WO2016051437A1

WO2016051437A1 - Ferritic stainless steel and method for producing same

Info

Publication number: WO2016051437A1
Application number: PCT/JP2014/005038
Authority: WO
Inventors: 正崇吉野; 映斗水谷; 光幸藤澤; 彩子田
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2014-10-02
Filing date: 2014-10-02
Publication date: 2016-04-07
Also published as: TW201614079A; JPWO2016051437A1; KR20170044741A; JP5904310B1; CN106715740A; TWI560283B; US20180265951A1; CN106715740B; KR101941066B1

Abstract

Provided is a ferritic stainless steel which has sufficient corrosion resistance, while having excellent formability and ridging resistance. A ferritic stainless steel according to the present invention contains, in mass%, 0.005-0.035% of C, 0.25% or more but less than 0.40% of Si, 0.05-0.35% of Mn, 0.040% or less of P, 0.01% or less of S, 15.5-18.0% of Cr, 0.001-0.10% of Al and 0.01-0.06% of N, with Si and Mn satisfying 29.5 × Si - 50 × Mn + 6 ≥ 0 (wherein Si and Mn represent the contents (mass%) of the respective elements), and with the balance made up of Fe and unavoidable impurities.

Description

Ferritic stainless steel and manufacturing method thereof

The present invention relates to a ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance and a method for producing the same.

Since ferritic stainless steel is inexpensive and has excellent corrosion resistance, it is used in various applications such as building materials, transportation equipment, home appliances, kitchen equipment, and automobile parts, and its application range has been further expanded in recent years. In order to be applied to these applications, not only corrosion resistance but also sufficient formability (high elongation and average Rankford value (hereinafter sometimes referred to as average r value)) that can be processed into a predetermined shape is required. .

On the other hand, ferritic stainless steel is often applied to applications that require a good appearance, and is also required to have excellent ridging resistance. Ridging is surface irregularities that occur due to molding distortion. Ferritic stainless steel may produce a group of crystal grains (colonies) with similar crystal orientations during casting and / or hot rolling, and in steel sheets with colonies remaining, the amount of strain at the colony and other parts during forming Since a large difference occurs in the surface, surface irregularities (ridging) occur after molding. When excessive ridging occurs after molding, there is a problem that a polishing step is required to remove surface irregularities, and the manufacturing cost of the molded product increases.

On the other hand, in Patent Document 1, in mass%, C: 0.02 to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less , Ti: 0.005% or less, Cr: 11-30%, Ni: 0.7% or less, and 0.06 ≦ (C + N) ≦ 0.12, 1 ≦ N / C and 1.5 × 10 ⁻³ ≦ (V × N) ≦ 1.5 A ferritic stainless steel excellent in formability and ridging resistance, characterized by satisfying × 10 ⁻² (C, N, and V each represents mass% of each element) is disclosed. However, in Patent Document 1, it is necessary to perform so-called box annealing (for example, annealing at 860 ° C. for 8 hours) after hot rolling. Such box annealing takes about one week when heating and cooling processes are included, and productivity is low.

On the other hand, in Patent Document 2, C: 0.01 to 0.10%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, Ni: 0.01 to 0.50%, Cr: 10 to 20%, Mo: 0.005 to 0.50%, Cu: 0.01 to 0.50%, V: 0.001 to 0.50%, Ti: 0.001 to 0.50%, Al: 0.01 to 0.20%, Nb: 0.001 to 0.50%, N: 0.005 to 0.050%, and B: 0.00010 to 0.00500 % Hot-rolled steel, hot-rolled sheet annealing in the ferrite single-phase temperature range using a box furnace or AP line (annealing and pickling line) continuous furnace, followed by cold rolling and finish annealing A ferritic stainless steel excellent in workability and surface properties characterized by being performed is disclosed. However, when a box furnace is used, there is a problem that productivity is low as in Patent Document 1 described above. Although no mention is made of elongation, when hot-rolled sheet annealing is performed in a ferrite single-phase temperature range using a continuous annealing furnace, recrystallization becomes insufficient due to the low annealing temperature, and the ferrite single-phase temperature is low. The elongation decreases compared to the case where box annealing is performed in the region. In general, ferritic stainless steel as in Patent Document 2 generates a group of crystal grains (colonies) having a similar crystal orientation during casting and / or hot rolling. If done, the ferrite phase colonies cannot be destroyed sufficiently. Therefore, there is a problem that the colony expands and remains in the rolling direction by cold rolling after hot-rolled sheet annealing, and ridging occurs after forming.

As described above, a technology for producing ferritic stainless steel having high formability and ridging resistance with high efficiency using a continuous annealing furnace has not been established.

Japanese Patent No. 3588281 (Japanese Patent Publication No. 2000-60134) Japanese Patent No. 3582001 (JP 2001-3143)

An object of the present invention is to solve such problems, and to provide a ferritic stainless steel having sufficient corrosion resistance, excellent formability and ridging resistance, and a method for producing the same.

In the present invention, sufficient corrosion resistance refers to a salt spray cycle test ((salt spray (35)) on a steel plate whose surface is polished with # 600 emery paper and then the end face is sealed. ℃, 5% NaCl, spray 2h) → drying (60 ° C, relative humidity 40%, 4h) → wet (50 ° C, relative humidity ≥ 95%, 2h)))) It means that the rusting area ratio (= (rusting area / total steel sheet area) × 100 (%)) on the steel sheet surface is 25% or less.

Excellent formability means that the elongation at break (El) in a tensile test in accordance with JIS Z2241 is 28% or more in a specimen perpendicular to the rolling direction, and a strain of 15% in a tensile test in accordance with JIS Z2241. Means that the average Rankford value (hereinafter referred to as the average r value) calculated by the following equation (1) is 0.70 or more.
Average r value = (r _L + 2 × r _D + r _C ) / 4 (1)
Here, r _L is an r value when a tensile test is performed in a direction parallel to the rolling direction, r _D is an r value when a tensile test is performed in a direction of 45 ° with respect to the rolling direction, and r _C is a direction perpendicular to the rolling direction. The r value when a tensile test is performed.

Furthermore, excellent ridging resistance means that the ridging height measured by the method described below is 2.5 μm or less. For measuring the ridging height, first, a JIS No. 5 tensile test piece is taken in parallel with the rolling direction. Next, the surface of the collected specimen is polished with # 600 emery paper, and then 20% tensile strain is applied. Next, the arithmetic average waviness (Wa) defined by JIS B 0601 (2001) is measured with a surface roughness meter in the direction perpendicular to the rolling direction on the polished surface at the center of the parallel part of the test piece. The measurement conditions are a measurement length of 16 mm, a high cut filter wavelength of 0.8 mm, and a low cut filter wavelength of 8 mm. This arithmetic mean swell is defined as the ridging height.

As a result of studying to solve the problems, hot rolled sheet annealing was performed in a temperature range in which the ferrite phase and the austenite phase are two phases before hot rolling and cold rolling on ferritic stainless steel having an appropriate component. In addition, by performing cold-rolled sheet annealing at a higher temperature than in the conventional single-phase temperature range, a ferritic stainless steel with sufficient corrosion resistance and excellent formability and ridging resistance can be obtained. I found.

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] By mass%, C: 0.005 to 0.035%, Si: 0.25 to less than 0.40%, Mn: 0.05 to 0.35%, P: 0.040% or less, S: 0.01% or less, Cr: 15.5 to 18.0%, Al: Contains 0.001 to 0.10%, N: 0.01 to 0.06%, and Si and Mn are 29.5 × Si-50 × Mn + 6 ≧ 0 (where Si and Mn indicate the content (% by mass)) Ferritic stainless steel characterized by being filled with the balance being Fe and inevitable impurities.
[2] By mass%, further containing one or more selected from Cu: 0.1 to 0.5%, Ni: 0.1 to 0.6%, Mo: 0.1 to 0.5%, Co: 0.01 to 0.3% The ferritic stainless steel as described in [1] above.
[3] By mass%, V: 0.01 to 0.10%, Ti: 0.001 to 0.05%, Nb: 0.001 to 0.05%, Ca: 0.0002 to 0.0020%, Mg: 0.0002 to 0.0050%, B: 0.0002 to 0.0050% REM: The ferritic stainless steel according to the above [1] or [2], comprising one or more selected from 0.01 to 0.10%.
[4] Any one of the above [1] to [3], wherein the elongation at break in the direction perpendicular to the rolling direction is 28% or more, the average Rankford value is 0.70 or more, and the ridging height is 2.5 μm or less. The ferritic stainless steel described.
[5] A method for producing a ferritic stainless steel according to any one of [1] to [4] above, wherein the steel slab is hot-rolled and then subjected to a temperature range of 900 to 1050 ° C. Manufacture of ferritic stainless steel characterized by annealing for 5 seconds to 15 minutes, followed by cold rolling, followed by annealing for 5 seconds to 5 minutes in the temperature range of 800 to 950 ° C Method.
In the present specification, “%” indicating the component of steel is “% by mass”.

According to the present invention, a ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance can be obtained.

It is a figure which shows the evaluation result of the ductility arranged by Si and Mn content.

Hereinafter, the present invention will be described in detail.

The ferritic stainless steel of the present invention is used for various applications such as building material parts, home appliance parts, kitchen appliances, and automobile parts by press working. In order to be applied to these applications, sufficient formability (elongation and average r value are large) is required.

For example, in the case of a spherical vent hood that is stretched and formed, if the elongation characteristics are insufficient, it cannot be molded due to necking or breakage in the direction of the worst elongation during molding. In addition, the appearance of the product may be deteriorated due to the fact that the thickness of the overhang portion after forming varies greatly depending on the direction of the steel plate before forming. Alternatively, large pans manufactured by drawing or the like are necked or broken when the average r value is low, and cannot be formed into a predetermined product shape. The plate thickness of the body of the pan varies greatly depending on the location, which may cause problems in heat transfer characteristics. Thus, it is desired that the elongation and the average r value are large.

Among ferritic stainless steels, SUS430LX (16mass% Cr-0.15mass% Ti or 16mass% Cr-0.4mass% Nb), SUS436L (18mass% Cr-1.0mass% Mo-0.25) specified in Japanese Industrial Standard JIS G4305 mass% Ti) contains a large amount of Ti and Nb, has a high El and average r value, has excellent formability, and is used in many applications. However, since these steel types contain a large amount of Ti and Nb, there is a problem that the raw material cost and the manufacturing cost are high and the price is high. On the other hand, SUS430 (16 mass% Cr), which is most produced among ferritic stainless steels, is less expensive than SUS430LX and SUS436L because it does not contain a large amount of Ti or Nb, but its formability is SUS430LX or SUS436L. Inferior. Therefore, SUS430 having improved formability has been demanded.

On the other hand, as described above, ferritic stainless steel generates surface irregularities called ridging on the surface of the steel sheet due to forming strain, so a product that requires surface aesthetics requires a polishing step to remove the surface irregularities, There is a problem that the manufacturing cost increases. The colonies that cause ridging contain Ti and Nb, and steels with less solid solution carbon are more likely to form. Therefore, the above SUS430LX and SUS436L have poor ridging resistance properties compared to SUS430.

As described above, the manufacturing technology for ferritic stainless steel, which has sufficient corrosion resistance, excellent formability, and excellent ridging resistance and is inexpensive, has not been sufficiently established.

Therefore, the inventors have El ≧ 28%, an average r value ≧ 0.70, and a ridging height of an appropriate component of ferritic stainless steel (particularly SUS430 (16 mass% Cr)) that does not contain a large amount of Ti or Nb. The method of obtaining ferritic stainless steel satisfying 2.5μm or less was studied intensively. In addition, there are box annealing (batch annealing) and continuous annealing as methods for annealing (hereinafter referred to as hot rolled sheet annealing) before cold rolling the ferritic stainless steel sheet after hot rolling. In short, instead of box annealing with low productivity, it was studied to obtain a predetermined formability by continuous annealing with high productivity.

The problem with the prior art using a continuous annealing furnace is that annealing is performed in a temperature range in which the metal structure becomes a single phase of ferrite, so that sufficient recrystallization does not occur, sufficient elongation cannot be obtained, The ridging resistance was not obtained because it remained even after finish annealing. Therefore, the inventors performed hot rolling sheet annealing in the two-phase region of the ferrite phase and austenite phase, then cold-rolled by a conventional method, and further performed finish annealing (cold rolling sheet annealing) at a higher temperature than conventional, We finally devised a ferrite single-phase structure again. Specifically, it is as follows. By performing hot-rolled sheet annealing in a two-phase region of ferrite phase and austenite that is higher than the ferrite single-phase temperature range, when the austenite phase is generated from the ferrite phase in hot-rolled sheet annealing, the austenite phase is the ferrite before annealing. It has a crystal orientation different from that of the phase. In addition, the metal structure after hot-rolled sheet annealing becomes a martensite phase generated during cooling due to transformation from the ferrite phase and austenite phase, and then at the heterogeneous interface between the soft ferrite phase and the hard martensite phase during cold rolling. Rolling strain is introduced more concentratedly and becomes a recrystallization site during cold-rolled sheet annealing. As a result, the ferrite phase colonies are effectively destroyed and the ridging resistance is improved. By cold rolling and further cold-rolled sheet annealing in the ferrite single-phase temperature range, the martensite phase is decomposed into carbonitride and ferrite phases, resulting in excellent ridging resistance of 2.5 μm or less in ridging height Is obtained.

However, it has been found that excellent moldability cannot be stably obtained only by the above technique. Therefore, the effects of various components on the moldability and production conditions on the moldability were examined in detail. As a result, it has been found that excellent formability can be obtained stably by adjusting the steel component and the cold-rolled sheet annealing temperature to a suitable range. That is, Mn is Si and austenite formers a ferrite forming element is adjusted to a suitable range, the lower limit of the temperature at which austenite phase is produced (hereinafter, sometimes referred to as T _A point) of the shifts to the high temperature side. Thereby, the cold-rolled sheet blunt temperature is further increased, and the grain growth is further promoted. As a result, a ferrite single-phase structure with sufficient grain growth in the metal structure after cold-rolled sheet annealing is obtained, and has excellent formability with an elongation at break (El) of 28% or more and an average r value of 0.70 or more, It has been found that it is compatible with ridging resistance.

Next, the component composition of the ferritic stainless steel of the present invention will be described.
Hereinafter, unless otherwise specified,% means mass%.

C: 0.005-0.035%
C promotes the formation of the austenite phase and has the effect of expanding the two-phase temperature range where the ferrite phase and the austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, 0.005% or more needs to be contained. However, if the C content exceeds 0.035%, the steel sheet becomes hard and ductility decreases. Therefore, the C content is in the range of 0.005 to 0.035%. Preferably it is 0.010 to 0.030% of range. More preferably, it is in the range of 0.015 to 0.025%.

Si: 0.25 to less than 0.40%
Si is an element to increase the T _A point. In order to obtain this effect, a content of 0.25% or more is necessary. However, when the Si content is 0.40% or more, the steel sheet becomes hard and the rolling load during hot rolling increases, and the ductility after annealing of the cold-rolled sheet decreases, and a predetermined breaking elongation cannot be obtained. Therefore, the Si content is in the range of 0.25% to less than 0.40%. Preferably it is 0.25 to 0.35% of range. More preferably, it is in the range of 0.25 to 0.30%.

Mn: 0.05-0.35%
Mn, like C, promotes the formation of an austenite phase and has the effect of expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, 0.05% or more needs to be contained. However, Mn amount is not obtained predetermined breaking elongation is too low T _A point exceeds 0.35%. Therefore, the Mn content is in the range of 0.05 to 0.35%. Preferably it is 0.10 to 0.30% of range. More preferably, it is in the range of 0.15 to 0.25%.

29.5 × Si-50 × Mn + 6 ≧ 0 (However, Si and Mn in the formula indicate the content (% by mass))
In the present invention, adjustment of the temperature of the lower limit austenite generated (T _A point) becomes an extremely important element. As described above, in the present invention, hot-rolled sheet annealing is performed in a two-phase temperature range of ferrite phase + austenite phase, and the structure after hot-rolled sheet annealing is a two-phase structure of ferrite phase + martensite phase. A great feature is that the cold-rolled sheet annealing to be performed later is performed in the ferrite single-phase temperature range, and the final structure is a ferrite single-phase. In order to obtain an excellent elongation at break by this method, the martensite phase generated by hot-rolled sheet annealing is decomposed and removed into a ferrite phase and carbide by cold-rolled sheet annealing, and the metal structure is converted into a single ferrite phase. It is necessary to increase the grain size of the ferrite crystal grains by causing sufficient grain growth. As a result of investigations by the inventors, the martensite phase generated by hot-rolled sheet annealing rapidly decomposes in a temperature range of about 750 ° C. or higher in the cold-rolled sheet annealing after cold rolling, and the temperature range of 800 ° C. or higher. Thus, ferrite crystal grains were formed by recrystallization.

It is well known that the growth of crystal grains proceeds as the temperature increases or the annealing time increases. However, since cold-rolled sheet annealing is performed in a continuous annealing furnace, if the annealing time is increased, the production efficiency is significantly reduced. On the other hand, when the cold-rolled sheet annealing temperature is increased with the steel components of the prior art, annealing is performed in a two-phase region of a ferrite phase and an austenite phase. In that case, an austenite phase is newly generated in the metal structure, and the austenite phase is transformed into a martensite phase after cooling, so that the steel sheet becomes hard and a predetermined elongation at break cannot be obtained.

Therefore, in the present invention, the temperature of the lower generating austenite by adjusting the steel component (T _A point) by a high temperature than the conventional, made it possible to more ferrite single phase region annealing at a high temperature. Thereby, ferrite crystal grains can be sufficiently grown without generating an austenite phase.

Specifically, the temperature increase at the T _A point is realized by adjusting the balance of the Si and Mn contents within a suitable range. Si increases the T _A point with increasing and content ferrite forming element. Meanwhile, Mn lowers the T _A point with increasing and content austenite forming element. A result of the study, 29.5 × Si-50 × Mn + 6 if less than 0, T _A point is not sufficiently high temperature, it was found that not be obtained predetermined breaking elongation. Therefore, in the present invention, 29.5 × Si-50 × Mn + 6 ≧ 0.

P: 0.040% or less
P is an element that promotes grain boundary fracture due to grain boundary segregation, so a lower value is desirable, and the upper limit is made 0.040%. Preferably it is 0.035% or less. More preferably, it is 0.030% or less.

S: 0.01% or less
S is an element that exists as sulfide inclusions such as MnS and reduces ductility, corrosion resistance, and the like, and particularly when the content exceeds 0.01%, the adverse effects thereof are remarkably generated. Therefore, it is desirable that the S amount be as low as possible. In the present invention, the upper limit of the S amount is 0.01%. Preferably it is 0.007% or less. More preferably, it is 0.005% or less.

Cr: 15.5-18.0%
Cr is an element having an effect of improving the corrosion resistance by forming a passive film on the steel sheet surface. In order to obtain this effect, the Cr content needs to be 15.5% or more. However, if the Cr content exceeds 18.0%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and a predetermined ridging resistance characteristic cannot be obtained. Therefore, the Cr content is in the range of 15.5 to 18.0%. Preferably it is 16.0 to 17.5% of range. More preferably, it is in the range of 16.0 to 17.0%.

Al: 0.001 to 0.10%
Al, like Si, is an element that acts as a deoxidizer. In order to acquire this effect, 0.001% or more needs to be contained. However, when the Al content exceeds 0.10%, Al-based inclusions such as Al ₂ O ₃ increase, and the surface properties tend to deteriorate. Therefore, the Al content is set in the range of 0.001 to 0.10%. Preferably it is 0.001 to 0.05% of range. More preferably, it is in the range of 0.001 to 0.03%.

N: 0.01-0.06%
N, like C and Mn, promotes the formation of the austenite phase and has the effect of expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing. In order to obtain this effect, the N content needs to be 0.01% or more. However, when the N content exceeds 0.06%, the ductility is remarkably lowered and the corrosion resistance is lowered by promoting the precipitation of Cr nitride. Therefore, the N content is in the range of 0.01 to 0.06%. Preferably it is 0.01 to 0.05% of range. More preferably, it is in the range of 0.02 to 0.04%.

The balance is Fe and inevitable impurities.

Although the effects of the present invention can be obtained by the above component composition, the following elements can be contained for the purpose of further improving manufacturability or material properties.

One or more selected from Cu: 0.1 to 0.5%, Ni: 0.1 to 0.6%, Mo: 0.1 to 0.5%, Co: 0.01 to 0.3% Both Cu and Ni are elements that improve corrosion resistance. In particular, it is effective to contain it when high corrosion resistance is required. Further, Cu and Ni have an effect of promoting the formation of the austenite phase and expanding the two-phase temperature range in which the ferrite phase and the austenite phase appear during hot-rolled sheet annealing. These effects become significant when the content is 0.1% or more. However, if the Cu content exceeds 0.5%, formability may be deteriorated, which is not preferable. Therefore, if Cu is contained, the content is made 0.1 to 0.5%. Preferably it is 0.2 to 0.3% of range. If the Ni content exceeds 0.6%, the moldability is lowered, which is not preferable. Therefore, when Ni is contained, the content is made 0.1 to 0.6%. Preferably it is 0.1 to 0.3% of range.

Mo is an element that improves corrosion resistance, and it is effective to contain it particularly when high corrosion resistance is required. This effect becomes significant when the content is 0.1% or more. However, when the Mo content exceeds 0.5%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and predetermined material characteristics cannot be obtained. Therefore, if it contains Mo, the content is made 0.1 to 0.5%. Preferably it is 0.2 to 0.3% of range.

Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, if the content exceeds 0.3%, the moldability is lowered. Therefore, if Co is contained, the content is made 0.01 to 0.3%.

V: 0.01-0.10%, Ti: 0.001-0.05%, Nb: 0.001-0.05%, Ca: 0.0002-0.0020%, Mg: 0.0002-0.0050%, B: 0.0002-0.0050%, REM: Of 0.01-0.10% One or more selected from V: 0.01-0.10%
V combines with C and N in the steel to reduce solute C and solute N. This improves the average r value. Furthermore, the surface behavior is improved by controlling the precipitation behavior of carbonitrides on the hot-rolled sheet to suppress the occurrence of linear flaws caused by hot rolling and annealing. In order to obtain these effects, the V content needs to be 0.01% or more. However, if the amount of V exceeds 0.10%, the workability decreases and the manufacturing cost increases. Therefore, when V is contained, the content is made 0.01 to 0.10%. Preferably it is 0.03 to 0.08% of range.

Ti: 0.001-0.05%, Nb: 0.001-0.05%,
Ti and Nb, like V, are elements with a high affinity for C and N, and precipitate as carbides or nitrides during hot rolling, reducing solid solution C and solid solution N in the matrix, and cold rolling. There is an effect of improving the workability after sheet annealing (after finish annealing). In order to obtain these effects, it is necessary to contain 0.001% or more of Ti or 0.001% or more of Nb. However, if the Ti content or Nb content exceeds 0.05%, it is not possible to obtain good surface properties due to precipitation of excess TiN and NbC. Therefore, when Ti is contained, the range is 0.001 to 0.05%, and when Nb is contained, the range is 0.001 to 0.05%. The amount of Ti is preferably in the range of 0.003 to 0.020%. The amount of Nb is preferably in the range of 0.005 to 0.020%. More preferably, it is in the range of 0.010 to 0.015%.

Ca: 0.0002-0.0020%
Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. In order to acquire the effect, 0.0002% or more needs to be contained. However, if the Ca content exceeds 0.0020%, CaS is generated and the corrosion resistance decreases. Therefore, when Ca is contained, the content is made 0.0002 to 0.0020%. The range is preferably 0.0005 to 0.0015. More preferably, it is in the range of 0.0005 to 0.0010%.

Mg: 0.0002-0.0050%
Mg is an element that has the effect of improving hot workability. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the Mg content exceeds 0.0050%, the surface quality decreases. Therefore, when Mg is contained, the content is made 0.0002 to 0.0050%. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is in the range of 0.0005 to 0.0020%.

B: 0.0002-0.0050%
B is an effective element for preventing low temperature secondary work embrittlement. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the amount of B exceeds 0.0050%, the hot workability decreases. Therefore, when B is contained, the content is made 0.0002 to 0.0050%. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is in the range of 0.0005 to 0.0020%.

REM: 0.01-0.10%
REM is an element that improves the oxidation resistance, and in particular has the effect of suppressing the formation of an oxide film on the weld and improving the corrosion resistance of the weld. In order to obtain this effect, a content of 0.01% or more is necessary. However, if the content exceeds 0.10%, productivity such as pickling at the time of cold rolling annealing is lowered. Moreover, since REM is an expensive element, excessive inclusion causes an increase in manufacturing cost, which is not preferable. Therefore, when REM is contained, the content is made 0.01 to 0.10%.

Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.
The ferritic stainless steel of the present invention is a steel slab having the above component composition, hot-rolled and then annealed at 900 to 1050 ° C. for 5 seconds to 15 minutes, After cold rolling, it is obtained by annealing in a temperature range of 800 to 950 ° C. for 5 seconds to 5 minutes.

First, molten steel is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method. This slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled as cast without heating to form a hot-rolled sheet.
Thereafter, hot rolled sheet annealing is performed for 5 seconds to 15 minutes at a temperature of 900 to 1050 ° C., which is a two-phase region temperature of a ferrite phase and an austenite phase.

Then, pickling is performed as necessary, and cold rolling and cold rolled sheet annealing are performed. Furthermore, pickling is performed as necessary to obtain a product.

Cold rolling is preferably performed at a rolling reduction of 50% or more from the viewpoints of extensibility, bendability, press formability, and shape correction. In the present invention, cold rolling and annealing may be repeated twice or more.

In addition, in order to further improve the surface properties, grinding or polishing may be performed.

The reasons for limiting the manufacturing conditions will be described below.

Hot-rolled sheet annealing held at a temperature of 900 to 1050 ° C. for 5 seconds to 15 minutes Hot-rolled sheet annealing is an extremely important step for the present invention to obtain excellent formability and ridging resistance. If the hot-rolled sheet annealing temperature is less than 900 ° C., sufficient recrystallization does not occur and the ferrite single-phase region is obtained, so that the effects of the present invention that are manifested by two-phase region annealing may not be obtained. However, when the annealing temperature exceeds 1050 ° C, solid solution of carbide is promoted, so C concentration in the austenite phase is promoted, and a hard martensite phase is generated after hot-rolled sheet annealing, resulting in poor surface properties. There is a case. When the annealing time is less than 5 seconds, even if annealing is performed at a predetermined temperature, generation of austenite phase and recrystallization of the ferrite phase do not occur sufficiently, so that desired formability may not be obtained. On the other hand, if the annealing time exceeds 15 minutes, C concentration in the austenite phase is promoted, and surface properties may be deteriorated by the same mechanism as described above. Therefore, hot-rolled sheet annealing is held at a temperature of 900 to 1050 ° C. for 5 seconds to 15 minutes. Preferably, the holding is performed at a temperature of 920 to 1020 ° C. for 15 seconds to 5 minutes. More preferably, the temperature is maintained at 920 to 1000 ° C. for 30 seconds to 3 minutes.

Cold-rolled sheet annealing, which is held at a temperature of 800-950 ° C for 5 seconds to 5 minutes. Cold-rolled sheet annealing is important for making the two-phase structure of the ferrite phase and martensite phase formed by hot-rolled sheet annealing into a single-phase structure. It is a difficult process. When the cold-rolled sheet annealing temperature is less than 800 ° C., sufficient recrystallization does not occur and a predetermined elongation at break and average r value cannot be obtained. On the other hand, when the cold-rolled sheet annealing temperature exceeds 950 ° C, the steel component becomes hard because the martensite phase is formed after the cold-rolled sheet annealing in the steel component in which the temperature is a two-phase temperature range of the ferrite phase and the austenite phase. A predetermined elongation at break cannot be obtained. Moreover, even if the temperature is a steel component in the ferrite single-phase temperature range, the glossiness of the steel sheet is lowered due to marked coarsening of crystal grains, which is not preferable from the viewpoint of surface quality. When the annealing time is less than 5 seconds, even if annealing is performed at a predetermined temperature, the ferrite phase is not sufficiently recrystallized, so that a predetermined breaking elongation and an average r value cannot be obtained. If the annealing time exceeds 5 minutes, the crystal grains become extremely coarse and the glossiness of the steel sheet is lowered, which is not preferable from the viewpoint of surface quality. Therefore, cold-rolled sheet annealing is held at 800 to 950 ° C for 5 seconds to 5 minutes. The holding is preferably performed at 850 ° C. to 900 ° C. for 15 seconds to 3 minutes. In order to obtain more gloss, BA annealing (bright annealing) may be performed.

Hereinafter, the present invention will be described in detail with reference to examples.
Stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated at 1150 ° C. for 1 h and then hot rolled to form 3.5 mm thick hot rolled sheets. Subsequently, these hot-rolled sheets were subjected to hot-rolled sheet annealing under the conditions described in Table 2. Next, the surface was descaled by shot blasting and pickling. Furthermore, after cold rolling to a sheet thickness of 0.7 mm, after performing cold-rolled sheet annealing (finish annealing) under the conditions shown in Table 2, descaling by pickling is performed to obtain a cold-rolled pickled and annealed sheet It was.

The cold roll pickling annealed plate thus obtained was evaluated as follows.

(1) Evaluation of ductility JIS 13B tensile test specimens were sampled from cold-rolled pickled and annealed sheets at right angles to the rolling direction, tensile tests were performed in accordance with JIS Z2241, and elongation at break was measured. A case where the elongation was 28% or more was judged as acceptable (◯), and a case where the elongation was less than 28% in one direction was regarded as unacceptable (x).

(2) Evaluation of average r value JIS 13B tensile test specimens in the direction parallel to the rolling direction (L direction), 45 ° (D direction) and perpendicular to the rolling direction (C direction) from the cold rolled pickling annealed plate Then, the tensile test based on JIS Z2411 was interrupted to a strain of 15%, the r value in each direction was measured, and the average r value (= (r _L + 2r _D + r _C ) / 4) was calculated. Here, r _L , r _D , and r _C are r values in the L direction, the D direction, and the C direction, respectively. As for the average r value, 0.70 or more was regarded as acceptable (◯), and less than 0.70 was regarded as unacceptable (x).

(3) Evaluation of ridging resistance properties JIS No. 5 tensile test specimens were collected from cold-rolled pickled and annealed sheets parallel to the rolling direction, and the surface was polished with # 600 emery paper, and then 20% tensile. Using a surface roughness meter in the direction perpendicular to the rolling direction on the polished surface at the center of the parallel part of the test piece, strain was applied, and the arithmetic average waviness (Wa) defined in JIS B 0601 (2001) was applied. Measured with a measurement length of 16 mm, a high-cut filter wavelength of 0.8 mm, and a low-cut filter wavelength of 8 mm. Arithmetic mean waviness (Wa) of 2.5 μm or less was accepted (◯), and the case of over 2.5 μm was rejected (×).

(4) Evaluation of corrosion resistance A 60 x 100 mm test piece was taken from a cold rolled pickled and annealed sheet, and the test piece was prepared by polishing the surface with # 600 emery paper and sealing the end face. Subjected to the prescribed salt spray cycle test. The salt spray cycle test consists of 1 cycle of salt spray (5 mass% NaCl, 35 ° C, spray 2h) → dry (60 ° C, 4h, relative humidity 40%) → wet (50 ° C, 2h, relative humidity ≥95%) As a result, 8 cycles were performed.
Photograph the surface of the specimen after 8 cycles of the salt spray cycle test, measure the rusting area on the specimen surface by image analysis, and determine the rusting rate ((development in the specimen) from the ratio to the total area of the specimen. Rust area / total area of test piece) × 100 [%]) was calculated. Rust rate of 10% or less passed with excellent corrosion resistance (◎), more than 10% 25% or less passed (○), and more than 25% rejected (×). It is shown in 2. Further, FIG. 1 shows a graph in which ductility evaluation results are arranged by Si and Mn contents for Nos. 1 to 25 where the Cr content satisfies the scope of the present invention.

Nos. 1-17 (steel AA-AQ) whose steel components satisfy the scope of the present invention have excellent formability and ridging resistance such as breaking elongation of 28% or more, average r value of 0.70 or more, and ridging height of 2.5 μm or less. The characteristics were confirmed. Furthermore, regarding corrosion resistance, the rusting rate on the surface of the test piece after 8 cycles of the salt spray cycle test is 25% or less, and good characteristics are obtained.

In particular, No.3 containing 0.4% Ni (steel AC), No.4 containing 17.7% Cr (steel AD), No.6 containing 0.4% Cu (steel AF), and 0.4% Mo In No. 7 (steel AG), the rusting rate after the No. 1 salt spray cycle test was 10% or less, and the corrosion resistance was further improved.

On the other hand, in No. 26 (steel BI) whose Cr content is lower than the range of the present invention, although predetermined formability and ridging resistance were obtained, the predetermined corrosion resistance was not obtained because the Cr content was insufficient. .

In No. 27 (steel BJ), whose Cr content exceeds the range of the present invention, sufficient corrosion resistance was obtained, but since it contained excessive Cr, an austenite phase was not generated during hot-rolled sheet annealing, and the prescribed resistance to resistance was obtained. The ridging characteristics could not be obtained.

No. 18 (steel BA) and 25 (steel BH), whose Si content is below the range of the present invention, produced an austenite phase during cold-rolled sheet annealing because the Si content was insufficient, and this austenite phase was cooled after cooling. Since the steel sheet was transformed into the martensite phase, the steel plate was hardened and a predetermined elongation at break could not be obtained.

In No. 19 (steel BB) having a Si content exceeding the range of the present invention, the steel sheet hardened due to excessive Si content, and a predetermined breaking elongation could not be obtained.

The Si and Mn contents are within the scope of the present invention, but No. 20 to 23 (steel BC to BF) in which 29.5 × Si-50 × Mn + 6 falls below the scope of the present invention has a balance of Si and Mn contents. Since it was not appropriate, an austenite phase was generated during the cold-rolled sheet annealing, and this austenite phase was transformed into a martensite phase after cooling, so that the steel sheet was hardened and a predetermined breaking elongation could not be obtained.
In No.24 (steel BG), whose Mn content exceeds the range of the present invention, 29.5 × Si-50 × Mn + 6 falls below the range of the present invention due to excessive Mn content, and an austenite phase is formed during cold-rolled sheet annealing. Since the austenite phase was transformed into a martensite phase after cooling, the steel sheet was hardened and a predetermined elongation at break could not be obtained. Of the above evaluation results, FIG. 1 is a graph in which ductility evaluation results are arranged by Si and Mn contents for Nos. 1 to 25 in which the Cr content satisfies the scope of the present invention. It can be seen that the predetermined elongation at break is obtained when 29.5 × Si−50 × Mn + 6 satisfies the scope of the present invention in addition to the Si and Mn contents.

In Nos. 28-30, steel BI was used that had the prescribed formability and ridging resistance but did not have the prescribed corrosion resistance due to insufficient Cr content. The effects of temperature on formability and ridging resistance were investigated. No. 28 having an annealing temperature of 880 ° C., which is lower than the range of the present invention, did not generate an austenite phase during the hot rolling annealing, and a predetermined ridging resistance characteristic could not be obtained. No. 29, which has a cold rolled sheet annealing temperature of 780 ° C below the scope of the present invention, has insufficient ferrite crystal grain growth during cold rolled sheet annealing, and the predetermined formability (breaking elongation and average r value) cannot be obtained. It was. No. 30 of 970 ° C, which exceeds the cold rolled sheet annealing temperature of the present invention, the austenite phase was generated during cold rolled sheet annealing, and the steel sheet became hardened because this austenite phase was transformed into a martensite phase after cooling. The elongation at break could not be obtained.

The ferritic stainless steel obtained in the present invention is particularly suitable for press-molded products mainly composed of a drawing and applications requiring high surface beauty, such as kitchen utensils and tableware.

Claims

In mass%, C: 0.005 to 0.035%, Si: 0.25 to less than 0.40%, Mn: 0.05 to 0.35%, P: 0.040% or less, S: 0.01% or less, Cr: 15.5 to 18.0%, Al: 0.001 to 0.10 %, N: 0.01-0.06%, Si and Mn satisfy 29.5 × Si-50 × Mn + 6 ≧ 0 (where Si and Mn indicate content (mass%)), the balance A ferritic stainless steel characterized by comprising Fe and inevitable impurities.
It is characterized by containing one or more selected from Cu: 0.1 to 0.5%, Ni: 0.1 to 0.6%, Mo: 0.1 to 0.5%, and Co: 0.01 to 0.3%. The ferritic stainless steel according to claim 1.
In mass%, V: 0.01-0.10%, Ti: 0.001-0.05%, Nb: 0.001-0.05%, Ca: 0.0002-0.0020%, Mg: 0.0002-0.0050%, B: 0.0002-0.0050%, REM: The ferritic stainless steel according to claim 1 or 2, comprising one or more selected from 0.01 to 0.10%.
The ferrite system according to any one of claims 1 to 3, wherein the elongation at break in the direction perpendicular to the rolling direction is 28% or more, the average Rankford value is 0.70 or more, and the ridging height is 2.5 µm or less. Stainless steel.
The method for producing a ferritic stainless steel according to any one of claims 1 to 4, wherein the steel slab is hot-rolled and then subjected to a temperature range of 900 to 1050 ° C for 5 seconds to 15 seconds. A method for producing a ferritic stainless steel, characterized in that annealing is performed for a minute, followed by cold rolling, followed by annealing at a temperature range of 800 to 950 ° C. for 5 seconds to 5 minutes.