CN112513303B

CN112513303B - Ferritic stainless steel sheet

Info

Publication number: CN112513303B
Application number: CN201880096252.5A
Authority: CN
Inventors: 木村谦; 田村真市; 伊藤力; 田口笃史; 石丸咏一朗; 加贺祐司; 大村圭一; 山岸昭仁
Original assignee: Nippon Steel and Sumikin Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2018-11-09
Filing date: 2018-11-09
Publication date: 2022-05-24
Anticipated expiration: 2038-11-09
Also published as: WO2020095437A1; BR112021000940B1; WO2020095437A8; EP3878993A1; KR102515016B1; EP3878993A4; BR112021000940A2; CN112513303A; KR20210034054A

Abstract

The ferritic stainless steel sheet contains Cr: 11.0% -25.0%, C: 0.001% -0.010%, Si: 0.01% -1.0%, Mn: 0.01% -1.0%, P: 0.10% or less, S: 0.01% or less, N: 0.002% -0.020%, further comprising Ti: 1.0% or less and Nb: 1.0% or less, and the balance containing Fe and impurities, and having a ferrite single-phase structure with a crystal grain size number of more than 9.0, wherein the random strength ratio of crystal orientation in a plane parallel to the rolling plane at the position 1/2 and 1/10 is I_{554}<225>≥7.0、I_{411}<148>≥0.9、I_{211}<011>≥1.0。

Description

Ferritic stainless steel sheet

Technical Field

The present invention relates to a ferritic stainless steel sheet, and particularly to a ferritic stainless steel sheet excellent in formability during forming and surface properties after forming.

Background

Austenitic stainless steels represented by SUS304(18Cr-8Ni), which is a typical steel type, are widely used for home appliances, kitchen products, building materials, and the like because of their excellent corrosion resistance, workability, aesthetic properties, and the like. However, austenitic stainless steel is expensive because a large amount of Ni, which is expensive and has drastic fluctuation in price, is added, and therefore, from the viewpoint of economy, cheaper stainless steel is desired.

On the other hand, ferritic stainless steel contains no Ni or a very small amount of Ni, and therefore, demand for a material having an excellent cost performance has been increasing in recent years. However, when ferritic stainless steel is used for forming applications, there are problems of the forming limit and deterioration of surface properties due to formation of surface irregularities after forming.

First, if the forming limit is compared, in the case of austenitic stainless steel, the bulging property is excellent, whereas in ferritic stainless steel, the bulging property is low, and large deformation cannot occur. However, since deep drawability can be controlled by adjusting crystal orientation (texture) in steel, when ferritic stainless steel is used for forming applications, a forming method mainly involving deep drawing is often used.

Next, the surface characteristics (surface irregularities) after the molding process will be described. Here, "surface irregularities" refer to fine irregularities (surface roughness) that are formed on the surface of the steel sheet after machining or forming, and correspond to crystal grains, and the surface irregularities become more prominent as the crystal grain size is larger.

In the case of austenitic stainless steel, since the work hardening property is excellent and the fine grain structure is relatively easy to produce, a steel sheet having a grain size number of about 10 is produced. Therefore, the surface unevenness (surface roughness) after the molding is small, and there is almost no problem. On the other hand, as for the crystal grain size of ferritic stainless steel, SUS430 is about 9, and SUS430LX is about 7, which are smaller than those of austenitic stainless steel. Here, a small particle size number indicates a large crystal particle size.

As one of the reasons why ferritic stainless steel is likely to be coarse, ferritic stainless steel tends to have a large recrystallized grain size, and particularly, high-purity ferritic stainless steel, such as SUS430LX, in which C, N is reduced to improve workability and formability, tends to have a larger crystal grain size than austenitic stainless steel because grain growth is likely to proceed.

In the case where relatively severe formability is required such as housings of home electric appliances and utensils, a high-purity ferritic stainless steel such as SUS430LX is often used as the ferritic stainless steel. Further, the thickness of the stainless steel sheet used for securing the strength after forming is usually 0.6mm or more in most cases, but since the crystal grain size is large as described above, the surface roughness after forming is large, and the surface irregularities are usually removed by polishing after forming.

From the above background, a method for reducing surface roughness of a high-purity ferritic stainless steel is disclosed.

Patent document 1 discloses a ferritic stainless steel and a method for producing the same, in which a high-purity ferritic stainless steel is used to control the size of precipitated particles and the crystal grain size, thereby reducing the roughness of a processed surface and improving formability. However, the method described in patent document 1 has a problem that although a steel sheet having a small crystal grain size is obtained, deep drawability at the time of forming is not sufficient, and surface roughness after forming is likely to occur even if the crystal grain size is small.

Patent document 2 discloses a technique of producing a stainless steel having excellent surface roughness resistance at the time of forming by performing hot rolling at a low temperature in a ferritic stainless steel containing Ti and Nb and making fine particles by adopting a high cold rolling ratio. With this technique, although the stainless steel sheet described in patent document 2 has a fine grain structure with a grain size number of 9.5, the surface roughness resistance after cup drawing (cup drawing) forming is not necessarily sufficient.

Patent document 3 discloses a ferritic stainless steel having improved deep drawability, wrinkle resistance, and surface roughening resistance by controlling the crystal grain size of a steel having a composition containing Nb and/or Ti before final cold rolling. However, the crystal grain size of the final product was 15 μm (8.8 in terms of the number of crystal grains), and the surface roughening resistance was not sufficient.

In addition, conventionally, in order to reduce surface irregularities of high-purity ferritic stainless steel, a method of reducing surface irregularities by increasing the number of cold rolling to make crystal grain sizes fine in the production of ferritic stainless steel sheet has been studied. However, in practice, surface irregularities may be formed on the product sheet, and the reason for this is not necessarily clear, and a technique capable of stably maintaining high quality of the steel sheet surface is desired.

As described above, when forming processing of a ferritic stainless steel is considered, it is very difficult to form the ferritic stainless steel into a predetermined shape and to satisfy surface properties after forming. Therefore, in the case of a ferritic stainless steel having ensured formability, a polishing step is required to remove surface irregularities generated after forming, but the polishing time is required, which increases the production cost, and there are environmental problems such as generation of a large amount of dust generated by polishing.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 4749888

Patent document 2: japanese laid-open patent publication No. 7-292417

Patent document 3: japanese patent No. 3788311

Non-patent document

Non-patent document 1: r.k.ray, j.j.jonas and r.e.hook: international Materials reviews. vol.39, No.4(1994), p131

Non-patent document 2: the inter-ear scion high, intermediate village, tomu, gilong tree: iron と stainless steel, vol.90, No.7(2004), p510-517

Non-patent document 3: coated article "Jizhi" with Changdao jin-edison, Wanshan (1984), p23

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made in view of the above problems, and provides a ferritic stainless steel sheet having excellent formability and surface properties after forming.

Means for solving the problems

The gist of an aspect of the present invention is as follows.

[1]A ferritic stainless steel sheet characterized by containing, in mass%, Cr: 11.0% -25.0%, C: 0.001% -0.010%, Si: 0.01% -1.0%, Mn: 0.01% -1.0%, P: 0.10% or less, S: 0.01% or less, N: 0.002% -0.020%, further comprising Ti: 1.0% or less and Nb: 1.0% or less, and the balance containing Fe and impurities, and having a ferrite single-phase structure with a crystal grain size number of more than 9.0, wherein the random strength ratio of crystal orientation in a plane parallel to the rolling plane at the position 1/2 and 1/10 is I_{554}<225>≥7.0、I_{411}<148>≥0.9、I_{211}<011>≥1.0。

Wherein, I_{hkl}<uvw>Denotes { hkl }<uvw>Random intensity ratio of orientation.

[2] The ferritic stainless steel sheet according to the above [1], which further comprises, in mass%, B: 0.0001 to 0.0025 percent of Sn: 0.005-0.50%, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 2.0% or less, Al: 1.0% or less, W: 1.0% or less, Co: 0.50% or less, V: 0.50% or less, Zr: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, Sb: 0.50% or less of 1 or 2 or more.

Effects of the invention

According to an aspect of the present invention, a ferritic stainless steel sheet excellent in formability and surface properties after forming can be provided.

Detailed Description

Hereinafter, an embodiment of the ferritic stainless steel sheet according to the present invention will be described.

Ferritic stainless steel of the present embodimentThe steel sheet contains, in mass%, Cr: 11.0% -25.0%, C: 0.001% -0.010%, Si: 0.01% -1.0%, Mn: 0.01% -1.0%, P: 0.10% or less, S: 0.01% or less, N: 0.002% -0.020%, further comprising Ti: 1.0% or less and Nb: 1.0% or less, and the balance containing Fe and impurities, and having a ferrite single-phase structure with a crystal grain size number of more than 9.0, wherein the random strength ratio of crystal orientation in a plane parallel to the rolling plane at the position 1/2 and 1/10 is I_{554}<225>≥7.0、I_{411}<148>≥0.9、I_{211}<011>≥1.0。

Hereinafter, each element will be described in detail.

First, the reasons for the limitation of the components will be described below. The expression "%" of the content of each element means "% by mass".

Cr is an element that improves corrosion resistance, which is a basic characteristic of stainless steel. Since sufficient corrosion resistance cannot be obtained below 11.0%, the lower limit is set to 11.0% or more. On the other hand, the upper limit is set to 25.0% or less because excessive addition promotes the formation of intermetallic compounds corresponding to the σ phase (intermetallic compounds of Fe — Cr) and promotes cracking during production. From the viewpoint of stable productivity (yield, rolling defects, etc.), it is preferably 14.0% to 22.0%. More preferably 16.0% to 20.0%.

Since C is an element that lowers the important formability (r value) in the present embodiment, C is preferably small, and the upper limit is set to 0.010% or less. However, since an excessive reduction leads to an increase in the refining cost, the lower limit is set to 0.001% or more. Considering both the refining cost and the moldability, the content is preferably 0.002% to 0.008%, and more preferably 0.002% to 0.006%.

Si is an oxidation resistance-improving element, but since excessive addition causes a reduction in formability, the upper limit is set to 1.0% or less. The Si content is preferably low from the viewpoint of formability, but since an excessive reduction leads to an increase in raw material cost, 0.01% or more is set as the lower limit. From the viewpoint of manufacturability, the preferable range is 0.05% to 0.60%, and more preferably 0.05% to 0.30%.

Since addition of a large amount of Mn causes a reduction in formability as in Si, the upper limit is set to 1.0% or less.

The Mn content is preferably low from the viewpoint of formability, but since an excessive reduction leads to an increase in raw material cost, 0.01% or more is set as the lower limit. From the viewpoint of manufacturability, the preferable range is 0.05% to 0.40%, and more preferably 0.05% to 0.30%.

P is an element that lowers the moldability (r value and product elongation), and is preferably low, and the upper limit is limited to 0.10% or less. However, since an excessive reduction leads to an increase in the cost of raw materials, the lower limit is preferably set to 0.005% or more. In consideration of both moldability and production cost, the range is preferably 0.007% to 0.030%, more preferably 0.010% to 0.025%.

S is an inevitable impurity element, and is preferably low because it promotes cracking during production, and the upper limit is limited to 0.01% or less. The lower the S content is, the more preferable is 0.0030% or less. On the other hand, since an excessive reduction leads to an increase in refining cost, the lower limit is preferably set to 0.0003% or more. From the viewpoint of manufacturability and cost, a preferable range is 0.0004% to 0.002%.

N is an element that lowers moldability (r value) in the same manner as C, and the upper limit is set to 0.020% or less. However, since an excessive reduction leads to an increase in the refining cost, the lower limit is set to 0.002% or more. From the viewpoint of moldability and manufacturability, the preferable range is 0.005% to 0.015%.

Either or both of Ti and Nb are contained as described below.

Ti binds to C, N to fix C, N (increase the purity) as precipitates such as TiC and TiN, thereby improving the formability (r value) and the product elongation. In order to obtain these effects, the lower limit is preferably set to 0.01% or more. On the other hand, the upper limit is set to 1.0% or less because excessive addition leads to an increase in alloy cost and a decrease in manufacturability associated with an increase in recrystallization temperature. From the viewpoint of moldability and manufacturability, the preferable range is 0.05% to 0.50%. Further, a preferable range in which the above-described effects of Ti are positively and effectively utilized is 0.10% to 0.30%.

Similarly to Ti, Nb is a stabilizing element for fixing C, N, and increases the formability (r value) and the product elongation by increasing the purity of steel by this action. In order to obtain these effects, the lower limit is preferably set to 0.01% or more. On the other hand, the upper limit is set to 1.0% or less because excessive addition leads to an increase in alloy cost and a decrease in manufacturability associated with an increase in recrystallization temperature. From the viewpoint of alloy cost and manufacturability, a preferable range is 0.02% to 0.30%. Further, a preferable range in which the above-described effects of Nb are positively and effectively utilized is 0.04% to 0.15%. More preferably 0.06% to 0.10%.

The ferritic stainless steel sheet of the present embodiment may optionally contain 1 or 2 or more of the following element groups in addition to the basic composition described above.

B is an element for improving secondary processability. Since 0.0001% or more is necessary to exert its effect, the lower limit is set. On the other hand, since excessive addition causes deterioration in manufacturability, particularly castability, the upper limit is set to 0.0025% or less. The preferable range is 0.0002 to 0.0020%, and more preferably 0.0003 to 0.0012%.

Since Sn is an element having an effect of improving corrosion resistance, Sn may be added in accordance with a corrosive environment at room temperature. Since the effect is exhibited when the content is 0.005% or more, the lower limit is set. On the other hand, since a large amount of addition causes deterioration in manufacturability, 0.50% or less is set as the upper limit. The preferable range is 0.01 to 0.20%, and more preferably 0.02 to 0.10% in view of the productivity.

Ni, Cu, Mo, Al, W, Co, V, and Zr are effective elements for improving corrosion resistance and oxidation resistance, and are added as necessary. However, excessive addition of these elements not only reduces formability, but also increases alloy cost and hinders manufacturability. Therefore, the upper limits of Ni, Cu, Al and W are set to 1.0% or less. Since Mo causes a reduction in manufacturability, the upper limit is set to 2.0% or less. The upper limit of Co, V and Zr is set to 0.50% or less. The lower limit of the more preferable content of any element is set to 0.10% or more.

Ca. Mg is an element for improving hot workability and secondary workability, and is added as necessary. However, since excessive addition of these elements hinders productivity, the upper limit of Ca and Mg is set to 0.0050% or less. The preferable lower limit is set to 0.0001% or more. In consideration of the manufacturability and hot workability, the preferable range is 0.0002 to 0.0020% for both Ca and Mg, and more preferably 0.0002 to 0.0010%.

Y, Hf and REM are effective elements for improving hot workability, purity of steel and oxidation resistance, and may be added as needed. In the case of addition, Y, REM was set to 0.10% or less and Hf was set to 0.20% or less, respectively, with respect to the upper limit. With respect to the preferable lower limit, Y, Hf, and REM are all set to 0.001% or more. Here, "REM" in the present embodiment refers to an element (lanthanoid element) having an atomic number of 57 to 71, and examples thereof include Ce, Pr, Nd, and the like.

Sb is an element having an effect of improving corrosion resistance, like Sn, and may be contained as needed. However, since the addition of Sb in a large amount causes deterioration in manufacturability, the upper limit is set to 0.50% or less. On the other hand, the effect of improving corrosion resistance is exhibited when 0.005% or more, and therefore, the lower limit is set.

The ferritic stainless steel sheet of the present embodiment contains Fe and impurities (including unavoidable impurities) in addition to the above elements, but may contain elements other than the elements described above within a range that does not impair the effects of the present invention. In the present embodiment, Bi, Pb, Se, H, Ta, and the like may be contained, for example, but in this case, it is preferable to reduce the content as much as possible. On the other hand, the content of these elements is controlled within limits to solve the problems of the present invention, and 1 or more of Bi. ltoreq.100 ppm, Pb. ltoreq.100 ppm, Se. ltoreq.100 ppm, H. ltoreq.100 ppm, and Ta. ltoreq.500 ppm may be contained as necessary.

The metal structure will be described next.

The ferritic stainless steel sheet of the present embodiment is formed of a ferrite single-phase structure having a grain size number exceeding 9.0.

The number of crystal grain size is set to be more than 9.0. The lower limit is set to the surface roughness after molding because it is more difficult to generate surface roughness as the crystal grain size number is larger, that is, as the grain size of ferrite grains is smaller. In order to further suppress surface unevenness, it is preferably more than 9.5, and more preferably more than 10.0.

The crystal grain size number can be determined by the line segment method of JIS G0551 (2013). Further, the particle size number: 9 corresponds to an average segment length of 14.1 μm per 1 crystal grain transversely cut within the crystal grain, and a grain size number: 10 corresponds to an average segment length of 10.0 μm per 1 crystal grain transversely cut in the crystal grain. In the measurement of the crystal grain size, the number of crystal grains crossing 1 sample was set to 500 or more from an optical microscopic structure photograph of a cross section of the test piece. The etching solution is preferably aqua regia or reverse aqua regia, but may be other solutions as long as crystal grain boundaries can be determined. Further, since grain boundaries may not be clearly seen depending on the orientation relationship of adjacent crystal grains, it is preferable to perform erosion more intensively. In addition, the twin crystal grain boundary was not measured in the measurement of the crystal grain boundary.

Generally, it is known that crystal orientation has a strong correlation with formability (r value), but in the present embodiment, texture is defined as follows based on new findings obtained by the present inventors. That is, the present inventors have found that the crystal orientation greatly affects the surface unevenness after molding. At the position 1/2 in the plate thickness and the position 1/10 in the plate thickness, the random strength ratio of the crystal orientation in the plane parallel to the rolling plane was set as follows.

I_{554}<225>≥7.0

I_{411}<148>≥0.9

I_{211}<011>≥1.0

In addition, I_{hkl}<uvw>Denotes { hkl }<uvw>Random intensity ratio of orientation.

It is known that the {554} <225> orientation is generated as a Recrystallization orientation of a high-purity ferritic stainless steel and is an orientation having good formability (non-patent document 1). Therefore, it is required to increase the {554} <225> orientation in the forming process centered on drawing.

On the other hand, {411} <148> orientation is generated when the cold rolling rate is increased (for example, non-patent document 2), but is not preferable for formability. The {211} <011> orientation is an orientation formed by rolling, but is an orientation which is eaten by silkworms at the time of recrystallization, and therefore does not substantially remain after the end of recrystallization (non-patent document 3). Therefore, in order to ensure formability, it is considered that it is effective to increase the degree of aggregation of the {554} <225> orientation (random intensity ratio) and decrease the degree of aggregation of the {411} <148> orientation and the {211} <011> orientation, and thus control the degree of aggregation.

However, the present inventors have found that: by not only increasing the aggregation of the {554} <225> orientation, which is a preferred orientation for moldability, but also increasing the aggregation of the {411} <148> orientation, which is not preferred for moldability, and the {211} <011> orientation, which is difficult to remain after recrystallization, and controlling the aggregation together with the crystal grain size (crystal grain diameter), surface irregularities (surface roughness) after molding can be stably suppressed.

That is, in the present embodiment, the random strength ratio is set to 7.0 or more in consideration of forming the steel sheet into various shapes with respect to the {554} <225> orientation. As described above, the random strength ratio of the {554} <225> orientation is preferably 8.0 or more because it is preferable to increase the forming limit.

The {411} <148> orientation is an orientation important for suppressing surface unevenness, and the random intensity ratio is set to 0.9 or more. Preferably 1.0 or more. In the case of manufacturing a ferritic stainless steel sheet by a conventional method, it generally becomes lower than 0.7. Therefore, in the present embodiment, in order to increase the {411} <148> orientation, control of the manufacturing method as described below is required.

The degree of aggregation of {211} <011> orientation is set to 1.0 or more. As described above, the {211} <011> orientation is difficult to remain after the completion of recrystallization, and the orientation is generally 0.8 or less even when a ferritic stainless steel sheet is produced by a conventional method. Therefore, as in the case of the control of the {411} <148> orientation, an effort of manufacturing conditions becomes necessary.

The method of measuring the random intensity ratio of crystal orientation will be described.

X-ray diffraction was performed on a plane parallel to the rolling surface of the steel sheet at the 1/2 position and 1/10 position of the sheet thickness. The 1/2 position shows the average texture of steel material in many cases, and can be an index of formability. Further, since surface irregularities (surface roughness) after molding are generated on the surface, the distribution of crystal orientation in the vicinity of the surface becomes important, and therefore the 1/10 position is also measured.

Three-dimensional orientation analysis was performed on the obtained data. As the analysis method, a well-known "Bunge" method can be used. The random intensity ratios at the eligible orientations were read from the crystallographic orientation profiles. Local orientation analysis using EBSD may be used, but in this case, it is necessary to obtain average texture information by examining a region where the number of crystal grains is 1000 or more.

The reason why both the formability and the surface unevenness (surface roughness) after forming are improved by the above-mentioned regulation of the texture is intensively investigated, but it is presumed that the following is made.

During the forming of the steel material, each crystal grain is deformed according to the respective crystal orientation. The active slip at this time is considered to be different in each crystal orientation. In general, an orientation with a high r value and an orientation with a low r value are different in terms of the active slip system (direction). Therefore, it is considered that when oriented crystal grains having a high r value and oriented crystal grains having a low r value are adjacent to each other on the surface of the steel material, surface variations (concavities or convexities) caused by sliding of one crystal grain are offset by different surface variations (convexities or concavities) of the adjacent crystal grains, and as a result, surface irregularities are suppressed. However, since the combination of adjacent grain orientations in the steel surface is enormous, further studies are required for elucidation of the mechanism thereof.

The metallic structure of the ferritic stainless steel sheet of the present embodiment is formed of a ferrite single-phase structure. This means that no austenite phase and no martensite structure are present. When the austenite phase or the martensite structure is included, it is relatively easy to make the crystal grain size fine, and the austenite phase exhibits high formability by the TRIP effect. However, since the raw material cost is high and the yield is likely to be lowered by edge cracking during production, the metal structure is a ferrite single-phase structure. In addition, although precipitates such as carbonitrides are present in the steel, the effect of the present invention is not largely influenced, and therefore the structure of the main phase is described without taking these into consideration.

The thickness of the ferritic stainless steel sheet according to the present embodiment is not particularly limited, but is preferably 0.5mm or more, and preferably 0.6mm or more, from the viewpoint of securing strength. This is due to: when the thickness is small, the strength of the formed member may become insufficient. The plate thickness needs to be designed in consideration of the size, shape, load resistance, and the like of the member to be manufactured.

Next, although the method for producing a ferritic stainless steel sheet according to the present embodiment is described above, the hot rolling, the cold rolling, and the respective heat treatments (annealing) are combined, and pickling is performed as appropriate as necessary. That is, as an example of the manufacturing method, for example, a manufacturing method including steps of steel making, hot rolling, hot rolled sheet annealing, cold rolling, and cold rolled sheet annealing can be used.

The points to be controlled so as to satisfy both the crystal grain size and the crystal orientation (texture) important in the present embodiment as described above are the heat treatment conditions after hot rolling, the cold rolling reduction, and the heat treatment conditions after cold rolling, and the other steps and conditions are not particularly limited.

In the heat treatment after hot rolling (hot-rolled sheet annealing), the recrystallization temperature T of the hot-rolled sheet₁The temperature (DEG C) varies depending on the thickness, composition and reduction ratio of hot rolling, but the maximum reaching temperature must be controlled to T₁～(T₁+35) (° c). This is due to: if the maximum reached temperature of the hot-rolled sheet is lower thanT₁When the temperature is lower than the above range, no recrystallized grains remain, and the resulting product is poor in crease resistance and moldability. On the other hand, this is due to: if the maximum reached temperature exceeds (T)₁+35) ° c, the crystal grain size is coarsened by grain growth, the crystal grain size is coarsened after cold rolling and cold-rolled sheet annealing, or the above-mentioned crystal orientation important for surface roughness is not obtained after cold rolling and cold-rolled sheet annealing.

The cold rolling rate is set to 93% or more. Generally, the cold rolling reduction is set to about 90% at maximum in the conventional method, but in the present embodiment, it is necessary to increase the amount of strain to be introduced in order to make the recrystallized grain size after cold rolling fine. Recrystallization starts from a portion where strain is introduced more. That is, the larger the amount of work (the larger the rolling reduction), the more the recrystallization starts, the smaller the recrystallized grain size, and it is also important to control the {554} <225> orientation, {411} <148> orientation, and {211} <011> orientation, which are important for the surface roughness, within the above-mentioned ranges after recrystallization. For these reasons, it is important to set the rolling reduction to 93% or more in the present embodiment. The upper limit of the rolling reduction is not particularly limited, and may be set to 97% or less from the viewpoint of the capacity of the rolling mill.

In addition, other rolling conditions for the cold rolling of the present embodiment can be appropriately selected and set.

Regarding the maximum reaching temperature in the heat treatment (cold-rolled sheet annealing, final annealing) after cold rolling, when the recrystallization temperature of the cold-rolled sheet is T₂In the case of (. degree. C.), it is necessary to control the temperature to be (T)₂-10)～(T₂+30) deg.C. This is due to: if the maximum reached temperature for annealing the cold-rolled sheet is less than (T)₂When the temperature is-10) DEG C, the material may be hardened and easily cracked, thereby deteriorating the moldability. On the other hand, this is due to: if the maximum reached temperature exceeds (T)₂+30) ° c, the crystal grain size increases and a predetermined crystal grain size number or a predetermined crystal orientation is not obtained, resulting in surface roughness after molding.

In the present embodiment, intermediate annealing may be inserted during cold rolling. That is, the cold rolling in the present embodiment may be a single rolling or two or more times with intermediate annealing interposed therebetween. The intermediate and final annealing may be batch annealing or continuous annealing. Each annealing may be bright annealing in which annealing is performed in a non-oxidizing atmosphere such as hydrogen or nitrogen, or annealing may be performed in the atmosphere, if necessary.

Recrystallization temperature T₁、T₂It can be determined by observing the structure of the metal after heat treatment using a hot-rolled sheet or a cold-rolled sheet while changing the temperature.

The ferritic stainless steel sheet according to the present embodiment can be obtained by the manufacturing method described above.

Examples

The following examples of the present invention are described, but the conditions in the examples are only examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to the conditions adopted in the following examples. Various conditions may be adopted in the present invention as long as the object of the present invention can be achieved without departing from the essential elements of the present invention.

Moreover, underlining in the tables shown below indicates a range that deviates from the present embodiment.

Stainless steel having the composition shown in table 1 was melted and cast into a slab, and the slab was hot rolled and rolled. Thereafter, hot-rolled sheet annealing, cold-rolled sheet annealing and cold-rolled sheet annealing were performed to manufacture stainless steel sheets (product sheets) Nos. 1 to 28 having a thickness of 0.6 mm. The conditions of each step were changed as shown in table 2. The annealing time (holding time) in the hot-rolled sheet annealing and the cold-rolled sheet annealing is set to be in the range of 1 to 60 seconds, and in the present embodiment, the intermediate annealing is omitted.

Next, the Grain Size Numbers (GSN) of the stainless steel sheets Nos. 1 to 28 obtained were measured in accordance with JIS G0551 (2013).

Further, the texture of each of the plate thickness centers (1/2t position) and 1/10t position of stainless steel plate Nos. 1 to 28 was measured by X-ray diffraction, which is the above-mentioned method, to obtain {554}<225>Orientation, {411}<148>Orientation, {211}<011>Random intensity ratio of orientation I_{554}<225>、I_{411}<148>、I_{211}<011>。

Further, samples having a diameter of 100mm were cut out from stainless steel plate Nos. 1 to 28, and a cup forming test having a limit drawing ratio of 2.0 was conducted by a hydroforming tester. It is found that the ultimate drawing has a large influence on the surface roughness after cup forming, but other forming conditions do not have an influence. In addition, the cup forming test conditions carried out this time were: a rust preventive OIL "DAPHONE OIL COAT Z3 (registered trademark)" made by Kyoto K.K. was applied as a lubricant between the sample and the punch while the punch diameter was 50mm, the punch shoulder R was 5mm, the die diameter was 53mm, the die shoulder R was 8mm, and the blank holding pressure was 10 tons. Then, in order to protect the surface of the steel sheet after the forming, a lubricating sheet "NAFLON tape TOMBO9001 manufactured by nicolas Corporation" was attached.

The surface roughness after cup forming was evaluated for a sample formed at a limit drawing ratio of 2.0. Specifically, the surface roughness of the sample after cup forming was measured by a two-dimensional contact surface roughness measuring instrument for a length of 5mm parallel to the height direction in the height center portion of the vertical wall portion. The surface roughness evaluation was judged to be good (". smallcircle") when the arithmetic average roughness Ra described in JIS B0031 (2003) was 2.0 μm or less, and poor (". xx") when the Ra exceeded 2.0 μm.

The results of the above property evaluations of stainless steel sheet Nos. 1 to 28 are shown in Table 3. The stainless steel sheets of the present examples all had a ferrite single phase (containing no austenite phase and no martensite structure).

As shown in table 3, according to the examples of the present invention, the grain size number and texture were controlled, and ferritic stainless steel sheets having excellent surface roughening resistance and formability were obtained.

In the case of the comparative example in which Ra exceeded 2.0 μm, the surface irregularities were significant, and the irregularities were finally removed by polishing, so that the evaluation was also poor in terms of production cost.

TABLE 2

TABLE 3

Industrial applicability

According to the present embodiment, a ferritic stainless steel sheet excellent in formability and surface properties after forming can be provided. The ferritic stainless steel sheet of the present embodiment has excellent surface properties after forming. Conventionally, a polishing step is performed after forming for the purpose of removing surface irregularities, but the ferritic stainless steel sheet of the present embodiment can be sufficiently enjoyed in terms of manufacturing cost because the polishing step can be omitted. Therefore, the ferritic stainless steel sheet according to the present embodiment is suitably used for forming applications.

Claims

1. A ferritic stainless steel sheet characterized by comprising, in mass%:

Cr：11.0％～25.0％、

C：0.001％～0.010％、

Si：0.01％～1.0％、

Mn：0.01％～1.0％、

p: less than 0.10 percent,

S: less than 0.01 percent,

N：0.002％～0.020％，

Further comprising Ti: 1.0% or less and Nb: 1.0% or less, and the remainder comprising Fe and impurities,

the steel sheet is composed of a ferrite single-phase structure having a grain size number of more than 9.0, and the random strength ratio of the crystal orientation in a plane parallel to the rolling plane at the position of 1/2 sheet thickness and the position of 1/10 sheet thickness is

I_{554}<225>≥7.0、

I_{411}<148>≥0.9、

I_{211}<011>≥1.0，

Wherein, I_{hkl}<uvw>Denotes { hkl }<uvw>Random intensity ratio of orientation.

2. The ferritic stainless steel sheet according to claim 1, further comprising, in mass%, B: 0.0001 to 0.0025 percent of Sn: 0.005-0.50%, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 2.0% or less, Al: 1.0% or less, W: 1.0% or less, Co: 0.50% or less, V: 0.50% or less, Zr: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, Sb: 0.50% or less of 1 or 2 or more.

3. The ferritic stainless steel sheet according to claim 1 or 2, characterized in that the Cr content is 16.0% to 20.0%.

4. The ferritic stainless steel sheet according to claim 1 or 2, characterized in that, in the case of containing Ti, the Ti content is 0.10% to 0.30%; when Nb is contained, the Nb content is 0.06-0.10%.

5. The ferritic stainless steel sheet according to claim 3, characterized in that, in the case of containing Ti, the Ti content is 0.10% to 0.30%; when Nb is contained, the Nb content is 0.06-0.10%.

6. The ferritic stainless steel sheet according to claim 1 or 2, characterized by a grain size number exceeding 9.5.

7. The ferritic stainless steel sheet according to claim 3, characterized by a crystal size number exceeding 9.5.

8. The ferritic stainless steel sheet according to claim 4, characterized by a grain size number exceeding 9.5.

9. The ferritic stainless steel sheet according to claim 5, characterized by a grain size number exceeding 9.5.