JPH02417B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a thin sheet of ferritic stainless steel with excellent workability, and more particularly, to a method for manufacturing a thin sheet of ferritic stainless steel, typified by SUS430 steel containing 0.08 to 0.5% Al. Normal SUS430 type ferritic stainless steel sheet with Al content of 0.08% or less is hot rolled sheet 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 usually at a temperature of 820~850℃, and the r value and ridging properties of the product are It is known that the soaking temperature and soaking time hardly change. In this hot-rolled sheet annealing step, the α' phase present in the hot-rolled sheet changes into a ferrite phase + carbide, and becomes a ferrite phase and a carbide phase after the hot-rolled sheet annealing. If such a material is heated after cold rolling, recovery and recrystallization will occur, and the microstructure after recrystallization will become equiaxed grains, and the grain size will remain constant with almost no effect on soaking temperature or time. Become. The temperature at which recrystallization is completed varies slightly depending on the ingredients, hot rolling conditions, and cold rolling reduction ratio, but if the material temperature reaches 800°C or higher, recrystallization will occur regardless of the soaking time; It was found that almost constant r value and ridging characteristics were obtained regardless of temperature and time. Furthermore, it was found that almost constant mechanical properties such as yield strength, yield point elongation, tensile strength, total elongation, and uniform elongation can be obtained by recrystallization. However, when a hot-rolled sheet of SUS430 type ferritic stainless steel containing a large amount of Al is cold-rolled and recrystallized without hot-rolled sheet annealing, the recrystallization temperature is the same as that of cold-rolled after hot-rolled sheet annealing. The temperature is higher than that in the case where the crystal grains become mixed grains. The r value and ridging properties vary greatly depending on the soaking temperature and time.The annealing conditions that simultaneously satisfy the r value and ridging properties are a soaking temperature of 850 to 900℃ and a soaking time of 60 seconds or less at a high temperature and short time. The present invention was completed after confirming that time annealing was used. Furthermore, even after recrystallization is completed, the mechanical properties change significantly depending on the soaking temperature and time. We found that the higher the temperature in the temperature range of ℃, and the longer the soaking time was 60 seconds or less, the more effective it was.
This completes the present invention. In other words, the gist of the present invention is Al0.08%
When a hot rolled sheet of ferritic stainless steel containing ~0.5% is cold rolled without hot rolling sheet annealing and then finish annealed, 60°C is applied in the temperature range of 850 to 900℃.
The present invention provides a method for producing a thin ferritic stainless steel plate with excellent workability, which is characterized by soaking within seconds. The present invention will be explained in detail below. In the present invention, when a hot-rolled sheet of Al-containing ferritic stainless steel, which is the target steel of the present invention, is cold-rolled without hot-rolled sheet annealing and then finish annealed, the soaking temperature and time of finish annealing are performed. 850
The reason for limiting the temperature to ~900°C and within 60 seconds is as follows. The relationship between the above-mentioned soaking conditions and product r value is that in the soaking temperature range up to 875°C, the longer the temperature increases, the higher the R value becomes.
It deteriorates slowly above 900°C, but at 850°C
This is at a higher level than when the soaking temperature is below ℃. Furthermore, the longer the soaking time is, the more remarkable the improvement in r-value is, but this is limited to cases where the soaking temperature is approximately 875°C or lower; when the soaking temperature exceeds approximately 875°C, an improvement in r-value is seen. The soaking time is up to 10 seconds, and even if the soaking time is increased beyond that, the r value will only improve slightly. The effect of the final annealing cycle on the ridging properties is complex, but when annealing at various temperatures with a constant soaking time, the least ridging occurs at a certain soaking temperature, and when lower than that temperature There is also a tendency that the optimal temperature shifts to the higher temperature side as the soaking time is shorter, and that the optimal temperature range is narrower and the reduction in ridging is large. For example, if the soaking time is long, 60 seconds, the relative value of ridging is relatively large, 850 ~
The value is almost constant in the range of 875℃, and at temperatures above 875℃ or below 850℃, the ridging tends to deteriorate.If the soaking time is as short as 10 seconds, the soaking temperature is 875℃. In a relatively narrow temperature range of ~900℃, the ridging shows a low value, and this absolute value is better than the ridging value obtained by annealing at the optimum temperature when the soaking time is as long as 60 seconds. be. The relationship between the soaking conditions for the steel of the present invention and the mechanical properties of the product is as follows. Figure 1 schematically shows the typical relationship between soaking time, soaking temperature, and yield strength. The comparative product in the figure is regular 430 with an Al content of 0.08% or less.
This material is made by box annealing a hot rolled steel plate and then cold rolling annealing it.
As is clear from the figure, the influence of the soaking time and temperature is small on the comparative product, but in the case of the steel of the present invention, it can be seen that there is an optimal combination of soaking time and temperature in order to obtain a low yield strength. The optimum combination conditions also vary depending on the hot rolling conditions, particularly the slab heating temperature and winding conditions. In other words, when the slab heating temperature is 1150℃ or lower, the lower the temperature, the more the optimal soaking conditions will change to the lower temperature and shorter time compared to materials heated to high temperature slabs of 1200℃ or higher. In addition, the influence of soaking temperature conditions is reduced, and the product exhibits behavior similar to that of comparative products. The effect of winding temperature is not as pronounced as that of slab heating temperature, but increasing the winding temperature
It has the same effect as lowering the slab heating temperature. Although the influence is smaller, the heating rate and cooling rate until the soaking temperature is reached also has an effect, and slow heating and cooling rates have the same effect as lengthening the soaking time. As mentioned above, the optimum combination of temperature and time to reduce yield strength varies depending on the material's previous history, but it is best to heat the slab at a high temperature of 1200℃ or higher, which is considered to require long-term annealing at the highest temperature. In order to obtain a low yield strength of 35 Kgf/mm ² or less when the starting material is a hot-rolled sheet that has been rolled at a low temperature of 600°C or less and is rapidly heated by sold bath heating, etc., at a soaking temperature of 825°C, 60 More than seconds, 850℃~875℃
In this case, soaking time should be 30 seconds to 60 seconds or more, and in case of 900℃ to 925℃, soaking time should be 20 seconds or more. At 950°C, soaking for about 10 seconds will lower the yield point, but if the soaking time is longer than this, the yield point will increase again, which is not preferable. This is due to γ phase precipitation. Figure 2 schematically shows a typical relationship between soaking time, soaking temperature, and elongation at yield point. It can be seen from the figure that the behavior of yield point elongation is almost similar to that of yield point. The effects of hot rolling conditions, temperature increase, and cooling rate also roughly correspond to the effects on yield point. In FIG. 2, when the soaking temperature is high, the elongation at yield point disappears in both the comparative product and the product of the present invention due to γ phase precipitation. In this case, the yield strength increases and the elongation decreases, which is not preferable. For the total elongation, uniform elongation, and tensile strength, the influence of the soaking temperature condition is extremely small for conventional products, but for the steel of the present invention, the influence of the soaking temperature and time is large, and the annealing is performed at a higher temperature for a longer period of time than conventional annealing. This will improve the results. Specifically, considering the previous history of the material, the temperature is 850℃.
As mentioned above, if soaking is performed for a maximum of 60 seconds at a temperature of 900°C or less, good values on the same level as conventional products can be obtained. In the case of a thin plate obtained by cold rolling a SUS430 type ferritic stainless steel hot-rolled plate containing Al without hot-rolled plate annealing, the r value, ridging properties, and mechanical properties are significantly improved even after recrystallization. The reason for the change is thought to be as follows. SUS430 type ferritic stainless steel containing Al has fewer hard phases such as martensite and bainite phases in the as-hot-rolled state compared to regular SUS430 type ferritic stainless steel that does not contain Al. No, it exists in about 10%. This hard phase is elongated in the rolling direction in the subsequent cold rolling process, and decomposed in the final annealing process to form a ferrite phase and carbide, and this ferrite phase also recrystallizes at a temperature higher than the recrystallization temperature. First of all, the reason why the r value improves with high temperature annealing is
This is because the carbides separated from the hard phase and the fine carbides that were already present agglomerate, and these carbides no longer have the ability to hinder the activity of dislocations that are active during plastic deformation, making plastic deformation easier. , it is thought that the r value improves because AlN precipitates during annealing. When hot-rolled sheets are annealed, the size and dispersion of carbides are determined during the hot-rolled sheet annealing process, and solid solution N and nitride Cr
In the final annealing process, the dispersion of these precipitates does not change depending on the annealing method, so once recrystallization is complete, the r value does not change regardless of the annealing conditions. It is something. When the hot-rolled sheet annealing temperature exceeds 875℃, the r value gradually deteriorates because AlN begins to dissolve into solid solution again, and carbides dissolve into solid solution again and become finer again.
This is because the amount of solid solution C and N in the matrix increases. Next, regarding ridging properties, the reason for deterioration when the annealing temperature is low is that in the case of the target steel of the present invention, AlN precipitates during annealing, and the recrystallization completion temperature becomes high, so complete recrystallization does not occur. , this is due to insufficient randomization of the texture. Furthermore, as the temperature rises, the ridging properties tend to deteriorate.
As the fine carbides re-dissolve, the fine crystal grains that existed at the grain boundaries of the old ferrite phase disappear, and the adjacent elongated ferrite phase is often made up of small-angle grain boundaries, so that the grain crystals are substantially reduced. It acts in the same way as when the pores become coarse, and the ridging characteristics deteriorate. Next, the reason why the mechanical properties of the steel of the present invention vary significantly depending on the annealing conditions will be explained. The main metallurgical changes in the final annealing process of the steel of the present invention are recrystallization, separation from α' phase to α+ carbide, and the release of free N accompanying the separation of Cr ₂ N, etc. , precipitation of AlN due to the reaction between the N released during the process and the free N that was in supersaturated solid solution before the main annealing, and the change in solubility of N, solid solution C in the ferrite matrix, and agglomeration or re-solid solution of microcarbides. , precipitation of γ phase (when the annealing temperature is high) and transformation of γ phase into α' phase due to cooling. First, the relationship between changes in yield strength, these metallurgical changes, and changes in annealing conditions will be explained. When the annealing temperature is low (for example, 775°C to 800°C, equivalent to Figure 1 a), the yield point is significantly higher than that of conventional products, and as the annealing time increases, the yield point decreases mainly because This is related to the recrystallization behavior of the steel, and while the conventional steel completely recrystallizes even when annealed at such low temperatures, the steel of the present invention has a high recrystallization temperature, so recrystallization does not occur at low annealing temperatures. This is because the temperature is insufficient, and the reason why it decreases as the soaking time increases is because the recrystallization rate increases. If the annealing temperature is slightly high (e.g. 850℃~875℃)
℃, corresponding to Fig. 1 b) mainly corresponds to the change in solid solution N in the ferrite matrix due to . In the conventional product, the α' phase has disappeared due to long box annealing in the ferrite region of the hot-rolled sheet, and most of the N is fixed in the form of AlN and Cr ₂ N.
The solid solution C and N are almost constant and low values (C, N, equivalent amounts that are almost in equilibrium with the box annealing temperature (≒840℃)), and at this level of annealing temperature condition, Cr ₂ N decomposes slightly. As a result, free N is released, but since the amount thereof is small, the increase in solid solution N is small, and it is thought that the increase in yield strength is proportionally small. In the case of the steel of the present invention, free N increases in the ferrite matrix due to the release of free N due to the decomposition of the α' phase and the release of free N due to the decomposition of Cr ₂ N, and as a result, the amount of free N increases in proportion to the soaking time. yield strength increases. The reason why the yield strength decreases as the soaking time increases further is because the strength decreases due to the transformation from α' phase to α phase, and the free N is fixed as AlN by reacting with Al. The main cause is thought to be a decrease in solid solution N in the ferrite matrix, and it is also thought to be due to a decrease in strength due to agglomeration of fine carbides. If the annealing temperature is higher (e.g. 900℃~925℃)
℃, corresponding to FIG. 1c) mainly corresponds to the change in solid solution N in the ferrite matrix. In this case, if the soaking time is longer than a certain degree, a reverse phenomenon occurs in which the yield strength of the product of the present invention is lower than that of the comparative product. This is the conventional product
The Al content is low, and the free N released by decomposition of Cr ₂ N is less than 0.08% Al.
Since it is not fixed in the form of AlN, the solid solution N in the matrix increases and the yield strength increases.
In the product of the present invention, the N released by the decomposition of the α' phase and the decomposition of Cr ₂ N has a high Al content and is in a temperature range where AlN tends to precipitate, so it is fixed as AlN.
Since the solid solution N in the matrix is lower than that of the conventional product, a reverse phenomenon occurs in which the yield strength is lower than that of the conventional product. In this case, the reason why the yield strength decreases as the soaking time increases is due to the decomposition of the α′ phase.
This is thought to be due to the fact that the longer the soaking time, the more AlN precipitates. When the annealing temperature is extremely high (e.g. 950℃~
At temperatures above 1000°C (corresponding to Figure 1 d), the longer the soaking time, the higher the yield strength of both the conventional product and the product of the present invention. This is because in addition to the above-mentioned →, → is also involved. In other words, even in the temperature range below γ phase precipitation, when heated at high temperature for a long time, the solubility of C and N increases,
C and N are re-dissolved and the cooling rate is fast (more than air cooling)
In this case, the yield strength increases mainly due to an increase in supersaturated C and N in the ferrite matrix and precipitation of a fine carbide phase during the cooling process. Furthermore, at high temperatures, the γ phase precipitates and becomes the α' phase during the cooling process, and the yield strength increases mainly due to the α' phase. Like the yield strength, the yield point elongation also changes mainly based on changes in solid solution N in the ferrite matrix, and can be summarized using the same metallurgy as the yield strength. In other words, the reason why no yield point elongation is observed when the annealing temperature is low is mainly due to the unrecrystallized portion, and the reason why there is no yield point elongation even when the annealing temperature is extremely high is due to the precipitation of the γ phase during annealing. This is due to the formation of the α' phase, and in the intermediate temperature range, the slight change in yield point elongation depending on the annealing conditions is due to the α' phase formed during hot rolling, and the release of free N due to the decomposition of Cr ₂ N. This is determined by the relationship between the increase in free N and the decrease in free N due to the precipitation of AlN. Changes in tensile strength, total elongation, and uniform elongation are mainly due to the decomposition of the α′ phase → ferrite + carbide.The longer the temperature and the longer the time, the more the ferrite matrix is cleaned and the carbides aggregate and coarsen, resulting in an increase in elongation. , the tensile strength will decrease. However, it goes without saying that if the annealing temperature is too high, the elongation decreases and the tensile strength increases due to the formation of the α' phase based on the reprecipitation of the γ phase. As is obvious from the above explanation of metallurgy, the lower the slab heating temperature is, the more AlN is already precipitated during slab heating, and the lower the slab heating temperature is about 1150°C or lower, the more AlN is present during slab heating. As the γ phase decreases, the α' phase decreases in the as-hot-rolled state, and a hot-rolled sheet hot-rolled in such a state has several 10% of the total N in the form of AlN in the as-hot-rolled state. The amount of N to be fixed in the final annealing process is small, and it is easy to transform the α' phase into ferrite + carbide, so it has better mechanical properties compared to high-temperature slab heating materials. In order to obtain this, the soaking temperature required is lower and the soaking time is shorter.
The longer the time from extraction after heating the slab until hot rolling and winding, and the higher the winding temperature, the more precipitation of AlN will be promoted and the α' phase will also be reduced, so that good mechanical properties can be obtained. The required soaking temperature is lower and the soaking time is shorter. If the rough rolled piece is heat-retained in a coil box etc. after the completion of rough rolling and before the start of finish rolling, or if so-called delay rolling is performed, such as leaving it for 60 seconds or more without active heating, γ will be reduced in this process. It goes without saying that the annealing conditions in the final annealing step can be relaxed because the →α transformation is promoted and the precipitation of AlN also progresses. Although it is not as noticeable as in the hot rolling condition, the slower the temperature increase and cooling rate during final annealing, especially the speed from 800℃ or higher to the soaking temperature, the lower the soaking temperature and the better the machine even if the soaking time is short. It becomes possible to obtain the following properties. The reason why the Al content of the target steel of the present invention is set to 0.08% or more is that if it is less than 0.08%, the mechanical properties of the product, especially the yield point, will increase regardless of the annealing temperature and time selected. The reason for limiting the content to 0.5% or less is that the r value is low even after heat treatment under these conditions, and surface defects called sparkling scratches are likely to occur.The reason for limiting the content to 0.5% or less is that even if the content is higher than this, the actual effect will not change. First, it is not economical. As mentioned above, the mechanical properties of the product, especially the yield strength, elongation at yield point, total elongation, uniform elongation, etc., also change depending on the hot rolling conditions, especially the slab heating temperature and winding temperature conditions. However, even under the most severe hot rolling conditions (high temperature slab heating, low temperature rolling), it takes about 60 seconds at 850℃ and about 20 seconds at 900℃.
At 950℃, good mechanical properties can be obtained by soaking for less than 10 seconds. When the hot rolling conditions are severe like this, the heat treatment conditions that provide the best mechanical properties are shifted toward higher temperatures and longer times than the heat treatment conditions that provide the best r value and ridging.
If we focus only on mechanical properties, it is sufficient to anneal for a short time even if the soaking temperature is 950°C, but in this case, it is not preferable for the r value and ridging. With this invention, the soaking temperature can be adjusted
The reason why we limited the temperature to 900℃ or less was with this in mind, and the reason why we limited it to 850℃ or higher was because of r-value, ridging,
Three points of mechanical properties are taken into consideration. The reason why we set the soaking time to within 60 seconds is because mechanical properties can be satisfied if the soaking temperature is selected appropriately depending on the material's previous history.
As mentioned above, this is because even if the soaking time is 60 seconds or more, there is little effect on the r value and ridging, and it is not economical. As explained above, SUS430 steel containing 0.08% or more and 0.5% or less of Al can be cold rolled to the final thickness in one round without performing the usual hot rolled sheet annealing, and then subjected to finish annealing. ,850~900
By performing soaking within 60 seconds in the temperature range of °C, it is possible to provide good workability, that is, good r value, ridging, and mechanical properties. The present invention will be specifically described below with reference to Examples. Example 1 Containing Al with a thickness of 2.70 mm shown in Table 1
SUS430 type ferritic stainless steel hot rolled plate (A)
The thickness of the hot-rolled sheet is 1.0mm and 0.4mm without annealing.
It was made into a cold-rolled sheet. These cold rolled sheets were heat treated in a salt bath, and after the heat treatment the r value and ridging were measured. For comparison, hot rolled sheet (B) of SUS430 type ferritic stainless steel containing a small amount of Al is as follows.
After hot-rolled sheets were annealed at 840°C for 4 hours, cold-rolled sheets with thicknesses of 1.0 mm and 0.4 mm were subjected to the same heat treatment, and the r value and ridging height were measured. Riding height and r
The measurement results are shown in Table 2, and it can be seen that when the target steel of the present invention was heat treated in the temperature range of 850 to 900°C for 60 seconds or less, both the r value and ridging properties were good.

【table】

【table】

[Table] Example 2 SUS430 with a thickness of 200 mm and the chemical components shown in Table 3
A molded ferrite stainless steel slab was heated at a temperature of 1220℃ for 2 hours and then roughly rolled in 6 passes to a thickness of 20mm.
Subsequently, it was made into a 2.3 mm hot-rolled coil by 6 passes. 2.3mm
The temperature at the time of finishing was 900°C, and immediately after finishing hot rolling, it was rapidly cooled and rolled at a low temperature of 600°C. For the conventional low Al material, the hot-rolled coil manufactured in this way was box annealed at 840°C for 4 hours.
It was cold rolled to 0.4mm by 2 times cold rolling. The steel of the present invention containing Al was cold-rolled to 0.4 mm in one cold rolling without hot-rolled plate annealing. Next, these cold-rolled sheets were heated at each temperature between 775℃ and 1000℃ for 0 soaking time.
The heating time was varied from seconds to a maximum of 2 minutes, and then the temperature was air cooled.
Table 3 shows the mechanical properties of the thus heat-treated materials. As is clear from Table 4, the mechanical properties of the conventional material show good mechanical properties under any heat treatment conditions, but in the case of the steel of the present invention, even when hot-rolled sheets are not annealed, the patent claims of the present invention As shown in the above range, it can be seen that by applying a soaking time of 30 seconds to 60 seconds at a temperature of 850°C or higher and 900°C, it is possible to provide good mechanical properties equivalent to or better than conventional products.

【table】

[Table] Example 3 A SUS430 type ferritic stainless steel slab (thickness 250 mm) containing Al shown in Table 5 was heated at a temperature of 1250° C. for 2 hours, and then rough rolled in 7 passes to a thickness of 20 mm. The rough rolling completion temperature was 1150°C.
Then, finish rolling was performed in 6 passes to obtain a hot rolled sheet with a thickness of 3.0 mm. The temperature at the end of finish rolling was 920°C.
The hot-rolled sheet rolled in this way is immediately quenched to 633°C.
After rolling at a temperature of Heat treatment was performed under heat treatment conditions commonly used for steel, and mechanical properties were measured and are shown in Table 6. As shown in Table 6, by treating the steel of the present invention under the heat treatment conditions specified in the present invention, good mechanical properties can be obtained. Table 5 Main chemical components of sample materials (wt%) C Si Mn Cr Al Fe 0.047 0.15 0.15 16.8 0.14 Remaining

【table】

【table】 [Brief explanation of drawings]

Figure 1 is a schematic diagram showing the relationship between yield strength, soaking time, and temperature of the steel of the present invention, and Figure 2 is a schematic diagram showing the relationship between yield point elongation, soaking time, and temperature of the steel of the present invention. It is.

Claims (1)

[Claims] 1 When finishing annealing a hot-rolled ferritic stainless steel sheet containing 0.08% to 0.5% Al without hot-rolling the hot-rolled sheet annealing, the hot-rolled sheet must be equalized within 60 seconds in a temperature range of 850-900℃. A method for producing ferrite-based stainless steel sheets with excellent workability, which involves heating.