FIELD OF THE INVENTION
The present invention relates to a process for controlling mechanical properties
of rebars produced in rolling mills by controlling unique Finish Rolling
Temperature for individual Heat based on Carbon Equivalent in Heat.
BACKGROUND OF THE INVENTION
In a process of rolling billets to rebar, the final Mechanical properties of rebar
depend on Input material chemistry and the adapted rolling process parameters.
Input material billet cast in steel plant, has C. Mn, etc, as the alloying elements
in steel. C plus one-sixth of Mn is termed as Carbon Equivalent (CE). In the
rolling Mill, during the process of rolling billets into rebars through Thermo
Mechanically Treated (TMT) universally known as Quenching and Self Tempering
(QST) process, the Finish Rolling Temperature (FRT) plays a major role to
develop the Mechanical Properties (Yield Strength, Ultimate Tensile Strength,
etc.) of the produced steel. When CE in a batch of billet (Heat) is fixed, the
Mechanical Properties can be varied by changing the finish rolling temperature
(FRT) only. Similarly, when the FRT is fixed, the Mechanical Properties can be
varied by changing the carbon equivalent (CE) only.
It is known that the Merchant Mill used to roll rebars in different grades (Fe500D,
Fe500SD, and Fe500CRSD) and in different sizes (20mm, 25mm, 28mm, 32mm,
36mm, and 40mm diameter). For different grades and sizes, input material billet
chemistry varies as per chemistry design. According to prior art, the rolling of
rebars is done as per Process Charts guidelines, where a range of CE (e.g. 0.34
to 0.38) and range of FRT (560°C to 600°C) are stipulated for a particular grade

(e.g. Fe500D) and section (e.g. 25mm). So one can roll Fe500D, 25mm diameter
rebar at FRT, 560°C or 600°C having CE ~ 0.34. Another can roll Fe500D, 25mm
rebar at FRT, 560°C or 600°C having CE ~ 0.38. By doing this there is no
violation of process charts. But, there is a risk of failure in Low Yield Strength (YS
for Fe500D is 500Mpa) if Fe500D, 25mm diameter rebar is rolled at 600°C. And
in the second case, there is a risk of failure in minimum specific Ultimate Tensile
Strength/Yield Strength (UTS/YS) ratio (ratio for Fe500D is 1.10) if Fe500D,
25mm diameter rebar is rolled at 560°C. Apart from the limitations explained
above, the Steel Making units fail to cast Heats as per the given range of CE
mentioned in the process charts. Sometimes, the CE goes beyond either lower
specification limit (LSL) (e.g. CE <0.34) or Upper specification limit (USL) (e.g.
CE>0.38). There is no guideline in the process charts as to the FRT, with which
the Heats are to be rolled. Either these Heats are to be rejected or rolled with
risks of failure in YS or in ratio.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention is to propose a process for controlling
mechanical properties of rebars produced in rolling mills by controlling unique
Finish Rolling Temperature for individual Heat based on Carbon Equivalent in
Heat.
Another object of the invention is to propose a process for controlling mechanical
properties of rebars produced in rolling mills with heats beyond standard process,
which allows rolling of heats beyond both upper specification limit (USL) and
lower specification limit (LSL) as per standard process charts.

This invention is to propose a process for controlling mechanical properties of
rebars produced in rolling mills, which enables to roll Heat with given carbon
equivalent (CE) at a given finish rolling temperature (FRT) such as Fe500D,
25mm rebar, for Heat CE 0.36 FRT is 590 C.
A further object of the invention is to propose a process for controlling
mechanical properties of rebars produced in rolling mills with heats beyond
standard process, which allows rolling Heats with variable CE to a given FRT to
obtain Mechanical Properties with less variation.
SUMMARY OF THE INVENTION
According to the prior art, the process control for rolling rebar has been based on
the process charts which includes a Finish Rolling Temperature (FRT) range for a
Carbon(C) and Manganese (Mn) range in the Heats. For such a condition, for
example for C range 0.21% to 0.24%, Mn 0.80% to 0.90%, the FRT range is
560°C to 600°C for rolling 25mm Fe500D rebar. In this case, any rolling for any
combination of C and Mn within the given range is correct if the FRT is between
560°C and 600°C. Lowest CE (C+ Mn /6) 0.34 can be rolled at highest FRT, i.e.
600°C which will achieve the lowest Yield Strength (YS). Similarly, the highest
CE0.38 can be rolled at lowest FRT, i.e. 560°C which will achieve high YS in the
finished product. Demerits of the above cases are that the mechanical properties
achieved in finished product is a chance phenomenon and cases of failure in
achieving desired property in the finished product cannot be explained. Thus, the
prior art of FRT measurement system cannot measure FRT range. In contrast
thereto, the present Invention identities CE vs FRT (Maximum). Maximum FRT is

easily measurable from HMI display. For each CE (0.32 to 042 step 0.01), there
is specified one FRT (Maximum). Process control is customized and also takes
care of deviated Heats as per process charts.
DETAIL DESCRIPTION OF THE INVENTION
According to the invention, a relationship Matrix between Finish Rolling
Temperature and Carbon Equivalent for each of different billet sections rolled
into rebar in a rolling Mill, has been developed and presented in the table 1
below.


Thus, the data in Table shows a relationship matrix between Carbon Equivalent
(CE) in billet and Finish Rolling Temperature (FRT) for rolling each section. Each
column in X axis presents the billet sections and for each section, the FRT and
CE are presented side by side in Y axis. The FRT shown in Table 1, is the
maximum FRT.
True Sampling
A relation exists among Yield Strength (YS), Finish Rolling Temperature (FRT)
and Carbon Equivalent (CE) of Heats for rolling the billets to rebar. YS are the
dependable variable, wherein the FRT and CE are independent variables. In the
known rolling process, it has been noted that both FRT and CE vary
independently. To find a correlation between the YS and FRT (considering CE is
constant), the true sampling is a very difficult task. For example, a 9m billet
(cross section 130mm X 130mm square) is rolled down to a rebar of length
450mm with 20mm diameter, the FRT varies along the length of the rebar
between 20 degree Centigrade to 30 degree Centigrade. Due to this FRT
variation, the YS may vary 15Mpa to 20MPa along the length. Thus, any sample
taken for correlation between the YS and FRT has built-in error of 20Mpa. To
overcome this disadvantage, according to this invention, data was collected from
already rolled rebar for YS and FRT for experiment purpose instead of selectively
taking samples. This has additionally eliminated intermittent stoppage of the
rolling mill leading to production loss.
Data collection
The CE vs FRT (Maximum) chart shown in Table-1, has been developed for six
sections (20, 25, 28, 32, 36 and 40mm) and for each section, there are eleven

different CE (0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42). To
obtain an optimum correlation for each CE, at least twenty data for YS and FRT
is required. YS were obtained from Physical property testing Laboratory; FRT was
obtained from Shift inspection Report of Technical Services, wherein the CE was
obtained from Steel Making unit chemical analysis report. The present inventors
accordingly recognized that the YS depend both on FRT and CE significantly.
During the rolling process, once the Heat details are received, the CE can be
derived from Steel Making unit chemical analysis report online. Once the CE is
known, the corresponding FRT is set for rolling. Once the rolling is over, the YS
can be found through Tensile Testing. Therefore, all the data were grouped CE
wise and FRT and YS were aligned in the group. This allows determining a
correlation between CE and FRT in a simple way. This has further eliminated the
complex multiple regression analysis resorted in the prior art.
As the FRT depends on flow and pressure of water in the header for water
treatment/ heat treatment in post rolling process, the FRT can be changed in
permutation and combination of operation of water valve which is opening the
water header; mill speed and water pump rpm. This can easily be done
through HMI control.

WE CLAIM
1. A process for controlling mechanical properties of rebars produced in
rolling mills with heats beyond standard process chart, comprising the
steps of:-
- generating a relationship matrix between carbon equivalent (CE) in
the billets being rolled into rebars, and finish rolling temperature
(FRT) for rolling each sections of rebars, the data being collected
through true sampling, wherein the dimension of each section of
the rebars recorded in X-axis, wherein the corresponding values of
FRT and CE for each section placed in Y-axis, wherein the yield
strength (YS) of the produced rebars varies between 15Mpa to
20Mpa along the length when said relationship matrix is generated
through true sampling;
- collecting data in respect of YS and FRT for experimental purpose
from already rolled rebars;
- obtaining a correlation for each CE by determining a plurality value
of YS from test laboratory, a plurality value of FRT from Technical
Services reports, wherein the CE was collected from Steel Making
unit chemical analysis report online.
- deriving the CE.upon receipt of heat detail in real time, setting the
FRT corresponding to derived CE-value, and the YS is ascertained
after rolling of the rebars from tensile testing, and

- grouping all the data CE-wise wherein the FRT and YS are aligned
in the group to establish a correlation between the CE and FRT.

ABSTRACT

Rolling Mills roll billets to rebars of high Yield Strength (YS 500Mpa) as per
IS1786:2008. Billets cast from a single cycle of steel making in LD is called 'Heat'.
Heat chemistry, mainly the Carbon plus one sixth of Manganese together called
Carbon Equivalent (CE) which determines the final YS in the finished product
(rebar). Finish Rolling Temperature (FRT) is the single most rolling process
parameter that determines the final YS in the rebar while the CE including other
process parameters remain unchanged and vice versa. As the FRT range and CE
range is specified in process charts, if a low CE Heat is rolled in high FRT, the
rebar will fail in low YS and vice versa. FRT vs CE (0.32 to 0.42 step 0.01) Matrix
was developed for each section of rebar rolled in Rolling Mill considering extreme
deviations in Heat CE, based on past six months data. The inventive process is to
roll heats at unique FRT for unique Heat CE and section of rebar and mainly to
reduce variation in YS due to variable chemistry in Heats.