JP2001355049A - Martensitic stainless steel sheet and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel sheet having stable intermediate temperature region strength and small in blanking crack sensitivity and to provide its production method. SOLUTION: This steel sheet has a chemical composition containing, by mass, 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 and containing at least one kind selected from <=0.1% Ti, <=0.1% Al and <=0.1% Nb and 0.02 to 1.0% Mo, and the balance Fe with unavoidable impurities, has a metallic structure composed of ferrite and carbide, and in which the following MP value satisfies -18<=MP<=-7, and in the production, hot rolled sheet annealing is performed at 600 deg.C to less than the Ac1 transformation point, and then, cooling is performed to 500 deg.C at >=5 deg.C/sec: MP(%)=180[C(%)+N(%)]+3Mn(%)+6Ni(%)-3Si(%)-2Cr(%)-2 Mo(%)-Nb(%).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic system which exhibits excellent strength when used or processed in a medium temperature range of 300.degree. C. to 600.degree. C. and has low end face cracking susceptibility when punched. The present invention relates to a stainless steel plate and an inexpensive manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, martensitic stainless steel sheets have been widely used as components of vessels, tableware, blades, turbines and screws of ships, etc., from the viewpoint of their excellent corrosion resistance, hardenability, wear resistance and the like. It is used.

However, in recent years, martensitic stainless steel sheets have been used even in applications where ferritic stainless steel sheets, austenitic stainless steel sheets, or aluminum killed steel sheets are conventionally used from the viewpoint of cost reduction and functionality of materials used. Cases are increasing.

For example, materials for automobile exhaust gas systems such as exhaust manifolds and mufflers have excellent oxidation resistance and high temperature resistance because they are used in a high temperature environment exceeding 800 ° C. or in a wet environment due to condensation or the like. Corrosion and corrosion resistance are required, and a ferritic stainless steel sheet or an austenitic stainless steel sheet has been mainly used conventionally.

[0005] Automotive exhaust gas-related members such as an exhaust manifold, a converter case, and a muffler are formed by processing a plate-shaped stainless steel material for each part by press molding or roll forming and joining the joints by caulking or welding. It is generally formed into a pipe shape. At one end of such a pipe-like member, a plate-like metal fitting called a so-called flange is joined by welding or the like. A hole corresponding to the outer diameter of the pipe is formed in the flange by punching or cutting in order to join the pipe-like member.
More than one place has been created. Pipe-shaped parts 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 to 600 ° C. than the exhaust gas material body, has less influence of dew condensation, etc., has a corrosive environment, and has a corrosive environment. Therefore, a martensitic stainless steel sheet that is less expensive than a ferritic stainless steel sheet or an austenitic stainless steel sheet is used, and material cost is reduced.

As described above, the flange portion is for mechanically joining the automobile exhaust gas member formed into a pipe shape, but the joining is not necessarily performed in a linear joining form (each shaft center of the flange and the pipe). Are arranged in a line in a straight line), and some stress such as bending stress or the weight of each member is applied to the flange portion even at normal temperature. When this is incorporated into an automobile, a thermal stress is repeatedly applied to the flange portion every time the automobile runs and stops, in addition to the stress described above, and furthermore, a stress due to engine vibration or the like is applied. Therefore, such a flange material is 30
The material must be able to withstand various stresses in the temperature range of 0 to 600 ° C.

[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 to 600 ° C. If the material strength in this temperature range is weak, the flange buckles or deforms during running, exhaust gas partially leaks, oxidation of the flange material due to local temperature rise, and corrosion of the flange material due to crevice corrosion And other troubles may occur. Therefore, as a countermeasure from the structural aspect, the thickness of the flange material is 4 mm to 8 mm to secure the strength of the joint,
Usually, a thick material of 6 mm or more is used.

[0009]

However, recently, there has been a demand for such a flange material for automobile exhaust gas to ensure a higher level of strength. In other words, from the viewpoint of improving the material yield or reducing the weight of the automobile, the thickness of the flange material is required to be reduced, and accordingly, the demand for a material having a higher strength in the middle temperature range than the current state is increasing. .

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 may be considered as a countermeasure. By the way, a general method for manufacturing a martensitic stainless steel sheet as described above comprises the following steps (i) to (iv). (i) A slab is formed by continuously casting molten steel to which a predetermined alloy element has been added and the components have been adjusted. (ii) The slab is hot-rolled under predetermined heating conditions and rolling conditions to obtain a hot-rolled steel strip having a predetermined thickness. (iii) The obtained hot-rolled steel strip is subjected to a heat treatment in a box annealing furnace at a temperature range of 700 ° C. or more for the purpose of annealing. (iv) After the heat treatment, the surface scale is removed by pickling, and if necessary, slit to a predetermined size and shipped as a product.

As described above, in order to maintain excellent material strength in an intermediate temperature range of 300 ° C. to 600 ° C. (also referred to simply as a medium temperature range), that is, to have a certain level of material strength even at room temperature, The heat treatment step (iii) is extremely important in determining the strength of the material.

Generally, in a material having an Ac ₁ transformation point such as a martensitic stainless steel sheet, the annealing of the hot-rolled sheet is shortened by a continuous line like a high-purity ferritic stainless steel sheet or an austenitic stainless steel sheet. You can't do it in time. This is because the annealing becomes incomplete. Therefore, performing box annealing as shown in the above step (iii) is a commonly used means at present.

Usually, the martensitic stainless steel sheet used for such an application is 700 to 85 in view of production cost, equipment life of an annealing furnace, or material strength after annealing.
Generally, long-time annealing for 4 to 8 hours is performed in a temperature range of 0 ° C. Therefore, it is considered that the annealing 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 room temperature.

However, when annealing is performed under such conditions, the difference in the material properties of the obtained martensitic stainless steel sheet in the width direction and in the longitudinal direction becomes very large. It has been found that the strength may exceed an acceptable range.

That is, the hot-rolled steel sheet obtained by hot rolling has a very large capacity of 8 to 18 tons at a normal industrial level. When the low-temperature short-time annealing is performed in the steel sheet, the reached temperature and the holding time in the width direction and the longitudinal direction of the steel sheet become more non-uniform than before, and therefore, the difference in material properties in the steel sheet after annealing becomes very large.

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 characteristic difference in the steel sheet cannot be easily avoided even if the annealing temperature or the like is changed in the box-type annealing furnace.

Further, 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. In general, when the heating temperature and finishing temperature of hot rolling are lowered, the probability of surface defects such as scratches and nicks on the steel sheet surface increases, and such surface defects increase. When this occurs, it is necessary to remove surface defects by grinding the steel sheet surface or the like, which is not preferable from the viewpoint of material yield or an increase in the number of steps.

[0018] As means for solving such a problem, for example, in JP-A-9-249942, Ac ₁ point +100 ° C. or higher
There is disclosed a method of heating and maintaining a temperature in a two-phase region of ferrite + austenite of 1200 ° C. or less and then cooling the temperature to 100 ° C. or less. The material obtained at this time consists essentially of a ferrite phase and a martensite phase, and is 450-500
It is a duplex stainless steel that exhibits high proof stress in the medium temperature range of ° C, and is used for aperture frames.

The invention disclosed in this publication relates to a base material for a television frame, and is an application requiring strength in a medium temperature range, similarly to a material for a flange portion, and is annealed in a box type annealing furnace. Therefore, no difference in properties occurs in the steel sheet.

Certainly, according to the method disclosed in the above-mentioned publication, the strength in the medium temperature range is extremely excellent. However, however, a composite structure of a soft ferrite phase and a hard martensite phase is obtained. Therefore, in the case of employing the above-described processing steps for the aperture frame and the flange material, in particular, in the case of employing the punching processing, there is a possibility that the following problems may occur.

That is, like the aperture frame material, the flange material used for joining the automobile exhaust gas material is usually formed into a predetermined size and shape by cutting, laser cutting, or punching a base material. The flange material is further formed by cutting, laser cutting, or punching a predetermined hole for joining a pipe and for penetrating a bolt. In this manufacturing process, from the viewpoint of being able to introduce equipment at relatively low cost and from the viewpoint of productivity, manufacturing by punching is common, but when such a punching process is performed, as shown in JP-A-9-249942, In the case of a material having a hard martensite phase in the base material, the punching die wears quickly, the frequency of die maintenance and die replacement increases, and this is a factor of hindering economy and productivity.

Further, in the fracture surface of the shearing process by punching, when the material breaks, stress is concentrated on the interface between the ferrite phase and the martensite phase, and cracks are generated from the interface, so-called end face cracking may occur. Get higher.

In practice, when a martensitic stainless steel plate manufactured by a conventional manufacturing method is also punched, this end face cracking often occurs. Conventionally, a martensitic stainless steel sheet has been used in a temperature range of 700 to 850 ° C. for 4 to 8 minutes, particularly from the viewpoint of securing material strength.
As described above, it is common to perform long-time annealing for a long time. The thus obtained martensitic stainless steel sheet has a ferrite structure remaining from the slab stage during casting and a martensite phase generated in a cooling process after hot rolling has been decomposed into ferrite and carbide by annealing. It is characterized by a layered structure of ferrite and carbide. Further, it is becoming clear that such a material having a layered structure has high end face cracking susceptibility when punching is performed, and this 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 and longer time than in the past, the layered structure changes to a sized structure, and end face cracking tends to be suppressed.

However, in the case of a martensitic stainless steel sheet having such a grain-sized structure, the properties at room temperature after annealing are significantly softened, so that the strength in the medium temperature range is greatly deteriorated.

[0025] Further, as for the knowledge about the problem at the time of punching a martensitic stainless steel sheet, see Japanese Patent Application Laid-Open No.
No. 9458 discloses a martensitic stainless steel sheet with a small amount of blanking, characterized by appropriately adjusting the cold-rolled base material and the cold-rolling rolling reduction. End face cracking at the time of punching is not improved, and it is premised that cold rolling is performed. Therefore, if it is applied to the martensitic stainless steel sheet in the present invention, the number of steps increases, and the production cost increases, which is not preferable.

Therefore, the strength in the middle temperature range, which is more excellent than conventionally required, can be stably secured without variation between the coils, and when punching is performed, there is a problem at the time of processing such as end face cracking as described above. At present, no martensitic stainless steel sheet satisfying both conflicting performances, in which no cracking occurs, has not yet been obtained.

[0027] It is an object of the present invention to provide a martensitic stainless steel sheet having excellent strength in a medium temperature range of 300 to 600 ° C and having reduced end face cracking susceptibility, and a method for producing the same.

Still another object of the present invention is to provide a martensitic stainless steel sheet which has a small variation between coils, has a stable strength at a middle temperature range, and has a small susceptibility to punching cracks, which has not been obtained conventionally. It is to provide a manufacturing method.

[0029]

In order to secure the conflicting performances as described above, the present inventors made an effort to secure the strength in the middle temperature range by stabilizing the chemical composition of the martensitic stainless steel sheet. As a result of various studies on the heat treatment conditions, the following conclusions were reached. (1) By adjusting the chemical composition of the martensitic stainless steel sheet within a certain range, annealing can be performed in a relatively short time without performing long-time annealing such as conventional box-shaped annealing. (2) The martensitic stainless steel sheet that has been annealed in such a short time can secure a higher strength level than that obtained by annealing in a conventional box-shaped annealing furnace.

That is, since the conventional martensitic stainless steel sheet has an Ac ₁ transformation point, short-time annealing in a continuous line may result in (i) insufficient annealing and the material may not be softened enough to withstand use. (Ii) In the case where annealing is performed at a temperature exceeding the Ac ₁ transformation point, the steel sheet is rapidly cooled thereafter, resulting in a hardened state, and the material strength is significantly increased. However, we have:
By adjusting the alloying elements within a certain range, they have found that sufficient annealing can be achieved even when annealing is performed at a temperature equal to or lower than the Ac ₁ transformation point.

The martensitic stainless steel sheet whose composition has been adjusted as described above can be used without using a conventional box-shaped annealing furnace.
Since annealing in a continuous annealing pickling line becomes possible,
The strength variation between the coils is smaller than before, and a higher strength level than before can be secured.

Next, the present inventors have conducted intensive studies to clarify the mechanism of occurrence of end face cracks generated during punching, and have reached the following conclusions. (1) When the structure of a martensitic stainless steel sheet having a layered structure was observed in detail after annealing of the hot-rolled sheet, there was a structurally non-uniform portion. 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. (2) End face cracks tend to occur preferentially along grain boundaries of different forms as described above. (3) The structural unevenness is caused by 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 low precipitation density of carbides, and have an austenite phase during hot rolling and a 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-uniformity of the structure of the conventional martensitic stainless steel sheet having a layered structure is caused by the difference between the ferrite structure from the solidification stage and the prior austenite structure. 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 end face susceptibility is very high. In other words, the ferrite structure has low strength from the solidification stage where the crystal grains are coarse, and the strength is high in the old austenite structure where the crystal grains are fine (the part that became a martensite phase in the cooling process and decomposed into ferrite and carbide during annealing). It was confirmed that shear stress was concentrated on the crystal grain boundaries, 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 performed while ensuring sufficient temperature and time, unevenness in structure is eliminated and a uniform ferrite grain size structure is obtained, so that end face cracking is suppressed. However, if such a method is used, 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 a more detailed study from the viewpoint 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 the heat treatment. It has been found that if C contained in the steel is present in a state of being dissolved in the steel to some extent, the bonding force of the crystal grain boundaries is strengthened, and it has been found that it is very effective against end face cracking.

In order for C to be in a solid solution state in the steel, it is necessary to increase the cooling rate after annealing of the hot-rolled sheet. If sufficient, C can be sufficiently left in the steel in a solid solution state.

That is, by adjusting the components of the martensitic stainless steel sheet to a certain range, annealing in a continuous annealing pickling line, which could not be performed in the past, becomes possible, and a higher strength level than in the conventional method is reduced between coils. Pressing can be secured stably, and since C remains in the solid solution state in the steel, the bonding force of the crystal grain boundaries is strengthened,
They also found that the susceptibility to edge cracking during punching was reduced.

The present invention has been obtained based on such findings, and its gist lies in the following martensitic stainless steel sheet and its manufacturing method. (1) In mass%, C: 0.02% or more and 0.10% or less, Si: 1.0%
Mn: 0.05% or more and 1.0% or less, S: 0.01% or less,
P: 0.1% or less, Cr: 10.5% or more and 13.5% or less, N: 0.05
%, At least one of Ti: 0.1% or less, Al: 0.1% or less, and Nb: 0.1% or less, and Mo: 0.02% or more and 1.0% or less, with the balance being Fe and inevitable parts. It has a chemical composition consisting of various impurities, is systematically composed of ferrite and carbide, and has the following MP value of −18 ≦ MP ≦
A martensitic stainless steel sheet having excellent strength in a medium temperature range, satisfying a range of −7.

MP (%) = 180 [C (%) + N (%)] + 3Mn (%) + 6Ni (%)-3S
i (%)-2Cr (%)-2Mo (%)-Nb (%) (2) The chemical composition is further in mass%, Ni: 0.01% or more
The martensitic stainless steel sheet according to the above (1), containing 0.5% or less.

(3) The chemical composition further comprises, by mass%,
0.0005 to 0.005%, Mg: 0.0005 to 0.005%, La: 0.0
02% to 0.05%, Ce: 0.002% to 0.05%, and Y: 0.00
The martensitic stainless steel sheet according to the above (1) or (2), containing at least one element selected from the group consisting of 2% to 0.05%.

(4) The chemical composition further comprises, by mass%,
Any of the above (1) to (3) containing at least one element selected from the group consisting of B: 0.0001% to 0.010%, V: 0.02% to 0.30%, and W: 0.02% to 1.0%. The martensitic stainless steel sheet according to 1.

[0043] (5) below A ₁ value is excellent excellent and punching crack sensitivity to the intensity of the intermediate temperature range, characterized in that it contains solid solution C in the range satisfying the A ₁ value ≧ 20 (MPa) (1 No)
The martensitic stainless steel sheet according to any of (4).

A ₁ value = YS _AGE- MS _RT 8 _% YS _AGE : 8% tensile strain + 300 ° C. × 30 minute proof stress MS _RT 8%: Material strength when 8% tensile strain is applied at room temperature (6) A hot-rolled sheet having the chemical composition according to any of the above (1) to (4) is subjected to hot-rolled sheet annealing at a temperature not lower than 600 ° C and not exceeding the Ac ₁ transformation point, and thereafter up to 500 ° C. 5 ℃
A method for producing a martensitic stainless steel sheet, characterized by cooling at a cooling rate of at least / sec.

[0045]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail. In the following, chemical composition% means% by 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. In order to suppress the end face cracks and secure the strength in the medium temperature range, an addition amount of 0.02% or more is necessary.

However, if the content exceeds 0.10%, the corrosion resistance of steel deteriorates, which is not preferable. Therefore, the upper limit of the C content is specified to be 0.10% or less. 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 workability deteriorates with an increase in the added amount. Therefore, the Si content is set to 1.0% or less. Preferably,
0.8% or less. Mn: Since Mn has a deoxidizing effect on steel, it is 0.1%.
It is contained in the range of not less than 05% and not more than 1.0%. Content 0.05
%, The 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 it exceeds 1.0%, it becomes a starting point of rust and pitting corrosion, and not only deteriorates the corrosion resistance, but also increases the cost of steel and is disadvantageous from the economical point of view. Preferably, the lower limit is 0.1
%, And the upper limit is 0.8%.

S: Since S becomes a starting point of rusting and pitting and deteriorates corrosion resistance, it is preferable that S is as low as possible. If the content exceeds 0.01%, the corrosion resistance deteriorates, so the upper limit was made 0.01%.

P: It is desirable that the steel be as low as possible in order to reduce 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.
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, a content of 10.5% or more is required. Preferably, it is at least 11%. 13.5
If the content exceeds 30%, the cost will increase.
13.5%. Preferably, it is at most 13%.

Ni: Ni is added as necessary to improve the toughness of the steel. In order to stably obtain the effect of improving toughness, 0.01% or more is added. However, if added over 0.5%, the cost of steel will increase, so the upper limit is 0.5%.
And Preferably, it is 0.4% or less.

N: N has the effect of increasing the strength of steel, but if it exceeds 0.05%, the corrosion resistance of steel deteriorates. Therefore, the upper limit is set to 0.05%. Preferably, 0.03%
It is as follows.

Ti, Nb, Al: At least one of these elements is added to improve strength. These elements are C
And N and precipitate in steel as precipitates. The precipitation of these precipitates increases the strength of the steel, and ensures the strength in a medium temperature range.

However, if each content exceeds 0.1% alone, not only the cost of the steel increases, but also the solute C and N in the steel are excessively precipitated as precipitates, and the bonding strength of the crystal grain boundaries decreases. It is not preferable because end face cracking susceptibility during punching increases. Therefore, the upper limit of each element is 0.
1%.

Mo: Mo is added to increase the strength in the medium temperature range. In addition, Mo also has the property of generating a P compound and increasing the strength. To stably exert its effect, add 0.02% or more. Preferably, it is at least 0.1%. However, Mo is an expensive alloy element, and its upper limit is set to 1.0% from the viewpoint of suppressing cost increase. Preferably, it is 0.8% or less.

The other component elements are not particularly limited in the present invention, but in order to enhance the hot workability, one or more of Ca, Mg, La, Ce, and Y elements are required.
Seeds or more may be added.
One, two or more of the elements B, V and W may be added. In particular, B is effective for increasing the strength of the crystal grain boundary and thus suppressing end face cracking, and V and W are effective for more stably securing the strength in the medium temperature range.

The preferred content of each is 0.0005 to 0.005% for each of Ca and Mg, and 0.002 for each of La, Ce and Y.
% To 0.05%, B: 0.0001% to 0.010%, V:
0.02% to 0.30%, W: 0.02% to 1.0%.

(MP value) The MP value is an index indicating the systematic stability of a martensitic stainless steel sheet and is expressed by the following equation. In the present invention, the MP value is set so as to satisfy the range of −18 ≦ MP ≦ −7. The steel composition is adjusted.

MP (%) = 180 [C (%) + N (%)] + 3Mn (%) + 6Ni (%)-3S
i (%)-2Cr (%)-2Mo (%)-Nb (%) If the MP value is smaller than -18, the material tends to soften, and it is difficult to secure sufficient strength in the middle temperature range. Further, when the MP value is larger than -7, the strength of the material is remarkably increased, which is not preferable. Therefore, the MP value was specified in the range of −18 to −7. A preferred MP value is from -16 to -8.

(A ₁ value) The A ₁ value is an index simply representing the amount of solid solution C in steel, and in the present invention, the material strength when a steel is subjected to a tensile strain of 8% at normal temperature ( MS _RT8% ) and the tensile strength (YS _AGE ) after aging heat treatment.
(YS _AGE- MS _{RT 8%} ). The conditions of the aging heat treatment are as follows:
Although not specifically defined in the present invention, 200 ° C.
In general, it is carried out in a temperature range of up to 400 ° C. for a holding time of about 10 minutes to 1 hour.

That is, the aging temperature is set to 200 ° C. in order to surely fix solid solution C to dislocations given by 8% tension.
The above is appropriate, and 400 ℃ to suppress the recovery of dislocation
The reaction is performed in the following temperature range. In this temperature range, the retention time is 10 minutes to 1 hour, preferably 20 minutes to 1 hour, in order to surely fix solid solution C to dislocations.

Since it is known that the accuracy as a measured value of the amount of solute C is deteriorated if the above condition is not satisfied, the aging heat treatment condition is set to 300 ° C. × 30 minutes in the present invention. Such conditions are measured using the A ₁ value = YS _AGE -
_If the value of MS _RT8% is large, the amount of solid solution C in the steel is large, and it can be said that it is effective for suppressing end face cracking and increasing the strength in a medium temperature range.

[0063] To obtain a further improved effect in the present invention shall not be less than 20Mpa the A ₁ value. Preferably it is 30 Mpa or more. In order to produce the martensitic stainless steel sheet according to the present invention exhibiting such excellent effects, for example, as described above, continuous casting, slabs by conventional means of hot rolling, and then producing a hot-rolled sheet. The following hot-rolled sheet annealing and cooling may be performed.

(Hot Rolled Sheet Annealing)
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 of _one transformation point or more, a martensite phase is generated 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 and the Ac ₁ transformation point −30 ° C. or lower.

In the present invention, the annealing time of the hot-rolled steel sheet is not particularly limited. However, in an actual production level, the annealing time is usually 10 minutes or less, which is a much shorter time than in the past. Temperature.

(Cooling rate) 500 hours after the completion of hot-rolled sheet annealing
Cool to 5 ° C at a cooling rate of 5 ° C / sec or more. 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. In the present invention, the upper limit of the cooling rate is not particularly limited, but is limited to about 30 ° C./sec from a practical viewpoint.

After cooling to 500 ° C. in this manner, there is no particular limitation. For example, it may be cooled to room temperature by air cooling. By doing so, a metal structure composed of ferrite and carbide can be obtained.

[0068]

Next, the present invention will be described in more detail with reference to examples.

(Example 1) In the actual production process, steels having the respective component compositions shown in Table 1 were smelted, slabs having a thickness of 200 mm were produced by continuous casting, and heated at 1200 ° C for 1.5 hours. A hot-rolled steel sheet having a thickness of 5.0 mm was obtained by hot rolling.

Then, from each hot-rolled steel sheet, a thickness of 5.0 mm × a width of 30 mm
Sample several hot-rolled sheets of 0 mm x 100 mm in length,
The hot rolled sheet was annealed at 800 ° C for 10 minutes in a laboratory box-type electric furnace, and then cooled to 500 ° C at a cooling rate of 15 ° C / sec. Thereafter, air cooling was performed.

From each heat-treated material thus obtained,
Each of the 13B test pieces specified in JIS Z2201 from the ° direction
The samples were collected and subjected to a tensile test at room temperature by a method specified in JIS Z2241 to measure the strength at room temperature.

In order to measure the A ₁ value with the remaining one, 8%
The material strength (MS _RT8% ) when the tensile strain was _applied was measured, and then, aging heat treatment was performed in a laboratory box-type electric furnace at 300 ° C for 30 minutes, and then a tensile test was performed as before. the aging after yield strength (YS _AGE) was measured to determine the a ₁ value = YS _AGE -MS _RT8%.

In order to measure the strength of the steel in the medium temperature range, a plate-like high-temperature tensile test specimen specified in JIS Z2201 was also sampled from the 90 ° direction of rolling, and the stress and heat history acting on a processed product such as a flange were measured. After applying a tensile strain of 7.5% to easily reproduce the above, heat-treating at 550 ° C for 30 minutes, the air-cooled test specimen was subjected to a tensile test at a temperature of 450 ° C by the method specified in JIS G0567. Then, the strength at 450 ° C. was determined. The higher the 450 ° C strength, the better the strength in the medium temperature range.

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. Using a die with a diameter of 10.8mm (clearance 8%), punch three consecutive sheets under non-lubricated condition, visually observe the fractured base material on the punched side after punching, An evaluation was performed. Those having end face cracks were cut out, the cross sections of the cracks were observed with an optical microscope, and the crack depth was measured.

In order to confirm the effect of the elements generally added, 90 ° bending was performed on some test materials under the condition of bending radius = 0.4 × plate thickness (bending radius = 2.0 mm per 5.0 t of plate thickness). After processing, the end and the presence or absence of cracks in the base material were observed, and the workability was evaluated. Further, the pitting potential of the base material was measured by the method shown in JIS G0577, and the corrosion resistance was compared. The results are shown in Table 2.

As is clear from the above, the martensitic stainless steel sheet of the present invention has an A ₁ value of 20 Mpa or more, a higher strength at 450 ° C. than the conventional method (comparative example), and a stable strength of 420 Mpa or more. Can be secured. Also, the end face cracking at the time of punching was slight, and only slight end face cracks occurred in Nos. 2 and 11, but no end face cracks occurred.

On the other hand, No. 15 in which the MP value is smaller than -18
And No. 17 is not preferable because the strength at 450 ° C. does not satisfy 400 MPa. In Nos. 16 and 19, in which the MP value was larger than -7, the material strength was remarkably increased, fracture occurred at the time when aging strain was imparted, and the base material cracked during punching. Then, No.14 small amount of C, low strength of 450 ° C., and, for lower A ₁ value, the end surface cracking occurs. Low Mo
No. 18 and Nos. 20 to 22 with a large amount of Ti, Al and Nb added
Has a reduced strength at 450 ° C.

Nos. 23 to 29, in which C, Si, Mn, P, S, Cr and N are out of the applicable range of the present invention, cannot simultaneously satisfy the bending property (workability) and corrosion resistance. (Example 2) In the actual production process, steel having the composition shown in Table 1-No. 1 was melted, slabs having a thickness of 200 mm were formed by continuous casting, and heated at 1200 ° C for 1.5 hours. Thickness by cold rolling
It was a hot-rolled steel sheet of 5.0 mm.

Thereafter, a 5.0 × 30
Samples of hot-rolled sheets having a length of 0 x 100 mm were taken, and were subjected to annealing in a laboratory box-type electric furnace under the conditions shown in Table 3.
Cooling down to 500.

Thereafter, the characteristics under the respective heat treatment conditions were examined in the same manner as in Example 1. The results are shown in Table 3. As is obvious from, martensitic stainless steel sheet subjected to heat treatment in the present invention embodiment, A ₁ value is 20
MPa, the strength at 450 ° C can be stabilized and 420MPa or more can be secured. In addition, the end face cracking at the time of punching was slight, and no end face cracks occurred at all except for Nos. 2 and 5.

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. The annealing temperature is Ac ₁
The No.13 to be more point strength at room temperature is very high, it is difficult to intensity measurements of A ₁ value and 450 ° C. to be broken by the tensile 7.5%. Further, at the time of punching, a base material crack has occurred.

[0082] In addition, A ₁ value for the cooling rate is slower at No.12 drops end surface cracking occurs. For No. 15, since the annealing temperature is high and the cooling rate is slow, a grain-sized structure is formed, and the end face cracking during punching is suppressed, but the strength at 450 ° C decreases.

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

[0084]

[Table 1]

[0085]

[Table 2]

[0086]

[Table 3]

[0087]

The martensitic stainless steel sheet according to the present invention does not crack at the end face at the time of punching.
Since it has excellent strength in the middle temperature range of 00 to 600 ° C, it is also used in applications where processing is performed in the middle temperature range, such as a flange of an automobile exhaust gas or a frame base material of a flat screen television. Conventionally, long-time annealing using box-shaped annealing and then pickling were performed.However, annealing and pickling can be performed simultaneously in a continuous line, and box-shaped annealing can be omitted. For this reason, a material having extremely superior performance compared to the conventional one is provided at a much lower cost than the conventional one, and the industrial effect is great.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Shinji Tsuge 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka F-term in Sumitomo Metal Industries, Ltd. (reference) 4K037 EA01 EA02 EA05 EA09 EA12 EA14 EA15 EA17 EA18 EA19 EA23 EA25 EA27 EA31 EA32 EA33 EA36 EB06 EB08 EB09 FF02 FF03

Claims (6)

[Claims] 1. mass%, C: 0.02% to 0.10%, Si: 1.0% or less, Mn: 0.05%
% To 1.0%, S: 0.01% or less, P: 0.1% or less, Cr: 10.5% to 13.5
%, N: 0.05% or less, further Ti: 0.1% or less, Al:
0.1% or less, and Nb: at least one kind of 0.1% or less, and Mo: 0.02% or more and 1.0% or less, with the balance being a chemical composition consisting of Fe and unavoidable impurities. A martensitic stainless steel sheet comprising carbide and having the following MP value satisfying a range of −18 ≦ MP ≦ −7, and having excellent strength in a medium temperature range. MP (%) = 180 [C (%) + N (%)] + 3Mn (%) + 6Ni (%)-3Si (%)-2Cr (%)-
2Mo (%)-Nb (%) 2. The method according to claim 1, wherein said chemical composition further comprises Ni: 0.0% by mass.
The martensitic stainless steel sheet according to claim 1, containing 1 to 0.5%. 3. The chemical composition further includes, by mass%, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, La:
The martensitic stainless steel sheet according to claim 1 or 2, containing at least one element selected from the group consisting of 0.002% to 0.05%, Ce: 0.002% to 0.05%, and Y: 0.002% to 0.05%. 4. The chemical composition further comprising at least one member selected from the group consisting of B: 0.0001% to 0.010%, V: 0.02% to 0.30%, and W: 0.02% to 1.0% by mass%. The martensitic stainless steel sheet according to any one of claims 1 to 3, further comprising an element. 5. The steel according to claim 1, wherein said alloy contains solid solution C in a range in which the following Al value satisfies A ₁ value ≧ 20 (MPa).
A martensitic stainless steel sheet according to any one of the above. Al value = YS _AGE- MS _RT 8 _% YS _AGE : 8% tensile strain + 300 ° C x 30 minutes proof stress after heat treatment MS _RT 8%: Material strength when 8% tensile strain is applied at room temperature 6. A hot-rolled sheet having the chemical composition according to any one of claims 1 to 4 is subjected to hot-rolled sheet annealing at a temperature not lower than 600 ° C. and not exceeding an Ac ₁ transformation point.
A method for producing a martensitic stainless steel sheet, comprising cooling to 500 ° C. at a cooling rate of 5 ° C./sec or more.