JPS6030728B2 - Manufacturing method of ferritic stainless steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to continuous and process-saving methods for manufacturing ferritic stainless steel sheets.

Conventionally, the manufacturing method for ferritic stainless steel sheets (SUS430) containing 16 to 18 chromium involves a long process of heating a hot-rolled sheet to 800 to 85 pC in a box Akatsuki anchor furnace, holding it for at least 2 hours, and then slowly cooling it to room temperature. After time solidifying and primary cold rolling to an intermediate thickness, intermediate dulling, secondary cold rolling to product thickness, and final dulling (this process is called R) to obtain the final product. ing. There are two main points in the manufacturing process of the above-mentioned conventional manufacturing method. The first point is the hot-rolled plate box dowel. In other words, unlike ordinary steel, the hot-rolled structure of ferritic stainless steel sheets is similar to that of cold-rolled steel, and if the cold-rolling process is started without competitive hammering, the material quality of the final product will deteriorate significantly, so it can be omitted. In order to obtain a good final product material, it was necessary to maintain the temperature at 800 to 85 mm for more than 2 hours. Phosphorus annealing at high temperatures exceeding 85 mm causes an austenite transformation phase, leading to material deterioration, and furthermore, if the cooling rate is too slow, intergranular corrosion occurs during pickling after hammering, significantly impairing the surface quality of the final product. Therefore, it is necessary to cool the furnace.

For these reasons, competitive anchors made of ferritic stainless steel hot-rolled steel sheets had no choice but to use box brick anchors. The second point is to perform the cold rolling process using XR. In other words, when a final product manufactured by a process that omits primary cold rolling (this process is called ICR) is deep drawn by pressing, severe ridging occurs and the r-value, which is a controlling factor for deep drawability, is generated. ICR of the R process has not been put to practical use because of problems such as deterioration of the steel and extremely poor deep drawing workability. The purpose of the present invention is to provide a method for obtaining a final product of a hot-rolled ferritic stainless steel plate using an ICR process that is equivalent to or better than the conventional box racing anchor-XR process after being subjected to a Rentsugi sintering process. be. That is, the gist of the present invention is as follows.

[1- After heating a ferritic stainless steel slab containing more than 0.10% to 0.25% peaks to a temperature of 1050 to 125 mm, hot rolling it into a steel strip, and converting this hot rolled steel strip into a 900% After heating to a temperature range of 0 to 1,050 q0 and holding for a short time of 20 minutes or less, continuous racing is applied to rapidly cool it down to room temperature, and then cold rolled to the final product thickness without intermediate annealing. A method for producing a ferritic stainless steel sheet, which comprises the steps of: recrystallization and dulling. '21 NO. A ferritic stainless steel slab containing more than 10% to 0.25% is heated to a temperature of 1050 to 125 degrees Celsius, then hot rolled into a steel strip, and this hot rolled steel strip is heated to a temperature of 900 degrees Celsius or more and less than 1050 degrees Celsius. After heating to a temperature range of 20 minutes or less and holding it for a short time of 20 minutes or less, in the cooling process to room temperature, a Rentsugi ware process is performed in which the temperature castle is 850 qo or less at a cooling rate of 50 pm C/sec or more, and then, A method for manufacturing a ferritic stainless steel sheet, characterized by rolling the plate to the final product thickness without intermediate annealing, and then recrystallization annealing.

The first problem in continuous hammering of rolled ferritic stainless steel sheets is, as mentioned earlier, intergranular corrosion during pickling after annealing the hot rolled sheets. In other words, in order to perform Akatsuki Tsuchi continuously, it is necessary to finish Akatsuki Tsuchi in a short period of time, but short-time slowing requires a heating temperature of 800 to 850°C and a holding time of 2 hours or more, which are the conventional box moth dulling conditions. In order to give the same thermal history as that, it is necessary to engrave more than 85 bi○. Moreover, when ordinary ferritic stainless steel sheets are heated to a temperature exceeding 850°C and then air cooled, the carbon that dissolves during heating and concentrates at the grain boundaries combines with Cr during the air cooling process, precipitating chromium carbide at the grain boundaries and forming grains. Since Cr near the boundaries is consumed, the Cr content near the grain boundaries decreases, and as a result, the grain boundaries are selectively dissolved during pickling after anchor sintering, resulting in intergranular corrosion. This has been one of the reasons why it has not been possible to achieve continuous production of ferritic stainless steel thermoformed steel plates in the past. The inventors of the present invention have investigated various methods for preventing intergranular corrosion, and have found that AI is extremely effective against intergranular corrosion that occurs during annealing of hot-rolled ferritic stainless steel sheets. Figure 1 shows the results.
The presence or absence of intergranular corrosion due to pickling after air cooling was investigated on ferritic stainless steel sheets with various amounts of N added.

From FIG. 1, when the amount of N added is 0.07 wt% or more, the dependence of intergranular corrosion on the dawn anchor temperature is improved, and especially when the amount of nail added is increased to 0.07 wt% or more, the dependence of intergranular corrosion on the dawn anchor temperature is improved.
If it exceeds 10%, the dawn anchor temperature will be 900℃ or higher and 1050qo.
It is possible to increase it to below. On the other hand, the ridging height of the final product manufactured by cold rolling twice from 800 to 850 qo steel material, which is a normal box racing cycle, is 2.
It is 5 to 3 tsubo. In order to obtain an IJ jing height equivalent to that of a product manufactured by this secondary method, a hot-rolled plate competitive anchor temperature of 90,000 is sufficient, as is clear from the results of the example shown in FIG. The shaded area in the figure is the level according to the conventional method. It is clear as shown in FIG. 6 or as will be described later that if the temperature of the hot-rolled sheet hammer is further increased, a product with even better ridging properties can be manufactured by the ICR process. However, when the hot-rolled sheet annealing temperature is set to 1050 oo or higher, the IJ jingability becomes extremely good, but the Akatsuki Anchor at this temperature range is the first. As shown in the figure, even if the amount of nail added is increased, intergranular corrosion will occur and the surface properties of the final product will be damaged.
It must be less than 50 qo. Next, the annealing and rolling processes of hot-rolled sheets are extremely important to ensure the quality of the final product.

In other words, in order to ensure the required ridging properties and r-values for the final cold-rolled hammered products used for deep drawing of utensils, etc., the following steps are usually taken: ■ Thoroughly perform hot-rolling and ■ Cold-rolling process is carried out in two steps.
It is produced by two processes: double cold rolling and double Akatsuki Anchor (next R). In other words, the hot-rolled sheet sintered anchor is insufficient, resulting in poor rolling properties, and if the cold rolling process is made once cold rolling and once sintered, both the IJ jingability and r value will decrease, so hot rolling is difficult from the material standpoint. It was difficult to make the board into Akatsuki and ICR. The present inventors separately considered the influence of manufacturing conditions on the ridging property and r value and clarified the following.

That is, the ridging property is greatly influenced by the hot-rolled sheet sintering process and the cold rolling process. If the branding temperature of the hot-rolled sheet is further raised above 850 oo, the ridging property will be significantly improved, and it will be possible to produce a final product that is fully satisfactory in terms of material quality even through the ICR process. However, due to the structure of the furnace, sluggishing at a temperature higher than 850 qo would cause severe damage to the furnace, making it impractical. Furthermore, in order to overcome this drawback of the Akatsuki Anchor, a method is known in which the Akatsuki Anchor temperature is raised to 100,000 ℃ using a continuous mound blunt furnace and the Akatsuki Hammer is completed in a short time, but the transformation point of the conventional component system is low and this At such high temperatures, 7 phases are formed, which transforms into martenzite during the cooling process, and if left heated to 1000°C, stretch cracks will occur during cold rolling in the next process. However, these methods cannot be put to practical use. The present invention aims to obtain the effect of improving ridging property by high-temperature annealing, and to suppress seven phases generated during high-temperature annealing.

That is, more than 0.10% in ferritic stainless steel
By adding 0.25% AI, the transformation point is raised, and the holding time at a high temperature of 900 qo or more and less than 105,000 qo during burning is as short as 20 minutes or less, and the 7-phase is cooled before the 7-phase is formed. This suppresses the formation of For this reason, although it is a high-temperature dawn anchor of 900 qo or more and less than 1050 qo, it is possible to obtain a cold rollability similar to that of conventional box annealing, and to obtain a material with good formability and ridging property. This makes it possible to manufacture final products with excellent ridging properties. Next, we investigated the influence of manufacturing conditions on the r value of the final product, which is a moldability parameter.

As a result, the r value was increased from the solid solute N before cold rolling ([total N-Na in the steel sheet
sNN] (tentatively referred to as solid solution N) was found to be greatly influenced by the amount.

This relationship is shown in FIG. Figure 2 shows solid solution N and I before cold rolling.
This figure shows the r value of the CR material, and the r value increases as solid solution N decreases. In the conventional manufacturing method using the box annealing primary R process, the solid solution N before the secondary cold rolling is such that the box annealing temperature is 800 to 850 pm, which promotes the precipitation of nitrogen, and the cooling rate in box annealing is slow. However, when hot-rolled sheet annealing is continued, the high-temperature annealing associated with the short-time branding described above causes the steel sheet to deteriorate. Furthermore, since the cooling rate of the Rentsugi sintered anchor is slow and there is almost no precipitation of N during the cooling process, the solid solute N before cold rolling increases. If the cold rolling process is the main R process, most of the solid solute N can be precipitated during intermediate rolling, reducing the solute N before the secondary cold rolling and increasing the r value. However, in the ICR process, since there is no process to reduce solid solution N before cold rolling, the r value becomes low and the material is unsatisfactory. The present invention obtains a sufficiently high r value through the Rentsugi annealing-ICR process, and the solid solution N in a normal hot-rolled plate is almost the same as the equilibrium value in the heating furnace. This is based on the discovery that precipitation of hardened N does not occur during the hot rolling process from when the sheet is rolled out to when it is turned into a hot-rolled sheet.

Therefore, in order to reduce the field twill N in the hot rolled sheet, it is necessary to sufficiently lower the equilibrium value of solid solution N in the heating furnace before hot rolling. For this purpose, AI addition and heating temperature control are extremely important. It is generally known that the equilibrium precipitation amount of N and AIN (solid solution N) in N-added steel is a function of the amount of AI added, N in the steel, and temperature. The relational expression between the three parties has not yet been determined. As a result of repeated experiments, the present inventors found that the following so-called Dark
We found that the formula for en was in relatively good agreement with experimental data. [N] [N] = exp (1.95-7400/T
) [N]: N in steel (M%) [N]: AI in steel (wt%) T: Temperature (K) Furthermore, the r of ordinary commercially available ferritic stainless steel sheet
It is clear that the value is 1.0 to 1.5, and from Figure 2 that even if the solid solute N before cold rolling is less than 8, the r value is 1.0 or more. , it can be seen that it is sufficient to suppress the solid solution N in the heating furnace to less than 8 degrees.

Figure 3A shows the equilibrium value calculated at each heating temperature for Okayuri N within the heating sequence using the Arken equation described above.

From these figures, in order to suppress the solid solution N in the heating furnace to 80 feet, it is possible to reduce the solid solution N to 8 cm or less even when heated to 1300°C by increasing the amount of AI added. AI
Considering the cost increase due to addition, the amount of AI added should be 0.
The limit is 25%. For this reason, it is industrially advantageous to set the heating temperature to 125°C or less. Also, from the above point of view, a lower heating temperature is preferable in order to suppress solid solution N, but if the heating temperature is lowered too much, the load during hot rolling will increase rapidly, and the surface properties of the hot rolled sheet will deteriorate. It is industrially desirable to set the lower limit of the heating temperature to 105 pi because of the possibility of extreme deterioration. FIG. 4 shows an example of hard lacquer N in a hot rolled sheet produced by balancing the amount of AI added and the heating conditions. Figure 4 and Figure 3 A,
As can be inferred, if the heating temperature is 1250 qo or less, the intermolten N in the hot rolled sheet is sufficiently reduced, and the r value of the ICR material is 1. It is possible to irrigate the area. In addition, by setting the upper limit of the continuous competitive annealing temperature of the hot rolled sheet to be lower than the heating temperature in the heating furnace, the re-solid solution of AIN does not occur, but rather a temperature range is created in which precipitation of NN is promoted. The above consideration of the fixation of solid solution N by the addition of AI and the observation that the cooling time during the Rentsugi sintered anchor process allows for natural cooling. If left to cool, 500q as shown in Figure 5
The cooling rate at a temperature below 0.0°C decreases rapidly, and it takes 10 minutes or more to reach room temperature. Although this time is shorter than the cooling time for conventional box scorching anchors, it is an extremely long time for continuous scorching equipment, and it takes quite a long time to cool down naturally to room temperature. The zone must be re-established within the continuous broiling equipment. During the cooling process of the continuous anchoring process, the temperature of the steel plate was 100,000, 900℃, 700℃, 60℃.
At the time when the steel sheets reached 500 qo, the steel sheets were rapidly cooled in water, and the final cold-rolled steel sheets were obtained through an ICR process, and the r value was measured. The results are shown in FIG. From Figure 8 1
Even when cooled in water from 000qo and 900°C, an r value of 1.0 or more can be maintained and a material no different from conventional materials can be obtained.
By setting the underwater cooling start temperature to 850 oo or less, r
The value becomes 1.2 or more, making it possible to obtain a more excellent r value. As mentioned above, in the cooling process of the Rentsugi sintered anchor process, the
If the material is rapidly cooled from a temperature range of 50 qo or less to room temperature, it is possible to obtain an extremely excellent material, and it is also possible to make equipment extremely advantageous industrially, such as shortening the cooling time and shortening the length of the cooling equipment.

Here, the cooling rate of 85 qo/sec or less may be higher than natural cooling in the atmosphere, but is preferably 50 qo/sec or higher. The present invention will be explained below with reference to Examples. Example A slab having the components shown in Table 1 was heated to 1200°C to a thickness of 3. Manufactured hot-rolled plates for the deck.

Table 1 (wt turtle) 90piC to 1050℃ for all steel types
After being kept at a temperature of 100°C, it was air-cooled to 800°C, and then subjected to continuous phosphorous annealing by cooling at a cooling rate of 50°C/sec or more, and then a 0.5 side cold-rolled chick plate was obtained by an ICR process. .

In addition, the continuous knee obtuse curve at this time is shown in FIG. In this process, the presence or absence of intergranular corrosion during pickling after Rentsugi sintered anchor was observed using a scanning electron microscope, and the results are shown in Table 2.
Table 2 Note: 0 means intergranular corrosion exists, It is shown together with the amount of solid solution N in the rolled sheet.

As shown in Table 2, grain boundary corrosion during pickling after Rentsugi sintered anchor shows satisfactory values for both the comparative example and the present invention when the Akimei temperature is 850 qo, but when the Akimei temperature is 900°C or higher, Comparative example (
Steel numbers A and B) suffer from severe intergranular corrosion.

However, in the steel numbers C to F of the present invention, as the AI content increases, intergranular corrosion does not occur even when subjected to high temperature dawn anchoring of 900 qo or more, and among them, steel numbers D, E, and F do not cause intergranular corrosion even when subjected to high temperature dawn anchoring of 1000 qo or more. Intergranular corrosion also showed satisfactory values. However, if the sintered anchor temperature is 1050 qo, intergranular corrosion will occur, so the sintered anchor temperature should be less than 1050 qo from the viewpoint of intergranular corrosion. Figure 6 shows the relationship between the dawn chick temperature and the IJ ring height according to the present invention, and from this figure, the competitive anchor temperature can be set to 90
If it is 0:0 or more, the sliding height is the same as the standard according to the conventional method (
(shaded range) or less.

Moreover, FIG. 7 shows the relationship between dawn anchor temperature and r value.

From this figure, the steel numbers C to F of the present invention have a sintering anchor temperature of 1000℃.
It can be seen that the r value is 1.0 below, and the r value deteriorates rapidly when the phosphorous temperature becomes 1050 qo. Next, by controlling the cooling rate during competitive cooling in the present invention within a specific range, the r value can be increased to 1. It can be stably secured on the premises.

This was confirmed for Steel No. D and E by examining the effect of cooling conditions after continuous dawn anchoring on the ICR material r value. Steel number D, E plate thickness 3
．． After holding the hot-rolled sheet on the 0 side at 100,000 for 2 hours, the temperature of the steel sheet decreased to 1,000 pm and 0,900 q○ during the natural cooling process.
, 800 q 0, 700 q 0, 600 pm, and 50 pm, respectively, after cooling in water, a 0.5 side cold rolled sintered anchor plate was obtained by an ICR process.

From the start of water cooling at this time, the temperature of the steel plate becomes the water temperature (15℃)
It took about 2 seconds to reach this point. The r-value of the cold-rolled annealed sheet is plotted on the horizontal axis with the water-cooling start temperature shown in the eighth chart.

As is clear from FIG. 8, if the water cooling start temperature is 850 qo or less, the r value is 1. It is clear that it is possible to secure the above a. The cooling rate at this time is 50 seconds or more. As described above, according to the present invention, a long-time sintering anchor of a hot-rolled steel sheet can be changed to a short-time continuous sintering anchor, and furthermore, the intermediate and intermediate cold rolling,
Even if the intermediate phosphorous dulling is omitted, the intergranular corrosion, ridging property, and r value of the resulting stainless steel sheet can be made to values superior to those produced by conventional methods, which is a remarkable effect.

[Brief explanation of the drawing]

・Figure 1 shows the presence or absence of intergranular corrosion when the mountain content and false anchor temperature of a hot-built ferritic stainless steel sheet are changed. Figure 2 shows the solid solution of a ferritic stainless steel sheet before cold rolling. A diagram showing the relationship between N and r value, Figure 3 A and 3 are diagrams showing the relationship between heating temperature and solid solution N in a heating furnace by changing the AI content, and Figure 4 shows the AI content in steel. 5 is a diagram illustrating the cooling rate in the Rentsugi hammer according to the present invention, and FIG. 6 is a diagram showing the relationship between
Figure 7 shows the relationship between the J-jing height, Figure 7 shows the relationship between anchor temperature and r value, Figure 8 shows the relationship between water cooling start temperature and r value during the competitive anchor cooling process, Figure 9 FIG. 2 is a diagram illustrating the steepening speed in the continuous phosphor dulling according to the present invention. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9

Claims (1)

[Scope of Claims] 1 A ferritic stainless steel slab containing more than 0.10% but not more than 0.25% of AI is heated to a temperature range of 1050 to 1250°C and then hot rolled into a steel strip, and this hot rolled steel After heating the strip to a temperature range of 900°C or more and less than 1050°C and holding it for a short time of 20 minutes or less, continuous annealing is performed by rapidly cooling it to room temperature, and then it is cooled to the final product thickness without intermediate annealing. A method for producing a ferritic stainless steel sheet, which comprises rolling and then recrystallizing annealing. 2 A ferritic stainless steel slab containing more than 0.10% of AI and less than or equal to 0.25% is heated to a temperature range of 1050 to 1250°C, then hot rolled into a steel strip, and this hot rolled steel strip is heated to a temperature of 900°C to 1050°C. Heating to a temperature range below ℃,
After holding for a short time of 20 minutes or less, continuous annealing is performed in the cooling process to room temperature in a temperature range of 850°C or less at a cooling rate of 50°C/sec or more, and then the final product thickness is reached without intermediate annealing. 2. The method for producing a ferritic stainless steel sheet according to claim 1, which comprises cold rolling and then recrystallization annealing.