Martensitic stainless steel sheet and method for producing same

Abstract

Provided is a stainless steel sheet that does not require a quenching treatment and that is suitable as a base material steel sheet for a disc brake in a variety of two-wheeled vehicles. The component composition is properly controlled, the structure balance index F is controlled to be in the range of 100-250, and the area fraction of ferrite grains having a grain average misorientation of not more than 1° and a grain diameter of not more than 50 μm is 0.01-50.00%.

Description

Martensitic stainless steel sheet and method for producing the same

[0001] The present invention relates to a martensitic stainless steel sheet and a method for producing the same.

[0002] For the disk brake of a two-wheeled vehicle, wear resistance is required to maintain the performance as a brake for a long period of time. Wear resistance generally improves as the hardness increases. On the other hand, toughness decreases as the hardness increases.

[0003] Therefore, for members such as disk brakes that require both high wear resistance and high toughness, steel having a Rockwell hardness (C scale) (hereinafter also referred to as HRC) measured in accordance with JIS Z 2245:2016 in the range of 30 to 40, for example, a martensitic stainless steel sheet as disclosed in Patent Document 1 is often used as a material.

[0004] Patent No. 5700172, Patent No. 4300672

[0005] When manufacturing a member such as a disk brake using a martensitic stainless steel sheet as disclosed in Patent Document 1 as a material, the steel sheet used as the material (hereinafter also referred to as the material steel sheet) is punched into a predetermined shape, and then quenching treatment is generally performed to increase the hardness.

[0006] By the way, in recent years, from the viewpoint of preventing global warming, further reduction of CO ２ emissions is required. In the quenching treatment after punching, a large amount of CO ２ is emitted. Therefore, there is a demand for the development of a material steel sheet that does not require quenching treatment and has an HRC in the range of 30 to 40.

[0007] Furthermore, Patent Document 2 discloses "a stainless steel sheet for motorcycle disc brakes that is used as cold-rolled and does not require quenching, characterized by a steel composition consisting of C: 0.1% or less, N: 0.1% or less, Cr: 10.0 to 20.0%, Nb: 0.05 to 1.0%, Ni: 0.1 to 2.0% by mass%, with the remainder being Fe and unavoidable impurities, and having a processed ferrite structure and a Vickers hardness (Hv) of 250 to 300." However, the stainless steel sheet disclosed in Patent Document 2 was difficult to apply as a base steel sheet for disc brakes on large motorcycles and the like due to its hardness and thickness.

[0008] The present invention was developed to meet the above-mentioned requirements and aims to provide a stainless steel sheet that does not require heat treatment and is suitable as a material steel sheet for disc brakes of various motorcycles. The present invention also aims to provide a suitable method for manufacturing the above-mentioned stainless steel sheet. In this disclosure, any numerical range expressed using "~" means a range that includes the numerical values ​​written before and after "~" as the lower limit and upper limit, respectively.

[0009] Now, the inventors diligently conducted research to achieve the above objective. As a result, the inventors obtained the following findings: (a) First, the inventors attempted to adjust the HRC to a range of 30 to 40 (hereinafter also referred to as the desired HRC) by using steel having the component composition described in Patent Document 1 and lowering the winding temperature in the hot rolling process. (b) However, the stainless steel sheet manufactured as described in (a) above had shape defects, and in particular, the waves and warp heights that occurred in the steel sheet became large, and the flatness was greatly reduced. Therefore, the stainless steel sheet manufactured as described in (a) above was unsuitable as a material steel sheet for disc brakes. (c) In other words, disc brakes are usually manufactured by punching out a material steel sheet to achieve the desired shape. However, the stainless steel sheet manufactured as described in (a) above had low flatness and could not be used for punching out as is. Furthermore, it was extremely difficult to correct the shape of the stainless steel sheet using skin pass rolling, tension leveling, or other shape correction methods. Furthermore, even if shape correction was possible, it resulted in a significant decrease in productivity and a deterioration of the product characteristics of the disc brakes.

[0010] Therefore, the inventors manufactured various stainless steel sheets by changing the component composition and manufacturing conditions, and conducted repeated studies to achieve both the desired HRC and suppression of the decrease in flatness without performing a heat treatment. As a result, the inventors obtained the following findings.

[0011] In other words, the component composition and the microstructure balance index F, defined by the following equation (1), are appropriately controlled. At the same time, in the microstructure of the stainless steel sheet, the area fraction of ferrite grains (hereinafter simply referred to as α-ferrite) with an average intragranular orientation difference of 1° or less and a particle size of 50 μm or less is controlled to be in the range of 0.01 to 50.00%, and the microstructure other than α-ferrite is basically composed of martensite. As a result, a stainless steel sheet with the desired HRC and suppressed reduction in flatness can be obtained without quenching treatment. F = 460 - 350 × ([%C] + [%N]) - 18 × [%Cr] - 25 × [%Mn] - 17 × [%Ni] - 8 × [%Cu] ... (1) In the equation, [%x] is the content (mass%) of element x in the component composition. Also, if element x is not contained in the component composition, [%x] is set to 0 when calculating F.

[0012] Furthermore, based on the above findings, the inventors conducted further studies and discovered that by appropriately controlling the component composition and microstructure balance index F, and simultaneously controlling the hot rolling conditions as follows, the above stainless steel sheet can be manufactured without quenching. The microstructure balance index F related to the winding temperature is also defined by the above formula (1). Slab heating temperature: 1100 to 1250°C Finishing rolling completion temperature: 800°C or higher Winding temperature: [Microstructure balance index F] to 600°C

[0013] This invention was completed based on the above findings and further investigations. Specifically, the gist of this invention is as follows:

[0014] 1. A martensitic stainless steel sheet having a composition in mass%, where C: 0.03-0.20%, Si: 0.01-0.50%, Mn: 0.1-2.0%, Cr: 10.0-14.5%, P: 0.040% or less, S: 0.010% or less, and N: 0.005-0.060%, with the remainder being Fe and unavoidable impurities; a microstructure balance index F defined by the following formula (1) is 100-250; an average intragranular orientation difference of 1° or less; and an area fraction of ferrite grains with a particle size of 50 μm or less is 0.01-50.00%. F = 460 - 350 × ([%C] + [%N]) - 18 × [%Cr] - 25 × [%Mn] - 17 × [%Ni] - 8 × [%Cu] ... (1) In the formula, [%x] is the mass %) content of element x in the above component composition. If element x is not contained in the above component composition, [%x] is set to 0 when calculating F.

[0015] 2. The martensitic stainless steel sheet according to 1, wherein the component composition further contains, by mass%, at least one of the following groups (Group A), (Group B), and (Group C): (Group A) One or two selected from Ni: 5.00% or less and Cu: 2.00% or less (Group B) One or more selected from Al: 0.100% or less, Mo: 1.00% or less, W: 1.00% or less, Co: 0.20% or less, and Sn: 0.50% or less (Group C) One or more selected from Ti: 0.40% or less, Nb: 0.40% or less, Zr: 0.40% or less, V: 1.0% or less, B: 0.010% or less, Mg: 0.0030% or less, REM: 0.100% or less, and Ca: 0.0030% or less

[0016] 3. A martensitic stainless steel sheet according to 1 or 2 above, wherein the Rockwell hardness (C scale) measured in accordance with JIS Z 2245:2016 is 30 to 40.

[0017] 4. A martensitic stainless steel sheet according to any one of items 1 to 3 above, wherein the flatness measured in accordance with JIS G 3193:2019 is 200 mm or less.

[0018] 5. A hot-rolled steel sheet, a martensitic stainless steel sheet as described in any of items 1 to 4 above.

[0019] 6. A martensitic stainless steel sheet according to any of items 1 to 5 above, for use with disc brakes.

[0020] 7. A method for manufacturing a martensitic stainless steel sheet according to any one of the above 1 to 6, wherein the method comprises a hot rolling step of hot rolling a slab to obtain a hot-rolled steel sheet, and in the hot rolling step, the slab heating temperature is 1100 to 1250°C, the finish rolling completion temperature is 800°C or higher, and the coiling temperature is [structural balance index F] to 600°C.

[0021] According to the present invention, a stainless steel sheet is obtained that does not require heat treatment and is suitable as a material steel sheet for disc brakes of various motorcycles, thus improving productivity and CO ２ Further reductions in emissions are possible, which is extremely advantageous from an industrial perspective.

[0022] The present invention will be described based on the following embodiments.

[0023] [1] Martensitic Stainless Steel Sheet First, the component composition of a martensitic stainless steel sheet according to one embodiment of the present invention will be described. Note that all units in the component composition are "mass%", and unless otherwise specified, they will be simply referred to as "%".

[0024] C: 0.03-0.20% Carbon (C) is an effective element for increasing hardness and improving wear resistance. To obtain the desired HRC, the C content should be 0.03% or more. On the other hand, if the C content exceeds 0.20%, the desired HRC cannot be obtained. Therefore, the C content should be in the range of 0.03-0.20%. Preferably, the C content is 0.04% or more. Preferably, the C content is 0.18% or less, more preferably 0.16% or less.

[0025] Si: 0.01-0.50% Si is an element that generates ferrite at high temperatures, improving hot workability. This effect is observed when the Si content is 0.01% or higher. On the other hand, if the Si content exceeds 0.50%, the hardness decreases and the toughness is negatively affected. Therefore, the Si content should be in the range of 0.01-0.50%. The Si content is preferably 0.05% or higher, more preferably 0.10% or higher. Furthermore, the Si content is preferably 0.45% or lower, more preferably 0.40% or lower.

[0026] Mn: 0.1-2.0% Mn is an effective element for suppressing the formation of δ-ferrite at high temperatures. Mn is an effective element for increasing hardness and obtaining a good shape. Furthermore, Mn is an element that makes it possible to control the microstructure balance index F, which will be described later, within an appropriate range. If the Mn content is less than 0.1%, δ-ferrite, which will be described later, will be formed, making it difficult to obtain the desired HRC. On the other hand, if the Mn content exceeds 2.0%, it promotes the formation of oxide scale and leads to an increase in polishing load. For this reason, the Mn content should be in the range of 0.1-2.0%. The Mn content is preferably 0.5% or more, more preferably 1.0% or more. Also, the Mn content is preferably 1.8% or less, more preferably 1.7% or less.

[0027] Cr: 10.0-14.5% To maintain corrosion resistance, the Cr content should be 10.0% or more. On the other hand, if the Cr content exceeds 14.5%, δ-ferrite, described later, will be formed during the hot rolling process, and the desired HRC cannot be obtained. Therefore, the Cr content should be in the range of 10.0-14.5%. The Cr content is preferably 10.5% or more, more preferably 11.0% or more. Also, the Cr content is preferably 14.0% or less, more preferably 13.5% or less.

[0028] P: 0.040% or less. P is an element that is inevitably present in steel. Here, P is an element that improves the hardness of the matrix and decreases its toughness due to its high solid solution strengthening ability. Therefore, it is preferable to reduce P as much as possible. Thus, the P content should be 0.040% or less. Also, reducing the P content makes it easier to suppress the decrease in toughness. Therefore, the P content is preferably 0.030% or less. There is no particular lower limit to the P content. However, since excessive removal of P increases costs, a P content of 0.010% or more is preferable.

[0029] S: 0.010% or less. S, like P, is an element that is inevitably present in steel. Here, S is an element that is detrimental to corrosion resistance and workability. Therefore, it is preferable to reduce S as much as possible. In particular, if the S content exceeds 0.010%, corrosion resistance decreases significantly. Therefore, the S content should be 0.010% or less. The S content is preferably 0.006% or less, more preferably 0.003% or less. There is no particular lower limit to the S content. However, since excessive removal of S increases costs, an S content of 0.0005% or more is preferable.

[0030] N: 0.005 to 0.060% N, like C, is an effective element for increasing hardness. If the N content is less than 0.005%, the hardness required for disc brakes cannot be obtained. If the hardness is insufficient, the disc brakes will be prone to deformation during use. On the other hand, if the N content exceeds 0.060%, air bubbles will be generated inside the steel during casting, leading to the occurrence of surface defects. Therefore, the N content should be in the range of 0.005 to 0.060%. The N content is preferably 0.008% or more, more preferably 0.010% or more. Furthermore, the N content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0031] The basic elements (hereinafter also referred to as basic component elements) of the component composition of a martensitic stainless steel sheet according to one embodiment of the present invention have been described above. In addition, the martensitic stainless steel sheet according to one embodiment of the present invention may contain, in addition to the above basic component elements, at least one of the following groups (Group A), (Group B), and (Group C) as optional additive elements. (Group A) One or two selected from Ni: 5.00% or less and Cu: 2.00% or less (Group B) One or more selected from Al: 0.100% or less, Mo: 1.00% or less, W: 1.00% or less, Co: 0.20% or less and Sn: 0.50% or less (Group C) One or more selected from Ti: 0.40% or less, Nb: 0.40% or less, Zr: 0.40% or less, V: 1.0% or less, B: 0.010% or less, Mg: 0.0030% or less, REM: 0.100% or less and Ca: 0.0030% or less

[0032] Ni: 5.00% or less. Ni, like Mn, is an effective element for suppressing the formation of δ-ferrite at high temperatures. Ni is an effective element for increasing hardness and obtaining a good shape. Furthermore, by including Ni together with Mn, it becomes more advantageous to control the microstructure balance index F, described later, within an appropriate range. For this reason, the Ni content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Ni content exceeds 5.00%, it promotes the formation of oxide scale and leads to an increase in polishing load. For this reason, when Ni is included, the Ni content is preferably 5.00% or less, more preferably 4.80% or less, and even more preferably 4.50% or less.

[0033] Cu: 2.00% or less. Like Mn, Cu is an effective element for suppressing the formation of δ-ferrite at high temperatures. Cu is an effective element for increasing hardness and obtaining a good shape. Furthermore, by including Cu together with Mn, it becomes more advantageous to control the microstructure balance index F, described later, within an appropriate range. For this reason, the Cu content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Cu content exceeds 2.00%, ε-Cu precipitates during cooling after coiling in the hot rolling process, reducing corrosion resistance. For this reason, when Cu is included, the Cu content is preferably 2.00% or less, more preferably 1.80% or less, and even more preferably 1.50% or less.

[0034] Al: 0.100% or less. Al is an effective deoxidizing agent. Furthermore, Al has a stronger affinity for N than Cr. Therefore, Al has the effect of suppressing a decrease in corrosion resistance by causing N to precipitate as Al nitride instead of Cr nitride during cooling after winding. These effects are preferably obtained when the Al content is 0.010% or more. On the other hand, if the Al content exceeds 0.100%, excessive precipitation of N may occur, potentially leading to a decrease in hardness. Therefore, when Al is included, the Al content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less.

[0035] Mo: 1.00% or less. Mo is an element that improves corrosion resistance. This effect is preferably obtained when the Mo content is 0.01% or more. On the other hand, if the Mo content exceeds 1.00%, the formation of austenite at high temperatures may be suppressed, which may lead to a decrease in hardness. Therefore, when Mo is included, the Mo content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.60% or less.

[0036] W: 1.00% or less. W, like Mo, is an element that improves corrosion resistance. This effect is preferably obtained when the W content is 0.01% or more, more preferably 0.05% or more. On the other hand, if the W content exceeds 1.00%, the strength will increase excessively, which may lead to a decrease in manufacturability due to increased rolling load, etc. Therefore, when W is included, the W content is preferably 1.00% or less, more preferably 0.80% or less.

[0037] Co: 0.20% or less. Co is an element that improves toughness. This effect is preferably obtained when the Co content is 0.01% or more. On the other hand, if the Co content exceeds 0.20%, the processability decreases. Therefore, when Co is included, the Co content is preferably 0.20% or less.

[0038] Sn: 0.50% or less. Sn is an element that improves corrosion resistance. This effect is preferably obtained when the Sn content is 0.001% or more. On the other hand, if the Sn content exceeds 0.50%, toughness and hot rolling properties may decrease due to grain boundary segregation. Therefore, when Sn is included, the Sn content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.20% or less.

[0039] Ti: 0.40% or less. Ti is an element that improves corrosion resistance. This effect is preferably obtained when the Ti content is 0.01% or more. On the other hand, if the Ti content exceeds 0.40%, the hardness decreases. Therefore, when Ti is included, the Ti content is preferably 0.40% or less, and more preferably 0.10% or less.

[0040] Nb: 0.40% or less. Nb is an element that improves the tempering softening resistance of steel and suppresses softening due to temperature rise when disc brakes are used. This effect is preferably obtained when the Nb content is 0.01% or more. On the other hand, if the Nb content exceeds 0.40%, the hardness decreases. Therefore, when Nb is included, the Nb content is preferably 0.40% or less, more preferably 0.10% or less.

[0041] Zr: 0.40% or less. Zr is an element that improves corrosion resistance. This effect is preferably obtained when the Zr content is 0.01% or more. On the other hand, when the Zr content exceeds 0.40%, the hardness decreases. Therefore, when Zr is contained, the Zr content is preferably 0.40% or less, more preferably 0.10% or less.

[0042] V: 1.0% or less. V is an element that improves the tempering softening resistance of steel. This effect is preferably obtained when the V content is 0.01% or more. On the other hand, when the V content exceeds 1.0%, the hardness decreases. Therefore, when V is contained, the V content is preferably 1.0% or less, more preferably 0.8% or less.

[0043] B: 0.010% or less. B is an element that segregates at grain boundaries to improve the strength of grain boundaries and is effective for improving toughness. This effect is preferably obtained when the B content is 0.0002% or more. On the other hand, when the B content exceeds 0.010%, borides may crystallize during casting and the toughness may decrease. Therefore, when B is contained, the B content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.006% or less.

[0044] Mg: 0.0030% or less. Mg improves the equiaxed crystal ratio of slabs and makes recrystallization during hot rolling easier. That is, Mg is an element effective for improving punching workability. This effect is preferably obtained when the Mg content is 0.0002% or more. On the other hand, when the Mg content exceeds 0.0030%, the surface properties of the steel deteriorate. Therefore, when Mg is contained, the Mg content is preferably 0.0030% or less, more preferably 0.0004% or less.

[0045] REM: 0.100% or less. REM (rare earth metals) improves oxidation resistance and suppresses the formation of oxide scale. Thereby, REM suppresses the formation of a Cr-deficient region immediately below the temper color. Here, the temper color is a region where the oxide film on the surface of the stainless steel sheet has changed color due to welding or the like, and can be formed, for example, with the temperature rise during the use of a disk brake. This effect is preferably obtained when the REM content is 0.001% or more. On the other hand, when REM is contained in excess, manufacturing properties such as pickling properties are deteriorated, and an increase in manufacturing cost is caused. Therefore, when REM is contained, the REM content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less. Here, REM is a general term for Sc, Y, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content referred to here is the total content of these elements.

[0046] Ca: 0.0030% or less. Ca is an effective element for preventing the clogging of the nozzle due to the crystallization of Ti-based inclusions that are likely to occur during continuous casting. This effect is preferably obtained when the Ca content is 0.0002% or more. On the other hand, when the Ca content exceeds 0.0030%, the corrosion resistance may be reduced due to the formation of CaS. Therefore, when Ca is contained, the Ca content is preferably 0.0030% or less, more preferably 0.0010% or less.

[0047] The balance other than the above elements is Fe and unavoidable impurities. Examples of the unavoidable impurities include O, As, Sb, Pb, Bi, Ta, and Hf. These elements are acceptable if each is 0.01% or less. The content of these elements may all be 0%. Also, any of the above optionally added elements may be 0%. In addition, when the content of each of the above optionally added elements is less than the preferred lower limit value, it can also be said that the element is included as an unavoidable impurity.

[0048] Furthermore, as mentioned above, in order to obtain a stainless steel sheet with the desired HRC and suppressed reduction in flatness without heat treatment, the following points are important: • Control the component composition as described above and appropriately control the microstructure balance index F defined by the above formula (1). • Simultaneously, in the microstructure of the stainless steel sheet, control the area fraction of α-ferrite to the range of 0.01 to 50.00%, and ensure that the microstructure other than α-ferrite is basically composed of martensite.

[0049] The inventors believe the above reasons are as follows:

[0050] The decrease in flatness is caused by the superposition of rolling factors and transformation factors. Here, rolling factors include edge elongation and mid-section elongation due to hot rolling, as well as thermal stress due to temperature deviations during cooling after hot rolling. Transformation factors include martensitic transformation that occurs before and after coiling after hot rolling.

[0051] To obtain the desired HRC, martensite is necessary in the microstructure of the stainless steel sheet. Therefore, it is not possible to completely eliminate the factors that cause transformation. However, when martensitic transformation occurs, if α-ferrite is present around it, the soft α-ferrite deforms, and the internal stress generated by the martensitic transformation is relieved. This suppresses the decrease in flatness.

[0052] Furthermore, δ-ferrite may precipitate during the manufacturing process of martensitic stainless steel sheets. However, δ-ferrite does not contribute to improving flatness. The inventors believe the reason for this is as follows: δ-ferrite precipitates, for example, during slab heating. Therefore, the dispersion morphology of δ-ferrite is not uniform. Also, δ-ferrite is coarse. Therefore, even if δ-ferrite deforms when martensitic transformation occurs, the internal stress remains non-uniform and does not contribute to improving flatness. Here, δ-ferrite is mainly coarse ferrite grains that are generated during heating in hot rolling. δ-ferrite can also be described as ferrite grains other than α-ferrite. α-ferrite, as will be described later, is mainly generated during cooling after coiling.

[0053] Furthermore, the inventors conducted various experiments and studies, such as those exemplified below (i) to (v), and found that the following points are important in obtaining the microstructure of the stainless steel sheet described above: • In the above component composition, the microstructure balance index F, defined by formula (1) above, is controlled to be in the range of 100 to 250. • Then, the hot rolling conditions are controlled to be: Slab heating temperature: 1100 to 1250°C, Finish rolling completion temperature: 800°C or higher, and coiling temperature: [Microstructure balance index F] to 600°C.

[0054] (i) In stainless steel sheets manufactured from steel materials with a structural balance index F of less than 100, a large amount of untransformed austenite (hereinafter also referred to as retained austenite) remained in the microstructure, and the desired HRC could not be obtained. (ii) In stainless steel sheets manufactured from steel materials with a structural balance index F of more than 250, the amount of solid solution of C and N, which increase the hardness of martensite, decreased due to self-tempering, and the desired HRC could not be obtained. (iii) When the coiling temperature was less than [structural balance index F]°C, the flatness of the stainless steel sheet obtained after the hot rolling process decreased significantly. In the microstructure of the stainless steel sheet in question, no α-ferrite was observed, and it consisted only of martensite, or martensite and δ-ferrite. Therefore, it is estimated that the structural balance index F corresponds to the martensitic transformation completion temperature. (iv) When the coiling temperature exceeded 600°C, the ferrite precipitate region remained for a long time during cooling after coiling, resulting in an excess of α-ferrite, and the desired HRC could not be obtained. (v) Even when the microstructure balance index F and winding temperature were controlled within an appropriate range, stainless steel sheets with the desired HRC and suppressed reduction in flatness could not be manufactured without quenching unless the finish rolling completion temperature was controlled to 800°C or higher.

[0055] The inventors believe that the reason for (v) above is due to a shift in the ferrite deposition temperature-time curve toward the short-time side, which occurs due to the recrystallization of austenite during hot rolling.

[0056] In other words, α-ferrite precipitates using austenite grain boundaries as precipitation nuclei. This means that a structure with many fine austenite grains can form α-ferrite in a shorter time. By setting the finish rolling temperature of the hot rolling process to a high temperature, specifically 800°C or higher, the recrystallization of austenite is promoted, and a structure with many fine austenite grains is obtained in the steel sheet after the finish rolling is completed. By winding a steel sheet with such a structure at the above winding temperature, appropriate amounts of α-ferrite and martensite are generated during cooling after winding, achieving both the desired HRC and suppression of a decrease in flatness.

[0057] The following explains the organizational balance index F and the organization itself.

[0058] Organizational balance index F: 100 to 250 If the organizational balance index F is less than 100, the desired HRC cannot be obtained, as described above. On the other hand, if the organizational balance index F exceeds 250, the desired HRC cannot be obtained, as described above. Therefore, the organizational balance index F should be in the range of 100 to 250. The organizational balance index F is preferably 120 or higher, more preferably 130 or higher. The organizational balance index F is preferably 240 or lower, more preferably 230 or lower.

[0059] Area fraction of α-ferrite: 0.01 to 50.00% As described above, when martensitic transformation occurs, the presence of α-ferrite in the surrounding area causes the soft α-ferrite to deform, relieving the internal stress generated by the martensitic transformation. This suppresses the decrease in flatness. To obtain this effect, the area fraction of α-ferrite is 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, if the area fraction of α-ferrite is excessive, the desired HRC cannot be obtained. Therefore, the area fraction of α-ferrite is 50.00% or less, preferably 48.00% or less, and more preferably 45.00% or less.

[0060] Here, ferrite grains with an average intragranular orientation difference of 1° or less and a particle size of 50 μm or less are defined as α-ferrite because these ferrite grains provide sufficient suppression of the decrease in flatness mentioned above.

[0061] Furthermore, in the microstructure of a martensitic stainless steel sheet according to one embodiment of the present invention, the microstructure other than α-ferrite is basically martensite. Here, "martensitic stainless steel sheet" refers to a steel sheet in which the area fraction of martensite is 50.00 to 99.99%. The area fraction of martensite is preferably 52.00% or more, more preferably 55.00% or more. The area fraction of martensite is preferably 99.98% or less, more preferably 99.95% or less. Furthermore, the microstructure of a martensitic stainless steel sheet according to one embodiment of the present invention may include the remaining microstructure other than martensite and α-ferrite (hereinafter simply referred to as the remaining microstructure). The area fraction of the remaining microstructure is preferably 10.00% or less, more preferably 8.00% or less. The area fraction of the remaining microstructure may be 0%. Examples of the remaining microstructure include retained austenite and δ-ferrite. The area fraction of each phase is the area ratio occupied by each phase to the entire microstructure.

[0062] The area fraction of α-ferrite is measured by electron backscatter diffraction (hereinafter also referred to as the EBSD method).

[0063] Specifically, a sample is cut from a martensitic stainless steel sheet so that the cross section perpendicular to the rolling direction (C section) becomes the observation surface. Next, the observation surface of the sample is mirror-polished, and then surface strain is removed by polishing with colloidal silica. Then, microstructure observation is performed using this sample by the EBSD method. For microstructure observation by the EBSD method, for example, a scanning electron microscope: JSM-6490LA manufactured by JEOL Ltd. and its accompanying crystal orientation analyzer: Orion1000P manufactured by TSL Solutions Co., Ltd. are used. For data acquisition, for example, APEX ver. 2.1 manufactured by EDAX is used. For analysis, for example, OIM Data Analysis ver. 8.6 manufactured by EDAX is used.

[0064] In one example of microstructure observation using the EBSD method, a 2 mm square area is set as the observation field of view in the sample, such that the center of the steel plate in the thickness direction is the center of that direction. If the thickness of the steel plate is less than 2 mm, the area is the same as above (4 mm). ２ The observation field of view is widened in the width direction of the plate so that the above observation field of view is achieved. For example, the observation magnification is set to 1000x, and tissue observation is performed using the montage mapping function of APEX ver. 2.1 to achieve the above observation field of view. The step size is set to 0.5 μm. The acceleration voltage of JSM-6490LA is set to 25 kV. The spot size is set to 70. The Gain of Orion1000P is set to High Speed. The exposure time is adjusted so that the maximum brightness of the camera is approximately 0.450. As a guideline, the exposure time is approximately 1.5 ms. It is also preferable that the Kikuchi pattern can be analyzed by checking the Kikuchi pattern at several points in the field of view and adjusting the parameters.

[0065] Next, the obtained measurement data is opened using OIM Data Analysis ver. 8.6, and a cleanup process is performed as the first preprocessing step before analysis. In the cleanup process, partitioning is performed to exclude points with a CI value of less than 0.1, and a dilation of one iteration is applied. In addition, if the obtained measurement data includes points where parts other than the sample, such as resin, were measured, partitioning is performed to remove those points. To ensure the validity of the measurement, if the average CI value of the partition is less than 0.5, the measurement data is not used.

[0066] Next, an analysis is performed on the partition obtained by partitioning as described above (hereinafter also referred to as Partition 1) to calculate the area fraction of α-ferrite. First, the orientation difference that EBSD recognizes as a crystal grain is set to 10°. Next, the region occupied by crystal grains with a particle size (equivalent circle diameter) of more than 50 μm is excluded from Partition 1 to obtain Partition 2. Next, in Partition 2, crystal grains with an average intra-grain orientation difference (Grain Average Misorientation: GAM) of 1° or less are highlighted as α-ferrite, and the highlighted region is designated as Partition 3. Then, the area fraction of α-ferrite is calculated using the following equation (2): [Area fraction of α-ferrite (%)] = [Area of ​​Partition 3] / [Area of ​​Partition 1] × 100 ... (2)

[0067] Furthermore, the area fraction of martensite can be calculated using the following formula (3). The area fraction of the remaining tissue can be calculated using the following formula (4). [Area fraction of martensite (%)] = ([Area of ​​partition 2] - [Area of ​​partition 3]) / [Area of ​​partition 1] × 100 ... (3) [Area fraction of the remaining tissue (%)] = 100 - [Area fraction of martensite (%)] - [Area fraction of α-ferrite (%)] ... (4)

[0068] In other words, the area of ​​partition 1 represents the total area of ​​the tissue, the area of ​​partition 2 represents the combined area of ​​martensite and α-ferrite, and the area of ​​partition 3 represents the area of ​​α-ferrite. It should be noted that it is difficult to determine and distinguish the presence or absence of α-ferrite by observing the tissue using an optical microscope.

[0069] HRC: 30-40 To obtain the excellent wear resistance required for disc brakes, the HRC is preferably 30 or higher, more preferably 31 or higher, and even more preferably 33 or higher. On the other hand, if the hardness is excessively high, squeaking will occur during braking, adversely affecting the characteristics of the disc brake. For this reason, the HRC is preferably 40 or lower, more preferably 39 or lower, and even more preferably 37 or lower.

[0070] HRC is measured in accordance with JIS Z 2245:2016. The specific measurement procedure is as described in the examples below.

[0071] Flatness: 200 mm or less. Flatness affects not only the processability for disc brakes but also the product characteristics of disc brakes, such as warping and wobble during use. Flatness can be improved by skin pass rolling, tension leveling, and other shape correction methods. However, shape correction of steel plates with an HRC of 30 to 40 is difficult. In particular, if the flatness exceeds 200 mm, shape correction becomes extremely difficult, and processing into disc brakes also becomes extremely difficult. Furthermore, even if shape correction is possible, it leads to a significant decrease in productivity and deterioration of the product characteristics of the disc brakes. For this reason, the flatness is preferably 200 mm or less, more preferably 180 mm or less, even more preferably 150 mm or less, and even more preferably 100 mm or less. The lower limit of flatness is not particularly limited and may be 0 mm.

[0072] Flatness is measured in accordance with JIS G 3193:2019. The specific measurement procedure is as described in the examples below.

[0073] Hereinafter, a material having an HRC in the range of 30 to 40 without undergoing quenching treatment, and a flatness of 200 mm or less (more preferably 180 mm or less, even more preferably 150 mm or less, and even more preferably 100 mm or less) is also referred to as having excellent compatibility between hardness and flatness.

[0074] The thickness of the martensitic stainless steel sheet according to one embodiment of the present invention is not particularly limited. For example, the thickness of the martensitic stainless steel sheet according to one embodiment of the present invention is preferably 3.0 to 12.0 mm. The thickness of the martensitic stainless steel sheet according to one embodiment of the present invention is more preferably 4.0 mm or more, and even more preferably 5.0 mm or more. Furthermore, the thickness of the martensitic stainless steel sheet according to one embodiment of the present invention is more preferably 11.5 mm or less, and even more preferably 11.0 mm or less.

[0075] Furthermore, the martensitic stainless steel sheet according to one embodiment of the present invention is preferably, for example, a hot-rolled steel sheet. Here, the hot-rolled steel sheet includes not only the as-hot-rolled steel sheet but also a steel sheet obtained by subjecting the as-hot-rolled steel sheet to an oxide scale removal treatment such as pickling. However, the hot-rolled steel sheet does not include a steel sheet obtained by subjecting the as-hot-rolled steel sheet to a heat treatment such as hot-rolled annealing or quenching (a heat treatment that causes a change in HRC). Moreover, the martensitic stainless steel sheet according to one embodiment of the present invention is particularly suitable for use in disc brakes.

[0076] [2] Method for manufacturing martensitic stainless steel sheets Next, a method for manufacturing martensitic stainless steel sheets according to one embodiment of the present invention will be described.

[0077] A method for manufacturing a martensitic stainless steel sheet according to one embodiment of the present invention comprises a hot rolling step of hot rolling a slab to obtain a hot-rolled steel sheet, characterized in that the hot rolling step is controlled to: slab heating temperature: 1100 to 1250°C, finish rolling completion temperature: 800°C or higher, and winding temperature: [structural balance index F] to 600°C.

[0078] A method for manufacturing a martensitic stainless steel sheet according to one embodiment of the present invention is a method for manufacturing a martensitic stainless steel sheet according to the above embodiment of the present invention. Here, unless otherwise specified, the temperature related to the manufacturing method is based on the surface temperature of the slab or steel sheet. The slab (steel material) to be subjected to hot rolling can be obtained, for example, as follows: Molten steel having the above-mentioned component composition is melted using a known method such as a converter, electric furnace, or vacuum melting furnace, and then formed into a slab (steel material) by continuous casting or ingot-splitting method.

[0079] The following describes in detail a method for manufacturing a martensitic stainless steel sheet according to one embodiment of the present invention, particularly the hot rolling conditions.

[0080] Slab heating temperature: 1100-1250°C. From the viewpoint of controlling the finish rolling completion temperature within a predetermined range during hot rolling, the slab heating temperature is 1100°C or higher, preferably 1120°C or higher, and more preferably 1140°C or higher. On the other hand, if the slab heating temperature exceeds 1250°C, it induces slab sagging in the heating furnace, leading to operational problems. Therefore, the slab heating temperature is 1250°C or lower.

[0081] Finishing rolling completion temperature: 800°C or higher As described above, in order to obtain a stainless steel sheet that is excellent in both hardness and flatness, it is extremely important to control the area fraction of α-ferrite in the microstructure of the stainless steel sheet to the above range and to ensure that the microstructure other than α-ferrite is basically composed of martensite. α-ferrite precipitates using austenite grain boundaries as precipitation nuclei. Therefore, in order to secure an appropriate amount of α-ferrite in the microstructure of the stainless steel sheet, it is necessary to promote the recrystallization of austenite grains during hot rolling and to obtain a microstructure with fine austenite grains in the steel sheet after the completion of finishing rolling. In order to obtain such a microstructure, the finishing rolling completion temperature is 800°C or higher, preferably 810°C or higher, and more preferably 815°C or higher. There is no particular upper limit to the finishing rolling completion temperature. However, if the finishing rolling completion temperature is excessively high, the slab heating temperature will become excessively high, which may induce slab sagging in the heating furnace and lead to operational problems. Therefore, for example, the finish rolling completion temperature is preferably 1000°C or lower.

[0082] Winding temperature: [Structural balance index F] to 600°C As described above, in order to obtain a stainless steel sheet that is excellent in both hardness and flatness, it is extremely important to control the area fraction of α-ferrite in the microstructure of the stainless steel sheet to the above range and to basically make up the microstructure other than α-ferrite with martensite. In order to obtain such a microstructure, it is important to control the winding temperature according to the structural balance index F after controlling the slab heating temperature and the finish rolling end temperature as described above. If the winding temperature is below [Structural balance index F]°C, an appropriate amount of α-ferrite cannot be secured and the flatness will decrease. On the other hand, if the winding temperature exceeds 600°C, α-ferrite will be generated excessively and the desired HRC cannot be obtained. For this reason, the winding temperature is [Structural balance index F]°C or higher, preferably [Structural balance index F + 5]°C or higher, and more preferably [Structural balance index F + 10]°C or higher. Also, the winding temperature is 600°C or lower, preferably 575°C or lower, and more preferably 550°C or lower.

[0083] Other than the conditions mentioned above, there are no particular limitations, and conventional methods should be followed. For example, hot rolling should be performed by rough rolling and finish rolling, with 5 to 10 rolling passes for rough rolling and 5 to 8 rolling passes for finish rolling. The total reduction ratio for hot rolling should be, for example, 90 to 99%. In addition, the hot-rolled steel sheet may optionally undergo oxide scale removal treatments such as pickling or shot blasting.

[0084] Molten steel having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a 150-ton converter and refined using a strongly agitated, vacuum oxygen decarburization (SS-VOD) treatment. The resulting molten steel was then continuously cast to form slabs with a width of 1000 mm and a thickness of 200 mm. The slabs were then heated at 1200°C for 1 hour, and hot-rolled under the conditions shown in Table 2 to form hot-rolled steel sheets with a width of 1000 mm and a thickness of 5.0 mm. In this process, the first 7th rolling passes were performed as rough rolling, and the 8th to 14th rolling passes were performed as finish rolling. Rough rolling was performed using a 3-stage stand reverse-type rough rolling mill, and finish rolling was performed using a 7-stage stand tandem-type finish rolling mill. Other conditions followed conventional methods.

[0085] The area fraction of α-ferrite was measured in the hot-rolled steel sheets obtained in the manner described above. The results are shown in Table 2. Note that the microstructure of all hot-rolled steel sheets consisted primarily of martensite, apart from α-ferrite, and the area fraction of the remaining microstructure was 10.00% or less in all cases.

[0086] Furthermore, (1) HRC and (2) flatness were measured according to the following test method, and the compatibility of hardness and flatness was evaluated according to the following evaluation criteria. The evaluation results are shown in Table 2. Pass A (Excellent): HRC of 30-40 and flatness of 100 mm or less Pass B (Very Good): HRC of 30-40 and flatness of more than 100 mm and 150 mm or less Pass C (Good): HRC of 30-40 and flatness of more than 150 mm and 200 mm or less Fail (Poor): HRC less than 30 or more than 40, and / or flatness more than 200 mm

[0087] (1) Near the leading edge of the HRC hot-rolled steel sheet (coil), a 50 mm square (50 mm x 50 mm) sample was taken so that the center of the width direction was 200 mm from the center of the sheet width and from the edge in the width direction. Similarly, near the trailing edge of the hot-rolled steel sheet (coil), a 50 mm square sample was taken so that the center of the width direction was 200 mm from the center of the sheet width and from the edge in the width direction.

[0088] Next, the surface of each sample was ground to a thickness of approximately 50 μm to remove the oxide scale. Then, the Rockwell hardness (C scale) was measured at five arbitrary points on each sample in accordance with JIS Z 2245:2016. The average value of the Rockwell hardness (C scale) measured at a total of 20 points (5 points per sample) was taken as the HRC of the steel plate.

[0089] (2) Flatness A sample with a length of 4 m in the rolling direction was taken near the leading edge of the hot-rolled steel sheet (coil). However, the region from the leading edge of the hot-rolled steel sheet to 10 m in the rolling direction was excluded from the sampling location because the coiling tendency was strong and the sample may bend strongly on the surface plate. Similarly, a sample with a length of 4 m in the rolling direction was taken near the trailing edge of the hot-rolled steel sheet (coil). However, the region from the trailing edge of the hot-rolled steel sheet to 10 m in the rolling direction was also excluded from the sampling location because the coiling tendency was strong and the sample may bend strongly on the surface plate. If the hot-rolled steel sheet is curved and it is difficult to take a sample with a length of 4 m parallel to the rolling direction, leveler straightening may be performed. In this case, it shall be separately confirmed that the change in flatness due to leveler straightening is less than 10 mm.

[0090] Next, the collected samples were placed on a surface plate and their flatness was measured in accordance with JIS G 3193:2019. Here, if the wave pitch of the sample was 4 m or less, the length between the wave pitches was used as the reference length. If the wave pitch of the sample exceeded 4 m, the length between both ends of the sample in the longitudinal direction (rolling direction) was used as the reference length. In addition, a straight ruler was used to measure the reference length, and the value obtained by subtracting the thickness of the sample from the distance between the straight ruler and the surface plate was used as the flatness. Furthermore, considering that ear waves and antinode elongation are factors that reduce flatness, the flatness was measured at three locations in the width direction of each sample: the center and both ends, and the maximum value was taken as the flatness of the steel plate.

[0091]

[0092]

[0093] As shown in Table 2, all of the inventive examples exhibited excellent balance between hardness and flatness. In other words, in all of the inventive examples, stainless steel sheets suitable as material steel sheets for disc brakes of various motorcycles were obtained without heat treatment, that is, stainless steel sheets having the desired HRC and suppressing a decrease in flatness.

[0094] On the other hand, in the comparative example, it was not possible to achieve both hardness and flatness, and a stainless steel sheet suitable as a material for disc brakes on various motorcycles could not be obtained.

[0095] In other words, in No. 14, the carbon content was below the appropriate range, so the desired HRC could not be obtained, and both hardness and flatness could not be achieved. In No. 15, the carbon content exceeded the appropriate range, so the desired HRC could not be obtained, and both hardness and flatness could not be achieved. In No. 16, the microstructure balance index F was below the appropriate range, so the desired HRC could not be obtained, and both hardness and flatness could not be achieved. In No. 17, the microstructure balance index F exceeded the appropriate range, so the desired HRC could not be obtained, and both hardness and flatness could not be achieved. In No. 18, because the finish rolling completion temperature was below the appropriate range in conjunction with the slab heating temperature, the area fraction of α-ferrite was below the appropriate range, the flatness decreased, and both hardness and flatness could not be achieved. In sample 19, the winding temperature exceeded the appropriate range, causing the area fraction of α-ferrite to exceed the appropriate range, resulting in an inability to obtain the desired HRC and achieve both hardness and flatness. In sample 20, the winding temperature was below the appropriate range, causing the area fraction of α-ferrite to fall below the appropriate range, resulting in a decrease in flatness and an inability to achieve both hardness and flatness.

Claims

1. A martensitic stainless steel sheet having a composition in mass%, where C: 0.03-0.20%, Si: 0.01-0.50%, Mn: 0.1-2.0%, Cr: 10.0-14.5%, P: 0.040% or less, S: 0.010% or less, and N: 0.005-0.060%, with the remainder being Fe and unavoidable impurities; a microstructure balance index F defined by the following formula (1) is 100-250; an average intragranular orientation difference of 1° or less; and an area fraction of ferrite grains with a particle size of 50 μm or less is 0.01-50.00%. F = 460 - 350 × ([%C] + [%N]) - 18 × [%Cr] - 25 × [%Mn] - 17 × [%Ni] - 8 × [%Cu] ... (1) In the formula, [%x] is the mass %) content of element x in the above component composition. If element x is not contained in the above component composition, [%x] is set to 0 when calculating F.

2. The martensitic stainless steel sheet according to claim 1, wherein the component composition further contains, by mass%, at least one of the following groups (Group A), (Group B), and (Group C): (Group A) One or two selected from Ni: 5.00% or less and Cu: 2.00% or less (Group B) One or more selected from Al: 0.100% or less, Mo: 1.00% or less, W: 1.00% or less, Co: 0.20% or less, and Sn: 0.50% or less (Group C) One or more selected from Ti: 0.40% or less, Nb: 0.40% or less, Zr: 0.40% or less, V: 1.0% or less, B: 0.010% or less, Mg: 0.0030% or less, REM: 0.100% or less, and Ca: 0.0030% or less 3. A martensitic stainless steel sheet according to claim 1 or 2, wherein the Rockwell hardness (C scale) measured in accordance with JIS Z 2245:2016 is 30 to 40.

4. A martensitic stainless steel sheet according to any one of claims 1 to 3, wherein the flatness measured in accordance with JIS G 3193:2019 is 200 mm or less.

5. A martensitic stainless steel sheet according to any one of claims 1 to 4, which is a hot-rolled steel sheet.

6. A martensitic stainless steel sheet according to any one of claims 1 to 5, for use with disc brakes.

7. A method for manufacturing a martensitic stainless steel sheet according to any one of claims 1 to 6, the method comprising a hot rolling step of hot rolling a slab to obtain a hot-rolled steel sheet, wherein the hot rolling step is: slab heating temperature: 1100 to 1250°C, finish rolling completion temperature: 800°C or higher, and winding temperature: [structural balance index F] to 600°C.

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