KR19990057390A - Process for producing acid-free alloyed hot-dip galvanized hot rolled steel sheet with excellent powder resistance and surface quality - Google Patents

Abstract

The present invention is a reduction pretreatment process by reducing the hot rolled steel sheet without removing the scale layer to reduce the hot rolled scale layer to a layer composed of porous reduced iron and FeO and invading the zinc plating bath with an appropriately adjusted Al concentration. Powder resistance consisting of a plating process that transforms the porous hot-rolled scale formed from to a ductile Zn-Fe-Al and Fe-Al compound and the upper part becomes a pure zinc plating layer, and an alloying process that continuously alloys it by low temperature heating and The manufacturing method of the acid-free alloying hot-dip galvanized hot-rolled steel sheet which is excellent in surface quality is made into a summary.

Description

An acid-free alloyed hot dip galvanized hot rolled steel sheet with excellent powder resistance and surface quality

The present invention relates to a method for producing an acid-free alloyed hot-dip galvanized hot rolled steel sheet having excellent powdering resistance and surface quality. More particularly, the hot rolled steel sheet, which has not been scaled out, is subjected to reduction heating, and the hot rolled scale layer is composed of porous reduced iron and FeO. Omitting the descaling process and reducing the alloying temperature of the hot-rolled steel sheet, which is a plating material, by the plating process for reducing the layer, the plating process for adjusting the Al concentration of the plating bath, and the alloying process for alloying by continuously heating at low temperature. The present invention relates to a method for producing an alloyed hot dip galvanized hot rolled steel sheet.

Since alloyed hot-dip galvanized steel sheet has better paintability, weldability, and post-coating corrosion resistance than hot-dip galvanized steel sheet, its demand is increasing as a high grade steel sheet for home appliances and automobiles. However, when thick materials are required in these fields, alloyed hot-dip galvanized hot-rolled steel sheets are used in which the hot dip galvanized hot-rolled hot-dip galvanized hot-rolled steel sheet is continuously alloyed and heat-treated to alloy the plating layer with Fe-Zn-based intermetallic compounds. have.

However, until now, the scale layer of hot-dip steel sheet, which is a plating material of hot-dip galvanized hot-rolled steel sheet and alloyed hot-dip galvanized hot-rolled steel sheet, greatly reduces plating adhesion, and thus, it is completely removed from hydrochloric acid, sulfuric acid, or a mixed solution thereof in the pickling process before plating. . However, since the oxide film of about 10-570 에서 still remains on the hot-rolled steel plate pickled in the pickling process, it inhibits the plating adhesion during hot dip galvanizing, so that it is heated to a temperature of up to 550 ° C. in a hydrogen reduction atmosphere of about 15% H ₂ -N ₂ . The remaining oxide film was reduced in a reduction heating zone and then deposited in a molten zinc plating bath to perform plating.

Since the pickling method has a large difference in pickling properties depending on the scale composition formed, some of the base structures are over-acidified to expose ferrous iron, resulting in rough steel pipe surfaces, hydrogen brittleness, iron loss, and acid loss. In addition, since pickling must be completed within a short time, it is difficult to maintain operating conditions such as heating conditions and acid concentration control. In addition, since hydrochloric acid or sulfuric acid, which is highly toxic and corrosive, is used as an acid solution, it causes an increase in costs and an environmental pollution problem in an acid recovery treatment facility and waste solution treatment.

In addition, the quality characteristics of conventional alloyed hot-dip galvanized steel sheet is excellent in spot weldability, corrosion resistance and coating adhesion after coating, but over alloying material causes a problem of powdering (powering) in which the plating layer falls into powder during processing. . Such powder resistance deteriorates in proportion to the increase in Fe concentration of the alloy layer, but improves the weldability. In consideration of the required properties as described above, the optimum Fe concentration of the plating layer is relatively narrowed to 8-12 wt%. Therefore, a conventional alloyed hot-dip galvanized steel sheet (No. 88580) using a cold rolled steel sheet as a plating material is diffused into the plating layer of Fe, and the plating layer is relatively hot to form Fe-Zn-based intermetallic compounds. After alloying at ℃ to secure the appropriate degree of alloying and air-cooled again to manufacture the alloyed hot-dip galvanized steel sheet, but in the case of using the plated material as a hot-rolled steel plate of the material material as in the present invention, the appropriate alloying degree is secured After the air-cooling treatment, the alloying proceeds during cooling due to the latent heat of the thick hot rolled steel sheet, so that superalloying tends to occur. Therefore, when the plated material uses a thick hot rolled steel sheet, rapid cooling fog cooling is adopted to prevent alloying during cooling. However, the quenching treatment of the alloy plating layer by the fog cooling method in which the aqueous solution and the air are sprayed induces a crack in the alloy layer due to thermal shock, and the crack is advanced and connected to the base steel, thus causing powdering. The surface of the alloy layer in direct contact with the particles has a problem of causing cratering surface defects, which are crater-shaped irregularities.

The present invention is to solve the above-mentioned problems, and the plating treatment process for reducing the hot rolled scale layer to a layer consisting of porous reduced iron and FeO by reducing the heat treatment of the hot rolled steel sheet without removing the scale and adjusting the Al concentration of the plating bath The purpose of the present invention is to provide a method for manufacturing an alloyed hot-dip galvanized hot-rolled steel sheet which can omit the scale removal process of the hot-rolled steel sheet, which is a plating material, and reduce the alloying treatment temperature by the plating process of plating by plating and the alloying process of alloying by continuously heating at low temperature. There is this.

The present invention is a reduction pretreatment process by reducing a hot rolled scale layer of a hot rolled steel sheet which is not pickled and reducing the hot rolled scale layer to a layer composed of porous reduced iron and FeO and a zinc plating bath in which the Al concentration is properly adjusted. The hot rolled scale layer of porous reduced iron is transformed into ductile Zn-Fe-Al and Fe-Al compound rolls, and the upper part is composed of a plating process of forming a pure zinc plating layer and an alloying process of alloying by continuously heating it at low temperature. Alloying hot-dip galvanized hot rolled steel sheet manufacturing method is characterized by.

The reduction behavior of the hot rolled scale layer according to the reduction heat treatment condition of the hot rolled steel sheet, the change of the hot rolled scale according to the Al concentration of the plating bath, and the alloying behavior according to the alloying condition were as follows.

1) The hot rolled scale layer is a secondary scale formed during the rough rolling process, and the outermost layer in contact with air is hematite (Fe ₂ O ₃ ), followed by magnetite (Fe ₃ O ₄ ) and whistite (Feo) It had a three-layer structure in contact with, and most of the scale layers were composed of magnetite (Fe ₃ O ₄ ), and the thickness of a typical scale layer was about 5 μm.

2) The hot rolled scale layer is cracked and reduced in the surface layer caused by volume expansion generated when Fe ₂ O ₃ `is reduced through low-dimensional oxides such as Fe ₃ O ₄ and FeO by reduction heat treatment as shown in the following reaction formula. Pores are formed by the release of the product by the reaction of the gas with the scale, and these are applied as a diffusion path for reducing the scale layer, and the dense hot rolled scale layer is composed of porous reduced iron and FeO. Transformed into layers. This phenomenon increased with increasing heating temperature and reducing hydrogen concentration.

Cracks or pores formed by the reheating treatment of the hot rolled scale layer are increased by increasing the heating temperature, heating holding time and hydrogen concentration, reducing gas.However, at the heating temperature of more than 820 ℃, the reduction reaction of the scale layer is accelerated. Porosity of the scale layer is promoted, but it is not preferable because it causes material deterioration of the plating material because it exceeds the recrystallization temperature. In addition, the reduction of the hot rolled scale layer was impossible even if the holding time was heated to 400 seconds or less at the existing reduction heating temperature of 55 ° C. or lower. In addition, the higher the hydrogen concentration of reducing gas, the better the reducing ability, which is not preferable because it causes a stability problem at 30% or more.

3) The Al concentration of the plating bath promotes the diffusion of zinc and Al along the cracks of the hot rolled scale layer formed by reduction heating treatment and the diffusion path of the vacancy, and the hot rolled scale layer is composed of ductile Zn-Fe-Al and Fe-Al compounds. It is formed to act as an anchor (anchor) connecting between the hot rolled scale layer and the base structure, and the upper part becomes a pure zinc plated layer, so that it is possible to secure the plating adhesion of the rinse-free hot dip galvanized hot rolled steel sheet.

However, when the Al concentration of the plating bath is 0.2wt% or less, the ductile Zn-Fe-Al and Fe-Al compounds are not completely formed in the hot rolled scale layer. Out-burst) structure is formed, so powdering property occurs during processing by disproportionation alloying and over alloying during continuous alloying treatment. In addition, when the Al concentration of the plating bath is 1.0 wt% or less, the inside and the surface of the hot rolled scale layer are completely formed with a flexible Zn-Fe-Al and Fe-Al compound film, and thus Zn-Al is added to the upper plating layer of the scale layer. The alloy is plated. In particular, when the Al concentration of the plating bath was 3wt% or more, the cross section of the plating layer showed an oval structure of the pure zinc plated layer, which is a typical plating layer structure of an An-Al alloy plated steel, and a comb pattern structure, which is an Al-rich region. Therefore, the optimum plating layer structure for continuous alloying after hot dip galvanizing is covered with a flexible Zn-Fe-Al and Fe-Al compound film on the inside and the surface of the thermal heat scale to form a non-uniform alloy layer on the upper zinc plating layer. Since it is not formed and the plating layer does not become an alloy plating layer of Zn-Al, the Al concentration of the plating bath is required to be managed so as to satisfy this condition. In the present invention, the above condition is satisfied at 0.2 to 1.0 wt% Al.

4) When the plating layer structure is continuously alloyed with a hot rolled scale layer composed of Zn-Fe-Al and Fe-Al compounds and a non-acid-free hot dip galvanized hot rolled steel sheet composed of a pure zinc plated layer, reduction is performed to the hot-line scale layer by reduction heating. Since the Fe component is converted into a pure zinc plated layer relatively easily to form a Zn-Fe alloy plated layer, the alloying hot dip galvanized hot-rolled steel sheet plated on a conventional scale picked hot-rolled steel sheet than the alloying temperature of 480-550 ℃ Alloying treatments were possible at low temperatures of 440-480 ° C. Therefore, because it does not need to be quenched to prevent the latent heat of the after-plating material, it was possible to prevent surface defects such as cracking and creering in the existing hot-dip galvanized hot-rolled steel sheet.

However, when the alloying treatment temperature is 430 ° C. or less, the Zn-Fe-Al and Fe-Al films formed at the interface between the reduced hot rolled scale layer and the zinc plated layer are not broken. Since the Fe component of the reduced hot rolled scale layer prevented diffusion into the galvanized layer, it was impossible to completely alloy the galvanized layer. This tendency increases with higher Al concentration of the plating bath, lower temperature of the plating bath, shorter deposition time, and lower alloying temperature.

In addition, the Zn-Al-Fe and Fe-Al films formed between the reduced hot rolled scale layer and the zinc plated layer are easily broken at the alloying treatment temperature of 490 ° C. or higher, so that the reduced Fe component diffuses rapidly into the soft plated layer, thereby overalloying and powdering. Will cause. Therefore, the alloying treatment temperature for producing an acid-free alloyed hot-dip galvanized hot rolled steel sheet excellent in powder resistance was 440-480 ° C. Since the alloying treatment temperature of the acid-free alloyed hot-dip galvanized hot rolled steel sheet is significantly lower than that of the conventional alloyed hot-dip galvanized hot-rolled steel sheet, the quenching treatment is performed to prevent the continuous alloying during the latent heat caused by conventional thick materials. It became unnecessary. Therefore, it was possible to prevent cracks and cretters, which are surface defects of the alloy plated steel sheet caused by the quenching treatment.

Hereinafter, embodiments of the present invention will be described.

Example

Hot rolled steel sheets of 2.3 mm thickness without hot rolled scales were cut into 100 mm horizontally and 200 mm vertically to obtain test specimens. The test piece was subjected to reduction heating under a condition of reducing heating treatment temperature of 550-820 ° C., holding time of 60-400 seconds, and hydrogen concentration of 10-30%. The pre-treated plating material was deposited for 3-5 seconds at a plating bath temperature of 450 ° C. in a plating bath of 0.2-5 wt% Al as shown in Table 1 below, followed by hot-dip galvanizing to obtain a cross-sectional deposit amount of 45-60 g / m ^2. Subsequently, alloying heat treatment was performed under alloying heat treatment conditions for 3-5 seconds at a temperature of 430-490 ° C. to prepare an acid-free alloyed hot dip galvanized hot rolled steel sheet. Table 1 shows the alloyability and powdering properties of the hot-rolled steel sheet of the acid-free alloyed hot-dip galvanized hot-rolled steel sheet thus prepared. Powdering property evaluation was a condition that evaluated the degree to which the powder dropped on the tate after the 60 ° bending test in the area subjected to the compressive stress. In this case, the first grade is unalloyed without powdering, the third grade is relatively good, and the fourth grade is less than the superalloyed powdering or the plating layer peeling occurs.

As shown in Table 1 below, using a pickling steel plate as a plating material, alloying at a temperature of 480-550 ° C. after plating in a plating bath having a reducing heating temperature of 550 ° C. or less and a plating bath Al concentration of 0.08-0.16 wt%. In the conventional case (No. 1) treated with fog after treatment, the powder resistance is excellent, but since the alloying temperature is high, the quenching treatment is required, resulting in surface defects such as cracking and cratering. In addition, when a hot rolled steel sheet without hot rolled scale is used as a plating material, the scale of the hot rolled steel sheet is reduced when the reduction heating treatment temperature is 550 ° C. or less (No. 2) or when the holding time is 65 seconds or less (No. 3), even at 65 ° C. Since the layer was not completely reduced, plating peeling occurred in the hot rolled scale layer.

On the other hand, the heating zone temperature is 650 ℃ and the holding time is 250 seconds or more, followed by reduction heating, and the plating bath is deposited by plating in an Al concentration of 0.2-1.0wt%, followed by alloying and air cooling at 440-480 ℃. The alloyed hot dip galvanized hot rolled steel sheet excellent in powder resistance (No. 5) can be manufactured. In addition, alloying with excellent powdering resistance even when the Al concentration of the plating bath is increased to 0.5-1wt% even though the hot rolled scale layer is not completely reduced by the reduction heating process having a heating table temperature of 650 ° C and a holding time of 120 seconds. It can be seen that hot-dip galvanized steel sheet can be manufactured. However, if the reduced heating treatment condition completely reduces the hot-rolled scale layer and the alloying heat treatment temperature satisfies the scope of the present invention, the Al concentration of the plating bath is 1.0 wt% or more (No. 8), and the plating layer becomes a Zn-Al alloy plating layer. Therefore, unplating occurred because diffusion of the Fe component prevented the diffusion.

In addition, when the temperature of the heating table is 750 ° C, the hot rolled scale layer is reduced even when the heating time is 60 seconds. Therefore, after plating in a plating bath having an Al concentration of 0.2-1.0wt% of the plating bath, the heat treatment is performed at 440-480 ° C (No. .10,12) Zinc alloy hot-dipped hot rolled steel sheets with excellent powdering properties can be produced. However, when the alloying temperature is 430 ° C. or less (No. 9) or 490 ° C. or more (No. 11), unalloyed and overalloyed is not preferable. In addition, even if the plating conditions and alloying conditions satisfy the scope of the present invention, it can be seen that unplating is not preferable because the Al concentration of the plating bath is 1.0 wt%.

No. Pickling treatment Reduction Heat Treatment Condition Plating bath Al concentration (wt%) Alloying temperature (℃) Cooling method Resistance to Powdering Remarks Temperature (℃) Retention time (seconds) Hydrogen concentration (vol%) One U 550 60 15 0.08-0.16 480-550 Fog 1-3 Example 2 radish 550 400 30 0.2 - Air cooling 5 Comparative example 3 radish 650 120 30 0.2 - Air cooling 4 Comparative example 4 radish 650 250 10-30 0.2 ≤430 Air cooling One Comparative example 5 radish 650 250-400 10-30 0.2 440-480 Air cooling 2-3 Inventive Example 6 radish 650 400 10-30 0.2 ≥490 Air cooling 4-5 Comparative example 7 radish 650 120 10-30 0.5-1.0 440-480 Air cooling 1-2 Inventive Example 8 radish 650 120-400 10-30 ≥1.0 440-480 Air cooling One Comparative example 9 radish 750 60 10-30 0.2 ≤430 Air cooling One Comparative example 10 radish 750 60-400 10-30 0.2 440-480 Air cooling 1-3 Inventive Example 11 radish 750 60-400 10-30 0.2 ≥490 Air cooling 4-5 Comparative example 12 radish 750 60-400 10-30 0.5-1.0 440-480 Air cooling 1-2 Inventive Example 13 radish 750 60-400 10-30 ≥1.0 440-480 Air cooling One Comparative example

division Manufacture process Conventional Alloying Hot-Dip Galvanized Steel Sheet Manufacturing Method Hot rolled steel sheet → Pickling treatment → Reduction heating treatment table (Max.550 ℃) → Plating (Al: 0.08 ~ 0.16) → Alloying heat treatment (480 ~ 550 ℃) → Cooling (fog) Alloying hot dip galvanized steel sheet manufacturing process of the present invention Hot-rolled steel sheet → Reduced heat treatment table (650 ~ 820 ℃) → Plating (Al: 0.2 ~ 1.0) → Alloying heat treatment (440 ~ 480 ℃) → Cooling (Air cooling)

It is possible to omit the hot rolled scale removal process by adjusting the Al concentration of the plating bath by adjusting the Al concentration of the plating bath by reducing the heating of the hot rolled steel before plating without removing the scale of the plated hot rolled steel sheet. Since the Fe component formed in the hot rolled scale reduction type diffuses into the plating layer during the alloying treatment, it is possible to greatly reduce the alloying treatment temperature, thereby preventing cracking or cruttering, which is a surface defect due to rapid cooling after the conventional alloying treatment. can do.

Claims (1)

In the production of alloyed hot-dip galvanized hot rolled steel sheet, the hot rolled steel sheet without scaling is reduced-heated under reduced heating temperature of 650 to 820 ° C., holding time of 60 to 400 seconds, and hydrogen concentration of 10 to 30 vol%. A plating pretreatment step of reducing iron; A plating step of plating by plating in a plating bath having an Al concentration of 0.2 to 1.0 wt% in the plating bath; A step of air cooling after performing an alloying treatment at a temperature of 440 to 480 ° C. for 3 to 5 seconds; An acid-free alloyed hot-dip galvanized hot-rolled steel sheet manufacturing method characterized in that made of excellent powder resistance and surface quality.