CA1331295C - Heat treatment method for strapping - Google Patents
Heat treatment method for strappingInfo
- Publication number
- CA1331295C CA1331295C CA000564912A CA564912A CA1331295C CA 1331295 C CA1331295 C CA 1331295C CA 000564912 A CA000564912 A CA 000564912A CA 564912 A CA564912 A CA 564912A CA 1331295 C CA1331295 C CA 1331295C
- Authority
- CA
- Canada
- Prior art keywords
- strapping
- steel
- matrix
- range
- heating
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
ABSTRACT OF DISCLOSURE
A method of heat treating cold rolled steel strapping comprising rapidly heating the strapping to the dual phase temperature range at a rate of the order of 100°C per second, with little or more soaking, and rapidly cooling the strapping at a rate of the order of 1000°C per second to form a microstructure comprising a matrix of recovery annealed cold work ferrite containing martensite and carbides dispersed through the matrix. A heat treatment line for performing this method is described. A heat treated cold rolled steel strapping produced by the method is also described. The steel from which the strapping is made preferably has less than 0.2%C and includes Ti in the range 0.06 to 0.15% preferably about 0.08% and Nb in the range 0.02 to 0.05% preferably about 0.04%.
A method of heat treating cold rolled steel strapping comprising rapidly heating the strapping to the dual phase temperature range at a rate of the order of 100°C per second, with little or more soaking, and rapidly cooling the strapping at a rate of the order of 1000°C per second to form a microstructure comprising a matrix of recovery annealed cold work ferrite containing martensite and carbides dispersed through the matrix. A heat treatment line for performing this method is described. A heat treated cold rolled steel strapping produced by the method is also described. The steel from which the strapping is made preferably has less than 0.2%C and includes Ti in the range 0.06 to 0.15% preferably about 0.08% and Nb in the range 0.02 to 0.05% preferably about 0.04%.
Description
~ . ~
` ' ~"` ~L33~29~
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 i ~.. ,. ~ :
~1 ' ~ ; ' ~; ; .
" ~331~3 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.
~331~9~
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 , 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 ~ ,' `` 133129a 1 under fast cooling to ferrite and marten~ite~ The ma~or . , 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 "i~
13~129a 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.
- 7 - ~ 33 12 9 5 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.
i ~
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 ,~
~ ~' - 7a - 1331~a -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 r a~=a r ~
':
~L33~93 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 ` - 9 -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~
~3 t~
~3~ ~9.~
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 ~. .) :~ :
~ 2 ~ti 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.
~ . ' !, ' ; , , . : ~
: `` 1 3 3 ~
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.
i 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~
; ~ . ., " ~ -_ 13 - ~33~9~
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 *Trade Mar~
~' .'.t~
. .... .. .. . ...
":,: ~' , . . '. ' ' `
1 ;,',, ' ` ' `: . .: :
~3~ 2~
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.
:
: :; : :.
, ~,
` ' ~"` ~L33~29~
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 i ~.. ,. ~ :
~1 ' ~ ; ' ~; ; .
" ~331~3 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.
~331~9~
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 , 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 ~ ,' `` 133129a 1 under fast cooling to ferrite and marten~ite~ The ma~or . , 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 "i~
13~129a 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.
- 7 - ~ 33 12 9 5 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.
i ~
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 ,~
~ ~' - 7a - 1331~a -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 r a~=a r ~
':
~L33~93 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 ` - 9 -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~
~3 t~
~3~ ~9.~
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 ~. .) :~ :
~ 2 ~ti 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.
~ . ' !, ' ; , , . : ~
: `` 1 3 3 ~
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.
i 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~
; ~ . ., " ~ -_ 13 - ~33~9~
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 *Trade Mar~
~' .'.t~
. .... .. .. . ...
":,: ~' , . . '. ' ' `
1 ;,',, ' ` ' `: . .: :
~3~ 2~
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.
:
: :; : :.
, ~,
Claims (15)
1. A method of heat treating cold rolled steel strapping comprising, rapidly heating the strapping to the dual phase temperature range, with little or no soaking, and rapidly cooling the strapping to form a microstructure comprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed throughout the matrix.
2. The method of claim 1, wherein the strapping is heated at a rate of between 70° to 140°C per second and the strapping as cooled at a rate greater than about 900°C per second.
3. The method of claim 2, wherein the strapping is heated at a rate of the order of 100°C per second and is cooled at a rate of the order of 1000°C per second.
4. The method of claim 1, wherein the strapping is heated to the Curie temperature and the heating period does not exceed about 20 seconds.
5. The method of claim 4, wherein the heating period is about 6 to 10 seconds.
6. The method of claim 5, wherein the steel has less than 0.2%C and includes Ti in the range 0.06 to 0.15% and Nb in the range 0.02 to 0.05%.
7. The method of claim 5, wherein the steel has less than 0.2%C and includes Ti in an amount of about 0.08% and Nb in an amount of about 0.04%.
8. A heat treatment line for cold rolled steel strapping comprising, heating means to rapidly heat the cold rolled steel strapping to the dual phase temperature range with little or no soaking, and cooling means to cool the strapping to form a microstructure comprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed throughout the matrix; wherein said heating means comprises a series of solenoid induction heating coils through which the strapping passes and said cooling means comprises a series of water nozzles directed at either side of the strapping as it emerges from the coils.
9. A heat treated cold rolled steel strapping which is characterized by a microstructure comprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed through the matrix.
10. The strapping of claim 9, wherein the steel comprises less than 0.02%C and contains one or more alloying elements which retard recrystallisation.
11. The strapping of claim 10, wherein the steel contains Ti in the range 0.06 - 0.15% and Nb in the range 0.02% to 0.05%.
12. The strapping of claim 10, wherein the steel contains Ti in the amount of about 0.08% and Nb in the amount of about 0.04%.
13. The method of claim 1, wherein said steel contains 1 or more alloying elements which retard recrystallization.
14. The method of claim 1, wherein said steel contains alloying elements which restrict the propagation of dislocations particularly in the ferrite phase, whereby the ferrite-phase is essentially in cold-worked form.
15. The strapping of claim 9, wherein the steel contains alloying elements which restrict the propagation of dislocations particularly in the ferrite phase, whereby the ferrite phase is essentially in cold-worked form.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU62991/86A AU600449B2 (en) | 1985-09-16 | 1986-09-10 | Heat treatment method for strapping |
| US07/071,293 US4816090A (en) | 1986-09-10 | 1987-07-09 | Heat treated cold rolled steel strapping |
| CA000564912A CA1331295C (en) | 1985-09-16 | 1988-04-22 | Heat treatment method for strapping |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPH243485 | 1985-09-16 | ||
| AU62991/86A AU600449B2 (en) | 1985-09-16 | 1986-09-10 | Heat treatment method for strapping |
| US07/071,293 US4816090A (en) | 1986-09-10 | 1987-07-09 | Heat treated cold rolled steel strapping |
| CA000564912A CA1331295C (en) | 1985-09-16 | 1988-04-22 | Heat treatment method for strapping |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1331295C true CA1331295C (en) | 1994-08-09 |
Family
ID=27423621
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000564912A Expired - Lifetime CA1331295C (en) | 1985-09-16 | 1988-04-22 | Heat treatment method for strapping |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1331295C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115011914A (en) * | 2022-08-08 | 2022-09-06 | 西北工业大学 | Preparation method of medical cobalt-based alloy seamless pipe |
-
1988
- 1988-04-22 CA CA000564912A patent/CA1331295C/en not_active Expired - Lifetime
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115011914A (en) * | 2022-08-08 | 2022-09-06 | 西北工业大学 | Preparation method of medical cobalt-based alloy seamless pipe |
| CN115011914B (en) * | 2022-08-08 | 2022-10-25 | 西北工业大学 | Preparation method of medical cobalt-based alloy seamless pipe |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4466842A (en) | Ferritic steel having ultra-fine grains and a method for producing the same | |
| EP0429094B1 (en) | High strength low carbon steels, steel articles thereof and method for manufacturing the steels | |
| CA1082950A (en) | Dual-phase hot-rolled steel strip | |
| RU2001119981A (en) | High strength three-phase steels with excellent toughness at cryogenic temperatures | |
| RU2001119979A (en) | HEAVY DUTY AUSTENITIC-AGING STEELS WITH EXCELLENT VISCOSITY AT CRYOGENIC TEMPERATURES | |
| RU2000104833A (en) | METHOD FOR PRODUCING ULTRA HIGH STRENGTH WELDABLE STEELS | |
| CA2470388C (en) | Triple-phase nano-composite steels | |
| MXPA04005744A (en) | Nano-compsite martensitic steels. | |
| US6673171B2 (en) | Medium carbon steel sheet and strip having enhanced uniform elongation and method for production thereof | |
| RU2005119192A (en) | STEEL WITH PACKET-GRID MARTENSIT-AUSTENITIC MICROSTRUCTURE SUBJECT TO COLD PROCESSING | |
| US4816090A (en) | Heat treated cold rolled steel strapping | |
| JPH11507103A (en) | Method of producing hot-rolled high-strength steel containing copper and containing easy transformation-induced plasticity | |
| CA1138756A (en) | Method for producing steel | |
| CA1331295C (en) | Heat treatment method for strapping | |
| JPH039168B2 (en) | ||
| TW346506B (en) | Method of cooling steel sections which are hot from rolling | |
| EP0020357B1 (en) | Lower bainite alloy steel article | |
| US3502514A (en) | Method of processing steel | |
| AU600449B2 (en) | Heat treatment method for strapping | |
| JPS63241120A (en) | Manufacture of high ductility and high strength steel sheet having composite structure | |
| US4483722A (en) | Low alloy cold-worked martensitic steel | |
| JPS56133423A (en) | Manufacture of composite structure type high-tensile high-ductility steel plate | |
| JPS62139821A (en) | Production of high-ductility high-strength cold rolled steel sheet | |
| US4737202A (en) | Method of producing steel components that simultaneously have high strength and high ductility and which retain these properties even after a hot forming operation | |
| JPH02267219A (en) | Production of steel plate excellent in carburizability |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |
Effective date: 20110809 |