JPS61245912A - Descaling method for austenitic stainless steel sheet or strip - Google Patents

Abstract

PURPOSE:To descale a hot rolled austenitic stainless steel sheet at a high speed by subjecting the sheet to mechanical descaling then preheating the sheet to an adequate temp. and pickling the sheet with an aq. soln. contg. nitric acid and hydrofluoric acid prepd. to an adequate concn. CONSTITUTION:The hot rolled sheet or strip of the austenitic stainless steel is subjected to mechanical descaling of a shot blast method, etc. Said steel sheet or strip is then preheated to 40-150 deg.C surface temp. and is thereafter pickled by using the aq. soln. consisting of nitric acid and hydrofluoric acid contg. 100-400g/l NHO3 and 75-400g/l HF. The reaction between the above- mentioned steel sheet or strip is thereby initiated right after the immersion and thereafter the reaction is stably continued, by which the steel sheet or strip is quickly descaled and the excellent surface free from intergranular corrosion is obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is a method for rapidly descaling the surface oxidation scale of austenitic stainless steel hot-rolled steel sheets or hot-rolled steel strips (hereinafter collectively referred to as hot-rolled sheets). [Prior art] In general, this is a method for manufacturing stainless steel plates and steel strips.

Continuously cast slab is soaked and hot rolled 2~8■
First, the hot-rolled sheet is annealed as necessary, or descaled as hot-rolled, and then cold-rolled to the product thickness.

Descaling of hot-rolled sheets is carried out by mechanical treatment such as shot blasting, followed by pickling with a mixed acid of nitric acid and hydrofluoric acid.1 The acid concentration usually used is: Nitric acid (HNO3) Ka5-15
%, hydrofluoric acid (HF) is 1 to 5 es ("Stainless Steel Handbook" 1978 edition P, -842). However, since descaling is usually carried out in a continuous line, the acid solution is heated at 35 to 70°C.
heat, and use methods such as dipping and spraying.

[Problem that the invention seeks to solve]

The present invention relates to a method for producing an austenitic stainless steel plate or steel strip, which aims to shorten the descaling time of hot rolled sheets and complete pickling descaling at high speed.

[Means/effects for solving the problem] The present inventor has 1
8 If hot-rolled sheets of Cr-8Ni type austenitic stainless steel are mechanically descaled and then pickled with nitric fluoride acid, the descaling time is long, and furthermore, due to the effects of hot coil slab heating, changes in hot rolling conditions, etc. It has been found that the scale thickness can prolong the descaling time.

By the way, in order to complete the descaling of the hot-rolled sheet at high speed, 1) the reaction with the pickling solution must be started quickly after the hot-rolled sheet is immersed, and 2) the reaction with the hot-rolled sheet must be stable and continuous after the start of the reaction. We thought that it was important for the acid solution to react.

Therefore, as a result of research on the initiation of pickling in nitric-fluoric acid of mechanically descaled steel sheets, it was found that the surface temperature of the steel sheet has a large effect.9.

In other words, in a pickling experiment in nitric-fluoric acid at about 50°C, when a hot-rolled sheet is immersed without preheating, the reaction does not start for 20 seconds, and then hydrogen is generated and pickling progresses rapidly and ends in 60 seconds. did. On the other hand, when the surface of the steel plate was preheated to 70° C. and immersed in the pickling solution, the pickling reaction started immediately after the immersion, and the pickling was completed in 25 seconds.

As described above, when high-speed pickling with an immersion time of 30 seconds or less is targeted, the effect of preheating the surface of the steel plate becomes noticeable and the pickling time can be significantly shortened.

The effect of preheating the steel plate surface is greater when the preheating temperature is 40°C or higher, and the higher the temperature, the more effective it is, but if it exceeds 150°C, the temperature distribution on the steel plate surface becomes uneven and the effect is saturated.

A dipping method was used as the pickling method and the above results were obtained. In the spray method, the effect of preheating the steel plate surface was even more pronounced, and a significant reduction in pickling time was achieved.

Further, the present inventor investigated the liquid composition of a high-speed pickling solution when a hot-rolled sheet of 18 Cr-8Nl type austenitic stainless steel is preheated, and as a result, the following became clear.

HNO3 in the nitric-fluoric acid mixture is 10θ to 4ooII/l.

HF is 75-4001! K if the concentration range is /l,
The descaling time is short and the surface texture after descaling is excellent. In other words, steel sheets that are hot rolled and then annealed in a nitric hydrofluoric acid mixture having a concentration range of nitric hydrofluoric acid, If the raw steel plate is preheated and pickled, descaling will be completed in a short time and an excellent surface free of intergranular corrosion will be obtained after descaling.

The temperature of the pickling solution may be a normal temperature, and is preferably 90°C or lower.

The action of mechanical descaling is also important in this high-speed pickling method according to the present invention, which is different from the conventionally used shot blasting method and black skin cold rolling method.

A high-pressure water spraying method containing iron sand particles is effective, especially in the case of hot-rolled sheet annealed materials with thin scales, and is effective with lighter application than conventional methods.

〔Example〕

Examples of the present invention will now be described.

After annealing a hot coil of austenitic stainless steel hot-rolled by a conventional method, or a steel plate without annealing, it was mechanically descaled by shot blasting or high-pressure water spraying containing iron sand particles under conventional conditions. After that, a pickling tank is used in the pickling line, and the temperature of hot water or an aqueous solution containing a neutral salt is raised to preheat the surface temperature of the steel plate shown in Table 1. Washing was carried out, the time taken to complete pickling was measured, and intergranular corrosion on the surface of the pot plate after pickling was investigated. As a comparative method, the pickling results without preheating are shown.

From these results, it is clear that according to the present invention, pickling is completed in half the time compared to the case without preheating, and within 25 seconds, and intergranular corrosion does not occur. A significant effect of the washing method was confirmed.

〔Effect of the invention〕

The present invention enables high-speed pickling of hot-rolled sheets without causing intergranular corrosion when producing austenitic stainless steel sheets, and provides products with high productivity and excellent surface properties, making it suitable for the industry. The effect is significantly large.

[Brief explanation of the drawing]

Figure 1 shows H that does not cause intergranular corrosion in the nitric-fluoric acid pickling method.
Figure 2 is a diagram showing the appropriate concentration range of NO, IF. Figure 2 is a micrograph diagram showing the metal structure of the surface of an austenitic stainless steel plate. (a) is a surface without intergranular corrosion, (b) is a surface with intergranular corrosion. This is the surface where . 6 Hydrocyanic acid A (degree (1/l) 1a) (JIS (l...) Figure 2 (force)

Claims (1)

[Claims] After performing mechanical descaling on a hot-rolled steel plate or hot-rolled steel strip of austenitic stainless steel, and then preheating the surface temperature of the steel plate or steel strip to 40 to 150°C, H
100-400g/l as NO_3 and 75 as HF
A method for descaling an austenitic stainless steel plate or steel strip, which comprises pickling using an aqueous solution of nitric and hydrofluoric acid containing up to 400 g/l.