(19)
(11)EP 4 026 920 A1

(12)EUROPEAN PATENT APPLICATION
published in accordance with Art. 153(4) EPC

(43)Date of publication:
13.07.2022 Bulletin 2022/28

(21)Application number: 20859662.7

(22)Date of filing:  24.08.2020
(51)International Patent Classification (IPC): 
C21D 8/02(2006.01)
C21D 9/18(2006.01)
C22C 38/00(2006.01)
C22C 38/50(2006.01)
C21D 9/00(2006.01)
C21D 9/46(2006.01)
C22C 38/46(2006.01)
(52)Cooperative Patent Classification (CPC):
C22C 38/00; C22C 38/46; C21D 9/18; C21D 9/00; C21D 9/46; C21D 8/02; C22C 38/50
(86)International application number:
PCT/JP2020/031886
(87)International publication number:
WO 2021/044889 (11.03.2021 Gazette  2021/10)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(30)Priority: 03.09.2019 JP 2019160432

(71)Applicant: NIPPON STEEL Stainless Steel Corporation
Tokyo 100-0005 (JP)

(72)Inventors:
  • INOUE Yoshiharu
    Tokyo 100-8071 (JP)
  • YAMADA Yoshihito
    Tokyo 100-8071 (JP)

(74)Representative: Vossius & Partner Patentanwälte Rechtsanwälte mbB 
Siebertstraße 3
81675 München
81675 München (DE)

  


(54)MARTENSITIC STAINLESS STEEL PLATE AND MARTENSITIC STAINLESS STEEL MEMBER


(57) There are provided martensitic stainless steel sheet and member exhibiting an excellent corrosion resistance even after air quenching used for manufacturing Western tableware knives, weaving machines, tools, disk brakes and the like. The martensitic stainless steel sheet contains, by mass%, C of 0.100 to 0.170%, Si of 0.30 to 0.60%, Mn of 0.10 to 0.60%, Cr of 11.0 to 15.0%, Ni of 0.05 to 0.60%, Cu of 0.010 to 0.50%, V of 0.010 to 0.10%, Al of 0.05% or less, N of 0.060 to 0.090%, and C+1/2N of 0.130 to 0.190%, and γp represented by a formula (1) is 120 or more. When the martensitic stainless steel sheet is quenched and tempered, an area ratio of δ ferrite (δFe) existing in a central part in a thickness direction of the steel sheet in a cross section in the thickness direction thereof is from 0.1 to 1%.




Description

TECHNICAL FIELD



[0001] The present invention relates to a martensitic stainless steel sheet and a martensitic stainless steel member which exhibit an excellent corrosion resistance after quenching. More specifically, the invention relates to martensitic stainless steel exhibiting an excellent corrosion resistance even after air quenching used for manufacturing Western tableware knives, weaving machines, tools, disk brakes and the like.

BACKGROUND ART



[0002] Martensitic stainless steel sheets such as SUS420J1 and SUS420J2 steel are generally used for tools such as Western tableware knives (table knives), scissors, weaving machines, and calipers. In such applications, it is difficult to use plating, painting, and rust preventive oil, therefore, a material itself is required to have rust resistance. It is also important that the material does not wear easily, so that the material is required to have a high hardness.

[0003] The manufacturing process of a Western tableware knife or the like is usually performed by die-cutting from a steel sheet, heating, quenching, and then polishing to obtain a knife. The quenching is often performed by air cooling, which also depends on the characteristics of the martensitic stainless steel sheet having excellent quenching properties.

[0004] Patent Literature 1 discloses martensitic stainless steel having an excellent corrosion resistance at the time of quenching by air cooling. Here, N is added to reach a content of approximately 0.06% as an element that improves corrosion resistance.

[0005] Patent Literature 2 discloses steel further added with N. Moreover, Patent Literature 3 discloses steel in which N is further improved by using special equipment.

[0006] In recent years, demand for corrosion resistance of Western tableware has become more sophisticated, especially in Europe. In a rust resistance evaluation test, rusting is sometimes observed on the back or tip of a blade of a table knife and in the center of a handle thereof. Accordingly, rust resistance of the steel is required to be improved.

CITATION LIST


PATENT LITERATURE(S)



[0007] 

Patent Literature 1: JP2008-163452A

Patent Literature 2: JP2005-163176A

Patent Literature 3: JP2005-248263A


NON-PATENT LITERATURE(S)



[0008] Non-Patent Literature 1: Journal of Japan Institute of Metals, 1962, vol. 26, No. 7, pp. 472-478

SUMMARY OF THE INVENTION


PROBLEM(S) TO BE SOLVED BY THE INVENTION



[0009] In recent years, with the sophisticated demand for corrosion resistance of Western tableware, mainly in Europe, there is an increasing demand for improvement in rusting on the back or tip of the blade of the table knife and the central part of the handle thereof in a strict corrosion resistance test. An object of the invention is to provide a martensitic stainless steel sheet and a martensitic stainless steel member which have an excellent corrosion resistance while keeping a sufficient hardness to withstand use of the martensitic stainless steel for Western tableware such as table knives.

MEANS FOR SOLVING THE PROBLEM(S)



[0010] In order to achieve the above object, the inventors first investigated a rusting state of a table knife in detail. As a result, it was revealed that a starting point of a rusting part was an end surface of a steel sheet, more specifically, a central part of the steel sheet in a thickness direction. Further, formation of a δ ferrite phase (δFe phase) due to macro-segregation was confirmed. Grain boundaries of the δFe phases became accumulation sites of carbides, and the carbides were melted by heating during quenching, and precipitated in the grain boundaries during subsequent cooling, resulting in sensitization and intergranular corrosion, which was turned out to be a mechanism of rusting.

[0011] It was also found that the rusting depended on a cooling rate during quenching. The rusting was evaluated under the condition that the cooling rate, which varies significantly depending on quenching equipment, was an average cooling rate from a quenching temperature to 600 degrees C that is a temperature at which precipitation of carbides are almost completed. Consequently, it was found that water quenching provides a cooling rate exceeding 100 degrees C/s, and therefore suppresses precipitation of carbides and causes rusting to be unlikely to occur, whereas air cooling often used in a manufacturing process of table knives provides a cooling rate of only approximately 5 degrees C/s and therefore cannot suppress precipitation of carbides and causes rusting to be likely to occur.

[0012] As a result of dedicatedly studying an improvement method on a basis of these findings, the inventors have found that addition of N to the steel sheet composition, can suppress the rusting in Western tableware knives after being subjected to forming and heat treatment.

[0013] Subsequently, the inventors conducted more detailed study to arrive at the invention.
  1. (1) A martensitic stainless steel plate according to an aspect of the invention includes:

    by mass%

    C of 0.100 to 0.170%,

    Si of 0.25 to 0.60%,

    Mn of 0.10 to 0.60%,

    P of 0.035% or less,

    S of 0.015% or less,

    Cr of 11.0 to 15.0%,

    Ni of 0.05 to 0.60%,

    Cu of 0.010 to 0.50%,

    V of 0.010 to 0.10%,

    Al of 0.05% or less,

    N of 0.060 to 0.090%,

    C+1/2N of 0.130 to 0.190%, and

    a balance consisting of Fe and impurities, and γp represented by a formula (1) below is 120 or more,

    when the martensitic stainless steel sheet is held at 1050 degrees C for 30 minutes, followed by air quenching, and tempered at 150 degrees C for 30 minutes, an area ratio of δ ferrite (δFe) existing in a central part in a thickness direction of the martensitic stainless steel sheet in a cross section in the thickness direction thereof is from 0.1 to 1%,

            γp = 420C + 470N + 30Ni + 7Mn + 9Cu - 11.5Cr - 11.5Si - 12Mo - 23V - 47Nb - 52Al + 189 ...     Formula (1)

    where element symbols in the formula (1) mean contents (mass%) of the respective elements.

  2. (2) The martensitic stainless steel plate according to the above aspect of the invention further includes: by mass%, in place of a part of Fe, one or two or more of

    Mo of 0.01 to 1.0%,

    Ti of 0.005 to 0.050%, and

    Nb of 0.005 to 0.050%.

  3. (3) The martensitic stainless steel plate according to the above aspect of the invention further includes: by mass%, in place of a part of Fe, one or two of Sn of

    0.01% to 0.10%, and

    Bi of 0.01% to 0.20%.

  4. (4) A martensitic stainless steel member according to another aspect of the invention

    has the steel composition according to the above aspect of the invention, in which γp represented by a formula (1) below is 120 or more, and

    an area ratio of δ ferrite (δFe) existing in a central part in a thickness direction of the martensitic stainless steel member in a cross section in the thickness direction thereof is from 0.1 to 1%,

    where element symbols in the formula (1) mean contents (mass%) of the respective elements.



[0014] A martensitic stainless steel sheet of the invention is excellent in corrosion resistance, especially, end-surface corrosion resistance while keeping a sufficient hardness to withstand use of the martensitic stainless steel for Western tableware such as table knives. Accordingly, when the martensitic stainless steel sheet is used as a martensitic stainless steel member for Western tableware knives, the martensitic stainless steel member can be expected to impart an improved corrosion resistance and also a prolonged lifetime to a product.

BRIEF DESCRIPTION OF DRAWING(S)



[0015] Fig. 1 is a representative example of a cross section structure of the present steel sheet etched using a modified Murakami's reagent.

DESCRIPTION OF EMBODIMENT(S)



[0016] An exemplary embodiment of the invention will be described in detail. Chemical Composition of Steel Sheet and Steel Member (% means mass%.)

C: 0.100 to 0.170%



[0017] C, as well as N, is an element that determines quenching hardness. A C content needs to be 0.100% or more in order to obtain hardness required for Western tableware knives. The C content is preferably 0.110% or more, or 0.120% or more. On the other hand, an excessive addition of C increases quenching hardness more than required, resulting in an increase in a load at the time of polishing and a decrease in toughness. Moreover, the C content is defined as 0.170% or less since Cr carbides are likely to be precipitated at the time of air quenching to impair corrosion resistance even according to the exemplary embodiment of the invention. The C content is preferably 0.155% or less.

Si: 0.25 to 0.60%



[0018] Si is required for deoxidation in steelmaking and also effective for suppressing generation of oxide scales after a quenching heat treatment, a Si content is defined as 0.25% or more. At the Si content of less than 0.25%, oxide scales are excessively generated, thereby increasing the final polishing load. However, an excessive addition of Si suppresses generation of austenite to impair hardenability, so that the Si content is defined as 0.60% or less.

Mn: 0.10 to 0.60%



[0019] Mn is an austenite-stabilizing element and is required for securing hardness and a martensite content during quenching. Accordingly, a Mn content is defined as 0.10% or more. However, since Mn promotes generation of oxide scales during quenching to increase a subsequent polishing load, the Mn content is defined as 0.60% or less. An excessive addition of Mn generates a large content of MnS to lower corrosion resistance.

P: 0.035% or less



[0020] P is an element contained as an impurity in a raw material that is molten pig iron and alloys such as ferrochrome. Since P is an element harmful to toughness of a steel sheet after hot-rolled annealing and after quenching, a P content is defined as 0.035% or less. An excessive addition of P lowers hot workability and corrosion resistance.

S: 0.015% or less



[0021] S has a small solid solubility in an austenite phase and segregates in grain boundaries to promote lowering hot workability. Accordingly, a S content is defined as 0.015% or less. An excessive addition of S generates a large content of MnS to lower corrosion resistance.

Cr: 11.0 to 15.0%



[0022] A Cr content needs to be at least 11.0% or more in order to maintain corrosion resistance as Western tableware knives. Since Cr also has an effect of narrowing a range of an austenite-stabilizing temperature, the Cr content is defined as 15.0% or less. The Cr content is preferably 12.0% or more. The upper limit of the Cr content is preferably 14.0% or less. A favorable range of the Cr content is from 12.0 to 14.0%.

Ni: 0.05 to 0.60%



[0023] Similar to Mn, Ni is an austenite-stabilizing element and is required for securing hardness and a martensite content during quenching. Ni also has an effect of improving corrosion resistance. Accordingly, Ni is contained at 0.05% or more. However, since Ni is more expensive than other elements, a Ni content is defined as 0.60% or less.

Cu: 0.010 to 0.50%



[0024] Similar to Mn and Ni, Cu is an austenite-stabilizing element and also a corrosion resistance-improving element. Although Cu is also an element inevitably mixed from scrap in steelmaking, Cu is contained at 0.010% or more in order to improve corrosion resistance. On the other hand, since an excessive Cu content lowers hot workability and the like, the Cu content is defined as 0.50% or less. Since Cu is relatively expensive although being cheaper than Ni, addition of Cu is preferably as low as possible.

V: 0.010 to 0.10%



[0025] V is an element that is often inevitably mixed from ferrochrome (i.e., alloying elements) and the like. It is difficult to reduce V. V is contained at 0.010% or more. However, an excessive V content narrows a temperature region in which austenite is formed, so that a V content is defined as 0.10% or less. Moreover, the excessive addition of V forms VN to fix N, which is not preferable because of lowering hardness and corrosion resistance.

Al: 0.05% or less



[0026] Al is an element effective for deoxidation, however, an excessive Al content generates CaS (i.e., soluble inclusion) during hot rolling to lower corrosion resistance. Accordingly, the Al content is defined as 0.05% or less. Al is not necessarily contained.

N: 0.060 to 0.090%



[0027] N is, as well as C, an element that determines quenching hardness. N is herein an element significant for improving corrosion resistance. Accordingly, N is herein contained at 0.060% or more. A N content is preferably 0.065% or more. However, an excessive N content is likely to generate gas defects in slab to increase manufacturing costs due to VOD and the like in secondary refining, so that the N content is defined as 0.090% or less. The N content is preferably 0.085% or less.

C+1/2 N: 0.130 to 0.190%



[0028] Elements that determine hardness of the martensite phase in steel are C and N. A total content of C and N contributes to the hardness. According to the study of the inventors, the contribution of N to hardness is half that of C, and a C+1/2N content needs to be 0.130% or more in order to obtain hardness required for Western tableware knives. The C+1/2N content is preferably 0.150% or more. On the other hand, an excessive C+1/2N content increases quenching hardness excessively to impair toughness of products and intermediate materials (such as cast steel) in a manufacturing process, so that the C+1/2N content is defined as 0.190% or less. The C+1/2N content is preferably 0.180% or less, and may be 0.175% or less.

[0029] Further, in order to stably exhibit hardness in quenching, it is necessary to adjust the relative contents of the elements described in the formula (1) so that γp is 120 or more. When γp is less than 120, fluctuation of hardness becomes large depending on quenching conditions and δFe in steel also increases. In the exemplary embodiment of the invention, γp may be adjusted to 130 or more, or alternatively, 140 or more. In the exemplary embodiment of the invention, γp may be 170 or less, or alternatively, 150 or less.

[0030] Each of the steel sheet and the steel member in the exemplary embodiment of the invention has a steel composition with the balance consisting of Fe and impurities. Moreover, in the exemplary embodiment of the invention, in addition to the above-described elements, elements of Mo, Nb, Ti, Sn, and Bi may be added in place of a part of Fe in order to improve rust resistance and corrosion resistance.

Mo: 0.01 to 1.0%



[0031] Mo is an element that improves corrosion resistance. This effect is produced at a Mo content of 0.01% or more. However, Mo is an expensive element and the effect thereof is not clear even if Mo is added excessively. The upper limit of the Mo content is thus defined as 1.0%.

Ti: 0.005 to 0.050%



[0032] Ti is an element that, by forming carbonitrides, suppresses a sensitization phenomenon due to precipitation of chromium carbonitride and a decrease in corrosion resistance in stainless steel. This effect is produced at a Ti content of 0.005% or more. However, an excessive addition of Ti destabilizes a martensite phase to lower hardness, so that the upper limit of the Ti content is defined as 0.050%.

Nb: 0.005 to 0.050%



[0033] Nb is an element that, by forming carbonitride, suppresses a sensitization phenomenon due to precipitation of chromium carbonitride and a decrease in corrosion resistance in stainless steel. This effect is produced at a Nb content of 0.005% or more. However, an excessive addition of Nb destabilizes a martensite phase to lower hardness, so that the upper limit of the Nb content is defined as 0.050%.

Sn: 0.01% to 0.10%



[0034] Sn is an element effective for improving corrosion resistance after quenching. A Sn content is preferably 0.01% or more, or alternatively, the Sn content of 0.05% or more is preferably added as needed. However, since an excessive addition of Sn promotes edge cracking at hot rolling, the Sn content is preferably defined as 0.10% or less.

Bi: 0.01% to 0.20%



[0035] Bi is an element that improves corrosion resistance. Although the mechanism is not clarified, it is deduced that addition of Bi decreases probability that MnS becomes a starting point of rusting because Bi micronizes MnS that is likely to be the starting point. The effect is produced at a Bi content of 0.01% or more. Even if Bi exceeding 0.20% is added, the effect only becomes saturated. Accordingly, the upper limit of the Bi content is defined as 0.20%.

δ Ferrite Phase Ratios of Steel Sheet and Steel Member



[0036] The inventors have found that δ ferrite (δFe) existing in a central part in a thickness direction of a steel sheet significantly influences end-surface corrosion resistance of the steel sheet. When the steel sheet is quenched at a cooling rate as slow as air cooling, it is considered that grain boundaries between the δFe phase and a matrix phase (γ phase) become a precipitation site for Cr carbides during cooling, thereby causing sensitization in the vicinity of the precipitated Cr carbides to lower end-surface corrosion resistance. Moreover, it is presumed that N improves the end-surface corrosion resistance because of also having an effect of suppressing the precipitation of Cr carbides.

[0037] Accordingly, according to the exemplary embodiment of the invention, it is effective for improving the end-surface corrosion resistance to contain N and suppress δFe in steel.

[0038] It would be favorable if δFe existing in the steel sheet before quenching could be measured, but δFe is difficult to measure because all the surroundings thereof are ferrite phases. In contrast, δFe existing in the steel sheet after quenching and tempering is relatively easy to measure because the surroundings are martensite phases. Therefore, the steel sheet according to the exemplary embodiment of the invention is subjected to quenching and tempering and then evaluated in terms of the content of δFe. For evaluation, the quenching conditions are heating to 1050 degrees C, holding for 30 minutes, and then air cooling, and the tempering condition is heating at 150 degrees C for 30 minutes. An excessively low temperature and an excessively short time for quenching are unfavorable since a ferrite phase remains and cannot be distinguished from a δFe phase. An excessively high temperature and an excessively long time for quenching are unfavorable since a δFe phase is changed into a state different from an initial state. A quenching method is air quenching. After the steel sheet is subjected to quenching and tempering under the conditions for evaluation, the steel sheet is evaluated in terms of an abundance area ratio in a cross section of the steel sheet in a thickness direction. A favorable end-surface corrosion resistance can be obtained at δFe of 1% or less. At δFe of less than 0.1%, any steel sheet, regardless of the invention, exhibits an excellent corrosion resistance. However, δFe of less than 0.1% is not preferable since a heat treatment for a long time is required for decreasing δFe, resulting in an increase in costs.δFe of more than 1% is not preferable since a corrosion resistance cannot be sufficiently improved and hardness is also insufficient even according to the exemplary embodiment of the invention. The upper limit of δFe is further preferably 0.5%. A favorable range of δFe is from 0.1% to 0.5%.

Manufacturing Method of Steel Sheet



[0039] A conventional method is used as a manufacturing method of a steel sheet in the exemplary embodiment of the invention. A slab having adjusted components is obtained by melting and casting. The slab is hot-rolled, then box-annealed, shot, and pickled to provide a product.

[0040] The slab is preheated in order to control δFe. Heating conditions here is preferably heating at a temperature from 1100 to 1150 degrees C for a soaking time from 1 hour to 50 hours. When the heating temperature exceeds 1150 degrees C., a duplex phase (γ+δ) becomes stable and the content of δFe rapidly increases, which is not preferable. In addition, the rapidly increased δFe remains in a large content even in the subsequent process, which causes a decrease in hardness. On the other hand, when the heating temperature is lower than 1100 degrees C, δFe does not decrease even after heating for a long time, which is not preferable. Since the content of δFe is smaller than that at the heating temperature of more than 1150 degrees C, the hardness may be maintained depending on the subsequent process. Further, the soaking time of less than 1 hour is not preferable since the content of δFe becomes excessively large, and the soaking time exceeding 50 hours is not preferable because of high costs.

[0041] This preheating may be conducted as slab heating before hot rolling and directly followed by hot rolling.

Manufacturing Method of Steel Member



[0042] According to the exemplary embodiment of the invention, the obtained steel sheet is punched, quenched, tempered, and polished to prepare a member. After punched, the steel sheet is forged to adjust a shape. Preferable quenching and tempering conditions are as follows. A quenching temperature preferably ranges from 1000 to 1150 degrees C. The quenching temperature of less than 1000 degrees C is not preferable since a content of an austenite phase is small at a high temperature and hardness after quenching is lowered. The quenching temperature exceeding 1150 degrees C is not preferable since a δ phase and a stable austenite phase are increased, and also in this case, hardness is lowered. A holding time during quenching is preferably from one minute to one hour. The holding time of less than one minute is not preferable since the content of the austenite phase is small at a high temperature and hardness after quenching is lowered. The holding time exceeding one hour is not preferable since a stable austenite phase is increased, and also in this case, hardness is lowered. A cooling rate during quenching is preferably equal to or more than 1 degree C/sec at an average cooling rate from the quenching temperature to 600 degrees C. The cooling rate less than 1 degree C/sec is not preferable since hardness is lowered. The above preferable cooling rate can be achieved by quenching through air cooling. A tempering temperature preferably ranges from 100 to 250 degrees C. The tempering temperature of less than 100 degrees C is not preferable because of poor tempering effects. The tempering temperature exceeding 250 degrees C is not preferable because of an excessively large reduction in hardness.

Example(s)



[0043] The effects of the invention will be described below with reference to Examples. However, the invention is by no means limited to conditions used in the following Examples.

[0044] In Examples, first, steels having compositions shown in Tables 1 and 2 were melted and cast into slabs each having a thickness of 250 mm. Next, these slabs were preheated at 1150 degrees C for 40 hours to provide the content of δFe falling within a predetermined range. However, A2 steel was preheated at 1175 degrees C for 40 hours to provide A2' steel and was preheated at 950 degrees C for 40 hours to provide A2" steel.

[0045] Subsequently, the A2 steel was heated at 1150 degrees C and hot-rolled to provide a hot-rolled steel sheet having a sheet thickness of 3 to 8 mm. Subsequently, the hot-rolled steel sheet was annealed by box annealing. The maximum heating temperature was set in a temperature range of 800 degrees C to 900 degrees C. Scale on a surface of each steel sheet after annealing was removed by shot blasting and pickled.






Example 1



[0046] In order to evaluate the obtained steel sheets, a sample for evaluation was cut out from each steel sheet, and the sample was subjected to quenching and tempering: heated to 1050 degrees C, held for 30 minutes, then air-cooled, and tempered at 150 degrees C for 30 minutes. Subsequently, the measurement of the content of δFe, the measurement of hardness, and the evaluation of the end-surface corrosion resistance were performed. The obtained results are shown in Table 3.





[0047] An end surface of each sample was mirror-polished and etched to reveal a structure, where the content of δFe was measured. Although δFe can appear in aqua regia or the like as an etching solution, it is preferable to use a reagent called a modified Murakami's reagent described in Non-Patent Literature 1 because δ Fe is deeply etched in brown by the reagent. Accordingly, evaluation was performed using this reagent. Fig. 1 shows a representative example.

[0048] The structure revealed by the modified Murakami's reagent was examined under a microscope. A photograph of δFe was taken with a certain width (2 mm in this example) over a total thickness. An area of δFe was obtained by an image analysis, and then an area ratio (δFe area (mm2)/2 mm x total thickness (mm) x 100 (%)) was calculated. In order that the steel member with the above components according to the exemplary embodiment of the invention exhibits an excellent corrosion resistance, a value of the area ratio needs to be from 0.1 to 1%. Further the value of the area ratio is preferably from 0.1 % to 0.5%. For the area ratio of δFe, 0.1 to 1% was defined as Pass (A) and a value falling out of this range was defined as Fail (X).

[0049] For hardness, after finish polishing of a top surface of the sample using #80, the surface hardness (quenching hardness) was evaluated using a Rockwell hardness tester C scale in accordance with JIS Z 2245, and 50 or more was defined as Pass (A) and a value falling out of this range was defined as Fail (X).

[0050] To evaluate the end-surface corrosion resistance, after polishing the top surface and an end surface of the sample using #600, the end surface, which was directed upward and used as the evaluation surface, was subjected to a salt spray test for 24 hours (JIS Z 2371 "salt spray test method"), where the number of rusting points was counted. A sample having two or less rusting points was defined as Pass (A). A sample having more than two rusting points was defined as Fail (X). Especially, a sample having no rusting point was defined Pass (S). Even if the salt spray test was conducted for 24 hours or more, rust did not develop any further. Therefore, the end-surface corrosion resistance was judged based on the results obtained for 24 hours.

[0051] All the steel sheets according to the exemplary embodiment of the invention exhibit not only an excellent end-surface corrosion resistance but also excellent other properties, therefore, are preferable as a steel sheet for a Western tableware knife. In contrast, comparative steels exhibit a poor end-surface corrosion resistance or poor other properties, therefore, are apparently not preferable as a steel sheet for a Western tableware knife.

Example 2



[0052] A member cut out from each of the obtained steel sheets was quenched and tempered under conditions shown in Table 4 to provide a steel member. In quenching, the member was heated at a temperature from 1050 to 1150 degrees C and then was cooled at a controlled cooling rate described in Table 4 from the quenching temperature to 600 degrees C. Further, the member was tempered at a temperature from 150 to 250 degrees C for one to two hours to provide a steel member. The A2' steel and A2" steel were processed in the same manner.

[0053] The measurement of the content of δFe, the measurement of hardness, and the end-surface corrosion resistance evaluation of the obtained steel members are shown, together with heat treatment conditions, in Table 4. The same evaluation method and evaluation standard as in Example 1 were used.





[0054] All the steel members according to the exemplary embodiment of the invention exhibit not only an excellent end-surface corrosion resistance but also excellent other properties, therefore, are preferable as a steel member for a Western tableware knife. In contrast, comparative steels exhibit a poor end-surface corrosion resistance or poor other properties, therefore, are apparently not preferable as a steel member for a Western tableware knife.

INDUSTRIAL APPLICABILITY



[0055] According to the invention, a martensitic stainless steel member having an excellent end-surface corrosion resistance after air quenching can be manufactured with a high productivity. The martensitic stainless steel member is industrially very useful since the martensitic stainless steel member improves corrosion resistance of a Western tableware knife manufactured therefrom.


Claims

1. A martensitic stainless steel sheet with a steel composition comprising: by mass%

C of 0.100 to 0.170%,

Si of 0.25 to 0.60%,

Mn of 0.10 to 0.60%,

P of 0.035% or less,

S of 0.015% or less,

Cr of 11.0 to 15.0%,

Ni of 0.05 to 0.60%,

Cu of 0.010 to 0.50%,

V of 0.010 to 0.10%,

Al of 0.05% or less,

N of 0.060 to 0.090%,

C+1/2N of 0.130 to 0.190%,

a balance consisting of Fe and impurities, and γp represented by a formula (1) below being 120 or more, wherein

when the martensitic stainless steel sheet is held at 1050 degrees C for 30 minutes, followed by air quenching, and tempered at 150 degrees C for 30 minutes, an area ratio of δ ferrite (δFe) existing in a central part in a thickness direction of the martensitic stainless steel sheet in a cross section in the thickness direction thereof is from 0.1 to 1%,

        γp = 420C + 470N + 30Ni + 7Mn + 9Cu - 11.5Cr - 11.5Si - 12Mo - 23V - 47Nb - 52Al + 189 ...     Formula (1)

where symbols of elements in the formula (1) mean contents (mass%) of the respective elements.


 
2. The martensitic stainless steel sheet according to claim 1, further comprising: by mass%, in place of a part of Fe, one or two or more of Mo of 0.01 to 1.0%, Ti of 0.005 to 0.050%, and Nb of 0.005 to 0.050%.
 
3. The martensitic stainless steel sheet according to claim 1 or 2, further comprising: by mass%, in place of a part of Fe, one or two of Sn of 0.01% to 0.10%, and Bi of 0.01% to 0.20%.
 
4. A martensitic stainless steel member comprising the steel composition according to any one of claims 1 to 3, wherein
γp represented by a formula (1) below is 120 or more, and an area ratio of δ ferrite (δFe) existing in a central part in a thickness direction of the martensitic stainless steel member in a cross section in the thickness direction thereof is from 0.1 to 1%,

where symbols of elements in the formula (1) mean contents (mass%) of the respective elements.
 




Drawing







Search report










Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description




Non-patent literature cited in the description