JPH0379426B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention has excellent hardenability leading to good wear resistance, as well as high toughness and high corrosion resistance, and is a low carbon material suitable for use as a material for motorcycle brake discs. This relates to martensitic stainless steel. <Prior art and its problems> In recent years, the demand for motorcycles has been increasing more and more, and at the same time, customer demands for performance, appearance, etc. are becoming more sophisticated. For example, in the case of braking devices, in order to meet these demands, the use of disc brakes has become commonplace, and various innovations and improvements have also been made to the materials etc. of the brakes. By the way, brake discs used on motorcycles are of course required to have excellent wear resistance and toughness, but in addition, due to the diversification of usage environments and aesthetic factors, excellent corrosion resistance and resistance are required. Having toughness is also becoming an essential requirement. Conventionally, in order to deal with this situation, motorcycle brake discs have been made of high carbon 1Cr steel (0.3wt%C-15.5wt%Cr steel) or SUS 420J2 steel (0.3wt%C-13wt%Cr steel). High-carbon martensitic stainless steels such as those used for this purpose were used, and the hardness was controlled to about H _R C35±3 through heat treatment. Adjusting the hardness of the brake disc to the value mentioned above is difficult because the hardness is too high considering only the wear resistance (the wear resistance of the disc is almost proportional to its hardness, so if the hardness is too high) This is because not only does this result in brake squeal, but also has a negative effect on braking stability. However, with the conventional materials, there are difficulties in adjusting the hardness, and improvements in this are strongly awaited.
It was not necessarily of the quality that would be achieved at present or in the near future. This is because the quenching hardness of the high carbon steel materials that have been used for motorcycle brake discs changes greatly depending on the content of C and Cr, etc. and slight variations in the quenching temperature.
Obtaining a hardness in the narrow range of H _R C35 ± 3 by “quenching” alone requires extremely strict material control and heat treatment control, and in practice, it is difficult to manufacture products of stable quality with good workability. It was extremely difficult. Therefore, in general, we give up on hardness adjustment by "quenching" alone, and instead achieve the target hardness by "tempering" at an appropriate temperature (550-650℃) after quenching. This process had no choice but to be taken. Moreover, the above-mentioned method involving "tempering" had the following problems. In other words, in metallurgical terms, "tempering" in this case is a transformation phenomenon from martensite to sorbite due to the precipitation of Cr carbides, but it is undeniable that the Cr concentration locally decreases around the precipitation of Cr carbides. However, there was nothing that could be done to prevent the corrosion resistance of the disk material from decreasing due to this. Due to these circumstances, we have recently been working on a steel that can easily obtain the characteristics required for brake discs only by "quenching", and we have developed Mn.
Proposals have also been made regarding low carbon martensitic stainless steel for motorcycle disc brakes with increased carbon content (Japanese Patent Application Laid-open No. 198249/1983). However, the low carbon martensitic stainless steel proposed in JP-A No. 57-198249 stably achieves the desired corrosion resistance, etc., as is clear from the description of the examples in the application specification. It is necessary to set the Cr content as high as 12 to 14% (hereinafter, % is by weight), which not only deteriorates the toughness of the steel material, but also makes it difficult to maintain austenite even if the Mn content is increased. Subsequent research by the present inventors has revealed that the stabilization of the steel is insufficient and there are problems such as problems with hardening characteristics and corrosion resistance. <Means for Solving the Problems> Therefore, the inventors of the present invention have discovered that a hardness of H _R C35±3 can be stably and easily obtained only by "quenching" without strictly controlling the heat treatment conditions, and that a brake for motorcycles can be obtained. In order to provide a steel material with excellent toughness and corrosion resistance that can be used as a disc material, we continued basic testing and research from various perspectives, and as a result, (a) martensitic stainless steel standardized as SUS403. Based on this, by regulating the total amount of C and N, keeping the Cr content low, and adding an appropriate amount of Mo, it is possible to obtain steel with well-balanced improvements in hardenability, toughness, and corrosion resistance. (b) It is possible to stably adjust the hardness of such steel to a suitable hardness (H _R C35±3) for motorcycle brake disc materials only by ordinary quenching; (c) The findings as shown in (a) to (c) above were obtained that when an appropriate amount of Nb is further added, the corrosion resistance is further improved. This invention was made based on the above knowledge, and stainless steel material is made of C: 0.08% or less, N: 0.04% or less (however, the total amount of C + N is 0.06 to 0.09%), and Si: 0.5% or less. , Mn: 0.4 to 1.0%, Cr: 10.0 to 11.5%, Ni: 0.5 to 1.0%, Mo: 0.05 to 0.5%, and if necessary, Nb: 0.04 to 0.1%, and the remainder is a low carbon martensitic steel that has excellent hardenability, toughness and corrosion resistance by having a composition consisting of Fe and unavoidable impurities (the above % indicates weight %), It has the following characteristics. Next, the reason why the component proportions of the low carbon martensitic stainless steel of the present invention are numerically limited as described above will be explained. (a) C and N Both C and N components are elements that determine the quenching hardness of steel, but even if the total amount of C + N is less than 0.06%, C and N are 0.08% and 0.04%, respectively. , and the total amount of C+N is 0.09%
Even if the hardness is exceeded, it will not be possible to obtain a hardness of H _R C35±3 during quenching from the austenite region (from the normal quenching temperature of 850 to 1000℃), and the content of C and N The amount has a significant effect on the toughness of steel, and if the content exceeds C: 0.08% and N: 0.04%, and the total amount of C + N falls outside the range of 0.06 to 0.09%, the toughness will deteriorate. Therefore, the content should be reduced to C: 0.08% or less, N: 0.04%
% or less, and the total amount of C+N was set at 0.06 to 0.09%. Figure 1 shows 0.3%Si-0.8%Mn-11.5%Cr-
FIG. 2 is a diagram showing the relationship between the [C+N] amount and quenching hardness in 0.8%Ni-0.3%Mo steel, and FIG. 2 is a diagram showing the relationship between the [C+N] amount and impact value in a similar steel. However, from these diagrams, in order to stably achieve a hardness of H _R C35±3 and obtain a steel that exhibits good toughness, [C+N]
It is clear that the amount should be between 0.06 and 0.09%. (b) Si Si is a ferrite-forming element that not only lowers the quenching hardness of steel but also has a negative effect on toughness, so it is not preferred as an alloying element. When the content exceeds 0.5%, the above-mentioned adverse effects become particularly severe, so Si
The content was set at 0.5% or less, similar to SUS 13Cr steel (for example, SUS403), but from the viewpoint of steel manufacturing, the lower the content, the better, as long as sufficient deoxidation can be achieved. (c) Mn Since the Mn component is an austenite-forming element, it has the effect of stabilizing hardening hardness, and is also essential from the viewpoint of deoxidation. However, in the case of the steel of the present invention, as described later, as an element that stabilizes the quench hardenability,
The Mn content is set at 1.0.
% or less. Although the desired effect can be obtained even if the Mn content is extremely small, it is preferable to add 0.4% or more in order to obtain a sufficient effect for the above-mentioned effect. (d) Cr The Cr component has the effect of ensuring the corrosion resistance of stainless steel, but if the content is less than 10.0%, the desired effect cannot be obtained in this effect; on the other hand, if the content exceeds 11.5%, The Cr content should be 10.0% or more, as this will cause deterioration of toughness.
It was set at 11.5%. Figure 3 shows 0.3%Si-0.8%Mn-0.8%Ni-0.3
%Mo steel is a diagram showing the relationship between the amount of Cr and corrosion resistance (indicated by a 10-level rating rating based on the salt spray test specified in JIS Z2371), and Figure 4 shows the amount of Cr in similar steel. It is clear from these diagrams that good corrosion resistance and toughness are achieved when the Cr content is in the range of 10.0 to 11.5%. Furthermore, since the martensitic stainless steel of this invention has a low Cr content, it has high austenite stability even at high temperatures, as is clear from the Fe-Cr phase diagram, and has excellent hardenability, toughness, and corrosion resistance. However, this point is also one of the major features of this invention. (e) Ni Since Ni is a strong austenite-forming element, it has an effective effect on improving quenching stability and also has an effect on improving toughness, but if its content is less than 0.5%, it will cause hardening. The desired stability improvement effect was not obtained, while 1.0%
The Ni content was determined to be 0.5 to 1.0% because the effect would be saturated and it would be economically unfavorable if the Ni content exceeded 0.5% to 1.0%. Figure 5 shows 0.3%Si−0.8%Mn−11.5%Cr−
FIG. 6 is a diagram showing the relationship between Ni content and quenching hardness in 0.3% Mo steel, and FIG. 6 is a diagram showing the relationship between Ni content and impact value in similar steel.
It is clear from these diagrams that Ni addition of 0.5 to 1.0% is effective. (f) Mo The Mo component has the effect of significantly improving the corrosion resistance of steel, and it is essential to compensate for the corrosion resistance by adding Mo in the stainless steel of this invention with reduced Cr content. The content is
If it exceeds 0.5%, the effect of improving corrosion resistance will be saturated, so the Mo content was set at 0.5% or less. Although Mo exhibits a certain effect of improving corrosion resistance even in a very small amount, it is preferably contained in an amount of 0.05% or more. Figure 7 shows 0.3%Si−0.8%Mn−11.5%Cr−
Mo content and corrosion resistance in 0.8% Ni steel (JIS
This is a diagram showing the relationship between the corrosion resistance (denoted by a 10-level latency number based on the salt spray test specified in Z2371), and it can be seen from the diagram that corrosion resistance is significantly improved by adding Mo. . (g) Nb Since the Nb component also has the effect of improving the corrosion resistance of the stainless steel of this invention, it is added when higher corrosion resistance is required.
If the Nb content exceeds 0.1%, it not only significantly impairs economic efficiency but also saturates the effect of improving corrosion resistance, so the Nb content was set at 0.1% or less. Note that Nb also exhibits a certain degree of corrosion resistance improvement effect when added in a very small amount, but preferably 0.04
% or more is preferable. The low carbon martensitic stainless steel of this invention is characterized by the above-mentioned composition, and although it goes without saying that it is better to have as few impurity elements as possible that are inevitably mixed in, it is especially desirable to reduce S. It is preferable to suppress the S content to 0.001% or less. Next, the present invention will be explained by examples. <Example> First, steel with the chemical composition shown in Table 1 was prepared.
The steel ingots were melted in the atmosphere in a 100Kg high-frequency melting furnace, and then subjected to conventional peeling, hot rolling, and annealing to obtain hot rolled sheets with a thickness of 6 mm. Next, these were sufficiently softened and annealed at 750℃,

【table】

[Table] Disc brake materials and laboratory test pieces were collected. The hardenability, toughness, and corrosion resistance of the test pieces thus obtained were investigated, and the results were reported in the 8th section.
, FIG. 9, FIG. 10, and Table 2. In general, brake disc materials are subjected to induction hardening to improve efficiency and prevent oxidation.
6mmφ It clearly shows that it has extremely excellent hardenability, reaching the target hardness and having almost uniform hardness in both the surface layer and the center.From this point of view, it is suitable as a brake disc material. It can be seen that this is suitable. Note that such quenching characteristics are not limited to Invention Steel 1;
It was confirmed that the same holds true for Invention Steels 2 to 15. In addition, when performing Cu brazing bonding of decorative materials to brake discs and heat treatment for hardening at the same time, heating to approximately 1150 to 1200°C is required, but the results shown in Figure 8 are based on such heat treatment. It also clearly shows that a hardening hardness of H _R C35±3 can be obtained even by hardening at a high temperature. This means that it is possible to have a large degree of freedom in material selection. Invention steels 1 and 15 and conventional steel 1
These facts are clearly supported by FIG. 9, which shows the high temperature quenching test results for No. 6. On the other hand, conventional steel 16 usually has a
At 1200℃, the austenite phase does not form as a single phase, but the ferrite phase precipitates, resulting in a two-phase structure of "martensite phase + ferrite phase" in the quenched state. This resulted in a decrease in the toughness and toughness. Furthermore, the reason why the steel of the present invention can obtain sufficiently stable quenching hardness is that the ferrite phase does not precipitate even at high temperatures exceeding 1200°C due to the low Cr content, and only austenite phase exists in the above temperature range. This is because martensite 1 phase can be sufficiently secured even after quenching. FIG. 10 is a diagram showing the relationship between quenching temperature and toughness for Inventive Steels 1 and 15 and Conventional Steel 16. The Inventive Steels are all 5Kg- However, conventional steel 16 has low toughness, especially when quenched at 850°C. It shows an impact value of only about 2 kg-m, which indicates that there is a problem with safety. Furthermore, Table 2 shows the results of the Charpy impact test at various temperatures (quenched at 950°C) and the salt spray test results (according to the salt spray test specified in JIS Z 2371) of the present invention steels 1 to 15 and conventional steel 16.
Table 2 shows that the steel of the present invention has sufficient toughness even at a low temperature of -25°C, making it suitable for use as a motorcycle disc. case

It can be seen that the [Table] can be made to run more stably and reliably in cold regions, and that it also has excellent corrosion resistance, less occurrence of rust, and is sufficiently superior in appearance retention performance and durability. <Overall Effects> As explained above, according to this invention, 850
The hardness can be stably adjusted to H _R C35 ± 3 only by quenching from a wide temperature range of ~1200℃, and even rapid quenching can achieve uniform hardness and improve toughness. In addition, extremely useful effects are brought about industrially, such as the production of a low carbon martensitic stainless steel that has excellent corrosion resistance and is suitable as a brake disc material for motorcycles.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between [C+N] amount and hardening hardness, Figure 2 is a diagram showing the relationship between [C+N] amount and impact value, and Figure 3 is a diagram showing the relationship between Cr content and corrosion resistance. Figure 4 is a diagram showing the relationship between Cr content and impact value, Figure 5 is a diagram showing the relationship between Ni content and quenching hardness, and Figure 6 is a diagram showing the relationship between Cr content and impact value. ,
Figure 7 is a diagram showing the relationship between Ni content and impact value, Figure 7 is a diagram showing the relationship between Mo content and corrosion resistance, and Figure 8 is a diagram showing the relationship between quenching temperature and surface layer and center area in the rapid quenching test. Figure 9 is a diagram showing the relationship between quenching temperature and hardness in a high temperature quenching test. Figure 10 is a diagram showing the relationship between quenching temperature and toughness. .

Claims (1)

[Claims] 1% by weight: C: 0.08% or less, N: 0.04% or less (however, the total amount of C + N is 0.06 to 0.09%), Si: 0.5% or less, Mn: 0.4 to 1.0%, Cr: 10.0 to 11.5%, Ni: 0.5 to 1.0%, Mo: 0.05 to 0.5%, and the remainder is Fe and other unavoidable impurities. Low carbon martensitic stainless steel with excellent corrosion resistance. 2% by weight: C: 0.08% or less, N: 0.04% or less (however, the total amount of C + N is 0.06 to 0.09%), Si: 0.5% or less, Mn: 0.4 to 1.0%, Cr: 10.0 to 11.5% , Ni: 0.5 to 1.0%, Mo: 0.05 to 0.5%, and further contains Nb: 0.04 to 0.1%, with the remainder consisting of Fe and other inevitable impurities. , low carbon martensitic stainless steel with excellent hardenability, toughness, and corrosion resistance.