Martensitic stainless steel sheet and production method for same

Abstract

The present invention provides a martensitic stainless steel sheet that makes it possible to set quenching hardness to a prescribed target range and that has excellent punch machining properties. In the present invention, the component composition is suitably controlled, and the DEA area ratio at a 1 / 2 sheet thickness position is not more than 30%.

Description

Martensitic stainless steel sheet and method of manufacturing the same

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

[0002] Motorcycle disc brakes require wear resistance to maintain braking performance over a long period of time. Wear resistance generally improves as hardness increases, while toughness decreases as hardness increases.

[0003] Therefore, for components such as disc brakes that require both high wear resistance and high toughness, steel having a Rockwell hardness (C scale) (hereinafter also referred to as HRC) in the range of 30 to 40, such as martensitic stainless steel plate as disclosed in Patent Document 1, is often used as the material.

[0004] Patent No. 1322454

[0005] Incidentally, when manufacturing components such as disc brakes using martensitic stainless steel sheets as the raw material, it is common to punch a hot-rolled steel sheet or the like into a predetermined shape and then quench it to increase its hardness. Hereinafter, the hardness of the martensitic stainless steel sheet after quenching is also referred to as quenched hardness.

[0006] However, the above-mentioned punching process often causes a double-plate-shaped crack in the hot-rolled steel sheet, which can result in a significant yield loss. Therefore, there is a strong demand for improved punching workability in martensitic stainless steel sheets such as those disclosed in Patent Document 1. Note that a double-plate-shaped crack is a crack occurring near the center of the sheet thickness in the cross section created by punching, forming two plates (or layers).

[0007] The present invention was developed to meet the above-mentioned demand, and aims to provide a martensitic stainless steel sheet that can achieve a quenching hardness within a predetermined target range (HRC of 30 to 40, preferably 33 to 37) and has excellent punching workability, along with a suitable manufacturing method thereof. Note that in this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively.

[0008] In order to achieve the above object, the inventors have conducted extensive research as follows.

[0009] (1) First, the inventors conducted a detailed study on the cause of double-plate cracks that occur during punching of martensitic stainless steel sheets. As a result, the inventors obtained the following findings: Double-plate cracks occur when they start from a dark etched area (hereinafter also referred to as a DEA area) that is observed under an optical microscope after etching with aqua regia and progress into the steel sheet.

[0010] (2) Based on the above findings, the inventors conducted further research. As a result, the inventors obtained the following findings. That is, by appropriately controlling the component composition and setting the area ratio of the DEA region at the half-thickness position (hereinafter also referred to as the DEA area ratio) to 30% or less, it becomes possible to make the distribution of plastic deformation in the thickness direction during punching more uniform, and to significantly suppress the occurrence of two-plate cracks. In other words, punching workability is significantly improved.

[0011] (3) The inventors have also found the following. That is, in order to make the DEA area ratio at the half-thickness position 30% or less, it is important to properly control the composition of the steel and then to properly control the hot rolling conditions. In particular, it is important to perform the following first rolling pass-hold combination, and then to perform the following second rolling pass-hold combination. First rolling pass-hold combination: A combination of one rolling pass at a rolling temperature of 1100°C or more and a reduction of 20% or more, and a hold immediately after the rolling pass at a hold temperature of 1080°C or more and a hold time of 20 seconds or more. Second rolling pass-hold combination: A combination of one or more rolling passes at a rolling temperature of less than 1100°C and 1050°C or less and a total reduction of 35% or more, and a hold immediately after the final rolling pass of the rolling passes at a hold temperature of 900°C or more and a hold time of 30 seconds or more.

[0012] The inventors believe that the reason for the above is as follows.

[0013] When comparing the DEA region with the region other than the DEA region (hereinafter also referred to as the non-DEA region), the DEA region has a higher hardness than the non-DEA region. In other words, the DEA region is less likely to deform during shearing due to punching than the surrounding non-DEA region. Therefore, during punching, high stress is generated locally in the DEA region, which becomes the starting point for two-plate cracks.

[0014] When crystal grains constituting DEA regions (hereinafter also referred to as DEA grains) are observed using a bright-field optical microscope, a large number of Cr carbides are precipitated within the crystal grains and at the grain boundaries in the DEA grains compared to crystal grains constituting non-DEA regions (hereinafter also referred to as non-DEA grains). Furthermore, the DEA grains have a higher Mn content than non-DEA grains, resulting in Mn segregation. In other words, the DEA regions correspond to regions that become austenite during hot-rolled sheet annealing, where the austenite phase is stable down to low temperatures during cooling after hot-rolled sheet annealing, due to their higher Mn content. Therefore, the DEA regions corrode more than the non-DEA regions when etched with aqua regia, and are identified as black when observed with an optical microscope. It should be noted that there have been no reported cases where regions that serve as starting points for bilamellar cracks can be revealed by etching with aqua regia.

[0015] Furthermore, eliminating (or alleviating) Mn segregation is effective in reducing the DEA area ratio. To achieve this, it is important to perform the above-mentioned first rolling pass-hold combination in hot rolling, followed by the above-mentioned second rolling pass-hold combination. Although it is not possible to directly observe the structural changes and Mn segregation behavior during hot rolling, this is thought to be because Mn segregation is eliminated in conjunction with structural changes such as recrystallization and grain growth that occur during hot rolling under the above-mentioned conditions.

[0016] In particular, in slabs (before hot rolling), Mn segregates between columnar crystals and in the final solidification area (hereinafter, the region where Mn segregates is also referred to as the Mn segregation layer). The grain boundaries that form during hot rolling serve as paths (hereinafter, also referred to as the diffusion paths) through which solid solution elements such as Mn diffuse at high speed. Grain boundary migration due to grain growth also promotes Mn diffusion. However, the Mn segregation layer in the slab is thick. Therefore, to eliminate the Mn segregation layer, it is effective to quickly diffuse Mn once the slab thickness has been reduced to a certain extent. Here, the combination of the second rolling pass and holding corresponds to the reduction and holding at a stage when the slab thickness has been reduced to a certain extent. The combination of the second rolling pass and holding then promotes recrystallization and grain growth, eliminating the Mn segregation layer.

[0017] However, it is difficult to sufficiently eliminate the Mn segregation layer simply by performing the second rolling pass-holding combination. The reason for this is thought to be that even if the second rolling pass-holding combination is performed while the coarse cast structure (hereinafter referred to as the coarse cast structure) generated during casting remains, the coarse cast structure cannot be completely recrystallized, and the Mn segregation layer remains between the columnar crystals.

[0018] In order to destroy such coarse cast structures, it is effective to roll the slab at a high reduction rate in a higher temperature range where recrystallization is more likely, and hold the slab for a certain period of time after rolling, before performing the second rolling pass-hold combination. In other words, it is effective to perform the first rolling pass-hold combination.

[0019] That is, by performing the above-mentioned first rolling pass-holding combination and then the above-mentioned second rolling pass-holding combination, the Mn segregated layer is more effectively eliminated without remaining between the columnar crystals, and as a result, it becomes possible to significantly reduce the DEA area ratio.

[0020] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0021] 1. A martensitic stainless steel sheet having a chemical composition, by mass%, of C: 0.03 to 0.09%, Si: 0.01 to 0.50%, Mn: 1.0 to 2.0%, Cr: 10.0 to 14.5%, P: 0.040% or less, S: 0.010% or less, Al: 0.001 to 0.100%, and N: 0.005 to 0.060%, with the balance being Fe and unavoidable impurities, and having a DEA area ratio at the half-thickness position of 30% or less, where the DEA area ratio is the area ratio of black regions observed with an optical microscope after etching with aqua regia.

[0022] 2. The martensitic stainless steel sheet according to item 1, wherein the chemical composition further contains, in mass %, one or both of the following (Group A) and (Group B): (Group A) one or more selected from Ni: 0.60% or less, Cu: 0.50% or less, Mo: 0.30% or less, W: 0.20% or less, Co: 0.20% or less, and Sn: 0.50% or less; and (Group B) one or more selected from Ti: 0.40% or less, Nb: 0.40% or less, Mg: 0.0030% or less, and Ca: 0.0030% or less.

[0023] 3. A method for producing the martensitic stainless steel sheet according to 1 or 2 above, comprising: hot rolling a slab heated to 1100 to 1250°C using two or more rolling passes to produce a hot-rolled steel sheet; and in the hot rolling, performing the following first rolling pass-hold combination, and then performing the following second rolling pass-hold combination. - First rolling pass-hold combination: A combination of one rolling pass in which the rolling temperature is 1100°C or higher and the reduction is 20% or higher, and a hold immediately after the rolling pass in which the hold temperature is 1080°C or higher and the hold time is 20 seconds or longer. - Second rolling pass-hold combination: A combination of one or more rolling passes in which the rolling temperature is lower than 1100°C but 1050°C or higher and the total reduction is 35% or higher, and a hold immediately after the final rolling pass of the rolling passes in which the hold temperature is 900°C or higher and the hold time is 30 seconds or longer.

[0024] 4. The method for producing a martensitic stainless steel sheet according to 3 above, wherein the first rolling pass-hold combination is performed two or more times.

[0025] According to the present invention, it is possible to obtain a martensitic stainless steel sheet having a quench hardness within a predetermined target range and excellent punchability. Furthermore, when the martensitic stainless steel sheet of the present invention is used to manufacture components such as disc brakes, yield loss is significantly reduced, which is extremely advantageous from an industrial perspective.

[0026] 2 is an example of an optical microscope image, and is a binarized image of the optical microscope image of FIG.

[0027] The present invention will be described based on the following embodiments.

[0028] [1] Martensitic Stainless Steel Sheet First, the chemical composition of a martensitic stainless steel sheet according to one embodiment of the present invention will be described. Note that the units for the chemical composition are all "mass%", and hereinafter, unless otherwise specified, they will be simply referred to as "%".

[0029] C: 0.03 to 0.09% C is an element effective in increasing quench hardness and improving wear resistance. Here, in order to achieve a predetermined target range for quench hardness, the C content is set to 0.03% or more. On the other hand, if the C content exceeds 0.09%, the quench hardness cannot be achieved within the predetermined target range. Therefore, the C content is set to the range of 0.03 to 0.09%. The C content is preferably 0.04% or more. The C content is preferably 0.08% or less, more preferably 0.06% or less.

[0030] Si: 0.01 to 0.50% Si is an element that generates ferrite at high temperatures and improves hot workability. This effect is manifested when the Si content is 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the quench hardness decreases. It also has a negative effect on toughness. Therefore, the Si content is set to the range of 0.01 to 0.50%. The Si content is preferably 0.05% or more, more preferably 0.10% or more. The Si content is also preferably 0.45% or less, more preferably 0.40% or less.

[0031] Mn: 1.0 to 2.0% Mn is an element effective in suppressing the formation of δ-ferrite at high temperatures. If the Mn content is less than 1.0%, δ-ferrite will form, making it difficult to achieve a quenched hardness within the desired target range. In other words, the temperature range required for quenching to achieve a quenched hardness within the desired target range becomes extremely narrow, making temperature control extremely difficult. Therefore, the Mn content is set to 1.0% or more. On the other hand, if the Mn content exceeds 2.0%, Mn segregation during casting becomes significant, and even if hot rolling is performed according to the above conditions, the DEA area ratio at the 1 / 2 thickness position cannot be sufficiently reduced. As a result, the desired punching workability cannot be achieved. Therefore, the Mn content is set to the range of 1.0 to 2.0%. The Mn content is preferably 1.2% or more, more preferably 1.4% or more. The Mn content is preferably 1.8% or less, more preferably 1.7% or less.

[0032] Cr: 10.0 to 14.5% To maintain corrosion resistance, the Cr content is set to 10.0% or more. On the other hand, if the Cr content exceeds 14.5%, δ-ferrite is generated during quenching, making it impossible to achieve the desired quench hardness range. Therefore, the Cr content is set to the range of 10.0 to 14.5%. The Cr content is preferably 10.5% or more, more preferably 11.0% or more. The Cr content is also preferably 14.0% or less, more preferably 13.5% or less.

[0033] P: 0.040% or less P is an element that is inevitably contained in steel. Here, P is an element that improves the hardness of the matrix and reduces toughness due to its high solid solution strengthening ability. Therefore, it is preferable to reduce P as much as possible. Therefore, the P content is set to 0.040% or less. Furthermore, reducing the P content makes it easier to suppress the reduction in toughness. Therefore, the P content is preferably 0.030% or less. There is no particular lower limit for the P content. However, since excessive dephosphorization increases costs, the P content is preferably 0.010% or more.

[0034] S: 0.010% or less Like P, S is an element that is inevitably contained in steel. Here, S is an element that is harmful 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 will be significantly reduced. Therefore, the S content is set to 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 for the S content. However, since excessive desulfurization increases costs, the S content is preferably 0.0005% or more.

[0035] Al: 0.001 to 0.100% Al is an effective element as a deoxidizer. Furthermore, Al has a stronger affinity for N than Cr. Therefore, Al precipitates N as Al nitrides rather than Cr nitrides during cooling after hot rolling and coiling, thereby suppressing a decrease in corrosion resistance. These effects are achieved when the Al content is 0.001% or more. On the other hand, an Al content exceeding 0.100% may cause excessive N precipitation, resulting in a decrease in hardness. Therefore, the Al content is set to the range of 0.001 to 0.100%. The Al content is preferably 0.005% or more, more preferably 0.010% or more. The Al content is preferably 0.060% or less, more preferably 0.040% or less.

[0036] N: 0.005 to 0.060% Like C, N is an effective element for increasing hardness after quenching. If the N content is less than 0.005%, the hardness required for disc brakes cannot be achieved. Insufficient hardness makes disc brakes more susceptible to deformation during use. On the other hand, if the N content exceeds 0.060%, bubbles form inside the steel during casting, resulting in surface defects. Therefore, the N content is limited to the range of 0.005 to 0.060%. The N content is preferably 0.008% or more, more preferably 0.010% or more. The N content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0037] The basic elements (hereinafter also referred to as basic element) of the composition of a martensitic stainless steel sheet according to one embodiment of the present invention have been described above. Furthermore, the martensitic stainless steel sheet according to one embodiment of the present invention may contain, in addition to the basic element, one or both of the following (Group A) and (Group B) as optional additional elements: (Group A) One or more elements selected from Ni: 0.60% or less, Cu: 0.50% or less, Mo: 0.30% or less, W: 0.20% or less, Co: 0.20% or less, and Sn: 0.50% or less; and (Group B) One or more elements selected from Ti: 0.40% or less, Nb: 0.40% or less, Mg: 0.0030% or less, and Ca: 0.0030% or less.

[0038] Ni: 0.60% or less Ni is an element that improves corrosion resistance. This effect is obtained when the Ni content is 0.01% or more. Therefore, when Ni is contained, the Ni content is preferably 0.01% or more. On the other hand, when the Ni content exceeds 0.60%, the strength increases excessively and the punching workability decreases. Therefore, when Ni is contained, the Ni content is preferably 0.60% or less. The Ni content is more preferably 0.40% or less.

[0039] Cu: 0.50% or less Cu is an element that improves corrosion resistance. This effect is obtained when the Cu content is 0.01% or more. Therefore, when Cu is contained, the Cu content is preferably 0.01% or more. On the other hand, when the Cu content exceeds 0.50%, precipitation of ε-Cu occurs, reducing corrosion resistance. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.30% or less.

[0040] Mo: 0.30% or less Mo is an element that improves corrosion resistance. This effect is obtained when the Mo content is 0.01% or more. Therefore, when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, when the Mo content exceeds 0.30%, the formation of austenite at high temperatures is suppressed, and hardenability is reduced. Therefore, when Mo is contained, the Mo content is preferably 0.30% or less. The Mo content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0041] W: 0.20% or less Like Mo, W is an element that improves corrosion resistance. This effect is obtained when the W content is 0.01% or more. Therefore, when W is contained, the W content is preferably 0.01% or more. The W content is more preferably 0.05% or more. On the other hand, when the W content exceeds 0.20%, the strength increases excessively, which may lead to a decrease in manufacturability due to an increase in rolling load, etc. Therefore, when W is contained, the W content is preferably 0.20% or less. The W content is more preferably 0.15% or less.

[0042] Co: 0.20% or less Co is an element that improves toughness. This effect is obtained when the Co content is 0.01% or more. Therefore, when Co is contained, the Co content is preferably 0.01% or more. On the other hand, when the Co content exceeds 0.20%, the workability decreases. Therefore, when Co is contained, the Co content is preferably 0.20% or less.

[0043] Sn: 0.50% or less Sn is an element that improves corrosion resistance. This effect is obtained when the Sn content is 0.001% or more. Therefore, when Sn is contained, the Sn content is preferably 0.001% or more. On the other hand, when the Sn content exceeds 0.50%, the toughness and hot rolling property may deteriorate due to grain boundary segregation. Therefore, when Sn is contained, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.30% or less, and even more preferably 0.20% or less.

[0044] Ti: 0.40% or less Ti is an element that improves the corrosion resistance of steel after quenching. This effect is obtained when the Ti content is 0.01% or more. Therefore, when Ti is contained, the Ti content is preferably 0.01% or more. On the other hand, when the Ti content exceeds 0.40%, the quench hardness decreases. Therefore, when Ti is contained, the Ti content is preferably 0.40% or less. The Ti content is more preferably 0.10% or less.

[0045] Nb: 0.40% or less Nb is an element that improves the temper softening resistance of steel after quenching. This effect is obtained when the Nb content is 0.01% or more. Therefore, when Nb is contained, the Nb content is preferably 0.01% or more. On the other hand, when the Nb content exceeds 0.40%, the quench hardness decreases. Therefore, when Nb is contained, the Nb content is preferably 0.40% or less. The Nb content is more preferably 0.10% or less.

[0046] Mg: 0.0030% or less Mg improves the equiaxed crystal ratio of the slab and facilitates recrystallization during hot rolling. That is, Mg is an element effective in improving punching workability. Therefore, when Mg is contained, the Mg content is preferably 0.0002% or more. On the other hand, if the Mg content exceeds 0.0030%, the surface quality of the steel deteriorates. Therefore, when Mg is contained, the Mg content is preferably 0.0030% or less. The Mg content is more preferably 0.0004% or less.

[0047] Ca: 0.0030% or less Ca is an element effective in preventing nozzle clogging due to the crystallization of Ti-based inclusions, which are likely to occur during continuous casting. This effect can be achieved by setting the Ca content to 0.0002% or more. Therefore, when Ca is contained, the Ca content is preferably 0.0002% or more. On the other hand, if the Ca content exceeds 0.0030%, the corrosion resistance decreases due to the formation of CaS. Therefore, when Ca is contained, the Ca content is preferably 0.0030% or less. The Ca content is more preferably 0.0010% or less.

[0048] The balance other than the above elements is Fe and inevitable impurities. Note that any of the above optional added elements may be 0%. Furthermore, when the content of each of the above optional added elements is less than the preferable lower limit, it can be said that the element is contained as an inevitable impurity.

[0049] In the martensitic stainless steel sheet according to one embodiment of the present invention, it is extremely important to make the DEA area ratio at the half-thickness position 30% or less, as described above.

[0050] DEA area ratio at half thickness position: 30% or less As described above, the DEA region has a higher hardness than the non-DEA region. In other words, the DEA region is less likely to deform during shearing during punching than the surrounding non-DEA region. Therefore, during punching, high stress is generated locally in the DEA region, which becomes the starting point for double-plate cracks. In order to prevent such double-plate cracks and improve punching workability, it is extremely important to set the DEA area ratio at half thickness position to 30% or less. The DEA area ratio at half thickness position is preferably 25% or less, more preferably 20% or less. There is no particular limit to the lower limit of the DEA area ratio at half thickness position. However, it is difficult to completely eliminate the Mn segregation layer. Furthermore, attempting to completely eliminate the Mn segregation layer would increase manufacturing costs. Therefore, the DEA area ratio at half thickness position is preferably 1% or more.

[0051] Here, the DEA area ratio is the area ratio of black regions observed under an optical microscope after etching with aqua regia. The DEA area ratio at the half-thickness position may be measured, for example, as follows.

[0052] Specifically, a sample is cut from a martensitic stainless steel plate so that the observation surface is a cross section parallel to the plate thickness direction and rolling direction (a cross section including the plate thickness direction and rolling direction of the martensitic stainless steel plate, hereinafter also referred to as an L-cross section). The sample is then mirror-polished and placed on a petri dish in an air-conditioned laboratory at 25°C. Next, 10 ml or more of aqua regia is dropped onto the sample, and the sample is immersed in the aqua regia for 10 seconds to reveal the metal structure. The sample is then subjected to optical microscope observation. For example, a DSX1000 manufactured by Evident Corporation may be used as the optical microscope. An image of an 800 μm × 600 μm area is then taken at an observation magnification of 100x at any location at half the plate thickness. If the brightness levels of each crystal grain are divided into multiple levels, the contrast can be adjusted so that the brightness of the brightest crystal grain is approximately 160 and the average brightness of the field of view is approximately 100. For reference, FIG. 1 shows an example of an optical microscope image. Image analysis is performed on the obtained optical microscope image using, for example, WinROOF2015 manufactured by Mitani Shoji Co., Ltd. As preprocessing for image analysis, monochrome imaging and histogram flattening are performed. Next, binarization is performed so that the crystal grain group with the lowest brightness in the optical microscope image is selected. The threshold value is, for example, 55. Next, an opening process, which is a morphological transformation, is performed once on the binarized image to remove noise such as grain boundaries. The area ratio of the selected crystal grain group with the lowest brightness is then defined as the DEA area ratio at that location. For reference, FIG. 2 shows a binarized image of the optical microscope image in FIG. 1. The above measurement is performed at five arbitrary locations at the 1 / 2 position of the plate thickness, and the average of the DEA area ratios at each location is defined as the DEA area ratio at the 1 / 2 position of the plate thickness.

[0053] Aqua regia is a liquid obtained by mixing concentrated hydrochloric acid and concentrated nitric acid in a molar ratio of 3: 1. Aqua regia can be obtained, for example, by mixing 35% by mass of concentrated hydrochloric acid and 60% by mass of concentrated nitric acid to achieve the above molar ratio.

[0054] Furthermore, the martensitic stainless steel sheet according to one embodiment of the present invention includes not only the steel sheet before quenching treatment but also the steel sheet after quenching treatment.

[0055] Here, the structure of the steel sheet before quenching is a structure mainly composed of ferrite phase. Specifically, the structure of the steel sheet before quenching is a structure in which the ferrite phase accounts for 80% or more, preferably 90% or more, more preferably 95% or more, and even more preferably 98% or more, by volume fraction. The volume fraction of the ferrite phase may be 100%. Examples of the remaining structure other than the ferrite phase include martensite phase, retained austenite phase, precipitates, and inclusions. The volume fraction of the remaining structure is preferably 20% or less, more preferably 10% or less, even more preferably 5% or less, and even more preferably 2% or less. The volume fraction of the remaining structure may be 0%. Examples of steel sheets before quenching include hot-rolled steel sheets and hot-rolled annealed steel sheets. Hot-rolled steel sheets include as-hot-rolled steel sheets as well as steel sheets obtained by subjecting as-hot-rolled steel sheets to oxide scale removal treatment such as pickling. Furthermore, hot-rolled annealed steel sheets include not only steel sheets obtained by subjecting hot-rolled steel sheets to hot-rolled sheet annealing, but also steel sheets obtained by further subjecting the steel sheets obtained by hot-rolled sheet annealing to an oxide scale removal treatment such as pickling.

[0056] Furthermore, the structure of the steel sheet after quenching treatment is a structure mainly composed of martensite phase. Specifically, the structure of the steel sheet after quenching treatment is a structure in which the martensite phase accounts for 80% or more, preferably 90% or more, more preferably 95% or more, and even more preferably 98% or more, by volume fraction. The volume fraction of the martensite phase may be 100%. Examples of the remaining structure other than the martensite phase include a ferrite phase, a retained austenite phase, precipitates, and inclusions. The volume fraction of the remaining structure is preferably 20% or less, more preferably 10% or less, even more preferably 5% or less, and even more preferably 2% or less. The volume fraction of the remaining structure may be 0%. Examples of steel sheets after quenching treatment include hot-rolled steel sheets and steel sheets obtained by quenching hot-rolled annealed steel sheets. Furthermore, since the steel sheet is hardened by quenching treatment, the HRC of the steel sheet after quenching treatment is in the range of 30 to 40, preferably 33 to 37.

[0057] The volume fractions of the ferrite and martensite phases can be measured using conventional methods. For example, a sample for microstructure observation is taken from the center of the width of a martensitic stainless steel plate used as a test material. Next, the L-section of the sample is mirror-polished and then etched using a picrate aqueous solution. Ten optical microscope images at 500x magnification are taken. In the obtained microstructure photographs, the martensite and ferrite phases are distinguished based on the microstructure shape and etching intensity (note that the martensite phase is etched more deeply than the ferrite phase. Therefore, the martensite phase has a darker contrast than the ferrite phase). Next, the volume fractions of the ferrite and martensite phases are calculated for each field of view using image processing. The arithmetic mean values ​​of the volume fractions of the ferrite and martensite phases obtained for each field of view are then calculated, and these values ​​are used as the volume fractions of the ferrite and martensite phases.

[0058] The thickness of the martensitic stainless steel plate according to one embodiment of the present invention is not particularly limited. For example, the thickness of the martensitic stainless steel plate according to one embodiment of the present invention is preferably 3.0 to 12.0 mm. The thickness of the martensitic stainless steel plate 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. The thickness of the martensitic stainless steel plate 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. In addition, the quench hardness of the martensitic stainless steel plate according to one embodiment of the present invention is in the range of HRC 30 to 40, preferably 33 to 37.

[0059] [2] Method for Manufacturing Martensitic Stainless Steel Sheet Next, a method for manufacturing a martensitic stainless steel sheet according to one embodiment of the present invention will be described.

[0060] In a method for producing a martensitic stainless steel sheet according to one embodiment of the present invention, a slab heated to 1100 to 1250°C is hot rolled using two or more rolling passes to produce a hot rolled steel sheet. The hot rolling is characterized in that after the first rolling pass-hold combination is performed, the second rolling pass-hold combination is performed.

[0061] Furthermore, a method for producing a martensitic stainless steel sheet according to one embodiment of the present invention is a method for producing the martensitic stainless steel sheet according to the above-mentioned embodiment of the present invention. Here, unless otherwise specified, all temperatures in the production method are based on the surface temperature of the slab, steel sheet, or the like. The slab (steel material) to be subjected to hot rolling can be obtained, for example, as follows. That is, molten steel having the above-mentioned chemical composition is melted by a known method such as a converter, electric furnace, or vacuum melting furnace, and then formed into a slab (steel material) by a continuous casting method or an ingot-making and blooming method. The method for producing a martensitic stainless steel sheet according to one embodiment of the present invention, particularly the hot rolling conditions, will be described in detail below.

[0062] Slab heating temperature: 1100 to 1250°C In hot rolling, from the viewpoint of performing the first rolling pass-hold combination and the second rolling pass-hold combination, the slab heating temperature is set to 1100°C or higher. The slab heating temperature is preferably 1120°C or higher, and more preferably 1140°C or higher. On the other hand, if the slab heating temperature exceeds 1250°C, slab sagging in the heating furnace will be induced, leading to operational problems. Therefore, the slab heating temperature is set to 1250°C or lower.

[0063] [First Rolling Pass-Hold Combination] As described above, in order to destroy the coarse cast structure, it is effective to reduce the slab at a high reduction rate in a higher temperature range where recrystallization is more likely, and hold the slab for a certain period of time after reduction, before performing the second rolling pass-hold combination. In particular, the rolling temperature and reduction rate of the rolling pass used to perform the reduction, as well as the holding temperature and holding time in the hold immediately after the rolling pass, have a significant effect on the progress of recrystallization during hot rolling. Therefore, it is important to perform the first rolling pass-hold combination under the above-described conditions. Below, the conditions for one rolling pass and the hold immediately after the rolling pass related to the first rolling pass-hold combination will be explained.

[0064] Rolling temperature of rolling pass: 1100°C or higher If the rolling temperature of the rolling pass relating to the first rolling pass-hold combination is less than 1100°C, the time required to complete recrystallization becomes longer, and rolling efficiency decreases. In addition, an increase in rolling resistance induces cracking of the material. Therefore, the rolling temperature is set to 1100°C or higher. The rolling temperature is preferably 1120°C or higher, more preferably 1140°C or higher. Furthermore, there is no particular upper limit to the rolling temperature. For example, the rolling temperature is preferably 1250°C or lower. The rolling temperature of a rolling pass refers to the entry temperature of the rolling pass. The same applies hereinafter.

[0065] Reduction ratio of rolling pass: 20% or more In order to introduce the strain necessary for recrystallization, the reduction ratio of the rolling pass (rolling temperature of rolling pass: 1100°C or more) relating to the first rolling pass-hold combination is set to 20% or more. The reduction ratio is preferably 21% or more, more preferably 22% or more. There is no particular upper limit to the reduction ratio. However, if the reduction ratio is excessively high, warping or the like may occur in the rolled material, which may cause problems. Therefore, the reduction ratio is preferably 50% or less. The reduction ratio of a rolling pass per pass can be calculated using the following formula. [Reduction ratio of rolling pass (%)] = ([Slab thickness at the entry side of the rolling pass (mm)] - [Slab thickness at the exit side of the rolling pass (mm)]) ÷ [Slab thickness at the entry side of the rolling pass (mm)] × 100

[0066] Holding temperature in the holding immediately after the rolling pass: 1080°C or higher During the holding immediately after the rolling pass, recrystallization occurs throughout the slab. Therefore, the holding temperature in the holding immediately after the rolling pass is set to 1080°C or higher. There is no particular upper limit to the holding temperature. For example, the holding temperature may be set to the delivery temperature of the rolling pass or lower.

[0067] Holding time in the holding immediately after the rolling pass: 20 seconds or more During the holding immediately after the rolling pass, recrystallization occurs throughout the slab. To promote sufficient recrystallization and subsequent grain growth, the holding time in the holding immediately after the rolling pass is set to 20 seconds or more. The holding time is preferably 22 seconds or more, more preferably 24 seconds or more. There is no particular upper limit to the holding time. For example, the holding time is preferably 60 seconds or less.

[0068] Here, the holding immediately after a rolling pass refers to the holding of the slab from the exit of the rolling pass to the entrance of the next rolling pass (when hot rolling is not being performed). The holding temperature during the holding immediately after a rolling pass is the lowest temperature of the slab from the exit of the rolling pass to the entrance of the next rolling pass (generally corresponding to the entry temperature of the next rolling pass). The holding time during the holding immediately after a rolling pass refers to the time during which the slab moves from the exit of the rolling pass to the entrance of the next rolling pass. However, if the temperature of the slab at the exit of the rolling pass is 1080°C or higher and the temperature of the slab drops to less than 1080°C while the slab moves from the exit of the rolling pass to the entrance of the next rolling pass, the holding temperature during the holding immediately after a rolling pass is 1080°C. In this case, the holding time during the holding immediately after a rolling pass refers to the time during which the slab moves from the exit of the rolling pass to a position where the temperature of the slab becomes 1080°C.

[0069] Furthermore, the first rolling pass-hold combination may be performed only once, but the more times it is performed, the more effective it is in eliminating the Mn segregation layer. Therefore, the number of times the first rolling pass-hold combination is performed is preferably two or more. There is no particular upper limit on the number of times the first rolling pass-hold combination is performed. However, if the number of times the first rolling pass-hold combination is performed exceeds five, the temperature of the slab will decrease, which may induce cracking of the material due to increased rolling resistance in subsequent rolling passes. Therefore, it is preferable that the number of times the first rolling pass-hold combination is performed is five or less.

[0070] [Second Rolling Pass-Holding Combination] After the above-mentioned first rolling pass-holding combination is performed, the second rolling pass-holding combination is performed. As a result, the Mn segregation layer is effectively eliminated without remaining between the columnar crystals. In particular, by performing the rolling passes according to the second rolling pass-holding combination, a sufficient driving force for recrystallization is obtained, and the Mn segregation layer is effectively eliminated by the recrystallization and grain growth in the subsequent holding. As a result, it is possible to significantly reduce the DEA area ratio. Below, the conditions for one or more rolling passes according to the second rolling pass-holding combination and the holding immediately after the final rolling pass among the rolling passes will be explained.

[0071] Rolling temperature of rolling pass: less than 1100°C, 1050°C or higher If the rolling temperature of the rolling pass related to the second rolling pass-holding combination is less than 1050°C, recrystallization and grain growth are retarded, and the Mn segregation layer cannot be sufficiently eliminated. As a result, it becomes impossible to achieve a DEA area ratio of 30% or less at the half thickness position. On the other hand, due to the conditions of the above-mentioned first rolling pass-holding combination, it is difficult to set the rolling temperature to 1100°C or higher. Therefore, the rolling temperature is set to a range of less than 1100°C, 1050°C or higher.

[0072] Total reduction of rolling passes: 35% or more By setting the total reduction of the rolling passes (rolling temperature: less than 1100°C and 1050°C or more) relating to the second rolling pass-hold combination to 35% or more, a sufficient driving force for recrystallization can be obtained. Therefore, the total reduction is set to 35% or more. The total reduction is preferably 38% or more, more preferably 40% or more. There is no particular upper limit to the total reduction. However, if the total reduction is excessively high, it becomes difficult to implement the first rolling pass-hold combination described above. Therefore, the total reduction is preferably 70% or less. The total reduction of the rolling passes can be calculated using the following formula: where ε ｔ : Total reduction rate of rolling passes (rolling temperature: less than 1100 ° C and 1050 ° C or more) Π: Direct product ε ｉ : Reduction ratio of the i-th rolling pass in the rolling pass (rolling temperature: rolling pass of less than 1100 ° C and 1050 ° C or more) n: Number of rolling passes (rolling temperature: rolling pass of less than 1100 ° C and 1050 ° C or more) i: Integer from 1 to n.

[0073] Number of rolling passes: 1 or more The number of rolling passes (rolling passes at a rolling temperature of less than 1100°C and 1050°C or more) related to the second rolling pass-holding combination may be 1 or more. The number of rolling passes is preferably 2 or more. There is no particular upper limit on the number of rolling passes. However, if the number of rolling passes exceeds 5, the temperature of the slab will decrease, which may induce material cracking in subsequent rolling passes due to increased rolling resistance. Therefore, the number of rolling passes is preferably 5 or less.

[0074] Holding temperature in holding immediately after the final rolling pass of the above rolling passes: 900°C or higher If the holding temperature in holding immediately after the final rolling pass of the above rolling passes is less than 900°C, recrystallization and grain growth are retarded, and the Mn segregation layer cannot be sufficiently eliminated. As a result, it becomes impossible to achieve a DEA area ratio of 30% or less at the half-thickness position. Therefore, the holding temperature is set to 900°C or higher. The upper limit of the holding temperature is not particularly limited. For example, the holding temperature may be set to be equal to or lower than the outlet temperature of the final rolling pass of the above rolling passes.

[0075] Holding time immediately after the final rolling pass among the above rolling passes: 30 seconds or more In order to sufficiently eliminate the Mn segregation layer through recrystallization and grain growth, the holding time immediately after the final rolling pass among the above rolling passes is set to 30 seconds or more. The holding time is preferably 35 seconds or more, more preferably 40 seconds or more. There is no particular upper limit to the holding time. However, if the holding time exceeds 200 seconds, it becomes difficult to maintain the slab temperature at 900°C or higher. Therefore, the holding time is preferably 200 seconds or less.

[0076] Here, the holding immediately after the final rolling pass among the rolling passes refers to the holding of the slab from the exit of the final rolling pass among the rolling passes (the final rolling pass among the rolling passes with a rolling temperature of less than 1100°C and equal to or greater than 1050°C) to the entrance of the next rolling pass (in a state where hot rolling is not being performed). Furthermore, the holding temperature during the holding immediately after the final rolling pass among the rolling passes is the lowest temperature of the slab from the exit of the final rolling pass among the rolling passes to the entrance of the next rolling pass (generally equivalent to the entry temperature of the next rolling pass). The holding time during the holding immediately after the final rolling pass among the rolling passes refers to the time it takes for the slab to move from the exit of the final rolling pass among the rolling passes to the entrance of the next rolling pass. However, if the temperature of the slab at the exit of the final rolling pass is 900°C or higher and the temperature of the slab drops to less than 900°C while the slab is moving from the exit of the final rolling pass to the entrance of the next rolling pass, the holding temperature immediately after the final rolling pass of the rolling passes shall be 900°C. In this case, the holding time immediately after the final rolling pass of the rolling passes shall be the time it takes for the slab to move from the exit of the final rolling pass to a position where the temperature of the slab reaches 900°C.

[0077] Hot rolling conditions other than those described above are not particularly limited and may be those of the ordinary method. For example, hot rolling may be performed by rough rolling and finish rolling, with the number of rolling passes in the rough rolling being 5 to 10 and the number of rolling passes in the finish rolling being 5 to 8. Furthermore, the above-mentioned first rolling pass-hold combination and second rolling pass-hold combination may be performed in either rough rolling or finish rolling, but are preferably performed in rough rolling. Furthermore, the finishing temperature is preferably 800 to 1000°C and the coiling temperature is preferably 400 to 800°C.

[0078] The hot-rolled steel sheet may also be optionally subjected to hot-rolled annealing to obtain a hot-rolled annealed steel sheet. The hot-rolled steel sheet annealing conditions may be in accordance with conventional methods. For example, batch annealing may be performed at a hot-rolled steel sheet annealing temperature of 750 to 900°C. The obtained hot-rolled annealed steel sheet may also be optionally subjected to an oxide scale removal treatment by pickling or shot blasting. The conditions for quenching the hot-rolled steel sheet or the hot-rolled annealed steel sheet are not particularly limited, and may be in accordance with conventional methods.

[0079] Molten steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was produced by refining in a 150-ton converter and by strong stirring and vacuum oxygen decarburization (SS-VOD). The resulting molten steel was then continuously cast into a slab with a width of 1000 mm and a thickness of 200 mm. The slab was then heated at 1200°C for 1 hour and hot rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet with a thickness of 5.0 mm. Among the rolling passes in the hot rolling, passes 1 to 7 were rough rolling passes, and passes 8 to 14 were finish rolling passes. Rough rolling was performed using a three-stand reverse-type rough rolling mill, and finish rolling was performed using a seven-stand tandem-type finish rolling mill. In Table 2, No. With the exception of No. 15, the representative conditions are the third rolling pass of rough rolling and the holding time immediately thereafter among the rolling passes in which the rolling temperature is 1100°C or higher (No. 15 lists the first rolling pass of rough rolling and the holding time immediately thereafter as the representative conditions). Furthermore, in Table 2, the representative conditions are the seventh rolling pass, which is the final rolling pass of rough rolling, and the holding time immediately thereafter among the rolling passes in which the rolling temperature is lower than 1100°C and higher than 1050°C. The sheet bar thickness in Table 2 is the thickness of the slab (sheet bar) at the end of rough rolling. The obtained hot-rolled steel sheet was then subjected to hot-rolled sheet annealing by batch annealing at a hot-rolled sheet annealing temperature of 750 to 900°C to obtain a martensitic stainless steel sheet. Other conditions were the same as those described above.

[0080] The martensitic stainless steel sheets thus obtained were subjected to measurement of the DEA area ratio at the half-thickness position in the same manner as described above. The results are shown in Table 2.

[0081] Furthermore, (1) quench hardness and (2) punching workability were evaluated according to the following test methods. The evaluation results are also shown in Table 2.

[0082] (1) Quench Hardness Near the leading edge of a martensitic stainless steel sheet (coil), 50 mm square (50 mm x 50 mm) samples were taken so that the widthwise center was at the center of the sheet width and at a position 200 mm from the widthwise end. Similarly, near the rear end of the martensitic stainless steel sheet (coil), 50 mm square samples were taken so that the widthwise center was at the center of the sheet width and at a position 200 mm from the widthwise end. Next, the four samples were subjected to a heat treatment under the following conditions simulating a quenching treatment. Heat Treatment Conditions Average heating rate from room temperature to heating temperature: 30°C / sec Heating temperature: 1000°C Holding time at heating temperature: 10 seconds Average cooling rate from heating temperature to 100°C: 70°C / sec

[0083] Next, the surface of each sample was ground to about 50 μm to remove scale. Then, Rockwell hardness (C scale) was measured at any five 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 (five points per sample) was taken as the quenched hardness, and the sample was judged as pass or fail according to the following criteria: Pass: The average value of the Rockwell hardness (C scale) was within the target range (HRC 30 to 40); Fail: The average value of the Rockwell hardness (C scale) was outside the target range (less than HRC 30 or more than 40).

[0084] In all of the inventive examples, the structure of the steel sheet before the heat treatment was mainly composed of a ferrite phase, and the structure of the steel sheet after the heat treatment was mainly composed of a martensite phase. Furthermore, in all of the inventive examples, the DEA area ratio of the steel sheet after the heat treatment was 30% or less.

[0085] (2) Punching Workability Multiple 100 mm square (100 mm × 100 mm) samples were taken near the leading edge of the martensitic stainless steel sheet (coil), with the widthwise center at the center of the sheet width and at a position 200 mm from the widthwise end. Similarly, multiple 100 mm square samples were taken near the rear end of the martensitic stainless steel sheet (coil), with the widthwise center at the center of the sheet width and at a position 200 mm from the widthwise end. Next, 100 randomly selected locations on the taken samples were punched with a 10 mm diameter hole at a clearance of 0.5 mm to simulate the punching process for disc brakes. The punched holes were then observed under an LED with an illuminance of 300 lumens to confirm the presence or absence of a bilamellar crack. A dark void observed in the center of the sheet thickness was determined to indicate the presence of a bilamellar crack, while a sample without such a void was determined to indicate the absence of a bilamellar crack. The punching workability was evaluated according to the following criteria: Pass (excellent): No double-plate-shaped cracks in all 100 locations Fail (bad): Double-plate-shaped cracks in one or more locations

[0086]

[0087]

[0088] As shown in Table 2, in all of the invention examples, the quenched hardness was within the target range, and excellent punching workability was obtained.

[0089] On the other hand, in the comparative examples, the quenching hardness was outside the target range or the punching workability was insufficient.

[0090] That is, in No. 8, the C content was below the appropriate range, so the quenched hardness was below the target range. In No. 9, the C content exceeded the appropriate range, so the quenched hardness exceeded the target range. In No. 10, the Mn content was below the appropriate range, so the quenched hardness was below the target range. In No. 11, the Mn content exceeded the appropriate range, so the DEA area ratio exceeded 30%, resulting in insufficient punching workability. In No. 12, the first rolling pass-hold combination was not performed, so the DEA area ratio exceeded 30%, resulting in insufficient punching workability. In No. 13, the second rolling pass-hold combination was not performed, and in particular, the total reduction rate of the rolling passes at a rolling temperature of less than 1100°C and 1050°C or higher was below the appropriate range, so the DEA area ratio exceeded 30%, resulting in insufficient punching workability. In No. 14, the second rolling pass-holding combination was not performed, and in particular, the holding time immediately after the final rolling pass among the rolling passes at a rolling temperature of less than 1100°C and 1050°C or higher was not within the appropriate range, resulting in a DEA area ratio of more than 30%, and punching workability was insufficient. In No. 15, the first rolling pass-holding combination was not performed, and in particular, a rolling pass at a rolling temperature of 1100°C or higher was not performed, resulting in a DEA area ratio of more than 30%, and punching workability was insufficient. In No. 16, the first rolling pass-holding combination was not performed, and in particular, a rolling pass at a rolling temperature of 1100°C or higher and a reduction ratio of 20% or higher was not performed, resulting in a DEA area ratio of more than 30%, and punching workability was insufficient.

Claims

1. A martensitic stainless steel sheet having a chemical composition, by mass%, of C: 0.03 to 0.09%, Si: 0.01 to 0.50%, Mn: 1.0 to 2.0%, Cr: 10.0 to 14.5%, P: 0.040% or less, S: 0.010% or less, Al: 0.001 to 0.100%, and N: 0.005 to 0.060%, with the balance being Fe and unavoidable impurities, and having a DEA area ratio at the half-thickness position of 30% or less, where the DEA area ratio is the area ratio of black regions observed with an optical microscope after etching with aqua regia.

2. The martensitic stainless steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, one or both of the following (Group A) and (Group B): (Group A) one or more selected from Ni: 0.60% or less, Cu: 0.50% or less, Mo: 0.30% or less, W: 0.20% or less, Co: 0.20% or less, and Sn: 0.50% or less; and (Group B) one or more selected from Ti: 0.40% or less, Nb: 0.40% or less, Mg: 0.0030% or less, and Ca: 0.0030% or less.

3. A method for producing a martensitic stainless steel sheet as set forth in claim 1 or 2, comprising: hot rolling a slab heated to 1100-1250°C using two or more rolling passes to produce a hot-rolled steel sheet; and in the hot rolling, the following first rolling pass-hold combination is performed, and then the following second rolling pass-hold combination is performed. - First rolling pass-hold combination: A combination of one rolling pass in which the rolling temperature is 1100°C or higher and the reduction is 20% or higher, and a hold immediately after the rolling pass in which the hold temperature is 1080°C or higher and the hold time is 20 seconds or longer. - Second rolling pass-hold combination: A combination of one or more rolling passes in which the rolling temperature is lower than 1100°C but 1050°C or higher and the total reduction is 35% or higher, and a hold immediately after the final rolling pass of the rolling passes in which the hold temperature is 900°C or higher and the hold time is 30 seconds or longer.

4. The method for producing a martensitic stainless steel sheet according to claim 3, wherein the first rolling pass-hold combination is performed two or more times.

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