Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The presen-t inverl,ioll re]ates to a metho~ oi trea~;iDg
rolled steel. produc-ts~ the method bei.ng applicable to
concrete~reililorci.ng rod, wire rod, or bars o-f circular
or non-circular cross-section, all these being referred ~o
as "rods" below, and being appli.cable -to sections
co~stituted by one web or by seve:ral in-tegral webs, the
term "sections" also being used.in this specification to
incl.ude rolled p.roducts such as sheet bars and sheets.
The main quali-ties required by users of rolled steel
products, among others, are as high as possible a ].imit
of elastici-ty for the grade of steel used, as well as
(depending on the circ-~mstances and the use for which the
~ products is intended) sa-tisfactory fatigue strength,
: ductility, weldability, or drawability.
~o this end, we have already suggested subjecting a
rolled product emerging from the finishing s-tand of the
rolling mill to cooling by means of a fluid so as to
produce martensitic and/or bainitic quenching of the
sur~ace layer of all or part of the product. Moreover,
we have already suggested that i-t is desirable that, at
the outlet of the fluid cooling zone, the non-quenched
portion of the rolled product is at a temperat-ure sufficient
to permit, during subsequent air cooling~ tempering of
the surface layer of martensite and/or bainite to take
place.
~he cooling fluid employed for carrying out the
method is usually water with or without conventi.onal
additives, or aqueous of mineral salts, for example. ~he
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fl.uid mc-~y be a mist, for example obtained by s~sp*llding
water in a gas, or it may be a gas, such ac s-team (wa-ter
vapour).
~rom -the practical view poi.nt, desired cooling of
the rol].ed products depends on the cooling d.evices used,
alld on suita.ble choice of the leng-th and the flow rate
characteristics of the cooling means.
The present invention relates particularly to the
determination of the opera-ting conditions of quenching and
~ 10 self--tempering of the surface layer of the rolled products,
,~ as referred to above.
In practice, the method in question is performed in
an install.ation whose main elements (finishing stand of
-the rolling mill and air cooling region) are located at
predetermined fixed positions, this determining the available
~' length Or the quenching zone. It is only rarely that it
is possible to adapt the whole installation to the possible
requirements of the method.
In general, the speed and the temperature of the rolled
, 20 products at the outlet of the last stand of the rolling mill
are data of the process too. On the other hand, the method
is applicable to one product at a time, the dimensions of
the product being known as well as the composition of the
steel, and thus its CCl (continuous cooling transforma-tion)
diagram. Knowing this diagram, it is possible to determine
a nu~ber of conditions governing the treatment of the
product in question, such as the temperature at which
formation of martensite occurs and the maximum time available
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fo.r performing su:rface quenching, particularly to -the
desired depth.
~o determine the cha:racteristi.cs of the cooling
devices, such as the cooli.ng ram~s or sprays, and the flow
rates of the fluid to be applied by means of such devices,
it is possible to draw curves of the temperature gradien-ts
in the core and the skin of the rolled produ.ct subjected
to the treatment in question, and thus the amoun-t of heat
to be removed may be determincd. From known da-ta of the
treatment installation and the product to be -treated and
by estimating the cooling requirements to be me-t, it is
then possible to de-termine the characteristics of the
cooiing devices and the flow rates of the fluid applied by
the devices.
; 15 The present invention relates to a method which is
particularly suitable for subjecting the products to
martensitic and/or bainitic surface quenching followed by
self-tempering, and adapted to ensure ready and effective
course of the treatment. The method comprises determining
an important datum to choose the conditions for carrying
out the treatment.
The method is based on the observation of the evolution
of the skin temperature of the product starting from the
end of the martensitic and/or bainitic quenching: during
the period following quenching, the skin temperature rises
(the temperature at the core continuously decreases af-ter
the product has emerged from the last stand of the rolling
mill). The skin temperature and the core temperature in a
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gi.ven crosx-section, developing independently of one
anothcr~ converge towa.rds a point called the "equalizatior.
tempelat-lre" from wh^re the -two curves contiIlue
substc~ntially parallel to one another. Ihe "equaliza-tion
temperature" is readily dete:rmined since it is the
temperat-ure after which the skin -temperature starts
: decreasing again: it is thus amaximum in the curve of the skin
temperature agains-t time.
It should be noted that th.e e~ualization temperature
is not necessarily that prevailing throughout the cross-
` section, because a difference of many degrees, e.g 40 to
50C, might exis-t between -the skin and the core.
; ~'he method of treating rolled steel products according
to the present invention is characterised in that, as a
convergence point for the curves of the course of the
temperature at the skin after quenching and the temperatures
at the core during cooling, one chooses a tempera-ture in
the range of 600C to 730C.
According to a particular way of carrying out the
- 20 method of the present invention, one chooses the level of
the equalization temperature, in the range 600 to 730C,
from a dlagram showing the course of the elastic limit as
a function of the equalization temperature. We have
unexpectedly found tha-t a direct relationship exists
between the equalization temperature and the mechar.ical
properties of the product subjected to surface quenching
and self-tempering as described above, whatever the grade
of the steel, even in the presence of a]loying elements
(e.g. ~b).
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- ~he de<,i.red. col~ver.genc:e of the temperat;-u~cs a-'- theskin a~te:r quenc~ing and a-t the core during cooling i.s
obtair~.ed by suitably choosing -the cooling devices and -the
flow rates of the cooling fluid.
,.
; 5 F~MPIE
Steel rod was treated, having a diameter of 14 mm
and containing, by weight:
carbon 0.1 5%
manganese 0.97~o
silicon 0~05%O
~he temperature at the end o~ the rolling operation was
950C. (~he untreated rod, i.e. air cooled without surface
quenching and self-tempering, had an elastic limit of
33 kg/mm .)
When treated according to the method in accordance
with the present invention, the rod had an elastic limit
of 46 kg/mm af-ter treatment in conditions such that the
equalization temperature was 680C, and had an elastic limit
of 49 kg/mm2 when the equalization temperature was 650C.
A particularly advantageous modification of the method
of the invention is easy to carry out, simple, and effective.
According to such modifications, one determines the surface
temperature to be presented by the said rolled product at
a given place along its path so that the skin temperature
after quenching and the temperature of the non-quenched
part during cooling converge to a value ranging from 600C
to 730C, one measures the surface temperature of the
product at the said given place, and one adjusts the cooling
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conditions in the insta]..lation so as to ~nainta.in the
sllrface temeratl~.re oi -the product in a range sat:isfyirlg
the conditlons for convergence of the temperatures.
Moreover, according to this variant, one determines
the surface temperatuxe the rolled pxod.uct has to possess
at the said given place, while taking into account, in
particular, the dimensions of -the rolled product and the
dura-tion of dwelling of the product in the quenching
devices.
` 10 ~urthermore, one preferably chooses as the place
where the surface temperature of the rolled produc-t is
; me&sured, a region located at the inlet of a still-air
cooling region.
~inally, it has been found to be advantageous to adjust
the cooling conditions of the rolled product by regulating
~` the flow rate of the fluid in the cooling devices and/or
the length oi` the cooling ra p.s or sprays.
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