CA1178182A

CA1178182A - Process for preparing aluminum-plated steel sheets having low yield strength and high oxidation resistance

Info

Publication number: CA1178182A
Application number: CA000368870A
Authority: CA
Inventors: Arihiko Morita; Seiichi Hamanaka; Norio Tsukiji
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1980-01-22
Filing date: 1981-01-20
Publication date: 1984-11-20
Also published as: DK22381A; IT8167019A0; ES498742A0; NL182414C; SE449758B; DK157690B; NZ196063A; DE3101850A1; AU6629181A; NL182414B; JPS633929B2; AU538073B2; SE8100310L; JPS56102523A; GB2069001A; FR2474060B1; IT1143260B; ZA8167B; BR8100316A; DE3101850C2

Abstract

Abstract of the Disclosure A process for preparing molten-aluminum-plated steel sheets having good formability and high oxidation resistance at elevated temperatures is disclosed. Said process comprises producing a steel containing 0.001 -0.020 % of C, 0.05 - 0.30 % of Mn, 0.05 - 0.50 % of Cr, 0.01 - 0.10 % of Al, and 0.10 - 0.50 % and not less than 10 times the C content of Ti, and making the steel into a hot coil with the coiling temperature not lower than 700°C, cold-rolling said hot coil and plating the cold-rolled sheet by heating it to a temperature not lower than 850°C using an in-line annealing type plating apparatus with non-oxidizing furnace.

Description

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Ti-tle of the Invention A process for preparing aluminum-plated steel sheets having low yield strength and high oxidation resistance T hnical Field of the Invention This invention relates to a process for preparing molten-aluminum-plated steel sheets which have low yield strength and exhibit low oxidation weight gain when subjected to oxidation at high temperatures.
Back~round of the Inventio Heretofore, molten-aluminum-plated steel sheets (hereinafter simply referred to as "aluminum-plated steel sheets") for use wherein heat resistance and corrosion resistance are required have been mainly made of cold-rolled sheets of low carbon rimmed steels. However, it 1.5 is well known that aluminum-plated steel sheets made of rimmed steel substrate sheets incur degradation in quality due to quench aging caused by rapid cooling at -the time of plating, which hardens the material.
The main measures that can be employed to prevent the above mentioned degradation are:
(1) To use substrate sheets made of a steel from which C and N, which cause quench aging, have been removed as completely as possible;

(2) To add a carbide-forming element such as Ti to fix C and N in the substrate material, which cause quench aging;

(3) To ef~ect over-aging of the aluminum-plated sheets in which quench aging has occurred;

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and so forth.
S~owever, the measure (1) is not economical in the case when steel sheets are plated by the in-line annealing type hot dip plating apparatus with a non oxidizing furnace, although it can be realized in the ordinary steel-making process by employment of decarburizing annealing.
The measure (2) is economical per se, but when it is applied to low carbon steels which are to be obtained by the ordinary converter process only, a considerable amount of Ti must be used and highly oxidizable Ti produces not a small amount of oxide inclusion which results in degradation of the surface quality of the product. Therefore, this measure is not, in the f.inal analysis, either economically or technically desirable.
The measure (3) is economical. But this treat-ment may increase the Fe-Al alloy layer, which has poor formability, even if the al~lminum pla-ting has been carried out under the conditions that control formation of the Fe-Al alloy layer. Therefore, by this treatment, although the property of the substrate material is improved, the formability of the aluminum-plated layer is impaired.
The assignee of this application previously obtained a Japanese patent for an invention on a process for preparing aluminum-plated steel sheets comprising:
?5 hot-rolling a steel essentially consisting of C: 0.001 -0.020 %, Si: ' 0.05%, Mn: 0.05 - 0.40 ~0, Cr: 0.10 - 0.30 %, effective Ti ttotal Ti less Ti in the oxide form): 0.03 -0.40 ~ and that not less than 4 times (C ~ N) %, N: ~ o.oo6 %~

o: c 0.020 ~, and the balance of Fe and inevitable incidental impurities at a temperature not lower than 800C; cold-rolling the hot coil at a reduction rate of 40 ~ or more; annealing the rolled sheet at 800 - 950C, and immersing the sheet in an Al-Si alloy bath (Si: ~ 10 %) maintained at 640 - 700C for not more than 10 minutes (Japanese Patent Publication No. 35532/76).
The aluminum-plated steel sheet obtained by this process has a yield point strength of 13 - 17 kg/mm2 and an elongation of 44 - 47 % in the state of a cold-rolled sheet of 0.8 mm thickness. Today, however, demand for more easily formable materials is rising.
Disclosure o~ the Invention We have studied ways fo]^ meeting this demand and have arrived at the idea of employing the following three measures to obtain improved aluminum-plated steel sheets which have good formabilit~ with low yield strength and are very low in the high temperature oxidation weight gain.
(1) To lower the content of C and 0 in the substrate steel sheet by employing vacuum degassing treatment and preliminarily deoxidizing with Al in the stage of steel-making and thus reducing the Ti con-tent which is necessary to fix C and N and controlling formation of oxides.
(2) To reduce the content of Mn which raises yield strength and to add Cr which lowers yield strength.
(3) To promote agglomeration and growth of the ~ ~ " ".. ...

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precipitated Ti carbide by coiling the finished hot coil at a temperature as high as 700C and thus to preventing hardening by formation of carbide of the Ti added to the substrate material. And to further promotion of agglomeration of the Ti carbide by heating the cold-rolled sheet at a temperature of 850C or more when it is passed through an in-line annealing type hot dip plating apparatus with a non-oxidizing furnace (which is commonly called NOF type - plating apparatus in Japan).
Additionally, the invention of this application is characterized in that high temperature oxidation resistance of the material is improved by combined addition of Ti and Cr. That is, oxidation weight gain at high temperatures is remarkably reduced by decarburizing the substrate material, and it is further improved by addition of Ti. The reasons are~ (1) By decarburizing or addition of Ti, cleanliness of the iron material is improved, and Al of the aluminum layer easily diffuses into the Fe substrate and an Al diffusion layer, having excellent high temperature oxidation resistance is formed; and (2) The Ti in the material diffuses toward the surface and forms a ~i-concentrated layer under the Al diffusion layer when the material is subjected -to high temperature, and thus prevents the further diffusion of Al into the interior, thereby retarding decrease in the Al concentration in the surface layerl and also fixes oxygen which has penetrated into the iron.
~ hus according -to this invention a process for preparing molten-aluminum-plated steel sheets having low ' . , , ~ 17~ 3Z

yield streng-th and high resistance to high-temperature oxidation comprising: producing a steel the chemical composition of which essentially consisting of C: 0.001 -0.020 %, Mn: 0.05 - 0.30 ~, Cr: 0.05 - S%, Al: 0.01 -0.10 %, Ti: 0.10 - 0.50 % and that not less than 10 times the precentage of C, the balance consisting of Fe and inevitable incidental impurities, by the ordinary converter-refining and vacuum degassing; making it into a slab by the ordinary casting and slabbing or the ordinary continuous casting; continuously hot-rolling said slab coiling it at a temperature not lower than 700C; cold-rolling the resulting hot coil after the ordinary pickling treatment; heating the cold-rolled sheet at a temperature not lower than 850C; and plating it with molten aluminum by means of an in-lining annealing type hot dip plating apparatus with a non-oxidizing furnace is provided.
In t`he pre~erred embodiment, the C content is 0cOOl - 0.010 %, the Mn conten-t is 0.05 - 0.20 %, the Cr content is 0.07 - 0.45 %, the Al content is 0.02 -0.05 %, and the Ti content is 0.15 - 0.40 % and not less than 20 times the C content.
In the more preferred embodiment, the C content is 0.001 - 0u007 %, the Mn content is 0.10 - 0,17 %, the Cr content is 0.07 - 0.42 ~0, the Al content is 0.03 - 0.041 %, and the Ti content is 0.19 - 0.23 % and not less than 30 times the C content, the hot coil coiling temperature is 720 - 730C and the heating temperature at plating is 860 - 900C.

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The reasons for the numerical limitations defined in the main claim are as follows.
The lower the carbon content is, the more the effect of quench aging is reduced. Therefore, i-t is desirable to reduce the C content as much as possible.
But it is not easy to reduce the C content to less than 0.001 % even by the modern steel-making process in which vacuum degassing is employed. Even if it can be achieved, it is not an economical operation. Therefore, the lower limit of the C content is defined as 0.001 %. The reason why its upper limit is defined as 0.020 % is that if the C content is over this limit, the amount of Ti to be added for prevention of the undesirable effect of C inducing quench aging must be uneconomically increased.
The reason for defining the Mn content as 0 05 -0.30 % is that it is difficult to obtain a steel the Mn content of which is less than 0.05 % by the ordinary steel-makin~ process and when the Mn content exceeds 0.30 %, the steel becomes hard and, as a consequence, has high yield strength.
The reason for defining the Cr content as 0.05 -0.50 % is that Cr in the amount of less than 0.05 % does not give sufficient effect in reducing yield ratio, and on the other hand, more than 0.50 % of Cr also reduces said effect.
Al is used for deoxidation of the mol-ten steel and, especially in this invention, it plays an important role as the preliminary deoxidation material which prevents ~L~7~:18 was-teful use of Ti. From this point of view, the lower limit of the Al content is defined as 0.01 %. However, if Al is added in an amount over 0.10 %, the surface properties and formability of the resulting steel sheet are impaired.
The reason why the Ti content is defined as 0.10 - 0.50 % and 10 times the C content is as follows.
If the Ti content is less than 0.10 %, the effect of improving high temperature oxidation resistance as represented by oxidation weight gain is not sufficient, although the yield strength is rather low, On the other hand, if the Ti content is in excess of 0.50 %, the material becomes hard and loses its low yield strength characteristic, although the oxidation weight gain becomes smaller. If the Ti content is less than 10 times the C content, the fixation o~`C with Ti is not sufficient, resulting in a rise in the yield strength and an increase in the oxidation weight gain, which eliminate the charac-teristics of this invention~
In the process of this invention, Si, P and S
as the inevitable impurities can be present to the extent that is ordinary and common in steels of this kind. Nitrogen and O can, without any inconveniences, be present at the levels usually attained by the vacuum degassing process.
The reason why the coiling temperature is defined as not lower than 700C is that at temperature lower than this, softening of the material owing to agglomeration and growth of the Ti precipitate, which has been formed by fixation of C with Ti, is not sufficient, and thus one of the features - :~L17~3182 of this invention is lost. Further it is required that in the aluminum plating line the cold-rolled sheet must be heated at not lower than 850C before entering the plating bath in order to give the cold-rolled sheet an annealing effect so that the Ti precipitate further agglomerates to larger particles and thus softens the material.
In the process of this invention, the composition of the substrate steel is different from that of the substrates steel of the method of the aforementioned Japanese Patent Publication r~O. 3~532/76 only in that the steel of this invention contains Al added intentionally. But the procedure is completely different from -that of said invention.
Description of Embodiments of the Inven-tion Now the invention is ex]?lained in detail specifi-cally on the basis o~ the~results of experiments, Steel samples, the compositions of which are indicated in Table 1, were prepared by converter refining and vacuum degassing. The ingots were subjected to slabbing and continuous hot-rolling and the rolled products were coiled at varied temperatures as indicated therein and hot coils of 2.5 mm thickness were obtained. After ordinary pickling, the hot coils were made into cold-rolled sheets of 0.8 mm thickness. The thus prepared cold-rolled sheets were heated at varied temperatures and plated with aluminum (60g/m2) by means of an in-line annealing type hot dip plating apparatus with a non-oxidi~ing furnace (practically, a modified Sendzimir apparatus), under the ordinary conditions. The aluminum-plated - ~ :
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-~L7~ 32 sheets were subjected to the material test and oxidationtest. The results (mechanical properties and oxidation weight gains) are shown in the same table. The material tests were carried out with specimens prepared according to JIS (Japanese Industrial Standards) Z-2201 No. 5 Cllt in the direction of rolling, and the oxidation test with five repeated runs of a cycle of holding the samples at 830C in -the atmosphere for 48 hours and cooling to room temperature.
In Table 1, samples D, E and F are within the composition range defined in this invention and F is within the scope of the invention in the heating temperature before the plating. This table shows that materials excellent both in formability and oxidation resistance are obtained only when all three fac-tors of the composition of -the substrate s-teel, the coiling temperature and the plating temperature satisfy the conditions defined in this invention.
When sample A and B are compared, it is learned that decrease in the ~ content contributes to lowering of the yield strength. Comparison of B and C shows that addition of Ti contributes to lowering of the yield strength, too. Comparison of C and F shows tha-t combined addition of Ti and Cr also contributes to lowering of the yield strength.
2~ Table 2 shows mechanical properties of the aluminum-plated steel sheet samples which were prepared by obtaining the steels with the compositions indicated therein and treating them in the same way as above.

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According to -this table, it is learned that Cr is effective for lowering the yield strength when contained in the amount of about 0.05 - 5 %
Table 3 shows mechanical properties and oxidation weight gains of the aluminum-plated steel sheet samples which were prepared by obtaining the steels the compositions of which are indicated therein and treating them in the same way as above. According to this table, it is learned that when the amount of the added Ti is 0.1 % or more and that more than 10 times the amount of C, the effects of lowering the yield strength and oxidation weight gain are achieved.
But as seen in sample G, when the Ti content exceeds 0.5 %, the yield strength again rises.
Table 4-2 shows the results of the same tes-ts as above carried out with respect to -the samples prepared in the same way as above from steels with -the compositions indicated in Table ~-1. Accordln~ to -this -table, it is learned that when the coiling temperature is lower than 700C, the softening of the material is not sufficient, and when the heating temperature before plating is 850C or higher, the softening is remarkable.
Preferred embodiments of this invention are described below.
Example 1 Ingots, the compositions of whlch are indicated in Table 5 as sample No. 1 and No. 2, were obtained by vacuum-degassing molten steel prepared by an LD converter so as to reduce the contents of C and 0, and thereafter ~`

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adjusting the composition by addition of ferro alloys such as ferrochromium, ferrotitanium, ferromanganese, etc. The ingots were made into slabs and the slabs were hot-rolled into 2.5 mm thick hot coils under the conditions indicated in Table 6. After pickling, the hot coils were cold-rolled into 0.8 mm thick sheets. The cold~rolled sheets were plated with aluminum by means of an in-line annealing type hot dip plating apparatus with a non-oxidizing furnace (a modified Sendzimir apparatus) under the conditions indicated -Table 6. The mechanical properties and oxidation weightgain of the thus obtained aluminum plated steel sheets as tested in the same manner as above are shown in Table 6.
Both sheets exhibit excellent properties --- low yield strength and high oxidation resistance at high temperatures.
1~ Example 2 Slabs, the compositions of which are indicated in Table 5 as sample No. 3 and No~ L~, were obtained by continuous cas-ting after smelting in the same way as in Example 1. The slabs were made into aluminum-plated steel sheets in the same way as described in Example 1. Mechanical properties and the test results of the thus obtained steel sheets as tested in the same way as in Example 1 are shown in Table 6. These sheets have excellent characteristics comparable to those of the sheets of Example 1.
2~ Industrial ApPlicability The products of this invention are suitable for manufacturing parts with complicated shapes used at high temperatures such as exhaust gas treating apparatuses for internal combustion engines.

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Claims

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

1. A process for preparing molten-aluminum-plated steel sheets having low yield strength and high resistance to high-temperature oxidation comprising: producing a steel the chemical composition of which essentially consists of C: 0.001 - 0.020 %, Mn: 0.05 - 0.30 %, Cr: 0.05 - 0.50 %, Al: 0.01 - 0.10 %, Ti: 0.10 - 0.50 %, the Ti content being not less than 10 times the percentage of C, the balance consisting of Fe and inevitable incidental impurities, by the ordinary converter-refining and vacuum degassing;
making it into a slab by the ordinary casting and slabbing or the ordinary continuous casting; continuously hot-rolling said slab coiling it at a temperature not lower than 700°C; cold-rolling the resulting hot coil after the ordinary pickling treatment; heating the cold-rolled sheet at a temperature not lower than 850°C; and plating it with molten aluminum by means of an in line annealing type hot dip plating apparatus with a non-oxidizing furnace.

2. The process as claimed in claim 1, wherein the C content is 0.001 - 0.010 %, the Mn content is 0.05 -0.20 %, the Cr content is 0.07 - 0.45 %, the Al content is 0.02 - 0.05 %, and the Ti content is 0.15 - 0.40 %
and not less than 20 times the C content.

3. The process as claimed in claim 2, wherein the C content is 0.001 - 0.007 %, the Mn content is 0.10 -0.17 %, the Cr content is 0.07 - 0.42 %, the Al content is 0.03 - 0.041 %, and the Ti content is 0.19 - 0.23 % and not less than 30 times the C content, the hot coil coiling temperature is 720 - 730°C and the heating temperature at the plating is 860 - 900°C.

4. Molten-Aluminum-plated steel sheets having low yield strength and high resistance to high-temperature oxidation produced by the process as described in claim 1, 2 or 3.