JP2017171990A - Martensitic stainless steel sheet and metal gasket manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease reduction of strength and corrosion resistance due to tempering softening or sensitization in a martensitic stainless steel sheet.SOLUTION: There is provided a martensitic stainless steel sheet having a chemical composition consisting of, by mass%, C:0.100 to 0.180%, Si:0.20 to 1.50%, Mn:0.10 to 2.00%, Ni:0.01 to 1.00%, Cr:11.00 to 18.00%, Cu:0.20 to 1.60%, Mo:0 to 0.50%, N:0.030 to 0.100%, V:0 to 0.50%, Nb:0 to 0.50%, Ti:0 to 0.50%, B:0 to 0.020% and the balance Fe with inevitable impurities, with total content of C and N of 0.130% or more, C/N content ratio of 5.0 or less and γmax value of 80.0 or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a martensitic stainless steel plate suitable for a metal gasket, a method for producing the same, and a method for producing a metal gasket using the same.

  Engine cylinder head gaskets and exhaust manifold gaskets in automobiles, motorcycles, and the like are exposed to repeated pressure fluctuations under the high temperature, high pressure, and high vibration specific to the engine. In particular, since a cylinder gasket of an automobile engine is subjected to a high pressure during compression, it needs to be in contact with both contact counterparts with a high contact pressure (surface pressure) in order to maintain the sealing performance. A metal gasket used for an engine or an exhaust gas path is generally formed with a bead (continuous raised portion) having a certain height in order to ensure a sufficient contact pressure. The bead is usually formed by bead molding by a press. This type of metal gasket is required to have high strength and high corrosion resistance.

  Conventionally, work-hardening type metastable austenitic stainless steel (SUS301 series or the like) is frequently used for gaskets applied to automobile engines and exhaust gas paths thereof. This type of steel is intended to increase strength by generating work-induced martensite by cold rolling. However, it is necessary to increase the cold rolling rate in order to raise the strength level. An increase in the cold rolling rate is a factor that decreases toughness, fatigue resistance, and workability.

  In addition, a painted metal gasket may be used for industrial piping or the like. This type of metal gasket also requires coating film adhesion.

  On the other hand, there is martensitic stainless steel as a material for increasing the strength without depending on the increase in the cold rolling rate. Patent Document 1 describes that martensitic stainless steel is applied to a gasket. Patent Document 2 describes a technique for improving the fatigue characteristics of a gasket using martensitic stainless steel by optimizing the inclusion composition.

JP 2000-109957 A Japanese Patent Laid-Open No. 2006-6222

  Martensitic stainless steel has a lower raw material cost than metastable austenitic stainless steel containing a large amount of expensive Ni. Further, since there is no need to impart a high cold rolling rate and work hardening, problems of toughness reduction due to work hardening and anisotropy due to texture are unlikely to occur. According to the technique of Patent Document 2, fatigue characteristics are also improved. However, since the martensitic stainless steel plate is strongly influenced by the cooling rate in the heat treatment for obtaining high strength, it is not easy to make the material uniform for the entire steel plate. According to the inventors' investigation, when viewed under strict evaluation criteria, there is a decrease in strength, a change in mechanical properties, and a decrease in corrosion resistance, which are thought to be due to a decrease in solid solution C and N due to temper softening and sensitization. I found it could be a problem. These characteristics affect the performance of the metal gasket.

  In the metal gasket, making the hardness (strength) distribution of the beads uniform contributes to the maintenance of a stable surface pressure, which leads to improvement of leak characteristics and the like. In addition, reducing the variation in characteristics among metal gasket products is also important for ensuring stable quality. In order to meet these requirements, it is extremely advantageous to make the material properties uniform in the width direction and the longitudinal direction of the steel strip in the steel plate as the material. However, in the conventional disclosed technique, it has been difficult to sufficiently uniform the material characteristics in the width direction and the longitudinal direction when continuously producing a wide steel strip.

  An object of this invention is to reduce the fall of the intensity | strength by a temper softening and sensitization, and corrosion resistance in a martensitic stainless steel plate. In addition, the coating film adhesion is improved.

The strength of the martensite phase is strongly influenced by the components and heat treatment conditions.
[Influence of ingredients]
The higher the amount of interstitial elements C and N, the higher the strength. However, since excessive C and N contents cause a decrease in toughness, their contents are limited. Further, among C and N present in the steel, it is C and N dissolved in the martensite phase that contribute to the strength. Excessive formation of carbonitrides consumes C and N in steel excessively and causes strength reduction.
[Effect of heat treatment]
In martensitic stainless steel, the austenite phase is transformed into a martensite phase by cooling after heating to Ac ₁ point or higher, and high strength is obtained (quenching). The degree of strengthening by quenching is affected by the heating temperature and the cooling rate. In steels with poor hardenability (that is, hard martensite is less likely to be produced when the cooling rate is slow), slight fluctuations in operating conditions tend to lead to variations in properties when producing steel strips.
These are factors that make it difficult to make the material uniform in the steel strip.

  In stainless steel, there are a large amount of metal elements such as Cr and Fe, which are easily combined with C and N. Comparing the case where these metal elements form carbide and the case where nitride is formed, it has been found that the case where carbide is formed is greatly affected by the cooling rate. That is, if the cooling rate is the same, the carbide forming reaction proceeds more rapidly than the nitride forming reaction. Therefore, at a location where the cooling rate is relatively slow in the steel strip, the solid solution C is easily consumed for carbide formation, and the contribution to strengthening by the solid solution C is reduced. This is a factor causing variations in characteristics within the steel strip. On the other hand, since the nitride formation reaction does not proceed as rapidly as the carbide formation reaction, the abundance of solid solution N is easily maintained even at locations where the cooling rate is relatively slow, and the contribution to strengthening by solid solution N is unlikely to be lost. Therefore, in the present invention, a component design is performed based on the idea of making N contribute as much as possible to the strengthening of the martensite phase, thereby realizing a steel sheet with high uniformity of material characteristics. In the component design, it is important to limit the C / N content ratio and to optimize the content of alloy elements such as Cr, Mn, Cu, etc. to ensure a sufficient solid solubility limit of N.

That is, in the present invention, by mass%, C: 0.10 to 0.180%, Si: 0.20 to 1.50%, Mn: 0.10 to 2.00%, Ni: 0.01 to 1. 00%, Cr: 11.00 to 18.00%, Cu: 0.20 to 1.60%, Mo: 0 to 0.50%, N: 0.030 to 0.100%, V: 0 to 0 .50%, Nb: 0 to 0.50%, Ti: 0 to 0.50%, B: 0 to 0.020%, remaining Fe and inevitable impurities, and the total content of C and N is 0.5. Provided is a martensitic stainless steel sheet having a chemical composition of 130% by mass or more, a C / N content ratio (mass% ratio) of 5.0 or less, and a γmax value determined by the following formula (1) of 80.0 or more. Is done.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1).

  Of the steel component elements, Mo, V, Nb, Ti, and B are arbitrarily added elements. The hardness of the plate surface (rolled surface) of this steel plate is, for example, 400 to 570 HV. The plate thickness can be set to 0.05 to 0.5 mm, for example, and can be controlled to 0.1 to 0.3 mm. This steel plate is suitable for a metal gasket that is used by pressing the top of the bead against a contact partner material. "Bead head top" means the top of the bead convex portion in contact with the contact partner material.

The steel sheet has particularly good coating film adhesion, and has pits with an opening diameter of 1.0 μm or more on the surface with a number density of 10 pieces / 0.01 mm ² or more, and a surface roughness Ra in the direction perpendicular to the rolling direction. A steel sheet having a thickness of 0.5 μm or less is provided.

The pits are formed by the precipitation particles falling off during the pickling treatment after finish annealing. The precipitated particles are mainly M ₂₃ C ₆ (M is a transition metal element such as Cr) type carbide particles. Although carbonitride may be formed, in this specification, carbonitride including carbonitride is called. The surface roughness Ra is an arithmetic average roughness Ra defined in JIS B0601: 2013. The rolling perpendicular direction means a direction perpendicular to the rolling direction. The opening diameter of a pit means the diameter (major diameter) of the longest part of the opening surrounded by the outline of the pit in an SEM (scanning electron microscope) image in which the steel plate surface is viewed in the plate thickness direction.

  As a manufacturing method of the steel sheet, the intermediate product steel sheet having the above chemical composition is heated to a two-phase temperature range in which the austenite single-phase temperature range or the austenite phase + 20 vol% or less ferrite phase is in the range of 800 to 1100 ° C. After that, a manufacturing method is provided which includes cooling and transforming the austenite phase into a martensite phase (finish annealing step). More preferably, the cooling is performed under the condition that the average cooling rate from 800 ° C. to 200 ° C. is 1 to 150 ° C./sec.

  In particular, as a technique for obtaining a steel sheet having good coating film adhesion, heating in the finish annealing step is performed in an oxidizing atmosphere, and further, the surface oxidized scale is removed by pickling the steel plate after the finish annealing step. In addition, there is provided a method for producing a steel sheet having a step (pickling step) in which carbide particles present on the surface are dropped to form pits on the surface.

  According to the present invention, by making full use of N, which is less susceptible to the cooling rate than C, and strengthening the martensite phase, it is possible to make the material uniform by improving the quenching stability, as well as tempering softening resistance and sharpness resistance. It has become possible to reduce material changes due to improved chemical properties. As a result, when a metal gasket made of this steel plate is used, the contact surface pressure applied to the top of the bead is more uniformly maintained, and a metal gasket excellent in leak resistance is realized.

The graph which shows the relationship between C / N content ratio and hardness variation | change_quantity (DELTA) H.

[Chemical composition]
The chemical composition of the steel that is the subject of the present invention will be described. Hereinafter, “%” in the steel composition means “% by mass” unless otherwise specified.
C is an austenite forming element and is an element effective for strengthening the ferrite phase and the martensite phase. If the C content is too small, the above-described strengthening effect cannot be fully exhibited, and it is difficult to adjust the component (optimization of γmax) for controlling the austenite generation amount at a temperature of Ac ₁ point or higher to an appropriate range. This is disadvantageous in securing the amount of martensite. As a result of various studies, the C content needs to be 0.100% or more. However, excessive C content tends to cause grain boundary precipitation of Cr-based carbides in the cooling process from the austenite generation temperature range, and causes a decrease in corrosion resistance. In addition, the influence of the cooling rate fluctuation on the strength variation in the steel strip increases. The C content is adjusted in the range of 0.180% or less.

  N is an austenite-forming element and is effective for strengthening the ferrite phase and the martensite phase in the same manner as C. As described above, N is likely to remain in a solid solution state in the matrix even when the cooling rate during finish annealing is relatively slow as compared to C. Therefore, in order to suppress the strength variation in the steel strip, the present invention utilizes the quenching strengthening action by N. In this case, the N content needs to be 0.030% or more. Excessive N content causes nitrides to form during the cooling process after annealing, which causes a decrease in corrosion resistance and fatigue resistance. N content is limited to 0.100% or less.

  In order to stably secure the strength required for the metal gasket, it is necessary to secure a total content of C and N of 0.13% or more. In addition, the upper limit of the C content that easily affects the variation in strength due to the cooling rate is strictly limited as described above, and the N content is sufficiently secured. As a result of various studies, it was found that setting the C / N content ratio in mass% to 5.0 or less is extremely effective for improving the quenching stability. It is more effective to set the C / N content ratio to 4.0 or less.

  Si is added as a deoxidizer during steelmaking. It is necessary to add Si so that the Si content is 0.20% or more. However, Si dissolves in the ferrite phase and the martensite phase, and has a particularly large effect of hardening the martensite phase. Moderate hardening is effective for increasing the strength of the gasket, but excessive hardening is a factor that reduces workability and toughness. Further, excessive Si content induces hot cracking. The Si content is limited to a range of 1.50% or less.

  Mn is an austenite generating element and expands the austenite phase region at high temperatures. Increasing the Mn content is effective for increasing the martensite content. It is also effective for securing the solid solubility limit of N. The Mn content is 0.10% or more. However, when the Mn content increases, the austenite phase generated at a high temperature becomes stable, and the austenite phase that cannot be fully martensitic transformed during the cooling process to room temperature remains. As a result of various studies, the Mn content is set to a range of 2.00% or less. You may manage to less than 1.00%.

  Ni is an austenite-forming element and is effective in securing a sufficient amount of martensite. It is effective that the Ni content is 0.01% or more. However, if the Ni content is excessive, a residual austenite phase tends to exist, which is disadvantageous for improving the strength. The Ni content is limited to 1.00% or less, and more preferably 0.65% or less.

  Cr is an essential element for imparting the necessary corrosion resistance as stainless steel. It is also effective for securing the solid solubility limit of N. However, in order to secure a sufficient amount of martensite, it is necessary to increase the content of austenite forming elements such as C, N, Ni and Mn as the Cr content increases, resulting in an increase in steel material costs. It becomes a factor of toughness fall. In the present invention, steel with a Cr content of 11.00 to 18.00% is targeted.

  Cu is an austenite generating element and is effective in securing a sufficient amount of martensite. It is also effective for securing the solid solubility limit of N. It is effective that the Cu content is 0.20% or more. However, if the Cu content is excessive, a residual austenite phase is likely to be present, which is disadvantageous for improving the strength and ensuring hot workability. The Cu content is 1.60% or less.

  Mo is effective in improving the corrosion resistance, and can be added as necessary. In that case, it is more effective to set the addition amount to 0.10% or more. Excessive Mo addition increases costs. When adding Mo, it is performed within a range of 0.50% or less.

  V, Nb, Ti, and B are effective elements for improving manufacturability, strength, fatigue resistance, and the like. One or more of these may be added as necessary. V is 0.50% or less, Nb is 0.50% or less, Ti is 0.50% or less, and B is 0.020% or less. More effective content ranges are: V: 0.01 to 0.50%, Nb: 0.01 to 0.50%, Ti: 0.01 to 0.50%, B: 0.0005 to 0.5. 020%.

The content of each element is adjusted so that the γmax value determined by the following equation (1) is 80.0 or more.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the value of mass% of the element is substituted for the element symbol in the formula (1).
The γmax is an index representing the maximum amount of austenite (volume%) generated when the temperature is raised to a temperature range of Ac ₁ point or higher. In the steel in which the content of each element is in the above-described range, it can be seen that almost all of the austenite phase at a high temperature is transformed into a martensite phase in the course of cooling to room temperature. Therefore, the steel base (matrix) of the cold-rolled annealed material that is the subject of the present invention is an amount (volume%) in which the martensite amount is substantially equal to γmax, and the balance is the ferrite phase. When γmax exceeds 100, the steel substrate has an almost 100% martensite structure.

  If the proportion of the ferrite phase in the steel substrate is too large, it will be difficult to stably realize high strength suitable for a metal gasket. In addition, the strength difference between the ferrite phase and the martensite phase tends to cause cracks from the phase interface, increasing the anisotropy of workability and fatigue resistance. As a result of various studies, the present invention employs a steel composition in which γmax is 80.0 or more.

〔Production method〕
A typical manufacturing method is illustrated below. The steel adjusted to the above-mentioned chemical composition is melted by normal stainless steel making equipment to obtain a slab. The slab is hot-rolled in the same manner as in the production of a normal martensitic stainless steel plate to obtain a hot-rolled steel plate.

The hot-rolled steel sheet is annealed, and then cold-rolled to reduce the sheet thickness. If necessary, intermediate annealing is performed during cold rolling. The final product plate thickness may be, for example, 0.05 to 0.5 mm. Finish annealing is performed on the cold-rolled steel sheet having a predetermined final product thickness. The finish annealing temperature is an austenite generation temperature range of Ac ₁ point or higher. Specifically, it is preferable to heat to an “austenite single-phase temperature range” or “two-phase temperature range in which an austenite phase + 20 vol% or less ferrite phase” in the range of 800 to 1100 ° C. If it is the steel composition range prescribed | regulated by this invention, the austenite phase of the quantity corresponding to (gamma) max can be normally produced | generated in the temperature range of 900-1050 degreeC. The holding time at the finish annealing temperature may be set in the range of 0 to 60 seconds.

  After the finish annealing, almost all of the austenite phase is transformed into the martensite phase during the cooling process to room temperature. In general, the martensite phase is hardened by the solid solution of C and N in supersaturation and the presence of a large amount of dislocations. The greater the cooling rate during the martensitic transformation, the greater the degree of hardening and the higher the strength. However, the martensite phase produced during quenching has poor toughness and requires post-treatment such as tempering heat treatment. According to the study by the inventors, in order to obtain a martensitic structure with good toughness without performing post-treatment such as tempering, the “austenite single phase temperature range” or “the austenite phase + 20% by volume or less of ferrite phase” After heating to a “two-phase temperature range”, it is effective to cool at a relatively slow (not rapid) cooling rate. However, when the cooling rate is excessively slow, the solid solution amount of C and N decreases and the strength of the martensite phase is reduced. Further, when the amount of C and N, which are austenite-generating elements, decreases, the amount of ferrite phase generated easily increases, and the strength is thereby reduced. As a result of various studies, it is preferable to adjust the cooling rate so that the average cooling rate from 800 ° C. to 200 ° C. is 1 to 150 ° C./s. The cooling rate in this range is easily realized by air cooling, but water cooling can also be adopted. The cooling at the relatively slow cooling rate described above is effective not only for imparting toughness to the martensite phase, but also for the generation of carbides that will be the pit formation source described later.

In order to improve coating film adhesion, (i) finish annealing is performed in an oxidizing atmosphere such as in the air, and (ii) the average cooling rate from 800 ° C. to 200 ° C. is 1 to 150 ° C./s, iii) An annealing pickling process in which descaling is performed in the subsequent pickling is extremely effective.
An oxide scale is formed on the surface of the steel sheet by heating in an oxidizing atmosphere. When the steel plate in this state is cooled at a cooling rate of 1 to 150 ° C./s from 800 ° C. to 200 ° C., the time margin for precipitation of carbides during the cooling process increases, and the matrix (metal A textured state in which spherical carbide particles are dispersed in the substrate is obtained. When pickling for the purpose of descaling is applied to an annealed steel sheet in which spherical carbide particles are dispersed, the spherical carbide particles that exist directly under the scale on the steel sheet surface are likely to fall off the steel sheet surface as the scale is removed. I understood it. A circular pit is formed as a drop mark at a portion where the spherical carbide particles are dropped by pickling. The circular pits exert an anchor effect on the coating film, and the coating film adhesion is improved.

The pickling for forming the falling marks of the carbide particles may be performed in the same manner as the pickling for descaling. For example, typical pickling techniques such as (a) electrolysis using a neutral salt, sulfuric acid, nitric acid, etc. and (b) immersion in a mixed acid bath of hydrofluoric acid and nitric acid can be mentioned. Either one or both of the above (a) and (b) can be employed. When the pickling conditions are increased, so-called peracid cleaning is performed, and the surface roughness increases due to dissolution of the matrix (metal substrate). In terms of coating film adhesion, it is generally advantageous that the surface roughness is large. However, if the surface roughness is excessive, the characteristics (workability, fatigue resistance, sealability) required for the gasket material are reduced. Therefore, in the present invention, by making the surface form in which the above-mentioned pits (drop-off marks) are dispersed in the surface having a metal substrate with high smoothness, both the characteristics required for the gasket material and the coating film adhesion can be achieved. Plan. Specifically, it has pits with an opening diameter of 1.0 μm or more formed on the surface at a number density of 10 / 0.01 mm ² or more formed by dropping the precipitated particles in the pickling treatment after finish annealing, and rolling. It is desirable to have a surface form in which the surface roughness Ra in the perpendicular direction is 0.500 μm or less. The rolling perpendicular direction Ra is more preferably 0.200 to 0.500 μm.
The number density of pits having an opening diameter of 1.0 μm or more formed by dropping the precipitated particles by pickling treatment can be measured as follows.

[Measurement method of pit number density]
Count the number of omission marks with an opening diameter of 1.0 μm or more present in an observation area with a total area of 0.1 mm ² or more in one or more observation fields randomly defined on the surface of the steel sheet. Divide by the total area of the observation area and convert to the number per 0.01 mm ² . Whether or not a pit existing on the boundary line of the set observation area falls into a drop mark having an opening diameter of 1.0 μm or more because the pit is surrounded by the opening outline and the boundary line on the observation area side. Determine.

As described above, the opening diameter of the pit means the diameter (long diameter) of the longest portion of the opening surrounded by the outline of the pit, but the opening of the pit formed by dropping the spherical carbide particles is It is characterized by a circular shape. In the pit opening, the diameter of the longest part of the opening measured in a direction perpendicular to the long diameter is called “short diameter”, and the ratio of the long diameter / short diameter is called the aspect ratio of the pit opening. The pits formed by dropping off the carbide particles have a circular shape with an opening aspect ratio of approximately 2.0 or less.
With a number density of 10 / 0.01 mm ² or more of pits having an opening diameter of 1.0 μm or more and an aspect ratio of 2.0 or less formed by the removal of carbide particles by pickling after finish annealing. The steel plate on the surface is a more suitable target in the present invention.

  The cold-rolled annealed steel sheet thus obtained is free from strength reduction caused by temper softening and sensitization, and is suitable for various press working applications including metal gaskets. Moreover, the cold-rolled annealed steel sheet in which the above-mentioned precipitate particle drop marks are dispersed on the surface is excellent in coating film adhesion. In the process of manufacturing the metal gasket, a bead having a certain height is formed by bead press molding. The obtained pressed product can be subjected to an aging treatment at 100 to 500 ° C. as necessary.

Example 1
Steel having the chemical composition shown in Table 1 was melted to obtain a slab. The slab was hot-rolled to obtain a hot-rolled steel plate having a thickness of 3.0 mm. Each hot-rolled steel sheet was subjected to furnace-cooled heat treatment at 800 ° C. for 24 hours, and then the sheet thickness was reduced by cold rolling. During the cold rolling, intermediate annealing at 800 ° C. × soaking for 60 seconds was performed once or a plurality of times to obtain a cold rolled steel sheet having a final thickness of 0.2 mm. Samples cut from each cold-rolled steel sheet were heated for 60 seconds at a temperature shown in Table 2, and then the average cooling rate from 800 ° C. to 200 ° C. was 150 ° C./s or 5 as shown in Table 2. It was cooled to room temperature under the control of ° C./s to obtain two types of cold-rolled annealed steel sheets having different cooling rates for each annealing temperature.

  About the plate | board surface (rolling surface) of said cold-rolled annealing steel plate, Vickers hardness was measured by test force 9.8N (hardness symbol HV1) according to JISZ2244: 2009. And the difference of the hardness of average cooling rate 5 degree-C / s and the hardness of average cooling rate 150 degree-C / s was calculated | required as hardness variation | change_quantity (DELTA) H. If the absolute value of ΔH is 10 HV or less, a predetermined strength is maintained even at a location where the cooling rate is relatively slow in actual operation, and the characteristic variation in the steel strip is extremely small. Can be evaluated. Therefore, a case where the absolute value of ΔH was 10 or less was evaluated as ◯ (quenching stability; remarkably improved), and other cases were evaluated as x (quenching stability; insufficient improvement), and a ◯ evaluation was determined as acceptable. The results are shown in Table 2.

In all the steels of the invention, the quenching stability was remarkably improved.
In contrast, steels No. 1 and No. 2, which are comparative steels, have a low N content, a low total content of C and N, and a high C / N content ratio, so the quenching stability is poor. It was. Steel No. 3 had a higher C content, a lower N content, and a higher C / N content ratio, and therefore the quenching stability was even worse.

  FIG. 1 shows the relationship between the C / N content ratio and the hardness change ΔH for these examples. It can be seen that when the C / N content ratio is 5.0 or less, ΔH is −10 or more, and the quenching stability is significantly improved when the cooling rate in the finish annealing is lowered.

Example 2
Table 3 shows the average cooling rate from 800 ° C. to 200 ° C. after cutting out a sample from the cold-rolled steel sheet (before finish annealing) prepared in Example 1 and heating at 1050 ° C. for 60 seconds. Thus, a cold-rolled annealed steel sheet was obtained. Cooling after the finish annealing was air cooling, and the cooling rate was controlled by adjusting the furnace temperature or the air blowing amount. The temperature change at the time of cooling was measured with the thermocouple attached to the sample surface, and the average cooling rate from 800 ° C. to 200 ° C. was determined based on the cooling curve. After the plate temperature dropped to near room temperature, pickling treatment was performed by dipping the sample in a pickling solution of 3% by mass hydrofluoric acid + 12% by mass nitric acid and 60 ° C. When the oxide scale could be removed, the pickling was finished, and the sample material (pickling material) was obtained through normal water washing. Each specimen was mirror-polished on a cross section (L cross section) parallel to the rolling direction and the plate thickness direction, and subjected to electrolytic etching in a 10% by mass oxalic acid aqueous solution for 6 V for 15 seconds using an optical microscope. Tissue observation was performed to examine the presence or absence of sensitization. The case where no sensitization was observed was evaluated as ◯ (sensitization; none), and the case where sensitization was observed was evaluated as x (sensitization; present). The results are shown in Table 3.

  The steels of the invention had improved sensitization resistance when the cooling rate was slow. In contrast, the comparative steels were sensitized when the cooling rate was slow. According to the present invention, it is considered that the precipitation of Cr carbide at the grain boundaries is suppressed even when the cooling rate of the finish annealing is slow, and the intergranular corrosion is suppressed.

Example 3
A cold-rolled steel sheet having a final thickness of 0.2 mm was obtained in the same manner as in Example 1 using No. 4 steel in Table 1. The cold-rolled steel sheet was subjected to finish annealing under the conditions shown in Table 4. In the same manner as in Example 2, the average cooling rate from 800 ° C. to 200 ° C. after finish annealing was controlled to obtain a cold-rolled annealed steel plate. The obtained cold-rolled annealed steel sheet was pickled by a method of dipping in 3% by mass hydrofluoric acid + 12% by mass nitric acid and 60 ° C. pickling solution. When the oxide scale could be removed, the pickling was finished, and the sample material (pickling material) was obtained through normal water washing. For comparison, a BA-treated material that was subjected to finish annealing in a reducing atmosphere was also prepared. For each specimen, the surface roughness Ra in the direction perpendicular to the rolling direction of the steel sheet surface was measured with a stylus type surface roughness meter. By observing the steel plate surface with an SEM for the pickling material, in accordance with the above-mentioned “Method for measuring the number density of pits”, pits having an opening diameter of 1.0 μm or more formed by the precipitation particles dropping out by pickling treatment are used. The number density was determined. At that time, SEM images of 12 visual fields were examined for each specimen.

  An epoxy-based primer was applied to the surface of each steel plate and baked and dried at 200 ° C. for 40 seconds, and then a polyester-based paint was applied thereon and baked and dried at 215 ° C. for 50 seconds to obtain a coated steel plate sample. Each coated steel plate sample was subjected to a bending test specified in JIS 3320: 1999. The bending test piece was sampled so that the longitudinal direction was the rolling direction, and was bent 180 ° at room temperature so that the bending axis was the direction perpendicular to the rolling direction and the outer surface of the bending was the painted surface. Observe the presence or absence of coating film peeling at the bending ridge line after the bending test, evaluate ○ for coating film peeling (good adhesion); evaluate x for coating film peeling observed (coating Film adhesion; poor). The results are shown in Table 4.

The atmosphere of finish annealing is an oxidizing atmosphere (atmosphere), and the average cooling rate from 800 ° C. to 200 ° C. after finish annealing is 1 to 150 ° C./sec (No. 4-2, 4-3, 4- In 4), the number density of pits having an opening diameter of 1.0 μm or more formed by dropping the precipitated particles by pickling treatment was 10 / 0.01 mm ² or more, and the coating film adhesion was good. . The surface roughness Ra in the direction perpendicular to rolling is 0.500 μm or less, and a gasket with high sealing properties can be obtained.

  On the other hand, in No. 4-1, since the cooling rate after the finish annealing was large, the martensite phase became excessively hard, and the bending test could not be performed up to 180 °. Further, precipitation growth of carbide was insufficient, and the number density of pits having an opening diameter of 1.0 μm or more was small. No. 4-5 was an example in which the cooling rate after finish annealing was extremely slow, and the hardness was low. Further, in pickling, peroxidation was performed at the stage where the oxide scale was removed, and the surface roughness Ra in the direction perpendicular to the rolling increased beyond 0.500 μm. No. 4-6 was an example in which finish annealing was performed in a reducing atmosphere, and since pickling was not performed, the surface smoothness was high and the coating film adhesion was inferior.

That is, in the present invention, by mass%, C: 0.10 to 0.180%, Si: 0.20 to 1.50%, Mn: 0.10 to 2.00%, Ni: 0.01 to 1. 00%, Cr: 11.00 to 18.00%, Cu: 0.18 to 1.60%, Mo: 0 to 0.50%, N: 0.030 to 0.100%, V: 0 to 0 .50%, Nb: 0 to 0.50%, Ti: 0 to 0.50%, B: 0 to 0.020%, remaining Fe and inevitable impurities, and the total content of C and N is 0.5. Provided is a martensitic stainless steel sheet having a chemical composition of 130% by mass or more, a C / N content ratio (mass% ratio) of 5.0 or less, and a γmax value determined by the following formula (1) of 80.0 or more. Is done.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1).

Cu is an austenite generating element and is effective in securing a sufficient amount of martensite. Further, it is also effective to ensure the solid solubility limit of N. However, it is retained austenite phase content of Cu becomes excessive is easily exist, which is disadvantageous for improving the strength and hot workability ensured. The Cu content is 0.18% or more and 1.60% or less.

Claims (10)

In mass%, C: 0.100 to 0.180%, Si: 0.20 to 1.50%, Mn: 0.10 to 2.00%, Ni: 0.01 to 1.00%, Cr: 11.0 to 18.00%, Cu: 0.20 to 1.60%, Mo: 0 to 0.50%, N: 0.030 to 0.100%, V: 0 to 0.50%, Nb : 0 to 0.50%, Ti: 0 to 0.50%, B: 0 to 0.020%, balance Fe and inevitable impurities, the total content of C and N is 0.130% or more, C A martensitic stainless steel sheet having a chemical composition having a / N content ratio of 5.0 or less and a γmax value determined by the following formula (1) of 80.0 or more.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1).   The martensitic stainless steel sheet according to claim 1, wherein the hardness of the plate surface (rolled surface) is 400 to 570 HV. The martens according to claim 1 or 2, wherein the surface has a number density of pits having an opening diameter of 1.0 µm or more and a number density of 10 pieces / 0.01 mm ² or more, and a surface roughness Ra in a direction perpendicular to the rolling is 0.500 µm or less. Site stainless steel sheet.   The martensitic stainless steel sheet according to any one of claims 1 to 3, wherein the plate thickness is 0.05 to 0.5 mm.   The martensitic stainless steel sheet according to any one of claims 1 to 4, wherein the martensitic stainless steel sheet is used for a metal gasket that is used by pressing a bead head portion against a contact partner material. In mass%, C: 0.100 to 0.180%, Si: 0.20 to 1.50%, Mn: 0.10 to 2.00%, Ni: 0.01 to 1.00%, Cr: 11.0 to 18.00%, Cu: 0.20 to 1.60%, Mo: 0 to 0.50%, N: 0.030 to 0.100%, V: 0 to 0.50%, Nb : 0 to 0.50%, Ti: 0 to 0.50%, B: 0 to 0.020%, balance Fe and inevitable impurities, the total content of C and N is 0.130% or more, C An austenite single-phase temperature of an intermediate product steel sheet having a chemical composition having a / N content ratio of 5.0 or less and a γmax value determined by the following formula (1) of 80.0 or more in a range of 800 to 1100 ° C. Or austenite phase + 20 volume% or less of the two-phase temperature range, which is a ferrite phase, and then cooled to cool the austenite phase to martensite. Step of transformation to (finish annealing process),
Method for producing martensitic stainless steel sheet having
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1). In mass%, C: 0.100 to 0.180%, Si: 0.20 to 1.50%, Mn: 0.10 to 2.00%, Ni: 0.01 to 1.00%, Cr: 11.0 to 18.00%, Cu: 0.20 to 1.60%, Mo: 0 to 0.50%, N: 0.030 to 0.100%, V: 0 to 0.50%, Nb : 0 to 0.50%, Ti: 0 to 0.50%, B: 0 to 0.020%, balance Fe and inevitable impurities, the total content of C and N is 0.130% or more, C An austenite single-phase temperature of an intermediate product steel sheet having a chemical composition having a / N content ratio of 5.0 or less and a γmax value determined by the following formula (1) of 80.0 or more in a range of 800 to 1100 ° C. The average cooling rate from 800 ° C to 200 ° C is 1-1 to Step of cooling such that 0 ° C. / sec (final annealing step),
Method for producing martensitic stainless steel sheet having
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1). In mass%, C: 0.100 to 0.180%, Si: 0.20 to 1.50%, Mn: 0.10 to 2.00%, Ni: 0.01 to 1.00%, Cr: 11.0 to 18.00%, Cu: 0.20 to 1.60%, Mo: 0 to 0.50%, N: 0.030 to 0.100%, V: 0 to 0.50%, Nb : 0 to 0.50%, Ti: 0 to 0.50%, B: 0 to 0.020%, balance Fe and inevitable impurities, the total content of C and N is 0.130% or more, C An intermediate product steel sheet having a chemical composition having a / N content ratio of 5.0 or less and a γmax value determined by the following formula (1) of 80.0 or more is in the range of 800 to 1100 ° C. in an oxidizing atmosphere. After heating to an austenite single phase temperature range or a two-phase temperature range that becomes an austenite phase + 20% by volume or less ferrite phase, the temperature ranges from 800 ° C. to 200 ° C. Step of precipitating carbide particles by cooling as average cooling rate is 1 to 150 ° C. / sec (final annealing step),
Removing the surface oxide scale by pickling the steel plate after the finish annealing step, dropping the carbide particles present on the surface to form pits on the surface (pickling step),
Method for producing martensitic stainless steel sheet having
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49 (Ti + Nb + V) + 470N + 189 (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1).   The manufacturing method of the metal gasket which presses the steel plate of any one of Claims 1-4, and forms a bead by pressing a bead head top part against a contact other material.   The metal which presses the steel plate of any one of Claims 1-4, forms a bead, and performs an aging treatment of 100-500 degreeC, presses a bead head top against a contact partner material, and uses it Manufacturing method of gasket.

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