JPH08311557A - Production of ferritic stainless steel sheet free from ridging - Google Patents

Abstract

PURPOSE: To provide a ferritic stainless steel free from occurrence of ridging. CONSTITUTION: A slab is heated to 1090-1200 deg.C, and rolling reduction in the latter stage of roughing is regulated to >=35%; per pass. After holding at 1000-1100 deg.C for 60-90sec, finish rolling is started and finish rolling finishing temp. is regulated to 960-1000 deg.C. The resulting plate is cooled to <=600 deg.C at >=25 deg.C/s cooling rate, coiled at <=600 deg.C, and subjected to water immersion treatment in the coiled state. Then, the plate is reheated to 800-850 deg.C while regulating the temp. rise rate from 400 deg.C to (50 to 100) deg.C/hr and annealed, followed by cold rolling. By specifying rolling and annealing conditions as mentioned above to make a crystal orientation random and refine the structure, the occurrence of ridging at the time of deep drawing can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel sheet, and more particularly to a method for producing a ferritic stainless steel sheet without ridging that occurs during deep drawing.

[0002]

2. Description of the Related Art Ferritic stainless steel is steel containing 14 to 19% by weight of chromium (Cr). Among them, the material specified as SUS 430 in the Japanese Industrial Standards (JIS) (hereinafter referred to as 430 steel) has excellent workability such as deep drawing and can withstand a considerable corrosive environment. Therefore, it is mainly used as a board material for building interior materials, daily durable equipment, kitchen equipment, automobile decorations, gas burners, equipment parts for nitric acid plants, and the like. However, when the 430 steel sheet is subjected to tensile deformation and deep drawing, a wrinkle-like (concave and convex) distortion pattern is likely to appear parallel to the rolling direction of the sheet. This phenomenon is called ridging or roping, and requires finish polishing to impair the appearance of the product, or causes severe work cracking.

The following method has been proposed as a method of reducing this phenomenon, that is, a method of improving ridging resistance.

A method of increasing the content of austenite-forming elements to increase the austenite phase during hot rolling so as to make the structure uniform, and a sheet after hot rolling or in the middle of cold rolling is temporarily heated to a temperature higher than the transformation point. Method of heating to a temperature and air cooling to homogenize the structure, method of electromagnetic stirring at the solidification stage of continuous cast slab to form fine equiaxed crystals and not to generate coarse columnar crystals, heating temperature of slab in hot rolling Of the austenite + ferrite two-phase structure to disperse and precipitate the martensite phase from the austenite + ferrite dual-phase structure and recrystallize after cold rolling. To accelerate the occurrence of ridging.

However, the above method leads to an increase in the material cost, the method causes the deterioration of the mechanical properties of the material, and it is difficult to form a perfect fine equiaxed crystal even by the method, and the columnar crystals remain. Unavoidable. If the heating temperature of the slab is raised by the method (1), the growth of the ferrite band which causes the occurrence of ridging is promoted. In addition, since the method of (1) has to carry out high reduction rolling, new rolling equipment is required.

Japanese Unexamined Patent Publication (Kokai) No. 1-111816 discloses a method of suppressing the occurrence of ridging by performing cold rolling while precipitating a martensite phase. However, the material in which the martensite phase is precipitated has a large deformation resistance, the number of rolling passes increases, and the production efficiency decreases.

[0007] Japanese Patent Publication No. 19688/1986 discloses that the temperature is 900 to 1200 ° C.
In the rough rolling, the rolling method of at least 2 to 7 passes having a time between passes of 15 to 60 seconds is performed, and a method for producing a ferritic stainless steel sheet having excellent workability (r value, ridging resistance) is disclosed. There is. It is said that by increasing the time between passes, static recrystallization of the rolled material is promoted and the cast structure becomes finely randomized. However, although recrystallization occurs in the processed portion, it is difficult for the processing strain to propagate to the center of the plate thickness, and only the surface layer portion is recrystallized.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a ferritic stainless steel sheet in which ridging does not occur in deep drawing due to the synergistic effect of hot rolling conditions and annealing conditions. .

[0009]

Means for Solving the Problems The present inventor has conducted extensive research on the ridging of ferritic stainless steel (for example, SUS 430 steel) thin plates, and in order to improve the ridging resistance,
It was found that it is preferable to combine and manufacture the hot rolling condition and the annealing condition in a specific range.

The present invention has been completed based on this finding, and the gist of the invention lies in the method for producing a ferritic stainless steel sheet shown below.

After heating the slab to 1090 to 1200 ° C., the reduction amount in the latter stage of rough rolling is set to 35% or more per pass,
After holding at 1000 to 1100 ° C for 60 to 90 seconds, finish rolling is started and the finish rolling end temperature is set to 960 to 1000 ° C.
It is cooled to below at a cooling rate of 25 ℃ / s or more, wound at 600 ℃ or less, immersed in water while being wound, and heated at a temperature of 400 ℃ to 50 〜 100 ℃ / hr 800 〜 850 ℃ A method for producing a ridging-free ferritic stainless steel sheet, which comprises reheating to a sheet, annealing, and then cold rolling.

The term "second stage of rough rolling" is usually 4-6.
It is the latter half of the pass in which rough rolling of the pass is performed, that is, the final 1 to 3 passes.

[0013]

According to the present invention, in order to suppress the occurrence of ridging, the rough rolling of hot rolling, finish rolling and annealing conditions are combined in an optimum range, so that ridging does not occur as a non-uniform texture and fine structure. This is a method for producing a ferritic stainless steel sheet. The manufacturing conditions will be described below.

1. Slab heating: Slab 1090 ~ 1200 ℃
In order to perform static recrystallization after rough rolling, which is one of the features of the method of the present invention, the temperature is lower than 1090 ° C.
It will be difficult to maintain the temperature above 1000 ° C and to secure the finish rolling end temperature above 960 ° C. Also 1200
If the temperature exceeds ℃, the grain growth of columnar crystals at the time of casting occurs, coarsening occurs, and the ridging resistance deteriorates, which is not desirable.

2. Amount of reduction in rough rolling: If the amount of reduction in the latter stage of rolling is increased, static recrystallization of the rolled material can be promoted and ridging is suppressed. But,
When the reduction amount is less than 35% per pass, the strain energy accumulated is small, and static recrystallization is small. In addition,
The upper limit of this reduction is about 45% due to the rolling mill capacity.

3. Hold after rough rolling: 1000 ~ after rough rolling
Up to 1100 ℃ (Two-phase region temperature of austenite + ferrite)
Allow static recrystallization by holding for 60-90 seconds.
However, if the holding time is less than 60 seconds, the progress of static recrystallization becomes insufficient. Further, if it exceeds 90 seconds, the temperature of the rolled material decreases, the deformation resistance during finish rolling increases, and surface defects occur. If the holding temperature is less than 1000 ° C, the finish rolling temperature may be lower than 960 ° C, which is not desirable. When the temperature after rough rolling becomes 1000 ° C. or lower, recrystallization heating may be performed. However, the upper limit of the holding temperature is lower than the heating temperature of the slab described in 1 above, and the temperature is 1100 ° C.
Is.

4. Finish rolling and subsequent quenching: By setting the finish rolling finish temperature to 960 ° C or higher, the structure of the rolled material becomes austenite + ferrite two-phase structure. After that, rapid cooling is performed to precipitate a martensite structure, and rapid thermal annealing is performed to randomize the crystal orientation and refine the structure. This is intended to improve ridging resistance. Since the coarsening of crystal grains occurs when the finish rolling end temperature becomes high, the upper limit is 10
Set to 00 ° C.

The cooling rate after finishing rolling may be any cooling rate that precipitates a martensite structure from the austenite + ferrite two-phase structure, and is a rapid cooling rate of 25 ° C./s or more.

5. Winding temperature: The rolling temperature of the finish rolled material is set to 600 ° C or lower because if the temperature exceeds 600 ° C, martensite is not sufficiently formed and the crystal orientation of the rolled material cannot be randomized. However, at a temperature lower than 400 ° C., the steel plate hardens, making it difficult to wind it up. Even if it is wound up, the coil is hardened, so that a flaw due to a gap between the coils occurs at the time of rewinding. Therefore, it is desirable that the coiling temperature be 400 ℃ or higher. 6. Immersion in water: If left unrolled, the latent heat causes a tempering phenomenon in which martensite turns into ferrite and carbide, and the cold-rolled steel sheet of the product tends to rid. Therefore, the martensite phase is surely precipitated and the recovery to the ferrite phase is suppressed by the water immersion treatment while being wound.

7. Annealing of the winding material: The winding material subjected to the water immersion treatment in the above 6 is annealed with the maximum heating temperature up to 800 to 850 ° C. At this time, the rate of temperature rise from 400 ° C to the maximum heating temperature shall be 50 ° C / hr or more. The rolled material obtained by the above finish rolling and quenching has accumulated rolling strain energy, and recrystallization is promoted by heating at 800 ° C or higher,
Randomization of crystal orientation and refinement of structure can be achieved. However, if the temperature exceeds 850 ° C, abnormal oxidation occurs, which may lead to deterioration of the surface properties during pickling after annealing. Also, the rate of temperature rise is
With slow cooling below 50 ° C / hr, static recovery proceeds, strain energy decreases, and grain refinement does not proceed sufficiently. The higher the heating rate, the better, but it is desirable to set the upper limit to 100 ° C / hr to prevent deformation of the winding material. Furthermore, the rate of temperature increase from 400 ℃ or more to 50 ℃ / h
The reason for setting r or higher is that recrystallization does not occur at a temperature lower than 400 ° C. and the purpose of grain refinement is not achieved. Since this annealing is usually performed in a coil state, it is 30 minutes to several hours, and cooling after annealing may be air cooling.

8. Cold rolling: The material annealed in 7 above is subjected to normal cold rolling at a reduction rate of 30 to 90%. afterwards,
If necessary, perform final annealing at 800 to 900 ° C for 10 to 60 seconds.

FIG. 1 is a diagram showing an example of a heat pattern in the manufacturing process of the present invention. The slab is heated to 1090 ~ 1200 ℃, and then rough rolling is carried out by applying strong rolling at the latter stage.
Hold at ~ 1100 ℃ for 60 ~ 90 seconds, finish rolling temperature 960 ~
After finish rolling to 1000 ℃, it is rapidly cooled to 600 ℃ or less and wound into a coil. The wound coil is immersed in water, rapidly heated to 400 ℃ or more and annealed at 800 to 850 ℃, then cold rolled and finish annealed.

[0023]

EXAMPLES The method of the present invention will be described below with reference to examples.

Typical SUS of ferritic stainless steel
430 Steel (C: 0.06wt%, Al: 0.10wt%, Cr: 16.2wt%,
N: 0.036 wt%, other slabs of Fe and unavoidable impurities (thickness 50 mm, width 60 mm, length 70 mm) were used. After heating to 1050 ° C., 1100 ° C., 1150 ° C., 1170 ° C. and 1250 ° C., respectively, rough rolling was performed for 6 passes. Rough rolling is performed when the reduction amount of each pass is equal in all passes as shown in Table 1.
(D) and the latter stage (from the 4th pass onward) when the amount of strong reduction is (A to C).

[0025]

[Table 1]

After these rough rolling, the holding temperature was set to 1100 to 1000.
After holding at ℃ for 50 to 100 seconds while changing the holding time, finish rolling was performed to a plate thickness of 3.6 mm. The finish rolling temperature shown in Table 2 was changed to 510 to 700 ° C. by water cooling (the cooling rate was changed to 20 to 35 ° C./s), followed by winding. The wound coil,
It was immersed in water as it was. Then reheat to 400 ° C
To the maximum heating temperature (750 to 900 ° C) at a heating rate of 50 ° C / hr and 25 ° C / hr and hold for 12 hours.
Annealing for cooling was performed. Then, after cold rolling to a thickness of 0.5 mm, finish annealing was carried out at 900 ° C. for 1 minute to obtain a test material.

From the obtained test material, thickness 0.5 mm, width 25
A test piece having a length of mm and a length of 250 mm was sampled, a tensile strain of 20% was applied, the roughness was measured in the tensile direction, and the maximum roughness was taken as the ridging height. A ridging height of 15 μm or less was defined as the scope of the present invention. The test results are shown in Table 2.

[0028]

[Table 2]

In Test Nos. 1 to 8 of the invention examples, the slab heating temperature was 1090 to 1200 ° C., a strong reduction of 36.2% or more was applied in the latter pass of the rough rolling, and after holding for 60 to 90 seconds, finish rolling was started. Finishing temperature: Rapid cooling at a cooling rate of 960 ℃ or more to 28 ℃ / s or more, winding at 510 to 600 ℃, soaking in water as the coil, and then annealing by heating at a heating rate of 50 ℃ / hr. Is. Both are in the range determined by the method of the present invention, and the ridging height is as good as 14 μm or less.

In the test number 9 of the comparative example, the heating temperature of the slab was as low as 1050 ° C., so that the finish rolling finish temperature was as low as 950 ° C. and the ridging height was 27 μm.

Test number 10 indicates that the heating temperature of the slab is 1250 ° C.
Therefore, the strain energy is less accumulated during rough rolling, which hinders the promotion of recrystallization by holding before finish rolling,
The ridging height is now 31 μm.

In Test No. 11, since rough rolling was performed with the same rolling reduction (27.0 to 27.5%), strain energy was not accumulated much during rough rolling, which hindered promotion of recrystallization by holding before finish rolling and ridging. The height was 28 μm.

Test number 12 is the holding time before finish rolling.
Since it was as short as 50 seconds, static recrystallization did not proceed and the ridging height was 25 μm.

Test No. 13 has a larger amount of reduction in the latter stage of rough rolling than in the former stage, but since the amount of reduction in the latter stage was 32.5% or less, the strain energy was less accumulated and was retained before the finish rolling. Static recrystallization did not proceed, and the ridging height became 26 μm.

Test number 14 is the holding time before finish rolling
Since it is long for 100 seconds, the finish rolling finish temperature is as low as 920 ℃, the amount of austenite is reduced by the γ → α transformation, the crystal structure cannot be randomized, and the deformation resistance increases, causing rolling defects. It was Also, the ridging height is 24 μm
And the ridging resistance is poor.

Test No. 15 has a cooling rate after finish rolling.
It is as low as 20 ° C / s, and because the martensite formation is insufficient, randomization of crystal orientation does not occur and the ridging height is 3
It became 0 μm.

In Test No. 16, since the winding temperature of the finish rolled material was as high as 700 ° C., the martensite was transformed into ferrite and the ridging height was 26 μm.

In Test No. 17, the annealing heating temperature was as low as 750 ° C., and recrystallization did not occur, so that the structure became coarse and the ridging height became 26 μm.

In Test No. 18, the heating temperature for annealing was as high as 900 ° C., so the surface quality deteriorated during pickling, so ridging could not be measured.

In Test No. 19, the annealing heating rate was low at 25 ° C./sec, and static recovery proceeded, so that microstructure did not occur and the ridging height was 25 μm.

In Test No. 20, the coil was not immersed in water, so that the ferrite phase was recovered and the ridging height was 31 μm.

[0042]

The present invention is a method for producing a ferritic stainless steel sheet in which the occurrence of ridging is suppressed by randomizing the crystal orientation and refining the structure by specifying the rolling conditions and the annealing conditions. The obtained ferritic stainless steel is a household product whose appearance is important,
It can be used for kitchen supplies, building materials and automobile materials.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a heat pattern in a manufacturing process of the present invention.

Claims (1)

[Claims] 1. After heating the slab to 1090 to 1200 ° C., the reduction amount in the latter stage of rough rolling is set to 35% or more per pass,
After holding at 1000 to 1100 ° C for 60 to 90 seconds, finish rolling is started and the finish rolling end temperature is set to 960 to 1000 ° C.
It is cooled to below at a cooling rate of 25 ℃ / s or more, wound at 600 ℃ or less, immersed in water while being wound, and heated at a temperature of 400 ℃ to 50 ℃ to 100 ℃ / 850 ℃ A method for producing a ridging-free ferritic stainless steel sheet, which comprises reheating to a sheet, annealing, and cold rolling.