JPS5856013B2 - Method for producing cold-rolled austenitic stainless steel plate with good surface gloss and corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing cold-rolled austenitic stainless steel sheets with good surface gloss and corrosion resistance.

Generally, in the manufacturing process of this type of cold-rolled sheet, it has been customary to carry out so-called solution heat treatment, which is heating and holding at a high temperature of over 1000° C. immediately after hot rolling.

In recent years, due to social demands for resource and energy conservation, reduction or complete elimination of the above-mentioned high-temperature heat treatment has been planned. Moreover, even if scale removal is achieved, the surface color becomes blackish and discolored, which impairs the luster of the cold-rolled product, making it impractical as a stainless steel.

In addition to this, pickling with a mixed acid of nitric acid and hydrofluoric acid has the problem of generating NOx gas, which is a problem related to pollution control.

Therefore, the present invention provides an advantageous solution to such problems, and the components and composition of the pickling bath are
After careful consideration, we found that the pickling ability of the conventional mixed acid of nitric acid and hydrofluoric acid depends on the temperature of the solution treatment applied to the hot-rolled strip, and when it drops below that level, The pickling treatment time increased rapidly, and it was determined that the reason for this was that the Cr concentration immediately below the scale was insufficiently reduced due to the solution heat treatment of the hot rolled strip.

As a result of continued research and consideration, it was found that under heat treatment conditions such that the formation of a Cr-depleted layer immediately below the scale is much reduced compared to the case of solution heat treatment, or under the abolition of solution heat treatment, it is possible to use hydrochloric acid alone. It has been found that a pickling bath of

Furthermore, it has been found that even more advantageous adaptation can be achieved by adding pickling treatment using nitric acid subsequent to this pickling treatment using only hydrochloric acid.

As a result, resource saving can be achieved by heat treatment at a much lower temperature than the above-mentioned solution heat treatment or by abolishing such heat treatment.
A manufacturing process for cold-rolled stainless steel sheets has been established that satisfies the demand for energy conservation and avoids pollution sources such as the generation of NOx due to the use of the above-mentioned mixed acid.

Now, when manufacturing conventional austenant stainless steel rolled sheets on a continuous line, the following steps are performed: melting → slab manufacturing → heating → hot rolling → solution treatment → short blasting → pickling → cold rolling → finishing solution heat treatment → It goes through processes such as pickling or bright annealing → temper rolling.

Austenitic stainless steel, represented by SUS 304, not only contains a large amount of Ni, which is generally more expensive than ferritic stainless steel, but also requires the solution heat treatment shown in the manufacturing process above to be performed at high temperatures exceeding 1000°C. In addition to the large amount of energy consumed, which is a disadvantage in terms of production costs, the need to conserve resources and energy cannot be ignored, so we decided to reduce or omit the solution heat treatment to improve the above-mentioned methods. For example, descaling, which replaces the increasingly difficult pickling process with mechanical grinding, negates the benefits of simplifying heat treatment due to the consumption of grinding materials and a decrease in material yield.

The inventors have developed an austenitic stainless steel hot-rolled steel I
After conducting the above study on the relationship between the heat treatment temperature prior to cold rolling of J-tube and the subsequent pickling properties, we found that conventional 1ooo This technology has made it possible to reduce or even eliminate the solution heat treatment temperature, which previously required temperatures above ℃, without deteriorating the quality of the finished product.

This invention firstly hot-rolls an austenitic stainless steel strip, then performs a mechanical preliminary descaling treatment, then pickles it in a pickling bath containing only hydrochloric acid, and then cold-rolls the austenitic stainless steel strip. This is a method for producing cold-rolled austenitic stainless steel sheet with good surface gloss and corrosion resistance by subjecting it to rolling and finishing solution heat treatment.Secondly, after hot rolling an austenitic stainless steel strip, 700-1
The surface gloss is achieved by solution heat treatment at 000℃, followed by mechanical preliminary descaling treatment, pickling in a pickling bath containing only hydrochloric acid, and then cold rolling and final solution heat treatment. , a method for producing cold-rolled austenitic stainless steel sheets with good corrosion resistance.

FIG. 1 shows the relationship between the heat treatment temperature of hot-rolled austenitic stainless steel strip and the average Cr concentration in the layer directly below the scale (approximately 2 mm deep).

As the heat treatment temperature increases, the Cr concentration in the layer directly below the scale tends to decrease, and the decrease in the Cr concentration directly below the scale is particularly significant in the temperature range exceeding 1000° C. where austenitic stainless steel becomes a solution.

In other words, when heat treatment is performed in a temperature range of just below 1000° C., the formation of a Cr-depleted layer just below the scale can be significantly reduced.

The pickling properties of these heat-treated hot-rolled strips were investigated using various acid solutions, as shown in Figure 2 (1010
When heat treated at temperatures above ℃, oxidizing, nitric acid,
It was confirmed that a mixed acid of butic acid is effective, but on the other hand, non-oxidizing hydrochloric acid is more effective when heat treatment is performed at a lower temperature or when heat treatment is omitted.

In other words, the pickling ability of the conventional mixed acid of nitric acid and hydrofluoric acid is
The pickling treatment time depends on the solution treatment temperature applied to the hot-rolled strip, and when the temperature drops below 1000℃, the pickling treatment time increases rapidly. This is due to insufficient reduction in Cr concentration directly under the scale due to chemical heat treatment, and on the other hand, at 1000°C, the formation of a Cr-depleted layer directly under the scale is much reduced compared to the case of solution heat treatment. It has been found that pickling with hydrochloric acid is particularly advantageous under the following heat treatment conditions or without solution heat treatment.

Also, when performing this hydrochloric acid pickling, it is important to note that if the hydrochloric acid pickled hot-rolled strip IJ strip is left for a long period of time, chloride adhering to the eaves of the strip surface may be removed. This causes pitting corrosion.

This problem can be solved advantageously by pickling the hot-rolled strip with hydrochloric acid and then pickling with nitric acid.At the same time, the finished color tone of the surface after pickling is further improved, and the steel sheet produced by subsequent cold rolling is improved. It was confirmed that the surface gloss and corrosion resistance of the material were also improved.

According to the present invention, the solution heat treatment conventionally performed before cold rolling of hot rolled IJ tubes can be simplified or omitted, so it is possible to save energy in the manufacturing process of cold rolled austenitic stainless steel sheets. can.

Regarding the application of the present invention, it is suitable for all austenitic stainless steels such as 5US304.304L and 316°316L, and in any case there is no need to restrict the hot rolling process, and it can be applied under known normal conditions. It will be done.

Next, in the heat treatment of the hot-rolled strip, in order to reduce or avoid the formation of a Cr-depleted layer directly under the scale, it is an essential requirement in this invention to perform the heat treatment at a temperature of 700 to 1000°C or to omit the heat treatment completely. be.

Here, the reason why the heat treatment conditions were limited as described above will be explained below.

The solution heat treatment temperature usually carried out after hot rolling is 101
0 to 1120'C, and its role is to improve the pickling with a mixed acid of nitric acid and hydrofluoric acid as mentioned above, and also to improve the corrosion resistance ratio by converting chromium carbonitrides precipitated during hot rolling into a solid solution. It is also considered to improve the properties of the steel, and to soften it during cold rolling in the subsequent process.

When the heat treatment temperature of the hot-rolled strip is 700°C or higher, improvements in the corrosion resistance ratio and signs of material softening are observed as the temperature increases, while when the temperature is lower than 1000°C, an energy-saving effect is observed compared to the conventional manufacturing process. Therefore, the temperature range of 700 to 1000°C was limited as the low temperature solution heat treatment.

Furthermore, since heat treatment at temperatures below 700°C has no effect on improving the corrosion resistance ratio or softening the material, it is better to omit heat treatment than to perform heat treatment unnecessarily, resulting in greater energy savings. In addition, even if heat treatment after hot rolling is omitted, the final solution heat treatment step after cold rolling fully plays a role in solutionizing precipitates, and the problem of softening can be solved by using austenitic stainless steel. In the case of steel, since there is no particularly large failure rate, we limited ourselves to omitting heat treatment after hot rolling.

Descaling after hot rolling is carried out by a method that combines mechanical pretreatment such as shot blasting, light rolling, and bending rolls with hydrochloric acid pickling or hydrochloric acid pickling followed by nitric acid pickling.

As for the surface color tone of the hot-rolled strip after pickling, it is generally assumed that the whiteness is 60 or more because it is necessary to improve the gloss when the product is made into a cold-rolled product.

The pickling conditions for satisfying the 1"L in hydrochloric acid pickling vary slightly depending on the steel type and heat treatment conditions, but an example is shown in FIG. 3 for the case of 5US304 in the as-hot-rolled state.

In this case, the hydrochloric acid pickling conditions are preferably Hc#15
% by weight or more, the immersion time is about 3 minutes or less at a liquid temperature of about 70 to 90°C, but in pickling lines where the pickling tank is long and the pickling time can be longer than 3 minutes, the HCl concentration or liquid temperature is It is also possible to make it lower.

However, if the temperature exceeds 90°C, the hydrochloric acid content in the acid bath will evaporate and the acid solution will be wasted.

Next, the nitric acid pickling conditions as a measure to prevent pitting corrosion after hydrochloric acid pickling are preferably HNO3:5 to 15 and weight source 50 to 70°C.

Conditions for cold rolling and final solution heat treatment after cold rolling do not need to be particularly limited, and any known conventional method may be used.

. Next, examples of the present invention will be described.

Example A 5US304 hot-rolled strip and a 5US316 hot-rolled strip that had been hot-rolled under normal conditions were heat treated and pickled in a continuous annealing pickling line under the conditions shown in Table 1, and then cold rolled through a normal cold rolling process. JIS when used as a product plate
The results of investigating the surface gloss of Z-8741 and the corrosion resistance of D-0201 by CAST test are shown in the same table.

Note that Table 1 also lists similar investigation results for cold-rolled product sheets manufactured by conventional manufacturing processes.

For both SUS 304 and SUS 316, the bath treatment after hot rolling is omitted, or after heat treatment at a temperature lower than the commonly known solution treatment temperature, hydrochloric acid pickling or hydrochloric acid-nitric acid pickling is performed. In this case, it can be seen that the surface gloss and corrosion resistance after cold-rolling are superior to those obtained by conventional nitric acid and pickling.

Furthermore, it has been found that in the hydrochloric acid pickling or hydrochloric acid→nitric acid pickling process according to the present invention, no NOx gas is particularly generated, and the pickling cost can be significantly reduced.

As described above, according to the present invention, the following effects can be achieved in the manufacturing process of cold-rolled austenitic stainless steel sheets.

(b) By simplifying or omitting the heat treatment of the hot-rolled strip, it is possible to achieve significant energy savings, as well as reduce process costs and reduce product costs.

(0) No NOx gas is generated in the pickling process of hot-rolled slab I-IJ tubes, which greatly contributes to environmental improvement.

(/→ Increased productivity in a continuous annealing and pickling line for hot-rolled strip can be achieved relatively easily.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the heat treatment temperature of austenitic stainless steel hot-rolled strip and the average Cr concentration in the layer directly below the scale, and Figure 2 is the relationship between the heat treatment temperature of hot-rolled strip and pickling time in various acid solutions. FIG. 3 is a graph showing the relationship between hydrochloric acid pickling conditions and finished whiteness after pickling.

Claims (1)

[Claims] 1. After hot rolling an austenitic stainless steel strip, it is subjected to mechanical preliminary descaling treatment, then pickled in a pickling bath containing only hydrochloric acid, and then cold rolled. and a method for producing a cold-rolled austenitic stainless steel sheet with good surface gloss and corrosion resistance, which comprises subjecting it to finishing solution heat treatment. 2 After hot rolling an austenitic stainless steel strip, it is subjected to solution heat treatment at 700 to 1000°C, followed by mechanical preliminary descaling treatment, and then pickled in a pickling bath containing only hydrochloric acid. , followed by cold rolling and final solution heat treatment.