Note: Descriptions are shown in the official language in which they were submitted.
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The production of high strength steel is performed
at present through the following methods :
- a - controlled rolling,
- b - normalization,
- c - conventional quenching and tempering.
For the purpose of increasing the cost/quality ratio
of the product, the method of controlled rolling, for a well
defined range of thicknesses, is preferable as it permits the
best balance between costs for the chemical composition, produc-
tion times and quality.
The normalization method has remarkable limits in
productivity, in a thermal treatment cycle which is performed
after rolling in a 'cold' sheet. In fact, as is well known,
in the controlled rolling method, the mechanical characteristics
are obtained at the same time as the physical transformation
through a thermo-mechanical treatment.
Even for compromise between cost, chemical composi-
tion and mechanical properties, the normalized product is less
advantageous than a product obtained through the controlled
rolling method.
On the contrary, the quenching and tempering treatment
obtains quality levels absolutely higher than those of both
the controlled rolling and the normalization methods, and
further for the same quality has a cost balance for the
chemical composition/mechanical properties much more favourable
than the other two methods.
However, in consequence of the double thermal cycle
on a 'cold' sheet after the rolling and the quick cooling of
the quenching step, the ratio (production cost/quality) is not
competitive with the ratios deriving from the other two
treatments, at 1 ast for the quality levels corresponding to
an ultimate tensile strength (U T S) greater than 60 kgs/mm2
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on thicknesses up to 50 mm for the controlled rolling and
100 mm for the normalization. For higher levels, no recourse
to other methods other than the quenching and temper one is
known in the industry.
Recent studies of the Metallurgy Division of the
British Iron and Steel Research Association and other
laboratories have pointed out the possibility of obtaining,
in particular types of steels, improvements in the strength and
toughness by a quenching treatment performed right after the
controlled rolling (direct quenching), with respect to natural
cooling. The favourable effects remain even after tempering,
so as to consider the characteristics are improved even with
respect to the quenching and tempering method performed through
a prolonged heating in austenization furnaces before the forced
cooling.
That involves the performance of the quenching within
the first seconds after the last passage of the controlled
rolling, within a period of time such as to allow ~he temperature
of the material to remain within the asutenitic range.
The invention has for its object a thermal treatment
of intermediate quenching and quick tempering by induction
and a device for applying said treatment to high productivity
rolling plants for flat products.
According to the invention, the quenching treatment
is effected, independently of the interval of time between
rolling and quenching, through a possible quick heating of the
material in an induction furnace.
The process according to the present invention
comprises receiving a rolled steel plate after it has been
rolled to a final thickness developing an uniform temperature
distribution within the rolled steel plate greater than a
critical temperature of the steel plate, and quenching the
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steel plate while the temperature distribution thereof is
still uniform and still exceeds the critical temperature.
The critical temperature may be Arl or Ar3. If, before
quenching of the steel plate,it cools below the critical
temperature, the method according to the present invention
further comprises heating the rolled steel plate to develop
an uniform temperature distribution therein greater than the
critical temperature. Heating is by fast induction heating,
and after the heating the rolled steel plate is quenched down
to an ambient temperature.
It may be defined as 'intermediate quenching', and
the quenching treatment may be carried out in water with
quenching machines like the-Drever one or the like (e.g. by
the method of a patent by C.S.M., Rome), starting from :
1) Temperatures greater than Ar3, reached during the natural
cooling after the hot rolling in times between 30 seconds
and 10 minutes;
2) temperatures comprised between the interval Ar3 + Arl,
reached during the natural cooling after the hot rolling in
times between 30 seconds and 25 minutes;
3) temperatures greater than Acl, reached through a quick
induction heating starting from temperatures grea~er than Arl
obtained during the natural cooling after the hot rolling
process in times greater than 30 seconds.
On the contrary, the quenching treatment after
heating in a conventional furnace at temperatures Ac3 starting
from room temperature is known, as well as the immediate direct
quenching treatment from the temperature at the end of the
rolling or from the temperature reached after natural cooling
after the hot rolling process in times less than 30 seconds.
The main advantages of the invention are considered
to be :
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the decrease of the problems involved with the controlled roll-
ing, technique resulting from the reduction of the limits of
the rolling requirements to be met,
the total or partial exploitation of the rolling heat and the
héat furnished in a quick heating cycle by an induction furnace,
the execution of tempering from temperatures Arl or Acl.
In order that the invention may be clearly under-
stood, it will now be described, by way of example, with
reference to the accompanying drawings, wherein figs 1, 2 and
3 show diagrammatically the mechanical characteristics of
steels obtained by the method claimed, said characteristics
being compared to those obtainable by the known methods.
Example N 1 C - Mn steel
The steel, whose composition is shown in Table N 1,
was heated up to 1250C, rolled to the thickness of 12 mm
with end temperature.s, the rolling being comprised in the
interval 800 - 1000C.
Table N 1
C Nn Si S P Al
.17 1.60 .30 .020 .012 .04
Table N 2
T Without heating or Heated and homogenized
homogenizationat 900C
Tfl 50 100 150 200 250 300 400 500 600
800 x
900 x x x ' x Y~ x
1000 x ~ x x x x x x
Tfl = rolling end temperature
T = decrease in temperature starting from the rolling
temperature, before -ubjecting the material to
the method claimed.
The sheets obtained were cooled in the air for such
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periods of time as to obtain the ~ T shown in Table N2,
then after the possible heating, quenched and afterwards
tempered at 600C , this temperature was kept for
one hour.
The results are shown in figs. 1 and 2, compared
to those of the direct and classical quenching. Fig. 1 shows
as ordinates the tensile strength Rm and the yield point on
two separate scales, so as to render the diagrarns clearer,
and as abscissa the temperature of the rolling end. Further,
the heavy shaded zones refer to the tensile stress Rm, while
the light shaded ones to the yield points. ~ -
The other symbols denote respectively :
DQR direct quenching, that is performed on the material
' coming out of the rollir~g plant without any intermediate
heating, but within 30 seconds from the'exit. That is a known
method.
IQl : intermediate quenching without heating, according to
the invention.
IQ2 : intermediate quenching with heating, according to the
invention.;
NQ : normal quenching, that is with cooling in the air dowr
to the room temperature and then heating in a furnace, this
obviously is a known method.
~' AR : 'as rolled', that is without any treatment.
The temperatures shown in the shaded zones represent
those of the rolling beginning for the IQ1 treatment and'
those reached by the sheets after rolling for the IQ2 treatment.
Fig. 2 shows as ordinates the resiliency (CVNL
20C) performed on a Charpy test element with a V-shaped
intake, at 20C, the diagrams concerning the resiliency are
heavy-shaded while'the light-shaded diagrams refer to the
transition temperature concerning the level : 3.5 kgs/cm2.
i
Example N 2 'Dispersoids' C - Mn steel
Various steels of this kind were considered, Their
composition is shown in Table N 3. After heating up to
1250C, they were rolled to a 12 mm thickness with end rolling
temperatures between 950 and 1000C, then quenched in line
from 900C and subject to a temper treatment at 600C/one
hour. Fig. 3 shows the results of their mechanical characte-
ristics, compared to those of a burnt steel obtained through
a classical quenching from 900C; with obviously the same
composition.
Table N 3
C Mn Si Nb V N Al
1.071.2/1.3 .25 - - - .030
2.lS1.2/1.3 .30 - - - .030
3.17 1.6 .30 - - - .030
4.071.2/1.3 .30 - - - .030
5.071.2/1.3 .30 .05 - - 030
6.07 1.6 .25 .10 - - .030
7.071.2/1.3 .30 .07 .08 - .030
8.071.2/1.3 .30 - .15 - .030
9.071.2/1.3 .30 - .08.015 .030
~; 10 .071.2/1.3 .30 - .15.015 .030
The same figure shows also, as comparison, the
results of some tests on C - Mn steels.
The present invention considers also an improved
treatment of temper to be performed after the quenching
operation in water.
The invention comprises effecting the temper at
/ Acl temperature on an even moving element through a quick
heating in an induction furnace.
Therefore, as 'quick temper' is defined a heating
and temperature keeping treatment performed wholly in 15
minutes, in competition with the conventional treatment
comprising temperature keeping times, in addition to the
heating times, of about 2 minutes for each mm of thickness.
The results are obtained at the end of two sub-
sequent steps:
l) heating from room temperature until reaching
the surface temper temperature in a section of the furnace
operating through eddy currents in times - 5 minutes;
2) temper temperature keeping for times -- 15
minutes, in a section of the furnace operating through the
conventional method and with a length proportioned to the
permanence time as a function of the element thickness and
translation speed.
B~ usinq this technique, the followinq results
were obtained :
Exam~Ie N3
A 20 mm sheet of the steel of Table N 4 was quenched
from 920C after heating in a conventional furnace. The
subsequent temper treatment was performed in both a two-section
furnace according to the invention with a permanence time lower
than 15 minutes, and a conventional furnace with a permanence
time higher than 30 minutes. Table N 5 shows the results
of that comparison.
Table N 4
C Mn Si S P Al
.12 1.15 .30 .009 .027 .040
Table N 5
- Rs () R () CVN MAX Transition
~ 2 ~ temperature
(kg/mm ) (kg/mm ) (kgm/cm~) 50% Cr (C)
Normal temper
(650C) 46.7 56.3 28.4 -llO
Table N 5 (Cont'd)
Rs (o) R t) CVN MAX Transition
2 2 2 temperature
(kg/mm ) (k~/mm ) (kgm/cm ) 50% Cr (C)
Quick temper
(650C) 46.6 57.9 28.7 - 85
() value on a test element obtained in longitudinal
direction.
The method claimed is intended to be used on sheets
with the following sizes:
- thickness 8 50 mm
- width 1000 4800 mm
- length 5 50 meters
according to two alternative cases.
CASE A : is performed through a plant consisting of :
1. an induction furnace and a quenching machine positioned
in line downstream the hot fairing machine, at a distance from
the finishing plant between 100 and 150 meters. Said induction
furnace consists of multiple sections operating sa as to assure
a complete homogenization of the temperature on the whole
shet, and able of increasing the sheet temperature of 300C
max. in a short time ( ~ 2 minutes).
2. a temper furnace of about 300 meters from the finishing
~plant and consisting of a first portion with induction multiple
:: ~
; ~ sections and a second portion formed by-a conventional furnace
with a length between 80 and 150 meters.
The main feature of the present invention is the
use of induction furnaces consisting of multiple sections
of inductors passed through by the moving piece. The variable
electromagnetic field originated by said inductors causes
the sheet heating through the phenomenum of the eddy currents.
As for the flow of the material in production, five
possible cycles are considered :
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1. untreated material;
2. burnt material through the treatment exploiting the
rolling heat;
3. only quenched material through the treatment exploiting
the rolling heat;
4. only tempered material,
5. material normalized through the use of a temper furnace.
1. Untreated material : it follows the normal cycle of rolling,
fairing, hot shearing, plate cooling and finishing, by-passing
the quenching step.
2. Burnt material : after the rolling, fairing and quenching
steps, it goes direct to the temper step and than to the
shearing, cooling and finishing.
3. Only quenched material : it follows the same cycle as
the burnt material until the quenching, than the same cycle
as the untreated material.
4. Normalized or 'only tempered' material : it comes from
the stock finishing and goes on to the temper or normalization
step and the subsequent ones.
CASE B : is embodied through a plant consisting of :
1. an induction furnace and a quenching machine positioned
not in line and down-stream the fairing machine and the hot
shearing machine, connected to the roller course in line with
the reversible through a transferring plate;
2. a temper furnace in line with the quenching machine,
formed as in case A;
3. a conventional heating and austenization furnace positioned
upstream the quenching machine.
As for the flow of the material in production, six
possible cycles are considered :
1. untreated material;
2. material burnt through the treatment exploiting the
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rolling heat;
3. material only quenched through the treatment exploiting
the rolling heat,
4. témpered material;
5. normalized material,
J 6. material burnt after heating from room temperature.
1. Untreated material : it follows the normal cycle without
involving the treatment line.
2. Burnt material (exploiting the rolling heat~ : it follows
the cycle corresponding to Case A with interposed the steps
or transfer from the reversible line to the treatment one, and
to the finishing cycle.
3. Only quenched material : it comes from the conventional
heating furnace and from the rolling line and goes on to the
quenching step and the finishing cycle.
4. Normalized or 'only tempered' material : it comes from the
stock finishing and goes on to the temper or normalization
step and the subsequent ones.
5. Material burnt after heating from room temperature : it
comes from the conventional heating furnace and goes on to
the quench, temper and subsequent steps.
It is to be understood that the invention is not
limited to the examples shown. It is intended to cover all
modifications and equivalents within the scope of the appended
claims.
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