WO2022085708A1

WO2022085708A1 - Ferritic stainless steel, and method for manufacturing ferritic stainless steel

Info

Publication number: WO2022085708A1
Application number: PCT/JP2021/038703
Authority: WO
Inventors: 祐太吉村; 直樹平川; 詠一朗石丸
Original assignee: 日鉄ステンレス株式会社
Priority date: 2020-10-23
Filing date: 2021-10-20
Publication date: 2022-04-28
Also published as: KR20230015982A; JP7374338B2; CN115917029A; JPWO2022085708A1

Abstract

Through the present invention, ferritic stainless steel that is superior both in deep drawability and ridging resistance is realized at lower cost relative to the prior art.　In this ferritic stainless steel, the average height of undulating curve elements in ridging formed on the surface of the ferritic stainless steel is 15 µm or less, and the r value thereof is 0.9 or greater, and the area ratio of a martensitic phase in a cross section cut in a plane parallel to a rolling direction and perpendicular to a transverse direction is 0% to less than 1.0%.

Description

Manufacturing method of ferritic stainless steel and ferritic stainless steel

The present invention relates to a ferritic stainless steel and a method for manufacturing a ferritic stainless steel.

Ferritic stainless steel has excellent corrosion resistance and heat resistance, and is used in various fields such as home appliances, cooking utensils, and construction applications. On the other hand, ferritic stainless steel is inferior in ductility to austenitic stainless steel. Further, the ferritic stainless steel has a problem that rigging occurs during the molding process, and this rigging impairs the surface quality of the molded product and the polishability of the ferritic stainless steel after the forming process.

Here, rigging is a surface defect that occurs on the surface of ferritic stainless steel, and specifically refers to striped or streaky undulations that occur on the surface of ferritic stainless steel in a direction parallel to the processing direction. The "machining direction" is a direction in which a steel strip of ferritic stainless steel is stretched by forming. Further, as the forming process that causes rigging, press working, tensile processing, drawing processing and the like can be exemplified.

It is generally known that it is effective to reduce the content of C and N in the ferritic stainless steel in order to improve the ductility, particularly the deep drawing property of the ferritic stainless steel. On the other hand, it is also generally known that reducing the content of C and N in ferritic stainless steel lowers the rigging resistance. For these reasons, it has been a conventional issue to realize a ferritic stainless steel having excellent both deep drawing resistance and rigging resistance.

Various researches have been conducted in order to solve the above problems. For example, in Patent Document 1, a ferritic stainless steel sheet having an improved r value and excellent rigging resistance is manufactured by adding an amount of Ti satisfying a predetermined condition to control the amount of precipitate deposited. The technology to be used is disclosed. The r value (Rankford value) is a characteristic value indicating the general anisotropy of the plate material, and is an index indicating the superiority or inferiority of the deep drawing property of the ferritic stainless steel. It is said that the larger the r value, the better the deep drawing property of the ferritic stainless steel. Further, for example, Patent Document 2 discloses a technique for producing a ferritic stainless thin steel sheet having a large r value by cold rolling using a work roll having a predetermined roll diameter. The work roll is a component of a cold rolling mill that comes into direct contact with a metal plate to be rolled during cold rolling.

Japanese Patent Application Laid-Open No. 10-130786 Japanese Patent Application Laid-Open No. 59-107030

However, the technique disclosed in Patent Document 1 adds Ti, which is an expensive element, so that the manufacturing cost of the ferritic stainless steel sheet is high. Further, since the technique disclosed in Patent Document 2 does not specify the manufacturing conditions and the component composition for improving the rigging resistance, the ferritic stainless steel sheet manufactured based on this technique has the rigging resistance. It cannot be said that it is sufficient in terms of.

One aspect of the present invention has been made in view of the above-mentioned problems, and is to realize a ferritic stainless steel having excellent both deep drawing property and rigging resistance at a lower cost than the conventional one.

In order to solve the above-mentioned problems, the ferritic stainless steel according to one aspect of the present invention has a mass% of C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ferritic stainless steel containing 1.0% or less, Cr: 12.0% or more and 18.0% or less, N: 0.10% or less and Al: 0.50% or less, and the balance is Fe and unavoidable impurities. It is a ferritic stainless steel, the average height of the waviness curve element in the rigging formed on the surface of the ferritic stainless steel is 15 μm or less, the r value is 0.9 or more, and in the rolling direction. A martensite phase having an area ratio of 0% or more and less than 1.0% in a cross section cut in a plane that is parallel and perpendicular to the width direction is included.

In order to solve the above-mentioned problems, the method for producing ferritic stainless steel according to one aspect of the present invention is, in terms of mass%, C: 0.12% or less, Si: 1.0% or less, Mn: 1.0. % Or less, Ni: 1.0% or less, Cr: 12.0% or more and 18.0% or less, N: 0.10% or less and Al: 0.50% or less, and the balance is Fe and unavoidable impurities. A hot rolling process that hot-rolls a steel slab made of ferritic stainless steel to produce a hot-rolled steel strip.
By baking and softening the hot-rolled steel strip produced in the hot-rolling step, the martensite phase in the cross section cut in a plane parallel to the rolling direction and perpendicular to the width direction of the hot-rolled hardened steel strip. A softening and annealing step for producing the hot-rolled annealed steel strip having an area ratio of 5.0% or more and 30.0% or less and the balance excluding the martensite phase containing a ferrite phase.
A cold rolling step of cold-rolling the hot-rolled annealed steel strip produced in the softening annealing step to produce a cold-rolled steel strip is included.
In the cold rolling process,
The total cold-rolling ratio, which is the ratio of the difference between the thickness of the hot-rolled annealed steel strip and the thickness of the cold-rolled steel strip to the thickness of the hot-rolled annealed steel strip, is set to 60% or more.
The hot-rolled annealed steel strip
(I) Using the first work roll having a roll diameter of 200 mm or more, the cold rolling rate per pass is 15% or more, and the cold rolling rate after the completion of all passes is 50% or more of the cold rolling rate. After cold rolling to
(Ii) Further cold rolling is performed using a second work roll having a roll diameter of less than 200 mm.

According to one aspect of the present invention, a ferritic stainless steel having excellent both deep drawing resistance and rigging resistance can be realized at a lower cost than before.

It is a flowchart which shows the manufacturing method of the ferritic stainless steel which concerns on one Embodiment of this invention.

Hereinafter, an embodiment of the present invention will be described in detail. In this specification, the content of each element in ferrite stainless steel (hereinafter abbreviated as "stainless steel") is simply referred to as the content. In addition, the indication of "%" regarding the content rate shall mean "mass%" unless otherwise specified. Further, with respect to the numerical values X1 and X2 (however, X1 <X2), "X1 to X2" means "X1 or more and X2 or less".

[Mechanism for improving deep drawing resistance and rigging resistance]
As a result of diligent studies, the present inventors have found an effective measure for realizing stainless steel having excellent deep drawing resistance and rigging resistance at a lower cost than before. Specifically, it is effective to appropriately set each of (i) the area ratio of the martensite phase after softening and annealing, and (ii) the roll diameter and cold rolling conditions of the work roll in cold rolling. I found that there was. This will be described below.

<Improved ductility and rigging resistance>
First, the results of studies by the present inventors regarding the measure (i) described above will be described. As a general finding, it is known that the martensite phase is dispersed by softening and annealing the strip of stainless steel after hot rolling (details will be described later). Hereinafter, the stainless steel strip is referred to as a "stainless steel strip". "Dispersion of martensite phase" means that the austenite phase in the stainless steel strip is transformed into the martensite phase, and the martensite phase is dispersed in the ferrite phase in the stainless steel strip. Dispersion of the martensite phase has the effect of separating colonies of the ferrite phase (organization of similar crystal orientations) in cold rolling after softening and annealing.

Here, since rigging occurs due to the continuous presence of colonies in the rolling direction in stainless steel, the dispersion of the martensite phase is an effective phenomenon for improving the rigging resistance. The "rolling direction" is a direction in which the strip is passed through the rolling apparatus when the strip of stainless steel is rolled.

However, if the martensite phase is excessively dispersed, the amount of the martensite phase will be excessively increased in the stainless steel strip after softening and annealing. Since the martensite phase has a hard and high-strength structure and the ferrite phase has a soft and excellent ductility structure, the ductility of stainless steel cannot be improved. It also causes edge cracking and coil (steel strip) breakage when the stainless steel strip after softening and annealing is cold-rolled.

On the other hand, if the dispersion of the martensite phase is small, in the cold rolling after softening annealing, the separation of the ferrite phase colonies by the martensite phase becomes insufficient, and the rigging resistance of the stainless steel cannot be improved. ..

As a result of diligent studies based on these facts, the present inventors have appropriately dispersed the martensite phase in order to improve the ductility and rigging resistance of stainless steel while maintaining the strength at the conventional level. It was found that it is effective to cause it. Specifically, it was found that it is effective to cause the dispersion of the martensite phase so that the area ratio of the martensite phase in the stainless steel strip after softening and annealing is 5.0 to 30.0%. rice field.

"Area ratio of martensite phase after softening annealing" is the ratio of the total area of the martensite phase region contained in the cut cross section to the area of the cut cross section of the stainless steel strip after softening annealing. This cut cross section is a cross section formed when the stainless steel strip after softening and annealing is cut in a plane parallel to the rolling direction and perpendicular to the width direction of the stainless steel strip. Hereinafter, the area ratio of the martensite phase after softening and annealing is referred to as "first martensite area ratio".

The first martensite area ratio can be calculated using, for example, EBSD (electron back scattering diffraction) crystal orientation analysis. Specifically, first, an EBSD detector mounted on a scanning electron microscope (SEM) is used to acquire an EBSD pattern on the measurement surface of a stainless steel strip after softening and annealing. For the acquisition of the EBSD pattern, the acquisition conditions are set as follows, for example.
-Measurement surface: L cross section (cut cross section: cross section formed when a stainless steel strip after softening and annealing is cut in a plane parallel to the rolling direction and perpendicular to the width direction of the softened and annealed stainless steel strip).
・ Measurement magnification: 100 to 800 times ・ Measurement area: 100 to 1000 μm square ・ Measurement pitch (step size): 0.3 to 0.8 μm
Next, an IQ (Image Quality) image is generated from the acquired EBSD pattern using OIM (Orientation Imaging Microscopy) analysis software. The IQ image is an image map showing the high and low sharpness of each structure formed on the measurement surface of the hot-rolled annealed steel strip. The martensite phase has a more complicated internal structure and lower sharpness than the ferrite phase. Therefore, the martensite phase region on the measurement surface appears relatively dark in the IQ image. On the other hand, the ferrite phase has a simpler internal structure and higher sharpness than the martensite phase. Therefore, the ferrite phase region on the measurement surface appears relatively bright in the IQ image. The first martensite area ratio can be calculated by binarizing this IQ image and dividing the total area of the martensite phase region by the area of the measurement surface.

By setting the first martensite area ratio to 5.0% or more, the present inventors have made the waviness height of the rigging formed on the surface of the stainless steel (details will be described later) lower than before. It has been found that the surface texture of stainless steel is improved and the molding process becomes easier. Further, by setting the first martensite area ratio to 30.0% or less, the present inventors do not reduce the ductility of the stainless steel strip after softening and annealing, and edge cracks and coils during cold rolling. It was found that poor cold ductility such as breakage is less likely to occur.

<Improvement of r value>
Although the ductility of stainless steel is improved only by adopting the measure (i) above, it cannot be said that it is sufficient in terms of improving the deep drawing property. Therefore, the present inventors further studied and found that the measure (ii) described above is effective in improving the deep drawing property of stainless steel. Hereinafter, the results of the study by the present inventors regarding (ii) will be described. As a general finding, the r value, which is an index of superiority or inferiority of deep drawability in stainless steel, is the crystal orientation (hereinafter, "{111} crystal orientation") produced in stainless steel and having a Miller index of {111}. It is known that the larger the number of (abbreviation), the larger the value. Since the {111} crystal orientation tends to be generated at a place where rolling strain occurs, it is likely to be generated at a grain boundary where rolling strain is concentrated in stainless steel.

Here, when a stainless steel strip after softening and annealing is cold-rolled with a work roll having a roll diameter of 50 to 100 mm, which is generally used in cold rolling, the concentration of rolling strain is concentrated on the stainless steel after cold-rolling. It tends to stay at the end of the steel strip in the plate thickness direction. In other words, in the central portion of the stainless steel strip after cold rolling in the plate thickness direction, the concentration of rolling strain is unlikely to occur and the grain boundaries tend to be difficult to be generated. Hereinafter, the central portion of the stainless steel strip in the plate thickness direction is referred to as a "plate thickness center portion", and the end portion in the plate thickness direction is referred to as a "plate thickness surface layer portion". In order to increase the r value, it is necessary that a large number of {111} crystal orientations are generated in the entire portion from the surface layer portion of the plate thickness to the center portion of the plate thickness. It is difficult to increase the r-value of stainless steel because many {111} crystal orientations cannot be generated in the central part of the thickness.

From these facts, the present inventors tend to make the roll diameter larger than that of the above-mentioned general work roll, so that the concentration of rolling strain is likely to occur even in the central portion of the plate thickness, and the crystal grain boundaries are likely to be generated. , {111} I came up with the idea that many crystal orientations might be generated. The idea is that if the distance between the surface of the stainless steel strip and the axis of rotation is the same for the general work roll and the work roll with a larger roll diameter, the work roll with a larger roll diameter is the plate. It is based on the fact that it can be rolled to a part closer to the thick center.

As a result of diligent studies based on the above idea, if the roll diameter of the work roll is set to 200 mm or more, the desired number of {111} is obtained in the entire portion from the plate thickness surface layer portion to the plate thickness center portion. It turned out that it may be possible to generate a crystal orientation. However, as a result of further studies, even if the roll diameter of the work roll is set to 200 mm or more, the cold rolling rate per pass in cold rolling is low, and cold rolling using the above-mentioned work roll is performed. It has been found that if the cold rolling ratio is low, it is difficult to generate a desired number of {111} crystal orientations at the center of the plate thickness. The cold spreading rate per pass is the thickness of the stainless steel strip one pass before and the thickness of the stainless steel strip after one pass with respect to the thickness of the stainless steel strip one pass before for any pass. It is the ratio of the difference from the thickness.

Therefore, the present inventors further studied. As a result, in order to improve the r value of stainless steel, the size of the roll diameter of the work roll is set to 200 mm or more, the cold rolling rate per pass is set to 15% or more, and all. It was found that it is effective to set the cold rolling rate after the end of the pass to 50% or more of the total cold rolling rate. Here, the total cold-rolled ratio is the ratio of the difference between the thickness of the hot-rolled annealed steel strip and the thickness of the cold-rolled steel strip to the thickness of the hot-rolled annealed steel strip before cold rolling. .. The cold-rolled steel strip, which is the basis for calculating the total cold-rolling ratio, is after all the processing in cold rolling is completed (in this embodiment, after the first cold rolling and the second cold rolling described later are completed). Refers to the steel strip of. The cold rolling rate after the completion of all passes will be described later.

That is, the size of the roll diameter of the work roll is set to 200 mm or more, the cold rolling rate per pass is set to 15% or more, and the cold rolling rate after the completion of all passes is the total cold rolling rate. It was found that by setting it to 50% or more, a desired number of {111} crystal orientations can be generated in the entire portion from the surface layer portion of the plate thickness to the center portion of the plate thickness.

In the following description, a work roll having a roll diameter of 200 mm or more is referred to as a "large diameter roll", and a work roll having a roll diameter of less than 200 mm is referred to as a "small diameter roll". According to these definitions, a work roll having a roll diameter of 50 to 100 mm, which is generally used in cold rolling, belongs to a "small diameter roll".

<Summary>
If stainless steel is manufactured based on the measures (i) and (ii) described above, expensive elements such as Ti are used as in the conventional manufacturing method in order to improve both deep drawing resistance and rigging resistance. Eliminates the need to add. In addition, it is not necessary to provide special manufacturing equipment for improving both deep drawing resistance and rigging resistance. Therefore, each of the above-mentioned measures (i) and (ii) is effective in realizing stainless steel having excellent deep drawing resistance and rigging resistance at a lower cost than before.

[Mechanism for improving corrosion resistance and workability]
As a result of further studies, the present inventors have eliminated the martensite phase in the cold-rolled steel strip obtained based on the measures (i) and (ii) described above, thereby resulting in corrosion resistance and processability. We have found that it is possible to realize excellent stainless steel at a lower cost than before.

Specifically, in the finish annealing after cold rolling, the cold-rolled steel strip is heated to 800 ° C. or higher and less than Ac1 at a heating rate of 50 ° C./s or lower. Ac1 is a measure of the temperature at which the formation of the austenite phase is started. By this heat treatment, the disappearance of the martensite phase starts when the cold-rolled steel strip reaches a certain temperature (about 700 ° C.). Next, the martensite phase in the cold-rolled steel strip is substantially eliminated by soaking the heated cold-rolled steel strip in a temperature range of 800 ° C. or higher and lower than Ac1 for 5 seconds or longer. Next, by cooling the cold-rolled steel strip after soaking at a cooling rate of 50 ° C./s or less, the disappearance of the martensite phase in the cold-rolled steel strip is aimed at.

The "disappearance of the martensite phase" in the present specification basically means that the area ratio of the martensite phase contained in the stainless steel as the final product is 0%, that is, the martensite in the stainless steel as the final product. It means that the site phase has completely disappeared. The "area ratio of martensite phase contained in stainless steel as a final product" is the ratio of the total area of the martensite phase region contained in the cut cross section to the area of the cut cross section of stainless steel as a final product. This cut cross section is a cross section formed when stainless steel as a final product is cut in a plane parallel to the rolling direction and perpendicular to the width direction of the stainless steel.

However, the "disappearance of the martensite phase" in the present specification is actually an area ratio of stainless steel as a final product as a result of finish annealing aiming at complete disappearance of the martensite phase (area ratio 0%). It is a concept that allows less than 1.0% of the martensite phase to remain. When the remaining martensite phase has an area ratio of less than 1.0%, the corrosion resistance and workability of stainless steel as a final product are both excellent. Hereinafter, the area ratio of the martensite phase contained in stainless steel as a final product is referred to as "second martensite area ratio".

The present inventors applied the above-mentioned finish annealing to the cold-rolled steel strip to complete the recrystallization of the ferrite phase, which is the parent phase, and the martensite phase (first martensite phase) intentionally produced in the softening annealing. It was found that the site area ratio (5.0 to 30.0%) can also be eliminated. Recrystallization refers to the formation of new grain grains free of dislocations in the cold-rolled steel strip. Dislocations are an example of lattice defects that occur inside a crystal. The present inventors have also found that by applying the above-mentioned finish annealing to the cold-rolled steel strip, it is possible to prevent the formation of a new martensite phase in the cold-rolled steel strip. The series of methods from softening annealing to finish annealing described above is different from the general stainless steel manufacturing method aiming at the positive disappearance of the martensite phase from the time of softening annealing.

[Ingredient composition]
The stainless steel according to the embodiment of the present invention has a mass% of C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 1.0% or less, Cr: 12.0 to 18.0%, N: 0.10% or less, Al: 0.50% or less, Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V : 0.15% or less, B: 0.10% or less, Ti: 0.50% or less, Co: 0.01 to 0.50%, Zr: 0.01 to 0.10%, Nb: 0.01 ~ 0.10%, Mg: 0.0005 ~ 0.003%, Ca: 0.0003 ~ 0.003%, Y: 0.01% ~ 0.20%, rare earth metals (REM) excluding Y: total 0.01 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, Pb: 0.01 to 0.10% and W: 0.01 to 0. .Contains 50%. In the following description, the stainless steel according to the embodiment of the present invention is abbreviated as "the present stainless steel".

The rest of this stainless steel consists of Fe and unavoidable impurities. Each of Mo, Cu, O, V, B, Ti, Co, Zr, Nb, Mg, Ca, Y, REM, Sn, Sb, Pb, and W is not an essential element of this stainless steel. Each of these elements is an arbitrary element as long as it contains at least one of these elements, if necessary. Hereinafter, each element contained in this stainless steel will be described.

<C: 0.12% or less>
C is an important element that forms a carbide with Cr to form an interface that is a source of dislocations when the stainless steel is deformed. However, if C is added in excess, the martensite phase is excessively generated, and the ductility of the present stainless steel is lowered. Therefore, the content of C is set to 0.12% or less.

<Si: 1.0% or less>
Si has an effect as a deoxidizing agent in the melting stage. However, if Si is added in excess, the stainless steel is hardened and the ductility is lowered. Therefore, the Si content is set to 1.0% or less.

<Mn: 1.0% or less>
Mn has an effect as a deoxidizing agent. However, if Mn is excessively added, the amount of MnS produced increases and the corrosion resistance of the present stainless steel decreases. Therefore, the Mn content is set to 1.0% or less.

<Ni: 1.0% or less>
Ni is an austenite-forming element and is an effective element for controlling the first martensite area ratio and the strength of the present stainless steel. However, if Ni is excessively added, the austenite phase is stabilized more than necessary, the ductility of the present stainless steel is lowered, and the raw material cost of the present stainless steel is increased. Therefore, the Ni content is set to 1.0% or less. In the following description, the strip of this stainless steel after softening and annealing is referred to as "hot-rolled annealed strip". The steel strip of the present stainless steel after softening and annealing is an example of the hot-rolled annealed steel strip according to the present invention.

<Cr: 12.0 to 18.0%>
Cr is required to form a passivation film on the surface of the steel strip of this stainless steel after cold rolling to improve corrosion resistance. However, if Cr is added in excess, the ductility of the present stainless steel is lowered. Therefore, the Cr content is set to 12.0 to 18.0%. In the following description, the steel strip of this stainless steel after cold rolling is referred to as "cold rolled steel strip". The steel strip of the present stainless steel after cold rolling is an example of the cold-rolled steel strip according to the present invention.

<N: 0.10% or less>
N is an important element that forms a nitride with Cr to form an interface that is a source of dislocations when the stainless steel is deformed. However, if N is added in excess, the martensite phase is excessively generated, and the ductility of the present stainless steel is lowered. Therefore, the content of N is set to 0.10% or less.

<Al: 0.50% or less>
Al is an element effective for deoxidation and can reduce _A2 inclusions which adversely affect press workability. However, if Al is added in excess, the surface defects of the present stainless steel increase. Therefore, the Al content is set to 0.50% or less.

<Mo: preferably 0.5% or less>
Mo is an element effective for improving corrosion resistance. However, if Mo is added in excess, the raw material cost of this stainless steel increases. Therefore, the Mo content is preferably set to 0.50% or less.

<Cu: preferably 1.0% or less>
Cu is an element effective for improving corrosion resistance. The Cu content is preferably set to 1.0% or less.

<O: preferably 0.01% or less>
O produces non-metal inclusions, which reduces the impact value and fatigue life of the present stainless steel. Therefore, the content of O is preferably set to 0.01% or less.

<V: preferably 0.15% or less>
V is an element effective for improving hardness and strength. However, if V is added in excess, the raw material cost of the present stainless steel increases. Therefore, the V content is preferably set to 0.15% or less.

<B: preferably 0.10% or less>
B is an element effective for improving toughness. However, this effect saturates above 0.10%. Therefore, the content of B is preferably set to 0.10% or less.

<Ti: preferably 0.50% or less>
Ti is an element that forms a carbonitride, and suppresses grain boundary precipitation of Cr carbonitride during heat treatment to improve the corrosion resistance of this stainless steel. Further, by fixing the solid solution C and the solid solution N in the stainless steel as carbonitride, the content of the solid solution C and the solid solution N is reduced and the r value of the stainless steel is improved. Further, by fixing the solid solution C and the solid solution N in the stainless steel as carbonitrides, the ductility of the stainless steel can be improved and the stretcher strain can be reduced. Stretcher strains are tiny irregularities formed on the surface of stainless steel that occur due to the yield elongation of several percent that occurs during stamping of stainless steel.

However, since Ti is an expensive element, if Ti is added in excess, the raw material cost of this stainless steel will increase. Therefore, the Ti content is preferably set to 0.50% or less.

<Co: preferably 0.01 to 0.50%>
Co is an element effective for improving corrosion resistance and heat resistance. However, if Co is added in excess, the raw material cost of this stainless steel increases. Therefore, the Co content is preferably set to 0.01 to 0.50%.

<Zr: preferably 0.01 to 0.10%>
Zr is an element effective for denitrification, deoxidation and desulfurization. However, if Zr is excessively added, the raw material cost of the present stainless steel increases. Therefore, the Zr content is preferably set to 0.01 to 0.10%.

<Nb: preferably 0.01 to 0.10%>
Similar to Ti, Nb reduces the content of solid solution C and solid solution N by fixing the solid solution C and solid solution N in the stainless steel as carbonitride, and reduces the r value of the stainless steel. Improve. Further, by fixing the solid solution C and the solid solution N in the stainless steel as carbonitrides, the ductility of the stainless steel can be improved and the stretcher strain can be reduced. However, since Nb is an expensive element, if Nb is excessively added, the raw material cost of the present stainless steel increases. Therefore, the Nb content is preferably set to 0.01 to 0.10%.

<Mg: preferably 0.0005 to 0.003%>
Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizing agent. However, if excess Mg is added in excess, the toughness of the present stainless steel is lowered, and as a result, the manufacturability is lowered. Therefore, the Mg content is preferably set to 0.0005 to 0.003%, more preferably 0.002% or less.

<Ca: preferably 0.0003 to 0.003%>
Ca is an element effective for degassing. The Ca content is preferably set to 0.0003 to 0.003%.

<Y: preferably 0.01 to 0.20%>
Y is an element effective for improving hot workability and oxidation resistance. However, these effects saturate above 0.20%. Therefore, the Y content is preferably set to 0.01 to 0.20%.

<REM: preferably 0.01 to 0.10% in total>
REMs (Rare Earth Metals) such as Sc and La are effective in improving hot workability and oxidation resistance, as in Y. However, these effects saturate above 0.10%. Therefore, the total content of REM is preferably set to 0.01 to 0.10%.

<Sn: preferably 0.001 to 0.50%>
Sn is an element effective for improving corrosion resistance. However, if Sn is added in excess, the hot workability and tenacity are lowered. Therefore, the Sn content is preferably set to 0.001 to 0.50%.

<Sb: preferably 0.001 to 0.50%>
Sb is effective in improving workability by promoting the formation of a deformed zone during rolling. However, when Sb is excessively added, this effect is saturated, and the processability is lowered depending on the amount of Sb added excessively. Therefore, the Sb content is preferably set to 0.001 to 0.50%, more preferably 0.20% or less.

<Pb: preferably 0.01 to 0.10%>
Pb is an element effective for improving free-cutting property. The Pb content is preferably set to 0.01 to 0.10%.

<W: preferably 0.01 to 0.50%>
W is an element effective for improving high temperature strength. However, if W is added in excess, the raw material cost of the present stainless steel increases. Therefore, the W content is preferably set to 0.01 to 0.50%.

<Others>
In this stainless steel, the balance other than the above-mentioned components is Fe and unavoidable impurities. The unavoidable impurities are impurities that are mixed from the raw materials and the manufacturing process, and are mixed within a range that does not affect the characteristics of each of the above-mentioned components.

[Rising swell height]
In this stainless steel, the waviness height of the rigging formed on the surface is 15 μm or less. In the present specification, "the undulation height of the rigging formed on the surface of the present stainless steel (hereinafter, abbreviated as" the undulation height of the present stainless steel ")" means the undulation of the rigging measured by the method shown below. Means height.

First, a JIS No. 5 tensile test piece (hereinafter abbreviated as "first tensile test piece") specified in JIS Z 2201 is collected from the final product of this stainless steel. Next, using an Instron type tensile tester, the first tensile test piece is pulled so that the direction parallel to the rolling direction is the tensile direction, with the distance between the gauge points being 50 mm. Then, by this tensile test, a tensile strain of 16% is applied to the first tensile test piece. Next, using a surface roughness measuring machine, the measurement length in the direction orthogonal to the rolling direction (in other words, the width direction of the first tensile test piece) in the portion between the gauge points of the first tensile test piece was set to 18 mm. Measure the swell height.

The swell height is the average height of the swell curve element measured by the surface texture measurement specified in JIS B 0601: 2001 or the like. In the present embodiment, the average height of the waviness curve element of the first tensile test piece is measured by the surface texture measurement specified in JIS B 0601: 2001. The average height of the waviness curve elements measured by this method is the waviness height of this stainless steel.

For conventional stainless steel, when the undulation height of the rigging formed on the surface is measured by the above method, the undulation height is 20 to 50 μm, which is higher than the undulation height of this stainless steel. From this, it can be said that this stainless steel has improved rigging resistance as compared with the conventional stainless steel.

[R value]
This stainless steel has an r value of 0.9 or more. In the present specification, the "r value of the present stainless steel" means the r value calculated by the method shown below.

First, the JIS13B tensile test piece specified in JIS Z2201 is collected from the final product of this stainless steel. Specifically, from the above-mentioned final product, a second tensile test piece whose tensile direction is parallel to the rolling direction, a third tensile test piece whose tensile direction is a direction forming an angle of 45 ° with the rolling direction, and rolling. Each of the fourth tensile test pieces whose direction orthogonal to the direction is the tensile direction is collected. Next, each of the second to fourth tensile test pieces is pulled using an Instron type tensile tester with the distance between the gauge points set to 20 mm. Then, by this tensile test, a tensile strain of 14.4% is applied to each of the second to fourth tensile test pieces.

Next, the r value of each of the second to fourth tensile test pieces is calculated using the following equation (1). r = ln (W / W1) / ln (t / t1) ... (1)
Here, W is the width before the tensile test, W1 is the width after the tensile test, t is the thickness before the tensile test, and t1 is the thickness after the tensile test. The "width" is the width of the portion between the gauge points in each of the second to fourth tensile test pieces. The "thickness" is the thickness of the portion between the gauge points in each of the second to fourth tensile test pieces.

Next, using the following equation (2), the average r value obtained by averaging the r values of the second to fourth tensile test pieces is calculated. This average r value is the r value of this stainless steel.
Average r value = (r0 + 2r45 + r90) / 4 ... (2)
Here, r0 is the r value of the second tensile test piece, r45 is the r value of the third tensile test piece, and r90 is the r value of the fourth tensile test piece.

For conventional stainless steel, when the r value is calculated by the above method, the r value is 0.6 to 0.8, which is smaller than the r value of this stainless steel. From this, it can be said that this stainless steel has improved deep drawing property as compared with the conventional stainless steel.

[Second martensite area ratio]
This stainless steel has a second martensite area ratio of 0% or more and less than 1.0%. The second martensite area ratio is preferably 0% from the viewpoint of improving the corrosion resistance and workability of the present stainless steel. However, if the second martensite area ratio is less than 1.0%, even if the area ratio is higher than 0%, this stainless steel has not only rigging resistance and deep drawing resistance but also corrosion resistance and workability. Excellent. The second martensite area ratio can be calculated using EBSD crystal orientation analysis in the same manner as the first martensite area ratio.

[Index value]
In this stainless steel, the index value represented by the following equation (3) is 15 to 50. This index value is an index showing the maximum amount of austenite phase produced by annealing. In the following equation (3), each element symbol represents the mass% concentration of the element.

(Index value) = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-52Al + 470N + 189 ... (3)
The austenite phase produced during annealing can be transformed into a martensite phase during the cooling process. Therefore, by adjusting the mass% concentration of each element in the above formula (3) so that the index value becomes 15 to 50, the amount of martensite phase produced in the cooling process can be appropriately controlled. .. "Properly managed" specifically means that the amount of martensite phase produced during the cooling process is sufficient to achieve both improved deep drawing and improved rigging resistance. , Refers to controlling the maximum amount of austenite phase produced by annealing. Further, since the amount of the martensite phase is appropriately controlled when the index value is 15 to 50, it becomes easy to control the first martensite area ratio to 5.0 to 30.0%.

[Manufacturing method of stainless steel]
A method for manufacturing stainless steel according to an embodiment of the present invention will be described with reference to FIG. 1. As shown in FIG. 1, the present stainless steel is manufactured by going through each of the melting step S1, the hot rolling step S2, the softening annealing step S3, the cold rolling step S4, and the baking step S5. Hereinafter, each step will be described, but the method for manufacturing the present stainless steel is not limited to the method shown in FIG.

<Melting process S1 and hot rolling process S2>
In order to produce this stainless steel, first, in the melting step S1, the stainless steel containing each of the above-mentioned components is melted to produce a steel slab. In the melting step S1, a general melting apparatus for stainless steel can be used, and general melting conditions can be set. Next, in the hot rolling step S2, the hot rolled steel strip is manufactured by hot rolling the steel slab produced in the melting step S1. This hot-rolled steel strip is an example of the hot-rolled steel strip according to the present invention. In the hot rolling step S2, a general hot rolling apparatus and hot rolling conditions for stainless steel can be used.

<Softening annealing step S3>
Next, in the softening annealing step S3, the hot-rolled annealed steel strip produced in the hot rolling step S2 is softened and annealed to produce a hot-rolled annealed steel strip. The softening annealing is a heat treatment in which the hot-rolled steel strip is annealed by setting the maximum temperature during the soaking process to Ac1 or higher in order to soften the hot-rolled steel strip. By softening and annealing to soften the hot-rolled steel strip, it becomes easy to adjust the thickness of the hot-rolled annealed steel strip in the subsequent cold rolling step S4.

Generally, when the temperature during the soaking process in softening annealing exceeds Ac1, the amount of austenite phase in the steel strip begins to increase. Then, when the temperature of the softening annealing becomes higher, the amount of the austenite phase in the steel strip increases to the peak amount and then starts to decrease. Since the austenite phase can be transformed into a martensite phase during the cooling process in the softening annealing, the first martensite area ratio is affected by the austenite phase that is increased by the softening annealing. Therefore, in order to make the first martensite area ratio 5.0 to 30.0%, it is necessary to set the maximum annealing temperature to a temperature equal to or higher than Ac1 and a temperature at which the amount of the austenite phase does not increase too much. The maximum annealing temperature is the maximum temperature during the soaking process in softening annealing.

However, Ac1 is just a guideline temperature in the regression equation and does not match the actual temperature at which the austenite phase begins to form. Therefore, as a result of diligent studies by the present inventors, when Ac1 is less than 921, the maximum annealing temperature is set to 0.76 × Ac1 + 201 ° C. or higher and 1.10 × Ac1-56 ° C. or lower to obtain the first martensite area. It was found that the ratio can be set to 5.0 to 30.0%.

On the other hand, when Ac1 is 921 or more, the peak amount of the austenite phase itself is small, so even if the total amount of the peak amount is transformed from the austenite phase to the martensite phase, the first martensite area ratio is 5.0 to 30. I was able to set it to 0.0%. Therefore, when Ac1 is 921 or more, it is not necessary to set the upper limit of the maximum annealing temperature to 1.10 × Ac1-56 ° C. Here, since softening and annealing at an excessively high temperature causes coarsening of crystal grains in the hot-rolled annealed steel strip and deterioration of characteristics such as rough skin during processing, the upper limit of the maximum annealing temperature is set to 1050 ° C. I decided to set it.

Specifically, the maximum annealing temperature is set as follows. First, Ac1 represented by the following formula (4) is calculated based on the composition of the steel slab. In the following equation (4), each element symbol represents the mass% concentration of the element. The Ac1 may be calculated in advance.
Ac1 = 35 × (Cr + 1.72Mo + 2.09Si + 4.86Nb + 8.29V + 1.77Ti + 21.4Al + 40B-7.14C-8.0N-3.24Ni-1.89Mn-0.51Cu) +310 ... (4)
When Ac1 calculated by the above equation (4) is less than 921, the maximum annealing temperature is set to 0.76 × Ac1 + 201 ° C. or higher and 1.10 × Ac1-56 ° C. or lower. On the other hand, when Ac1 calculated by the above equation (4) is 921 or more, the maximum annealing temperature is set to 0.76 × Ac1 + 201 ° C. or more and 1050 ° C. or less.

The rate of temperature rise in the temperature rise process in softening annealing is preferably set to 10 ° C./sec or higher. If the temperature rise rate is 10 ° C./sec or more, the temperature rise time in the temperature rise process can be shortened to a practically meaningful degree, so that the total time required for manufacturing this stainless steel is also to a practically meaningful degree. Can be shortened. Therefore, the productivity of this stainless steel can be improved. Further, the heat equalization time in the heat equalization process in the softening annealing is preferably set to 5 seconds or more. If the heat soaking time is 5 seconds or more, the austenite phase can be reliably generated during the heat soaking process. Since the austenite phase transforms into the martensite phase during the cooling process after the soaking process, the first martensite area ratio is set to 5.0 to 30.0% by setting the soaking time to 5 seconds or longer. It will be easier to manage.

Furthermore, the cooling rate in the cooling process in softening annealing is set to 5.0 ° C./sec or higher. If the cooling rate is less than 5.0 ° C./sec, the cooling time in the cooling process becomes longer than necessary, and the austenite phase is transformed into a stable ferrite phase. Therefore, the area ratio of the first martensite is lowered to less than 5.0%, and the rigging resistance of the present stainless steel is lowered to be lower than that of the conventional stainless steel. For this reason, the cooling rate is set to 5.0 ° C./sec or higher in order to maintain good rigging resistance of the present stainless steel.

<Cold rolling process S4>
Next, in the cold rolling step S4, the hot-rolled annealed steel strip produced in the softening annealing step S3 is cold-rolled to produce a cold-rolled steel strip. As a cold rolling condition, the total cold rolling ratio after the completion of the cold rolling step S4 is set to 60% or more.

In the cold rolling in the cold rolling step S4, first, the first cold rolling is performed by passing a hot-rolled annealed steel strip through a large-diameter roll having a roll diameter of 200 mm or more. A large-diameter roll having a roll diameter of 200 mm or more is an example of a first work roll according to the present invention. As the cold rolling conditions for the first cold rolling, the cold rolling rate per pass is set to 15% or more, and the cold rolling rate after the completion of the first cold rolling (after the completion of all the passes in the first cold rolling). Is set to 50% or more of the total cold rolling ratio. The cold rolling ratio after the completion of the first cold rolling (after the completion of all passes in the first cold rolling) is the hot rolling annealing with respect to the thickness of the hot-rolled annealed steel strip before the first cold rolling. It is the ratio of the difference between the thickness of the steel strip and the thickness of the steel strip after the completion of all passes.

After the completion of the first cold rolling, the steel strip passed through the large diameter roll is subjected to the second cold rolling through the small diameter roll having a roll diameter of 50 to 100 mm. A small diameter roll having a roll diameter of 50 to 100 mm is an example of a second work roll according to the present invention. In the second cold rolling, only the remaining band thickness that could not be rolled in the first cold rolling is rolled. The steel strip after the completion of the second cold rolling becomes a cold-rolled steel strip.

Here, the reason for performing the second cold rolling after the completion of the first cold rolling will be described. That is, when comparing the first cold rolling and the second cold rolling, assuming that both cold rollings have the same rolling ratio, the first cold rolling using a large diameter roll is better. A larger rolling load is required than in the second cold rolling using a small diameter roll. In general, stainless steel is harder than ordinary steel, and in cold rolling, work hardening progresses in the latter half of the treatment, and the strength of the steel strip increases. From these facts, if cold rolling is performed only with a large diameter roll in the cold rolling step S4, the rolling load that must be applied to the steel strip in order to obtain a cold rolled steel strip having a desired strip thickness is the present stainless steel. It exceeds the acceptable range in terms of manufacturability and productivity. Therefore, as in the present embodiment, the first cold rolling is performed on the large diameter roll, and then the second cold rolling is performed on the small diameter roll. In the present embodiment, among the small diameter rolls having a roll diameter of less than 200 mm, a work roll having a roll diameter of 50 to 100 mm, which is generally used as a small diameter roll, is used as the second work roll.

In the present embodiment, in the cold rolling step S4, the second cold rolling is performed after the first cold rolling, but the first cold rolling is performed after the second cold rolling. May be good. However, stainless steel is generally harder than ordinary steel. Further, in cold rolling, work hardening progresses in the latter half of the treatment, and the strength of the steel strip increases. Therefore, when the first cold rolling is performed after the second cold rolling, the first cold rolling is performed in order to cause the concentration of rolling strain at the center of the plate thickness of the steel strip after the second cold rolling. A larger rolling load is required than in the case of performing the second cold rolling after performing the above. From these facts, considering the manufacturability and productivity of the present stainless steel, it is preferable to perform the first cold rolling and then the second cold rolling as in the present embodiment.

<Annealing step S5>
Next, in the annealing step S5, the cold-rolled steel strip produced in the cold rolling step S4 is annealed at a temperature equal to or higher than the start temperature of recrystallization and lower than Ac1. The annealing performed in the annealing step S5 is a finish annealing for the purpose of achieving both the completion of recrystallization of the ferrite phase in the cold-rolled steel strip and the disappearance of the martensite phase. The finish annealing performed in the annealing step S5 is composed of a temperature raising process, a soaking process, and a cooling process, similarly to the softening annealing in the softening annealing step S3.

In the heating process, the cold-rolled steel strip is heated to a temperature equal to or higher than the start temperature of recrystallization and lower than Ac1 at a heating rate of 50 ° C./s or less. By setting the temperature rise rate to 50 ° C./s or less, the martensite phase can be eliminated in the process of temperature rise. In the heat equalization process, the cold-rolled steel strip after the temperature raising process is heated at a temperature equal to or higher than the start temperature of recrystallization and lower than Ac1 for 5 seconds or longer. In this embodiment, the start temperature of recrystallization is set to 800 ° C. By setting the recrystallization start time to 800 ° C., the recrystallization of the ferrite phase is completed in a short soaking time. However, the start temperature of recrystallization is not limited to 800 ° C., and the start temperature of recrystallization may be set to a temperature lower than, for example, 800 ° C. On the other hand, by setting the upper limit of the soaking temperature to a temperature of Ac1 or less, the martensite phase remaining in the cold-rolled steel strip is substantially eliminated while preventing the formation of a new martensite phase in the cold-rolled steel strip. be able to.

In the cooling process, the cold-rolled steel strip after the soaking process is cooled at a cooling rate of 50 ° C./s or less. By cooling at a cooling rate of 50 ° C./s or less, the martensite phase can be eliminated even in the course of the cooling process. By applying the finish annealing composed of each of these treatments to the cold-rolled steel strip, it is possible to efficiently achieve both the completion of recrystallization of the ferrite phase in the cold-rolled steel strip and the disappearance of the martensite phase in the annealing step S5. can do. When the annealing step S5 is completed, the present stainless steel as a final product is obtained, and the production of the present stainless steel is completed.

〔summary〕
The ferritic stainless steel according to one aspect of the present invention has Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V: 0.15% or less, B in mass%. It may further contain one or more selected from: 0.10% or less and Ti: 0.50% or less.

The ferritic stainless steel according to one aspect of the present invention has Co: 0.01% or more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Nb: 0.01% or more in mass%. 0.10% or less, Mg: 0.0005% or more and 0.003% or less, Ca: 0.0003% or more and 0.003% or less, Y: 0.01% or more and 0.20% or less, rare earth metals excluding Y : Total 0.01% or more and 0.10% or less, Sn: 0.001% or more and 0.50% or less, Sb: 0.001% or more and 0.50% or less, Pb: 0.01% or more and 0.10 % Or less and W: One or more selected from 0.01% or more and 0.50% or less may be further contained.

In the method for producing ferritic stainless steel according to one aspect of the present invention, the steel slab has Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V in mass%. It may further contain one or more selected from: 0.15% or less, B: 0.10% or less, and Ti: 0.50% or less.

In the method for producing ferritic stainless steel according to one aspect of the present invention, the steel slab has Co: 0.01% or more and 0.50% or less and Zr: 0.01% or more and 0.10% or less in terms of mass%. , Nb: 0.01% or more and 0.10% or less, Mg: 0.0005% or more and 0.003% or less, Ca: 0.0003% or more and 0.003% or less, Y: 0.01% or more and 0.20 % Or less, rare earth metal excluding Y: 0.01% or more and 0.10% or less in total, Sn: 0.001% or more and 0.50% or less, Sb: 0.001% or more and 0.50% or less, Pb: One or more selected from 0.01% or more and 0.10% or less and W: 0.01% or more and 0.50% or less may be further contained.

[Additional notes]
The present invention is not limited to the present embodiment, and various modifications can be made within the scope of the claims, and the embodiment obtained by appropriately combining the technical means disclosed in the present embodiment is also available. , Included in the technical scope of the present invention.

〔Example〕
The results of evaluation of the ferrite-based stainless steel according to the examples and comparative examples of the present invention will be described below. Hereinafter, the ferritic stainless steel according to the embodiment of the present invention will be referred to as "invention steel", and the ferritic stainless steel according to the comparative example of the present invention will be referred to as "comparative steel". In this example, first, five kinds of steel slabs having any of the compositions A to E shown in Table 1 below were produced by melting on an actual operation line. The content of each element constituting the compositions A to E was within the numerical range of the content of each element contained in the ferritic stainless steel according to one aspect of the present invention. In addition, Table 1 also shows the numerical value of Ac1 in each case of composition A to E.

The index value of the invention steel and the comparative steel produced based on the steel slab having the composition E was 85 as shown in Table 1 above, which exceeded the upper limit value 50 of the preferable numerical range in the present invention. It is presumed that such a result was obtained because the content of Cr in the composition E, which affects the index value, was 12.5%.

Next, by hot rolling the steel slabs of each composition, hot-rolled steel strips of each composition having a plate thickness of 3 mm and a plate width of 150 mm were manufactured. Next, the hot-rolled steel strips of each composition are subjected to softening annealing and cold rolling under the "actual conditions" shown in Table 2 below, so that the hot-rolled steel strips of each composition having a plate thickness of 1 mm and a plate width of 150 mm are cold-rolled. Manufactured steel strips. Then, the cold-rolled steel strips having each composition were finish-annealed to produce the 1st to 7th invention steels and the 1st to 18th comparative steels.

Table 2 above also shows "recommended conditions" for softening annealing and cold rolling. The conditions described in the “Recommended conditions” column in Table 2 above are the same as those in the present embodiment. Further, the "lower limit temperature" in the "softening annealing" column of the "recommended conditions" indicates the lower limit of the maximum annealing temperature, and the "upper limit temperature" in the same column indicates the upper limit of the maximum annealing temperature. The numerical values described in the "lower limit temperature" and "upper limit temperature" columns are 0.76 × Ac1 + 201 ° C. or higher and 1050 ° C. or lower for each invention steel and each comparative steel of composition A. It was calculated by substituting the numerical value (942). On the other hand, for each of the invention steels of compositions B to E and each comparative steel, the numerical values of each Ac1 of compositions B to E (811, 855,) in the formula of 0.76 × Ac1 + 201 ° C. or higher and 1.10 × Ac1-56 ° C. or lower. It was calculated by substituting 920 and 710).

Table 2 above also shows "characteristic evaluation" and "comprehensive evaluation" for each of the 1st to 7th invention steels and the 1st to 18th comparative steels. The column of "waviness height" of "characteristic evaluation" shows the measurement result of the waviness height of rigging. Further, the column of "r value" of "characteristic evaluation" shows the calculation result of r value. The method for measuring the swell height of the rigging and the method for calculating the r value were the same as those in the present embodiment. "Comprehensive evaluation" is "○" when the swell height of the rigging is 15 μm or less, the r value is 0.9 or more, and the second martensite area ratio is 0% or more and less than 1.0%. And said. On the other hand, if the swell height of the rigging is higher than 15 μm, the r value is less than 0.9, or the second martensite area ratio is 1.0% or more, it is evaluated as “x”. ..

The underlined numerical values in Table 2 above indicate numerical values outside the preferable numerical range in the present embodiment. Further, the underlined “x” in Table 2 above indicates that the cold rolling rate after the completion of all passes was less than 50% of the total cold rolling rate.

As shown in Table 2 above, for the 1st, 9th, 11th, 13th, and 16th comparative steels, the swell height of the rigging was higher than 15 μm, so the overall evaluation was “x”. It became. The swell height of the rigging was higher than 15 μm because the first martensite area ratio was less than 5.0% for all of the 1st, 9th, 11th, 13th, and 16th comparative steels. It is inferred that. That is, in all of the 1st, 9th, 11th, 13th, and 16th comparative steels, the amount of increase in colonies in the steel exceeded the permissible range for improving the rigging resistance, so that the swell height of the rigging was high. It is presumed that the height was higher than 15 μm.

Further, as shown in Table 2 above, for each of the comparative steels other than the 1st, 9th, and 18th comparative steels, the r value was less than 0.9, so that the overall evaluation was “x”. .. The reason why the r value is less than 0.9 is presumed to be explained below. First, for the 2nd and 3rd comparative steels, the 5th to 8th comparative steels, the 10th to 13th comparative steels, and the 16th comparative steels, the total cold rolling ratio is less than 60%, and the cold rolling ratio per pass is high. It is presumed that this is because it corresponds to at least one of less than 15% and the cold rolling rate after the completion of all passes is less than 50% of the total cold rolling rate. That is, in all of the second and third comparative steels, the fifth to eighth comparative steels, the tenth to thirteenth comparative steels, and the sixteenth comparative steels, the rolling strain is concentrated in the center of the plate thickness of these comparative steels. It is presumed that the r value was less than 0.9 because it did not occur sufficiently.

Next, it is presumed that the 1st martensite area ratio was larger than 30.0% for the 14th and 15th comparative steels. That is, it is presumed that the r value of both the 14th and 15th comparative steels was less than 0.9 because the martensite phase increased more than necessary and the ductility decreased. Next, for the 17th comparative steel, the total cold rolling ratio was less than 60%, and the first martensite area ratio was larger than 30.0%, so the r value was less than 0.9. It is presumed that it has become.

Furthermore, as shown in Table 2 above, for the 14th, 15th, 17th, and 18th comparative steels, the second martensite area ratio was 1.0% or more, so the overall evaluation was "x". It became.

On the other hand, for the 1st to 7th invention steels, the characteristic evaluations of the 3rd and 4th invention steels were the best overall results among all the invention steels. Specifically, the steel of the third invention had the third lowest swell height of rigging among all the steels of the invention (2.39 μm). It is presumed that such a result was obtained because the first martensite area ratio was the third highest among all the invention steels (9.44%). In addition, the steel of the third invention had the largest r value among all the steels of the invention (1.12). It is presumed that such a result was obtained because the total cold rolling ratio was the highest among all the invention steels (85%). Furthermore, the steel of the third invention had the fourth lowest ratio of the area of the second martensite among all the steels of the invention (0.17%).

The swell height of the rigging of the 4th invention steel was the lowest among all the invention steels (2.28 μm). In addition, the steel of the fourth invention had the fourth largest r value among all the steels of the invention (0.93). It is presumed that such a result was obtained because the total cold rolling ratio was the fourth highest (69%) of all invention steels. Furthermore, the 4th invention steel had the third lowest (0.15%) of the 2nd martensite area ratio among all the invention steels.

The present invention can be used for manufacturing ferritic stainless steel and ferritic stainless steel.

S1 Melting process S2 Hot rolling process S3 Softening annealing process S4 Cold rolling process S5 Annealing process

Claims

By mass%, C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 1.0% or less, Cr: 12.0% or more and 18.0% or less, N Ferrite-based stainless steel containing 0.10% or less and Al: 0.50% or less, with the balance being Fe and unavoidable impurities.
The average height of the waviness curve element in the rigging formed on the surface of the ferritic stainless steel is 15 μm or less, the r value is 0.9 or more, and it is parallel to the rolling direction and perpendicular to the width direction. A ferritic stainless steel containing a martensite phase in which the area ratio of the martensite phase in a cross section cut on a flat surface is 0% or more and less than 1.0%.
By mass%, Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V: 0.15% or less, B: 0.10% or less and Ti: 0.50% The ferrite-based stainless steel according to claim 1, further comprising one or more selected from the following.
By mass%, Co: 0.01% or more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Nb: 0.01% or more and 0.10% or less, Mg: 0.0005% or more 0.003% or less, Ca: 0.0003% or more and 0.003% or less, Y: 0.01% or more and 0.20% or less, rare earth metals excluding Y: 0.01% or more and 0.10% or less in total , Sn: 0.001% or more and 0.50% or less, Sb: 0.001% or more and 0.50% or less, Pb: 0.01% or more and 0.10% or less and W: 0.01% or more and 0.50 The ferritic stainless steel according to claim 1 or 2, further containing one or more selected from% or less.
By mass%, C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 1.0% or less, Cr: 12.0% or more and 18.0% or less, N A hot rolling step of hot rolling a steel slab containing 0.10% or less and Al: 0.50% or less, the balance of which is Fe and unavoidable impurities, to produce a hot-rolled steel strip.
By baking and softening the hot-rolled steel strip produced in the hot-rolling step, the martensite phase in the cross section cut in a plane parallel to the rolling direction and perpendicular to the width direction of the hot-rolled hardened steel strip. A softening and annealing step for producing the hot-rolled annealed steel strip having an area ratio of 5.0% or more and 30.0% or less and the balance excluding the martensite phase containing a ferrite phase.
A cold rolling step of cold-rolling the hot-rolled annealed steel strip produced in the softening annealing step to produce a cold-rolled steel strip is included.
In the cold rolling process,
The total cold rolling ratio, which is the ratio of the difference between the thickness of the hot-rolled annealed steel strip and the thickness of the cold-rolled steel strip to the thickness of the hot-rolled annealed steel strip, is set to 60% or more.
The hot-rolled annealed steel strip
(I) Using a first work roll having a roll diameter of 200 mm or more, the cold rolling rate per pass is 15% or more, and the cold rolling rate after the completion of all passes is 50% or more of the total cold rolling rate. After cold rolling to
(Ii) A method for producing ferritic stainless steel, which is further cold-rolled using a second work roll having a roll diameter of less than 200 mm.
In terms of mass%, the steel slab has Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V: 0.15% or less, B: 0.10% or less, and Ti. : The method for producing a ferritic stainless steel according to claim 4, further containing one or more selected from 0.50% or less.
In terms of mass%, the steel slab has Co: 0.01% or more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Nb: 0.01% or more and 0.10% or less, Mg: 0.0005% or more and 0.003% or less, Ca: 0.0003% or more and 0.003% or less, Y: 0.01% or more and 0.20% or less, rare earth metals excluding Y: 0.01% or more in total 0.10% or less, Sn: 0.001% or more and 0.50% or less, Sb: 0.001% or more and 0.50% or less, Pb: 0.01% or more and 0.10% or less and W: 0.01 The method for producing a ferritic stainless steel according to claim 4 or 5, further comprising one or more selected from% or more and 0.50% or less.