PROCESS TECHNOLOGY FOR THE PRODUCTION OF MICRO ALLOYED
HOT ROLLED HIGH STRENGTH STEEL STRIP AND PRODUCT
THEREFROM
FIELD OF THE INVENTION
The present invention is directed to a method for production of micro alloyed high
strength steel strips in hot rolled condition and a product therefrom and in
particular to a process technology for producing high strength steel strips
(thickness 3-10mm with YS > 500 MPa) in hot rolled condition using a controlled
alloy chemistry and controlled rolling and cooling conditions to produce high
strength steel strip capable of meeting mechanical and compositional
requirements for a number of ASTM specifications which are normally produced
through quenching & tempering technology.
BACKGROUND ART
Market survey has shown that there is huge demand for high strength hot rolled
steel plates (YS: 550 MPa min) in India with guaranteed impact property.
Therefore it is a prior need to develop suitable technology for production of
such high strength steel plates in hot rolled condition. This process technology
will help in producing high strength steel plates in hot rolled condition, which are
normally produced through quenching & tempering technology.
In the prior art, lower carbon, high strength (or High Performance Steel, HPS)
grade steels are being increasingly employed for application in line pipes for
transportation of oil and natural gases, auto chasis, pre-fabricated building
manufacturers etc and other high strength applications. These steel materials
offer three advantages over concrete and other types of steel materials. First, the
use of higher strength materials can reduce the overall weight of the structure
being built and can also reduce the material cost.
The use of these types of steels is guided by ASTM specifications. For a medium
strength application, e.g., ASTM A588-Grade B or A709-Grade 50 W, weathering
steels having a 50 KSI minimum yield strength is specified. These steels typically
employ about 0.16% by weight of carbon. Other ASTM specifications for steels
which are commonly used for bridge and pole applications include A709-Grades
70 W and HPS 70 W for bridge applications and A871 -Grade 65 for pole or
tubular applications. The specifications require that these grades be produced by
rolling, quenching, and tempering. The conventional 70 W grade is a higher
carbon grade (0.12% by weight), whereas the newer HPS 70 W grade utilizes a
lower carbon level (0.10% by weight). The HPS 70 W grade is generally
produced in plates up to 3" thickness.
The U.S. Pat. No. 5,514,227 to Bodnar et al. (herein incorporated in its entirety
by reference) This patent describes a method of making a steel to meet ASTM
A572, Grade Gr.F Q&T steel, a 50 KSI minimum yield strength specification. The
alloy chemistry in this patent specifies low levels of vanadium and 1.0 to 1.25%
manganese.
Another Japanese Patent publication JP2001259703 discloses a method and
equipment for temper rolling by which the generation of cross buckle is prevented
even in the case that the outflow angle of a steel strip from a temper rolling mill is
hard to secure. The rolling is performed by controlling the distance between the
contact points of a work roll for the steel strip and auxiliary roll within 100 mm by
pushing up or down the auxiliary roll which is provided on the outlet side of the
work rolls of the temper rolling mill.
Yet another Japanese Patent publication JP2005230900 discloses a temper
rolling method and a temper rolling mill which can manufacture a steel strip
excellent in surface property by suppressing the occurrence of roll marks on the
surface of the steel strip by preventing the occurrence of both dint flaws and the
adherence of foreign matters onto the surfaces of work rolls. The steel strip is
temper-rolled by using the work rolls and back-up rolls respectively taking the
Shore hardness A (Hs) of the work roll and the Shore hardness B (Hs) of back-up
rolls as the ranges A >= 90, B > 75 and (A-20) < B <= (A-10).
OBJECTS OF INNOVATION
In present innovation, a specially designed innovative alloy chemistry as shown
in Table 1 has been used. Processing technology which included the reheat
practices, draft schedules and control of thermal regime of rolling and coiling
ensured finely dispersed nano sized precipitates of microalloying elements in the
ferrite - pearlite matrix along with globularised shapes of oxy-sulphide inclusions
obtained as a result of Ca-Si treatment.
Often the strengthening mechanisms adopted for enhancing strength cause loss
in ductility and toughness. Further, achieving a low ratio of yield strength (YS) to
tensile strength (UTS) is very difficult especially in high strength steels. Designing
of steel chemistry and suitable control of rolling parameters have been vital to
achieve the yield strength above 500 MPa, YS / UTS ratio below 0.9 and impact
toughness above 100 J at 0 C simultaneously.
Achieving above properties in a ferrite-pearlite based microstructure with nano-
precipitates of microalloying elements through BOF-CC-HSM route was the
object of innovation.
For the above and other purposes, and notably for attainment of high strength
low alloy steel products having superior properties like strength, limited rolling
load and yet characterized by economy of cost and ease of processing, the
invention, in an important aspect, consists of steel characterized by additions of
the microalloying elements Niobium, Vanadium and Titanium (singly or in
combination) in low to moderate amounts, with critical, very low content of carbon
and a relatively high manganese, silicon and phosphorus.
With such proportions of elements, the balance of the steel being iron and
incidental substances, and actual numerical ranges for the above elements and
also nominal values for normal minor elements such as aluminium, manganese,
silicon and phosphorus being as given herein below, a paramount feature of the
invention is the attainment of desired strength and formability in an unusually
lean alloy, with respect both to the so-called microalloying elements such as
Niobium, Vanadium and Titanium.
The process for manufacturing of a hot rolled high strength steel strips
comprising the steps of : charging and casting of the different compositions of
raw materials in Basic Oxygen furnace; refining of the castables using LF /
Vacuum Arc Degassing (VAD) Unit; continuous casting of the alloy strips;
soaking of strips in Reheating furnaces; controlled rough rolling of the strips in
the Plate mill with Hot working of the Cast alloy; controlled finished rolling of the
strips in the Plate mill; controlled air-cooling of the steel strips during rolling
controlled coiling of the finished strips.
The cost effective High Strength steel as disclosed consist essentially of, in
weight percent, weight percent of about 0.08% (max) Carbon, 1.4 - 1.5%
Manganese, 0.35% (max) silicon, 0.012% (max) Phosphorus, 0.005% (max)
Sulphur, 0.02 - 0.04% Aluminium, 0.05% (max) Niobium, 0.15% (max)
Vanadium and 0.02% (max) Titanium and balance essentially iron.
A more specific finding is that in the new compositions, the yield strength is
directly related to the specific percentages of these elements Niobium, Vanadium
and Titanium. Thus in the stated compositions, with the total of Niobium,
Vanadium and Titanium at maximum percentages of 0.05%, 0.05% and 0.02%
respectively, with the thickness of strip between (3-1 Omm) it is possible to obtain
yield strengths (e.g. in both directions) in the range of YS: 500 Mpa (min), UTS:
580 Mpa (min), El: 25% (min), Charpy Impact properties: 100 J (min) at 0°C and
% Shear area in DWTT : 85% min. at 0°C respectively, in all types of plate mills
and further achieving above properties in a ferrite-pearlite based microstructure
with nano-precipitates of microalloying elements through BOF-CC-HSM route.
A second important aspect of the present invention is that with the stated
microalloyed compositions, especially having the prescribed or preferred levels of
Carbon, Manganese, silicon, phosphorus with Nb-V-Ti combination, the rolled
products are found to exhibit superior strength and impact properties, Abrasion
resistance property and good toughness without special additions or processing /
rolling.
Another objective of the present invention is to provide flexibility in terms of
impurity contents i.e. both S & P to the tune of 0.005 and 0.012% max for
enhancing Charpy impact toughness and % shear area requirement in drop
weight tear test.
Another object of the present invention is to achieve both grain refinement due to
controlled rolling and to add Nb & V for nano-size precipitates for precipitation
hardening.
Another exemplary aspect of the invention is to provide CaSi treatment during
steel making for the globularisation of inclusions which helps in improving impact
toughness properties.
The heat treatment processes involved in imparting high strength to the steel or
its components are normalizing followed by hardening and tempering. The last
phase of the heat treatment i.e. hardening and tempering makes the steel /
component quite expensive at these involves huge investments in terms of
facilities, high operating cost on energy, time and skilled human contributions.
Moreover, these steels are highly susceptible to Temper-embrittlement. Hence
special caution is required in selection of tempering temperature and processes.
The reheat temperature is selected between 1200 - 1280 C with 2 hours soaking
ensures dissolution of microalloying elements into austenitic matrix. R5
Temperature of 1050 + 10 ensured entry of rough rolled transfer bar to finishing
strand at temperatures below temperature of no recrystallisation.
Yet another objective of the present invention is to provide a good combination of
high strength, superior impact properties with abrasive resistant alloy at room
temperature.
As per another embodiment of the present invention there is provided thermo-
mechanically controlled processing, keeping finish rolling temperature (FRT) in
the range of 880-860 degree Celsius, coiling temperatures between 640 - 660
deg Cel coupled with cumulative reduction of more than 67% deformation.
The invention also includes a steel strip made by the inventive method as a hot
rolled and high strength steel plate. The strip can have all of (1) a strip thickness
of 3 - 10mm and a yield strength of 500 MPa, UTS of 713 - 742 Mpa, YS/UTS
<0.9 with Charpy Impact properties: >100 J at 0°C. The alloy chemistry or
composition is also part of the invention, in terms of its broad and preferred
rangesi
Given the following detailed description, it should become apparent to the person
having ordinary skill in the art that the invention herein provides a novel
engineered tile and method permitting exploitation of significantly augmented
efficiencies while mitigating problems of the prior art
BRIEF DESCRIPTION OF THE ACCPMPANYING DRAWINGS
Fig. 1 illustrates secondary electron image of a globular inclusion inclusion and
X-ray mapping of various elements in it in accordance with the present invention;
Fig. 2 illustrates a typical ferrite - pearlite microstructure observed in the hot
rolled stock in accordance with the present invention.
DETAILED DESCRIPTION
The present invention provides a significant advancement in producing high
strength steel strips (3-1 Omm) in terms of cost-effectiveness, improved mill
productivity, flexibility, improved formability and castability, and energy
efficiency. The inventive method produces an improved high strength grade
steel strip in an hot rolled condition, thereby eliminating the need for quenching
and tempering (i.e., saving production cost and shortening delivery time) as is
used in present day weathering grade steel strips. With the inventive
processing, the chemical and mechanical requirements for a variety of ASTM
specifications can be met so that the invention produces a multi-purpose steel
strips. High strength steel grade is intended to mean alloy chemistries as
exemplified by the above-referenced ASTM specifications that employ effective
levels of Nb-V-Ti and silicon to achieve favourable characteristics in some
applications. The Nb-V-Ti chemistry with 0.05 (max) % Nb, 0.05 (max)%V and
0.02 (max) %Ti is disclosed. A controlled vanadium content facilitated control of
mill load well within the mill capability. Adoption of TMCP technology with air-
cooling during plate rolling facilitated grain refinement and precipitation of Nb &
V(CN) to increase the strength of the steel strips. Carbon and vanadium
contents in steel were optimized in such a manner to achieve both desired
strength with guaranteed Charpy impact energy
Microalloying elements of titanium, niobium, and vanadium are used for ensuring
high YS, low YS / UTS ratio and high impact toughness in the material, addition
of Ni and Cr in the alloy is advocated while a small amount of retained austenite
or bainite in final microstructure is a desirable feature. The balance of the
alloying chemistry is iron, other basic steel making elements such as sulfur,
phosphorous, aluminum and those other incidental impurities commonly found in
these types of steels. The concept of shifting the TTT diagram to the left through
addition of Mo, or B has also been exploited for the above reason. As revealed
through the search, there is no reference; however, for ferrite-pearlite
microstructures based hot rolled steels having high YS (> 500 MPa), YS / UTS
ratio < 0.9 and Charpy impact toughness > 100 J at 0 C.
Vanadium content was kept on higher since addition of vanadium imparts
strength to the steel without increasing the rolling load. Nb & V were added
(0.05% max. each) for achieving both grain refinements due to controlled rolling
and nano-size precipites for precipitation hardening. Vanadium addition intended
to provide precipitation hardening without increasing the mill load. Carbon is
restricted below 0.08% to avoid peritectic reaction during solidification and to
optimized carbon content achieves desired strength without impairing the
toughness property. Nb is added to achieve grain refinement during TMCP rolling
of strips. Ti is added to ensure defect free casting of strips. In view of adverse
segregation tendency and the beneficial solid solution strengthening associated
with Mn, its range was selected to be 1.4 - 1.45%. For enhancing Charpy impact
toughness and % shear area requirement in drop weight tear test, S & P is
restricted below 0.005% & 0.012% respectively.
The hot rolled steel coils in thickness range of 3 - 10 mm having high strength
(YS > 500 MPa) and Charpy impact toughness of 100 J at 0 C finds application in
line pipes for transportation of oil and natural gases, auto chasis, pre-fabricated
building manufacturers etc. Development of such grades has been focus of many
steel makers. In view of the high YS (500 MPa or more) and high toughness
(more than 100 J at 0 C), role of C, Mn and Si towards beneficial solid solution
strengthening, adverse impairment of toughness, and the adverse effect on
segregation were considered. Accordingly, these elements were adjusted in the
range shown in Table 1

The inventive method links the selection of a minimum yield strength: strip
thickness target to a sequence of first casting a shape, e.g., a slab or ingot,
having a controlled alloy chemistry and subsequent controlled rolling into a strips.
It is preferred to continuously cast slabs to fully achieve the benefits of titanium
nitride technology. That is, continuous casting produces a fine dispersion of
titanium nitride particles that restrict grain growth during reheating and after each
austentite recrystallization. Following controlled rolling, the final gauge rolled
plate product is subjected to cooling, either air cooling or accelerated cooling,
depending on the minimum yield strength and strip thickness target and thereby
performing the coiling of the strips at a controlled temperature environment.
The slabs were charged into reheating furnaces and soaked at 1200 - 1280°C for
2 hrs (min). These slabs were then rolled into 3-10 mm thick strips in a plate mill.
During rolling, controlled rolling technology was adopted. During this rough
rolling, the coarse grains of the cast slab are refined by austenite recrystallization
for each rolling pass. The level of reduction can vary depending on the final
gauge plate target and the thickness of the cast slab. The temperature after
roughing rolling is kept between 1050-1080°C. Also keeping finish rolling
temperature (FRT) in the range of 880-860 degree Celsius, coiling temperatures
between 640 - 660 deg Cel coupled with cumulative reduction of more than 67%
deformation. The cumulative reduction more than 67% below temperature of no
recrystallisation in the finishing strands led to desired refinement of ferrite grains
along with nano-sized precipitation of micro alloying elements which in turn
ensured the properties of the hot band.
Innovative alloy design included the consideration of metallurgical requirements
of final HR coils on one hand and the process requirements of steel making,
concast and hot strip mill on the other. BOF-LF-CC defines the broad process
route. Process technology has been developed at BSL, which ensures all the
quality requirements. The steel making and casting combined with reheating,
controlled hot rolling, post finish lamellar cooling followed with coiling comprise
the process technology route.
Further the CaSi treatment during steel making ensures globularisation of
inclusions which helps in improving impact toughness properties. Table 2 shows
the processing parameters of the developed technology along with respective
purpose. Fig. 1 shows a typical globular shape of inclusions observed in the hot
rolled stocks. EPMA analysis confirmed that Ca-Si treatment caused the
globularisation of oxy-sulphide inclusions. Fig. 2 shows the typical ferrite -
pearlite microstructure. Ferrite grain size averaged between 9-11 Dm. Fine
spacing of pearlite and evidence of broken pearlite testified right selection of post
cooling parameters.

Fine dispersion of nano-sized precipitates of microalloying elements and
evidence of dislocation - pinning by these precipitates confirmed that the
selection of rolling parameters viz., reheating, R5 temperature (which is rough-
rolling end temperature), deformation below temperature of no recrystallisation
(Tnr) during finish rolling, post cooling and coiling temperature were properly
selected.
Mechanical properties are shown in Table 3. All the properties are in the targeted
range primarily because of desirable microstructure both at optical and TEM
levels which the process technology could ensure. Hot rolled stocks processed
with recommended parameters showed toughness in excess of 170 J at 0 C with
the ductile brittle transition temperature below - 40 C.

Visual and ultrasonic inspection showed that the rolled strips were free from
surface and internal defects. The tensile properties of strips were evaluated and
were found in the range as shown in Table - 3.
As a summary the innovative steps by following the important innovative
technological intervention resulted in successful development of HR coils
(thickness: 3-10 mm) of microalloyed high strength (> 500 MPa) at BSL with YS
/ UTS < 0.9 and charpy impact toughness > 100 J at 0 C .
Innovative Alloy Design based on consideration of end property of product
and the BOF-CC-HSM route is shown in Table 1.
C was restricted below 0.08% to avoid peritectic reaction during
solidification. In view of adverse segregation tendency and the beneficial
solid solution strengthening associated with Mn, its range was selected to
be 1.4 -1.45%.
For enhancing Charpy impact toughness and % shear area requirement in
drop weight tear test, S & P was restricted below 0.005% & 0.012%
respectively.
Nb & V were added for achieving both grain refinements due to controlled
rolling and nano-size precipites for precipitation hardening.
CaSi treatment during steel making ensured globularisation of inclusions
which helps in improving impact toughness properties.
Reheat Temperature 1250 - 1260 C with 2 hours soaking ensures
dissolution of microalloying elements into austenitic matrix. R5 Temperature
of 1050 + 10 ensured entry of rough rolled transfer bar to finishing strand at
temperatures below temperature of no recrystallisation.
Finish Rolling Temperature between 880 - 860 C, coiling temperatures
between 640 - 660 C, and cumulative reduction more than 67% below
temperature of no recrystallisation in the finishing strands led to desired
refinement of ferrite grains along with nano-sized precipitation of
microalloying elements which in turn ensured the properties of the hot band.
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. The process for manufacturing of a hot rolled high strength steel strip
comprising the steps of:-
charging and casting of the different compositions of raw materials in
basic oxygen furnace;
refining of the castables using LF / vacuum arc degassing (VAD) unit;
continuous casting of the alloy strips;
soaking of strips in reheating furnaces;
controlled rough rolling of the strips in the hot strip mill with hot working of
the cast alloy;
controlled finished rolling of the strips in the hot strip mill;
controlled air-cooling of the steel strips during rolling.
controlled coiling of the finished strips.
2. The process as claimed in claim 1, wherein the said melt consists of in
weight percent of about essentially of, in weight percent, weight percent of about
0.8% (max) Carbon, 1.40% to 1.50% Manganese, 0.35% (max) Silicon, 0.012%
(max) Phosphorus, 0.005% (max) Sulphur, 0.02% - 0.04% Aluminium, 0.05%
(max) Niobium, 0.05% (max) Vanadium and 0.02% (max) Titanium and balance
essentially iron.
3. The process as claimed in claim 2, wherein the said melt is completely
deoxidized with Silicon, Manganese and Aluminium.
4. The process as claimed in claim 1, wherein the strips are soaked at 1200 -
1280°C for 2 hrs in reheating furnaces.
5. The process as claimed in claim 1, wherein the controlled rough rolling is
carried out at temperatures 1050-1080°C and the cumulative reduction of
thickness is more than 67% in a controlled manner below temperature of no
recrystallisation in the finishing strands.
6. The process as claimed in claim 1, wherein the controlled finish rolling is
carried out at temperatures less than 880°C.
7. The process as claimed in claim 1, wherein R5 Temperature is controlled
at 1050+10 in the entry of rough rolled transfer bar to finishing strand at
temperatures below temperature of no recrystallisation.
8. The process as claimed in claim 6, wherein the controlled coiling
temperature is less than 680°C
9. The process as claimed in claim 1, wherein the controlled thermal regime
of rolling and coiling ensures finely dispersed nano sized precipitates of
microalloying elements in the ferrite - pearlite matrix along with globularised
shapes of oxy-sulphide inclusions obtained as a result of Ca-Si treatment.
10. The cost effective as-rolled high strength steel strips comprises essentially
of, in weight percent, weight percent of about 0.08% (max) Carbon, 1.40% to
1.50% Manganese, 0.35% (max) Silicon, 0.012% (max) Phosphorus, 0.005%
(max) Sulphur, 0.02% - 0.04% Aluminium, 0.05% (max) Niobium, 0.05% (max)
Vanadium and 0.02% (max) Titanium and balance essentially iron.
11. The alloy as claimed in claim 1, wherein the said alloy provides good
impact toughness YS: 500 Mpa (min), UTS: 580 Mpa (min), YS/UTS 0.90 max
El: 25% min, Charpy Impact properties: 100 J min at 0°C and % Shear area in
DWTT: 85% min. at 0°C
12. The process for manufacturing of hot-rolled high strength steel strips,
substantially as herein described with particular reference to the accompanying
drawings.
13, The cost effective hot-rolled high strength steel strips, substantially as
herein described with particular reference to the accompanying drawings.

The present invention relates to process for manufacturing of a hot rolled high
strength steel strip comprising the steps of charging and casting of the different
compositions of raw materials in basic oxygen furnace; refining of the castables
using LF / vacuum arc degassing (VAD) unit; continuous casting of the alloy
strips; soaking of strips in reheating furnaces; controlled rough rolling of the strips
in the hot strip mill with hot working of the cast alloy; controlled finished rolling of
the strips in the hot strip mill; controlled air-cooling of the steel strips during
rolling; controlled coiling of the finished strips, and cost effective as-rolled high
strength steel strips produced by said process.