christopher michael sellars 4 august 1935- 15 … · danieli-1 (danieli-2) mitsubishi csp rougher...
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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
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