JPS61195924A - Production of ferritic stainless steel strip having excellent workability without ridging - Google Patents

Abstract

PURPOSE:To obtain a steel strip having excellent workability without ridging by controlling simultaneously the heating and cooling of the hot rolled steel strip of a ferritic stainless steel in a specific temp. range. CONSTITUTION:The hot rolled steel strip of the stainless steel having the prescribed compsn. is first heated at a heating rate of <75 deg.C/hr from 700 deg.C to the Ac1 point - 1,000 deg.C in a heat treatment of the heat treatment of said strip. The strip is cooled at a cooling rate of 20-200 deg.C/hr in the temp. region from the heating temp. down to 700 deg.C and is successively allowed to cool. The strip is otherwise cooled at a cooling rate of >=200 deg.C/hr in the 775-725 deg.C region from the above-mentioned heating temp. and is then allowed to cool after the strip is held in succession at 775-725<= for >=2hr. The coarse ferrite particles and ferrite bands existing in the central part of the strip thickness are annihilated by such heat treatment. The thin cold rolled ferritic stainless steel sheet having the excellent workability without ridging is stably supplied by the above- mentioned method.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a ferritic stainless steel strip that is free from ridging and has excellent workability.

(Prior art) When ferritic stainless steel sheets such as SUS 430 are subjected to processing such as deep drawing or overhanging, wrinkles parallel to the rolling direction, called ridging or roving, occur. Can be seen. Such wrinkles, or ridging, are a serious defect in processed stainless steel plate products that require a beautiful surface, so when such steel plates are subjected to forming processes such as press working,
There is a drawback that ridging must be removed by processing such as polishing after processing.

By the way, the latest manufacturing process for stainless steel sheets involves first melting raw materials in an electric furnace, then refining them using the AOD method or VOD method, followed by continuous casting to form a slab, which is then hot-rolled to form a 3. ～6 thick hot-rolled steel strips (
(hereinafter referred to as the tropics).

After annealing and pickling, cold rolling, annealing, and pickling are performed once or twice to form a cold rolled steel strip, which is then temper rolled and then cut into specified dimensions. and make it into a product.

During the manufacturing process, a so-called ferrite band in which coarse ferrite grains and coarse ferrite grain groups having macroscopically similar crystal orientations extend in a layered manner is formed at the center of the tropical sheet thickness. Even after subsequent tropical annealing, cold rolling, and final annealing, this ferrite band portion is difficult to recrystallize and remains as a group of ferrite grains with similar crystal orientations, making it appear as if it were a single crystal during deep drawing or stretching. Because of this deformation behavior, it is thought that ridging occurs due to plastic anisotropy with other heptagonite grains that come into instant contact with the grains.

As mentioned above, one of the reasons for the formation of ferrite bands in the center of the plate thickness in the tropics is thought to be the coarse structure of the continuously cast slab. It is known that since processing such as blooming is performed before forming a slab, the coarse structure is fragmented and ridging is improved.

In other words, in the manufacturing method of ferritic stainless steel sheets, the problem of such ridging occurring was rare in the days when the ordinary ingot forming method was used before the continuous casting method was developed. However, in terms of productivity, it is now completely unthinkable to discontinue the continuous casting method in order to avoid ridging.

)Yo. 7. -t'1, engineering, 3. Wow. A□Koyu 1. Therefore, the reason why the coarse ferrite grains or ferrite bands at the center of the sheet thickness cannot be subdivided sufficiently is that the processing strain at the center of the sheet thickness during hot rolling is smaller than that at the surface layer, and also because of the temperature during hot rolling. This decrease is thought to be due to the fact that the center part of the plate is smaller than the surface part, so the dislocations that multiply with each roll pass are easily recovered, and there is little opportunity for microstructure refinement by recrystallization. , In order to reduce ridging, a possible measure to be taken in the hot rolling process is to increase the probability of recrystallization during hot rolling by applying as large a strain as possible to the center of the sheet thickness. It is known that increasing the rolling reduction per pass and using rolls of different diameters are effective. In addition to the above-mentioned means, in order to promote recrystallization in tropical annealing, there is also known a method of lowering the finishing temperature during hot rolling to leave strain in the center of the thickness of the tropical sheet.

However, it is difficult to fully implement the above measures in the actual hot rolling process due to equipment constraints. The types of mills currently used for hot rolling stainless steel include tandem mills, which are commonly used for mild steel, and Steckel mills and planetary mills, which are exclusively used for stainless steel. Among these, planetary mills in particular complete rolling within a distance of only a few meters and in an extremely short time, making it even more difficult to implement the above measures.

According to the invention described in Japanese Patent Publication No. 47-1878, a method for manufacturing a ferritic stainless steel sheet that does not cause ridging and has excellent workability is proposed. The gist of this method is as stated in the claims: [In ferritic stainless steel containing 0.15% or less carbon and 13 to 25% chromium by weight,
After annealing the stiff hot-rolled sheet within 10 minutes at a temperature range of 930 to 990°C where austenite and 7-erite phases coexist, the sheet is cooled in air or at a faster rate than air cooling to form a ferritic structure containing a martensite dispersed phase. , A ferritic stainless steel sheet having excellent workability without causing ridging, characterized in that an annealed sheet having this structure is cold-rolled in multiple stages including steps with a reduction ratio of 30% or more and intercalated annealing, and then annealed. agricultural methods. ”. In other words, according to this method, coarse ferrite grains and ferrite bands existing in the center of the tropical plate thickness are not only fragmented during high-temperature heating, but also cold-rolled into a heptite structure containing a hard martensite dispersed phase. As a result, the ferrite grains around the martensite phase are divided, and the occurrence of ridging can be almost completely prevented.

Furthermore, according to the invention described in Japanese Patent Publication No. 57-58416,
A method for preventing gold dust on cold-rolled thin sheets of chromium-containing ferritic stainless steel is disclosed. This method involves heat-treating a hot-rolled steel strip at a temperature range of 850 N and 1000°C and then cooling it at a rate of 200°C/hr or less to reach 700°C in the production of cold-rolled ferritic stainless steel sheets. A specified invention is a method characterized by adding cooling control to suppress the above heat treatment, and replenishing the Or-deficient region once generated in the steel by the diffusion of Or from the surrounding ground, and further In the above cooling process, 200℃/h while reaching the temperature range of 750 to 650℃
At a cooling rate exceeding r, cooling control is applied to maintain the cooling temperature at a temperature of 700°C or higher within the temperature range for at least 15 minutes, for a longer period of time at a much lower temperature, and for 60 minutes or more at the lower limit temperature. , due to the diffusion of Cr from the surrounding ground into the Or-deficient region once formed in the steel by the above heat treatment. The second invention is a method characterized in that the area is compensated for by seven points.

(Problems to be Solved by the Invention) In the invention described in Japanese Patent Publication No. 47-1878, the martensite dispersed phase is lost, so the strength in the tropics increases and the ductility deteriorates. Therefore, coil edge breakage and coil breakage occur during cold rolling. Another disadvantage is that the presence of the martensitic phase in the tropics leads to a deterioration of the i-value, which is an indicator of the ductility and maximum deep drawing depth of the product.

Furthermore, according to the above method, grain boundary KO/carbide precipitates during the cooling process after heating in the tropics (high-temperature oil), and because the cooling rate is fast, an Or-depleted layer is generated around it.

For this reason, grain boundary corrosion occurs during the subsequent pickling process, and after cold rolling, there are many minute scratches and cracks, and when the film such as PvC attached to protect the surface is removed when the product is used, it peels off. This causes a problem in that it causes diffuse reflection in the form of dots, resulting in glittering surface defects that impair the surface appearance of the product.

On the other hand, according to the invention described in Japanese Patent Publication No. 57-58416,
In addition to preventing gold dust, cooling control also suppresses the formation of martensite, which prevents coil breakage during cooling and deterioration of product ductility. Since division is not performed, it is insufficient to prevent ridging, and furthermore, since the heat treatment is annealed at a high temperature of 850 to 1000°C, the r value is more likely to deteriorate than in normal annealing. Therefore, although the above-mentioned publication is an appropriate method for preventing gold dust, which is the object of the invention, it is inappropriate as a method for manufacturing a ferritic stainless steel strip without ribbing.

In summary, 1. Among the conventional manufacturing processes for ferritic stainless steel, problems inherent in the tropical high temperature annealing method,
The method disclosed in Japanese Patent Publication No. 47-1878 causes the coil edges to be cut during the process, deterioration of the r-value of the product, and generation of gold dust, while the invention described in Japanese Patent Publication No. 57-58416 does not sufficiently rid the product. The problem is that it cannot be prevented.

(Means for solving the problems) The present invention solves the above problems at once by simultaneously controlling the heating and cooling processes of tropical high-temperature annealing. It is an object of the present invention to provide a method for manufacturing a stainless steel strip. Next, the present invention will be described in detail. Conventionally, tropical annealing of ferritic stainless steel has been carried out in a box furnace or a continuous annealing furnace. , the heating rate is mainly determined by the burner capacity of the furnace, but for box-type furnaces it is
50~b・'' is 100~b. We investigated the effects of ridging and 〒＠[1] on products that

As a result, by controlling the heating means in the tropical high temperature annealing method, product ridging can be significantly reduced.
Furthermore, the present invention was completed based on the new finding that by controlling the cooling rate, it is possible to simultaneously prevent three-value deterioration and the generation of gold dust caused by conventional methods.

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a diagram showing the relationship between temperature and time in the heating and cooling process of the specific invention of the present invention.
75℃/h from temperature to Ac1 point to 1000℃
After heating at a heating rate of less than r, the heating temperature is reduced to 70
By cooling in the temperature range down to 0°C at a cooling rate of 20 to 200°C/hr and then allowing it to cool, a ferritic stainless steel strip with no ridging and excellent workability can be produced.

Ukiru.

゛°During the heating process, the heating rate was controlled to 700℃.
By heating from °C to AC+ point to 1000 °C,
It is possible to completely eliminate the coarse ferrite grains and ferrite bands that are present in the center of the plate thickness in the tropics, which are the causes of ridging in the products mentioned above.

The reason for this is that an austenite phase is generated in the coarse ferrite grains and ferrite bands to separate them. Therefore, the first requirement of the present invention is to cause the austenite phase to appear by heating above the AOt point, but it is also an essential requirement to randomize the position where the austenite phase occurs.

Here, the microscopic structure after hot rolling consists of a ferrite phase with a high amount of Or, a martensite phase with a low amount of Or, and an Or carbonitride phase, and the martensite phase transforms into a ferrite phase during the subsequent annealing heating process. If the heating rate is not controlled as in the conventional method, the austenite phase generated above the AOt point will concentrate in the original martensite phase, resulting in the separation of the coarse heptite grains and ferrite bands. It weakens.

One of the features of the present invention is that the heating rate is controlled to less than 75°C/hr in the temperature range from 700°C to the AC+ point, where the diffusion rate is fast, to reduce the heterogeneous structure after hot rolling. This makes it possible to uniformly generate the austenite phase in the coarse ferrite grains and ferrite bands in the following AC+C origin temperature range, and to completely divide the austenite phase.

On the other hand, if the cooling rate at 700' Ct exceeds 200°C/hr in the cooling process after heating, the austenite phase at high temperature will transform into a brittle martensite phase, causing problems such as edge breakage in the subsequent cold rolling process. In addition, as disclosed in the invention described in Japanese Patent Publication No. 57-58416, an ir-deficient layer (ir-deficient layer) is generated at grain boundaries, which causes surface flaws, i.e., gold dust, when the product is manufactured. Therefore, the cooling rate up to 700°C is 200°C in the present invention.
It is necessary to limit the amount to less than /hr.

' Next, the conventional tropical high-temperature annealing method had the disadvantage that the i value, which is an index of product workability, easily deteriorated. We newly discovered that crystal grains tend to coarsen, and as a method to prevent this, we suppressed the maximum heating temperature in the annealing process to 1000°C or less, and in the subsequent cooling process, heated up to 700°C, where crystal grains tend to coarsen. Temperature range 20℃/
The specific invention was completed by discovering a method for cooling at a rate of more than 1 hour.

As mentioned above, the specific invention of the present invention (hereinafter referred to as the first invention, and the present inventions related to the specific invention described below are expressed as the second invention, third invention, etc.) has been explained. The second invention will be explained.

The purpose of cooling control in the present invention is to completely transform the austenite phase to the ferrite phase and to
The purpose is to prevent the formation of an r-depleted layer and the coarsening of crystal grains, but the cooling rate is 200°C/h in the cooling process after heating.
The second invention was completed based on the knowledge that even if the temperature exceeds r, the above object can be sufficiently achieved by maintaining the temperature within the temperature range of 775 to 725°C for 2 hours or more, where the transformation rate is high and grain coarsening does not occur. . Note that the heating process of the second invention is the same as that of the first invention, but the above-mentioned cooling process is included.

In other words, the second invention is shown in Figure 2, and the tropical area is 700
℃ temperature to Ac+ point to 1000℃ temperature 75℃/
After heating at a heating rate of less than 7 hr from the above heating temperature
It is characterized in that the temperature range of 75 to 725°C is cooled at a cooling rate exceeding 200″cAr, and then the temperature range of 775 to 725°C is maintained for 2 hours or more, and then allowed to cool.

Next, the purpose of heating control in the present invention is to
The aim is to achieve compositional homogenization using diffusion phenomena in the temperature range from 00°C to Ac1 point, but even if heated at a temperature of 700°C or higher at a rate exceeding 75°C/hr, the diffusion rate is high. The third invention was completed by discovering the temperature range from °C to Ac1 point.

According to the third aspect of the present invention, as shown in FIG. Subsequently, after holding within the temperature range of 800°C to A (+ point) for more than 4 hours, further heating within the temperature range of AC3 point to 1000'C, and then heating from the above heating temperature to 700'C at 20 to 200°C. It is generally recommended that the sample be cooled at a cooling rate of /hr and then allowed to cool.

Next, the object of the present invention can be sufficiently achieved by combining the heating process using the method of the third invention and the cooling process using the method of the second invention. That is, according to the fourth aspect of the present invention, as shown in FIG.
After heating at a heating rate exceeding 75 ° C / hr to the temperature of the Act point, after holding in the temperature range of 800 ° C to Ac1 point for 4 hours or more, and further heating to the temperature range of AOt point to 1000 ° C., 775 to 72 from the above heating temperature
It is characterized by cooling in a temperature range of 5°C at a cooling rate of over 200°C/hr, subsequently maintaining the temperature within a temperature range of 775 to 725°C for 2 hours or more, and then allowing it to cool.

Next, the present invention will be explained with reference to examples.

EXAMPLE Slabs formed by the continuous casting method were rolled in a planetary mill to a thickness of 5 cm. SUS 430 Tropical material was used and annealed in a laboratory Elema furnace at various temperatures for one hour. The temperature point of the sample E31JE3430 in the tropics was 880°C. After annealing, the tropical plate is pickled to remove scale, and then finished in a 4-stage mill with a thickness of 1.6 mm at 830℃ x 2
Intermediate annealing with air cooling for 2 min, pickling, and then using a 4-stage mill to obtain a board with a thickness of 4 cm, final annealing with air cooling at 830°C x 2 min, and pickling. Served.

The evaluation method for ridging is to apply 15% tensile strain in the direction parallel to the rolling direction, and use a surface roughness meter to measure JISB-060.
1, measure the cross-sectional curve defined by 1, and measure the standard length 2.5
Evaluation was made based on the value of the maximum waviness height WaM (2). Note that this method does not cause ridging.) When measuring the product of AsUs 304 with a thickness of 0.4, the W2N value is 4 to 5 μm.
If it is 5 μm or less, it can be said that there is substantially no ridging. r* is JI35
The measurement was carried out by applying a tensile strain of 15% to the No. 1 test piece. Plate thickness 0
．． If the i value of one 45 mg product is 1.2 or more, normal deep drawing is sufficient. In addition to ridging and r-value, gold dust generated by normal tensile tests and cellophane tape peeling was evaluated. The rating for Gold Dust is 0 if it has not occurred at all, and 1 to 3 if it has occurred depending on the degree of persistence.
classified into.

Gold dust scores up to 1 are allowed. Next, the above test results are shown in the table.

It is clear from this table that according to the present invention, a ferritic stainless steel strip with no ridging (and high F value) can be obtained, troubles such as coil breakage during the manufacturing process are prevented, and gold dust is removed. It can be seen that the occurrence of is also prevented.

On the other hand, among the conventional methods, the generally employed low temperature diffusion annealing method performed at a temperature below the Ac1 point has a markedly inferior ridging of the product. Also, tropical high temperature annealing method,
- According to the 18 conditions included in the invention described in No. 1878, r
Gold dust is generated along with deterioration of value and elongation, and heating control is insufficient (Effects of the invention) The present invention adds heating control to ridging, which has been a drawback when manufacturing ferritic stainless steel strips. This method uses a tropical high-temperature annealing method to prevent ridging, and eliminates the factors that increase costs and cause instability in effectiveness compared to conventional ingot-forming methods or methods to prevent ridging by controlling hot-rolling conditions. Moreover, compared to the conventional tropical high-m annealing method, troubles in the following cold rolling process can be prevented from occurring, and the deterioration of the i value, which was a drawback, can be prevented.

As described above, according to the present invention, it is possible to economically and stably supply ferritic stainless steel with no ridging and excellent workability, and the economic effect thereof is enormous.

[Brief explanation of the drawing]

1 to 4 are views showing the heat treatment method of the present invention in tropical annealing, respectively. Patent applicant: Nippon Yakin Kogyo Co., Ltd. Agent: Patent attorney Mura 1) Masaru Oil quantity
Patent Attorney Takuya Hatano Figure 2 Figure 4 Shun Maki

Claims (1)

[Claims] 1. In the process of heat-treating a hot-rolled steel strip in the manufacturing process of cold-rolled ferritic stainless steel thin sheets: The hot-rolled steel strip is heated from a temperature of 700°C to a temperature of A_C_1 to 1000°C. After heating at a heating rate of less than 75°C/hr;
A method for manufacturing a ferritic stainless steel strip having no ridging and excellent workability, characterized by: cooling at a cooling rate of 00° C./hr and then allowing it to cool. 2. In the process of heat-treating a hot-rolled steel strip in the manufacturing process of cold-rolled ferritic stainless steel thin sheets: The hot-rolled steel strip is heated from a temperature of 700°C to a temperature of 1000°C from point A_C_1 at a rate of less than 75°C/hr. After heating at the heating rate; from the above heating temperature to 775 to 725°C.
Cooling at a cooling rate greater than 0°C/hr; followed by 775
A method for producing a ferritic stainless steel strip having no ridging and excellent workability, characterized in that the strip is maintained within a temperature range of 2 hours or more and then allowed to cool. 3. In the process of heat-treating a hot-rolled steel strip in the manufacturing process of cold-rolled ferritic stainless steel thin sheets: The hot-rolled steel strip is heated from a temperature of 700°C to 800°C to A_
Heated to a temperature of point C_1 at a heating rate of 75°C/hr or more; Subsequently held within the temperature range of 800°C to point A_C_1 for 4 hours or more; further heated to a temperature range of point A_C_1 to 1000°C. After: Cooling from the heating temperature to 700°C at a cooling rate of 20 to 200°C/hr, and then allowing it to cool. A method for producing a ferritic stainless steel strip having no ridging and excellent workability. 4. In the process of heat-treating a hot-rolled steel strip in the manufacturing process of cold-rolled ferritic stainless steel thin sheets: The hot-rolled steel strip is heated from a temperature of 700°C to 800°C to A_
After heating to the temperature of point C_1 at a heating rate of 75°C/hr or more; After maintaining the temperature within the temperature range of 800°C to point A_C_1 for 4 hours or more; After further heating to the temperature range of point A_C_1 to 1000°C; The temperature range from 775 to 725 degrees Celsius is 20 degrees from the above heating temperature.
Cooling at a cooling rate exceeding 0°C/hr; subsequently, 77
A method for manufacturing a ferritic stainless steel strip having no ridging and excellent workability, characterized by: maintaining the temperature within a temperature range of 5 to 725° C. for 2 hours or more and then allowing it to cool.