JP2007239060A - Surface hardened ferritic stainless steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stainless steel sheet which is improved in slidability and flawing resistance while maintaining its satisfactory workability. <P>SOLUTION: The surface hardened ferritic stainless steel sheet has a hard surface layer including a martensitic phase, and in which the inside in the sheet thickness direction therefrom has a ferritic phase, surface hardness is ≥250 HV, and the hardness at the central part in the sheet thickness is ≤230 HV. The ferritic structure has a composition containing, by mass, 0.005 to 0.1% C, <0.5% Si, 0.1 to 1% Mn, 11 to 25% Cr, ≤0.1% N, and the balance substantially Fe, and in which γmax defined by formula: γmax=420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-50Nb-52Al+470N+189 is -5 to 30. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a surface-hardened ferritic stainless steel sheet suitable for processing into articles that require slidability and scratch resistance.

  Stainless steel typified by SUS430 and SUS304 is widely used as a processing material for kitchen equipment and various items by utilizing the excellent corrosion resistance due to the passive film. Conventionally, many coated steel sheets have been used for the outer panels of household electrical appliances and commercial electric / electronic equipment, but stainless steel is also increasingly used in applications that emphasize corrosion resistance.

  In recent years, in the above-mentioned members constituting the appearance part of articles, there has been an increasing demand for materials that take into consideration not only corrosion resistance but also “slidability” and “scratch resistance”. However, in stainless steel such as SUS430 and SUS304, it is necessary to finish softly in order to ensure good workability. In the case of stainless steel that has been finished to be soft, slidability and scratch resistance are reduced.

  Increasing the surface hardness is effective in improving the slidability and scratch resistance. In that sense, it is advantageous to use high-strength stainless steel such as SUS301 or SUS630. Further, even with general-purpose stainless steels such as SUS430 and SUS304, it is possible to improve slidability and scratch resistance to some extent by increasing the strength using work hardening. However, such a high-strength stainless steel material sacrifices workability and may make it difficult to process an article having a predetermined shape. Further, with regard to slidability and scratch resistance itself, it is not always possible to achieve a sufficient level of improvement simply by increasing the strength.

  On the other hand, in a surface-treated steel sheet having a surface coated or the like, the surface-treated layer often cannot withstand severe processing, and it is generally difficult to form an article with a high degree of processing. In addition, the unique metallic feeling exhibited by the solid metal surface is impaired.

  As a stainless steel that simultaneously realizes high strength and high ductility, martensite + ferrite dual phase stainless steel as disclosed in Patent Document 1 has been developed. However, this steel is not intended to improve the slidability and scratch resistance of the surface. If the strength is increased before sufficient slidability and wrinkle resistance can be obtained using this multiphase technique, the workability is still sacrificed.

Japanese Patent Laid-Open No. 7-138704

  Thus, it is difficult to achieve both good workability, slidability and scratch resistance at a high level in a stainless steel material. In view of such a current situation, the present invention is excellent in stainless steel sheets having good workability that can be sufficiently processed into outer plates of kitchen equipment, home appliances, commercial electric / electronic equipment, various kinds of equipment, interior building materials, etc. It is intended to develop and provide products with added slidability and scratch resistance.

As a result of various studies, the inventors have found that the above object can be realized by forming a hard surface layer containing a martensite phase on the surface of a ferritic stainless steel sheet.
That is, in the present invention, it has a hard surface layer containing a martensite phase, the inside of the plate thickness direction is a ferrite structure, the surface hardness is 250 HV or more, the hardness of the plate thickness center is 230 HV or less, and the ferrite The structure contains, in mass%, C: 0.005-0.1%, Si: less than 0.5%, Mn: 0.1-1%, Cr: 11-25%, N: 0.1% or less, Further, if necessary, Ni: 1% or less, Cu: 1.5% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.5% or less, Mo: 1.5% or less There is provided a surface-hardened ferritic stainless steel sheet that includes one or more of the following, the balance being substantially Fe, and having a composition of γmax defined by the following formula (1) of −5 to 30: The
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (1)

  Here, as the surface hardness, a value measured by a method in which a cone is pushed into the steel plate surface at 0.98 N using a micro Vickers hardness meter can be adopted. As the hardness of the central portion of the plate thickness, a value measured by the same method as described above for the central portion of the plate thickness having a cross section (L cross section) parallel to the rolling direction and the plate thickness direction can be adopted. The value of the content of each element expressed in mass% (0% for an additive-free element) is substituted for the element symbol in the above formula (1). “The balance is substantially Fe” means that elements other than those specified above are allowed within the range that does not impair the effects of the present invention, and includes the case where the balance is made of Fe and inevitable impurities. .

As the martensite phase constituting the hard surface layer, those transformed from the austenite phase generated by absorbing nitrogen in the atmosphere during annealing are suitable targets.
Such a hard surface layer containing a martensite phase may be formed by performing bright (BA) annealing at 800 to 1100 ° C. in a reducing atmosphere of N ₂ : 15 to 35% by volume and H ₂ : 65 to 85% by volume. it can.

  According to the present invention, slidability and weather resistance can be maintained while maintaining good processability that can be sufficiently processed into outer panels of kitchen equipment, home appliances, commercial electric / electronic equipment, various types of equipment, interior building materials, and the like. The sticking property was stably improved remarkably. Workability, slidability, and scratch resistance are inherently in a trade-off relationship, and it has been extremely difficult to achieve both in stainless steel. In the present invention, a method of absorbing nitrogen from the surface by using bright annealing can be adopted, and implementation is relatively easy using existing equipment. Further, since an inexpensive ferritic steel type can be used for the base steel, an increase in cost can be suppressed.

  The inventors have made various studies on methods for improving slidability and scratch resistance while maintaining high workability of the stainless steel sheet. As a result, it was found that a method of hardening only the vicinity of the surface layer portion is effective in a steel sheet based on a soft and good workability.

  Conventionally, heat treatment such as carburizing and nitriding is known as a treatment for hardening the surface layer of steel. These heat-treat machine parts (such as gears) after processing in a special atmosphere to give the surface layer a characteristic that hardens and hardens during quenching and tempering (carburization), and generates hard nitrides (nitriding) It is a technique to do. However, in the present invention, at the stage of the “material steel plate” before being processed into a product, it is aimed to achieve surface hardening while ensuring processability and to maintain a beautiful glossy surface unique to stainless steel. For this reason, carburizing and nitriding means cannot be employed.

  Therefore, in the present invention, a technique is adopted in which nitrogen as an atmospheric gas component is absorbed in the vicinity of the surface of the stainless steel plate using bright annealing. At that time, the chemical composition of the base steel plate (steel plate for heat treatment for forming the hardened surface layer) is strictly adjusted in advance, and only the surface layer portion where the nitrogen concentration has increased in the annealing temperature range is higher than the normal temperature at the Ms point. The austenite phase with a nitrogen concentration and the inside where the nitrogen concentration does not change should be a ferrite phase. If it does so, only the surface layer part can be hardened using a martensitic transformation in the cooling process of the bright annealing concerned. The inside becomes an annealed soft ferrite phase, so that good workability is also ensured. Moreover, since bright annealing is employed, a beautiful metallic gloss surface can be obtained.

  The chemical composition of the base steel plate (steel plate for the heat treatment for forming the hardened surface layer) is adjusted to the following range. The chemical composition of the base steel sheet is directly reflected in the chemical composition of the ferrite structure in the thickness direction of the hardened surface layer in the steel sheet of the present invention.

  C is an element that increases the strength of the ferritic stainless steel, and is preferably smaller from the viewpoint of ensuring good workability, and is limited to 0.1% by mass or less in the present invention. However, C is also an austenite forming element, and it is advantageous to contain 0.005% by mass or more of C in order to sufficiently generate the austenite phase in the surface layer that has absorbed nitrogen during the heat treatment. More preferably, the content is 0.01% by mass or more.

  Although Si is effective as a deoxidizing element for steel, it exhibits a solid solution strengthening action and hardens the steel, so it is limited to less than 0.5% by mass from the viewpoint of ensuring workability.

  Mn is an element suitable for adjusting the later-described γmax while maintaining the ferritic stainless steel soft, and it is desirable to ensure a content of 0.1% by mass or more. However, since a large amount of Mn leads to a decrease in corrosion resistance, the Mn content is set to a range of 1% by mass or less.

  Cr is an essential element for securing the corrosion resistance required for stainless steel, and it is necessary to contain at least 11% by mass or more. However, a large amount of Cr is disadvantageous in hardening the steel and maintaining workability. Further, since Cr is a ferrite forming element, it also becomes a factor that inhibits the formation of martensite on the surface layer. Therefore, the Cr content is suppressed to 25% by mass or less. A more preferable Cr content is 13 to 20% by mass.

  N, like C, is an element that increases the strength of ferritic stainless steel, and is limited to 0.1% by mass or less in the present invention from the viewpoint of ensuring good workability. In addition, when the N content in the base steel sheet is 0.1% or less, the austenite balance of the surface layer can be optimized together with nitrogen absorbed during bright annealing.

  Ni, like Mn, is an element that can adjust γmax while keeping the steel soft, and can be contained as necessary. However, since a large amount of Ni causes an increase in cost, when Ni is contained, the content is desirably 1% by mass or less, and more preferably 0.5% by mass or less.

  Cu, like Mn and Ni, is an element suitable for adjusting γmax while keeping the steel soft, and can be contained as required. However, since containing a large amount of Cu is disadvantageous for corrosion resistance, when Cu is contained, the content is desirably set to 1.5% by mass or less, and more preferably 0.7% by mass or less.

  Nb and Ti have the effect of fixing solid solution C and N, and the amount of C and N solid solution in the final annealing state is reduced to improve workability and corrosion resistance. Therefore, Nb and Ti can be contained as necessary. . However, if excessively contained, solid solution Ti and Nb increase and productivity decreases, so the content of Nb and Ti is both 0.5% by mass or less.

  Al is effective as a deoxidizing element for steel, and can be contained in a range of 0.5% by mass or less as necessary.

  Mo is an element that improves the corrosion resistance of the Cr-containing steel, and can be contained as necessary. However, since Mo is an expensive element and adding a large amount causes an increase in cost, when Mo is contained, it is performed in a range of 1.5 mass% or less.

  In addition, V: 0.1 mass% or less, W: 1 mass% or less, Zr: 1 mass% or less, REM (rare earth element): total 0.1 mass% or less, Ca: 0.1 mass% or less, Y: Various elements such as 0.1% by mass or less and Mg: 0.1% by mass or less are allowed to be mixed within a range that does not impair the function of the present invention.

Furthermore, in the present invention, it is necessary to adjust the components so that the relationship in which γmax represented by the following formula (1) is −5 to 30 is established between the components of the base steel plate.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (1)
γmax is an index for evaluating the maximum amount of austenite at a high temperature. In the case of a base steel sheet in which this value is in the range of −5 to 30, in the “inside” that is not affected by the atmospheric gas components during bright annealing, an austenite phase is not generated in the annealing temperature range, or even if generated. Therefore, a martensite phase is not generated in such an amount that hardening occurs in the cooling process after annealing, and a soft ferrite structure is obtained after bright annealing. On the other hand, in the surface layer that has absorbed nitrogen by bright annealing, the composition with the absorbed nitrogen added causes an increase in γmax, and a large amount of austenite phase is generated in the annealing temperature range. This austenite phase transforms into a martensite phase during cooling and contributes to curing. That is, when the γmax of the base steel plate is adjusted to the above range, an amount of martensite sufficient to cause remarkable hardening is secured in the surface layer portion, and hardening only in the surface layer region is achieved.

  When considering the workability to various articles as described above, the region inside the plate thickness direction from the hard surface layer needs to have a ferrite structure having a hardness of 230 HV or less. The hardness can be known by measuring the cross-sectional hardness at the center of the plate thickness. The ferrite structure is a metal structure composed of a matrix mainly composed of a ferrite phase, specifically, a metal structure in which the ferrite phase occupies 90% by volume or more. In addition to the ferrite phase, it may contain a small amount of martensite phase, inclusions and precipitates found in general ferritic stainless steel sheets.

  On the other hand, the hard surface layer is a hardened surface layer containing a martensite phase, and the thickness of the hard surface layer does not need to be particularly defined, but the surface hardness needs to be 250 HV or more. If it is softer than this, it will be difficult to ensure sufficient slidability and scratch resistance. This surface hardness is a hardness in a depth region within approximately 5 μm from the surface. In general, the thickness of the hard surface layer is about several μm to several tens of μm. A passive film is present on the outermost surface of the hard surface layer, and the corrosion resistance of the stainless steel is maintained.

  The difference (Δh) between the surface hardness and the hardness at the center of the plate thickness is preferably 50 or more in terms of HV value. When Δh is large, the effect of improving the surface hardness is remarkably exhibited according to each application (workability level), which is preferable. More preferably, Δh is 75 or more.

The stainless steel plate having the above characteristics can be produced as follows.
The steel adjusted to the above-mentioned chemical composition is melted by a general stainless steel making process, and a base steel plate that is subjected to bright annealing through hot rolling, annealing, and cold rolling is manufactured. The plate thickness of the base steel plate can be about 0.5 to 2.0 mm. Further, “annealing and cold rolling” can be appropriately performed a plurality of times according to the target plate thickness. Each of these steps may be performed in accordance with a normal method for producing a ferritic stainless steel sheet. It is desirable that the cold rolling rate (the rolling rate in the last cold rolling when cold rolling is performed a plurality of times) of the base steel sheet to be subjected to bright annealing is 50 to 80%.

Next, the base steel sheet is subjected to bright annealing. The bright annealing atmosphere may be a reducing atmosphere containing N ₂ : 15 to 35% by volume. When the N ₂ concentration of the atmospheric gas is set to 10% by volume or more, nitrogen in the atmosphere is absorbed into the steel and becomes a solid solution, and sufficiently contributes to an increase in γmax. However, if the N ₂ concentration of the atmospheric gas exceeds 35% by volume, nitrogen absorbed in the steel reacts with the steel components, and it is easy to form nitrides that cause surface defects and stress concentration, which is not preferable. As a specific atmosphere gas, a mixed gas of N ₂ : 15 to 35% by volume + H ₂ : 65 to 85% by volume can be adopted. The annealing temperature may be 800-1100 ° C. It is preferable that the annealing time is 0 to 2 minutes. The austenite phase formed in the surface layer during the cooling process after bright annealing transforms into the martensite phase. Unlike the quenching treatment of ordinary steel, the cooling rate does not need to be increased so much and may be in the range of 1 to 10 ° C./sec. This bright annealing can be performed in a continuous annealing line.

  The steel plate after bright annealing can be subjected to temper rolling as necessary. At that time, the rolling rate is adjusted so that the surface hardness and the hardness at the center of the plate thickness are in the above ranges. For example, the rolling rate is preferably 10% or less.

  In this way, a ferritic stainless steel sheet imparted with slidability and scratch resistance while maintaining good workability can be realized, and this can be molded into a product and used. Depending on the application, it can also be used for coating and other surface treatments. Even in this case, the excellent slidability and scratch resistance of the base steel sheet result in improved durability on the surface after the surface treatment. However, it is necessary to select the surface treatment means within a range that can withstand the required molding process.

A ferritic stainless steel having the composition shown in Table 1 is melted, cast into a 30 kg ingot, hot-rolled, and thereafter annealed and cold-rolled repeatedly to obtain a cold-rolled steel sheet having a thickness of 0.5 mm (after annealing, The rolling ratio in the last cold rolling was about 65% for each steel type. Each cold-rolled steel sheet was then subjected to final annealing under various conditions. The final annealing was a bright annealing in a H ₂ + N ₂ mixed gas except for some samples. The annealing conditions are listed in Table 2. The bright annealing time is 0.5 minutes (common) for soaking. Some samples employed atmospheric annealing instead of bright annealing and were subjected to pickling. Furthermore, 2% temper rolling was applied to some samples.

Using the material after final annealing or temper rolling obtained in this way as the test material, the surface hardness, the hardness at the center of the plate thickness, slidability, scratch resistance, and workability were investigated. It was.
The surface hardness was determined by measuring the surface of the plate with a micro Vickers hardness meter at 0.98N.
The hardness at the center of the plate thickness was determined by measuring a cross section (L cross section) parallel to the rolling direction and the plate thickness direction at 0.98 N with a micro Vickers hardness meter.

The slidability is determined by pressing the load (static friction) generated at the start when the steel ball of JIS B1501 (1/4 inch) is pressed against the surface of the specimen steel plate with a load of 3.92 N and moved with a stroke of 20 mm. When the static friction was 0.59 N or less, ○ (good), 0.59 to 0.98 N or less was evaluated as Δ (slightly defective), and 0.98 N or more was evaluated as × (defective). .
With respect to scratch resistance, JIS B1501 steel balls (1/4 inch) are pressed against the surface of the test steel plate with a load of 0.49 N, 50 strokes are made with a stroke of 15 mm, and no wrinkles are observed after the test. A thing with ◎ (excellent) and wrinkles was observed, and a width of less than 0.1 mm was evaluated as ◯ (good), and a wrinkle with a width of 0.1 mm or more was evaluated as x (bad).

As for workability, 90 ° bending is performed with the direction perpendicular to the rolling direction as the bending axis, and the inner surface R = 0.2 mm is bent with no cracks on the surface of the processed part. Cracks are observed on the surface of the machined part by bending 0.2 mm, but no cracks are found on the inner R = 0.5 mm bend (good), and bent on the inner part R = 0.5 mm on the machined part surface. Those with cracks were evaluated as x (defect).
The results are shown in Table 2.

  The steel sheets of the examples of the present invention all had a hard surface layer containing a martensite phase, and it was confirmed by structural observation that the inside in the thickness direction was a ferrite structure. The average thickness of the hard surface layer was in the range of 8 to 35 μm as a result of observing a sample cross section etched with a mixed solution of hydrofluoric acid + nitric acid + glycerin with an optical microscope after polishing. As can be seen from Table 2, the steel sheet of the present invention has a surface hardness of 250 HV or more and a center thickness of 230 HV or less, with excellent slidability and wrinkle resistance while maintaining good workability Exhibited sex.

On the other hand, No. 1-3 which is a comparative example uses SUS430 with high Si content, and realizes excellent slidability even if the final annealing condition is changed or temper rolling is performed. I could not. No. 4 is a steel equivalent to SUS430J1L, and No. 5 is a steel equivalent to SUS430LX, and since γmax is low, a hard surface layer is not formed and the sliding property and scratch resistance are inferior. No. 6 was inferior in slidability and scratch resistance because the hard surface layer was not formed because the N ₂ concentration in the bright annealing atmosphere was low. In No. 7, since the N ₂ concentration in the bright annealing atmosphere was high, a hard nitride was formed on the surface layer, which caused deterioration of workability. No. 8 was inferior in slidability and scratch resistance because a hard surface layer was not formed due to a low bright annealing temperature. Since No. 9 annealed in air | atmosphere, a hard surface layer was not formed but it was inferior to slidability and an abrasion resistance.

Claims (4)

It has a hard surface layer containing a martensite phase, and the inside in the thickness direction is a ferrite structure, the surface hardness is 250 HV or more, the hardness at the center of the thickness is 230 HV or less, and the ferrite structure is in mass%. C: 0.005-0.1%, Si: less than 0.5%, Mn: 0.1-1%, Cr: 11-25%, N: 0.1% or less, the balance being substantially Fe, and A surface-hardened ferritic stainless steel sheet having a composition in which γmax defined by the following formula (1) is −5 to 30.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (1)   In the above composition, Ni: 1% or less, Cu: 1.5% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.5% or less, Mo: 1.5% or less The surface-hardened ferritic stainless steel sheet according to claim 1, comprising one or more of the following.   The surface-hardened ferritic stainless steel sheet according to claim 1 or 2, wherein the martensite phase of the hard surface layer is transformed from an austenite phase generated by absorbing nitrogen in the atmosphere during annealing. The hard surface layer containing a martensite phase is formed by bright annealing at 800 to 1100 ° C in a reducing atmosphere of N ₂ : 15 to 35% by volume and H ₂ : 65 to 85% by volume. 2. The surface-hardened ferritic stainless steel sheet according to 2.