LOW CARBON BORON-ADDED MILD STEEL FOR APPLICATION IN
NUCLEAR REACTORS AND THE PROCESS OF MANUFACTURING
THEREOF
FIELD OF INVENTION
This invention relates generally to Boron-added mild steel, and more particularly
to mild steel which is capable of shielding against the effect of both gamma rays
and neutrons. It also lowers the cost of production in comparison to boron-added
stainless steel, has tensile strength in the range of 450 to 525 MPa and shows
martensitic/ bainitic phases.
BACKGROUND ART
As performance demanded as a charge of nuclear fuel basket material of
transportation of spent nuclear fuel and the task for storage, they are (1) thermal-
neutron absorption power, (2) processability, (3) toughness, and (4) corrosion
resistance. Since Boron has high thermal neutron absorption power with Boron
0.3 - 3.0 wt%. Austenitic stainless steel is used for the above-mentioned basket.
Boron is a potent alloying element which is cheap and is used for replacing
mainly chromium and molybdenum. The presence of boron in ppm levels steel
remarkably increases the hardenability of low alloy steel. Boron has high neutron
absorption capability whereas steel is a good gamma ray shielding material.
Boron steel is therefore, extensively used in nuclear reactors for shielding
neutrons and gamma rays. World over, boron-added 304 grade austenitic
stainless steel is the most popular shielding material used in nuclear reactors.
Boron-added mild steel has been developed, which exhibits satisfactory
shielding against both gamma rays and neutrons. The cost of production of
boron added mild steel is lower in comparison to boron-added stainless steel.
A number of steel materials proposed for use in applications which require a
combination of hardenability, toughness, and temper resistance properties, have
compositions which include relatively high amounts of boron. The element boron

has numerous uses in the construction of nuclear reactors, since it possesses a
high thermal neutron absorption cross section. This property makes this element
particularly useful for controlling neutron flux and is therefore employed as an
effective shielding element in various compositions for reactor control rods.
When exposed to neutron bombardment the boron isotope, 10B, about 20 % of
which are present in naturally occurring boron, is broken down into stable atomic
nuclei of 'Li' and 'He', without the emission of gamma rays.
This property of boron is utilised for the construction of thermal shields for the
purpose of preventing certain components of a nuclear reactor from being
heated excessively. For example, in nuclear reactors in which the fuel is in the
form of loosely packed spheres, the withdrawal tubes for the fuel spheres are
enclosed in a layer of a boron containing material. The provision of such shields
permits the wall thickness of other reactor components to be reduced. It also
allows the pressure vessel of the reactor to be examined under less hazardous
conditions during a shut-down period.
Conventionally, boron is incorporated in control rods are that of B.C, as
metallurgical^ produced powder mixture or as a metallic alloy. However, the
powder mixture suffers from a serious disadvantage in that it is attacked by
vapour and rapidly disintegrates. It must therefore be encased in sleeves of
stainless steel. This, however, is not altogether satisfactory solution since the
control rods may expand owing to the accumulation of helium and lithium atoms
formed by decay. The production-of control rods in which the boron is present in
the form of a powder mixture also suffers from technical difficulties.
The use of boron in the form of metallic alloys has been known for some time.
Cast steels with a boron content of 5 % are already being industrially produced.
Austenitic chromium-nickel steels having a eutectic boron content of 2.1% can
be forged. Although an increase in the boron content improves the tensile
strength and yield point of the alloy, it does, however, simultaneously reduce the
elongation, notch impact strength and necking prior to fracture. The use of steel

alloys with higher boron contents would not therefore have appeared to,
commend itself.
U.S. Pat. No. 5,131,965 issued July 21, 1992 to J. Mc Vicker discloses a steel
having high hardenability and toughness. However, patent 5,131,965 uses
higher chromium to attain high hardenability and temper resistance without
exploiting the hardenability and precipitation effect of boron to obtain high
fracture toughness, as has been done in the present invention. In addition, the
present invention uses boron to lower grain boundary energy and, thus, improve
fracture toughness.
SUMMARY OF INVENTION
The principle object of the present invention is thus directed to developing a new
product as High Boron Mild Steel. It is low carbon boron-added mild steel
containing 0.20 to 1.0 % boron.
Another object of the present invention is directed to developing high boron steel
such that it is made in medium frequency 100 kg Air Induction furnace using soft
iron and with calculated amount of ferro-alloys.
Another object of the present invention is directed to developing high boron steel
by hot rolling of cast ingots in Experimental Rolling Mill. Before rolling, ingots
were soaked at 1080 C for two hours. The finish rolling temperature was kept
900 C. Finally, ingots were hot rolled into plate of dimension 455x265x16 mm.
Another object of the present invention is directed to developing the chemistry of
the high boron steel plate was found as C (0.06 - 1.0%), Si (0.20 - 0.80%), S
(0.03% max), P (0.03% max), Mn (0.50 -1.50 %), Cu (0.20 - 0.25%), B (0.25 -
1.0%) and rest being Fe.
A still further object of the present invention is directed to developing high boron
steel which possesses yield strength 300 to 425 MPa, ultimate tensile strength in
the range of 450 to 525 MPa and elongation in the range of 30 to 10 %.

A still further object of the present invention is directed to developing high boron
steel, having impact toughness value 35 to 5 Joules.
A still further object of the present invention is directed to developing high boron
steel which possesses hardness (HRB) in the range of 60 to 85.
A still further object of the present invention is directed to developing high boron
steel which microstructure shows precipitates of iron boride at the grain
boundaries along with boron carbides and boron nitrides phases in the matrix of
ferrite.
The high boron steel can be used to shield the gamma rays and neutron
especially in nuclear reactors used for the production of nuclear energy.
The high boron steel may be used to cover lattice tube of end fittings of
Pressurized Heavy Water Reactor in the form of split sleeve.
The high boron steel may be utilized in the fabrication of storage of spent nuclear
fuel, reactor shielding, nuclear waste disposals, castor transport vessels and fuel
shipment containers, etc.
TECHNICAL DISCLOSURE ON INVENTION
Steel making
It is the method of manufacturing mild steel containing Boron continuously and
effectively, by hot-rolling cast steel at the finishing rolling temperature of not less
than 900 degrees C.
Hereafter, it is described in detail about the present invention. Since an inner
layer part is in molten state forever, and Boron cannot cause a breakout if the
inner layer part and outer layer part containing B are made into the same steel
type in order to make the melting point of steel be deteriorated, it needs to
eliminate a melting point difference. So, to an inner layer part, since plain steel

with the melting point high about 100 degrees C is used and it has sufficient
neutron absorption ability rather than stainless steel, at least 0.2-1% or more of B
is added. The resulting boron mild steel whose melting point is [ excelling in
corrosion resistance in view of corrosion resistance, since it is dissolution of a
melting point difference although use of boron steel is indispensable ] low to an
outer layer part.
For studies on development of high boron steel, melting experiments were
carried out in medium frequency 100 kg air induction furnace using 50 kg low
carbon steel scrap with measured quantities of ferroalloys such as manganese
metal, silicon metal, ferro-boron and copper metal so as to achieve the desired
chemistry. Since boron combines aggressively with oxygen and nitrogen
dissolved in steel, care has been taken during experimental heat making and
during addition practices. The liquid steel was killed fully with aluminium before
boron addition because aluminium offers dual advantages. Firstly, it provides
better recovery of boron and secondly, it prevents boron from combining with
nitrogen. Recovery of boron was observed to be 65% up to 1.0% boron in steel.
Liquid steel was cast into 25 Kg ingots of size (110x100) mm cross section. Two
ingots were obtained from each heat.
Hot rolling of boron steel ingots
The ingots thus produced were dressed and cropped from the top to exclude the
pipes and other solidification defects. The Steel composition was as follows:
C (0.05 - 1.5%), Si (0.20 - 1.5%), S (0.03% max), P (0.03% max), Mn (0.50 -2.0
%), Cu (0.20 - 0.25%), B (0.20-1.0%) and rest being Fe. The ingots were soaked
at 1080 C for two hours and were hot rolled into 16 mm thick plates. Initially, six
passes were applied to the hot ingot longitudinally; there after the ingot was
rotated by 90° and was subjected another thirteen passes with reheating of the
plate after first eleven passes. The ingots were finally rolled into plates of
dimension 460x330x16 mm. Initial and finishing rolling temperatures were
maintained between 1000 - 1025°C and 900 - 925°C respectively. Rolling

speed was kept at 10 meter/ min and plates were cooled in air. The
manufacturing process of herein disclosed high boron mild steel comprising
following steps:
• Boron steel was made in medium frequency 100 Kg Air Induction Furnace
and cast in to 25 Kg ingots.
• Ingots were reheated/ soaked in a reheating furnace to 1050 to 1080°C for 2
hours.
• Reheated ingots were hot cross rolled in to 16 mm thick plates.
• Rolling was conducted at the speed of 10 m/ minute.
• The final dimension of the plate was obtained as 455x265x16 mm.
• The final rolling reduction was observed about 86%.
• The finish rolling temperature was maintained at 900°C.
• Rolled plates were cooled in air.
Evaluation of mechanical properties
Evaluation of mechanical properties of boron steels was carried out. All the high
boron steels exhibited increasing UTS and YS values with increasing percentage
of boron in steel, whereas the percentage of elongation and reduction in area
values found to decrease with increasing percentage of boron. The higher
strength and lower ductility of boron steels is attributed to the formation of
martensitic/ bainitic phases in boron steel owing to an increase in hardenability of
steel. It was observed that steel with low boron had higher Charpy V-notch
(CVN) impact toughness. The impact toughness value was found decreased by
almost half when tested at -20°C.

Metallographic examination
Metallographic examination of boron steel revealed the presence of precipitates
of iron boride at the grain boundaries. Boron carbides and boron nitrides phases
were also observed in the matrix of ferrite which was confirmed through energy
dispersive x-ray analysis in a Scanning Electron Microscope.
Although the foregoing description of the present invention has been shown and
described with reference to particular embodiments and applications thereof, it
has been presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the particular embodiments
and applications disclosed. It will be apparent to those having ordinary skill in the
art that a number of changes, modifications, variations, or alterations to the
invention as described herein may be made, none of which depart from the spirit
or scope of the present invention. The particular embodiments and applications
were chosen and described to provide the best illustration of the principles of the
invention and its practical application to thereby enable one of ordinary skill in
the art to utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. All such changes,
modifications, variations, and alterations should therefore be seen as being
within the scope of the present invention as determined by the appended claims
when interpreted in accordance with the breadth to which they are fairly, legally,
and equitably entitled.

We Claim:
1. A low carbon boron-added mild steel for application in nuclear reactors
comprising of: C (0.06 - 1.0%), Si (0.20 - 0.80%), S (0.03% max), P (0.03%
max), Mn (0.50 -1.50 %), Cu (0.20 - 0.25%), B (0.25 -1.0%) and rest being Fe.
2. A low carbon boron-added mild steel as claimed in claim 1, wherein the said
steel possess yield strength 300 to 425 MPa, ultimate tensile strength in the
range of 450 to 525 MPa and elongation in the range of 30 to 10 %.
3. A low carbon boron-added mild steel as claimed in claim 1, wherein the said
steel possesses impact toughness value 35 to 5 Joules.
4. A low carbon boron-added mild steel as claimed in claim 1, wherein the said
high boron steel possesses hardness (HRB) in the range of 60 to 85.
5. A low carbon boron-added mild steel as claimed in claim 1, wherein the said
high boron steel microstructure shows martensitic/ bainitic phases precipitates of
iron boride at the grain boundaries along with boron carbides and boron nitrides
phases in the matrix of ferrite.
6. A process of manufacturing low carbon boron-added mild steel comprising
the steps of:

- Casting of boron steel into ingots;
- Reheating and soaking the said ingots in a reheating furnace to 1050 to
1080°C for 2 hours.
- Performing hot cross rolling of reheated / soaked ingots in to 16 mm thick
plates wherein the rolling is conducted at the speed of 10 m/ minute.
- Performing final rolling till the reduction is observed about 86% at
temperature maintained at 900°C.

- Cooling the rolled plates in air.
7. A process of manufacturing low carbon boron-added mild steel as claimed in
claim 6 wherein the boron steel comprises the composition: C (0.06 - 1.0%),
Si (0.20 - 0.80%), S (0.03% max), P (0.03% max), Mn (0.50 -1.50 %), Cu
(0.20 - 0.25%), B (0.25 -1.0%) and rest being Fe.
8. A process of manufacturing low carbon boron-added mild steel as claimed in
claim 6 wherein the casting is carried out in medium frequency 100 Kg Air
Induction Furnace.
9. A process of manufacturing low carbon boron-added mild steel as claimed in
claim 6 wherein said high boron steel microstructure shows martensitic/
bainitic phases including precipitates of iron boride at the grain boundaries
along with boron carbides and boron nitrides phases in the matrix of ferrite.
10. A process of manufacturing low carbon boron-added mild steel as claimed in
claim 6 wherein the liquid steel in the casting process is killed fully with
aluminium before boron addition and the recovery of boron is attained to 65%
i.e. up to 1.0% boron in boron steel.

ABSTRACT

The present invention relates to Boron-added mild steel, and more particularly to
mild steel which is capable of shielding against the effect of both gamma rays
and neutrons, it also lowers the cost of production in comparison to boron-added
stainless steel, has tensile strength in the range of 450 to 525 MPa and shows
martensitic/ bainitic phases.