Development_Hstrength_rebar.pptx

Development
of High
Strength TMT
Rebars
LISBOAMETALS
Orientada ao Sucesso do Cliente
25 de junho de 2021

The structural segment
https://www.worldsteel.org/steel-by-topic/steel-markets.html

Classic application of Niobium in
low–C steels for flat products:
• Cost reduction
• Better performance:
Higher strength and higher elong.(%)

Introduction
“The need for special designs
and materials to enable buildings
to withstand earthquakes has
been recognised and understood
for many years, yet we still
occasionally witness the terrifying
impact of unpredictable seismic
events.”
4 Análise Projeto 11 may 2023

Options for producing high strength reinforcing bars
High strength hot rolled rebars are normally produced by two different processes;
• In-line heat treatment of low alloy C-Mn steels - sometimes referred as Quench and Self Tempered (QST)
process
• Microalloying and air cooling – by the addition of V, Nb or Ti
• Recently, a new technology, which uses deformation induced ferrite transformation (DIFT) mechanism to
produce rebars, with a very fine ferrite grain size, has also been developed but is not yet widely used.
• From a performance perspective, the microstructure, consisting of a hard tempered martensite outer layer
and softer core, results in a lower tensile strength to yield strength ratio compared with micro alloyed rebars.
As shown in table (next slide), this is an important characteristic in most seismic rebar standards, and the
relatively worse performance of QST rebar limits its use for seismic design applications.
• In the microalloying process, small amounts of elements such as Vanadium, Niobium and/or Titanium are
added, which promote strength by precipitation strengthening and grain refinement. Despite the additional
cost of microalloying, this rebars possess certain advantages making them an appropriate choice for many
high strength applications including seismic.
5

Mechanical property requirements for seismic resistant
rebars
6
In summary, the essential properties for high performance seismic resistant rebars are listed below, and the values
specified in relevant countries’ standards are given in Table 1.
• High yield strength
• Low variation in yield strength
• Good ductility
• High strain low cycle fatigue properties
• Weldability
• s

Strengthening mechanism of V-N rebar

Microalloying technology
• Microalloying utilizes two advanced technologies which are fine-grain strengthening and precipitation
strengthening to improve strength. Micro alloying elements are added to rebar such as titanium,
niobium and vanadium with a content ranging from 0.02% to 0.15%.
• Three kinds of technologies, namely Ti-microalloying, Nb-microalloying and V-microalloying are
introduced for rebar production and emphasis is given to the research achievements of V-N
microalloyed rebars.
• The microalloying process is mainly used to develop high strength weldable rebars around the world.
During rebar production process, the high rolling speed and high finishing temperature are quite
desirable for application of vanadium microalloying technique. The rebar standard issued in China also
recommends the use of vanadium microalloying to produce grade 3 rebars with yield strength 500MPa.
• Effect of nitrogen Fig.4 shows the effect of nitrogen on strength of rebar containing vanadium. With
almost the same content of vanadium, the strength of V-N microalloyed rebars is much higher than that
of vanadium microallyed steel. It can be seen that with enhancement of 100ppm of nitrogen, the yield
strength and tensile strength of V-N microalloyed rebar increase by 117.5MPa and 135MPa,
respectively, compared to vanadium containing steel. Experimental result shows that nitrogen
markedly improves the strengthening effectiveness of vanadium in steel, which means nitrogen is a
very effective strengthening element for rebars containing vanadium.
20-08-2023
9

V effects
It can be seen that with the same strength, V-
N rebars save V resources substantially
compared with rebars containing vanadium
only , with V consumption decreasing from
0.06% ～ 0.08% to 0.03% ～ 0.04% for
400MPa rebars, and from 0.07% ～ 0.12%
to 0.06%～ 0.08% for 500MPa rebars. By
accelerated cooling process, the consumption
of vanadium can be reduced to 0.02%～
0.03% for 400MPa V-N rebars and 0.04% ～
0.05% for 500MPa V-N rebars. It follows
that FeV consumption and costs will be
saved by adopting microalloying technique
combined with accelerated cooling process.
1.2.4 Technology of fine-grain

Mo addition of high-strength seismic rebars subjected to
Tempcore
processes for tensile strength enhancement
20 de agosto de
2023
Análise Projeto
13
• Addition of 0.11 wt% Mo improved
the seismic performance of
Tempcore rebars.
• Low yield ratio of Tempcore rebars
was achieved by bainite formation.
• Mo-complex carbide formation was
minimal for self-tempering of the
Tempcore rebar.
• The solute drag effect of Mo
contributed more to improving the
tensile strength rebar than
precipitation hardening.

14
JSW Bar Mill Dolvi Plant
Characteristics  Technology – Danieli Morgardshammar
 Commissioned -December 2015
 Mill capacity-1.4 MTPA
 Roughing (4H+4V), Pre-finishing (3H+2V+1H) &
8 NTM/FFB; Total 22 stands.
 Asia’s first high speed bar mill 45 m/sec
 Production Rate: 245 TPH
 Product Range: 8,10,12,16,20,25,28,32,36 and 40 mm
 Length of cooling bed is 102 meters.

Manufacturing Process of High
Strength TMT Bar
• Manufacturing process of TMT bars can be better described as “Hot
Rolling” where heated iron billets continuously pass through the rollers of
decreasing diameter.
• After coming out of the last rolling mill bars are water cooled in a
microcontroller controlled WATER BOX where temperature gradient is
produced in from core to surface.
• After the intensive cooling, the bar is exposed to air and the core re-heats
the quenched surface layer by conduction, therefore tempering the
external martensite.
• When the bars are taken out of the cooling system, the heat flows from
the core to the outer surface, further tempering of the bars, which helps
them attain a higher yield strength.

Ishikawa Diagram of TMT rebar Quality
Chemical
composition

Quenching and Self Tempering (QST)
process
Quenchin
g Self
Tempering Core Formation
A
B
A
C
C B

Philosophy for High Strength TMT Bar
• Uniform heating of Billet at Reheating Furnace at 1130-1150
Deg C to ensure Std. 1 entry temp 1030-1050 Deg C.
• Billet rolled in particular section in mill.
• After rolling temp is about 1060-1080 Deg C before entering
water box.
• Temperature at cooling bed maintained about 550-570 Deg. C.
• Water flow and pressure kept accordingly.

Rolling Parameter for High Strength TMT
Bar
Section
(mm)
Mill
Speed(m/s)
Stand #1 entry
temp.(Deg C)
Water Box
entry
temp.(Deg
C)
Cooling bed
entry temp.
(Deg C)
FLOW
(M3/h)
PRESSURE
(Bar)
RB 8 45
1030-1050 1060 - 1080 550-570
500 9
RB 10 45 850 10
RB 12 44.37 1350 12.5
RB 16 24.96 1200 14
RB 20 15.97 1350 14
RB 25 20.44 1800 10
RB 28 16.3 1800 11
RB 32 12.48 2000 11
RB 36 9.86 1800 11
RB 40 7.99 1700 10

Influence of Process Parameters on Microstructure
Sample (I) Sample (II) Sample (III)
High Yield Strength Normal Yield Strength Low Yield Strength

Tempered Martensite
Centre Tempered Martensite
Centre
Sample (I) Sample (III)
Influence of Process Parameters on Microstructure Continued......

Influence of Microstructure on Mechanical Property

Sample
YS(MPa
) UTS(MPa)
%
Elongation
Martensite Ring
Length (in micron)
Martensite Area
(mm²)
Ferrite +
Pearlite
Area(mm²)
I 1003 1096 16.1 827 30.3 19.9
II 819 1054 9.7 1002 31.4 18.8
III 663 745 17.7 998 31.4 18.9
IV 600 710 22.7 723 29.7 20.6
V 573 680 20.9 657 29.2 21.0
VI 538 649 22.5 660 29.3 21.0
VII 494 594 25 563 28.7 21.6
VII 456 573 22.5 431 27.8 22.4
IX 470 581 23.8 383 27.5 22.7
Influence of Microstructure on Mechanical Property Continued......

Experimental results & Conclusion
Present study is helpful to optimize the process parameters to
achieve mechanical property with desired microstructure.
Rolling speed and the water flow rate totally changes the
microstructure as a result mechanical property.
Future aspect is to recommend industries for developing rebar of higher
properties without modifying the chemical composition by optimizing the
mill process parameters.

Gaps
 N2 analisys at Melt Shop;
 N2 ferroalloys;
 Micrografic analisys capacity in laboratory;
 Determination statistic of process capacity;
 Laboratory benchmarking;

Niobium technology in rebar steels -
The strengthening mechanisms &
Latest results
August 2018

Confidential Content -
Objective of this presentation
The purpose of the presentation is show the contributions of Nb application to rebars.
This will be done by directing discussion to the following questions:
• What is the reheating temperature necessary? Nb in rebar steels
• Is it necessary to modify any process parameters to apply Nb in rebar steels? Nb
contribution during the hot rolling
• Is it cost effective? Cost analyses of compositions using Nb
The latest results concerning laboratory investigations and industrial cases will be
commented.

Classical application of Nb in low-C steels for flat products
Roughing
(reversible
mill)
Reheating Finishing
(continuous mill)
Austenitization Austenite conditioning Phase
transformation
Mechanical
properties
Final product
Plate, Strip….
Cooling bed
Recrystallization
Finer and more
homogeneous austenite
“Tnr”
Strain accumulation
Elongated austenite
with inside defects

Classical application of Nb in low-C steels for flat products
Reheating
Nb (C,N)
dissolution
Plate, Strip….
Roughing
(reversible
mill)
Finishing
(continuous mill)
Final product
Cooling bed
Static recrystallization
Nbso
l
Solute drag effect
Nb in solid
solution
Static Recrystallization-
Precipitation interaction
Nb(C,
N)
Nb strain-
induced
precipitation
Phase transformation
Grain
refinement
Nb remaining in
solution
Hardenability
(precipitation
strengthening)
Grain boundary

What changes in the hot rolling of rebars
Plate, Strip….
Reheating
C < 0,15 %
Rolling speed < 10 m/s
Bar,
Rebar…
C: 0,20% ~ 0,40%
Roughing
(reversible
mill)
Finishing
(continuous mill)
Final product
Cooling bed
Rolling speed > 50 m/s
 High strain levels
 High strain rates
 Short interpass times

Nb contribution on rebars
Bar,
Rebar…
Metallurgical
mechanisms:
• DRX
• MDRX
• Nb in solid solution
• Nb(C,N) strain
induced precipitation
? Phase transformation:
• Phase balance
• Precipitation strengthening
?
Solubility of NB ?

Discharge
temperature of the
billet
1070-
1100°
C
Temperature of
the furnace
1150-
1180°
C
1020-
1050°
C
Temperature of he
first pass
0,20% C
100 ppm N
0,35% C
100 ppm N
Usual reheating temperatures for hot rolling of rebars
Solutility curves for NbC and NbCN

What is the optimum reheating temperature?
• Above 1150°C the austenite grains will being to coarsen.
• This happens as the as-cast Nb-precipitates go into
solution at higher temperatures and stop pinning the
austenite grain boundaries.
• In the “hatched-area” a mixed grain size is seen, as
some grains coarse first.
For illustration purposes only

Is it best to dissolve all added niobium? If part of the niobium is not dissolved, what is
the role?
What is the optimum reheating temperature?
• Reheating tests show 1150℃,
austenite grain sizes start to
coarsen above 1150°C, and
• When reheating temperature reach
at 1200℃, no pinning effect for
austenite, which mean all added
Nb are in solution.

Nb in rebar steels
What is the reheating temperature necessary?
Bar,
Rebar…
Reheating temperature range practiced in rebars (1150-1180°C furnace, that means
1070-1100°C dropout) is sufficient to ensure the application of up to 0,03% Nb in steels
in the carbon range of 0,20 – 0,30%

Nb contribution
during the hot
rolling

Austenite microstructures after bar rolling simulation by multipass hot torsion tests…
 20mm
Dmean= 12.1 ± 0.3
µm
0.24C-0.03Nb steel 0.25C-Mn steel
Dmean= 22.3 ± 0.6
µm
0.24C-0.03Nb steel 0.25C-Mn steel
 8mm
Dmean= 14.4 ± 0.5
µm
Dmean= 21.2 ± 1 µm
0
50
100
150
200
250
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Stress
(MPa)
Strain
Sinobras, 8 mm, 1220ºC
Tonghua, 8 mm, 1220ºC
0.25C-Mn
0.24C-0.03Nb
Finish: 995ºC to 1030ºC in 3.5 s
0
50
100
150
200
250
0 1 2 3 4 5 6 7 8
Stress
(MPa)
Strain
Sinobras, 20 mm, 1220ºC
Tonghua, 20 mm, 1220ºC
0.25C-Mn
0.24C-0.03Nb
955ºC to 1010ºC in 5.5 s

Nb contribution during the hot rolling
(no TMCP)
Is it necessary to modify any process parameters to apply Nb in rebar steels?
Bar,
Rebar…
Considering world wide specifications for rebar steel grades and conventional diameter
ranges, that is, 8 mm to 32 mm, no modifications are necessary in the process
Austenite refinement

Bar,
Rebar…
Parameters
influencing the
final
microstructure

Finer Austenite grains contribute to finer Ferrite grains…
(a) 0%Nb (b) 0.02%Nb
d = 4.0 ± 0.4 m
d = 5.8 ± 0.9 m
Cooling at 2ºC/s from 900ºC to 560ºC (0.3%C)
Source: Laboratory results to be published.
By Hall-Petch relation + 60 MPa YS

0.25%C, 0.03%Nb
Cu
Mn
Fe
Fe Nb
Cu
Nb
Ni
Cu
Ni
Mn
Fe
Nb
Mn
Nb
Ni
C
0 2 4 6 8 10 12 14 16 18 20
keV
Full Scale 439 cts Cursor: 19.799 (0 cts)
Spectrum 3
Cu Nb
Nb Fe
Ni
Cu Nb
Cu
Fe
Ni Fe
O
Nb
Ni
C
0 2 4 6 8 10 12 14 16 18 20
keV
Full Scale 194 cts Cursor: 19.558 (0 cts)
14 aglo
TEM analysis shows fine Nb(C,N) particles
formed during cooling
Source: Laboratory results to be
published.
During the final cooling (phase transformation), Niobium also contributes to
precipitation strengthening…

Increase in Yield and Tensile Strength by precipitation
Source: W. Murphy & R. Yeo, Metal. Prog., Sep. 1969
During the final cooling (phase transformation), Niobium also contributes to
precipitation strengthening…

 Control of austenite grain size during the reheating
 Grain refinement of austenite and ferrite
 For steels used in conventional world wide rebar specifications, Niobium contributes
substantially to strengthening mechanisms:
 Fine Nb precipitates were noted after final cooling which contribute to
precipitation strengthening
 These effects have been observed without any change of the process parameters,
that is, without TMCP.
Conclusions so far…
 A reheating temperature in the range of 1150-1180°C is sufficient to ensure the
application up to 0,030% Nb in steels in the carbon range of 0,20 - 0,30%

46
Last results from
Industrial cases in
China

 GB/T 1499.2-2018: Steel for the reinforcement of concrete – Part 2: Hot rolled ribbed
bars, in which some changes creates opportunity for Nb-bearing rebars.
 GB1499.2 requires both mechanical properties and microstructure of ferrite plus pearlite.
New version of GB1499.2
Aug.24th , 2016, testing requirement on
microstructure was proposed and added.
Mar. 26th, 2018, new version of
GB1499.2 was approved.
Nov. 1st, 2018, new version of
GB1499.2 will take effect.
Main changes:
• remove HRB335
• add HRB600
• increase “E” grade
• increase requirement for microstructure testing
• increase requirement for macrostructure, section hardness,
microstructure testing method. F+P F + P + tempered structure
Among all changes, requirements on microstructure and hardness testing are most for FeNb
consumption.

New version of GB1499.2
• Requirements of mechanical properties( Here we only consider earthquake resistant grades)
Grade YS, MPa TS, MPa A, % Agt,% TS/YS YSa/YSm
HRB400 ≥400 ≥540 ≥16 ≥7.5 ---- ----
HRB400E ≥400 ≥540 ≥16 ≥9.0 ≥1.25 ≤1.30
HRB500 ≥500 ≥630 ≥15 ≥7.5 ---- ----
HRB500E ≥500 ≥630 ≥15 ≥9.0 ≥1.25 ≤1.30
HRB600 ≥600 ≥730 ≥14.0 ≥7.5 ---- ----
• Chemical compositions (Matrix: 20MnSi)
Grade C Si Mn P S Ceq Nb, V, and Ti
HRB400/E
0.25
0.80 1.60 0.045 0.045
0.54
Rebar makers are free to choose Nb, V and Ti.
HRB500/E 0.55
HRB600 0.28 0.58

Industrial results: Nb in HRB400
 Seven new Chinese mills applied Niobium in industrial heats of HRB400 for cost
optimization since the end of 2017
Industrial results of Niobium application in seven mills, HRB400

Cost analysis: Nb in HRB400
Assuming 92% recovery of Fe-Alloy addition
Price base: FeNb at US$ 33.00/kg
NitroVan(80) at US$ 60.00/kg
FeMn at US$ 1.40/kg
SAVING
US$ 8.80 / tonne
Existing
HRB400
Vanadiu
m
Base
steel
+ 0.030% V
+US$ 47.02 /
tonne
+ 1.3% Mn
+ US$ 19.57 / t
+ US$ 27.45 / t
HRB400
Niobium
Mill E
Base
steel
+US$ 38.21 /
tonne
+ 0.030% Nb
+ 1.3% Mn
+ US$ 10.76 / t
+ US$ 27.45 / t
 No Cracks at continuous casting
 Using the same reheating temperature
NO EXTRA COSTS

1. Nb will also boost the
effect of V when used
in combination.
3.1. Rebars
Yield strength of microalloyed ferrite-
pearlite rebar steel
Microalloy Content in %
Yield
Strength
in
N/mm
2
Base composition in mass %
C 0.18, Mn 1.2, Si 0.25
Nb
V
0.05 % Nb + V

16mm: ~ 0.20%C, 0.025%Nb, 0.035%V

Traditionally, vanadium grades are used to achieve HRB500 or higher strengths
Existing
HRB500
Vanadium
Base
steel
+ 0.060% V
+ 1.4% Mn

Cost analysis: Nb in HRB500
Assuming 92% recovery of Fe-Alloy addition
Price base: FeNb at US$ 33.00/kg
NitroVan(80) at US$ 60.00/kg
FeMn at US$ 1.40/kg
SAVING
US$ 7.34 / tonne
Existing
HRB500
Vanadiu
m
Base
steel
+ 0.060% V
+ 1.4% Mn
+US$ 68.69 /
tonne
+ US$ 39.13 / t
+ US$ 29.56 / t
HRB500
Niobium
Base
steel
+US$ 61.36 /
tonne
+ 0.035% V
+ 1.4% Mn
+ US$ 22.83 / t
+ US$ 29.56 / t
+ 0.025% Nb + US$ 8.97 / t

Fixed rolling
schedule and roll
pass design
Process
characteristics
Steel alloy
constraints
Increasing Mn =
higher CEq and
increased alloy cost
Increasing C =
higher CEq and
reduced elongation
Specifications and
demands
Target properties:
YS, TS, Agt, TS/YS,
CEq
LOW COST
REQUIREMENT
Summarizing…
Nb addition
advantages
Refines grain size even without TMCP;
Higher YS and TS;
Higher TS/YS
MORE COST EFFECTIVE with better
effects with smaller amounts than
Vanadium
NO REDUCTION IN PRODUCTION
OUTPUT (tonne/hour)
NO REDUCTION IN
PRODUCTION
OUTPUT
(tonne/hour)

Specifications in Brazil
NBR 7480
Grade YS, MPa TS, MPa A, % TS/YS
CA50 ≥ 500 ≥ 660 ≥ 8 ≥ 1.08
CA60 ≥ 600 ≥ 660 ≥ 5 ≥ 1.05
Requirements of mechanical properties
Chemical composition
No specifications/restrictions on chemical composition

10 µm 1 µm 100 nm 10 nm 1 nm
In liquid
Post-
solidification
(continuous
casting)
Reheating Roughing
Strain
induced
precipitation
(during hot
rolling)
During
transformation
(at the exit of hot
rolling)
Detrimental
for
toughness
Austenite grain size control Delay static rex
Austenite
pancaking
Precipitation +
hardenability
TiN TiN, NbN Nb in solution (partial or total) Nb(C,N)
NbC, TiC, V(C,N),
Nbsolution
New approaches with Nb that lead to new
markets
Long products (bars and
rebars): no required all Nb in
solution
Low Mn-low Nb
concept

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