HIGH STRENGTH CORROSION RESISTANT BAINITIC LOW ALLOY STEELS AND
THEIR PROCESS OF MANUFACTURE
FIELD OF INVENTION
The present invention relates to novel high strength weather resistant low alloy steel
composition and the process for production of superior quality corrosion resistant low alloy
steel with microstructure consisting of ferrite-pearlite-bainite.
PRIOR ART AND DRAWBACKS
For certain types of steel structures, application of weather resistant (WR) steel is
especially recommended. These are electrical transmission towers, masts, bridges and other
structures used in outdoor exposure, where brown colour of weathering steel rust patina
gives pleasing appearance and needs minimum maintenance. Additionally, plates and
sheets have got extensive applications in the manufacture of railway coaches and wagons,
marine containers, etc. The basic requirements for these applications in addition to the
weather resistance are high yield strength, better weldability and low temperature
toughness. Therefore, Cu, P, Cr and Ni are generally added in the weather resistant steel for
improving the atmospheric corrosion resistance by developing a relatively stable layer of
hydrated iron oxide under alternate dry and wet climatic conditions. Weather resistant steel
(SAILCOR) is regularly produced at Bokaro Steel Plant. The nominal chemistry is C: 0.12%,
Mn: 0.35%, S: 0.015%, P: 0.10%, Si 0.50%, Cr: 0.45%, Ni: 0.32%, Cu: 0.45% and Al: 0.03%.
The patent document US2011259478 discloses a high-strength steel and a high-
strength, weldable steel pipe. The invention relates to a high-strength steel and to a high-
strength, weldable steel pipe. The high-strength, low-alloy steel for seamless steel pipes with
outstanding weldability and resistance to stress corrosion cracking with minimum yield
strength of 620 MPa and a tensile strength of at least 690 MPa.
The JP2009221555 document describes an ultra-high-strength thin steel sheet which
is excellent in workability and corrosion resistance and has tensile strength of > 980 MPa.
The ultra-high-strength thin steel sheet which is excellent in workability and corrosion
resistance and has tensile strength of > 980 MPa includes, by mass, 0.10-0.18% C, 1.0-
3.0% Si, 1.0-3.5% Mn, 0.05-0.2% P, < 0.02% (not including 0%) S, < 0.5% (not including
0%) Al and the balance being Fe and unavoidable impurities and has such a composite
structure that bainitic ferrite and martensite occupy > 80% of the area in total and retained
austenite occupies > 1 % of the area, wherein the average axial ratio (long axis/short axis) of
the retained austenite crystal grains is > 5 and the average length of the short axis of the
retained austenite crystal grains is = 1 pm.
The document WO2008062984 discloses steel having superior corrosion resistance
to condensed sulphuric acid, and a method for manufacturing the same. The steel having
corrosion resistance to sulphuric acid includes, by weight: C: 0.15% or less, Si: 1.0% or less,
Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co:
0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe
and other inevitable impurities. Accordingly, the steel may have a polygonal ferrite structure,
or have at least one selected from the group consisting of low temperature structures of
accicular ferrite, bainitic ferrite and bainite. Also, the steel including a low temperature
structure exhibits excellent mechanical properties in a wide temperature range from a room
temperature to 500°C.
The document JP2007197758 discloses high tensile strength marine steel exhibiting
corrosion resistance effective even if not being subjected to coating and electrolytic
protection (particularly, excellent durability to crevice corrosion in a wet air atmosphere of the
upper part in a ballast tank, the upper deck plate of a crude oil tank or the like on which
electrolytic protection is not acted), and further exhibiting excellent base metal toughness.
The marine steel has a composition comprising, by mass, 0.01 to 0.2% C, 0.01 to 0.5% Si,
0.01 to 2% Mn, 0.05 to 0.5% Al, 0.010 to 1.5% Cu and 0.010 to 1% Cr, in which the content
of P is suppressed to = 0.02% (exclusive of 0%) and the content of S is suppressed to =
0.01% (exclusive of 0%), respectively, and the balance Fe with inevitable impurities, and in
which the fraction of insular martensite is = 1.1 %, and the balance bainitic structure.
The document JP9067619 discloses high strength hot rolled steel plate excellent in
workability and corrosion resistance by subjecting a continuously cast slab of low carbon
steel, free from segregation and having a specific composition, to hot rolling, cooling and
coiling under respectively specified temperature conditions. .A low carbon steel, which has a
composition containing, by weight, 0.02-0.08% C, = 2.0% Mn, = 1.5% Si, 0.05-0.1% P, =
0.005% S, 0.10-0.60% Cu, 0.05-0.60% Ni, and = 0.06% Cr or further containing specific
small amounts of Mo, Nb, and Ti, is cast into a slab by a continuous casting method, and the
un solidified part with high Mn and P contents in the central part of the slab is removed by,
e.g. pushing it upward, by which a segregation-free continuously cast slab is prepared. This
slab is finish-hot-rolled in a temp, range between Ar3 and (Ar3 +100°C) and the resultant
plate is cooled without delay at = 50 deg.C/sec cooling rate and successively coiled in a
bainitic transformation region of 300 to 500 deg.C, not higher than (550-1000P) deg.C
(where P represents the content of P in the steel plate).
The document TW200920860 discloses iron base corrosion resistance abrasion
resistance alloy and is low C - high Si - high Cr-B-Nb series, which has overwhelmingly
excellent corrosion resistance and abrasion resistance corresponding to 304 stainless steel
or high chrome carbide, high carbon - high chrome cast iron series material; and has high
corrosion resistance that absolutely impossible obtained from the high carbon - high chrome
carbide elution type iron base corrosion resistance abrasion resistance alloy; simultaneously,
has abrasion resistance superior to the said metal. Further, the said iron base corrosion
resistance abrasion resistance alloy has high performance which the specific brittleness
peeling property of high Si steel would not be easily produced, and cheap. The said alloy
has C: 0.5 - 2.0%, Si: 2.5 - 4.5%, Mn: 0 - 10% or less, Cr: 15 - 31%, Ni: 0 - 16%, Cu: 7% or
less, Mo: 10% or less, B: 0.5 - 3.5%, 0 = Nb+V= 8% in terms of wt%. The said alloy satisfies
(SixB) = 2014/Cr2+0.083Cr+1.05 when in the range of 15% = Cr = 27%; satisfies 1.25% =
(SixB) = 6.0% when in range of 27% = Cr = 31%; satisfies (SixB) = 570/Cr2-0.066Cr+1.145
when in the range of 15% = Cr = 20%; and satisfies (SixB) = 1.25 when in the range of 20%
=Cr=31%.
The document CN102168229 discloses a weather resistant steel plate, which
comprises components of, by mass percent, 0.02 to 0.10 of C, 0.10 to 0.40 of Si, 1.0 to 1.6
of Mn, less than or equal to 0.025 of P, less than or equal to 0.015 of S, 0.20 to 0.50 of Cu,
0.30 to 0.60 of Cr, 0.10 to 0.50 of Ni, less than or equal to 0.40 of Mo, less than or equal to
0.060 of Nb, less than or equal to 0.060 of V, 0.010 to 0.035 of Ti, less than or equal to
0.0030 of B, less than or equal to 0.0050 of Ca, 0.015 to 0.050 of Al, and the balance Fe and
other unavoidable impurities. Correspondingly, the present invention also provides a
manufacturing method of the weather resistant steel plate. Through reasonable distribution
ratio of alloy components, the method provided in the invention can obtain a high-strength
and high-toughness weather resistant steel plate with good corrosion resisting performance,
high yield strength and tensile strength, and excellent low temperature impact toughness.
The steel plate also possesses good weldability, being able to carry out welding without
preheating or employ lower preheating temperature welding. The steel plate provided in the
invention can be widely applied to large-scale steel structure engineering such as building
structures, bridges, etc.
Thermo mechanical treatment (TMT) is a technique for improving the mechanical
properties of the steels by controlling the hot deformation process. Controlled rolling is a
typical example of thermo mechanical treatment that has played a vital role in the
development of the as-rolled high strength low alloy steels.
Corrosion resistance is closely related to the ferrite grain size. Depending on the
hardenability of the weather resistant steel, which is influenced by microalloying also,
acicular ferrite and different types of bainite also form after rolling. The influence of ferrite
grain size and different microstructures on mechanical properties is well established.
However, the corrosion behaviour of such microalloyed thermo mechanically treated weather
resistant steel is yet to be systematically studied.
In view of above, a new corrosion resistant low alloy steel composition having
superior quality corrosion resistance and also the process for production of superior quality
corrosion resistant low alloy steel with microstructure consisting of ferrite-pearlite-bainite is
developed in the present invention.
OBJECT OF INVENTION
The main objective of the invention is to provide innovative high strength weather
resistant low alloy steel of composition (by weight %) containing: C: 0.04 to 0.06%, Mn: 1.15
to 1.25%, S: 0.025 to 0.030%, P: 0.025 to 0.030%, Si: 0.25 to 0.30%, Cu: 0.50 to 0.55%, Cr:
0.45 to 0.50%, Ni: 0.30 to 0.35%, Al: 0.035 to 0.045% and Nb: 0.018 to 0.022%.
It is therefore objective of the invention to provide innovative high strength weather
resistant low alloy steel with ferrite-pearlite-bainite microstructure.
It is therefore objective of the invention to provide innovative high strength weather
resistant low alloy with YS: 352-393 MPa, UTS: 531-580 MPa and % El: 24-26 on 50 mm GL
after austenitisation at 1200°C and subsequent 50-70% deformation at 800-900°C.
It is therefore objective of the invention to provide innovative high strength weather
resistant low alloy with better weldability, formability, and good low temperature toughness.
It is further object of the invention to provide a process for production of innovative
high strength corrosion resistant low alloy steel composition resulting in ferrite-pearlite-
bainite microstructure with YS: 352-393 MPa, UTS: 531-580 MPa and % El: 24-26 on 50 mm
GL after austenitisation at 1200°C and subsequent 50-70% deformation at 800-900°C.
These and other objects of the invention will be clear from the following paragraphs.
DETAILED DESCRIPTION OF THE INVENTION
"Weathering" means that due to their chemical compositions, these steels exhibit
increased resistance to atmospheric corrosion compared to other steels. This is because the
steel forms a protective layer on its surface under the influence of the weather.
The corrosion-retarding effect of the protective layer is produced by the particular
distribution and concentration of alloying elements in it. The layer protecting the surface
develops and regenerates continuously when subjected to the influence of the weather. In
other words, the steel is allowed to rust in order to form the 'protective' coating.
High-strength low-alloy steels, or microalloyed steels, are designed to provide better
mechanical properties and/or greater resistance to atmospheric corrosion than conventional
carbon steels. They are not considered to be alloy steels in the normal sense because they
are designed to meet specific mechanical properties rather than a chemical composition.
High-strength low-alloy steels have yield strengths greater than 275 MPa, or 40 ksi. The
chemical composition of specific High-strength low-alloy steel may vary for different product
thicknesses to meet mechanical property requirements. The High-strength low-alloy steels in
sheet or plate form have low carbon content in order to produce adequate formability and
weldability, and they have manganese content up to 2.0%. Small quantities of chromium,
nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used
in various combinations.
High-strength low-alloy steels are classified as a separate steel category, which is
similar to as-rolled mild-carbon steel with enhanced mechanical properties obtained by the
addition of small amounts of alloying elements and, perhaps, special processing techniques
such as controlled rolling and accelerated cooling methods.
High-strength low-alloy steels can be divided into six categories:
Weathering steels, which contain small amounts of alloying elements such as
copper and phosphorus for improved atmospheric corrosion resistance and solid-solution
strengthening.
Microalloyed ferrite-pearlite steels, which contain very small (generally, less than
0.10%) additions of strong carbide or carbonitride forming elements such as niobium,
vanadium, and/or titanium for precipitation strengthening, grain refinement, and possibly
transformation temperature control.
• As-rolled peariitic steels, which may include carbon-manganese steels but which
may also have small additions of other alloying elements to enhance strength, toughness,
formability, and weldability.
• Acicular ferrite (low-carbon bainite) steels, which are low-carbon (less than 0.05%
C) steels with an excellent combination of high yield strengths, (as high as 690 MPa, or 100
ksi) weldability, formability, and good toughness.
• Dual-phase steels, which have a microstructure of martensite dispersed in a
ferritic matrix and provide a good combination of ductility and high tensile strength.
Inclusion-shape-controlled steels, which provide improved ductility and through-
thickness toughness by the small additions of calcium, zirconium, or titanium, or perhaps
rare earth elements so that the shape of the sulphide inclusions is changed from elongated
stringers to small, dispersed, almost spherical globules.
Applications of HSLA steels include oil and gas pipelines, heavy-duty highway and
off-road vehicles, construction and farm machinery, industrial equipment, storage tanks,
mine and railroad cars, barges and dredges, snowmobiles, lawn mowers, and passenger car
components. Bridges, off- shore structures, power transmission towers, light poles, and
building beams and panels are additional uses of these steels.
The choice of specific high-strength steel depends on a number of application
requirements including thickness reduction, corrosion resistance, formability, and weldability.
For many applications, the most important factor in the steel selection process is the
favourable strength-to-weight ratio of HSLA steels compared with conventional low-carbon
steels.
Effect of alloying elements on steel properties:
Alloying is changing chemical composition of steel by adding elements with purpose
to improve its properties as compared to the plane carbon steel. Alloying improves the ferrite
stabilizing effect. In the present invention following elements have ferrite stabilizing effect:
chromium (Cr), vanadium (V), aluminium (Al) and silicon (Si). Alloying also increases
corrosion resistance. Aluminium (Al), silicon (Si), and chromium (Cr) form thin strong oxide
film on the steel surface, protecting it from chemical attacks.
The principal function of alloying elements in this ferrite-pearlite high strength low
alloy steels, other than corrosion resistance, is strengthening of the ferrite by grain
refinement, precipitation strengthening, and solid-solution strengthening. Solid-solution
strengthening is closely related to alloy contents, while grain refinement and precipitation
strengthening depend on the complex effects of alloy design and thermo-mechanical
treatment.
Alloying elements are also selected to influence transformation temperatures so that
the transformation of austenite to ferrite and pearlite occurs at a lower temperature during air
cooling. This lowering of the transformation temperature produces a finer-grain
transformation product, which is a major source of strengthening. At the low carbon levels
typical of high strength low alloy steels, elements such as silicon, copper, nickel, and
phosphorus are particularly effective for producing fine pearlite. Element such as,
manganese and chromium, which are present in both the cementite and ferrite, also
strengthen the ferrite by solid-solution strengthening in proportion to the amount, dissolved in
the ferrite.
Characteristics of alloying elements
• Carbon (C) -- In the presence of alloying elements, the practical maximum carbon
content at which High strength low alloy steels can be used in the as-cooled
condition is approximately 0.20%. Higher levels of carbon tend to form martensite or
bainite in themicrostructure of as-rolled steels, although some of the higher-strength
low-alloy steels have carbon contents that approach 0.30%.
• Manganese (Mn) - improves hardenability, ductility and wear resistance. Mn
eliminates formation of harmful iron sulphides, increasing strength at high
temperatures. Manganese is the principal strengthening element in plain carbon
high-strength structural steels. It functions mainly as a mild solid-solution
strengthened in ferrite, but it also provides a marked decrease in the austenite-to-
ferrite transformation temperature.
• Nickel (Ni) - increases strength, toughness, and impart corrosion resistance in
combination with other elements.
• Chromium (Cr) - improves hardenability, strength and wear resistance, sharply
increases corrosion resistance at high concentrations (= 12%).
• Vanadium (V) - increases strength, hardness, creep resistance and impact
resistance due to formation of hard vanadium carbides, limits grain size. Vanadium
strengthens HSLA steels by both precipitation hardening the ferrite and refining the
ferrite grain size. The precipitation of vanadium carbonitride in ferrite can develop a
significant increase in strength that depends not only on the rolling process used, but
also on the base composition. Carbon contents above 0.13 to 0.15% and manganese
content of 1% or more enhances the precipitation hardening, particularly when the
nitrogen content is at least 0.01%.
• Silicon (Si) - improves strength, elasticity, acid resistance and promotes large grain
sizes, which cause increasing magnetic permeability. One of the most important
applications of silicon is its use as a deoxidizer in molten steel. Silicon has a
strengthening effect in low-alloy structural steels. In larger amounts, it increases
resistance to scaling at elevated temperatures. Silicon has a significant effect on
yield strength enhancement by solid-solution strengthening and is widely used in
HSLA steels for riveted or bolted structures.
• Copper (Cu) - Copper in levels in excess of 0.50% also increases the strength of
both low- and medium-carbon steels by virtue of ferrite strengthening, which is
accompanied by only slight decreases in ductility. Copper can be retained in solid
solution even at the slow rate of cooling obtained when large sections are
normalized, but it is precipitated out when the steel is reheated to about 510 to 605°C
(950 to 1125°F). At about 1% copper, the yield strength is increased by about 70 to
140 MPa regardless of the effects of other alloying elements. Copper in amounts up
to 0.75% is considered to have only minor adverse effects on notch toughness or
weldability. Copper precipitation hardening gives the steel the ability to be formed
extensively and then precipitation hardened as a complex shape or welded
assembly.
• Phosphorus (P) — The atmospheric-corrosion resistance of steel is increased
appreciably by the addition of phosphorus, and when small amounts of copper are
present in the steel, the effect of the phosphorus is greatly enhanced. When both
phosphorus and copper are present, there is a greater beneficial effect on corrosion
resistance than the sum of the effects of the individual elements.
• Aluminium (Al) - deoxidizer, limits austenite grains growth. Aluminium is widely used
as a deoxidizer and was the first element used to control austenite grain growth
during reheating. During controlled rolling, niobium and titanium are more effective
grain refiners than aluminium.
Most high strength low alloy steels are furnished in the as-hot-rolled condition with
ferrite-pearlite microstructure. The exceptions are the controlled-rolled steels with an acicular
ferrite microstructure and the dual-phase steels with martensite dispersed in a matrix of
polygonal ferrite.
Pearlite is generally an undesirable strengthening agent in structural steels because
it reduces impact toughness and requires higher carbon contents. Moreover, yield strength is
largely unaffected by a higher pearlite content.
Strengthening Mechanisms in Ferrite
The ferrite in high strength low alloy steels is typically strengthened by grain
refinement, precipitation hardening, and, to a lesser extent, solid-solution strengthening.
Grain refinement is the most desirable strengthening mechanism because it improves not
only strength but also toughness.
Grain refinement is influenced by the complex effects of alloy design and processing
methods. For example, the various methods of grain refinement used in the three different
stages of hot rolling (that is, reheating, hot rolling, and cooling) include:
(a) The addition of titanium or aluminium to retard austenite grain growth when the steel
is reheated for hot deformation or subsequent heat treatment
(b) The controlled rolling of microalloyed steels to condition the austenite so that it
transforms into fine-grain ferrite
(c) The use of alloy additions and/or faster cooling rates to lower the austenite-to-ferrite
transformation temperature.
The use of higher cooling rates for grain refinement may require consideration of its
effect on precipitation strengthening and the possibility of undesirable transformation
products.
Precipitation strengthening occurs from the formation of finely dispersed
carbonitrides developed during heating and cooling. Because precipitation strengthening is
generally associated with a reduction in toughness, grain refinement is often used in
conjunction with precipitation strengthening to improve toughness. Precipitation
strengthening is influenced by the type of carbonitride, its grain size, and, of course, the
number of carbonitrides precipitated. The formation of MC is the most effective metal carbide
in the precipitation strengthening of microalloyed niobium, vanadium, and/or titanium steels.
The number of fine MC particles formed during heating and cooling depends on the solubility
of the carbides in austenite and on cooling rates.
Steel making
Precise steelmaking operations are also essential in controlling the properties and
chemistry of high strength low alloy steels. Optimum property levels depend on such factors
as the control of significant alloying elements and the reduction of impurities and nonmetallic
inclusions.
Controlled Rolling
The hot-rolling process has gradually become a much more closely controlled
operation, and controlled rolling is now being increasingly applied to microalloyed steels with
compositions carefully chosen to provide optimum mechanical properties at room
temperature. Controlled rolling is a procedure whereby the various stages of rolling are
temperature controlled, with the amount of reduction in each pass predetermined and the
finishing temperature precisely defined. This processing is widely used to obtain reliable
mechanical properties in steels for pipelines, bridges, offshore platforms, and many other
engineering applications. The use of controlled rolling has resulted in improved combinations
of strength and toughness and further reductions in the carbon content of microalloyed
HSLA steels.
In view of the above, similar heats (Table 1) with microalloy (Nb and V) additions
have been made in 100 Kg induction furnace. The defective portions of the ingots from top
and bottom were discarded. The remaining portions of the ingots were soaked at 1200°C for
2 hours and.hot rolled to plates with a final thickness of 12 mm over a temperature range of
1150-900°C. Plates were subsequently controlled rolled giving 50, 60 and 70% reductions at
800 and 900°C. Thus microalloyed high strength weather resistant steels with YS: 352-536
MPa, UTS: 531-830 MPa and %EI: 17-26 were successfully produced. Controlled rolling of
microalloyed steels has resulted in development of fine ferrite-pearlite-bainite microstructure.
Such microstructure is desirable not only for high strength but also for better toughness and
wedability of the as hot rolled structural steel.
Subsequently, electrochemical corrosion performance of thermo mechanically
treated microalloyed weather resistant steel samples vis-a-vis SAILCOR steel in 3.5 % NaCI
solution was evaluated using Tafel polarization and extrapolation technique. The results
show significantly lower corrosion rate for 50% deformed microalloyed weather resistant
steel samples than that of SAILCOR steel. However, when % deformation was more,
corrosion rates of microalloyed steels increased. This is attributed to the ferrite grain
refinement of microalloyed steel at higher deformation which causes increase in grain
boundary area where more corrosion takes place.
A marginal 0.020% Nb microalloying and 0.70% Mn addition over SAILCOR when
deformed 50 and 60 % at 900 and 800°C each showed charge transfer resistance superior
or comparable to SAILCOR. The complex plane Nyquist Bode magnitude and Bode phase
plots of this steel roiled 50 % at 800 and 900°C also show comparable electrochemical
impedances with that of SAILCOR.
Weight losses and corrosion rates of microalloyed weather resistant steels along with
SAILCOR in 5% salt fog over an exposure period of 98 days have been studied. The
experimental steels exhibited substantially lower weight losses, slower corrosion kinetics and
impeding of corrosion rates over the entire exposure period of 98 days, which is an
indication of superior corrosion resistance of experimental steels. The pore resistance of the
rust coatings on microalloyed experimental steels was found either comparable or less than
that of SAILCOR. Correspondingly, the Nyquist plots of rust patinas on experimental steels
show smaller capacitive semicircles indicative of lower impedances than that of SAILCOR
presumably due to lesser rust formation during the initial 15-day exposure to 5% salt fog.
This also indicates the slower corrosion kinetics prevailing in the microalloyed weather
resistant steel.
Table: 1 Chemical compositions of steels fwt%)

Salient features of innovation:
In the present innovation, the modification of chemistry of SAILCOR weather resistant
steel on the laboratory scale was carried out. This has resulted in higher strength and better
corrosion properties in the weather resistant steel after controlled rolling at 800 and 900°C.
The salient features are as follows:
• Weather resistant steel with chemical composition C: 0.04-0.06%, Mn: 1.15-
1.25%, S: 0.025-0.030%, P: 0.025-0.030%, Si: 0.25-0.30%, Cu: 0.50-0.55%, Cr:
0.45-0.50%, Ni: 0.30-0.35%, Al: 0.035-0.045% and Nb: 0.018-0.022% results in
ferrite-pearlite-bainite microstructure with YS: 352-393 MPa, UTS: 531-580 MPa
and % El: 24-26 on 50 mm GL after austenitisation at 1200°C and subsequent
50-70% deformation at 800-900°C.
• The above steel which has lower C (0.05%), no P alloying as in SAILCOR, but
additional alloying with Mn (0.65%) and Nb microalloying (0.020%) (Steel 1) has
resulted in corrosion properties superior to SAILCOR. The corrosion rates of
experimental steel rolled 50 and 60% at 900°C and SAILCOR steel were - 6.5
mpy and 10.2 mpy respectively under Tafel polarization test in 3.5% NaCI. As in
the case of polarization, among the steels studied, this steel deformed 50 and
60% at both 800 and 900°C showed charge transfer resistance superior to
SAILCOR. In 5% salt fog (as per ASTM B117) test also, the corrosion rates of the
above samples were in the range of 10.7-12.5 mpy against 17.6 mpy for
SAILCOR steel after 98 days of test exposure.
• In the same steel when C and Nb were increased from 0.05 to 0.20% and 0.020
to 0.054% respectively and additionally microalloyed with 0.046% V (Steel 2), the
microstructure became ferrite-bainite resulting in YS and UTS 536 and 830 MPa
respectively after 70% deformation at 900°C. The Tafel polarization test of this
steel along with the SAILCOR steel carried out in 3.5% NaCI showed comparable
corrosion behaviour of both the steels. Thus, microalloying results in higher
strength with improved corrosion properties of the steel.
While this invention has been described with an emphasis upon preferred
embodiments, it will be obvious to those of ordinary skill in the art that variations in the
preferred methods may be used and that it is intended that the invention may be practiced
otherwise than as specifically described herein. Accordingly, this invention includes all
modifications encompassed within the spirit and scope of the invention as defined by the
claims that follow.
WE CLAIM:
1. A weather resistant bainitic high strength low alloy steel composition having excellent
resistance to corrosion, comprising, (by weight %) - C: 0.04 to 0.06%, Mn: 1.15 to 1.25%, S:
0.025 to 0.030%, P: 0.025 to 0.030%, Si: 0.25 to 0.30%, Cu: 0.50 to 0.55%, Cr: 0.45 to
0.50%, Ni: 0.30 to 0.35%, Al: 0.035 to 0.045% and Nb: 0.018 to 0.022%.
2. A weather resistant bainitic high strength low alloy steel compositiori as claimed in
claim 1, where in, the said alloy results in ferrite-pearlite-bainite microstructure.
3. A weather resistant bainitic high strength low alloy steel composition as claimed in
claim 1, where in, the alloy is having the following properties-YS: 352-393 MPa, UTS: 531-
580 MPa and % El: 24-26 on 50 mm GL after austenitisation at 12000C and subsequent 50-
70% deformation at 800-9000C.
4. A weather resistant bainitic high strength low alloy steel composition as claimed in
claim 1, where in, the alloy is having high strength weather resistant low alloy with better
weldability, formability, and good low temperature toughness.
5. A process for manufacturing weather resistant bainitic high strength low alloy steel
composition having excellent resistance to corrosion, the method comprising:
(a) hot-rolling a steel comprising, by weight%- C:0.04 to 0.06%, Mn: 1.15 to 1.25%, S:
0.025 to 0.030%, P: 0.025 to 0.030%, Si: 0.25 to 0.30%, Cu: 0.50 to 0.55%, Cr: 0.45 to
0.50%, Ni: 0.30 to 0.35%, Al: 0.035 to 0.045% and Nb: 0.018 to 0.022%;
(b) the alloy in the step a is taken in 100 kg induction furnace, where ingots are formed;
(c) the defective portions of the ingots obtained from step b is discarded from top and
bottom;
(d) then remaining portions of the ingots are soaked at 1200°C for 2 hours and hot rolled
to plates with a final thickness of 12 mm over a temperature range of 1150-900°C;
(e) the plates obtained in step d were subsequently controlled rolled giving 50, 60 and
70% reductions at 800 and 900°C.
6. A process for manufacturing weather resistant bainitic high strength low alloy steel
composition having excellent resistance to corrosion, as claimed in claim 5, wherein, the
micro alloyed high strength weather resistant steels have YS: 352-536 MPa, UTS: 531-830
MPaand%El: 17-26.
7. A process for manufacturing weather resistant bainitic high strength low alloy steel
composition having excellent resistance to corrosion, as claimed in claim 5, wherein, the
micro alloyed high strength weather resistant steels have fine ferrite-pearlite-bainite
microstructure.
8. A process for manufacturing weather resistant bainitic high strength low alloy steel
composition having excellent resistance to corrosion, as claimed in claim 7, wherein, the
micro alloyed high strength weather resistant steels have better toughness and wedability of
the as hot rolled structural steel.
9. A weather resistant bainitic high strength low alloy steel composition as claimed in
claim 1, where in, weight % of C is 0.05, an additional alloying with Mn 0.65% and Nb
microalloying 0.020% with no P as alloying component (Steel 1), have superior corrosion
properties.
10. A weather resistant bainitic high strength low alloy steel composition as claimed in
claim 1, where in, weight % of C and Nb were increased from 0.05 to 0.20% and 0.020 to
0.054% respectively and additionally microalloyed with 0.046% V (Steel 2), the
microstructure became ferrite-bainite resulting in YS and UTS 536 and 830 MPa respectively
after 70% deformation at 900°C.
11. A weather resistant bainitic high strength low alloy steel composition having excellent
resistance to corrosion substantially as herein described.
12. A process for manufacturing weather resistant bainitic high strength low alloy steel
composition having excellent resistance to corrosion substantially as herein described.

ABSTRACT

A weather resistant bainitic high strength low alloy steel composition having
excellent resistance to corrosion, comprising, (by weight%)- C: 0.04 to 0.06%, Mn: 1.15 to
1.25%, S: 0.025 to 0.030%, P: 0.025 to 0.030%, Si: 0.25 to 0.30%, Cu: 0.50 to 0.55%, Cr:
0.45 to 0.50%, Ni: 0.30 to 0.35%, Al: 0.035 to 0.045% and Nb: 0.018 to 0.022% with
microstructure consisting of ferrite-pearlite-bainite, and the process for manufacturing
weather resistant bainitic high strength low alloy steel composition having excellent
resistance to corrosion thereof.