Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 Field of the Invention
2 Thi~ invention relates to heat treated steel strapping,
3 and more particularly to a method of and an apparatus for
4 heat treating steel strapping, and the improved strapping so
produced.
6 Backqround of the Invention
7 Steel strapping is formed by slitting cold rolled steel
8 strip into the required width and is used in a variety of
9 applications which require a range of properties
Generally, the properties which must be considered when
11 produclng strapping are tensile strength, ductility, notch
12 properties and work hardening. These properties are
13 dependent on the composition of the steel and the heat
14 treatment proce~ses applied to the strappinq.
The minimum tensile strength of steel strapping varies
16 between 500 and 1250 MPa. Strapping having tensile
17 strengths in the range 500 to 800 ~Pa is manufactured and
18 sold by the applicant as 'standard' strapplng and strapping
19 havlng tensile strengths in excess of 300 MPa is
manufactured and sold by the applicant as '~uper' strapping.
21 Standard strapping is generally formed from low carbon
22 steels and may be used in its cold rolled and slit form
23 without heat treatment in applications requiring moderate
2~ strength levels, for example in the securing of cardboard
cartons to pallets. In some lnstances standard strapping is
26 formed from medium carbon steels and ls sub~ected to a
27 stress relief annealing treatment or a blueing heat
28 treatment in order to improve ductllity.
29 Super ~trapping ls qenerally formed from medium carbon
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1 steel~ and the strapping i~ ~ub~ecte~ to heat treatment to
2 provide the required properties. Super strapping is used
3 in heavy duty applications requirlng medium to high tensile
4 strength and good ductility, notch propertles ~nd work
hardening. Uses lnclude unitising of steel pipe into
6 bundles, the fastening o~ heavy loads to pallets and
7 containing high density wool and cotton bales.
8 The conventional heat treatment process for super
9 strapping, which is a vèrsion of the so-called Austemper
process~ compris~s:
11 (a) heating cold rolled steel strapping (generally
12 having a carbon content between 0.20 and 0.60~ to between
13 80QC and 900, to transform the structure to austenite,
14 (b) fast cooling the strapping in a lead or salt bath
to a temperature between 350 and 500C, to initiate
16 tra~sformation from austenite to bainite,
17 (c) air cooling the strapp~ng for a short period of
18 time to allow transformation of any remaining austenite, and ~
19 (d) quenching the strapping to ambient temperatures. ;~-
It is known that bainite has acceptable properties for
21 medium to high tensile strength strapping. However, the
22 Austemper process has a number of disadvantages.
23 Fir~t, there is a ~ubstantlal capital coRt associated
24 wlth the use of lead, as well as costs to replace lead lost
25~ ~through oxidation and lead 'idrag-out'~ on the strip, costs
2~ as~ociated wlth loss of product due to lntermlttent lead
27 contamlnation of the strip, cost of maintenance of the lead
28 bath~ and costs as30ciated with minimislng environmental and
29 health problems generally associated w1th lead.
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1 Second, the speed of the heat treatment process is
2 1 imited by the cooling power of the lead bath and the need
3 to allow sufficient time at the transformatlon temperature
4 range for transformation of au~tenite to bainite. The
required increa~e in the length of the lead quench bath
6 necessary to allow sufficient time at the quench temperature
7 to enable complete transformation of austenite to bainite at
8 higher speeds would be cost prohibitive.
9 A third disadvanta~e is associated with the need to use
sufficiently high carbon and manganese le~els to avoid
11 martensite formation during heat treatment with the
12 countervailing requirements to keep the analysis lean to
13 minimise steelmaklng problems. In this regard, it is the
14 de~ire of the steel maker to keep the carbon content of his
steel as low as possible to avoid steel making problems.
16 Howe~er, the lower the carbon content, the more difficult it
17 is to produce bainite because the temperature at which
18 martensite forms increases, and the Austemper proce~s
19 becomes less and les3 useful.
Another heat treatment process for producing hlg~er
21 strength steels, known as the Continuous Annealinq line
22 process, may appear at first sight to overcome certain of
23 the problems associated with the Au~temper proces~, but the
24 process still has some shortcoming~ The proces~ involves a
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25 to 40 ~ec heat up perlod, a 10 to 120 sec 30aklng period
: 26 followed by a 0.5 to 30 sec coolinq period. This process
27 results in a dual phase ferrite/marten~ite steel, which
28 requlre~ a ~oaklng period of at least 10 seconds for Rtable
29 formation of ferrlte and autenite phases which tranYform
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1 under fast cooling to ferrite and marten~ite~ The ma~or
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2 streng~hening factor is the amount of hard marten~ite phase
3 (15 to 60%) which may be assisted by ferrite strengtheners
4 such as cold worked structure. ~ 15~ structure would
S require other strengthening factors to achleve properties
6 which are achieved according to the present invention.
7 Summary of the Invention
8 The object of the present invention is to provide an
9 improved 3teel strapping and method of and apparatus for
producing the strapping by-an improved heat treatment
11 process, which at least ameliorates the disadvantages
12 described in the preceding paragraphs and which results in a
13 treated steel strapping havlng a novel microstructure and
14 irnproved properties.
In accordance with the present invention there is
16 provided a method of heat treatlng cold rolled steel
17 strapping comprising, rapidly heating the strapping to the
18 dual phase temperature range, with little or no soaking, and
19 rapldly cooling the strapping to form a microstructure
comprising a matr1x of recovery annealed cold worked ferrite
21 contalning martensite and carbides dispersed throughout the
22 matrix.
23 The term "dual phase" as used herein is under~tood to
~24 mean the pha~e equllibrium region where au~tenite and
25~ ferrlte phases co-exist.
26 In another aspect, the invention provide~ a heat
27 treated cold rolled steel strapping which is characterized
~; 28 by a micro~tructure compri~ing a matrix of recovery annealed
29 cold worked ~errite containing martensite and carbides
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1 dispersed through the matrix.
2 The martensite and carbldes preferably comprises less
3 than 20~ ~y volume of the microstructure.
4 The composition of the steel preferably comprises less
than 0.2~C and is characterised by alloying elements which
6 retard recrystallisation and act as precipitation
7 strengtheners, for example titanium and preferably also
8 Niobium. Titanium may be present in the range 0.06-0.15%
9 and preferably 0.08% whlle Niobium may be present in the
range 0.02 to 0~05% and preferably about 0.04%. The steel
11 also preferably contains manganese in the range 1 to 2%,
12 preferably about 1.45% and silicon in the range 0.2 to 0.4%,
13 preferably about 0.33%.
14 The method according to the inYention results in a tri-
p h a s e r e c o v e r y a n n e a l e d c o l d w o r k e d
16 ferrite/martensite¦carbide steel and ls characterised by a
17 short heating/soaking cycle. This result~ in only some
18 carbides with favourable compositions transforming to
19 austenite and thus to martensite on rapid cooling. ~any
carbides go throuqh the transformation without appreciable
21 change. With the microalloying elements (such as Ti, Nb3
22 present, thls cycle result~ in a tri-phase structure of
23 recovery annealed cold worked ferrite containing marten~ite
24 in the region of 5% and carbides ln the region of 10~.
In each of the above aspects, the carbides are pre~ent
26 in the form of flne spheroidal cementite and fine fragmented
27 cementite. The fine fragmented cementite is a consequence
28 of the rapid heating step and the ab~ence of any apprec~able
29 soaklng.
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2 In accordance with the present invention there is also
3 provided a heat treatment line for cold rolled steel
4 strapping comprising, heating means to rapidly heat the
cold rolled steel strapping to the dual phase temperature
6 range with little or no soaking, and cooling means to cool
7 the strapping to form a microstructure comprising a matrix
8 of reco~ery annealed cold worked ferrite containing
9 martensite and carbides dispersed throughout the matrix;
wherein said heating means comprises a series of sol~noid
ll induction heating coils through which the strapping passes
12 and said cooling means comprises a series of water nozzles
13 directed at either side of the strapping as it emerges from
14 the coils.
Since the Austemper process can be used only to form
16 a structure containing bainite, a new type of heat
17 treatment line which enables the required short rapid
18 heating time and rapid cooling necessary to achieve the new
19 microstructure is provided by the present invention. In
its presently preferred form, the heating line comprises a
21 series of solenoid induction heating coils followed by a
22 rapid cooling station including a series of water nozzles
23 directed at either side of the heated strapping as it
24 emerges from the heating coils.
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25~ ~ The heating period is selected to provide a balance
26 between the consumption of power by the heating coils and
27 the required speed which the heat treating line is to
: 28 operate. In most existing plants, the speed of operation
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2 will be dictated by considerations relating to the coiling
3 and painting plants which are usually already present at
4 the plant and accordingly substantial speed gains will not
be possible. However, in the case of a fresh site, line '.
6 speeds of up to 600m/min will be possible using the
7 apparatus to be described further below. Thus, heating
8 periods as low as two seconds and as high as 16 seconds may
9 be required, but
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1 using heating rates of between ~0 to 140C per second, and
2 preferably about 100C per second, the most likely heating
3 period range should be about s~x to ten seconds. The heating
4 period comprises little or no soaki~g period although
periods of from one to several seconds may occur without
6 adverse results.
7 The cooling rate should be sufficiently high to ensure
8 that the required regions of martensite are formed in the
g matrix. A cooling rate greater than 900C per second, and
preferably desirably at least 1000C per second should
11 achieve acceptable results.
12 Brief Description of the Drawings
13 Further detailed description of the present invention
14 will now be provided wlth reference to the accompanying
drawings in which:
16 Figure 1 is a schematic diagram showing a preferred
17 embodiment of the hsat treatment line;
18 Figure 2 shows one preferred cooling arrangement for
19 the line of ~igure 1;
Figure 3 shows schematically the process stageq for a
21 typical continuous annealing line and for the line embodying
22 the invention;
~3 Flgures 4A and 4B respectively show transmision
24 electron micro-grap~s (X 4100 and X 25,000 respectively) of
2S the micro-structure after heat treatment.
26 ~hc~leeio~ of Preferred Embodiment
27 The present invention is based on the realization that
28 steel having a mlcrostructure compris~ng a matrix of
29 recovery annealed cold worked ferrite with martensite and
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1 carbides dispersed throughout the matrlx exhibits ~uitable
2 properties for u~e as 'super' strapping. As a consequence,
3 the micro-structure ls an acceptable substitute for balnite
4 which is the predominant constituent in strapping formed by
the Austemper processO
6 In he preferred embodiment the microstructure is
7 formed by a heat treatment method which is based on the use
8 of induction heating to heat the strapping.
9 With reference to Figure 1, in the heat treatment line
of the preferred embodiment, the cold rslled steel strapping
11 2 is fed from a coil unwinding station 3 through an
12 induction heating station 5 and a cooling station 7 to a
13 coil winding station 9.
14 The induction heating station 5 comprises a number of
solenoid induction heating coils 1 1, for example six,
16 connected in serles and arranged to allow the strapping to
17 pass through the coils. Each induction heating coil 11 is
18 preferably connected to a 5 to 25 kilohertz Statipak STK 4
19 power unit manufactured by Inductoheat Pty. Ltd. It will be
~0 appreciated that any other ~uitable induction heating coil
~1 and power supply combinatlon may be used although a 5 to 25
22 kilohertz power supply is preferred.
23 As shown in greater detail in Figure 2 of the drawings,
24 the cooling statlon 7 compr~ses a plurality of nozzles 10
posltioned withln a houslng or tank 12 to direct sprays of
26 water onto the surfaces of the strapping 2. In the
27 em~odiment shown, the nozzles 10 are arranged in four groups
28 A to D with the nozzles 10 in groups A and B being angularly
29 ad~usted ~o as to be directed away from the heatlng coil~
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1 11, that ls, in the direction of travel of the strip 2, and
2 the nozzles 10 ln groups C and D being flxed perpendicul;arly
3 to the surface~ of the strapping 2~ The angular adj ustment
4 of the nozzleq in groups A and B reduces the l~kelihood that
S cooling water will travel along the strapping and enter the
6 induction heating coils 11.
7 The mains water supply to the headers supporting the
8 nozzles 10 includ~s separate control valves 13 for the upper
9 and lower headers so that flow to the upper and lower
noz~les 10 may be separately controlled. The number of
11 nozzles 10 and the controlled flow rate are selected to
12 achieve the desired coollng rate discussed in greater detall
13 below.
14 In use, the cold rolled steel strapping 2 fed from the
coil unwindlng station 3 ls heated to the dual phase
16 temperature range as it passes through the solenoid
17 induction coils 11 and ls then quenched by water sprayed at
18 the cooling station 7.
19 It is preferred that the composition of the steel is
selected to comprise less than 0.2~C and alloying elements
21 which retard recrystallisation and which act as
22 precipitation strengtheners, such as tltanium and niobium
23 In one Example, the steel ha~ the followlng composition:
24 C: 0.16% P: 0.023% Mn 1.45% Si 0.33% S: 0.010% Ni
0.028~ Cr 0.030% Mo:0.005% Ca 0.013% Al 0.029~ Nb:0.041
26 ~i 0.080% N 0.0075%
27 In the above Example, the values given for each element
28 are not critlcal or es~entlal. For example Tl may vary
2g between 0.06% and 0.15% while Nb may vary between 0.02~ and
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1 0.05%, Mn may vary between 0.5~ and 2~ and Sl may vary between
2 0.2~ to 105~- ~
3 The solenoid induction heating coils 11 heat the steel
4 to the Curie Temperature, which is within the dual phase
temperature range, at a heating rate of between 70 to 140C
6 per second, preferably approximately 100C per second. The
7 overall heating period before cooling should not exceed
8 about 20 seconds and includes little if any soaking to
9 minimize significant recrystallization of the ferrite. In
this regard, Figure 3 shows a comparison of a process
11 prepared on a typical continuous annealing line with the
12 process of the present invent~on. Under the conditions of
13 the presPnt invention some of the pearlite and a proportion
14 of the ferrite in the steel transforms to austenite and the
remaining ferrite stress relief anneals. Further, on a
16 micro-scale there is some recrystallization of the ferrite
17 (}imited by the short heating time and the absence of
18 soaking~, although on a macro-scale he ferrite retains its
19 oriented elongate grains reflecting the previous cold
rolling. The minimal recrystallization on the macro-scale
21 is due princlpally to the titanium and niobium additions.
22 TiC in particular retard~ recrystallisation considerably.
23 As mentioned above, the flow rate of water and the
24 number of nozzles 10 used in the cooling station 7 is
selected so that the cooling rate is greater than about
26 900C per second and preferably about 1000C per second.
27 This cooling rate is sufficiently high to transform the
28 austenite to martensite to achieve the preferred micro-
29 structure in the ~trapping.
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1 The composition of the steel and the operating
2 conditions of the heat treatment llne, such as the heating
3 rate and th2 speed of the strapping through the line, are
4 selected so that on heating to the dual phase range from
about 5 to 20% pearlite and ferrite are transformed to
6 austenite. The reason for this is that it has been found
7 that where the micro-structure contains more than about 20%
8 martensite, the martensite can form as a continuous
9 constituent with the result that the notch strength of the
strapping is significantly reduced to unacceptable levels.
11 The control of the heating conditions is significantly
12 simplified by reliance on the Curie temperature, which is
13 approximately 770C and therefore within the dual phase
14 range. The Curie temperature as used herein is understood
to mean the critical temperature above which steel is non-
16 magnetic and below which steel is magnetic. As a
17 consequence of heatlng of the strapping by the solenoid
18 induction heating coils, the heating efficiency
19 substantially reduces above the Curie temperature.
This phenomenon is used in two ways. Fir~t, it
21 prevents the strapping from overheating much beyond the
22 Curie temperature, thereby eliminating overheating problems.
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23 Second, by choosing the C~rie temperature as the anneallng
24 temperature (and varying the chemlcal composition of the
steel in order to achieve thiP necessary propertlç3 for the
26 strapping based on the Curie temperature as the annealing
27 temperature) the change ln heating efficiency enables
28 natural control of the strapping temperature both on a macro
29 and mlcro scale. For example, this control eliminate~
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1 overheating of any part of the cross-section of the
2 strapping and the constant annealing temperature enables the
3 production of consistent and reproducible properties.
4 The resultant microstructure will be seen from the
micrographs of Figures 4A and 4B to comprise matrix of
6 recovery annealed cold worked ferrite with martensite m and
7 carbides c dispersed discontinuously through the matrix~
8 This microstructure has been found to exhibit the required
9 properties for super strapping. In particular, it has been
found that it is possible to form strapping having tensile
11 strengths ranging from about 800 ~Pa to 1000 MPa with
12 acceptable ductility and notch properties throughout the
13 range of tensile strengths.
14 Furthermore, it has been found in laboratory tests thus
~ar conducted that there is only a mlnimal variation in the
1~ order of 20 Npa in tensile strengths of strapping of the
17 same composition but thicknesses varying between 1.5 mm and
18 0.5 mm~ On the other hand, strapping heat treated in
19 accordance with the Austemper process has been found to
exhibit a variatlon in the order of 300 MPa over the same
21 thic~ness range.
22 In the steel composition described in the Example 1 the
23 following propertles were produced. For comparison
24 purposes, th~ propertias of Super Strapping produced by the
Austemper process are ~uoted in bracket~.
26 Ultimate Tensile Strenqth
27 Average 955 MPa (935 Mpa)
~8 Range 890-1070 MPa ~831-1000 ~pa)
29 Elonqation
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1 Average 14~i (10%)
2 Ranga 12-16% (7-144)
3 R~verse Bends to Failure
4 Average 11 (Typical - 6)
~ange 8~15
6 The ultimate tensile strengths of the new product are
7 similar and satisfactory for this product. However, the
8 elongation and reverse bend values of the new proiuct are
9 superior to the Austemper productO Thls may give a better
performance in use than the conventional product, which
11 already performs satisfactorily in use.
12 The described heat treatment line enables the heat~ng
13 treatment of cold rolled steel to form microstructures
14 havlng suitable properties for use as strapping and thus
represents an alternative to the Austemper process.
16 Further, the heat treatment line is not sub~ect to the
17 disadvantages of the Austemper proc~ss associated with the
18 use of lead. ~owever, lt should be appreciated that whilst
19 the use of induction heating is most preferred since it
enables excellent "automatlc" control of the heating of the
21 strapping in the described heat treatment line, any suitable
22 means for rapidly heating the strapp~ng to the dual phase
23 temperature range could be used.
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