JP3110238B2 - Method for producing hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet used for an automobile body or the like.

[0002]

2. Description of the Related Art Conventionally, various surface-treated steel sheets having excellent corrosion resistance have been used as steel sheets for automobiles. Among them, hot-dip galvanized steel sheet manufactured in continuous hot-dip galvanizing equipment that performs recrystallization annealing and plating on the same line is capable of high corrosion resistance and inexpensive manufacturing. The applied galvannealed steel sheet is widely used because it has excellent weldability and press workability in addition to corrosion resistance.

[0003] On the other hand, in recent years, global environmental problems have been highlighted, and reduction in weight is required for improving fuel efficiency of automobiles.
Therefore, a high-strength steel plate with increased strength has been developed,
At present, hot dip galvanizing and alloying hot dip galvanizing are required for corrosion resistance. High-strength steel sheets are generally obtained by adding elements such as P, Si, Mn, and Cr.

[0004] It is known that, in a steel sheet containing P, recrystallization annealing in a hot-dip galvanizing apparatus (hereinafter, referred to as CGL) causes precipitation of P at a surface crystal grain boundary to slow alloying. This is thought to be because iron is eluted into the zinc plating during alloying mainly at the grain boundaries, but in the steel sheet containing P, the grain boundaries P block the iron elution path.

Further, it is known that in steel sheets containing Si, Mn, Cr, etc., these elements are concentrated on the surface during recrystallization annealing and impair plating wettability. This is because, in the recrystallization annealing atmosphere, iron is not generated because Fe is reducible, but Si, Mn, and Cr are oxidizing atmospheres, so these elements become oxides on the steel sheet surface. This is because the oxide film is concentrated and the contact area between the molten zinc and the steel sheet is reduced.

As a conventional method for improving the plating wettability, a method of performing electroplating before introducing a steel sheet into CGL (Japanese Patent Laid-Open No. 2-194156) or a steel having a low content of Si, Mn or the like by a cladding method is used. (JP-A-3-199363) has been devised to improve the plating wettability by using as a surface layer. On the other hand, a method of improving the wettability with molten zinc by further adding Ti to steel (Japanese Unexamined Patent Application Publication No.
148,073) has also been devised.

On the other hand, due to the remarkable delay in alloying, it is necessary to increase the alloying temperature or decrease the line speed and lengthen the alloying time in order to sufficiently alloy the plated layer. However, by increasing the alloying temperature, it becomes difficult to control the iron content in the plating layer, and it tends to be unnecessarily high. Connect. Further, when alloying is performed for a long time by lowering the line speed, the productivity is deteriorated and the production cost is increased.

A conventional method for accelerating alloying is disclosed in
As disclosed in US Pat.
By performing Ni or Cu plating before reduction annealing,
The purpose is to promote alloying of Fe-Zn. However, in this method, it is necessary to install equipment for Ni or Cu plating before reduction annealing.

Japanese Patent Publication No. 64-11111 discloses a method of changing the Al concentration and the bath temperature in a Zn bath to keep the alloying conditions constant. In this method, however, it is necessary to change the Zn bath conditions. It takes a long time and it is necessary to stop the line, which deteriorates productivity. Further, in the embodiment disclosed in the same gazette, it is disclosed that the present invention can be applied to a steel sheet having a Si content of 0.2% or less and a P content of 0.1% or less. However, up to 2.0% of Si, which is the object of the present invention, P: 0.
It cannot be applied to steel sheets with a large addition of up to 2%.

Japanese Patent Application Laid-Open No. Hei 3-243751 discloses a method of accelerating alloying by removing a P-enriched layer by annealing and pickling P-added steel. The technique disclosed in this publication is aimed at promoting alloying of P-added steel generally having a tensile strength of 35 kgf / mm ² class, and is an object of the present invention. A steel sheet with a complex addition of Si, Mn and Cr, so-called tensile strength of 4
It is not a technique for improving non-plating defects in steel sheets of 5 kgf / mm ² or more.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art.
In producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet by using a high-strength steel sheet containing i, Mn, and Cr as the base steel sheet, it is necessary to minimize the complexity of the process and reduce productivity as much as possible, It is an object of the present invention to provide a method of manufacturing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that can be manufactured well at low cost.

[0012]

Means for solving the above problems in the present invention are as follows. We have:
The steel sheet surface concentration state after recrystallization annealing in the steel sheet to which P and Si, Mn, and Cr are added as a target of the present invention, and the P concentration of the crystal grain boundary exposes the crystal grain boundary by brittle fracture. Auger analysis (AES) was performed, and the concentration of Si, Mn, and Cr on the steel sheet surface was measured by glow discharge spectroscopy (GDS). FIG. 1a) shows an AES spectrum of a surface grain boundary after recrystallization annealing, and FIG. 1b) shows an AES spectrum of a crystal grain boundary inside the steel sheet. Also,
FIG. 2a) shows a GDS spectrum of the steel sheet surface after the recrystallization annealing. From these results, P and Si, Mn, C
It was found that in the composite steel sheet of r, all these elements were concentrated on the surface.

Therefore, in order to improve the wettability of the plating and to accelerate the alloying speed, it is considered that the amount of the surface-concentrated layer of these elements should be reduced when the steel sheet is introduced into the hot-dip galvanizing bath. .

The inventors of the present invention have studied in detail the conditions of the reduction annealing, the amount of the surface-concentrated layer, the plating wettability and the alloying rate. As a result, after annealing the cold-rolled high strength steel sheet at the recrystallization temperature, When the surface concentrated layer is removed by washing, the surface concentration of P, Si, Mn, and Cr is less likely to occur again in the re-reduction annealing before hot-dip galvanizing, and the effect of improving the plating wettability and accelerating the alloying speed is reduced. It was found that it was seen.

FIG. 1c) shows the results of AES spectrum of grain boundaries on the surface of a steel sheet which was pickled after annealing and then reduced and annealed again. Compared to FIG. 1a), it can be seen that the amount of grain boundary P is reduced by annealing after pickling. Also, GDS is shown in FIG.
The state of surface concentration of the high-strength steel sheet further reduced again after the annealing and pickling obtained by the method is shown. FIG. 3 shows the effect of the annealing temperature or the re-reduction temperature after the annealing and pickling on the surface concentration by taking Mn as an example. From these results, it was found that the surface-enriched layer was removed by pickling after annealing and then re-annealed to enable immersion in a hot-dip galvanizing bath with a small amount of surface-enriched layer.

Accordingly, the present inventors have conducted an annealing treatment using a continuous annealing equipment (hereinafter referred to as CAL) capable of producing cold-rolled and annealed steel sheets with high productivity, and then, after P, Si,
After removing the concentrated layer of Mn, Cr and the like by pickling, it was found that plating without non-plating defects could be easily performed by CGL and that alloying could be performed more quickly. That is, the present invention has been made for the first time based on the above findings,
And P: not less than 0.03% and not more than 0.2%, and
Continuous steel sheet containing at least one of i: 0.1% or more and 2.0% or less, Mn: 0.5% or more and 2.0% or less, and Cr: 0.1% or more and 2.0% or less. After recrystallization annealing in an annealing facility, after cooling, the concentrated layer of the components in the steel on the steel sheet surface is removed by pickling, and the steel sheet is recrystallized in a continuous hot-dip galvanizing facility at 650 ° C. or higher and in a continuous annealing facility. An object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet, wherein hot-dip galvanizing is performed by heating at or below an annealing temperature.

The present invention also provides a method for producing an alloyed hot-dip galvanized steel sheet, wherein the hot-dip galvanized steel sheet obtained by the above-mentioned manufacturing method is further alloyed.

Here, it is preferable that the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet obtained by the above-mentioned respective production methods are subjected to upper layer plating.

[0019]

According to the present invention, P: 0.03% by weight, respectively.
Not less than 0.2% and not more than Si: 0.1% and not more than 2.0%, Mn: not less than 0.5% and not more than 2.0%, and Cr: not less than 0.1% and not more than 2.0%. When a steel sheet containing at least one or more of them is used as a base steel sheet,
The steel sheet is annealed at a recrystallization annealing temperature in a continuous annealing equipment, and after cooling, a concentrated layer of a steel component on the surface of the steel sheet is removed by pickling, and the steel sheet is heated again by a continuous hot-dip galvanizing equipment in a CGL. Is a method of performing hot-dip galvanizing by heating (reducing) at 650 ° C. or higher and the annealing temperature of CAL or lower, and a method of further performing an alloying treatment on the hot-dip galvanized steel sheet manufactured as described above. Further, in the heat treatment at the time of alloying, if the temperature is lower than 460 ° C., long-time heating is required and productivity is lowered. The hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet obtained as described above may be further plated on the upper layer as necessary.

Hereinafter, the present invention will be described in more detail. First, a method for performing galvanizing and subsequent alloying on a P-added high-strength steel sheet using CAL and CGL used in the present invention will be described. After the thickness of a steel sheet to be a plating material is adjusted by hot rolling and cold rolling, the steel sheet is annealed by CAL at a recrystallization temperature. The atmosphere of CAL requires reducing properties of the steel sheet in order to prevent generation of scale, and generally, N ₂ gas containing several% H ₂ may be used. C
Although the ultimate temperature of the steel sheet at the AL varies depending on the components in the steel and the target material, it is generally in the range of 750 ° C to 950 ° C.

In the steel sheet annealed at the recrystallization temperature in CAL, P precipitates on the surface at the crystal grain boundary of the steel sheet due to a component in the steel,
Si, Mn, Cr and the like are concentrated as oxides. After removing the surface thickened layer by pickling, a steel sheet is introduced into CGL. The reduction annealing by CGL is sufficient at about 600 ° C. for a steel sheet with little addition of Si, Mn, and Cr (hot-rolled finish), and plating is possible, but recrystallization annealing after cold rolling with addition of Si, Mn, and Cr was performed. In the case of a steel sheet, from the viewpoint of plating wettability and alloying speed, an improvement effect appears when the reannealing reduction temperature is 650 ° C. or higher, and it is in a suitable range at 700 ° C. or higher. But,
In order to prevent re-surface enrichment and on the material of the steel sheet, the re-annealing reduction temperature is preferably not more than the recrystallization annealing temperature in CAL, and more preferably not more than (the re-crystallization annealing temperature in CAL-30) ° C (see FIGS. 4 and 5). ). The atmosphere of re-annealing in CGL is
Like CAL, N ₂ containing several% H ₂ may be used.

The steel sheet re-annealed at the above temperature is cooled to about 500 ° C. in the same manner as ordinary hot-dip galvanizing.
About 0-500 ° C, dissolved Al concentration 0.13-0.14w
It is introduced into a hot-dip galvanizing bath of about t% and is galvanized, and the weight per unit area is adjusted by gas wiping when rising from the bath. Thus, a hot-dip galvanized steel sheet is manufactured. If necessary, it is immediately heat-alloyed to produce an alloyed hot-dip galvanized steel sheet. The alloying temperature is 460 ° C. or higher for productivity and 560 ° C. or less for plating adhesion at the time of press molding. After hot-dip galvanizing or alloyed hot-dip galvanizing, it is possible to improve the plating properties by performing upper layer plating as necessary. For example, as the upper layer plating, Fe-Zn or Fe-P plating performed for improving the sliding property at the time of pressing may be performed. This upper plating may be any plating depending on the application.

The components in the base steel sheet used in the present invention will be described below. P, Si, Mn, and Cr are added to impart strength to the steel. P can be made strong by adding a small amount, and it is relatively inexpensive.
Since secondary working embrittlement is likely to occur and deep drawability is adversely affected, the content is set to 0.03% or more and 0.2% or less.

Si has the effect of increasing the strength of steel.
The content is set to 1% or more, and set to 2.0% or less to form an oxide film on the surface and reduce the adhesion to the plating bath. Mn is set to 0.5% or more at which the effect of increasing the strength of steel is exhibited, and is set to 2.0% or less because it has an adverse effect on deep drawability. Cr is set to 0.1% or more at which the effect of increasing the strength of the steel appears, and is set to 0.1% or more and 2.0% or less from the viewpoint of saturation of the effect of improving strength and economy.

The present invention is effective in steel sheets to which Si, Mn, and Cr are added in addition to P. Ti and Nb, which are carbonitride forming elements added to these steel sheets to improve formability, are provided. The present invention is also effective in steel sheets to which is added. Further, a steel sheet to which P, Si, Mn, Cr, Ti, and Nb are added to which B is added for improving the brittleness in secondary working and weldability may be used.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Table 1 shows the composition of the test steel sheet. The steel sheet which has been previously cleaned is subjected to the conventional treatment of only annealing or the treatment of the present invention, ie, annealing-concentrated layer removal (hydrochloric acid pickling) -reduction annealing treatment, followed by hot-dip galvanizing. Then, a hot-dip galvanized steel sheet was obtained. Thereafter, the galvanized steel sheet was alloyed to obtain an alloyed galvanized steel sheet. The plating appearance of the obtained steel sheet, the iron content in the plating layer and the powdering resistance were evaluated.

Here, an example of hot dip galvanizing without removing the concentrated layer after the first annealing (conventional method), and an example of performing the second annealing after removing the concentrated layer after the first annealing (this method) Inventive method) is shown in Table 2. The annealing conditions, the thickening layer removing treatment conditions, the galvanizing conditions, and the alloying treatment conditions, and the evaluation methods of the obtained steel sheets are shown below.

Annealing conditions (including first and second times) Atmosphere 5% H ₂ -N ₂ gas (dew point −20 ° C.) Temperature Table 2 Time 20 seconds In the one-time annealing method, the steel sheet is subjected to a predetermined temperature after annealing. When it becomes, put it into the plating bath. In the twice annealing method, after annealing, the steel sheet is once cooled to room temperature, a concentrated layer is removed, then annealed again, and the steel sheet is put into a plating bath when the temperature is lowered to a predetermined temperature.

・ Concentrated layer removal treatment condition Hydrochloric acid pickling concentration 5% HCl aqueous solution Temperature 60 ° C. Immersion time 6 seconds

Plating conditions Plating bath Al concentration 0.13 wt% Bath temperature 475 ° C Plate temperature 475 ° C Immersion time 3 seconds Weight per unit area 45g / m ² Alloying conditions Temperature Table 2 Time Table 2

Evaluation method The determination of non-plating defects was made by visual observation. A sample having no non-plating defects was rated "1", and a sample having the most non-plating defects was rated "5". The iron content in the plating layer was measured by dissolving the plating layer with sulfuric acid and measuring by atomic absorption. The powdering resistance was measured by a fluorescent X-ray after the 90 ° C. bending back test, after the zinc powder attached to the cellophane tape. Table 2 shows the results.

[0032]

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

[0040]

[Table 8]

[0041]

As described in detail below, according to the present invention, a high-strength steel sheet which is difficult to be galvanized and which is alloyed with P, Si, Mn, and Cr, which is extremely slow in alloying. Even if it does, a steel sheet free from non-plating defects can be obtained, and further, low-temperature alloying is possible and alloying can be easily controlled. In addition, the line is not complicated and the productivity is not reduced. Further, according to the present invention, a conventional facility can be used to obtain the above-mentioned effects, so that there is also an effect that no capital investment is required.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram in which P content of a grain boundary on a steel sheet surface is measured by AES, and (a) shows a surface grain boundary after annealing;
(B) is a spectrum of an internal crystal grain boundary after annealing, and (c) is a spectrum of a surface crystal grain boundary after annealing-hydrochloric acid-reduction annealing.

FIG. 2 is a diagram showing a state of surface concentration of a high-strength steel sheet, which is obtained by glow discharge spectroscopy. FIG.
(B) is a figure after annealing-hydrochloric acid pickling-re-reduction annealing.

FIG. 3 is a view showing the influence of annealing and re-reduction annealing temperatures on the surface concentration of Mn.

FIG. 4 is a diagram showing the effect of the reduction annealing temperature on non-plating defects.

FIG. 5 is a diagram showing the effect of the reduction annealing temperature on the alloying speed.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Nobuo Totsuka 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (72) Inventor Nobuyuki Morito Kawasaki Chuo-ku, Chiba No. 1 town Kawasaki Steel Corp. Technical Research Division (56) References JP-A-6-207259 (JP, A) JP-A-6-88193 (JP, A) JP-A-6-17124 (JP, A) ( 58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40 C22C 38/00 301

Claims (4)

(57) [Claims] (1) In weight%, P: 0.03% or more and 0.2% or less, Si: 0.1% or more and 2.0% or less, M
n: 0.5% or more and 2.0% or less, and Cr: 0.1%
A steel sheet containing at least one of at least 2.0% or less is recrystallized and annealed in a continuous annealing equipment, and after cooling, a concentrated layer of steel components on the steel sheet surface is removed by pickling, and the continuous hot-dip galvanizing equipment is used. Hot-dip galvanizing by heating the steel sheet again at a temperature of 650 ° C. or higher and a recrystallization annealing temperature of a continuous annealing facility or lower. 2. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the hot dip galvanizing is followed by an upper layer plating. 3. A method for producing a galvannealed steel sheet, further comprising alloying the galvanized steel sheet produced by the production method according to claim 1 or 2. 4. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 3, wherein an upper layer is further plated after alloying.