JPH03274224A - Pretreatment for heat treatment of stainless steel - Google Patents

Abstract

PURPOSE:To facilitate descaling after heat treatment, to speed up the heat treatment and the descaling stage of a stainless steel strip, and to improve productivity by previously electrodepositing Ca or Mo onto the surface of a stainless steel strip at the time of applying heat treatment, such as annealing and ageing treatment, to the steel strip. CONSTITUTION:At the time of applying process annealing or finish annealing to a stainless steel strip prepared by means of hot rolling and cold rolling, electrolytic treatment is applied to this steel strip in an electrolyte containing Ca ions or further Mo ions at >=1coulomb/dm<2> current density to electrodeposit Ca and Mo onto the surface of the stainless steel strip with superior adhesive strength. By this method, the stainless steel strip having superior descaling property at the time of subsequent annealing and excellent in brightness characteristics and corrosion resistance after descaling can be produced with high productivity by a simplified method.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a pretreatment method for heat treatment such as annealing and aging of stainless steel.

<Conventional technology> Stainless steel is used after being subjected to various heat treatments, such as solution heat treatment and aging heat treatment, in order to fully utilize its characteristics. Since it impairs the aesthetic appearance and significantly deteriorates corrosion resistance, it is usually removed by methods such as acid cleaning.

In the manufacturing process of stainless steel, it is common to process the stainless steel into a steel strip or steel plate of a predetermined thickness by hot rolling and annealing, or by repeating cold rolling and annealing. Here, as the annealing method, a so-called continuous annealing method is widely used, in which a copper strip wound into a coil is continuously fed into a direct-fired heating furnace while being unwound.
This annealing method produces oxide scale on the surface.

As mentioned above, it is necessary to completely remove this oxide scale from the viewpoint of aesthetics, corrosion resistance, and workability. For this purpose, descaling treatment such as acid cleaning is performed after annealing. Further, the same applies when aging treatment is performed in the above manufacturing process.

Methods for removing oxide scale generated during finish annealing after cold rolling include electrolytic descaling, which involves electrolysis in an electrolyte such as an aqueous sodium sulfate solution, sulfuric acid, or nitric acid, a mixed acid of nitric acid and hydrofluoric acid, or an alkaline molten salt. There is an immersion descaling method in which the product is immersed in water, etc., and usually a combination of the above descaling methods is used, taking into consideration the removability of oxidized scale and the surface quality after scale removal.

In recent years, with the growth in demand for stainless steel, its production has increased, and the annealing and descaling lines mentioned above are tending to be faster. The oxidized scales that occur are Cr2O3 and FeCr2
Since it contains a large amount of O4, there is a limit to speeding up the descaling process even if it is chemically removed.

<Problems to be Solved by the Invention> In order to meet these demands for speeding up the annealing/descaling line, for example, as disclosed in Japanese Patent Publication No. 63-28965, the surface of stainless steel is coated as a pretreatment for annealing. There are examples of applying Cr plating, but in this case the equipment and maintenance costs required for plating become enormous.

Therefore, even if it becomes possible to omit part of the descaling equipment, the cost will increase if you consider the additional plating equipment and the cost of plating. In addition, there are problems such as increasing the acid concentration and temperature to shorten the descaling time, such as roughening the substrate surface.

Furthermore, as disclosed in JP-A No. 55-47318, there are cases in which an aqueous solution containing Fe chloride is applied, but in this case, uneven coating occurs, resulting in non-uniform oxide scale. After descaling, it remains as a pattern that not only impairs the aesthetic appearance, but also causes environmental pollution due to the generation of harmful chlorine gas, and is problematic because it significantly shortens the life of equipment. Moreover, if chlorine gas recovery equipment is provided, a large amount of cost will be required.

In addition, as disclosed in JP-A-61-127823, there are also examples in which an alkali metal compound or alkaline earth metal compound is suspended in an organic solvent or oil and then applied and annealed. Also, as mentioned above, uneven coating occurs.

Furthermore, since organic substances are used, carburization occurs, significantly deteriorating the surface quality, and the coating material adheres to and accumulates on support rolls in the annealing furnace, causing problems such as scratches and uneven annealing.

In addition, the technique disclosed in JP-A No. 61-56721 is an example of applying oil, but in this case as well, as mentioned above, uneven coating tends to occur, carburization occurs, and the coated material adheres to the support roll. There are problems such as

The present invention has been made by focusing on the problems of the prior art.
Alternatively, by electrodepositing ions containing Mo, the target ions can be deposited uniformly and with good adhesion, and there is no accumulation of deposits on the support roll in the furnace, and acid cleaning or electrolytic treatment after heat treatment The purpose is to speed up the heat treatment and descaling process by changing the form into a form that is easy to descale.

<Means for Solving the Problems> In order to achieve the above object, according to the present invention, a t't' of 1 coulomb/d m' or more is applied to the surface of stainless steel in an aqueous solution containing Ca and/or Mo. A pretreatment method for heat treatment of stainless steel is provided, characterized in that Ca and/or Mo are electrodeposited by electrolysis at ftV degrees.

The present invention will be explained in more detail below. In addition, although the case where it is applicable to the manufacturing process of stainless steel mV is demonstrated as a representative example, it is not limited to this.

When applying the present invention to intermediate annealing or final annealing of stainless steel m-band, electrodeposition is carried out by electrolysis in an aqueous solution containing ions from Ca and/or Mo immediately before annealing.

The concentration of the electrolytic solution (electrodeposition solution) may be dilute, but the concentration of ions containing Ca and/or Mo is 1.
It is preferable to set it to 100 pp or more.

Regarding the polarity during electrodeposition electrolysis, when the ions containing Ca and/or Mo are cations, cathodic electrolysis is performed, and when the ions are anions, anodic electrolysis is performed. Furthermore, when using a solution containing both cations and anions, cathodic electrolysis and positive 8i
i Perform electrolysis. At this time, it is preferable to perform anodic electrolysis last. Further, when the polarity of electrodeposition electrolysis is cathode, it is preferable to perform anodic electrolysis after cathodic electrolysis in order to improve the descaling property.

The reason for this is thought to be that an anodic oxide film with a thickness of several nm is formed on the ions electrodeposited on the stainless steel surface, which improves the adhesion of the ions.

Here, at least 1 coulomb/d m is required to electrodeposit enough ions to improve descaling performance.
It is necessary to electrolyze and electrodeposit at a current density higher than 100%.The effect of electrodeposition increases as the current density increases, but in the case of aqueous solutions with relatively high electrical resistance, there is a limit to the amount of electricity that can be passed. The lower limit is set to 1 coulomb.The upper limit is not limited, but 100 coulombs/dm'' or less is appropriate.

In order to make the amount of these ions adhered uniform throughout, it is necessary to wash with water after electrodeposition to remove excess aqueous solution.

Ions introduced to the stainless steel surface by electrodeposition are not removed even when washed with water, so they do not accumulate on support rolls or the like.

By performing annealing after such treatment, Ca and/or Mo are incorporated into the oxide scale generated on the stainless steel surface, changing the structure of the scale to a form that facilitates descaling. Therefore, descaling can be carried out in a short time by electrolytic descaling in neutral salts or by immersion in a mixed acid of nitric acid and hydrofluoric acid, which is normally carried out in the manufacturing process of stainless steel strips. This not only makes it possible to reduce running costs or increase production by increasing speed and omitting part of the descaling process, but also
It can improve surface gloss and produce high quality stainless steel.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) Steel plates made from hot-rolled annealed sheets of 5US430 and 5US304 each having a thickness of 4mm and hardened to a hard material having a thickness of 1mm using a Sendzimir rolling mill were used as raw materials, and were made into flat test pieces using a leveler. This test piece was degreased with an organic solvent.

A Ca(OH)2 aqueous solution, a H3M004 aqueous solution, and a mixed aqueous solution of these □ were prepared, and the supernatant liquid was used as an electrodeposition liquid. The electrodeposition conditions were a solution temperature of 50°C, and -3A/clrn'' in the case of a solution containing Ca and both Ca and Mo.
+3 after negative electrolysis for 5 seconds (-15 riron/dm2)
In the case of Mo, only the above-mentioned anodic electrolysis was performed. After electrodeposition, the electrodeposition was washed with water and lightly rubbed with gauze to remove excess aqueous solution, followed by drying. Annealing is 5US43
850℃ for 0, 110℃ for 5US304
I went at 0'C. The atmosphere used was nitrogen gas containing 3 voj 2% oxygen and having a dew point of +60°C.

Descaling properties are determined by the nitric acid electrolysis time required for complete descaling by performing nitric acid electrolysis after neutral salt electrolysis treatment for 5U3430, and by the nitric acid electrolysis time until complete descaling for 5U304 in a mixed acid of nitric acid and hydrofluoric acid after neutral salt electrolysis treatment. Each sample was evaluated based on the time required for complete descaling when immersed in water (Inventive Examples 1 to 6).
For comparison, a test piece with only degreasing, Ca at 50℃ after degreasing
(OH)2, partially in the supernatant mixture of H2MoO4,
5 A/drn', 1 sec (-0,5 R1ron/d
m") after negative electrolysis +〇, 5 ^/dm",
A test piece was subjected to positive electrolysis for 1 second (+0.5 R/dm"), a test piece was coated with rolling oil after degreasing, and a test piece was coated with a mixture of Ca(OH)2 and H2MoO4 in oil. Test pieces were also subjected to testing (Comparative Examples 1 to 10).
The adhesion of the pretreatment agent was determined to be poor if a part of the test piece was rubbed with gauze, annealed, and descaled, and the descaling time was the same as in the case without pretreatment. Regarding the evaluation of the pattern after descaling, if a clear pattern (unevenness) was visually confirmed, it was determined that there was a pattern. Especially 5
For U3430, the surface gloss after complete descaling was also measured. The gloss level of 5US430 immediately after degreasing was 390.

Tables 1 and 2 are 5US430° and 5US304, respectively.
This is the result. From the table, when the amount of electricity during electrodeposition is 0.5 coulomb/d m'', a sufficient descaling property improvement effect is not obtained (Comparative Example 2.7).
OH) 2+ alcohol (1g/fill”), H2
It can be seen that when M O04 + alcohol (1 g/m") was applied, the adhesion of the applied material was poor and a pattern was formed on the surface after descaling due to pickling (Comparative Examples 3 to 5)
．． 8-10). Furthermore, in 5US430, Comparative Examples 3~
No. 5 has a significantly deteriorated gloss level. This is because the surface after descaling was roughened due to carburization. On the other hand, those in which Ca and/or Mo electrodeposition treatment was performed as a pretreatment for annealing had good adhesion of the electrodeposit and did not come off even if rubbed (Examples 1 to 6 of the present invention). The time required for descaling is short and there is no pattern after descaling. Furthermore, for 5US430, in the case of only degreasing in Comparative Example 1, the gloss after descaling was lower than before annealing, whereas in Inventive Examples 1 to 3, the gloss was lower than before annealing. It can be seen that the gloss level is maintained.

Table 1 Table 2 <5US430><5US304><Effects of the Invention> Since the present invention is configured as explained above, C
By electrodepositing ions containing a and/or Mo on the stainless steel surface before heat treatment, these elements can be introduced to the stainless steel surface with good adhesion, and the oxide scale generated on the stainless steel surface after heat treatment can be reduced to a form with good scale properties. , the descaling time can be shortened, and the gloss before annealing can be maintained even after descaling. By utilizing the present invention in the manufacturing process of stainless steel strips, annealing and
It is possible to increase the speed of the descaling line, or to omit part of the descaling process.

Claims (1)

[Claims] (1) Ca and/or Mo on the surface of stainless steel
1. A pretreatment method for heat treatment of stainless steel, which comprises electrodepositing Ca and/or Mo by electrolyzing at a current density of 1 coulomb/dm^2 or more in an aqueous solution containing the above.