CN111575626A - A kind of hot-dip galvanized hot-rolled steel and preparation method thereof - Google Patents

Abstract

The invention particularly relates to a hot-dip galvanized hot-rolled steel and a preparation method thereof, belonging to the technical field of coating. : Mg: 0.5-4%, Al: 3-20%, newly added elements ≤ 1%, and the remainder is zinc; the newly added elements include at least one of the following mass fractions: Ti: 0.01-0.1%, B: 0.005‑0.02%, Ca: 0.002‑0.02%, Si: 0.01‑0.5%, Cu: 0.02‑0.2%; the hot-dip galvanized hot-rolled steel provided by the embodiment of the present invention has a thickness of the steel base of 4 mm, and a surface defect rate of the coating at the same time Significantly reduced, strong coating adhesion, strong coating corrosion resistance.

Description

A kind of hot-dip galvanized hot-rolled steel and preparation method thereof

technical field

The invention belongs to the technical field of coating, in particular to a hot-dip galvanized hot-rolled steel and a preparation method thereof.

Background technique

Hot dip galvanizing is the reaction of molten zinc and its alloys with the steel substrate to form a strong metallurgically bonded coating. Hot-dip galvanized steel has the advantages of strong coating adhesion, long service life, simple manufacturing process, and low product price, and its demand is increasing in many different industries such as automobile industry, electrical industry and construction industry.

The steel substrates used in hot-dip galvanized steel sheets can usually be divided into two categories: hot-rolled steel substrates and cold-rolled steel sheets. Cold-rolled steel sheets have the characteristics of good mechanical properties, high dimensional accuracy, good surface quality, and excellent shape control. application. However, compared with the cold-rolled steel plate, the hot-rolled steel matrix also has advantages that cannot be ignored, including that the thickness of the hot-rolled steel matrix can be more than 3mm, the production process of the hot-rolled steel matrix is shorter, the efficiency is higher, and the cost is lower, etc. Therefore, hot-dip galvanized hot-rolled steel is also widely used in many fields that do not require high surface quality and dimensional accuracy, including construction, power brackets, communication base stations, automobile chassis, etc.

Commonly used types of hot-dip galvanized coatings are pure zinc coating, zinc-5% aluminum alloy coating and zinc-55% aluminum alloy coating. These coatings have a good anodic protection effect on the steel substrate, but they each have several weaknesses in terms of their own corrosion resistance and workability in the atmosphere, such as poor incision corrosion resistance, brittleness of zinc-5% aluminum alloy coatings Larger, more silicon elements must be added to the zinc-55% aluminum alloy coating, and the cost is higher.

The common hot-dip galvanized hot-rolled steel is pure zinc coating. With the improvement of the corrosion resistance requirements of hot-dip galvanized hot-rolled steel, the traditional pure zinc-coated steel plate can no longer meet the corrosion resistance requirements. In addition, the thickness of the hot-rolled steel substrate is relatively thick, and the conventional pure zinc coating has poor protection ability to the side of the steel plate, and it is difficult to meet the requirements of use. However, the alloy coating often contains more alloying elements, which increases the brittleness of the coating. The coating is prone to cracking and peeling during use, and surface defects such as surface leakage and inclusions are prone to occur during the manufacturing process, which seriously deteriorates corrosion resistance. Especially for the hot-rolled steel substrate, the surface quality is often worse than that of the cold-rolled steel sheet, resulting in a much worse uniformity of the alloy coating than that of the cold-rolled steel sheet.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, the present invention is proposed to provide a hot-dip galvanized hot-rolled steel and a method for producing the same which overcome the above-mentioned problems or at least partially solve the above-mentioned problems.

An embodiment of the present invention provides a hot-dip galvanized hot-rolled steel, including a hot-rolled steel substrate and a coating, and the coating is composed of the following chemical components by mass fraction:

Mg: 0.5-4%, Al: 3-20%, new elements ≤ 1%, the balance is zinc;

The newly added element includes at least one of the following mass fractions:

Ti: 0.01-0.1%, B: 0.005-0.02%, Ca: 0.002-0.02%, Si: 0.01-0.5%, Cu: 0.02-0.2%.

Further, the single-sided galvanizing solution adhesion amount of the coating layer is 30-300 g/m ² .

Based on the same inventive concept, an embodiment of the present invention also provides a method for preparing hot-dip galvanized hot-rolled steel, comprising:

obtaining a galvanizing bath containing the chemical composition;

Heating the galvanizing solution until the temperature of the galvanizing solution is 20-50°C higher than the melting point of the galvanizing solution to obtain a heated galvanizing solution;

Obtain hot rolled steel matrix;

heating the hot rolled steel substrate;

The heated hot-rolled steel substrate is immersed in the heated galvanizing solution for coating treatment to obtain hot-dip galvanized hot-rolled steel.

Further, the heating of the hot-rolled steel substrate includes:

The hot-rolled steel substrate is heated to 500-600° C. and kept for 30-120 s, and then the temperature of the hot-rolled steel substrate is controlled to be 10-50° C. higher than the temperature of the heated galvanizing solution.

Further, the time of the coating treatment is 0.1-3s.

Further, the obtaining of the hot-rolled steel substrate includes:

The molten steel is smelted to obtain billets;

heating, hot-rolling, cooling and pickling the steel billet to obtain a hot-rolled pickled plate;

rolling the hot-rolled pickled sheet, and then performing surface degreasing and cleaning to obtain a hot-rolled steel substrate; in the rolling, the surface roughness Ra of the roll is 2.0-4.0, and the total rolling reduction ratio is 5- 20%, the rolling pass is 2-4 times, and the rolling reduction rate is 2-10% for a single rolling.

Further, the surface roughness Ra of the hot-rolled steel substrate is 1.0-2.0um.

Further, the area percentage of oxides on the surface of the hot-rolled steel base does not exceed 20%.

One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

In the hot-dip galvanized hot-rolled steel provided by the embodiment of the present invention, the thickness of the steel substrate reaches 4 mm, the surface defect rate of the coating layer is significantly reduced, the coating layer has strong adhesion, and the coating layer has strong corrosion resistance.

The above description is only an overview of the technical solutions of the present invention, in order to be able to understand the technical means of the present invention more clearly, it can be implemented according to the content of the description, and in order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand , the following specific embodiments of the present invention are given.

Detailed ways

The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly presented therefrom. It should be understood by those skilled in the art that these specific embodiments and examples are used to illustrate the present invention, but not to limit the present invention.

Throughout the specification, unless otherwise specified,

Terms used herein should be understood as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification takes precedence.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or can be prepared by existing methods.

Words such as "first" and "second" used in the present invention do not represent a sequence relationship, but can be understood as nouns.

The technical solutions of the embodiments of the present application are to solve the above-mentioned technical problems, and the general idea is as follows:

According to a typical embodiment of the present invention, a hot-dip galvanized hot-rolled steel is provided, comprising a hot-rolled steel substrate and a coating, and the coating is composed of the following chemical components in mass fractions:

Mg: 0.5-4%, Al: 3-20%, new elements ≤ 1%, the balance is zinc;

The newly added element includes at least one of the following mass fractions:

Ti: 0.01-0.1%, B: 0.005-0.02%, Ca: 0.002-0.02%, Si: 0.01-0.5%, Cu: 0.02-0.2%.

The coating of traditional hot-dip galvanized hot-rolled steel sheet is pure zinc coating, but the corrosion resistance of pure zinc coating itself in the atmosphere is poor, because the corrosion products of pure zinc in the atmosphere are loose zinc oxide and hydrozinc ore. Adding a certain amount of aluminum and magnesium to the pure zinc coating can transform the corrosion products of the coating into dense galena and double-layer hydroxides, which significantly improves the corrosion resistance of the coating itself. At the same time, the chlorhexite formed can also cover the side surface of the steel plate, which improves the corrosion resistance of the side surface of the steel plate, and has a particularly important benefit for the hot-rolled steel plate with a thickness of more than 3 mm.

In order to achieve the above purpose, the aluminum content in the coating should not be less than 3%. This is because the thickness of hot-rolled steel sheets is usually relatively thick, and the maximum thickness can reach 6mm. Therefore, more aluminum content is required to form sufficient protection on the side. However, if the aluminum content is too high, the brittleness of the coating will increase, which will lead to cracking and leakage of the coating. At the same time, during the hot-dip process, aluminum easily reacts with the substrate to form compounds such as zinc dross. These compounds are wrapped in the coating to form inclusion defects. The aluminum content cannot be higher than 20%. In addition, the study believes that a certain amount of magnesium should be added to the coating, and magnesium can be precipitated first during the corrosion process to form magnesium hydroxide, which can inhibit the rapid rise of surface pH, thereby inhibiting the formation of loose zinc oxide and hydrozinc ore. The content of magnesium added in the coating should exceed 0.5% to achieve this effect. However, magnesium is an active metal, and it is easy to react with air to form magnesium oxide during hot-dip galvanizing, which floats on the surface of the zinc liquid and will be wrapped in the coating to form inclusion defects such as zinc slag. Therefore, the magnesium content should not be too high and should not exceed 4%.

Ti can play the role of refining aluminum grains and improve the corrosion resistance of the coating. The content of Ti added should not be less than 0.01%. Generally speaking, the amount of Ti added is 3/1000 to 5/1000 of the aluminum content. Too much will easily deteriorate the toughness of the coating and reduce the adhesion between the coating and the substrate. Low effect is not obvious.

B can play a similar role as Ti, and addition of B and Ti together is also beneficial to lower the melting point temperature of the Ti-Al compound, so that Ti will not form a high melting point compound with Al. The amount of B added is usually 0.005% or more, but too much is not beneficial, so the maximum value is limited to 0.02%.

Ca is an active metal element, and trace amounts of Ca can react with trace oxygen in the plating solution to form spherical oxides and float on the surface, thereby eliminating residual oxygen in the plating solution and improving the toughness of the coating. However, if too much, inclusions will be formed in the coating, which will worsen the corrosion resistance of the coating and cause leakage plating defects. The range of the addition amount of Ca specified in this application is 0.002% to 0.02%.

Cu element can improve the corrosion resistance of zinc-aluminum-magnesium coating, and at the same time, Cu element and magnesium element can form a complex, improve the bonding force between the coating and the substrate, and avoid peeling and cracking of the coating during use.

Si element is a strong aluminum compound element, which can inhibit the reaction between aluminum and iron matrix and avoid the formation of too many defects such as zinc slag. In addition, Si and Al can form a dense Si-Al-Fe compound layer with the substrate to improve the corrosion resistance of the coating. However, adding too much will inhibit the reaction between the zinc liquid and the substrate, resulting in a decrease in the adhesion of the coating.

In the zinc plating solution, if too much Mg, Al, Ti, B, Ca, Si and Cu are added, a large number of precipitates will appear in the coating, which will deteriorate the corrosion resistance and reduce the toughness of the coating.

As some preferred embodiments, the single-sided galvanizing solution adhesion amount of the coating layer is 30-300 g/m ² . The range of the galvanizing solution adhesion amount is determined according to the hot-dip galvanizing process and the properties of the coating material. The amount of adhesion on one side is too small, which obviously cannot meet the requirements of corrosion resistance. However, if the amount of adhesion is too large, on the one hand, it is difficult to achieve a completely uniform thickness, and the corrosion resistance is determined by the position with the thinnest thickness. Plating corrosion resistance.

According to another typical embodiment of the present invention, a preparation method of hot-dip galvanized hot-rolled steel is provided, comprising:

obtaining a galvanizing bath containing the chemical composition;

Heating the galvanizing solution until the temperature of the galvanizing solution is 20-50°C higher than the melting point of the galvanizing solution to obtain a heated galvanizing solution;

Obtain hot rolled steel matrix;

Heating the hot-rolled steel substrate to 500-600°C and keeping the temperature for 30-120s, and then controlling the temperature of the hot-rolled steel substrate to be 10-50°C higher than the temperature of the heated galvanizing solution to obtain a heated hot-rolled steel substrate;

The heated hot-rolled steel substrate is dipped into the heated galvanizing solution for coating treatment to obtain hot-dip galvanized hot-rolled steel.

The temperature of the zinc liquid is higher than the melting point temperature of the zinc liquid, in order to ensure that the zinc liquid has good fluidity and obtain a uniform coating thickness control, but if it is too high, it will cause a lot of loss of alloying elements, and at the same time cause leakage plating, so the zinc liquid temperature is higher than that of the zinc liquid. The melting point temperature is 20-50°C higher.

Before galvanizing, it is necessary to first heat the hot-rolled steel sheet to 500-600°C for 30-120 seconds, which can eliminate the internal stress and work hardening effect generated during the rolling of the hot-rolled sheet, and at the same time make the surface of the hot-rolled sheet more stable. Porous sponge iron is formed during the heating process, which is beneficial to improve the adhesion between the alloy coating and the steel plate. The heating temperature is too low and the time is too short to eliminate work hardening. If the temperature is too high and the time is too long, it is easy to cause the surface holes to be too large, which will make the coating uneven and reduce the corrosion resistance.

The temperature of the hot-rolled sheet before galvanizing is controlled to be 10-50 °C higher than the temperature of the zinc liquid, which can promote the metallurgical reaction between the hot-rolled sheet and the zinc liquid to form a dense alloy layer, including Fe-Al, Fe-Si and other compounds, Improve corrosion resistance and coating adhesion. If the temperature is too low, the reaction is too slow, which reduces the adhesion and corrosion resistance. If the temperature is too high, it is easy to dissolve Fe into the core and form inclusion defects, and at the same time, it will also cause the volatilization loss of magnesium in the local zinc solution.

As some preferred embodiments, the coating treatment time is 0.1-3s.

The hot-rolled sheet galvanizing time is 0.1-3s. If the time is too long, an excessively thick compound layer will be formed between the substrate and the zinc liquid, which will cause the brittleness of the coating. If the time is too short, a dense compound layer cannot be formed, so that the adhesion between the plating layer and the substrate decreases. It also reduces corrosion resistance.

As some preferred embodiments, the obtaining of the hot-rolled steel substrate includes:

The molten steel is smelted to obtain billets;

heating, hot-rolling, cooling and pickling the steel billet to obtain a hot-rolled pickled plate;

rolling the hot-rolled pickled sheet, and then performing surface degreasing and cleaning to obtain a hot-rolled steel substrate; in the rolling, the surface roughness Ra of the roll is 2.0-4.0, and the total rolling reduction ratio is 5- 20%, the rolling pass is 2-4 times, and the rolling reduction rate is 2-10% for a single rolling.

The production of hot-rolled steel substrates can use hot-rolled sheets obtained by conventional continuous casting methods, or hot-rolled sheets produced by continuous casting and rolling. The above method is to prepare the hot-rolled steel matrix by the traditional continuous casting method.

Due to the thick iron oxide scale on the surface of the hot-rolled sheet, it is still difficult to completely remove it even after pickling. Therefore, a subsequent rolling process is required to remove the oxide scale, otherwise there will be obvious leakage plating. At the same time, a certain roughness can be obtained on the surface of the hot-rolled sheet by rolling. However, the rolling reduction rate of the hot-rolled sheet should not be too large, otherwise it will seriously deteriorate the mechanical properties. Therefore, the total reduction ratio is not more than 20% and not less than 5%. The surface roughness of the rolls used for rolling is in the range of 2-4 microns. In order to break the iron oxide scale as much as possible during the rolling process, multiple passes of rolling should be used, no less than 2 passes, but if too much is not economical, it will also cause surface hardening and affect the quality of the galvanized surface, so the pass No more than 4 times. In order to have the effect of breaking the iron oxide scale in each rolling, the rolling elongation of each pass is not less than 2%. In order to reduce the deterioration of mechanical properties, the reduction ratio of each rolling should not exceed 10%.

As some preferred embodiments, the surface roughness Ra of the hot-rolled steel substrate is 1.0-2.0um.

In order to improve the adhesion between the coating and the hot-rolled steel sheet and avoid cracking, the surface roughness Ra of the hot-rolled steel sheet cannot be too low, but if the Ra is too high, a violent reaction will occur during hot-dip galvanizing, and the roughness peaks are easily formed by a rapid reaction Zinc slag is mixed, so the surface roughness Ra of the hot-rolled steel sheet before galvanizing is in the range of 1.0-2.0um.

As some preferred embodiments, the area percentage of oxides on the surface of the hot-rolled steel substrate does not exceed 20%.

The area percentage of oxides on the surface of the hot-rolled sheet before galvanizing shall not exceed 20%. This is because if there are too many surface oxides, the zinc liquid cannot react with the substrate during galvanizing, and aluminum and magnesium react with oxides, resulting in local depletion of aluminum and magnesium, and a dense compound layer cannot be formed between the zinc liquid and the substrate. Thereby resulting in coating leakage plating defects.

The hot-dip galvanized hot-rolled steel of the present application and the preparation method thereof will be described in detail below with reference to the embodiments, comparative examples and experimental data.

Examples 1-9 and Comparative Examples 1-12

The traditional hot-rolled steel plate with a thickness of 4mm is used as the steel matrix, and the steel plate material is CQ grade. Table 1 lists the process parameters for preparing steel substrates in Examples 1-9 and Comparative Examples 1-12. All coatings are 10 microns thick.

Table 1

Experiment 1

Surface defect evaluation was carried out on the hot-dip galvanized hot-rolled steel substrate prepared according to the process parameters in the above-mentioned Examples 1-9 and Comparative Examples 1-12 and the preparation method of the present invention, and the slag defects on the surface were automatically identified by the image method, Calculate the number of zinc dross per unit area and the number of missing plating spots, the unit is pcs/square meter.

The substrate microhardness test was carried out on the above-mentioned samples, the microhardness test pressure was 100 g, and the measurement position was the section of the sample.

The coating adhesion test was carried out on the above samples. The sample method was a bending test. The execution standard was GB/T 232, the bending radius was 8 mm, and the bending angle was 180°. After bending, observe the surface morphology of the outer edge of the sample:

Good: No cracks occurred

Qualified: There are cracks within 3mm from the edge, and no cracks in other positions

Unqualified: Cracks appear outside the range of 3mm from the edge.

The coating corrosion resistance test was carried out on the above samples. First, the samples were subjected to a bending test. The implementation standard was GB/T 232, the bending radius was 4 mm, and the bending angle was 90°. Then, the neutral salt spray test was used to evaluate for 480 hours. Observe Corrosion condition at the outer edge of the bend and the location of the cut.

Good: no red rust

Qualified: red rust area does not exceed 5%

Unqualified: red rust area exceeds 5%

The experimental evaluation results are shown in Table 2.

Table 2

Examples 10-18 and Comparative Examples 13-24

The continuous casting and rolling hot-rolled steel plate with a thickness of 4mm is used as the steel matrix, and the steel plate material is CQ grade. Table 3 lists the process parameters for preparing steel substrates in Examples 10-18 and Comparative Examples 13-24. The thickness of the coating is 10 microns.

table 3

Experiment 2

Surface defects were evaluated for the hot-dip galvanized hot-rolled steel sheets prepared according to the process parameters in the above-mentioned Examples 10-18 and Comparative Examples 13-24 and the preparation method of the present invention, and the slag defects on the surface were automatically identified by the image method. The number of zinc dross per unit area and the number of missing plating spots, in units/square meter.

The substrate microhardness test was carried out on the above-mentioned samples, the microhardness test pressure was 100 g, and the measurement position was the section of the sample.

The coating adhesion test was carried out on the above samples. The sample method was a bending test. The execution standard was GB/T 232, the bending radius was 8 mm, and the bending angle was 180°. After bending, observe the surface morphology of the outer edge of the sample:

Good: No cracks occurred

Qualified: There are cracks within 3mm from the edge, and no cracks in other positions

Unqualified: Cracks appear outside the range of 3mm from the edge.

The coating corrosion resistance test was carried out on the above samples. First, the samples were subjected to a bending test. The implementation standard was GB/T 232, the bending radius was 4 mm, and the bending angle was 90°. Then, the neutral salt spray test was used to evaluate for 480 hours. Observe Corrosion condition at the outer edge of the bend and the location of the cut.

Good: no red rust

Qualified: red rust area does not exceed 5%

Unqualified: red rust area exceeds 5%

The experimental evaluation results are shown in Table 4.

Table 4

sample Zinc dross defect Leak plating defects Microhardness Plating Adhesion Plating corrosion resistance Example 10 1 1 299 good good Example 11 1 2 267 qualified good Example 12 0 0 290 good good Example 13 2 0 308 good qualified Example 14 1 1 301 good qualified Example 15 1 0 298 good good Example 16 1 2 278 qualified good Example 17 2 1 310 qualified good Example 18 2 0 297 qualified good Comparative Example 13 1 20 267 Failed Unqualified (bending red rust) Comparative Example 14 10 12 246 Failed Unqualified (bending red rust) Comparative Example 15 20 12 254 Failed qualified Comparative Example 16 twenty three 1 400 Failed qualified Comparative Example 17 3 19 400 Failed Unqualified (bending red rust) Comparative Example 18 34 20 299 Failed good Comparative Example 19 39 8 290 qualified Unqualified (bending red rust) Comparative Example 20 28 8 350 qualified Unqualified (bending red rust) Comparative Example 21 31 1 370 Failed Unqualified (cut red rust) Comparative Example 22 43 10 279 Failed qualified Comparative Example 23 40 9 289 Failed qualified Comparative Example 24 twenty one 1 286 qualified Unqualified (cut red rust)

Finally, it should also be noted that the terms "comprising", "comprising" or any other variation thereof are intended to encompass a non-exclusive inclusion such that a process, method, article or device comprising a series of elements includes not only those elements, but also Also included are other elements not expressly listed or inherent to such a process, method, article or apparatus.

Although the preferred embodiments of the present invention have been described, additional changes and modifications to these embodiments may occur to those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be construed to include the preferred embodiment and all changes and modifications that fall within the scope of the present invention.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Thus, provided that these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims (9)

1. The hot-dip galvanized hot-rolled steel is characterized by comprising a hot-rolled steel substrate and a coating, wherein the coating comprises the following chemical components in percentage by mass:

mg: 0.5-4%, AI: 3-20%, new additive element less than or equal to 1% and the balance of zinc;

the new added elements comprise at least one of the following components in percentage by mass:

Ti：0.01-0.1％，B：0.005-0.02％，Ca：0.002-0.02％，Si：0.01-0.5％，Cu：0.02-0.2％。

2. the hot-dip galvanized and hot-rolled steel according to claim 1, wherein the coating has a single-side galvanizing flux of 30-300g/m²。

3. A method for producing a hot-dip galvanized hot-rolled steel according to claim 1 or 2, comprising:

obtaining a galvanizing solution containing the chemical components;

heating the galvanizing solution until the temperature of the galvanizing solution is 20-50 ℃ higher than the melting point of the galvanizing solution to obtain a heating galvanizing solution;

obtaining a hot-rolled steel matrix;

heating the hot-rolled steel substrate;

and immersing the heated hot rolled steel matrix into the heating galvanizing solution for coating treatment to obtain the hot galvanized hot rolled steel.

4. The hot-dip galvanized hot-rolled steel according to claim 3, wherein the heating the hot-rolled steel substrate comprises:

heating the hot rolled steel matrix to 500-600 ℃, preserving heat for 30-120s, and controlling the temperature of the hot rolled steel matrix to be 10-50 ℃ higher than that of the heating zinc plating solution.

5. A hot-dip galvanized hot-rolled steel according to claim 3, characterized in that the coating treatment time is 0.1 to 3 seconds.

6. The hot-dip galvanized hot-rolled steel according to claim 3, wherein the hot-rolled steel substrate is obtained by the method comprising the following steps:

smelting the molten steel to obtain a steel billet;

heating, hot rolling, cooling and acid washing the steel billet to obtain a hot-rolled acid-washed plate;

and rolling the hot-rolled pickled plate, and then degreasing and cleaning the surface of the hot-rolled pickled plate to obtain a hot-rolled steel matrix.

7. The hot-dip galvanized hot-rolled steel according to claim 6, wherein in the rolling, the surface roughness Ra of the roller is 2.0-4.0, the total rolling reduction is 5-20%, the number of rolling passes is 2-4, and the single rolling reduction is 2-10%.

8. A hot-dip galvanized hot-rolled steel according to claim 3 or 6, characterized in that the hot-rolled steel substrate has a surface roughness Ra of 1.0 to 2.0 μm.

9. A hot-dip galvanized hot-rolled steel according to claim 3 or 6, characterized in that the area percentage of surface oxides of the hot-rolled steel substrate does not exceed 20%.