JPS58126993A - Molten salt bath for continuous annealing and pickling device - Google Patents

Abstract

PURPOSE:To decrease the quantity of molten salt bath taken out in annealing by making NaOH a main component of molten salt bath used in continuous annealing and pickling process of stainless steel hoop, adding NaNO3, KOH thereto and limiting the quantity of Na2CO3, an inpurity, below a specified value. CONSTITUTION:In rolling a stainless steel hoop, a beltlike intermediate product is continuously annealed and pickled, and cold rolled. In this case, dense oxide scales formed in annealing process cannot be removed perfectly by direct pickling process. Accordingly, they are changed to scales soluble in acid by passing through molten salt bath composed mainly of NaOH and containing 10-40wt% NaNO3 and 5-20wt% KOH and containing <=2% Na2CO3 as an impurity. Then, scales on the surface of the hoop are perfectly removed by pickling. In this case, by making the molten salt bath above-mentioned composition, the quantity taken out adhering to the steel hoop is decreased remarkably and the cost can be reduced by decrease of consumption of the molten salt bath.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to molten salt baths used in continuous annealing and pickling processes for stainless steel strip and the like.

When manufacturing thin sheets of stainless steel, titanium, etc. using the so-called strip method, a strip-shaped intermediate product is usually continuously annealed and pickled. If an attempt is made to directly remove the oxidized scale by pickling, it takes a long time to sufficiently remove the oxide scale, and often results in a product with a rough surface. For this reason, before pickling, a process is usually carried out in which the plate is passed through a molten salt bath containing sodium hydroxide as its main component, and as a result, scales with uneven degrees of oxidation are further oxidized to forms that are easily soluble in acids. Instead, they can be removed by a short pickling process, resulting in a product with a good surface condition.

Conventionally, as a molten salt bath agent with excellent decooling ability, there is NaN019, which is called DG8.
~15%, Na0j 5~15%, iOH8
A molten salt bath consisting of NaOH 50-20% and NaOH 50-78% is known, and is used for descaling stainless steel and the like.

The present inventors found that when stainless steel copper strips are continuously treated in a molten salt bath, a large amount of copper is attached to the copper strips and is carried out of the bath and is lost (amount carried out), which causes an increase in costs. After paying attention to this fact and conducting detailed research on the amount of outflow, they succeeded in inventing a molten salt bath that has a much smaller amount of outflow than conventional molten salt baths.

That is, in the present invention, NaOHt is the main component, and 10 to
The basic composition is a book containing 40% NaNO3 and 5-20% KOH, and Na2 mixed as an impurity.
This is based on our findings that if 003 is limited to 2- or less, the amount carried out will be significantly smaller than in conventional salt baths when used at temperatures below 450°C.

The above-mentioned DG8 bath is usually used at a temperature of 480 to 500°C, but attempts have been made to lower the temperature of the bath for thermoeconomic reasons. However, lowering the temperature inevitably increases the viscosity of the bath and increases the amount carried out. Even when the molten salt bath of the present invention is used at a relatively low temperature such as 450° C. or lower, the amount carried out is smaller than that of conventional baths.

Next, the reason for limiting the components of the molten salt bath according to the present invention will be described, and the characteristics of the present invention will also be explained.

N a N Os acts as an oxidizing agent and is necessary to oxidize scales with insufficient oxidation degree.
Although it varies depending on the type of material to be treated with IkFi, in general, when it is less than 10, the oxidizing ability of the bath becomes easier to understand.

On the other hand, as for the amount taken out, as shown in Figure 1, 10~
Within the range of 40%, there is almost no difference in the amount taken out, so Na
The amount of NO3 is 10-40%.

KOH, like NaOH, is an alkali necessary to melt chromium in oxide scale as chromate, but
As shown in FIG. 2, an amount added in the range of 5 to 20% has the effect of reducing the amount taken out, and an amount greater or less than this range increases the amount taken out. Therefore, the addition range is 5 to 2
The alkali necessary to melt the salt as
It is sufficient if the total amount with OH is 50 or more.

Note that N ai O03 is not added,
O is mixed as an impurity or is generated when the salt bath absorbs COt gas in the air, but as its amount increases, the amount taken out increases as shown in FIG.

If the percentage exceeds 2-, the amount taken out increases rapidly, so the amount is limited to 2- or less.

In the DG8 solution mentioned above, around 10 Is of NaC4 is added to lower the viscosity of the molten salt bath, but as shown in Figure 4, when the temperature of the salt bath is 480°C, riMao#
The amount taken out will decrease slightly in the range of o to 10%, or 4
In the case of 50°C, the opposite effect occurs, and as the amount added increases, the amount carried out increases, and in particular, it was found that it is better not to add MaCA Fi because the amount carried out increases rapidly at Ma (! The figure shows MaCA O%,
This figure shows how the amount taken out of a 5% to 10% salt bath changes depending on the temperature. As is clear from this figure, when the bath temperature is below 450°C, the MaCjO- bath has the smallest amount taken out.

Based on the above-mentioned findings, the present invention does not contain Na0j,
The predetermined ranges are 0 g of NaOH, KOH, and NaN.

The effects of the present invention will be explained below using examples.

450 for a molten salt bath having a composition as shown in Table 1
Table 2 and FIG. 6 show the results of the carry-out 1iiik measurements at 450°C and 450°C.

Table 1 Table 2 The amount taken out of the above molten salt bath O measurement method O outline is as follows υ
It is.

A stainless steel plate test piece (8U8s O4) was immersed in an oxidizing molten salt bath for 10 seconds, then pulled out, cooled in a desiccator, weighed, and the amount taken out was determined from the difference in weight before and after immersion.Main measurement In order to improve measurement accuracy, 10 test pieces were measured at once for one weight measurement, and the temperature of the salt bath was controlled using a Pt-Rh thermoelectric system that automatically controlled the power supply. It was measured using a pair.

In addition, since the lifting amount of the plate is greatly affected by the speed, an electric motor was used to accurately control the pulling speed of the test piece, and the test piece was pulled at a constant speed (189 m/m1n).

As is clear from Table 2 and FIG. 6, the amount taken out of the salt bath of the present invention is smaller than the amount taken out of any of the comparative salt baths. In particular, at 450°C or 450°C, the amount of take-out of the comparative salt bath is 5 to 20 more than that of the salt bath of the present invention, which is advantageous when the salt bath of the present invention is used at a relatively low temperature of 450°C or lower. It is shown that.

In addition, in order to confirm the descaling ability of the salt bath of the present invention, S
U85Ω4 stainless steel plate test piece (α6 (e)
0 Qf)X 50 (Avm) in an electric furnace at 110
430.450 and 48 annealed at 0°C for 80 seconds
After immersing it in a salt bath of the present invention at 0°C for 10 seconds, it was put into water, and after pickling for 1 minute in a 60°C O 15% HNO3-1-HP pickling solution, it was washed with water, dried, and examined for the presence of scale using an optical microscope. Ta. The results are shown in Table 3.

Table 5 As is clear from Table 3, no scale was observed at any temperature, confirming that the salt bath of the present invention had sufficient descaling ability at 430 to 480°C.

As mentioned above, the salt bath of the present invention can be heated at 430 to 480°C, especially at 45°C.
It has the characteristic that when used at a relatively low temperature of 0° C. or lower, the amount taken out is small, and is very useful industrially for the purpose of energy reduction.

[Brief explanation of the drawing]

In the accompanying drawings, FIGS. 1 to 6 are charts showing the relationship between the salt bath of the present invention, the amount of discharge, and O.

Claims (1)

[Claims] Weight - Mallollo ~4011, KOH5~2
A molten salt bath for continuous annealing and pickling equipment, characterized in that the content of Ma*OO1 mixed as impurities is t2- or less.