DK157690B - PROCEDURE FOR MAKING ALUMINUM COATED STEEL PLATE WITH LOW VOLUME STRENGTH AND GREAT OXIDATION RESISTANCE - Google Patents

Description

DK 157690 B

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a process for producing an aluminum-coated steel plate of low carbon content, low yield strength and high resistance to high temperature oxidation, wherein a process first produces a steel plate containing carbon, manganese, chromium, aluminum and titanium and iron with the inevitable , random impurities and a titanium content of at least ten times the carbon content, that the sheet form is provided by casting and propagating to sheets or by usual continuous casting and continuous hot rolling and rolling at a temperature of at least 700 ° C, after which the sheet is coated by immersion in molten aluminum.

Heretofore, aluminum-coated steel sheets made by the application of molten aluminum (hereinafter referred to as aluminum-coated steel sheets) for use in locations where heat and corrosion resistance is required are mainly made of cold-rolled low-carbon non-corrosive steel. . However, it is well known that aluminum-coated steel sheets made of uncoated steel substrate sheets are subject to deterioration in quality due to aging by quenching caused by rapid cooling at the time of application and hardening of the material.

25 The most important measures that can be used to avoid the above degradation are: 1) The use of substrate steels made of steel from which C and N, which cause quenching aging, are removed as completely as possible, 30 2) addition of a carbide-forming substance such as

Ti for bonding C and N in the substrate material which causes quench cooling, 3) producing over-aging of the aluminum-coated sheets wherein quenching has occurred,

DK 157690B

2 and so on.

However, the precaution 1) is not economical in cases where the steel sheets are coated by an on-line annealing device of a type 5 for hot dipping and with a non-oxidizing furnace, although it can be realized in a normal steel manufacturing process at use of decarbonizing tarnishing.

By an in-line stove device, it is to be understood herein and hereinafter that the heated plate is passed through a non-oxidizing furnace and then passed through the hot-dip bath. In addition, a so-called Sendzimir apparatus is also modified, as described in "Handbook of Metal Surface Treatment ", Kinzokh Hyomen Gijut-zu Binran, page 490. This apparatus has an oxidation furnace, a reduction furnace, and a cooling zone.

The precaution 2) is economical in itself, but when used in the case of low carbon steels that can only be reached by a normal converter process, a considerable amount of Ti must be used, and highly oxidizable Ti produces a small amount of oxide inclusions. which results in degradation of the surface quality of the product. Therefore, this measure is neither economically nor technically desirable in the final analysis.

25 Precaution 3) is economical. However, this treatment can enhance the Fe-Al layer, which has poor forming properties, even though the aluminum coating is carried out under conditions where the formation of Fe-Al layers is controlled. Although the properties of the substrate material are improved by this treatment, the forming properties of the aluminum coated layer are impaired.

JP Publication No. 35 532/76 discloses a process for producing aluminum-coated steel sheets, the method comprising the following 35.

the steps:

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3 is a hot rolling of a steel substantially constituting 0.001-0.020% C, 0.05% Si, 0.05-0.40% Mn, 0.10-0.30% Cr, effective Ti (total Ten minus Ti in oxidized form 0.03-0.40% and at least 4 times (C + N)%, 0.006% N, 5 ^ 0.020% 0 and the remainder being Fe and inevitably random impurities with a temperature of at least 800 ° C, cold rolling of the hot roll to a reduction ratio of 40% or more, annealing of the rolled plate at 800-950 ° C, and immersing the plate in a 10 Al-Si bath 10% Si) maintained at 640-700 ° C for a maximum of 10 minutes.

The aluminum-coated steel sheet obtained by this process has a yield point stress of 13-17 kp / mm and an extension of 44-47% in a cold rolled state with a sheet thickness of 0.8 mm. Today, however, the demand for malleable material has increased.

DE Patent Specification No. 22 58 286 describes flux steel blanks coated with aluminum or aluminum alloys which have a low carbon content, 20 usually below 0.1%, and are resistant to substrate oxidation at 815 ° C. The stated carbon content is, according to that described in the preamble of claim 1, set to "usually below 0.1%", however, the preferred composition is stated according to claim 25 and according to the embodiment in column 8 having a carbon content of from 0.05% by weight. The reduction of carbon content is already a major problem in itself.

Therefore, assuming that a carbon content of from 0.001 to 0.02%, as defined in the present invention, is encompassed by this prior art, it is unsustainable from a metallurgy point of view. Already reducing the carbon content of steel to less than 0.05% presents difficulties.

According to U.S. Patent No. 39,057,780, the carrier steel must contain 0.5-3% chromium. As will be explained later, the reason for the present invention is to determine

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4 chromium content to 0.05-0.5% that chromium in this area acts to reduce the yield stress, but this effect disappears if the chromium content exceeds 0.5%. No teachings can be drawn from the aforementioned U.S. Patent No. 39 05 5 780 on the importance of chromium content to the flow conditions underlying the present invention, although the addition of chromium is described and precisely in a range of 0.5 -3%.

Furthermore, according to the aforementioned US Patent Specification No. 39 05780, the steel must contain 0.8-3% aluminum, while the steel disclosed in the present invention must have an aluminum content of 0.01-0.1%.

Column 3, lines 10-21 of the aforementioned US patent states the lower limit value for chromium and aluminum, and it is also stated in lines 22-27 that a minimum silicon content of 0.4% is significant.

Both of the latter literature sites make no reference to the method used in the present invention. According to the invention, the 20 hot-rolled sheets or strips must be rolled up before their temperature drops to 700 ° C. According to the prior art, the hot roll is wound at a temperature below 700 ° C or at a temperature above 800 ° C, as stated in the aforementioned JP Publication No. 35 532/76.

25 In the commonly used coating device (Sendzimir device) with continuous cold rolling and hot dipping, the final temperature of the hot rollers is usually below 700 ° C. Therefore, when it is proposed that the hot coiling should end at a temperature above 30 700 ° C, further precautions are necessary. However, it is clear that it is easier and more economical to keep the hot rolling temperature before finishing above 700 ° C than to keep the final temperature above 800 ° C.

According to the invention, the cold rolled plate is to be raised

heated further to a temperature of at least 850 ° C. IN

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5 discloses JP disclosure no. 35532/76 discloses a process in which the cold rolled plate is not heated to a temperature of at least 850 ° C. The effect of heating the cold rolled plate to a temperature of at least 850 ° C is clearly shown in Tables 4 - 1 and 4 - 2.

It is an object of the invention to provide an extensible, easily deformable steel sheet with aluminum coating, i.e. a low carbon steel sheet 10 with good formability, low yield strength and high resistance to high temperature oxidation.

This is achieved by the following three criteria: 1) Reduction of the Gr.and O content in substrate steel using vacuum degassing and preliminary deoxidation with Al in the steelmaking phase, thus reducing the Ti content, which is necessary for binding of C and N and to control the formation of oxides.

2) Reduction of the content of Mn, which increases the yield strength and the addition of Cr, which reduces the yield strength.

3) Induction of agglomeration and the growth of separated Ti carbide by cooling the finished hot roll at a temperature as high as 700 ° C, thereby preventing curing of carbide formation as a result of the added Ti to the substrate. In addition, an aclglomeration of Ti carbide is induced by heating the cold-rolled plate at a temperature of 850 ° C or more as it passes through an in-line annealing apparatus of a kind for application by hot dipping oxidizing furnace.

30

Further particular features of the invention are that the high temperature oxidation resistance of the material is improved by a combined addition of Ti and Cr. This means that weight gain due to oxidation at high temperatures is considerably reduced by a decarbonization of the substrate material and that the resistance is increased more by the addition of Ti. The reasons for this are:

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6 1) that the purity of the iron material is improved by decarbonization or addition of Ti, and that Al in the aluminum layer readily diffuses into the Fe substrate and that an Al diffusion layer of excellent high temperature oxidation resistance 5 is formed and 2) that Ti in the material diffuses out against the surface, forming a Ti-concentrated layer under the Al diffusion layer when the material is exposed to high temperature, thus preventing further diffusion of Al into the interior, thereby delaying the decrease of 10 in the Al concentration in the surface layer, and that oxygen which is penetrated into the iron is also bonded.

Thus, according to the invention, there is provided a process for the production of aluminum-coated steel plate, which is characterized in that a steel is used for the preparation of the steel plate, the chemical composition of which is substantially 0.001 - 0.02% C, 0.05 - 0.30% Mn 0.05 - 0.50% Cr, 0.01 - 0.10% Al 0.10 - 0.5% Ti and the remainder Fe, and the steel, in connection with the converter process, is subjected to yet another vacuum degassing, 2 that the rolled plate is cold rolled after a seminal ordinary dressing and that the cold rolled plate is again heated to a temperature of at least 850 ° C, after which the plate is coated with molten aluminum, the heated plate passed through a non-oxidizing furnace and then passed through the hot-dip bath.

A first method according to the invention is characterized in that the hot rolling is carried out up to a thickness of 2.5 mm, after which the hot rollers are, after normal pickling, rolled down to cold rolled sheets of 0.8 mm thickness and then further processed.

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7

Another method of the invention is characterized in that a steel having a chemical composition of 0.001 - 0.01% C 0.05 - 0.20% Mn 0.07 - 0.45 Cr 0 is used for the preparation of the steel plate. O 2 - 0.05% Al 0.15 - 0.40% Ti and the remainder Fe, 10 and wherein the titanium content is at least twenty times the carbon content.

A third process according to the invention is characterized in that a steel having a chemical composition of 0.001 - 0.007% C is used to prepare the steel plate.

0.10 - 0.17% Mn 0.07 - 0.42 Cr 0.03 - 0.041% Al 0.19 - 0.23 Ti and the residue Fe and the titanium content is at least thirty times the carbon content that the plate is rolled up by a temperature of 720 -730 ° C and the heating temperature of the aluminum coating is 860 - 900 ° C.

The reasons for the numerical constraints according to the main claim are due to the following.

The lower the carbon content, the more the effect of the quench cooling aging is reduced. Therefore, it is desirable to reduce the C content as much as possible.

But it is not necessary to reduce the C content to less than 0.001%, even in the modern steelmaking industry.

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8 processes using vacuum degassing. Although a reduction can be achieved, it is not an economical method. Therefore, the lower limit of C content is defined as 0.001%. The reason that the upper limit of C 5 is set to 0.020% is that an increase in the C content above this limit will increase the amount of Ti added to prevent the undesirable effect of C s induced abrasion cooling aging. in an uneconomical direction.

10 The reason for defining the Mn content as 0.05-0.30% is that it is difficult to obtain a steel whose Mn content is less than 0.05% by a normal steelmaking method and that an excess of Mn the content of 0.30% will cause the steel to become hard and as a result it has a high yield strength.

The reason for defining the Cr content as 0.05-0.50% is that Cr in amounts less than 0.05% will not give sufficient effect to reduce the flow ratio, and on the other hand, more than 20 0.50% Cr also reduce this effect.

Al is used for deoxidizing the molten steel and in particular according to this invention Al plays an important role as the preliminary deoxidizing material, preventing uneconomical use of Ti. From this point of view 25, the lowest limit of Al content is defined as 0.01%. However, if Al is added in amounts greater than 0.10%, the surface properties and forming ability of the final steel plate are impaired.

The reason why the Ti content is defined as 0.10-30 0.50% and 10 times the C content is as follows. If the Ti content is less than 0.10%, the effect of improved high temperature oxidation resistance, expressed by weight gain due to oxidation, is not sufficient, although the liquid strength is rather low. On the other hand, 35, the material becomes hard and loses its low yield strength if the Ti content increases above 0.50%, although the weight gain due to oxidation becomes smaller. If the Ti content is less than 10 times the C content 9

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the bonding of C with Ti is not sufficient, resulting in the strength of the liquid being increased and the weight gain upon oxidation increasing, eliminating the particular features of this invention.

In the process of this invention, Si, P

and S are considered to be the inevitable impurities present in a degree corresponding to the normal and usual for steel of this kind. Nitrogen and 0 may be present in amounts normally obtained by the vacuum degassing process without any difficulty.

The reason for the rewinding temperature to be at least 700 ° C is that, at a lower temperature than this, insufficient softening of the material is not achieved due to the agglomeration and growth of the Ti excretion, which has occurred in the bonding of C with Ti, and thus, one of the features of this invention would be lost. In addition, on a line for applying aluminum, it is required that the cold rolled plate is heated to at least 850 ° C before entering the application bath in order to impart a tempering effect to the cold rolled plate so that the Ti separation further agglomerates. to larger particles, thereby softening the material.

In the process of this invention, the composition of the substrate steel differs only from the substrate steel used in the method of the aforementioned Japanese Publication No. 35532/76 in that the steel of this invention contains Al added intentionally.

But the two approaches are vastly different.

The invention is now further explained on the basis of test results.

Steel samples whose compositions are listed in Table 1 were prepared by the converter process and vacuum degassing. The blocks were subjected to coarsening and continuous hot rolling, and the rolled products were rolled up at 35 different temperatures, as indicated in the table, and hot rolls of 2.5 mm thickness were prepared. After normal pickling, the hot rollers were changed to cold rolled sheets of 0.8 mm thickness. Thus manufactured

DK 157690B

Ten cold-rolled sheets were heated to various temperatures and coated with aluminum (60 g / m) using a line-in-line annealing apparatus with a non-oxidizing furnace (practically a modified Sendzimir apparatus) under normal conditions. The aluminum coated sheets were subjected to a material and an oxidation test. The results (mechanical properties and weight gain upon oxidation) are shown in the same table. The material tests were performed with samples manufactured in accordance with JIS (Japanese Industry Standard) Ζ-220Ί No. 5 and cut in the rolling direction. The oxidation test included five repetitions of the cycle where the samples were kept at 830 ° C in the atmosphere for 48 hours and cooled to room temperature.

In Table 1, samples D, E and F are within the composition range of the invention, and P is within the scope of the invention with respect to the heating temperature prior to application. This table shows that materials which must be excellent in both forming ability and oxidation resistance are obtained only if all 3 factors, namely steel substrate composition, roll-up temperature and coating temperature, meet the conditions defined by the invention.

Comparison of samples A and B shows that a decrease of 25 in the Mn content causes a decrease in the yield strength. Comparison between B and C shows that addition of Ti also causes a decrease in the yield strength. Comparison between C and F shows that a combined addition of Ti and Cr also causes the yield strength to be lowered.

30 Table 2 shows the mechanical properties of the aluminum coated steel sheet samples which were made from steel with the compositions shown and by treating them in the same manner as indicated above. According to this table, it is seen that Cr is effective in lowering the buoyancy when it occurs in amounts of about 0.05-0.50%.

Table 3 shows the mechanical properties and weight gain due to oxidation of the aluminum coated steel.

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1 1 sheet samples, which were made on the basis of steel with the compositions shown, and by treating them in the same manner as indicated above. According to this table, it is seen that if the amount of Ti added is 0.1% or 5 more and more than 10 times the amount of C, then effects are obtained which cause both a lower yield strength and a lower weight gain due to oxidation. However, as shown in Sample G, the yield strength increases again when the Ti content exceeds 0.5%.

Table 4-2 shows the results of the same test as above, carried out on similar samples prepared in the same way as above steel, whose compositions are shown in Table 4-1. From this table it can be seen that the softening of the material is not sufficient when the roll-up temperature is lower than 700 ° C and that the softening is significant when the heating temperature before application is 850 ° C or higher.

The following is a preferred method of the invention.

Example 1

Blocks whose compositions appear in Table 5 and are marked with Sample Nos. 1 and 2 were obtained by a vacuum degassing of molten steel prepared by an LD converter to reduce the content of C and O, and 25 by later, an adjustment of the composition was made by adding alloys such as ferrochrome, ferro-titanium, ferromangan, etc. The blocks were changed to plates and the plates were hot rolled to 2.5 mm thick hot rolls under conditions set forth in Table 6 After pickling, the hot rolls were cold rolled to 0.8 mm thick sheets. The cold-rolled sheets were applied to aluminum by means of a hot dip immersion apparatus and with a non-oxidizing furnace (a modified Sendzimir apparatus) under the conditions shown in Table 6. The mechanical properties and weight gain of due to oxidation of the thus obtained aluminum coated steel sheets which were tested in the same way as above is

DK 157690B

12 shown in Table 6. Both plates exhibit excellent properties, namely low flow strength and high oxidation resistance at high temperatures.

Example 2 Plates, the composition of which is given in Table 5 as Sample Nos. 3 and 4, were prepared by continuous casting after melting in the same manner as in Example 1. Of the plates, aluminum-coated steel sheets were prepared in the same manner as described in Example 1. The 10 mechanical properties and test results of the steel sheets thus obtained, which are examined in the same manner as in Example 1, are shown in Table 6. These sheets have excellent characteristics comparable to the plates of Example 1.

Materials manufactured in accordance with this invention * are suitable for the production of components of complicated shapes and used at high temperatures, for example, for exhaust gas treatment appliances from internal combustion engines.

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Claims (4)

A process for producing an aluminum-coated steel plate with low carbon content, low yield strength and high resistance to high temperature oxidation, wherein a process first produces in a normal conversion process a steel plate containing carbon, manganese, chromium, aluminum and titanium and iron with the unavoidable random impurities and a titanium content of at least ten times the carbon content, the plate form is provided by molding and propagating to 10 sheets or by usual continuous casting and continuous hot rolling and winding at a temperature of at least 700 ° C, after which the sheet is coated at molten aluminum immersion, characterized in that a steel is used for the manufacture of the steel sheet, the chemical composition of which is substantially 0.001 - 0.02% C, 0.05 - 0.30% Mn, 0.05-0, 50% Cr, 0.01 - 0.10% Al, 20 0.10 - 0.5% Ti and the remainder Fe, and the steel in connection with the converter process is subjected to yet another vacuum exhaust show that the rolled plate is cold rolled after a conventional pickling treatment and that the rolled plate is again heated to a temperature of at least 850 ° C, after which the plate is coated with molten aluminum, the heated plate passing through a non-oxidizing furnace and then passed through the hot dip bath. Process according to claim 1, characterized in that the hot rolling is carried out up to a thickness of 2.5 mm, after which the hot rolls are, after normal pickling, rolled down to cold rolled plates of thickness of 0.8 mm and then further processed. DK 157690B 19 Process according to Claim 1 or 2, characterized in that a steel is used for the preparation of the steel plate, the chemical composition of which is: 0.001 - 0.01% C 5 0.05-0.20% Mn 0.07 - 0, 45% Cr 0.02 - 0.05% Al 0.15 - 0.40% Ti and the rest Fe, 10 and the titanium content is at least twenty times the carbon content. Process according to claim 1 or 2, characterized in that a steel is used for the preparation of the steel plate, the chemical composition of which is 0.001 - 0.007% C 0.10 - 0, T7% Mn 0.07 - 0.42% Cr 0.03 - 0.041% Al 0.19 - 0.23% Ti and 20 the residue Fe, and where the titanium content is at least thirty times the carbon content, that the plate is rolled up at a temperature of 720 -730 ° C and that the heating temperature at aluminum coating The temperature is between 860 and 900 ° C.