DE2105391A1 - Ferrous enamel mass with a low carbon content and process for their production - Google Patents

Description

AHMCO STEEL CORPORATION

703 Curtis Street
Middletown, Ohio / V.St.A.

Our reference: A I36I

Ferrous enamelling compound with a low carbon content and methods of making them

The invention relates to a deformable, ferrous Low carbon enamel composition and a process for its manufacture.

White enamel (porcelain enamel) coatings have been used for many years. Originally they were called Decorative layer used on various metal substrates. Coatings of this type were modified and provided for application to iron-based metals a rust and corrosion protection. Because of the advantages of protection and decoration (good looks) were used for manufacturing many larger household appliances, e.g. B. from cooking stoves, Refrigerators, washing machines and dryers, materials coated with enamel are used.

So far, the surface of the molded steel blank has been

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Thoroughly freed from mill scale, oxide, grease, etc. before applying a coating. The cleaned object was then provided with a primer layer, ie an enamel layer based on cobalt, either by immersion or by spraying. After drying, the article for several minutes was heated to a temperature of about 85O ⁰ G · long or baked at this temperature could then further coatings are applied to the base layer.

The current trend, however, is one, straightforward to use coating layer applied to the base metal. For this purpose a decarburized or carbon stabilized one must be used Steel can be used. Böi is the production of a tool part, when it is drawn out or formed in tension in steel. During the firing of the enamel, a critical grain growth can occur with a simultaneous decrease the yield strength and hardness occur. Since this loss of strength has the consequence that the steel after Applying the porcelain enamel can be bent more strongly, the steel can deform and the enamel can either Chip off during assembly, shipping or use (operation).

So far, attempts have been made to address these difficulties avoid adding a number of expensive stabilizing elements such as vanadium, niobium and titanium. These additives provided after the critical pecking and burning through the formation of carbides and / or nitrides additional strength. Although the problem of strength could be solved by adding expensive additives, other difficulties were introduced by these procedures. With the added strength came a simultaneous one Loss of ductility or deformability of the machinable

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processed material. That is, it became difficult to use such materials where they were strong Were exposed to tensile forces. Another difficulty encountered with those stabilized with the above elements Steels finally occurred, was that within the cast ingots inconsistent (uneven) Properties exhibited. This resulted in a non-uniformity of properties within a coil a single block.

The present invention overcomes the above difficulties by using relatively inexpensive additives, ie aluminum and nitrogen, to produce a more uniform ingot and by controlling the machining to produce a steel with considerably better "ductility". which, however, after stretching by up to 20 % and after firing the enamel, has a minimum tensile strength of at least 130 MN / m (MN = meganewtons).

When carrying out the invention, the steel starting material, which is used for possible enamelling, is melted to be machined using standard procedures for a low carbon aluminum killed steel. The changes in the standard melting process and in the subsequent processing depend on the quantity and the way in which the aluminum and nitrogen additives are introduced into the steel.

The composition of the steel raw material produced by the process described below, was as follows, all percentages being based on weight are:

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Composition broad range preferred range

Carbon 0.008% or less 0.003% or less

Manganese at least 0.10% at leastO.10 %

Phosphorus 0.020% or less 0.020 % or less

Sulfur 0.025% or less 0.025 % or less

Silicon not more than 0.025% not more than 0.015 %

Copper not more than 0.10% not more than 0.02%

Aluminum 0.02 - 0.15% 0.04 - 0.06 %

Nitrogen at least 0.006% at least 0.012 %

Iron remainder remainder

The inventive method for producing a deformable, iron-containing enamel with a high yield strength after stretching by up to 20 % and after firing the enamel is that an iron-containing alloy with an initial carbon content between about 0.03 and 0.10 wt .-%, at least 0.10% by weight manganese and the remainder essentially iron with remaining impurities, melts, this alloy is hot-rolled to an average thickness, the hot-rolled material is cold-rolled to essentially the final thickness and the finished-rolled material Glows at a temperature between about 625 and 800 ⁰ C and it is characterized in that the carbon content is reduced to a maximum value of 0.008 wt .-%, the steel aluminum and nitrogen in amounts between about 0.02 and 0.15, at least in each case 0.006% by weight is added and the coiling temperature following the hot rolling is set to a maximum value of about 735.degree.

Although the composition to achieve the desired results, especially with regard to the aluminum and The addition of nitrogen is critical, there are various methods to bring about at least the addition of nitrogen into the steel.

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For example, the nitrogen can be in the form of a solid Additive in the form of a nitrided (nitrided) manganese introduced into the furnace, the ladle or the mold will. A second method of adding the nitrogen is to make a dry one during the annealing process Maintains an atmosphere containing nitrogen. These different procedures can be based on the workflow a typical procedure can be demonstrated:

Addition of solid stickies Addition of gaseous nitrogen

material

1.) Production of a melt 1.) Production of a melt (batch) of steel with (batch) of steel with low carbon low carbon

2.) Addition of aluminum shot 2.) Addition of aluminum shot and nitrogen in the form of a nitride-hardened ladle
Alloy to the ladle

3.) Casting in bar molds 3) Casting in bar molds

4.) Hot rolling to a 4.) Hot rolling to a medium thickness, coiling medium thickness, coiling temperature of about 650 C temperature of about 650 C

5.) Removal of the mill scale. 5.) Removal of the mill scale

6.) Cold rolling to practically 6.) Cold rolling to practically the final thickness, 35 to table the final thickness, 35 to 85% 85 %

7.) decarburization 7 under application) Normal Offenbundglühung dung a conventional disclosure at 625-800 C in a waistband (open-coil) annealing, nitrogen atmosphere, where-625-800 ⁰ C in

a) for nitriding part of the cycle at a dew point of -18 C or less and

b) for decarburization, another part of the cycle is carried out at a dew point of +15 C or more.

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If you look at the invention in more detail, so there are several different methods by which the composition of the steel can be modified, to in connection with the control of the processing according to the invention to achieve desired results. The chemical change consists in reducing the carbon content the initial melt and the increase in the normal aluminum and nitrogen content.

Before these changes are discussed in more detail, it is expedient to consider the changes that follow one another according to the invention To explain the stages again. A typical process involves the following stages:

Melting the steel in a conventional furnace to a predetermined chemical composition, Casting the steel in ingot molds or continuous casting and, if necessary, hot rolling to a medium one Strength,

Pickling to remove the surface scale, cold rolling to practically the final thickness, Open collar annealing with decarburization, skin passaging,

critical stretching by deforming and applying an enamel coating and firing.

As already stated above, the machined steel is stretched when it is drawn or deformed. During the firing of the enamel, critical grain growth can occur with a simultaneous loss of yield strength and hardness. The steel feedstock must have two important properties, that is, it must be ductile enough so that the part can be properly drawn out, and at the same time, after the critical stretching and firing of the enamel, it must have sufficient strength to withstand the impact of the one

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Cause bending of the molded part. This then constitutes an aim of the present invention; H. the Achievement of good deformability for the manufacturer together with high yield strength and hardness after the manufacture and after firing of the enamel,

According to the invention, this aim is achieved by adding controlled amounts of aluminum and nitrogen to the steel. The aluminum is in the form of aluminum shot (aluminum granules) from the ladle and / or the casting mold added while the steel is essentially molten. This procedure ensures a proportionate even distribution and good recovery (balance). Although I according to the invention the aluminum content between about 0.02 and 0.15% by weight is the preferred amount between 0.04 and 0.06 wt%.

During the investigations which led to the development described above, it was found that the nitrogen can be added to the molten steel in two ways, ie in the form of a solid additive or in the form of a gaseous additive during the annealing process. In the former case a nitrogen-containing material, such as nitrided manganese, the molten steel in the furnace, the ladle or the shape of i is added. In the latter method, the nitrogen can be added during the annealing, by performing annealing in a nitrogen-containing atmosphere whose dew point is -18 ⁰ C or less. In the latter case, nitrogen can be added either before or after the decarburization. In any event, however, it is possible in any process to incorporate the desired minimum nitrogen content of 0.006% by weight, or preferably a minimum nitrogen content of 0.012% by weight.

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Following the above procedures, a third procedure was used to add the nitrogen to the steel found. In this case, the nitrogen is introduced into the molten steel in gaseous form. This method can appropriately be referred to as the "gas mixing process". For example, it can be a nozzle in the molten steel or a porous plug in the bottom of the molten steel Steel-containing ladle are used to release the gaseous nitrogen for a predetermined period of time thereby blowing in to bring the nitrogen content of the steel to the desired value given above.

Another chemical modification that forms a part of the invention, finally, is that the carbon content to less than 0.008, preferably less than 0.003 wt -.%, Reduces to the carbon boil (carboxylic boiling) and the formation of a fibrous surface to avoid if only a single enamel coating is applied. Another advantage of making a steel feedstock that has no yield point elongation is obtained by keeping the carbon content less than about 0.001 weight percent and having virtually all of the nitrogen bound to the metallic aluminum is under the formation of aluminum nitride. In this way, the steel starting material contains no or relatively few unbound interstitial atoms. Such a material would be resistant to aging and would not have stretch lines for those cases where appearance is important. With such a material, the problem does not arise that kinks (surface defects) arise or must be trained to eliminate the elongation at the yield point. For example, a batch of a material processed according to the invention with a final composition of 0.0009%

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Carbon, 0.012% nitrogen and 0.052% metallic aluminum no elongation at the yield point in the soft expansion. stood up.

The preferred method of achieving this result is to have the annealing cycle in a moist or decarburizing atmosphere with a dew point of at least + 15 ° C. However, it was found that the Decarburxerungscyclus can be omitted if one to lower the carbon content of the molten Subjected steel to a degassing treatment. In each case, however, the carbon content is of a nominal value from 0.10% by weight to the values given above set.

The typical processing steps, as indicated above, insofar as they relate to the successive steps of hot rolling, removal of the mill scale, cold rolling and annealing, are customary and therefore do not represent a limitation of the invention. However, it has been found that that considerably better properties can be achieved if one makes a regulation in this process, namely regulates the Goiling temperature following the hot rolling. The processing of the material according to the invention is therefore modified in such a way that the coiling temperature is kept below about 735.degree. "If the coiling temperature is kept below this value, it was found that the tensile strength of the enamelled product made from the material according to the invention was kept at a value above 110 MN / m after the critical stretching and after the enamel had been fired.

Although aluminum and nitrogen are the two main materials added to the steel of the invention,

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this does not mean that the remaining elements of the steel are not critical. It was z. B. found that then, if the final carbon content exceeds 0.008% by weight, the risk of "staining", a surface defect Sj caused by the carbon foaming inside the steel. If on the other hand in the steel If there is not enough manganese present, the phenomenon known as "hot brittleness" can occur. Although phosphorus and sulfur are generally present in residual amounts in most steels, it has been found that in the present steel, when excessive amounts are present, a loss of ductility occurs at the same time.

Silicon and copper are two of the elements whose maximum content is preferably below the specified values should. In the case of silicon, amounts in excess of 0.025% by weight have been found to result in a product with poor surface qualities deliver. On the other hand, it has been found that a copper content of more than 0.10% by weight improves the pickling rate of steel inhibits.

To come back to the main additions, it should be noted that aluminum must be present in the steel in an amount of at least 0.02 wt .-% in order to ensure a killed steel. On the other hand, if the aluminum is present in an amount exceeding the maximum value of 0.15 % by weight, the result is a product with a poor surface quality and in a condition which leads to excessive inclusions. It will be recalled that one of the objects of the invention is to produce formable steel while maintaining a desired level of yield strength after critical stretching and burning. It is due to the interaction of nitrogen with

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to attribute to the aluminum in the steel that a resistance against the critical grain growth arises,
which in turn determines the strength value after deformation
and after firing the enamel increases or maintains, therefore the nitrogen must be in an amount of at least
0.006% by weight are present.

The methods discussed above are exemplified below some exemplary embodiments explained in more detail. In the following examples steels were used whose chemical composition is given in Table I below.

Tabel I. Stole " B " done composition Stole " A " in the pan 0.0010 in the pan done 0.032 0.33 carbon 0.066 0.0018 0.31 0.007 manganese 0.48 0.48 0.008 . 0.014 phosphorus 0.007 0.007 0.011 0.019 sulfur 0.021 0.021 0.015 0.03 silicon 0.015 0.015 0.03 0.049 copper 0.046 0.046 0.04 0.025 aluminum 0.046 0.055 0.0057 rest nitrogen 0.014 0.014 rest iron rest rest example 1

A Siemens-Martin batch with the ladle analysis of steel "A", for which 545 kg of aluminum shot and 2OOokg of nitrided Manganese had been added, was rolled down from the thickness of a slab at a coiling temperature of 59O ° C to 2.16 mm hot. The hot rolled strip was pickled and

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Cold rolled to practically the final thickness of 0.86 mm. Then, the strip was subjected to Offenbundglühung at 7OO ° C in an atmosphere of 20% Hg / 80% N ₂ exposed with a dew point of +35 ⁰ C, to reduce the carbon content to 0.0018%, and finally trained 0.5% forming a steel that had the following mechanical properties when shipped:

Tensile strength 305 MN / m ²

Yield strength 157 MN / m ²

Strain . 44%

Hardness ß _B 49

After a perk of 0 to 20 % and a simulated 5-minute firing of the enamel at a temperature of 790 ° C »to test the critical grain growth, the following minimum yield strength was found:

Yield strength 153 MN / m ² (minimum) Example 2

In a modification of Example 1, a degassed, continuous casting batch with the ladle analysis of steel ¹¹ B ″, to which a total of 111 kg of aluminum shot had been added to the ladle and the degassing device, at a coiling temperature of 610 ° C. to 2.29 The hot rolled strip was then pickled and cold rolled to essentially the final thickness of 0.91 mm.

Although the order of the finishing treatment can be reversed as indicated above, in this example the cold rolled strip was first subjected to an open beam anneal at 790 ° C. in an atmosphere of 20 % Hp / 80 % Np with a dew point

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of about -73 ° C and then soaked for about 20 hours, the furnace was cooled to 700 ° G and water vapor was introduced to raise the dew point to about + 35 ° C. The material was soaked for an additional 5 hours to decarburize to the indicated value. Before and after stretching up to 20 % and firing, the following mechanical properties were found:

Mechanical properties After decarburization after the CS. and firing tensile strength 286 MN / m ²

Yield strength 158 MN / m ² Yield strength 135 MN / m ² i elongation 47.5 % (minimum)

Hardness R-n

To compare the steels according to the invention with other commercially available steels Steels became the enamelling steel raw material without the additives mentioned above and without control of the Coiling temperature processed similarly to Example 1, unless specifically stated. The coiling temperature varied between about 700 and 760 ° C.

Commercially available steels Typical mechanical properties

- Tensile strenght when shipping according to CS. and burning "CQ" Yield strength 296 MN / m ² strain 193 MN / M ² 83 MN / m ² (minimum) hardness 40% - Tensile strenght R _B 40 "DQ" Yield strength 276 MN / m ² strain 172 MN / m ² 83 MN / m ² (minimum) hardness 45 % Η _Ώ 35

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- 14 - 30th MN / m ² 2105 391 "EB" * - tensile strength 379 ^R B MN / m ² Yield strength 510 635 ⁰ C % 172 MN / m ² (Minimum) strain 65 hardness * Coiling temperature =

If the above typical properties of commercially available Steels compared with the typical properties of steels according to the invention, in particular the following will

when shipping to CS and burning tensile strength 290 MN / m ²

Yield strength 172 MN / m ² I58 MN / m ² (minimum) elongation 43%

Hardness R _ß - 43

it follows from this that the last-mentioned steel has all the advantageous characteristics of the steels mentioned above.

It should be noted that the type of material to which the present invention relates is characterized by changes in mechanical properties which occur primarily as a result of the final heat treatments. The final heating stage is the firing of the enamel and although this is not considered a heat treatment, it has the effect of modifying the mechanical properties, especially the yield strength. Up to now this loss has been up to 57 % in the case of the above-mentioned commercial steels. h, it was about 8

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This loss of strength can therefore normally be expected to be less than about 25%. this is also achieved without sacrificing the forming properties of the steel "in shipping". This is special important for the manufacturer »who must first bring the steel into the desired shape before applying the enamel coating can muster and burn.

Patent claims:

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Claims (15)

Claims ^r 1. ) Deformable, iron-containing enameling compound with a low carbon content, which consists essentially of about 0.02 to 0.15% by weight of aluminum, at least 0.006% by weight of nitrogen and essentially of iron as the remainder, characterized in that it retains a high tensile strength after stretching by up to 20 % and after firing the enamel. 2. Iron-containing enamelling compound according to claim 1, characterized in that that their maximum carbon content is 0.008% by weight. 3. Iron-containing enamelling compound according to claim 1, characterized in that that their maximum carbon content is 0.003% by weight. 4 ·. Iron-containing enamelling mas6e according to claim 1, characterized in that that their maximum carbon content is 0.001% by weight. 5. A process for the production of a malleable, iron-containing enamelling compound with a high yield strength after stretching by up to 20 % and after firing the enamel according to one of claims 1 to 4, in which an iron-containing alloy with an initial carbon content between about 0.03 and 0.10% by weight, at least 0.10% by weight manganese and the remainder essentially iron with residual impurities melted, hot-rolled to a medium thickness, then cold-rolled to practically the final thickness and finally at a temperature between about 625 is annealed uad 800 ^ C, characterized in that the carbon content is reduced to a maximum value of 0.008 wt .-%, to the 109833/1441 ~ 17 - Steel, aluminum and nitrogen in amounts between about 0.02 and 0.15 »at least 0.006% by weight each and the coiling temperature is regulated to a value of at most about 735 C after hot rolling. 6. The method according to claim 5 »characterized in that the carbon in an amount of at most 0.003 wt .-%, the aluminum in an amount of 0.04 to 0.06 wt .-% and the nitrogen in an amount of at least 0.012 wt -.% are present. 7. The method according to claim 6, characterized in that the carbon content to a value not exceeding 0.001 | % By weight is maintained. 8. The method according to claim 5 »characterized in that at least part of the annealing in a dry nitrogen-containing atmosphere with a dew point of -18 ° C or less is carried out for a sufficient time to cause nitrogen uptake to the specified value. 9. The method according to claim 8, characterized in that another! Part of the annealing in a humid atmosphere with a dew point of at least + 1 ^ C sufficiently long is carried out to decarburize the material to effect the specified content. 10. The method according to claim 5 »characterized in that the nitrogen in the form of a nitrided alloy is added to the molten steel. 11. The method according to claim 5 »characterized in that the nitrogen in gaseous form in the molten Steel is introduced. 109833 / UA1 12. The method according to claim 5 »characterized in that the steel is degassed in the molten state in order to reduce the carbon content to a maximum of 0.008 % . 13. Method according to claim 8, 10, 11 or 12, characterized in that the carbon in an amount of at most 0.003 %, the aluminum in an amount of 0.04 to 0.06 % and the nitrogen in an amount of at least 0.012 % are present, 14. The method according to claim 13 »characterized in that 'the carbon content is kept at a value of at most 0.001 % . 15. The method according to claim 5 »characterized in that the coiling temperature is kept at at least about 58O ⁰ C. 109833 / U41