Microstructural Evolution in Microalloyed Steels with High-Speed ... · Microstructural Evolution...

Proceedings of International Federation of Heat Treating and Surface Engineering, ASM, April 18-21, 2016, Savannah, GA

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 1.7 – 2.0 times the area reduction for

oval passes and 2.0 – 2.5 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 in-

line 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]


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 electro-

mechanical 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 (Tnr) with

limited strains and strain rates. [4] Tnr 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 Tnr as multiple passes are applied before

recrystallization occurs. [5] Tnr 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]

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

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-0.0062% 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 0.1-0.2% 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

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


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 1150-1200 oC for the 1045

Steels and 1080-1125 oC 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.

Table 1: Chemical Compositions of Test Steels in wt. pct.

Steel C Mn Si Ni Cr Mo

16MnCr5 0.18 1.13 0.21 0.10 1.05 0.04

1045 Al 0.45 0.72 0.24 0.08 0.12 0.04

10V45 0.45 0.82 0.28 0.07 0.15 0.03

10V45Nb 0.46 0.85 0.27 0.08 0.14 0.03

Steel Al V Nb Ti B

(ppm)

N

(ppm)

16MnCr5 0.029 NA* NA NA NA

1045 Al 0.021 0.003 0.001 0.001 NA 97

10V45 0.000 0.084 0.001 0.001 NA 127

10V45Nb 0.000 0.092 0.020 0.001 NA 124

*NA = none added

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 oC for 38 mm

diameter bars. The addition of intermediate water cooling

lowered the final rolling temperature to 886 oC. Calculated Tnr

for Steel 16MnCr5 according to the Boratto equation is

estimated to be about 906 oC. [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(a). The lower finish rolling

temperature resulted in a microstructure consisting of ferrite

and pearlite with well dispersed pearlite colonies as shown in

Figure 6(b). The area fraction of free ferrite increased from

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.

(a) (b)

(c) (d)

Figure 6: Microstructures of 16MnCr5 hot rolled bars finish

rolled at 1018 oC (a, c) and 886

oC (b, d) as revealed by light

optical (a, b) and scanning electron(c, d) microscopy. (2%

picral etch) [15]

Figure 7: Effect of austenitizing temperature (solid line

1050 oC and dashed line 870

oC) 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


diagrams for austenitizing temperatures of 870 oC and

1050 oC. [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 ferrite-

pearlite microstructure is more readily hardened than coarse

ferrite-pearlite microstructures. [17]

10

45

Al

(a) (b)

10

V4

5

(c) (d)

10

V4

5 N

b

(e) (f)

Figure 8: Microstructures of SAE 1045 as- rolled bars after

finish rolling at 1000 oC (a, c, e) and 800

oC (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 oC (HR) to

800 oC (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

to simulate the time-temperature-deformation sequence for the

industrial rolling cycles. Specimens were quenched after

reheating to 1200 oC, and immediately after final deformations

at 1000 oC and 800

oC. 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 oC deformation

temperature, all three steels showed full recrystallization. [13]

(a)

(b)

(c)

Figure 9: Microstructures of Gleeble®

specimens quenched

after final deformation at 800 oC showing prior austenite

grain sizes, shapes, and amount of recrystallization (saturated

picric etch) for (a) Steel 1045 Al, (b) 10V45, and (c) 10V45

Nb. [13]


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 oC 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 10: Prior austenite grain sizes measured on Gleeble ®

torsion simulation specimens quenched after reheating

(austenitizing), HR deformation (1000 oC) and TMP

deformation (800 oC). [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 oC 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

(a) (b)

(c) (d)

Figure 11: Microstructures of 16MnCr5 specimens annealed

at 692 oC for 6 hours; (a, c) finish rolled at 1018

oC, (b, d)

finish rolled at 886 oC, (a, b) light optical micrographs, (c, d)

scanning electron micrographs. (2% picral etch) [15]

(a) (b)

(c) (d)

Figure 12: Effects of lamellar pearlite (LP) annealing on the

microstructures of 1045 steel specimens: (a) Steel 1045 Al HR

at 1000 oC, (b) Steel 1045 Al HR and LP annealed, (c) Steel

10V45 Nb TMR at 800 oC, and (d) 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 oC and for Steel

10V45Nb that was TMR at 800 oC. After rolling, specimens of

each steel were lamellar pearlite (LP) annealed by heating to


805 oC (Ac3 + 50

oC) for 1 hour followed by furnace cooling at

0.06 oC/s (in the range of 800 – 300

oC). Comparison of HR

Steel 1045 Al in Figure 12 (a) and (b) shows that the LP

anneal reduced the pearlite colony size and introduced a

banded structure parallel to the rolling direction (vertical in

(b)). LP annealing of TMR Steel 10V45 Nb imparted a limited

effect on the microstructure after the LP anneal as shown in

Figures 12 (c) and (d). The overall result is that the LP

annealed microstructure is substantially modified by the initial

microstructure and the addition of micro alloys (Figures 12 (b)

and (d).

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® 3500. 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 oC

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 oC/s. Gleeble®

specimens processed to the HR and TMP conditions were

subsequently heat treated to the simulated induction hardening

schedule in Figure 13.

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.

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

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


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 (>100-200

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.

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