Microstructural Evolution in Microalloyed Steels with High-Speed Thermomechanical Bar and Rod Rolling

Transcription

1 Microstructural Evolution in Microalloyed Steels with High-Speed Thermomechanical Bar and Rod Rolling Robert Cryderman, Blake Whitely, and John Speer Advanced Steel Processing and Products Research Center, George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden Colorado, USA Abstract Bars and rods are rolled at high total deformations, high strain rates, and short inter-pass times compared to products such as plates or structural sections where extensive studies have been conducted to understand the effects of microalloying and the limited range of thermomechanical process parameters. Data are presented to illustrate how microalloying and high-speed thermomechanical processing affect the as hot-rolled microstructures for a variety of steel grades and applications. Simulations on a Gleeble 3500 using torsional deformation and controlled time-temperature schedules as well as interrupted quenching have allowed examination of the evolution of prior austenite grain size and morphology. The austenite condition, in combination with the final cooling schedule, influence final hot-rolled microstructures and can lead to significant effects on microstructures and mechanical properties after subsequent heat treatment. Introduction There have been many studies leading to models of austenite grain development in flat rolled plates and strip where deformation essentially occurs in two dimensions (plane strain) such that the thickness is reduced and the length is increased. In contrast, rolling to long products such as bars and rods provides deformation in three dimensions and increased total deformation. For example rolling a 250 mm slab to 16 mm plate achieves an elongation of about 16:1 as compared to rolling a 200 mm square billet to a 40 mm round bar for an elongation of 32:1. Smaller finished sizes are also possible using the same semi-finished sections to produce light gauge strip at 1.5 mm for an elongation of 167:1 or to produce 5.5 mm diameter rod for an elongation of 1680:1. Average actual strains in long products are further increased by the redundant deformation that occurs during the bar forming process. A simplified analysis by Lee et al demonstrates that actual strain per pass is times the area reduction for oval passes and times the area reduction for round passes in the oval-round pass sequences commonly utilized for bar and rod rolling. [1] Consequently, total strains in bar rolling are on the order of 4 times higher than plates and 20 times higher in rod as compared to light gauge strip. Figure 1: Bar speed increases exponentially as size is reduced through in-line rolling stands. Example shown is for rounds at 150 ton/hour rolling rate. Modern bar and rod mills are typically designed to utilize inline rolling stand arrangements that necessitate a constant mass throughput rate for all rolling stands. Consequently, the linear speed through consecutive rolling stands increases as the size becomes smaller as illustrated in Figure 1. Figure 2: Inter-pass times and strain rates (initial-final passes) for wire rod and thin strip compared to Gleeble 3800 capability above and left of the dashed line. [2]

2 As the rolling speed increases, the inter-pass time is reduced and the strain rate is increased as illustrated by Kuziak in Figure 2. [2] Development of Gleeble and similar systems equipped with hot torsional deformation capability has allowed simulation of the pass strains, inter-pass times, and strain rates for hot rolling bars and thin plates. However, the limitations of the electromechanical torsion equipment prevent replication of the strain rate and interpass times encountered in the high speed rolling of small diameter rods as illustrated in Figure 2. [2] Strains, strain rates, and inter-pass times determine the characteristics of austenite grains (austenite condition ) at the completion of hot rolling. As shown in Figure 3, large equiaxed austenite grains created during heating prior to rolling are deformed to a distorted shape during deformation at each rolling pass. After a single rolling pass, the distorted grains recrystallize and then begin to grow. The amount of strain, strain rate, and temperature affect the rates of recrystallization and grain growth. [3] At high strains, strain rates, and temperatures recrystallization occurs dynamically (DRx) during the rolling pass. At lower strains, strain rates, and temperatures the recrystallization occurs statically (SRx) after the rolling pass. Recrystallization can be completely suppressed at temperatures below a critical value (T nr ) with limited strains and strain rates. [4] T nr can be affected by the strain and strain rates. Short inter-pass times limit the amount of recrystallization and grain growth between stands or increase T nr as multiple passes are applied before recrystallization occurs. [5] T nr is heavily affected by the addition of small amounts of V, Nb, and Ti as these additions slow grain boundary movement by solute drag and by combining with C and N to form precipitates that limit recrystallization and grain growth kinetics. [6-8] of strain and strain rates at various temperatures on recrystallization and grain growth rates. The test data have been assembled into kinetic equations that allow calculation of grain sizes at the entry and exit of each rolling pass. [5, 9, 10] These equations can be used to show that lowering the temperature during the final few rolling passes decreases the austenite grain size and can change the shape of austenite grains for a given steel chemical composition. [5] Additional investigations have been conducted on low carbon plate steels to show that the prior austenite grain size and shape modifies the microstructure that is formed during subsequent cooling. [11] Generally, the increased grain boundary area associated with smaller deformed austenite grains increases the rate of nucleation of small ferrite grains and reduces the time for ferrite nucleation as illustrated in Figure 4. [11] Figure 4: Comparison of CCT diagrams for transformation from recrystallized and un-recrystallized austenite in a 0.06% C- 1.20% Mn % Nb-0.053% V plate steel. [11] Similar effects are anticipated in bar steels with higher carbon and alloy contents compared to the amounts common in plate steels. For example, reducing the austenite grain size has been shown to reduce the size of ferrite grains and increase the amount of ferrite formed in a series of 0.4% C steels containing about 1% Mn and % V. [12] The following sections describe experiments that were conducted to illustrate the effects of thermomechanical rolling of bars on the microstructures, and the influence of these microstructures on subsequent heat treatment. Thermomechanically Rolled Bars Figure 3: Schematic diagram showing deformation, dynamic recrystallization, static recrystallization, and growth of austenite grains during multi-pass hot rolling. Steels with different chemical compositions have been tested using the Gleeble or similar equipment to establish the effects Bars of chemical compositions listed in Table 1 were hot rolled from 152 mm square billets in a rolling mill described schematically in Figure 5. The rolling mill consists of a reheat furnace, eight roughing stands in H-V arrangement, water cooling boxes for cooling before finish rolling, space to allow temperature equalization in the bar cross section, an eight

3 stand 3-roll type reducing mill, and a three stand 3-roll type precision sizing block. [13] The bars were cooled in air as straight bars on a walking beam cooling bed after rolling. Initial heating temperatures were o C for the 1045 Steels and o C for the 16MnCr5 steel. The final rolling temperatures were adjusted to the desired levels by utilizing the water cooling boxes located immediately after the rough rolling stands. 16% to 48% with the lower final rolling temperature. Examination by scanning electron microscopy, Figures 6(c, d) confirmed the change from dispersed carbides to fine lamellar pearlite. Table 1: Chemical Compositions of Test Steels in wt. pct. Steel C Mn Si Ni Cr Mo 16MnCr Al V V45Nb Steel Al V Nb Ti B (ppm) N (ppm) 16MnCr NA* NA NA NA 1045 Al NA 97 10V NA V45Nb NA 124 *NA = none added Figure 6: Microstructures of 16MnCr5 hot rolled bars finish rolled at 1018 o C (a, c) and 886 o C (b, d) as revealed by light optical (a, b) and scanning electron(c, d) microscopy. (2% picral etch) [15] Figure 5: Schematic diagram of rolling mill for evaluating effects of final rolling temperature. [13] Steel 16MnCr5 is a low carbon hardenable steel commonly utilized to produce driveline components by forging and machining followed by carburizing and hardening. The chemical composition includes controlled amounts of Mn and Cr to achieve the hardenability required for hardening after carburizing. Conventional rolling without intermediate cooling resulted in a final rolling temperature of 1018 o C for 38 mm diameter bars. The addition of intermediate water cooling lowered the final rolling temperature to 886 o C. Calculated T nr for Steel 16MnCr5 according to the Boratto equation is estimated to be about 906 o C. [14] Prior austenite grain size is expected to be considerably smaller with the lower final rolling temperature. The microstructures from cross sections at the mid radius of the 38 mm diameter bars after air cooling are shown in Figure 6. [15] The higher, conventional finish rolling temperature resulted in a microstructure consisting of proeutectoid ferrite at prior austenite grain boundaries and bainite as shown in Figure 6. The lower finish rolling temperature resulted in a microstructure consisting of ferrite and pearlite with well dispersed pearlite colonies as shown in Figure 6. The area fraction of free ferrite increased from Figure 7: Effect of austenitizing temperature (solid line 1050 o C and dashed line 870 o C) on the continuous cooling transformation of 16MnCr5 steel. [16] The effects of 16MnCr5 prior austenite grain size, ( as determined by austenitizing temperature) on continuous cooling transformation kinetics are illustrated in Figure 7 which was redrawn to provide a direct comparison of the CCT

4 10V45 Nb 10V Al Proceedings of International Federation of Heat Treating and Surface Engineering, ASM, April 18-21, 2016, Savannah, GA diagrams for austenitizing temperatures of 870 o C and 1050 o C. [16] The cooling curve shown in Figure 7 is for air cooling 38 mm bars after finish rolling. The finer austenite grain size accelerated the formation of grain boundary ferrite, leading to a higher ferrite fraction, and increased the size of the pearlite transformation field so that pearlite was formed rather than bainite. SAE 1045 steels are commonly used for production of shafts that are induction hardened to resist fatigue at high torsional and bending loads. Attainment of the desired final microstructures and properties with the short reheating times for induction surface hardening is highly influenced by the starting microstructure. It has been shown that a finer ferritepearlite microstructure is more readily hardened than coarse ferrite-pearlite microstructures. [17] to simulate the time-temperature-deformation sequence for the industrial rolling cycles. Specimens were quenched after reheating to 1200 o C, and immediately after final deformations at 1000 o C and 800 o C. An example showing the austenite grain shape and partial recrystallization is shown in Figure 9. A tangential sectioning technique was utilized to reveal the prior austenite grain deformation on the shear plane during torsion. [13] The deformed prior austenite grains for the TMP simulations on the three 1045 Steels are shown in Figure 9. Analysis of the microstructures showed the same amount of shear deformation in the unrecrystallized grains for all three steels with 40% recrystallization in Steel 1045 Al, 8% recrystallization in Steel 10V45, and <1% recrystallization in Steel 10V45 Nb. At the higher 1000 o C deformation temperature, all three steels showed full recrystallization. [13] (e) (f) Figure 8: Microstructures of SAE 1045 as- rolled bars after finish rolling at 1000 o C (a, c, e) and 800 o C (b, d, f) as revealed by light optical microscopy (2% nital etch). [13] The effects of microalloy additions (Table 1) and low temperature thermomechanical rolling (TMR) on the microstructures of 1045 steel are illustrated in Figure 8. In all three 1045 steels, the grain size was substantially reduced by lowering the final rolling temperature from 1000 o C (HR) to 800 o C (TMR). The lower rolling temperature also increased the amount of free ferrite. Specimens from the three 1045 steels were processed on a Gleeble 3500 thermomechanical simulator utilizing torsion Figure 9: Microstructures of Gleeble specimens quenched after final deformation at 800 o C showing prior austenite grain sizes, shapes, and amount of recrystallization (saturated picric etch) for Steel 1045 Al, 10V45, and 10V45 Nb. [13]

5 The quenched Gleeble specimens were re-sectioned to show the equiaxed cross-sections of the prior austenite grains for all three test conditions and facilitate measurement of the grain diameters. As shown in Figure 10, the large prior austenite grains present after heating to 1200 o C were refined by the HR simulation and further refined by the TMR simulation. After the rolling simulations, Steel 1045 Al consistently exhibited the largest prior austenite grain size and Steel 10V45 Nb exhibited the smallest prior austenite grains. Compared to the HR simulation, the TMR simulation provided reductions in grain size of about 30% for Steel 1045 Al and 50% for the two microalloyed steels. Figure 11: Microstructures of 16MnCr5 specimens annealed at 692 o C for 6 hours; (a, c) finish rolled at 1018 o C, (b, d) finish rolled at 886 o C, (a, b) light optical micrographs, (c, d) scanning electron micrographs. (2% picral etch) [15] Figure 10: Prior austenite grain sizes measured on Gleeble torsion simulation specimens quenched after reheating (austenitizing), HR deformation (1000 o C) and TMP deformation (800 o C). [13] Heat Treatment of Thermomechanically Rolled Bars The following examples show that the as- rolled microstructure can substantially influence the resulting microstructure after final heat treatment. The 16MnCr5 steels in the two rolling conditions from Figure 6 were sub-critically annealed at 692 o C for 6 hours and the resulting microstructures are shown in Figure 11. After 6 hours of annealing, the carbides for both prior rolling conditions were fully spheroidized as shown Figure 11 (c, d). However, the distribution of the spheroidized carbides within the overall microstructure was substantially different as shown in Figures 11 (a, b). These microstructural differences could be expected to have an influence on microstructures and properties after the final carburizing and hardening of the finished parts. The response of the 1045 steels as-rolled microstructures to supercritical heat treatments varied depending on the Figure 12: Effects of lamellar pearlite (LP) annealing on the microstructures of 1045 steel specimens: Steel 1045 Al HR at 1000 o C, Steel 1045 Al HR and LP annealed, Steel 10V45 Nb TMR at 800 o C, and Steel 10V45 Nb TMR and LP annealed. microalloys, the prior rolling conditions, and the type of heat treatment. Two examples are illustrated in Figure 12 for Steel 1045 Al that was initially HR at 1000 o C and for Steel 10V45Nb that was TMR at 800 o C. After rolling, specimens of each steel were lamellar pearlite (LP) annealed by heating to

6 805 o C (A c o C) for 1 hour followed by furnace cooling at 0.06 o C/s (in the range of o C). Comparison of HR Steel 1045 Al in Figure 12 and shows that the LP anneal reduced the pearlite colony size and introduced a banded structure parallel to the rolling direction (vertical in ). LP annealing of TMR Steel 10V45 Nb imparted a limited effect on the microstructure after the LP anneal as shown in Figures 12 and. The overall result is that the LP annealed microstructure is substantially modified by the initial microstructure and the addition of micro alloys (Figures 12 and. As noted previously, 1045 steel is commonly utilized to produce induction hardened shafts. Specimens obtained from the three 1045 steels (Table1) were heat treated to simulate an induction hardening thermal cycle on a Gleeble The thermal cycle, calculated using Elta 2-D, is shown in Figure 13 and was developed to simulate a 2 mm deep case depth on a 38 mm diameter shaft using a 2-turn coil. [18] The heating portion of the cycle shows an interruption at about 700 o C which reflects the space between the first and second turn during scan hardening. The cooling after heating was first accelerated with forced air to simulate heat transfer to the core and then water quenched at a rate of 3500 o C/s. Gleeble specimens processed to the HR and TMP conditions were subsequently heat treated to the simulated induction hardening schedule in Figure 13. Figure 14: Effects of prior rolling practice (HR vs. TMP) and microalloy content on the prior austenite grain size after induction hardening of 1045 steels to a 2 mm case depth. The prior austenite grain size after simulated induction hardening was measured for all test specimens and the results are summarized in Figure 14. For the combinations of microalloy contents and rolling conditions evaluated, the prior austenite grain size was reduced by the induction hardening simulation. For Steel 10V45 Nb, the induction hardening simulation reduced the prior austenite grain size by about 50% for the HR and TMP conditions, and consistently attained the smallest prior austenite grain size for all conditions. For Steel 1045 Al the induction hardening simulation reduced the HR and TMP prior austenite grain sizes by about 20% and consistently exhibited the largest grain size for each condition. Summary and Conclusions Figure 13: Thermal profile for a Gleenle 3500 simulation of induction hardening a 38 mm diameter shaft to a 2 mm case depth showing modeled (solid line) and experimental (dashed line) time-temperature profiles. Bars and rods are being thermomechanically rolled to modify the prior austenite grain size and shape. Implementation of the thermomechanical rolling for bars and rods is different than processes developed for flat rolled or structural steel forms due to the higher strains associated with bi-dimensional reduction and the higher rolling speeds. Suitable rolling mill designs are necessary to impart the high strains, high strain rates, and short inter-pass times. Careful selection of microalloys consistent with the specific base chemical compositions can enhance the effectiveness of thermomechanical rolling in achieving refined final microstructures. Modification of austenite morphology by thermomechanical rolling changes the final microstructure of the as rolled product by affecting the transformation kinetics during subsequent cooling. The finer austenite grain size influences the CCT diagram and can accelerate the nucleation of ferrite. In steels like 16MnCr5 phase fractions can be substantially modified so

7 that bainite is suppressed in favor of pearlite formation. In steels like 1045 that readily transform to ferrite and pearlite, the finer austenite grain size leads to finer ferrite-pearlite microstructures and an increased fraction of free ferrite. The effects of thermomechanical rolling on microstructures after subsequent heat treatment are not well documented in the literature. For subcritical annealing, the long range (> microns) microstructural features are relatively unaffected by the heat treatment, even though the short range features (<20 microns) may be modified. The microstructure after supercritical heating such as normalizing or LP annealing is heavily influenced by the microstructure in the bars after hot rolling. A similar effect occurs during the short time higher austenitizing temperatures attained during induction heating. Fine prior austenite grain sizes like the 10 micron sizes observed for the 10V45 Nb after TMP and induction hardening have been shown to increase the fatigue strength of case hardened components. [19] An understanding of the prior microstructural effects from TMP on the microstructures after final heat treatment is needed to optimize the heat treatment process parameters and the performance of the finished parts. Acknowledgements The authors gratefully acknowledge the continued support of the sponsors of the Advanced Steel Processing and Products Research Center, an industry/university cooperative research center at the Colorado School of Mines. References [1] Lee, Y. Choi, S., and Hodgson, P.D., Analytical Model of Pass-by-Pass Strain in Rod (or Bar) Rolling and its Applications to Predictions of Austenite Grain Size, Materials Science and Engineering, A336 (2002), pp [2] Kuziak, R., Physical Simulation of Thermomechanical Treatment Employing Gleeble 3800 Simulator, Metal 2006, Hradek Nad Moravici. [3] Bianchi, J. H. and Karjalainen, Modelling of Dynamic and Metadynamic Recrystallization During Bar Rolling of Medium Carbon Bar Steel, Journal of Materials Processing Technology, Vol 160 (2005) pp [4] Kozasu, I., Shimuzu, I. T. and Kubota, H., Trans. Iron and Steel Institute of Japan, Vol. 11 (1971), pp [5] Maccagno, T.M.,Jonas, J.J., and Hodgson, P.D., Spreadsheet Modelling of Grain Size Evolution during Rod Rolling, ISIJ International, Vol. 36 (1996), No. 6, pp [6] Homsher, C.N. and Van Tyne, C.J., Empirical Equations for the No-Recrystallization Temperature in Hot Rolled Steel Plates, Proceedings Materials Science and Technology Conference, Montreal Quebec, Canada, Oct 27-31, 2013, pp [7] Speer, J. G. and Hansen, S. S. Austenite Recrystallization and Carbonitride Precipitation in Niobium Microalloyed Steels, Metall. Trans. A, vol. 20, (1989), pp ,. [8] Vervynckt, S., Verbeken, K., Thibaux, P. and Houbaert Y., Recrystallization-Precipitation Interaction during Austenite Hot Deformation of a Nb Microalloyed Steel, Materials Science and Engineering A, Vol. 528 (2011), pp [9] El Mehtedi, F. et al., Predicition Models of the Final Properties of Steel Rods Obtained by Thermomechanical Rolling Process, La Metallurgia Italiana, n. 3 (2013), pp [10] Kruse, M., Schwarz, M., Schuck, M., Kocks Microstructure Simulator (KMS) New Technical Tool for Process Simulation of Long Products, La Metallurgia Italiana, n. 1 (2014), pp [11] Olasolo, M., Uranga, P., Rodriguez-Ibabe, J.M., and López, B., Effect of Austenite Microstructure and Cooling Rate on Transformation Characteristics in a low carbon Nb-V Microalloyed Steel, Materials Science and Engineering, A 528 (2011), pp [12] Cryderman, R.L. et al. Effects of Chemical Composition, Heat Treatment, and Microstructure in Splittable Forged Steel Connecting Rods, SAE Int. J. Mater. Manf. 8(3):2015, doi: / [13] Whitley, B. M., Easter, C. T., Cryderman, R. L., and Speer, J. G., Thermomechanical Simulation and Microstructural Analysis of Microalloyed Medium Carbon Bar Steels, Proceedings of International Conference on Advances in Metallurgy of Long and Forged Products, AIST, July 12-15, 2015, Vail Colorado, USA. [14] Barbosa, R., Boratto, F., Yue, S., and Jonas, J.J., The Influence of Chemical Composition on the Recrystallization Behavior of Microalloyed Steels, Processing, Microstructure and Properties of HSLA Steels, (1988), pp [15] Schaneman, R.A., The Effects of Prior Microstructure on Spheroidizing Kinetics and Cold Workability in Bar Steels, M. S. Thesis, Colorado School of Mines, (2009). [16] Totten, G.E. and Howes, M.A.H., Steel Heat Treatment Handbook, New York, NY: Marcel Dekker, 1997, pp [17] Matlock, D.K. Metallurgy of Induction Hardening of Steel, ASM Handbook, Vol. 4C, 2014, pp [18] Goldstein, R, Fluxtrol, Auburn Hills, MI 48326, private communication, July 27, [19] Hyde R.S., Matlock, D.K. and Krauss, G "Quench Embrittlement: Intergranular Fracture Due to Cementite and Phosphorous in Quenched Carbon and Alloy Steels", Proceedings of the 40th Mechanical Working and Steel Processing Conference, 1998, pp