EP1793009A1 - Hot-dip galvanic bath for steel parts - Google Patents

Abstract

The hot galvanizing bath, especially for silicon and/or phosphorous steel components that have been previously treated to remove grease, scale, and pre-treated with acid and flux, uses a zinc alloy containing the following elements in weight per cent: 0.1 - 1.5 bismuth; 0.1 - 1.5 tin;0.04 - 0.15 vanadium; 0.10 - 0.30 manganese, and at least 0.002 aluminium, with the remainder of zinc.

Description

The present invention relates to a hot-dip galvanizing bath of parts in any grade of steel that may or may not contain silicon and / or phosphorus.

It is well known that in all areas of the industry it is necessary to protect iron, cast iron or steel parts against corrosion.

Galvanizing, particularly hot dip galvanizing which consists of covering the parts to be protected with a zinc-based layer is one of the most commonly used methods for obtaining such protection.

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

Before carrying out this operation, it is necessary to prepare the parts to be treated so that they are able to receive the galvanizing layer and allow uniform deposition of this layer over their entire surface.

This pretreatment conventionally consists of successive stages of degreasing, generally carried out in an alkaline medium, acid pickling combined with an etching inhibitor, and fluxing in a pretreatment bath containing, as a rule, zinc chloride and sodium chloride. ammonium.

Between the stages of degreasing, stripping and fluxing, the parts to be treated are generally rinsed with water.

In addition, between these preliminary steps and the treatment step in the galvanizing bath can be implemented a stoving step of drying the interface layer obtained at the end of the fluxing step and to mount the parts to be treated in temperature.

The galvanizing coatings must have a uniform non-marbled and glossy appearance, good adhesion to steel and furthermore a homogeneous thickness generally of the order of 10 to 70 microns.

To improve these characteristics and obtain fully satisfactory galvanizing deposits it has already been proposed to add to the molten zinc other elements such as, for example, nickel, copper, lead, iron, cobalt or else aluminum.

It is known in particular that the addition of aluminum improves the gloss of galvanizing coatings, reduces oxidation superficial zinc, improves the fluidity of the bath and allows to control the zinc / iron reaction which contributes to obtaining the thickness.

However, if unalloyed steels and malleable cast irons can satisfactorily undergo treatment in conventional galvanizing baths, especially those containing aluminum, the same is not the case for certain alloy steels, particularly metal sheets. steel with high silicon and / or phosphorus contents.

Indeed, in the case of such steels, the conventional hot dip galvanizing operation results in obtaining coatings having a greyish appearance unsatisfactory from the point of view of aesthetics, abnormally high thickness, which can go up to 400 or even 500 μm and furthermore not very adherent and not very resistant to shocks (risk of flaking under occasional shocks).

These problems are mainly related to the fact that the presence of silicon and / or phosphorus increases the reactivity of the steel and promotes the rapid formation of fragile intermetallic compounds.

To study this phenomenon more precisely, the specialists defined the notion of silicon equivalent of a steel (Si equivalent = Si + 2.5P) and analyzed the variations of the thickness of a galvanizing layer deposited on a steel part. depending on the equivalent Si content of this steel.

They have thus been able to establish, by drawing up the so-called Sandelin curve which is represented in FIG. 1, that in the case of silicon and / or phosphorus steels, the thickness of the galvanizing layer is not a linear function. equivalent Si content.

The Sandelin curve is characterized by a thick peak known as the "Sandelin peak" whose presence proves that the growth of the galvanizing layer is very fast approaching an equivalent Si content equal to 0.1%.

As a result, the conventional hot-dip galvanizing baths make it possible to obtain satisfactory results for so-called "hypo-Sandelin" steel grades with a low silicon or equivalent silicon content (equivalent Si <0.01%) but not for grades of steel having a higher equivalent Si content.

However, new steels have recently been developed which are distinguished by a significant content of silicon and / or phosphorus, such as high-strength (HLE) or very high-limit steels. elastic (THLE) which contain up to 2% Si equivalent and whose mechanical characteristics are particularly interesting.

It would therefore be advantageous to have a dip hot dip galvanizing bath of a kind to obtain satisfactory deposits in terms of appearance, adhesion and thickness on all steel grades including steels with very low equivalent Si content and HLE or THLE steels, that is to say, to "smooth" the Sandelin curve.

The object of the invention is to propose a hot-dip galvanizing bath of steel parts having a particular composition making it possible to achieve this goal.

Such a hot-dip galvanizing bath is suitable for the treatment of parts in any grade of steel having undergone pretreatment prior to degreasing, acid pickling and fluxing.

It is characterized 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.

It has been surprisingly found, in accordance with the invention, that the addition of bismuth and tin to a hot dip galvanizing bath improves the fluidity of this bath and therefore to promote the penetration of coating on the surface of the parts to be treated and therefore its adhesion.

According to a preferred feature of the invention, the galvanizing bath also contains at least one metal selected from the group consisting of vanadium, manganese and aluminum.

This bath may in particular 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 aluminum.

In such a bath, the complement to 100% by weight is constituted, besides by bismuth and tin by zinc of commercial purity (quality Z1 or Z2 having a minimum zinc content of 99.995% or 99.95% respectively) .

It has been verified, in accordance with the invention, that the addition of manganese and / or vanadium and / or aluminum to the hot dip galvanizing bath surprisingly reduces the reactivity of the zinc and therefore the thicknesses of the deposits. for a wide range of steels with high levels of silicon and / or phosphorus.

Such a bath makes it possible in parallel to satisfactorily treat steels containing only a low silicon content. and / or in phosphorus and also obtain, on such steels deposits of aesthetic appearance, thickness and adhesion able to meet the requirements.

According to another characteristic of the invention, the hot-dip galvanizing bath contains between 0.06 and 0.12% by weight of vanadium.

According to another characteristic of the invention, the hot-dip galvanizing bath contains between 0.15 and 0.25% by weight of manganese.

According to another characteristic of the invention, the hot-dip galvanizing bath contains between 0.0040 and 0.020% by weight of aluminum.

The hot dip galvanizing bath according to the invention may advantageously contain in proportions by weight between 0.5 and 1.5% of bismuth, between 0.5 and 1.5% of tin, between 0.06 and 0.12 % vanadium, between 0.15 and 0.25% manganese and between 0.004 and 0.20% aluminum.

In particular, it has been possible to obtain, over a wide range of steel grades with or without high silicon and / or phosphorus contents, galvanizing deposits that are totally satisfactory in terms of their appearance, adhesion and thickness. using a hot dip galvanizing bath containing in weight proportions: 0.10% of vanadium, 0.17% of manganese, 0.006% of aluminum, 0.2% of tin and 0.2% of bismuth, the complement to 100% consisting of zinc of commercial purity.

It has also been established that the use of the galvanizing bath according to the invention improves the mechanical characteristics and in particular the fatigue strength of HLE and THLE steels.

It is known that the higher the mechanical properties of a material (which is the case of HLE and THLE steels) minus the thickness of the hot dip galvanizing coating must be important to not influence its fatigue strength.

The KITAGAWA diagram shows, for a given steel, the variations in the maximum stress at break after a cyclic loading of one million cycles, depending on the thickness of a conventional galvanizing coating.

In one example, the maximum tensile strength of the steel was 350 MPa in the raw state, and remained essentially constant for galvanizing coatings with a thickness of less than about 80 μm before decreasing significantly.

As a result, for the steel thus analyzed, the maximum permissible thickness of the galvanizing coating was about 80 μm.

To characterize the influence of a galvanizing coating on the fatigue strength of different grades of steel HLE or THLE referenced A to E, the maximum tensile strengths were determined after a cyclic loading of one million cycles of these steels in the raw state and coated with a coating of galvanization.

The percentage of the fatigue strength loss between the raw and the coated samples was then calculated and defined according to the KITAGAWA diagram the maximum thickness of the galvanizing coating to not affect the fatigue strength.

The results obtained are collated in Table 1 below. HLE or THLE steels Coating thickness (μm) Maximum stress at break at 1-10 ⁶ cycles on raw steel Maximum tensile strength at 1-10 ⁶ cycles on coated steel % loss of fatigue strength at 1-10 ⁶ cycles Thickness limit of coating according to Kitagawa AT 61.5 μm / / 0 > 80 μm B 65 μm 380 MPa 380 MPa 0 80 μm VS 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

It has thus been observed that for certain grades of steel no loss of fatigue strength was observed after one million cycles, so that the galvanizing coating did not affect the mechanical characteristics of the steel (for example steel B ) whereas for other steel grades such as for example steel E, more than 20% loss of fatigue strength may be present in the presence of a 72 μm galvanizing coating, which imposes a thickness maximum not to exceed that is 50 microns.

In parallel, a comparative test was carried out on the fatigue strength of a THLE steel sample coated with a conventional galvanizing coating (Galva A) and with a galvanization coating obtained after treatment in a bath according to the invention. (Invention).

The results obtained are collated in Table 2 below. State of the Material Maximum stress at break Thickness coating % of fatigue resistance loss Gross state 480 MPA Without / Galva A 400 MPa 40 μm 20% Invention 450 MPa 40 μm 7%

It has thus been established that, for an identical coating thickness, the loss of fatigue strength with respect to the uncoated raw sample amounted to 20% for the Galva A sample compared with only 7% for that treated in the sample. galvanizing bath according to the invention.

This result is such as to prove that the galvanizing bath according to the invention makes it possible to obtain a specific deposition structure likely to favor a limitation of the reduction in fatigue strength of the steel.

The particularly advantageous nature of the galvanizing bath according to the invention has also been confirmed by the example below.

18 samples of steels having a variable content of silicon and phosphorus were prepared.

The compositions of these steels are specified in Table 3 below. Steel mark Chemical composition by weight (%) If equivalent IF P VS mn S al Or Ti 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 preliminary treatment of degreasing, rinsing, pickling, fluxing and steaming.

They were then immersed for 7 minutes in a galvanizing bath according to the invention heated to a temperature of 450 ° C. and containing 0.10% of vanadium, 0.17% of manganese, 0.2% of bismuth and , 2% tin, and 0.0060% aluminum, the 100% complement being zinc of commercial purity.

The hot-dip galvanizing deposits thus obtained were then analyzed and in particular their average thicknesses and their weights calculated.

The characteristics of these deposits are summarized in Table 4 below. Steel n ° Thickness (μm) Weight of layer (g / m ² ) Aspect of the coating Avg. mini max Delta (maxi-mini) difference 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 representing the variations in the average thickness (Figure 2) and the weight (Figure 3) of the galvanizing deposits were also plotted against the equivalent Si content (Si + 2.5P) of the steel samples.

These curves prove incontestably that the galvanizing bath according to the invention has "smoothed" the Sandelin curve and obtain satisfactory galvanizing deposits regardless of the steel grade of the sample.

Claims (6)

Hot-dip galvanizing bath of parts in any grade of steel, in particular silicon and / or phosphorus steel parts having undergone preliminary pretreatment of degreasing, pickling, acid and fluxing
characterized in that
it is constituted by 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 aluminum, the complement to 100% by weight consisting of zinc of commercial purity. Hot-dip galvanizing bath according to claim 1,
characterized in that
it contains between 0.06 and 0.12% by weight of vanadium. Hot-dip galvanizing bath according to claim 1,
characterized in that
it contains between 0.15 and 0.25% by weight of manganese. Hot-dip galvanizing bath according to claim 1,
characterized in that
it contains between 0.0040 and 0.20% by weight of aluminum. Hot-dip galvanizing bath according to claim 1,
characterized in that
it contains in weight proportions 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 % manganese and between 0.004 and 0.020% aluminum. Hot-dip galvanizing bath according to claim 1,
characterized in that
it contains, in weight proportions, 0.10% of vanadium, 0.17% of manganese, 0.006% of aluminum, 0.2% of tin and 0.2% of bismuth.

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