WO2016043050A1

WO2016043050A1 - Martensitic stainless steel for brake disk and method for producing said steel

Info

Publication number: WO2016043050A1
Application number: PCT/JP2015/074912
Authority: WO
Inventors: 慎一寺岡; 井上　宜治; 祐司小山; 濱田　純一; 利男田上
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2014-09-17
Filing date: 2015-09-02
Publication date: 2016-03-24

Abstract

　Provided is a martensitic stainless steel for use in a two-wheeled-vehicle brake disk, said steel characterized in containing 0.025-0.080% of C, 0.05-0.8% of Si, 0.5-1.5% of Mn, no more than 0.035% of P, no more than 0.015% of S, 11.0-13.5% of Cr, 0.01-0.50% of Ni, 0.01-0.08% of Cu, 0.01-0.30% of Mo, 0.01-0.10% of V, no more than 0.05% of Al, and 0.015-0.060% of N (all percentages being given with respect to mass); having a DFE value of 5 to 30 (inclusive) (where DFE=12(Cr+Si)－430C－460N－20Ni－7Mn－89 (formula 1)); and having a δ ferrite content observed in the cross-sectional structure of 5 to 30% (inclusive) by area. Ti, B, Nb, Sn, or Bi may be added.

Description

Martensitic stainless steel for brake disc and its manufacturing method

The present invention relates to a stainless steel plate for a brake disc of a motorcycle and a manufacturing method thereof, and relates to a martensitic stainless steel plate for a motorcycle brake disc having excellent surface and end surface properties.

¡Brake discs for motorcycles are required to have wear resistance, weather resistance, toughness and other characteristics. Abrasion resistance generally increases with increasing hardness. On the other hand, if the hardness is too high, a so-called brake squeak occurs between the brake and the pad. Therefore, the hardness of the brake is required to be 32 to 38 HRC (Rockwell hardness C scale). Because of these required characteristics, martensitic stainless steel plates are used for the brake discs of motorcycles.

Conventionally, SUS420J2 was quenched and tempered to adjust to a desired hardness and used as a brake disk, but in this case, there was a problem that required two heat treatment steps of quenching and tempering. On the other hand, Patent Document 1 discloses an invention relating to a steel composition that can stably obtain a desired hardness in a wider quenching temperature range than conventional steel that is SUS420J2 steel and that is used as-quenched. It was. This is because, like SUS410, SUS403, and SUS410S steel, the reduction of C and the reduction of the austenite single-phase temperature range due to the reduction of C, that is, the quenching heating temperature range is narrowed by the addition of Mn, an austenite stabilizing element. It is a thing.

Patent Document 2 discloses an invention related to a steel plate for motorcycle disc brakes that is used as-quenched with low Mn steel. This steel sheet is obtained by adding Mn and simultaneously adding Ni and Cu having the same effect as an austenite forming element.

Recently, there has been a demand for weight reduction of motorcycles in motorcycles, and weight reduction of motorcycle brake discs is being studied. In this case, the problem is the deformation of the disk due to the softening of the disk material due to the heat generated during braking. In order to solve this problem, it is necessary to improve the heat resistance of the disk material. One solution is to improve the temper softening resistance. Patent Document 3 discloses an invention relating to a method for improving heat resistance by adding Nb and Mo. Patent Document 4 discloses an invention relating to a disk material having excellent heat resistance by performing a quenching process from a temperature exceeding 1000 ° C. As a brake disk excellent in tempering softening resistance, Patent Document 5 discloses a brake disk having a martensite structure in which the average particle size of prior austenite grains is 8 μm or more, and Patent Document 6 discloses that the area ratio of the structure after quenching is 75. % Or more is martensite and Nb is 0.10% or more and 0.60% or less.

Such low-C martensitic stainless steel has low hot workability and cracks at the end of the width during hot rolling, so that so-called ear cracks are likely to occur, so the components are controlled within a limited range where cracks are unlikely to occur. This is disclosed in Patent Document 7.

Patent Document 8 relates to a method for manufacturing a ferritic stainless steel strip. In this document, the optimum conditions for heating the seat bar are disclosed, particularly with respect to a production method capable of producing a ferritic stainless hot-rolled steel strip excellent in forming processability and material uniformity with high productivity.

JP-A-57-198249 JP-A-8-60309 JP 2001-220654 A JP 2005-133204 A JP 2006-322071 A JP 2011-12343 A JP 2008-285692 A JP 2000-61524 A

With this technology, low C martensitic stainless steel has become widespread as a material for disc brakes in motorcycles. On the other hand, in recent years, improvement in productivity when manufacturing a disc brake has been demanded. For example, shortening of the heating time at the time of heat quenching and shortening of the polishing time after the heat quenching are required. It is also required to improve the yield by using the steel strip up to the width end.
If the amount of polishing per unit time is increased in order to shorten the polishing time, the wear of the jig increases due to heat generated by processing friction, and temper softening of the material occurs. Therefore, it is common to reduce the polishing thickness for the purpose of shortening the polishing time without causing processing frictional heat generation. Therefore, an edge seam ridge at the end of the steel strip has become a problem.
FIG. 1A shows an appearance of an edge seam ridge in the actual product, and FIG. 1B shows a micrograph of a cross section of the edge seam ridge in the actual product. In the process of manufacturing a hot-rolled steel strip, a slab having a thickness of 150 to 250 mm is heated to 1100 to 1300 ° C. and rolled to a coarse bar having a thickness of 20 to 40 mm with a rough hot rolling mill, and then subjected to a plate with a finishing hot rolling mill. In general, it is rolled to a thickness of 3 to 6 mm and wound. Since rough hot rolling does not apply tension, the width is widened, and a part of the end face of the slab becomes the surface of the rough bar. Since the end face of the slab does not contact the rolling roll at the initial stage of rough hot rolling, the roughness is large, and when it comes into contact with the rolling roll after that, it causes wrinkles.

Edge seams are recognized in many hot-rolled steel strips of steel materials. In the laboratory, 80 mm thick steel ingots of various types of stainless steel were hot-rolled to 20 mm, and a photograph of the end face observed was shown in FIG. 2, but it can be seen that the degree of roughness of the end face varies greatly depending on the steel type. Moreover, in SUS410 steel, it turns out that the rough skin of an end surface changes greatly with hot rolling heating temperature. The rough surface on the end surface of the slab during rough hot rolling is caused by the difference in deformation mode due to the difference in crystal orientation of each crystal grain of the slab, and thus becomes prominent when the crystal grain size is coarse. For example, when ordinary steel is cooled to room temperature after solidification, it undergoes two transformations, δ / γ and γ / α, and the structure becomes finer. Here, δ is δ ferrite, γ is austenite, and α is α ferrite, but when expressed as ferrite, it usually means α ferrite. δ ferrite is ferrite precipitated at the A4 transformation point or higher, and α ferrite is ferrite precipitated at the A3 transformation point or lower.

In ordinary steel, the structure is refined by α / γ transformation again during hot rolling, and the coarse hot rolling is performed in the γ single-phase region where recrystallization is easy. It becomes fine grain and edge seam wrinkle is hard to occur. On the other hand, when ferritic stainless steel is maintained until hot-rolling heating without transforming the ferrite grains at the time of solidification, edge seam wrinkles are likely to occur because of the coarse grains. In steels such as this ferritic stainless steel that do not become a γ single phase after solidification, it is common not to distinguish between δ ferrite and α ferrite.

Even in martensitic stainless steel, if the component is 13% Cr-0.2% C as in SUS420J1, it is an austenite single phase during hot rolling and heating, and the structure is refined by transformation and recrystallized from austenite. The edge seam wrinkle is hard to come out by miniaturization.

However, in the low-C martensitic stainless steel, the temperature range in which the austenite single phase is formed is narrow, and a two-phase structure of δ ferrite and austenite is formed during hot rolling heating. Edge seam wrinkles are likely to occur due to δ ferrite at this time, and a polishing thickness exceeding the edge seam wrinkle depth is required in the polishing process after quenching of the disc brake, which hinders productivity.

When the austenite ratio is increased by lowering the hot rolling heating temperature, there is a problem that hot workability is lowered due to an increase in deformation resistance, and ear cracks occur during hot rolling. When the C content was increased to increase the γ phase fraction, the quenching hardness became too high. When an austenite stabilizing element such as Mn, Ni, Cu or the like is further added, there is a problem that the raw material cost is increased, and the annealing cooling time is prolonged in the hot-rolled sheet annealing process, and the productivity is impaired. When the amount of Cr was lowered and the austenite fraction was raised, there was a problem that the corrosion resistance was impaired.

To control the δ ferrite fraction, it is necessary to know the change in the δ ferrite fraction during hot rolling, but the δ ferrite fraction of the hot rolled sheet could not be measured. The δ ferrite fraction of the slab during hot rolling can be measured by a phase diagram calculation method or a heat treatment test in a laboratory. When quenched from the two-phase structure of austenite and martensite, the austenite phase is easily distinguished from the martensite structure, and the δ ferrite phase is easily distinguished as a δ ferrite phase with less strain. However, in the actual hot rolling process, it was not possible to know how the amount of δ ferrite changed during hot rolling after the slab exited the hot rolling furnace. The hot-rolled steel strip wound after finishing hot rolling has a low toughness because it includes a martensitic structure in which austenite is transformed, and is difficult to rewind as it is. Although the hot-rolled sheet annealing was performed in a box annealing furnace and the martensite was tempered into ferrite and carbide, it was possible to rewind, but the hot-rolled sheet structure before annealing could not be investigated. After hot rolling annealing, as shown in FIG. 3, it was a structure of ferrite and carbide, and the δ ferrite fraction could not be measured.

The present inventors examined a method of examining the δ ferrite fraction in the ferrite matrix in a hot rolled annealed steel sheet of low C martensitic stainless steel. As a result of trying various etching solutions for microstructure analysis and optical microscope observation by electron backscatter diffraction (EBSD), it was found that δ ferrite can be colored by Murakami reagent. Murakami's reagent is an aqueous solution of potassium ferricyanide, which is etched by heating the liquid and immersing the sample in it. Usually, it is used to distinguish between austenite and δ ferrite phase by coloring δ ferrite mixed in the austenite matrix like the solidification structure of austenitic stainless steel. Although it was not initially imagined that δ ferrite could be identified in a hot-rolled annealed steel sheet of martensitic stainless steel in which δ ferrite and ferrite were mixed, it could be clearly identified as shown in FIG. The gray contrast portion in FIG. 4 is the δ ferrite portion. The mechanism by which δ ferrite is colored and identified by Murakami's reagent is not clear. However, as a result of investigation by the present inventors, the Cr concentration in the δ ferrite phase and the austenite phase (martensite phase at room temperature) during hot rolling is about 1 It is estimated that the high Cr δ ferrite phase was colored and identified by the Murakami reagent because of a difference of 5%. There is no example of using Murakami's reagent in the appearance of δ ferrite in such a low Cr martensitic stainless steel, and it was a new finding that a Cr amount difference of only about 1.5% could be identified. By using this method, the behavior of δ-ferrite, which had not been understood before, became clear. For example, it was found that the amount of ferrite in the hot-rolled sheet was greatly reduced compared to the amount of δ ferrite at the hot rolling heating temperature. In addition, the amount of ferrite at the width end is larger than that at the width center of the steel strip, and there may be a temperature difference between the width end of the slab and the width center, and the amount of δ ferrite increases due to decarburization at the surface layer. The possibility of doing it was also considered. For hot-rolled steel sheets that are not subjected to hot-roll annealing, δ ferrite can be identified by evaluating the steel sheet samples as described above.

FIG. 5 shows the relationship between the edge seam wrinkle and the amount of δ ferrite, but no edge seam wrinkle was observed in the austenitic stainless steel having a δ ferrite fraction of 0%. The depth of the seam ridge increases as the δ ferrite fraction increases, but the increase in the seam ridge depth is small up to a δ ferrite fraction of 30%. However, it can be seen that when the δ ferrite fraction exceeds 30%, the seam depth increases rapidly.

On the other hand, the edge cracks at the end of the steel sheet tend to occur when the δ ferrite fraction falls below 5%. Fig. 6 shows the end of a steel hot rolled (δ ferrite fraction 4%, 20% (11% Cr, 12% Cr) -0.04% C-1.4% Mn-0.03% N steel). Although the shape of the part is shown, remarkable ear cracking occurs when the δ ferrite fraction decreases.

Thus, in both hot-rolled annealed steel sheets and hot-rolled steel sheets, the δ ferrite fraction has a strong correlation with the edge seam flaw depth and ear cracks at the width end of the steel plate, and the martensitic stainless steel with a controlled δ ferrite fraction is used. In the case of steel, there are no ear cracks and the edge seam wrinkle depth is shallow, so that it is possible to reduce the grinding depth in the brake disk manufacturing process, thereby improving the productivity of the brake disk. Furthermore, since the end of the steel plate can be used, the yield is improved.

Thus, as a method of controlling the δ ferrite fraction that greatly affects the surface quality, it is considered effective to control (1) chemical composition and (2) hot rolling heating temperature. However, when the chemical composition (1) is controlled by the amount of C and N, it is not preferable because the quenching hardness required for the disc brake cannot be obtained. In addition, Si, Mn, Cr, Ni, Cu, etc. affect the thickness of hot-rolled scale, affect temper softening resistance and corrosion resistance, and if added in a large amount, there is a problem that the alloy cost increases, and there is a range that can be controlled. limited. When adjusting the amount of δ ferrite at the hot rolling heating temperature in (2), if the heating temperature is set to 1150 ° C. or less in order to reduce the amount of δ ferrite, the difference in strength between the austenite phase and the slightly remaining ferrite phase will occur. Cracking was likely to occur, and surface quality was not easily improved by controlling δ ferrite.

The present inventors have conducted a detailed study on the amount of δ ferrite, hot-rolling operation conditions, and chemical composition of the hot-rolled annealed steel sheet, achieving both surface quality and ear cracks, and the hardness and corrosion resistance required for disc brakes. In order to satisfy the above, an effective method has been found. That is, (1) the amount of δ ferrite during hot rolling heating and (2) component adjustment that satisfies various characteristics, and (3) due to the temperature drop during rough rolling out of the hot rolling heating furnace In order to prevent a decrease in the amount of δ ferrite, it is necessary to heat and heat the coarse bar by a method such as induction heating between the coarse hot rolling and the finish hot rolling.

Based on these findings, we provide martensitic stainless steel hot-rolled steel sheets and hot-rolled annealed steel sheets for brake disks that reduce edge seam wrinkles and prevent edge cracks at the ends of hot-rolled steel strips along with their microstructure control methods. It became possible to do.

The present invention has been made on the basis of these findings, and is a means for solving the problems of the present invention, that is, martensitic stainless steel for a motorcycle brake disk according to the present invention (hot rolled steel sheet (hot-rolled annealing is performed). No), including hot-rolled annealed steel sheet) and its manufacturing method is as follows.
(1) By mass%, C: 0.025 to 0.080%, Si: 0.05% to 0.8%, Mn: 0.5 to 1.5%, P: 0.035% or less, S : 0.015% or less, Cr: 11.0 to 13.5%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.08%, Mo: 0.01 to 0.30% , V: 0.01 to 0.10%, Al: 0.05% or less, N: 0.015 to 0.060%, remaining Fe and inevitable impurities, defined by the formula (1) A martensitic stainless steel for motorcycle brake discs having a DFE value of 5 or more and 30 or less, and a δ ferrite fraction observed in a cross-sectional structure of 5% or more and 30% or less in terms of area ratio.
DFE = 12 (Cr + Si) -430C-460N-20Ni-7Mn-89 (1) Formula In the formula (1), Cr, Si, C, N, Ni, and Mn are the content of each element ( Mass%).
(2) The martensitic stainless steel for a motorcycle brake disc according to the present invention further comprising one or two of Ti: 0.03% or less and B: 0.0050% or less in mass%. .
(3) The martensitic stainless steel for motorcycle brake discs according to the present invention, further comprising, by mass%, Nb: 0.30% or less.
(4) The martensitic stainless steel for a motorcycle brake disk according to the present invention further comprising one or two of Sn: 0.1% or less and Bi: 0.2% or less in mass%. .
(5) The martensitic stainless steel for the motorcycle brake disk, wherein the rough bar is heated at 10 ° C. or more and 50 ° C. or less between the rough hot rolling and the finish hot rolling. Manufacturing method of martensitic stainless steel for motorcycle brake discs.
(6) The martensitic stainless steel for motorcycle brake discs according to the present invention, wherein the martensitic stainless steel for motorcycle brake discs is a hot-rolled steel plate not subjected to hot-rolled sheet annealing.
(7) The martensitic stainless steel for motorcycle brake discs according to the present invention, wherein the martensitic stainless steel for motorcycle brake discs is a hot-rolled annealed steel plate.

By the structure and composition control technology of the present invention, it is possible to obtain a hot-rolled steel sheet and a hot-rolled annealed steel sheet for a motorcycle brake disk that reduce edge seam wrinkles at the width end of the hot-rolled steel strip and prevent ear cracks at the width end. It becomes possible. The quality is preferable from the viewpoint of improving the productivity and yield of brake discs.

The external appearance of the edge seam flaw in the width | variety edge part of the hot rolled annealing steel strip of martensitic steel for brake discs is shown. The microscope image observed from the cross section of the edge seam fistula in the width end part of the hot-rolled annealing steel strip of martensitic steel for brake discs is shown. In order to show the generation process of edge seam wrinkles, a steel ingot of 300 L × 180 w × 80 t (mm) cast in a laboratory is rolled to a thickness of 20 mm with a laboratory hot rolling mill, and a width end face is observed. The structure after hot-rolling annealing (where ferrite grain boundaries and carbides are mainly revealed), which is an 11% Cr-1% Mn-0.04% C-0.04% N steel hot-rolled annealing plate It is a photograph which shows a general cross-sectional structure. This is a short-time etching with aqua regia. 11% Cr-1% Mn-0.04% C-0.04% N Martensitic stainless steel hot-rolled annealed steel strip with TD ferrite distribution in the TD cross section where no edge seam flaws or ear cracks were observed This is a photograph (edge seam wrinkle, δ ferrite structure of hot-rolled annealed plate with good ear cracking quality). Martensitic stainless steel grades for motorcycle disc brakes are hot rolled to a thickness of 3.8 mm after changing the hot rolling heating temperature from 1100 to 1280 ° C, and after hot rolling annealing, It is the figure which expand | extracted the extended coil and extract | collected the sample and investigated the relationship between the depth of an edge seam ridge and the amount of (delta) ferrite. 11-12% Cr-0.04% C-0.5-1.4% Mn-0.03% N steel lab 50mm thick steel ingot is heated to 1250 ° C in the lab and hot rolled to 3mm thickness FIG. 6 is a photograph in which the relationship between the amount of δ ferrite affecting the edge cracks on the end face is examined.

Hereinafter, embodiments of the present invention will be described. First, the reason which limited the steel composition of the stainless steel plate of this embodiment is demonstrated. In addition, the description of% about a composition means the mass% unless there is particular notice.

C: 0.025 to 0.080%
C is an essential element for obtaining a predetermined hardness after quenching, and is added in combination with N so as to obtain a predetermined hardness level. In order to avoid excessive addition of C and make the best use of the effect of N, the upper limit is made 0.080% in the present invention. It is because hardness will be too hard when added exceeding this, and malfunctions, such as a squeal of a brake and a toughness fall, will be produced. The upper limit of the C content is preferably 0.060% from the viewpoint of hardness control and corrosion resistance improvement. On the other hand, if the C content is less than 0.025%, N must be excessively added to obtain hardness, so 0.025% is made the lower limit of the C content. From the viewpoint of the stability of quenching hardness, it is desirable to set it to 0.040% or more.

Si: 0.05% to 0.8%
Si is necessary for deoxidation at the time of melting and refining, and is also useful for suppressing the formation of oxide scale during quenching heat treatment, and the effect is manifested at 0.05% or more. 0.05% or more. However, since Si is mixed from a raw material such as hot metal, an excessive decrease leads to an increase in cost. Further, Si narrows the austenite single phase temperature range and impairs the quenching stability, so that the Si content is set to 0.8% or less. In order to reduce the addition amount of the austenite stabilizing element and reduce the cost, 0.6% or less is desirable.

Mn: 0.5 to 1.5%
Mn is an element added as a deoxidizer, and contributes to improving the hardenability by expanding the austenite single phase region. Since the effect clearly appears at 0.5% or more, the Mn content is set to 0.5% or more. In order to ensure the hardenability stably, it is desirable to make it 1.1% or more. However, Mn promotes the generation of oxide scale during quenching heating and increases the subsequent polishing load. Therefore, the upper limit of the Mn content is set to 1.5% or less. Considering a decrease in corrosion resistance due to granulated materials such as MnS, 1.3% or less is desirable.

P: 0.035% or less P is an element contained as an impurity in main raw materials such as hot metal and ferrochrome. Since it is an element harmful to toughness after quenching of the hot-rolled annealed sheet, the P content is set to 0.035% or less. In addition, Preferably it is 0.030% or less. Since excessive reduction leads to an increase in cost, such as making it necessary to use a high-purity raw material, the lower limit of P is preferably 0.010%.

S: 0.015% or less S forms sulfide inclusions and degrades general corrosion resistance (full corrosion and pitting corrosion) of steel materials, and S decreases hot workability and hot rolled steel sheets. In order to increase the ear cracking susceptibility, the upper limit of the S content is preferably small, and the S content upper limit is set to 0.015%. A more preferable upper limit is 0.008%. Further, the smaller the S content, the better the corrosion resistance. However, since the desulfurization load increases and the production cost increases for lowering the S content, the lower limit is preferably made 0.001%.

Cr: 11.0-13.5%
In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If the Cr content is less than 11.0%, these effects are not exhibited. On the other hand, if the Cr content exceeds 13.5%, the austenite single phase region is reduced and the hardenability is impaired. 11.0 to 13.5%. In consideration of the stability of corrosion resistance, the Cr content is desirably 12.0% or more. In consideration of press formability, the Cr content is desirably 13.0% or less.

Ni: 0.01 to 0.50%
Ni is mixed as an inevitable impurity in the ferritic stainless steel alloy raw material, and is generally contained in the range of 0.01 to 0.10%. Moreover, since it is an element effective in suppressing the progress of pitting corrosion and the effect is stably exhibited by addition of 0.03% or more, it is preferable that the lower limit of Ni content is 0.03%. On the other hand, addition of a large amount may cause a decrease in press formability due to solid solution strengthening in the hot-rolled annealed steel sheet, so the upper limit of the Ni content is 0.50%. In consideration of the alloy cost, the Ni content is preferably 0.15% or less.

Cu: 0.01 to 0.08%
Cu is effective in improving the corrosion resistance of the martensite structure containing δ ferrite, and the effect is manifested at 0.01% or more. In some cases, the austenite stabilizing element is positively added to improve hardenability. However, excessive addition leads to a decrease in hot workability and an increase in raw material cost, so 0.08% or less is made the upper limit of the Cu content. In consideration of the occurrence of acid rain and the like, the lower limit of the Cu content is preferably 0.02% or more. Further, considering the press formability of the hot-rolled sheet fired steel sheet, 0.08% or less is preferable.

Mo: 0.01-0.30%
Mo is effective for improving the corrosion resistance of the martensite structure containing δ ferrite, and the effect is manifested at 0.01% or more, so the lower limit of the Mo content is set to 0.01%. 0.02% or more is preferable because it is effective for improving the hardenability and improving the heat resistance after quenching. The steel may be tempered by heating after quenching, resulting in a decrease in hardness. Here, the improvement in heat resistance after quenching means that the hardness reduction margin is small. Also called temper softening resistance. The disc brake is used by quenching, but the disc material is heated by resistance heat generated by braking during use. This characteristic is therefore important.

Mo is a ferrite phase stabilizing element, and excessive addition impairs the quenching characteristics by narrowing the austenite single phase temperature range, so the upper limit of the Mo content is 0.30% or less.

In order to improve the heat resistance after quenching, it is desirable to add together with Nb. In the case of simultaneous addition, Mo: 0.05 to 0.20%, Nb: 0.05 to 0.20% are particularly preferable ranges. is there.

V: 0.01 to 0.10%
V is generally contained in the range of 0.01 to 0.10% because it is mixed as an inevitable impurity in the ferritic stainless steel alloy raw material and is difficult to remove in the refining process. V is an element that forms fine carbonitrides and improves the wear resistance of the brake disk and also has an effect of improving the corrosion resistance. Therefore, V is intentionally added as necessary. . Since the effect is stably exhibited by addition of 0.02% or more, the V content lower limit is preferably 0.02%. 0.03% or more is more desirable. On the other hand, if added excessively, the precipitates may be coarsened. As a result, the toughness after quenching is lowered, so the V content upper limit is made 0.10%. In view of manufacturing cost and manufacturability, the V content is preferably 0.08% or less.

Al: 0.05% or less Al is an element that improves oxidation resistance in addition to being added as a deoxidizing element. Since the effect is obtained at 0.001% or more, the lower limit of the Al content is preferably set to 0.001% or more. On the other hand, the upper limit of the Al content is set to 0.05% because the toughness of the brake disk is impaired by solid solution strengthening and the formation of large oxide inclusions. Preferably it is 0.03% or less. Al may not be contained.

N: 0.015 to 0.060%
N is one of the very important elements in the present invention. Like C, it is an essential element for obtaining a predetermined hardness after quenching, and is added in combination with C so as to obtain a predetermined hardness level. In addition, when quenching as a two-phase structure of austenite and ferrite during quenching heating, precipitation of Cr carbide, that is, sensitization phenomenon is likely to occur and corrosion resistance may be reduced, but nitrogen suppresses precipitation of Cr carbide and has corrosion resistance. The improvement effect may be shown. Since the effect is manifested at 0.015% or more, the N content is set to 0.015% or more. On the other hand, the effect is saturated at 0.060%, and it is feared that the yield decreases due to the formation of bubble defects, so 0.060% is made the N content upper limit. In consideration of the effect of improving the corrosion resistance by strengthening the passive film, the N content is preferably 0.030% or more. Moreover, it is desirable to make it into the range of 0.050% or less.

The amount of δ ferrite (δ ferrite fraction) observed in the hot-rolled steel sheet or hot-rolled annealed steel sheet is 5% or more and 30% or less in terms of area ratio.
The amount of δ ferrite in the steel affects edge seam defects and hot-ear cracks during hot rolling. If the δ ferrite fraction is less than 5%, the hot workability deteriorates and the ear cracks are liable to occur. On the other hand, when the δ ferrite fraction exceeds 30%, edge seam wrinkles are likely to occur due to the coarsening of the crystal grain size, and in the polishing process after quenching the brake disk, a large grinding thickness is used to remove the edge seam wrinkles. Therefore, the δ ferrite fraction should be 30% or less. Note that δ ferrite during hot rolling is observed in the cross section of the hot rolled annealed steel sheet and hot rolled steel sheet, and is evaluated by ordinary microscopic observation. However, the structure etching of δ ferrite is performed by Murakami reagent (potassium ferricyanide). It is desirable to immerse the sample in a heated solution.

The DFE value defined by the formula (1) (DFE = 12 (Cr + Si) -430C-460N-20Ni-7Mn-89) is 5 or more and 20 or less.
If the DFE value is low, the amount of δ ferrite decreases and the frequency of occurrence of ear cracks during hot rolling increases, so it is set to 5 or more. Further, if the DFE value is high, δ ferrite increases and edge seam flaws are likely to occur, so it is set to 20 or less. In addition, Cr, Si, C, N, Ni, and Mn in the formula (1) mean the content (mass%) of each element.

In the present invention, in addition to the above elements, the following elements can be added in order to improve weather resistance, heat resistance, hot workability, and the like.

Ti: 0.03% or less Ti is an element that suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitride in stainless steel by forming carbonitride. The Ti content is preferably 0.001% or more. However, in the brake disk, forming a large TiN causes a decrease in toughness and squealing, so the upper limit of the Ti content is 0.03% or less. Considering toughness in winter, it is desirable to make it 0.01% or less. Ti may not be contained.

B: 0.0050% or less B is an element effective for improving hot workability, and its effect is manifested at 0.0002% or more. Therefore, B may be added at 0.0002% or more. In order to improve the hot workability in a wider temperature range, the content is preferably 0.0010% or more. On the other hand, excessive addition impairs hardenability due to combined precipitation of boride and carbide, so 0.0050% is made the B content upper limit. In consideration of corrosion resistance, 0.0025% or less is desirable.

Nb: 0.3% or less Nb is an element that suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitride in stainless steel by forming carbonitride. Nb content is preferably 0.001% or more. Furthermore, it is an element that greatly improves the heat resistance after quenching. Here, the heat resistance is also referred to as the degree of resistance to softening when subjected to heat after quenching, that is, the temper softening resistance.

However, when Nb is added excessively, formation of NbN in the brake disc causes a decrease in toughness and squeal, which is not preferable, and 0.3% is made the upper limit of the Nb content.

In order to improve heat resistance after quenching, composite addition with Mo is desirable, and in the case of simultaneous addition, Mo: 0.05 to 0.20%, Nb: 0.05 to 0.20% are particularly preferable ranges. is there.

Sn: 0.1% or less Sn is an element effective for improving the corrosion resistance after quenching, and is preferably 0.001% or more, and preferably 0.02% or more if necessary. However, excessive addition promotes ear cracking during hot rolling, so it is preferably made 0.10% or less.

Bi: 0.2% or less Bi is an element that improves corrosion resistance. Although the mechanism is not clarified, it is believed that MnS, which is likely to become a starting point, has an effect of miniaturization by adding Bi, and therefore the probability of becoming a starting point is lowered. The effect is exhibited by adding 0.01% or more of Bi. Even if added over 0.2%, the effect is only saturated, so the upper limit of Bi content is set to 0.2%.

In addition to the elements described above, an impurity element is contained within a range not impairing the effects of the present invention. It is preferable to reduce as much as possible Zn, Pb, Se, Sb, H, Ga, Ta, Ca, Mg, Zr, etc. as well as the above-mentioned general impurity elements P and S. On the other hand, the content ratio of these elements is controlled within the limits of solving the problems of the present invention, and the content is Zn ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, Sb ≦ 500 ppm, H ≦ 100 ppm, Ga ≦ 500 ppm, Ta ≦ 500 ppm, Ca ≦ 120 ppm, Mg ≦ 120 ppm, Zr ≦ 120 ppm.

In the hot rolling step, it is preferable to heat a rough bar having a thickness of 20 to 40 mm between 10 ° C. and 50 ° C. using an induction heating device (bar heater) between rough rolling and finish rolling. When the heating temperature of the coarse bar is less than 10 ° C., the amount of δ ferrite is small, and ear cracks are likely to occur due to a decrease in hot workability. On the other hand, when the heating temperature exceeds 50 ° C., the amount of δ ferrite becomes too large, the crystal grain size becomes coarse, the rough surface of the rough bar end face becomes large, and deep edge seams are likely to occur. Even if the slab heating temperature is increased without heating with the coarse bar heater, the temperature of the coarse bar increases. However, when the heating temperature exceeds 1250 ° C, the crystal grain size becomes coarse, and the coarse bar becomes rough during the rough rolling process. Since the roughness of the end face becomes large and the edge seam wrinkle becomes deep, the hot rolling heating temperature is preferably 1250 ° C. or lower. In addition, when the hot rolling heating temperature is less than 1150 ° C., the deformation resistance concentrates in a small amount of δ ferrite phase due to an increase in deformation resistance of the austenite matrix phase and a decrease in the amount of δ ferrite. Since this occurs and the yield decreases, the hot rolling heating temperature is desirably 1150 ° C. or higher.

The quality specified in each claim can be realized by having the components and the δ ferrite fraction described in each claim. The martensitic stainless steel for motorcycle brake discs of the present invention can exhibit an effect in both hot-rolled steel sheets and hot-rolled annealed steel sheets that are not subjected to hot-roll annealing.

Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

In this example, first, steels having the component compositions shown in Table 1-1 and Table 1-2 were melted and cast into slabs having a thickness of 200 mm. This slab was heated to 1150 to 1250 ° C., then subjected to rough hot rolling and finish hot rolling to obtain a hot-rolled steel plate having a thickness of 4 mm and wound in a temperature range of 750 to 900 ° C. Between rough hot rolling and finish rolling, a rough bar heater using induction heating was used, and heating was performed at a temperature rise condition of 10 to 50 ° C. The hot-rolled steel sheet was subsequently annealed in a box-type annealing furnace with respect to the hot-rolled coil. The maximum heating temperature was set to a temperature range of 800 ° C. or higher and 900 ° C. or lower. The scale on the surface of the hot-rolled annealed steel sheet was removed by shot blasting, pickled, and then evaluated for edge seam flaws and ear cracks. When the depth of the edge seam wrinkles was less than 150 μm, it was determined that the edge seam wrinkles could not be confirmed by visual inspection. The case where there was an edge seam wrinkle having a depth of 150 μm or more was regarded as rejected (judgment C).

Moreover, the case where the cracks of the depth of 10 mm or more did not occur was determined as pass (determination A), and the case where the ear crack occurred was determined as failure (determination B). Moreover, the thing in which the ear crack generate | occur | produced continuously was set as the disqualification (judgment C).

Also, the cross-sectional structure was observed using an optical microscope, and the amount of δ ferrite was measured by image analysis. Murakami reagent was used for the appearance of δ ferrite.

Subsequently, after hot-rolled annealing-pickled plate was quenched and the surface was polished by # 80, JIS surface hardness (quenched hardness) was evaluated with the Rockwell Hardness Tester C scale, passing 32-38, otherwise rejected It was. The conditions for quenching the disc brake were that the average heating rate was about 50 ° C./s, the temperature was raised to 1000 ° C., held for 1 second, and cooled to room temperature at an average cooling rate of about 70 ° C./s.

Also, as an evaluation of heat resistance after quenching, after tempering at 500 ° C. for 1 h and finishing the surface by # 80 polishing, the JIS surface hardness (quenching hardness) is evaluated with a Rockwell hardness meter C scale, less than 32 Was rejected (B), and 32 or more was determined to be acceptable (A). Further, a test at a tempering temperature of 530 ° C. was performed in the same manner, and 32 or higher was accepted (S), and entered in the “Temper softening resistance” column of Tables 2-1, 2-2, and 2-3.

Corrosion resistance was evaluated by finishing the surface of hot-rolled annealed pickled plate with # 600, followed by a salt spray test for 4 hours (JIS Z 2371 “Salt spray test method”), measuring the rust area ratio, and rust area ratio of 10 % Or more was rejected (B), and less than that was determined to be pass (A). In particular, those having a rust area ratio of zero were regarded as acceptable (S).

Regarding abrasiveness, the depth of the edge seam wrinkles was 150 μm or less as pass (A), and over 150 μm was rejected (B).

As a comparative example, the same evaluation was performed for a sample in which the composition outside the present invention, the hot rolling heating conditions, and the area ratio of δ ferrite in the hot rolled annealed plate were outside the present invention.

As is apparent from Table 1-1, Table 1-2, Table 2-1, Table 2-2, and Table 2-3, the component composition to which the present invention is applied, and the δ ferrite area ratio is 5% or more and 30% or less. In the inventive examples having the above, the quality of the edge seam wrinkles was acceptable, the ear cracking quality was also acceptable, and the quenching hardness, heat resistance, and corrosion resistance were also good. Further, when the heating temperature of the coarse bar by the coarse bar heater is within the range of the present invention, the depth of the edge seam wrinkles is further reduced, and the polishing time of the disc after quenching can be shortened. In addition, the ear cracking quality was further improved and was no longer recognized. On the other hand, it is difficult to control the amount of δ ferrite in the hot-rolled annealed plate with a component composition outside the present invention, and one or more of edge seam flaw quality, ear crack quality, quenching hardness, and post-quenching corrosion resistance are not satisfactory. It was a pass. Thereby, it turns out that the characteristic of the brake disc in a comparative example is inferior.

Specifically, test no. Nos. 38 and 56 are high in C and N. Since C and N were low in Nos. 42 and 55, the quenching hardness was outside the target range. NO. No. 40 is low in Si. No. 54 had poor polishability in the polishing step after quenching because of high V. No. 41, 42, 45, 46, 57, 58, 59, 60, 61, 62 had poor edge seam flaw quality or ear crack rating because the amount of δ ferrite of hot-rolled annealed plate was more than 30% or less than 5% Met. No. No. 43 has a high P. No. 44 has a high S. No. 50 had a poor ear crack rating because Cu was high.

No. Since 45, 47, 49, 51, and 53 had low Cr, Ni, Cu, Mo, and V, respectively, their corrosion resistance was poor. No. Nos. 48 and 50 have a large amount of Ni and Cu, so that the press formability is improved. 46 and 60 to 62 are Cr, No. Nos. 39 and 52 had a large amount of Si and Mo, respectively, so that the hardenability was lowered and the quenching hardness was lowered. No. Nos. 48, 50, 52, and 54 were judged to be economically poor because of high raw material costs. No. No. 57 had a low DFE value, so the fraction of δ ferrite was low, and the ear cracking was poor.

From these results, the above-mentioned knowledge could be confirmed, and the grounds for limiting the above-mentioned steel composition and composition could be supported.

As is clear from the above description, the martensitic stainless steel plate for brake discs of the present invention is optimized for the amount of δ ferrite observed in hot-rolled and annealed steel plates and hot-rolled steel plates, and controls component design and hot rolling conditions. As a result, good edge seam wrinkle quality and ear cracking quality are obtained, and a high-quality brake disc without deterioration in hardness and corrosion resistance after quenching is obtained. Furthermore, the edge seam wrinkle quality and the ear crack quality were further improved by heating the rough bar between the rough hot rolling and the finish hot rolling under the optimum conditions according to the composition. By applying the material to which the present invention is applied to a motorcycle brake disc, the yield is improved, the inspection load is reduced, and the productivity can be improved by shortening the grinding time. Can be increased. In other words, the present invention has sufficient industrial applicability.

Claims

In mass%, C: 0.025 to 0.080%, Si: 0.05% to 0.8%, Mn: 0.5 to 1.5%, P: 0.035% or less, S: 0.0. 015% or less, Cr: 11.0 to 13.5%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.08%, Mo: 0.01 to 0.30%, V: It contains 0.01 to 0.10%, Al: 0.05% or less, N: 0.015 to 0.060%, the remaining Fe and inevitable impurities, and the DFE value defined by the formula (1) A martensitic stainless steel for motorcycle brake discs, wherein the δ ferrite fraction observed in the cross-sectional structure is 5% or more and 30 or less, and the area ratio is 5% or more and 30% or less.
DFE = 12 (Cr + Si) -430C-460N-20Ni-7Mn-89 (1) Formula In the formula (1), Cr, Si, C, N, Ni, and Mn are the content of each element ( Mass%).
The martensitic stainless steel for a motorcycle brake disk according to claim 1, further comprising one or two of mass%, Ti: 0.03% or less, B: 0.0050% or less. .
The martensitic stainless steel for motorcycle brake disks according to claim 1 or 2, further comprising Nb: 0.30% or less in terms of mass%.
The composition according to any one of claims 1 to 3, further comprising one or two of Sn: 0.1% or less and Bi: 0.2% or less in mass%. Martensitic stainless steel for motorcycle brake discs.
The martensitic stainless steel for the motorcycle brake disc, wherein the rough bar is heated at 10 ° C or more and 50 ° C or less between the rough hot rolling and the finish hot rolling. 5. The method for producing martensitic stainless steel for motorcycle brake disks according to any one of 4.
The martensitic system for a motorcycle brake disc according to any one of claims 1 to 4, wherein the martensitic stainless steel for a motorcycle brake disc is a hot-rolled steel plate that is not subjected to hot-rolled sheet annealing. Stainless steel.
The martensitic stainless steel for motorcycle brake discs according to any one of claims 1 to 4, wherein the martensitic stainless steel for motorcycle brake discs is a hot-rolled annealed steel plate.