TITLE:
Hot Rolled Quenched and Nonisothermally Partitioned Low Alloy Steel with High
Strength, Impact Toughness and Abrasion Resistance
Field Of the lnvention
The invention relates to a method of manufacturing a low alloy steel grade with
improved strength, abrasion resistance and impact toughness and the invention
further relates to a Quenching and non-isothermal Partitioning (Q&P) process for
producing a low alloy steel grade with improved strength, abrasion resistance and
Impact toughness.
Background of the Invention
The process of wear involves gradual and progressive loss of material from the
surface of a component due to relative motion between the actrve and counter body
species either by mechanical action or chemical reaction. Depending upon the
environment of application, the material interacts with different kinds of abrasives
and as a result wears out Therefore, abrasive wear resistant materials become
highly desirable in the industrial applications such as agriculture, earth moving,
excavation, mining, mineral processing, transportation etc.

The increasing cost of replacing worn parts in mining and earth moving equipment
confronts a continual challenge to materials development. As per the report of a
national survey in 1997, for UK industries who have wear problem, the cost of wear
was typically about 0.25% of their turnover Out of this, abrasive wear alone
contributes to around 63%[1]. This undesirable loss, particularly due to the process
of wear, can be reduced to at least half by replacing the currently used materials with
new ones and/or by selecting a better design of the components. However, a new
design of a component is often coupledwith other associated risk, and also does not
always make an economically viable alternative, To the contrary, the use of new
material can be considered as a better option.
Components in wear resistant applications, particularly in mining and earthmoving
sectors, are essentially required to have adequate abrasion resistance along with the
ability to resist chemical attacks As impact loading is mostly unavoidable in such
applications, the requirement for good impact toughness also becomes one of the
major concerns. Typically, the material with high abrasion resistance is hard and
brittle Hence, these materials are limited by low impact toughness, which leads to
the generation of cracks while experiencing sudden impact loading[2]. Thus, there is
a need to develop new grade of material which is highly resistant to abrasion
together with good impact toughness,

At present, the materials in abrasion resistant applications are either used in full or
tempered martensitic condition[3]. Fully martensitic structure can provide high
hardness without any additional heat treatment after hot strip mill However, the
carbon content in these materials is generally high which subsequently pose
problems during welding and also results in reduced impact toughness The
tempering process is used to improve toughness with a compromise on hardness
Also, this process requires an additional facility after hot strip mill, which adds up to
an extra cost The heat treatment process for direct quenching and tempering
technique is schematically shown in Fig. 1
Another application of hot rolled steel containing retained austenite could be in
automotive. The development of advanced high strength steels (AHSS) is one of the
major focus areas in automobile industries The excellent combination of strength
and formability in AHSS allows the use of thinner gauges as the structural parts in
automobile for weight reduction, improvement in fuel efficiency, reduction in
environment emissions and enhancement in collision safety.The presence of
retained austenite in microstructure delays the crack prpogation and hence,
expected to improve the ductility, work hardening and shock absorption capacity of
steel.

Objects of the Invention
An object of the present invention is to propose a method of manufacturing a low
alloy steel grade with improved strength, abrasion resistance and impact toughness
Another object of the present invention is to propose a process for quenching and
non-isothermal partitioning process for low alloy grade steel.
Further, object of the present invention is to propose a process which restricts
progressive loss of material from the surface of a component due to relative motion
between active and counter body species.
Still further object of the present invention is to propose a method of manufacturing a
low alloy steel grade wherein the austenite structure is retained in the microstructure
which delays the crack prpogation and hence, expected to improve the ductility, work
hardening and shock absorption capacity of steel.

Brief Description of the Invention
According to this invention there is provided a method for manufacturing a low alloy
steel grade, the method comprising:
casting a steel with composition in wt. % Carbon: 0 10 - 0.25, Manganese: 1.0 - 1.8,
Silicon: 0.20 - 0 6, Aluminium: 0.10 - 0.30, Chromium: 0.15 - 0.25, Molybdenum:
0.10 - 0.15, Nickel: 04 - 0.6, Vanadium: 0-01, Carbon equivalent: 0.34 - 0.69,
balance being Iron and residual impurities,
homogenizingthe steel by austenitizing at 1150-1200°C for 100-120 minutes;
rollingthe steel about 90%; and
quenching the steel on a run out table to 200°C-250°C at 50-70°C/s,
The present invention also focuses on stabilizing an optimum amount of austenite
through Quenching and non-isothermal Partitioning (Q&P) process Two different
alloys were designed and cast at 80 kg scale. The cast steel slab with 83 mm
thickness was homogenized at 1200°C for 2 h and then hot rolled to 6 mm in 10
passes. Immediately after hot rolling, the steel plate was quenched to - 200-250°Cat
cooling rate of 50°C and 70°C/s followed by slow cooling (0 25-0 50°C/min) to room
temperature The result shows that stabilization of retained austenite is possible in
one of the alloys, which depicts higher impact toughness, good strength and superior
abrasion resistance.However, the hardness is somewhat compromised due to the
presence of retained austenite, ferrite and a littletempering of martensite. The other

alloy does not show any presence of retained austenite The possible reason could
be an excessive amount of tempering during slow cooling, which is evident in the
microstructure. The tempered structure of this alloy resulted in improved abrasion
resistance, superior strengthand hardness However, the impact toughness was
reduced in comparision to alloy containing retained austenite.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows schematic of (a) hot rolling and direct quenching; (b) tempering
process.
Figure 2 shows a process with various steps of Quenching and non-isothermal
Partitioning (Q&P) process,
Figure 3: shows typical layout of hot rolling for Quenching and non-isothermal
Partitioning (Q&P) process along with hot rolled coil cooling profile.
Figure 4: shows the actual process schedule for both the alloys,
Figure 5: shows the SEM Micrograph of Alloy-1 quenched at (a) 50oC/s ;(b) 70°C/s
Figure 6 shows the SEM Micrograph of Alloy-2 quenched at (a) 50°C/s ; (b) 70°C/s
Detailed Description of the Invention
The present inventions based on studies and experimentations noted that the
requirement of good impact toughness along with superior abrasion resistance can
be fulfilled by obtaining some amount of retained austenite along with martensite in

the microstructure. This can be achieved through Quenching and Partitioning (Q&P)
process, Q&P involves partial transformation of austenite to martensite, followed by
carbon diffusion from martensite into untransformed austenite, which further
stabilizes the remaining austenite to room temperature The final microstructure
obtained from aforementioned process, constitutes finely distributed retained
austenite between the martensitic laths.
The concept of conventional Quenching and isothermal Partitioning (Q&P) process
was proposed by Speer et al in 2003[4]. The process requires isothermal holding of
material, containing a combination of martensite and unstable austenite, at relatively
high temperature for carbon partitioning. Till date, this process has been
investigated by several research groups mainly targeting cold rolled products for
automotive applications Also, the above process calls for an additional facility after
hot strip mill to perform heat treatment,
The idea of carbon partitioning during non-isothermal slow cooling of hot rolled coil
was proposed by Thomas et al in 2009[5]. This process does not require any
additional facility to carry out heat treatment It mainly utilizes the coil heat for carbon
partitioning Hence, it is an integrated Quenching and non-isothermal Partitioning
(Q&P) process.
A process (200) comprising various steps shows the Quenching and non-isothermal
Partitioning (Q&P) process in Fig. 2.

At step (204) steel is casted with composition (all in wt %) 0.10 - 0.25 carbon, 1.0 -
1.8 Manganese, 0.20 - 0.6 Silicon, 0.10-0.30 Aluminium, 0.15 - 0.25 Chromium,
0.10 - 0.15 Molybdenum, 0.4 - 0.6 Nickel, 0-0.1 Vanadium, 0.34 - 0.69 Carbon
equivalent, balance being iron and residual impurities (in wt.%),
At step (208) the steel alloy is homogenized by austenitizing at 1150-1200X for 120
minutes and then rolled from 83 mm to 6 mm thickness in 10 passes.
At step (212) the steel alloy is quenched on run out table to 200°C-250°C at 50-
70°C/s and
At step (216) steel alloy is cooled to room temperature by extremely slow rate of
0.25-0.50 °C/min.
The carbon partitioning from martensite to austenite happens during slow non-
isothermal cooling of the hot rolled coil after quenching at the run out table The
austenite has sufficiently higher carbon content as compared to the initial alloy
carbon concentration (i e Cy> Ci) and remains stable. It does not transform to
martensite on cooling to room temperature. Here it is worth mentioning that the
quench temperature (QT) is same as the coiling temperature (CT) and the
partitioning start temperature (PT). Thus, a single temperature (i e, QT = CT = PT)
controls both the amount of untransformed initial austenite as well as the driving

force for carbon partitioning that is in other words the partitioning kinetics Hence, the
choice of optimum quench temperature is fairly more critical in the non-isothermal
Q&P process as compared to the isothermal Q&P process, where QT and PTi are
different The thermal profile (T vs t) of a hot rolled coil is also shown in Fig. 3, which
was calculated by B. Nelson of ArcelorMittal[5]. Here it can be noted that
temperature drop with time is extremely slow. Hence, depending upon the quench
temperature and alloy composition, it may provide sufficient driving force for carbon
partitioning.
The next step was to design an alloy composition considering the following factors:
1. Higher Mstemperature to provide more driving force for carbon partitioning at
the same quench temperature.
2. Low carbon equivalent to eliminate the problem of weldabihty and low carbon
content toimprove impact toughness.
3. Higher hardenability to eliminate bainite or pearlite formation during
quenching below Mstemperature,
4 Addition of less micro-alloying elements to have more solute carbon for
partitioning.
5. The optimum amount of Si and/or Al addition to prevent carbide formation
during carbon partitioning.

The chemical composition of the proposed alloys, their carbon equivalent
(CE) values and Mstemperature (calculated empihcally)are shown in Table 1. In the
Alloy-1, small amount of Si and Al was added to prevent carbide formation Earlier
research shows that 1.5wt% Si addition is required to completely avoid carbide
formation[6]. However, the present investigation does not aim at stabilizing very high
level of retained austenite Hence, to avoid harmful effect of higher Si addition, such
as brittleness and poor surface properties[7], 0 3 wt% Si was added to check the
possibility of retained austenite stabilization Vanadium was included in order to
eliminate the detrimental effect ofnitrogen. The Alloy-2 contains an increased amount
of carbon and Mn Ni was added to improve impact toughness and also to act as an
austenite stabilizer.
Table 1: Chemical composition, carbon equivalent (CE) and Ms temperatures of
Alloy 1 & 2.


Theoretical calculation of retained austenite variation with quench temperature was
performed using the model developed by Speer et al [4]. Based on the results of
these calculations a quench temperature was selected which predicted the maximum
amount of retained austenite. In the present case the temperature was - 200°C, For
both the alloys, mechanical properties were evaluated for sample quenched to
200oC with quenching rate of 50-70°C/s followed by carbon partitioning through
extremely slow cooling.
Mechanical properties (on ASTM standard samples) and the retained austenite
volume fraction (through XRD) for above mentioned experiments are shown inTable
2
In case of Alloy-1, Q&P treated sample shows the presence of retained austenite
The run out table quenching at 50°C/s led to formation of 40-50 % ferrite along with
martensite and retained austenite. However, at 70°C/s the ferrite fraction was
significantly reduced. The microstructure for both cooling rates is shown in Figure 5
The retained austenite lead to higher impact toughness, good strength and superior
abrasion resistance However, the hardness value is somewhat compromised
In case of Alloy-2, the retained austenite is not present in the Q&P treated sample
The run out table quenching at 50°C/s led to formation of small amount of ferrite,
tempered martensite and bainite, if any. The run out table quenching at 70°C/s led to
formation of tempered martensite and bainite, if any. The presence of ferrite was not

observed for quenching at 70oC/S as in case of alloy-1. The micro-structure for both
the cooling rates is shown in Figure 6. The tempered structure of this alloy resulted
in improved abrasion resistance, superior strengthand hardness, However, the
impact toughness was reduced in comparision to alloy containing retained austenite-
Table 2: Mechanical properties and retained austenite volume fractions of Alloy 1 &
2

With reference Table 2 it is could be well accepted that the low alloy steel have the
following properties:
Having 0-8% (by volume) austenite and varying amount of carbides, ferrite,
bainite and martensite.
Having abrasive wear volume loss (in 10 mins) for low alloy steel is 210-250
mm3 on a dry sand rubber wheel test machine.

HavingCharpy impact toughness of low alloy steel at room temperature is 20-
70 J (for sub size sample with 5×10 mm2 cross section).
Having hardness (at 1 kg load) of low alloy steel is 300-500Hv
Having Yield Strength of low alloy steel is 800-1450 MPa
Having ultimate Tensile Strength of low alloy steel is 950-1550 MPa.
HavingTotal elongation of low alloy steel is 11 -15 %.

WE CLAIM:
1. A method for manufacturing a low alloy steel grade, the method
comprising:
casting a steel with composition in wt % Carbon: 0.10 - 0,25,
Manganese: 1,0 - 1.8, Silicon: 0.20 - 0,6, Aluminium: 0.10 - 0 30,
Chromium: 0.15 - 0 25, Molybdenum. 0.10 - 0.15, Nickel: 0.4 - 0.6,
Vanadium: 0-0.1, Carbon equivalent: 0.34 - 0.69, balance being Iron
and residual impurities;
homogenizingthe steel by austenitizing at 1150-1200°C for 100-120
minutes;
rollingthe steel about 90%; and
quenching the steel on a run out table to 200°C-250°C at 50-70°C/s.
2 The method as claimed in claim 1, wherein the cooling the steel to room
temperature at a rate of 0 25-0.50 °C/min,
3 The method as claimed in claim 1, wherein the composition is C-0 20, Mn-
1.50, Si- 0.3, Cr-0 2, Mo-0.1, Ni-0.5, CE-0.54.
4, A low alloy steel grade with improved strength, comprising:
Hardness (HV 1 kg): 290-500; Retained austenite (%): 0-8%; Abrasive Wear
volume loss (mm3): 210 - 250, Charpy Impact toughness at room temperature
(J): 20 - 70, Yield Strength (MPa)- 800 - 1450, Ultimate Tensile Strength
(MPa): 950 - 1550, Total elongation (%): 11 - 15.0.

5 The process as claimed in claim 4, wherein the low alloy steel grade
comprises 0-8% (by volume) austenite and varying amount of carbides,
ferrite, bainite and martensite