JPS61136621A - Manufacture of ferritic stainless steel sheet having superior ridging resistance - Google Patents

Abstract

PURPOSE:To destroy effectively the cast structure over the whole sheet thickness and to improve ridging resistance, by casting the ferritic stainless steel contg. a specific amount of Cr and by hot-rolling the plate after holding in a specific temp. CONSTITUTION:An ingot of ferritic stainless steel contg. >=10% Cr is cast, which is held at 700 deg.C - Ac3 point for >=30min, without cooling to <=300 deg.C. This heat treatment accelerates the transformation of austenitic phase into a ferritic phase and into a carbide. This ingot is reheated to be hot-rolled, annealed, if necessary, and cold-rolled. In this way, ridging resistance of the ferritic stainless steel sheet can be improved.

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.

[Conventional technology]

Although ferritic stainless steel has the advantage of being inexpensive because it does not contain Ntt, its use has been limited due to its generally poor corrosion resistance and workability. However, recently, as a result of rigorous study of the effects of additive elements and manufacturing conditions, a steel type with corrosion resistance and workability, especially press formability, comparable to austenitic stainless steel has been produced. Despite this, ferritic stainless steel has a unique glue-song 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. As a means for this, for example, a method of applying electromagnetic stirring during casting (Japanese Unexamined Patent Publication No. 50-16616), a method of repeating strong reduction during rolling (Japanese Unexamined Patent Publication No. 52-47)
513), a method of lowering the hot rolling finishing temperature (U.S. Pat. No. 3,128,211), and a method of destroying the hot rolled structure by once generating an austenite phase after hot rolling (U.S. Pat. No. 2,772,992). etc. have been proposed.

[Problem that the invention seeks to solve]

Although all of these known methods have been recognized to have a corresponding effect, 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, which hardens during the process, into the ferrite phase, long annealing is required, making it impractical.

An object of the present invention is to advantageously improve the fracture and song properties of a cast structure over the entire plate thickness.

Note that so-called 1391Cr steel is called martensitic stainless steel, but steel types with low C such as SUS 410 steel are similar to ferritic stainless steel in terms of structure and use, so they are not suitable for use in the present invention. It was decided to include it in ferritic stainless steel.

[Means for solving problems]

The present inventors investigated a method for destroying the cast structure after casting a ferritic stainless steel slab, and found that the Ac3 It has been discovered that by heating and holding just below the transformation point to promote the transformation of the austenite phase into the ferrite phase (and carbide), the cast structure can be destroyed and song properties can be improved. Furthermore, after examining the influence of the components in the steel, it was found that by adding Al, the heating time at the ACl transformation point can be shortened.

That is, the present invention provides (1) a step of hot rolling a ferritic stainless steel slab containing 101 or more Cr and cold rolling it without or after annealing to produce a stainless steel sheet. , after casting the slab, 300
Risong property characterized by holding at a temperature of 700°C or more and below the Acg point for 30 minutes or more without cooling to below ℃ to promote transformation from austenite phase to ferrite single phase and carbide, and then hot rolling. and (2) hot rolling a ferritic stainless steel slab containing 101 or more Cr and 0.06% or more and 0.3% or less Al and annealing it. In the process of manufacturing a stainless steel plate by cold rolling the slab without or after applying it, the slab is
℃ or less (temperature of 700'C or higher and AC8 or lower) for 30 minutes or more to promote the transformation of austenite phase into ferrite phase and carbide, and then hot rolling. The gist of this paper is a method for manufacturing a ferritic stainless steel sheet with excellent properties.

[Effect]

The present invention will be explained in detail below.

A cooled ferritic stainless steel slab has a two-phase structure of coarse ferrite grains and a fine martensitic phase.

Therefore, it can be easily estimated that there is a two-phase structure of 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) by maintaining the temperature in a temperature range above and below the A e g point and the Ms point. 1st
The figure shows SUS 4 having the chemical composition shown in A1 of Table 1.
30 steel was reheated at 670°C during cooling after casting, and it was heated to 95°C.
This is an optical microscopic structure of a sample that was heated for 3 hours at 0° C. and then cooled in air. Structure is single phase ferrite (and carbide)
The part of the structure that is estimated to have been an austenite phase at high temperatures is a fine ferrite structure, and a relatively large number of precipitates (presumed to be Cr carbides) are precipitated along the grain boundaries. Figure 2 shows that after casting SUS 430 steel having the chemical composition shown in 41 in Table 1, it was reheated and held at various temperatures without being cooled below 300°C, resulting in the formation of austenite phase, ferrite phase (and This is the result of investigating the transformation rate to 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°C, and the test steel (JftLl in Table 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 adding Al shortens the time until completion of transformation. Figure 3 shows that 17Cr steel containing various amounts of Al was cast, reheated before being cooled to below 300°C, and kept at 950°C for various times to transform the austenite phase into ferrite phase (and carbide). The results of investigating the rate of metamorphosis are shown. In the figure, an X mark indicates that the metamorphosis has not been completed, and an O mark indicates that the metamorphosis has been completed. That is, the Al content is O,0
At 3%, the austenite phase transformation requires approximately 3% at 950°C.
It was found that holding for 0 minutes is required for steel containing 0.06% Al, and about 5 minutes for steel containing 0.131 L.

Note that since the heating for hot rolling is usually above the Ac3 transformation point, the cast slab that has been transformed into a single ferrite phase will return to a two-phase structure of ferrite and austenite, but once transformed into a ferrite structure, the cast structure will change. It is quite conceivable that destruction will progress. Furthermore, carbides that precipitate by transforming the austenite phase of the slab gather in large numbers 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 effect on the material are shown below. 5 having the chemical components shown in Mu 1 of Table 1
After casting US430 steel, the slab was held at 950°C for 2 hours without cooling below 600°C, then heated again to 1200°C, then hot rolled, and then hot rolled sheet annealed.
Cold rolling annealing was performed using the CR method to obtain a cold rolled annealed plate with a thickness of 0.5 m. At the same time, a cast slab that was not held at 950°C was also hot rolled under the same conditions and made into a cold rolled annealed plate with a thickness of 0.5 wr and used as a comparison material.

These thin plates were stretched by 20% in the L direction, and the ridging property expressed by the maximum waviness height was measured. As a result, 95
The comparative material that was not retained at 0°C had a diameter of 35.41 erL, whereas the sample that was retained at 950°C had a diameter of 22.6 μm, indicating that the retention at 950°C improved the ridging property. It was done.

Next, cold-rolled annealed sheets were produced using 17cr steels with different Al contents in a 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 O indicates the ridging height is 20 μm.
30 μm or more, and the mark X indicates a ridging height of 301 Mn or more. As is clear from the figure, the retention time just below the hag point, where the effect on ridging properties appears, is shorter in steel with Al added than in steel without M, and the level of improvement is also large. Admitted.

As shown in Figure 1, the effect of retention at 950°C is that the austenite phase is transformed at the slab stage to destroy the casting structure that is harmful to ridging properties, and that once the ferrite single phase (and carbide) It is presumed that this is due to the two effects of refining the austenite phase that is generated again by IC.

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. , IOchi was set as the lower limit.

The amount of Al added does not matter because the effect of the present invention is observed regardless of whether Al is added, but as shown in Figure 4, the addition of Al shortens the time it takes for the improvement in ridging properties to become apparent. Since further effects were observed, the application to steel to which Al was added was designated as the second invention. The amount of Al added in the second invention is 0.06, which has the effect of shortening the time it takes for the improvement in ridging property to become apparent, as shown in Figure 4.
The lower limit was %. However, if more than 0.3% is added, 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 Ma point, because it is necessary to maintain the temperature at a temperature that does not transform the austenite phase to martensite. As a matter of course, it is necessary to transform the austenite phase into the ferrite phase, so the upper limit of the temperature for the transformation treatment to the ferrite phase is the ACl transformation point. But 70
At temperatures below 0°C, transformation takes an extremely long time, so 70°C
The lower limit was 0°C.

The holding time just below the Ac3 point is not particularly limited as long as it is sufficient time to transform the austenite phase into a single ferrite phase (and carbide), but if the heating temperature is 9.
At temperatures above 00°C, at least 30 minutes are required, and from 700°C to 900°C, low-temperature rooting requires a long time. The upper limit of the retention time just below the Ac2 point is not particularly limited because it is more effective for a longer time. However, heating for an unnecessarily long time is not only disadvantageous in terms of cost, but also causes problems such as decarburization and deoring, resulting in deterioration of corrosion resistance, so it is preferable to heat the product within 30 hours.

The basic idea of the present invention is to temporarily transform the austenite phase of the slab into a single ferrite phase (
The temperature history of the slab after being transformed into a single phase of ferrite (and carbide) is the effect of the present invention. has no effect on the Therefore, Ac1, 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, a method in which the slab is held in a temperature range of 700°C or below the Ac3 point during cooling and then hot rolled until then, has the advantage of I (remarkable energy saving effect as a CR process). In addition, the temperature of the slab during hot rolling becomes uniform, which has the advantage of reducing the rolling load.

〔Example〕

1 in Table 1. After SUS 430 steel having the chemical composition shown in Figures 2 and A3 was made into slabs by continuous casting,
It was heated in a temperature range of 900 to 950°C for 1.5 hours or in a temperature range of 800 to 750°C for 4 hours without cooling to below 600°C. This slab is once cooled to room temperature or immediately heated to 1180°C without cooling and hot-rolled, then hot-rolled plate annealed by a conventional method, and IC
A cold rolled annealed plate having a thickness of 0.4 mm was prepared using the R method. This cold-rolled annealed plate was pulled 20 inches in the L direction, and the height of the thickest ridge was measured and evaluated, and the songability is shown in Table 2. As a comparative example, data is also shown for a sample in which a continuously cast slab was cooled to room temperature without holding in a specific temperature range during cooling, then heated to 1180°C, and similarly bound to a cold-rolled annealed plate. During the heating process of the comparative material, the temperature between 750 and 1000°C was approximately 20°C.
It only took minutes. As shown in Table 2, it can be seen that the cold rolled annealed sheet according to the present invention has better songability than the sample shown in the comparative example.

Table 1 [wt] Table 2 [Effects of the invention] As detailed above, according to the present invention, A during cooling of the slab
The ridging properties of the cold-rolled annealed sheet are significantly improved by simply maintaining the temperature in the temperature range just below the a-z point for a certain period of time.

Furthermore, in the second aspect of the present invention regarding steel to which Al is added, the holding time at a temperature just below the Ae1 point can be shortened, and the stress required for the process is further reduced. In particular, when applied to an HCR process in which hot rolling is performed without cooling after casting, it is possible to obtain the quality improvement effect of the present invention in addition to the energy saving effect of HCR. If conventional ridging measures are taken, manufacturability will be significantly degraded (for example, low-temperature finishing of hot-rolled steel increases rolling load and causes a significant number of surface scratches, and the transformation to austenite phase after hot-rolling is However, the present invention has the disadvantage of requiring a very long annealing time.
It only requires ripening energy for retention at a temperature below the s point, and does not have any adverse effects on manufacturability 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 productivity will be improved as described above.

Furthermore, since the present invention is a process that has been overlooked by conventional ridging countermeasures, there is no problem in combining the present invention with conventional ridging countermeasures.
By doing so, it is possible to achieve even greater effects.

As described above, according to the present invention, the ridging properties of ferritic stainless steel are greatly improved, making it possible to apply inexpensive ferritic stainless steel to applications that could not previously be used for ridging. The benefits obtained in terms of resources and economy are significant.

[Brief explanation of drawings]

Figure 1 is a cross-sectional optical micrograph of a cast slab that was cooled to 670°C after casting, then reheated and held at 950°C for 3 hours. Figure 2 is the holding temperature for the transformation of the austenite phase to the ferrite phase. Figure 3 is a graph showing the effect of Al on the transformation rate of austenite phase to ferrite phase, and Figure 4 is a graph showing the effect of Al on the transformation time of austenite phase to ferrite phase. FIG. 3 is a diagram showing the influence in terms of the height of ridging. Fig. 1 50 urn Fig. 2 time FJl [min] ○: Mugi et al. 3 A no soe Karo t [Wf %1 ■;
Rigging collision and Opm or more 3θ μm end ×: Rigging height - t3
AI attached gas σ amount of 0μm or more Cttt1%J Procedure amendment (method) April 12, 1985

Claims (2)

[Claims] (1) In the process of hot rolling a ferritic stainless steel slab containing 10% or more of Cr, cold rolling it without annealing or after annealing to produce a stainless steel sheet, After casting, the piece is heated to 30°C at a temperature of 700°C or higher and Ac_3 points or lower without cooling to 300°C or lower.
A method for producing a ferritic stainless steel sheet with excellent ridging properties, which comprises holding the sheet for more than a minute to promote the transformation of an austenite phase into a ferrite phase and carbide, and then hot rolling. (2) After hot rolling a ferritic stainless steel slab containing 10% or more Cr and 0.06% or more and 0.3% or less Al, without annealing or after annealing,
In the process of manufacturing stainless steel sheets by cold rolling,
After casting, the slab was heated to 70°C without being cooled to below 300°C.
A ferritic stainless steel sheet with excellent ridging properties, which is maintained at a temperature of 0°C or more and Ac_3 points or less for 30 minutes or more to promote the transformation of austenite phase into ferrite phase and carbide, and then hot rolled. Production method.