JPH0116288B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a ferritic stainless steel strip that is free of lysine and has excellent workability. (Prior technology) When ferritic stainless steel sheets such as SUS430 are subjected to processing such as deep drawing or overhanging, wrinkles parallel to the rolling direction called ridging or roping occur. . Such wrinkles, or ridging, can be a serious defect in stainless steel plate products that require a beautiful surface, so if such steel plates are subjected to forming processes such as press working, they must be polished after processing. There is a drawback that ridging must be removed by processing such as By the way, the latest manufacturing process for stainless steel sheets is to first melt raw materials in an electric furnace, then refine them using the AOD or VOD method, then continuously cast them into a slab, and then hot-roll this slab. ~6mm thick hot-rolled steel strip (hereinafter referred to as tropical). After annealing and pickling this tropical
Cold rolling, annealing, and pickling are sequentially performed once or twice to obtain a cold rolled steel strip, which is then subjected to skin pass rolling and then cut into predetermined dimensions to produce 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, ridging is thought to occur due to plastic anisotropy with other adjacent ferrite grain groups. 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, there was little problem of such ridging 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 currently unthinkable to discontinue the continuous casting method in order to avoid ridging. By the way, the reason why coarse ferrite grains or ferrite bands at the center of the thickness cannot be sufficiently refined during the hot rolling process of continuously cast slabs is that the processing strain at the center of the thickness is smaller than that at the surface layer during hot rolling. Instead, the temperature drop during hot rolling is
This is thought to be because the central part of the plate thickness is smaller than the surface part, so dislocations that multiply with each roll pass are easily recovered, and there is little opportunity for microstructure refinement by recrystallization. Therefore, 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 also used for mild steel, and stetskell mills and planetary mills, which are exclusively 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, as stated in the claims, is that ``This hot-rolled sheet is made into austenitic steel in a ferritic stainless steel containing 0.15% or less carbon and 13 to 25% chromium by weight. After annealing within 10 minutes at a temperature range of 930 to 990°C where a ferrite phase coexists, the annealed plate is cooled in air or at a faster rate than air cooling to form a ferrite structure containing a martensite dispersed phase, and the annealed plate with this structure is rolled. The method is characterized by performing one-stage cold rolling followed by annealing at a rolling reduction ratio of 30% or more, or multi-stage cold rolling including intermediate annealing and followed by annealing. ``Production method for superior ferritic stainless steel sheets.'' In other words, according to this method, the coarse ferrite grains and ferrite bands present in the center of the tropical sheet thickness are not only fragmented during high-temperature heating, but also cold-rolled with a ferrite structure containing a hard martensite dispersed phase. By dividing the ferrite grains around the martensite phase, 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 to 1000°C and then cooling it during the production of cold-rolled ferritic stainless steel thin sheets.
By adding cooling control to suppress the cooling rate that reaches 700°C to 200°C/hr or less, we compensate for the Cr-deficient regions that once formed in the steel through the above heat treatment with Cr diffusion from the surrounding soil. The method characterized by
~650°C, at a cooling rate of over 200°C/hr, at temperatures above 700°C within the temperature range for at least 15 minutes, longer at lower temperatures, and at least 60 minutes at the lower temperature limit. A second invention provides a method characterized by adding cooling control to maintain the temperature during the heat treatment, and replenishing the Cr-deficient region once formed in the steel by Cr diffusion from the surrounding ground. It is something to do. (Problems to be solved by the invention) Regarding the invention described in Japanese Patent Publication No. 47-1878,
Because the martensite dispersed phase remains, the strength in the tropics increases and the ductility deteriorates. Therefore, coil edge breakage and coil breakage occur during cold rolling. There is also the disadvantage that the presence of the martensitic phase in the tropics leads to a deterioration of the values indicative of the ductility and maximum deep drawing depth of the product. Furthermore, according to the above method, Cr carbides are precipitated at grain boundaries during the cooling process after heating the tropics to high temperatures, and a Cr-depleted layer is generated around the grain boundaries because the cooling rate is fast.
For this reason, intergranular corrosion occurs during the subsequent pickling process, resulting in numerous minute scratches after cold rolling. There is a problem in that it causes diffused reflection, 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,
Not only is gold dust prevented, but cooling control also suppresses the formation of martensite, preventing coil breakage during cooling and deterioration of product ductility. Preventing ridging is insufficient as fragmentation does not take place, and further tropical
By annealing at a high temperature of 850 to 1000°C, the value tends to deteriorate compared to normal annealing. Therefore, although the above 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 ridging. In summary, among the conventional manufacturing processes for ferritic stainless steel, problems inherent in the tropical high-temperature annealing method, namely, the method of Japanese Patent Publication No. 1878-1878, cause coil edge breakage during the process and deterioration of product value. There was also the occurrence of gold dust, and the
According to the invention described in No. 58416, there was a problem that ridging of the product could not be sufficiently 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, and produces a ferritic stainless steel with no ridging and excellent workability. The object of the present invention is to provide a method for manufacturing a strip. Next, the present invention will be explained in detail. Conventionally, tropical annealing of ferritic stainless steel has been carried out in a box furnace or continuous annealing furnace, and the heating rate is mainly determined by the burner capacity of the furnace, but in the case of a box furnace, the heating rate is 150 to 200℃. /hr, and 100 to 1000°C/min in the case of a continuous annealing furnace.The present inventors comprehensively investigated the relationship between the heating means including the heating rate of the SUS430 tropical, the cooling means, and the structural changes in the tropical. The effects on the ridging and value of the products thus obtained were investigated.
As a result, by controlling the heating means in the tropical high temperature annealing method, product ridging can be significantly reduced, and by controlling the cooling rate, it is possible to simultaneously prevent value deterioration and the generation of gold dust caused by conventional methods. The present invention was completed based on the new finding that this is possible. Next, the present invention will be explained with reference to the drawings. FIG. 1 is a diagram showing the heating and cooling process of the specific invention of the present invention in terms of the relationship between temperature and time.
After heating the tropical zone from a temperature of 700℃ to a temperature of 1000℃ from A _C1 point at a heating rate of less than 75℃/hr, the temperature range from the above heating temperature to 700℃ is heated from 20 to 200℃/hr.
By cooling at a cooling rate of hr and then allowing it to cool, a ferritic stainless steel strip with no ridging and excellent workability can be produced. In the heating process, the heating rate is controlled.
By heating from 700°C to A _C1 point to 1000°C, it is possible to completely eliminate the coarse ferrite grains and ferrite bands that exist in the tropics and the center of the plate thickness, which cause ridging in the product described 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 to the A _C1 point or higher, but it is also an essential requirement to randomize the position where the austenite phase occurs. Here, the microstructure after hot rolling consists of a ferrite phase with a high Cr content, a martensitic phase with a low Cr content, and
It consists of a Cr carbonitride phase, and the martensite phase transforms into a ferrite phase during the subsequent annealing and heating process.
Unless the heating rate is controlled as in the conventional method, A _C1
The austenite phase generated above this point is concentrated in the original martensite phase, and as a result, the breaking action of the coarse ferrite grains and ferrite bands is weakened. One of the features of the present invention is that the heating rate of the heterogeneous structure after hot rolling is controlled to less than 75°C/hr in the temperature range of 700°C to A _C1 point where the diffusion rate is fast.
This control first achieves compositional homogenization, and then
It has become possible to generate an austenite phase evenly in the coarse ferrite grains and ferrite bands in a temperature range of A _C1 point or higher and to completely separate the austenite phase. On the other hand, if the cooling rate to 700℃ exceeds 200℃/hr in the cooling process after heating, the austenite phase at high temperatures will transform into a brittle martensite phase, causing problems such as edge breakage in the subsequent cold rolling process. It also causes Cr-depleted layers at grain boundaries as disclosed in the invention described in Japanese Patent Publication No. 57-58416.
It also leads to surface flaws, that is, gold dust, when the product is manufactured. Therefore, the cooling rate up to 700°C must be limited to 200°C/hr or less in the present invention. Next, the conventional tropical high-temperature annealing method has the disadvantage that the value, which is an index of product workability, tends to deteriorate.
The inventors of the present invention investigated the cause of this problem and discovered that the crystal grains of the product tend to become coarse due to tropical high-temperature annealing.As a way to prevent this, the maximum heating temperature in the annealing process was increased. 1000℃
The specific invention was completed by discovering a method for cooling at a rate of 20°C/hr or more in the temperature range up to 700°C, where crystal grains tend to coarsen during the subsequent cooling process. As mentioned above, the specific invention of the present invention (hereinafter referred to as the first invention, and the present invention related to the specific invention described below will be expressed as the second invention, third invention, etc.)
The second invention will now be described. The purpose of cooling control in the present invention is to completely transform the austenite phase to the ferrite phase and to prevent the formation of grain boundary Cr-depleted layers and coarsening of crystal grains. Even if the cooling rate exceeds 200℃/hr, the transformation rate is fast and grain coarsening does not occur between 775 and 725℃.
The second invention was completed based on the finding that the above object can be sufficiently achieved by maintaining the temperature within the temperature range of 2 hours or more. Note that the heating process of the second invention is the same as that of the first invention, but the cooling process is different as described above. In other words, the second invention is as shown in Figure 2, when the temperature of the tropics is increased from 700℃ to 1000℃ from point A _C1 to 75℃.
After heating at a heating rate of less than ℃/hr, cooling in the temperature range of 775 to 725℃ from the above heating temperature at a cooling rate of more than 200℃/hr, and then cooling within the temperature range of 775 to 725℃ for 2 hours. It is characterized in that it is allowed to cool after being held for a longer period of time. Next, the purpose of the heating control in the present invention is to achieve compositional homogenization by utilizing the diffusion phenomenon in the temperature range from 700°C to A _C1 point as described above.
Even if heated at a temperature higher than 75°C/hr, the _first and second
The third invention was completed after discovering that an effect equal to or greater than that of the invention can be obtained. According to the third aspect of the present invention, as shown in _FIG . Subsequently, after holding the temperature within the temperature range of 800℃ to A _C1 point for more than 4 hours, further heating within the temperature range of A _C1 point to 1000℃, and then heating from the above heating temperature to 700℃ for 20 to 200℃/hr. The feature is that it is cooled at a cooling rate of , and then left 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, the fourth aspect of the present invention
According to the invention, as shown in FIG.
From the temperature of 700℃ to 800℃ to the temperature of point A _C1 to 75℃/
800℃ or more after heating at a heating rate exceeding hr.
After maintaining the temperature within the temperature range of point A _C1 for more than 4 hours, and further heating to the temperature range of point A _C1 to 1000℃,
200°C in the temperature range from 775 to 725℃ from the above heating temperature.
Cooled at a cooling rate exceeding 775 °C/hr, followed by
It is characterized by being kept within a temperature range of ~725°C for 2 hours or more and then allowed to cool. Next, the present invention will be explained with reference to examples. EXAMPLE Slabs formed by continuous casting were rolled in a planetary mill to a thickness of 5 mm using SUS430 Tropical as a material, and annealed at various temperature and time patterns in a laboratory Elema furnace. Note that the A _C1 point of the tropical sample SUS430 was 880°C. After annealing, the scale was removed by pickling, and the plate thickness was reduced to 1.6mm using a four-stage mill.
830℃×2min air cooling intermediate annealing, pickling, and then a 4-stage mill to reduce the plate thickness to 0.4mm, 830℃×
After final annealing with air cooling for 2 minutes and pickling, the product was subjected to various tests. The evaluation method for ridging is parallel to the rolling direction.
Apply 15% tensile strain and pass JISB by surface roughness tester.
- The cross-sectional curve defined by 0601 was measured and evaluated based on the value of the maximum waviness height W _CM with a reference length of 2.5 mm. Note that this method does not cause ridging.
When measuring a SUS304 product with a thickness of 0.4 mm, the W _CM value is 4 to 5 μm, and if it is 5 μm or less, it can be considered that there is virtually no ridging. The value was measured by applying 15% tensile strain to a JIS No. 5 test piece. Plate thickness
For 0.4mm products, 1.2 or more is sufficient for normal deep drawing processing. In addition to the ridging and value, gold dust generated by normal tensile tests and cellophane tape peeling was evaluated. The score for gold dust was 0 if no gold dust was generated, and 1 to 3 if it was generated. Gold dust scores up to 1 are allowed. Next, the above test results are shown in the table.

【table】

[Table] As is clear from this table, according to the present invention, a ferritic stainless steel strip with no ridging and a high value can be obtained, and troubles such as coil breakage during the manufacturing process are prevented, and gold dust is removed. It can be seen that this can also be prevented. On the other hand, among the conventional methods, the commonly adopted
A Low-temperature diffusion annealing performed at temperatures below _{the C1} point results in significantly poor ridging of the product. In addition, the tropical high temperature annealing method is included in the invention described in Japanese Patent Publication No. 47-1878.
According to No. 8 conditions, gold dust was generated along with deterioration of value and elongation, and No. 9 where heating control was insufficient.
It can be seen that No. 10 and No. 10 have inferior ridging. (Effects of the Invention) The present invention prevents ridging, which has been a drawback when manufacturing ferritic stainless steel strips, by using a tropical high temperature annealing method with added heating control. Compared to the ridging prevention method by controlling the rolling conditions, the factors that increase costs and cause instability in effectiveness are eliminated. Moreover, compared to the conventional tropical high temperature annealing method, troubles in the following cold rolling process can be prevented from occurring and deterioration of values, 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 which is free from ridging and has excellent workability, and its economic effects are enormous.

[Brief explanation of drawings]

1 to 4 are views showing the heat treatment method of the present invention in tropical annealing, respectively.

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 point A _C1 to
After heating to a temperature of 1000°C at a heating rate of less than 75°C/hr; cooling the temperature range from the above 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 that has no ridging and has excellent workability. 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 point A _C1 to
After heating to a temperature of 1000℃ at a heating rate of less than 75℃/hr; from the above heating temperature to a temperature range of 775 to 725℃
Ferritic stainless steel with no ridging and excellent workability, characterized by cooling at a cooling rate of over 200℃/hr; subsequently maintaining the temperature within a temperature range of 775 to 725℃ for 2 hours or more, and then cooling it; Method for manufacturing steel strips. 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.
After heating to the temperature of A _C1 point at a heating rate of 75℃/hr or more; Subsequently, maintaining the temperature within the temperature range of 800℃ to A _C1 point for 4 hours or more; and then within the temperature range of A _C1 point to 1000℃ After heating to; 20 to 200℃/hr from the above heating temperature to 700℃
A method for producing a ferritic stainless steel strip having no ridging and excellent workability, characterized by: cooling at a cooling rate of , and then allowing it to cool. 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.
After heating to the temperature of point A _C1 at a heating rate of 75℃/hr or more; After maintaining the temperature within the temperature range of 800℃ to point A _C1 for 4 hours or more; Further heating to the temperature range of point A _C1 to 1000℃ After that; from the above heating temperature to a temperature range of 775 to 725℃.
A ferrite type with no ridging and excellent workability, characterized by cooling at a cooling rate of over 200℃/hr; subsequently, keeping it within the temperature range of 775 to 725℃ for 2 hours or more, and then cooling it; Method for manufacturing stainless steel strips.