FIELD OF THE INVENTION
The present invention relates to hot rolled steel. More particularly, the invention is concerned
about chemical composition of high strength hot rolled steel alloy with low DBTT (Ductile
brittle Transition Temperature) and a method for producing the high strength hot rolled steel
alloy with low DBTT.
BACKGROUND OF THE INVENTION
The demand for steel grades with increased strength levels and well balanced formability has
strongly been increasing over the years in a wide range of application fields such as
automotive and truck but also construction engineering and crane industry. In addition,
toughness of material is another very important parameter for these applications, which is the
ability of a metal to deform plastically and to absorb energy in the process before fracture.
Normally, the strengthening mechanism increases strength of material at the expense of
toughness except grain refinement, which enhances both the parameters concurrently with
lowering of DBTT (Ductile brittle Transition Temperature).
US 2010186856 teaches a steel product or thin steel cast strip including, by weight, less than
0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than
0.01% aluminum, and at least one of niobium between 0.01% and 0.20% and vanadium
between 0.01% and 0.20%, and a microstructure of a majority bainite and acicular ferrite, and
more than 70% niobium and/or vanadium in solid solution. The steel product may have an
increase in elongation and an increase in yield strength after age hardening. The age hardened
steel product may have niobium carbonitride particles with an average particle size of 10
nanometers and less, and may have substantially no niobium carbonitride particles greater
than 50 nanometers. The steel product may have a yield strength of at least 380 MPa, a
tensile strength of at least 410 MPa, or both. The product may have a total elongation of at
least 6% or 10%.
US 6,066,212 teaches that Niobium (Nb) is added to promote grain refinement of the rolled
microstructure of the steel, which improves both the strength and toughness. Niobium carbide
precipitation during hot rolling serves to retard recrystallization and to inhibit grain growth,
thereby providing a means of austenite grain refinement. For these reasons, at least about
0.02 wt % Nb is preferred. However, Nb is a strong BCC stabilizer and thus raises DBTT.
Too much Nb can be harmful to the weldability and HAZ toughness, so a maximum of about
0.1 wt % is preferred. More preferably, the upper limit for Nb content is about 0.05 wt %.
Silicon (Si) is added to steel for deoxidation purposes and a minimum of about 0.01 wt % is
preferred for this purpose. However, Si is a strong BCC stabilizer and thus raises DBTT and
also has an adverse effect on the toughness. For these reasons, when Si is added, an upper
limit of about 0.5 wt % Si is preferred. More preferably, the upper limit for Si content is
about 0.1 wt %. Silicon is not always necessary for deoxidation since aluminum or titanium
can perform the same.
The problems of the prior art are that the strengthening mechanism increases the strength of
material at the expense of toughness except grain refinement, which enhances both the
parameters concurrently. Conventionally addition of micro-alloying elements like niobium,
vanadium and titanium coupled with thermo-mechanical controlled processing lead to grain
refinement in hot rolled strip and plates. However there is limitation of grain refinement to
the tune of 5-6 micron in hot rolled microalloyed steel employing roll finish in austenitic
range. The use of Silicon is also traditionally known and has been used effectively for its
strengthening effect. However in the present claim, silicon in combination with niobium has
been used as grain refiner.
Thus, there is a need to overcome the disadvantages of the prior art. Therefore, to overcome
the problem of the prior art, the inventors have developed the process technology for
production of high strength hot rolled steel alloy with low DBTT and also provided the steel
alloy composition having excellent combination of high strength, formability and toughness
properties together.
OBJECTS OF THE INVENTION
An object of the present invention is to overcome the problems/disadvantages of the prior art.
Another object of the present invention is to provide a high strength hot rolled steel alloy with
low Ductile brittle transition temperature (DBTT).
Another object of the present invention is to provide a method for producing a high strength
hot rolled steel alloy with low Ductile brittle transition temperature (DBTT).
Another object of the present invention is to provide a high strength hot rolled steel alloy (>
500 MPa) with low (<-80 QDuctile brittle transition temperature (DBTT).
Another object of the present invention is to provide a high strength hot rolled steel alloy with
low Ductile brittle transition temperature (DBTT) having ferrite grain size is less than3
microns.
Another object of the present invention is to provide a high strength hot rolled steel alloy with
low Ductile brittle transition temperature (DBTT) having YS/UTS ratio less than 0.9
Another object of the present invention is to provide a high strength hot rolled steel alloy with
low Ductile brittle transition temperature (DBTT) having higher elongation greater than 25%
Yet, another object of the present invention is to provide a high strength hot rolled steel alloy
with low Ductile brittle transition temperature (DBTT) having hole expansion ration greater
than 40% with 14 mm dia spherical punch and greater than 150 % with conical punch.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a high strength hot rolled
steel with low Ductile brittle transition temperature (DBTT), comprising (by weight %):
0500 MPa) hot rolled steel with low DBTT (<- 80
C) of hot rolled steel comprises that LD converter of 300 T is used to make the steel through
Basic Oxygen furnace (BOF). The alloyed steel is heated in LD converter and processed
through ladle refining before being continuously cast in the slab of 210 mm thickness. The
innovative alloy design with reduced Nb and increased silicon has led to substantial reduction
in cost of production. The Concast slabs are processed into 3.8 mm hot rolled coils in hot
strip mill by adhering to finish rolling and coiling temperatures of 860 + 10 C and of 630 +
10 C respectively and minimum draft of 85 % in the finishing zone.
The process steps for production of high strength (>500 MPa) hot rolled steel with low DBTT
(<- 80 C) of hot rolled steel comprising as follows:
1) Heating the elements of alloyed steel in LD converter.
2) Processing of the composition of alloyed steel through ladle refining.
3) Casting continuously the alloyed steel in the slab of 210mm thickness.
4) Processing the concast slabs into 3.8 mm hot rolled coils in hot strip mill at finishing
temperature of 860±10C and coiling temperature of 630±10C respectively.
In the present invention, Carbon, Manganese and niobium are to provide strength in steel,
Aluminium is added for killing oxygen and Silicon is added for grain refinement. By varying
Niobium (Nb) and Silicon (Si) contents from 0.035-0.045 and 0.25-0.35 respectively have
resulted in synergetic effect and contributed to the larger availability of carbon for enhanced
precipitation of Nb(CN) during hot rolling resulting into fine ferrite grains of size 2-3
microns. Ferrite grain size of 2 to 3 microns throughout the thickness could be achieved
uniformly across the thickness of hot band as shown in figure la. Production of such fine
grained steel in industrially produced hot rolled coil is a breakthrough towards development
of cost effective high strength steels (YS > 500 MPa) with low DBTT (<- 80 C) and good
forming and toughness properties. A definite trend emerged while co-relating silicon vis-a-vis
grain size as shown in figure lb, showing an excellent correlation coefficient (R2 = 0.98).
The effect of silicon in presence of niobium has played an important role in achieving the
invention. Silicon enhances the activity of C and N, and enhances precipitation of Nb(CN)),
which in turn resulted in such a unique combination of properties in hot rolled micro-alloyed
steel. Role of Carbonitride precipitation influences microstructural evolution during hot
rolling of micro-alloyed steels. The synergetic effect of silicon in presence of Nb has
contributed to larger availability of carbon for enhanced precipitation of Nb(CN) during hot
rolling resulting into fine ferrite grains. Silicon enhanced the precipitation of Nb(CN)
precipitation and contributed in ferrite grain refinement which in turn contributed in strength
and impact properties concurrently. The Parameters such as strain (0.3-0.4), Inter-pass time
(1.5-2.5 sec) and strain rate (30-55/sec) in the finishing zone have provided enhanced strain
induced precipitation of Nb(CN) which thereby contributed in grain refinement. In plate
rolling, these conditions are not favourable for acceleration of Nb(C,N) precipitation.
Production of such fine grained steel in industrially produced hot rolled coil is a technical
breakthrough.
The properties like tensile property is achieved found to be most interesting especially
because of excellent combination of strength and forming properties, like lower YS/UTS
ratio (<0.9) and higher elongation (>25 %). In general, higher percentage elongation coupled
with lower YS/UTS ratio of sheet material favours an improved strain distribution during
press forming, enabling more complex parts to be formed. The hole expansion ratio, which is
commonly used to measure the formability of hot rolled steel, is found to be appreciably
higher in case of newly developed steel (43-45 %) as compared to that of conventional steel
(32-34 %) as shown herein below table.
The steel is found to posses very low ductile to brittle transition temperature (< - 80 C),
although there are much not appreciable difference observed in charpy impact value with
decreasing the temperature from as high as 30 C to -80 C as in figure 2. The excellent
combination of strength and DBTT is attributed to fine ferrite grain (< 3 micron).
ADVANTAGES OF THE INVENTION
1. Reduction of cost of production.
2. Cost effective high strength steel.
The invention has been described in a preferred form only and many variations may be made
in the invention which will still be comprised within its spirit. The invention is not limited to
the details cited above and all the details may furthermore be replaced with elements having
technical equivalence.
WE CLAIM
1. A high strength hot rolled steel with low Ductile brittle transition temperature
(DBTT), comprising (by weight %):
0