JP2000256748A - Manufacture of ferritic stainless steel sheet excellent in ridiging resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a ferritic stainless steel sheet excellent in ridging resistance which can not be obtained under the conventional SUS 430 manufacturing technology. SOLUTION: A ferritic stainless steel slab containing 0.02-0.10 wt.% C and 50-70% γp (γ potential) is heated to the temperature of the ferrite-austenite two-phase (α+γ) area, and then, subjected to the low-temperature hot rough rolling in which the total draft is >=50% with at least two passes, and the number of passes whose draft is >=20% is >=1/2 of the number of total passes in the two-phase temperature range. At the low temperature free from re- crystallization, the low-temperature hot finish rolling in which the total draft is >=50% with at least two passes is achieved, and then, the slab is cooled to the non-recovery temperature range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a ferritic stainless steel sheet having extremely little ridging and excellent ridging resistance, and more particularly, to extracting a slab by heating and extracting it in the (α + γ) two-phase region, The present invention relates to a method for producing a ferritic stainless steel sheet excellent in deep drawability and resistance to surface roughening due to processing by low-temperature finish rolling.

[0002]

2. Description of the Related Art Ridging (also called roping) is a ridge-like undulation (elongated striped shape) that occurs along the rolling direction on the surface of a ferritic stainless steel sheet when it is stretched or deep drawn. (Irregularities)
This phenomenon is unique to ferritic stainless steel sheets.

[0003] Generally, cold rolled ferritic stainless steel sheets
(JIS-SUS430, etc.) has excellent corrosion resistance, maintains beautiful surface gloss over a long period of time, has good workability, and is less expensive than austenitic stainless steel, etc. Used in a wide range of fields such as electrical equipment for automobiles and automobile parts. in this way,
Ferritic stainless steels are often used mainly for applications requiring decorativeness, so that not only corrosion resistance and press workability but also beautiful surface properties after processing are important requirements. In particular, ferritic stainless steel is used as a material for drawing such as cylinders and square cylinders, but if the material properties due to the manufacturing method are poor, the above-described ridging appears during molding and impairs the appearance of the surface. Not only that, in severe cases, this causes a problem that cracks occur during molding.

[0004] Therefore, in the art, it has been a major research subject to produce a ferritic stainless steel capable of reducing or eliminating ridging. There have been many studies on such ridging prevention technology, and a method of producing a hot-rolled sheet having a uniform recrystallized structure for preventing ridging has been particularly noted. For example, Japanese Patent Publication No. 45-34016 discloses that ridging resistance is improved by performing hot rolling at a low temperature, then performing box-shaped annealing at 800 to 830 ° C., and then performing cold rolling and finish annealing. Is disclosed. Japanese Patent Publication No. Sho 57-61096 discloses a method in which hot rolling at a rolling reduction of 20% or more is performed by a deformed roll rolling mill, followed by hot-rolled sheet annealing, cold rolling, and finish annealing. In JP-A-1-111816, hot rolling is performed at 850 ° C. or more, immediately cooled at a rate of 10 ° C./sec or more, and
A method has been proposed in which a two-phase structure of ferrite and martensite is formed by winding at a temperature of 50 ° C. or less, and then cold rolling is performed with a cumulative draft of 50% or more. In JP-A-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 sheet thickness is cast.
A method has been proposed in which this slab is subjected to hot rolling at a reduction rate of 40% or more at 1100 to 1000 ° C., and then to hot rolled sheet annealing, cold rolling, and finish annealing. Japanese Patent Application Laid-Open No. Hei 7-118754 discloses a gamma potential (γ
p) and heating is carried out at a temperature of 1100-1220 ° C.
Hot finish rolling at a temperature of 950-150 ° C, and 450
A method has been proposed in which winding is performed at a temperature of up to 800 ° C., descaling is performed, and then cold rolling is performed at a total draft of 70% or more.

[0005] However, all of these methods are:
During the entire manufacturing process, local countermeasures, that is, improvement proposals for any of the processes such as the casting process, the hot rolling process, and the annealing process, have not been made sufficiently. In addition, the content of each of these processes is not a technology that directly addresses ridging resistance, and therefore cannot be said to be sufficient as a measure for reducing ridging.

[0006]

The cause of ridging which occurs in thin steel sheets of ferritic stainless steel is as follows.
There is a common perception that it is mainly due to heterogeneous tissue (colony tissue) present in the plate. For example,
The columnar crystals of the continuously cast slab cannot be sufficiently broken only by ordinary hot rough rolling or hot finish rolling. Therefore, even if such a hot-rolled steel sheet is subjected to hot-rolled sheet annealing or cold rolling, it is difficult to obtain a steel sheet that can reliably prevent ridging as long as a colony structure remains. It is. Therefore, an object of the present invention is to establish a technique for manufacturing a ferritic stainless steel sheet having excellent ridging resistance, which was not obtained under the conventional SUS 430 manufacturing technology, by locating the cause of ridging. .

[0007]

Means for Solving the Problems As a result of intensive studies aimed at realizing the above-mentioned object, the present inventors have improved the equiaxed crystal ratio of slabs of ferritic stainless steel, It has been found that the occurrence of the ridging can be reduced or prevented if the columnar crystals of the cast slab are surely divided (destructed). Therefore, in the present invention, mainly by controlling the precipitation of austenite phase in hot rolling and not to recrystallize, while controlling the amount of the precipitated austenite phase, the band-like structure causing ridging is reduced. The slicing was severed, which greatly reduced ridging.

In order to more reliably realize such effects, the present invention has further studied the following points. In other words, the composition, casting structure, heating conditions, hot rolling conditions (coarse hot rolling conditions, finishing hot rolling conditions), and cold rolling conditions required to further suppress the occurrence of ridging were examined. To increase the gamma potential (γp), which is an index indicating the maximum precipitation amount of the γ phase in ferritic stainless steel with C ≧ 0.02wt%. The condition is that more γ phase is precipitated by heating in the two-phase (α + γ) region of ferrite and austenite, and the rough rolling is performed in the two-phase (α + γ) region as the hot rolling condition. By dividing the cast structure (columnar crystals)
By applying a low-temperature finish rolling, a predetermined strain is accumulated in the hot-rolled sheet, and recrystallization is performed in the next step of annealing the hot-rolled sheet to randomize the structure of the hot-rolled sheet. The present inventors have found that the present invention is effective in producing a ferritic stainless steel sheet having excellent ridging properties, and have completed the present invention.

Further, in the present invention, by increasing the total rolling reduction in the cold rolling step, the accumulation of strain is further increased, and the product is recrystallized in the final annealing process, whereby the crystallization of the product sheet is achieved. By realizing orientation randomization and crystal grain refinement, and then performing temper rolling as necessary, ridging resistance can be further improved.

The present invention completed under such a concept is as follows: C: 0.02 to 0.10 wt%, Si: 0.1 to 1.0 wt%, Mn: 0.
1 to 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.
A continuous cast slab of ferritic stainless steel containing 1 to 0.6 wt%, the balance being Fe and unavoidable impurities, and having a component composition in which γp satisfies the following formula (1), is extracted at the time of extraction from the mold. 15% or more of one thickness (preferably 20%
A continuous cast slab having a solidified structure in which the above is more preferably 25% or more is equiaxed is prepared, and the slab is subjected to hot rough rolling and hot finish rolling to form a hot-rolled sheet, and then to a normal rolled sheet. In a method for producing a ferritic stainless steel sheet by annealing and cold rolling, a slab of the ferritic stainless steel is heated to a temperature in a ferrite and austenite two phase (α + γ) region, and then in the two phase temperature region, Perform hot rough rolling in which at least two passes or more have a total draft of 50% or more and a draft of 20% or more is 1/2 or more of the total passes, and then at least in a temperature range where recrystallization does not occur. A ferrite-based steel with excellent ridging resistance characterized by performing hot finish rolling in which the total draft is 50% or more in two passes or more, and then cooling to a temperature range where it does not recover. This is a method for manufacturing a stainless steel sheet. 50.0 ≦ γp = 288C-54Si + 7.5Mn + 22Ni-18.75Cr + 350N + 338.5 ≦ 70.0 …… (1)

Further, the present invention provides the above slab,
Heat at a temperature of 50 ° C, perform multi-pass hot rough rolling with a rolling start temperature of 1000 ° C or more, a rolling end temperature of 950 ° C or more, and an interval of each pass of 10 seconds or less. rear,
1.2 ° C / mi before starting hot finish rolling in the next process
Cooling at a speed not lower than n, at a temperature where the rolling temperature does not exceed 900 ° C, and at intervals of each pass of 5 seconds or less, perform hot finish rolling so that recrystallization by reheating does not occur, and It is preferable to produce a ferritic stainless steel having excellent ridging resistance by winding at a temperature of 700 ° C. or less.

Further, the present invention provides a method of manufacturing a steel sheet according to the present invention, wherein after the completion of the above-mentioned hot rolling, in one or two or more cold rolling steps including an intermediate annealing step, the cold rolling is performed at a rolling reduction of 60% or more in total. It is a preferable method to produce a ferritic stainless steel sheet excellent in ridging resistance by performing rolling, performing final annealing and descaling treatment, or performing bright annealing, and further performing temper rolling as necessary.

In the present invention, hot finish rolling is performed by a steckel mill provided with a furnace coiler on the entrance and exit sides of a rolling stand, that is, in a normal method, the coil wound on the coiler for each pass has a recrystallization temperature. It is preferable to apply the method to a hot finish rolling method using a Steckel mill in which the heat is recovered as described above, the processing strain is naturally removed, and the crystal is recrystallized.
Further, it can be applied to a tandem mill.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the manufacturing method according to the present invention, a steel type to be applied, that is, a starting material (slab) having the following component composition is continuously cast and used. C: 0.04 to 0.10 wt% C has a large effect on γp. To design the component to satisfy the above equation (1), if it is less than 0.04 wt%, a large amount of other austenite forming elements must be added. However, the cost is increased, the strength is reduced, or the manufacturability and moldability are deteriorated. On the other hand, when the content is 0.1 wt% or more, the precipitation of more Cr carbide deteriorates the corrosion resistance and also deteriorates the workability. Therefore, C
Is 0.04 to 0.10 wt%. Preferably, 0.05
~ 0.07 wt%.

Si: 0.1-1.0 wt% Si, which is a component necessary for deoxidation, is 0.1-1.0 wt% of the level contained in ordinary ferritic stainless steel.
And Preferably it is 0.1-0.4 wt%.

Mn: 0.1 to 1.0 wt% Mn is an element necessary for deoxidation, and is set to 0.1 to 1.0 wt% in order to ensure good hot workability and strength. Preferably it is in the range of 0.2-0.7 wt%.

P: 0.040 wt% or less P deteriorates toughness, hot workability and corrosion resistance, so it is desirable to make P 0.040 wt% or less.

S: 0.020 wt% or less S deteriorates toughness, hot workability and corrosion resistance, so is made 0.020 wt% or less, more preferably 0.010 wt% or less.

Cr: 15.0 to 18.0 wt% Cr is an indispensable component in stainless steel for corrosion resistance and high-temperature oxidation resistance, and at least 15 wt% must be added. However, when the content exceeds 18 wt%, γ
p becomes smaller and the cost becomes higher.

N: 0.02 to 0.06 wt% N forms ferrite-based stainless steel sheets in the same manner as C, and forms Cr nitrides, which causes intergranular corrosion. Also, γ
The effect on p is also large, and it is necessary to be 0.02 to 0.06 wt% in order to satisfy the above equation (1). Preferably, it is 0.03 to 0.05 wt%.

Ni: 0.1-0.6 wt% Ni is an element which forms the γ phase, but its value is very high, and its addition range is 0.1-0.6 wt%. Preferably, 0.02
~ 0.05 wt%.

In the present invention, the slab of ferritic stainless steel described above is not sufficient to have the above-mentioned composition, and the slab of γp is used to precipitate an amount of γ phase necessary for dividing columnar crystals. Control is effective. This γ
p is controlled so that the numerical value represented by the following equation (1) becomes 50 to 70. 50.0 ≦ γp = 288C-54Si + 7.5Mn + 22Ni-18.75Cr + 350N + 338.5 ≦ 70.0… (1)

The reason is that, as shown in FIG.
If it is less than γ, the precipitation amount of γ is small, and the effect of randomizing the hot-rolled sheet structure by precipitation of γ phase is small. On the other hand, if γp is more than 70, the precipitation amount of gamma phase is too large and the productivity is deteriorated. Therefore, .gamma.p must be in the range of 50 to 70 in order to obtain a level of ridging resistance that satisfies the required quality. The ridging level A in the figure is WCM <25 μm, and B is 25 μm.
m ≦ WCM <35 μm, C is 35 μm ≦ WCM <45 μm,
D is WCM ≧ 45 μm. (Measurement length: 20mm)

According to the present invention, when the slab is extracted from the continuous casting mold, the slab has an equiaxed solidification structure in at least a central portion of the slab.
It is necessary to use a continuously cast slab occupying the above. Generally, ridging is caused by a columnar crystal solidification structure formed in a continuously cast slab, and this structure exists as a cast structure without phase transformation in a cooling process, and this structure remains completely after hot rolling and cold rolling. This is considered to be due to forming a texture by remaining without being destroyed. Therefore, in the present invention, as a measure for reducing ridging, the columnar crystal structure of the continuously cast slab, which is the root cause of ridging, is reduced, and the proportion of the equiaxed crystal solidification structure is increased. In particular, when the equiaxed crystal ratio of this slab is 15% or less of the plate thickness, the effect of reducing ridging is small.
15% or more. Preferably at least 20%, more preferably
It is desirable to make it 25% or more.

In the present invention, the slab is heated to 1050 ° C. or more.
It is characterized by heating to a temperature of 1150 ° C or less. As shown in the Fe-Cr phase diagram of FIG. 2, the ferritic stainless steel according to the composition of the above components is C: 0.05 wt%,
In the case of N: 0.03 wt% and Cr: 16 wt%, the temperature range of the γ phase exists at 850 to 1200 ° C. Heating it to a temperature range of 1050 to 1150 ° C means heating in a two-phase (α + γ) region of ferrite and austenite (≧ 1200 ° C in α single phase region extraction). The reason for heating in such a region is that, by precipitating a large amount of the γ phase, the columnar crystal is divided and broken at the stage of hot rolling, and the generation of ridging is easily prevented. If the slab is heated to a temperature range other than the above range, the amount of the γ phase during the rough hot rolling is reduced, and the ridging reduction effect is reduced.

In the present invention, in order to cut the columnar crystals of the continuously cast slab, in the two-phase (α + γ) temperature region, the total draft is at least 50% in at least 2 passes or more (preferably about 5 to 7 passes). Preferably, a path having a rolling reduction of at least 65%, more preferably at least 75%, and a reduction rate of at least 20% is at least 1/2 of the total number of paths (preferably, at least 25% is at least 1/2 of the total number of paths, more preferably Performs hot rough rolling where 30% or more passes are more than 1/2 of the total number of passes). The reason for this is (α + γ)
This is because the necessary amount of plastic deformation is secured in the two-phase region and the columnar crystal is more effectively divided.

In the above-mentioned hot rough rolling, specifically, the rolling start temperature is 1000 ° C. or higher, preferably 1050 ° C. or higher, the rolling end temperature is 950 ° C. or higher, preferably 1000 ° C. or higher, and the interval between each pass is set. In particular, in a multi-pass in which the pass time until the plate thickness becomes 1/4 or less of the initial slab thickness is 10 seconds or less, the rolling reduction is 20% or more (preferably 25% or more, more preferably
(30% or more) hot rough rolling to make the pass more than 1/2 of the total pass, and after the hot rough rolling, the time between the start of the hot finishing rolling in the next process is 1.2 ° C / min or more. Performs multi-pass rolling, cooling at a speed. The reason for performing such multi-pass hot rough rolling is that when the temperature at the start of rolling is 1000 ° C. or lower, a large amount of γ phase is precipitated, and it becomes difficult to recrystallize the α phase. The reason for setting the time between passes to 10 seconds or less is that if the time is longer than 10 seconds, the temperature is lowered, the content of the γ phase is reduced, and the effect of reducing ridging is reduced.
Before or after the final pass of the hot rough rolling, the reason for performing the hot finish rolling by cooling to a temperature of 850 ° C or less at a rate of 1.2 ° C / min or more is that the hot finish rolling is performed at a low temperature. It is. In other words, if the temperature is not cooled to 850 ° C or less, the maximum temperature of the hot-rolled sheet becomes 900
This is because the temperature exceeds ℃, a part of the structure of the hot-rolled sheet is recovered, and the ridging resistance is deteriorated.

In the present invention, hot finish rolling at a low temperature is performed. The reason for this is that during hot-rolled sheet annealing, deformation strain required for recrystallization is accumulated. 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 a total of 50% or more (preferably 65% or more, more preferably 75% or more) in two or more passes (preferably 5 to 7 passes).
% Or more) in order to carry out multi-pass hot finish rolling at an accumulated draft of 0.1% or more. If the total draft at this time is less than 50%,
Since the amount of deformation strain accumulated in the hot-rolled sheet is small and uniform recrystallization cannot be obtained in the next hot-rolled sheet annealing, ridging resistance deteriorates.

In the present invention, after completion of hot finish rolling,
In order to minimize the strain relief of the obtained hot-rolled sheet, it is necessary to wind at a winding temperature of 700 ° C or lower (preferably 650 ° C or lower, more preferably 600 ° C or lower).

In the present invention, after the completion of the hot rolling as described above, the cumulative rolling reduction of two or more times including one cold rolling or intermediate annealing through hot strip annealing is performed.
Ferritic stainless steel with excellent ridging resistance, that is, no ridging, by performing cold rolling of 60% or more, performing final annealing and descaling, or performing bright annealing, and then performing temper rolling as necessary. Manufacture steel sheet.

[0031]

EXAMPLES Examples of producing ferritic stainless steel according to the production method according to the present invention will be described below. A ferritic stainless steel having the chemical composition shown in Table 1 was melted in a converter (AOD) and cast continuously, and the center portion was a continuous 200 mm thick plate in which 30% or more of the plate thickness was equiaxed. It was a cast slab. Next, hot rolling was performed by performing hot rolling under the conditions shown in Table 2. In this hot rolling, rough rolling was performed by rolling from 5 mm to 7 mm to 200 mm to 25 mm, and finishing rolling was performed from 5 mm to 25 mm to 6 mm. The obtained hot-rolled sheet is bell-annealed at a temperature of 820 ° C.
A cold rolled sheet having a thickness of 0.6 mm was manufactured by cold rolling. Table 2 also 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 criteria described above.

[0032]

[Table 1]

[0033]

[Table 2]

The results were, as evident in the description in Table 2,
The waviness (WCM) of the present invention example is 25 μm
Although the occurrence of ridging is extremely small in the following, the heating temperature is out of the range of the present invention in the comparative example: No. 5,
9, 10, high finish rolling temperature over 900 ℃: 6,
Nos. 9 and 11, long pass time intervals and low end temperature of rough rolling: No. 7 and high winding temperature: No. 8 WCM exceeded 30 μm, and ridging was observed. .

[0035]

As described above, according to the present invention, a 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 C: 0.02-0.10 wt%, Cr: 15-18 wt% Fe-Cr
FIG.

Continued on the front page F term (reference) 4K037 EA05 EA12 EA15 EA18 EA20 EA23 EA25 EA27 EC01 EC04 FA02 FB04 FB06 FB07 FC03 FE02 FE03 FG03 FM02 HA06 JA06

Claims (5)

[Claims] 1. A slab of ferritic stainless steel having a C content of 0.02 to 0.10 wt% is heated to a temperature in a two-phase (α + γ) region of ferrite and austenite. Perform hot rough rolling in which two or more passes have a total reduction of 50% or more and a reduction of 20% or more is 1/2 or more of the total passes. A method for producing a ferritic stainless steel sheet having excellent ridging resistance, comprising performing low-temperature hot finish rolling at a total reduction ratio of 50% or more at a pass or more, and thereafter cooling to a temperature range where no recovery occurs. 2. The method according to claim 1, wherein the slab of ferritic stainless steel has an equiaxed solidification structure at a central portion, and the equiaxed portion occupies 15% or more of the plate thickness. A method for producing a ferritic stainless steel sheet having excellent ridging resistance, comprising using a continuously cast slab. 3. The two-phase zone heating of the slab is performed at 1050 to 1150 ° C.
The hot rough rolling is performed by multi-pass rolling with a rolling start temperature of 1000 ° C or more, a rolling end temperature of 950 ° C or more, and an interval between each pass of 10 seconds or less. Then, cool at a rate of at least 1.2 ° C / min until the next hot finish rolling starts, and then set the temperature at not more than 900 ° C, and set the interval between each pass to 5 seconds or less. 3. The production method according to claim 1, wherein hot finish rolling is performed so that recrystallization does not occur, and winding is performed at a temperature of 700 [deg.] C. or lower, which is a temperature at which recovery does not occur. 4. A total rolling reduction of one or more times of cold rolling including hot-rolled sheet annealing or two or more times of intermediate annealing after hot rolling as set forth in any one of claims 1 to 3. Production of ferritic stainless steel sheet with excellent ridging resistance, characterized in that cold rolling of 60% or more is performed, followed by final annealing or bright annealing, and then, if necessary, temper rolling. Method. 5. The composition of a ferritic stainless steel slab is as follows: C: 0.02 to 0.10 wt%, Si: 0.1 to 1.0 wt%, Mn:
0.1-1.0 wt%, P: 0.040 wt% or less, S: 0.020 wt%
Hereinafter, Cr: 15.0 to 18.0 wt%, N: 0.02 to 0.06 wt%, Ni:
5. The composition according to claim 1, which contains 0.1 to 0.6 wt%, with the balance being Fe and unavoidable impurities and satisfying a gamma potential (γp) represented by the following formula (1). The method according to any one of the preceding items. 50.0 ≦ γp = 288C-54Si + 7.5Mn + 22Ni-18.75Cr + 350N + 338.5 ≦ 70.0 …… (1)