JPH03134147A - Production of alloying hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To produce an alloying hot dip galvanized steel sheet excellent in surface chracteristics by degreasing and pickling a steel sheet, forming specific amounts of ferrous oxide on the surface, performing reducing annealing, and then subjecting the steel sheet to hot dip galvanizing and to alloying. CONSTITUTION:Degreasing and pickling are previously applied to a steel sheet to remove impurities, such as Si oxide, in particular. It is desirable to carry out the above pickling by means of electrolytic pickling using H2SO4, etc. Subsequently, heating is applied to the steel sheet up to 400-750 deg.C in a weakly oxidizing atmosphere to form uniformly a ferrous oxide on the surface by 0.1-3 g/m<2> expressed in terms of Fe. At this time, the uniformity of the above ferrous oxide layer can be improved by applying, after the above degreasing and pickling, precoating with one or more elements among Ni, Co, Fe, and Cu by 2-70 mg/m<2> coating weight to at least one side of the steel sheet. After reducing annealing, this steel sheet is immersed into a molten zinc bath to undergo galvanizing and then heated, by which the galvanizing layer is alloyed with the base steel. By this method, the smooth alloying hot dip galvanized steel sheet suitable for automobile trim us and excellent in surface characteristics can be obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a surface-treated steel sheet with excellent smoothness on the surface of a plating film, and in particular, an alloyed molten zinc alloy with excellent surface properties suitable for use in automobile exteriors. The present invention relates to a method for producing plated steel sheets.

(Conventional technology) Alloyed hot-dip galvanized steel sheets are produced by heat-diffusion treatment of hot-dip galvanized steel sheets to alloy the galvanized layer with the base steel.
Due to its relatively excellent weldability and corrosion resistance after painting,
It has traditionally been used in the field of floor desks. In particular, in recent years, steel sheets for automobile bodies, especially steel sheets for exterior use, are being used.

In order to apply alloyed hot-dip galvanized steel sheets to automobile bodies, which require severe processing, the plating base plate must be a steel type with excellent deep drawability. In the case of hot-dip plating, if low-carbon ρ-killed steel for general processing is used as the plating base plate, supersaturated carbon remains in solid solution and hardens due to the thermal history of rapid heating and rapid cooling during annealing in the plating line. ,-C
Ultra-low carbon steel, especially ultra-low carbon Ti-added steel and ultra-low carbon Nb-added steel, which are made by adding Ti and Nb to ultra-low carbon steel to fix carbon and nitrogen in the steel, are used.

However, although these ultra-low carbon steel types have excellent deep drawability, their ferrite grain boundaries have a high degree of cleanliness, so the Fe-Zn alloying reaction occurs more easily at the grain boundaries than inside the ferrite grains, and the alloying process For example, if the amount of zinc deposited is 100 g/m or more, the growth of the alloy layer will occur unevenly and the surface of the film after alloying will become rough. As the Fe-Zn alloying reaction also progresses, a smooth plating layer l is finally obtained as shown in Figure 1 (a), but if the zinc adhesion is less than 80 g/m'', As shown in Figure (b), the plating layer tends to have many irregularities, especially when the amount of zinc coating is less than 40g/n'', the depressions (craters) are extremely large as shown in Figure (C). The plating layer 1 tends to be large.In addition, 2 in the figure is a steel plate.

If such defects exist, coating defects are likely to occur during electrocoating, and even during electrostatic powder coating, defects may occur due to moisture adsorbed to some of the craters in the plating layer during baking. In addition to high corrosion resistance, alloyed hot-dip galvanized steel sheets used for the exterior of automobile bodies, which are susceptible to corrosion, are required to have aesthetic elements, so it is desirable to have a smooth coating surface with few irregularities.

As a method of suppressing unevenness defects caused by such non-uniform alloying reactions, 0.1 to 0.0. 2% Al is added, but this A
j! Effective AI for concentration! ,? A possible method is to reduce the content to 0.08% or less in a degree (analytical value of Al - analytical value of Fg). However, when the Al concentration in the galvanizing bath is lowered, although uneven defects are reduced, the elution of Fe from the steel sheet is significantly increased, and a large amount of bottom dross mainly composed of δ and phase (FeZnt) is generated in the galvanizing bath. A problem occurs in that this dross floats in the bath and adheres to the surface of the steel sheet, increasing surface defects.

(Problems to be Solved by the Invention) As mentioned above, in the conventional manufacturing method of alloyed hot-dip galvanized steel sheets, many crater-like defects occur on the surface of the plating layer, which has a negative impact on the finished state after painting. There is a problem of giving In particular, IF is made by adding Ti and Nb to ultra-low carbon steel, which is often used for automobile bodies.
(rntersLiLial Free) This problem becomes noticeable when steel is used as the plating base plate.

The present invention has been made to solve the above-mentioned problems, and in particular, the purpose of the present invention is to produce an alloyed hot-dip galvanized steel sheet with excellent surface properties suitable for automotive exterior use. The goal is to provide a way to do so.

(Means for Solving the Problems) The present inventors conducted a detailed study on the surface condition of a steel sheet before reduction annealing and the smoothness of the plating film after alloying. As a result, the following findings were obtained.

■If Fe-based oxide is formed with an appropriate thickness on the surface of the steel sheet before reduction annealing, the smoothness of the surface of the plating film after alloying will be improved.

(2) However, simply forming an Fe-based oxide may result in little improvement in smoothness. This is mainly due to the thermal history of the upper process (hot rolling process), and is thought to be because the formation of Fe-based oxides is suppressed by the remaining Si oxides and the like.

(2) The above problem (2) can be solved by degreasing and pickling the surface of the steel plate in advance. By degreasing and pickling in advance, the Fe-based oxide can be stably and uniformly formed regardless of the thermal history of the previous step.

■Furthermore, Ni, Co, and F are added to the surface of the steel plate after degreasing and pickling.
If Fe-based oxide is formed after pre-plating with one or more of e and Cu, the smoothness of the surface of the plated film after alloying is further improved.

The present invention has been made based on the above findings, and has the following gist (i) and (ii) t7.

(i) A steel plate that has been previously degreased and pickled is heated to 400 to 750'C in a slightly oxidizing atmosphere to form 0.1 to 3 g/m'' of Fe-based oxide as metal Fe on the surface. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises performing reduction annealing, then galvanizing the steel sheet by immersing it in a hot-dip zinc bath, and then alloying the galvanized layer with the base steel by heating.

(ii) At least one side of the steel plate, which has been previously degreased and pickled, is coated with Ni, Co, and F at an adhesion amount of 2 to 70 s+g/m.
e. After plating with one or more types of Cu, it is heated to 400 to 750°C in a weakly oxidizing atmosphere to coat the surface with metal F.
Claim (1) characterized in that after forming the Fe-based oxide of 0.1 to 3 g/+w'' as e, reduction annealing is performed.
The method for manufacturing the alloyed hot-dip galvanized steel sheet described.

(effect) The present invention will be explained in detail below.

The process according to the invention is carried out in an in-line hot-dip annealing type, so-called Sendzimer type and similar types of hot dipping lines.

The feature of the method of the present invention is that in the step before reduction annealing the steel plate, 1
ii1 The surface of the plate is degreased and pickled to form an Fe-based oxide with an appropriate thickness on the surface.

If impurities such as Si oxide generated in the previous process remain on the steel sheet surface, the formation of Fe-based oxides will be suppressed and become non-uniform during heating in a weakly oxidizing atmosphere, which will be described later. Fe-based oxide can be uniformly formed by degreasing and pickling.

This degreasing can be carried out by any method, but it is preferable to carry out electrolytic treatment using an alkaline electrolyte, and it is more preferable that the alkaline solution does not contain silicate. Acid solutions commonly used in industry can be used, but among these, 11□S
Electrolytic pickling using O4 is preferred; this method allows pickling to be carried out within 3 seconds.

After the pickled steel plate is washed with water and dried, it is heated in a weakly oxidizing atmosphere in order to form an Fe-based oxide with an appropriate thickness.

If an appropriate thickness of Fe-based oxide is formed on the surface of the steel sheet before reduction annealing, the smoothness of the film after alloying will be improved. This is because the Fe-based oxide is removed from the metal F in the next reduction annealing step.
It is thought that this metal Fe imparts uniform reactivity to the surface of the steel sheet in the alloying reaction with zinc and improves the smoothness of the film surface after alloying.

In order to improve smoothness, the steel plate after degreasing and pickling is heated to 400 to 750°C in a slightly oxidizing atmosphere, and the surface is coated with metal Fe.
It is necessary to form 0.1 to 3 g/m'' of Fe-based oxide.

When the heating temperature is less than 400'C, P of 0.18/m" or more
It takes a long time to form an e-based oxide, and if the temperature exceeds 750°C, Fe will be excessively oxidized, making the Fe-based oxide unnecessary.
It forms thickly. Fe-based oxide is 0 as metal Fe
．． If it is less than 1 g/n", the effect is insufficient to smooth the film after alloying, and if it is more than 3 g/m", the re-Zn alloy layer formed during hot-dip plating may develop excessively. However, in addition to making thinning difficult, problems such as an increase in the amount of dross generated in the plating bath occur.

The weakly oxidizing atmosphere for forming the Fe-based oxide having the above thickness is (02): 100 to 2000 ppm of oxygen or a mixed gas having an oxygen potential equivalent to this.

After that, use a reducing atmosphere as before, e.g. dew point -25.
If reduction and annealing are performed at a recrystallization temperature according to the steel type in a 11□ + inert gas atmosphere of 11 □ or below ℃, zinc plating is applied by immersion in a hot-dip zinc bath, and then alloying is performed at 470 to 600 ℃. An alloyed hot-dip galvanized steel sheet with a smooth coating surface can be obtained.

In addition, in conventional melt-dipping lines equipped with non-oxidizing furnaces, cold-rolled steel sheets or hot-rolled wA sheets after pickling are heated in a weakly oxidizing atmosphere to degrease them, and then reduction annealing is performed in a non-oxidizing atmosphere. However, in the conventional method, the formed Fe-based oxide is relatively non-uniform, and the amount is small at less than 0.1 g/m'' as metal re.This is a small amount in the conventional method. This is because the formation of Fe-based oxides was thought to be the best from the viewpoint of plating adhesion in hot-dip plating (non-alloyed material).

Further, conventionally, no investigation has been conducted regarding the appropriate amount of oxidation from the viewpoint of uniformity of the alloyed film.

The method of the first invention described above has a large smoothness improvement effect in the case of a galvanizing bath in which the effective AP concentration in the bath is less than 0.12%. However, in the case of this method, when the effective Al concentration is 0.12% or more, the smoothness improving effect may be insufficient. When the ^l concentration in the bath becomes high, the progress of alloying becomes extremely uneven, and the surface of the film after alloying tends to become uneven.

When using such a galvanizing bath with a high effective N concentration, it is preferable to produce an alloyed hot-dip galvanized steel sheet by the method of the second invention of the present application. That is, F is applied to the surface of the steel plate after the degreasing and pickling.
Before forming the e-based oxide, apply a coating amount of 2 to 70ff1g/
At least one of 112 Ni, Go, Pe, Cu
This is a method of pre-plating a seed or more, followed by reduction annealing, hot-dip galvanizing and alloying. With this method, for example, an alloyed hot-dip galvanized steel sheet with good smoothness can be produced even from a galvanizing bath with an effective^2 concentration of 0.15%.

Although the reason for this is not clear, such pre-plating usually has a high AI! , Fe2^2. phase, that is, Fe, Al, which has the effect of suppressing the Fe-Zn reaction
This is thought to be due to the effect of suppressing the formation of five phases and activating the Fe-Zn reaction.

However, the adhesion amount of the pre-plating was 2 mg/+++"
If it is less than 7, the smoothness improving effect is the same as that of the first invention of the present application.
If it exceeds 0mg/+”, PE-7 during hot-dip galvanizing,
n reaction becomes excessive and dilution becomes difficult.

Note that this pre-plating may be applied to both sides of the steel plate, or may be applied only to one side, which is the outer surface where smoothness is highly required, such as an exterior steel plate for an automobile.

Further, this pre-plating is generally performed by electroplating, but except for Fe, Ni, Co, and Cu may be performed by electroless plating.

The present invention will be further explained below with reference to Examples.

(Example 1) C: 0.003%, Si: tr, Mn: 0
．． 16%, P: 0.008%, S 70.008%
, Sol, AI: 0.023%, Ti:
Ultra-low carbon Ti-Nb containing 0.038%, Nb; 0.008%, with the balance consisting of Fe and inevitable impurities.
A full hard steel plate of additive IF steel (thickness = 0.8+am) was used as a test material, and this test material was 100m+m x 250a.
After cutting to +m, the test material was electrolytically degreased in an aqueous solution of soda carbonate and sodium hydroxide, and after washing with water,
A pickling treatment was performed for 10 seconds in 10% hydrochloric acid. After pickling,
The product was washed with water and dried, and then heated under various conditions shown in Table 1 in a hot-dip plating simulator having a vertical atmosphere furnace with adjustable atmosphere to form Fe-based oxides on the surface. Then, reduction annealing was performed for 45 seconds at a temperature of 850'C to 26%+1
．． -1-N, (dew point knee 35°C) atmosphere, 50
After cooling to 0℃, 0.11% ^l at bath temperature 460℃
After applying galvanization by immersing it in a hot-dip galvanizing bath containing 2 seconds, the coating weight was adjusted to about 60 g/m'' using a gas wiper.

A part of the obtained as-plated test material was subjected to chemical analysis.
The reaction (alloying) Pejil during hot-dip galvanizing was measured. The remaining as-plated test material was alloyed at 500° C., and then the smoothness of the plating layer was investigated using a scanning electron microscope using a cross-sectional embedded sample.

These results are also listed in Table 1.

The smoothness was evaluated on a five-point scale, with 1 being extremely poor, 2 being poor, 3 being fair, 4 being good, and 5 being extremely good.

(Hereinafter, blank space) As is clear from Table 1, all of the examples of the present invention have extremely good surface smoothness of the plating film after alloying, and the reaction Fe1i during plating is also small. Comparative examples manufactured using methods that deviate from the conditions specified in the present invention have poor smoothness or have a high reaction Fel during plating, exceeding 1.5 g/11".The reaction Fetl during plating is If it exceeds 1.5 g/m", there is a problem that it becomes difficult to thin the material and the amount of dross generated increases, but in all of the examples of the present invention, the amount of reacted Fe is 1.5 g/m" or less. Therefore, such problems will not occur.

(Example 2) Using a test material with the same chemical composition as in Example 1, it was degreased in the same manner, and then heated at a current density of 40 A/dm for 8% + 12 s.
o, cathodic electrolytic pickling was performed in the solution for 0.8 seconds. After pickling, electroplating with one of Ni, 'Co%Fe, or Cu under the following conditions was carried out, followed by ambient temperature up to 600'C at 8°C/sec in the hot-dip plating simulator described above, and N, +500p.
It was heated in pm [0,] to form an Fe-based oxide on the surface.

Thereafter, reduction annealing and Ft'4i galvanizing were performed in the same manner as in Example 1. In this example, the Al14 degree in the hot-dip galvanizing bath was 0.14%.

(1) Ni plating condition bath composition NiSO4.611zO: 240g/1
113BO-: 24g/I NazSOa: 75g/j! pH: 2.
o, Bath temperature: 50°C Anode = Ni, Liquid flow rate: 0.34 m/sec Current density:
30A/dm” (2) Co plating condition bath composition CO3O4・71120: 330g/
ICoCe z: 45g/11IJO+
: 3Bg/l pH: 3. o, Bath temperature: 50°C Anode: Carbon, Liquid flow rate: 0.1 m/sec Current density: 0.5 A/dm” (3) Pe plating conditions Bath composition Fe5On 711zO: 250 g/
1 (N114)! So4: 120g/1 citric acid: 0.6g#! pH 72.0, Bath temperature: Anode: Fe, Liquid flow rate: Current density: 25A/dta” (4) Cu plating conditions Bath composition Copper birophosphate Potassium pyrophosphate: Ammonia N03 PH: 8.5, Bath temperature: Anode: Cu , Liquid flow rate: Current density: 5A/da'' A portion of the as-plated test material thus obtained was subjected to chemical analysis in the same manner as in Example 1, and the reaction (alloying) during plating was evaluated.
In addition to measuring the amount of Fe, the remaining as-plated test material was alloyed at 500''C, and then the smoothness of the plating layer was investigated.These results are also listed in Table 2. The evaluation criteria are the same as in Example 1.

(Hereafter, blank space) 50℃ 50℃ 65g/ 1 230g/ Q 2g/1 15g/ ff1 0.1 dawn/sec 0.34 m/sec Looking at the example of the present invention in Table 2, the Al concentration is 0.14% Even in a galvanizing bath of It turns out that there are few.

On the other hand, as comparative examples, there is one without pre-plating (Ni11) or one with a small amount of coating (Ni2).
, Nα3, Nα12) have a small smoothness improvement effect, and those with too much adhesion (Nα6, Nα9 to Nα11) have excellent smoothness but have a large amount of reactive Fe during plating, and Nα17
Even if an appropriate amount of pre-plating is applied as in 18 and
If Fe oxide is not formed, no improvement in smoothness is observed.

(Effects of the Invention) As explained above, according to the method of the present invention, a high quality alloyed hot-dip galvanized steel sheet with excellent smoothness can be manufactured. In particular, the method of the present invention is applicable to IFl containing Ti and Nb.
The effect is great when Iil is used as the plating base material,
This method is extremely effective as a method for producing alloyed hot-dip galvanized steel sheets with surface properties suitable for use as plated steel sheets for automobile exteriors.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a cross section of an alloyed hot-dip galvanized layer.

Claims (2)

[Claims] (1) A steel plate that has been previously degreased and pickled is heated to 400 to 750°C in a slightly oxidizing atmosphere, and the surface is heated to 0.0
．． After forming 1 to 3 g/m^2 of Fe-based oxide, reduction annealing is performed, and then galvanization is performed by immersion in a molten zinc bath, and the galvanized layer is alloyed with the base steel by heating. A method for manufacturing an alloyed hot-dip galvanized steel sheet, characterized by: (2) At least one side of a steel plate that has been previously degreased and pickled,
Ni, Co, Fe, Cu with adhesion amount of 2 to 70 mg/m^2
After plating with one or more of the following, it is heated to 400 to 750°C in a weakly oxidizing atmosphere, and the surface is coated with zero metal Fe.
．． 2. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1, wherein reduction annealing is performed after forming 1 to 3 g/m^2 of Fe-based oxide.