JP2022079072A - Ferritic stainless steel sheet and method for manufacturing the same - Google Patents

Abstract

To provide a ferritic stainless steel sheet which is excellent in moldability and surface characteristics after molding, and a method for manufacturing the same.SOLUTION: A ferritic stainless steel sheet comprises, by mass%, 11.0% or more and 30.0% or less Cr, 0.001% or more and 0.030% or less C, 0.01% or more and 2.00% or less Si, 0.01% or more and 2.00% or less Mn, 0.005% or more and 0.100% or less P, 0.0100% or less S, 2.00% or less Al, 0.030% or less N, further comprises one or two of 0.50% or less Ti and 1.00% or less Nb, and the balance Fe with inevitable impurities, is composed of a ferrite monolayer structure having a crystal grain size number measured according to JIS G 0551 of 9.0 or more, and has a random intensity ratio in a crystal orientation in a surface parallel to a rolled surface at a plate thickness 1/2 position of I{111}<112>≥9, I{111}<110>≥6 and I{411}<148>≤1, where I{hkl}<uvw> represents a random intensity ratio of an {hkl}<uvw> orientation.SELECTED DRAWING: None

Description

The present invention relates to a ferrite-based stainless steel sheet steel sheet having excellent formability during molding and polishing after molding, and a method for producing the same. In particular, it is suitable for applications that require polishing to remove surface irregularities of steel sheets after molding.

SUS304 (18Cr-8Ni), which is a representative steel grade of austenitic stainless steel, is widely used in home appliances, kitchen products, building materials, etc. because of its excellent corrosion resistance, workability, and beauty. However, it is said that the price of steel sheet is high because a large amount of Ni, which is expensive and the price fluctuates sharply, is added. On the other hand, since ferritic stainless steel does not contain Ni or has an extremely low content, demand is increasing as a material having excellent cost performance.

When used for molding purposes, austenitic stainless steel is easy to work harden and has excellent overhangability. Further, since austenitic stainless steel is relatively easy to form a fine grain structure, a fine grain steel sheet having a crystal grain size number of about 10 is manufactured. Therefore, the surface unevenness (rough skin) after the molding process is small, and there is almost no problem. As described above, the austenitic stainless steel has both formability and surface unevenness resistance after forming.

On the other hand, when ferritic stainless steel is used for molding, the problems are formability and surface unevenness after molding. The overhangability of ferritic stainless steel is low and cannot be changed significantly. However, since it is possible to control the deep drawing property by changing the crystal orientation (aggregation structure), in many cases, a forming method mainly for deep drawing is used for ferritic stainless steel.

The r value (Rankford value) is used as an index of the deep drawing characteristics. As a method for improving the r value of a ferritic stainless steel sheet, it is generally known to reduce C and N and add stabilizing elements such as Ti and Nb, and to grow crystal grains. Ferritic stainless steel sheets of value are manufactured.

However, ferritic stainless steel sheets tend to have coarser grains than austenitic stainless steel sheets. The reason for this is that the recrystallized grain size tends to increase and the addition of Ti or Nb facilitates the growth of crystal grains. In addition, when the high r value is guaranteed by the grain growth, the r value is lowered unless the grain is grown in order to obtain fine grain steel.

As described above, it is difficult for a ferritic stainless steel sheet to have both deep drawability and rough surface resistance after molding.

When relatively strict formability is required such as a housing of a home electric appliance or an instrument, a high-purity ferritic stainless steel such as SUS430LX is often used as the ferritic stainless steel. In most cases, the thickness of the stainless steel sheet used to ensure the strength after molding is 0.6 mm or more, and as described above, since the crystal grain size is large, the rough skin after molding is large. Therefore, the unevenness is usually removed by polishing.

Patent Document 1 describes a method for improving deep drawability while reducing rough skin of high-purity ferritic stainless steel, specifically, crystals in the final annealing step of ferritic stainless steel containing Ti and / or Nb. A method for improving the r value by grain growth is disclosed. However, the obtained crystal grain size number is as coarse as 7 or less, and rough processing is likely to occur.

Patent Document 2 describes a ferritic stainless steel containing Ti and / or Nb, which has a ferritic single-phase structure having a crystal grain size number of more than 9.0, and has a plate thickness of 1/2 position and a plate thickness of 1/10 position. Disclosed is a technique relating to a ferritic stainless steel sheet having excellent formability during molding and surface characteristics after molding by controlling a random strength ratio of crystal orientations in a plane parallel to the rolled surface. This technique is carried out by a manufacturing method having a high-pressure rolling ratio of 93% or more in a cold rolling ratio.

Japanese Patent Application Laid-Open No. 2003-73782 Japanese Unexamined Patent Publication No. 2019-26913

Hiroshi Inoue; Light Metal, vol42, No6 (1992), p358.

Considering the molding process of ferritic stainless steel sheets, there is currently no steel sheet that can be formed into a predetermined strict shape and can satisfy the surface texture after forming. For this reason, in the case of a ferritic stainless steel sheet, there is a problem that polishing time is required in the polishing process after molding, and a large amount of dust generated by polishing is generated.

In view of the above, it is an object of the present invention to provide a ferritic stainless steel sheet having excellent moldability and surface characteristics after molding and a method for producing the same.

The present inventors have carefully investigated the recrystallization process in the final annealing of ferritic stainless steel sheets. As a result, in the final stage of recrystallization, the rolled structure having an orientation close to {100} <011>, which remains unrecrystallized, is eroded by the surrounding recrystallized grains until the recrystallization is completed. It was found that the recrystallized grain size increases as the surrounding recrystallized grains grow.

Next, the production conditions for reducing the {100} <011> oriented grains in the stage after cold rolling and facilitating recrystallization were searched for. As a result, by combining cold rolling with rolling in the diagonal direction instead of rolling in only one direction, {100} <011> azimuth grains are reduced and recrystallization is easy, and the r value after recrystallization. Was found to improve dramatically.

By applying a part of cold rolling in an oblique direction, it is possible to obtain a recrystallized structure as fine grains without the growth of surrounding recrystallized grains by the time the recrystallization is completed. It is possible to manufacture a metal structure having an r value, and it is possible to achieve both deep draw formability and processing rough skin resistance.

Further, by making the structure of the surface of the steel sheet a fine grain structure, it is possible to reduce rough skin after processing. At the same time, the r value, which is an index of deep drawing workability, is increased by increasing the degree of integration in the {111} direction and decreasing the degree of integration in the other position. In particular, by reducing the degree of integration in {411} <148>, which tends to accumulate together with {111}, a high r value can be obtained, and strict deep drawing can be performed.

In order to obtain the above structure, rolling in multiple directions is combined during cold rolling. Ideally, by combining the 0 ° direction and the 45 ° direction as the same reduction ratio with respect to the original rolling direction, it is a cold-rolled texture that is disadvantageous for recrystallization during cold-rolled annealing {100} <011. > Accumulation during cold rolling can be suppressed, and accumulation in directions other than {111} after annealing can be suppressed.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In terms of mass%, Cr: 11.0% or more, 30.0% or less, C: 0.001% or more, 0.030% or less, Si: 0.01% or more, 2.00% or less, Mn : 0.01% or more, 2.00% or less, P: 0.005% or more, 0.100% or less, S: 0.010% or less, Al: 2.00% or less, and N: 0.030% The following is included, and one or two of Ti: 0.50% or less and Nb: 1.00% or less are contained, the balance is Fe and impurities, and the metallographic structure is measured according to JIS G 0551. It is a ferrite single layer structure with a crystal grain size number of 9.0 or more, and the random intensity ratio of the crystal orientation on the plane parallel to the rolled surface at the plate thickness 1/2 position is I _{{111} <112>} ≧ 9, I. A ferrite-based stainless steel plate characterized in that _{{111} <110>} ≧ 6 and I _{{411} <148>} ≦ 1. Here, I _{{hkl} <uvw>} indicates the random intensity ratio of the {hkl} <uvw> orientation.

(2) The ferrite-based stainless steel sheet of the above (1), which is characterized by containing 1 group or 2 or more groups of the following groups A to C in mass%.
Group A:
Sn: 0.50% or less,
Ni: 1.00% or less,
Cu: 1.00% or less,
Mo: 2.00% or less,
W: 1.00% or less,
Co: 0.50% or less,
V: 0.50% or less,
Zr: 0.50% or less, and Sb: 0.50% or less, 1 type or 2 or more types B group:
B: 0.0025% or less,
Ca: 0.0050% or less,
Mg: 1 type or 2 types or more of 0.0050% or less Group C:
Y: 0.20% or less,
Hf: 0.20% or less,
REM: 1 type or 2 types or more of 0.10% or less

(3) The ferrite-based stainless steel sheet according to (1) or (2) above, wherein the average r value is 1.5 or more.

(4) The ferrite-based stainless steel sheet according to any one of (1) to (3) above, which is a housing or an instrument of a home electric appliance.

(5) A method for producing a ferrite-based stainless steel plate according to any one of the above (1) to (3), wherein a steel slab having the component of the above (1) or (2) is hot-rolled and hot-rolled steel plate. A cold rolling step of cold-rolling the hot-rolled steel plate to obtain a cold-rolled steel plate, and a heat treatment step of heat-treating the cold-rolled steel plate to obtain a heat-treated steel plate are provided. In the rolling process, the cold rolling ratio RT is set to 50% or more and 90% or less, and the cold rolling ratio is diagonally applied to a part of the cold rolling at an angle of 20 ° or more and 70 ° or less with respect to the direction of the hot rolling. A method for manufacturing a ferrite-based stainless steel plate, which comprises cold rolling at RC 20% or more.

(6) The ferrite-based stainless steel according to (5), wherein a part of the cold rolling further includes cold rolling in the same direction as the hot rolling with a cold rolling ratio of RL 20% or more. Steel plate manufacturing method.

(7) Manufacture of the ferrite-based stainless steel sheet according to (6), which comprises a hot-rolled sheet annealing step for annealing the hot-rolled steel sheet between the hot-rolled step and the cold-rolled step. Method.

According to the present invention, it is possible to obtain a ferritic stainless steel sheet having excellent moldability and surface characteristics after molding.

Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(I) Chemical composition

The ferritic stainless steel sheet of the present invention has the following chemical components. The reasons for limiting the chemical composition will be described below.

Cr: 11.0% or more and 30.0% or less

Cr is an element that improves corrosion resistance, which is a basic characteristic of stainless steel. If it is less than 11.0%, sufficient corrosion resistance cannot be obtained, so the lower limit is 11.0%. On the other hand, excessive addition promotes the formation of an intermetallic compound equivalent to σ and promotes cracking during production and deterioration of moldability, so the upper limit is set to 30.0%. From the viewpoint of stable manufacturability (yield, rolling defects, etc.), 14.0 to 25.0% is preferable. More preferably, it is 16.0 to 20.0%.

C: 0.001% or more, 0.030% or less

Since C is an element that lowers the formability (r value), which is important in the present invention, it is preferably less, and the upper limit is 0.030%. However, the lower limit is set to 0.001% because excessive reduction causes an increase in refining cost. Considering both refining cost and moldability, 0.002 to 0.018% is preferable.

Si: 0.01% or more, 2.00% or less

Si is an element for improving oxidation resistance, but its upper limit is 2.0% because excessive addition causes deterioration of moldability. A lower value is preferable from the viewpoint of moldability, but if the content is excessively lowered, the raw material cost will increase, so 0.01% is set as the lower limit. From the viewpoint of manufacturability, the preferable range is 0.05 to 0.30%.

Mn: 0.01% or more, 2.00% or less

As with Si, adding a large amount of Mn causes deterioration of moldability, so the upper limit is set to 2.00%. A lower value is preferable from the viewpoint of formability, but an excessive decrease causes an increase in raw material cost, so the lower limit is 0.01%. From the viewpoint of manufacturability, the preferable range is 0.05 to 0.30%.

P: 0.005% or more and 0.100% or less

Since P is an element that lowers moldability (r value and product elongation), it is preferably low, and the upper limit is 0.100%. However, the lower limit is set to 0.005% in order to utilize the effect of improving the r value of phosphide precipitation. Excessive reduction leads to an increase in raw material cost, and when both formability and manufacturing cost are taken into consideration, the preferable range is 0.010 to 0.070%, more preferably 0.020 to 0.050%. be.

S: 0.010% or less

S is an unavoidable impurity element, and it is preferably low because it promotes cracking during production, and the upper limit is 0.010%. The lower the amount of S, the more preferably 0.003% or less. The lower limit is 0. However, since an excessive decrease causes an increase in refining cost, 0.001% may be the lower limit. From the viewpoint of manufacturability and cost, the preferable range is 0.001 to 0.002%.

Al: 2.00% or less

Al is an element effective for enhancing corrosion resistance and oxidation resistance. The upper limit is set to 2.00% because excessive addition not only causes a decrease in moldability but also leads to an increase in alloy cost and an impediment to manufacturability. The content of Al is not essential and the lower limit is 0. In order to obtain the effects of improving corrosion resistance and oxidation resistance, the lower limit of the preferable content is 0.01%.

N: 0.030% or less

Like C, N is an element that lowers formability (r value), and the upper limit is 0.030%. The smaller the amount of N, the more preferable, and the lower limit is 0. However, since excessive reduction leads to an increase in refining cost, 0.002% may be the lower limit. The preferable range is 0.005 to 0.015% from the viewpoint of moldability and manufacturability.

Further, one or two kinds of Ti and Nb are contained in the following range.

Ti: 0.50% or less Ti brings improvement in r value and product elongation through high purification in which C and N are fixed as precipitates. Although these effects can be obtained even with a small amount of content, the lower limit is preferably 0.03% in order to effectively obtain the effects. On the other hand, excessive addition causes an increase in alloy cost and a decrease in manufacturability due to an increase in recrystallization temperature, so the upper limit is set to 0.50%. From the viewpoint of moldability and manufacturability, the preferable range is 0.05 to 0.40%. Further, a suitable range for positively utilizing the effect of Ti is 0.10 to 0.30%.

Nb: 1.00% or less Nb also brings about improvement in r value and product elongation through high purification of steel by the action of stabilizing elements that fix C and N like Ti. In order to obtain these effects, it is preferable to set the lower limit to 0.03% when adding. On the other hand, excessive addition leads to an increase in alloy cost and a decrease in manufacturability due to an increase in recrystallization temperature, so the upper limit is set to 1.00%. From the viewpoint of alloy cost and manufacturability, the preferable range is 0.03 to 0.50%. Further, a suitable range for positively utilizing the effect of Nb is 0.04 to 0.30%. More preferably, it is 0.06 to 0.10%.

In addition to the above basic composition, the following elements may be selectively added.

Group A elements:
Sn: 0.50% or less,
Ni: 1.00% or less,
Cu: 1.00% or less,
Mo: 2.00% or less,
W: 1.00% or less,
Co: 0.50% or less,
V: 0.50% or less,
Zr: 0.50% or less,
Sb: 0.50% or less

Sn, Ni, Cu, Mo, W, Co, V, Zr, and Sb are effective elements for enhancing corrosion resistance and oxidation resistance, and one or more are added as necessary. However, excessive addition not only lowers the formability but also increases the alloy cost and impairs the manufacturability. Therefore, the upper limit of Ni, Cu and W is 1.00%, and the upper limit of Mo is 2.00. %. The upper limit of Sn, Co, V, and Zr is 0.50%. The effect of improving corrosion resistance and oxidation resistance can be obtained even with a small amount of content. In particular, in the case of Sn and Sb, it is preferable to contain 0.005% or more in order to surely obtain the effect. In the case of Ni, Cu, Mo, W, Co, V and Zr, the lower limit of the preferable content is 0.05%.

Group B elements:
B: 0.0025% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less

B, Ca, and Mg are elements that improve hot workability and secondary workability, and one kind or two or more kinds are added as necessary. However, since excessive addition leads to inhibition of manufacturability, the upper limit of B is 0.0025% and the upper limit of Ca and Mg is 0.0050%. The effect of improving hot workability and secondary workability can be obtained even with a small amount of content, but in the case of B, it is preferable to contain 0.0001% or more in order to surely obtain the effect. Considering manufacturability and hot workability, the more preferable range of B is 0.0003 to 0.0012%, and the preferable range of Ca and Mg is 0.0002 to 0.0010%.

Group C elements:
Y: 0.20% or less,
Hf: 0.20% or less,
REM: 0.10% or less

Y, Hf, and REM are elements effective for improving hot workability, cleanliness of steel, and improving oxidation resistance, and one kind or two or more kinds may be added as needed. When added, the upper limit of Y and Hf is 0.20%, and the upper limit of REM is 0.10%. The effect of improving hot workability and cleanliness of steel can be obtained even with a small amount of the content, but the preferable lower limit is 0.001%. Here, REM is an element belonging to atomic numbers 57 to 71, and is, for example, La, Ce, Pr, Nd, or the like.

In addition to the elements described above, they can be contained within a range that does not impair the effects of the present invention. It is preferable to reduce Bi, Pb, Se, H, Ta and the like as much as possible, but Bi ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, as necessary, as long as the effect of the present invention is not affected. It may contain one or more of H ≦ 100 ppm and Ta ≦ 500 ppm.

(II) Metallographic structure

The metallographic structure will be described.

The ferritic stainless steel sheet of the present invention has a ferritic single layer structure in which the metal structure is measured according to JIS G 0551 and has a crystal grain size number of 9.0 or more.

Since the roughened processed surface after molding is less likely to occur as the crystal grain size number is larger, the crystal grain size number is set to 9.0 or more. In order to further suppress rough skin, it is preferably 9.5 or more, and more preferably 10.0 or more.

The method for measuring the crystal grain size number is determined by the line segment method of JIS G 0551 (2013). The particle size number: 9 corresponds to the average line segment length of 14.1 μm per crystal grain crossing the inside of the crystal grain, and the particle size number: 10 corresponds to the average line segment length of 10.0 μm per crystal grain crossing the inside of the crystal grain. do. The crystal grain size is measured by using an optical microscope microstructure photograph of the cross section of the test piece, and the number of crystal grains crossed per sample is 500 or more. The etching solution may be aqua regia or reverse aqua regia, but other solutions may be used as long as the grain boundaries can be determined. Further, since the grain boundaries may not be clearly visible depending on the orientation relationship of the adjacent crystal grains, it is preferable to perform deep etching. In addition, the twin grain boundaries are not measured when measuring the grain boundaries.

The ferrite single-phase structure of the metal structure means that it does not contain an austenite phase or a martensite structure. Including these phases, the austenite phase exhibits high formability due to the TRIP effect in addition to the fact that it is relatively easy to make the crystal grain size finer, but in addition to the high raw material cost, the ear at the time of manufacture. Yield reduction such as cracking is likely to occur.

Whether or not the structure is ferrite single phase can be examined by EBSD (Electron BackScatter Diffraction) measurement. In the present invention, in addition to the determination of the austenite phase by EBSD measurement of the cross section, the region where the IQ value is lower than 3000 when the EBSD pattern is collected under the conditions of an acceleration voltage of 15 kV and a focal distance of 19 mm is set as a martensite phase, and austenite and martensite are used. If the area ratio of the site is 1/100 or less of the whole, it is judged to be a ferrite single phase.

(III) Aggregate organization

The collective organization will be described.

In the ferrite-based stainless steel sheet of the present invention, the workability can be improved by controlling the random intensity ratio of the crystal orientation in the plane parallel to the rolled plane at the plate thickness 1/2 position within the following range.

I _{{111} <112>} ≧ 9
I _{{111} <110>} ≧ 6
I _{{411} <148>} ≦ 1

The {111} <112> orientation is known to be a orientation that is generated as a recrystallization orientation of high-purity steel and improves formability. It is required to increase when performing molding processing centered on deep drawing. Since the random intensity ratio is most likely to be strong when a sufficient cold rolling ratio is secured, the random intensity ratio is set to 15 or more. The higher the random strength ratio of {111} <112>, the more advantageous the formability, so it is preferably 18 or more, more preferably 20 or more.

Similar to the {111} <112> orientation, the {111} <110> orientation is also generated as a recrystallization orientation of high-purity steel to improve formability. The {111} <110> direction is a direction in which the degree of integration is less likely to increase as compared with the {111} <112> direction. When the degree of integration (random intensity ratio) of {111} <112> is high and the degree of integration of {111} <110> is low, the anisotropy of the r value becomes large, and when molding by deep drawing is performed. The height of the ears increases, leading to a decrease in yield. Therefore, it is desirable that the degree of integration of the {111} <110> orientation is high, and the random intensity ratio of {111} <110> is set to 10 or more. It is preferably 12 or more, and more preferably 14 or more.

Since the {411} <148> orientation is not preferable for moldability, its random intensity ratio is set to 1 or less. It is preferably 0.7 or less, more preferably 0.5 or less. {411} <148> is easily generated as a recrystallization orientation, and until now, the degree of integration could not be sufficiently reduced, and high formability could not be obtained.

In the present invention, it is ideal to increase the degree of integration of the {111} <112> and {111} <110> orientations that improve the above-mentioned formability, and to decrease the degree of integration of the {411} <148> orientations. It is characterized by establishing crystal orientation control and simultaneously satisfying these random intensity ratios.

Further, by suppressing the accumulation in the {411} <148>, the degree of accumulation in the {111} orientation, which is advantageous for improving the moldability, can be increased.

The random intensity ratio of the crystal orientation is measured by the following method.

X-ray diffraction at the 1 / 2t position of the plate thickness t is performed on the surface parallel to the rolled surface of the steel sheet. The 1 / 2t position often indicates the average texture of the steel material and can be an index of formability. Using the obtained data, a three-dimensional orientation analysis is performed using the Bunge method described in Non-Patent Document 1. From the crystal orientation distribution map, read the random intensity ratio in the corresponding orientation.

(IV) r-value

The r value will be described.

The ferrite-based stainless steel sheet of the present invention preferably has an average r value (Rankford value) of 1.5 or more. By setting the average r value to 1.5 or more, the formability of the ferritic stainless steel sheet can be improved and strict processing can be performed, and at the same time, the load during molding can be reduced and the consumption of the mold can be suppressed. can. The average r value is more preferably 1.7 or more.

The average r value is measured by the plastic strain ratio test method of JIS Z 2254 (2008), and can be obtained by the following formula (1) according to JIS Z 2254 (2008).

Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 ... Equation (1)

However, in equation (1), r ₀ indicates the r value in the rolling direction, r ₉₀ indicates the r value in the rolling perpendicular direction, and r ₄₅ indicates the r value in the rolling 45 degree direction.

Next, a method for manufacturing the ferritic stainless steel sheet of the present invention will be described.

(I) Hot rolling process

After heating the stainless steel slab having the above-mentioned components, hot rolling consisting of rough rolling and finish rolling is carried out to obtain a hot-rolled steel sheet. As for the slab heating temperature, if the heating temperature is too high, Ti charcoal sulfide (Ti ₄ C ₂ S ₂ ) melts during heating, and recrystallization occurs due to an increase in solid-dissolved carbon and reprecipitation during the hot rolling process. A phenomenon such as delay occurs. These phenomena suppress the development of the recrystallized texture of the product board and deteriorate the workability. In addition, the crystal grains are remarkably enlarged, coarse wrought grains are formed in the hot rolling process, and the workability of the product plate is deteriorated. In addition, an excessive decrease in temperature causes surface scratches, resulting in deterioration of corrosion resistance due to rusting from the scratched portion. Therefore, the slab heating temperature is preferably 1100 to 1250 ° C. Further, considering the decrease in productivity due to the seizure of the rolled roll, the slab heating temperature is preferably 1130 to 1230 ° C.

(II) Hot-rolled plate annealing process

Following hot rolling, hot-rolled sheet annealing for the purpose of recrystallization may be carried out. Annealing of hot rolled plates is not essential.

By performing hot-rolled sheet annealing, surface defects that occur during molding, which is called product rigging, can be reduced. The hot-rolled sheet annealing is preferably performed in the range of T to T + 50 ° C. with the recrystallization temperature as T. If the temperature is too low, recrystallization defects will occur and the rigging reduction effect will not be sufficiently obtained. If the temperature is too high, the crystal grains will be enlarged and the random intensity ratio of the predetermined crystal orientation cannot be improved after cold rolling annealing. The workability of the product deteriorates. However, since it is possible to reduce rigging as compared with the conventional method by performing cold rolling as described later, hot-rolled sheet annealing may be omitted.

After hot-rolled sheet annealing, if hot-rolled sheet annealing is omitted, surface scale is formed after hot rolling, and if problems such as surface defects occur in the post-process, removal by pickling, etc., if necessary. It may be scaled.

(III) Cold rolling process

Cold rolling rate RT: 50-90%

Cold rolling is carried out following the descaling treatment carried out as necessary. If the cold rolling ratio RT is too low, the metal structure is coarsened and the random intensity ratio in the {111} direction is lowered. Therefore, the cold rolling ratio RT is set to 50% as the lower limit. It is preferably 55% or more, more preferably 60% or more. Further, if the cold rolling ratio RT is too high, the moldability is rather lowered, so the upper limit is 90%. It is preferably 87% or less, more preferably 85% or less.

Diagonal rolling angle: 20-70 °
Total cold rolling ratio for diagonal rolling RC: 20% or more

The cold rolling of the present invention is characterized in that it is not cold-rolled only in the hot-rolling direction but is rolled in the diagonal direction under the above conditions. As a result, the random intensity ratio of the {111} <112> and {111} <110> orientations is improved, the random intensity ratio of the {411} <148> orientations is lowered, and the granulation can be achieved at the same time.

By combining cold rolling in the diagonal direction, the random intensity ratio of the {111} <112> and {111} <110> orientations is improved, the random intensity ratio of the {411} <148> orientation is decreased, and the random intensity ratio is decreased. The factors that can achieve both fine graining are considered as follows.

It is known that during cold rolling, it accumulates in cold rolling priority directions such as {001} <110>, {112} <110>, and {111} <110>. Among these, it is known that the {001} <110> direction does not easily accumulate the strain of cold rolling, so that it remains until the end of recrystallization and becomes the rate-determining factor for the completion of recrystallization. By changing the direction of cold rolling, the {001} <110> orientation can be changed to an orientation such as {100} <001> where strain of cold rolling is likely to accumulate and recrystallization is likely to occur. A fine-grained structure is obtained to expedite the completion of recrystallization. Further, it is known that the rolling orientation of {100} <001> is likely to be recrystallized in an orientation disadvantageous to formability such as {411} <148> after recrystallization, and the {411} <148> orientation is reduced. Therefore, it is considered that the proportion of {111} oriented grains, which is advantageous for formability, increases.

As for the direction of cold rolling, diagonal rolling in a direction of an angle of 20 to 70 ° with respect to the direction of hot rolling in the previous step is performed at a cold rolling ratio RC of 20% or more. When the rolling angle in the diagonal direction is smaller than 20 °, the effect of lowering the random intensity ratio of {411} <148>, the effect of improving the random intensity ratio of the {111} orientation, and the effect of grain refinement are obtained. Therefore, the lower limit of the rolling angle in the diagonal direction is 20 ° because it is not possible to improve the formability and suppress the roughening of the processed surface. Similarly, even if the temperature is 70 ° or higher, the moldability is improved and the crystal grains cannot be refined. Therefore, the upper limit is 70 °. Further, when the cold rolling ratio RC in the diagonal direction is smaller than 20%, the lower limit of the cold rolling ratio RC is set to 20% because the moldability is similarly improved and the crystal grains cannot be refined.

Total cold rolling ratio RL in the hot rolling direction: 20% or more

The effect of diagonal rolling in the present invention is obtained by combining rolling in a plurality of directions, not simply by rolling diagonally. Therefore, in the case of a steel sheet that has been annealed by hot rolling, in addition to cold rolling at a cold rolling ratio RC in the diagonal direction, cold rolling at a cold rolling ratio RL in the hot rolling direction is performed by 20% or more. ..

However, when hot-rolled sheet annealing is omitted, the desired effect can be obtained by combining cold rolling in the hot rolling direction and diagonally, so cold rolling in the hot rolling direction is omitted. You may.

(IV) Heat treatment process (finish annealing process)

The cold-rolled steel sheet obtained by cold rolling as described above is heat-treated for the purpose of recrystallization to obtain a heat-treated steel sheet. The heat treatment temperature is preferably in the range of T to T + 30 ° C. when the recrystallization temperature is T. If the temperature is too low, the structure becomes unrecrystallized and the moldability is deteriorated. If the temperature is too high, the grain growth causes the processed skin to become rough.

Next, an embodiment of the present invention will be shown.

The stainless steels shown in Tables 1 and 2 are melted, rolled to the plate thickness shown in Tables 3 to 4 by hot rolling, cold rolled to a plate thickness of 1.0 mm, and then annealed by cold rolling. Manufactured a product board. Some hot-rolled plates were annealed at 980 ° C. for 2 minutes, then cold-rolled and cold-rolled. Each process condition was changed as shown in Table 2.

The following evaluations were made on the manufactured stainless steel sheets.

<Crystal particle size number>

The crystal particle size number was determined by the line segment method of JIS G 0551 (2013). The pass was 9.0 or higher, which is mild after molding.

<Random intensity ratio>

The random intensity ratio was determined by performing X-ray diffraction at a position 1 / 2t of the plate thickness t on a surface parallel to the rolled surface of the steel sheet, and using the obtained data, using the Bunge method described in Non-Patent Document 1. It was obtained by three-dimensional orientation analysis. I {111} <112> ≧ 9, I {111} <110> ≧ 6, and I {411} <148> ≦ 1, which enable deep drawing with a drawing ratio of 2.4 or more, were accepted.

<Average r-value>

The average r value was measured by the plastic strain ratio test method of JIS Z 2254 (2008), and was calculated by the formula (1) according to JIS Z 2254 (2008).

Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 ... Equation (1)

Passed 1.50 or more, which is a guideline for deep drawing with a drawing ratio of 2.4 or more, and rejected less than 1.50.

<Formability>
A test piece having a diameter of 120 mm was cut out from the product plate, and a cup forming test was carried out with a drawing ratio of 2.4, which enables the forming of a cylinder having a height larger than the diameter in one step. The conditions implemented this time are: punch diameter is 50 mm, punch shoulder R is 5R, clearance is 1.2 mm on one side, and for lubrication between the steel plate and punch, rust preventive oil Z3 is applied and then a PTFE (polytetrafluoroethylene) sheet is attached. rice field.

The moldability of the sample for which the cup forming test with the drawing ratio of 2.4 was completed was evaluated as “◯”, and the formability of the sample broken during molding was evaluated as “x”.

<Rough skin after molding>

For the sample that could be molded with a drawing ratio of 2.4, the rough skin after cup molding was evaluated by the arithmetic mean roughness Ra.

Surface roughness measurement according to JIS B 0601 using a contact-type surface roughness measuring machine for a length of 5 mm parallel to the height at a position 15 mm to 20 mm from the bottom of the cup inside the vertical wall of the cup. Was performed, and the arithmetic average roughness Ra was calculated.

Rough skin after molding is mild and it is not necessary to remove unevenness by rough polishing of about 80 count. When the arithmetic average roughness Ra ≤ 1.2, the rough skin after molding is passed and the removal of unevenness by rough polishing is indispensable. When Ra> 1.2, it was rejected.

The evaluation results are shown in Table 5.

According to the present invention, a ferritic stainless steel sheet having excellent formability and polishing resistance after molding can be obtained by controlling the random strength ratio by appropriately performing the cold rolling direction and the cold rolling ratio. As shown in Table 5, in the example of the invention, the molding process could be completed without breaking at a drawing ratio of 2.4, and Ra ≦ 1.2 μm, so that the roughened surface was suppressed.

In reference numerals c1, c4, c5, and c9, which are comparative examples shown in Table 5, the crystal grain size, random intensity ratio, and average r value are within the claimed range, but c1 is Cr, c4 is Si, c5 is Mn, and c9 is Al. The content of each was excessive, and the strength and ductility were increased, so that the molding process could not be completed due to breakage during molding. Further, with reference numeral c14, the average r value was high and cup molding could be performed, but the crystal grain size number was large and the processed surface roughness after molding was large.

According to the present invention, as compared with the conventional method for manufacturing a stainless steel sheet, only a part of rolling is carried out in an oblique direction, and characteristics remarkably superior to those in the conventional method can be obtained, so that the industrial applicability is great. Further, regardless of the form of the product, a cutting plate or a coil may be used. In particular, it is suitable for applications that require relatively strict formability, such as housings for home appliances or equipment.

Claims (7)

By mass%,
Cr: 11.0% or more, 30.0% or less,
C: 0.001% or more, 0.030% or less,
Si: 0.01% or more, 2.00% or less,
Mn: 0.01% or more, 2.00% or less,
P: 0.005% or more, 0.100% or less,
S: 0.010% or less,
Al: 2.00% or less, and N: 0.030% or less,
Including
moreover,
Ti: 0.50% or less and Nb: 1.00% or less are contained, and the balance is Fe and impurities.
The metal structure is a ferrite single layer structure having a crystal grain size number of 9.0 or more as measured according to JIS G 0551.
The random intensity ratio of the crystal orientation in the plane parallel to the rolled plane at the plate thickness 1/2 position is
I _{{111} <112>} ≧ 9,
I _{{111} <110>} ≧ 6,
I _{{411} <148>} ≦ 1
Ferritic stainless steel sheet characterized by being.
Here, I _{{hkl} <uvw>} indicates the random intensity ratio of the {hkl} <uvw> orientation. The ferritic stainless steel sheet according to claim 1, further comprising one group or two or more groups of the following groups A to C in mass%.
Group A:
Sn: 0.50% or less,
Ni: 1.00% or less,
Cu: 1.00% or less,
Mo: 2.00% or less,
W: 1.00% or less,
Co: 0.50% or less,
V: 0.50% or less,
Zr: 0.50% or less, and Sb: 0.50% or less, 1 type or 2 or more types B group:
B: 0.0025% or less,
Ca: 0.0050% or less,
Mg: 1 type or 2 types or more of 0.0050% or less Group C:
Y: 0.20% or less,
Hf: 0.20% or less,
REM: 1 type or 2 types or more of 0.10% or less The ferrite-based stainless steel sheet according to claim 1 or 2, wherein the average r value is 1.5 or more. The ferrite-based stainless steel sheet according to any one of claims 1 to 3, which is a housing or an instrument of a home electric appliance. The method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 3.
A hot-rolling step of hot-rolling a steel slab having the component according to claim 1 or 2 to obtain a hot-rolled steel sheet,
A cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet,
It is provided with a heat treatment step of heat-treating the cold-rolled steel sheet to obtain a heat-treated steel sheet.
In the cold rolling step, the cold rolling ratio RT is set to 50% or more and 90% or less, and a part of the cold rolling is obliquely oriented at an angle of 20 ° or more and 70 ° or less with respect to the direction of the hot rolling. A method for manufacturing a ferrite-based stainless steel sheet, which comprises cold rolling at a cold rolling ratio of RC 20% or more. The ferrite-based stainless steel sheet according to claim 5, wherein a part of the cold rolling further includes cold rolling in the same direction as the hot rolling direction with a cold rolling ratio of RL 20% or more. Production method. The method for manufacturing a ferrite-based stainless steel sheet according to claim 6, further comprising a hot-rolled sheet annealing step for annealing the hot-rolled steel sheet between the hot-rolled step and the cold-rolled step.