PROCESS TECHNOLOGY FOR THE PRODUCTION OF AS-ROLLED HIGH
STRENGTH STEEL PLATES WITH IMPACT TOUGHNESS GUARANTEE
FIELD OF THE INVENTION
The present invention is directed to a method for production of high strength steel
plates in as-rolled condition and a product there from and in particular to a
process technology will help in producing high strength steel plates in hot rolled
condition using a controlled alloy chemistry and controlled rolling and cooling
conditions to produce an as-rolled and cooled high strength steel plate capable of
meeting mechanical and compositional requirements for a number of ASTM
specifications which are normally produced through quenching & tempering
technology. The present invention is also related to high strength high resistant
low alloy steels adapted to use for the material in manufacture of earth moving
equipments, heavy machineries, bridges, impellers dams, bridges etc.
BACKGROUND ART
Market survey has shown that there is huge demand for high strength as-hot
rolled steel plates (YS: 550 MPa min) in India with guaranteed sub-zero impact
property. Therefore it is a prior need to develop suitable technology for
production of such high strength steel plates in as-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 bridge, pole 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
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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 are 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. Table 1 lists the ASTM specifications with
comparative results.
These high strength ASTM specifications are not without their disadvantages.
First, processing whereby the hot rolled, quenched and tempered product is
energy intensive. Second, these quenched and tempered grades are limited by
plate length due to furnace length restrictions. In other words, only certain length
plates can be heat treated following the quenching operation since the furnaces
will accept only a set length, in some instances, only up to 600". Bridge builders
particularly are demanding ever-increasing lengths (to reduce the number of
splicing welds required and save fabrication cost) of plate for construction; such
demands are not being met by current plate manufacturing technology for high
strength steels.
The use of accelerated cooling and hot rolling is disclosed in 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
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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.
SUMMARY OF THE INVENTION
For the above and other purposes, and notably for attainment of high strength
low alloy steel products having superior properties like strength, sub zero impact
properties, 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 micro alloying elements Niobium, Vanadium
and titanium 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 micro alloying metals and to
elements such as Niobium, Vanadium and Titanium.
The process for manufacturing of a cost effective High Strength steel in as-rolled
condition as disclosed in the present specification comprising the steps of:
Charging and Casting of the different compositions of raw materials in Basic
Oxygen furnace; refining of the castables using Vacuum Arc Degassing (VAD)
Unit; Continuous casting of the alloy slabs; Soaking of slabs in Reheating
furnaces; Controlled rough rolling of the slabs in the Plate mill with Hot working of
the Cast alloy; Controlled finished rolling of the slabs in the Plate mill; air-cooling
of the steel plates during rolling for facilitating the grain refinement.
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The cost effective High Strength steel as disclosed consist essentially of, in
weight percent, weight percent of about 0.16% to 0.18% Carbon, 1.50% to 1.60%
Manganese, 0.40% to 0.60% silicon, 0.02% (max) Phosphorus, 0.01% (max)
Sulphur, 0.02% (min) Aluminium, 0.03% to 0.04% Niobium, 0.14% to 0.16%
Vanadium and 0.01% to 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 0.04%, 0.16% and 0.02%
respectively, it is possible to obtain yield strengths (e.g. in both directions) in the
range of YS: 562-611 MPa, UTS: 713-742 MPa, El: 17-30% Av. Charpy Impact
properties: 30-101 J at -20°C, respectively, in all types of plate mills including the
plate mill with limited rolling load capacity of 4500 tons max.
A second important aspect of the present invention is that with the stated micro
alloyed 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 sub zero impact properties, Abrasion
resistance property and good toughness without special additions or processing /
rolling.
The heat treatment processes involved in imparting ultra-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. In
spite of these heat treatment operations, these steel generally suffer from low
toughness and poor ductility. Moreover, these steels are highly susceptible to
Temper-embitterment. Hence special caution is required in selection of
tempering temperature and processes.
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Accordingly, the another exemplary aspect of the present invention is to eliminate
the expensive hardening and tempering treatment process.
Another object of the present invention is to provide a cost effectiveness by
producing the alloy in all types of plate mills including the plate mill with limited
rolling load capacity of 4500 tons max.
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.
Another objective of the present invention is to provide flexibility in terms of
impurity contents i.e. both S & P to the tune of 0.01 and 0.02% max.
As per another embodiment of the present invention there is provided thermo-
mechanically controlled processing, keeping finish rolling temperature (FRT) in
the range of 800-830 degree Celsius coupled with < 20% deformation.
Another objective of the present invention is to provide 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 plates with
polygonal ferrite-pearlite microstructures.
The invention also includes a plate made by the inventive method as an as-hot
rolled and high strength steel plate, not a quenched and tempered plate product.
The plate can have all of: (1) a plate thickness of upto 0.25 mm and a yield
strength of 562 - 611 Mpa, UTS of 713 - 742 Mpa, Elongation of 17 - 30% with
Charpy Impact properties: 30-101 J at -20°C. The alloy chemistry or composition
is also part of the invention, in terms of its broad and preferred ranges.
In the following description, reference is made to the accompanying drawing, and
which is shown by way of illustration to the specific embodiments in which the
invention may be practiced. The following illustrated embodiments are described
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in sufficient detail to enable those skilled in the art to practice the invention. It is
to be understood that other embodiments may be utilized and that structural
changes based on presently known structural and/or functional equivalents may
be made without departing from the scope of the invention.
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 ACCOMPANYING DRAWINGS
Reference is now made to the drawings of the invention wherein:
FIG. 1 is a flowchart for the manufacture of the high strength steel;
FIG. 2 is a chart showing the weight of different constituents of the alloy in
percentage.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a significant advancement in producing high
strength steel plate 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 plate in an
as-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 plates. 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 plate. 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
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silicon to achieve favourable characteristics in some applications. The Nb-V-Ti
chemistry with 0.03-0.04Nb, 0.12-14%V and 0.01-0.02%Ti is disclosed. A high
vanadium content facilitated control of mill load well within the plate 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 plates. Carbon and vanadium contents in steel were
optimized in such a manner to achieve both desired strength with guaranteed
Charpy impact energy
To explore the possibility of producing such high strength steel plates with
guaranteed sub zero impact property in all types of plate mills including the plate
mill with limited rolling load capacity of 4500 tons max., it was found necessary to
design a suitable chemistry. Taking into consideration the influence of various
micro alloying elements on the strength, impact properties and rolling load,
following steel chemistry was designed:
Micro alloying elements of titanium, niobium, and vanadium are also used along
with an effective amount of nitrogen. 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.
Vanadium content was kept on higher since addition of vanadium imparts
strength to the steel without increasing the rolling load. It was also decided to
optimize the carbon content in order to achieve desired strength without
impairing the toughness property. Nb was added to achieve grain refinement
during TMCP rolling of slabs. Ti was added to ensure defect free casting of slabs.
More than twenty heats have been made at Bhilai Steel Plant in a 150 ton Basic
Oxygen Furnace. The heats were processed through Vacuum Arc Degassing
(VAD) Unit to lower the sulphur content and to minimize the dissolved gases.
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During VAD treatment, the vacuum level was maintained at ~1 mbar. These
heats were cast into slabs of 242x1500 mm size in continuous casting machine.
The inventive method is tailored in both alloy chemistry and controlled
rolling/cooling to produce a discontinuous yielding plate to assure that the
minimum yield strengths and required tensile strengths in the various ASTM
specifications are met in the final gauge steel plate.
The inventive method links the selection of minimum yield strength: plate
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 plate.
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
austenite 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 plate thickness target.
Slabs were charge into reheating furnaces and soaked at 1230±10°C for 3 hrs.
These slabs were then rolled into 25 mm thick & 2500 mm wide plates in a two-
stand four high plate mill. The flow chart for producing the aforesaid product is
shown in Fig 1. During rolling, controlled rolling technology was adopted.
During this rough rolling, the coarse grains of the as-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 as-cast slab.
In roughing rolling, thickness of slab was brought down from 242 mm to 65 mm
in 8 passes with reduction varying between 10-20%. The temperature after
roughing rolling is kept between 1000-1030°C. During finish rolling, total
reduction of ~62% was given in 8 passes to bring down the thickness of plates
to 25 mm. The reduction per pass varied between 5-15% during finish rolling.
Finish rolling temperature was maintained at 800-830°C. The maximum load is
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kept at 3800 tons during rough rolling and 4200 tons during finish rolling and
therefore, this process can be used in the plate mills with 4500 tons rolling load
capacity.
Visual and ultrasonic inspection showed that the rolled plates were free from
surface and internal defects. The tensile properties of plates were evaluated
and were found in the following range: YS: 562-611 MPa, UTS: 713-742 MPa,
El: 17-30% Average. Charpy Impact properties: 30-101 J at-20°C
Keeping in view its end-application for manufacture of earthmovers and
excavator components, the abrasion property of this steel was evaluated in
Wear & Friction Machine by pin and disk method. The results revealed that the
abrasion resistance of this steel (Wt. Loss: 0.0388 gm. at 5000 revolutions) is
superior to that of ASTM A 517 Gr.F Q&T steel (Wt. Loss: 0.077 gm. at 5000
revolutions)
The through thickness microstructure of these plates comprised polygonal ferrite-
pearlite near surface and banded ferrite-pearlite in the mid thickness. These
plates were supplied to a customer for manufacturer of penstocks and
performance of these plates was found to be satisfactory during fabrication.
Based on successful performance, repeat order of this steel has been received
and commercial production of these high strength plates has commenced at
Bhilai Steel Plant under the brand name of SAILMA 550HI.
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
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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.
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We claim:
1. The process for manufacturing of a as-rolled high strength steel plates
comprising the steps of:-
charging and casting of the different compositions of raw materials in
Basic Oxygen furnace;
refining of the castables using Vacuum Arc Degassing (VAD) Unit;
continuous casting of the alloy slabs;
soaking of slabs in Reheating furnaces;
controlled rough rolling of the slabs in the Plate mill with Hot working of the
cast alloy;
controlled finished rolling of the slabs in the Plate mill;
air-cooling of the steel plates during rolling.
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
0.16% to 0.18% Carbon, 1.50% to 1.60% Manganese, 0.40% to 0.60% silicon,
0.02% (max) Phosphorus, 0.01% (max) Sulphur, 0.02% (min) Aluminium, 0.03%
to 0.04% Niobium, 0.14% to 0.16% Vanadium and 0.01% to 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 3, wherein the said melt is refined for
making final addition of alloying elements by a preferred Ladle / VAD treatment
the vacuum level is maintained at ~1 mbar.
5. The process as claimed in claim 1, wherein the slabs are soaked at
1230±10°C for 3 hrs in reheating furnaces.
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6. The process as claimed in claim 1, wherein the controlled rough rolling is
carried out at temperatures 1000-1030°C and the level of reduction of the as-cast
slab is varied between 10-20% depending on the final gauge plate target and the
thickness of the as-cast slab.
7. The process as claimed in claim 6, wherein the controlled finish rolling is
carried out at temperatures 800-830°C and the level of reduction of the as-cast
slab is varied between 5-15% depending on the final gauge plate target and the
thickness of the as-cast slab.
8. The process as claimed in claim 6, wherein the controlled Thermo-
mechanically (TMCP) rough rolling is carried out at 3800 tons (max).
9. The process as claimed in claim 7, wherein the controlled Thermo-
mechanically (TMCP) finish rolling is carried out at 4200 tons (max).
10. The process as claimed in claim 1, wherein the controlled air cooling for
the slabs is done during the rolling to facilitate grain refinement of the slab.
11. The process as claimed in claim 1, wherein the said heat cycle does not
consist of any expensive hardening or tampering processes.
12. The cost effective as-rolled high strength steel plates comprises
essentially of, in weight percent, weight percent of about 0.16% to 0.18%
Carbon, 1.50% to 1.60% Manganese, 0.40% to 0.60% silicon, 0.02% (max)
Phosphorus, 0.01% (max) Sulphur, 0.02% (min) Aluminium, 0.03% to 0.04%
Niobium, 0.14% to 0.16% Vanadium and 0.01% to 0.02% (max) Titanium and
balance essentially iron.
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13. The alloy as claimed in claim 12, wherein the said alloy provides good
impact toughness YS: 562-611 MPa, UTS: 713-742 MPa, El: 17-30% average,
charpy Impact properties: 30-101 J at -20°C.
14. The process for manufacturing of a as-rolled high strength steel plates,
substantially as herein described with particular reference to the accompanying
drawings.
15. The cost effective as-rolled high strength steel plates, substantially as
herein described with particular reference to the accompanying drawings.

To,
The Controller of Patents,
The Patent Office, Kolkata.
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The present invention relates to The process for manufacturing of a as-rolled high strength steel plates comprising the steps of charging and casting of the different compositions of raw materials in Basic Oxygen furnace; refining of the castables using Vacuum Arc Degassing (VAD) Unit; continuous casting of the alloy slabs; soaking of slabs in Reheating furnaces; controlled rough rolling of the slabs in the Plate mill with Hot working of the cast alloy; controlled finished rolling of the slabs in the Plate mill; air-cooling of the steel plates during rolling and the cost effective as-rolled high strength steel plates comprises essentially of, in weight percent, weight percent of about 0.16% to 0.18% Carbon, 1.50% to 1.60%
Manganese, 0.40% to 0.60% silicon, 0.02% (max) Phosphorus, 0.01% (max) Sulphur, 0.02% (min) Aluminium, 0.03% to 0.04% Niobium, 0.14% to 0.16%
Vanadium and 0.01% to 0.02% (max) Titanium and balance essentially iron.