JP7374338B2 - Ferritic stainless steel and manufacturing method of ferritic stainless steel - Google Patents

Description

The present invention relates to ferritic stainless steel and a method for manufacturing 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 architectural applications. On the other hand, ferritic stainless steel has inferior ductility compared to austenitic stainless steel. Further, ferritic stainless steel has a problem in that ridging occurs during molding, and this ridging impairs the surface quality of the molded product and the polishability of the ferritic stainless steel after molding.

Here, ridging is a surface defect that occurs on the surface of ferritic stainless steel, and specifically refers to striped or streak-like undulations that occur on the surface of ferritic stainless steel in a direction parallel to the processing direction. The "processing direction" is the direction in which the ferritic stainless steel strip is stretched by forming. In addition, examples of the forming process that causes ridging include press working, tension working, drawing process, and the like.

It is generally known that in order to improve the ductility of ferritic stainless steel, particularly its deep drawability, it is effective to reduce the content of C and N in 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 ridging resistance. For these reasons, it has long been a challenge to realize ferritic stainless steel that is excellent in both deep drawability and ridging resistance.

In order to solve the above-mentioned problems, various researches have been carried out in the past. For example, Patent Document 1 discloses that by adding Ti in an amount that satisfies predetermined conditions and controlling the amount of precipitates, a ferritic stainless steel sheet with improved r value and excellent ridging resistance is manufactured. A technique for doing so has been disclosed. The r value (Lankford value) is a characteristic value that represents the anisotropy of sheet materials in general, and serves as an index that indicates the superiority or inferiority of the deep drawability of ferritic stainless steel. It is said that the larger the r value, the better the deep drawability of the ferritic stainless steel. Further, for example, Patent Document 2 discloses a technique for manufacturing a ferritic stainless thin steel sheet with a large r value by performing cold rolling using work rolls having a predetermined roll diameter. A 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 Publication No. 10-130786 Japanese Patent Publication No. 59-107030

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

One aspect of the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to realize a ferritic stainless steel that is excellent in both deep drawability and ridging resistance at a lower cost than conventional ones.

In order to solve the above-mentioned problems, the ferritic stainless steel according to one embodiment of the present invention contains, in 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: 0.5
0% or less, with the remainder consisting of Fe and unavoidable impurities, the average height of the waviness curve elements in the ridging formed on the surface of the ferritic stainless steel is 15 μm or less, and a martensite phase having an r value of 0.9 or more, and an area ratio of the martensite phase in a cross section cut along a plane parallel to the rolling direction and perpendicular to the width direction of 0% or more and less than 1.0%. including.

In order to solve the above problems, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention includes, in 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 A
A hot rolling step of manufacturing a hot rolled steel strip by hot rolling a steel slab containing 0.50% or less of l: with the remainder consisting of Fe and unavoidable impurities;
By annealing and softening the hot-rolled steel strip produced in the hot-rolling process, martensite phase in a cross section cut along a plane parallel to the rolling direction and perpendicular to the width direction of the hot-rolled annealed steel strip is reduced. A softening annealing step of producing the hot rolled annealed steel strip having an area ratio of 5.0% or more and 30.0% or less, and the remainder excluding the martensitic phase containing a ferrite phase;
a cold rolling step of manufacturing a cold rolled steel strip by cold rolling the hot rolled annealed steel strip produced in the softening annealing step,
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 with a roll diameter of 200 mm or more, the cold rolling rate per pass is 15% or more, and the cold rolling rate after all passes is 50% or more of the cold rolling rate. After cold rolling,
(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 that is excellent in both deep drawability and ridging resistance can be realized at a lower cost than before.

1 is a flowchart showing a method for manufacturing ferritic stainless steel according to an embodiment of the present invention.

Hereinafter, one embodiment of the present invention will be described in detail. Note that in this specification, the content rate of each element in ferritic stainless steel (hereinafter abbreviated as "stainless steel") is simply referred to as content rate. Further, unless otherwise specified, the expression "%" regarding the content rate means "mass %". Furthermore, regarding the numerical value X1 and the numerical value X2 (where X1<X2), "X1 to X2" shall mean "more than or equal to X1 and less than or equal to X2."

[Mechanism of improving deep drawability and ridging resistance]
As a result of intensive studies, the present inventors have discovered an effective method for realizing stainless steel that is excellent in both deep drawability and ridging resistance at a lower cost than before. Specifically, (i)
It has been found that it is effective to appropriately set the area ratio of the martensitic phase after softening annealing, and (ii) the roll diameter of the work roll and cold rolling conditions in cold rolling. This will be explained below.

<Improvement of ductility and ridging resistance>
First, the results of the inventors' study regarding the measure (i) above will be explained. As a general knowledge, it is known that dispersion of the martensitic phase occurs by softening annealing a stainless steel strip after hot rolling (details will be described later). Hereinafter, the stainless steel strip will be referred to as a "stainless steel strip.""Dispersion of the martensitic phase" refers to the transformation of the austenite phase in the stainless steel strip into the martensite phase, and the martensitic phase being dispersed within the ferrite phase in the stainless steel strip. The dispersion of the martensitic phase has the effect of breaking up ferrite phase colonies (texture with similar crystal orientation) during cold rolling after softening annealing.

Here, since ridging occurs due to colonies existing in a row in the rolling direction in stainless steel, dispersion of the martensitic phase is an effective phenomenon in improving ridging resistance. The "rolling direction" is a direction in which a stainless steel strip is passed through a rolling device when the steel strip is rolled.

However, if the dispersion of the martensitic phase occurs excessively, the amount of the martensitic phase will increase excessively in the stainless steel strip after softening annealing. Since the martensitic phase is a hard and high-strength structure, and the ferrite phase is a soft and highly ductile structure, it becomes impossible to improve the ductility of stainless steel. Furthermore, when cold rolling a stainless steel strip after softening annealing, it may cause edge cracking and coil (steel strip) breakage.

On the other hand, if the martensite phase is poorly dispersed, the martensite phase will not divide the ferrite phase colonies sufficiently during cold rolling after softening annealing, making it impossible to improve the ridging resistance of stainless steel. .

As a result of intensive studies based on these matters, the present inventors have determined that in order to improve the ductility and ridging resistance of stainless steel while maintaining the strength at the conventional level, it is necessary to appropriately disperse the martensitic phase. We have found that it is effective to allow this to occur. Specifically, we found that it is effective to disperse the martensite phase so that the area ratio of the martensite phase in the stainless steel strip after softening annealing is 5.0 to 30.0%. Ta.

The "area ratio of martensitic phase after softening annealing" is the ratio of the total area of the martensitic phase region included in the cut cross section to the area of the cut cross section in the stainless steel strip after softening annealing. This cut cross section is a cross section formed when the stainless steel strip after softening annealing is cut along 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 annealing will be 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, using an EBSD detector mounted on a scanning electron microscope (SEM), an EBSD pattern of the measurement surface of the stainless steel strip after softening annealing is obtained. Regarding the acquisition of the EBSD pattern, the acquisition conditions are set as follows, for example.
・Measurement surface: L cross section (cut cross section: a cross section formed when the stainless steel strip after softening annealing is cut in a plane parallel to the rolling direction and perpendicular to the width direction of the stainless steel strip after softening annealing)・Measurement magnification: 100 to 800 times ・Measurement area: 100 to 1000μm square ・Measurement pitch (step size): 0.3 to 0.8μm
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 that shows each structure formed on the measurement surface of the hot-rolled annealed steel strip with high and low clarity. The martensitic phase has a more complex internal structure and lower clarity than the ferrite phase. Therefore, the martensitic 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 clarity than the martensitic 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.

The present inventors have found that by setting the first martensite area ratio to 5.0% or more, the waviness height of the ridging formed on the surface of stainless steel (details will be described later) is lower than before, It has been found that the surface properties of stainless steel are improved and molding becomes easier. In addition, the present inventors have found that by setting the first martensite area ratio to 30.0% or less, the ductility of the stainless steel strip after softening annealing does not decrease, and edge cracks and coils do not occur during cold rolling. It has been found that poor cold rollability such as breakage is less likely to occur.

<Improvement of r value>
Although the ductility of stainless steel was improved by simply adopting the measure (i), it was not sufficient to improve the deep drawability. Therefore, the present inventors further investigated and found that the measure (ii) described above is effective in improving the deep drawability of stainless steel. The results of the inventors' study regarding (ii) above will be explained below. As a general knowledge, the r value, which is an index of the superiority or inferiority of deep drawability in stainless steel, is based on the crystal orientation that is generated in stainless steel and has a Miller index of {111} (hereinafter referred to as "{111} crystal orientation"). It is known that the larger the number of (abbreviated), the larger the value. Since the {111} crystal orientation tends to be generated at locations where rolling strain occurs, it is likely to be generated at grain boundaries where rolling strain is concentrated in stainless steel.

Here, when a stainless steel strip after softening annealing is cold rolled with a work roll having a roll diameter of 50 to 100 mm, which is commonly used in cold rolling, the concentration of rolling strain is It tends to stay at the ends of the steel strip in the thickness direction. In other words, rolling strain concentration tends to be less likely to occur and grain boundaries are less likely to be generated at the center in the thickness direction of the stainless steel strip after cold rolling. Hereinafter, the central part of the stainless steel strip in the thickness direction will be referred to as the "thickness center", and the end in the thickness direction will be referred to as the "thickness surface layer". In order to increase the r value, it is necessary to generate many {111} crystal orientations in the entire area from the surface layer to the center of the sheet thickness. It is difficult to increase the r value of stainless steel because many {111} crystal orientations cannot be generated at the center of thickness.

Based on these facts, the present inventors believe that if the roll diameter is made larger than the above-mentioned general work roll, rolling strain will be more likely to be concentrated even in the center of the plate thickness, and grain boundaries will be more likely to be formed. , we have come to the idea that many {111} crystal orientations may be generated. This idea is based on the idea that if the distance between the surface of the stainless steel strip and the rotating shaft is the same between the above-mentioned general work roll and a work roll with a larger roll diameter, the work roll with a larger roll diameter has a higher platen surface. This is based on the fact that it can be rolled closer to the center of thickness.

As a result of intensive studies based on the above idea, we found that if the diameter of the work roll is set to 200 mm or more, the desired number of {111} It was found that it is possible to generate crystal orientation. However, as a result of further investigation, 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 the cold rolling using the above-mentioned work roll is It has been found that it is difficult to generate a desired number of {111} crystal orientations at the center of the sheet thickness if the cold rolling rate is low. Note that the cold rolling rate per pass is determined by the thickness of the stainless steel strip before one pass and the thickness of the stainless steel strip after one pass with respect to the thickness of the stainless steel strip before one pass for any given pass. This is the ratio of the difference from the thickness.

Therefore, the present inventors conducted further investigation. As a result, in order to improve the r value of stainless steel, it is necessary to set the work roll diameter to 200 mm or more, set the cold rolling rate per pass to 15% or more, and It has been 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 rolling rate 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 with respect 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 rate, is processed after all cold rolling processes are completed (in this embodiment, after the first cold rolling and second cold rolling described below are completed). Refers to the steel strip. The cold rolling rate after all passes will be described later.

In other words, 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 all passes is the total cold rolling rate. It has been found that by setting the ratio to 50% or more, a desired number of {111} crystal orientations can be generated in the entire region from the surface layer to the center of the sheet thickness.

In the following description, a work roll with a roll diameter of 200 mm or more will be referred to as a "large diameter roll", and a work roll with a roll diameter of less than 200 mm will be referred to as a "small diameter roll". According to these definitions, work rolls with a roll diameter of 50 to 100 mm, which are commonly used in cold rolling, belong to "small diameter rolls."

<Summary>
If stainless steel is manufactured based on each of the measures (i) and (ii) above, expensive elements such as Ti are not used in conventional manufacturing methods in order to improve both deep drawability and ridging resistance. There is no need to add. Further, there is no need to provide special manufacturing equipment to improve both deep drawability and ridging resistance. Therefore, (i) and (ii) above
Each of these measures is effective in producing stainless steel with excellent both deep drawability and ridging resistance at a lower cost than before.

[Mechanism of improving corrosion resistance and workability]
As a result of further studies, the present inventors found that by eliminating the martensitic phase in the cold-rolled steel strip obtained based on the measures (i) and (ii) described above, corrosion resistance and workability were improved. We have discovered that it is possible to create stainless steel with superior properties at a lower cost than conventional methods.

Specifically, in finish annealing after cold rolling, the cold rolled steel strip is heated to 800° C. or more and less than Ac1 at a temperature increase rate of 50° C./s or less. Ac1 is a measure of the temperature at which the austenite phase starts to form. Through this heat treatment, the martensitic phase starts to disappear from the time when the cold rolled steel strip reaches a certain temperature (approximately 700° C.). Next, the heated cold-rolled steel strip is soaked for 5 seconds or more in a temperature range of 800° C. or higher and lower than Ac1, thereby substantially eliminating the martensite phase in the cold-rolled steel strip. Next, the soaked cold-rolled steel strip is cooled at a cooling rate of 50° C./s or less to eliminate the martensite phase in the cold-rolled steel strip.

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

However, "disappearance of the martensitic phase" in this specification refers to finish annealing aimed at complete disappearance of the martensitic phase (area ratio of 0%), and in reality, the area ratio of stainless steel as a final product is reduced. The concept is to allow less than 1.0% of the martensite phase to remain. If the area ratio of the remaining martensitic phase is less than 1.0%, the stainless steel as a final product will have excellent corrosion resistance and workability. Hereinafter, the area ratio of the martensite phase contained in the stainless steel as a final product will be referred to as "second martensite area ratio."

The present inventors completed the recrystallization of the ferrite phase, which is the parent phase, by subjecting the cold-rolled steel strip to the above-mentioned finish annealing, while completing the martensite phase (first martensite phase) intentionally generated during softening annealing. It has been found that the site area ratio (5.0 to 30.0%) can also be eliminated. Recrystallization refers to the generation of new crystal grains that do not contain dislocations in a cold rolled steel strip. Dislocations are an example of lattice defects that occur inside a crystal. Furthermore, the present inventors have discovered that by subjecting the cold-rolled steel strip to the above-described finish annealing, it is possible to prevent the generation of a new martensitic phase in the cold-rolled steel strip. The series of methods from softening annealing to finish annealing described above is different from a general stainless steel manufacturing method that aims to actively eliminate the martensitic phase from the time of softening annealing.

[Component composition]
The stainless steel according to an embodiment of the present invention has, in mass %, C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 1.0% or less, Cr: 12.0-18.0%,
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 to 0.10%, Mg: 0 .0005 to 0.003%, Ca: 0.0003 to 0.003%, Y: 0.01% to 0.20%, Rare earth metals (REM) excluding Y: 0.01 to 0.10% in total , Sn: 0.001-0.50%, Sb: 0.001-0.50%, Pb: 0.01-0.10% and W: 0.01-0.50%. In addition, in the following description, the stainless steel according to one embodiment of the present invention will be abbreviated as "this stainless steel".

The remainder of the stainless steel consists of Fe and unavoidable impurities. Note that 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 the present stainless steel. Each of these elements is an arbitrary element as long as at least one or more of these elements is included as necessary. Each element contained in this stainless steel will be explained below.

<C: 0.12% or less>
C is an important element that forms a carbide with Cr, thereby creating an interface that becomes a source of dislocations when the stainless steel is deformed. However, when C is added in excess, an excessive amount of martensitic phase is generated, resulting in a decrease in the ductility of the stainless steel. 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, when Si is added in excess, the stainless steel becomes hard and its ductility decreases. 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, when Mn is added excessively, the amount of MnS produced increases and the corrosion resistance of the 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 stainless steel. However, when Ni is added excessively, the austenite phase is stabilized more than necessary, the ductility of the stainless steel decreases, and the raw material cost of the stainless steel increases. Therefore, the Ni content is set to 1.0% or less. In the following description, the steel strip of the present stainless steel after softening annealing will be referred to as a "hot rolled annealed steel strip." The steel strip of the present stainless steel after softening annealing is an example of the hot rolled annealed steel strip according to the present invention.

<Cr:12.0-18.0%>
Cr is necessary to form a passive film on the surface of the steel strip of the stainless steel after cold rolling and to improve corrosion resistance. However, when excessive Cr is added, the ductility of the stainless steel decreases. Therefore, the Cr content is set to 12.0 to 18.0%. In addition, in the following description, the steel strip of this stainless steel after cold rolling is referred to as a "cold rolled steel strip." The stainless steel strip 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, thereby creating an interface that becomes a source of dislocations when the stainless steel is deformed. However, when N is added excessively, an excessive amount of martensitic phase is generated, resulting in a decrease in the ductility of the stainless steel. Therefore, the N content is set to 0.10% or less.

<Al: 0.50% or less>
Al is an effective element for deoxidation and can reduce A2 _- based inclusions that adversely affect press workability. However, when Al is added excessively, surface defects of the 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 in improving corrosion resistance. However, when Mo is added excessively, the raw material cost of the stainless steel increases. Therefore, it is preferable that the Mo content is set to 0.50% or less.

<Cu: preferably 1.0% or less>
Cu is an element effective in improving corrosion resistance. It is preferable that the content of Cu is set to 1.0% or less.

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

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

<B: Preferably 0.10% or less>
B is an element effective in improving toughness. However, this effect is saturated 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 carbonitrides, and improves the corrosion resistance of the stainless steel by suppressing grain boundary precipitation of Cr carbonitrides during heat treatment. Furthermore, by fixing the solid solute C and solid solute N in the stainless steel as carbonitrides, the contents of the solid solute C and solid solute N are reduced, thereby improving the r value of the stainless steel. Furthermore, by fixing solid solution C and solid solution N in the stainless steel as carbonitrides, the ductility of the stainless steel can be improved and stretcher strain can be reduced. Stretcher strain is minute irregularities formed on the surface of stainless steel due to several percent yield elongation that occurs during press working of stainless steel.

However, since Ti is an expensive element, when Ti is added in excess, the raw material cost of the stainless steel increases. Therefore, it is preferable that the Ti content is set to 0.50% or less.

<Co: preferably 0.01 to 0.50%>
Co is an element effective in improving corrosion resistance and heat resistance. However, when Co is added excessively, the raw material cost of the 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 effective element for denitrification, deoxidation, and desulfurization. However, when Zr is added excessively, the raw material cost of the stainless steel increases. Therefore, the Zr content is preferably set to 0.01 to 0.10%.

<Nb: Preferably 0.01 to 0.10%>
Like Ti, Nb fixes the solute C and N in the stainless steel as carbonitrides, thereby reducing the contents of the solute C and N and increasing the r value of the stainless steel. Improve. Moreover, by fixing the solid solution C and solid solution N in the stainless steel as carbonitrides, the ductility of the stainless steel can be improved and stretcher strain can be reduced. However, since Nb is an expensive element, when Nb is added in excess, the raw material cost of the 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, when excessive Mg is added, the toughness of the stainless steel decreases, and as a result, the manufacturability decreases. 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 effective element for degassing. The content of Ca is preferably set to 0.0003 to 0.003%.

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

<REM: preferably 0.01 to 0.10% in total>
Like Y, REM (Rare Earth Metal) such as Sc and La is effective in improving hot workability and oxidation resistance. However, these effects become saturated 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 in improving corrosion resistance. However, when Sn is added in excess, hot workability and tenacity decrease. 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 deformation bands during rolling. However, if too much Sb is added, this effect is saturated, and workability decreases depending on the amount of Sb added in excess. 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 in improving free machinability. The content of Pb is preferably set to 0.01 to 0.10%.

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

<Others>
In this stainless steel, the remainder other than the above-mentioned components is Fe and inevitable impurities. Unavoidable impurities are impurities that are mixed in due to raw materials and manufacturing processes, and are mixed in to the extent that they do not affect the characteristics of each component described above.

[Riding swell height]
In this stainless steel, the undulation height of the ridging formed on the surface is 15 μm or less. In this specification, "undulation height of the ridging formed on the surface of this stainless steel (hereinafter abbreviated as "undulation height of this stainless steel")" refers to the undulation of the ridging 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 taken from the final product of this stainless steel. Next, using an Instron type tensile testing machine, the first tensile test piece is pulled so that the gauge distance is 50 mm and the tensile direction is parallel to the rolling direction. Through 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 perpendicular to the rolling direction (in other words, the width direction of the first tensile test piece) in the part between the gauge marks of the first tensile test piece was set to 18 mm. Measure the undulation height.

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

When the waviness height of the ridging formed on the surface of conventional stainless steel is measured by the above method, the waviness height is 20 to 50 μm, which is higher than the waviness height of the present stainless steel. From this, it can be said that this stainless steel has improved ridging resistance compared to conventional stainless steel.

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

First, a JIS No. 13B tensile test piece specified in JIS Z 2201 is taken from the final product of this stainless steel. Specifically, from the 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 making an angle of 45° with the rolling direction, and a rolled A fourth tensile test piece whose tensile direction is perpendicular to the above direction is taken. Next, each of the second to fourth tensile test pieces is pulled using an Instron type tensile tester with a gauge distance of 20 mm. Through 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. "Width" is the width of the portion between the gauge marks in each of the second to fourth tensile test pieces. "Thickness" is the thickness of the portion between the gauge marks in each of the second to fourth tensile test pieces.

Next, the average r value of the respective r values of the second to fourth tensile test pieces is calculated using the following equation (2). This average r value becomes 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.

When the r value of conventional stainless steel is calculated using the method described above, the r value is 0.6 to 0.8, which is smaller than the r value of the present stainless steel. From this, it can be said that this stainless steel has improved deep drawability compared to conventional stainless steel.

[Second martensite area ratio]
The present 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 stainless steel. However, if the secondary martensite area ratio is less than 1.0%, even if the area ratio is higher than 0%, this stainless steel will not only have good ridging resistance and deep drawability, but also corrosion resistance and workability. Excellent. The second martensite area ratio can be calculated using EBSD crystal orientation analysis in the same way as the first martensite area ratio.

[Indicator value]
This stainless steel has an index value of 15 to 50 expressed by the following formula (3). This index value is an index representing the maximum amount of austenite phase produced by annealing. In the following formula (3), each element symbol represents the mass % concentration of the element.

(Indicator value) = 420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-52Al+470N+189...(3)
The austenite phase produced during annealing can transform into a martensitic phase during the cooling process. Therefore, by adjusting the mass % concentration of each element in equation (3) above so that the index value is between 15 and 50, the amount of martensitic phase generated during the cooling process can be appropriately controlled. . Specifically, "appropriately manage" means that the amount of martensite phase generated during the cooling process is such that it can improve both deep drawability and ridging resistance. , refers to controlling the maximum amount of austenite phase produced by annealing. Further, since the amount of martensite phase is appropriately controlled by setting the index value to 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. As shown in FIG. 1, this stainless steel is manufactured by performing the following steps: melting step S1, hot rolling step S2, softening annealing step S3, cold rolling step S4, and annealing step S5. Each step will be explained below, but the method for manufacturing this stainless steel is not limited to the method shown in FIG. 1.

<Smelting process S1 and hot rolling process S2>
In order to manufacture this stainless steel, first, in a melting process S1, stainless steel containing the above-mentioned components is melted to produce a steel slab. In the melting process S1, a general stainless steel melting apparatus can be used, and general melting conditions can be set. Next, in hot rolling process S2, a hot rolled steel strip is manufactured by hot rolling the steel slab manufactured in ingot process 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, common hot rolling equipment and hot rolling conditions for stainless steel can be used.

<Softening annealing step S3>
Next, in the softening annealing step S3, the hot rolled steel strip produced in the hot rolling step S2 is softened and annealed to produce a hot rolled annealed steel strip. 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 the hot-rolled steel strip by performing softening annealing, the thickness of the hot-rolled annealed steel strip can be easily adjusted 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. When the softening annealing temperature is further increased, the amount of austenite phase in the steel strip increases to a peak amount, and then begins to decrease. Since the austenite phase can transform into the martensite phase during the cooling process during softening annealing, the first martensite area ratio is influenced by the austenite phase, which increases due to 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 Ac1 or higher and a temperature at which the amount of 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 only a guideline temperature based on the regression equation, and does not match the actual temperature at which the austenite phase begins to form. Therefore, as a result of intensive study by the present inventors, in the case where Ac1 is less than 921, the maximum annealing temperature is set to 0.76×Ac1+201°C or more and 1.10×Ac1−56°C or less, thereby reducing the first martensite area. It has been found that the ratio can be set between 5.0 and 30.0%.

On the other hand, when Ac1 is 921 or more, the peak amount of the austenite phase itself is small. It was possible to set it to .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 annealing at excessively high temperatures causes coarsening of crystal grains in the hot-rolled annealed steel strip and deterioration of properties such as surface roughness during processing, the upper limit of the maximum annealing temperature is set at 1050°C. I decided to set it.

Specifically, the maximum annealing temperature is set as follows. First, Ac1 expressed by the following equation (4) is calculated based on the composition of the steel slab. In the following formula (4), each element symbol represents the mass % concentration of the element. The Ac1 need only 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)
If Ac1 calculated by the above equation (4) is less than 921, the maximum annealing temperature is set to 0.76×Ac1+201°C or more and 1.10×Ac1−56°C or less. 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 temperature increase rate in the temperature increase process in softening annealing is preferably set to 10° C./sec or more. If the heating rate is 10°C/sec or higher, the heating time during the heating process can be shortened to a practically meaningful degree, and the total time required to manufacture this stainless steel can also be reduced to a practically meaningful degree. Can be shortened. Therefore, the productivity of this stainless steel can be improved. Further, the soaking time in the soaking process in softening annealing is preferably set to 5 seconds or more. If the soaking time is 5 seconds or more, an austenite phase can be reliably generated during the soaking process. Since the austenite phase transforms into the martensite phase during the cooling process after the soaking process, by setting the soaking time to 5 seconds or more, the first martensite area ratio can be increased to 5.0 to 30.0%. even easier to manage.

Furthermore, the cooling rate in the cooling process in softening annealing is set to 5.0° C./sec or more. 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 transforms into a stable ferrite phase. Therefore, the first martensite area ratio decreases to less than 5.0%, and the ridging resistance of this stainless steel decreases to a level lower than that of conventional stainless steel. Therefore, in order to maintain good ridging resistance of the present stainless steel, the cooling rate is set at 5.0° C./sec or more.

<Cold rolling process S4>
Next, in the cold rolling step S4, a cold rolled steel strip is manufactured by cold rolling the hot rolled annealed steel strip manufactured in the softening annealing step S3. As the cold rolling conditions, the total cold rolling rate after the end of the cold rolling step S4 is set to 60% or more.

In the cold rolling in the cold rolling step S4, first cold rolling is performed in which the hot rolled annealed steel strip is passed 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 the first work roll according to the present invention. As the cold rolling conditions of the first cold rolling, the cold rolling rate per pass is set to 15% or more, and the cold rolling rate after the end of the first cold rolling (after the end of all passes in the first cold rolling). is set to 50% or more of the total cold rolling rate. The cold rolling rate after the first cold rolling (after all passes in the first cold rolling) is 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 completing all passes.

After the first cold rolling is completed, a second cold rolling is performed in which the steel strip passed through the large-diameter roll is passed through a 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 the second work roll according to the present invention. In the second cold rolling, the remaining strip thickness that was not completely rolled in the first cold rolling is rolled. The steel strip after the second cold rolling becomes a cold rolled steel strip.

Here, the reason why the second cold rolling is performed after the first cold rolling is completed will be explained. In other words, when comparing the first cold rolling and the second cold rolling, assuming that both cold rollings have the same rolling rate, the first cold rolling using large diameter rolls is better. A larger rolling load is required than in the second cold rolling using small diameter rolls. Generally, stainless steel is harder than ordinary steel, and in cold rolling, work hardening progresses toward the latter half of the process, increasing the strength of the steel strip. From these facts, if cold rolling is performed using only large-diameter rolls in the cold rolling process S4, the rolling load that must be applied to the steel strip to obtain the cold rolled steel strip of the desired strip thickness will be lower than that of this stainless steel. This exceeds the acceptable range in terms of manufacturability and productivity. Therefore, as in this embodiment, after the first cold rolling is performed using a large diameter roll, the second cold rolling is performed using a small diameter roll. In this embodiment, among small diameter rolls with a roll diameter of less than 200 mm, a work roll with a roll diameter of 50 to 100 mm, which is commonly used as a small diameter roll, is used as the second work roll.

Note that 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. Good too. However, stainless steel is generally harder than ordinary steel. Furthermore, in cold rolling, work hardening progresses toward the latter half of the process, increasing the strength of the steel strip. Therefore, when performing the first cold rolling after the second cold rolling, in order to cause the concentration of rolling strain at the center of the thickness of the steel strip after the second cold rolling, it is necessary to A larger rolling load is required than when performing the second cold rolling after performing the above. For these reasons, in consideration of the manufacturability and productivity of the stainless steel, it is preferable to perform the second cold rolling after the first cold rolling as in this embodiment.

<Annealing step S5>
Next, in the annealing step S5, the cold rolled steel strip manufactured in the cold rolling step S4 is annealed at a temperature higher than the recrystallization start temperature and lower than Ac1. The annealing performed in the annealing step S5 is finish annealing that aims to achieve both the completion of recrystallization of the ferrite phase and the disappearance of the martensitic phase in the cold rolled steel strip. The finish annealing performed in the annealing step S5 includes a temperature raising process, a soaking process, and a cooling process, similar to the softening annealing in the softening annealing process S3.

In the temperature raising process, the cold rolled steel strip is heated at a temperature raising rate of 50° C./s or less to a temperature higher than the recrystallization start temperature and lower than Ac1. By setting the temperature increase rate to 50° C./s or less, the martensite phase can be eliminated during the temperature increase process. In the soaking process, the cold rolled steel strip after the temperature raising process is soaked for 5 seconds or more at a temperature higher than the recrystallization start temperature and lower than Ac1. In this embodiment, the recrystallization start temperature is set at 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 recrystallization start temperature is not limited to 800°C, and the recrystallization start temperature may be set to a temperature lower than 800°C, for example. On the other hand, by setting the upper limit of the soaking temperature to Ac1 or lower, the martensite phase remaining in the cold rolled steel strip is almost eliminated while preventing the generation 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 during the cooling process. By subjecting the cold-rolled steel strip to finish annealing consisting of each of these treatments, it is possible to efficiently achieve both the completion of recrystallization of the ferrite phase and the disappearance of the martensitic phase in the cold-rolled steel strip in the annealing step S5. can do. By completing the annealing step S5, the stainless steel as a final product is obtained, and the production of the stainless steel is completed.

〔summary〕
The ferritic stainless steel according to one embodiment of the present invention includes, in 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% or less may further be contained.

The ferritic stainless steel according to one embodiment of the present invention contains, in mass %, Co: 0.01% or more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, and Nb: 0.01% or more. 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 of 0.01% to 0.10%, Sn: 0.001% to 0.50%, Sb: 0.001% to 0.50%, Pb: 0.01% to 0.10 % or less and W: 0.01% or more and 0.50% or less.

In the method for manufacturing ferritic stainless steel according to one aspect of the present invention, the steel slab has, in 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% or less.

In the method for manufacturing ferritic stainless steel according to one aspect of the present invention, the steel slab contains Co: 0.01% or more and 0.50% or less, and Zr: 0.01% or more and 0.10% or less, in 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 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: It may further contain one or more selected from 0.01% to 0.10% and W: 0.01% to 0.50%.

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

〔Example〕
The results of evaluating ferritic stainless steels according to Examples and Comparative Examples of the present invention will be described below. Hereinafter, ferritic stainless steels according to examples of the present invention will be referred to as "inventive steels", and ferritic stainless steels according to comparative examples of the present invention will be referred to as "comparative steels". In this example, first, five types of steel slabs having any of the compositions A to E shown in Table 1 below were manufactured by melting on an actual production line. The content of each element constituting compositions A to E was all within the numerical range of the content of each element contained in the ferritic stainless steel according to one embodiment of the present invention. Note that Table 1 also shows the values of Ac1 for each of the compositions A to E.

組成Ｅを有する鋼スラブに基づいて製造された発明鋼および比較鋼の指標値は、前記の表１に示すように８５となり、本発明における好ましい数値範囲の上限値５０を超える結果となった。このような結果になったのは、組成Ｅのうち、指標値に影響を与えるＣｒの含有率が１２．５％だったためであると推察される。

The index value of the invention steel and comparative steel manufactured based on the steel slab having composition E was 85 as shown in Table 1 above, which exceeded the upper limit of 50 of the preferred numerical range in the present invention. The reason for this result is presumed to be that in composition E, the content of Cr, which affects the index value, was 12.5%.

Next, the steel slabs of each composition were hot rolled to produce hot rolled steel strips of each composition with a thickness of 3 mm and a width of 150 mm. Next, the hot-rolled steel strips of each composition were subjected to softening annealing and cold rolling under the "actual conditions" shown in Table 2 below. Manufactured steel strips. Then, the first to seventh invention steels and the first to 18th comparative steels were manufactured by final annealing the cold rolled steel strips of each composition.

前記の表２には、軟質化焼鈍および冷間圧延の「推奨条件」も示されている。前記の表２における「推奨条件」の欄に記載された各条件は、本実施形態と同一の条件とした。また、「推奨条件」の「軟質化焼鈍」の欄における「下限温度」は最高焼鈍温度の下限値を示し、同欄の「上限温度」は最高焼鈍温度の上限値を示す。「下限温度」および「上限温度」の各欄に記載された数値は、組成Ａの各発明鋼および各比較鋼については、０．７６×Ａｃ１＋２０１℃以上１０５０℃以下の式に組成ＡのＡｃ１の数値（９４２）を代入して算出した。一方、組成Ｂ～Ｅの各発明鋼および各比較鋼については、０．７６×Ａｃ１＋２０１℃以上１．１０×Ａｃ１－５６℃以下の式に組成Ｂ～Ｅの各Ａｃ１の数値（８１１、８５５、９２０、７１０）を代入して算出した。

Table 2 above also shows "recommended conditions" for softening annealing and cold rolling. The conditions listed in the "recommended conditions" column in Table 2 above were the same as those in this embodiment. Further, "lower limit temperature" in the "softening annealing" column of "recommended conditions" indicates the lower limit of the maximum annealing temperature, and "upper limit temperature" in the same column indicates the upper limit of the maximum annealing temperature. The values listed in the "Lower limit temperature" and "Upper limit temperature" columns are calculated using the formula 0.76 x Ac1 + 201°C or more and 1050°C or less for each invention steel of composition A and each comparative steel. Calculated by substituting the numerical value (942). On the other hand, for each invention steel and each comparison steel of compositions B to E, the numerical values of each Ac1 of compositions B to E (811, 855, 920, 710).

Table 2 also shows "characteristic evaluation" and "overall evaluation" for each of the 1st to 7th invention steels and the 1st to 18th comparison steels. The "Waviness Height" column of "Characteristic Evaluation" shows the measurement results of the ridging waviness height. Further, the "r value" column of "characteristic evaluation" shows the calculation result of the r value. Note that the method for measuring the undulation height of ridging and the method for calculating the r value were the same as in this embodiment. "Comprehensive evaluation" is "○" when the ridging waviness height is 15 μm or less, the r value is 0.9 or more, and the secondary martensite area ratio is 0% or more and less than 1.0%. And so. On the other hand, if the ridging waviness height 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 was marked as "x". .

The underlined numerical values in Table 2 above indicate numerical values outside the preferred numerical range in this embodiment. Further, the underlined "x" in Table 2 indicates that the cold rolling rate after 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 ridging waviness heights were all higher than 15 μm, so the overall evaluation was "x". It became. The reason why the ridging waviness height was higher than 15 μm is because the primary martensite area ratio was less than 5.0% in all of the 1st, 9th, 11th, 13th, and 16th comparison steels. It is assumed that. In other words, for all of the No. 1, No. 9, No. 11, No. 13, and No. 16 comparison steels, the amount of increase in colonies in the steel exceeded the allowable range for improving the ridging resistance, so the waviness of the ridging It is presumed that the height was higher than 15 μm.

In addition, as shown in Table 2 above, for each comparative steel other than the 1st, 9th, and 18th comparative steels, the r value was less than 0.9, so the overall evaluation was "x". . The reason why the r value became 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 rate is less than 60%, and the cold rolling rate per pass is It is presumed that this is because at least one of the following applies: less than 15%, and the cold rolling ratio after all passes being less than 50% of the total cold rolling ratio. In other words, for all of the 2nd and 3rd comparative steels, the 5th to 8th comparative steels, the 10th to 13th comparative steels, and the 16th comparative steels, rolling strain is concentrated at the center of the plate thickness of these comparative steels. It is presumed that the r value was less than 0.9 because it was not sufficiently generated.

Next, regarding the 14th and 15th comparative steels, it is presumed that this is because the first martensite area ratio was larger than 30.0%. In other words, it is presumed that in both the 14th and 15th comparative steels, the martensitic phase increased more than necessary and the ductility decreased, resulting in the r value being less than 0.9. Next, regarding the 17th comparative steel, in addition to the total cold rolling rate being less than 60%, the first martensite area ratio was greater than 30.0%, so the r value was less than 0.9. It is presumed that this has happened.

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 "×" ”.

On the other hand, regarding the first to seventh invention steels, the characteristics evaluation of the third and fourth invention steels gave the best overall results among all the invention steels. Specifically, the third invention steel had the third lowest ridging waviness height (2.39 μm) among all the invention steels. The reason for this result is presumed to be that the first martensite area ratio was the third highest (9.44%) among all the invented steels. Furthermore, the third invention steel had the largest r value (1.12) among all the invention steels. This result is presumed to be because the total cold rolling rate was the highest (85%) among all the invented steels. Furthermore, the third invention steel had the fourth lowest second martensite area ratio (0.17%) among all invention steels.

The fourth invention steel had the lowest ridging waviness height (2.28 μm) among all the invention steels. Furthermore, the fourth invention steel had the fourth largest r value (0.93) among all invention steels. This result is presumed to be because the total cold rolling rate was the fourth highest (69%) of all invented steels. Furthermore, the fourth invention steel had the third lowest (0.15%) of all invention steels in terms of second martensite area ratio.

INDUSTRIAL APPLICATION This invention can be utilized for the manufacturing method of 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 (6)

In mass %, C: 0.039% or more and 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 1.0% or less, Cr: 12.0% or more18. 0% or less, N: 0.10% or less and Al: 0.50% or less, with the balance consisting of Fe and inevitable impurities,
(i) When measuring the surface texture of the ridging formed by performing a tensile test in which a 16% tensile strain is applied to the ferritic stainless steel sheet so that the tensile direction is parallel to the rolling direction. The undulation height of the ridging represented by the average height of the undulation curve elements is 15 μm or less,
(ii) the r value is 0.9 or more, and
(iii) A ferritic stainless steel sheet containing a martensitic phase having an area ratio of 0% or more and less than 1.0% in a cross section cut along a plane parallel to the rolling direction and perpendicular to the width direction. In 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 ferritic stainless steel sheet according to claim 1, further comprising one or more selected from the following. In 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 sheet according to claim 1 or 2, further comprising one or more selected from the following. In 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, the remainder being Fe and inevitable impurities, a hot rolling step of producing a hot rolled steel strip;
By annealing and softening the hot-rolled steel strip produced in the hot-rolling process, martensite phase in a cross section cut along a plane parallel to the rolling direction and perpendicular to the width direction of the hot-rolled annealed steel strip is reduced. A softening annealing step of producing the hot rolled annealed steel strip having an area ratio of 5.0% or more and 30.0% or less, and the remainder excluding the martensitic phase containing a ferrite phase;
a cold rolling step of manufacturing a cold rolled steel strip by cold rolling the hot rolled annealed steel strip produced in the softening annealing step;
an annealing step of annealing the cold rolled steel strip at a temperature higher than the recrystallization start temperature and lower than Ac1 expressed by the following formula,
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;
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 with a roll diameter of 200 mm or more, the cold rolling rate per pass is 15% or more, and the cold rolling rate after all passes is 50% or more of the total cold rolling rate. After cold rolling,
(ii) further cold rolling using a second work roll with a roll diameter of less than 200 mm,
The ferritic stainless steel sheet obtained by the annealing process is a ferritic stainless steel sheet in which the area ratio of the martensitic phase in a cross section taken along a plane parallel to the rolling direction and perpendicular to the width direction is 0% or more and less than 1.0%. Method of manufacturing steel plates . The steel slab contains, in 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. The method for producing a ferritic stainless steel sheet according to claim 4, further comprising one or more selected from: 0.50% or less. The steel slab has, in 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 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 sheet according to claim 4 or 5, further comprising one or more selected from % or more and 0.50% or less.

Images