AU600449B2

AU600449B2 - Heat treatment method for strapping

Info

Publication number: AU600449B2
Application number: AU62991/86A
Authority: AU
Inventors: Robert K Armstrong; Jeffery W Doonan; Peter I. Unicomb
Original assignee: Broken Hill Pty Co Ltd
Current assignee: Illinois Tool Works Inc
Priority date: 1985-09-16
Filing date: 1986-09-10
Publication date: 1990-08-16
Anticipated expiration: 2006-09-10
Also published as: AU6299186A

Description

if~ V AUSTRALI!A4 0 AAI 60 0 4 4o,

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PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: 62q9/ 486 i; dOCuimnt contjin the atne1]dflmeIs made uinder tioj 1 49 and is correct for Sprin Lin2.

RElated Art: Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: TO BE COMPLETED BY APPLICANT THE BROKEN HILL PROPRIETARY COMPANY LIMITED BHP House, 140 William Street, Melbourne, Victoria, 3000. Australia JEFFERY W. DOONAN, PETER I. UNICOMB ROBERT K. ARMSTRONG SANDERCOCK, SMITH BEADLE, Patent Attorneys, 207 Riversdale Road, Hawthorn, Victoria, 3122,

AUSTRALIA

Complete Specification for the invention entitled: HEAT TREATMENT METHOD FOR STRAPPING The following statement is a full dcscription of this invention, including the best method of performing it known to me:-' SNote: The description is to be typed in double spacing, pica type face, in an area not Ixceeding 250 mm in depth and 160 mm in width, on tough white paper of good quality and it is to be inserted inside this form.

141m6677-L L, Printed by C. J. THoMPsoN, Acting Commwealth Government Printer, Canberr

I

_I i. I Signature(s) of declarant(s).

To: The Commissioner of Patents, '4I 1 This invention relates to heat treated steel strapping, 2 and more particularly to a method of and an apparatus for 3 heat treating steel strapping, and the improved strapping so 4 produced.

Steel strapping is formed by slitting cold rolled steel 6 strip into the required width and is used in a variety of 7 applications which require a range of properties.

8 Generally, the properties which must be considered when 9 producing strapping are tensile strength, ductility, notch properties and work hardening. These properties are 11 dependent on the composition of the steel and the heat 12 treatment processes applied to the strapping.

13 The minimum tensile strength of steel strapping varies 14 between 500 and 1250 MPa. Strapping having tensile strengths in the range 500 to 800 MPa is manufactured and 16 sold by the applicant as 'standard' strapping and strapping 17 having tensile strengths in excess of 800 MPa is 18 manufactured and sold by the applicant as 'super' strapping.

19 Standard strapping is generally formed from low carbon steels and may be used in its cold rolled and slit form 21 without heat treatment in applications requiring moderate 22 strength levels, for example in the securing of cardboard 23 cartons to pallets. In some instances standard strapping is 24 formed from medium carbon steels and is subjected to a stress relief annealing treatment or a blueing heat 26 treatment in order to improve ductility.

27 Super strapping is generally formed from medium carbon 28 steels and the strapping is subjected to heat treatment to 29 provide the required properties. Super strapping is used 860910,!tspe.009,bhpl.spe, _L I 3- 1 in heavy duty applications requiring medium to high tensile I 2 strength and good ductility, notch properties and work 3 hardening. Uses include unitising of steel pipe into 4 bundles, the fastening of heavy loads to pallets and containing high density wool and cotton bales.

6 The conventional heat treatment process for super 7 strapping, which is a version of the so-called Austemper 8 process, comprises: 9 heating cold rolled steel strapping (generally having a carbon content between 0.20 and 0.60%) to between 11 8000C and 9000, to transform the structure to austenite, 12 fast cooling the strapping in a lead or salt bath 13 to a temperature between 350 and 500 0 C, to initiate 14 transformation from austenite to bainite, air cooling the strapping for a short period of 16 time to allow transformation of any remaining austenite, and 17 quenching the strapping to ambient temperatures.

18 It is known that bainite has acceptable properties for 19 medium to high tensile strength strapping. However, the Austemper process has a number of disadvantages.

21 First, there is a substantial capital cost associated 22 with the use of lead, as well as costs to replace lead lost S23 through oxidation and lead "drag-out" on the strip, costs 24 associated with loss of product due to intermittent lead contamination of the strip, cost of maintenance of the lead 26 baths and costs associated with minimising environmental and 27 health problems generally associated with lead.

28 Second, the speed of the heat treatment process is 29 limited by the cooling power of the lead bath and the need 860910,!tbspe.009,bhpl.spe, 4 1 to allow sufficient time at the transformation temperature 2 range for transformation of austenite to bainite. The 3 required increase in the length of the lead quench bath 4 necessary to allow sufficient time at the quench temperature to enable complete transformation of austenite to bainite at 6 higher speeds would be cost prohibitive.

7 A third disadvantage is associated with the need to ase 8 sufficiently high carbon and manganese levels to avoid 9 martensite formation during heat treatment with the countervailing requirements to keep the analysis lean to 11 minimise steelmaking problems. In this regard, it is the 12 desire of the steel maker to keep the carbon content of his 13 steel as low as pjss blt to avoid steel making problems.

14 However, the lower the carbon content, the more difficult it is to produce bainite because the temperature at which 16 martensite forms increases, and the Austemper process 17 becomes less and less useful.

18 Another heat treatment process for producing higher 19 strength steels, known as the Continuous Annealing line process, may appear at first sight to overcome certain of 21 the problems associated with the Austemper process, but the 22 process still has some shortcomings. The process involves a 23 25 to 40 sec heat up period, a 10 to 120 sec soaking period 24 followed by a 0.5 to 30 sec cooling period. This process results in a dual phase ferrite/martensite steel, which 26 requires a soaking period of at least 10 seconds for stable 27 formation of ferrite and at ia phases which transform 28 under fast cooling to ferrite and martensite. The major strengthening factor is the amount of hard martensite phase 910,!tbspe.009,bhp.pe, 860910, !tbspe.009,bhpl.spe, to 60%) which may be assisted by ferrite strengtheners such as cold worked structure. A 15% structure would require other strengthening factors to achieve properties which are achieved according to the present invention.

The object of the present invention is to provide an improved steel strapping and method of and apparatus for producing the strapping by an improved heat treatment process, o G which at least ameliorates the disadvantages described in the preceding paragraphs and which results in a treated steel strapping having a novel microstructure and improved properties.

0 C In accordance with the present invention there is provided a method of heat treating cold rolled steel strapping comprising, rapidly heating the strapping to the Curie temperature in the dual phase temperature range, with little or no soaking, such that the total heating period does not exceed about 20 seconds c

C'

and rapidly cooling the strapping to form a microstructure Scomprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed throughout the matrix.

c The term "dual phase" as used herein is understood to mean the phase equilibrium region where austenite and ferrite phases co-exist.

f In another aspect, the invention provides 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.

tbspe.048/bhpl 90 5 The martensite and carbides preferably comprises less than by volume of the microstructure.

L 048/bhpl 90 5 _j 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 S 22 23 24 26 27 28 29 6 The composition of the steel preferably comprises less than 0.2%C and is characterised by alloying elements which retard recrystallisation and act as precipitation strengtheners, for example titanium and preferably also Niobium. Titanium may be present in the range 0.06-0.15% and preferably 0.08% while Niobium may be present in the range 0.02 to 0.05% and preferably about 0.04%. The steel also preferably contains manganese in the range 1 to 2%, preferably about 1.45% and silicon in the range 0.2 to 0.4%, preferably about 0.33%.

The method according to the invention results in a triphase recovery annealed cold worked ferrite/martensite/carbide steel and is characterised by a short heating/soaking cycle. This results in only some carbides with favourable compositions transforming to austenite and thus to martensite on rapid cooling. Many carbides go through the transformation without appreciable change. With the microalloying elements (such as Ti, Nb) present, this cycle results in a tri-phase structure of recovery annealed cold worked ferrite containing martensite in the region of 5% and carbides in the region of In each of the above aspects, the carbides are present in the form of fine spheroidal cementite and fine fragmented cementite. The fine fragmented cementite is a consequence of the rapid heating step and the absence of any appreciable soaking.

In accordance with the present invention there is also provided a heat treatment line for cold rolled steel strapping comprising, heating means to rapidly heat the cold 8 60910,!tbspe.009,bhpl.spe,

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7 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.

SSince the Austemper process can be used only to form a structure containing bainite, a new type of heat treatment line 1C which enables the required short rapid heating time and rapid cooling necessary to achieve the new microstructure is provided by the present invention. In its presently preferred form, the heating line comprises a series of solenoid induction heating coils followed by a rapid cooling station including a series of water nozzles directed at either side of the heated strapping as it emerges from the heating coils.

The heating period is selected to provide a balance between the consumption of power by the heating coils and the required speed which the heat treating line is to operate. In most existing plants, the speed of operation will be dictated by considerations relating to the coiling and painting plants which are usually already present at the plant and accordingly substantial speed gains will not be possible. However, in the case of a fresh site, line speeds of up to 600m/min will be possible using the apparatus to be described further below.

Thus, heating periods as low as two seconds and as high as about seconds may be required, but using heating rates of between 700 t b pe.048/bhpl 90 5 j::k od ~I 1-U~to 140 0 C per second, and preferably about 100 0 C per second, the most likely heating period range should be about six to ten seconds. The heating /f Lbspe.048/bhpl A

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r 90 5 8 1 period comprises little or no soaking period although 2 periods of from one to several seconds may occur without 3 adverse results.

4 The cooling rate should be sufficiently high to ensure that the required regions of martensite are formed in the 6 matrix. A cooling rate greater than 900 0 C per second, and 7 preferably desirably at least 1000 0 C per second should 8 achieve accepta' e results.

9 Further detailed description of the present invention will now be provided with reference to the accompanying 11 drawings in which: 12 Figure 1 is a schematic diagram showing a preferred 13 embodiment of the heat treatment line; 14 Figure 2 shows one preferred cooling arrangement for the line of Figure 1; 16 Figure 3 shows schematically the process stages for a 17 typical continuous annealing line and-for the line embodying 18 the invention; 19 Figures 4A and 4B respectively show -trancmiicin electron micro-graphs (X 4100 and X 25,000 respectively) of 21 the micro-structure after heat treatment.

22 The present invention is based on the realization that 23 steel having a microstructure comprising a matrix of 24 recovery annealed cold worked ferrite with martensite and carbides dispersed throughout the matrix exhibits suitable 26 properties for use as 'super' strapping. As a consequence, 27 the micro-structure is an acceptable substitute for bainite 28 which, is the predominant constituent in strapping formed by pU 9 the Austemper process.

zcl.N 60910,!tbspe.009,bhpl.spe, .^fl 9- 1 In the preferred embodiment the microstructure is 2 formed by a heat treatment method which is based on the use 3 of induction heating to heat the strapping., 4 With reference to Figure 1, in the heat treatment line of the preferred embodiment, the cold rolled steel strapping 6 2 is fed from a coil unwinding station 3 through an 7 induction heating station 5 and a cooling station 7 to a 8 coil winding station 9.

9 The induction heating station 5 comprises a number of solenoid induction heating coils 11, for example six, 11 connected in series and arranged to allow the strapping to 12 pass through the coils. Each induction heating coil 11 is 13 preferably connected to a 5 to 25 kilohertz Statipak STK 4 14 power unit manufactured by Inductoheat Pty. Ltd. It will be appreciated that any other suitable induction heating coil 16 and power supply combination may be used although a 5 to 17 kilohertz power supply is preferred. 18 As shown in greater detail in Figure 2 of the drawings, 19 the cooling station 7 comprises a plurality of nozzles positioned within a housing or tank 12 to direct sprays of 21 water onto the surfaces of the strapping 2. In the 22 embodiment shown, the nozzles 10 are arranged in four groups 23 A to D with the nozzles l0 in groups A and B being angularly 24 adjusted so as to be directed away from the heating coils 11, that is, in the direction of travel of the strip 2, and 26 the nozzles 10 in groups C and D being fixed perpendicularly 27 to the surfaces of the strapping 2. The angular adjustment 28 of the nozzles in groups A and B reduces the likelihood that 29 cooling water will travel along the strapping and enter the 86 0910,!tbspe.009,bhpl.spe, 1 induction heating coils 11.

2 The mains water supply to the headers supporting the 3 nozzles 10 includes separate control valves 13 for the upper 4 and lower headers so that flow to the upper and lower nozzles 10 may be separately controlled. The number of 6 nozzles 10 and the controlled flow rate are selected to 7 achieve the desired cooling rate discussed in greater detail 8 below.

9 In use, the cold rolled steel strapping 2 fed from the coil unwinding station 3 is heated to the dual phase 11 temperature range as it passes through the solenoid 12 induction coils 11 and is then quenched by water sprayed at 13 the cooling station 7.

14 It is preferred that the composition of the steel is selected to comprise less than 0.2%C and alloying elements 16 which retard recrystallisation and which act as 17 precipitation strengtheners, such as- titanium and niobium.

18 In one Example, the steel has the following composition: 19 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 21 Ti 0.080% N 0.0075% 22 In the above Example, the values given for each element 23 are not critical or essential. For example Ti may vary 24 between 0.06% and 0.15% while Nb may vary between 0.02% and 0.05%, Mn may vary between 0.5% and 2% and Si may vary between 26 0.2% to 27 The solenoid induction heating coils 11 heat the steel 28 to the Curie Temperature, which. is within the dual phase 29 temperature range, at a heating rate of between 70° to 140 0

C

860910,!tbspe.009,bhp1l.spe, worked ferrite containing martensite and carbides dispersed throughout the matrix.

11 1 per second, preferably approximately 100 0 C per second. The 2 overall heating period before cooling should not exceed 3 about 20 seconds and includes little if any soaking to 4 minimize significant recrystallization of the ferrite. In this regard, Figure 3 shows a comparison of a process 6 prepared on a typical continuous annealing line with the 7 process of the present invention. Under the conditions of 8 the present invention some of the pearlite and a proportion 9 of the ferrite in the steel transforms to austenite and the remaining ferrite stress relief anneals. Further, on a 11 micro-scale there is some recrystallization of the ferrite 12 (limited by the short heating time and the absence of 13 soaking), although on a macro-scale the ferrite retains its 14 oriented elongate grains reflecting the previous cold rolling. The minimal recrystallization on the macro-scale 16 is due principally to the titanium and niobium additions.

17 TiC in particular retards recrystallisation considerably.

18 As mentioned above, the flow rate of water and the 19 number of nozzles 10 used in the cooling station 7 is selected so that the cooling rate is greater than about 21 900 0 C per second and preferably about 1000°C per second.

22 This cooling rate is sufficiently high to transform the 23 austenite to martensite to achieve the preferred micro- 24 structure in the strapping.

The composition of the steel and the operating 26 conditions of the heat treatment line, such as the heating 27 rate and the speed of the strapping through the line, are 28 selected so that on heating to the dual phase range from 29 about 5 to 20% pearlite and ferrite are transformed to 86 0910,!tbspe.009,bh.pl .spe, I -12- 1 austenite. The reason for this is that it has been found 2 that where the micro-structure contains more than about 3 martensite, the martensite can form as a continuous 4 constituent with the result that the notch strength of the strapping is significantly reduced to unacceptable levels.

6 The control of the heating conditions is significantly 7 simplified by reliance on the Curie temperature, which is 8 approximately 770 0 C and therefore within the dual phase 9 range. The Curie temperature as used herein is understood to mean the critical temperatire above which steel is non- 11 magnetic and below which steel is magnetic. As a 12 consequence of heating of the strapping by the solenoid 13 induction heating coils, the heating efficiency 14 substantially reduces above the Curie temperature.

This phenomenon is used in two ways. First, it 16 prevents the strapping from overheating much beyond the 17 Curie temperature, thereby eliminating overheating problems.

18 Second, by choosing the Curie temperature as the annealing 19 temperature (and varying the chemical composition of the steel in order to achieve the necessary properties for the 21 strapping based on the Curie temperature as the annealing 22 temperature) the change in heating efficiency enables 23 natural control of the strapping temperature both on a macro 24 and micro scale. For example, this control eliminates overheating of any part of the cross-section of the 26 strapping and the constant annealing temperature enables the 27 production of consistent and reproducible properties.

28 The resultant microstructure will be seen from the 29 micrographs of Figures 4A and 4B to comprise matrix of 860910,!tbspe.009,bhpl.spe, I- 13 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 recovery annealed cold worked ferrite with martensite m and carbides c dispersed discontinuously through the matrix.

This microstructure has been found to exhibit the required properties for super strapping. In particular, it has been found that it is possible to form strapping havLng tensile strengths ranging from about 800 MPa to 1000 MPa with acceptable ductility and notch properties throughout the range of tensile strengths.

Furthermore, it has been found in laboratory tests thus far conducted that there is only a minimal variation in the order of 20 Mpa in tensile strengths of strapping of the same composition but thicknesses varying between 1.5 mm and 0.5 mm. On the other hand, strapping heat treated in accordance with the Austemper process has been found to exhibit a variation in the order of 300 MPa over the same thickness range.

In the steel composition described in the Example 1 the following properties were produced. For comparison purposes, the properties of Super Strapping produced by the Austemper process are quoted in brackets.

Ultimate Tensile Strength Average 955 MPa (935 Mpa) Range 890-1070 MPa (831-1000 Mpa) Elongation Average 14% Range 12-16% (7-14%) Reverse Bends to Failure Average 11 (Typical 6) Range 8-15 8 60910,!tbspe.009,bhp1.spe, iUa~IL 14 1 The ultimate tensile strengths of the new pr du, t are 2 similar and satisfactory for this product. However, the 3 elongation and reverse bend values of the new product are 4 superior to the Austemper product. This may give a better performance in use than the conventional product, which 6 already performs satisfactorily in use.

7 The described heat treatment line enables the heating 8 treatment of cold rolled steel to form microstructures 9 having suitable properties for use as strapping and thus represents an alternative to the Austemper process.

11 Further, the heat treatment line is not subject to the 12 disadvantages of the Austemper process associated with the 13 use of lead. However, it should be appreciated that whilst 14 the use of induction heating is most preferred since it enables excellent "automatic" control of the heating of the 16 strapping in the described heat treatment line, any suitable 17 means for rapidly heating the strapping to the dual phase 18 temperature range could be used.

.19 Thc ontir- contents -f leeprovisionan pccificatins lodged with Australian Patent Applications of this is 21 the complete specification are h-e y imported into this 22 specification and f part of the disclosure of this 23 specific 'on. The claims form part of the disclosure of 24 thi-4s eG..atoto 4

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860910,!tbspe.009,bhpl.spe,

Claims

1. A method of heat treating cold rolled steel strapping comprising, rapidly heating the strapping to the Curie temperature in the dual phase temperature range, with little or no soaking, such that the total heating period does not exceed about 20 seconds 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 heating period is about 6 to 10 seconds.

The method of any preceding claim, wherein the steel has less than 0.2%C and includes Ti as a carbide forming element in the range 0.06 to 0.15%.

6. Tha method of claim 5, wherein said steel includes about 0.08% Ti. H a-nboit 0,Q nh_4=A

7. The method of claim 5 or 6 wherein the steel further includes niobium in the range 0.02 to 0.05%.

8. The method of claim 5 or 6 wherein the steel includes about 0.04% Nb.

9. A heat treatment line for cold rolled steel strapping amspe.003/bhp2 90 5 b- L i 16 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.

The line of claim 9, wherein said heating means is controlled to heat the strapping at a rate of between 70° to 140°C per second and the cooling means is controlled to cool the strapping at a rate greater than about 900 C per second.

11. The line of claim 9, wherein said heating rate is controlled to be of the order of 100°C per second and said cooling rate is controlled to be of the order of 1000°C per second.

12. The line of claim 9, 10 or 11, wherein said cooling means comprises a series of water nozzles directed at either side of the strapping as it emerges from the coils.

13. Steel strapping when produced by the method of any one of claims 1 to 8.

14. 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.

15. The strapping of claim 14 wherein the steel comprises less than 0.2%C and contains one or more alloying elements which retard recrystallisation. amspe.003/bhp2 90 5 0v 7 17

16. The strapping of claim 15, wherein the steel contains Ti as a carbide forming element in the range 0.06 0.15%.

17. The strapping of claim 16 wherein said steel includes about 0.08% Ti.

18. The strapping of claim 16 or 17 wherein the steel further includes niobium in the range 0.02 to 0.05%.

19. The strapping of claim 16 or 17 wherein the steel further includes about 0.04% Nb.

The strapping of any one of claims 14 17 wherein the carbides in the microstructure are in the form of fine spheroidal cementite and fine fragmented cementite.

21. The method of any one of claims 1 8 wherein the carbides in th3 microstructure are in the form of fine spheroidal cementite and fine fragmented cementite.

22. The line of any one of claims 9 12 wherein the carbides in the microstructure are in the form of final spheroidal cementite and fine fragmented cementite. 3Q,24 A method of heat treating steel strapping substantially as described with reference to the accompanying drawings. 2^ A heat treatment line substantially as described with reference to the accompanying drawings. c. A heat treated strapping product substantially as 1, hereinbefore described with reference to the Example. da pe.003/bhp2 90 5 31 i 18 DATED this May 31, 1990 SMITH SHELSTON BEADLE Fellows Institute of Patent Attorneys of Australia Patent Attorneys for the Applicant: THE BROKEN HILL PROPRIETARY COMPANY LIMITED amspe.003/bhp2 90 5 31 t.