AU600402B2 - Process for preparing alloyed-zinc-plated titanium-killed steel sheet having excellent deep-drawbility - Google Patents

Description

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AUSTRALIA

Patents Act 6 0 0 4 0 2 CCM~PLETE SPECIFICATICNI

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: 'I a.ir~C2..

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APPLICANT'S PE RI'NCE: FNq2P224 "Name(s) of Applicant(s): Nisshin Steel Company, Ltd Address(es) of Applicant(s): 4-1 Marunauchi 3-chome, Chiyoda-ku, Tokyo,

JAPAN.

S Address for Service is: PHILLIP.) ORMNDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Complete Specification for the invention entitled: PrOCESS FOR PREPARJ14G ALLOYED- ZINC-PLATED TITANIUM-KILLED STMEI SHMEET HAVING EXCELLNT DE1E -DRAWBILI1TY Our Ref 122903 POF Code: 1317/49098 The following statement is a full description of this invention, including the best method of performinig it known to applicant(s): 6003q/1 1- L711- Title of the Invention i Process for preparing alloyed-zinc-plated titanium-killed steel sheet having excellent deep-drawability Field of the Invention This invention relates to a process for preparing alloyedzinc(Zn)-plated titanium(Ti)-killed steel sheet having excellent deepdrawability (good workability and powdering resistance) by vacuum vapor deposition.

Background of the Invention An alloyed plated steel sheet such as an alloyed-Zn-plated steel sheet is better than ordinary (Zn-)platpd steel sheet in that continuous spot-welding can be easily carried out, the adhesion of o' electrophoretic coating film is good, the corrosion resistance is Sbetter, etc. and, therefore, sheet of this type is being widely used in the automobile industry and other fields.

i Conventionally, alloyed-Zn-plated steel sheet is manufactured by Sheat-treating Zn-plated steel sheet prepared by hot-dip plating or electrolytic plating. However, hot-dip plating is not suitable for Soo thin coating of less than 30g/m 2 and one side plating, and hot-dipo 0 20 plated steel sheet is inferior in uniformity in thickness of the plating layer in the londitudinal direction and the transverse t c I direction. Electrolytic plating is not suitable for plating of thicker than 50g/m 2 because the plating cost rises steeply with the increase in coating weight. In contrast, the vacuum vapor deposition 25 process is advantageous in that the coating weight can be relatively easily controlled and uniform thickness is easily achieved.

Therefore, processes for preparing alloyed Zn-plated steel sheet utilizing vacuum vapor deposition have been proposed and being practiced (JP-A-61-195965 for instance). According to this process, cold-rolled steel coil is plated with Zn on one or both sides by vacuum vapor deposition and, thereafter, is heat-treated for alloying at 250-350'C for 1-15 hours in a non-oxidizing or weakly reducing atmosphere in a batch annealing furnace.

In the meanwhile, for manufacturing steel sheets for deepdrawing, aluminum(Al)-killed steel and Ti-killed steel are used.

With Al-killed steel sheet, alloyed-Zn-plated steel sheet can be manufactured by vacuum vapor deposition and heat treatment in accordance with the above-described known process. With Ti-killed

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steel sheet, however, there is a problem that the formed alloy layer is inferior in powdering resistance and easily scales off.

For ordinary deep drawing purposes, Al-killed steel sheet suffices. Recently, however, more and more complicated shaping is being required. For such purposes, Ti-killed steel, which has better workability and formability, must be used. Therefore, alloyed-Znplated-steel sheet made of Ti-killed steel sheet also must be provided with good powdering resistance.

We have conducted studies to determine the cause of the inferior S 10 powdering resistance of alloyed-Zn-plated steel sheet made of Tikilled steel sheet focusing our attention on the structure of alloyed plating layer. As a result of our studies, we have found that in the case of Ti-killed steel, very brittle intermetallic compounds of iron(Fe) and Zn are formed when the Zn-plated steel sheet is heated to temperatures outside of a specific temperature range and that this specific temperature range differs depending upon the Ti content.

That is, the temperature is in excess of 3207 when the Ti content is S0.05% and in excess of 260'C when the Ti content is Further, once a brittle intermetallic compound layer is formed, deterioration S0 of powdering resistance is inevitable irrespective of the alloying conditions uner which the plated steel sheet is treatled. Thus we o, have found that the aforesaid problem can be solved by maintaining the substrate Ti-killed steel sheet at a low temperature set relative to the Ti content during the vacuum vapor deposition and heating the plated steel sheet in a non-oxidizing atmosphere, preferably in a batch heat treatment annealing furnace, at a temperature within the temperature range in which the Fe content of the formed alloyed layer is wisthin a prescribed range.

Summary of the Invention 30 -Th-i -invpn-nri prnvi rip q pprnrrP Sfnr prnparing an rld zinc plated stte-I eet having excellent deep drawability, which comprises adjusting the temperat of substrate steel sheet of titanium-killed steel essentially consisting o Qa01%, Si<0. 15%, Mn: 0.15-0.85%, Ti: 0.05-0.30%, P0.02%, S0.02%, Alg0.05,t balance of Fe, to a temperature within a range T(C) defined as 180=T -24 92, wherein is the Ti content of the substrate steel sheet, subjecting steel sheet to Zn-plating by vacuum vapor deposition, and thereafter mai-n-t-a-i-nip-ng-att-e--led-t- e---het f-alyig-e-h i inc -2- L 04 SAccording to the present invention there is provided a process for preparing alloyed-zinc-plated steel sheet having excellent deep drawability, which comprises adjusting the temperature of a steel sheet substrate of titanium-killed steel to within the range defined as 180 C) j 240xW 292, where is the Ti content of the steel, subjecting the substrate to zinc plating to coat a surface of the substrate with Zn by vacuum vapour deposition, and thereafter subjecting the Zn-plated substrate to a heat treatment for alloying the Zn with a surface layer of the substrate, the steel having C <0.01%, Si from 0.15 to 0.85% Mn, from 0.05 to 0.30% Ti, P S <0.02% and Al <0.05% with the balance, apart from incidental impurities, o substantially being Fe and the heat treatment comprising heating the Zn-coated substrate at from 220 to 320°C for a period of from 1 to 50 hours in accordance with the shaded area enclosed by and ,o°o including the dashed line of Figure 2 of the O accompanying drawings, with the maximum temperature a of the heat treatment increasing inversely with Ti content from 280°C for a Ti content of 0.3% to 320°C for a Ti content of 0.05%.

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L0 C -2a- J.aypr Pnd the ch;ihqtrtt1 ftR7.l at 9.9fl-.9n fnr 1-5n hbollrs in a go-r-rance with Ti cofe-ntent.

Throughout the specification, percent(%) means that by weight.

Ti-killed steel for manufacturing steel sheet suitable for deep drawing is a low carbon(C) steel, the C content of which is usually not more than 0.01%. The silicon(Si) content should preferably be less than 0.15% bevcause with a Si content in excess of 0.15% the adhesion of plating layer is unsatisfactory. Manganese(Mn), an element which enhances the strength of steels, should preferably be contained in the range of 0.15-0.85%. When the Mn content is less than 0.15%, satisfactory strength is not obtained. On the other hand, the strength of steels is not further improved by a Mn content in excess of 0.85%. The Ti content in Ti-killed steel for manufaca turing sheet for deep drawing is usually 0.05-0.30%. Ti is an 5 element which fixes C in steel and improves workability of the steel Sn"o, and is thought to be required in an amount of at least about four times the content of C. This element also fixes nitrogen(N) which is an impurity in steel. Therefore, Ti is contained in an amount in excess of 0.05% or more in consideration of the N content. On the on' 30 other hand, no advantage commensurate with the rise in the manufacturing cost is obtained if the Ti content is in excess of 0.3%.

As impurities, phosphorus(P), sulfur(S) and Al may be contained respectively in an amount of <0.02% and 0.05%. These amounts are the same as the ordinary impurity contents of plain carbon steels.

The above-described Ti-killed steel sheet is maintained at a temperature T(C) defined as 180=T<-240xW+292 in accordance with the STi content and then subjected to plating with Zn by vacuum vapor deposition.

S 30 In vacuum vapor deposition plating, Zn vapor which condenses on the surface of the steel strip to form a plating layer raises the temperature of the steel strip by 40"0 at most. If the temperature of the substrate steel of Ti-killed steel exceeds the temperature defined as 180_TS-240xW+292 in accordance with the Ti content before being subjected to vacuum vapor deposition, during vacuum vapor deposition, the temperature of the steel sheet reaches a temperature at which brittle intermetallic compounds are formed because of temperature rise caused by heat of condensation of the Zn vapor.

-3- Thus the powdering resistance of the formed alloyed layer deteriorates.

The upper limit of the substrate temperature differs depending upon Ti content as described above. Specifically, when the Ti content is it is not higher than 220'C as calculated by the above formula; and when the Ti content is 0.05%, it is not higher than 280C. That is, the substrate temperature must be adjusted so as not to exceed the upper limit as defined by the above formula prior to vacuum vapor deposition.

If the substrate temperature is too low, the adhesion of the formed plating Zn layer is poor. The lower limit is 180'C which is the same as in the case of plain carbon steel containing no Ti.

The steel strip plated with Zn by vacuum vapor deposition is heat-treated for alloying. The formed alloyed layer should preferably contain 8-12% of Fe. If the Fe content is less than 6%, unalloyed 7-Zn remains on the surface of the plating layer and affects the coatability, weldability, etc. of the plated steel sheet.

If the Fe content exceeds 14%, the powdering resistance of the alloyed layer deteriorates.

o2.o 0 As the heating means, a batch annealing furnace can be used. In the alloying treatment with a batch annealing furnace, generally steel sheets are heated in a non-oxidizing atmosphere in order to prevent oxidation of the steel strip (or sheet). Heating can be carried out with a strip in the form of a coil, a tight coil or an open coil.

The heating temnperature and time can be varied in accordance with intended coating weight and average Fe content, but the heating temperature must be lower by 30'C than the alloying temperature for Zn-plated Ti-free steel sheet. The upper limit of the heating 30 temperature varies depending upon the Ti content. It is 280'C when the Ti content is and is 320°C when the Ti content is 0.05%. If the heating temperature exceeds the above-described upper limit, powdering resistance deteriorates, since brittle alloyed layer is formed. On the other hand, if the heating temperature is below the above-described temperature range, the Zn plating layer is not well alloyed. Heating for shorter than one hour does not fully raise the substrate temperature and satisfactory heating cannot be effected.

On the other hand, heating for longer than 50 hours lowers -4- 4 I productivity.

It is possible to continuously carry out heating by means of an annealing furnace provided in a continuous plating line, but the heating conditions will differ from those in the case of a batch annealing furnace. Employment of a batch annealing furnace is preferred since temperature control is easy.

The temperature raising rate and cooling rate are not specifically limited and these conditions are determined with consideration to the performance of ordinary industrial batch annealing furnaces.

Brief Explanation of the Attached Drawings Fig. 1 is a diagram showing the ranges of substrate sheet temperature and Ti contents in substrate steel in which brittle Fe-Zn internetallic compounds are not formed at the interface of the steel substrate and the plating layer when the substrate is plated with Zn by vacuum vapor deposition.

Fig. 2 is a diagram showing ranges of treating temperature and time in which the average Fe in the alloyed layer can be controlled to 8-12.0%.

Specific Description of the Invention Now the invention will be specifically described by way of experimental results, working examples and comparative examples.

We plated ordinary Ti-killed steel strips containing 0.05-0.3% Ti with Zn by vacuum vapor deposition using a continuous vacuum vapor deposition apparatus as disclosed in Nisshin Giho No. 51,1954, 1984 with various substrate temperatures.

The other operation conditions were as follows: Steel strip: 0.8mm thick and 1200mm w ide Ti-killed steel strips Operation speed: Pressure in the deposition chamber: 0. OTorr Alloying temperature: 270' Thus we checked the relation between Ti content of substrate steels and substrate temperature for obtaining an alloyed layer containing no intermetallic compounds. The results are summarized in Fig. 1.

Fig. 1 shows ranges of substrate temperature and Ti content in substrate steel in which formation of Fe-Zn intermetallic compounds does not occur at the interface of the substrate and the plating layer. In the drawing, the solid line a represents upper limit te..iperatures below 5 L7F which brittle Fe-Zn intermetallic compounds are not formed. The line is expressed by the relation T=-240XW+292(), wherein T is the substrate temperature in *C and W is Ti content in The solid line b represents lower limit temperatures above which the adhesion of the formed plating layer is satisfactory. This temperature is constantly 180C regardless of Ti content.

The Zn-plated steel strips prepared as described above (not yet alloyed) were maintained at various temperatures for various times and Fe contents of the formed alloy layers were checked. It was found that there is a relation between the Ti content and the upper limit temperature. The results are summarized in Fig. 2.

0 Fig. 2 is a diagram which shows ranges of heat-treating temperatures and times in which the average Fe content in the alloyed layer can be controlled to 8-12.0%. In the drawing, the area surrounded by broken lines is the range in which the Fe content of the formed alloy layer falls between 8% and 12%. As seen in this figure, the strip can be heated to 320'C when the Ti content is 0.05% and 280C, when the Ti content is Those skilled in the art will be able to determine the upper limit temperature for Ti contents between 0.05% and 0.3% from this figure. The Fe content of the alloyed layer of Tikilled steel sheet falls between 8% and 12% when the Zn-plated steel sheet is heated at 220-320'C according to the Ti content.

In Fig. 2, the area surrounded by solid lines is the range known I for Al-killed steel. That is, the sloped line on the bottom left side is known by those skilled in the art.

Examples and Comparative Examples The above-described Ti-killed steel strips containing 0.05% and 0.3% were plated with Zn on both sides by vacuum vapor deposition as described above under various conditions indicated in Table 1.

S 30 The thus Zn-plated steel coils were heat treated for alloying in Sa batch annealing furnace in an atmosphere consisting of 3% H 2 and 97% NZ and having a dew point of -25C under the temperature and time conditions indicated in Table 1.

The surface condition and powdering resistance of the thus produced alloyed Zn-plated steel sheets were checked. The powdering resistance was judged by bending specimens with a radius of curvature of 3 times the thickness of the sheet up to 180° (6t bending) and observing whether scaling-off of the plating layers took place inside 6 or not.

The results are summarized in Table 1. The terms used in Table 1 means as follows: With respect to surface condition, Good: Homogencous alloy were formed.

Zn remaining: n-Zn remained.

With respect to powdering resistance, Good: No powdering occurred.

Poor: Powdering occurred.

Poor adhesion: Plating layer easily peeled off.

The asterisked values are outside the conditions defined in the claim.

As is apparent from the above-described results, the products of the present invention w.ere provided with good surface conditions and 15 powdering resistance, while the products of the comparative examples S, r'ere inferior because of the existence of unalloyed Zn or formation of intermetallic compounds.

a 44 O0 I 3 7 Table 1 (1) Coating Ti cont. of TUpper limit of Substrate Substrate Alloying Alloying Surface Powdering Run W'eight Substrate jsubstrate temp. temp. temp. after temp. time condi- resistance (g/m 2 (-240W4+292) 0 C 0 C) plating 0 C) 0 C) (hr) tion EiX. 1 10/10 0.05 280 180 184 280 1 good good 2 10/10 0.05 280 180 184 320 1II 3 10/10 0.05 280 180 184 320 4 10/10 0.05 280 180 184 220 10/10 0.05 280 180 184 220 6 10/10 0.05 280 280 284 280 1II 7 10/10 0.05 280 280 284 320 1 I 8 10/10 0.05 280 280 284 320 9 10/10 0.05 280 280 284 220 10/10 0.05 280 280 284 220 11 10/10 0.30 220 180 184 260 3 I 12 10/10 0.30 220 180 184 260 13 10/10 0.30 220 180 184 220 14 10/10 0.30 220 180 184 220 10/10 0.30 220 220 224 260 3II 16 10/10 0.30 220 220 224 260 504 17 10/10 0.30 220 220 224 220 15 good good Table 1 C6ating Ti cont. of Upper limit of Substrate Substrate Alloying Alloying Surface Powdering Run Weight Substrate substrate temp. temp. temp. after temp. time Icondi- resistance (g/m 2 (-240W+292)°C plating (hr) tion Ex. 18 10/10 0.30 220 220 224 220 50 good good 19 50/50 0.05 280 180 200 280 1 I I 50/50 0.05 280 180 200 320 1 I 21 50/50 0.05 280 180 200 320 50 I I 22 50/50 0.05 280 180 200 220 15 I I 23 50/50 0.05 280 180 200 220 50 I I 24 50/50 0.05 280 280 300 280 1 I I 50/50 0.05 280 280 300 320 1 I I 26 50/50 0.05 280 280 300 320 50 I I 27 50/50" 0.05 280 280 300 220 15 I I 28 50/50 0.05 280 280 300 220 50 I I 29 50/50 0.30 220 180 200 260 3 I I 50/50 0.30 220 180 200 260 50 I I 31 50/50 0.30 220 180 200 220 50 I I 32 50/50 0.30 220 180 200 220 15 I I 33 50/50 0.30 220 220 240 260 3 I I 34 50/50 0.30 220 220 240 260 50 4 50/50 0.30 220 220 240 220 15 good good Table 1 (3) Coating Ti cont. of Upper limit of Substrate Substrate Alloying Alloying Surface Powdering Run Weight Substrate substrate temp. temp. temp. after temp. time condi- resistance (g/m 2 (-240W+292)"C plating (hr) tion Ex. 36 50/ 50 0.30 220 220 240 220 50 good good 37 100/100 0.05 280 180 220 280 1 38 100/100 0.05 280 180 220 320 1 39 100/100 0.05 280 180 220 320 100/100 0.05 280 180 220 220 41 100/100 0.05 280 180 220 220 42 100/100 0.05 280 280 320 280 1 43 100/100 0.05 280 280 320 320 1 I 44 100/100 0.05 280 280 320 320 100/100 0.05 280 280 320 220 46 100/100 0.05 280 280 320 220 47 100/100 0.30 220 180 220 260 3 48 100/100 0.30 220 180 220 260 49 100/100 0.30 220 180 220 220 100/100 0.30 220 180 220 220 51 100/100 0.30 220 220 260 260 3 I 52 100/100 0.30 220 220 260 260 50 53 100/100 0.30 220 220 260 220 15 good good L r I I Table 1 (4) Coating Weight (g/m 2 Ti cont. of Substrate (wt%) Upper limit of substrate temp.

(-240W+292) °C Substrate temp.

(IC)

Substrate temp. after plating Alloying temp.

Alloying time (hr) Surface condition Ex. 54 100/100 0.30 220 220 260 220 50 good Comp. 1 Ex. 2 3 4 6 7 8 9 11 12 13 14 16 17 10/ 10 10/ 10 10/ 10 10/ 10 10/ 10 10/ 10 10/ 10 10/ 10 5Q/ 50 50/ 50 50/ 50 50/ 50 50/ 50 50/ 50 50/ 50 50/ 50 100/100 0.05 0.05 0.05 0.05 0.30 0.30 0.30 0.30 0.05 0.05 0.05 0.05 0.30 0.30 0.30 0.30 0.05 160 330 240 240 160 330 200 200 160 330 240 240 160 330 200 200 160 Zn remains good good Zn remains Zn remains good good Zn remains Zn remains good good Zn remains Zn remains good good Zn remains Zn remains Poor adhesion poor poor good Poor adhesion poor poor good Poor adhesion poor poor good Poor adhesion poor poor good Poor adhesion

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Table 1 Comp.18 Ex. 19 21 22 23 24 Coating Weight (g/m 2 100/100 100/100 100/100 100/100 100/100 100/100 100/100 Ti cont. of Substrate (wt%) 0.05 0.05 0.05 0.30 0.30 0.30 0.30 Upper limit of substrate temp.

(-240W+292) °C Substrate temp.

330 240 240 160 330 200 200 Substrate temp. after plating Alloying temp.

Alloying Surface time (hr) condition good good Zn remains Zn remains good good Zn remains Powdering resistance poor poor good Poor adhesion poor poor good Outside of the claimed condition i

Claims (2)

1. A process for preparing alloyed-zinc-plated steel sheet having excellent deep drawability, which comprises adjusting the temperature of a steel sheet substrate of titanium-killed steel to within the range defined as 180 C) i- 240xW 292, where is the Ti content of the steel, subjecting the substrate to zinc plating to coat a surface of the substrate with Zn by vacuum vapour deposition, and thereafter subjecting the Zn-plated substrate to a heat treatment for alloying the Zn with a surface layer of the substrate, the steel having C _0.01%, Si from 0.15 to 0.85% Mn, from 0.05 to 0.30% Ti, P S (0.02% and Al (0,05% with the balance, apart from incidental impurities, 0o substantially being Fe and the heat treatment 0 0 oo comprising heating the Zn-coated substrate at from 0000 0 0 220 to 320 C for a period of from 1 to 50 hours in o o accordance with the shaded area enclosed by and .0 C including the dashed line of Figure 2 of the 00 C c accompanying drawings, with the maximum temperature 0 C of the heat treatment increasing inversely with Ti content from 280°C for a Ti content of 0.3% to 320 C for a Ti content of 0.05%.

2. A process according to claim 1, substantially cc as herein described with reference to any one of S Examples 1 to 53. C C cc C CCCCC C C DATED: 6 JANUARY, 1990 C C cccc PHILLIPS ORMONDE FITZPATRICK Attorneys For: NISSHIN STEEL COMPANY, LTD -13- Ce;: I--