JPH02419B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a ferritic stainless steel thin plate with no surface flaws and excellent workability by omitting a hot-rolled plate annealing process. (Prior art) Ferritic stainless steel thin sheets are generally produced by hot-rolled sheet annealing, cold rolling once or twice with intermediate annealing in between, and then finish annealing. It is. If the above-mentioned hot-rolled sheet annealing is not performed, surface defects will occur in the final product.The yield strength will be high, the elongation at the yield point will be large, and the elongation will be small. The r value is small and the ridging is large. Such defects are more likely to occur. As a result of research into a manufacturing method for ferritic stainless steel thin sheets that can prevent these defects without hot-rolled sheet annealing, the inventor found that 0.08%~
After cold rolling a hot-rolled ferritic stainless steel plate containing 0.5% Al at a rolling reduction of 40% or more,
After heating to a temperature of 700℃~1000℃ for less than 10 minutes,
Furthermore, we have discovered a method of cold rolling at a rolling ratio (reduction ratio) of 40% or more, followed by recrystallization annealing. (Structure of the Invention) The present invention further improves and develops this technology, and its gist is that in the hot rolling process, the winding temperature of the hot rolled sheet is set to 750°C or higher, and the intermediate annealing conditions are set to 700°C or higher. It is characterized by heating within 10 minutes at a temperature of ℃ to 1050℃. The metallurgical reason why such high-temperature winding results in no surface flaws, high workability, ie, high r-value, and reduced ridging, will be explained in detail below. In the manufacturing process of cold rolling a thin sheet once without annealing a hot rolled sheet, when the ferritic stainless steel of the present invention contains 0.08% to 0.5% Al, the winding temperature is high. Although the r value improves as the value increases, it has the disadvantage that ridging deteriorates. However, in a manufacturing process in which hot-rolled sheets are not annealed but are immediately cold-rolled, then intermediate annealed, and then cold-rolled annealed, the higher the winding temperature, the higher the r value and the better the ridging. The reason is,
In high-temperature rolling, a {111}<112> texture, which is advantageous for deep drawability, is significantly developed after intermediate annealing than in low-temperature rolling, and such a texture is used as a starting material for cold rolling annealing. In this case, the development of {111}<110> texture, which is advantageous for deep drawability, becomes noticeable in the final product. In the case of low-temperature rolling, the {111} <112> texture, which is advantageous for deep drawability, is not sufficiently developed after intermediate annealing, resulting in a relatively random texture, and it is difficult to use this texture as a starting material. When cold-rolling and annealing, compared to high-temperature rolling, the {111} <110> texture, which is advantageous for deep drawability, is insufficiently developed, resulting in poor deep drawability. However, in the case of the single cold rolling method, the {111}-based texture, which is advantageous for deep drawability even after high-temperature rolling, is {111}<
112> system is the main orientation, but in the two-time cold rolling method according to the present invention, the {111} system texture, which is advantageous for deep drawability even in low temperature rolling, is {111}
112> and {111}<110> are the main directions, and the absolute value of the r value is larger than that in the hot rolling single-step cold rolling method. In the one-time cold-rolling method that omits hot-rolled sheet annealing, the ridging deteriorates due to high-temperature winding, whereas in the two-time cold-rolling method of the present invention, the reason why the ridging improves during high-temperature winding is as follows. This is due to the fact that the {100}<110> texture does not develop in the final annealing process compared to low-temperature rolling. In order to preferentially develop the {111}-based texture in recrystallization annealing, it is necessary that the grain boundary area is large and the matrix is cleaned before cold rolling. In the case of high-temperature rolling in the one-step cold rolling process that omits hot-rolled sheet annealing, the {111}-based recrystallized texture develops because there are fewer hard phases and the matrix is cleaned compared to low-temperature rolling. However, in the as-hot-rolled state, the grain boundary area is smaller than that after one cold-rolling annealing, and the grain boundary area is smaller in the as-hot-rolled state, and the {100}<110> system is one of the main orientations of the hot-rolled texture. Even during the first recrystallization annealing, the texture of The reduction in texture is relatively small. In the case of low-temperature rolling, there are many so-called hard phases in the as-hot-rolled state, and during the first recrystallization annealing, {111}<
Although the texture of the 112> system is insufficiently developed, microscopic shear deformation and shear strain occur around the hard phase, and in the first recrystallization, the texture of the {110} system develops,
The texture of the {100}<110> system is relatively reduced. In the intermediate annealing process, the so-called hard phase remaining in the as-hot-rolled state is transformed into α+ carbide, and the N released in this process is fixed in the form of AlN. These changes occur more markedly in high-temperature rolled material with less hard phase Cr ₂ N. Therefore, the recrystallized structure after one cold rolling in high-temperature rolling has more {100}-based texture than in low-temperature rolling, but the matrix is cleaner and there is less solid solution N. The grain boundary area is significantly increased compared to the as-hot-rolled state. When cold rolling recrystallization is performed using such a recrystallized texture as a starting material, many textures with {111} planes parallel to the rolling surface develop from the grain boundaries, making it difficult to recrystallize the {100} system. direction will be eaten by these directions, resulting in {100}
High-temperature winding also improves ridging because the texture of the system is reduced. Compared to high-temperature winding, low-temperature winding has less development of the {100}-based texture that deteriorates ridging after intermediate annealing, but it also has less {111}-based texture, more solid solution N, and a higher concentration of matrix. Because the cleaning is insufficient, even after the final annealing, the development of the {111}-based texture is insufficient compared to high-temperature rolling, and the amount of decrease in the {100}-based texture is relatively small. Therefore, the improvement in ridging will be small. For the above reasons, the winding temperature is set to 750°C or higher. As is clear from the above explanation of metallurgy, it is preferable that the winding temperature is higher than 750℃.
If the normal slab heating temperature is, for example, 1200°C, the finish rolling end temperature is about 900°C.
Even if a close coiler is installed, the upper limit is considered to be around 900℃. Even when wound at a temperature of 750°C, the innermost and outermost turns of the coil, which are equivalent to several tens of meters in length, tend to have slightly inferior r-value and ridging compared to the central part of the coil because the temperature drops rapidly. To prevent this, it is desirable to take measures to prevent heat dissipation, such as covering the coil with a heat insulating material immediately after winding. Therefore, in order to prevent heat loss from the winding machine, it is particularly important to take measures such as using a winding machine with a heat insulating sleeve or heating the sleeve. In the present invention, the reason why the first or second cold rolling ratio is set to 40% or more is to sufficiently develop the {111} texture in the recrystallized structure, and if it is less than this, the {111} texture will not develop. However, the higher the cold rolling rate, the better, but the maximum thickness of the hot rolled sheet is 6 m/m, and the minimum thickness of the final product is about 0.3 m/m.
m is normal, and the upper limit is determined automatically. The reason why the intermediate annealing temperature was set at 700°C to 1050°C for less than 10 minutes is that this annealing sufficiently promotes recrystallization, decomposes the hard phase into α+ carbide, and decomposes Cr ₂ N. The aim is to precipitate AlN through the process and reduce the amount of solid solute N. The reason for limiting the temperature to 700°C or higher is that at temperatures below this, recrystallization is insufficient and AlN precipitation is also insufficient.
This is because the mechanical properties of the product, especially the yield strength, are high, the elongation is low, and the r value and ridging are also degraded. This is because the r value of . The longer the annealing time, the more AlN precipitates, the more recrystallization progresses, and the more purified the matrix becomes, improving the mechanical properties, r-value, and ridging of the product. The heating time was limited to 10 minutes or less. In the conventional manufacturing process of hot-rolled plate annealing followed by cold-rolling annealing, the lower the slab heating temperature and the lower the finish rolling temperature, the better the r value and ridging of the final product, but such low-temperature slab heating, When low-temperature finish rolling is performed, there is a strong tendency for surface flaws, so-called scale flaws, to occur during the hot rolling process. However, in a process without hot-rolled plate annealing as in the present invention, there is no need to promote recrystallization in the hot-rolled plate annealing process by low-temperature slab heating and low-temperature finishing hot rolling to improve r value and ridging. High-temperature slab heating and high-temperature finishing hot rolling are possible, and a hot-rolled sheet with a good surface can be obtained without the occurrence of scale defects during hot rolling. When pickling without annealing a hot-rolled sheet, the higher the slab heating temperature and the lower the rolling temperature, the transformation of the prior γ phase called the α′ phase or “hard phase” occurs in the as-hot-rolled state. There are many phases, and when these phases are pickled using an acid solution mainly composed of nitric acid, there is a high probability that these parts will corrode abnormally and surface defects called so-called glitter defects will occur in the final product. However, if high-temperature winding is performed, even if an acid solution mainly composed of nitric acid is used, the occurrence of such abnormal corrosion will be reduced, and the final product will be free from surface defects. The reason for setting the Al content to 0.08% or more is that if the Al content is less than this, precipitation of AlN will be insufficient in any process and the yield point of the final product will be high, which is undesirable, and there is little room for improvement in the r value. This is because surface flaws called glitter flaws are more likely to appear. The upper limit of Al was set at 0.5% because even if added in excess of this, the effect would be saturated and it would be uneconomical. The present invention will be specifically described below with reference to Examples. Example 1 A continuously cast slab with a thickness of 200 mm having the chemical components shown in Table 1 was heated to a temperature of 1200°C, and then hot-rolled to a thickness of 4.0 mm using a hot rolling mill consisting of a rough rolling mill and a continuous rolling mill. The rolled plate was rolled up at a temperature of 760°C, immediately covered with a heat insulating material after being rolled up, and after 1 hour the heat insulating material was removed and cooled in air. For comparison, it was rapidly cooled to 620℃ immediately after hot rolling.
A coil was also created that was wound at a temperature of . These hot rolled coils are 2.5mm thick without hot rolled plate annealing.
(cold rolling ratio: 37.5%), 2.0mm (cold rolling ratio: 50%), 1.0mm (cold rolling ratio: 75%), intermediate annealing at 840°C for 2 minutes, and cold rolling to 0.4mm. Final annealing was performed at ℃ for 2 minutes. The measurement results for the r value and ridging of the material thus produced are shown in Table 2, and it can be seen that both the r value and ridging are better when the material is rolled at a high temperature according to the present invention, compared to when it is rolled at a low temperature. .

【table】

[Table] Example 2 A continuously cast slab with a thickness of 200 mm having the chemical components shown in Table 3 was heated to a temperature of 1250°C, and then hot rolled in a hot rolling mill consisting of a rough rolling mill and a continuous rolling mill to reduce the thickness. A hot-rolled sheet of 4.0 mm was formed and rolled at a temperature of 800°C. Immediately after winding, it was covered with a heat insulating cover. After 1 hour, the heat insulating cover was removed, and the coil was immersed in a water tank to cool. For comparison, we also created a coil that was cooled immediately after hot rolling, rolled up at 650°C, and cooled in the air. These hot-rolled coils were pickled without annealing the hot-rolled sheets and cooled to a thickness of 2.0 mm (reduction ratio of 50%). Then, intermediate annealing was performed under various heat treatment conditions shown in Table 4, and after cold rolling to two thicknesses of 0.7 mm and 0.4 mm, final annealing was performed at 840° C. for 2 minutes. It can be seen that the material rolled at a high temperature according to the method of the present invention has better r value and ridging than the material rolled at a low temperature, and the influence of intermediate annealing conditions is relatively small.

【table】

【table】

[Table] As described above, the present invention has a significantly higher r value than a thin sheet produced by conventional hot-rolled sheet annealing and one cold-rolling annealing under high-temperature slab heating and high-temperature rolling conditions that do not cause surface defects.
A thin plate with good ridging, that is, a thin plate with good r value and ridging equivalent to or better than a thin plate produced by hot-rolled plate annealing and then cold-rolling annealing twice, is cold-rolled twice without hot-rolled plate annealing. This technology provides manufacturing technology using an annealing process, eliminating the need for hot-rolled plate annealing equipment.
A great invention is that it is economically profitable.

Claims (1)

[Claims] 1 A hot-rolled ferritic stainless steel sheet containing Al: 0.08% to 0.5% is processed without annealing the hot-rolled sheet.
After cold rolling with a rolling reduction of 40% or more, 700℃ ~
After heating at a temperature of 1050 ° C for no more than 10 minutes, an additional 40
% or more, followed by recrystallization annealing to form a thin sheet, in which the hot-rolled sheet is rolled up at a temperature of 760°C or more in the hot rolling process, without surface defects. , a method for manufacturing thin ferritic stainless steel sheets with excellent workability.