HOT-DIP GALVANIZATION BATH FOR PARTS MADE FROM ANY STEEL COMPOSITION

Hot-dip galvanisation bath for parts made from any steel composition, in particular parts made from steel containing silicon and/or phosphorus, having undergone a prior degreasing, acid dipping and fluxing treatment, characterised in that it contains zinc as well as 0.1 to 1.5% by weight of bismuth and 0.1 to 1.5% by weight of tin.

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Description

The present invention relates to a hot-dip galvanisation bath for parts made from any composition of steel, which may or may not contain silicon and/or phosphorus.

It is a known fact that parts made from iron, cast iron or steel have to be protected against corrosion in all fields of industry.

Galvanisation, in particular hot galvanisation based on dipping, which involves covering the parts to be protected with a zinc-based coating, is one of the most common processes used to obtain such protection.

To this end, the parts to be treated are dipped in a bath of molten zinc or a zinc alloy at a temperature in the order of 400 to 500° C.

Before carrying out this operation, the parts to be treated must be prepared so that they are in a state ready to receive the galvanised coating, enabling this coating to be deposited uniformly on their entire surface.

This initial treatment conventionally involves successive steps of degreasing, generally carried out using an alkaline medium, acid dipping using a corrosion inhibitor and fluxing in a pre-treatment bath which usually contains zinc chloride and ammonium chloride.

Between the degreasing, acid dipping and fluxing steps, the parts to be treated are usually rinsed with water.

In addition, a stoving step may be incorporated between these preliminary steps and the treatment step in the galvanisation bath, which involves drying the interface layer obtained at the end of the fluxing step by subjecting the parts to be treated to an increased temperature.

The galvanised coatings must be uniform, non-marbled and shiny in appearance, must exhibit good adhesion with respect to the steel and should also have a homogeneous thickness in the order of 10 to 70 μm as a rule.

In order to improve these characteristics and obtain totally satisfactory galvanised coatings, it has already been proposed that other elements should be added to the molten zinc, such as nickel, copper, lead, iron, cobalt or alternatively aluminium, for example.

In particular, it is known that adding aluminium improves the sheen of the galvanised coatings, reduces superficial oxidation of the zinc, improves the fluidity of the bath and enables the zinc/iron reaction to be controlled, which contributes to obtaining the requisite thickness.

However, whilst non-alloyed steels and malleable cast irons can be satisfactorily treated in conventional galvanisation baths, especially those containing aluminium, the same is not true of certain alloyed steels, in particular steel sheets with high contents of silicon and/or phosphorus.

In the case of such steels, the conventional hot-dip galvanisation operation in effect produces coatings which have a greyish appearance which is not satisfactory from an aesthetic point of view, an abnormally high thickness which can be as much as 400 or even 500 μm and which also are not very adherent and not very resistant to impacts (risk of peeling under the effect of localised impacts).

These problems are essentially attributable to the fact that the presence of silicon and/or phosphorus increases the reactivity of the steel and encourages the rapid formation of fragile inter-metallic compounds.

In order to study this phenomenon in more detail, specialists defined the concept of silicon equivalent for a steel (Si equivalent=Si+2.5P) and analysed the variations in the thickness of a galvanised coating deposited on a steel part as a function of the amount of Si equivalent contained in this steel.

By plotting what is known as the Sandelin curve illustrated in FIG. 1, they were therefore able to establish that, in the case of steels containing silicon and/or phosphorus, the thickness of the galvanised coating is not a linear function of the content of Si equivalent.

In effect, the Sandelin curve is characterised by a thickness peak known as the “Sandelin peak”, the presence of which proves that growth in the galvanised coating is very rapid if the content of Si equivalent is in the vicinity of 0.1%.

As a result, conventional hot-dip galvanisation baths enable satisfactory results to be obtained for steel compositions known as “hypo-Sandelin steels” with a low content of silicon or silicon equivalent (Si equivalent<0.01%) but not for steel compositions with a high content of Si equivalent.

New steels have recently been developed, which are distinctive due to a not inconsiderable content of silicon and/or phosphorus, such as high elastic limit steels (HEL) or very high elastic limit steels (VHEL) which contain up to 2% of Si equivalent, the mechanical characteristics of which are particularly interesting.

Accordingly, it would be of advantage to devise a hot-dip galvanisation bath of a nature that would enable satisfactory coatings to be obtained in terms of appearance, adhesion and thickness for all steel compositions, including steels with a very low Si equivalent content and HEL or VHEL steels, i.e. to be able to “smooth” the Sandelin curve.

The objective of the invention is to propose a hot-dip galvanisation bath for steel parts, based on a specific composition enabling this objective to be achieved.

Such a hot-dip galvanisation bath is adapted for the treatment of parts of any steel composition which have previously undergone a degreasing, acid dipping and fluxing pre-treatment.

It is characterised in that it contains zinc as well as 0.1 to 1.5% by weight of bismuth and 0.1 to 1.5% by weight of tin.

Surprisingly, it has been found that adding bismuth and tin to a hot-dip galvanisation bath as proposed by the invention enables the fluidity of this bath to be improved, thereby promoting penetration of the coating on the surface of the parts to be treated and hence their adhesion.

By virtue of a preferred characteristic of the invention, the galvanisation bath additionally contains at last one metal selected from the group comprising vanadium, manganese and aluminium.

This bath may specifically contain 0.04 to 0.15% by weight of vanadium and/or 0.10 to 0.30% by weight of manganese and/or at least 0.002% by weight of aluminium.

The rest of such a bath making up the 100% by weight comprises, in addition to the bismuth and tin, zinc of commercial purity (Z1 or Z2 quality with a minimum zinc content of 99.995% or 99.95% respectively).

It has been confirmed that, as a result of the invention, adding manganese and/or vanadium and/or aluminium to the hot-dip galvanisation bath surprisingly reduces the reactivity of the zinc and hence the thicknesses of the coatings for a large range of steels with high contents of silicon and/or phosphorus.

Such a bath enables steels which have only a low content of silicon and/or phosphorus to be treated satisfactorily and, at the same time, also makes it possible to produce coatings on such steels which satisfy requirements in terms of aesthetic appearance, thickness and adhesion.

As a result of one characteristic of the invention, the hot-dip galvanisation bath contains between 0.06 and 0.12% by weight of vanadium.

As a result of another characteristic of the invention, the hot-dip galvanisation bath contains between 0.15 and 0.25% by weight of manganese.

As a result of another characteristic of the invention, the hot-dip galvanisation bath contains between 0.0040 and 0.020% by weight of aluminium.

The hot-dip galvanisation bath proposed by the invention may advantageously contain, as a proportion by weight, between 0.5 and 1.5% of bismuth, between 0.5 and 1.5% of tin, between 0.06 and 0.12% of vanadium, between 0.15 and 0.25% of manganese and between 0.004 and 0.20% of aluminium.

It has proved possible to obtain, over a broad range of steel compositions which may or may not have high contents of silicon and/or phosphorus, galvanised coatings that are totally satisfactory in terms of appearance, adhesion and thickness using a hot-dip galvanisation bath containing, as a proportion by weight: 0.10% of vanadium, 0.17% of manganese, 0.006% of aluminium, 0.2% of tin and 0.2% of bismuth, the rest making up the 100% being zinc of commercial purity.

It has also been established that using a galvanisation bath proposed by the invention enables mechanical characteristics to be improved and in particular the fatigue resistance of HEL and VHEL steels.

It is a known fact that the higher the mechanical properties of a material are (which is the case with HEL and VHEL steels) the less the need for a thick hot-dip galvanised coating to avoid affecting its fatigue resistance.

KITAGAWA's diagram illustrates, for a given steel, variations in the maximal stress before breaking after one million cycles of being cyclically subjected to stress, as a function of the thickness of a conventional galvanised coating.

In one example, the maximal stress prior to the steel breaking was equal to 350 MPa in the untreated state and remained essentially constant for galvanised coatings with a thickness of less than approximately 80 μm before decreasing sharply.

Consequently, for the steel thus analysed, the maximum admissible thickness of the galvanised coating was approximately 80 μm.

In order to characterise what effect a galvanised coating has on the fatigue resistance of different HEL or VHEL steel compositions denoted by A to E, the maximal stress before break was determined after these steels had been subjected to cyclical stress of one million cycles in the untreated state and coated with a galvanised coating.

The percentage loss of fatigue resistance was then calculated for the untreated samples and the coated samples, and the maximum thickness of the galvanised coating without affecting fatigue resistance was defined on the basis of the KITAGAWA diagram.

The results are set out in Table 1 below.

Maximal Maximal % loss in Limit of stress at stress at fatigue coating break after break after resistance thickness HEL or Coating 1-106 cycles 1-106 cycles after according VHEL thickness for untreated for coated 1-106 to steels (μm) steel steel cycles Kitagawa A 61.5 μm   / / 0 >80 μm  B 65 μm 380 MPa 380 MPa 0 80 μm C 63 μm 440 MPa 422 MPa −5% 60 μm D 65 μm 460 MPa 420 MPa −8% 55 μm E 72 μm 525 μm  400 MPa −23%  50 μm

Accordingly, it was observed that in the case of certain steel compositions, there was no loss of fatigue resistance after one million cycles, which means that the galvanised coating did not affect the mechanical characteristics of the steel (for example steel B), whereas in the case of other steel compositions, such as steel E for example, there may be a loss of more than 20% in fatigue resistance in the presence of a galvanised coating with a thickness of 72 μm, which means that a maximum thickness of 50 μm must not be exceeded.

A comparative test for fatigue resistance was run in parallel on a sample of VHEL steel coated with a conventional galvanised coating (Galva A) and a galvanised coating produced following treatment in a bath proposed by the invention (Invention).

The results are set out in Table 2 below.

State of the Maximal stress at % loss of fatigue material break Coating thickness resistance Untreated state 480 MPA None / Galva A 400 MPa 40 μm 20% Invention 450 MPa 40 μm  7%

It was found that for an identical coating thickness, the loss in fatigue resistance compared with the untreated, non-coated sample was 20% for the Galva A sample compared with only 7% in the case of that treated in the galvanisation bath proposed by the invention.

This result proves that the galvanisation bath proposed by the invention enables a specific structure to be deposited, of a type conducive to limiting the drop in the steel's fatigue resistance.

The particularly advantageous nature of the galvanisation bath proposed by the invention was also demonstrated by the example described below.

18 samples of steel with a variable content of silicon and phosphorus were prepared.

The compositions of these steels are specified in Table 3 below.

Steel Chemical composition by weight (%) Si equi- No. SI P C Mn S Al Ni Ti valent 1 0.010 0.008 0.070 0.310 0.004 0.030 0.030 2 0.236 0.008 0.226 1.143 0.003 0.039 0.018 0.035 0.256 3 0.013 0.011 0.055 0.342 0.027 0.003 0.041 4 0.013 0.017 0.082 1.452 0.005 0.029 0.040 5 0.056 0.017 0.130 1.155 0.002 0.031 0.099 6 0.365 0.018 0.113 1.395 0.002 0.040 0.410 7 0.207 0.016 0.141 1.916 0.001 0.024 0.247 8 1.707 0.020 0.226 1.654 0.004 0.043 0.020 0.004 1.756 9 <0.01 0.017 0.087 1.570 0.004 0.039 0.0425 10 0.210 0.010 0.120 1.500 0.004 0.029 0.090 0.002 0.235 11 0.220 0.013 0.240 1.210 0.003 0.042 0.030 0.033 0.253 12 0.010 0.008 0.050 0.200 0.003 0.039 0.040 0.017 0.030 13 0.350 0.009 0.056 0.630 0.003 0.039 0.020 0.003 0.372 14 <0.01 0.011 0.028 15 0.063 0.014 0.098 16 0.061 0.012 0.122 1.448 0.002 1.370 0.021 0.005 0.092 17 0.328 0.008 0.121 1.274 0.013 0.040 0.024 0.349 18 0.663 0.015 0.149 1.891 0.003 0.047 0.030 0.112 0.700

These 18 samples were subjected to a conventional prior degreasing, rinsing, acid dipping, fluxing and stoving treatment.

They were then immersed for 7 minutes in a galvanisation bath proposed by the invention, heated to a temperature of 450° C. and containing 0.10% of vanadium, 0.17% of manganese, 0.2% of bismuth, 0.2% of tin and 0.0060% of aluminium, the rest making up the 100% being zinc of commercial purity.

The hot-dip galvanised coatings thus obtained were analysed and in particular their mean thicknesses and weights calculated.

The characteristics of these coatings are set out in Table 4 below.

Thickness (μm) Coating Appear- Delta weight ance of Steel n° mean mini maxi (maxi-mini) Diff. (g/m2) coating 1 59.3 48.8 70.8 22.0 5.7 472 Satin 2 70.8 64.6 76.8 12.2 4.2 523 (qq grains) satin 3 56.4 46.9 75.8 28.9 8.3 428 (qq grains) satin 4 56.9 49.2 63.3 14.1 4.4 437 satin 5 60.1 50.8 66.0 15.2 4.6 435 satin 6 58.9 50.9 66.4 15.5 3.9 478 satin 7 75.1 67.9 80.6 12.7 3.7 519 satin 8 71.5 61.1 78.7 17.6 6.0 528 satin 9 55.9 52.3 60.9 8.6 2.7 411 satin 10 66.0 57.7 77.8 20.1 6.2 469 satin 11 70.5 63.7 76.1 12.4 3.5 — satin 12 58.9 55.4 68.2 12.8 4.1 426 (qq grains) satin 13 63.8 58.3 74.0 15.7 4.6 454 satin 14 53.8 45.3 65.4 20.1 5.6 387 (qq grains) satin 15 57.3 48.5 61.9 13.4 4.2 403 satin 16 56.9 48.6 64.0 15.4 5.2 — satin 17 64.2 56.3 67.5 11.2 3.3 467 satin 18 66.6 60.0 71.8 11.8 3.3 476 satin

Curves were also plotted for the variations in mean thickness (FIG. 2) and weight (FIG. 3) of the galvanised coatings as a function of the content of Si equivalent (Si+2.5P) for the steel samples.

These curves incontestably show that the galvanisation bath proposed by the invention enables the Sandelin curve to be “smoothed”, thereby producing satisfactory galvanised coatings irrespective of the composition of the steel sample.

Claims

1) Hot-dip galvanisation bath for parts made from any steel composition, in particular parts made from steel containing silicon and/or phosphorus, which have undergone a prior degreasing, acid dipping and fluxing treatment,

characterised in that
it is made up of a zinc alloy containing:
0.1 to 1.5% by weight of bismuth,
0.1 to 1.5% by weight of tin,
0.04 to 0.15% by weight of vanadium,
0.10 to 0.30% by weight of manganese and
at least 0.002% by weight of aluminium,
the rest making up the 100% by weight being zinc of commercial purity.

2) Hot-dip galvanisation bath as claimed in claim 1,

characterised in that
it contains between 0.06 and 0.12% by weight of vanadium.

3) Hot-dip galvanisation bath as claimed in claim 1,

characterised in that
it contains between 0.15 and 0.25% by weight of manganese.

4) Hot-dip galvanisation bath as claimed in claim 1,

characterised in that
it contains between 0.0040 and 0.20% by weight of aluminium.

5) Hot-dip galvanisation bath as claimed in claim 1,

characterised in that
it contains, as a proportion by weight, between 0.5 and 1.5% of bismuth, between 0.5 and 1.5% of tin, between 0.06 and 0.12% of vanadium, between 0.15 and 0.25% of manganese and between 0.004 and 0.020% of aluminium.

6) Hot-dip galvanisation bath as claimed in claim 1,

characterised in that
it contains, as a proportion by weight, 0.10% of vanadium, 0.17% of manganese, 0.006% of aluminium, 0.2% of tin and 0.2% of bismuth.
Patent History
Publication number: 20070128067
Type: Application
Filed: Nov 30, 2006
Publication Date: Jun 7, 2007
Applicant: GROUPE ELECTROPOLI (St. James)
Inventor: Thierry Schmitz (Avranches)
Application Number: 11/565,233
Classifications
Current U.S. Class: 420/520.000
International Classification: C22C 18/00 (20060101);