DE2732269A1 - METHOD FOR SURFACE TREATING STEEL BEFORE COLD FORMING - Google Patents

Description

The invention relates to a method for the surface treatment of carbon steel with less than 1 % carbon or alloy steel with less than 10 % chromium and less than 2 % molybdenum before cold forming.

To facilitate the cold forming of steel workpieces, it is common to carry out a chemical surface treatment and the resulting coating with a lubricant, for example a soap solution containing essentially sodium stearate, to be treated in order to remove a metal soap containing and to get smear layer. In practice, mainly phosphate coatings are used as chemical coatings. Although oxalate coatings have long been used as suitable separating layers and lubricant carriers are known, find oxalation processes only when cold forming alloyed steels that do not react with phosphating solutions, Use. This is due to the fact that it is more difficult to obtain a sufficient oxalate layer To produce strength and with satisfactory adhesive strength. In addition, the bath management for oxalization processes, in order to obtain a consistent quality of the coating formed, it is generally very expensive. Oxalization processes could therefore not gain acceptance with other steels because of the disadvantages mentioned.

The formation of oxalate layers on the surface of steel is known to be based on the reaction of iron with

709884/0907

Oxalic acid, a layer that is sparingly soluble in water and more or less firmly bonded to the metal surface of iron (II) oxalate is formed. In order to form oxalate coatings on stainless steels, it is necessary that the Treatment solutions in addition to oxalic acid and / or oxalates as the main component, special additives such as activators and / or Oxidizing agents. Numerous shacks for such Additions for the treatment of stainless steels have been made. For example, U.S. Patent 3,121,033 Thiosulfate and iodide and in US-PS 2,550,660 sodium thiosulfate, Bifluoride and chloride are described as additives to oxalic acid. According to DT-AS 1 103 715 and DT-PS 1 179 437 fluoride or chlorate are added to the oxalic acid.

However, the known methods have proven to be unsuitable for treating the steels mentioned at the beginning. When using solutions which only contain oxalic acid and the subsequent treatment with the lubricant solution, it is still partially dissolved, so that the layer present is in no way sufficient to produce an adequate release effect during cold forming. On the other hand, the known additives for the steels under consideration cannot be used either. This is because, when treating carbon steel with less than 1 % carbon and alloy steel with less than 10 % chromium and less than 2 % molybdenum, they have the disadvantage that the surfaces are overpickled. This increases the iron concentration in the treatment bath, which on the one hand leads to a deterioration in the initially good layer and on the other hand also to the undesired formation of iron oxate sludge in the bath. Since oxalic acid is lost as a result of the slurry formation, the consumption of chemicals increases and the service life of the treatment solution is also shortened. Especially when using

709884/0907

of fluoride as an activator, which is in the treatment bath enriched, this then results in a deterioration in the adhesive strength of the oxalate layer and thus a shortening the service life of the treatment solution.

The object of the present invention is therefore to develop a method which, when treating carbon steel with less than 1 % carbon or alloy steel with less than 10% chromium and less than 2 % molybdenum, results in firmly adhering oxalate coatings suitable for subsequent cold forming and does not have the aforementioned disadvantages.

The object is achieved according to the invention in that the workpieces are immersed in an aqueous solution, which Oxalic acid and 0.5 to 3.0 g of chloride ions per liter of solution. It was surprising found that when the steels mentioned were treated, the content of 0.5 to 3.0 g / l chloride ions resulted in overpickling the surfaces avoided and a coating that is well suited for cold forming is obtained in a simple manner will.

The proposed procedure is not suitable for treating carbon steel with more than 1 % carbon or alloy steel with more than 10 % chromium and more than 2 % molybdenum, since only a thin and unusable coating is then obtained.

The oxalic acid is expediently in the treatment solution in a concentration of 10 to 100 g, preferably 20 to 40 g / l, used. The chloride can be added to the oxalic acid solution as a solid agent or in the form of an aqueous solution of ammonium chloride, potassium chloride or preferably of

709884/0907

Sodium chloride can be added.

When adding chloride ions, it behaves differently than when adding fluoride ions, since they take place without increasing accumulation by adding them to the treatment solution can. The use of more than 3 g of chloride ions per liter of solution leads to difficulties, on the other hand, is with a chloride ion concentration below 0.5 g / l no usable coating was obtained. Compliance with the specified Concentration is therefore important.

The procedure according to the invention is carried out in the immersion process. At temperatures of 70 to 90 ⁰ C are obtained upon treatment of 10 to 20 minutes finely crystalline adherent coatings.

By using the method for surface coating according to the invention, a very good, finely crystalline reinforced oxalate coating can be obtained. By avoiding overpickling, the iron content is kept low and the stability of the solution is increased.

example 1

Various steels of the type shown in Table 1 were immersed in treatment solutions of the following composition :

Pattern 1 30th s / i Oxalic acid 2 g / l Sodium chloride Comparison sample 1

Oxalic acid 30 g / l

Conditions of use; 15 minutes at 80 ^° C

709884/0907

^steel C Kn Si Cr Ho designation

1 0.18 0.28 0.45 JIS-G-3461

3TB30

2 0.34 0.23 0.65 1.10 0.20 JIS-G-4105

SCII3

3 0.12 0.35 0.40 2.20 1.00 JIS-G-3462

STBA24

4 0.12 0.35 0.40 5.05 0.53 JIS-G-3462

STBA25

The test results were recorded in Table 2:

709884/0907

Tabel

Mat. No.

Layer weight Adhesion layer weight Adhesion layer weight Adhesion layer weight Adhesion g / m2 g / m ² g / m ² g / m ²

Pattern 1 10.67

9.47

10.11

9.53

Comparison pattern 1

8.12

7.68

7.59 O

7.12

“4 Note: The adhesion was determined by removing the layer through an applied adhesive tape

0 = very good Λ = slight layer removal χ = complete layer removal

ro

CD CD

It can be stated that especially with material IJr. 4 in comparison sample 1 only a thin coating is obtained, while in the case of sample 1, after the addition of chloride, a layer sufficient for cold forming is available.

Example 2

If the treatment solution is used continuously, the influence of the addition of sodium chloride on the layer is reduced and the accumulation of chloride ions in the solution are shown in Table 3.

Treated material

Cold rolled steel sheet JIS-G-314-1 - 50 100 χ 0.8 mm

Conditions of use

15 Hinuten at 80 ⁰ C

Composition of the treatment baths

Oxalic acid 30 g / l

Sodium chloride 2 g / l

Oxalic acid 30 g / l

Oxalic acid 50 g / l

sodium chloride 20 g / l

Ammonium bifluoride 10 g / l

Sodium thiosulphate 7 0 9 8 8 W 0 9 0 7 ^{2 g / l}

Table 3

Comparison sample 1 Comparison sample 2

Behan- layer- iron- chloride- Haf- layer- iron- chloride- Haf- layer- iron- chloride- Hafdelte weight content content device weight content content device weight content content device M.

m ² /! g / m '

ppm

g / l

ppm

g / l

₂ g / m

ppm

0
2
4th

10

9.48
9.32
10.15
10.72
11.46
11.18

114 128 132 146 159 183

0.89 0.93 1.10 0.99 1.04 1.07

0 0 0 0 0 0

98 110 124 130 144 158

O 14.38 290 12.6 0 1 O 15.92 318 13.4 Δ O 17.08 464 13.8 O 18.78 609 14.6 χ O Bad liability O Bad useless

ro

OJ

ro ro

CD

co

From the above results it can be clearly seen that the addition of chloride according to the invention is a very good one Coating is obtained and poor adhesion of the layer due to the accumulation of iron and high chloride content is avoided.

709884/0907

Claims (1)

METALLGESELLSCHAFT Frankfurt / M., July 12, 1977 Aktiengesellschaft DrBr / LWü 6000 Frankfurt / Main Prov. No. 8124 M. Claim Process for the surface treatment of carbon steel with less than 1 % carbon or alloy steel with less than 10 % chromium and less than 2 % molybdenum before cold forming, characterized in that the workpiece surfaces are immersed in one
aqueous solution, the oxalic acid and 0.3 to 3.0 g / l
Contains chloride ions. 709884/0907 ORIGINAL INSPiECTED