New Thermomechanical Hot Rolling Schedule for The Processing of High Strength Fine Grained Multiphase Steels

Transcription

1 New Thermomechanical Hot Rolling Schedule for The Processing of High Strength Fine Grained Multiphase Steels * A. Schmitz, J. Neutjens, J.C. Herman, and V. Leroy CRM, Rue E. Solvay, 11, B-4 Liège, Belgium, * currently at RDCS, Bvd de Colonster, B57, B-4 Liège, Belgium ABSTRACT The present paper deals with the metallurgical aspects of microstructural evolution of austenite prior to its transformation to ferrite during continuous cooling. The optimization of the processing route in terms of chemistry and rolling schedule is carried out by means of hot compression and torsion tests. The control of austenite recrystallization during the multipass deformation and the subsequent phase transformation provides fine grained ferrite-martensite microstructure. The combined use of a new thermomechanical rolling schedule with an appropriate continuous cooling down to martensite start temperature allows to produce a high strength level (75 MPa) based on a C-Mn-Nb steel chemistry which exhibits a very attractive resistance-ductility balance, a high toughness as well as interesting fatigue properties in the as hot-rolled condition. 1. INTRODUCTION Fuel economy and safety considerations constitute the driving force for the increasing demand of higher strength steels in the automotive industry. Concurrently with the use of microalloyed strip and sheet, dual phase steels whose microstructure is a mixture of ferrite and martensite are known to provide a good combination of (1) strength and ductility. As hot-rolled, dual phase steels are now accepted for automotive wheel disks because their cost and property performances are superior to those (2) of heat treated dual phase steels. Maid et al have put in evidence the correlation of grain size and properties of dual phase steels. A finer grained microstructure improves the elongation and tensile properties. As shown by Sudo (3) et al, the grain refinement is also beneficial for better hole expansivity, sufficient for application. Furthermore, wheel disks produced from fine grained dual phase steels exhibit superior fatigue life when compared to the microalloyed pearlite reduced steel grades of the same (4) tensile strength level. Different rolling schedules have been developed to produce microalloyed steels with ultrafine grain size, each of them being characterized by the way the austenite is prepared before the phase transformation to ferrite on the run-out table. The addition of Mo, Nb or Ti influences both static and dynamic recrystallization during hot deformation of austenite. Static recrystallization occurs in all steels with low alloying levels and high temperatures, and the austenite grain size is reduced through repeated static recrystallization between the different stands of a finishing train. This leads to the recrystallization controlled rolling process (RCR). If there is sufficient strain-induced precipitation of the microalloying elements, neither static nor dynamic recrystallization can take place and the austenite grains become pancaked through the finishing process. The pancaked austenite grain structure is the basis for conventional controlled rolling (CCR). When Mo, Nb or Ti are in solution in steels, static recrystallization is retarded. If the interpass times are short enough, the strain is accumulated from pass to pass in the absence of large amount of straininduced precipitation up to the initiation of dynamic recrystallization. In their study of dynamic recrystallization controlled rolling (DRCR) of strips, Samuel et (5) al have observed that this process produces fine grained austenite (below 5µm) and ferrite grain sizes of about 3µm when cooling rate is 1 C/s. On the other hand, the pancaked austenite obtained by the CCR route leads to ~ 7µm ferrite grain size after cooling. From the similar results of Kaspar et al, the dynamic recrystallization controlled rolling seems thus the most performant route to refine the ferrite grain size. (7) Bowden et al have subsequently studied the effect of interpass time on austenite grain refinement in presence of dynamic recrystallization. The possible metadynamic recrystallization which is expected to take place after initiation of dynamic recrystallization during the subsequent interpass times has been analyzed and (8) modelled by Roucoules et al, and extended later to (9) other alloying elements. (6) ISS Technical Paper, A. Schmitz Page 1 of 14

2 To achieve an appropriate microstructure in hotrolled dual phase steels, first alloy concepts were based on (1) Mn, Si, Cr and Mo alloyed steels. Owing to the rather high alloy content, these steels were expensive and alternative grades have been proposed with an optimized (11) stepwise cooling down to a low coiling temperature. The principal disadvantage of these complex thermal paths on the run-out table lies in the regulation of the cooling system to ensure a constant thermal profile along the length of the coil. Without any coil-box prior to finishing, this problem is amplified by the speed-up which is necessary to obtain a constant temperature at the exit of the finishing train. Non homogeneous microstructure with varying amounts of ferrite along the length of the coil results from the sensitivity of this rolling schedule to the process parameters. Another rolling procedure has been (4) put forward by Mizui and Takahashi in which lowcarbon Si-Mn steels containing no other alloying elements are hot-rolled at temperatures slightly below the Ar 3 transformation temperature. This helps to accelerate the ferrite formation and a continuous rapid cooling leads to a fine grained dual phase steel. However, this solution suffers from the loss of ductility related to the fact that ferrite is slightly strain hardened. The key idea of the present work combines the grain refinement of austenite by means of dynamic recrystallization with the resulting enhanced transformation kinetics in order to produce a dual phase steel by a continuous cooling to room temperature. Some results (12) of Kaspar et al. seem indeed to indicate the presence of high-carbon constituent in the microstructure as well as the absence of a sharp yield point, the both effects being characteristic of a dual phase steel. The aim of this work is the optimization of the process in terms of chemistry and rolling schedule to achieve a fine grained dual phase steel by a continuous cooling. A systematic study of the effect of different amounts of microalloying element on the kinetics of static and dynamic recrystallization is carried out in parallel with the influence of rolling parameters such as the rolling temperature, the amount of cumulative strain and the interpass time. An optimized process is proposed and the mechanical properties are evaluated on an as hotrolled strip elaborated in laboratory. 2. EXPERIMENTAL PROCEDURE Experiments were conducted in three phases. First, static recrystallization kinetics was analyzed by compression tests which simulate the critical interstand of a finishing train. Next, the dynamic recrystallization behaviour of the different steels was evaluated up to large deformation by means of torsion tests. Finally, an optimized rolling schedule was tested at the laboratory scale Materials The chemical compositions of the grades chosen to carry out this study are presented in Table I. Table I Chemical composition (1 wt.%) of the experimental Nb-containing steels Steel C Si Mn P S Ti Al Nb N To promote the initiation of dynamic recrystallization, the finishing rolling must be performed below a temperature T for which no static recrystallization is nr expected to occur between the interstands, and above the transformation temperature Ar to ensure deformation in 3 the austenitic temperature range. From a practical point of view, the temperature range between T and Ar must be nr 3 sufficient to enable a rolling schedule on an industrial line. This range is spread by means of an addition of Mn to lower Ar 3 and different levels of Nb to increase T nr. From the different atoms which retard static recrystallization by the solute drag effect, Nb constitutes (13) the best candidate in the austenitic temperature range. Its effect on final refinement of ferrite grain size is systematically analyzed for different contents ranging from to 83 1 wt.%. Finally, Ti is added to remove N in solution by the formation of TiN precipitates which also inhibit grain growth at high reheating temperatures. The steels were cast and hot-rolled to plates of 25mm and 12.5mm final thicknesses at the CRM pilot line plant. Compression samples of 2mm in diameter and 2mm in height were machined out of the 25mm plates. Torsion test specimens with a gage length of 24mm and a diameter of 6mm were machined from the 12.5mm hot-rolled plate with the orientation along the transverse direction. ISS Technical Paper, A. Schmitz Page 2 of 14

3 2.2. Double-hit compression tests To study the progress of static recrystallization during finishing rolling, several sets of double-hit compression tests were conducted on the CRM computerized MTS servohydraulic press. The specimens were austenitized at 125 C for 2 min which ensures complete dissolution of Nb precipitates, and then slowly cooled (~1-2 C/s) to the testing temperature (85, 9, 95, 1 and 15 C). They were subsequently hot compressed to a true strain of.5 at a strain rate of 1/s. After a constant unloading time of 5s, the specimens were reloaded at the same strain rate. The load-contraction data were converted into true stress σ and true strain ε, and the degree of softening X during the unloading period was fitted with the help of the first σ 1 and second σ 2 loading curves on the basis of the mixture low : σ (ε) = X. σ (ε -.5) + (1 - X). σ (ε) Hot torsion tests To investigate the softening behaviour up to large deformation and the resulting refinement of ferrite grain size, torsion tests were carried out on the CRM hottorsion machine equiped with an induction heating. After austenitization at 125 C for 3s, the specimens were cooled down at 5 C/s to 11 C and subjected to a first roughing strain of.5 at 1/s to refine the austenitic grain size. They were subsequently cooled to the test temperature at 5 C/s. True stress and true strain were derived from the torque-twist data using the method of Gräber and (14) Pöhland at the critical radius for a solid specimen. Isothermal torsion tests were performed at strain rate of 1/s and at temperatures ranging from 1 C down to 85 C. Different types of torsion tests were carried out : (a) The dynamic recrystallization behaviour was obtained for different Nb content from continuous curves up to a strain of 3 which is representative of the cumulative strains observed on industrial lines. b) The effect of interpass time was evaluated for the 83 1 wt.% Nb steel for which strain-induced precipitation is expected to take place. c) Interrupted torsion tests at various strain levels were performed to examine the influence on ferrite grain size. cooling to room temperature at a cooling rate of 2 C/s characteristic of most laminar cooling systems on the runout table. Metallographic samples were polished and etched in a Marshall solution to reveal the ferrite grain boundaries or in a Lepera solution to put high-carbon constituents in evidence. The retained austenite was detected by X-ray diffraction using Mo-K radiation Laboratory scale hot-rolling simulation From the above process analysis, an optimized chemistry and rolling schedule was proposed and tested on the instrumented CRM pilot scale rolling mill. Mechanical properties were determined from flat tensile specimens (5 x 2 x 15 mm³) and low temperature toughness was evaluated from Charpy specimens (5 x 1 mm² fracture section) machined out of the strip. The fatigue properties of this dual phase steel were measured by means of strain controlled fatigue tests. The tests were performed on a MTS closed loop system at a strain ratio of R = -1, and at a frequency of.3 Hz with specimens of 2mm in gauge length and 6mm in width. 3. RESULTS 3.1. Static softening during interpass time Figure 1 shows the thermal profile computed by the (15) STRIPCAM program which simulates the rolling of a transfer bar of 28mm thickness down to 3mm on an industrial finishing train. The corresponding equivalent strain reaches 2.9 value in this case. Due to the natural heat losses by convection and radiation between the stands, the mean temperature of the strip continuously decreases throughout the train. In parallel, a decreasing interpass time IPT translates the mass flow continuity which is characteristic of the plastic deformation of metals. From the point of view of kinetics of static recrystallization, the most critical interstand is located between stands 1 and 2 where the highest temperatures and the longest interpass times are observed. The influence of the different austenite microstructures generated by the various rolling schedules on the ferrite grain refinement was analyzed after the austenite to ferrite transformation during continuous ISS Technical Paper, A. Schmitz Page 3 of 14

4 TEMPERATURE PROFILE IN ROLLING ( C) IPT = 5.4 s IPT = 3.3 s IPT = 2.1 s IPT = 1.4 s IPT = 1. s Mean 1/2 thick. 1/4 thick. 1/8 thick. 1/16 thick. recrystallization is only initiated in the Nb free steel at the highest temperature (15 C). While in that case metadynamic recrystallization is supposed to take place, the other Nb steels never exhibit dynamic recrystallization under the simulated conditions. The softening curves as function of the temperature of pre-deformation are illustrated in figure 3 for the different Nb content. STATIC RECRYSTALLIZATION KINETICS Time since exit of furnace (s) Fig.1. Stress (MPa) Thermal profile through the strip thickness simulated for an industrial finishing train One can assume that static recrystallization will become more and more difficult downwards in the train, and only the thermomechanical conditions prevailing in the interstand 1-2 will be retained here for the softening study by means of compression tests. A first strain of.5 is applied to the samples, which corresponds to the mean strain value observed at the stand 1 of the train. An interpass time of 5s is adopted in this study. The static recrystallization behaviour of the different Nb content steels are compared at 95 C in figure 2. Fig.2. DOUBLE HIT COMPRESSION TEST 95 C - IPT = 5 s Strain 1/s Double-hit compression tests on Nb-containing steels for the critical interpass time of 5s (Nb content in 1 wt.%) During the prestrain, only positive slopes are observed at 95 C, and this corresponds to the dynamic recovery regime on which Nb has a slight effect by solid solution (around 83 percent increase of the flow stress per added weight percent of Nb). After a prestrain of.5, dynamic Relative softening X Fig.3. Strain=.5 1/s IPT=5 s Temperature ( C) Softening curves of the C-Mn and Nb steels corresponding to 5s unloading time (Nb content in 1 wt.%) Below 85 C, recrystallization is essentially arrested in all steels and strain accumulation is thus expected to take place under strip rolling conditions even for Nb free steel. While some softening is observed at 9 C for the C-Mn steel, the addition of.17 percent of Nb is enough to prevent static recrystallization to appear between the two first stands. The increase of the entry temperature to 95 C necessitates a Nb level of more than.34 percent to ensure complete strain accumulation while static recrystallization is complete for C-Mn steel. At 1 C, only the.83%nb steel is able to cumulate the deformation whereas for temperatures higher than 15 C, static recrystallization seems unavoidable for all the steels tested in the work Dynamic softening up to large deformation The torsion curves for the different Nb steels deformed at 95 C are presented in figure 4. ISS Technical Paper, A. Schmitz Page 4 of 14

5 Stress (MPa) Peak strain TORSION TEST 95 C Strain 1/s DYNAMIC RECRYSTALLIZATION KINETICS Temperature ( C) 1/s Nb 17 Nb 34 Nb 83 Nb Fig.4 Torsion flow curves for different Nb steels tested at 95 C and a strain rate of 1/s (Nb content in 1 wt.%) All the steels work harden to a maximum at the peak strain ε p and then soften to eventually reach a plateau with an approximatively constant flow stress. This softening is attributed to the dynamic recrystallization which spreads during straining up to completion at the steady-state stress. Nb addition has a marked retarding effect on the kinetics of dynamic recrystallization by shifting the peak strain to larger values. In the case of the.83%nb steel, the importance of the peak strain shift is such that no complete dynamic recrystallization is seen even up to a strain of 3. On the other hand, Nb slightly influences the level of the steady state stress once it is reached. Fig Dependence of the peak strain on the temperature for different Nb content (in 1 wt.%) The strains corresponding to the peak stress are reported in figure 5 as function of the temperature of deformation. Despite the decrease of the peak strain with the temperature, a lower bound of strain of.8 has to be obtained to observe dynamic softening in any steel under 1 C. The retarding sensitivity to Nb is pronounced at low levels while it seems to saturate for higher alloying levels, especially at the low temperature (85 C). The figure 6 illustrates the effect of the interpass time on the softening behaviour during multipass deformation at 9 C of steels with three levels of Nb content. Stress (MPa) Fig.6 1/s TORSION TEST 9 C - IPT=1s Strain Nb 17 Nb 83 Nb Interrupted stress-strain curves at 9 C for different Nb content with an interpass time of 1s (in 1 wt.%) The C-Mn flow curve exhibits a rounded shape for the individual curves which remain at approximatively the same level. This can be explained from the compression tests (fig.3) since an interpass time of 1s should be enough for complete static recrystallization of a C-Mn steel at 9 C. In sharp contrast to Nb free steel, complete strain accumulation from pass to pass is observed for the.83%nb steel. Furthermore, no softening of the envelope of the individual curves is noticed, which can be translated into a complete pancaking of the microstructure. Such an amount of Nb thus prevents static and dynamic recrystallization by solid solution and probably also by strain-induced precipitation of Nb that can take place during the 1s delay for this high alloying content. An intermediate situation is given by the.17%nb steel for which the overall stress level for the first two passes is the same than for the.83%nb case, but followed by a decrease of the mean stress for the subsequent deformation steps. After the first pass, the holding period is too short for static recrystallization in agreement with the compression test (Fig.3) of the.17% Nb steel. ISS Technical Paper, A. Schmitz Page 5 of 14

6 Consequently, the strain accumulates up to the initiation of dynamic recrystallization. This is possible since Nb certainly does not precipitate at this Nb level and retards dynamic recrystallization by the solid solution effect. The subsequent softenings have to be related to the fast metadynamic recrystallization which takes place within 1s after initiation of dynamic recrystallization. As shown in figure 8, the addition of.17% of Nb still allows dynamic recrystallization for any temperature of deformation ranging from 85 C to 1 C. After having been cooled down to room temperature, the metallographic specimens present a transition from acicular to fine equiaxed ferrite when the temperature of deformation is decreased from 1 C to 85 C. 2 TORSION TEST 9 C - 1/s 16 T8/5 = 15 s CR = 2 C/s Stress (MPa) 12 8 Nb 17 Nb 83 Nb 4 2 µm Strain Fig.7 Influence of the Nb content after straining at 9 C and on the as-cooled microstructure (Marshall etchant) 3.3. Influence of the rolling parameters on the microstructure The effect of Nb content on the grain refinement is displayed in figure 7 in which the flow curves at 9 C are presented with the corresponding ferrite microstructure after cooling down to room temperature at 2 C/s. The sensitivity of ferrite grain size to the Nb content is already marked for low Nb level such as.17%. While complete dynamic recrystallization is noticed for the C-Mn and.17% Nb steels, the finest grain size of around 2µm is obtained in the case of the.83%nb steel which does not recrystallize completely after a strain of 3. However, no great differences are observed for the lower temperatures at 85 C and 9 C. For the interpretation of the influence of the temperature of deformation, care must be taken over the time that the austenitic microstructure spend at high temperature during cooling down to 2 C. As a matter of fact, some metadynamic recrystallization is expected to take place at the higher temperatures after complete dynamic recrystallization. Indeed, the kinetics of metadynamic recrystallization is known to be faster than for static recrystallization, and this latter is already active after 5s above 9 C for the.17% Nb steel. ISS Technical Paper, A. Schmitz Page 6 of 14

7 2 TORSION TEST 17 Nb - 1/s C 9 C 95 C 1 C Stress (MPa) 12 8 T8/5 = 15 s CR = 2 C/s 4 2 µm Strain Fig.8. Temperature effect on the dynamic softening of the.17% Nb steel and on the resulting microstructure after cooling down to room temperature (Marshall etchant) Consequently, some metadynamic softening should appear for deformation above 95 C where sufficient time (a few seconds) is spent between deformation and transformation with a cooling rate of 2 C/s. The evolution of ferrite microstructure with the amount of continuous deformation at 9 C is illustrated in figure 9 in the case of.83% Nb. By straining up the austenite, the as-cooled microstructure transforms gradually from a pure bainitic state to a polygonal ferrite. The additional strain leads to the nucleation of small grains of ferrite along the grain boundary of parent austenite, and the volume fraction of these fine grains increases with prior deformation. A mixed ferrite-bainite microstructure is observed up to a strain of 3 where fine grained ferrite prevails throughout the sample. From a practical point of view, the above results confirm that a sufficient amount of strain has to be cumulated within the finishing train in order to enable the formation of ferrite with a fine equiaxed grain size, and this necessitates an entry temperature lower than T. nr presents the higher level of Nb supersaturation. This steel is able to pancake easily through copious strain-induced precipitation which should occur during straining at 9 C. An interpass time of s corresponds to a continuous deformation and leads to a nearly complete dynamic recrystallization after a strain of 3. The corresponding grain size of ferrite is very fine (2µm). For all the other values of the interpass time, the mechanical behaviour is characteristic of full pancaking of the austenitic microstructures, even for interpass time as low as.6s. However, the ferrite grain size after austenite pancaking increases from 2 µm for an interpass time of.6s towards a rougher value around 6-7 µm in the case of 1s interpass time. This experimental fact does not match the (6,7) interpretation of other searchers which identify a clear relationship between the fine ferrite and the occurrence of dynamic recrystallization of austenite prior to its transformation. Some complementary comments on the fine grained ferrite formation will be presented later in the discussion part of this paper. The effect of the interpass time (fig.1) has been more particularly studied for the.83% Nb steel which ISS Technical Paper, A. Schmitz Page 7 of 14

8 2 TORSION TEST 83 Nb - 9 C 1/s 16 Stress (MPa) 12 8 T8/5 = 15 s CR = 2 C/s Strain 2 µm Fig.9. Microstructural evolution of the as-cooled.83% Nb steel as function of the amount of strain applied at 9 C (Marshall echant) The etching of the metallographic samples with Lepera etchant reveals a fine distribution of small white islets which correspond either to martensite or to austenite. As a matter of fact, X-ray diffraction has put in evidence the presence of some retained austenite at room temperature which amounts to 4.3% volume fraction. The shift of the (22) austenite peak allows the determination of the carbon concentration in the retained austenite with the help of the extrapolation method of the austenite lattice (16) parameter. By means of the Ridley's expression for the austenite lattice parameter a as a function of the carbon concentration C, a (Å) = wt.%c a carbon content around.9 wt.% is estimated for the retained austenite. The micrographs also show that a refinement of the ferrite grain size leads to a more widespread distribution of smaller martensite-austenite islets. This refining effect should be beneficial for the mechanical properties, especially for what concerns ductility and toughness Optimized chemistry and rolling schedule From the previous results, it appears that the formation of a fine grained dual phase steel implies a cumulative deformation of austenite above a strain value around 2 (see fig.9) which corresponds to a minimum thickness reduction of 85%. On the other hand, the strain accumulation necessitates the absence of any static recrystallization within the finishing grain. To avoid static recrystallization of steels, the rolling temperature can be decreased below 85 C (Nb free steel in fig.3) but this is accompanied by an important rise of the rolling forces and powers. To increase the temperature at the entry of the finishing grain, the kinetics of static recrystallization can be slowed down by means of Nb addition through its retarding effect by solute drag. This temperature increase does not affect too much the ferrite grain refinement if it is limited to 95 C (see fig.8) to avoid any post softening during a subsequent cooling. A minimum of.3 wt.% of Nb has to be added to steels in order to reach an entry temperature of 95 C (fig.3) and below which static recrystallization is prevented. Furthermore, Nb addition favours a fine grain size (fig.7), even at a low level. ISS Technical Paper, A. Schmitz Page 8 of 14

9 2 16 1/s TORSION TEST 83 Nb - 9 C T8/5 = 15 s CR = 2 C/s Stress (MPa) 12 8 IPT= s IPT=.6 s IPT=2 s IPT=1 s 4 3 µm Strain Fig.1 Effect of interpass time on pancaking and microstructure of the.83%nb steel cooled to room temperature (Lepera etchant) Taking into account the temperature loss which takes place throughout an industrial finishing train (fig.1), it is necessary to extend the temperature range between T nr and A in order to avoid phase transformation within the r3 train. A temperature loss around 1 C is commonly observed on the lines for a strip of 3mm thickness, which leads to a maximum A r3 value around 8 C. This can be achieved by Mn addition which is also beneficial by its solid solution strengthening of ferrite. According to the (17) Ouchi formula of A r3 temperature for strain-hardened austenite, an A r3 of 8 C requires a minimum Mn addition of 1.4 wt.% on a basis of.7 wt.% of C. This C content should provide sufficient volume fraction of finely dispersed second phase particles in order to ensure performant mechanical properties. During the continuous cooling at a cooling rate of 2 C/s, strain-hardened austenite transforms into a fine grained ferrite (fig.1) which rejects its carbon into the residual austenite. The cooling rate must not be too high to enable complete ferrite transformation and a good carbon enrichment of residual austenite, but also not to low to prevent the austenite transformation into bainite or pearlite. The nature and hardness of the second phase issued from the transformation of enriched austenite are defined by the coiling temperature. To produce a ferritemartensite microstructure, the coiling temperature must be below the martensite start (M s) temperature. The previous evaluation of.9 wt.%c in the retained austenite allows to estimate an M s temperature around 17 C according (18) Kunitake. It is therefore advised here to continuously cool the strips down to room temperature to ensure sufficient transformation of residual austenite into martensite. Some retained austenite is however unavoidable but it can be beneficial for the mechanical properties through the Transformation Induced Plasticity (TRIP effect) during the tensile test at room temperature. ISS Technical Paper, A. Schmitz Page 9 of 14

10 From the above considerations, the laboratory rolling trial has been carried out on a steel whose chemical composition is given in table II. The optimized rolling schedule is summarized in table III. The interpass time does not exceed 1s for any pass. The cooling of the strip was performed by quenching in still water at room temperature.the objective was thus the production of a Dual Phase Continuous Cooling (DPCC) steel. Table II Chemical composition of the experimental steel (1 wt.%) C Mn P S Nb Al N The innovative aspect of this process of thermomechanical rolling is two-fold. First, it allows to produce dual phase steels from a lean chemical composition which is quite cheap. Second, the production schedule is based on a continuous cooling of the hot-strip on the run-out table down to room temperature.this way to process the hot-strip avoids the problems akin to heterogeneous microstructures along the length of the coil that result from the stepwise cooling paths proposed in the past for the elaboration of dual phase steels. The chemical composition and the optimized rolling schedule proposed hereabove are subjected to an European Patent which is currently under (19) examination. Table III Thermomechanical processing parameters of the laboratory hot-rolling trial Reheating temperature Finishing pass schedule 115 C (1h) Start rolling temperature 94 C Finish rolling temperature 84 C Cooling rate mm ~ 8 C/s Coiling temperature ~ 2 C 3.5. Mechanical properties of the laboratory strip The results of the tensile test as well as the microstructure of the experimental steel processed by the optimized process are presented in figure 11. TENSILE TEST 8 6 YS = 45 MPa TS = 753 MPa A5 = 23 % Stress (MPa) µm Strain (%) Fig.11 Tensile stress-strain curve and optical micrograph of the laboratory multiphase steel hotrolled according the optimized schedule (Lepera etchant) ISS Technical Paper, A. Schmitz Page 1 of 14

11 The laboratory strip gives a yield strength of 45 MPa and a tensile strength of 753 MPa associated with an important total elongation of 23%. This steel thus provides a good combination of strength and ductility with a product TS x A value which amounts to 17,3 5 Mpa. %. The stress-strain curve of such a steel does not exhibit any yield point, which results in a low YS/TS yield ratio of.6. The absence of sharp yield point is typical of dual phase microstructures where the presence of high-carbon constituents leads to a less effective locking of the increased dislocation population. This is confirmed by the Lepera etching of the metallographic sample which puts in evidence a fine distribution of white martensite-austenite particles dispersed in a fine grained ferrite of 4-5 µm. The interface between these martensite particles and the ferrite grain is particularly rich in dislocations that were created by the volume expansion accompanying the martensitic transformation of residual austenite during cooling below the M temperature. The s unlocked dislocations are considered to the origin of the stress-strain curve free from any yield point elongation. The market benefit of fine grained ferrite can also be observed by the measurement of the low temperature toughness (figure 12). high-carbon constituent whose fine homogeneous distribution is probably less damaging to fracture initiation and propagation despite its high hardness. In figure 13, the cyclic response curve at.25% strain shows an initial cyclic hardening followed by cyclic softening. The peak stress amplitude (42 MPa) is attained at around 1 percent of the number of cycles to failure which amounts to 5. Stress amplitude (MPa) OLIGOCYCLIC PLASTIC FATIGUE Strain =.25% Strain =.1 % N Cycles 2 Charpy test - subsize (5mm) Fig.13. Stress response curves at low cycle fatigue test on the laboratory multiphase steel Energy (J/cm2) The subsquent softening around 15% at the moment of failure demonstrates the relative stability of this steel. 4. DISCUSSION Fig Temperature ( C) Low temperature toughness curve by Charpy test on the laboratory multiphase steel While the impact energy at room temperature saturates to 2J/cm², the impact transition temperature ITT determined at half the upper shelf energy is as low as -13 C. This excellent result may be attributed to the microstructure since it is well known that the resistance to brittle fracture is greatly enhanced by grain refinement. But an additional reason seems to be the scale of the 4.1. Retardation effect of Nb on static and dynamic recrystallization The kinetics of recrystallization are retarded by Nb addition either by solid solution or under the form of precipitates. Different modes of precipitation can be distinguished depending on the state of austenite prior to precipitation. In a recrystallized austenite, the static precipitation is characterized by a sparse nucleation of precipitates on grain boundaries and around undissolved particles. On the other hand, strain-induced precipitation is considerably accelerated by a prestrain and is still further accelerated during a continuous deformation which generates dynamic precipitates. The rate increase ISS Technical Paper, A. Schmitz Page 11 of 14

12 can be attributed to the relatively dense nucleation of precipitates on dislocations and the enhanced growth rate by the increased Nb diffusion in the presence of excess (2) vacancies. In their investigation of the progress of precipitation in a.35%nb steel containing.5% of C and.42% (21) of Mn, Weiss and Jonas have determined the kinetics of the different precipitation mode by means of a mechanical method based on the determination of the peak stress of dynamic recrystallization during constant strain rate compression tests. Whereas static precipitation only takes place after 1s at 9 C, the imposition of 5 percent prestrain considerably speeds up the precipitation rate by about one order of magnitude and initiates straininduced precipitation after 1s at 9 C. The rate of dynamic precipitation is again one order of magnitude faster than strain-induced precipitation and already appears after 1s at the temperature nose which lies at 9 C. After a prestrain of.5 at 9 C, no important amount of strain-induced precipitation should occur within 5s for the Nb steels investigated by compression in this work (see fig.3), except perhaps for the.83%nb for which the high level of supersaturation is supposed to enhance precipitation. Consequently, Nb is assumed here to interact with static recrystallization after a single pass by the solute drag effect. While solute Nb certainly delays dynamic recrystallization during continuous deformation, some dynamic precipitation is possible after a strain of more than 1 since it corresponds to a deformation time of 1s with a 1/s strain rate. This precipitation should be more pronounced in presence of high Nb supersaturation and could eventually affect dynamic recrystallization. This is indeed put in evidence in figure 4 where the stress-strain curve of the.83%nb steel displays a plateau around a strain of 1 before softening. More complex situations are encountered in the case of multipass deformations separated by varying interpass times. In the case of long interpass times and a high degree of supersaturation (such as 1s and.83%nb) strain-induced precipitation delays both static and dynamic recrystallization, so that metadynamic recrystallization is also prevented and a full pancaking is observed (fig.6). As mentioned by (9) Roucoules et al., the introduction of fresh precipitates that nucleate during the successive interpass times compensates the coarsening which takes place in the subsequent passes and renders ineffective the retarding effect on the recrystallization. When lower levels of supersaturation or shorter interpass times are used (as in continuous mills), the density of fresh precipitates is apparently not high enough to prevent dynamic recrystallization which leads in turn to metadynamic recrystallization (see.17%nb in fig.6). The rate of the latter mechanism increases with accumulated strain because the amount of dynamic precipitate coarsening (22) increases with deformation. However, it should be remarked that at high supersaturation contents such as.83%nb, interpass times as short as.6s ensure a complete pancaking of austenite (fig.1). This can be explained by the solute drag effect of Nb on both static and dynamic recrystallization up to the peak strain (around 1.8 at 9 C), associated with a fine dynamic precipitation which is expected to take place during the corresponding 4s cumulative time Refinement of ferrite grain size by Nb (5,7) In agreement with some authors, the analysis of the effect of the interpass time on the microstructure (fig.1) for continuous (IPT = s) and long interruption time (IPT = 1s) would lead to conclude that it is dynamic recrystallization which is at the basis of ferrite grain refinement through the generation of fine recrystallized grains more efficient than pancaked grains. Nevertheless, this interpretation does not explain how the interrupted torsion test with.6s interpass time also provides a very fine ferrite grain although full pancaking is observed. Another explanation is given here where the accent is put on the effect of Nb in solid solution on the grain refinement of ferrite during the phase transformation. As shown in figure 7, the ferrite grain size is quite sensitive to the addition of Nb after direct cooling of the dynamically recrystallized austenite in which Nb has mainly remained in solid solution. Furthermore, Fujioka (23) et al have shown that retardation of ferrite transformation is weakened by the progress of straininduced precipitation in austenite and they concluded that ferrite transformation is significantly restrained by Nb in solution but unsubstantially as precipitates. As a consequence of these observations, it results that only Nb in solution plays an important role in the refinement of the ferrite grain size, probably by its effect on carbon (24) diffusivity. On the other hand, Herman et al have determined the isothermal kinetics of strain-induced precipitation after a single pass deformation by selective eletrolytic dissolution of a.8%nb steel with 1.5% of Mn. Their analysis revealed that strain-induced precipitation starts at 1s, in agreement with previous (21) works, and that.4%nb has precipitated after 1s. ISS Technical Paper, A. Schmitz Page 12 of 14

13 This situation should be amplified during multipass deformation where fresh precipitates are continuously nucleated and one can expect a monotonic decrease of the Nb solute content with the interpass time. Although the dynamic recrystallization of austenite is very beneficial to obtain a fine grained ferrite, the effect of the interpass time on the grain refinement has to be translated more in terms of Nb solute depletion of the austenitic matrix than due to dynamic recrystallization of austenite Effect of austenite hot-deformation on the kinetics of austenite to ferrite transformation As mentioned above, in itself Nb in solution in steels slows down the isothermal kinetics of transformation. Despite of that, the addition of Nb generally leads to a faster transformation given that it prevents from recrystallization and allows to retain work-hardened austenite up to the start of transformation. Umemoto et (25) al have found that the ferrite nucleation rate was substantially increased by the work-hardening of austenite which refines considerably the ferrite grain size. The mechanism of enhanced nucleation was formulated considering the ledge formation at grain boundaries, the increase in dislocation density within austenite grains, etc... Furthermore, the work-hardened austenite is rich in dislocations which favour the carbon diffusion ahead the front of transformation and thus increase the ferrite growth rate. The acceleration of transformation due to work-hardening of austenite is important in ferrite and pearlite transformations, whereas the bainite transformation is not much affected by austenite deformation (26). The enhanced kinetics of ferrite transformation from a work-hardened austenite allows the formation of a dual phase steel by continuous cooling since the ferrite rejects is carbon content into austenite. As a consequence, the mean carbon content of residual austenite increases with the volume fraction of polygonal ferrite, and this carbon enrichment slows down the kinetics of austenite transformation up to an extent where only martensite transformation remains possible. The enhanced ferrite kinetics thus avoids any stepwise cooling to allow sufficient volume fraction of ferrite to appear in the austenite microstructure. 5. CONCLUSIONS The influence of the chemical composition and rolling schedule parameters on the ferrite microstructure after continuous cooling have been investigated in this work. From the mechanical and metallographic results, the following conclusions can be drawn : 1. During multipass deformation in a finishing mill, the addition of.3 wt.%nb prevents the occurrence of static recrystallization for an entry temperature below 95 C. 2. During continuous deformation, the Nb addition retards considerably the initiation of dynamic recrystallization and a cumulative strain of more than 2 has to be applied below 95 C on a.3 wt.%nb steel in order to induce complete dynamic recrystallization. 3. After continuous cooling down to room temperature at 2 C/s, a polygonal fine grained ferrite is obtained in the absence of any static recrystallization by a prior deformation below 95 C up to a cumulative strain of 3. In these conditions, the Nb content in solid solution has a marked effect on refinement of ferrite grain size leading to 2µm ferrite grains with.83 wt.%nb addition. 4. In the case of multipass deformation in the presence of Nb, an increase of the interpass times gives more time for strain-induced precipitation to take place and thereby delays dynamic recrystallization. Long interpass times higher than 1s and high levels of Nb supersaturation such as.83 wt.% prevent dynamic recrystallization and lead to full pancaking of austenite microstructure. However, lower supersaturation and shorter interpass times allow metadynamic recrystallization to occur once dynamic recrystallization is initiated. 5. Longer interpass times provide rougher ferrite grain sizes after continuous cooling to room temperature. This fact is interpreted as a result of the loss of solute Nb due to strain-induced precipitation during the hot-deformation of austenite. 6. After the application of the optimized rolling schedule proposed in this work, a continuous cooling to room temperature provides a strip with a fine grained ferrite (4-5µm) in which small highcarbon constituents are homogeneously dispersed. The resulting dual phase steel exhibits excellent tensile properties (TS = 75MPa, A 5 = 23%) aswell as improved toughness and fatigue resistance. ISS Technical Paper, A. Schmitz Page 13 of 14

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