JPH037729B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a cold rolled steel strip of austenitic stainless steel, mainly 18Cr-8Ni steel. In general, 18Cr-8Ni steel, i.e., austenitic stainless cold-rolled steel strip mainly made of SUS304 steel, is hot-rolled by slab heating at around 1200 to 1300℃, and then heated to about 1010 to 1150℃ as a solution treatment. It is usually manufactured by subjecting it to a heat treatment of quenching and pickling, followed by cold rolling, final annealing, and pickling. However, in this conventional manufacturing method, a considerable amount of thermal energy is consumed for solution treatment, so recently, by softening the hot-rolled stainless steel strip during hot rolling, Several manufacturing methods have been proposed that save energy by omitting the subsequent solution heat treatment. What these proposals have in common is that by regulating the rolling reduction rate and finish rolling temperature during hot rolling within a certain range, the hot rolled steel strip is made soft, and in the subsequent cooling process, intergranular corrosion due to the precipitation of Cr carbides is prevented. In order to suppress this, rapid cooling is performed. The reason for rapid cooling after hot rolling is that when hot-rolled steel strips that have been sensitized by slow cooling are pickled with nitric-fluoric acid, which is the conventional pickling method, grain boundaries are This is because dissolution occurs and grain boundary peeling occurs in the subsequent cold rolling process, which significantly deteriorates the surface properties. However, as mentioned above, in the method of softening the hot-rolled steel strip during hot rolling without performing solution heat treatment, the in-plane anisotropy of the cold-rolled steel strip becomes large, and when it is subjected to processing such as deep drawing, it becomes difficult to make earrings. There is a problem in that the amount of paper becomes large, and as a result, there is a problem in that the yield rate decreases significantly when processing companies perform board cutting. Therefore, from the perspective of energy saving, the present inventors have developed a method that can omit the solution treatment of hot-rolled steel strips and at the same time improve the in-plane anisotropy of cold-rolled sheets to prevent the occurrence of earrings as much as possible. After careful consideration, we found that during hot rolling, the cumulative reduction ratio of rough rolling and finish rolling is 50% or more, and the finish rolling end temperature is 870℃ or higher, and hot rolling is carried out at a temperature of 650℃ or higher. By coiling and slow cooling from the coiling temperature to at least 550℃ at a cooling rate of 1℃/second or less, the hot-rolled steel strip becomes soft and solution heat treatment of the hot-rolled steel strip is performed. It has been found that the in-plane anisotropy of the cold-rolled sheet obtained through the subsequent pickling, cold rolling, and final annealing is significantly reduced. However, as mentioned above, if slow cooling is performed after winding the hot rolled steel strip, the conventional pickling with nitric hydrofluoric acid or sulfuric acid will selectively dissolve the grain boundaries and significantly deteriorate the surface quality. . In order to solve this problem, the inventors conducted further studies and found that instead of the conventional general-purpose pickling method, descaling was performed using hydrochloric acid pickling, hydrochloric acid electrolysis, or nitric acid electrolysis. For example, selective corrosion of grain boundaries does not occur, and as a result, grain boundary peeling does not occur during the cold rolling process, and a cold rolled sheet with good surface properties can be obtained, and this invention has been completed. be. Therefore, an object of the present invention is to provide a method for manufacturing an austenitic stainless steel cold-rolled steel strip that has less in-plane anisotropy and good surface properties while saving energy by omitting solution heat treatment. The gist is that when hot rolling an austenitic stainless steel slab, the cumulative reduction in rough rolling and finish rolling should be 50% or more, and the finish rolling end temperature should be 870°C or more. The hot-rolled steel strip is rolled at a temperature of 650°C or higher, and then slowly cooled from the coiling temperature to a temperature of 550°C or lower at a cooling rate of 1°C/second or lower. On the other hand, it is characterized in that scale is removed by one or more of hydrochloric acid pickling, hydrochloric acid electrolysis, or nitric acid electrolysis without performing solution treatment, followed by cold rolling. The method of the present invention will be explained in more detail below. In the manufacturing method of the present invention, a slab of austenitic stainless steel obtained by a known continuous casting method or an ingot-blowing rolling method is
When hot rolling after heating to about 1300℃,
Hot rolling is performed with a cumulative reduction ratio of 50% or more in rough rolling and finish rolling, and a finish rolling end temperature of 870°C. Such hot rolling conditions are essential for softening the hot rolled steel strip and omitting solution heat treatment. In other words, if the cumulative reduction ratio in hot rolling is less than 50%, the microstructure of the hot rolled steel strip not only becomes a mixed grain structure, but also becomes hard, making it easy for edge cracks to occur in the subsequent cold rolling process. Also, if the finish rolling end temperature is less than 870℃,
The hot-rolled structure becomes an unrecrystallized structure and becomes hard, causing difficulties in the cold rolling process as described above. Therefore, in the present invention, the cumulative reduction rate in hot rolling and the finish rolling temperature are determined as described above. If any one of these conditions is missing, the hot-rolled steel strip will become hard, and edge cracks will occur during the cold rolling process, resulting in a significant decrease in yield, so in practice, solution heat treatment cannot be omitted. Put it away. After finish rolling under the above-mentioned conditions, the hot-rolled coil is generally air-cooled or forced-air cooled for about 3 to 15 seconds on a hot run table, but the conditions at this time have little effect on the hardening of the hot-rolled coil. The hot-rolled steel strip is coiled at a temperature of 650℃ or higher, and in the subsequent cooling process, the coiling temperature is reduced to 550℃.
Slowly cool to the following temperature at a cooling rate of 1° C./second or less. Such hot-rolled steel strip coiling temperature conditions and cooling conditions are necessary to reduce in-plane anisotropy of cold-rolled sheets, and the details of these conditions were found by the inventors through experiments. This is what happened. In other words, the present inventors determined the coiling temperature of hot-rolled steel strip for SUS304 steel,
The cooling rate from the coiling temperature to 550°C was varied, and then pickling was performed without solution heat treatment.
The in-plane anisotropy of cold-rolled sheets finished by cold rolling was investigated using the earring ratio using a cylindrical drawing cup test. Figure 1 shows the effects of the coiling temperature of a hot rolled steel strip and the cooling rate from the coiling temperature to 550°C on the in-plane anisotropy expressed by the earring ratio. Here, the earring ratio is defined as Hmax, the height of the peak (ear) of the flange part, and Hmin, the height of the valley, in the cylindrical draw cup test, and earring ratio = (Hmax - Hmin) / (Hmax + Hmin) x 1 / 2×100(
%). From Fig. 1, the lower the coiling temperature of the hot rolled steel strip, the
In addition, the faster the cooling rate to 550°C after coiling, the stronger the in-plane anisotropy of the cold-rolled sheet, and the larger the earring ratio, ensuring a practically acceptable earring ratio of 5% or less. In order to achieve this, it is clear that the winding temperature must be 650° C. or higher and the cooling rate after winding must be 1° C./second or lower. Note that slow cooling of the hot rolled steel strip at a rate of 1°C/sec or less after coiling is required to at least 550°C, and at lower temperatures, the in-plane anisotropy of the cold rolled sheet will change depending on the cooling rate. Gender is not affected. In addition, the coiling temperature of the hot-rolled steel strip needs to be 650℃ or higher as mentioned above, but the higher the coiling temperature, the softer the hot-rolled steel strip becomes, and the in-plane anisotropy of the cooling plate also decreases. Since the winding temperature is small, there is no particular upper limit for the winding temperature. The hot rolled steel strip cooled after hot rolling as described above is subjected to a scale removal treatment. As this scale removal treatment, hydrochloric acid pickling, hydrochloric acid electrolysis, or nitric acid electrolysis is employed. As mentioned above, hot-rolled steel strips that have been slowly cooled have become sensitized, so pickling with nitric-fluoric acid or sulfuric acid, which is commonly used in the past, causes selective dissolution of grain boundaries, which affects the surface quality after cold rolling. However, in hydrochloric acid pickling, hydrochloric acid electrolysis, or nitric acid electrolysis, selective dissolution of grain boundaries can be prevented and a cold-rolled sheet with excellent surface properties can be obtained. Of course, scale removal may be performed by combining two or more of these treatments. The conditions for scale removal treatment are as follows:
In the case of hydrochloric acid pickling or hydrochloric acid electrolysis, it is desirable to use 5-30% hydrochloric acid at a liquid temperature of 30-90°C, and in the case of nitric acid electrolysis, it is preferable to use 5-70% nitric acid and carry out at a liquid temperature of 30-90°C. It is desirable to carry out at 90℃.
If these acid solution concentration conditions and solution temperature conditions are not met, overaciding or scale residue will occur, which will impair the surface properties of the product, which is not preferable. Furthermore, it goes without saying that each acid solution may contain a small amount of an oxidizing agent. Further, the electrolytic conditions for hydrochloric acid electrolysis or nitric acid electrolysis are preferably 150 coulombs/100 cm ² or more. If it is less than 150 coulombs/100 cm ² , a sufficient scale removal effect on the hot rolled steel strip cannot be obtained. Further, after the scale removal treatment and before the cold rolling, a passivation treatment such as a nitric acid immersion treatment may be performed as necessary. By immersion in nitric acid after such scale removal treatment, almost no intergranular corrosion occurs. Cold rolling conditions have a slight effect on the in-plane anisotropy of cold-rolled sheets, but compared to the effects of the coiling temperature of hot-rolled steel strips and the cooling conditions after coiling, this effect is small. much smaller. Therefore, for cold rolling, it is sufficient to apply one-time cold rolling, two-time cold rolling, or warm cold rolling according to the conventional method, and by any method, a cold-rolled sheet with small in-plane anisotropy can be obtained. Can be done. Next, an embodiment of the present invention will be described. Continuously cast 10 ton slab of 200mm thick SUS304 steel.
After heating to 1280℃, it is roughly rolled to a thickness of 23mm using an ordinary hot strip mill, and the rough rolling finish temperature is 1060℃, and then final rolled to a 3mm thick hot rolled steel strip and scaled. After removal treatment and cold rolling, it was made into a cold rolled sheet with a 2B finish. Table 1 shows the process conditions after finishing hot rolling. The cumulative reduction ratio in hot rolling is 50% or more in all cases. Furthermore, Table 2 shows the hardness of the hot-rolled sheets, the mechanical properties of the cold-rolled sheets, and the surface properties of the cold-rolled sheets.

【table】

[Table] *Water cooling after finish rolling to reduce coiling temperature

[Table] * Before solution heat treatment
(Hmax−H〓)
**Earring rate=

Claims (1)

[Claims] 1 When hot rolling an austenitic stainless steel slab, the cumulative reduction ratio of rough rolling and finishing rolling is 50% or more, and the finish rolling end temperature is 870°C or higher, and then hot rolling is carried out at 650°C or higher. The hot-rolled steel strip is coiled at a temperature of 1. A method for producing an austenitic stainless steel strip, which comprises performing a scale removal treatment by one or more of hydrochloric acid pickling, hydrochloric acid electrolysis, or nitric acid electrolysis without carrying out any process, followed by cold rolling.