JPH0227411B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a ferritic stainless steel thin plate with excellent workability.
The present invention relates to a method for producing thin plates of ferritic stainless steel, typically SUS430 steel containing . (Conventional technology) Normal SUS430 type ferritic stainless steel thin plate with an Al content of 0.08% or less is a hot rolled plate with approximately 800%
It is generally manufactured by box annealing at a temperature of ~850℃ for more than 2 hours, followed by cold rolling, and then annealing for a short time at a temperature of usually 820~850℃.
The value, ridging, mechanical properties, etc. hardly change depending on the hot-rolling temperature and final annealing conditions. However, in a manufacturing method such as that disclosed in Japanese Patent Publication No. 59-576, in which a thin plate is produced by cold rolling and annealing without annealing a hot rolled plate, the r
The effects of final annealing conditions on strength, ridging, and mechanical properties were not understood. (Object of the invention) The present invention provides the steps of omitting hot-rolled plate annealing.
In particular, by investigating the relationship between Al content, winding temperature conditions, final annealing conditions, and the mechanical properties of the product, we can appropriately control these conditions. This was done based on the new recognition that it can provide properties equivalent to or better than stainless steel thin sheets. (Structure of the Invention) The present invention involves rolling a ferritic stainless steel slab containing 0.08 to 0.5% Al by weight in a hot rolling facility consisting of a rough rolling mill and a plurality of continuous finishing mills, and then rolling the slab at 700°C. After rolling in the following temperature range and cold rolling the hot rolled sheet without annealing, 900
Excellent workability, characterized by finish annealing in a temperature range of more than 1000°C for less than 30 seconds and within the temperature/time relationship area indicated by diagonal lines in Figure 5 corresponding to the above-mentioned winding temperature. The gist of this paper is a method for manufacturing ferritic stainless steel thin plates. The present invention will be explained in detail below. First, the metallurgical basis and reasons for limitations of the present invention will be explained. The first reason for adding Al as a steel component is AlN.
The aim is to precipitate N and reduce solid solution N to improve the mechanical properties of the product, particularly lowering the yield point, elongation at yield point, and improving the r value. The step of precipitating AlN in order to lower the yield point and reduce the elongation at yield point may be any step of slab heating, rough to finish rolling, winding, intermediate annealing, or final annealing. Under normal hot rolling conditions, it is most effective to precipitate in the final annealing step, which is why the annealing temperature of the present invention is significantly higher than that of conventional materials. The reason why the lower limit of the amount of Al added was set at 0.08% is that if it is less than this, precipitation of AlN cannot be expected in any process, and the above effect cannot be expected. Limited to Al
When 0.08% or more of Al is added, the effect increases as the amount added increases, but the degree of effect decreases from about 0.2% or more, and it is almost saturated at 0.5% or more, and it is economically undesirable to add more than that. It is from. The second reason is to prevent abnormal corrosion during the pickling process and to prevent surface defects called "sparkle defects" from occurring in the final product. The present inventor conducted various studies on abnormal corrosion phenomena that occur when hot-rolled sheets are pickled without annealing. As a result, Al content,
The abnormal corrosion phenomenon varies greatly depending on the hot-rolling temperature conditions and the type of pickling liquid, but if the hot-rolling conditions and pickling liquid are constant, the higher the Al content, the less abnormal corrosion will occur. I found out that it can be done. The lower limit of the amount of Al added is set at 0.08% because if the amount of Al added is less than this, "sparkling defects" may occur in the final product due to abnormal corrosion phenomena. As the amount of Al added increases, this abnormal corrosion phenomenon is improved, but
Since the improvement effect is almost saturated even if it is added in excess of 0.5%, the upper limit of the amount of Al added is set at 0.5%. Next, the reason for limiting the winding temperature conditions when hot rolling ferritic stainless steel using a continuous hot rolling mill consisting of a rough rolling mill and a plurality of finishing mills will be described. Figure 1 shows the r-value, ridging and winding of hot-rolled sheets with conventional composition (Al<0.08%) and the present invention composition (Al0.08%), which were cold-rolled without hot-rolled sheet annealing and then annealed. This shows the relationship between temperature. As can be seen from the figure, the r value of the conventional component material is extremely low regardless of the winding temperature, but when compared at the same winding temperature, the r value of the steel of the present invention is significantly higher than that of the conventional component material, and the r value is higher. It was found that the r value increases as the temperature increases. Regarding ridging, both the steel of the present invention and the conventional steel deteriorate as the winding temperature increases, but in the case of the steel of the present invention, it is found that the ridging deteriorates rapidly especially when the winding temperature exceeds 700°C. The reason why the winding temperature is limited to 700℃ or less in the present invention is that
This is to prevent ridging from deteriorating. Next, we will explain why the final annealing conditions were limited to less than 30 seconds in a temperature range of over 900°C and below 1000°C. FIG. 2 schematically shows the relationship between the hot rolling winding temperature, the heating temperature (T° C.), and the heating condition (t seconds) in the final annealing step. It can be seen from the figure that the lower the winding temperature, the higher the heating temperature and the longer the heating time to obtain a yield strength of 35 Kg/mm ² or less. The reason for this is that in the state of the hot rolled sheet,
The lower the winding temperature, the more Cr ₂ N and α′ phase will be present.
As shown in FIG. 3, these decompose in the final annealing step and release N, and as the heating temperature increases, the amount of solid solute N in the steel sheet increases. Therefore, if the steel is cooled as it is, the yield strength will increase due to the increase in solute N, but in the case of the steel of the present invention, since a large amount of Al is added, it is fixed as AlN.
If the temperature and time range at which AlN can be precipitated is maintained
The amount of precipitated AlN increases, resulting in a decrease in solid solution N, resulting in a decrease in yield strength. When the heating temperature is further increased, the yield point rises again due to the redissolution of AlN, and at even higher temperatures, the solid solution N decreases due to the precipitation of the γ phase, and the yield point falls again. FIG. 4 schematically shows this relationship. In the present invention, the lower the winding temperature, the higher the temperature and the need for long-term heating.The reason is that the Cr ₂ N and α' phases are more present than in the high-temperature winding material. This is because it requires a long time. The winding temperature, material heating temperature (T°C) and time (t seconds) in the final annealing process are shown in Figure 2.
The relationship is shown in more detail in FIG. As is clear from FIG. 5, the temperature/time relationship of the material in the final annealing process is determined depending on the strip winding temperature level after hot rolling. In the steel strip containing 0.08 to 0.5% Al defined in the present invention, the AlN precipitation nose is near 900°C. If the material is kept in a temperature range above 900°C for a long time, the AlN will re-dissolve, the amount of solid solute N will increase, and the yield point of the resulting product will increase. Therefore, in the steel composition of the present invention, 900℃
The annealing condition is to hold the temperature in the ultra-1000℃ range for less than 30 seconds. That is, the lower the winding temperature is, the greater the amount of α' phase, which is a transformed prior γ phase contained in the hot rolled sheet, needs to be decomposed during final annealing. Therefore, the lower the strip winding temperature in the hot rolling process, the longer the material must be held at a higher temperature during final annealing. The lower limit of the winding temperature in normal hot rolling is approximately 500°C, and the upper limit is 700°C from the viewpoint of ripping (Figure 1). Incidentally, the α' phase also changes depending on the content of Al and C, and the smaller the Al content, the larger the larger the C content, so this must also be taken into consideration. Taking advantage of the present invention, C: 0.045%,
For a typical composition of Al: 0.12%, the relationship between winding temperature and final annealing conditions is as follows.

[Table] In this way, when the winding temperature is low, compared to high temperature winding, if the finishing annealing temperature is the same, longer heating is required, and if the heating time is the same, the material is heated to a higher temperature. This is necessary. however,
As is clear from the above table, if the material heating temperature in final annealing exceeds 900℃, a product with good mechanical properties, i.e., a yield strength of approximately 35Kg/ ^mm2 or less, can be obtained with heating for less than 30 seconds. be able to. The reason why the upper limit of the annealing temperature is set to 1000° C. or less is that at a high temperature exceeding this temperature, the γ phase reprecipitates and transforms into the α' phase during the cooling process, which significantly increases elongation and tensile strength, which is undesirable. In addition, in descaling, the main component is
When H ₂ SO ₄ or HCl or a mixed pickling solution thereof is used, the surface quality is improved. Also, it is better to avoid using pickling liquids containing NHO ₃ as the main component. The present invention will be specifically described below with reference to Examples. Example 1 Containing Al with a thickness of 200 mm shown in Table 1
SUS430 type ferritic stainless steel slab at 1220℃
After heating for 2 hours at a temperature of
It was rolled as a 2.40mm hot-rolled coil at a temperature of 650℃. For comparison, SUS430 steel with an Al content of 0.06% was hot-rolled under the same conditions to form a hot-rolled coil with a thickness of 3.0 mm. The steel of the present invention was then cold-rolled to a thickness of 0.4 mm without hot-rolled plate annealing.
The comparative steel has a temperature of 0.4 after hot-rolled sheet annealing at a temperature of 820℃ for 4 hours.
It was made into a cold-rolled sheet of mm. After annealing these cold-rolled sheets at various heating temperatures and times, their mechanical properties were measured. The measurement results are shown in Table 2. As is clear from the table, the comparative steel exhibits good mechanical properties even when heated for 10 seconds in a wide temperature range of 800 to 900°C, but in the case of the inventive steel, the optimum range for conventional materials is 10 seconds at 800 to 850°C. In the case of heating, the yield strength is high and the yield point elongation is large, but by annealing at a high temperature of over 900℃ to 1000℃ for a short time, it is possible to obtain mechanical properties equivalent to or better than conventional materials. can.

【table】

[Table] (Effects of the Invention) As described above, the present invention is a process-saving process that omits the hot rolling annealing process compared to conventional manufacturing methods, and enables the production of thin plates with excellent surface properties and workability. The economic effects are extremely large, and therefore the industrial benefits are extremely large.

[Brief explanation of drawings]

Figure 1 shows the relationship between winding temperature, r value, and ridging when the present invention steel and conventional steel are cold-rolled annealed without hot-rolled sheet annealing. Figure 2 shows the finished product of the present invention steel after final annealing. A schematic diagram showing the relationship between yield strength, hot-rolling temperature, and final annealing temperature x time. Figure 3 is a schematic diagram showing the change in nitrides in the final annealing process. Figure 4 shows the yield strength in the final annealing process. Figure 5 is a schematic diagram showing the relationship between changes in heating temperature, time, and reaction of nitrides. FIG. 2 is a diagram showing the relationship in more detail.

Claims (1)

[Claims] 1 A ferritic stainless steel slab containing Al: 0.08 to 0.5% by weight is rolled in a hot rolling facility consisting of a rough rolling mill and multiple continuous finishing mills, and then rolled in a temperature range of 700°C or less. , after cold rolling the hot-rolled sheet without annealing, the temperature exceeds 900°C,
A ferritic stainless steel with excellent workability, characterized in that finish annealing is performed in a temperature range of 1000°C or less for less than 30 seconds and within the temperature/time relationship area indicated by diagonal lines in Figure 5 corresponding to the above-mentioned winding temperature. Method for manufacturing thin steel sheets.