CA1190353A

CA1190353A - Process for preparing hot-dip zinc-plated steel sheets

Info

Publication number: CA1190353A
Application number: CA000414918A
Authority: CA
Inventors: Takehiko Ito; Kiichiro Katayama; Fumihiro Ida; Yorimasa Mitani; Yasushi Miyoshi
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1981-11-18
Filing date: 1982-11-04
Publication date: 1985-07-16
Also published as: DE3242625C2; FR2537161A1; KR840002463A; AU9022982A; JPS6330984B2; DE3242625A1; FR2537161B1; GB2110248A; KR890001829B1; AU540419B2; JPS5891162A; GB2110248B

Abstract

Abstract of tile Disclosure A process for preparing hot-dip zinc-plated steel sheets characterized by using a plating bath essentially consisting of 0.35 - 3.0 % A1 0.15 - 1.0 % Mg, balance Zn and inevitably impurities whereby the Pb content is not more than 0.015 % is disclosed. The products are better than the products of the process using the conventional composition plating bath both in corrosion resistance, and surface appearance without sacrificing workability.

Description

'I`l.tlc o[ ~ he .Lnventic)n E'rocess to~- prepdrln(3 hot-clip zinc-plated steel stlelLs I`ecl1nicc~ cld of the Invention I`hls invention relates to a process for preparirlc3 hot-dip zinc-plated steel sheets which have excellent corrosion resistance and excellent coatability.
Background of the Invention Ln recent years, use of hot-dip zinc-plated steeL
sheets has become rnore diversified and sophis-ticated.
Whereas these steel sheets have conventionally been used for roo~s and walls, they are now used as materials for autornobiles, as precoated materials for colored sheets, e-tc. That is, in these uses the steel sheets are subjected to severe plastic worlcing such as bending, deep drawing, etc. and the finished produc-ts must have excellent surface properties and appearance. So, there is increasing demand for products which are far better than the conventional ones not only in the formabilitv of the substrate sheet but also in the formability of -the alloyed layer, as well as in the corrosion resistance of the alloyed layer of the formed parts.
In order to improve the corrosion resistance of hot-dip zinc-plated steel sheets, two me-thods have hitherto been resorted to. One is to increase the thickness of plated zinc layer, and the other is to improve the quality of the plated zinc layer i-tself by incorporating o-ther elements in the plating bath The former will enhance the protec-tive effect of the zinc layer for the iron subs-trate, but a thick layer is susceptible to cracking when the plated sheet is worked, and flaking o~ the plated layer and deteriora-tion in the surface appearance rnay result Therefore, this method is prac-tically unacceptable.
~ q Regarding the latter rnethod, ~. S Patent No.
3'5 4,02~,~78 proposes use of a zinc bath containing 0.2 - 17 3~

Al, 0.003 - ().15 % M~ and 0.02 - 0.15 ~ Pb. Use of ti1iS
zinc bath ~ives considerdhly improved corrosion resistance.
~lowever, a close study revealed thcl~ the resultir1cJ corrosio resistance is not entirely sufficient and -that this method does not yive sheets with satisfactory coatability. The reason is surmised to be that the balance among the added elements such as Al, Mg and Pb is not completely satisfactory. More specifically -the Pb content is rather too high and the Mg content is insufficient rela-tive to the Pb content.
In order -to irnprove the surface appearance and mechanical properties of zinc plated steel sheets, it is also generally practiced to cool the plated sheet by spraying water mist immediately before the plated zinc solidifies after plated so as to minimize the size o~ the zinc crystal grains, or to smooth the surface of the sheet by stretcher leveling and temper rolling in combination.
However, rapid cooling of the plated layer or mechanical smoothing increases the number of ac-tivated points at which corrosion may be initiated. Therefore, these me-thods are not desirable from the viewpoint of corrosion resistance, and especially excessive temper rolling should be avoided.
Further, when use of zinc-plated steel sheets for precoated sheets or automobile materials is considered, a thinner plated layer is preferred because these materials have to be subjected to shaping and welding and thinly plated sheets are advantageous in this respect. Therefore, a steel sheet with a thinner plated layer and high corrosion resistance is desired. Modernly, the thickness of plated layers is controlled by gas-wiping under high speed operation (160 - 200 m/min). Under such high speed operation, the amoun-t of plated zinc is in the range of ~5 ~ 60 g/m~ per side (the amount of plated zinc will be indicated per surface hereinafter) with the conventional plating bath (0.15 - 0.18 ~ ~l*). This is far from the * All composition percentages yiven by weigh-t unless otherwise speci~ied.
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targe-t oE 30 g/m or less, which is desirable from the standpoin-t of working of the plated sheets. The measure generally employed in order to reduce the amount of plated ~:inc is to slow the plating speed by 20 - 30 % below the standard speed to increase gas wiping effect. However, this lowers the productivity and therefore is no-t desir-able in an industrial process.
We have studied -the above problem and ~ound that the key to the solution fundamen-tally resides in the property of the plating bath. Thus we have completed this invention.
Summary of the Invention According to this invention, there is provided a novel zinc plating bath essentially consisting oE 0.35 -3.0 % Al, 0.15 - 1.0 % Mg, and the balance of Zn and inevitable impurities, whereby the Pb content is not more than 0.015 %.
Further, a process for preparing hot-dip zinc-plated sheets characterized in that a plating bath essentially consisting of 0.35 - 3.0 % Al, 0.15 - 1.0 %
My, balance Zn and inevitable impurities, whereby the Pb content is no-t more than 0.015 %.
In the preferred embodiment, the amount of the plated zinc is not more than 30 g/m per side. Also the plated shee-ts are heated so as to form alloyed layers.
A preferred bath composition is: 0.35 - 2.0 %
Al, 0.15 - 0.8 % Mg, balance Zn and inevitahle impurities, whereby the Pb content is not more than 0.01 %
A more preferred bath composition is: 0.35 -1.0 % Al, greater than 0.20 - 0.5 % Mg, balance Zn and inevitable impurities, whereby the Pb content is not more than 0.007 %.
By inevitable impurities is meant impurities contained in th~ commercially available ~inc, that is, not more than 0.01 % Cd, not more than 0.01 % Sn and not more than 1.0 % Fe.
Lower limits for the amount of Mg in the bath composltion include greater than 0.20 %.

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~rllis inv~u~tion has the ~oLlowing advantages.
1. The~ corrosion resistance oE -the products is more than three times t~lat of the products produced with the~
conventional plating ba-tn containing 0.15 - 0.18 Al.

2. Smooth plated sheets with mlnimized spangles are obtained wi-thout particularly rapid cooling aEter the sheets have passed through the plating bath.

3. Produc-ts having excellent surface appearance and mechan:ical properties are obtained by sub~ecting the sheets to a light temper rolling (aroun~ 1 % reduction) aEter plating.

4. As a result of increasing the Al content in -the ba-th 2.5 - 20 times, -the Eluidity of the bath is increased and the wlping effect in gas wiping is enhanced under the same gas wiping condition at the same temperature and thus products with a thinner plated layer are obtained.
~ow each component of the bath composition and the concentration thereof are explained.
1. Aluminum As the Al content increases in the ho-t-dip zinc-plating bath, the fluidity of the bath at the same -temperature increases remarkably over that of -the conventional plating bath. When the Al content is increased from 0.15 % of the conventional bath to 0.3 % or more, the fluidity of the plating bath is increased by 1.5 - 2.0 %
or so. So -the lower limit of the Al content of the bath in this invention is defined as 0.35 %. The upper limit is defined as 3.0 %, since if the Al content exceeds this value, -the Zn-Al eutectic structure becomes remarkable and local cells are formed between the eutectic phase and the zinc phase and this induces deterioration in corrosion resistance.
2. Magnesium .~¢3~ 3 -- 5 ~

~ g is one of the elements added for the purpose of increasing corrosion resistance. The corrosion resistance of the ~ormed alloyed layer marked:Ly improved when about 0.15 ~ or more Mg is contained. As the Mg content increases, the corrosion resistance increases accordingly, but if it exceeds 1.0 %, deterioration in the surface appearance of the alloyed layer by ormation of wrinkles, and oxidation of the surface gradually becomes pronounced, and at the same time oxida-tion (formation of dross) on the surface of the pLa-ting bath is promoted, which results in wasting of -the pla-ting bath. The allowable upper limit of the Mg content is 1.0 %.
3. Lead Pb has almost no solid-solubility in Zn at ordinary temperature and therefore, it precipitates usually within the crystal grains or at the grain boundaries, as minu-te par-ticles which form local cells. This is one of the causes of deterioration in corrosion resistance. In this sense, it is desirable that the Pb content be as low as possible. It has been confirmed, as the result of close investigation, -that when the Pb content is not more than 0.015 %, microscopically almost no precipitation of Pb in grains and at grain boundaries of the alloyed layer is found, prac-tically no intergranular corrosion is found;
and almost no crystal spangles of Zn can be observed by the naked eye. Thus the Pb content is defined as not more than 0.015 ~.
By using the Zn plating ba-th of the above-mentioned composition, Zn-plated steel plates with a very -thin plated layer (not more than 30 g/m2) and having excellent appearance and corrosion resistance can be obtained under the conven-tional operation conditions.
It should also be noted that although the thinness of the plated layer is one of the characteristics of this inven-tion, the amount of the plated Zn can be varied by .33~3 6 ~

changing the conditions o~ gas wiping. Therefore, thicker plated layers can also he obt~ined.
Detailed Description of the Embodiments Now the invention is explained in detal:L b~ way of working exampl.es.
Example 1 Using a Zn plating bath, the composition oE which is shown in Table 1, non-annealed rimmed steel sheets 0.4 mm in thickness and 300 mm in wldth were plated with Zn with a conventional gas reduction plating apparatus under -the following conditions.
Pre-treatment:
Temperature of sheets at -the outle-t of the non-oxidizing furnace: 590 - 600C
Reduction furnace gas: H2 75 %I N2 25 %
Temperature of the gas at the outlet oE
the reduction furnace: 700 - 720C
Bath temperature: 460C + 5 C
Thickness of pla-ted layer: 120 g/m per side After-treatmen-t:
I'emper rolling: not employed After-treatment with chromic acid- not employed The methods for testing of charac-teristics of the obtained products and for evaluation of the test results were as follows:
Size of spangles: Observation by naked eye Adherence of plated layer: Plated sheets were lock-formed and a cellophane adhesive -tape was applied on the worked portion, whereafter flaking of the p.Lated layer was observed when the cellophane -tape was peeled off. No flaking is rated as 'good'.
Existence of Pb in grains and at grain boundaries:
Determined by means of a scanning electron microscope and rated as 'large', 'medium', 'small' and 'minute'.
Corrosion resistance:
(1) Time (hour) until generat.ion of red rust was observed in the salt water spray test (ASTM B117-73) was measured.
(2) Corros:ion weight loss (g/m2) was determined 200 hours after the salt water (ASTM B117-73).

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~ l~l3 3~3 lrhe test results are sumrnarized in Iable 1 together with the bath composition. The characteris-tics of the products of the process oE this invention and ~hose of the conventional process (comparative samples) are explained i.n comparison.
1. Spangle size:
The products of the process of t`r,is invention exhibited very minute spangles despite the Eact that the pla-ted sheets were allowed to spontaneously cool by themselves. The ra-ting 'minute' in the -tabl.e means that spangles were almost unrecognizable by naked eye and the surface appearance was very smoo-th. In contrast, appa:ren-t 'medium' and 'large' spangles could be observed by naked eye in all the comparative samples in wh.i.ch the plated sheets were cooled spontaneously, The spangles were 'small' only in sample No. 10 in which the sheets were forcibly cooled (water cooling). It is no-ted -that water cooling makes the spangles minute in all the comparative samples, although inferior in homogenuity. When water cooling was not employed, apparent spangles were observed and the surfaces were considerably rough. Therefore rather high level temper rolling was necessary ln order to smooth the surfaces.
2. Adherence of plated layer Although there was no problem in either the products of the comparative samples or those of -this invention in the adherence of the plated layer per se, occurrence of cracking in the plated layer at -the bent portion was very rare in the products of the process of this invention, while in the comparative samples considerably remarkable cracking in the plated layer within the crystal grains and at the grain boundaries were detected. The products of the process of this invention were much superior to the compara-ti,ve samples.
3. Existence of Pb in grains and at grain boundaries As the resuLt of restric-ting the Pb content to not more than 0.015 ~, existence of Pb in grair1s and at grain boundaries was very slight and could not be clearly discerned in the scal1ning electron microscopic photographs.

5 In contrast, in -the comparative samples in which the Pb content was high, exlstence oE Pb in grains and at grain boundaries was clearly observed, in all the samples except No. 4.
4. Corrosion resistance In dll the produc-ts of the process of -this invention, more than 1000 hours passed before red rust was generated, and the corrosion weight loss was less than 30 g/m2. On the other hand, red rust was generated within 800 hours in the rather better sample No. 11 and wi-thin 200 hours in the inferior samples No. 10 and 12. The corrosion weight loss of the comparative samples were 2 -

6 times as great as that of the products of the process of this invention.
Example 2 Using a hot-dip zlnc plating bath of the composition shown in Table 2, very thinly plated zinc-plated steel sheets were prepared with the same apparatus as used in Example 1 under the following conditions.
Substrate sheets: 0.4 mm thick rimmed steel sheets Pre-treatment: the same as in Example 1 Thickness of plated layer (condition of gas wiping) Gas pressure: 0.35 kg/cm Position of nozzle: 150 mm from the bath surface Distance between strip and nozzle tip. 6 mm After-treatment:
Temper rolling: not employed After treatment with chromic acid: not employed The test results are summarized in Table 2 together with the bath composition. The charac-teristics oE the products of the process of this invention and those o~ the convent.ional process (comparative samples) are explained in comparison.

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_ ~ o s a I dUI ~ S aA ,1 __ ._ _ 1. Amount oE pla-ted zinc It is apparent from the table that the dmount of the pla-ted zinc is around 10 g/m2 per side in the products of this invention, meaning that it is very thin plating, while the amount of the plated zinc is about twice that, in -the comparative samples. This means that the process of this inven-tion is very effective in controlling the thickness of the plated layer by gas wiping.
2. Corrosion resistance As the plated layer is very thin in the process of this invention, the corrosion resistance oE the products was a matter of concern. ~lowever, i-t was conEirmed tha-t considerably high level corrosion resistance was acquired as seen in the table. That is, the -time required for generation of red rus-t was 270 hours in Samples No. 17, 140 hours and 180 hours respective in Nos. 16 and 18. The time before the generated red rust spread to about 30 %
of the tested area was 840 hours, 520 hours and 580 hours respectively in Nos. 17, 16 and 18. That is, development of corrosion was considerably slow and good corrosion resistance was recognized. On -the other hand, the amount of the plated zinc in the comparative samples was about twice tha-t of -the samples of the process of -this invention, and yet the area covered by red rust exceeded 30 % in 100 hours in No. 19 and exceeded 30 % in 450 hours even in No.
20, which had a thick pla-ted layer. This fact means that the process of this invention is superior to the conventional process even in the corrosion resistance of the products.
It has been also confirmed that alloyed zinc plated steel sheets which are obtained by heat--treating the plated sheets made in accordance with the process of this invention are provided with far better corrosion resistance than the products produced by using the conventional plating bath.

3~3 Also i.t has been confirmed that single side zinc-plated steel shee-ts Eor automobiles produced on the trial base by using a plating inhibitor are e~cellent in bo-th corrosion resistance and workability.
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The zinc-plated steel shee-ts produced by the process of -this invention are expec-ted to find use as pre-coated sheets for colored sheets, automobi.le materials, materials for householcl electric appliances as well as in industrial fields which will be developed from now on, in addition to the conventional use as materials for roofs and walls. The invention of this application will be highly evaluated in its industrial application.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing hot-dip zinc plated steel sheets which comprises using a plating bath consisting of about 0.35 to 3.0 % Al, greater than 0.20 to less than 1.0 %
Mg, and the balance being Zn and inevitable impurities whereby any Pb content is not more than about 0.01 %, wherein all percentages are by weight.

2. A process as claimed in claim 1, wherein the A1 content is about 0.35 to 2.0 %, the Mg content is greater than 0.20 to about 0.80 %.

3. A process as claimed in claim 1, wherein the Al content is about 0.35 to 1.0 %, the Mg content is greater than 0.20 to about 0.5 % and the Pb content is not more than 0.007 %.

4. A process as claimed in claim 1, wherein the amount of plated zinc is not more than 30 g/m2 per side.

5. A process as claimed in claim 2, wherein the amount of plated zinc is not more than 30 g/m2 per side.

6. A process as claimed in claim 3, wherein the amount of plated zinc is not more than 30 g/m2 per side.

7. A process as claimed in claim 1, wherein only one side of the sheets is plated.

8. A process as claimed in claim 2, wherein only one side of the sheets is plated.

9. A process as claimed in claim 3, wherein only one side of the sheets is plated.

10. A hot-dip zinc-plated steel sheet which has been prepared by the process claimed in claim 1, 2 or 3.

11. A hot-dip zinc-plated steel sheet which has been prepared by the process claimed in claim 4, 5 or 6.

12. A hot-dip zinc-plated steel sheet which has been prepared by the process claimed in claim 7, 8 or 9.

13. A hot-dip zinc plating bath consisting of about 0.35 to 3.0 % A1, greater than 0.20 to less than 1.0 % Mg and the balance being Zn and inevitable impurities whereby any Pb content is not more than 0.01 %, wherein all percentages are by weight.

14. The hot-dip zinc plating bath as claimed in claim 13, wherein the A1 content is about 0.35 to 2.0 % A1, the Mg content is greater than 0.20 to about 0.8 %.

15. The hot-dip zinc plating bath as claimed in claim 14, wherein the A1 content is about 0.35 to 1.0 %, the Mg content is greater than 0.20 to about 0.5 % and the Pb content is not more than 0.007 %.

16. A process as claimed in claim 1 wherein the Pb content is not more than 0.007 %.

17. A process as claimed in claim 2 wherein the Pb content is not more than 0.007 %.

18. A process as claimed in claim 16 wherein the amount of plated zinc is not more than 30 g/m2 per side.

19. A hot-dip zinc-plated steel sheet which has been prepared by the process claimed in claim 16, 17 or 18.

20. A process as claimed in claim 1, 2 or 3 wherein the Mg content is not less than 0.25 %.

21. A process as claimed in claim 4, 5 or 6 wherein the Mg content is not less than 0.25 %.

22. A process as claimed in claim 7, 8 or 9 wherein the Mg content is not less than 0.25 %.

23. A process as claimed in claim 16 wherein the Mg content is not less than 0.25 %.

24. A process as claimed in claim 17 wherein the Mg content is not less than 0.25 %.

25. A process as claimed in claim 23 wherein the amount of plated zinc is not more than 30 g/m per side.

26. A hot-dip zinc-plated steel sheet which has been prepared by the process claimed in claim 23 or 24.

27. The process of claim 1, 18 or 25 including subjecting the sheets to a light temper rolling for about 1 % thickness reduction after plating.