JP3917320B2 - Method for producing ferritic stainless steel sheet with excellent ridging resistance - Google Patents

Description

【００３３】
【表２】

【００３４】
結果は、表２中の記載に明らかなように、本発明例についてはいずれもうねり (ＷＣＭ）が25μｍ以下でリジングの発生は極めて少ないが、比較例については、加熱温度が本発明の範囲を外れる例：No. ５，９，10、仕上圧延温度が 900℃を超えて高い例：６，９，11、パス時間間隔が長く、粗圧延の終了温度が低い例：No. ７、巻取り温度が高い例：No. ８は、ＷＣＭがいずれも30μｍを超え、リジングの発生が認められた。
【００３５】
【発明の効果】
以上説明したように本発明によれば、リジングが極めて少ないフェライト系ステンレス鋼を安価にかつ確実に製造することができる。
【図面の簡単な説明】
【図１】リジングレベルに及ぼすγｐの影響を示すグラフである。
【図２】Ｃ：0.02〜0.10wt％、Cr：15〜18wt％のFe−Crの状態図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a method for producing a ferritic stainless steel sheet having extremely low ridging generation and excellent ridging resistance, and in particular, by subjecting a slab to heat extraction in the (α + γ) two-phase region, followed by the high temperature rough rolling-low temperature finish rolling. Further, the present invention relates to a method for producing a ferritic stainless steel sheet having excellent deep drawability and resistance to rough skin by processing.
[0002]
[Prior art]
Ridging (also called roping) is a saddle-like undulation (elongated streak-like irregularities) that occurs along the rolling direction of a steel surface of a ferritic stainless steel when it is pulled or deep drawn. This is a phenomenon peculiar to ferritic stainless steel sheets.
[0003]
In general, ferritic stainless steel cold-rolled steel sheets (JIS-SUS430, etc.) have excellent corrosion resistance, maintain a beautiful surface gloss over a long period of time, have good workability, and are less expensive than austenitic stainless steel. For this reason, they are used in a wide range of fields such as kitchen appliances, electric appliances for home appliances, and automobile parts. Thus, since ferritic stainless steel is often used mainly for applications that require decorative properties, not only corrosion resistance and press workability, but also the beauty of the surface properties after processing is an important requirement. In particular, ferritic stainless steel is used as a drawing material such as a cylinder or a square tube. However, if the material properties resulting from the manufacturing method are poor, the above-mentioned ridging appears at the time of molding processing, and the appearance of the surface is impaired. Not only that, but in severe cases, this causes a problem that cracks occur during molding.
[0004]
Therefore, in this field, it has become a major research subject to produce ferritic stainless steel that can reduce or eliminate the generation of ridging. There have been many studies on the ridging generation prevention technology, and a method for producing a hot-rolled sheet having a uniform recrystallized structure is particularly attracting attention in order to prevent ridging generation. For example,
(1) Japanese Patent Publication No. 45-34016 discloses hot rolling at a low temperature, followed by box annealing at 800-830 ° C, followed by cold rolling and finish annealing to improve ridging resistance. A method of making it happen is disclosed.
(2) Japanese Patent Publication No.57-61096 discloses a method in which hot rolling with a rolling reduction of 20% or more is performed by a special shape rolling mill, followed by hot-rolled sheet annealing, cold rolling, and finish annealing. Yes.
(3) In JP-A-1-111816, hot rolling at 850 ° C. or higher, immediately cooling at a rate of 10 ° C./second or higher, and winding at a temperature of 550 ° C. or lower to obtain two ferrite and martensite There has been proposed a method of forming a phase structure and then performing cold rolling with a cumulative rolling reduction of 50% or more.
(4) In Japanese Patent Application Laid-Open No. 7-84617, a continuous cast slab in which equiaxed grains having a grain size of 0.9 mm or less occupy 70% or more of the plate thickness is cast, and this slab is reduced to a reduction rate of 40% at 1100 to 1000 ° C. There has been proposed a method of performing hot rolling sheet annealing, cold rolling, and finish annealing after performing the above hot rolling.
(5) In JP-A-7-118754, in designing the composition of ferritic stainless steel, the gamma potential (γp) is increased and heating is performed at a temperature of 1100 to 1220 ° C, and hot at a temperature of 950 to 1050 ° C. A method is proposed in which finish rolling is performed, winding is performed at a temperature of 450 to 800 ° C., descaling is performed, and then cold rolling is performed at a total reduction of 70% or more.
[0005]
However, all of these methods have been proposed to improve local measures during the entire manufacturing process, i.e., any of the processes such as the casting process, hot rolling process, or annealing process. is not. Moreover, the content of each of these processes is not a technique that makes ridging resistance a direct solution, and is therefore not a sufficient measure for reducing ridging.
[0006]
[Problems to be solved by the invention]
There is a common recognition that the cause of ridging that occurs in ferritic stainless steel sheet is mainly due to the heterogeneous structure (colony structure) present in the sheet. For example, the columnar crystals of a continuous cast slab cannot be sufficiently broken only by ordinary hot rough rolling and hot finish rolling processes. Therefore, even if hot-rolled sheet annealing or cold rolling is applied to such a hot-rolled steel sheet, it is difficult to obtain a steel sheet that can reliably prevent ridging as long as the colony structure remains. It is.
Accordingly, an object of the present invention is to establish a technique for producing a ferritic stainless steel sheet having excellent ridging resistance, which has not been obtained under the conventional SUS 430 production technique, by determining the cause of ridging. .
[0007]
[Means for Solving the Problems]
As a result of intensive research aimed at realizing the above-mentioned objectives, the inventors have improved the equiaxed crystal ratio of slabs of ferritic stainless steel, and reliably divided columnar crystals of continuous cast slabs during hot rolling. It was found that the generation of the ridging can be reduced or prevented by (destruction). Therefore, in the present invention, mainly in hot rolling, the austenite phase is controlled to be precipitated and not recrystallized, while the amount of the precipitated austenite phase is controlled, thereby forming a band structure that causes ridging. This was done so that ridging was greatly reduced.
[0008]
In the present invention, the following points have been further studied in order to realize such operational effects more reliably. In other words, we examined the component composition, casting structure, heating conditions, hot rolling conditions (coarse hot rolling conditions, finishing hot rolling conditions) and cold rolling conditions necessary to further suppress the generation of ridging.
(1) As the component composition, increasing the gamma potential (γp), which is an index representing the maximum precipitation amount of the γ phase for C ≧ 0.02 wt% ferritic stainless steel,
(2) For the cast structure, increase the equiaxed crystal ratio of the continuous cast slab,
(3) As heating conditions, more γ phase is precipitated by heating in the two-phase (α + γ) region of ferrite and austenite.
(4) Hot rolling conditions include rough rolling in the two-phase (α + γ) region to sever the cast structure (columnar crystals), and then cold rolling to a predetermined strain on the hot rolled plate. , And randomizing the structure of the hot-rolled sheet by recrystallization during the next process of hot-rolled sheet annealing,
Has been found to be effective in the production of ferritic stainless steel sheets having excellent ridging resistance, and the present invention has been completed.
[0009]
Further, in the present invention, by increasing the total rolling reduction in the cold rolling process, the accumulation of strain is further increased, and the crystal orientation of the product plate is randomized by recrystallization in the final product annealing process. Further improvement of the ridging resistance can be achieved by realizing crystallization and refinement of crystal grains and then subjecting to temper rolling as necessary.
[0010]
The present invention completed under such a concept,
C: 0.02-0.10 wt%, Si: 0.1-1.0 wt%, Mn: 0.1-1.0 wt%, P: 0.040 wt% or less, S: 0.020 wt% or less, Cr : 15.0 to 18.0 wt%, N: 0.02 to 0.06 wt%, Ni: 0.1 to 0.6 wt%, the balance is Fe and inevitable impurities, and γp is the following ( 1) When a continuous cast slab of ferritic stainless steel having a composition satisfying the formula is extracted from a mold, 15% or more (preferably 20% or more, more preferably 25% or more) of the slab thickness is equiaxed. A continuous cast slab having a solidified structure is prepared, and this slab is hot rough-rolled and hot-finished to form a hot-rolled sheet, and then subjected to normal annealing and cold rolling to obtain a ferritic stainless steel sheet. In the method to A slab of ferritic stainless steel is heated to a temperature of 1050 to 1150 ° C. in a ferrite and austenite two-phase (α + γ) region, and then hot rough rolling performed in the two-phase temperature region is performed at a rolling start temperature of 1000 ° C. or higher. In a multi-pass rolling in which the rolling end temperature is 950 ° C. or more and the interval between each pass is 10 seconds or less, there are two passes or more and a total reduction rate of 50% or more and a reduction rate of 20% or more. Cooling to 850 ° C. or lower at a rate of 1.2 ° C./min or higher until the next hot finish rolling is started, and then not exceeding 900 ° C. In the temperature range where recrystallization is not caused by recuperation for 5 seconds or less at intervals of each pass, hot finish rolling is performed in which the total reduction ratio is 50% or more after 2 passes or more, and then does not recover. temperature der It is a method for producing a ferritic stainless steel sheet excellent in ridging resistance, characterized by being cooled to 700 ° C. or lower and wound up .
50.0 ≦ γp = 288C−54Si + 7.5Mn + 22Ni-18.75Cr + 350N + 338.5 ≦ 70.0 (1)
[0012]
Furthermore, in the present invention, after the hot rolling is completed, cold rolling is performed at a reduction rate of 60% or more in total in one or more cold rollings including hot-rolled sheet annealing and two or more cold sandwiches. It is preferable to produce a ferritic stainless steel sheet having excellent ridging resistance by performing final annealing and descaling treatment or bright annealing, and further performing temper rolling as necessary.
[0013]
In the present invention, hot finish rolling is performed in a stickel mill having a furnace coiler on the entry / exit side of the rolling stand, that is, in a normal method, the coil wound in the coiler for each pass is restored to the recrystallization temperature or more. It is suitable to apply to a hot finish rolling method using a Steckel mill that heats and naturally removes processing strain and recrystallizes. It can also be applied to tandem mills.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
In the production method according to the present invention, the steel type to be applied, that is, the starting material (slab) is used by continuously casting the following component composition.
C: 0.04-0.10wt%
C has a large effect on γp, and in order to design the component so as to satisfy the above equation (1), if it is less than 0.04 wt%, it is necessary to add a large amount of other austenite-generating elements, resulting in an increase in cost and strength. Decreases or deteriorates manufacturability and moldability. On the other hand, when the content is 0.1 wt% or more, more Cr carbide precipitates, which deteriorates corrosion resistance and processability. Therefore, the C content for achieving an appropriate balance is 0.04 to 0.10 wt%. Preferably, it is 0.05-0.07 wt%.
[0015]
Si: 0.1 to 1.0 wt%
Si, a component necessary for deoxidation, was set to 0.1 to 1.0 wt% to the extent that it is contained in ordinary ferritic stainless steel. Preferably it is 0.1-0.4 wt%.
[0016]
Mn: 0.1 to 1.0 wt%
Mn is an element necessary for deoxidation, and is 0.1 to 1.0 wt% in order to ensure good hot workability and strength. Preferably it is the range of 0.2-0.7 wt%.
[0017]
P: 0.040 wt% or less Since P deteriorates toughness, hot workability and corrosion resistance, 0.040 wt% or less is desirable.
[0018]
S: 0.020 wt% or less Since S deteriorates toughness, hot workability and corrosion resistance, it is made 0.020 wt% or less, more preferably 0.010 wt% or less.
[0019]
Cr: 15.0-18.0wt%
Cr is an indispensable component for corrosion resistance and high temperature oxidation resistance in stainless steel, and it is necessary to add at least 15 wt%. However, if the content exceeds 18 wt%, γp decreases and the cost increases.
[0020]
N: 0.02 to 0.06 wt%
N, like C, produces Cr nitride and causes intergranular corrosion in ferritic stainless steel sheets. In addition, the influence on γp is large, and 0.02 to 0.06 wt% is necessary to satisfy the above equation (1). Preferably, it is 0.03-0.05 wt%.
[0021]
Ni: 0.1 to 0.6 wt%
Ni is a formation element of the γ phase, but its value is very high, and the addition range is 0.1 to 0.6 wt%. Preferably, it is 0.02 to 0.05 wt%.
[0022]
In the present invention, the slab of the ferritic stainless steel is not only sufficient to have the above-described component composition, and further, the control of γp is effective in order to precipitate the amount of γ phase necessary to break up the columnar crystals. It is. This γp is controlled so that the numerical value represented by the following formula (1) is 50 to 70.
50.0 ≦ γp = 288C-54Si + 7.5Mn + 22Ni-18.75Cr + 350N + 338.5 ≦ 70.0… (1)
[0023]
The reason for this is that, as shown in FIG. 1, when γp is less than 50, the amount of γ precipitates is small, and the effect of randomizing the hot-rolled sheet structure due to the precipitation of γ phase is small. Too much amount degrades manufacturability. Therefore, in order to obtain ridging resistance at a level that satisfies the required quality, γp must be in the range of 50 to 70.
In the figure, ridging level A is WCM <25 μm, B is 25 μm ≦ WCM <35 μm, C is 35 μm ≦ WCM <45 μm, and D is WCM ≧ 45 μm. (Measurement length: 20mm)
[0024]
In the present invention, when the slab is extracted from the continuous casting mold, the slab has an equiaxed crystal solidification structure at least in the center, and the equiaxed crystal portion particularly accounts for 15% or more of the plate thickness. It is necessary to use a slab. Generally, ridging is caused by a columnar solidified structure formed in a continuous cast slab, and this structure remains as a cast structure without undergoing phase transformation during the cooling process, and this structure remains completely after hot rolling and cold rolling. This is thought to be because it remains without being destroyed and forms a texture.
Therefore, in the present invention, as a countermeasure for reducing ridging, the columnar crystal structure of the continuous cast slab, which is the root cause of ridging, is reduced, and the ratio of equiaxed solidified structure is increased. In particular, if the equiaxed crystal ratio of this slab is 15% or less of the plate thickness, the effect of reducing ridging is small, so it was set to 15% or more. Preferably it is 20% or more, more preferably 25% or more.
[0025]
The present invention is also characterized in that the slab is heated to a temperature of 1050 ° C. or higher and 1150 ° C. or lower. As shown in the Fe-Cr phase diagram of FIG. 2, the ferritic stainless steel according to the composition of the above components is composed of C: 0.05wt%, N: 0.03wt%, Cr: 16wt%, The temperature range is 850-1200 ° C. Heating it to a temperature range of 1050 to 1150 ° C means heating in the two-phase (α + γ) region of ferrite and austenite (≧ 1200 ° C for α single-phase region extraction). The reason for heating in such a region is that by precipitating a large amount of γ phase, the columnar crystals are divided and broken at the stage of hot rolling to facilitate the prevention of ridging. If the slab is heated to a temperature range other than the above range, the amount of γ phase during hot rough hot rolling decreases, and the ridging reduction effect decreases.
[0026]
In the present invention, in order to sever the columnar crystals of the continuous cast slab, the total rolling reduction is at least 50%, preferably at least 2 passes (preferably about 5 to 7 passes) in the two-phase (α + γ) temperature range. %, More preferably 75% or more, and a path with a rolling reduction of 20% or more is 1/2 or more of the total path (preferably 25% or more of the path is 1/2 or more of the total number of paths, more preferably 30% Hot rough rolling is performed in which the above passes are 1/2 or more of the total number of passes). This is because the necessary amount of plastic deformation is secured in the (α + γ) two-phase region and the columnar crystals are more effectively divided.
[0027]
Specifically, the above-mentioned hot rough rolling is performed at a rolling start temperature of 1000 ° C. or higher, preferably 1050 ° C. or higher, a rolling end temperature of 950 ° C. or higher, preferably 1000 ° C. or higher, and the interval between each pass, particularly the plate. In a multi-pass in which the pass time until the thickness becomes 1/4 or less of the initial slab thickness is 10 seconds or less, the total number of passes with a rolling reduction of 20% or more (preferably 25% or more, more preferably 30% or more) Multi-pass rolling that performs hot rough rolling to 1/2 or more of the pass and cools at a rate of 1.2 ° C / min or more after the hot rough rolling until the start of hot finish rolling in the next process I do. The reason for performing such multipass hot rough rolling is that when the temperature at the start of rolling is 1000 ° C. or less, a large amount of γ phase is precipitated, making it difficult to recrystallize the α phase. The reason for setting the time between the passes to 10 seconds or less is that if it is 10 seconds or more, the temperature is lowered and the content of the γ phase is reduced, and the effect of reducing ridging is reduced.
The reason for hot finish rolling after cooling to a temperature of 850 ° C or lower at a rate of 1.2 ° C / min or higher before or after the final pass of hot rough rolling is to perform hot finish rolling at a low temperature. It is. That is, if the temperature is not cooled below 850 ° C, the maximum temperature of the hot-rolled sheet exceeds 900 ° C due to recuperation during the hot finish rolling, so that a part of the hot-rolled sheet structure is recovered and the ridging resistance deteriorates. It is.
[0028]
In the present invention, hot finish rolling is performed at a low temperature. The reason for this is to accumulate deformation strain necessary for recrystallization during hot-rolled sheet annealing. That is, the maximum temperature of the hot-rolled sheet during hot finish rolling is 900 ° C. or less (preferably 850 to 700 ° C.) and 2 or more passes (preferably 5 to 7 passes) in total 50% or more (preferably 65 % Or more, more preferably 75% or more) in order to carry out multi-pass hot finish rolling with a cumulative reduction ratio. In addition, if the total rolling reduction at this time is less than 50%, the accumulated amount of deformation strain in the hot-rolled sheet is small, and uniform recrystallization cannot be obtained in the next hot-rolled sheet annealing, so the ridging resistance is deteriorated. .
[0029]
In the present invention, after the hot finish rolling is completed, the coiling temperature is set to 700 ° C. or lower (preferably 650 ° C. or lower, more preferably 600 ° C. or lower) in order to minimize the strain release of the obtained hot rolled sheet. It is necessary to wind up.
[0030]
In the present invention, after the hot rolling is completed as described above, the total rolling reduction ratio of two or more sandwiching one cold rolling or intermediate annealing through hot rolled sheet annealing is 60% or more. Ferritic stainless steel sheet with excellent ridging resistance, that is, no ridging is produced by performing cold rolling, performing final annealing and descaling or bright annealing, and then performing temper rolling as necessary. .
[0031]
【Example】
Examples of producing ferritic stainless steel according to the production method of the present invention will be described below.
A ferritic stainless steel having the chemical components shown in Table 1 is melted in a converter (AOD) and continuously cast, and the central part is a continuous 200 mm thick whose equiaxed crystal is 30% or more of the plate thickness. A cast slab was used. Next, hot rolling was performed under the conditions shown in Table 2 to obtain a hot rolled coil. In this hot rolling, rough rolling was performed from 200 mm to 25 mm in 5 and 7 passes, and finish rolling was performed from 25 mm to 6 mm in 5 passes. The obtained hot-rolled sheet was subjected to bell annealing at a temperature of 820 ° C., and a cold-rolled sheet having a thickness of 0.6 mm was manufactured by cold rolling twice with intermediate annealing. Table 2 shows the cross-sectional structure of the hot-rolled sheet and the ridging score of the cold-rolled sheet. The ridging score is based on the aforementioned criteria.
[0032]
[Table 1]

[0033]
[Table 2]

[0034]
As is apparent from the description in Table 2, the undulation (WCM) is 25 μm or less for the example of the present invention, and the occurrence of ridging is extremely small. However, for the comparative example, the heating temperature is within the range of the present invention. Example: No. 5, 9, 10, High finish rolling temperature exceeding 900 ° C Example: 6, 9, 11, Example of long pass time interval and low end temperature of rough rolling: No. 7, Winding Example with high temperature: In No. 8, WCM exceeded 30 μm, and ridging was observed.
[0035]
【The invention's effect】
As described above, according to the present invention, ferritic stainless steel with very little ridging can be produced inexpensively and reliably.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of γp on ridging levels.
FIG. 2 is a phase diagram of Fe—Cr with C: 0.02 to 0.10 wt% and Cr: 15 to 18 wt%.

Claims (2)

C: 0.02-0.10 wt%, Si: 0.1-1.0 wt%, Mn: 0.1-1.0 wt%, P: 0.040 wt% or less, S: 0.020 wt% or less, Cr : 15.0 to 18.0 wt%, N: 0.02 to 0.06 wt%, Ni: 0.1 to 0.6 wt%, the balance is made of Fe and inevitable impurities, and the center is a plate A slab of ferritic stainless steel having an equiaxed crystal structure of 15% or more of the thickness and satisfying the gamma potential (γp) represented by the following formula (1) is used in the ferrite and austenite two-phase (α + γ) region. The hot rough rolling performed at a temperature of 1050 to 1150 ° C. and then in the two-phase temperature range has a rolling start temperature of 1000 ° C. or more, a rolling end temperature of 950 ° C. or more, and the interval between each pass is 10 seconds or less. a multi-pass rolling is 2 The total rolling reduction is 50% or more and the rolling reduction of 20% or more is 1/2 or more of the total pass, and the time until the next hot finish rolling is started is 1.2 ° C / Cooling to a temperature of 850 ° C. or less at a rate of min or more, then, at a temperature not exceeding 900 ° C., and with the interval between each pass being 5 seconds or less, the recrystallization due to reheating does not occur for 2 passes or more. A ferritic stainless steel sheet having excellent ridging resistance, characterized by performing low temperature hot finish rolling with a total rolling reduction of 50% or more, and then cooling and winding to 700 ° C. or less, which is a temperature that does not recover. Production method.
50.0 ≦ γp = 288C−54Si + 7.5Mn + 22Ni-18.75Cr + 350N + 338.5 ≦ 70.0 (1) After the hot rolling described in claim 1, perform one cold rolling including hot-rolled sheet annealing, or two or more sandwiching the intermediate annealing, and performing cold rolling to a total reduction ratio of 60% or more, A method for producing a ferritic stainless steel sheet having excellent ridging resistance, characterized in that final annealing or bright annealing is performed thereafter, and then temper rolling is performed as necessary.

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