Mechanicalproperities of anHSLAbainitic steel subjected to controlledrollingwith acceleratedcooling
P.C.M. Rodrigues * E.V. Pereloma b* D.B.Santos c
° Deparmest ef Mecihasical Engineering FUNREI Sao Jodo del Rei MG Bruzif Departmest of Merallrgical nd Materials Engimering UFMG Belo Horicoste MG Bruzi b Deparmenr of Mareriats Eegweermg Monash Unternry Clyton VIC 3800 Aastralia
Received 6 September 199; received in revised form 6 December 1999
Abstract
Controlled roling followed by accelerated cooling was utilised in laboratory simulations to study the microstructure andmechanical properties of an HSLA low carbon bainitic steel. The effects of proceing parameters such as cooling starttemperature and cooling rates on the final microstructure and mechanical properties were studied. Optical microscopy and transmision electron microscopy were used to evaluate the plex microstructures consisting of polygonal ferrite pearlite relationships between mechanical properties and acelerated cooling variables: cooling rates and cooling start temperatures. bainite and martensite/retained austenite constituent. The use of the multiple regression analysis allowed establishment of theC 2000 Elsevier Science S.A. All rights reserved.
Keyword: Bainitic steel; Controlled rolling: Accelerated cooling: Mechanical properties; Microstructural characterisation
Accelerated cooling after controlled rolling producesa refined final microstructure and favours the formationof low transformation temperature products like bainite and martensiteaustenite (MA) constituent[7 8]. The majority of works has been directed towards the study of microstructural evolution during acceler-ated cooling [4 9]. However only a limited amount ofresearch was carried out on the effect of accelerated cooling variables on the mechanical properties of steels[1 10].
1. Introduction
ture have been cxtensively studied in the last two HSLA low carbon steels with multiphase microstruc-decades [14]. These steels have high tensile strength good toughness and weldability. This bination of properties has led to their application in the automotiveindustry in manufacturing of large diameter pipes forgas and oil transportation in the areas of low tempera- ture and as plates for naval ships construction.
The objective of the present work was to study theinfluence of cooling rate and accclerating cooling starttemperature on the mechanical properties (Vickers mi- crohardness yield strength tensile strength and totalelongation) of a HSLA low carbon bainitic steel and toestablish the correlation between them.
ferrite pearlite bainite and martensite can be produced Multiphase microstructures consisting of polygonalin a great variety of HSLA low carbon steels as a resultof an appropriate bination of chemical posi- tion thermomechanical processing and acceleratedcooling conditions. The choice of optimum parametersfor accelerated cooling such as accelerated cooling start and finish temperatures and cooling rate also dependson the chemical position of the steel and the ther-momechanical processing (TMP) employed [5 6].
2. Experimental procedure
given in Table 1. This steel contains 21 pm of B and is The chemical position of the steel investigated ismicroalloyed with Nb Ti V and Ni. The presence of B and high Mn content increases hardenability of steel
Chemical position of stel (wt.%) Table 1
C Mn s P S A1 Nb V B Ti N0.08 1.70 0.25 0.021 0.002 6200 0.033 0.058 0.17 0.0024 0.026 0.0048
jet unit set at 60 V. Thin foils were examined usingtransmission clectron microscope Philips CM 20 oper- ated at 200 kV.
causing the formation of a multiphase microstructurecontaining bainite. This steel was developed as a - mercial grade of HSLA steel and was received as hotrolled.
model Dialux 22-RZD-DO microhardness tester with Vickers microhardness was measured using a Leitz load of 4.93 N. Each data point represents the mean of20 impressions from the analysed section and is ex- pressed in Vickers hardness numbers (VHN). Tensiletests were conducted in a MTS hydraulic machine
The controlled rolling process followed by controlled oo N o to 1no p sm min 1. Table 2 shows a rolling schedule. The speci- with 20 mm diameter rolls and rolling sped of 25 m20 min rolled with four pass schedule to 9 mm gauge mens (19 mm thick slabs) were reheated at 1200°C forsheets resulting in a total reduction of 74.7% truetemperature) were ~ 38% and below 7 were 37% The strain. The reductions above T (non-recrystallsationfinish rolling temperature was 825°C (Table 2).
Table 2Schedule of controlled rolling
Pass True Strain (c) Temperatures (°C)1 2 807 17.2 1152 105716.7 20.1 875 825
The values of critical temperatures such as T 4( →α transformation temperature) and B (bainite starttransformation temperature) were experimentally ob- tained for hot torsion tests using a technique developedby Boratto et al. [1l] and recorded from the dilation curves for conventional dilatometer tests without defor-ship between mean flow stress and temperature. mation. The Boratto method is based on the relation-
The rolled specimens were cooled down by an accel-work as shown in Fig. 1 [12]. This cooling system erated cooling device developed and assembled for thisnozzles. To study the effct of accelerated cooling consists of top and bottom double banks of water sprayparameters the following variables were used:
1. cooling rates (r ): 0.03 1.3 3.4 7 11 20 40 60 80 and 100 K s1 2. accelerated cooling start temperatures (7): 750 700and 650°C (Fig. 2).
These cooling rates are average values for the tem-perature range from 800 to 500°C and were registered in Omega Engineering Inc. interface with a thermocou-ple embedded in the mid-width/mid-thickness of thesample.
Transverse sections from the rolled specimens were(SEM). Samples were carefully prepared following the examined by optical and scanning electron microscopyusing 2% Nital and a special colour etchant used by standard procedure. The microstructure was revealedLePera [13]. Selected samples were studied using trans-prepared from 200 to 250 μum thick slices. These slices mission electron microscopy (TEM). Thin foils werewere mechanically thinned to about 120 μm thicknessand 3 mm diameter discs were punched out. Discs were electropolished in a solution of methanol with 5%perchloric acid at 30°C in a Struers Tenupol double
Fig. 1. Longitudinal schematic section of the cooling system device[12] where: (1) galvanized steel sheet box; (2) toolbox: (3) adjustable supports; (4) specimen bed; (5) water or air inlet hose; (6) water outlethose: (7) connecion hoses; (8) globe valve; (9) side valve; (10) manometer (pressure gauge); (11) plastic covering (I2) upper bank ofspray nozzles; (13) lower bank of spray nozzles; (14) rolled specimen.
Fig. 2. Schematic diagram of thermomechanical processing schedule.T - accelerated cooling start temperature
Heat treatments of specimens without deformation* Table 3
Heat Treatment Cooling rate (K s1) VHN MicrostructureAnnealing 0.03 167 PFPNormalising Quenching 300 1.4 224 436 8dd M
PF. polygonal ferte; P. pearite: GB. granular bainite; M.martensite.
model 814-14 at deformation rate of 10 ′ s. All tests were performed at room temperature and speci-[1] 68-0V NSV o Supoe pedd am suoTo pare the microhardness results three samples were heated at 1200°C for 20 min and subjected to fullannealing normalising and ice-brine quenching heat treatments without any previous deformation (Table 3).
3. Results and discussion
final microstructsure 3.1. Effect of accelerated cooling parameters on the
peratures were T = 945 ± 5°C and A s = 760 ± 5°C. The experimentally determined values of critical tem-1] s se9 =g sem amead For 1K s cooling rate bainite transformation start
Fig. 3 shows representative microstructures formedThere are substantial amounts of polygonal ferrite at as a result of various accelerated cooling schedules.g) (s 00 1e %06 01 dn) s uo mos and b) while microstructure in samples cooled withhigher rates contains signifcant amounts of bainite (Fig. 3cf) and martensite (Fig. 3h g) respectively.
During the cooling from 700°C after the controlledrolling as the rate increases from 1.3 to 11 K s the
crostructure also increcases up to ~ 6070% (cf. Fig. 3c amount of granular bainite formed in the final mi-e). Simultaneously the volume fraction of polygonalferrite decreases to 1220%.Rejected during the forma- tion of ferrite carbon diffuses into remaining austenitethus increasing its concentration. This leads to an in-Therefore the higher the cooling rate the shorter time crease in hardenability of untransformed austenite.the larger is the amount of austenite available for the available for diffusion controlled transformations andphase transformation with shear mode. This high C austenite transforms on cooling into MA constituent.These islands of MA were revealed by the coloration ofwhite tonality after etching with LePera solution. Ap- proximately 1216% of MA present in microstructureat these cooling rates. The formation of bainite and MA at higher cooling rates ( ≥ 11 K s1 Fig. 5d) isacpanied by the generation of high density mobiledislocations as a result of a shear ponent in the mechanism of these phase transformations.
a result of accelerated cooling at 3.4 K s from all The final microstructure of Nb VB steel formed asthree accelerated cooling start temperatures (Fig. 4)islands of MA constituent. In the lower part of Fig. 4a consists of polygonal ferrite and granular bainite withthese islands show cquiaxed (irregular) morphology without a preferential orientation while in the upperpart the MA constituent is of acicular shape and finer.Some MA laths (up to 0.3 μm in width) are of the same orientation and parallel to each other. Higher volumefraction of acicular ferrite was formed in the specimens cooled from 750 and 700°C than from 650°C acceler-ated cooling start temperature. Contrary the amountof polygonal ferrite was higher in the samples with lower accelerated cooling start temperature (Fig. 4c)the start of acccerated cooling. due to the longer time available for its formation before
More detailed observations of the microstructuralResults have confirmed that in samples cooled at slow evolution with cooling rate were obtained using TEM.cooling rates (111 K s') the dominant phase is polygonal ferrite and granular bainite with smallwith increasing cooling rate from 1.4 to 11 K s the amount of acicular bainite. It is clearly evident thatvolume fraction of granular bainite increases. Granularbainite appears as equiaxed ferrite grains with much higher dislocation density than in polygonal ferrite andislands of martensite and/or retained austenite betweenthem [1517] (Fig. 5a). Acicular ferrite consists of lath-like carbide-free ferrte grains with retained austen-ite or martensite layers between them [1618]. At inter- mediate cooling rate (20 K s1) bainitic phasesdominate. Both carbide-free acicular ferrite and car-bide-containing bainite (upper bainite and lower bainite) were observed (Figs. Sc and 5d). Further in-crease in cooling rate up to 40100 K s produces
observed in microstructure of studied samples:1. blocky type between granular bainite or polygonal ferrite grains (Fig. 5a b) and2. thin layers between acicular ferrite laths (Fig. 5c).
microstructure mainly consisted of lower bainite andupper bainite (Fig. 5e f). martensite with small amounts of acicular ferrite and
Two types of retained austenite morphology were
Fig. 3. Effet of cooling rate on final microstructure im samples afer controlled rolling and acoelerated cooling from 70oC: at 0.03 K s cooling rate (a b) at I3 K s (c d) at 11 K s (e) at 20 K s (I) and at 60 K s (g b). P pearlite; PF polygonal ferrite; MA martensitausteniteconstituent; AF acicular ferrite; GB granular bainite; UB upper bainite; B bainite; M martensite.
Fig. 4. Effect of acelerated cooling start temperature on the finalmicrostructure in samples coled at 3.4 K s: 7 750°C (a) 700°C (b) and 650°C (c). PF polygonal ferrite; MA martensiteausteniteconstituent; AF acicular ferite; GB granular bainite.
3.2. Effect of the processing parameters on the mechanical properties
Vickers hardness data for samples subjeeted to an-nealing normalising and quenching is given in Table 3.The microstructure of the sample after industrial con- trolled rolling and cooling at 1 K s1 is similar to thegranular bainite with MA and pearlite islands. How- normalised sample. It consists of polygonal ferrite ever the hardness of the industrially rolled sample isgrain structure produced as a result of the deformation higher than that of the normalised one due to the finerduring rolling.
ness (HV) [19] ultimate tensile strength (UTS) yield The effect of cooling rate on the Vickers microhard-strength (YS) and total elongation (El) for all three
Figs. 6 and 7. accelerated cooling start temperatures are shown in the
Three different regions could be identified on thecurves in Figs. 6 and 7 associated with the microstruc- tures obtained in the samples. At slowest cooling rates(≤1.0 K s) the Vickers hardness numbers yieldstrength and ultimate tensile strength have the lowest values correlated to the predominantly polygonal ferriteceeds 24%. microstructure. Correspondingly the elongation ex-
cooled at rates in the range 1.420 K s 1 corresponds Increase in microhardness and strength in samplesto the formation of multiphase microstructure consist-ing of polygonal ferrte granular bainite bainite and MA constituent. Presence of retained austenite in mi-crostructure at these cooling rates has significant effect on the mechanical properties. Stable retained austenite pe rn e o sn euandproduce transformation induced plasticity effect which is beneficial for the formability of steel.Due to the carbon enrichment microhardness of re- tained austenite is much higher than that of ferrite andit also contributes to increase of strength in the sampleswith granular bainite. However presence of bainite and MA in microstructure lcads to a decrease in steel(Fig. 7b). ductility as reflected in lower values of elongation
At high cooling rates ( > 20 K s ') the microstruc-ture consists predominantly of martensite and micro- hardness and tensile strength have corresponded values.that at cooling rates above 60 K sthe VHN and It could be seen from the curves for 7 = 700 and 750°CUTS have reached a plateau that is probably related to650°C a similar tendency is evident however the VHN the pletely martensite microstructure. For T =consisting of both bainite and martensite. In the sam- values are lower and associated with microstructuresthan 18% and reaches the value of 10% at 100 K s1 ples cooled at these rates the total elongation is lesscooling rate.
above 20 K s' higher HV and UTS values are It could also be pointed out that for cooling ratesfor the rates below 1 K s′ microhardness and UTS associated with higher cooling start temperature whileare higher for the lower cooling start temperature. Inthe interval of cooling rates between 1.0 and 20 K s1 the HV and UTS values increase with increasing of thecooling rate but are independent of the cooling starttemperature.
This tendency of increase in strength and hardnesswith increase in cooling rate and/or cooling start tem- perature is due to the fact that the amount of polygonalthe cooling rate or in the cooling start temperature ferrite in microstructure decreases with the increase in(Figs. 3 and 4). The temperature of ferrite formation is760°C and this allows the formation of a significant
