IMPROVED PROCESS FOR PRODUCING HIGH STRENGTH STEEL PLATES
WITH IMPROVED IMPACT VALUES USING RELAXED ROLLING
CONDITION
INTRODUCTION TO THE FIELD OF INVENTION
This invention relates to an improved process for producing high strength steel plates
with improved impact values. The high strength steel plates with improved impact values
are achieved by using modified alloy steel composition, and rolling at temperatures
higher than the conventional 800*'C, which can be up to 880 "C
This invention more particularly relates to a process for producing high strength steel
plates with improved impact values, using modified alloy steel composition combined
with relaxed rolling conditions. The alloy steel composition is modified by addition of
specific quantity of Nb and Ti, which helps in grain refinement thereby increasing the
strength and toughness of the steel.
PRIOR ART AND DRAWBACKS
The major limitation of underpowered mills in rolling high strength steel plates e.g.
higher grades of API are the inability to give higher reductions per pass and to roll at low
finish rolling temperature (800"C). In addition, the absence of accelerated cooling adds to
the problem of achieving desired grain size of 4-8 micron, which is optimum with respect
to strength, toughness and ductility.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose an improved process for producing
high strength steel plates by using a modified steel alloy composition.
It is another object of this invention to combine the use of modified steel alloy
composition with modified rolling conditions.
It is thus another object of this invention to propose dual improved conditions in the
manufacture of high strength steel plates in underpowered mills.
BACKGROUND OF THE INVENTION
After intensive studies, experiments and evaluation tests, it was found that the rolling at
the temperatures higher than the conventional 800"C even up to SSO^C it is possible to get
desired properties. We have also studied the behavioral characteristics of the various
alloying elements inter reactions, which was possible only after intensive research and
experiments.
We have also been able to achieve less deformation per pass at higher rolling
temperature, which is unique for producing high strength steel plates and has not been
anywhere at all.
Furthermore, our studies have been oriented to achieve restricted austenite grain size
during slab reheating
Our studies were oriented towards evaluating the behaviors of Nb and Ti and it has been
found that Niobium upto 0 05% increases strength and toughness through grain
refinement. Beyond 0.05% Nb increases the toughness of the steel and facilitates relaxed
rolling of plates (higher finishing temperature). It is made possible as austenite non-
crystallization temperature (1 nr) is raised due to Nb addition and it maximize %
reduction below Tnr which is prerequisite of thermomechanical control processing
(TMCP).
Titanium combines with Nitrogen to form TiN precipitates, which restrict austenite grain
size during slab reheating. It also reduces the free Nitrogen in the steel and improves
toughness property.
BRIEF STATEMENT OF THE INVENTION
Thus according to this invention there is provided a process for the production of high
strength steel plates with improved impact values which comprises.
(a) providing a molten steel having the following alloy steel composition comprising
in wt. %
C: 0.07-0.10% Mn; 1.40-1.50% Si; 0.25-0.35% S; 0.01%max
P; 0.02% max Al; 0.025-0.045% Nb: 0.06-0.09% 11:0.01-0.02%
(b) subjecting the same to Calcium treatment by adding Calcium silicate during
refming, whereby the calcium treatment is done to modify the inclusion
population in steel;
(c) casting into a slab,
(d) avoiding re-oxidation during casting using argon injection as necessary;
(e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating
furnace which serve as heating and buffer zone , the casts slabs are heated up to
pre determined rolling temperature in the furnace for the efficient rolling. The
schedule of the controlled rolling of the steel slab is as follows:
Soaking temp. And time : 1250+ lOT/ 5''^hrs.
Rough rolling : 1150-1050 ^C (10 passes)
Finish rolling : 1000-850"C (5 passes)
Cumulative reduction in the finish zone : > 65%
Reduction in final pass : > 10%
such that there is obtained a low reduction per pass, high finish roUing start
temperature and absence of accelerated cooling followed by;
(f) subjecting the finish rolled plate to air cooling wherein the finish rolling is
preferably carried out at temperature in the range of 830-880''C,
With low reduction per pass, high finish rolling start temperature and absence of
accelerated coling, ferrite grain size of 4-8 micron was achieved and mechanical
properties obtained upto 15mm thickness were: YS: 480Mpa min; UTS; 540 Mpa
min.; % El; 33 min and Charpy impact; 150 J min at O^'C.
DETAILED DESCRIPTION OF THE INVENTION
In thermomechanical treatments of low carbon microalloyed steels, the main goal is to
promote austenite grain refinement in the recrystalltzation temperature range, combined
with straining at temperatures below the startup of deformation-induced precipitation.
The latter process is very effective because, during low temperature hot deformation,
precipitates such as carbonitride particles stabilize the dislocation substructure and retard
recrystallization, thus facilitating ferrite refinement.
The main purpose of this steel processing is to produce the fine and homogeneous ferrite
grains as well as a high volume fraction of carbide/nitride precipitates during or after
austenite-ferrite transformation. This process results in superior mechanical properties
such as high strength, toughness, and good ductility and weldability. These purposes can
be achieved by controlling the processing stages as schematically described below:
1. The reheating prior to rolling;
2. The controlled rolling of austenite in the recrystallization region, T > Tnr
(non-recrystallization temperature);
3. The controlled rolling of austenite in the non-recrystallization region, T < Tnr
(non-recrystallization temperature);
4. The controlled cooling after rolling.
All the above processes interact in determining the final steel properties, and widely
known as Thermomechanical Controlled Processing (TMCP).
Recent high strength structural steel plates are produced by applying thermomechanical
controlled processing (TMCP), which includes controlled rolling followed by accelerated
cooling (AcC) or quenching and tempering (Q-T). Especially, advance in the accelecated
cooling process has enabled higher strength by transformation strengthening under higher
cooling rate. Low carbon steels are usually used for the steels that are required of high
strength and higher levels of base metal toughness and HAZ (Heat Affected Zone)
toughness, as well as good weldability.
Micro-alloying elements, such as Nb and Ti, are added for preventing grain coarsening
during heating at austenization temperature by pinning effect of carbo-nitride particles.
Nb is also a quite important element in controlled rolling process because solute Nb can
increase the non re-crystallization temperature and insure the grain refining of
transformed bainite microstruaure after controlled rolling and accelerated cooling.
Strength and toughness of steel is strongly affected by its transformation and precipitation
behavior, therefore, controlling those phenomena based on complete understanding of
transformation and precipitation mechanisms should be a key issue for materials
designing of high strength steels.
EFFECTS OF MICRO ALLOYING ELEMENTS
Carbon: Most of the microalloyed steels developed for forging have carbon contents
ranging from 0.07 to 0.10%, which is enough to form a large amount of pearlite. The
pearlite is responsible for substantial strengthening. This level of carbon also decreases
the solubility of the micro alloying constituents in austenite.
Niobium and Titanium: Formation of carbonitnde precipitates is the other major
strengthening mechanism of microallo\ed forging steels. Niobium and titanium enhance strength
and toughness b\- providing control of austenite grain size.
Niobium Microalloyed Steels: like vanadium, niobium increases yield strength by
precipitation hardening; the magnitude of the increase depends on the size and amount of
precipitated niobium carbides. However, niobium is also a more effective grain refiner
than vanadium. Thus, the combined effect of precipitation strengthening and ferrite grain
refinement makes niobium a more effective strengthening agent than vanadium. The
niobium addition is 0.06 to 0.09%, which is about one-third the optimum vanadium
addition. Strengthening by niobium is 35 to 40 MPa per 0.01% addition.
Titanium Microalloyed Steels: Titanium in low-carbon steels forms into a number of
compounds that provide grain refinement, precipitation strengthening, and sulfide shape
control. However, because titanium is also a strong deoxidizer, titanium can be used only
in fully killed (aluminum deoxidized) steels so that titanium is available for forming into
compounds other than titanium oxide. Commercially, steels precipitation strengthened
with titanium are produced in thicknesses up to 9.5 mm in the minimum yield strength
range from 345 to 550 MPa with controlled rolling required to maximize strengthening
and improve toughness. ¦
Titanium-Niobium Microalloyed Steels: Although precipitation-strengthened titanium steels
have limitations in terms of toughness and vanability of mechanical properties, research has
shown that an addition of titanium to low-carbon niobium steels results in an overall
improvement in properties. Titanium increases the efficiency of niobium because it combines
with the nitrogen-forming titanium nitrides, thus preventing the formation of niobium nitrides.
Manganese: Manganese is used in relatively large amounts (1.4 to 1.5%) in many microalloyed-
forging steels. It tends to reduce the cementite plate thickness while maintaining the interlamellar
spacing of pearlite developed; thus high manganese levels require lower carbon contents to retain
the large amounts of pearlite required for high hardness. Manganese also provides substantial
solid solution strengthening, enhances the solubility of vanadium carbonitridcs, and lowers the
solvus temperature for these phases.
Silicon: The silicon content of most commercial microalloyed forging steels is about 0.30%;
some grades contain up to 0.70%. Higher silicon contents arc associated ^vith significantly higher
toughness, apparently because of an increased amount of ferrite rclati\c to that formed in fcrritc-
pearlite steels with lower silicon contents.
Sulfur: It is undesirable as it reduces impact properties.
«
Phosphorus: Phosphorous is considered undesirable because they reduce elevated
temperature ductility and hence affect the stress rapture strength and thermal fatigue life
An excessive content of Phosphorous in steel makes it impossible to prevent defects form
being formed.
Aluminum: Aluminum is important for austenite grain size control in micro allo\ed steels. The
mechanism of aluminum grain size control is the formation of aluminum nitride particles
Aluminium is an element of deteriorating the index of cleanliness of steel. Moreover, Al
causes a nozzle to be stopped when a base material is produced in the continuous casting
Therefore it is necessary to set the Aluminium content not more than 0.05%. On the other
hand. Aluminium is effective for deoxidation, so that the Aluminium content should be
preferably not less than 0.02%,
PROCESSING DETAILS
Steel making: The steel is made through the Basic Oxygen Process, where pure Oxygen
is blown into a bath of molten blast-fijrnace iron and scrap. The Oxygen initiates a series
of intensively exothermic reactions, including the oxidation of such impurities as Carbon.
Silicon, Phosphorous and Manganese. Most of the alloying additions and deoxidation
carried out at Converter shop. The crude steel tapped from the converter contains
impurities, which are not desired in the final steel chemistry; hence, the steel is processed
through Steel Refining Unit where final additions are made to have desired chemistry.
Steel is purged by argon to facilitate floatation of inclusions and uniformity of chemistry.
Argon bubbling is applied to homogenize the steel composition and for avoiding re-
9
oxidation during casting. Calcium Silicate is added for inclusion modification Calcium
forms compounds with AI2O3, which are liquid and float to the top. Sulphur forms
Calcium Sulphide, which are of globular, shape The adverse affect of Sulphur is taken
care by this. ¦ • ^ .
Rolling: The normal controlled rolling process consists of three steps. The first step is
known as the roughing stage and is characterized by the application high strain degrees to
the rolling stock; this is possible due to the high rolling temperature. The second step is
knowTi as the holding stage; at that time the rolling stock is only cooled at the roller table,
without being submitted to hot rolling until its temperature reaches an ideal value to
resume the forming process. That is, there is no hot rolling at all during this step. The
third step consists in the finishing stage of the rolling stock, which now is under a lower
temperature. .As the hot strength is very high, the strain degree applied in each pass
became very low.
During the holding stage of a rolling stock other slabs are roughened. This concomitant
process, known as tandem rolling, increases the productivity of the plate mill line.
Formerly the rolling stock thickness after the first controlled rolling stage was defined
accordingly to the final plate thickness.
The control of grain size at high austcnitizing temperatures requires as fmc a grain boundan.-
precipitate as possible, and one which will not dissolve completely in the austeniie. c\cn at the
highest working temperatures (12()0-1300°C). The best grain refining elements are \crv strong
carbide and nitride formers, such as niobium, titanium and \'anadium, also alumiiuini that forms
onl\- a nitride. As both carbon and nitrogen are present in control-rolled steels, and as d\c nitrides
are even more stable than the carbides it is likel\- that the most effective grain refining compounds
are the respective carbo-nitrides, except in the case of aluminum nitride. As a result of the
combined use of controlled rolling and fine dispersions of carbo-nitrides in low allov steels, it has
been possible to obtain ferrite grain sizes between 5 and 10 micron.
The effect of the finishing temperature for rolling is important in determining the gr^in
size and, therefore, strength level reached for particular steel. It is now becoming
common to roll through the transformation into the completely ferritic condition, and so
obtain fine sub grain structures in the ferrite, which provide an additional contribution to
strength. Alternatively, the rolling is finished above the y/a transformation, and
increasing the cooling rate alters the nature of the transformation. Slow rates of cooling
obtained by coiling at a particular temperature will give lower strengths than rapid rates
imposed by water spray cooling following rolling.
Hence, in the modern control-rolled micro-alloyed steels, there are at least three strengthening
mechanisms, which contribute to the final strength achieved. The relative contribution fi^om each
is determined by the composition of the steei and, equalK' important, tlie details of the
thermomechanical treatment to which the steel is subjected. Firstly, there are the solid solution
strengthening increments from manganese, silicon and uncombined nitrogen. Secondly, the grain
size contribution to the yield stress is shown as a ven,- substantial component, the magnitude of
which is ver\' sensitive to the detailed thermomechanical history. Finally, a t>pical increment is
dispersion strengthening. The total result is a range of yields strengths between about 350 and 500
MPa..
Micro-alloyed steels produced by controlled rolling are a most attractive proposition in many
engineering applications because of their relatively low cost, moderate strength, and ven good
toughness and fatigue strength, together with their ability to be readily welded. They have, to a
considerable degree, eliminated quenched and tempered steels in many applications.
ADVANTAGES
With low reduction per pass, high finish rolling start temperature and absence of
accelerated coling, ferrite grain size of 4-8 micron was achieved and mechanical
properties obtained upto 15mm thickness were: YS; 480Mpa min; UTS; 540 Mpa min.,
% El: 33 min and Charpy impact: 150 J min at O^'C.
We Claim;
(1) A process for the production of high strength steel plates with improved impact
values which comprises. "
(a) providing a moUen steel having the following alloy steel composition comprising
in wt.%
«
C: 0,07-0.10% Mn: 1.40-1.50% Si: 0.25-0.35% S: 0.01%max
P: 0.02% max Al; 0.025-0,045% Nb; 0,06-0,09% Ti:0,01-0,02%
(b) subjecting the same to Calcium treatment by adding Calcium silicate during
refining, whereby the caJcium treatment is done to modify the inclusion
population in steel;
(c) casting into a slab;
(d) avoiding re-oxidation during casting using argon injection as necessary;
(e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating
furnace which serve as heating and buffer zone , the casts slabs are heated up to
pre determined rolling temperature in the furnace for the efficient rolling. The
schedule of the controlled rolling of the steel slab is as follows.
Soaking temp. And time : 1250+ lOT/ 5'^hrs,
Rough rolling : II50-1050T (10 passes)
Finish rolling : 1000-850 T (5 passes)
Cumulative reduction in the finish zone : > 65%
Reduction in final pass ; > 10%
such that there is obtained a low reduction per pass, high finish rolling start
temperature and absence of accelerated cooling followed by;
(f) subjecting the finish rolled plate to air cooling wherein the finish rolling is
preferably carried out at temperature in the range of 830-880 "C.
(2) A process as claimed in claim I, wherein, the air-cooling stage produces a steel plate
with the following characteristics:
low reduction per pass, high finish rolling start temperature and absence of
accelerated coling, ferrite grain size of 4-8 micron was achieved and mechanical
properties obtained upto 15mm thickness were: YS: 480Mpa min; UTS: 540 Mpa
min.; % El: 33 min and Charpy impact: 150 J min at 0°C.
(3) A process for the production of high strength steel plates with improved impact values
substantially as herein described.

The present invention relates to a process for the production of high strength steel
plates with improved impact values which comprises:
(a) providing a molten steel having the following alloy steel composition comprising
in wt.%
C: 0.07-0.10% Mn: 1.40-1.50% Si: 0.25-0.35% S: 0.01%max
P: 0.02% max .M: 0.025-0.045% Nb: 0,06-0.09% Ti:0.01-0.02%
(b) subjecting the same to Calcium treatment by adding Calcium silicate during
refining, whereby the calcium treatment is done to modify the inclusion
population in steel;
(c) casting into a slab:
(d) avoiding re-oxidation during casting using argon injection as necessary;
(e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating
furnace which serve as heating and buffer zone , the casts slabs are heated up to
pre determined rolling temperature in the furnace for the efficient rolling. The
schedule of the controlled rolling of the steel slab is as follows:
Soaking temp. And time : 1250± 10oC/ 5½ hrs.
Rough rolling : 1150-105oC (10 passes)
Finish rolling :1000-850 oC (5 passes)
Cumulative reduction in the finish zone : > 65%
Reduction in final pass : > 10%
such that there is obtained a low reduction per pass, high finish rolling start
temperature and absence of accelerated cooling followed by;
(f) subjecting the finish rolled plate to air cooling wherein the finish rolling is
preferably carried out at temperature in the range of 830-880 oC.