JP2001131714A - Martensitic stainless steel sheet and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel sheet realizing the thinning of a flange of an exhaust gas member and the reduction of the material cost and excellent in punchability and strength in a medium temperature region and to provided a method for producing the same. SOLUTION: This steel sheet has a steel composition containing 0.02 to 0.10% C, <=1.0% Si, 0.05 to 1.0% Mn. <=0.01% S, <=0.1% P, 10.5 to 13.5% Cr and <=0.05% N, containing one or two or more kinds among Ti, Al and Nb respectively by <=0.1% and also by the quantity in the range satisfying the following inequality (1), and the balance Fe with inevitable impurities, has a structure of ferrite and carbides, and in which A1 value showing the content of solid solution C satisfies the following inequality (2): O<Ti/48+Al/27+Nb/93<C/12+ N/14...1(1), and Al value = YSAGE-YSRT>=20 Mpa...(2), where YSAGE: 8% tensile strain + proof stress after 300 deg.C×30 min heat treatment, and YSRT: proof stress at ordinary temperature. As for the producing method, a hot rolled sheet is annealed in the temperature range of 600 deg.C to less than the Ac, transformation point in a short time and is thereafter cooled to 500 deg.C at a cooling rate of >=5 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel sheet having a low end face cracking susceptibility at the time of punching and having an excellent strength in a middle temperature range of 300.degree. C. to 600.degree.

[0002]

2. Description of the Related Art Conventionally, martensitic stainless steel sheets have been used as members of turbines, screws, etc. of vessels, tableware, cutlery, ships, etc. from the viewpoint of their excellent corrosion resistance, hardenability, wear resistance and the like. I have. However, in recent years, martensitic stainless steel sheets have been used in applications where ferritic stainless steel sheets, austenitic stainless steel sheets, or aluminum killed steel sheets have been used, from the viewpoint of cost reduction and functionality of the materials used. It is increasing. For example, automotive exhaust system materials, such as exhaust manifolds and mufflers, are used in high-temperature environments exceeding 800 ° C,
In order to be used in a wet environment due to condensation or the like, excellent oxidation resistance, high-temperature corrosion resistance, and corrosion resistance are required. Conventionally, ferritic stainless steel sheets and austenitic stainless steel sheets are mainly used.

[0003] Automotive exhaust system components (such as exhaust manifolds, converter cases and mufflers)
(Hereinafter, simply referred to as the exhaust gas member) is a plate-shaped stainless steel material processed for each part by press forming or roll forming, etc., and the joint portion is joined by caulking or welding to form a pipe shape, and It is common to do.

At one end of such a pipe-shaped member, a plate-shaped metal fitting called a so-called flange is joined by welding or the like. Holes corresponding to the outer diameter of the pipe are made by punching or cutting in order to join the pipe-shaped member to the flange, and in addition to that, usually two or more holes for passing bolts are made. . The pipe-shaped members such as the exhaust manifold and the front pipe or the converter and the center pipe can be mechanically joined by bolts or the like by overlapping the flanges.

The metal material used for such a flange has a lower operating temperature range of 300.degree. C. to 600.degree. C. than the exhaust gas member main body, has less influence of dew condensation, etc., and has a corrosive environment and an exhaust gas member. A martensitic stainless steel sheet, which is less expensive than a main body and is less expensive than a ferritic stainless steel sheet or an austenitic stainless steel sheet, is used to reduce material costs.

As described above, the flange is for mechanically joining the exhaust gas member formed into a pipe shape, but the joining is not always a straight joining form. Some stress such as the own weight of each member is applied. When this is incorporated into an automobile, a thermal stress is applied to the flange each time the automobile runs and stops, in addition to the above-described stress, and furthermore, a stress due to engine vibration or the like is applied.

Therefore, the material constituting such a flange (hereinafter, referred to as a flange material) must be a material capable of sufficiently withstanding various stresses in a temperature range of 300 ° C. to 600 ° C. No.

[0008] That is, the most important performance required of the flange material is strength in the temperature range used, that is, 30%.
This is the material strength in the so-called medium temperature range of 0 ° C to 600 ° C. If the material strength in this temperature range is weak, the flange will buckle or deform during running, partially exhaust gas leaks, oxidation of the flange material due to local temperature rise, and corrosion of the flange material due to crevice corrosion, etc. There is a possibility that the malfunction of the above will occur.

Therefore, as a measure against the structure, the thickness of the flange material is set to 4 mm to secure the strength of the joint.
A thick material of 〜8 mm, usually 6 mm or more is used.

[0010]

However, recently, there has been a demand for such a flange of an exhaust gas member to secure a higher level of strength. In other words, from the viewpoint of improving the material yield or reducing the weight of automobiles, thinner flanges have been required, and the demand for materials with higher strength in the middle temperature range than that of the current state has been increasing. is there.

In general, the strength in the medium temperature range tends to be substantially proportional to the strength of the material at room temperature. Therefore, a simple method of increasing the strength at room temperature is considered to be a good countermeasure.

A general method for producing a martensitic stainless steel sheet as described above is as follows. (1) A molten steel to which a predetermined alloy element is added and whose components are adjusted is formed into a slab by continuous casting. (2) The slab is subjected to hot rolling under predetermined heating and rolling conditions to form a hot-rolled steel strip having a predetermined thickness. (3) The obtained hot-rolled steel strip is heat-treated in a box-type annealing furnace at a temperature of 700 ° C or more. (4) After the heat treatment, the surface scale is removed by pickling, and if necessary, slit to a predetermined size and shipped as a product.

Here, in order to maintain excellent material strength in a temperature range of 300 ° C. to 600 ° C., in other words, to have a certain level of material strength at room temperature,
The heat treatment step shown in (3) is extremely important in determining the strength of the material.

Generally, for a material having an Ac ₁ transformation point such as a martensitic stainless steel sheet, the hot-rolled sheet annealing is performed at a high temperature and a short time in a continuous line like a high-purity ferritic stainless steel sheet or an austenitic stainless steel sheet. It is difficult to perform it in time, and performing box-shaped annealing as shown in the above (3) is a commonly used means at present.
Usually, the martensitic stainless steel sheet used for such an application has a long time of 4 to 8 hours in a temperature range of 700 ° C to 850 ° C from the viewpoint of manufacturing cost, equipment life of an annealing furnace, or material strength after annealing. Generally, annealing is performed. Therefore, it is considered that the temperature or the annealing time in the box-shaped annealing should be shifted to a lower temperature and a shorter time than the current one in order to have a certain or more material strength at normal temperature.

However, when annealing is performed under such conditions, the difference in material properties in the width direction and longitudinal direction of the obtained martensitic stainless steel sheet (hereinafter, referred to as
(Simply, the difference in properties) became very large, and in some cases, it was found that the strength could exceed the generally acceptable range of strength.

That is, the hot-rolled steel sheet has a very large capacity of 8 to 18 tons at a normal industrial level due to the hot rolling. When short-time annealing is performed, the ultimate temperature and holding time in the width direction and longitudinal direction of the steel sheet become more non-uniform than before, resulting in the phenomenon that the difference in properties of the material in the steel sheet after annealing becomes extremely large. It is.
In particular, in the case of a stainless steel sheet which is inferior in thermal conductivity as compared with general ordinary steel, the temperature difference between parts in the steel sheet tends to be very large.

Such a difference in characteristics in the steel sheet cannot be easily avoided even if the annealing temperature or the like is changed in the box type annealing furnace. As a method of increasing the material strength after annealing without changing the annealing conditions, a method of increasing the strength of the hot-rolled steel sheet itself by changing the hot-rolling conditions may be considered. For this purpose, by setting the heating temperature, finishing temperature or winding temperature in hot rolling lower than the current one, the dislocation density of the hot-rolled steel sheet increases, and the strength after annealing under the same annealing conditions as in the past. The general idea is that when the heating temperature or finishing temperature of hot rolling is lowered, the probability of surface defects such as scratches and nicks on the steel sheet surface increases, and the steel sheet surface is ground. For this reason, it is necessary to remove surface defects, which is not preferable from the viewpoint of material yield or increase in steps.

[0018] As means for solving such a problem, for example, in JP-A-9-249942, Ac ₁ point +100 ° C. or higher
After heating and holding at a temperature in the two-phase region of ferrite + austenite of 1200 ° C or less, it is cooled to a temperature of 100 ° C or less, and substantially consists of a ferrite phase and a martensite phase.
An aperture frame using a duplex stainless steel material exhibiting high proof stress in a medium temperature range of ℃ to 500 ℃ is disclosed.

The present invention relates to a base material of a television frame, which is an application requiring strength in a medium temperature range as in the case of a flange material, and is not annealed in a box-type annealing furnace. No characteristic difference occurs.

However, according to the above-mentioned method, the strength in the medium temperature range is very excellent, but since the structure has a complex structure of a soft ferrite phase and a hard martensite phase, a flange forming process is required. May cause problems. The flange used for joining the exhaust gas members is usually formed by punching a base material into a predetermined size, and further, by punching or cutting a hole for a predetermined pipe connection, bolt penetration, or the like. As described above. Therefore, if a hard martensite phase is present in the base material, the punching die wears quickly, and the frequency of die care and die replacement increases, which is a factor in hindering economy and productivity. In addition, in the fracture surface of the shearing process by punching, when the material breaks, stress concentrates on the interface between the ferrite phase (also called ferrite structure) and the martensite phase (also called martensite structure), and cracks occur from the interface. That is, the possibility of occurrence of so-called end face cracks increases. Actually, even in a martensitic stainless steel sheet manufactured by a conventional manufacturing method, this end face cracking often occurs.

Conventionally, martensitic stainless steel sheets are generally subjected to long-time annealing for 4 to 8 hours in a temperature range of 700 ° C. to 850 ° C., particularly from the viewpoint of securing material strength. As described above. The martensitic stainless steel sheet obtained in this way has a ferrite structure remaining from the slab stage during casting,
It is characterized in that the martensite phase formed in the cooling process after hot rolling has a layered structure of a ferrite phase and a carbide formed by decomposition by annealing. Further, it is becoming clear that such a material having a layered structure has high end face cracking susceptibility at the time of punching and can be improved by heat treatment at a high temperature for a long time. That is,
When the hot-rolled sheet annealing is performed at a higher temperature for a longer time than before, the lamellar structure changes to a sized structure, and the end face cracks tend to be suppressed. However, in the case of a martensitic stainless steel sheet having such a grain-size-regulated structure, the properties at room temperature after annealing are significantly softened, so that the strength at a medium temperature range is greatly deteriorated.

[0022] Further, as for the knowledge regarding the defect at the time of punching a martensitic stainless steel sheet, see Japanese Patent Laid-Open No. 10-25 / 1998.
No. 9458 discloses a martensitic stainless steel sheet with a small amount of blanking, which is characterized by appropriately adjusting the cold rolling base material and the cold rolling reduction ratio. End face cracking is not improved, and it is premised that cold rolling is performed. Therefore, application to a martensitic stainless steel sheet is not preferable because it increases the number of steps and the manufacturing cost.

Accordingly, a martensitic stainless steel sheet having no contradictory performance that does not cause such troubles at the time of processing such as end face cracking and secures superior strength in the middle temperature range more than conventionally required. Has not yet been obtained.

An object of the present invention is to provide a martensitic stainless steel sheet excellent in punching properties and strength in a medium temperature range and a method of manufacturing the same, which could not be obtained conventionally, by conducting a more detailed study on the components and annealing conditions. To provide.

[0025]

In order to secure the conflicting performances as described above, the present inventors first worked diligently to clarify the mechanism of generation of end face cracks generated during punching. Researched.

As a result, the following conclusions have been reached. (i) When the structure of a martensitic stainless steel sheet having a layered structure was observed in detail after annealing of a hot-rolled sheet, there were parts that were uneven in structure. That is, it was confirmed that a crystal grain having a large crystal grain and a small carbide precipitation density and a crystal grain having a small crystal grain size and a large carbide precipitation density were adjacent to each other. (ii) End face cracks tend to occur preferentially along grain boundaries of different morphologies as in (i) above. (iii) The structural unevenness is due to the history of the base material prior to hot rolled sheet annealing. That is, from the solidification stage, those having a ferrite structure have large crystal grains after annealing of a hot-rolled sheet and have low carbide precipitation density, and have austenite phase during hot rolling and martensite phase during cooling after hot rolling. The crystal grains having a small crystal grain size and a large carbide precipitation density after the hot-rolled sheet annealing are obtained.

That is, the non-uniform organization of the conventional martensitic stainless steel sheet having a layered structure is caused by the difference between the ferrite structure and the prior austenite structure from the solidification stage. Under the sheet annealing conditions, the martensitic phase in which the former austenite structure is transformed only decomposes into fine ferrite and carbide in situ, and from the viewpoint of the subsequent growth of crystal grains and diffusion of carbides, both the temperature and the time This is a state in which the effects of solidification or hot rolling remain as they are even after hot-rolled sheet annealing. In the structure in which the history before annealing remains as described above, the strength differs for each crystal grain, and as a result, the susceptibility to end face cracking is extremely high.

That is, the ferrite structure from the solidification stage where the crystal grains are coarse has low strength, and the old austenite structure where the crystal grains are fine (where the martensite phase becomes a martensite phase in the cooling process and decomposes into ferrite and carbide during annealing) has a low strength. It was confirmed that the shear stress was concentrated at the crystal grain boundaries due to the high temperature, cracks were generated along the crystal grain boundaries, and the cracks propagated to such crystal grain boundaries to form macroscopic end face cracks. .

Therefore, if annealing is carried out with sufficient temperature and time, unevenness in structure is eliminated and a uniform ferrite grain structure is obtained, so that end face cracking is suppressed.

However, with such a method, the fact that the strength in the medium temperature range is greatly impaired is not an effective solution as described above. Therefore, the present inventors conducted further detailed studies in view of the bonding strength of the crystal grain boundaries after the heat treatment, in addition to the components of the martensitic stainless steel sheet, the heat treatment conditions, and the structure after that. As a result, by appropriately adjusting the components to be added, and further by specifying the heat treatment conditions at that time, the end face cracking at the time of punching is eliminated, and it is superior to the conventional one in the middle temperature range of 300 to 600 ° C. They have come up with a way to ensure strength.

That is, if C contained in the steel is present in a state in which the C is dissolved in the steel to some extent, the bonding force of the crystal grain boundaries is strengthened, which is very effective in suppressing the end face cracking.
If a certain amount of C exists in the steel in a solid solution state, 30
In the temperature range of 0 ° C. to 600 ° C., excellent strength can be secured by the effect of so-called strain aging.

Further, such a martensitic stainless steel sheet can be easily manufactured by performing short-time annealing in a specific temperature range and then rapidly cooling without performing long-time annealing in a conventional box-type annealing furnace. For this reason, the conventional box-type annealing can be omitted, and a sufficiently excellent performance can be secured in the continuous annealing + pickling line, so that a material having characteristics superior to the conventional one can be manufactured at a lower cost.

The present invention has been obtained based on such findings, and its gist lies in the following martensitic stainless steel sheet.

That is, in the present invention, C: 0.02% by mass.
0.1% or less, Si: 1.0% or less, Mn: 0.05% or more and 1.0% or less, S: 0.01% or less, P: 0.1% or less, Cr: 10.5-13.5
%, N: 0.05% or less, one or two or more of Ti, Al, and Nb each in an amount of 0.1% or less and in an amount satisfying the following formula (1), with the balance being Fe and inevitable has a steel composition consisting of impurities, systematically ferrite and consists carbide, the intensity of the punching crack susceptibility is small intermediate temperature range, characterized in that a ₁ value satisfies the following expression (2) indicating the amount of solute C Excellent martensitic stainless steel sheet.

[0035] O <Ti / 48 + Al / 27 + Nb / 93 <C / 12 + N / 14 ··· (1) A 1 value _{= YS AGE -YS RT ≧ 20MPa ···} (2) However, YS _AGE: 8 % Tensile strain + 300 ° C x 30 minutes proof stress after heat treatment YS _RT : proof stress at room temperature In addition, another aspect of the present invention is that the hot rolled steel sheet having the above steel composition has an Ac ₁ transformation point of 600 ° C or more. This is a method for producing a martensitic stainless steel sheet, wherein annealing is performed for a short time in a temperature range not exceeding the above range, and thereafter cooling is performed to 500 ° C. at a cooling rate of 5 ° C./sec or more.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. In the following, the percentages of the chemical compositions mean mass% unless otherwise specified.

(Composition of Steel Sheet) C: C is an important element for enhancing the strength and the bonding strength of the grain boundaries. Add 0.02% or more to suppress end face cracks and ensure strength in the medium temperature range. However, 0.10
%, It is not preferable because the corrosion resistance of steel deteriorates. Therefore, the upper limit of the content of C is defined as 0.10%. The preferred C content is 0.03% or more and 0.08% or less.

Si: Si is a component effective as a deoxidizing agent for steel and improves the oxidation resistance of steel. However, if the content exceeds 1.0%, the content increases and the hardness increases and the workability deteriorates. Therefore, the Si content is set to 1.0% or less. The lower limit is not particularly limited, but generally 0.1% or more is added.

Mn: Since Mn has a deoxidizing effect on steel, it is contained in a range of 0.05% or more and 1.0% or less. If the content is less than 0.05%, deoxidation of the steel becomes insufficient and the cleanliness deteriorates, so the Mn content was set to 0.05% or more. On the other hand, if the content exceeds 1.0%, not only does MnS become a starting point of rust and pitting corrosion, thereby deteriorating the corrosion resistance, but also increases the cost of steel and is disadvantageous in terms of economy.

S: S is preferably as low as possible because it becomes a starting point of rusting and pitting and deteriorates corrosion resistance. If the content exceeds 0.01%, the corrosion resistance deteriorates, so the upper limit was made 0.01%. Preferably it is 0.008% or less.

P: As much as possible, it is desirable to lower the corrosion resistance and toughness of the steel. If the content exceeds 0.1%, the workability deteriorates, so the P content is set to 0.1% or less.
Preferably it is 0.05% or less.

Cr: Cr is a main component for maintaining corrosion resistance and oxidation resistance. Corrosion resistance and oxidation resistance improve as the Cr content increases. To ensure the desired corrosion resistance of steel 1
The content is 0.5% or more. On the other hand, if the content exceeds 13.5%, the manufacturability will deteriorate and the cost will increase.
13.5%.

N: N has the effect of increasing the strength of the steel, but if it exceeds 0.05%, the corrosion resistance of the steel deteriorates. Therefore, the upper limit is set to 0.05%. Preferably 0.035%
It is as follows.

Ti, Al, Nb: Ti, Al, Nb combine with C and N and precipitate in steel as carbide, nitride or carbonitride. The precipitation of these precipitates increases the strength of the steel, and ensures the strength in a medium temperature range. In order to stably obtain the effect of precipitation strengthening, one or more of Ti, Al, and Nb must be contained in excess of zero.
Therefore, it is necessary to satisfy O <Ti / 48 + Al / 27 + Nb / 93. The preferred content is one or more of each is 0.0001% or more. However, each 0.1
If it is contained in excess of%, the cost of steel increases, which is economically disadvantageous. Therefore, the upper limit of each of one or more of them is set to 0.1%.

The elements of Ti, Al and Nb are each 0.1%
Even if it is contained below, if it exceeds a certain amount with respect to the content of C and N, solid solution C and N in the steel will be depleted and the bonding strength of the grain boundary and the material strength will be reduced. Therefore these elements are Ti / 48 + Al / 2
7 + Nb / 93 <C / 12 + N / 14.

A₁value_:A₁The value simply shows the amount of solid solution C in steel.
The tensile strength of steel at room temperature (YS _RT) And 8%
Tensile strain is applied, and then after aging heat treatment
Strength (YS _AGE ) Is expressed as the absolute value of the difference.

The conditions for the aging heat treatment are not particularly limited in the present invention, but are generally carried out in a temperature range of 200 ° C. to 400 ° C. for a holding time of about 20 minutes to 1 hour.

That is, the aging temperature is suitably 200 ° C. or more in order to surely fix solid solution C to dislocations given by tension, and the aging temperature is 400 ° C. or less in order to suppress the recovery of dislocations. In this temperature range, the holding time is 20 minutes to 1 hour in order to securely fix the solute C to the dislocation.

It is generally known that if the above conditions are not met, the accuracy of the measured value of the amount of dissolved C is deteriorated. Therefore, in the present invention, the aging heat treatment conditions are 300 ° C. × 30 minutes. did.

A ₁ value measured under such conditions =
As the value of YS _AGE- YS _RT increases, the amount of solid solution C in the steel increases, which can be said to be effective in suppressing end face cracking and increasing strength in a medium temperature range.

To obtain a particularly remarkable effect in the present invention
The A ₁ value not less than 20 MPa, more preferably not less than 30 MPa.

(Hot rolled sheet annealing) Annealing of hot rolled sheet
Perform in a temperature range that does not exceed _one transformation point. If the annealing is performed in a temperature range lower than 600 ° C., the recrystallization of the steel is not performed and the strength of the annealed steel is extremely high, so that the workability is deteriorated. Also, Ac
_When annealing is performed in a temperature range exceeding _one point, a martensite phase is formed in a subsequent cooling step, and an end face crack occurs at the time of punching. The preferred temperature range is 700 ° C or higher (Ac ₁ transformation point −30
° C) or less.

In the present invention, the annealing time of the hot-rolled steel sheet is not particularly limited, but at an actual production level, the annealing temperature is usually 10 minutes or less, which is an extremely short annealing temperature compared with the conventional method. Become.

(Cooling rate) The cooling rate is specified to be 5 ° C./sec or more. If the cooling rate is less than 5 ° C./sec, C dissolved in the steel precipitates as carbides, so that it has no effect on suppressing end face cracking and the strength in the medium temperature range is reduced. A preferred cooling rate is 10 ° C./sec or more.

[0055]

Next, the present invention will be described in more detail with reference to examples. (Example 1) In an actual production process, steels having the respective component compositions shown in Table 1 were melted, formed into a slab having a thickness of 200 mm by a continuous casting method, and heated at 1200 ° C for 1.5 hours. 5.0 by rolling
mm hot-rolled steel sheet.

From each hot-rolled steel sheet, several hot-rolled steel sheets having a thickness of 5.0 mm × a width of 300 mm × a length of 100 mm were sampled, and heated at 800 ° C. for 10 minutes in a laboratory box-type electric furnace. The sheet was annealed and then cooled to 500 ° C. at a cooling rate of 15 ° C./sec to obtain a heat-treated material.

From each heat-treated material, JIS Z 2 rolled from 90 ° direction
Two 13B test pieces specified in 201 were sampled, and one of them was subjected to a normal temperature tensile test by the method specified in JIS Z2241, and the normal temperature proof stress (YS _RT ) and the normal temperature strength were measured. . Also, the remaining one gives 8% tensile strain,
Aging heat treatment is again performed in a laboratory box-type electric furnace at 300 ° C for 30 minutes. After that, a tensile test is performed in the same manner as above, and the aging resistance (YS _AGE ) is measured, and A ₁ value = YS _AGE − I asked for YS _RT .

Further, in order to measure the strength of the steel in the middle temperature range, a plate-like high-temperature tensile test specimen specified in JIS Z2201 was similarly taken from the 90 ° rolling direction, and the stress acting on the flange was reproduced. After applying tensile strain to 7.5%, 450%
A tensile test was carried out at a temperature of ° C. according to the method specified in JIS G 0567, and a strength of 450 ° C. was obtained.

The higher the 450 ° C. strength, the better the strength in the medium temperature range. Further, as an evaluation of end face cracking at the time of punching, a punching test was performed using a punch having a diameter of 10 mm with a crank press. Use a die with a diameter of 10.8mm
(Clearance: 8%) Three sheets were continuously punched under non-lubricated conditions, and the base material fracture surface on the punched side after punching was visually observed to evaluate end face cracking. With respect to those having an end face crack, the section of the crack was observed with an optical microscope and the crack depth was measured.

In addition, in order to confirm the influence of the elements added in general, the bending radius of some of the test materials was 0.4 × the thickness of the sheet (bending radius = 2.0 mm per 5.0 t of sheet thickness).
° Bending and observe the end and base material for cracks,
The workability was evaluated.

The pitting potential of the base material was measured by the method shown in JIS G 0577, and the corrosion resistance was evaluated. The results are shown in Table 2.

In Table 2, the strength at 450 ° C. is preferably 400 MPa or more, which is a value after imparting 7.5% tensile strain, which simulates the workability of the flange portion. Punching property evaluation: ：: No end face cracking ○: Mild cracking (crack depth ≦ 0.5 mm) △: Cracked (0.5 mm <depth ≦ 1.0 mm) ×: Deep crack (1.0 mm <depth) ◎ and の, which are no problematic levels, were evaluated as good.

Bendability: ：: no cracks △: fine cracks on end face (width 0.5 mm or less) X: clear cracks on end face (width over 0.5 mm)

Pitting potential: ：: 20 mV or more. ×: less than 20 mV.

As is evident from these results, the martensitic stainless steel sheet according to the example of the present invention has an A ₁ value.
The strength at 450 ° C. is higher than that of the conventional method (comparative example), and the strength of 400 MPa or more can be stably secured. Also, the end face cracking during punching is mild,
o. In 2, 4, and 10, only slight edge cracks occurred, and no edge cracks occurred.

On the other hand, Ti / 48 + Al / 27 + Nb / 93 <C
/ 12 + N / 14 No. 14~17 which does not satisfy the relational expression is not less than A ₁ value is 20 MPa, low strength of 450 ° C., and the end surface cracking occurs during punching.

[0067] Then, No.18 small amount of C, A ₁ value is low, although the strength of 450 ° C. is satisfied, the end surface cracking occurs and does not include Ti, Al, Nb is No.19, the end face No cracks are generated, but strength cannot be secured by precipitation strengthening.
° C strength is decreasing. Also, C, Si, Mn, P, S,
Nos. 20 to 26 where Cr and N are out of the applicable range of the present invention are
(Workability) and corrosion resistance cannot be satisfied at the same time. (Example 2) In the actual production process, steels having the respective component compositions shown in No. 1 of Table 1 were melted, slabs having a thickness of 200 mm were produced by continuous casting, and heated at 1200 ° C for 1.5 hours. And hot rolling 5.
It was a hot-rolled steel sheet of 0 mm.

From this hot rolled steel sheet, a thickness of 5.0 mm × a width of 300 mm ×
Several hot-rolled sheet samples having a length of 100 mm were sampled, and were subjected to hot-rolled sheet annealing and 500
Cooling down to ° C. was carried out.

The characteristics under the respective heat treatment conditions were examined in the same manner as in Example 1 according to Example 1. The results are shown in Table 3. The evaluation indices are the same as in Table 2.

As is clear from the above, the heat-treated martensitic stainless steel sheet in the present invention example is as follows.
A ₁ value becomes 20MPa or more, and the strength at 450 ℃ is stable and 400M
Pa or more can be secured. In addition, the end face cracking at the time of punching was slight. In particular, No. 4, 6, and 8 did not have any end face cracks.

On the other hand, the annealing temperature is lower than 600 ° C.
No.11 is, A ₁ value shows a 48MPa and high value, high punching resistance strength at room temperature is deteriorated. No. 13 in which the annealing temperature is higher than the Ac ₁ point has extremely high room temperature strength, and breaks even when pulled by 7.5%, making it difficult to measure the A ₁ value and the strength at 450 ° C. Further, at the time of punching, deep end face cracks occurred.

[0072] In No.12 cooling rate A ₁ value is lowered end surface cracking occurs slower. For No.15, annealing the temperature is high and the cooling rate slower, the end surface cracking during punching becomes grain structure is being suppressed, reduction in strength of 450 ° C. for solute C content A ₁ value is low is small I do.

No. 16 is excellent in both the strength at 450 ° C. and the punching property, but is not preferred in the present invention since the annealing cost is the same as that of the conventional one due to the long box annealing.

[0074]

[Table 1]

[0075]

[Table 2]

[0076]

[Table 3]

[0077]

The martensitic stainless steel sheet according to the present invention does not crack at the end face during punching,
Since it has excellent strength in the middle temperature range from 0 ° C to 600 ° C, it can be applied to applications where processing is performed in the middle temperature range, such as flanges of automobile exhaust gas or base materials for flat screen television frames. In addition, conventionally, long-time annealing using box-type annealing and then pickling were performed.However, annealing and pickling can be performed simultaneously in a continuous line, and omission of annealing using a box-type annealing furnace is omitted. Therefore, a material having extremely superior performance compared to the conventional one is provided at a much lower cost than the conventional one, and has a great industrial effect.

Claims (2)

[Claims] C .: 0.02 to 0.10%, Si: 1.0% by mass
% Or less, Mn: 0.05% or more and 1.0% or less, S: 0.01% or less,
P: 0.1% or less, Cr: 10.5-13.5%, N: 0.05% or less, one or more of Ti, Al and Nb are each 0.1% or less and satisfy the following formula (1). containing an amount of the balance has a steel composition consisting of Fe and unavoidable impurities, organizationally it consists ferrite and carbides, the a ₁ value indicating the amount of solute C satisfies the following formula (2) A martensitic stainless steel sheet with low punch cracking sensitivity and excellent strength in the medium temperature range. O <Ti / 48 + Al / 27 + Nb / 93 <C / 12 + N / 14 ··· (1) A 1 value _{= YS AGE -YS RT ≧ 20Mpa ···} (2) However, YS _AGE: 8% tensile strain +300 ° C. × Strength after heat treatment for 30 minutes YS _RT : Strength at room temperature 2. The hot-rolled sheet having the steel composition according to claim 1 is annealed for a short time in a temperature range not lower than 600 ° C. and not exceeding the Ac ₁ transformation point, and thereafter up to 500 ° C. at 5 ° C./sec. A method for producing a martensitic stainless steel sheet, characterized by cooling at the above cooling rate.