NL8100030A - METHOD FOR PREPARING A STEEL SHEET WITH ALUMINUM COATING - Google Patents

Description

* t -1- 21701 / Vk / yg

Applicant: Nisshin Steel Company, Ltd., Tokyo, Japan.

Short indication: Method for preparing a stable egg plate, provided with an aluminum layer.

The invention relates to a method for preparing a stable egg plate, provided with an aluminum layer with a low tensile stress and a high resistance to oxidation at an elevated temperature.

Heretofore, steel plates provided with an aluminum layer have been obtained by applying molten aluminum thereon, further referred to as 10 steel plates provided with an aluminum layer, which are used to obtain a high resistance to heating and a good corrosion resistance. manufactured primarily by cold rolling plates obtained from low carbon, mild steel. However, it is well known that steel plates coated with an aluminum layer and made of troubled steel are subject to a deterioration in quality due to quench aging, caused by rapid cooling during the manufacture of the plates, resulting in the material hardens.

The main measures that can be applied to prevent the above-mentioned deterioration in quality are: 1) the use of substrate plates, made of steel from which carbon and nitrogen, which effect the aging by quenching, have been removed, as completely as possible, 2) the addition of carbide-forming elements, such as titanium, to fix carbon and nitrogen in the substrate material, which accomplishes quench aging, 3) further aging the aluminum-coated plates with quenching aging.

However, the first measure is economically difficult to implement when the steel plates are coated by an located tempering and hot-dipping device with a non-oxidizing oven, although this measure can be accomplished by a known process for preparing steel using a decarburizing tempering operation. The second measure can be carried out economically, but when using low carbon steels obtained by the ordinary converter process, a considerable amount of titanium must be used and it gives very good x-oxidizable titanium a minor amount of oxide inclusion 8100030 f * ♦ -2-21701 / Vk / jg seis, resulting in a reduction in the surface quality of the product. Therefore, this measure has ultimately proved economically or technically inappropriate.

The third measure can be carried out economically, but this treatment can bring about an increase in the Fe-Al alloy layer, resulting in poor ductility, even when the aluminum coating is carried out under conditions such that the formation of the Fe-Al alloy layer can be controlled. Therefore, it can be stated that this treatment, although improving the property of the substrate material, adversely affects the deformability of the aluminum coated sheet.

According to the applicant's Japanese patent 35532/76, a method is known for preparing steel plates coated with aluminum, wherein steel mainly consists of 0.001-0.020% C, 15 <0.05% Si, 0.05-0 , 40% ffa, 0.10-0.30% Cr, an amount of titanium (the total titanium content minus the titanium in the oxide form) of 0.03-0.40% and which is not less than 4 times the content a carbon + nitrogen and <0.006% N and <0.020% 0, the remainder being, in addition to unavoidable impurities, iron, which plates are rolled hot at a temperature not lower than 800 ° C, then cold rolled, with the hot Spiro * undergoes a thickness reduction of 40% or more, tempering the rolled sheet at a temperature of 800-950 ° C and immersing the sheet in an Al-Si alloy bath less than 10%) and kept at a temperature of 640-700 ° C for no more than 10 minutes.

The aluminum clad steel sheet obtained by this process 2 has a yield strength of 13-17 kg / mm and an extensibility of 44-47% in the state of the cold rolled sheet with a thickness of 0.8 mm. Today, however, there is a greater demand for more easily deformable materials.

Therefore, further research has been carried out to meet this demand and the following three measures have been further investigated to obtain improved steel plates which are provided with an aluminum layer with a good deformability and with a low tensile stress and which have a low have weight gain from oxidation. These measures 35 are: 1) lowering the carbon and oxygen content in the steel sheet, which is started using a degassing treatment under reduced pressure and pre-deoxidizing with aluminum at 8100030

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-21- 21701 / Vk / yg steel making ai thus, it is possible to reduce the titanium content necessary to fix carbon and nitrogen and thereby control the formation of oxides, 2} decreasing the manganese content, which increases the tensile stress. and adding chromium, thereby reducing the tensile stress, 3) promoting agglomeration and growth of the precipitated tifcan carbide by allowing the final hot spiral shaped product to roll up at a temperature above 700 ° C, thereby curing prevented by the formation of titanium carbide, which titanium is added to the substrate material. Also, a promotion of the agglomeration of the titanium carbide is accomplished by heating the cold rolled plate at a temperature of 850 ° C or higher by passing it through an inline tempering and hot dipping device with a non-oxidizing furnace, which is commonly referred to as NOR type coating equipment.

Furthermore, according to the invention, the high temperature oxidation resistance of the material is improved by the combined addition of titanium and chromium. As a result, the increase in weight at high temperature by oxidation is markedly reduced by decarburizing the substrate material and further enhanced by the addition of titanium. The reasons for this are that 1) by decarburizing or adding titanium the purity of the iron material is improved and aluminum or the aluminum layer readily diffuses into the iron substrate and an aluminum-containing diffusion layer with excellent resistance to high temperature oxidation is formed and 2) the titanium in the material diffuses through the surface and forms a titanium-concentrated layer under the aluminum diffusion layer when the material is subjected to a high temperature and thus prevents the further diffusion of aluminum into the interior, thereby causing a delay. 30 is adjusted in the decrease of the aluminum concentration in the surface layer and oxygen is also fixed which penetrates into the iron. According to the invention, there is thus obtained a method for preparing steel plates which are provided with an aluminum layer with a low tensile stress and a high resistance to oxidation at a high temperature, which method is characterized in that a steel is prepared with the following composition, 0.001-0.020% C, 0.05-0.30% Mn, 0.05-0.50% Cr, 0.01-0.10% Al, 0.10-0.50% Ti, where the Ti content is not less than 10 times the percentage carbon content and the remainder, in addition to 8100030 τ τ * -4-21701 / Vk / yg, unavoidable impurities is iron, by known converter purification operation and degassing under reduced pressure, wherein a plate is manufactured by casting and plate-forming or continuous plate-forming according to a method known per se, continuous hot rolling of the plate, rolling up, at a temperature not lower than 700 ° C, cold rolling of the obtained hot spiral after a known stripping operation, heating the ko The rolled sheet at a temperature of not less than 850 ° C and the coating with molten aluminum by means of an inline tempering and hot dipping device with a non-oxidizing oven.

According to a preferred embodiment, the carbon content is 0.001-0.010%, the going content 0.05-0.20%, the chromium content 0.07-0.45%, the aluminum content 0.02-0.05% and the titanium content is 0.15-0.40% and not less than 20 times the carbon content. In a further preferred embodiment, the carbon content is 0.001-0.007%, the manganese content 0.10-0.17%, the chromium content 0.07-0.42%, the aluminum content 0.03-0.041% and the titanium content is 0 19-0.23% and not less than 30 times the carbon content, furthermore the roll-up temperature is 720t-730 ° C and the coating temperature is 20 860-900 ° C.

The reasons for the numerical restrictions set out in the main claim are as follows.

The lower the carbon content, the more the influence of quenching aging is reduced. Therefore, it is desirable to reduce the carbon content as much as possible. However, it is not easy to make the carbon content below 0.001%, even according to modern steelmaking techniques, where degassing under reduced pressure is employed. Even if this can be achieved, this is not an economically viable operation. Therefore, the lower limit for the carbon content is indicated as 0.001%. The reason for the upper limit, indicated as 0.020%, is that when the carbon content is higher than this value, the amount of titanium to be added in order for the undesirable effect of the carbon to quench aging must be uneconomically large.

35 The reason for keeping the manganese content at 0.05-0.30% is that it is difficult to obtain a steel grade whose manganese content is less than 0.05% according to the usual steel making process and when the manganese content is above 0.3f0 % is the steel hardens and a high tensile stress is obtained as a result of this.

The reason for keeping the chromium content at 0.05-0.50% is that chromium at an amount below 0.05% does not have sufficient activity to decrease the elongation ratio while also above 0.50% chromium this activity is reduced. . ·

Aluminum is used as a deoxidizing agent for the molten steel and in particular in the process of the invention it plays an important role as the first deoxidizing material, thereby avoiding wasteful use of titanium. In view of this, the lower limit for the aluminum content can be set at 0.01%. However, when aluminum is added in an amount of more than 0.10%, the surface properties and ductility of the obtained steel sheet are adversely affected.

The reason for keeping the titanium content within 0.10-0.50% and more than 10 times the amount of carbon is the following. If the carbon content is lower than 0.10%, the influence for improving the high temperature oxidation resistance is indicated if the weight increase by oxidation is not sufficient, although the tensile stress is rather low. On the other hand, when the titanium content exceeds 0.50%, the material hardens and loses its low tensile stress characteristic, although the weight increase due to oxidation decreases. When the titanium content is less than 10 times the carbon content, which results in an increase in the tensile stress and in an increase in the increase in weight by oxidation, as a result of which the characteristics according to the invention are not observed.

In the method of the invention, Si, P and S are unavoidable impurities and can occur in an amount usually. is present in normal steel grades. Nitrogen and oxygen can be present without any drawbacks at a level usually accomplished by degassing under reduced pressure.

The reason for indicating the roll-up temperature as not less than 700 ° C is that at a temperature lower than this value, the softening of the material by agglomeration and growth of the titanium precipitate formed by fixing carbon with titanium is not sufficient and thus one of the objects according to the invention is lost. Furthermore, it is required that when applying the aluminum layer, the cold rolled steel sheet must be heated to a temperature not lower than 850 ° C before it is brought into the 8100030 - * * -6- 21701 / Vk / yg dressing bath, in order to obtain a cold rolled sheet with a dampening action, so that the titanium deposit further agglomerates into larger particles, thereby softening the material.

In the method according to the invention, the composition of the substrate steel is different from the substrate steel as stated in Japanese patent 35532/76, the difference being that the steel according to the invention contains aluminum added to it. Furthermore, the method is completely different from that according to the invention.

The invention is further elucidated on the basis of the following description of the experiments and results thereof.

Steel samples, the compositions of which are listed in Table A, were prepared by purification in a converter and by degassing at reduced pressure. The castings were subjected to plate forming and continuous rolling under high temperature, and the rolled products were spiraled at various temperatures as indicated and hot coils 2.5 mm thick were obtained. After a stripping operation known per se, the hot coils were processed into cold-rolled layers with a thickness of 0.8 mm. The cold-rolled layers thus prepared were heated at different temperatures and provided with an aluminum layer 2 'in an amount of 60 g / m using an inline device for tempering and hot dipping with a non-oxidizing oven, for which a modified Sendzimir equipment was used under the usual conditions. The aluminum coated plates were subjected to various tests and an oxidation test. The results of this, such as the mechanical properties and weight gain by oxidation, are shown in Table A. The material tests were performed with samples prepared according to JIS (Japanese Industrial Standards) Z-2201 No. 5 »cut in the direction of rolling and the oxy-30 data test with 5 samples of a cycle, the samples being held at a temperature of 830 ° C in the atmosphere for 48 hours and cooled to room temperature.

In Table A, samples 4, 5 and 6 are within the composition limits indicated according to the invention and sample 6 is within the scope of the invention as regards the heating temperature before the coating is applied. This table shows that the materials have excellent formability and oxidation resistance, but only when all three factors are satisfied with regard to 8100030 -7- 21701 / Vk / yg • the composition of the steel forming the substrate, the temperature wherein the spiral formation (rolling up) takes place and the temperature at which the coating is applied, as required according to the invention.

5 When samples 1 and 2 are compared, this shows that a decrease in the manganese content contributes to the reduction of the tensile stress. When samples 2 and 3 are compared, this shows that the addition of titanium contributes to the reduction of the tensile stress. When the samples 3 and 6 are compared, it appears from this that by the combined addition of titanium and chromium, a contribution is also obtained for lowering the tensile stress.

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Table B shows the mechanical properties of the sample steel sheet coated with an aluminum layer prepared by starting from steel grades having a composition as listed in Table B and treating them in the same manner as indicated above. Table B shows that the chromium content is effective in reducing the tensile stress when chromium is present in an amount of 0.05 to 0.50%.

* TABLE B

monchemical composition (% J mechanical property note kin chin g C Mi Ti Cr Al TS * p YP * i El * 5 __ (kg / mm) (kg / mrn) (%) 7 0.007 0.14 0 , 19 - 0.03 32 16 47 A1 8 0.006 0.15 0.22 0.03 0.05 31 15 48 * 1 9 0.004 0.17 0.20 0.07 0.04 31 13 51 * 2 10 0.005 0.15 0.18 0.16 0.04 32 13 52 * 2 11 0.004 0.13 0.19 0.30 0.05 31 14 51 X2 12 0.007 0.14 0.22 0.42 0.03 32 14 49 # 2 13 0.004 0.11 0.19 0.58 0.05 32 17 46 # 1

Remarks: J: comparative steel type 2 * * steel according to the invention 3 10 *: tensile strength * 4 *: yield strength 5 *; extension

Table C shows the mechanical properties and weight gain by oxidation of the steel samples provided with an aluminum layer, the steel having a composition as indicated in Table C and treated in the same manner as indicated above. It can be seen from this table that when the amount of the titanium added is 0.1% or more and more than 10 times the amount of carbon, the effect of reducing the tensile stress and weight gain by oxidation is obtained. However, data from Sample 20 shows that when the titanium content is greater than 0.5%, the tensile stress increases again.

-TABLE C- 8100030

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-10-21701 / Vk / yg

TABLE C

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Remarks: * 1: comparative steel type f 2 #: steel according to the invention ^: tensile strength * 4 *: yield stress c 5 o *: elongation

Table D shows the results of such tests as performed above, but with respect to a sample whose composition is: 0.006% C, 0.02% Si, 0.12% Mn, 0.008% P, 0.006% S, 0 0.10% Cr, 0.027% Al, 0.21% Ti and wherein the Ti / C ratio is 35.10, which sample was prepared in the same manner as indicated above. Table D shows that when the roll-up temperature is less than 700 ° C, the softening of the material is not sufficient and when the temperature for applying the coating is 850 ° C or higher, the softening is remarkably good.

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The invention is further illustrated by the following examples.

Example I

Castings, the compositions of which are shown in Table 5E as samples 21 and 22, were obtained by vacuum degassing of molten steel prepared using an LD converter to reduce the carbon and oxygen contents and then the composition is set by adding iron alloys such as ferrochromium, ferrotitanium, ferromanganese and the like. The castings were turned into plates and the plates were hot rolled into a 2.5mm thick hot spiral under conditions as indicated in Table F. After the stripping treatment, the hot coils were cold rolled into layers of thickness 0.8 mm. The cold rolled layers were coated with an aluminum layer using an inline tempering and hot dipping device 15 with a non-oxidizing oven (a modified Sendzimir device), under conditions as listed in Table F. The mechanical properties and the increase in weight by oxidation of the steel plates thus obtained which are provided with an aluminum layer have been tested in the manner as indicated above and the results of this are stated in table F. Both plates gave excellent properties, namely, a low tensile stress and a high resistance to oxidation at high temperature.

Example II

Plates with a composition as shown in Table E as samples 23 and 24 were obtained by continuous casting after melting in the same manner as in Example I. The plates were made from Steel plates coated with an aluminum layer in the same manner as in Example 1. The mechanical properties and the results obtained from the tests are shown in Table F and the tests were performed in the same manner as in Example I. These plates have excellent characteristics, comparable to those of example I.

The products according to the invention are very suitable for manufacturing parts with complex shapes that are used at high temperatures, such as equipment for treating off-gases for internal combustion engines.

TABLES E AND F- 8100030 * -13- 21701 / Vk / jg

TABLE E

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monchemical composition (%) star -; --- ~

C Si Mn P S Cr Al Ti Ti / C

"1" I. . II. . 1 '' - - —......... '..... "" ~ "21 0.007 0.02 0.12 0.009 0.009 0.11 0.028 0.19 27 22 0.006 0.03 0, 11 0.008 0.007 0.10 0.034 0.21 35 23 0.008 0.04 0.13 0.009 0.006 0.14 0.038 .20.25 24 24 0.007 0.05 0.12 0.008 0.007 0.09 0.041 0.22 31

TABLE F

mon- conditions for temperature of mechanical rise of star hot rolling_the cold rolled properties weight final temp.rolling plate when adjusting the 2 tie (g / m) temperature at “TS ^ YP * 2 El * * (° C) (° c) coating Q [kg / mm) (k £ / mm) (%) _____ ________ (C) __________________ ___________ 21 850 730 900 31 13 50 46 22 830 730 880 30 12 51 48 23 840 720 890 30 13 52 39 24 860 730 880 31 13 51 54

Remarks:: tensile yield strength *: elongation -CONCLUSIONS- 8100030

Claims (3)

A method for preparing a steel plate provided with an aluminum plate having a low tensile stress and a high resistance to oxidation at a high temperature, characterized in that a steel type is prepared with the following composition: 0.001-0.020% C, 0.05-0.30% Mn, 0.05-0.50% Cr, 0.01-0.10% Al, 0.10-0.50% Ti, the Ti content being not less than 10 times the percentage carbon content, and the remainder in addition to unavoidable impurities is iron, by a converter purification process known per se and degassing under reduced pressure, in which a plate is manufactured by casting and plate forming or continuous plate forming in known ways, it is continuously hot rolling the plate, rolling up at a temperature not lower than 700 ° C, cold rolling the obtained hot spiral after a known stripping-off operation, heating the cold rolled plate at a temperature not lower than 850 ° C and coating with molten aluminum by means of el of an inline tempering and hot dipping device with a non-oxidizing oven. Method according to claim 1, characterized in that the carbon content is 0.001-0.010%, the manganese content is 0.05-0.20%, the chromium content is 0.07-0.45%, the aluminum content 0.02-0.05% and the titanium content is 0.15-0.40% and not less than 20 times the carbon content. 3. Process according to claim .2, characterized in that the carbon content is 0.001-0.007 $, the manganese content is 0.10-0.17%, the chromium content is 0.07-0.42%, the aluminum content is 0.03-0.041% and the titanium content is 0.19-0.23% and not less than '30 times the carbon content, the coil temperature is 720-730 ° C and the coating temperature is 860-900 ° C. 30 Eindhoven, January 1981.8100030