MODELLING OF THERMOMECHANICAL ROLLING, Niobium 2001 E.J. Palmiere, C.M. Sellars and S.V. Subramanian

Advances in thermo-mechanical rolling of Nb microalloyed steel in compact strip rolling, collaborative work with Mike Sellars since Niobium 2001

4 AUGUST 1935- 15 NOVEMBER 2012

CHRISTOPHER MICHAEL SELLARS

Charles Hatchett Seminar 2013

Dedication to Mike Sellars

1. What is the market driver for near net shape processing?

2. What are the mill options available currently to process thin slab and how they differ from

conventional hot strip mills?

3. What are the metallurgical perspective/ modeling capabilities we have to-date to produce

higher grade line pipe in thicker gages, using knowledge-base on TMCP of niobium

microalloying gained in plate rolling?

4. What are the engineering challenges to meet the metallurgical requirements?

5. What are the target parameters in terms of morphological and crystallographic structure

and how we have achieved X-70 using the currently available three mill designs?

6. What is a viable strategy to meet the market demand to produce X-80 and higher grades in

thicker gages?

Design of next generation mill should integrate mill design with metallurgical

requirements of microalloying grades

7. What next after 10 years of TMCP?

8. Acknowledgements

Outline

Charles Hatchett Seminar 2013

What are the existing MILL OPTIONS available for strip rolling and the niche each offers from metallurgical perspective?

Conventional HSM

Danieli-1 (Danieli-2) Mitsubishi

CSP

Rougher 5-7, Reversing 1, Non Reversing

2, Non Reversing

0

RM Temp, C 1180-1040 1120 1120-980 N.A.

RM Exit GS, µm 7-12 150 15-20 1000

FME Temp, C 1000 1050 950-900 1040

FME GS, µm 40-70 150-230 25-40 <1000

FME Thk, mm 30-50 45-50 30 70

Possible Redn below TNR for 12.5 mm

66% 40% 60% N.A.

What is the MARKET DRIVER? Thin slab direct rolling ( TSDR) offers Energy saving of 1080 kJ/kg of steel and CO2 saving of 0.3 kg/kg of steel. Source: Environmental protection agency (EPA), US Govt, Sep 2012

Charles Hatchett Seminar 2013

Holding furnace Rolling Stands

caster

Conventional Vs CSP

Conventional Hot Strip Mill

CSP type Strip Mill

In CSP mill ,coarse as-cast grain (1000µm) enters Finishing Mill as there is no roughing stand. This has to be refined and then pancaked. Rolling in partial recrystallization regime is difficult to avoid

1180C-1050C

FME=1040C GS=600-1000µm

Charles Hatchett Seminar 2013

Target Parameters at finish mill entry Temp<920C GS:25-40 µm

Danieli -First and second Generation design

Refine the austenite grain size to 180µm at the end of R1. Continue to refine in F1 and dummy F2, difficult to avoid rolling in partial recrystallization regime

Refine the austenite grain size to 30µm at the end of R2. FM entry temperature can be independently controlled to enter below TNR to avoid rolling in partial recrystallization regime

D-I t

D-II

Target Parameters at finish mill entry Temp<920C GS:25-40 µm

Thermo-mechanical controlled processing - to control microstructure evolution

Grain size effect

Solid solution strengthening

Precipitation strengthening

Thermo-mechanical rolling of plate of higher grade niobium micro-alloyed line pipe steel

Module-1

Module-2

Module-3

-------------

--------------

Grain refining

Grain coarsening

Strain

accumulation

Transformation

Hardening

Metallurgical perspective based on plate rolling of X-80 and above

Effect of grain size on the ductile-brittle transition temperature in bainite-martensite structures. (a) Effect of austenite grain size on a 700 MPa proof stress bainitic steel. (T. Gladman and F.B. Pickering )

Without OHTP With OHTP

Novel insight from plate rolling studies

Concept-1: Grain refinement austenite before pancaking

is the key to suppress competition from brittle fracture as

measured by %age shear in DWTT- Basis for OHTP process

(Charles Hatchett award paper- 2011)

Charles Hatchett Seminar 2013

Concept-1 continued: Control of crystallographic

HAGB is the key to control of brittle fracture.

You Yang, C. Shang and S.V. Subramanian, 2011 Miao Chengliang, C. Shang and S.V. Subramanian, 2010

Charles Hatchett Seminar 2013

Austenite grain refinement compensates for reduced total deformation inherent in thin slab technology to obtain the same Sv factor (surface to volume ratio of austenite grains) as in conventional processing of plates to give the same number density of nucleation sites for ferrite grain refinement. This offers a novel strategy to produce thicker gage in thin slab processing

Concept-2: Refinement of austenite grain upstream to

can be used to compensate for less pass reduction

available for strain accumulation in finish rolling in

thin slab processing using the well established concept

of Sv factor in Kozazu’s diagram for plate rolling.

Charles Hatchett Seminar 2013

Increases Sv factor without high deformation below TNR.

Advantage in rolling thicker sections and in thin slab rolling.

Novel use of Kozazu’s Diagram on Sv factor for compact

strip rolling Concept 2 contd.

1375 µm

As-Cast (before R1)

180 µm

After R1

Challenge-1 : How to control austenite grain size before pancaking

Dr.-Ing. Thomas Heller,

Challenge-2: Metallurgical control of microstructure is coupled to heat transfer control that is controlled by mill design

Solidification

Microstructure

Length Scales > 1000

to 1500 µm

Grain refinement

of austenite by

static

recrystallization -

after two stand

roughing –

Aim: 15 to 25 µm

Finish rolling carried

out Tnr) -pan caked

austenite grain

thickness 6 to 12µm

depending on entry

austenite grain size and

per centage total

reduction in finishing

mill allowable by gage

thickness)

Controlled accelerated

cooling and coiling temp.

to form acicular ferrite

transformed at low

temperature to meet target

acicular ferrite- bainitic

ferrite microstructure with

high density and

dispersion of high angle

boundaries to yield X-80

properties.

Laminar cooling

and solute Nb are

used to prevent

grain coarsening

of austenite and

enter finish rolling

below Tnr ( 920C)

to avoid rolling in

partial recrystalli-

zation regime –

typically 960-920C)

Refinement of austenite grain size by rolling reduction of 38% in R1 at 1070 oC & 34% in R2 at 1048 oC followed by coarsening till F1.

Prev. practice without IMC Our Recommendation with IMC

Mitsubishi- Hitachi Danieli-2nd Generation (2 Roughing Stands )

Coarsening Kinetics

Mitsubishi-Hitachi-Danieli-2nd Generation(for arctic grade), 2012

R2 F1

F2 F3 F4

Mitsubishi DSPC CSP

Charpy Impact, J 310 (12.2mm, 0C) 444 (12.86mm,-15C) 215 (3/4th,

11.1mm,0C)

YS 530 512 524, 45o

UTS 606 613 593, 45o

%El 32.7 41 34

R1

Charles Hatchett Seminar 2013

F. Borrato, J.J Jonas, S.V.Subramanian and C.M.Sellars

TMCP- What Next? Control of TNR

Growth of NbC on well dispersed TiN can obviate the need for strain induced precipitation and serve to raise Tnr which will allow rolling hot, thereby reducing the mill load. The key is good density and dispersion of TiN precipitates. Charles Hatchett Seminar 2013

Future direction • X70 in 12mm has been demonstrated in all three available near

net shape mill designs, CSP, Danieli and Mitsubishi-Hitachi.

• The feasibility of obtaining ultra fine grains of 1-2 um has been demonstrated which gives an optimum combination of strength and formability suitable for automotive grades.

• The modeling capability has allowed us to predict target parameters for producing higher grades of line pipe steel but the mill has to be designed around the metallurgical requirement of Time-Temp-Deformation schedule of pass reduction.

• Good dispersion of TiN can be used to advantage to promote epitaxial growth of Nb(C,N) so as to avoid strain induced precipitation of NbC, thereby raising the temperature of no recrystallization for the same chemistry. This will allow finish rolling at elevated temperature, which will reduce the rolling load in processing higher niobium grades. Charles Hatchett Seminar 2013

Acknowledgements 1. Late Dr. Mike Sellars 2. Dr Lutz-Meyer 3. Dr R. Kasper 4. Dr G.R.Purdy 5. Dr. John Jonas 6. Dr Francisco Borato 7. Dr.G.Zhu 8. Dr H. Zurob 9. Dr P. Hodgson 10. Dr C. Shang 11. Dr Kevin Banks 12. Dr.T. Zhou 13. Dr M. Chengliang 14. Kashif Rehman