JP2000273548A - Production of ferritic stainless steel sheet excellent in ridging resistance and crown shape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make excellent the ridging resistance and a crown shape by executing hot rolling in such a manner that the draft in the final pass in a rough rolling stage is controlled to the % equal to or below the specified one, and the draft in at least one pass other than the final pass is controlled to the % equal to or above the specified one. SOLUTION: A ferritic stainless steel sheet is subjected to hot rolling in such a manner that the draft in the final pass in a rough rolling stage is controlled to <=20%, and the draft in at least one pass other than the final pass is controlled to >=40% and is thereafter subjected to finish rolling by a hot finish rolling mill. For example, a sheet bar of 30 mm thickness subjected to rough rolling in such a manner that the draft in the pass before the final pass is controlled to 50%, and the draft in the final pass is controlled to 10% is rolled by using a rolling mill of 6 stands having roll cross as an actuator for controlling the crown shape. The sheet crown of 3.0 m final finish thickness, 1000 mm sheet width and 0 μm to the objective value of 50 μm of the sheet crown is obtd. It is also preferable that a plurality of sheet bars rolled in the rough rolling stage are joined, which is continuouslly rolled in the finish rolling stage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel sheet which has excellent workability, especially ridging resistance, and has a high quality crown shape.

[0002]

2. Description of the Related Art Ferritic stainless steel sheets have excellent corrosion resistance in various corrosive environments and are inexpensive because they do not contain Ni.
It is commonly used as a material for electrical parts. When the ferritic stainless steel sheet is subjected to drawing processing, wrinkling defects of uneven stripes along the rolling direction called ridging are apt to occur, which causes deterioration of the appearance of the product. For this reason, conventionally, suppression of ridging has been a major issue when manufacturing ferritic stainless steel sheets.

As a technique for suppressing this ridging, for example, Japanese Patent Application Laid-Open No. H7-126757 discloses a technique in which the total rolling reduction of the final pass of rough rolling and the preceding pass thereof is 70% or more. Japanese Patent Application Laid-Open No. 7-268461 discloses a technique in which at least one pass of rolling is performed at 40 to 75% in rough rolling, and at least one pass is reduced at a rolling reduction of 20 to 45% in finish rolling.

It is well known that by increasing the rolling reduction per pass in this way, the crystal grains of the ferritic stainless steel sheet become finer and a ferritic stainless steel sheet having excellent ridging resistance can be obtained.

[0005]

However, in finish rolling, it is necessary to satisfy not only quality standards for materials but also quality standards for the dimensions of steel sheets. As one of the quality standards, there is a quality standard for a thickness distribution in a width direction of a steel sheet called a sheet crown. The flat shape of the rolled steel sheet is required, and the flat shape between the finish rolling stands is also required for stable threading.

The finishing mill is provided with an actuator for controlling a crown shape such as a roll bender, a roll shift, and a roll cloth in order to achieve a target value of the plate crown and to make the plate shape flat. The setting is made so that the plate shape has an optimum value. The plate crown Cr is defined by equation (1).

Cr = hc-he (1) where hc is the thickness of the central portion of the plate, and he is the thickness of the crown quality evaluation point near the edge of the plate which is usually several tens of mm from the edge of the plate. It is. Further, in the finish rolling, the strip crown Cr _i mill i stands th equation (2), a plate shape (steepness) lambda _i is an index that represents the waviness of the strip caused due to the difference in elongation in the sheet width direction Yes, it is calculated by equation (3), and the optimum actuator setting value is obtained from these equations.

[0008] _{Cr i = α i × Crm i} + β i × Cr i-1 ... (2) λ i = 2 / π · {ξ i · (Cr i / hc i -Cr i-1 / hc i-1) ｝ ^1/2 (3) Here, Crm is a roll crown under load which can be obtained from a rolling load or roll deformation by an actuator. α is an influence coefficient of the roll crown at the time of load on the output side sheet crown, and β is an influence coefficient of the entry side sheet crown on the output side sheet crown. The suffix i indicates the stand number of the rolling mill.

Ξ is a shape change coefficient. It is not always necessary to calculate the shape using the crown at the same position as the quality evaluation point of the crown. By the way, when the rolling reduction is increased in the rough rolling, the rolling load increases, and the deflection and flatness of the work roll increase. Therefore, the sheet crown of the seat bar becomes large.

FIG. 8 shows an example of a sheet profile of a sheet bar when rolling is performed at a rolling reduction of 50% in the final pass of rough rolling.
(A). In FIG. 8A, the sheet crown of the sheet bar has a large value of about 400 μm. If the sheet crown of the sheet bar is large, the shape between the stands tends to be stretched in the finish rolling. If the shape between the stands is made flat for stable threading, the sheet crown on the exit side of the finishing mill must be large. Therefore, it is not possible to achieve the target value of the sheet crown on the exit side of the finishing mill and to make the shape between stands flat.

Therefore, conventionally, although it was possible to obtain a steel sheet having excellent ridging resistance, it was not possible to simultaneously achieve the target values of the sheet crown and the sheet shape (steepness). An object of the present invention is to solve the above-mentioned problems and to provide a method for producing a ferritic stainless steel sheet having excellent ridging resistance and a good crown shape with good quality of a sheet crown and a sheet shape (steepness). And

[0012]

In the rough rolling, the sheet profile on the incoming side has almost no effect on the sheet profile on the outgoing side, and the roll profile at the time of rough rolling is directly transferred to the sheet bar, and the sheet profile of the sheet bar is transferred. You can think of it as a crown. Here, factors that constitute the roll profile during rolling include roll deflection, roll flatness, roll wear, and roll thermal expansion.

Therefore, the larger the rolling reduction, the larger the deflection and flatness of the roll, and the larger the sheet crown of the sheet bar. The present inventors set the rolling reduction to at least 40% in at least one pass other than the final pass of the rough rolling,
By performing light reduction in the final pass of rough rolling just before finish rolling, the sheet crown of the sheet bar after rough rolling can be reduced, and finish rolling can be performed on a sheet bar with a small sheet crown. Was found.

[0014] The rolling reduction of 40% in the pass just before the final rough rolling
FIG. 8 (b) shows an example of the sheet profile of the sheet bar when rolling is performed at a rolling reduction of 10% in the final pass. In this case, it can be seen that the sheet crown of the seat bar is almost 0 μm. Here, the present inventors have found that it is preferable to reduce the pressure in the final pass by 20% or less.

In order to refine the crystal grains of the ferritic stainless steel sheet and obtain a ferritic stainless steel sheet having excellent ridging resistance, it is necessary to reduce the ferrite stainless steel sheet at least once at a reduction rate of 40% or more. It is essential. That is, the present invention is characterized in that, in hot rolling of a ferritic stainless steel sheet, the rolling reduction in the final pass of the rough rolling step is 20% or less, and the rolling reduction in at least one pass other than the final pass is 40% or more. This problem has been solved by a method for producing a ferritic stainless steel sheet having excellent ridging resistance and crown shape.

Further, the present invention has found that a plurality of sheet bars rolled in the rough rolling step may be joined, and may be continuously rolled in the finish rolling step.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION As an embodiment of the present invention, a case will be described in which rough rolling is performed and then finish rolling is performed by a six-stand hot finishing mill. Here, a roll cross method was employed as an actuator for controlling the crown shape. The schedule shown in Table 1 was applied to the sheet thickness schedule in the finish rolling.

[0018]

[Table 1]

A material having a sheet bar thickness of 30 mm at the time of the completion of the rough rolling is rolled by a finish rolling mill to a final finish thickness of 3.0 mm and a sheet width.
We manufacture 1000mm ferritic stainless steel sheet. At that time, the influence of the sheet crown of the sheet bar on the sheet crown between the stands and the steepness (plate shape) in the finish rolling was calculated using the equations (2) and (3). The calculation results are shown in FIGS. Here, the target value of the sheet crown after the finish rolling is 50 μm.

The quality evaluation point of the plate crown is 25 mm from the edge of the plate, and the steepness is 100 mm of the distance from the edge of the plate. In addition, when the steepness is +, the plate shape indicates ear extension, and when the steepness is-, the belly extension is indicated. The allowable range of the steepness is such that slight ear extension is provided at a later stage in order to secure the stability of the passing board. (A range indicated by a broken line is an allowable range). FIG.
Indicates that the target value of the plate crown of the final finishing stand is 50 μm
In the case where the cross angle of the finishing mill is adjusted so as to achieve the above.

When the sheet crown of the sheet bar is 150 μm, the steepness is almost equal to the lower limit. When the sheet crown of the seat bar is 300 μm, the steepness is −
The shape (steepness) is out of the permissible range due to (minus) and belly elongation. FIG. 3 shows the result when the finishing mill is adjusted so that the steepness falls within the allowable range.

In this case, the sheet crown of the seat bar is 300
In the case of μm, the sheet crown at the final stand of the finish rolling mill is 90 μm, and the target value of 50 μm cannot be achieved. In the case of FIG. 3, in order to achieve the target plate crown (50 μm) of the final finishing stand and to keep the steepness between the stands of the finishing rolling mill within an allowable range, the sheet bar plate immediately after the completion of rough rolling is required. Crown at least 150
It is necessary to be less than μm.

Therefore, in rough rolling, rolling is performed with a reduction ratio of 50% in a pass before the final pass, and then the final pass is reduced in a reduction ratio of 50%.
Rolled at 10%. FIG. 8B shows the measurement result of the sheet profile of the sheet bar at this time. In the case of FIG. 8B, the sheet crown of the seat bar is approximately 0 μm. The result of finish rolling this material is referred to as an example of the present invention, and the r value and ridging index are shown in FIGS. 9 (a) and 9 (b).

The r value is a Rankford value, and is a ratio of a width strain to a thickness strain when a longitudinal plastic strain is applied to a steel sheet. It is known that the larger this value is, the more excellent the steel sheet is in deep drawing formability. The ridging index is an index indicating the ridging resistance of the steel sheet. The definition of the ridging index is as follows.

A JIS No. 5 test piece was subjected to a 20% tensile strain.
Depending on the degree of wrinkles that occur on the specimen surface when added
Assign numbers from 1.0 to 3.0 in 0.1 increments. Wrinkled
The degree is visually determined. One without wrinkles 1.
0, 1.5 for ridging that is practically acceptable
2.5 for severe ridging that cannot withstand the heat. Rigi
Index and surface roughness maximum value (R_max) And almost
There is a linear relationship, with a ridging index of 1.5_maxIs about
10μm, Ridging index 2.5 _maxIs about 35 μm
You.

Here, in FIG. 9, the hatched portion shows the result of the conventional example in which the rolling reduction of the final pass of the rough rolling is not particularly defined.
As shown in FIG. 9, the r value and the ridging index of the example of the present invention are almost the same as those of the conventional example. If the rolling reduction of the final pass in rough rolling exceeds 20%, the sheet bar crown
It is preferable that the final rolling reduction in the rough rolling is set to 20% or less because the ridging index becomes 0.5 μm or more and the ridging index increases by 0.5 or more.

Rolling was performed by changing the rolling reduction of the final pass and the final pass of the rough rolling variously, and the ridging resistance and the shape of the rolled material were evaluated. The result is shown in FIG. Here, as a criterion for evaluation, a ridging index of 1.5 or less is defined as “good”, and a steepness allowable range of the rolled material at the final rolling final stand is 0.05% or more and 0.3% or less,
The range is defined as “good”.

From these results, the rolling reduction in the final pass of the rough rolling was calculated.
It can be seen that it is preferable to set the rolling reduction to 20% or less and the maximum draft in the rough rolling to 40% or more.

[0029]

EXAMPLE 1 The present invention was applied to the production of a ferritic stainless steel sheet in a hot rolling line in which sheet bars were individually finished and rolled one by one after rough rolling. The hot finishing rolling mill is a six-stand rolling mill having a roll cloth as an actuator for controlling the crown shape.

At the time of rolling, the method of the present invention in which the rolling reduction in the final pass of the rough rolling step is 20% or less and the rolling reduction in at least one pass other than the final pass is 40% or more, The rolling reduction of the pass is not particularly defined, and the conventional method in which each steepness in the finishing rolling stand is controlled to be within an allowable range is applied. Here, the finish rolling thickness schedule is the schedule shown in Table 1, and the finish thickness is 3.0mm and the width is 1000mm.
S430 was rolled.

FIG. 4 shows the relationship between the target value and the measured value of the sheet crown at the final stand of the finish rolling in the above operation. FIG. 4A shows the result of the conventional example, and FIG. 4B shows the result of the example of the present invention. In the conventional example of FIG. 4A, since the steepness is within the allowable value, the measured value of the sheet crown is 60 to 90 μm, which is considerably larger than the target value. On the other hand, in the example of the present invention, the actually measured value of the plate crown of the final finishing stand is almost as the target value.

FIG. 5 shows the relationship between the r value of these materials and the ridging index. From FIG. 5, it can be seen that the processing characteristics of the example of the present invention are almost equal to those of the conventional example. Example 2 The present invention was applied to the production of a ferritic stainless steel sheet in a hot rolling line in which sheet bars were joined after rough rolling and finish rolling was continuously performed.

The hot finishing rolling mill is a six-stand rolling mill having a roll cloth as an actuator for controlling the crown shape. In rolling, the method of the present invention in which the rolling reduction in the final pass of the rough rolling step is 20% or less, and the rolling reduction in at least one pass other than the final pass is 40% or more; The rate was not particularly specified, and the conventional method in which each steepness at the finishing rolling stand was controlled to be within an allowable range was applied. Here, the sheet thickness schedule for the finish rolling was the schedule shown in Table 1, and a SUS430 ferritic stainless steel sheet having a finished thickness of 3.0 mm and a width of 1000 mm was rolled. Here, the thickness schedule of the finish rolling is the schedule shown in Table 1, and the finish thickness
3.0mm, 1000mm wide ferritic stainless steel sheet SUS430
Was rolled.

FIG. 6 shows the relationship between the target value and the actually measured value of the sheet crown at the final rolling stand in the above operation. FIG. 6A shows the result of the conventional example, and FIG. 6B shows the result of the example of the present invention. In the conventional example shown in FIG. 6A, the measured value of the sheet crown is considerably larger than the target value of 30 μm.
8080 μm. On the other hand, in the example of the present invention, the actually measured value of the plate crown of the final finishing stand is almost the target value.
It is 30 μm.

FIG. 7 shows the relationship between the r value and the ridging index of these materials. From FIG. 7, it can be seen that the processing characteristics of the example of the present invention are almost equal to those of the conventional example.

[0036]

As described above, according to the present invention, it is possible to obtain a ferritic stainless steel sheet having excellent workability, especially ridging resistance, and having a high-quality crown shape.

[Brief description of the drawings]

FIG. 1 is a graph showing an evaluation of ridging resistance and shape (steepness) with respect to a combination of a final pass draft and a maximum draft in a rough rolling step.

FIG. 2 is a graph of (a) steepness and (b) strip crown in each rolling stand when shape control is performed so that the strip crown becomes a target value on the exit side of the finishing mill.

FIG. 3 is a graph of (a) steepness and (b) strip crown in each rolling stand when shape control is performed so that the steepness is within an allowable range on the finish rolling mill exit side.

FIG. 4 is a graph showing a relationship between a sheet crown target value and a sheet crown actual value when a sheet bar is rolled alone;
(A) shows a conventional example, and (b) shows an example of the present invention.

FIG. 5 is a graph showing a relationship between an r value and a ridging index when a sheet bar is rolled alone.

FIGS. 6A and 6B are graphs showing a relationship between a sheet crown target value and a sheet crown actual value when a sheet bar is joined and rolled, where FIG. 6A shows a conventional example and FIG. 6B shows an example of the present invention. .

FIG. 7 is a graph showing a relationship between an r value and a ridging index when a sheet bar is joined and rolled.

FIG. 8 is a graph showing a sheet profile of a sheet bar. FIG. 8 (a) shows an example of a sheet profile when the rolling is performed at a rolling reduction of 50% in a rough rolling final pass, and FIG. An example of a plate profile when a reduction is performed at a rate of 10% is shown.

FIG. 9 shows a ridging index (a) and r of the present invention example and the conventional example.
It is a graph which compares a value (b).

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masanori Kitahama 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba In the Technical Research Institute of Kawasaki Steel (72) Inventor Shinzo Uchimaki 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba 4K032 BA01 CB00 4K037 EB14 FB06 FB10

Claims (2)

[Claims] In the hot rolling of a ferritic stainless steel sheet, the rolling reduction in the final pass of the rough rolling step is set to 20% or less, and the rolling reduction in at least one pass other than the final pass is reduced.
A method for producing a ferritic stainless steel sheet having excellent ridging resistance and crown shape, characterized by being at least 40%. 2. The ridging resistance and crown shape according to claim 1, wherein a plurality of sheet bars rolled in the rough rolling step are joined and continuously rolled in a finish rolling step. Manufacturing method of ferritic stainless steel sheet.