CN111485188A - Method for improving surface platability of high-strength steel plate by adopting pre-oxidation technology - Google Patents

Abstract

The invention relates to a method for improving the surface platability of a high-strength galvanized steel sheet by using a pre-oxidation technology. During the continuous hot-dip galvanizing process of the high-strength steel sheet, the air-fuel ratio of the open flame heating section in the continuous annealing furnace is 1.0-1.1, and the surface of the high-strength steel sheet is slightly oxidized. The thickness of the final rust layer is 0.1-1.0 μm, and the surface oxide layer is reduced by hydrogen in the soaking section. By adjusting the air-fuel ratio of the open fire heating section of the annealing furnace, the invention actively causes micro-oxidation on the surface of the high-strength steel plate, ensures the thickness of the rust layer, ensures the occurrence of internal oxidation, and then reduces the surface oxide layer by hydrogen in the soaking section; effectively reduces the high-strength steel plate. Elements that are not conducive to galvanizing on the surface make the surface coating quality of the hot-dip galvanized sheet produced by the continuous galvanizing line of the high-strength steel sheet significantly improved.

Description

A method for improving the surface platability of high-strength steel plates by using pre-oxidation technology

technical field

The invention relates to the technical field of hot-dip galvanizing, in particular to a method for improving the surface platability of a high-strength steel plate by using a pre-oxidation technology.

Background technique

Continuous galvanizing refers to a surface treatment technology that coats a layer of zinc on the surface of metals, alloys or other materials for aesthetics and rust prevention. Now the main method is hot continuous galvanizing. According to the different pre-plating treatment methods, the hot continuous galvanizing process is divided into two categories: off-line annealing and in-line annealing. Among them, off-line annealing means that before the hot-rolled or cold-rolled steel plate enters the hot continuous galvanizing line, the bottom is first drawn. Recrystallization annealing is carried out in a annealing furnace or a bell annealing furnace, that is, a continuous galvanizing line does not include an annealing process. Before hot continuous galvanizing, the steel plate must maintain a clean pure iron active surface free of oxides and other impurities. Specifically, the annealed surface iron oxide scale is first removed by pickling, and then coated with a layer of chlorine. Zinc chloride or a solvent composed of a mixture of ammonium chloride and zinc chloride is used for protection, thereby preventing the steel plate from being oxidized again. In-line annealing is to provide coils directly from the cold rolling or hot rolling workshop as the original plate for hot continuous galvanizing, and conduct gas protection recrystallization annealing in the hot continuous galvanizing line. The hot-dip galvanizing methods that belong to this category include: Sendzimir method, improved Sendzimir method, American Steel Union method (same as Japan's Kawasaki method), Silas method and Sharon method.

In order to meet the requirements of lightweight automotive steel, high-strength galvanized automotive sheet has become the focus of development. In order to obtain a high-strength steel plate, it is necessary to add more elements such as Mn and Si, and the existence of these elements will lead to poor platability of the surface of the steel plate, which cannot meet the requirements of users. In order to solve this problem, some domestic research institutions and iron and steel enterprises have carried out some research on how to improve the surface platability of high-strength galvanized sheets, and proposed to pre-oxidize the surface first, so that Mn, Si and other elements are internally oxidized, and then The method of reducing the surface and reducing the presence of elements such as Mn and Si on the surface, thereby improving the quality of the coating. The method has been tested in the laboratory, and the parameters such as dew point and hydrogen content and the law of internal oxidation have been obtained. However, all laboratories use radiant tubes to simulate heating. In addition, according to the classic high-temperature oxidation theory, iron-silicon alloys generally do not undergo internal oxidation, and internal oxidation can only occur under the premise of reducing or preventing the inward movement of the outer rust layer.

SUMMARY OF THE INVENTION

The invention provides a method for improving the surface platability of a high-strength steel plate by using a pre-oxidation technology. The pre-oxidation of the high-strength steel plate is realized by increasing the air-fuel ratio of the open flame heating section of an annealing furnace, and then the surface oxide layer is reduced by hydrogen in the soaking section, effectively The elements that are not conducive to galvanizing on the surface of the high-strength steel sheet are reduced, and the surface galvanization of the hot-dip galvanized sheet produced by the continuous galvanizing line of the high-strength steel sheet is significantly improved.

In order to achieve the above object, the present invention adopts the following technical solutions to realize:

A method for improving the surface platability of high-strength steel plates by using pre-oxidation technology. During the continuous hot-dip galvanizing process of high-strength steel plates, the air-fuel ratio of the open flame heating section in the continuous annealing furnace is 1.0 to 1.1, and the thickness of the rust layer after micro-oxidation on the surface of the high-strength steel plate It is 0.1-1.0 μm, and the surface oxide layer is reduced by hydrogen in the soaking section.

The chemical composition of the high-strength steel plate is calculated as: C 0.001%-1%, Mn 0.1%-25%, Si 0.01%-0.5%, and the rest are Fe and other inevitable impurities; the incoming state of the high-strength steel plate For the chilled plate after rolling.

In the process of continuous hot-dip galvanizing, the outlet temperature of the open flame heating section of the continuous annealing furnace is 580-740 °C, the temperature of the soaking section is 650-850 °C, and the moving speed of the high-strength steel plate in the continuous annealing furnace is 30-180m/min.

When the hydrogen reduces the surface oxide layer, the hydrogen content is above 10%, and the chemical equation of the hydrogen reduction reaction is: MeO+H ₂ =Me+H ₂ 0, wherein MeO is a metal oxide.

Compared with the prior art, the beneficial effects of the present invention are:

By adjusting the air-fuel ratio of the open flame heating section of the annealing furnace, the surface of the high-strength steel plate is actively slightly oxidized, and the thickness of the rust layer is ensured to ensure the occurrence of internal oxidation, and then the surface oxide layer is reduced by hydrogen in the soaking section; effectively reducing the high-strength galvanized steel plate Elements that are not conducive to galvanizing on the surface make the surface platability of the hot-dip galvanized sheet produced by the continuous galvanizing line of the high-strength galvanized steel sheet significantly improved.

Description of drawings

FIG. 1 is a schematic diagram of the sampling position in the embodiment of the present invention.

FIG. 2 is a sample test result at C1 in the embodiment of the present invention.

Fig. 3 is the sampling test result at C2 in the embodiment of the present invention.

In the picture: 1. Preheating section 2. Soaking section 3. High strength steel plate

Detailed ways

The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings:

According to the method for improving the surface platability of high-strength steel sheets by using pre-oxidation technology, in the process of continuous hot-dip galvanizing of high-strength steel sheets, the air-fuel ratio of the open flame heating section in the continuous annealing furnace is 1.0-1.1, and the surface of the high-strength steel sheet is slightly oxidized. The thickness of the rust layer is 0.1 ~ 1.0μm, and the surface oxide layer is reduced by hydrogen in the soaking section.

The chemical composition of the high-strength steel plate is calculated as: C 0.001%-1%, Mn 0.1%-25%, Si 0.01%-0.5%, and the rest are Fe and other inevitable impurities; the incoming state of the high-strength steel plate For the chilled plate after rolling.

In the process of continuous hot-dip galvanizing, the outlet temperature of the open flame heating section of the continuous annealing furnace is 580-740 °C, the temperature of the soaking section is 650-850 °C, and the moving speed of the high-strength steel plate in the continuous annealing furnace is 30-180m/min.

When the hydrogen reduces the surface oxide layer, the hydrogen content is above 10%, and the chemical equation of the hydrogen reduction reaction is: MeO+H ₂ =Me+H ₂ 0, wherein MeO is a metal oxide.

The following examples are implemented on the premise of the technical solutions of the present invention, and provide detailed embodiments and specific operation processes, but the protection scope of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

【Example】

In this example, a continuous annealing furnace heated by an open flame with an improved Sendzimir method is used for production practice; by adjusting the air-fuel ratio of the heating section of the open flame, the surface of the high-strength steel plate 3 (strip steel) is actively slightly oxidized, and the thickness of the rust layer is ensured to ensure the occurrence of internal oxidation. Then, the surface oxide layer is reduced by hydrogen in the soaking section, so that the surface state of the high-strength steel plate 3 is good before being put into the zinc pot. In this example, samples were taken after micro-oxidation and reduction, respectively, to find out the rules.

In this embodiment, the chemical components of the high-strength steel sheet 3 pre-dip galvanized are C%1, Si 0.12%, Mn 1.5%, and the rest are Fe and other inevitable impurities.

The schematic diagram of the open flame annealing furnace used in the test is shown in Figure 1. The first stroke is the preheating section 1, the second stroke is the open flame heating section 2, and the latter is the soaking section. The outlet temperature of the open flame section of the continuous annealing furnace is 740°C, the temperature of the soaking section is 780°C, and the moving speed of the high-strength steel sheet 3 in the continuous annealing furnace is 100 m/min. The test requires that the air-fuel ratio of the open-fire heating section be adjusted to 1.02 before the furnace is shut down, the surface oxide layer is reduced by hydrogen in the soaking section, and the hydrogen content in the reduction process is 12% (the hydrogen content in the conventional hydrogen reduction process is about 5%). 3 Stop the line after the zinc pot is discharged, purge with large flow of nitrogen, and take samples at positions C1 and C2 in the figure after the furnace is cooled.

The samples were tested with a glow spectrometer. The specific test method was to analyze the depth along the surface after bombardment. For the convenience of observation, the contents of O and Mn in the test results were magnified by 10 times, and Si was magnified by 50 times. The test results of the sample after pre-oxidation (sample at C1) and the sample after reduction (sample at C2) are shown in Figure 2 and Figure 3 .

It can be seen from Figure 2 that the surface of the high-strength steel sheet is dominated by Fe and Fe oxides, with about 0.5% Mn, and basically no Si, and Si has obvious internal oxidation, reaching a peak at a depth of 0.3 μm. The oxidation is not as obvious as that of Si, but it is also dominated by internal oxidation, reaching a peak at 0.12 μm.

It can be seen from Figure 3 that after the hydrogen reduction in the soaking stage, the O on the surface of the high-strength steel plate is basically gone, and the Si and Mn elements did not diffuse to the surface during the reduction process, indicating that the Si and Mn elements on the surface of the high-strength steel plate were not diffused to the surface before entering the zinc pot. Mn is greatly reduced, and there is no presence of O on the surface, thereby improving the surface platability of the high-silicon and high-manganese steel sheet.

Generally, for Fe-Si alloys, SiO ₂ is formed on the alloy surface when the silicon content is low, and the distribution of SiO ₂ is very fine, which is not surrounded by the advancing rust layer, but spreads all over the alloy surface and gather together. At the same time, SiO ₂ reacts with FeO to form fayalite Fe ₂ SiO ₄ . The formation rate of the outer oxide layer is so fast that the thickness of the inner oxide region is negligible. It is reasonable to say that there will be a phenomenon of leakage plating during the hot-dip galvanizing process, but this phenomenon did not occur in the test of this embodiment, which should be the reason for the short oxidation time, that is to say, by reducing or preventing the inward movement of the outer rust layer , which can make Fe-Si have the characteristics of internal oxidation.

For Fe-Mn alloys, since the Gibbs free energy of oxides of Mn is lower _than that of SiO, and the selective oxidation ability relative to Fe is weak, the internal oxidation phenomenon is not as obvious as that of Si, but the occurrence of internal oxidation can still be observed. The content of surface Mn elements can also be reduced to a certain extent.

For the reduction process, from the test results, the reduction ability of the soaking section can meet the requirements of producing high surface quality galvanized automotive sheets.

This example proves that, through the process of pre-oxidation and re-reduction, the surface of the high-strength galvanized sheet is reduced in elements that are not conducive to galvanizing, and it is possible to produce high-quality products of high-silicon and high-manganese steel in a continuous galvanizing line.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims (4)

1. The method for improving the surface platability of the high-strength steel plate by adopting the pre-oxidation technology is characterized in that in the continuous hot galvanizing process of the high-strength steel plate, the air-fuel ratio of an open flame heating section in a continuous annealing furnace is 1.0-1.1, the thickness of a rust layer after the surface of the high-strength steel plate is subjected to micro oxidation is 0.1-1.0 mu m, and a surface oxidation layer is reduced by hydrogen in a soaking section.

2. The method for improving the platability of the surface of the high-strength steel plate by adopting the pre-oxidation technology as claimed in claim 1, wherein the chemical composition of the high-strength steel plate is as follows by weight percent: 0.001 to 1 percent of C, 0.1 to 25 percent of Mn, 0.01 to 0.5 percent of Si, and the balance of Fe and other inevitable impurities; the incoming material state of the high-strength steel plate is a cold-hard plate after rolling.

3. The method for improving the surface platability of the high-strength steel plate by adopting the pre-oxidation technology as claimed in claim 1, wherein in the continuous hot galvanizing process, the outlet temperature of an open flame heating section of a continuous annealing furnace is 580-740 ℃, the temperature of a soaking section is 650-850 ℃, and the moving speed of the high-strength steel plate in the continuous annealing furnace is 30-180 m/min.

4. The method for improving the platability of the surface of a steel plate by using a pre-oxidation technology as claimed in claim 1, wherein the hydrogen content is above 10% when the hydrogen is used for reducing the surface oxide layer, and the chemical equation of the hydrogen reduction reaction is as follows: MeO + H₂＝Me+H₂0, wherein MeO is a metal oxide.

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