Martensitic stainless steel with excellent hardening ability

Abstract

The object of the present invention is to provide a martensitic stainless steel which exhibits excellent hardenability due to a small hardness deviation. [Solution] The martensitic stainless steel of the present invention, which has excellent hardenability through control of its component system, contains, by weight, 0.01-0.1% C, 0.05-1.0% Si, 0.05-1.0% Mn, 11.0-14.0% Cr, 0.05-1.0% Ni, 0.05%-2.0% Cu, 0.04%-0.08% N, with the remainder being Fe and unavoidable impurities, and satisfies the following formula (1). (1) 1.0≦Mn+Ni+Cu≦2.5

Description

Technical Field

[0001] The present invention relates to a martensitic stainless steel having excellent hardening ability, and more particularly, to a martensitic stainless steel showing excellent hardening ability due to little hardness deviation.

Background Art

[0002] Generally, materials for discs used in motorcycles are required to have high hardness to prevent wear of the discs, and thus, martensitic stainless steels with high hardness are mainly used.

[0003] Martensitic stainless steel is composed of a ferrite phase and precipitates when manufactured as a sheet material, and is subjected to hardening heat treatment after punching in the form of a disc. The hardening heat treatment involves heating to a temperature at which the ferrite phase transforms into an austenite phase, maintaining for a certain period of time, and then rapidly cooling to form a martensite phase. When the martensite phase is formed, appropriate high hardness for motorcycle discs can be obtained.

[0004] However, in order to exhibit uniform disc performance, the hardness deviation must be small so that the hardness by position of the disc is uniform. When the hardness deviation is large, wear of the pads that rub against the disc occurs quickly, or appropriate braking performance cannot be obtained. Therefore, a martensitic stainless steel with uniform hardness by position of the disc is required.

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a martensitic stainless steel showing excellent hardening ability due to little hardness deviation.

Means for Solving the Problems

[0006] The martensitic stainless steel of the present invention, which has excellent hardening ability, is characterized by containing, by weight percent, C: 0.01~0.1%, Si: 0.05~1.0%, Mn: 0.05~1.0%, Cr: 11.0~14.0%, Ni: 0.05~1.0%, Cu: 0.05%~2.0%, N: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1).

[0007] (1) 1.0 ≤ Mn + Ni + Cu ≤ 2.5

[0008] (Here, Mn, Ni, and Cu represent the content (by weight %) of each element.)

[0009] Furthermore, the area fraction of the ferrite phase may be 20% or less in any cross-section of the martensitic stainless steel with excellent hardening ability according to the present invention.

[0010] Furthermore, in any cross-section, there are 2 precipitates per 100 μm with a major axis length exceeding 1 μm. 2 The following is also acceptable.

[0011] Furthermore, the Rockwell hardness deviation may be 2.0 or less in any cross-section. [Effects of the Invention]

[0012] The martensitic stainless steel of the present invention can reduce the area fraction of the ferrite phase or the number of coarse precipitates by controlling the composition, thereby reducing the hardness deviation and improving the hardening ability. [Brief explanation of the drawing]

[0013] [Figure 1] This is a photograph showing the ferrite and martensite phases in a cross-section of conventional martensitic stainless steel. [Figure 2] This is a photograph showing the ferrite phase and martensite phase in a cross-section of martensitic stainless steel according to one embodiment of the present invention. [Figure 3]This is a photograph showing precipitates on a cross-section of martensitic stainless steel according to one embodiment of the present invention. [Modes for carrying out the invention]

[0014] A martensitic stainless steel with excellent hardening ability according to one embodiment of the present invention contains, by weight percent, C: 0.01~0.1%, Si: 0.05~1.0%, Mn: 0.05~1.0%, Cr: 11.0~14.0%, Ni: 0.05~1.0%, Cu: 0.05%~2.0%, N: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities, and satisfies the following formula (1).

[0015] (1) 1.0 ≤ Mn + Ni + Cu ≤ 2.5

[0016] (Here, Mn, Ni, and Cu represent the content (by weight %) of each element.)

[0017] The following embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the concept of the present invention to those who have ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein and can be embodied in other forms. In order to clarify the present invention, the drawings may omit illustrations of parts not relevant to the description and may slightly exaggerate the sizes of components to aid understanding.

[0018] Furthermore, when we say that any part "includes" a certain component, this does not mean that it excludes other components, unless otherwise stated, but rather that it may include other components.

[0019] A singular expression includes plural forms unless the context clearly indicates otherwise.

[0020] The martensitic stainless steel with excellent curability according to an embodiment of the present invention contains, in weight %, C: 0.01 to 0.1%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 11.0 to 14.0%, Ni: 0.05 to 1.0%, Cu: 0.05% to 2.0%, N: 0.04% to 0.08%, and the balance consists of Fe and inevitable impurities.

[0021] Hereinafter, the reasons for the numerical limitations of the element contents of the alloy components in the examples of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight %.

[0022] The content of C (carbon) is 0.01 to 0.1%.

[0023] C is an element that greatly affects the hardness. If its content is less than 0.01%, the desired level of hardness cannot be obtained. If it exceeds 0.1%, the hardness becomes excessive and exceeds the level of hardness required for disk applications.

[0024] The content of Si is 0.05 to 1.0%.

[0025] Si is an element that can improve the corrosion resistance and is added at 0.05% or more. However, if its content exceeds 1.0%, it may inhibit the toughness during production, so its upper limit is limited to 1.0% or less.

[0026] The content of Mn (manganese) is 0.05 to 1.0%.

[0027] Mn is an element that helps form the austenite phase during hardening heat treatment and is added at 0.05% or more. If the content of Mn exceeds 1.0%, it may inhibit the corrosion resistance, so its upper limit is set to 1.0% or less.

[0028] The content of Cr (chromium) is 11.0 to 14.0%.

[0029] Cr is an element that improves the corrosion resistance of steel and should be added at a concentration of 11.0% or more. However, since excessive Cr content is a major factor in increasing the size of precipitates, its upper limit should be restricted to 14.0% or less.

[0030] The nickel (Ni) content is 0.05-1.0%.

[0031] Ni is an element that helps form the austenite phase during the hardening heat treatment, and it is added at a concentration of 0.05% or more. Since Ni is an expensive element, adding large amounts of Ni will increase costs, so the upper limit should be 1.0% or less.

[0032] The copper (Cu) content is 0.05-2.0%.

[0033] Cu is an element that helps form the austenite phase during the hardening heat treatment, and in this invention, it is added in an amount of 0.05% or more. However, since an excessive amount of Cu would increase costs, its upper limit is restricted to 2.0% or less.

[0034] The nitrogen (N) content is 0.04-0.08%.

[0035] N is an element that controls the hardness of the disk and is present in amounts of 0.04% or more. When the N content exceeds 0.08%, the hardness becomes excessive and exceeds the level of hardness required for disk applications.

[0036] The remaining stainless steel, after removing the alloying elements mentioned above, consists of Fe and other unavoidable impurities.

[0037] To improve the hardening ability of stainless steel, the positional hardness deviation of the stainless steel must decrease after the hardening heat treatment. The positional hardness deviation of stainless steel is due to the presence of other different phases in addition to the martensite phase in the hardened stainless steel. If the ferrite phase that constituted the stainless steel before the hardening heat treatment does not sufficiently transform into the austenite phase during the hardening heat treatment, the ferrite phase will remain after the hardening heat treatment, thereby increasing the hardness deviation.

[0038] Furthermore, in order to improve the hardening ability of stainless steel, there should be no coarse precipitates before the hardening heat treatment. If large precipitates are present, the transformation to the austenite phase will not occur sufficiently during the hardening heat treatment, resulting in the ferrite phase remaining after the hardening heat treatment and an increase in hardness deviation.

[0039] In this invention, formula (1) was used to derive a range of components that can reduce the area fraction of the residual ferrite phase after hardening heat treatment.

[0040] (1) 1.0 ≤ Mn + Ni + Cu ≤ 2.5

[0041] (Here, Mn, Ni, and Cu represent the content (by weight %) of each element.)

[0042] When the value of equation (1) is between 1.0 and 2.5, the ferrite phase can sufficiently transform into the austenite phase during the hardening heat treatment, and the area fraction of the ferrite phase falls below a certain level, thereby controlling the hardness deviation to an appropriate level or lower.

[0043] When the value of equation (1) is between 1.0 and 2.5, the area fraction of the ferrite phase remaining after the hardening heat treatment is 20% or less in any cross-section, preferably 10% or less. Here, any cross-section means a surface in which the martensitic stainless steel is cut in any direction after the hardening heat treatment, and specifically, any cross-section means a surface parallel to the longitudinal direction of a precipitate whose major axis length exceeds 1 μm.

[0044] Furthermore, if the value of equation (1) is between 1.0 and 2.5, the number of coarse precipitates generated before the hardening heat treatment can be reduced, and the persistence of the ferrite phase after the hardening heat treatment can be prevented, thereby reducing the hardness deviation.

[0045] If the value of equation (1) is between 1.0 and 2.5, then there are 2 precipitates per 100 μm in any cross-section with a major axis length exceeding 1 μm before the hardening heat treatment. 2 The following conditions may exist. Here, any cross-section means a surface cut in any direction before the hardening heat treatment of martensitic stainless steel.

[0046] Furthermore, the martensitic stainless steel according to one embodiment of the present invention may have a hardness deviation value of 2 or less, as expressed by formula (2).

[0047] TIFF0007900390000001.tif19128

[0048] (Here, [Hardness-HRC] is the Rockwell hardness (HRC) measured at any given cross-section, and m is the average of 10 Rockwell hardness measurements.)

[0049] If the value of equation (2) is 2 or less, the hardness of the martensitic stainless steel is uniform, which can reduce the wear of the pads that rub against the disc during braking, and ensure the target braking performance.

[0050] Examples

[0051] Stainless steel was cast using the alloy composition system described in Table 1 below and hot-rolled to a thickness of 4 mm. The hot-rolled thickness can be varied depending on the application. After hot-rolling, the temperature was maintained at around 750°C for about 20 hours to transform the austenite phase formed during hot-rolling into a ferrite phase.

[0052] [Table 1]

[0053] For stainless steel manufactured in this manner, the size (μm) and distribution density (pieces / 100μm) of precipitates are as follows: 2 The size and distribution density of precipitates were measured. The size and distribution density of precipitates can be determined by etching the remaining tissue after removing the precipitates and observing it with a scanning electron microscope. The etching method can be one that is common in academia and industry.

[0054] After processing into a disk shape, the material was maintained at 1000°C for 1 minute, followed by water cooling, and the area fraction (%) of the ferrite phase was measured. The area fraction of the ferrite phase can be confirmed by observing an arbitrary cross-section using electron backscatter diffraction (ESF) on a scanning electron microscope, and then displaying an Image Quality Map in the resulting photograph. This can be confirmed using methods commonly used in academia and industry, such as etching.

[0055] Furthermore, to confirm whether the hardness of the disc was appropriate, the Rockwell-C (HRC) was measured 10 times at arbitrary cross-sections, and the hardness deviation was calculated using equation (2). The results for each are shown in Table 2.

[0056] [Table 2]

[0057] As shown in Tables 1 and 2, the steel grades of Examples 1 to 8 satisfy the value of formula (1) between 1.0 and 2.5, and have 2 precipitates per 100 μm in any cross section before strengthening heat treatment, with a major axis length exceeding 1 μm. 2 The following conditions were met: it was confirmed that the area fraction of the ferrite phase was 20% or less in any cross-section after the strengthening heat treatment, and the hardness deviation was 2 or less.

[0058] On the other hand, in Comparative Examples 1 and 3, the value of formula (1) was 0.9 or less, and there were 3 precipitates per 100 μm with a major axis length exceeding 1 μm. 2In conclusion, the hardness deviation was 4 or higher, confirming that it is not suitable as a motorcycle disc, where a hardness deviation of 2 or less is recommended.

[0059] On the other hand, Comparative Examples 2 and 4 did not satisfy the component range of the present invention, with the value of formula (1) being 0.6 or less, the area fraction of the ferrite phase exceeding 20%, and 5 precipitates per 100 μm having a major axis length exceeding 1 μm. 2 That concludes the findings. Furthermore, the hardness deviation was also 10 or greater, and it was confirmed that the further the value of equation (1) deviates from the range of 1.0 to 2.5, the greater the hardness deviation.

[0060] Figure 1 is a photograph showing the ferrite phase and martensite phase in a cross-section of conventional martensitic stainless steel, and Figure 2 is a photograph showing the ferrite phase and martensite phase in a cross-section of martensitic stainless steel according to one embodiment 1 of the present invention.

[0061] In Figures 1 and 2, the bright regions represent the ferrite phase, while the dark, needle-filled regions represent the martensite phase.

[0062] As shown in Figure 1, the area fraction of the ferrite phase exceeds 20%. On the other hand, as presented in this invention, Figure 2 shows that the area fraction of the ferrite phase is 20% or less, and is almost nonexistent.

[0063] Figure 3 is a photograph showing precipitates on a cross-section of martensitic stainless steel according to one embodiment 1 of the present invention.

[0064] As shown in Figure 3, as presented in this invention, there are 2 precipitates per 100 μm with a major axis length exceeding 1 μm. 2 The following results indicate the presence of fine precipitates with a major axis length of 1 μm or less.

[0065] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person with ordinary skill in the art should understand that various modifications and variations are possible without departing from the concepts and scope of the claims described below. [Industrial applicability]

[0066] According to one example of the present invention, it is possible to provide a martensitic stainless steel that exhibits excellent hardening ability due to its small hardness deviation.

Claims

1. In mass percent, it contains C: 0.01–0.1%, Si: 0.05–1.0%, Mn: 0.05–1.0%, Cr: 11.0–14.0%, Ni: 0.05–1.0%, Cu: 0.05–2.0%, N: 0.04–0.08%, with the remainder consisting of Fe and unavoidable impurities. Two precipitates per 100 μm satisfy the following formula (1) and have a major axis length exceeding 1 μm in any cross-section. ２ The following: A martensitic stainless steel with excellent hardening ability, characterized in that the Rockwell hardness deviation expressed by the following formula (2) is 2.0 or less. (1) 1.0≦Mn+Ni+Cu≦2.5 (In equation (1), Mn, Ni, and Cu represent the content (by weight %) of each element.) (In equation (2), [Hardness - HRC] is the Rockwell hardness (HRC) measured at any cross-section, and m is the average of 10 Rockwell hardness measurements.)

2. The martensitic stainless steel with excellent hardening ability according to claim 1, characterized in that the area fraction of the ferrite phase in any cross-section is 20% or less.

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