (Ti) K J 44MnSiS6 2) 0,42 0,47 1,30 1,6 0, ,035 0,020 0,035 0,10 0,15 - (Ti) > H080SL 3) 0,41 0,48 0,6 1,0 0,15 0, 3 0,035 0,020 0,040 0,04 0,06 - >500 > H090SL 3) 0,48 0,54 0,8 1,1 0, ,035 0,020 0,040 0,04 0,06 - >500 > GN 4) 0,25 0,29 1,3 1,4 <0,35 0,022 0,010 0,030 0,10 0,15-0,020N >25 >740 * >830 - >45 C-Mn-Ti--N 5) 0,25 0,35 1,5 1,8 0,4 0,7 0,040 0,080 0,08 0,15 0,02 0,015 0,025N >620 >820 - >38 1) steels develped in France culd be prduced with increased cntents f S up t 0,045% r < 0,25% Si and 0,25% Pb t increase machinability; 2) steels prduced by Thyssen cmpany; 3) steels develped by Fiat Aut and Deltasider Cmpany; 4) steel wrked in Silesian University f Technlgy; 5) steels develped in Institute f Ferrum Metallurgy in Gliwice; DM energy t break a sample with intersectin 10x10 mm with U ntch with 3mm depth, * - frging with bainitic structure Especially usefull fr this are steels with micr-additives f B which increase hardenability, and Ti which makes a shield fr this element preventing f his cnnectin with N t stable nitride BN. Anyway, when the cncentratin f Ti is t small t absrbe cmpletely N in this case B is cut ut frm his effecting n hardenability f the steel and at the same mment nt enugh vlume f TiN will nt be a barrier fr grwing austenite grains in high temperature during heating up the charge. 2. Experimental prcedure prcedure The gal f this experiment is a structure and mechanical prperties f frged elements using therm-mechanical treatment, made frm C-Mn steel with micr-additins f Ti,, B and N (table 3) melted using: after furnace treatment f metal bath, and cntinuus casting ingts with intersectin mm. Ingts after their slidificatins were adjustly rlled fr bars with a dimentin abut 36 mm, and after using the same cnditins fr bars with dimentin f 17 mm. The range f adjusted rlling temperature were taken basing n calculatin f slubility in the austenite micradditins added t the steel. T make this kind f calculatin authrs used a kinetic equatin (1) and data frm table 1. It was fund that micr-additin f Ti intrduced t the steel type A with cncentratin f 0,005% is cmpletely disslved in austenite in temperature abut 1250 C (fig. 2) and during cling is nt absrbing whle N frm the steel t TiN. In this case excess f N is creating nitrides f AlN and BN during cling the steel (fig. 3). Brn is fixed t stable nitride BN and practically is nt increasing the hardenability f the steel. On the ther hand nitride frm steels B and C type is cmpletely fixed in TiN. Ttal disslving f that phase in the austenite is pssible nly in temperatures f 1400 and 1450 C, respectively fr steel type B and C. The rest f intrduced t thse steels amunt f Ti is fixing itself t carbnitrides Ti(C,N) and carbides TiC. Hwever, Al stays in disslved state and B is fixing t M 23 (C,B) 6 during cling f the steel which culd be disslved in the slid slutin in temperatures little higher then temperature A c3 f the steel. These data related with kinetics f precipitatin prcesses f MX phases in austenite, were used t determine cnditins f adjusting rlling f charge fr frging in a shape f bars within the temperature range frm 1200 up t 900 ºC [8, 9]. Engineering f frged prducts f micrallyed cnstructinal steels 155

4 Jurnal f Achievements in Materials and Manufacturing Engineering Mn 1,02 1,03 1,17 Si 0,25 0,23 0,25 P 0,018 0,018 0,015 Cntents Ti, % A B C C 0,21 0,21 0,32 S 0,005 0,013 0,007 Chemical cmpsitin, % Cr Ni M Ti 0,18 0,07 0,03 0,005 0,14 0,07 0,03 0,032 0,22 0,07 0,02 0,035 Cntents N, % Steel B 0,002 0,002 0,003 Cu 0,11 0,14 0,23 Alc 0,024 0,024 0,025 N 0,009 0,008 0,010 Temperature, C Austenitizing temperature, C Fig. 2. Slubility curve f TiN nitride in austenite f A type steel in functin f temperature Fig. 4. Influence f the austenitizing temperature n the grain size f austenite Al. = 0,024 Austenitegrain grainsize, size, Austenite mm B = 0,002 Cntents N, %, Cntents Al.% Steel A Steel B Steel C Isthermal hlding time, s Temperature, C Fig. 3. Slubility curves f AlN and BN nitrides in austenite f A type steel in functin f temperature Fig. 5. Influence f the isthermal hlding time f the specimens after cmpleting ht-wrking at a temperature f 900 C befre water quenching n the grain size f austenite Charge heating temperature fr frging was calculated basing n primary austenite grain size index f the samples quenched frm prgrammed increasing austenitizing temperature (fig. 4). As it culd be seen steel A type with small cncentratin f Ti and nt high vlume f TiN phase have distinct grw f austenite grains just after crssing 900 ºC temperature. Table 4. Results f the mechanical prperties and hardenability f the investigated steels Treatment type Rp0,2 Treatment Tempering Steel cnditins temperature, C A 900 C/3s/water B 900 C/12s/water C 900 C/16s/water * - specimen breaking energy tested at temperature f 20 C 156 0,008 0,026 Steel A Steel B Steel C Austenite grain size, m Table 3. Chemical cmpsitin f tested steels lume 15 Issue 1-2 March-April 2006 Research paper UTS A % Z % K J HB * 108* DI mm J. Adamczyk, M. Opiela

5 Manufacturing and prcessing 3 m Fig. 6. Fine-grained structure f the statically recrystallized austenite; finishing frging temperature 900 C; isthermal hlding time 12s; steel B Fig. 9. Lath martensite structure f steel C quenched frm a finishing ht wrking temperature f 900 C after hlding fr 16s a) b) 0,4 m 10 m Fig. 7. Fine-grained structure f the statically recrystallized austenite; finishing frging temperature 900 C; isthermal hlding time 16s; steel C Fig. 10. Lath martensite structure with the dispersive Fe3C precipitatins (steel C): a light field, b diffractin pattern t Fig. 10a [111]Fe and [012]Fe3C a) b) 3 m Fig. 8. Martensitic-bainitic structure f steel B water quenched frm a finishing ht wrking temperature f 900 C after hlding fr 12s Engineering f frged prducts f micrallyed cnstructinal steels Fig. 11. Dispersive M23(C,B)6 precipitatins at the grain bundaries f the primary austenite (steel C); a light field, b diffractin pattern t Fig. 11a [111]Fe and [001]M23(C,B)6 157

6 Jurnal f Achievements in Materials and Manufacturing Engineering lume 15 Issue 1-2 March-April 2006 Whereas grwth f grains f the γ phase f the steel type B prceed sftly up t 1100 C temperatures, and in steel type C even higher than 1150 C. Basing n a data which were shwn charge heating temperatures were adjusted fr steel A type C, and fr steel B and C respectively 1000 and 1150 C. Time needed fr ccuring static recrystallizatin f γ phase after finishing plastic defrmatin in the temperature f end frging was determined basing n a primary austenite grain size f upset frging samples with ε = 0,25 in a temperature f 900 C with defrmatin speed f 14 s -1, hld befre quenching in the water fr the time frm 0 up t 24 s (fig. 5). As it is shwn in that figure steel A type btains the finest austenite grain size after hlding samples in this temperature fr 3 s, whereas steels B and C type fr 12 s and 16 s respectively. Therm-mechanical treatment was realized thanks t pen die frging f experimental segments with 17 mm diameter and 150 mm length in the temperature range f 950 t 900 C and 1000 t 900 C - respectively frm steel A and B type t rds with intersectin 12 x 12 mm. Befre quenching in water these rds were hld at a temperature f end frging fr 3 and 12s. Quenched rds were tempered at a temperature f 600 C fr 1h. Whereas rd segments frm steel type C with intersectin 24 x 24 mm were frged in a range f temperature frm 1150 up t 900 C t the shape f rds f intersectin 12 x 12 mm. Befre quenching in water the rds were hld in a temperature f end frging fr 16s, and after were tempered in temperature f 500 and 600 C fr 1h. Investigatins shwed that examinated steels after thermmechanical treatment and after quenching have a fine-grain structure f primary austenite (fig. 6, 7) with a grain size abut 10, 5 and 8 and martensitic-bainitic structure (fig. 8) and hardness 42, 44 and 49 HRC respectively fr a steel A, B and C type. Hardness f the steels is decreasing after high tempering frm 220 t 250 HB, which nt create difficulties, during mechanical treatment f frged elements. Examinatins f the structure f thin fils made in TEM (transmissin electrn micrscpe) shwed that steel C type (quenched frm a temperature f end frging 900 C and hld befre that in this temperature fr 16s) have lath martensite structure (fig. 9). Inside f martensite laths it was apprved a presence f dispersive particles f cementite (fig. 10), whereas at the bundaries f primary austenite M 23 (C,B) 6 type dispersive particles were fund (fig. 11), which have ccure in a steel during self tempering prcess. Particular attentin shuld be fcused n high mechanical prperties f the steels in high tempered state and especially n their crack resistance als in lw temperatures (table 4). In this table als shwn ideal diameter D I, which characterizing their hardenability. The ideal diameter was calculated due t prcedure described in ASTM A standard. Micr-additive f brn intrduced t the steel A type is ttally fixed in nitride BN and because f this is nt increasing hardenability f the steel. On the cntrary in steels B and C type micr-additive f brn disslved in slid slutin is affecting n hardenability. It was nticed during calculated ideal diameter D I, which fr the steel type A is equal 28 mm. At the same time calculated ideal diameter fr steels type B and C taking int accunt influence f micr-additive f brn D IB is equal respectively - 55 and 96 mm. This sugests that steel A type is useful fr frging relatively small intersectins and the steels B and C type fr a big ne [10]. 3. Cnclusins Investigatins carried ut shwn full usability f micrallyed steels fr prducing frged machine parts, with high strength and gd cracking resistance, using energy-saving therm-mechanical treatment methd. This therm-mechanical treatment, allws t btain fine-grained austenite structure during ht plastic defrmatin, and gives us frged elements btaining yield pint R p0,2 ver 690, UTS ver 770, hardness 220 up t 250 HB and breaking energy K ver 180J. Thse prperties culd be reach after hlding frged elements in temperature f end plastic prcessing, fr the time needed t create at least 50% recrystallized austenite, after they have t be quenched frm this temperature and finally highly tempered. Designing f frging technlgy frm micr-allys steels needs accmmdatin f charge heating cnditins t the kinetics f precipitatin prcesses f MX phases in austenite, withut grwing grains f γ phase. Thanks t investigatins f influence f austenitizing temperature n a primary austenite grain size, and a kinetics f dislving interstitial MX phases in austenite it was fund that charge heating temperature fr frging elements made frm steel A type shuld nt exceed 950 C, and fr elements made frm steels B and C type culd have even 1150 C. This shws that a charge made frm steels B and C type culd be heated t temperature cnsiderably higher than A c3 fr that steel, with keeping fine-grained structure. This is increasing the live time f dies. Intrducing micr-additives f brn t the steel (which increase the hardenability) made frm fine-grained steel structure needs a Ti shield with quantity indispensable t fix all nitrgen t BN. When the cncentratin f Ti is t small t cnsume whle nitrgen frm the steel in that mment brn is turn ff frm effecting n hardenability f investigated steels. Als small amunt f TiN nitrides is nt prtecting steel frm austenite grains grw, in a high temperature used fr charge heating. References [1] T. Gladman, The Physical Metallurgy f Micrallyed Steels, The Institute f Materials, Lndn, [2] J. Adamczyk, Engineering f Steel Prducts, Wyd. Plitechniki Śląskiej, Gliwice, 2000, (in Plish). [3] J. Adamczyk, Engineering f Metallic Prducts cz.1, Wyd. Plitechniki Śląskiej, Gliwice, 2004, (in Plish). [4] J. Adamczyk, M. Opiela, Jurnal f Mater. Prcessing and Technlgy, v , 2004, s [5] J. Adamczyk, E. Kalinwska-Ozgwicz, W. Ozgwicz, R. Wusatwski, Jurnal f Mater. Prcessing and Technlgy, v. 53, 1995, s. 23. [6] M. Krchynsky, Micrallyed Frging Steel, Unin Carbide, GmbH, [7] S. Engineer, B. Huchteman, Prc. Symp. Fundamentals and Applicatins f Micrallying Frging Steels, Clrad, TMS, 1996, s. 61. [8] J. Adamczyk, M. Opiela, A. Grajcar, 10th Int. Cnf. AMME 2001, 2001, s. 5, (in Plish). [9] J. Adamczyk, M. Opiela, A. Grajcar, 11th Int. Cnf. AMME 2002, 2002, s. 7, (in Plish). [10] J. Adamczyk, M. Opiela, A. Grajcar, 12th Int. Cnf. AMME 2003, 2003, s. 21, (in Plish). 158 Research paper READING DIRECT: