CN115896667A - Method for hot dip galvanizing of low-alloy high-strength structural steel - Google Patents

Abstract

The invention relates to a hot-dip galvanizing method for low-alloy high-strength structural steel, which comprises the following steps of: pre-plating treatment: the surface of the plated steel is treated to prepare for hot-dip galvanizing of the steel, so that the condition that molten zinc cannot normally and completely react with steel in the subsequent hot-dip galvanizing process is avoided; hot dip galvanizing: immersing the steel subjected to the pre-plating treatment into a zinc bath, and reacting the steel with molten zinc to generate an alloy plating layer so as to complete hot dip galvanizing; wherein the zinc bath comprises the following raw materials in parts by weight: zn: 99.585-98.67 parts, V:0.04 to 0.1 part, sn: 0.3-1.0 part, ni: 0.06-0.2 parts of Ti: 0.01-0.02 parts of Al:0.005 to 0.01 portion. The invention improves the corrosion resistance and the adhesiveness of the plating layer and changes the appearance quality of the plating layer by changing the components and the proportion of the zinc bath when the steel is subjected to hot dip galvanizing.

Description

Method for hot dip galvanizing of low-alloy high-strength structural steel

Technical Field

The invention relates to the technical field of steel product galvanizing, in particular to a hot dip galvanizing method for low-alloy high-strength structural steel.

Background

Since birth, steel faces the problem of rust prevention, and the corrosion and rust of steel are the most common natural phenomena under the action of natural environment. At present, the hot-dip galvanizing process is generally applied to prevent steel from being corroded and rusted. The performance of the coating after the hot dip galvanizing of the steel is good, namely, the coating has good adhesiveness, and the excellent coating formability is influenced by a plurality of factors, such as the components of a zinc bath, the temperature of the hot dip galvanizing, the immersion speed, the galvanizing time, the lifting speed and the like.

The Si content in the low-alloy high-strength structural steel can reach 0.55 percent, which is far higher than that of the common carbon structural steel. The growth of the Fe-Zn compound layer is difficult to control due to the presence of silicon reactivity. Even after being lifted from the zinc bath, the coating still reacts rapidly, so that the thickness of the iron-zinc compound layer is obviously increased, the free zinc layer is consumed, and the thickness of the iron-zinc compound layer in the coating exceeds 95 percent, and even the coating has no free zinc layer. Therefore, the surface of the coating often has color difference defects, the coating is easy to blacken, and the corrosion resistance and the adhesion of the coating are greatly reduced, and the root cause is that the coating still reacts quickly after being lifted from a zinc bath.

Although the surface quality of the hot dip galvanized layer of the low alloy high strength structural steel can be improved by adding Al and Ti, the effect of inhibiting the reactivity of the silicon of the galvanized layer is not obvious. It is found that the addition of V, sn alloy element in zinc bath can slow down the silicon reactivity of low-alloy high-strength structural steel, however, since V, sn element is expensive, a large amount of V, sn needs to be added to completely inhibit the silicon reactivity, which leads to high cost, and the addition of Sn element has adverse effect on the corrosion resistance of the coating, and the addition amount needs to be strictly controlled.

The additive amount of alloy elements can be reduced through multi-component synergistic effect, the additive amount of elements such as V, sn can be reduced through adding Ni, silicon reactivity can be effectively inhibited through synergistic effect, the surface color difference of a coating is eliminated, the coating of a structural steel part is prevented from blackening, the corrosion resistance and the adhesion of the coating are improved, and the zinc bath cost is reduced.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: in order to overcome the defects in the prior art, the invention provides a hot-dip galvanizing method for low-alloy high-strength structural steel, which aims to solve the problems of surface blackening and corrosion resistance and adhesion reduction caused by silicon reactivity during hot-dip galvanizing of the low-alloy high-strength structural steel in the prior art.

The technical scheme adopted by the invention for solving the technical problem is as follows: a method of hot dip galvanising a low alloy high strength structural steel comprising the steps of:

step (1), pre-plating treatment: the surface of a structural steel matrix is treated, so that the condition that molten zinc cannot completely react with the matrix in the subsequent hot-dip galvanizing process is avoided;

step (2), hot dip galvanizing: immersing the structural steel substrate subjected to the pre-plating treatment into a zinc bath, so that the structural steel substrate reacts with molten zinc to generate an alloy plating layer on the surface of the substrate, thereby completing hot-dip galvanizing;

wherein the zinc bath comprises the following raw materials in parts by weight:

zn: 99.585-98.67 parts, V:0.04 to 0.1 part, sn:0.3 to 1.0 portion of,

ni: 0.06-0.2 parts of Ti: 0.01-0.02 parts of Al:0.005 to 0.01 portion.

Preferably, in the step (1), the pre-plating treatment comprises degreasing, acid washing, water washing, plating assisting and drying.

Further, the degreasing was alkaline degreasing, and the degreasing agent used was 15% NaOH solution.

Furthermore, the plating assistant agent adopted by the plating assistant is zinc ammonium chloride plating assistant solution, and the weight ratio of the ammonium chloride to the zinc chloride is (1.2-1).

Further, the drying temperature is 80-100 ℃.

Preferably, in the step (2), the temperature of the hot dip galvanizing is 430-460 ℃, and the time of the hot dip galvanizing is 180-300 s.

The beneficial effects of the invention are: the invention has good controllability, can enhance the corrosion resistance and the adhesiveness of the plating layer and change the surface quality of the plating layer by changing the components and the proportion of the zinc bath during the hot dip galvanizing of the steel, and the prepared workpiece has good corrosion resistance, and the plating layer has bright surface, excellent adhesiveness and proper thickness.

Drawings

The invention is further illustrated by the following examples in conjunction with the drawings.

FIG. 1 is a microstructure morphology of a hot dip galvanized alloy coating of the low alloy high strength structural steel prepared in example 1.

FIG. 2 is a microstructure morphology of a hot dip galvanized alloy coating of the low alloy high strength structural steel prepared in example 2.

FIG. 3 is a microstructure morphology of a hot dip galvanized alloy coating of the low alloy high strength structural steel manufactured in example 3.

FIG. 4 is a microstructure morphology of a hot dip galvanized alloy coating of the low alloy high strength structural steel prepared in example 4.

Fig. 5 is a microstructure morphology of the low alloy high strength structural steel hot dip coated with pure zinc prepared in comparative example 1.

Fig. 6 is a microstructure morphology of a hot-dip galvanized alloy coating of the low-alloy high-strength structural steel manufactured in comparative example 2.

Fig. 7 is a surface photograph of the low-alloy high-strength hot-dip galvanized structural steel manufactured in example 1, comparative example 1, and comparative example 2.

FIG. 8 is a diffusion channel model of Fe- (Zn, V) reaction.

FIG. 9 is a schematic diagram of a 450 ℃ zinc-rich angle-related system of a Zn-Fe-Si-V quaternary system and a diffusion channel model of interface reaction of active steel during dip plating in a Zn-V molten pool.

Detailed Description

Example 1

A method of hot dip galvanising a low alloy high strength structural steel comprising the steps of:

step (1), pretreatment before plating: the surface of the structural steel part is treated, so that the condition that molten zinc cannot completely react with the part in the subsequent hot-dip galvanizing process is avoided;

the pre-plating treatment comprises the following steps:

a. degreasing: immersing the structural steel workpiece into degreasing liquid for degreasing, wherein the degreasing liquid is 15 percent of NaOH solution, the degreasing temperature is 25-50 ℃, so as to remove grease adhered to the surface of the workpiece in the processing, storage and transportation processes;

b, acid washing: placing the workpiece into hydrochloric acid with the mass fraction of 10% -15% for acid washing, properly stirring the workpiece in the acid washing process, and controlling the acid washing temperature to be 18-21 ℃ so as to remove oxide skin and iron rust existing on the surface of the workpiece;

c. washing: washing the pickled workpiece with 95% alcohol;

d. plating assistance: immersing the acid-washed workpiece into a zinc ammonium chloride plating assistant solution with a certain component to finish plating assistant, wherein the temperature of the plating assistant solution is 60-80 ℃, and the plating assistant solution has the function of removing ferric salt or oxide remained on the surface of the workpiece after acid washing and cleaning, so that the workpiece has the maximum surface activity when entering a zinc bath; meanwhile, a salt film is deposited on the surface of the workpiece, so that the workpiece can be prevented from being rusted in the air within a period of time from the plating-assisting tank to the zinc pot, and the liquid-phase zinc immersed in the zinc bath of the workpiece can be purified, so that the workpiece and the liquid-phase zinc are quickly soaked and react;

the concentration of ammonium zinc chloride in the plating assistant solution is controlled to be 200 g/L-400 g/L, and the weight ratio of the ammonium chloride to the zinc chloride is 1.2-1.6, wherein W FeCl2 is less than 1%;

e. drying: drying the workpiece after plating assistant to ensure that the workpiece attached with the plating assistant is dried as thoroughly as possible, wherein the drying temperature is 80-100 ℃;

(2) Hot dip galvanizing: immersing the workpiece which is subjected to the pre-plating treatment into a zinc bath, reacting the workpiece with molten zinc, and generating an alloy coating on the surface of the workpiece, thereby completing hot-dip galvanizing, wherein the temperature of the hot-dip galvanizing is 430-460 ℃, and the galvanizing time is controlled within 180-300 s;

wherein the zinc bath comprises the following raw materials in parts by weight:

zn:99.294 parts, V:0.06 part, sn:0.5 part of (by weight),

ni:0.12 part, ti:0.02 part, al:0.006 part.

The microstructure morphology of the hot dip galvanized alloy coating of the low alloy high strength structural steel prepared in example 1 is shown in fig. 1.

The hot dip galvanizing methods of example 2, example 3, example 4, comparative example 1 and comparative example 2 are the same as those of example 1, and the differences are the compositions and the proportions of the zinc baths of example 2, example 3, example 4, comparative example 1, comparative example 2 and example 1 in the hot dip galvanizing.

The microstructure and morphology diagrams of the low-alloy high-strength structural steel hot-dip-coated with pure zinc prepared in the above examples and comparative examples are shown in fig. 2 to 6. Photographs of the surfaces of the low-alloy high-strength hot-dip galvanized structural steels manufactured in example 1, comparative example 1, and comparative example 2 are shown in fig. 7.

The specific components and formulation of the zinc baths of examples 1, 2, 3, 4, comparative examples 1, 2 are shown in table 1;

TABLE 1

Experimental detection

The structural steel products prepared in examples 1 to 4 and comparative examples 1 to 2 were subjected to tests on the properties and appearance qualities of the coating after hot dip galvanizing, including the thickness of the alloy layer, the appearance quality of the coating, the adhesion of the coating, and the corrosion resistance of the coating.

The thickness of the alloy layer is measured by a metallographic microscope, three different photos of the same coating are taken at 200 times, and the average thickness of the coating is measured by a ruler.

The appearance quality of the coating is specified in GB/T13912-2002 Hot galvanizing Standard: the appearance quality indexes of the hot-dip coating mainly comprise that the main surface is smooth, has no drop nodules, roughness and zinc burrs, has no peeling, has no plating leakage and has no residual flux slag, and the zinc nodules and zinc dust do not exist at the parts which possibly influence the use or the corrosion resistance of the hot-dip galvanizing work, and the appearance quality of the coating is observed by adopting a magnifying lens of 10 times.

The coating adhesion test is carried out according to the reference standard GB/T1732-93, and a paint film impact tester is used for testing the coating. The tested sample is placed on a base of a paint film impact testing machine, a heavy hammer with fixed mass is dropped from a certain height, and a punch is impacted, so that a plating layer and a matrix are deformed. Either positive (paint film facing up) or negative (paint film facing down). By gradually increasing the falling height of the heavy hammer, the value points of frequent breakage of the coating are measured.

The corrosion resistance of the coating is carried out according to the standard GB 10125-88. Performing neutral salt spray experiment with salt water spray tester, wherein the spray medium is 5% NaCl solution, pH value is 6.5-7.2, temperature in the salt spray box is controlled at (35 + -2) deg.C during experiment, continuously spraying for 480h, taking out the sample, recording the weight of the sample after removing corrosion products, and accurately measuring to 0.0001g, according to formula

The corrosion rate v is calculated.

The test results of the plating properties of the steel material prepared in this example are shown in table 2:

table 2:

as can be seen from table 2: structural steel parts are hot dip galvanized in zinc baths of different compositions and proportions to obtain coatings with different appearance qualities and properties. The performance and appearance of the plating layers of the steel materials prepared in the examples 1 to 4 and the comparative examples 1 to 2 of the present invention all meet the national standards, and the corrosion resistance, the surface finish, the uniformity and the adhesion are excellent, especially the examples 1 and 2.

Examples 1 to 4 in which the growth of the alloy layer was suppressed in the plating layers prepared by doping the elements V, sn, ni, ti, al in the zinc bath; the smoothness, the adhesive force and the corrosion resistance of the plated surface are excellent; in the comparative example 1, the alloy layer growth of the coating prepared by not doping the additional element into the zinc bath is not inhibited; the plating surface is dark, and the adhesion and corrosion resistance of the plating surface are poor; in comparative example 2, the coating prepared by doping Ti and Al elements into the zinc bath was bright, but the adhesion and corrosion resistance of the coating were general.

By comparing comparative example 1 and comparative example 2, it can be seen that: compared with the steel prepared by the comparative example 1 without adding extra elements, the steel plated by the comparative example 2 with Al and Ti elements added into the zinc bath has the advantages of improved appearance quality and corrosion resistance, unobvious improvement and no obvious change. Therefore, it is explained that the addition of small amounts of Al and Ti elements to the zinc bath has a certain effect on the appearance and performance of the coating, but the effect is small.

The comparison between examples 1 to 4 in which V, sn and Ni elements were added to the zinc bath and comparative example 2 in which V, sn and Ni elements were not added to the zinc bath shows that: the steel products prepared in the examples 1 to 4 and the comparative example 2 have large differences in alloy layer thickness, plating layer appearance quality, plating layer adhesion and plating layer corrosion resistance, and have significant changes, and the plating layer appearance quality, plating layer adhesion and plating layer corrosion resistance of the examples 1 to 4 are superior to those of the comparative example 2. Therefore, V, sn and Ni elements are doped into a zinc bath for hot dip galvanizing of steel materials, so that the adhesion, the smoothness and the corrosion resistance of a plated surface can be improved.

The comparison between examples 1 in which V, sn and Ni elements were added to the zinc bath and examples 2 to 4 in which V, sn and Ni elements were less than those of example 1 were added to the zinc bath shows that: the appearance quality and the adhesion of the plating layer of the steel prepared in the embodiment 1 and the steel prepared in the embodiments 2 to 4 are not very different and have no obvious change, but the thickness of the alloy layer and the corrosion resistance of the plating layer are different and are improved to a certain extent, and the thickness of the alloy layer and the corrosion resistance of the plating layer of the embodiment 1 are better than those of the embodiments 2 to 4. Therefore, the positive effect of inhibiting the growth of the alloy layer on the corrosion resistance of the coating when the steel is hot-dip galvanized is inhibited.

Based on the measured 450 ℃ isothermal section of the Zn-Fe-V ternary system, the influence of V on hot dip galvanizing is explained by adopting a diffusion channel theory. When Fe is immersion plated into Zn-V alloy, diffusion Path Path I is formed due to the metastable solubility of Fe in Zn close to the zeta phase component, as shown in FIG. 8; as the immersion plating time is prolonged, the T phase replaces the zeta phase to become an equilibrium phase with liquid zinc, so that the T phase is formed on the interface of zeta and liquid phase, a diffusion channel cuts a conjugate line of a T + Liq two-phase region, and then the T + zeta + Liq three-phase region is skipped to enter the zeta + Liq phase region (Path II), thereby effectively controlling the growth of the Fe-Zn alloy layer.

When a certain amount of Si is contained in the steel matrix, a Zn-Fe-Si ternary phase diagram is combined, and a phase relation of the Zn-Fe-Si-V quaternary system Zn-Fe-rich side is extrapolated, wherein the schematic diagram is shown in FIG. 9. In the figure, path0 is an explanation model of the diffusion channel of silicon reactivity, and since silicon has almost no solubility in the zeta phase, which leads to the enrichment of Si at the delta boundary, the diffusion channel cuts the zeta + Liq conjugate line, thereby bypassing the zeta phase. When a certain amount of V is contained in the zinc bath, V and Si have stronger binding energy, and the solubility of Si in the T phase is higher than that of the zeta phase, so that the tendency of the diffusion channel to deviate towards the Si-rich side is reduced, and the zeta + T can effectively prevent the direct erosion of the delta layer by the zinc liquid as long as the zeta + T + Liq three-phase region (Path 3) is not formed by the diffusion channel, thereby controlling the growth of the Fe-Zn alloy layer. However, once the diffusion Path deviates from the ζ + T + Liq triple-phase region (Path 1), silicon reactivity may not be inhibited. When the diffusion Path is just on the a-b line (Path 2), the matrix composition fluctuates slightly, and the silicon reactivity in a local region cannot be suppressed, and burst structure may be formed.

Therefore, when a certain amount of V is added to the zinc bath, the ternary compound T phase replaces the zeta phase to form a compound in equilibrium with liquid zinc, and V has a strong affinity with Si, so that the degree of deviation of the diffusion channel of the galvanizing reaction to the Si-rich angle is reduced, and the zeta or zeta + T layer can form a barrier layer as long as the diffusion channel does not deviate from the zeta + T + Liq three-phase region, thereby effectively inhibiting the occurrence of silicon reactivity and controlling the growth of the Fe-Zn alloy layer.

Meanwhile, V has the advantages of inactive chemical property at normal temperature, easy passivation, strong corrosion resistance, no reaction with air and water, and no reaction with non-oxidizing acid and strong alkali solution. The corrosion resistance of the plating layer can be effectively improved by adding V into the plating layer. Although V is an expensive alloying element, the production cost of the alloy is not increased significantly if V is added only in a small amount.

Therefore, the invention regulates and controls the dosage of V by adding Sn, ni, ti, al and other alloys on the basis of adding V in the zinc bath, thereby not only ensuring the thickness of the free layer and inhibiting the silicon reactivity, but also having low cost.

Therefore, the steel prepared by the invention has excellent coating performance, and the coating of the prepared steel has more excellent performance by doping V, sn and Ni elements into a zinc bath of hot dip galvanizing of the steel, so that the coating surface smoothness, the coating surface adhesive force and the coating surface corrosion resistance of the coating can be improved.

In light of the foregoing description of the preferred embodiment of the present invention, many modifications and variations will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims (6)

1. A method of hot dip galvanising a low alloy high strength structural steel, characterised by the steps of:

step (1), pre-plating treatment: the surface of a structural steel matrix is treated, so that the condition that molten zinc cannot completely react with the matrix in the subsequent hot-dip galvanizing process is avoided;

step (2), hot-dip galvanizing: immersing the structural steel substrate subjected to the pre-plating treatment into a zinc bath, so that the structural steel substrate reacts with molten zinc to generate an alloy plating layer on the surface of the substrate, thereby completing hot-dip galvanizing;

the zinc bath comprises the following raw materials in parts by weight:

zn: 99.585-98.67 parts, V:0.04 to 0.1 part, sn:0.3 to 1.0 portion of,

ni: 0.06-0.2 parts of Ti: 0.01-0.02 parts of Al:0.005 to 0.01 portion.

2. The method of hot-dip galvanizing structural steel as claimed in claim 1, wherein: in the step (1), the pre-plating treatment comprises degreasing, acid washing, water washing, plating assisting and drying.

3. The method for hot-dip galvanizing structural steel as set forth in claim 2, wherein: the degreasing is alkaline degreasing, and the degreasing agent is 15% NaOH solution.

4. The method of hot-dip galvanizing structural steel as claimed in claim 2, wherein: the plating assistant agent adopted by the plating assistant is zinc ammonium chloride plating assistant solution, and the weight ratio of ammonium chloride to zinc chloride in the zinc ammonium chloride plating assistant solution is 1.2-1.6.

5. The method for hot-dip galvanizing structural steel as set forth in claim 2, wherein: the drying temperature is 80-100 ℃.

6. The method of hot-dip galvanizing structural steel as claimed in claim 1, wherein: in the step (2), the temperature of the hot-dip galvanizing is 430-460 ℃, and the time of the hot-dip galvanizing is 180-300 s.

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