Martensitic stainless steel plate

Abstract

To provide a martensitic stainless steel sheet that has excellent rusting resistance.SOLUTION: Adopted is a martensitic stainless steel sheet that contains C, Si, Mn, P, S, Cr, Ni, Cu, N, Mo, V, O, Al, Mg and Ca, and the balance being Fe and impurities, and in which, on the surface of the steel sheet, among the oxysulfide having a circle equivalent diameter of 1 μm or more and containing S by 5% or more, the number ratio of (Mn,Cr)S-based oxysulfide is 70% or more and the number ratio of CaS-based oxysulfide is less than 30%, and, on the surface of the steel sheet, the number density of oxysulfide having a circle equivalent diameter of 5 μm or more and containing S by 5% or more is 0.50 / mm2 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a martensitic stainless steel sheet having excellent rust resistance. [Background technology]

[0002] Stainless steel is generally used in practical applications without being painted or otherwise treated. Martensitic stainless steel is a type of steel with a high carbon concentration to improve hardness after quenching. Applications include cutting tools such as table knives and scissors, tools such as loom parts and calipers, and structural components such as motorcycle disc brakes and reinforcing bars. For these applications, it is difficult to form a plating layer for rust prevention, paint, or apply rust-preventive oil.

[0003] In general, the corrosion resistance of stainless steel base materials is significantly affected by their chemical composition and is measured using a pitting resistance equivalent (PRE) index. The higher the PRE index, the higher the corrosion resistance. In this case, corrosion resistance refers to a neutral chloride aqueous solution environment, and methods for evaluation include the pitting potential measurement method for stainless steels specified in JIS G0577 and the salt spray test method specified in JIS Z2371. However, the corrosion resistance predicted from the chemical composition of the base material may not necessarily correspond to the actual corrosion resistance of the steel. Typical causes of corrosion resistance degradation in this case include the presence of CaS, a sulfide exposed on the steel surface, and the presence of a Cr-depleted zone (sensitization) formed around carbides during quenching.

[0004] Patent Document 1 describes a Ca-containing steel that suppresses the formation of CaS when the temperature of the molten steel is lowered or during solidification by controlling the equilibrium S concentration of inclusions present in the molten steel to a low level.

[0005] Patent Document 2 describes a method for producing high-Al stainless steel with excellent rust resistance, in which the CaO concentration in the slag at the end of refining is controlled to 35% or less, thereby suppressing the accumulation of S in CaO, and the MgO concentration in the slag is controlled to 30% or less, thereby generating solid-phase MgO that has good lattice matching with CaS, preventing the easy precipitation of CaS.

[0006] Patent Document 3 describes a ferritic stainless steel that is less susceptible to rusting, in which the value of the formula relating to the composition of inclusions, represented by the X value, is kept below a certain level and refining is performed so that the formula consisting of [Ca], [S], [Al], T, and [O] is satisfied, thereby suppressing the formation of CaS.

[0007] Patent Document 4 describes a method for preventing sensitization during hardening and reducing oxides of 10 μm or more to 0.2 particles / cm 2 The following martensitic stainless steel sheets with excellent manufacturability and corrosion resistance are described: In Patent Document 4, Al is reduced to 0.02% or less and O is reduced to 0.001 to 0.01% in order to reduce the amount of inclusions formed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-107178 [Patent Document 2] Japanese Patent Application Publication No. 5-339620 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-162948 [Patent Document 4] Japanese Patent Application Publication No. 2018-9231 Summary of the Invention [Problem to be solved by the invention]

[0009] Patent Document 1 does not take into consideration CaS that is generated when heating the slab before hot rolling, and therefore corrosion resistance may deteriorate.

[0010] The techniques of Patent Documents 2 and 3 have many restrictions on the slag composition and the concentrations of Ca and S in the molten steel. Furthermore, in martensitic stainless steel, the Al concentration must be reduced as much as possible because solid solution strengthening reduces toughness. Therefore, there is room for further improvement.

[0011] Furthermore, the technology described in Patent Document 4 reduces coarse inclusions, making rust less noticeable. However, the composition of sulfides generated in the refining process and subsequent processes cannot be controlled, leaving room for further improvement in corrosion resistance.

[0012] The present invention has been made in view of the above problems, and an object of the present invention is to provide a martensitic stainless steel sheet having excellent rust resistance. [Means for solving the problem]

[0013] The present inventors conducted a detailed investigation into factors affecting rust resistance in martensitic stainless steel sheets with high carbon contents manufactured by various methods. As a result, it was found that the composition and size of sulfides affect rust resistance in steel sheets that have undergone conventional quenching treatment. In this specification, sulfides are often formed around or inside oxides, making them difficult to distinguish, so inclusions containing 5% or more of sulfur are referred to as oxysulfides.

[0014] That is, on the steel sheet surface, the number of oxysulfides with a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 particles / mm 2 It was found that the rust resistance is improved when the number ratio of (Mn,Cr)S-based inclusions with an equivalent circle diameter of 1 μm or more among oxysulfides to the number of CaS-based inclusions is 70% or more.

[0015] The present invention has been made based on the above findings in order to solve the above problems, and the gist of the present invention is as follows.

[0016] [1] In mass %, C: 0.10~0.60%, Si: 0.05 to 1.0% Mn:[S]×100%~1.0% (where [S] is the S content (%)), P: 0.04% or less, S: 0.008% or less, Cr: 11-16%, Ni: 0.01 to 0.50% Cu: 0.01 to 0.26%, N: 0.01 to 0.10%, Mo: 0.01 to 1.0%, V: 0.01~0.5%, O: 0.0010~0.0080%, Al: 0.001%~0.025%, Mg: 0.0020% or less, Ca: 0.0020% or less, the balance being Fe and impurities, Among oxysulfides on the surface of a steel sheet that have an equivalent circle diameter of 1 μm or more and contain 5% or more of S, the concentrations (mass%) of MnS, CrS, and CaS are calculated from the respective concentrations (mass%) of S, Mn, Cr, and Ca in the oxysulfides, and when inclusions with a (Mn,Cr)S / CaS ratio of more than 1 and inclusions with a CaS ratio of 0 are defined as (Mn,Cr)S-based oxysulfides and other oxysulfides are defined as CaS-based oxysulfides, the number ratio of (Mn,Cr)S-based oxysulfides is 70% or more and the number ratio of CaS-based oxysulfides is less than 30%, On the surface of the steel sheet, the number density of oxysulfides with a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 particles / mm 2 A martensitic stainless steel sheet characterized by the following: [2] In mass %, C: 0.10~0.60%, Si: 0.05 to 1.0% Mn:[S]×100%~1.0% (where [S] is the S content (%)), P: 0.04% or less, S: 0.008% or less, Cr: 11-16%, Ni: 0.01 to 0.50% Cu: 0.01 to 0.26%, N: 0.01 to 0.10%, Mo: 0.01 to 1.0%, V: 0.01~0.5%, O: 0.0010~0.0080%, Al: 0.001%~0.025%, Mg: 0.0020% or less, Ca: 0.0020% or less, Ta: contains 0.0005 to 0.01%; the balance being Fe and impurities, Among oxysulfides on the surface of a steel sheet that have an equivalent circle diameter of 1 μm or more and contain 5% or more of S, the concentrations (mass%) of MnS, CrS, TaS, and CaS are calculated from the respective concentrations (mass%) of S, Mn, Cr, Ta, and Ca in the oxysulfides, and when inclusions with a (Mn,Cr,Ta)S / CaS ratio exceeding 1 and inclusions with a CaS ratio of 0 are defined as (Mn,Cr,Ta)S-based oxysulfides, inclusions that do not contain Ta are defined as (Mn,Cr)S-based oxysulfides, and the rest are defined as CaS-based oxysulfides, the total number ratio of the (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides is 70% or more and the number ratio of CaS-based oxysulfides is less than 30%, On the surface of the steel sheet, the number density of oxysulfides with a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 particles / mm 2 A martensitic stainless steel sheet characterized by the following: [3] In place of a part of Fe, further, by mass%, Co: 0.05 to 1.00%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.005% or less, Sn: 0.005% or less 0.20%、 REM: 0.002% or less. The martensitic stainless steel sheet according to [1] or [2], characterized in that it contains one or more selected from the following: [Effects of the Invention]

[0017] According to the present invention, a martensitic stainless steel sheet having excellent rust resistance can be provided. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a graph showing the rust area ratio relative to the number ratio of (Mn,Cr)S-based oxysulfides. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in detail below. As mentioned above, typical causes of deterioration in corrosion resistance of martensitic stainless steels with a high carbon content include the presence of a Cr-depleted zone (sensitization) that forms around carbides during quenching, and the presence of CaS, a sulfide exposed on the steel surface.

[0020] Sensitization in stainless steel is more accelerated with a higher carbon content, and it has been thought that martensitic stainless steels with a high carbon content are more susceptible to sensitization. Generally, during quenching, in order to prevent coarsening of austenite grains, the steel is heated at a temperature at which some chromium carbides do not dissolve, leaving undissolved chromium carbides, which inhibit the growth of austenite grains. Therefore, after quenching, the steel is left with finely dispersed undissolved carbides. However, the present inventors have confirmed that these carbides are relatively fine, and that no chromium-depleted zones are formed around them during quenching and cooling, thereby not impairing corrosion resistance.

[0021] CaS is one of the non-metallic inclusions in steel that can be the starting point for rusting. Although CaS is not present at the slab stage, i.e., at the steelmaking stage, it can be formed when the slab is heated before hot rolling, as mentioned above. The mechanism is thought to be that Ca separates from CaO in the inclusions, and S also separates from the inclusions, and these react to form CaS, or that S in the base metal reacts with CaO in the inclusions to form CaS. Therefore, if both the CaO in the inclusions and the S in the base metal are reduced before heating the slab, CaS formation will be suppressed.

[0022] In martensitic stainless steels with a high carbon content, ferrite is formed as the primary crystal when the molten steel is solidified, causing the carbon to concentrate in the liquid phase and lowering the solidus temperature. This results in a longer solidification time than other stainless steels, which tends to promote the growth of inclusions during solidification.

[0023] Therefore, it was found that in high-C martensitic stainless steels, when the contents of Ca, Al, and Mg are high, CaO-Al2O3-MgO inclusions tend to aggregate and coarsen, which is unfavorable for suppressing CaS. On the other hand, it was also found to have the advantage that S in the base material can be fixed as (Mn,Cr)S during solidification. Therefore, extensive research was conducted to control the composition of sulfides formed during solidification.

[0024] As a result, we found that by optimizing the amounts of Mn and S in the steel and controlling the concentrations of various deoxidizing elements and O, it is possible to modify CaS to (Mn,Cr)S and obtain steel sheets with excellent rust resistance and controlled low levels of (Mn,Cr)S. When the Mn and oxygen concentrations are high, MnO·Cr2O3 is formed first, making it impossible to fix the S in the base material.

[0025] <About steel composition> As described above, the present invention relates to the control of inclusion composition and the control of the components of martensitic stainless steel, and is applicable to commonly produced martensitic stainless steel. The following shows the ranges of components that can be suitably used, but the present invention is not limited to these.

[0026] C: 0.10 to 0.60% Carbon lowers the solidus temperature and promotes the growth of inclusions, and is necessary to obtain a martensite structure after quenching and achieve high strength. Therefore, the carbon content is set to 0.10% or more, preferably 0.20% or more. On the other hand, excessive carbon content generates coarse carbides that act as rust initiation sites, so the carbon content is set to 0.60% or less, preferably 0.55% or less.

[0027] Si: 0.05 to 1.0% Si is added to reduce and recover Cr oxides produced during the decarburization of stainless steel. For this reason, the Si content is set to 0.05% or more. For deoxidation, a content of 0.10% or more is preferable. On the other hand, if the Si content exceeds 1.0%, impurities in the Si raw material and slag during refining are reduced, increasing the Ca content in the molten steel, so the Si content must be set to 1.0% or less. However, to narrow the austenite single-phase temperature range and reduce hardening stability, a content of 0.80% or less is preferable.

[0028] Mn:[S]×100%~1.0% (where [S] is the S content in steel (mass%)) Mn, together with Cr, fixes S and suppresses CaS, and also expands the austenite single-phase region, contributing to improved hardenability. For this reason, the Mn content should be at least [S] × 100%, preferably at least [S] × 120%. On the other hand, if the Mn content exceeds 1.0%, the formation of MnO·Cr2O3 inhibits the formation of (Mn,Cr)S and promotes the formation of oxide scale during quenching heating, so the upper limit is set to 1.0%. Considering the coarsening of (Mn,Cr)S, it is preferable to keep the Mn content at 0.8% or less.

[0029] P:0.04% or less P is an element contained as an impurity in the main raw materials such as molten iron and ferrochrome. Since it is a harmful element to the toughness and corrosion resistance of hot-rolled annealed sheets and after quenching, its content is set to 0.04% or less, preferably 0.030% or less.

[0030] S: 0.008% or less S forms sulfide-based inclusions, which deteriorate the corrosion resistance of steel materials, and also reduces the number density of oxysulfides of 5 μm or more to 0.50 pieces / mm 2 In order to keep the content below 0.004%, the lower the upper limit of the content, the better, and the upper limit is set to 0.008%, preferably 0.004% or less.

[0031] Cr: 11-16% Cr is an important element that provides corrosion resistance to stainless steel, and together with Mn, it fixes S and suppresses CaS. For this reason, Cr must be at least 11% or more, and preferably 12% or more. On the other hand, to prevent the formation of residual ferrite after quenching, the upper limit is set to 16% and preferably 15% or less.

[0032] Ni: 0.01 to 0.50% Ni stabilizes austenite without solidification segregation. While other austenite-stabilizing elements such as C, N, and Mn may be depleted from the surface layer due to decarburization, denitrification, or oxidation during quenching, resulting in the formation of ferrite in the surface layer, Ni is an important element because it does not deplete from the surface layer. Ni is also effective in suppressing the progression of pitting corrosion, and this effect is apparent from 0.01% or more, so its content is set to 0.01% or more. On the other hand, since Ni increases the solidus temperature and suppresses the growth of inclusions, its upper limit is set to 0.50% or less. Since a large Ni content may cause a decrease in press formability due to solid solution strengthening in hot-rolled annealed steel sheets, its content is preferably set to 0.30% or less. Considering the effect of uniforming scale formation during quenching, its lower limit is preferably set to 0.05% or more.

[0033] Cu: 0.01 to 0.50% Cu suppresses active dissolution and ensures corrosion resistance, so a content of 0.01% or more is required. For steel types with a low pitting corrosion resistance index (PRE), a content of 0.02% or more is preferable as a countermeasure. On the other hand, excessive content reduces corrosion resistance and also deteriorates manufacturability, such as causing cracks in the cast slab, so the content should be 0.50% or less. Cu precipitates during quenching and tempering, which can impair the integrity of the passive film and reduce corrosion resistance, so a content of 0.20% or less is preferable.

[0034] N: 0.01 to 0.10% Like C, N lowers the solidus temperature, promoting the growth of inclusions and increasing quench hardness. It also strengthens the passive film and inhibits the precipitation of Cr carbides (suppressing the Cr-depleted layer). To achieve these effects, N content should be 0.01% or more. However, excessive N content can cause blowholes, so it should be 0.10% or less. To inhibit sensitization, N content is preferably 0.02% or more. Furthermore, N increases the hardness of hot-rolled annealed steel sheets, reducing their workability, so it is preferable to limit the N content to 0.05% or less.

[0035] Mo: 0.01 to 1.0% Mo is an element necessary for improving the corrosion resistance of sensitized parts, so its content should be 0.01% or more. 0.02% or more is preferable for suppressing the progression of pitting corrosion. On the other hand, Mo is an element that stabilizes the ferrite phase, so its content should be 1.0% or less to avoid promoting sensitization associated with the formation of residual ferrite. It is preferable to limit its content to 0.8% or less because it increases temper softening resistance, thereby lengthening the annealing time of hot-rolled sheets and thereby deteriorating manufacturability.

[0036] V: 0.01 to 0.5% V has the effect of improving corrosion resistance, and also has the effect of finely dispersing precipitates by dissolving V in carbonitrides, so its lower limit is set to 0.01% or more, preferably 0.02% or more. Since V has a strong effect of narrowing the austenite single-phase temperature range, its content is set to 0.5% or less. To avoid a decrease in toughness due to coarsening of precipitates, it is desirable to set it to 0.20% or less.

[0037] O: 0.0010 to 0.0080% O content is set to 0.0080% or less to suppress the formation of coarse inclusions, including CaO-based inclusions, and MnO Cr2O3. It is preferably set to 0.0050% or less. However, excessive deoxidation can easily cause Ca, Al, and Mg to be mixed from the slag into the molten steel during refining, so the O content is set to 0.0010% or more. It is preferably set to 0.0020% or more.

[0038] Al: 0.001% to 0.025% Al is contained for deoxidation and desulfurization. Furthermore, the presence of Al2O3 in CaO-based inclusions reduces the concentration of solute S, making it difficult for CaS to precipitate. Therefore, the lower limit is set to 0.001% or more. To stabilize deoxidation and desulfurization, 0.002% or more is preferred. However, excessive Al content impairs toughness through solid solution strengthening, so the upper limit is set to 0.025% or less. To avoid the reduction of Ca and Mg from slag to molten steel during refining, the Al content is preferably set to 0.020% or less. More preferably, the Al content is set to 0.015% or less.

[0039] Mg: 0.0020% or less Mg is an effective element for deoxidation and desulfurization, but excessive Mg content forms coarse inclusions, including CaO-based inclusions. It also combines with Al2O3 in the CaO-based inclusions to form MgO·Al2O3. As a result, the relative proportion of CaO inclusions increases, raising the upper limit of the solute S concentration and facilitating the precipitation of CaS. Therefore, the upper limit is set to 0.0020% or less, preferably 0.0010% or less. The lower limit for Mg may be 0.00005% or more, 0.0001% or more, or 0.0002% or more.

[0040] Ca:0.0020% or less Ca is an element effective for deoxidation and desulfurization, but excessive Ca content generates coarse inclusions, including CaO-based inclusions, and also makes CaS more likely to precipitate. Therefore, the upper limit is set to 0.0020% or less, preferably 0.0010% or less. The lower limit of Ca may be 0.00005% or more, 0.0001% or more, or 0.0002% or more.

[0041] The balance of the above steel components is Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial steel production due to various factors in the production process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0042] In the present invention, Ta can be contained in addition to the essential elements described above.

[0043] Ta: 0.0005 to 0.01% Ta is an element effective for deoxidation and desulfurization, and also improves corrosion resistance by dissolving in (Mn, Cr)S to form (Mn, Cr, Ta)S. To obtain this effect, a Ta content of 0.0005% or more is necessary, and preferably 0.0010% or more. However, excessive Ta content reduces room temperature ductility and toughness, so the upper limit is set to 0.01% or less, and preferably 0.005% or less.

[0044] Furthermore, in the present invention, in addition to the elements explained above, one or more of Co, Ti, Nb, B, Sn, and REM may be contained.

[0045] Co: 0.05 to 1.00% Co has the effect of increasing the strength of steel, so it can be added as needed. To obtain this effect, a content of 0.05% or more is required. However, excessive content leads to a decrease in toughness, so the upper limit is set to 1.00%.

[0046] Ti: 0.05% or less Ti is an element that forms carbonitrides and thereby suppresses sensitization and a decrease in corrosion resistance due to the precipitation of chromium mononitrides. To obtain this effect, a Ti content of 0.001% or more is preferred. However, if it is contained in excess, coarse oxides are formed, which leads to a decrease in toughness, so the upper limit is set to 0.05%.

[0047] Nb: 0.05% or less Nb is an element that forms carbonitrides and thereby suppresses sensitization and a decrease in corrosion resistance due to the precipitation of chromium mononitrides. To obtain this effect, the Nb content is preferably 0.001% or more. However, an excessive content of Mn makes the martensite phase unstable, reducing hardness, so the upper limit is set to 0.05%.

[0048] B: 0.005% or less B is an element effective in improving hot workability. To obtain this effect, it is preferable to include 0.0002% or more. However, if it is included in excess, it precipitates together with carbides, reducing hardenability, so the upper limit is set at 0.005%.

[0049] Sn: 0.005~ 0.20% Sn is a segregation element and is concentrated at the interface between the matrix and precipitates, suppressing the growth and coarsening of precipitates. This suppresses sensitization and improves corrosion resistance, so a content of 0.005% or more is preferable. However, Sn has a small solid solubility limit in the austenite phase and is known to cause hot-rolling cracks and defects in ordinary steel. Therefore, it is desirable to reduce the amount of Sn as much as possible, so a content of 0.20% or less is preferable.

[0050] REM: 0.002% or less REM (Rare-Earth Metals) have a high affinity with O and S, making them effective elements for deoxidation and desulfurization, and may be added as needed. However, adding too much can result in the formation of large amounts of oxides before or during casting, which can cause casting problems such as nozzle clogging and the formation of coarse inclusions, so the upper limit is set at 0.002%. REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these 17 elements.

[0051] <Method for measuring oxysulfides> The method for measuring inclusions in steel sheets will be described below. The steel sheet surface is mirror-finished for observation.

[0052] (oxysulfide number density) On the observation surface, inclusions with a circle equivalent diameter of 5 μm or more are randomly selected and analyzed using SEM-EDS to identify the composition of the inclusions. Inclusions containing 5% or more of S are defined as oxysulfides. 100 mm 2In the above observation area, 10 or more oxysulfides are counted. If the number of oxysulfides with a circle-equivalent diameter of 5 μm or more in the set observation area is less than 10, the observation area is enlarged. In this way, the number density of oxysulfides with a circle-equivalent diameter of 5 μm or more and containing 5% or more of S is measured.

[0053] (number ratio of oxysulfides) Next, 100 or more oxysulfides with an equivalent circle diameter of 1 μm or more and containing 5% or more S are randomly selected, and the composition of the inclusions is measured using an EDS (energy dispersive elemental analyzer).Then, based on the criteria described below, they are classified into (Mn, Cr)S-based oxysulfides, (Mn, Cr, Ta)S-based oxysulfides, and CaS-based oxysulfides, and the number of each is counted to determine the number ratio.

[0054] In addition, in JIS G0555, which is generally used as an evaluation method for inclusions, even if two or more inclusions are present at a distance from each other, they may be regarded as one inclusion depending on the type and distance, but in this embodiment, they are regarded as individual inclusions.

[0055] Next, the reasons for limiting the number of oxysulfides and the relationship between the composition of the sulfide portion and the corrosion resistance will be explained.

[0056] <The number of oxysulfides with a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 particles / mm 2 Below> The more sulfides formed around or inside coarse oxides, the more easily they are observed as rust. When investigating the relationship between inclusions before the corrosion resistance test and the rusted areas after the test, it was found that there was a high probability that rust would originate from oxysulfides with a circle equivalent diameter of 5 μm or more, so oxysulfides with a circle equivalent diameter of 5 μm or more were targeted. If there are many oxysulfides with a circle equivalent diameter of 5 μm or more, the number of rust initiation points increases and rust resistance decreases, so the upper limit of the number density was set at 0.50 particles / mm 2 The density should be less than 0.40 pieces / mm. 2 The following is the result.

[0057] <The number ratio of (Mn,Cr)S-based oxysulfides with a circle equivalent diameter of 1 μm or more is 70% or more among the oxysulfides> Sulfides form around or inside oxides, making their composition difficult to determine. The concentrations (mass%) of MnS, CrS, and CaS are calculated from the respective concentrations (mass%) of S, Mn, Cr, and Ca in each oxysulfide. Then, inclusions with a (Mn,Cr)S / CaS ratio greater than 1 and inclusions with a CaS ratio of 0 are considered (Mn,Cr)S-based oxysulfides. All other oxysulfides are considered CaS-based oxysulfides. The number of each is then counted. If 70% or more of the oxysulfides are (Mn,Cr)S-based oxysulfides and the CaS-based oxysulfides are less than 30%, it is determined that the S in the base material has been fixed and the CaS-based oxysulfides have been suppressed. As shown in Figure 1, by ensuring that the number ratio of (Mn,Cr)S-based oxysulfides is 70% or more of the oxysulfides, the rust area ratio can be reduced to 10% or less, resulting in a steel sheet with excellent rust resistance. The (Mn,Cr)S-based oxysulfides in the oxysulfides are preferably 80% or more. In Fig. 1, a rust area ratio of 10% or more is indicated by "x", and a rust area ratio of less than 10% is indicated by "o".

[0058] <The total number ratio of (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides with a circle equivalent diameter of 1 μm or more is 70% or more of all oxysulfides> When Ta is contained in steel, (Mn, Cr, Ta)S-based oxysulfides are generated in addition to (Mn, Cr)S-based oxysulfides. Ta is a highly reducing element, and its inclusion in (Mn, Cr)S-based oxysulfides can further improve corrosion resistance. The concentrations (mass%) of MnS, CrS, TaS, and CaS are calculated from the respective concentrations (mass%) of S, Mn, Cr, Ta, and Ca in each oxysulfide. Then, inclusions with a (Mn, Cr, Ta)S / CaS ratio greater than 1 and inclusions with a CaS ratio of 0 are considered (Mn, Cr, Ta)S-based oxysulfides. Inclusions that do not contain Ta are considered (Mn, Cr)S-based oxysulfides. All other inclusions are considered CaS-based oxysulfides. The number of each is then counted. If 70% or more of the oxysulfides are (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides and the CaS-based oxysulfides are less than 30%, it is determined that the S in the base material is fixed and the CaS-based oxysulfides are suppressed. The proportion of (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides in the oxysulfides is preferably 80% or more.

[0059] <CaS-based oxysulfides with a circle equivalent diameter of 1 μm or more make up less than 30% of the oxysulfides> Even if (Mn,Cr)S or (Mn,Cr,Ta)S is not formed in the steel at the slab stage, CaS may be formed during heating of the slab before hot rolling. Therefore, oxysulfides other than (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides, in which the ratio of (Mn,Cr)S / CaS or (Mn,Cr,Ta)S / CaS is 1 or less, are defined as CaS-based oxysulfides, and their number ratio is limited. If the CaS-based oxysulfides in the oxysulfides exceed 30%, rusting cannot be suppressed and the rust area ratio increases, which is undesirable, so the content is set to less than 30%. The CaS-based oxysulfides in the oxysulfides are preferably 20% or less.

[0060] <Manufacturing method> The martensitic stainless steel sheet of this embodiment is a hot-rolled steel sheet or a cold-rolled steel sheet manufactured using a normal method for manufacturing a martensitic stainless steel sheet. Hot-rolled steel sheets are manufactured through the steps of melting and casting, hot rolling, annealing the hot-rolled sheet, and pickling, while cold-rolled steel sheets are manufactured through the steps of cold rolling, annealing the cold-rolled sheet, and pickling.

[0061] First, steel is produced by melting it to have the above-mentioned specified composition. By reducing the S concentration and controlling various deoxidizing elements such as Al, Mg, and Si, as well as the oxygen concentration and elements that form oxysulfides such as Ca, Mn, and Cr, the number density of oxysulfides with a circle equivalent diameter of 5 μm or more is reduced to 0.50 particles / mm 2 It can be the following:

[0062] Also, the liquidus temperature (T LL ) and solidus temperature (T SL The temperature difference △T0 (solidification temperature range) between the two is 70°C or more, and (Mn,Cr)S-based oxysulfides or (Mn,Cr,Ta)S-based oxysulfides can be formed. The solidification temperature range △T0 can be easily calculated from the steel composition using a thermodynamic data collection or commercial thermodynamic calculation software.

[0063] In addition to the above, the cooling rate is controlled during casting. By controlling the average cooling rate between 1400 and 700°C near the surface of the slab to 50°C / min or less, the proportion of (Mn,Cr)S-based oxysulfides or (Mn,Cr,Ta)S-based oxysulfides with a circle equivalent diameter of 1µm or more can be increased to 70% or more. The average cooling rate is preferably in the range of 30°C / min or less, and more preferably in the range of 15°C / min or less.

[0064] After casting, steel sheets can be manufactured under known manufacturing conditions. For example, the slab is heated to 1100-1300°C, and then rough-rolled and finish-rolled to a thickness of 2-8m. The hot-rolled sheet annealing process involves annealing at 750-900°C using conventional box annealing. The sheet is then pickled to remove surface scale, resulting in a hot-rolled steel sheet. In the case of cold-rolled steel sheets, the product is then cold-rolled and final annealed. [Example]

[0065] The effects of the present invention will be explained below using examples, but the present invention is not limited to the conditions used in the following examples.

[0066] In this example, steels having the chemical compositions shown in Tables 1A to 1D were melted and continuously cast into 200 mm thick slabs. The cooling rate of the slabs was controlled to various speeds during casting. The average cooling rate near the slab surface between 1400 and 700°C was evaluated by numerical calculation using heat transfer analysis, and is shown in Tables 2 and 3. The slabs were heated to 1150°C to 1250°C for 2 hours, and then hot-rolled to produce hot-rolled steel sheets with a thickness of 5 mm. The hot-rolled sheets were annealed at 850°C, pickled, and cold-rolled to a thickness of 3.0 mm. The thin coils were then subjected to final annealing and pickling to produce cold-rolled steel sheets.

[0067] The inclusions were measured as follows: First, the steel sheet surface was mirror-finished for observation.

[0068] On the mirror-finished observation surface, inclusions with a circle equivalent diameter of 5 μm or more were randomly selected and analyzed by SEM-EDS to identify their composition. Inclusions containing 5% or more S were defined as oxysulfides. 2 In the observation area, 10 or more oxysulfides were counted. If the number of oxysulfides with a circle-equivalent diameter of 5 μm or more in the set observation area was less than 10, the observation area was enlarged. In this way, the number density of oxysulfides with a circle-equivalent diameter of 5 μm or more and containing 5% or more of S was measured.

[0069] Next, 100 or more oxysulfides with an equivalent circle diameter of 1 μm or more and containing 5% or more S were randomly selected, and the composition of the inclusions was measured using an EDS (energy dispersive elemental analyzer). Then, based on the criteria described below, the inclusions were classified into (Mn,Cr)S-based oxysulfides, (Mn,Cr,Ta)S-based oxysulfides, and CaS-based oxysulfides, and the number of each was counted to determine the number ratio. The "(Mn,Cr)S-based oxysulfide ratio" column in Tables 2 and 3 shows the total ratio of (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides.

[0070] In addition, in JIS G 0555:2015, which is a commonly used method for evaluating inclusions, even if two or more inclusions are present at a distance from each other, they may be regarded as one inclusion depending on the type and distance, but in this example, they were regarded as individual inclusions.

[0071] (When Ta is not contained) The concentrations (mass%) of MnS, CrS, and CaS were calculated from the concentrations (mass%) of S, Mn, Cr, and Ca in each oxysulfide. Inclusions with a (Mn,Cr)S / CaS ratio exceeding 1 and inclusions with a CaS ratio of 0 were defined as (Mn,Cr)S-based oxysulfides. Other oxysulfides were defined as CaS-based oxysulfides.

[0072] (When Ta is contained) The concentrations (mass%) of MnS, CrS, TaS, and CaS were calculated from the concentrations (mass%) of S, Mn, Cr, Ta, and Ca in each oxysulfide. Inclusions with a (Mn,Cr,Ta)S / CaS ratio exceeding 1 and inclusions with a CaS ratio of 0 were defined as (Mn,Cr,Ta)S-based oxysulfides. All inclusions other than (Mn,Cr,Ta)S-based oxysulfides and (Mn,Cr)S-based oxysulfides were defined as CaS-based oxysulfides. For Steel No. A28, the total number ratio of (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides is shown in Table 2.

[0073] Rust resistance was evaluated by polishing the surface with #600 sandpaper and then conducting a salt spray test based on JIS Z 2371:2015. A neutral salt spray test was used as the salt solution, and the test was conducted continuously for 24 hours. A rust area rate of 10% or more was considered a failure (×), and a rust area rate of less than 10% was considered a pass (◯). A rust area rate of 5% or less was considered a good material (◎).

[0074] As shown in Tables 1A, 1B and 2, the invention examples Nos. A1 to A30 had steel chemical compositions and oxysulfide number densities and number ratios that satisfied the ranges of the invention, and were excellent in rust resistance.

[0075] On the other hand, as shown in Tables 1C, 1D and 3, the comparative examples Nos. a1 to a27 did not satisfy the ranges of the present invention in terms of the chemical composition of the steel, the number density or the number ratio of oxysulfides, and therefore had poor rust resistance.

[0076] No. a1 had a low C content, ΔT0 was less than 70°C, the formation of (Mn, Cr)S-based oxysulfides was not promoted, and rust resistance was reduced. No. a2 had an excessive C content, which resulted in the formation of coarse carbides and reduced rust resistance.

[0077] No. a3 had a low Si content, and as a result, MnCr oxides were formed, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance. No. a4 had an excessive Si content, which resulted in excessive deoxidation, an increase in Ca in the molten steel, an increase in CaS-based oxysulfides, and a decrease in rusting resistance.

[0078] No. a5 had a low Mn content, which did not promote the formation of (Mn,Cr)S-based oxysulfides, resulting in a decrease in rust resistance. No. a6 had an excessive Mn content, which resulted in the formation of MnCr oxides, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance.

[0079] No. a7 had an excessive P content, which reduced the corrosion resistance of the steel and reduced rust resistance. No. a8 had an excessive S content, which resulted in the formation of a large amount of oxysulfides, increasing the number density and reducing rust resistance.

[0080] No. a9 had a low Cr content, which reduced the corrosion resistance of the steel and reduced rust resistance. In No. a10, the Cr content was excessive, and MnCr oxides were formed, preventing the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance.

[0081] In No. a11, the Ni content was excessive, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance. No. a12 had a low Ni content, which reduced the corrosion resistance of the steel and reduced rust resistance.

[0082] No. a13 had a low Cu content, which reduced the corrosion resistance of the steel and reduced rust resistance. No. a14 had an excessive Cu content, which reduced the corrosion resistance of the steel and reduced rust resistance.

[0083] No. a15 had a low N content, which reduced the corrosion resistance of the steel and reduced rust resistance. No. a16 had an excessive N content, which reduced the corrosion resistance of the steel and reduced rust resistance.

[0084] No. a17 had an excessive Mo content, which reduced manufacturability and also reduced rust resistance. No. a18 had a low Mo content, which reduced the corrosion resistance of the steel and reduced rust resistance.

[0085] No. a19 had a low V content, which reduced the corrosion resistance of the steel and reduced rust resistance. No. a20 had an excessive V content, which caused the precipitates to become coarse, resulting in a decrease in toughness and a decrease in rust resistance.

[0086] No. a21 had a low O content, which resulted in the formation of a large amount of CaS-based oxysulfides, and reduced rust resistance. No. a22 had an excessive O content, which resulted in the formation of MnCr oxides, preventing the formation of (Mn, Cr)S-based oxysulfides, and reduced rust resistance.

[0087] No. a23 had an excessive Al content, which resulted in the formation of a large amount of CaS-based oxysulfides, and reduced rust resistance. No. a24 had a low Al content, and a large amount of (Mn, Cr)S-based oxysulfides were generated and coarsened, resulting in a decrease in the number density and a decrease in rust resistance.

[0088] No. a25 had a low Ca content, which resulted in the formation of a large amount of CaS-based oxysulfides, and reduced rust resistance. No. a26 had an excessive Ca content, which resulted in the formation of a large amount of CaS-based oxysulfides, and reduced rust resistance.

[0089] No. a27 had chemical compositions that satisfied the ranges of the present invention, but the cooling rate of the slab was low, which resulted in the formation of a large amount of CaS-based oxysulfides and reduced rust resistance.

[0090] [Table 1A]

[0091] [Table 1B]

[0092] [Table 1C]

[0093] [Table 1D]

[0094] [Table 2]

[0095] [Table 3]

Claims

1. In mass%, C: 0.10-0.60%, Si: 0.05-1.0%, Mn: [S] × 100% to 1.0% (where [S] is the S content (%)), P: 0.04% or less, S: 0.008% or less, Cr: 11-16%, Ni: 0.01-0.50%, Cu: 0.01-0.26%, N: 0.01 to 0.10%, Mo: 0.01-1.0%, V: 0.01 to 0.5%, O: 0.0010 to 0.0080%, Al: 0.001% to 0.025%, Mg: 0.0020% or less, Ca: 0.0020% or less, the balance being Fe and impurities; Among oxysulfides on the surface of a steel sheet that have an equivalent circle diameter of 1 μm or more and contain 5% or more of S, the concentrations (mass%) of MnS, CrS, and CaS are calculated from the concentrations (mass%) of S, Mn, Cr, and Ca in the oxysulfides, and when inclusions with a (Mn, Cr)S / CaS ratio of more than 1 and inclusions with a CaS ratio of 0 are defined as (Mn, Cr)S-based oxysulfides and other oxysulfides are defined as CaS-based oxysulfides, the number ratio of the (Mn, Cr)S-based oxysulfides is 70% or more and the number ratio of the CaS-based oxysulfides is less than 30%, On the surface of the steel sheet, the number density of oxysulfides having a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces / mm ２ A martensitic stainless steel sheet characterized by the following:

2. In mass%, C: 0.10-0.60%, Si: 0.05-1.0%, Mn: [S] × 100% to 1.0% (where [S] is the S content (%)), P: 0.04% or less, S: 0.008% or less, Cr: 11-16%, Ni: 0.01-0.50%, Cu: 0.01-0.26%, N: 0.01 to 0.10%, Mo: 0.01-1.0%, V: 0.01 to 0.5%, O: 0.0010 to 0.0080%, Al: 0.001% to 0.025%, Mg: 0.0020% or less, Ca: 0.0020% or less, Ta: 0.0005 to 0.01%; the balance being Fe and impurities; Among oxysulfides on the surface of the steel sheet that have an equivalent circle diameter of 1 μm or more and contain 5% or more of S, the concentrations (mass%) of MnS, CrS, TaS, and CaS are calculated from the concentrations (mass%) of S, Mn, Cr, Ta, and Ca in the oxysulfides, and when inclusions with a (Mn, Cr, Ta)S / CaS ratio exceeding 1 and inclusions with CaS being 0 are defined as (Mn, Cr, Ta)S-based oxysulfides, inclusions not containing Ta are defined as (Mn, Cr)S-based oxysulfides, and the rest are defined as CaS-based oxysulfides, the total number ratio of the (Mn, Cr)S-based oxysulfides and (Mn, Cr, Ta)S-based oxysulfides is 70% or more and the number ratio of CaS-based oxysulfides is less than 30%, On the surface of the steel sheet, the number density of oxysulfides having a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces / mm ２ A martensitic stainless steel sheet characterized by the following:

3. 3. The martensitic stainless steel sheet according to claim 1, further comprising, in place of a portion of Fe, one or more selected from, by mass%, Co: 0.05 to 1.00%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.005% or less, Sn: 0.005 to 0.20%, and REM: 0.002% or less.

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