JPH02258917A - Heat treatment of stainless steel - Google Patents

Abstract

PURPOSE:To suppress a cost increase caused by the additional installation of coating equipment prior to annealing and to speed up annealing and descaling lines by coating the surface of a stainless steel with the oxide of Fe and Cr and then annealing the stainless steel. CONSTITUTION:The surface of the stainless steel is coated with the oxide of at least either of the Fe and the Cr. The surface is otherwise coated with at least one kind among the hydroxide of the Fe, the hydroxide of the Cr, the oxyhydroxide of the Fe, and the oxyhydroxide of the Cr. The stainless steel is thereafter annealed. The coating is executed by suspending the above- mentioned materials into liquid and applying the soln. on the steel. The endothermic property at the time of the annealing is enhanced in this way and the heating up time is shortened. The direct contact of the atmosphere gas in an annealing furnace and the surface of the stainless steel is hindered, by which the oxide scale on the surface of the stainless steel is thinned and the time for descaling is shortened.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for heat treating 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 demonstrate its characteristics, but these heat treatments generate oxide scale on the surface, which causes the stainless steel to Since the original beauty is impaired and the corrosion resistance is significantly deteriorated, the oxidized scale is removed by methods such as acid cleaning.

Further, in the manufacturing process, stainless steel is formed into a steel strip or steel plate of a predetermined thickness by hot rolling, annealing, and cold rolling and annealing. Here, as the annealing method, a so-called continuous annealing method is widely used in which a steel strip wound into a coil is continuously fed into a direct-fired heating furnace while being unwound. Oxidized scale is generated on the surface of the copper strip. For the reasons mentioned above, this oxide scale must be removed, so descaling treatment such as acid cleaning is performed after annealing, and the same applies when aging treatment is performed in the manufacturing process.

Methods for removing oxide scale generated during finish annealing after cold rolling include sodium sulfate aqueous solution, sulfuric acid,
There is an electrolytic descaling method in which electrolysis is carried out in an electrolyte such as nitric acid, and an immersion descaling method in which immersion is performed in a mixed acid of nitric acid and hydrofluoric acid or an alkaline molten salt. Usually, the removability of oxide scale and the surface quality after scale removal are determined. In consideration of the above, methods are being used to appropriately combine the above descaling methods.

In recent years, the production volume of stainless steel has increased with the growth in demand for stainless steel, and the annealing and descaling lines mentioned above are tending to speed up. There is a limit to increasing the speed in terms of this, and the oxide scale that is generated on the surface of the copper strip during annealing is difficult to chemically remove because it contains a large amount of CrzO=, so there is a limit to increasing the speed in the descaling process. There is.

[Problem to be solved by the invention]

In order to meet these demands for increasing the speed of annealing/descaling lines, there are examples of modifying oxide scale by controlling the atmosphere during annealing, and pre-annealing treatment as disclosed in Japanese Patent Publication No. 63-28965. There is an example of applying Cr plating to the surface of stainless steel, but in this case, the equipment required for plating and the cost required for maintenance are enormous. Therefore, although the above Cr plating method makes it possible to omit part of the descaling equipment, there is a problem in that the overall cost becomes high considering the addition of plating equipment and the increase in plating cost. In addition, increasing the acid concentration and temperature to shorten the dissolution time of the plating layer after annealing will damage the base surface of the stainless steel, so we fully respond to the request for increasing the speed of the annealing/descaling line. There is also the problem that it cannot be done.

Furthermore, as disclosed in Japanese Patent Application Laid-Open No. 55-47318, there are cases in which an aqueous solution containing Fe chloride is applied, but in this case, it not only roughens the stainless steel surface but also releases harmful chlorine gas. Since chlorine gas is generated, there are problems in that it not only causes environmental pollution but also significantly shortens the life of the equipment. On the other hand, there is also the problem that providing chlorine gas recovery equipment requires a large increase in cost.

The present invention has been made by focusing on the problems of the prior art, and aims to minimize the cost increase due to the addition of coating equipment before annealing and increase the speed of the annealing and descaling line. It is an object.

[Means to solve the problem]

Among the methods of heat treating stainless steel according to the present invention, the first invention is to coat the surface of stainless steel with an oxide of at least one of Fe and Cr, and then annealing the stainless steel.

As the oxide, it is preferable to use a suspension of oxide powder in a liquid, and apply the suspension to the surface of the stainless steel to perform the coating.

It is preferable to use an organic liquid as this liquid.

Further, the second invention provides Fe hydroxide, Cr hydroxide,
After the surface of stainless steel is coated with at least one of Fe oxyhydroxide and Cr oxyhydroxide, the stainless steel is annealed.

In order to coat the stainless steel surface uniformly and with good adhesion with the coating made of the hydroxide, it is preferable to use an organic liquid as the liquid, or, if water is used, to use it in the form of a sol that is difficult to separate. It is.

[Effect]

In the method using the oxides, Fe and Cr oxide powders are dark in color and therefore absorb heat quickly, and when applied to the surface of stainless steel, the rate of temperature rise during annealing is improved. In particular, FElxOa is black, and Fe, 0
．． Since the degree of oxidation is lower, not only the heat absorption power due to the black color, but also the oxidation heat due to the oxidation of Fe2O, to Pe, O, promotes the temperature rise of the steel, significantly increasing the temperature rise rate. .

When at least one of the oxide powders is suspended in a liquid and applied to the stainless steel surface, the applied material suppresses direct contact between the atmospheric gas and the steel surface in the annealing furnace, as shown in FIG.
As shown in a) to (d), the oxide scale on the stainless steel surface is thin and has a relatively large amount of Fe, which not only shortens the descaling time but also allows melting as a pretreatment for pickling. Alkaline salt treatment, neutral salt electrolytic treatment, etc. can also be omitted.

Examples of the liquid include water, oil, methyl cellulose, etc., but especially in the case of organic substances such as oil, the organic substances burn during annealing, thereby further promoting the temperature rise of the stainless steel. In this case, the quantity and quality of the organic liquid should be adjusted so that the organic substance is completely combusted in the annealing furnace.

During the annealing process, the oxide becomes Fe, 0. The oxidized scale on the surface of the steel plate becomes thin because it becomes a stable oxide such as Crz(h) and prevents direct contact between the stainless steel surface and the atmospheric gas.

The stainless steel that has been annealed is washed with water and brushed to remove the coating material.

Here, since the oxide used is a powder and coats the stainless steel surface, the oxide can be easily removed after annealing. Removal of this oxide facilitates descaling in the subsequent process.

In addition, in the method using hydroxide or oxyhydroxide, since Fe and Cr hydroxides and oxyhydroxides are brown or brownish in color, they absorb heat faster than stainless steel surfaces that are not coated with these. , the temperature increase rate during annealing is improved. When the temperature rises, these coatings generate water vapor and turn into oxides, but the reaction is gradual and the coatings do not become rough or peel off. Therefore, it has an excellent shielding effect between the stainless steel surface and atmospheric gas. Furthermore, since water vapor is generated, it does not pollute the environment or shorten the life of equipment.

Furthermore, if any of the above hydroxides or oxyhydroxides is used in sol form to coat the stainless steel surface, the fine particles will densely cover the stainless steel surface, which will also have an effect of blocking atmospheric gas during annealing. As in the case of oxides, the oxide scale on the stainless steel surface is extremely thin, and as shown in Figure 1(e).

As shown in (f), the oxide scale has a relatively large amount of Fe. The coating after annealing is removed by brushing while washing with water, and then the oxidized scale on the stainless steel surface is removed in a descaling process, but as mentioned above, the thickness is extremely thin and relatively thin compared to normal conditions. Since the oxide scale contains a large amount of Fe, the descaling time can be shortened.

These methods can be applied to either hot-rolled steel strip or cold-rolled steel strip. The important point in carrying out this method is the uniform application of the oxide, and the method is as follows:
There are spray coating methods, roll coating methods, etc.

Note that these oxides and hydroxides of Fe and Cr.

The effects of the present invention can be sufficiently obtained even when the oxyhydroxides are mixed together and used as a coating.

[First Example] A test piece was made of a hard material that was descaled from a hot-rolled annealed plate of S [15430 and 5US304 with a thickness of 4IIIl, and then made into a thickness of 1 round using a Sendzimir rolling mill, and this was degreased with alkali and then used for temperature measurement. A thermocouple was attached and the prescribed coating was applied.

A case in which the above-mentioned oxide is used as this coating will be described as a first example, and a case in which the above-mentioned hydroxide or oxyhydroxide is used as a second example will be described below.

As the oxide of the first example, Ferns, Feze
s, Cr, 0. Using three types of oxide powders, these were suspended in distilled water, rolling oil (mineral straight oil), and methyl cellulose (viscosity 50 cp) to give a concentration of about 30%.
The solution suspension was uniformly applied to the test piece by spraying. The combinations of the oxide powder and the liquid in which it is suspended are shown in Table 1.2. The amount of coverage was determined by weight measurement after drying when distilled water was used and immediately after application when other liquids were used.

Annealing is performed using nitrogen gas containing 3% by volume of oxygen at a dew point of 60° in the furnace.
In an atmosphere of C, for 5tlS430, furnace temperature 1000''
Furnace temperature 1200°C1 maximum plate temperature 1100°C
The heating time from 100°C to the maximum plate temperature was measured.

After annealing, the coated material was removed by rubbing with a nylon brush while washing with water, and the material was dried. For 5US430, nitric acid electrolysis was performed after neutral salt electrolysis treatment, and the removal of oxide scale was evaluated based on the time until complete descaling. Also 5
As for t15304, it was immersed in a mixed acid of nitric acid and hydrofluoric acid after the neutral salt electrolytic treatment, and the time until complete descaling was evaluated. In addition, as a comparative example, SiO□ and AjEzOIJ
Similar annealing and descaling evaluations were performed on the coated material and the test piece that had been degreased with alkali.

Table 1 shows the results of 5IJS430, and Table 2 shows the results of 5IJS430.
This is the result of S304.

In these tables, the heating time ratio is expressed as a percentage based on the heating time while defatted.

As shown in Table 1.2, in 5tlS430, the temperature increase rate is improved in the present invention example compared to the comparative example. In particular, it can be seen that when FexOa+rolling oil is used, the required time can be reduced to less than half.

Moreover, the pickling time is also sufficiently shortened by the pretreatment.

Similar results were obtained with S[l5304.

Therefore, by performing the annealing pretreatment, the temperature increase time and the descaling time are shortened.

``As a result, it is recognized that the temperature rise time and descaling time can be shortened even for steels other than stainless steel.

[Second Example] Next, a second example in which a hydroxide is used for the coating will be described.

This coating consists of Fe(O)I) 3 , Fe0OH,
Cr(OH)! , Cr00tl was suspended in water to a concentration of 300 g/l, acid was added to form a sol with a pH of 2, and this was uniformly applied to the test piece by spraying.
The amount of coverage was determined by weight measurement.

Annealing and descaling were performed in the same manner as in the first example.

Also, as a comparative example, A E (O)l) ff+water, N
Similar annealing and descaling evaluations were also performed on the i (OH) z sol coated material and the as-degreased test piece.

Table 3 shows the results for 5US430, and Table 4 shows the results for 5US3.
This is the result of 04.

(Table 3) (SUS430) (Table 4) (SUS304) As shown in Tables 3 and 4, 5tlS430.5US
For both 304 and the comparative example, when Fe hydroxide, Cr hydroxide, Fe oxyhydroxide, and Cr oxyhydroxide were applied, the heating time ratio was small and the objective was achieved in a short time. It can be seen that the temperature has been reached. Furthermore, the descaling time was similarly shortened, indicating that the descaling performance was improved.

As described above, in the stainless steel manufacturing process, by coating the stainless steel surface with the coating agent before annealing after cold rolling and removing the coating agent after annealing, annealing and descaling can be performed quickly. As mentioned above, it is possible to increase the speed of the annealing line and descaling line, and to omit part of the descaling process. In addition, the above is an example of passing through annealing in the manufacturing process of stainless steel, but in addition,
This method can be applied to aging treatment as well as a means for easily removing oxidized scale after aging.

〔Effect of the invention〕

As explained above, in both the methods using the oxides and the methods using the hydroxides and oxyhydroxides, powders of these substances are used as they are, or they are suspended in a liquid or made into a sol. By coating the stainless steel surface with this coating, it is possible to increase the heat absorption during annealing and shorten the temperature rise time. Reduces descaling time by thinning oxide scale,
Alternatively, it is a method for heat treatment of stainless steel that can omit part of the descaling step.

[Brief explanation of drawings]

FIG. 1 is a graph showing SIMS profiles in the depth direction of a stainless steel surface heat-treated using Fe oxide and oxyhydroxide, and a stainless steel surface heat-treated without using the coating. Figure (a) shows 5US heat treated without coating with oxide while degreased.
In the case of 430, O)) The figure shows the case of 5IIS304 which was heat treated without being coated with oxide while being degreased, and the figure (C) is the case of 5US43Q which was coated with oxide after degreasing and heat treated.
(d) The figure shows S[l5] coated with oxide and heat treated after degreasing.
In the case of 304, (e) the figure shows 5US430 which was coated with oxyhydroxide sol after degreasing and heat treated, and (f) figure shows 5US3 which was coated with oxyhydroxide sol after degreasing and heat treated.
04 cases are shown respectively.

Claims (2)

[Claims] (1) A method for heat treating stainless steel, which comprises coating the surface of stainless steel with an oxide of at least one of Fe and Cr, and then annealing the stainless steel. (2) At least one of Fe hydroxide, Cr hydroxide, Fe oxyhydroxide, and Cr oxyhydroxide
A method for heat treatment of stainless steel, characterized in that the surface of stainless steel is coated with a seed, and then the stainless steel is annealed.