EP0303035A1 - Cold-rolled strip electroplated with a nickel sheet with a great diffusion depth, and process for its manufacture - Google Patents

Abstract

To develop a cold-rolled strip and a process for the manufacture thereof, which strip shows a reduced tendency to stick, is readily formable, has great diffusion depths of the coating and advantageous corrosion behaviour, has improved electrochemical properties and can be produced economically, the invention proposes that the nickel layer has a thickness from 1  mu m to 6  mu m and carries an electrolytically applied cobalt layer of thickness from 0.01  mu m to 1.0  mu m, the cold-rolled strip, after coating, having been heat-treated at a temperature of between 580 DEG C and 710 DEG C.

Description

The invention relates to a cold strip with an electrolytically applied nickel coating. Cold strips of this type are used in a wide variety of fields of application where modern manufacturing processes place high demands on the material in terms of mechanical properties, surface, processability and the like, which can only be met by cold-rolled products. Cold strip according to DIN 1624 has smooth, dense and glossy or evenly slightly roughened surfaces after cold forming by means of the correspondingly prepared rollers. Cold rolled strip is free of pores and cracks in the surface types RP and RPG, so that it can be surface-finished, in particular nickel-plated, without any problems. A deep-drawn cold strip with an electrolytically applied nickel coating is therefore known.

For economic reasons, thinner galvanic coatings are generally used in the field of coil plating than is usual for piece plating. It can be achieved through suitable measures, such as shielding the anode, flooding, inserting perforated plates in front of the anodes bar that the deposition takes place in a uniform layer thickness and layer thickness differences are reduced to a minimum. However, it is disadvantageous that the thinner layers have a lower corrosion resistance than thicker galvanic layers. It is also disadvantageous that the cold-rolled strip or cold-rolled and galvanized strips has a "tendency to stick" when annealed in the closed coil. Such glue points are preferably created during the cold rolling of low-carbon cold strip, the surface of which has a minimal micro-roughness. After winding and heat treatment, glue spots are formed sporadically or over a large area and continuously, where the superimposed surfaces adhere to one another stubbornly and difficult to separate. When uncoiling is unwound, the adhesive points are separated / torn open, as a result of which the high-quality surface is damaged or destroyed. In addition, considerable glitches can occur due to the glue points, in addition to or rejects.

In the course of the production of cold strip, it is also known to subject the galvanized nickel-plated strips to a heat treatment before further processing, in order to increase the formability of the strip and coating composite system. During this thermal treatment, the deposited nickel diffuses into the base metal. The diffusion rate is disadvantageously relatively slow and the process is time-consuming and expensive if certain diffusion depths and the formation of certain mixed crystals are aimed for.

The object of the invention is to develop a galvanized cold strip and a method for its production which avoid none of the disadvantages mentioned above Tends to stick, is easy to form, has high diffusion depths in the coating and has favorable corrosion behavior, its electrochemical behavior is improved and it can be produced economically.

The object is achieved in that the nickel layer has a thickness of 1 to 6 microns and carries an electrolytically applied cobalt layer of a thickness of 0.01 to 1.0 microns, the cold strip after coating with a temperature between 580 ° C. and 710 ° C is heat treated. Such a coated and heat-treated cold strip surprisingly no longer tends to stick and shows a significantly more favorable corrosion behavior than the only nickel-coated strips which have the same total layer thickness. In terms of activity, polarizability and electrode potential, the electrochemical behavior of the cold strip according to the invention has values which are considerably more favorable than the only nickel-plated strips. Nickel coating, cobalt coating and heat treatment complement each other in a combinatorial manner to achieve an overall effect that goes beyond the sum effect, since a composite material is created which can be economically achieved with high-quality properties. Thermal treatment results in a significantly higher diffusion rate, which means that the quality of the formability of the composite system is improved and the penetration of the coating metals into the base material by diffusion shows a depth that is several times the coating thickness (including the nickel layer). Despite the extremely thin cobalt coating, a high technical and economic overall success is achieved.

As known, the patent literature describes the cobalt deposition for different tasks with different processes courses, however, it does not contain any teaching, the same objective and solutions as the present invention describes.

DE-OS 1 421 999 describes the cobalt coating of magnetic tapes which are made of plastic.

DE-OS 2 048 209 requests patent protection for the production of shiny co-layers with organic additives, preferably in low current density ranges (<0.5 A / dm²).

DE-OS 2 060 120 describes the co-deposition from iodide-containing electrolytes.

DE-OS 2 134 457 mentions four additives which enable co-deposition even in the presence of Zn impurities.

DE-OS 2 417 952 describes co-deposition (mainly co-alloys) with additions of mannitol, sorbitol etc.

DE-PS 25 22 130 patented the deposition of satin matt Ni-Co alloys with the help of polysiloxane-polyoxyalkylene block polymers.

DE-OS 2 642 666 describes the high-gloss Co and Ni-Co alloy deposition in order to save Ni.

DE-OS 2 718 285 has similar objectives as DE-OS 2 642 666.

DE-OS 3 112 919 describes the use of Co, Co alloy layers for better adhesion of the subsequent aluminum deposition.

In an expedient embodiment of the invention, the base material is a low-carbon steel strip, which is nickel layer of 1.5 to 5 µm in thickness and a cobalt layer of 0.1 to 0.5 µm in thickness, the final heat treatment being carried out at a temperature between 600 ° C and 710 ° C depending on the steel grade. The thickness of the applied nickel layer is preferably 2 μm and the thickness of the applied cobalt layer is 0.1 μm.

The base material of the cold strip is expediently characterized by a ferritic structure with embedded cementite with average grain sizes between 17.0 and 12.0 µm, the steel being 0.001 to 0.70% C, 0.170 to 0.350% Mn, 0.005 to 0.020% P, 0.005 to 0.020% S, 0.030 to 0.060% Al, 0.0015 to 0.0070% N, 0.003 to 0.006% B or instead of boron or additionally 0.005 to 0.15% Ti, balance iron with the usual impurities. (All data in% by weight). The base metal preferably has a steel analysis

C 0.030 - 0.060%
Mn 0.200 - 0.250%
P 0.005 - 0.020%
S 0.005 - 0.015%
Al 0.030 - 0.060%
N 0.0015 - 0.0070%
Ti 0.005 - 0.015%,
Remainder iron with the usual accompanying elements

on. After deep drawing, smooth surfaces are achieved due to the very fine grain diameter. The composition of the steel is particularly important in order to achieve the globular shape of the grain and the grain size specified above over the entire ring length in the cold strip, even in the start and end areas.

The process according to the invention for producing the cold strip described above is characterized in that a hot strip with a thickness of 1.8 to 2.8 mm is used as the starting material is used, the hot strip is cold rolled with or without intermediate annealing with such coordinated rolling degrees that a relative tip height of maximum 3% with a final thickness after cold rolling between 0.10 and 0.70 mm is reached, that the cold strip is then electrolytically in the alkaline degreasing bath at a temperature of 50 to 70 ° C, a current density of 5 to 60 A / dm², degreased for 5 to 30 seconds with or without polarity reversal, that after a rinsing process in 50 to 20% by weight sulfuric acid 3 to 8 seconds long is picked up, then electrolytically nickel-plated at a temperature of 50 to 80 ° C, at a current density of 5 to 70 A / dm² and at a pH value of 3.5 to 3.8, that after a rinsing process thereupon electrolytically a cobalt layer is deposited at a temperature of 50 to 70 ° C, at a current density of 5 to 30 A / dm² and a pH of 3.0 to 3.5 and finally, after rinsing and drying the cold strip, a thermal annealing treatment in a Sch Commercial gas atmosphere is carried out at a temperature of 580 ° C to 710 ° C. The cold strip obtained in this way has no tendency to stick, is distinguished by a current flow which is perceptible by chronoamperometric measurements and is of a considerably higher magnitude than is known, and is extremely economical due to the thin coating with expensive cobalt. During heat treatment, nickel and cobalt penetrate deeply into the base material through diffusion.

The following electrolyte compositions can advantageously be used for the electrolytic deposition of nickel and cobalt: Nickel deposition Electrolyte composition NiSO₄ · 6H₂O 150-300 g / l Cl (as NiCl₂ · 6H₂O) 15-30 g / l Boric acid 40-42 g / l Cobalt deposition Electrolyte composition CoSO₄.7H₂O 300-350 g / l CoCl₂ · 6H₂O 40 - 60 g / l NaCl 15-25 g / l Boric acid 40-42 g / l.

Further details, features and advantages of the subject matter of the invention result from the following description of four examples:

1.1 Base material (steel analysis)

Composition A Composition C C. 0.020 - 0.070 % By weight C. 0.020 % By weight Mn 0.170 - 0.350 % By weight Mn 0.170 % By weight P 0.005 - 0.020 % By weight P 0.005 % By weight S 0.005 - 0.020 % By weight S 0.005 % By weight Al 0.030 - 0.060 % By weight Al 0.030 % By weight B 0.003 - 0.006 % By weight N <0.0030 % By weight N <0.0070 % By weight Composition B Composition D C. 0.030 - 0.060 % By weight C. 0.001 - 0.01 % By weight Mn 0.200 - 0.250 % By weight Mn 0.150 - 0.200 % By weight P 0.005 - 0.020 % By weight P 0.005 - 0.020 % By weight S 0.005 - 0.015 % By weight S 0.005 - 0.015 % By weight Al 0.030 - 0.060 % By weight Al 0.030 - 0.060 % By weight Ti 0.005 - 0.015 % By weight Ti 0.05-0.15 % By weight N <0.0070 % By weight N <0.0070 % By weight

Texture: Ferritic structure with embedded cementite. The grain size is: 17.0 - 12.0 µm (expressed as the mean grain size), in the present case as globular grains, in order to achieve smooth surfaces after deep-drawing due to the very fine grain diameter.

The composition of the steel is of crucial importance in order to achieve this grain shape and grain size over the entire length of the ring, also in the start and end areas.

1.2 Cold rolling

The production of the finished steel strips according to the invention is based on 1.8-2.8 mm thick hot strips. The hot strip is cold rolled with or without intermediate annealing, with coordinated rolling degrees, to a relative tip height of max. To reach 3%. The final thickness after cold rolling is 0.10 - 0.70 mm.

1.3 Galvanic finishing

• 1.3.1 Electrolytically degrease in a commercially available alkaline degreasing bath, at a temperature of approx. 50 - 70 ° C, at a current density of 5 - 60 A / dm², for 5 - 30 seconds, with or without polarity reversal.
• 1.3.2 Rinsing
• 1.3.3 Pickling in 5-20% by weight sulfuric acid for 3-8 seconds.
• 1.3.4 Electrolytic nickel plating at a temperature of 50 - 80 ° C, at a current density of 5 - 70 A / dm², at a pH of 3.5 - 3.8.
  
  Electrolyte composition:
  NiSO₄ ^. 6H₂O 150-300 g / l
  Cl (as NiCl₂ ^6H₂O.) 15 - 30 g / l
  Boric acid 40 - 42 g / l
  Layer thickness approx. 1 µm
• 1.3.5 Rinse
• 1.3.6 Apply the electrolytic co-layer at a temperature of 50 - 70 ° C, at a current density of 5 - 30 A / dm², at a pH of 3.0 - 3.5.
  
  Electrolyte composition:
  CoSO₄ ^. 7H₂O 300-350 g / l
  CoCl₂ ^. 6H₂O 40 - 60 g / l
  NaCl 15-25 g / l
  Boric acid 40 - 42 g / l
  Layer thickness 0.01 - 0.8 µm
• 1.3.7 Rinse
• 1.3.8 drying

1.4 Thermal treatment (annealing)

The refined material is annealed with a defined protective gas (with up to 100% H₂) in order to achieve a stain-free surface.

The temperature is 580 - 710 ° C depending on the steel type and applied galvanic layer thickness. By optimizing the thermal treatment at different temperatures, you can achieve specific diffusion depths.

When testing a nickel-coated cold-rolled strip produced in this way and coated with thin Co and finally heat-treated, virtually no tendency to stick was found. The cold strips show very good resistance in an alkaline medium. Chronoamperometric measurements were carried out to measure the electrochemical behavior. This method of measurement is based on the fact that at a constant voltage (eg plus 100 mV), the more active the tested surface is, the faster an oxide layer is formed on the surface. The measurement is carried out with the so-called three-electrode system, the following electrodes being used:

Reference electrode: mercury oxide / mercury (HgO / Hg)
Auxiliary electrode: platinum wire
Working electrode: nickel-plated, cobalt-coated and heat-treated cold-rolled strip, surface: 283 mm²
Electrolyte: 35% potassium hydroxide solution

The measurement is carried out after preactivation, which removes the natural oxide layer from the surface immediately before the chronoamperometric measurement. The pre-activation voltage applied was approximately -550 mV.

It was surprisingly found that while the only nickel-plated working electrodes showed approx. 8-10 .mu.A current transfer, the current transfer for the working electrodes constructed according to the invention was 80-80 mA. Due to the rapid formation of oxide, the current decreased very quickly and tends to asymtotically after 3 minutes to 0 mA. With the only nickel-plated working electrodes, the near O value (current) was only reached after 15-20 minutes.

It was found that the electrode potential of the cold strip produced in accordance with the invention in alkaline electrolyte remains constant at least twice as long as the electrode consisting only of nickel-plated cold strip.

Finally, it was surprisingly found by means of metallographic cuts and surface analyzes with the glow discharge lamp that the diffusion depth of the coating metals nickel and cobalt was a multiple of the applied layer thickness. With a layer of 2 µm nickel and 0.1 µm cobalt applied, the diffusion depth - penetration of the coating metal into the base material steel - was 5 µm. From this it can be seen that a new composite material with special properties can be produced with the method according to the invention.

Claims (8)

1. Cold strip with electrolytically applied nickel coating,
characterized,
that the nickel layer has a thickness of 1 µm to 6 µm and carries an electrolytically applied cobalt layer of a thickness of 0.01 µm to 1.0 µm, the cold strip being finally heat-treated after the coating at a temperature between 580 ° C and 710 ° C is. 2. Cold strip according to claim 1, characterized in that the base material is a low-carbon steel strip, which carries a nickel layer of 1.5 µm to 5 µm in thickness and a cobalt layer of 0.1 µm to 0.5 µm in thickness, the final heat treatment is carried out at a temperature between 600 ° C and 710 ° C. 3. Cold strip according to claim 1 and 2, characterized in that the thickness of the applied nickel layer is 2 microns and the thickness of the applied cobalt layer is 0.1 microns. 4. Cold strip according to one of claims 1 to 3, characterized in that the base material has a ferritic structure with embedded cementite with average grain sizes between 17.0 and 12.0 microns, the steel 0.001 to 0.70% C, 0.170 to 0.350% Mn, 0.005 to 0.020% P, 0.005 to 0.020% S, 0.030 to 0.060% Al, 0.0015% to 0.0070% N, 0.003 to 0.006% B or 0.005 to 0.15 Ti instead of boron, the rest Contains iron with the usual accompanying elements. 5. Cold strip according to claim 4, characterized in that the base material is a steel analysis with 0.030 to 0.060% C, 0.200 to 0.250% Mn, 0.005 to 0.020% P, 0.005 to 0.015% S, 0.030 to 0.060% Al, 0.0015 to 0.0070% N, 0.005 to 0.015% Ti, balance iron with the usual impurities. 6. A method for producing a cold strip according to one of claims 1 to 5,
characterized,
that a hot strip with a thickness of 1.8 to 2.8 mm is used as the starting material, the hot strip with or without intermediate annealing is cold-rolled with such coordinated rolling degrees that a relative tip height of maximum 3% with a final thickness after cold rolling between 0.10 and 0.70 mm is achieved that the cold strip is then electrolytically in an alkaline degreasing bath at a temperature of 50 ° C to 70 ° C, a current density of 5 A / dm² to 60 A / dm² for 5 to 30 seconds with or without polarity reversal is degreased that after a rinsing process in 50 to 20% by weight sulfuric acid is decapitated for 3 to 8 seconds, then electrolytically at a temperature of 50 ° C to 80 ° C, at a current density of 5 A / dm² to 70 A / dm² and is nickel-plated at a pH of 3.5 to 3.8, that after a rinsing process thereupon electrolytically a cobalt layer at a temperature of 50 ° C to 70 ° C, at a current density of 5 A / dm² to 30 A / dm² and a pH of 3.0 to 3.5 is deposited and schli After rinsing and drying the cold strip, a thermal annealing treatment is carried out in a protective gas atmosphere at a temperature of 580 ° C to 710 °. 7. A process for the production of cold strip according to claim 6, characterized in that the following electrolyte compositions are used for the electrolytic deposition of nickel and cobalt: Nickel deposition Electrolyte composition NiSO₄ · 6H₂O 150-300 g / l Cl (as NiCl₂ · 6H₂O) 15-30 g / l Boric acid 40-42 g / l Cobalt deposition Electrolyte composition CoSO₄.7H₂O 300-350 g / l CoCl₂ · 6H₂O 40 - 60 g / l NaCl 15-25 g / l Boric acid 40-42 g / l.