JPH0233770B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a ferritic stainless steel sheet with excellent ridging properties. [Prior Art] Ferritic stainless steel has the advantage of being inexpensive because it does not contain Ni, but its use has been limited due to its generally poor corrosion resistance and workability. However, recently, as a result of rigorous studies on the effects of additive elements and manufacturing conditions, steel types with corrosion resistance and workability, especially press formability, that are comparable to austenitic stainless steels have been produced.
Despite this, ferritic stainless steel has a unique rizing phenomenon, which is a fatal flaw in applications that require a beautiful surface, so the reality is that it cannot necessarily be replaced with austenitic stainless steel. . Since the ridging phenomenon is presumed to be based on the solidified structure during casting, techniques for improving ridging properties are aimed at reducing the size of the structure during casting or destroying the structure during rolling or annealing. Examples of such methods include applying electromagnetic stirring during casting (Japanese Patent Application Laid-Open No. 50-16616), repeating strong reduction during rolling (Japanese Patent Application Laid-Open No. 52-47513), and lowering the hot-rolling finishing temperature. (U.S. Pat. No. 3,128,211), a method of destroying the hot-rolled structure by once generating an austenite phase after hot rolling (U.S. Pat. No. 3,128,211),
2772992) etc. have been proposed. [Problems to be Solved by the Invention] Although all of these known methods have been found to be effective, the reality is that none of them has been able to completely eliminate ridging. Furthermore, while ridging is a property that is affected throughout the thickness direction of a thin plate, countermeasures during casting only target the center of the thickness, and countermeasures during rolling only target the surface layer of the plate. One alone is not sufficient. As a countermeasure that targets the entire plate thickness, there is a method of generating an austenite phase after hot rolling, which is recognized to be much more effective than other methods, but if it remains, it will cause martensitic transformation. In order to transform the austenite phase that hardens into the ferrite phase, a long time annealing is required, which is not practical. An object of the present invention is to advantageously destroy the cast structure throughout the entire plate thickness and improve ridging properties. Note that so-called 13% Cr steel is called martensitic stainless steel, but steel types with low C such as SUS410 steel are similar to ferritic stainless steel in terms of structure and use, so the present invention It was decided to include it in the ferritic stainless steels. [Means for Solving the Problems] The present inventors investigated a method for destroying the cast structure after casting ferritic stainless steel pieces, and found that
After casting, the slab is heated and held at just below the Ac ₃ transformation point before being heated for hot rolling without being cooled to below 300°C to promote the transformation of the austenite phase to the ferrite phase (and carbide). It was discovered that the casting structure could be destroyed and the ridging properties could be improved. Furthermore, we investigated the effects of the components in the steel and found that adding Al can shorten the heating time just below the Ac ₃ transformation point. That is, the present invention includes (1) a process of hot rolling a ferritic stainless steel slab containing 10% or more of Cr and cold rolling it without or after annealing to produce a stainless steel sheet; After casting, the slab was heated to 700°C without being cooled below 300°C.
Production of a ferritic stainless steel sheet with excellent ridging properties, which is maintained at a temperature of Ac ₃ or below for 30 minutes or more to promote transformation from austenite phase to ferrite single phase and carbide, and then hot rolled. method, and (2) hot rolling of a ferritic stainless steel slab containing 10% or more Cr and 0.06% or more and 0.3% or less Al, and then cold rolling without annealing or after annealing. In the process of manufacturing stainless steel sheets, after casting the slab,
It is characterized by holding at a temperature of 700°C or more and Ac ₃ or less for 30 minutes or more without cooling to 300°C or less to promote the transformation of the austenite phase into a ferrite phase and hydrocarbons, and then hot rolling. A method for producing a ferritic stainless steel sheet with excellent ridging properties,
The main points are as follows. [Operation] The present invention will be explained in detail below. The cooled ferritic stainless steel slab consists of coarse ferrite grains and fine martensitic phase.
It is a phase organization. Therefore, it can be easily estimated that the two-phase structure is ferrite and austenite in the temperature range above the Ms point after casting. It is considered that it is possible to transform the austenite phase into a single ferrite phase (and carbide) if the temperature is maintained in a temperature range of Ac ₃ or lower and Ms point or higher. Figure 1 shows
This is a light-based microscopic structure of a sample of SUS430 steel having the chemical composition shown in No. 1 in Table 1, which was reheated at 670°C during cooling after casting, maintained at 950°C for 3 hours, and then air cooled. The structure is a single-phase ferrite (and carbide) structure, and the part that is estimated to have been an austenite phase at high temperatures is a fine ferrite structure, with a relatively large number of precipitates (estimated to be Cr carbides) along the grain boundaries. is precipitated. Figure 2 shows that after casting SUS430 steel having the chemical composition shown in No. 1 in Table 1, it was reheated before being cooled to below 300°C and held at various temperatures to form an austenite phase and a ferrite phase ( This is the result of investigating the transformation rate to carbide and carbide). In the diagram, the x mark indicates incomplete metamorphosis.
The ○ mark indicates the completion of metamorphosis. From the same figure, the time required for transformation is the shortest at 950℃, and the time required for transformation is the shortest at 950℃, and
In the case of No. 1), it can be seen that metamorphosis is completed in 60 minutes. Next, we investigated the effects of various additive elements on transformation and found that the time required to complete transformation was shortened when Al was added. Figure 3 shows that 17Cr steel containing various amounts of Al was cast and
The results are shown in which the transformation rate of the austenite phase to the ferrite phase (and carbide) was investigated by reheating the sample before cooling it below ℃ and holding it at 950℃ for various times. In the figure, an x mark indicates that the metamorphosis has not been completed, and an ○ mark indicates that the metamorphosis has been completed. That is, when the Al content is 0.03%, the transformation of the austenite phase requires holding at 950°C for about 30 minutes, but for steel containing 0.06% Al, it takes about 10 minutes, and for steel containing 0.13% Al, it takes about 30 minutes. I found out that it can be completed in about 5 minutes. Furthermore, since the heating for hot rolling is usually above the Ac ₃ transformation point, a cast slab that has been transformed into a single ferrite phase will return to a two-phase structure of ferrite and austenite, but once it is transformed into a ferrite structure, it cannot be cast. It is quite conceivable that tissue destruction will progress. Furthermore, many carbides that precipitate by transforming the austenite phase of the slab gather at the boundary between the old austenite phase and the ferrite phase from high temperatures, so the regenerated austenite phase will be finer than that of the slab. can be expected. Based on the above knowledge, the results of confirming the effects of materials are shown below. After casting SUS430 steel having the chemical composition shown in No. 1 in Table 1, the slab was kept at 950℃ for 2 hours without cooling below 600℃.
It was heated again to 1200°C, then hot rolled, then hot rolled and annealed, and then cold rolled and annealed using the ICR method to obtain a cold rolled annealed plate with a thickness of 0.5 mm. At the same time, slabs that were not held at 950°C were also hot rolled under the same conditions to 0.5mm
A thick cold-rolled annealed plate was used as a comparison material. These thin plates were stretched 20% in the L direction, and the ridging property expressed as the maximum waviness height was measured. As a result, 950℃
The comparative material without retention at 950°C had a diameter of 35.4μm, while the specimen retained at 950℃ had a diameter of 35.4μm.
It was observed that the ridging property was improved by holding at 22.6 μm and 950°C. Next, cold-rolled annealed sheets were manufactured using 17Cr steels with different Al contents in the same process including holding at 950°C, and the effect on ridging properties was investigated. The results are shown in Figure 4. In the figure, ◎ indicates the ridging height is less than 20 μm, and ○ indicates the ridging height is less than 20μm.
20μm or more and less than 30μm, × mark indicates ridging height of 30μm
m or more. As is clear from the figure, Ac ₃ has an effect on ridging properties in steel with Al added.
It was observed that the retention time just below the point was shorter than that of steel without Al addition, and the level of improvement was also significant. Regarding the effect of retention at 950℃, as shown in Figure 1, the austenite phase is transformed at the slab stage to destroy the cast structure that is harmful to ridging properties, and once the ferrite single phase (and carbide) It is presumed that this is due to two effects: the re-generated austenite phase is made finer by this process. Next, the reasons for limiting the constituent elements of the present invention will be described. If the Cr content is less than 10%, it is excluded because it lacks the basic corrosion resistance of stainless steel and is prone to recrystallization after hot rolling, so the ridging property is at such an excellent level that it is not a problem. ,Ten
The lower limit was %. The amount of Al added does not matter because the effect of the present invention is recognized regardless of whether Al is added.
As shown in FIG. 4, the addition of Al shortens the time it takes for the improvement in ridging property to become apparent, and further effects are observed. Therefore, the application to steel to which Al is added was designated as the second invention. The lower limit of the amount of Al added in the second invention was set at 0.06%, at which the effect of shortening the time for improvement in ridging property to become apparent as shown in FIG. 4 was set as the lower limit. However, if added in excess of 0.3%, hot workability deteriorates, so the upper limit was set at 0.3%. The lower limit of the temperature history of the slab before the transformation treatment to the ferrite phase was set at 300°C, which exceeds the Ms point, because it is necessary to maintain the temperature at a temperature that does not transform the austenite phase to martensite. The upper limit of the temperature for the transformation treatment into the ferrite phase is the Ac ₃ transformation point, since it is necessary to transform the austenite phase into the ferrite phase. However, below 700℃,
Since the transformation takes an extremely long time, the lower limit was set at 700°C. The holding time just below the Ac ₃ point is not particularly limited as long as it is enough time to transform the austenite phase into a single ferrite phase (and carbide), but if the heating temperature is 900°C or higher, at least 30 minutes is required. From 700℃ to 900℃, the lower the temperature, the longer the time required. The upper limit of the retention time just below Ac ₃ points is not particularly limited as it is more effective for a longer period of time. However, heating for an unnecessarily long time is not only disadvantageous in terms of cost, but also causes problems such as decarburization and dechromium removal, leading to deterioration of corrosion resistance, so it is preferable to heat within 30 hours. The basic idea of the present invention is to transform the austenite phase of the slab into a single ferrite phase (and carbide) without transforming it into martensitic material, and then heat and hot-roll it again into a mixed structure of austenite and ferrite. Therefore, the temperature history of the slab after it has been transformed into a single ferrite phase (and carbide) does not have any effect on the effects of the present invention. Therefore, Ac ₃ which is the technical gist of the present invention
After holding at a temperature of 700° C. or higher, it is possible to cool it once to room temperature, or it is also possible to directly raise the heating temperature for hot rolling.
Particularly in the latter case, Ac ₃ during cooling of the slab.
In addition to the effects of the present invention, the method of holding the temperature in the temperature range of 700℃ or higher and then hot rolling it as it is provides a significant energy saving effect as an HCR process, and also makes the temperature of the slab uniform during hot rolling. Therefore, there is an advantage that the rolling load is reduced. [Example] After SUS430 steel having the chemical composition shown in No. 1, No. 2, and No. 3 in Table 1 was made into a slab by continuous casting, it was heated to 900 to 950 °C without cooling to below 600 °C.
℃ for 1.5 hours or 800-750℃ for 4 hours. This slab is cooled to room temperature or immediately heated to 1180℃ without cooling.
The material was heated to 100% and hot-rolled, and then hot-rolled sheet annealing was performed by a conventional method, and a cold-rolled annealed sheet with a thickness of 0.4 mm was obtained by the ICR method. Table 2 shows the ridging properties measured and evaluated based on the maximum waviness height after this cold rolled annealed sheet was stretched by 20% in the L direction. As a comparative example, data is also shown for a sample in which a continuously cast slab was cooled to room temperature without any indefinite treatment in a specific temperature range during cooling, and then heated to 1180°C to produce a cold-rolled annealed plate in the same manner. In the heating process of the comparative material, only about 20 minutes were required between 750 and 1000°C. As shown in Table 2, it can be seen that the cold rolled annealed sheet according to the present invention has superior ridging properties compared to the samples shown in the comparative example.

【table】

〔Effect of the invention〕

As detailed above, according to the present invention, the ridging property of a cold-rolled annealed sheet is significantly improved simply by maintaining the temperature in the temperature range just below the Ac ₃ point for a certain period of time during cooling of the slab.
Furthermore, the second aspect of the present invention regarding steel added with Al
In the invention, the retention time at a temperature just below the Ac ₃ point can be shortened, and the cost required for the process can be further reduced. In particular, it is possible to obtain the quality improvement effect of the present invention in addition to the energy saving effect of HCR applied to the HCR process in which hot rolling is performed without cooling after casting. If conventional ridging countermeasures are taken together, productivity will be significantly degraded (for example, low-temperature finishing of hot-rolling increases rolling load and causes a significant number of surface scratches, and the transformation to austenite phase after hot-rolling However, the present invention is inexpensive, requires only thermal energy for holding at a temperature just below the Ac ₃ point, and is easy to manufacture. There are no negative effects on sex or anything else. Rather, if a process is adopted in which heating for hot rolling is performed immediately without cooling after heat holding treatment for transformation, the manufacturability will be improved as described above. Furthermore, since the present invention involves processing in a process that has been overlooked in conventional ridging countermeasures, there is no problem in combining the present invention with conventional ridging countermeasures, thereby achieving even greater effects. is possible. As described above, according to the present invention, the ridging properties of ferritic stainless steel are greatly improved.
It is now possible to apply inexpensive ferritic stainless steel to applications where it could not previously be used for ridging, resulting in significant resource and economic benefits.

[Brief explanation of drawings]

Figure 1 is a cross-sectional optical microscope metallographic photograph of a cast slab that was cooled to 670℃ after casting, then reheated and held at 950℃ for 3 hours.Figure 2 shows the retention during the transformation of the austenite phase to the ferrite phase. Figure 3 shows the relationship between temperature and retention time.
Figure 4 is a diagram showing the influence of Al on the transformation time of the austenite phase to the ferrite phase in terms of ridging height.

Claims (1)

[Claims] 1. A process of hot rolling a ferritic stainless steel slab containing 10% or more of Cr, followed by cold rolling without annealing or after annealing to produce a stainless steel sheet. After casting, the slab is kept at a temperature of 700°C or higher and Ac ₃ or lower for 30 minutes or more without cooling to 300°C or lower to promote the transformation of the austenite phase into a ferrite phase and carbide, and then hot rolled. A method for manufacturing a ferritic stainless steel sheet with excellent ridging properties. 2 A ferritic stainless steel slab containing 10% or more Cr and 0.06% or more and 0.3% or less Al is hot-rolled, and then cold-rolled to form a stainless steel plate without or after annealing. In the manufacturing process, after casting, the slab is held at a temperature of 700°C or higher and Ac ₃ or lower for 30 minutes or more without cooling to 300°C or lower to promote the transformation of the austenite phase into a ferrite phase and carbide. A method for producing a ferritic stainless steel sheet with excellent ridging properties, which is characterized by hot rolling.