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

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

of 7

  • date post

    25-Mar-2020
  • Category

    Documents

  • view

    3
  • download

    0

Embed Size (px)

Transcript of Microstructural Evolution in Microalloyed Steels with High-Speed ... Microstructural Evolution in...

  • 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]

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

    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

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

    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 o C for the 1045

    Steels and 1080-1125 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.

    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)