US20110061777A1

US20110061777A1 - Ferritic stainless steel sheet having superior punching workability and method for manufacturing the same

Info

Publication number: US20110061777A1
Application number: US12/673,651
Authority: US
Inventors: Tomohiro Ishii; Yoshimasa Funakawa; Shuji Okada; Masayuki Ohta
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2007-08-20
Filing date: 2008-06-18
Publication date: 2011-03-17
Also published as: JP2009068104A; EP2182085A4; ES2651023T3; JP5391609B2; WO2009025125A1; CN101784686A; EP2182085A1; TWI390049B; EP2182085B1; CN101784686B; TW200918676A

Abstract

A ferritic stainless steel sheet and a method for manufacturing the ferritic stainless steel sheet include a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities is hot-rolled at a finishing temperature of 900° C. or more and at a coiling temperature of 400 to 550° C., softening annealing is performed on an obtained hot-rolled steel sheet, picking is further performed, and cold rolling is subsequently performed.

Description

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/061498, with an international filing date of Jun. 18, 2008 (WO 2009/025125 A1, published Feb. 26, 2009), which is based on Japanese Patent Application No. 2007-213801, filed Aug. 20, 2007, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a ferritic stainless steel sheet having a superior punching workability and a method for manufacturing the same.

BACKGROUND

Since having a superior corrosion resistance and being easily worked, a ferritic stainless steel sheet has been used in various applications, such as architectural materials, transport machines, electric home appliances, and kitchen instruments. To manufacture these structures, after a ferritic stainless steel sheet is cut into a predetermined shape, a work, such as forming or welding, is further performed. For cutting of a ferritic stainless steel sheet, a shearing work, which has a high productivity, has been widely used.
In a shearing work, burrs are generated at a cross section of a ferritic stainless steel sheet. In the case in which the height of burrs is large, (a) when a ferritic stainless steel sheet which is cut out is transported to a forming machine (such as a press forming machine), trouble may arise due to the presence of the burrs, and
(b) when welding is performed, since a space may be generated at a burr position of a ferritic stainless steel sheet which is to be welded, for example, burn through may disadvantageously occur. Burrs are not only generated by a shearing work but are also generated by a punching work as shown in FIG. 1B. Hence, development of a punching technique and/or a shearing technique that generates no burrs has been desired.
In the punching work, since a cutting plane is also formed by shearing, the punching work and the shearing work are essentially the same. That is, a generation mechanism of burrs by the punching work is the same as that by the shearing work.
However, heretofore, an investigation to prevent the generation of burrs caused by the punching work and/or the shearing work has not been sufficiently performed, and an investigation to suppress the generation of burrs through an improvement in formability of a steel sheet has been performed.
For example, in Japanese Unexamined Patent Application Publication No. 10-204588, a technique has been disclosed in which recrystallization is facilitated by defining components of a hot-rolled steel sheet and a coiling temperature thereof. According to that technique, to improve the formability, the contents of C, P, and S are decreased to decrease precipitates of FeTiP, Ti₄C₂S₂, TiC, and the like. However, in the punching work and/or the shearing work, a large burr is generated.
In Japanese Unexamined Patent Application Publication No. 2002-249857, a technique has been disclosed in which crystal grains of ferrite are coarsened by defining components of a ferritic stainless steel sheet. In that technique, crystal grains of ferrite are coarsened (GSN 5.5 to 8.0) to improve stretch formability of a ferritic stainless steel sheet. However, in the punching work and/or the shearing work, a large burr is liable to be generated.
In Japanese Unexamined Patent Application Publication No. 2002-12955, a technique for improving press formability has been disclosed in which Ti is added, and TiO₂and Al₂O₃are suppressed from being precipitated. However, even when the ferritic stainless steel sheet according to this technique is used, in the punching work and/or the shearing work, a large burr is also liable to be generated.
It could therefore be helpful to provide a ferritic stainless steel sheet which can be processed by a punching work and/or a shearing work without generating burrs and a method for manufacturing the above ferritic stainless steel sheet. Hereinafter, the punching work and the shearing work are collectively called “punching work.”

SUMMARY

We researched causes of burrs generated when punching work is performed on a ferritic stainless steel sheet. As a result, we found the following:

- (A) When a NbTi complex carbonitride is precipitated in grain boundaries of ferrite crystal grains of a ferritic stainless steel sheet, cracks caused by a punching work are likely to be propagated, and as a result, the generation of burrs can be prevented.
- (B) When the average ferrite crystal grain size of a ferritic stainless steel sheet measured in accordance with ASTM E 112 is set to 20 μm or less, a NbTi complex carbonitride can be uniformly dispersed.
- (C) When the yield ratio of a ferritic stainless steel sheet is set to 0.65 or more, work hardening caused by a punching work is suppressed, and propagation of cracks is facilitated, so that the generation of burrs can be prevented. This disclosure is based on the above findings.

That is, we provide a ferritic stainless steel sheet having a superior punching work-ability, which comprises: a composition containing 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities; and a structure in which an average ferrite crystal grain size is 20 μm or less, and a ratio [Nb]/[Ti] of a Nb content to a Ti content contained in a NbTi complex carbonitride present in ferrite crystal grain boundaries is in the range of 1 to 10. In addition, the ferrite crystal grain size is an ASTM nominal grain diameter obtained in accordance with ASTM E 112.
In addition, according to the above ferritic stainless steel sheet, the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.
In addition, the ferritic stainless steel sheet further comprises 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.
In addition, we provide a method for manufacturing a ferritic stainless steel sheet having a superior punching workability, which comprises: performing hot-rolling of a slab having a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities at a finishing temperature of 900° C. or more and at a coiling temperature of 400 to 550° C.; performing softening annealing of an obtained hot-rolled steel sheet; then performing pickling; then performing cold rolling; and performing recrystallization annealing of an obtained cold-rolled steel sheet.
In addition, according to the above method for manufacturing a ferritic stainless steel sheet, the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.
In addition, the method for manufacturing a ferritic stainless steel sheet further comprises 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.
In addition, according to the method for manufacturing a ferritic stainless steel sheet, a slab heating temperature is 1,000° C. or less.
A ferritic stainless steel sheet can be manufactured which can be processed by a punching work without generating large burrs which cause an industrial problem.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show side views of a punching device before and after, respectively, a punching work is performed.

FIG. 2 shows a plan view and a side view of a punched-out hole formed by punching out a disc having a diameter of 10 mm.

DETAILED DESCRIPTION

First, the reasons for selecting components of a ferritic stainless steel sheet will be described. In addition, as described above, a punching work and a shearing work are collectively called a punching work.

C: 0.0030 to 0.012 Mass Percent

C is an element that binds to Cr, which will be described later, to form a Cr carbide which makes stainless steel sensitive to corrosion. Hence, by addition of Ti and Nb, C is fixed in the form of a NbTi complex carbonitride, and the NbTi complex carbonitride is dispersed and precipitated, so that the generation of burrs caused by a punching work is prevented. When the C content is less than 0.0030 mass percent, the above effect cannot be obtained. On the other hand, when the content is more than 0.012 mass percent, the generation of a Cr carbide cannot be suppressed, and the corrosion resistance is degraded. In addition, since the amount of the NbTi complex carbonitride is increased, and ferrite grains are liable to be expanded and coarsened, burrs are liable to be generated. Hence, the C content is set in the range of 0.0030 to 0.012 mass percent. More preferably, the content is 0.004 to 0.010 mass percent.

Si: 0.13 Mass Percent Or Less

Si is an element that hardens a ferritic stainless steel sheet by solid-solution hardening to degrade the ductility. When the Si content is more than 0.13 mass percent, the ductility of a ferritic stainless steel sheet is considerably degraded. Hence, the Si content is set to 0.13 mass percent or less. More preferably, the content is 0.10 mass percent or less.

Mn: 0.25 Mass Percent Or Less

Mn is an element that degrades the corrosion resistance of a ferritic stainless steel sheet. When the Mn content is more than 0.25 mass percent, in addition to the degradation in corrosion resistance, Mn binds to S which will be described later, and as a result, fine MnS is liable to be generated. MnS is precipitated in grain boundaries of ferrite crystal grains and expands the ferrite crystal grains by hot rolling and/or cold rolling, and as a result, burrs having a large height are generated in a punching work. Hence, the Mn content is set to 0.25 mass percent or less. More preferably, the content is 0.20 mass percent or less.

P: 0.04 Mass Percent Or Less

P is an element that hardens a ferritic stainless steel sheet by solid-solution hardening to degrade the toughness. When the P content is more than 0.04 mass percent, the toughness of a ferritic stainless steel sheet is considerably degraded. Hence, the P content is set to 0.04 mass percent or less. More preferably, the content is 0.03 mass percent or less.

S: 0.005 Mass Percent Or Less

S is an element that binds to Mn or Ti, which will be described later, to form MnS or TiS and disturb equiaxial crystallization of ferrite crystal grains. When the S content is more than 0.005 mass percent, since ferrite crystal grains are considerably expanded, burrs having a large height are generated in a punching work. Hence, the S content is set to 0.005 mass percent or less. More preferably, the content is 0.003 mass percent or less.

Al: 0.06 Mass Percent Or Less

Al is used as a deoxidizing agent in a steelmaking process for forming ferritic stainless steel. To obtain the above effect, the content is preferably 0.01 mass percent or more. When the Al content is more than 0.06 mass percent, Al binds to N, and AlN is liable to be generated. AlN expands ferrite crystal grains by hot rolling and/or cold rolling, so that burrs having a large height are generated in a punching work. Hence, the Al content is set to 0.06 mass percent or less. However, when the Al content is less than 0.02 mass percent, the deoxidizing effect cannot be obtained in a steelmaking process. Accordingly, the Al content is preferably in the range of 0.02 to 0.06 mass percent. More preferably, the content is 0.02 to 0.045 mass percent.

N: 0.0030 to 0.012 Mass Percent

N generates a NbTi complex carbonitride. When a NbTi complex carbonitride is uniformly dispersed in a ferritic stainless steel sheet, cracks generated by a punching work are likely to be propagated, so that the generation of burrs can be prevented. When the N content is less than 0.0030 mass percent, a sufficient NbTi complex carbonitride amount is not generated. On the other hand, when the content is more than 0.012 mass percent, a Cr nitride is precipitated, so that the corrosion resistance is degraded. Hence, the N content is set in the range of 0.0030 to 0.012 mass percent. More preferably, the content is 0.0040 to 0.010 mass percent.

Cr: 20.5 to 23.5 Mass Percent

Cr is an element for forming a passivation film on a surface of a ferritic stainless steel sheet to improve the corrosion resistance. When the Cr content is less than 20.5 mass percent, a superior corrosion resistance to that of a stainless steel containing 18% of Cr cannot be obtained. On the other hand, when the content is more than 23.5%, since a hard phase containing Cr and Nb is liable to be precipitated, the workability is degraded, and in addition, recrystallization by annealing after hot rolling (hereinafter referred to as “soft annealing”) and/or by annealing after cold rolling (hereinafter referred to as “recrystallization annealing”) is disturbed, so that ferrite crystal grains are liable to be expanded in a rolling direction. When the ferrite crystal grains are expanded, burrs having a large height are liable to be generated in a punching work. Hence, the Cr content is set in the range of 20.5 to 23.5 mass percent. More preferably, the content is 20.5 to 22.5 mass percent.

Cu: 0.3 to 0.6 Mass Percent

Cu has a function to further improve the corrosion resistance of a ferritic stainless steel sheet containing 20.5 mass percent or more of Cr. When the Cu content is less than 0.3 mass percent, the above effect cannot be obtained. On the other hand, when the content is more than 0.6 mass percent, Cu binds to S, and hence CuS is liable to be generated. CuS expands ferrite crystal grains by hot rolling and/or cold rolling and generates burrs having a large height in a punching work. Hence, the Cu content is set in the range of 0.3 to 0.6 mass percent. More preferably, the content is 0.3 to 0.5 mass percent. Even more preferably, the content is 0.3 to 0.45 mass percent.

Ni: 0.5 Mass Percent Or Less

Ni has a function to further improve the corrosion resistance of a ferritic stainless steel sheet. To obtain the above effect, the content is preferably 0.1 mass percent or more. However, when the Ni content is more than 0.5 mass percent, a ferritic stainless steel sheet is hardened, and as a result, the ductility thereof is degraded. Hence, the Ni content is set to 0.5 mass percent or less. More preferably, the content is 0.4 mass percent or less.

Nb: 0.3 to 0.5 Mass Percent

Nb has a function to generate a NbTi complex carbonitride in a ferritic stainless steel sheet and to facilitate propagation of cracks generated in a punching work, so that the generation of burrs can be prevented. When the Nb content is less than 0.3 mass percent, a large amount of Cr carbonitride is precipitated, and as a result, the corrosion resistance of a ferritic stainless steel sheet is degraded. On the other hand, when the content is more than 0.5 mass percent, a hard phase containing Cr and Nb is generated, the workability is degraded, and in addition, since the NbTi complex carbonitride is not likely to be generated, burrs having a large height are generated in a punching work. Hence, the Nb content is set in the range of 0.3 to 0.5 mass percent. More preferably, the content is 0.3 to 0.45 mass percent.
Ti: 0.05 to 0.15 mass percent
Ti has a function to generate a NbTi complex carbonitride in a ferritic stainless steel sheet and to facilitate propagation of cracks generated in a punching work, so that the generation of burrs can be prevented. When the Ti content is less than 0.05 mass percent, the NbTi complex carbonitride is not generated, and a Ti carbonitride and/or a Nb carbonitride is precipitated in ferrite crystal grains. As a result, burrs having a large height are generated in a punching work. On the other hand, when the content is more than 0.15 mass percent, a large amount of TiS is precipitated, equiaxial crystallization of ferrite grains is disturbed, and as a result, burrs having a large height are generated in a punching work. Hence, the Ti content is set in the range of 0.05 to 0.15 mass percent. More preferably, the content is 0.05 to 0.12 mass percent.
The balance other than the components described above contains Fe and inevitable impurities. The amount of the inevitable impurities is preferably decreased as small as possible.
Furthermore, in the ferritic stainless steel sheet, at least one selected from the group consisting of B, Mo, V, and Ca is preferably contained.
For example, 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca may be contained. B: 0.001 mass percent or less
When a very small amount of B is added, recrystallization nuclei are formed, and as a result, an effect of grain refining crystal grains is obtained. To obtain the above effect, the content is preferably 0.0001 mass percent or more. However, when more than 0.001 mass percent of B is added, the workability may be degraded due to steel hardening, and surface defects may occur. Hence, the B content is set to 0.001 mass percent or less.

Mo: 0.1 Mass Percent Or Less

Mo is an element to strengthen a passivation film, facilitate re-passivation after corrosion generation, and improve the corrosion resistance of stainless steel. To obtain the above effect, the content is preferably 0.01 mass percent or more. However, when more than 0.1 mass percent is added, the workability, such as press workability, is degraded by solid solution strengthening. Hence, the Mo content is set to 0.1 mass percent or less.

V: 0.05 Mass Percent Or Less

V is an element to improve the corrosion resistance of stainless steel. To obtain the above effect, the content is preferably 0.01 mass percent or more. However, when more than 0.05 mass percent is added, steel is hardened, and as a result, the workability is degraded. Hence, the V content is set to 0.05 mass percent or less.

Ca: 0.01 Mass Percent Or Less

Ca is an element to prevent molten steel from adhering to steelmaking devices, such as a nozzle. This effect can be obtained at a content of 0.001 mass percent or more. However, when more than 0.01 mass percent is added, Ca is precipitated in the form, for example, of CaO and CaS in steel. Since these inclusions are easily dissolved in water and increase a local pH, corrosion starts therefrom. Hence, the Ca content is set to 0.01 mass percent or less.
Next, a structure of the ferritic stainless steel sheet will be described. Average grains size of ferrite crystal grains: 20 μm or less
The size of ferrite crystal grains of a ferritic stainless steel sheet has a significant influence on the height of burrs generated by a punching work. When the grain size is more than 20 μm, deformation of each ferrite crystal grain is increased, and hence, burrs having a large height are liable to be generated. Accordingly, the grain size of ferrite crystal grains is set to 20 μm or less. Incidentally, the ferrite crystal grain size is an ASTM nominal grain diameter obtained in accordance with ASTM E 112.

Ratio [Nb]/[Ti] Between Nb Content And Ti Content Contained In Nbti Complex Carbonitride: 1 to 10

Cracks caused by a punching work are generated from interfaces between ferrite crystal grains and precipitates present in grain boundaries thereof and are propagated along the grain boundaries. Hence, when a NbTi complex carbonitride is made to be precipitated in grain boundaries of ferrite crystal grains, and when a great number of cracks are made to be generated from the carbonitride and are further made to be combined with each other, cutting can be easily performed. As a result, the generation of burrs in a punching work can be prevented. When the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride is less than 1, adhesion between ferrite grain boundaries and the NbTi complex carbonitride is increased in a punching work, cracks are not likely to be generated, and as a result, the height of burrs is increased. On the other hand, when the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride is more than 10, the NbTi complex carbonitride is particularized, and as a result, cracks are also not likely to be generated at interfaces formed with ferrite grain boundaries. Hence, the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride is set in the range of 1 to 10.
In addition, as a method for measuring the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride, after a thin film is formed from a central portion of a ferritic stainless steel sheet in a thickness direction by a twin jet method, the Nb content [Nb] and the Ti content [Ti] of the NbTi complex carbonitride (inclusions in which a Nb carbonitride and a Ti carbonitride are mixed together on an atomic level, or precipitated inclusions in which one carbonitride functions as precipitation sites and the other carbonitride adheres thereto) precipitated in grain boundaries are measured by a transmission electron microscope, and the [Nb]/[Ti] value is calculated.
Next, mechanical properties of the ferritic stainless steel sheet will be described.
Yield ratio: 0.65 Or More
When the yield ratio of a ferritic stainless steel sheet is less than 0.65, since work hardening is liable to occur by a punching work, deformation of each ferrite crystal grain is increased, and hence burrs having a large height are liable to be generated in a punching work. The ferritic stainless steel sheet has a yield ratio of 0.65 or more.
Next, a method for manufacturing the ferritic stainless steel sheet will be described.
After ferritic stainless steel having predetermined components is formed by melting and is then further formed into a slab, hot rolling (finishing temperature: 900° C. or more, coiling temperature: 400 to 550° C.) is performed by heating to 1,000° C. or more, so that a hot-rolled steel sheet is formed.

Heating Temperature Of Slab: 1,000° C. Or More

Carbides and nitrides are once melted by heating a slab, and the finishing temperature and the coiling temperature are defined, so that a NbTi complex carbonitride is made to be precipitated in grain boundaries of ferrite crystal grains. Hence, the heating temperature of the slab is preferably set to 1,000° C. or more. In this case, since the slab is deformed at a high temperature, and manufacturing cannot be easily performed, the upper limit of the slab heating temperature is 1,250° C. A more preferable range is 1,050 to 1,200° C.

Finishing Temperature: 900° C. Or More

When the finishing temperature is less than 900° C., recrystallization is disturbed during hot rolling, so that ferrite crystal grains are expanded in a rolling direction by hot rolling. Hence, burrs having a large height are liable to be generated when a ferritic stainless steel sheet is processed by a punching work. Accordingly, the finishing temperature is set to 900° C. or more. In addition, by the reason to prevent seizing with a rolling roll, the upper limit of the finishing temperature is 1,050° C. More preferably, the finishing temperature is in the range of 920 to 1,000° C.

Coiling Temperature: 400 to 550° C.

The coiling temperature of a hot-rolled steel sheet has an important function to precipitate a NbTi complex carbonitride in grain boundaries of ferrite crystal grains. When the coiling temperature is less than 400° C., the NbTi complex carbonitride is not precipitated. More preferably, the coiling temperature is in the range of 450 to 530° C.
On the other hand, when the coiling temperature of a hot-rolled steel sheet is more than 550° C., a hard phase containing Nb and Cr is precipitated, and as a result, the toughness is considerably degraded.
Hence, the coiling temperature of a hot-rolled steel sheet is set in the range of 400 to 550° C. When the coiling temperature is in this range, the NbTi complex carbonitride is precipitated in grain boundaries of ferrite crystal grains.
The hot-rolled steel sheet thus obtained is processed by softening annealing and is further processed by pickling. Conditions of the softening annealing and those of the pickling are not particularly limited, and these processes are performed in accordance with known methods. For example, as a preferable condition range for the softening annealing, the temperature is 900 to 1,100° C., and the time is 30 to 180 seconds. Next, cold rolling is performed, so that a cold-rolled steel sheet is obtained. The cold-rolled steel sheet thus obtained is processed by recrystallization annealing, so that a ferritic stainless steel sheet is obtained. Conditions of the cold rolling and those of the recrystallization annealing are not particularly limited, and these processes are performed in accordance with known methods. For example, as a preferable condition range for the recrystallization annealing, the temperature is 900 to 1,100° C., and the time is 30 to 180 seconds. In addition, the cold-rolled steel sheet may be processed by temper rolling. The draft of the temper rolling is preferably in the range of 0.5% to 1.5%.

EXAMPLES

After each ferritic stainless steel having components shown in Table 1 was formed by melting and was further molded into a slab, hot rolling was performed, so that a hot-rolled steel sheet having a thickness of 3 mm was obtained. The conditions of the hot rolling were shown in Table 2. The hot-rolled steel sheet thus obtained was processed by softening annealing (temperature: 900 to 1,100° C., time: 100 to 500 seconds) and was further processed by pickling. Subsequently, cold rolling was performed, so that a cold-rolled steel sheet having a thickness of 0.8 mm was obtained.
The cold-rolled steel sheet thus obtained was processed by recrystallization annealing (temperature: 900 to 1,100° C., time: 100 to 500 seconds) and was further processed by pickling.
After a thin film was formed from a central portion of the ferritic stainless steel sheet thus formed in a thickness direction by a twin jet method, a Nb content [Nb] and a Ti content [Ti] of a NbTi complex carbonitride precipitated in grain boundaries were measured by a transmission electron microscope, and the [Nb]/[Ti] value was calculated. After a structure was exposed by polishing a sheet-thickness cross section parallel to the rolling direction, the ferrite grain size was observed using an optical microscope. Next, 5 line segments each having an actual length of 500 μm were drawn on a photograph in each of a longitudinal direction and a lateral direction, and the number of intersections between the line segments and crystal grain boundaries shown in the photograph was counted. The ASTM nominal grain diameter was obtained in such a way that the total length of the line segments was divided by the number of intersections, and the value obtained thereby was multiplied by 1.13. The results are shown in Table 2. In addition, the measurement of the grain size was performed using one arbitrary viewing field.
In addition, a JIS-No. 13B tensile test piece was formed from the ferritic stainless steel sheet, and a tensile test was performed. The results are shown in Table 2. The tensile test piece was obtained so that a tensile direction was parallel to the rolling direction.
Furthermore, a punching test piece (100 mm by 100 mm) was obtained by cutting the ferritic stainless steel sheet, and a punching test was performed using a punching device shown in FIGS. 1A and 1B. After a round hole having a diameter of 10 mm was formed by a punching work at a central portion of the punching test piece, the height of burrs was measured. The results are shown in Table 2. In addition, in FIG. 2, a schematic view of a burr of a punched-out hole formed by punching out a disc having a diameter of 10 mm is shown. The height of the burr of one round hole was measured at 4 points at 90° regular intervals, and the average of the height was obtained therefrom.
Nos. 1 to 5 of Table 2 are examples in each of which the C content was changed. Although the height of the burr of Nos. 2 to 4 which were within our range was 50 μm or less, in Nos. 1 and 5 which were out of our range, a burr having a height of more than 100 μm was generated.
Nos. 6 to 10 are examples in each of which the Nb content was changed. The height of the burr of Nos. 7 to 9 which were within our range was 50 μm or less. In No. 6 in which the Nb content was lower than our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated. In No. 10 in which the Nb content was higher than our range, the ferrite crystal grains were expanded, and a burr having a height of more than 100 μm was generated.
Nos. 11 to 15 are examples in each of which the Ti content was changed. The height of the burr of Nos. 12 to 14 which were within our range was 50 μm or less. In No. 11 in which the Ti content was lower than our range, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Since the amount of precipitation of the NbTi complex carbonitride was small, a burr having a height of more than 100 μm was generated. In No. 15 in which the Ti content was higher than our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated.
Nos. 16 to 20 are examples in each of which the N content was changed. The height of the burr of Nos. 17 to 19 which were within our range was 50 μm or less. In No. 16 in which the N content was lower than our range, since the amount of the NbTi complex carbonitride was small, and the [Nb]/[Ti] value was small, a burr having a height of more than 100 μm was generated. In No. 20 in which the N content was higher than our range, in addition to a high [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated.
Nos. 21 to 25 are examples in which the conditions of the hot rolling were changed. The height of the burr of Nos. 23 and 24 which were within our range was 50 μm or less. In No. 21 in which the finishing temperature and the coiling temperature were out of our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated. In No. 22 in which the coiling temperature was lower than our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated. In No. 25 in which the coiling temperature was higher than our range, in addition to a high [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated.

	TABLE 1

	COMPONENT (MASS PERCENT)

No.

C

Si

Mn

P

S

Cr

Al

N

Cu

Ni

Nb

Ti

1	0.0011	0.13	0.16	0.028	0.002	20.8	0.041	0.0077	0.39	0.13	0.38	0.08
2	0.0048	0.13	0.16	0.028	0.002	20.8	0.041	0.0077	0.39	0.13	0.38	0.08
3	0.0085	0.13	0.16	0.028	0.003	20.8	0.041	0.0077	0.39	0.12	0.38	0.08
4	0.0105	0.13	0.16	0.028	0.002	20.8	0.041	0.0077	0.39	0.14	0.38	0.08
5	0.0023	0.13	0.16	0.028	0.002	20.8	0.041	0.0077	0.39	0.12	0.38	0.08
6	0.0066	0.06	0.21	0.032	0.001	22.3	0.024	0.0098	0.56	0.25	0.12	0.12
7	0.007	0.06	0.21	0.033	0.001	22.3	0.025	0.0098	0.56	0.25	0.35	0.12
8	0.0068	0.06	0.21	0.031	0.001	22.3	0.024	0.0098	0.56	0.25	0.43	0.12
9	0.0066	0.06	0.21	0.032	0.001	22.3	0.025	0.0098	0.56	0.25	0.48	0.12
10	0.0066	0.06	0.21	0.032	0.001	22.3	0.025	0.0098	0.56	0.24	0.65	0.12
11	0.0102	0.08	0.13	0.018	0.004	20.5	0.055	0.0065	0.43	0.42	0.41	0.001
12	0.0107	0.08	0.13	0.018	0.004	20.5	0.055	0.0065	0.43	0.42	0.41	0.07
13	0.0105	0.08	0.13	0.018	0.004	20.5	0.055	0.0065	0.43	0.42	0.41	0.11
14	0.0108	0.08	0.13	0.018	0.004	20.5	0.055	0.0065	0.43	0.42	0.41	0.14
15	0.0105	0.08	0.13	0.018	0.004	20.5	0.055	0.0065	0.43	0.42	0.41	0.26
16	0.0057	0.05	0.18	0.036	0.001	21.2	0.041	0.0011	0.41	0.33	0.48	0.07
17	0.0059	0.05	0.18	0.035	0.001	21.2	0.041	0.0039	0.41	0.33	0.48	0.07
18	0.0055	0.05	0.18	0.035	0.001	21.2	0.041	0.0066	0.41	0.33	0.48	0.07
19	0.0057	0.05	0.18	0.036	0.001	21.2	0.041	0.0105	0.41	0.33	0.48	0.07
20	0.0023	0.08	0.18	0.035	0.001	22.0	0.041	0.0212	0.41	0.33	0.48	0.07
21	0.0082	0.12	0.16	0.036	0.002	23.0	0.034	0.0107	0.34	0.44	0.34	0.09
22	0.0082	0.12	0.16	0.036	0.002	23.0	0.034	0.0102	0.34	0.44	0.34	0.09
23	0.0085	0.12	0.16	0.036	0.002	23.0	0.034	0.0105	0.34	0.44	0.34	0.09
24	0.0083	0.12	0.16	0.036	0.002	23.0	0.034	0.0107	0.34	0.44	0.34	0.09
25	0.0083	0.12	0.16	0.036	0.002	23.0	0.034	0.0108	0.34	0.45	0.34	0.09

	TABLE 2

	HOT ROLLING		FERRITE

	HEAT-	FINISH-			CRYSTAL		PUNCHING
	ING	ING	COILING	NbTi	GRAINS	MECHANICAL PROPERTIES	WORK

	TEMPER-	TEMPER-	TEMPER-	COMPOSITE	GRAIN		TENSILE	ELON-	BURR
	ATURE	ATURE	ATURE	CARBONITRIDE	SIZE	YIELD	STRENGTH	GATION	HEIGHT
No.	° C.)	(° C.)	(° C.)	[Nb]/[Ti]	(μm)	RATIO	(MPa)	(%)	(μm)	REMARKS

1	1200	940	420	4.4	32	0.61	440	36	123	COMPARATIVE
										EXAMPLE
2	1200	940	420	4.4	18	0.74	465	35	30	INVENTION
3	1200	940	420	4.4	15	0.75	476	34	31	EXAMPLE
4	1200	940	420	4.4	15	0.76	480	32	33
5	1200	940	420	4.4	26	0.80	539	26	135	COMPARATIVE
					(EXPANDED					EXAMPLE
					GRAIN)
6	1170	980	400	0.9	25	0.62	451	34	110
7	1170	980	400	2.7	16	0.75	468	34	36	INVENTION
8	1170	980	400	3.3	17	0.76	477	33	32	EXAMPLE
9	1170	980	400	3.7	17	0.76	484	33	33
10	1170	980	400	5.0	39	0.61	446	35	132	COMPARATIVE
					(EXPANDED					EXAMPLE
					GRAIN)
11	1150	900	400	—	26	0.63	451	34	105
12	1150	900	400	5.4	18	0.76	467	33	33	INVENTION
13	1150	900	400	3.4	17	0.77	473	33	34	EXAMPLE
14	1150	900	400	2.7	16	0.77	479	32	35
15	1150	900	400	0.8	28	0.61	443	33	122	COMPARATIVE
					(EXPANDED					EXAMPLE
					GRAIN)
16	1180	950	440	0.6	17	0.75	465	33	131
17	1180	950	440	6.3	16	0.74	466	32	45	INVENTION
18	1180	950	440	6.8	15	0.75	467	32	37	EXAMPLE
19	1180	950	440	7.1	15	0.76	465	33	34
20	1180	950	440	1.6	26	0.63	466	35	139	COMPARATIVE
21	1160	800	400	0.6	45	0.64	478	32	144	EXAMPLE
					(EXPANDED
					GRAIN)
22	1160	950	350	0.7	36	0.66	468	34	137
					(EXPANDED
					GRAIN)
23	1160	950	440	3.5	16	0.73	478	33	42	INVENTION
24	1160	950	500	3.5	17	0.72	481	32	43	EXAMPLE
25	1160	950	650	2.3	25	0.65	467	31	146	COMPARATIVE
					(EXPANDED					EXAMPLE
					GRAIN)

Claims

1. A ferritic stainless steel sheet comprising: a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities; and a structure in which a ferrite crystal grain size is 20 μm or less, and a ratio [Nb]/[Ti] of a Nb content to a Ti content contained in a NbTi complex carbonitride present in ferrite crystal grain boundaries is in the range of 1 to 10.

2. The ferritic stainless steel sheet according to claim 1, wherein the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.

3. The ferritic stainless steel sheet according to claim 1, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.

4. A method for manufacturing a ferritic stainless steel sheet comprising:

performing hot-rolling of a slab having a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities at a finishing temperature of 900° C. or more and at a coiling temperature of 400 to 550° C.;

performing softening annealing of an obtained hot-rolled steel sheet;

performing pickling;

performing cold rolling; and

performing recrystallization annealing of an obtained cold-rolled steel sheet.

5. The method for manufacturing the ferritic stainless steel sheet according to claim 4, wherein the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.

6. The method for manufacturing the ferritic stainless steel sheet according to claim 4, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.

7. The method for manufacturing a ferritic stainless steel sheet according to one of claim 4, wherein a slab heating temperature is 1,000° C. or more.

8. The ferritic stainless steel sheet according to claim 2, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.

9. The method for manufacturing the ferritic stainless steel sheet according to claim 5, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.

10. The method for manufacturing a ferritic stainless steel sheet according to one of claim 5, wherein a slab heating temperature is 1,000° C. or more.

11. The method for manufacturing a ferritic stainless steel sheet according to one of claim 6, wherein a slab heating temperature is 1,000° C. or more.

12. The method for manufacturing a ferritic stainless steel sheet according to one of claim 9, wherein a slab heating temperature is 1,000° C. or more.