JPH0375347A - Method for controlling operation in production of hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To rapidly and securely carry out switching by adding Mn so that a specific concentration is reached at the time of switching nonalloying plating by the use of a hot dip galvanizing bath containing specific amounts of Al to alloying plating. CONSTITUTION:At the time of switching operations from the production of a nonalloying hot dip galvanized steel sheet by the use of a hot dip galvanizing bath in which 0.05-0.2% Al is dissolved and contained to the production of an alloying hit dip galvanized steel sheet, Mn is added to the plating bath so that an Mn concentration satisfying an inequality [Mn(%)]13.5X[Al(%)]<1.5> is reached. By this method, the length of time required for switching is shortened, and a plated steel material having superior quality can be obtained with high efficiency in high yield.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an operational management method when manufacturing non-alloyed hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets using the same equipment and the same pot. It is.

<Prior art and its challenges> Hot-dip galvanized steel sheets, in which a molten zinc layer is adhered and solidified on the steel sheet surface, have been in wide demand as economical and inexpensive rust-proof steel sheets. This is alloyed with “hot-dip galvanized steel sheet (GI)” (thermal diffusion treatment)
The production volume of ``alloyed galvanized steel sheets (GA)'' with improved corrosion resistance is also increasing. Note that the alloyed hot-dip galvanized steel sheet (hereinafter referred to as GA) is produced using the same zinc pot in the same manufacturing equipment as the case of hot-dip galvanized steel sheet (hereinafter referred to as GI) due to equipment costs, etc. It is usually produced by hot-dip galvanizing, followed by heating and diffusion treatment at 450 to 650°C in a heating furnace.

By the way, in the case of manufacturing the above-mentioned GI, generally 0.08 to 0.2% (hereinafter, % representing the component proportion is expressed as weight %) of A1 is added to the hot-dip plating bath. just,
Some percentage of this added Al combines with Fe present in the molten zinc bath, forms a Pe-Zn-Af compound containing Zn, and is dispersed in the molten metal in a solid state (generally,
This is called "dross"), so the concentration of M dissolved in the Zn liquid phase is lower than the total A1f7% in the zinc bath. PJlfl1 degree [-Ai']. , as [u] −t
= [Aj! ] r [Fe] t and is used as basic data for bath management.

Here, the purpose of adding Al to the zinc bath is to suppress the Fe-Zn-based intermetallic compound layer that tends to be formed on the surface of the steel sheet during hot-dip plating. In other words, when the Fe-Zn-based intermetallic compound layer grows large, it has a considerable adverse effect on the workability of the product because it is essentially brittle and lacks ductility. Therefore, when aiming to ensure sufficient film workability for hot-dip galvanized steel sheets, the hot-dip zinc bath should be 0.14%
Above [Ai! ]-r. When the above-mentioned Al addition is made, "thin Fe," which suppresses the formation of an Fe-Zn diffusion layer, is formed on the surface of the steel sheet.
A1. phase" is formed, and as a result, the enlargement of the Fe--Zn-based intermetallic compound layer is suppressed, so that the workability of the film is ensured.

On the other hand, GA is made by diffusion-transforming a relatively thin galvanized layer into an Fe-Zn alloy layer by heat treatment, but when manufacturing this GA, [Al
If 1-r is high, the progress of alloying is delayed and manufacturability is seriously hindered, so the above-mentioned [Al1-v] needs to be as low as possible. Therefore, [Al1°, in the zinc bath when producing GA is generally adjusted to 0.11% or less.

Therefore, in the production of GI and GA, which generally share the same molten zinc bath pot, conventionally, when producing GI, the zinc bath was set to [Al1, f≧0.14%].
When manufacturing GA, switching and adjusting the conditions to [u] -rfaO and 11% have been carried out.

However, in actual work, [A
Although it is easy to increase l1 of, on the contrary, [A
There is no suitable measure to reduce [Ai'at] when switching from GI production to GA production.
] The current situation was to wait for AT to gradually decrease. Therefore, during this transition period when [U] and t decrease, that is, 0.11% <[AJ] * t <0, 14%, the bath conditions are unsuitable for the production of either Gl or GA. Even if we continued to manufacture GI, we could only obtain something that was rather inferior in terms of film processability. And this is GA
This was becoming a very serious problem as the performance requirements for

That is, in recent years, GA with excellent deep drawability has been used for automobile bodies.
In order to meet this demand, GA is being produced using ultra-low C-Ti additive steel as the material steel.
Compared to GA using other steel types, the plating layer is more likely to have unevenness, and there was a concern that the finish of the electrodeposition coating would be adversely affected.

However, this phenomenon occurs because alloying of ferrite grain boundaries progresses preferentially when the plated steel sheet base material is steel with an extremely low C-Ti content, and is caused by the preferential alloying of ferrite grain boundaries in the hot-dip galvanizing bath. [Aj! ]-r is further reduced (e.g. [Aj!
]−r≦0.08%), but this measure resulted in a larger gap between G[u]my during manufacturing, further aggravating the aforementioned problem. This led to this.

Therefore, an object of the present invention is to reduce the transition time when switching from GI production to GA production in a production line for zinc-based hot-dip galvanized steel sheets, compared to the conventional [Al1-r control method]. We can quickly and accurately manufacture GA using steel sheets with extremely low C-Ti content, and provide high-quality plated products with minimal film irregularities at high efficiency and high yield. It was placed on establishing the means to obtain.

Means for Solving the Problems> Therefore, the present inventors conducted research while repeating numerous experiments in order to achieve the above object, and as a result, they came to the following findings.

That is, conventionally, ``addition of Mn to a hot-dip galvanizing bath is
It was known that the amount of Mn added was effective in suppressing the formation of Fe-Zn compound phase at the interface between the plating film and the steel sheet, similar to the case of Al addition.However, in the presence of dissolved Al, the amount of Mn added When the amount exceeds Mn, Mn exhibits a completely opposite effect and promotes alloying between Fe and Zn. Therefore, when switching over to OA production, it is necessary to add Mn exceeding a certain amount in combination with conventional Al addition means during CI production. We have discovered a new fact that if this is implemented, the operational transition time for GA production = G② production will be significantly reduced.

This invention was made based on the above-mentioned knowledge, etc., and it is said that ``In the production of zinc-based hot-dip galvanized steel sheets,
．． Use a hot-dip galvanizing bath that dissolves and contains 2% of ^l, or a hot-dip galvanizing bath that dissolves and contains Mn at a concentration that satisfies 0.05 to 0.2% of A1 and formula %. At the same time, when switching from CI production to GA production, the transition time can be minimized by adding Mn to the plating bath to achieve a Mn concentration that satisfies the formula % and then proceeding to the production of alloyed plated steel sheets. It is characterized by the fact that it is possible to manufacture high-quality zinc-based hot-dipped steel sheets with high efficiency and high yield.

Here, [Mn(χ)'' is the analytical concentration of Mn in the hot-dip galvanizing bath, and 7 [Al(X)J ha[AlJ a
Each value is expressed in weight%.

Hereinafter, the reason why the conditions for adjusting the plating bath in the present invention are limited as described above will be explained together with the effect thereof.

<Effect> When Mn is added to a hot-dip galvanizing bath containing Al, as shown in Figure 1, the effect of suppressing alloying between Fe and Zn increases as the MJI degree increases until a certain concentration is reached. , it is possible to produce Gl products of good quality even if the dissolved Al content in the bath is low. However, when Mnt 74 degrees in the plating bath exceeds the above-described specific concentration, alloying is significantly promoted. Therefore, when switching to GA type production, simply by adding Mn to the plating bath and adjusting the Mn concentration above the specified concentration, the system for producing high quality GA products is quickly established, and there is concern that quality may deteriorate during the transition period. This allows smooth operations to continue without any problems. In addition,
FIG. 1 is a graph showing an example of the Mn concentration in the bath and the required alloying time (rate) of the resulting plated steel sheet.

The critical Mn11 degree at which the addition of Mn becomes effective in promoting alloying is given by the formula %, and if [Mn (X)] is lower than this value, alloying will be reversed. It will be suppressed. Therefore, in the production of zinc-based hot-dip coated steel sheets, Mn is not actively added to the plating bath during Gl production, or Mn is not actively added to the plating bath.
Adjust the n concentration within the range of the formula % formula %] and add Mn in an amount to obtain an MntM degree that satisfies the formula % formula %] into the plating bath when switching from GI production to GA production. It was determined that Here, there is no particular upper limit for the Mn concentration in the plating bath when switching to GA production, but since adding a large amount of Mn during GA production will generate compounds and increase dross in the bath, It is preferable to lower u4 degrees and lower Mnt within the above range.

In addition, in the present invention, dissolved Al in the hot-dip galvanizing bath
When the concentration (= [u], r) is lower than 0.05%, Gl
Deterioration of plating adhesion during manufacturing becomes significant. On the other hand, when the degree of dissolved AltM exceeds 0.2%, the required Mnfi required for GA increases, and 3
The original compound tends to precipitate as dross. Therefore, the dissolved Al concentration in the hot dip galvanizing bath is 0.05 to 0.2%.
It was decided that the adjustment should be made to

Now, Figure 2 shows the "conventional method of controlling the composition of the plating bath" (Figure 2 (a)
))" and "The method of controlling the plating bath composition according to the method of the present invention (FIG. 2(b))" is a conceptual diagram comparing the method.

As is clear from FIG. 2, in the present invention, [Ai
] As a CI manufacturing countermeasure (measure to suppress the formation of FeZn-based intermetallic compound layer) when sr decreases, alloying tendency can be reduced by implementing "Mn addition at a relatively low concentration (concentration below the critical concentration mentioned above)" On the other hand, when manufacturing the GA type, r
The alloying tendency is increased by increasing the amount of Mn added to raise the [Mn] ta degree above the critical concentration.

Therefore, the operation changeover from CI type manufacturing to GA type manufacturing can be performed extremely smoothly, and the deterioration of product performance that tends to occur during the transition period can be prevented as much as possible, and the material yield can be greatly improved.

Moreover, adding Mn to the galvanizing bath results in extremely low C-
Even when manufacturing GA using Tt-added steel sheets as a base material, the "problem of unevenness in the plating layer" is effectively resolved, making it possible to stably obtain GA type products with excellent plating film smoothness. .

Next, the present invention will be explained in more detail based on Examples.

<Example> Example 1 First, C: 0.04%, Si: 0.01%, M
n: 0.18%.

P: 0.010%, S: 0.008% and s
A full hard steel plate (thickness: 0.76 m) of low carbon aluminum killed steel containing 0.027% of ol, Af and the remainder being substantially Fe was prepared, and a hot-dip plating simulator [■ made by Resca] was prepared. After annealing at 720° C. for 60 seconds in a “26% H2 + N2 atmosphere”, hot-dip galvanizing was performed.

Note that the plating bath temperature at this time was 470° C.±3° C., and predetermined amounts of Af and Mn were added to the plating bath.

Then, the hot-dip plated steel sheet whose coating weight was adjusted to 65±10 g/rrr was subsequently alloyed in a 500° C. molten salt bath, and the time required to complete alloying was measured.

The results are shown in FIG.

Figure 3 is a graph with the coordinate axes of [Alco, f and [Mn] concentrations in the bath, and shows the required alloying time for each composition.
5 seconds or less) + G (more than 15 seconds and less than 30 seconds), ・ (more than 30 seconds), and from this figure 3, the Mn concentration in the plating bath is expressed by the formula % formula %)] It can be confirmed that alloying is promoted within the range where the conditions are satisfied.

Example 2 Chemical composition: C: 0.003%, Si: 0.01%,
Mn: 0.16%.

p: 0.008%, S: 0.008%, sol
, containing 0.021% Al and 0.066% Ti, with the remainder essentially consisting of Fe.
A full hard steel plate (thickness: 0.8On) of F steel was annealed at 850°C for 60 seconds in a "26% H2 + NZ atmosphere" using the same equipment as in Example 1, and then hot-dip galvanized. went.

Note that the plating bath temperature at this time was 470° C.±3° C., and predetermined amounts of All and Mn were added to the plating bath.

Then, the hot-dip galvanized steel sheet with the coating weight adjusted to 65±10 g/% was subsequently alloyed in a molten salt bath at 500°C, and the time required to complete alloying was measured, and the alloying process was The applied sample was embedded in resin and the cross section was observed to examine the occurrence of irregularities in the plating layer.

These results are shown in Table 1.

In Table 1, the "smoothness of the plating layer" is indicated by an ○ mark indicating the state in Fig. 4 (a) as "good" and an X mark indicating the state in Fig. 4 (b) as "poor". did.

From the results shown in Table 1 above, it is clear that GA with excellent quality can be produced smoothly when the conditions specified in the present invention are followed, whereas comparisons are made when the plating bath composition management deviates from the conditions specified in the present invention. In the example, it can be confirmed that the required alloying time is long, and the resulting GA plating layer has difficulty in smoothness.

Summary of Effects> As explained above, according to the present invention, it is possible to quickly switch from the production of GI to the production of GA in the production of zinc-based hot-dip coated steel sheets, and it is also possible to quickly switch from the production of GA to the production of GA in the production of CI.
This brings about extremely useful effects industrially, such as making it possible to stably obtain products of good quality even in the production of products.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the Mn content of a hot-dip galvanizing bath and the required alloying time. FIG. 2 is a conceptual diagram showing an example of controlling the plating bath composition in the production of hot-dip galvanized steel sheets. FIG. 2(a) shows the conventional example, and FIG. 2(b) shows the example of the present invention. This is related. FIG. 3 is a graph showing the investigation results of "alloying time required for [Mn] in plating bath and [Af]sr" in Examples. FIG. 4 is a schematic diagram explaining the "evaluation criteria for plating layer smoothness" in Examples, and FIG. 4(a) shows the state evaluated as "good", and FIG. The states that are evaluated as “poor” are shown respectively. Mn content (weight %) Figure 3 Comparison content (weight %) Good (○) Poor (×)

Claims (2)

[Claims] (1) In the production of zinc-based hot-dip galvanized steel sheets, a hot-dip galvanizing bath containing 0.05 to 0.2% Al by weight is used, and from the production of non-alloyed galvanized steel sheets to alloyed galvanized steel plates. When switching to production, Mn is added to the plating bath to obtain the formula [Mn (%)]≧3.5×Al (%)^1^. An operation management method for manufacturing zinc-based hot-dip galvanized steel sheets, characterized in that the production of alloyed galvanized steel sheets begins after the Mn concentration satisfies ^5. (2) The hot-dip galvanizing bath before switching to the production of alloyed plated steel sheets, together with Al, is calculated using the formula [Mn (%)] < 3.5 x [Al (%)]^1^. ^5
2. The operational management method for producing a zinc-based hot-dip plated steel sheet according to claim 1, wherein Mn is dissolved and contained at a concentration that satisfies the following.