JPH0418013B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a ferritic stainless thin steel sheet that is free from ridging and hot roll seizure flaws. Ferritic stainless thin steel sheets are less expensive than austenitic stainless steels and have a clearer color tone, so they are widely used in parts for automobiles and household appliances. However, ferritic stainless steel has the disadvantage that when it is subjected to drawing or tension processing in the manufacture of parts, an uneven striped pattern called ridging or roping occurs, which significantly impairs the appearance. A great deal of research has been conducted and various proposals have been made on methods to prevent this ridging, but the main idea is to retain as much strain as possible during hot rolling to promote recrystallization during subsequent hot-rolled sheet annealing. The purpose is to destroy the casting structure that causes ridging. Specifically, a method of hot rolling at a low temperature (see, for example, Japanese Patent Publication No. 45-34016) has been proposed, and a method of short-time annealing at a high temperature with the addition of Ti, Nb, etc. (for example, a special method of hot rolling) has been proposed. Publication No. 149116 (1972), Special Publication No. 149116 (1977)
-6086 Publication) has been proposed, and furthermore, proposals combining these methods (for example, Japanese Patent Application Laid-Open No. 1983-6086) have been proposed.
123527) has also been made. However, although these methods are effective in reducing ridging, they are not decisive for completely eliminating ridging as in the case of austenitic stainless steel. Although it is different from the above-mentioned idea, the method of once making the austenite phase appear during hot-rolled sheet annealing (Japanese Patent Publication No. 48-24611) is very effective as a measure to prevent ridging. It is necessary to perform very slow cooling or to heat for a long time below the transformation temperature, which significantly impedes productivity. Unlike the method described above, the present invention destroys the cast structure that causes ridging by repeatedly recrystallizing Nb-added ferritic single-phase stainless steel, including during hot rolling, and improves the ridging property. The aim is to completely eliminate ridging. That is, the present invention has C0.015% or less, N0.015% or more,
0.035% or less, Nb 0.2% or more, and C and N weight%
A ferritic stainless steel slab containing 8 times or more and 1.0% or less of the sum of
After hot rolling at 1000°C or higher with a reduction rate of 30% or more and less than 60% per pass at least once, heat to 1250°C or higher and at least 10 minutes at 900°C or higher and less than 1100°C. A ferrite system free from ridging and hot roll seizing defects, characterized by subjecting a hot rolled sheet to annealing, followed by cold rolling, or cold rolling with one or more intermediate annealing steps, followed by recrystallization annealing. The gist of this paper is a method for manufacturing stainless thin steel sheets. It is already known that Nb-added ferritic stainless steel has excellent ridging properties (Special Publication No. 38
However, it is well known that adding Nb alone is not decisive. The present invention takes advantage of the fact that Nb-added steel has a high recrystallization temperature, and instead of the conventional method of low-temperature hot rolling, the present invention promotes recrystallization during hot rolling by heating and hot rolling at a high temperature to prevent ridging. The feature is that it destroys the causative casting structure. In the case of steel that does not contain Nb, although the recrystallization temperature is low, hot rolling at high temperatures does not accumulate enough strain to cause recrystallization, and only recovery tends to occur, and hot rolling at low temperatures does accumulate strain, but Since recrystallization is less likely to occur during hot rolling, it is difficult to realize the features of the present invention. In steels with a high C content and in which an austenite phase appears at high temperatures, recrystallization is effective, but not to the extent that it destroys the cast structure. Next, the present inventors discovered carbides, which are Nb precipitates,
The effect of nitride was investigated. As a result, it was found that nitrides precipitated at temperatures above 1100°C, and it was assumed that these were the core of preventing crystal grain coarsening and strain accumulation. Based on this knowledge, C
and the amount of N. FIG. 1 is a diagram showing the influence of N on the ridging height of 19Cr-0.4Nb steel with carbon content of 0.01% or less.
The ● mark in the figure cannot be measured because the ridging height is less than 2 μm of the roughening height, so it is plotted uniformly at a point of 1.5 μm. The test conditions in Figure 1 are ○ and ● for slab heating at 1200℃, rolling reduction at a rolling reduction rate of 40 to 45% in the first pass of hot rolling, thickness of hot rolled sheet is 4 mm, and hot rolled sheet. The annealing was carried out at 980°C for 2 minutes, and the cold rolling was a so-called two-time cold rolling with one intermediate annealing, and the recrystallized annealed material of 0.5 mm was subjected to 20% tensile processing using a JIS No. 5 tensile test piece. The × marks are examples in which the hot rolling reduction ratio was all less than 30%. In FIG. 1, when the N content is 0.015% or more, the height of the ridging becomes less than 2μ, which is the level where no ridging occurs. Furthermore, the effect of N appears only when hot rolling is carried out under high pressure, indicating that the combined effect of both is large. FIG. 2 is a diagram showing the influence of the amount of C on the ridging properties of steels with an N amount of 0.015% or more. Contrary to N, the lower the C is, the better it is, 0.015%
It was found that the effect becomes clearer below 0.010%, and especially when the content is below 0.010%, ridging occurs below the height of roughened surface. The influence of C is also not clear unless a large reduction is applied during hot rolling. As shown above, the effect of the method of the present invention is that N
It can be seen that C appears at 0.015% or more, and C appears at 0.015% or less. Next, the results of an investigation into the recrystallization behavior during hot rolling of Nb-added steel will be explained. When Nb-added steel is rolled at a temperature of 1000°C or higher and a reduction rate of 30% or more per pass, recrystallization occurs during hot rolling, improving the ridging properties of the final product as shown in Figures 1 and 2. Admitted. In this case, there was an effect above 1200°C, but the degree of effect became smaller. This is presumed to be because the rate of strain recovery becomes faster at temperatures above 1200°C even in Nb-added steel, which has a high recrystallization temperature. moreover
It was observed that when the temperature exceeded 1250°C, recrystallization no longer occurred and rapid coarsening occurred at the same time as recovery. This is presumed to be because when the temperature exceeds 1250°C, most of the Nb precipitates dissolve into solid solution, so that the coarsening suppressing effect disappears. Next, we investigated recrystallization during hot-rolled sheet annealing and found that, as previously proposed in the present invention (Japanese Patent Publication No. 55-6086, Japanese Patent Application Publication No. 51-149116),
By performing high-temperature short-time annealing at less than 1100°C for 10 minutes or less, the stretched ferrite grains can be finely recrystallized, which is highly effective in improving ridging properties. However, conventionally, low-temperature hot rolling has been aimed at for this purpose (Japanese Unexamined Patent Publication No. 56-123327), but in the present invention, since recrystallization is repeated during hot rolling, it is possible to carry out particularly low-temperature hot rolling. Ordinary hot rolling is sufficient to achieve this purpose. As shown above, by adding N to Nb-added ferritic stainless steel and applying high pressure (reduction rate of 30% to less than 60% per pass) at 1000℃ or higher, hot-rolled sheet annealing can be performed at high temperature and for a short time. Combined with this effect, it has become possible to completely eliminate ridging. Moreover, an important feature of the present invention is that there is no need to lower the heating temperature or the hot-rolling finishing temperature to a particularly low temperature.
The industrial benefits are great because ridging can be completely eliminated without increasing roll seizure flaws on steel sheets or promoting wear of hot rolling rolls. Next, the reasons for limiting the composition and manufacturing conditions of the steel used in the present invention will be described. The upper limit of C in Nb-added ferritic stainless steel was set at 0.015% because a large amount of C deteriorates the corrosion resistance of the product as well as the ridging properties. As mentioned above, N is effective in improving ridging as a Nb nitride, so it should be set at 0.015% or more, and a large amount will deteriorate the corrosion resistance of the product, so the upper limit should be set.
It was set at 0.035%. Since Nb is an element that increases the recrystallization temperature,
Even considering the amount of precipitated carbides and nitrides, C must be at least 0.2% so that it can exist in solid solution in steel.
and at least 8 times the weight percent of N. If it exceeds 1.0%, the effect is saturated, so 1.0% is the upper limit. Furthermore, although Cr is not an element that limits the effects of the present invention, the lower limit was set at 10%, which is the minimum required for stainless steel, and the upper limit was set at 22%, which has no use as a thin steel sheet. However, the effects of the present invention can also be achieved with other amounts of Cr. Note that 13% Cr steel is generally treated as martensitic stainless steel, but as in the present invention, in a low C range, no austenite phase appears even at high temperatures and the ferrite single phase remains, so it is considered a ferritic stainless steel. was treated the same way. In addition, one or more of Cu (1.0% or less), Ni (1.5% or less), and Mo (3.0% or less) may be added for the purpose of improving corrosion resistance. The heating temperature of the slab was set at 1250°C as the upper limit because if it exceeded 1250°C, Nb nitrides would dissolve into solid solution and coarsen the grains, deteriorating the ridging properties. In addition, even if it is less than 1100°C, if it is above 1000°C, the effect of improving ridging will not change, but it will increase hot rolling defects and eliminate the benefit of hot rolling at high temperatures, so 1100°C was set as the lower limit. Strain is applied at high temperature and under large pressure to cause recrystallization during hot rolling, but recrystallization does not occur when the rolling temperature is less than 1000°C, so 1000°C was set as the lower limit. The effect decreases when the rolling temperature is 1200°C or higher, but the effect is observed up to 1250°C, so the upper limit of the rolling temperature is valid up to the upper limit of the heating temperature of the slab. The rolling reduction rate must be 30% or more per pass, and the greater the reduction rate, the more effective it is. However, if it is less than 30%, the effect will vary, so 30% is the lower limit, and 60%
In the above case, severe seizure flaws due to roll seizure occur in hot conditions, so the upper limit was set to less than 60%. As per conventional knowledge, fine recrystallization does not occur at temperatures below 900°C for annealing hot-rolled sheets, so
The upper limit was set because crystal grains begin to coarsen at temperatures above 1100°C. Furthermore, annealing for more than 10 minutes has no effect on quality, so 10 minutes was set as the upper limit. This high-temperature short-time annealing was particularly effective for steel whose recrystallization was promoted during hot rolling. Next, the present invention will be explained in more detail based on Examples and Comparative Examples. Steels having the components shown in Table 1 were hot-rolled and annealed under the conditions shown in Table 2, and the roll seizure flaws of the hot-rolled sheets, the ridging properties of the final products, and the r value were measured. According to the method of the present invention, ridging is visually eliminated without causing surface flaws on the hot-rolled sheet, and the height measured by roughness is also below the processing roughness height, which is substantially the same as that of the SUS304 steel shown as a comparative example. In the case of (No.
12), it is recognized that the level can be reached without ridging. Examples of charts for measuring the ridging heights of Nos. 3, 6, 11, and 12 are shown in FIGS. 3a to 3d. Further, the method of the present invention can be expected to have a high r value and high workability. We were able to reproduce that the conventionally oriented low-temperature hot-rolled material (No. 8) is superior to the high-temperature hot-rolled material (No. 7) of the same material, but the method of the present invention (for example, No. 3)
It can be seen that the effect is small compared to .

【table】

[Table] As detailed above, according to the method of the present invention, ridging is not merely reduced as in conventional measures against ridging, but is virtually eliminated, so its handling can be the same as that of SUS304 steel without ridging. become. Moreover, since the thin steel sheet produced by the method of the present invention has a high r value, it can be processed by taking advantage of its high workability without interrupting processing for ridging or spending a lot of effort and cost on polishing after processing. I can do it. As a result, Ni
Since it becomes possible to use the ferritic stainless steel produced by the method of the present invention in fields where austenitic stainless steel (for example, SUS304 steel) containing a large amount of ferrite is used, the industrial benefits are extremely large.

[Brief explanation of drawings]

Figure 1 shows the influence of N on ridging properties.
When the reduction is 30% or more per pass, the × mark indicates that there is no rolling, and the ● mark indicates that no ridging is visible to the naked eye.
The height of the ridging is also plotted at a point of 1.5μ for convenience in cases where it is impossible to measure because it is less than the height of roughened surface. FIG. 2 is a diagram showing the influence of C on ridging properties, and the symbols are the same as in FIG. 1. Figure 3 is a chart of ridging measurements of No. 3, No. 6, No. 11 and No. 12 in Table 2, where a is No. 3 (method of the present invention) and b is No. 3.
6 (comparison method), (c) is No. 11 (comparison method, SUS430),
(d) shows the measurement chart of No. 12 (comparison method, SUS304).

Claims (1)

[Claims] 1 Ferrite containing C: 0.015% or less, N: 0.015% or more and 0.035% or less, Nb: 0.2% or more and 8 times or more and 1.0% or less of the sum of the weight percentages of C and N, and Cr: 10% or more and 22% or less Stainless steel slabs over 1100℃
Hot-rolled sheet heated at 1250°C or lower, including one or more reductions at 1000°C or higher with a reduction rate of 30% or more and less than 60% per pass, and then hot-rolled at 900°C or higher and less than 1100°C for 10 minutes or more. Annealing and then cold rolling or 1
A method for producing a ferritic stainless thin steel sheet free from ridging and hot roll seizure defects, characterized by cold rolling including intermediate annealing more than once, followed by recrystallization annealing.