PRODUCTION DE BARRES ET TIGES EN NICKEL

PRODUCTION OF NICKEL BAR AND ROD

Abrégé anglais

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A B S T R A C T
PRODUCTION OF NICKEL BAR AND ROD
The invention concerns a method of producing nickel
steel rod or bar of a composition including at least 4%
by weight nickel and less than 0.2% by weight carbon.
The method comprises cooling a rod or bar of such
composition on leaving a rolling mill at a rate which is
controlled to produce in the cooled rod or bar a micro-
structure which is predominately martensitic.

Revendications

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:
1. A method of producing nickel steel rod or bar
of a composition including at least 4% by weight nickel
and less than 0.2% by weight carbon, which method com-
prises the step of cooling a rod or bar of such composition
on leaving a rolling mill at a rate which is controlled to
produce in the cooled rod or bar a micro-structure which
is predominantly martensitic.
2. A method as claimed in claim 1, wherein the as-
rolled rod or bar is subsequently tempered without inter-
mediate heat treatment.
3. A method as claimed in claim 1, wherein the
nickel content of the steel from which the rod or bar is
produced whilst within the range of 4% to 12% by weight
and wherein the carbon content of such steel is less than
0.15% by weight.
4. A method as claimed in claim 3, wherein the
nickel content of the steel lies within the range of
from 5% to 10% by weight and wherein the carbon content
is less than 0.15% by weight.
5. A method as claimed in claim 4, wherein the
nickel content of such steel lies within the range of
from 8% to 10% by weight.
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Description

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The invention relates to the production
of nickel steel bar and r~d. More especially, the
in~ention relates to the production of such bar and rod
for concrete reinforcement. By nickel steel is meant
a steel containing at least 4% by weight of nickel.
It is well known to ~se nickel steel
plate and sections in structures to be subjected to
low temperatures because of their enhanced toughness
and strength. These properties are generally developed
by an expensive postrolling heat treatment comprising
either double or single normalising followed by
tempering or reheating, ~uenching and tempering.
The normalising or quenching treatment is adopted to
provide a microstructure which, on tempering, gives
the desired combination of properties.
The present invention sets out an
alternative process route particularly suited for
the production of nickel steel bar and rod for
reinforcement of concrete structures to be subjected to
low temperatures, eg temperatures below -40~C.
According to the present invention
there is provided a method of producing nickel steel
bar or rod in ~Ihich a bar or rod of such steel is
cooled on leaving a rolling mill at a rate which is
controlled to produce in the cooled bar or rod a
micro-structure which is primarily martensitic.
The present invention in particular provides
a method of producing nickel steel rod or ~ar of a
composition including at least 4% by weight nickel and
less than 0.2% by weight carbon, which method comprises
the step of cooling a rod or bar of such composition
on leaving a rolling mill at a rate which is controlled
to produce in the cooled rod or bar a micro-structure which
is predominantly martensitic.
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The as-rolled bar o~ rod may subsequently
be tempered
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witho t intermediate heat treatrnerlt.
The nickel steel may inclucle adcli-tions of harclenability
elements such as chromium and rnolybclenum to increase the
hardenability of the rod or bar.
The nickel and carbon contents of the steel from which the
rod or bar is produced may respectively fall within the following
ranges: 4O to 12% ancl less than 0.15% by weight. Preferred
ranges for these elements are nickel 5 or 8 to 10% by weight
and carbon less than 0.10~ by weight.
In one example of a process in accorclance with the
invention billets of a nominally 9~6 nickel steel of the following
composition
C Si Mn P S Ni Al N
0.07 0.25 0.55 0.012 0.010 9.6 0.041 0.0055
were reheatecl at 1250 prior to rolling to 16 and 32 mm ribbed
reinforcing bar on a continuous mill. The bars emerged from the
last rolling stand at a~temperature of 1050C and 1070C respectively
and were subsequently cooled to ambient temperature using a combination
o~ water and air cooling at a rate sufficiently rapid to suppress any
20 extensive formation o~ undesirable transformation products e.g.
ferrite and bainite. Table I below shows the micro-structures
developed by this combination of rolling and cooling conditions:
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TABLE I Size mm Constituents
16 and 32 Martensite 95 ~
Bainite 2- 3%
Austenite I - 3%
The mechanical properties of the as-rolled bars are shown in
Table 2 below:
TABLE 2
Charpy 2 mm V Impacts-Joules Full Section Tensiles
Size
mm
Room O o Yield T ensile Eloll ,a~ioo
Ternperature -120 C -196 C N/mm2 N/n-m2 % on 5d
16 109/ 114 95193 57/63 630 1025 17
32 120/117 74/77 29/29 688 970 14
It can be seen that, even in this condition, the bars provide an
attractive combination of toughness and strength. Tempering of the
as-rolled bars at temperatures between 550-600C results in a further
15 improvement in the toughness of the product as exemplified in the data
shown in Table 3 below:

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TABLE 3
Charpy 2mm V Impac ts - Joules
Si ze
rnm R.T. -120 C -140C -196 C
16 220/213 1~4/200 1~3/180 115/110
32 220/220 196/200 190/ 196 108/77
Table 4 below shows the minimum properties specified in
British Standard BS1501-509 1970 for a steel of similar composi-tion
by the more conventional double normalise or quench and temper
route:
TABI~E 4
Charpy 2mm V Impact - Joules
Yield Tensile El
N/mm2 N/mrn2 % R. T. - 100 C - 196C
525 695 18 67 47 34
15 It can clearly be seen that the invention as disclosed in the
preceding paragraphs provides a product comparable to that achieved
by the conventional and more expensive heat treatment route.
It is to be understood that the invention is applica'ole to the
entire range of nickel steels for low temperature applications
20 ranging from 4~O -to 12% Nickel and that the selection of rolling and/or
controlled cooling conditions will be determined by the specific
composition. The combination of rolling and/or cooling conditions
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is selected to provide a substanticllly martensitic structure. It will
be understood that such a selection can be made from a i<nowledge
of the continuous cooling transformation diagram for a particular
steel taking in-to account the effects of roll finishing temperature.
5 It will also be understood that, where the cooling installation on a
particular mill is incapable of providing sufficiently rapid cooling
to prevent premature transformation, the hardenability of the
base alloy steel may be increased by the addition of hardenability
stabilising elements such as molybdenum and chromium to lower
10 tl-e critical cooling rate.
It will be appreciated that by controlling the cooling rate of
bar and rod directly from the hot rolling mill, conventional heat
treatments including normalising or quenching can be avoided thereby
providing a significant saving in processing costs. CorTtrol of the
15 cooling rate is effected to produce a micro-structure in the as-rolled
bar which consists primarily of martensite with only a smaJI volume
fraction of bainite. This structure is considered ideal for optimal
response to ~he -tempering operation and additionally confers a
combination of properties to the as-rolled product which are
20 sufficiently attractive to permit the product to be used in all
applications other than the very lowest service temperature without
resort to tempering.
. .