Martensitic stainless steel sheet and its manufacturing method

Abstract

To provide a martensitic stainless steel sheet that has excellent punching workability and can achieve a quenching hardness within a specified target range. The composition is appropriately controlled, and the DEA area ratio at the 1 / 2 position of the sheet thickness is set to 30% or less.

Description

[Technical field]

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

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

[0003] For this reason, components such as disc brakes, which require both high wear resistance and high toughness, are often made from steel with 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 1322454 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, when manufacturing components such as disc brakes using martensitic stainless steel sheet as the material as disclosed in Patent Document 1, it is common to punch a hot-rolled steel sheet or the like into a predetermined shape and then perform a quenching treatment to increase the 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 generates a double-plate-shaped crack in the hot-rolled steel sheet, which can lead to 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 generated by the punching process, forming two plates (or layers).

[0007] The present invention has been developed to meet the above 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, together 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. [Means for solving the problem]

[0008] In order to achieve the above object, the inventors have carried out extensive research as described below.

[0009] (1) First, the inventors conducted a detailed study on the cause of the double-plate cracks that occur during punching of martensitic stainless steel sheets. As a result, the inventors obtained the following findings. That is, the double-plate cracks occur when they start from the dark etched area (hereinafter also referred to as the DEA area) 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 studies. As a result, the inventors obtained the following findings. That is, by appropriately controlling the composition of the components and setting the area ratio of the DEA region at the 1 / 2 position of the sheet thickness (hereinafter also referred to as the DEA area ratio) to 30% or less, it is possible to make the distribution of plastic deformation in the sheet thickness direction during punching closer to uniform, and the occurrence of two-sheet cracks is significantly suppressed. In other words, punching workability is significantly improved.

[0011] (3) The inventors have also found the following: In order to make the DEA area ratio at the 1 / 2 thickness position 30% or less, it is important to properly control the composition and then to properly control the hot rolling conditions, in particular, to perform the following first rolling pass-hold combination and then the following second rolling pass-hold combination. First rolling pass-holding combination A rolling pass in which the rolling temperature is 1100°C or higher and the rolling reduction is 20% or higher; A combination of a holding temperature of 1080°C or higher and a holding time of 20 seconds or longer immediately after the rolling pass Second rolling pass - holding combination One or more rolling passes in which the rolling temperature is less than 1100°C and not more than 1050°C and the total reduction is 35% or more; A combination of a holding temperature of 900°C or higher and a holding time of 30 seconds or longer, and a holding time immediately after the final rolling pass of the rolling passes.

[0012] The inventors consider the above reasons as follows.

[0013] 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 caused by punching compared to the surrounding non-DEA region. Therefore, during punching, high stress is generated locally in the DEA region, which becomes the starting point of a two-plate crack.

[0014] When the crystal grains constituting the DEA region (hereinafter also referred to as DEA grains) are observed by the bright field method of an optical microscope, a large number of Cr carbides are precipitated in the crystal grains and at the grain boundaries in the DEA grains, compared to the crystal grains constituting the non-DEA region (hereinafter also referred to as non-DEA grains). In addition, the Mn content is higher in the DEA grains than in the non-DEA grains, and Mn is segregated. In other words, the DEA region corresponds to a region in which the austenite phase is stable up to a low temperature range during cooling after the hot-rolled sheet annealing, because the Mn content is higher among the regions that become austenite phase during hot-rolled sheet annealing. Therefore, the DEA region corrodes more than the non-DEA region when etched with aqua regia, and is identified as black when observed with an optical microscope. There have been no reports of the region that becomes the starting point of a two-plate crack being revealed by etching with aqua regia.

[0015] In addition, eliminating (or alleviating) Mn segregation is effective in reducing the DEA area ratio. For this purpose, 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 microstructural changes and Mn segregation behavior during hot rolling, this is believed to be because Mn segregation is eliminated in conjunction with the microstructural changes, such as recrystallization and grain growth, that occur during hot rolling under the above-mentioned conditions.

[0016] In particular, in the slab (before hot rolling), Mn is segregated between the columnar crystals and in the final solidification portion (hereinafter, the region where Mn is segregated is also referred to as the Mn segregation layer). The grain boundaries generated during hot rolling become paths (hereinafter, also referred to as the diffusion paths) along which solid solution elements such as Mn diffuse at high speed. In addition, grain boundary migration due to grain growth also promotes the diffusion of Mn. However, the Mn segregation layer of the slab is thick. Therefore, in order to eliminate the Mn segregation layer, it is effective to diffuse Mn quickly when the slab thickness has become thin to a certain extent. Here, the combination of the second rolling pass and holding corresponds to the reduction and holding when the slab thickness has become thin to a certain extent. The combination of the second rolling pass and holding promotes recrystallization and grain growth, and eliminates the Mn segregation layer.

[0017] However, it is difficult to sufficiently eliminate the Mn segregation layer by only performing the combination of the second rolling pass and holding. This is thought to be because, when the coarse cast structure generated during casting (hereinafter, referred to as the coarse cast structure) remains, even if the combination of the second rolling pass and holding is performed, 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 before performing the second rolling pass-hold combination described above, i.e., to perform the first rolling pass-hold combination described above.

[0019] That is, by carrying out the above-mentioned first rolling pass-holding combination and then the above-mentioned second rolling pass-holding combination, the Mn segregation 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. In mass percent, C: 0.03-0.09%, Si: 0.01 to 0.50%, Mn: 1.0-2.0%, Cr: 10.0-14.5%, P: 0.040% or less, S: 0.010% or less, Al: 0.001 to 0.100% and N: 0.005~0.060% and the balance being Fe and unavoidable impurities, A martensitic stainless steel plate having a DEA area ratio of 30% or less at the 1 / 2 position of the plate thickness. Here, the DEA area ratio is the area ratio of black regions observed under an optical microscope after etching with aqua regia.

[0022] 2. The martensitic stainless steel sheet according to 1 above, 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 (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, comprising the steps of: The slab is heated to 1100 to 1250°C and hot-rolled with two or more rolling passes to produce a hot-rolled steel sheet. In the hot rolling, After carrying out the following first pass-hold combinations: Performing the following second rolling pass-hold combinations: A method for manufacturing martensitic stainless steel sheet. First rolling pass-holding combination A rolling pass in which the rolling temperature is 1100°C or higher and the rolling reduction is 20% or higher; A combination of a holding temperature of 1080°C or higher and a holding time of 20 seconds or longer immediately after the rolling pass Second rolling pass - holding combination One or more rolling passes, the rolling temperature of which is less than 1100°C and is equal to or greater than 1050°C, and the total reduction is equal to or greater than 35%; A combination of a holding temperature of 900°C or higher and a holding time of 30 seconds or longer, and a holding time immediately after the final rolling pass of the rolling passes.

[0024] 4. The method for manufacturing the martensitic stainless steel sheet according to item 3, wherein the combination of the first rolling pass and retention is performed two or more times.

Advantages of the Invention

[0025] According to the present invention, it is possible to set the quenching hardness within a predetermined target range, and a martensitic stainless steel sheet having excellent punching workability can be obtained. Further, when a member such as a disc brake is manufactured using the martensitic stainless steel sheet of the present invention, the creep loss is greatly reduced, which is extremely advantageous industrially.

Brief Description of the Drawings

[0026] [Figure 1] This is an example of an optical microscope image. [Diagram 2] This is an image obtained by binarizing the optical microscope image of FIG. 1.

Embodiments for Carrying Out the Invention

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

[0028] [1] Martensitic Stainless Steel Sheet First, the component composition of the martensitic stainless steel sheet according to an embodiment of the present invention will be described. In the component composition, the unit is “mass%” in all cases, and hereinafter, unless otherwise specified, it is simply indicated by “%”.

[0029] C: 0.03 to 0.09% C is an element effective for increasing the quenching hardness and improving the wear resistance. Here, in order to set the quenching hardness within a predetermined target range, the C content is set to 0.03% or more. On the other hand, when the C content exceeds 0.09%, the quenching hardness cannot be set within the predetermined target range. Therefore, the C content is in 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] Silicon: 0.01 to 0.50% Silicon is an element that generates ferrite at high temperatures and improves hot workability. This effect is manifested when the silicon content is 0.01% or more. On the other hand, when the silicon content exceeds 0.50%, the quenching hardness decreases. In addition, it has an adverse effect on toughness. Therefore, the silicon content is set to a range of 0.01 to 0.50%. The silicon content is preferably 0.05% or more, more preferably 0.10% or more. In addition, the silicon content is preferably 0.45% or less, more preferably 0.40% or less.

[0031] Mn: 1.0-2.0% 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 is formed, making it difficult to set the quenching hardness within a predetermined target range. That is, the temperature range in the quenching process for setting the quenching hardness within a predetermined 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%, the segregation of Mn during casting becomes significant, and even if hot rolling is performed according to the above conditions, the DEA area ratio at the 1 / 2 position of the plate thickness cannot be sufficiently reduced. As a result, the desired punching workability cannot be obtained. Therefore, the Mn content is set to a 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: 10.0~14.5% In order 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, and the quenched hardness cannot be set within the predetermined target range. Therefore, the Cr content is set to a range of 10.0 to 14.5%. The Cr content is preferably 10.5% or more, more preferably 11.0% or more. In addition, the Cr content is 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 the toughness because of 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 decrease 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 de-P 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, when the S content exceeds 0.010%, the corrosion resistance is significantly reduced. Therefore, the S content is set to 0.010% or less. The S content is preferably 0.006% or less, and more preferably 0.003% or less. There is no particular lower limit for the S content. However, since excessive de-S increases costs, the S content is preferably 0.0005% or more.

[0035] Al: 0.001 to 0.100% Al is an element effective as a deoxidizer. Moreover, Al has a stronger affinity with N than Cr. Therefore, Al has the effect of precipitating N as Al nitrides instead of Cr nitrides during cooling after coiling in hot rolling, thereby suppressing the deterioration of corrosion resistance. These effects are obtained when the Al content is 0.001% or more. On the other hand, when the Al content exceeds 0.100%, N is excessively precipitated, which may reduce 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: 0.005~0.060% Like C, N is an element effective in increasing the quenching hardness. Here, if the N content is less than 0.005%, the quenching hardness required for the disc brake cannot be obtained. If the hardness is insufficient, the disc brake is likely to deform during use. On the other hand, if the N content exceeds 0.060%, bubbles are generated inside the steel during casting, which leads to the occurrence of surface defects. Therefore, the N content is set to a 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 component elements) of the composition of the martensitic stainless steel sheet according to one embodiment of the present invention have been described above. In addition to the above basic component elements, the martensitic stainless steel sheet according to one embodiment of the present invention may contain one or both of the following (Group A) and (Group B) as optional added elements. (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 (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

[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%, ε-Cu precipitates and the corrosion resistance decreases. 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 further 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 further 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 makes recrystallization during hot rolling easier. That is, Mg is an element that is 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 properties of the steel deteriorate. 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 that are likely to occur during continuous casting. This effect can be obtained by making the Ca content 0.0002% or more. Therefore, when Ca is contained, the Ca content is preferably 0.0002% or more. On the other hand, when 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. The above optional elements may each be 0%. When the content of each of the above optional elements is less than the preferable lower limit, the element can be said to be included as an inevitable impurity.

[0049] In the martensitic stainless steel sheet according to one embodiment of the present invention, as described above, it is extremely important that the DEA area ratio at the 1 / 2 sheet thickness position is 30% or less.

[0050] DEA area ratio at 1 / 2 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 due to punching compared to the surrounding non-DEA region. Therefore, during punching, high stress occurs locally in the DEA region, which becomes the starting point of a two-plate crack. In order to prevent such two-plate cracking and improve punching workability, it is extremely important to set the DEA area ratio at the 1 / 2 position of the plate thickness to 30% or less. The DEA area ratio at the 1 / 2 position of the plate thickness is preferably 25% or less, more preferably 20% or less. The lower limit of the DEA area ratio at the 1 / 2 position of the plate thickness is not particularly limited. However, it is difficult to completely eliminate the Mn segregation layer. In addition, if the Mn segregation layer is to be completely eliminated, the manufacturing cost will increase. Therefore, the DEA area ratio at the 1 / 2 position of the plate thickness is preferably 1% or more.

[0051] Here, the DEA area ratio is the area ratio of the black regions observed under an optical microscope when etching is performed with aqua regia. The DEA area ratio at the 1 / 2 plate thickness position may be measured, for example, in the following manner.

[0052] That is, a sample is cut out so that a cross-section parallel to the plate thickness direction and the rolling direction from the martensitic stainless steel plate (a cross-section including the plate thickness direction and the rolling direction of the martensitic stainless steel plate, hereinafter also referred to as the L cross-section) becomes the observation surface. Next, after the sample is mirror-polished, it is left standing on a petri dish in a laboratory under air conditioning at 25°C. Next, 10 ml or more of aqua regia is dropped onto the sample and the sample is immersed in aqua regia for 10 seconds to reveal the metal structure. Next, the sample is subjected to optical microscope observation. For the optical microscope, for example, DSX1000 manufactured by Evident may be used. Then, for an arbitrary location at the 1 / 2 plate thickness position, an image is taken at an observation magnification of 100 times in the range of 800 μm × 600 μm. When the brightness levels of each crystal grain are divided into a plurality, the contrast may be adjusted so that the brightness of the brightest crystal grain is about 160 and the average brightness of the field of view is about 100. For reference, FIG. 1 shows an example of an optical microscope image. For the obtained optical microscope image, for example, image analysis is performed using WinROOF2015 manufactured by Mitani Shosha. As preprocessing for the image analysis, monochrome imaging and histogram equalization 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, for the binarized image, an opening process, which is a morphological transformation, is performed once to remove noise such as grain boundaries. Then, the area ratio of the selected crystal grain group with the lowest brightness is taken as the DEA area ratio at that location. For reference, FIG. 2 shows an image obtained by binarizing the optical microscope image of FIG. 1. The above measurement is performed at five arbitrary locations at the 1 / 2 plate thickness position, and the average value of the DEA area ratios at each location is taken as the DEA area ratio at the 1 / 2 plate thickness position.

[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] Moreover, 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 has a volume fraction of 80% or more, preferably 90% or more, more preferably 95% or more, and further preferably 98% or more. 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, further preferably 5% or less, and further preferably 2% or less. The volume fraction of the remaining structure may be 0%. In addition, examples of the steel sheet before quenching include hot-rolled steel sheet and hot-rolled annealed steel sheet. The hot-rolled steel sheet includes, in addition to the hot-rolled steel sheet, a steel sheet obtained by subjecting the hot-rolled steel sheet to an oxide scale removal treatment such as pickling. In addition, the hot-rolled annealed steel sheet includes not only a steel sheet obtained by subjecting a hot-rolled steel sheet to hot-rolled sheet annealing, but also a steel sheet obtained by further subjecting the steel sheet obtained by subjecting the hot-rolled sheet annealing to an oxide scale removal treatment such as pickling.

[0056] The structure of the steel sheet after quenching is mainly composed of martensite phase. Specifically, the structure of the steel sheet after quenching is a structure in which the martensite phase has a volume fraction of 80% or more, preferably 90% or more, more preferably 95% or more, and further preferably 98% or more. 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, further preferably 5% or less, and further preferably 2% or less. The volume fraction of the remaining structure may be 0%. Examples of the steel sheet after quenching include a hot-rolled steel sheet and a steel sheet obtained by quenching a hot-rolled annealed steel sheet. Since the steel sheet is hardened by quenching, the steel sheet after quenching has an HRC in the range of 30 to 40, preferably 33 to 37.

[0057] The volume fractions of the ferrite phase and the martensite phase may be measured according to a conventional method. For example, a sample for microstructure observation is taken from the center of the width of a martensitic stainless steel plate to be used as a test material. Next, the L-section of the sample is mirror-polished, and then etched using an aqueous picrate acid solution, and 10 optical microscope images at a magnification of 500 times are taken. In the obtained microstructure photograph, the martensite phase and the ferrite phase are distinguished from each other based on the microstructure shape and etching strength (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 phase and the martensite phase are calculated for each field of view by image processing. Next, the arithmetic average value of the volume fractions of the ferrite phase and the martensite phase obtained for each field of view is calculated, and the value is taken as the volume fraction of the ferrite phase and the martensite phase.

[0058] 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 further preferably 5.0 mm or more. 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 further preferably 11.0 mm or less. In addition, the quench hardness of the martensitic stainless steel sheet according to one embodiment of the present invention is in the range of HRC 30 to 40, preferably 33 to 37.

[0059] [2] Manufacturing method for martensitic stainless steel sheets 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 an embodiment of the present invention, a slab heated to 1100 to 1250°C is subjected to hot rolling by two or more rolling passes to obtain a hot-rolled steel sheet. After the first rolling pass-hold combination described above is performed, performing the second rolling pass-hold combination described above; It is characterized by the above.

[0061] The manufacturing method of the martensitic stainless steel sheet according to one embodiment of the present invention is a method for manufacturing the martensitic stainless steel sheet according to the above-mentioned one embodiment of the present invention. Here, unless otherwise specified, the temperatures in the manufacturing method are all based on the surface temperature of the slab, steel sheet, etc. 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 composition is melted by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and a slab (steel material) is made by a continuous casting method or an ingot casting-blooming method. The manufacturing method of the 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: 1100~1250℃ In the hot rolling, from the viewpoint of performing the above-mentioned first rolling pass-hold combination and the above-mentioned 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, more preferably 1140°C or higher. On the other hand, if the slab heating temperature exceeds 1250°C, slab sagging in the heating furnace is induced, resulting in operational trouble. 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 roll the slab with a high reduction rate in a higher temperature range where recrystallization is likely to occur and hold for a certain period of time after rolling before performing the second rolling pass-hold combination. In particular, the rolling temperature and reduction rate of the rolling pass in which the reduction is performed, and the holding temperature and holding time in the hold immediately after the rolling pass, greatly affect the progress of recrystallization during hot rolling. Therefore, it is important to perform the first rolling pass-hold combination under the above-mentioned conditions. Below, the conditions of the first 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℃ or higher If the rolling temperature of the rolling pass related to the first rolling pass-holding combination is less than 1100°C, the time required to complete recrystallization becomes long, and the rolling efficiency decreases. In addition, the rolling resistance increases, which induces material cracking. 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. In addition, the upper limit of the rolling temperature is not particularly limited. For example, the rolling temperature is preferably 1250°C or lower. The rolling temperature of a rolling pass means the inlet temperature of the rolling pass. The same applies to the following.

[0065] Reduction rate of rolling pass: 20% or more In order to introduce strain required for recrystallization, the rolling pass (rolling temperature of rolling pass: 1100°C or higher) for the first rolling pass-hold combination has a rolling reduction of 20% or more. The rolling reduction is preferably 21% or more, more preferably 22% or more. There is no particular upper limit to the rolling reduction. However, if the rolling reduction is excessively high, warping or the like may occur in the rolled material, which may cause problems. Therefore, the rolling reduction is preferably 50% or less. The rolling reduction per pass can be calculated by the following formula. [Rolling pass reduction rate (%)] = ([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 immediately after the rolling pass: 1080℃ or higher In the above-mentioned holding immediately after the rolling pass, recrystallization occurs throughout the slab. Therefore, the holding temperature in the above-mentioned holding immediately after the rolling pass is set to 1080°C or higher. The upper limit of the holding temperature is not particularly limited. For example, the holding temperature may be set to the exit temperature of the above-mentioned rolling pass or lower.

[0067] Holding time immediately after the rolling pass: 20 seconds or more In the above-mentioned holding immediately after the rolling pass, recrystallization occurs throughout the slab. In order to promote sufficient recrystallization and subsequent grain growth, the holding time in the above-mentioned 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 time immediately after a rolling pass refers to the holding time 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 time immediately after a rolling pass is the minimum temperature of the slab from the exit of the rolling pass to the entrance of the next rolling pass (generally corresponds to the entrance temperature of the next rolling pass). The holding time during the holding time immediately after a rolling pass is 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 time immediately after a rolling pass is 1080°C. In this case, the holding time during the holding time immediately after a rolling pass is 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] The number of times the first rolling pass-hold combination is performed may be one, but the more times it is performed, the more effective it is at 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 to 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 decreases, and there is a risk that material cracks will be induced in the subsequent rolling passes due to increased rolling resistance. Therefore, the number of times the first rolling pass-hold combination is performed is preferably five or less.

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

[0071] Rolling temperature of rolling pass: less than 1100℃, more than 1050℃ If the rolling temperature of the rolling pass in 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, the DEA area ratio at the 1 / 2 sheet thickness position cannot be set to 30% or less. 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 and not less than 1050°C.

[0072] Total reduction of rolling passes: 35% or more By setting the total reduction rate of the rolling passes (rolling temperature: rolling passes of 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 rate is set to 35% or more. The total reduction rate is preferably 38% or more, more preferably 40% or more. There is no particular upper limit to the total reduction rate. However, if the total reduction rate is excessively high, it becomes difficult to implement the above-mentioned first rolling pass-hold combination. Therefore, the total reduction rate is preferably 70% or less. The total reduction rate of the rolling passes can be calculated by the following formula.

number

[0073] Number of rolling passes: 1 or more The number of rolling passes (rolling temperature: rolling passes less than 1100°C and 1050°C or more) in 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 to the number of rolling passes. However, if the number of rolling passes exceeds 5, the temperature of the slab will decrease, and there is a risk that material cracks will be induced in the subsequent rolling passes due to increased rolling resistance. Therefore, the number of rolling passes is preferably 5 or less.

[0074] Holding temperature immediately after the final rolling pass among the above rolling passes: 900°C or higher If the holding temperature immediately after the final rolling pass among 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, the DEA area ratio at the 1 / 2 sheet thickness position cannot be set to 30% or less. Therefore, the holding temperature is set to 900°C or more. 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 exit temperature of the final rolling pass among 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 by 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 temperature of the slab 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 of the rolling passes refers to the holding of the slab from the exit of the final rolling pass of the rolling passes (the final rolling pass of the rolling passes with a rolling temperature of less than 1100°C and not less than 1050°C) to the entrance of the next rolling pass (in a state where hot rolling is not being performed). The holding temperature in the holding immediately after the final rolling pass of the rolling passes is the minimum temperature of the slab from the exit of the final rolling pass of 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 in the holding immediately after the final rolling pass of the rolling passes refers to the time during which the slab is held from the exit of the final rolling pass of 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 moves 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 slab temperature becomes 900°C.

[0077] Other hot rolling conditions are not particularly limited and may be in accordance with conventional methods. For example, hot rolling is 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. The above-mentioned first rolling pass-hold combination and second rolling pass-hold combination may be performed in either the rough rolling or the finish rolling, but are preferably performed in the rough rolling. Furthermore, the finishing temperature is preferably 800 to 1000°C, and the coiling temperature is preferably 400 to 800°C.

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

[0079] Molten steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was produced by refining using a 150-ton converter and strong stirring / vacuum oxygen decarburization (SS-VOD). The obtained 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. Here, among the rolling passes of the hot rolling, the first to seventh passes were performed as rough rolling, and the eighth to fourteenth passes were performed as finish rolling. The rough rolling was performed using a three-stand reverse-type rough rolling device, and the finish rolling was performed using a seven-stand tandem-type finish rolling device. In Table 2, except for No. 15, the third rolling pass of rough rolling and the holding immediately thereafter are listed as representative conditions among the rolling passes with a rolling temperature of 1100 ° C or more (in No. 15, the first rolling pass of rough rolling and the holding immediately thereafter are listed as representative conditions). In addition, in Table 2, the seventh rolling pass, which is the final rolling pass of rough rolling, and the holding immediately thereafter are listed as representative conditions among the rolling passes with a rolling temperature of less than 1100 ° C and 1050 ° C or more. In addition, the sheet bar thickness in Table 2 is the plate thickness of the slab (sheet bar) at the end of rough rolling. Next, the obtained hot-rolled steel sheet was 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. The other conditions were in accordance with the usual method.

[0080] The DEA area ratio of the martensitic stainless steel sheets thus obtained was measured at the 1 / 2 sheet thickness position in the same manner as described above. The results are shown in Table 2.

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

[0082] (1) Quenching hardness Near the leading edge of the martensitic stainless steel sheet (coil), 50 mm square (50 mm x 50 mm) samples were taken so that the center of the width direction was the center of the sheet width and 200 mm from the width end. Similarly, near the trailing edge of the martensitic stainless steel sheet (coil), 50 mm square samples were taken so that the center of the width direction was the center of the sheet width and 200 mm from the width end. The four samples were then subjected to the following heat treatment simulating quenching. Heat treatment conditions Average heating rate from room temperature to heating temperature: 30℃ / sec Heating temperature: 1000℃ Hold time at heating temperature: 10 seconds Average cooling rate from heating temperature to 100℃: 70℃ / sec

[0083] Next, the surface of each sample was ground to remove scale by about 50 μm. Then, the Rockwell hardness (C scale) was measured at any five points of 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 quenching hardness, and the pass / fail judgment was made according to the following criteria. Pass: The average value of Rockwell hardness (C scale) is within the target range (HRC30~40). Fail: The average value of Rockwell hardness (scale C) is outside the target range (less than HRC30 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 ferrite phase, and the structure of the steel sheet after the heat treatment was mainly composed of martensite phase. In addition, 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 In the vicinity of the leading edge of the martensitic stainless steel plate (coil), multiple 100 mm square (100 mm x 100 mm) samples were taken so that the center of the width direction was the center of the width direction at the center of the width direction and at a position 200 mm from the width end. Similarly, multiple 100 mm square samples were taken in the vicinity of the rear end of the martensitic stainless steel plate (coil) so that the center of the width direction was the center of the width direction at the center of the width direction and at a position 200 mm from the width end. Next, in 100 random locations of the taken samples, a hole with a diameter of 10 mm was punched with a clearance of 0.5 mm to simulate the punching process of a disk brake. Next, the punched hole was observed while illuminating it with an LED with an illuminance of 300 lumens, and the presence or absence of a bilaminar crack was confirmed. Here, a sample in which a dark void was observed in the center of the plate thickness was judged to have a bilaminar crack, and a sample in which no void was observed was judged to have no bilaminar crack. The punching workability was evaluated according to the following criteria. Pass (Excellent): No bifacial cracks in all 100 locations Failed (defective): One or more cracks in two plates

[0086] [Table 1]

[0087] [Table 2]

[0088] As shown in Table 2, in all of the inventive examples, the hardening 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, and therefore 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%, and punching workability was insufficient. In No. 12, the combination of the first rolling pass and holding was not performed, so the DEA area ratio was over 30%, and punching workability was insufficient. In No. 13, the combination of the second rolling pass and holding was not performed. In particular, the total reduction ratio of the rolling passes at a rolling temperature of less than 1,100°C and 1,050°C or higher was below the appropriate range. As a result, the DEA area ratio exceeded 30%, and punching workability was insufficient. In No. 14, the combination of the second rolling pass and holding was not performed. In particular, the holding time immediately after the final rolling pass among the rolling passes with a rolling temperature of less than 1,100°C and 1,050°C or higher was not within the appropriate range. As a result, the DEA area ratio exceeded 30%, and punching workability was insufficient. In No. 15, the first rolling pass-hold combination was not performed, in particular, a rolling pass at a rolling temperature of 1100°C or higher was not performed, so the DEA area ratio exceeded 30% and punching workability was insufficient. In No. 16, the first rolling pass-hold combination was not performed, in particular, a rolling pass at a rolling temperature of 1100°C or higher and a rolling reduction of 20% or more was not performed. As a result, the DEA area ratio exceeded 30%, and punching workability was insufficient.

Claims

1. In mass percent, C: 0.03-0.09%, Si: 0.01-0.50%, Mn: 1.0 to 2.0%, Cr: 10.0-14.5%, P: 0.040% or less, S: 0.010% or less, Al: 0.001 to 0.100% and N: 0.005-0.060% and the balance being Fe and unavoidable impurities, A martensitic stainless steel plate having a DEA area ratio of 30% or less at the 1 / 2 position of the plate thickness. Here, the DEA area ratio is the area ratio of black regions observed under 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 (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 the martensitic stainless steel sheet according to claim 1 or 2, comprising the steps of: The slab heated to 1100 to 1250°C is subjected to hot rolling by two or more rolling passes to obtain a hot-rolled steel sheet; In the hot rolling, After performing the following first pass-hold combination:

2. Performing the following second rolling pass-hold combinations: A method for manufacturing martensitic stainless steel sheet. First rolling pass-hold combination One rolling pass, in which the rolling temperature is 1100° C. or more and the rolling reduction is 20% or more; A combination of a holding temperature of 1080° C. or more and a holding time of 20 seconds or more immediately after the rolling pass. Second rolling pass-hold combination One or more rolling passes, the rolling temperature of which is less than 1100°C and is 1050°C or more, and the total reduction is 35% or more; A combination of a holding temperature of 900° C. or more and a holding time of 30 seconds or more, and a holding time immediately after the final rolling pass among the rolling passes.

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.