JP7758927B2 - Martensitic stainless steel plate - Google Patents

Description

The present invention relates to a martensitic stainless steel sheet with excellent rust resistance.

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, and its 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. In these applications, it is difficult to form a plating layer to prevent rust, paint, or apply rust-preventive oil.

Generally, the corrosion resistance of stainless steel base material is greatly influenced by its composition and is measured using the pitting resistance equivalent (PRE) index. The higher the value, the higher the corrosion resistance. In this case, corrosion resistance refers to a neutral chloride aqueous solution environment, and evaluation methods include the stainless steel pitting potential measurement method specified in JIS G0577 and the salt spray test method specified in JIS Z2371. However, the corrosion resistance predicted from the composition of the base material may not 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 layer (sensitization) that forms around carbides during quenching.

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.

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 forming solid-phase MgO with good lattice matching with CaS and preventing the easy precipitation of CaS.

Patent Document 3 describes a ferritic stainless steel that is less susceptible to rusting, where 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 carried out so that the formula consisting of [Ca], [S], [Al], T, and [O] is satisfied, thereby suppressing the formation of CaS.

Patent Document 4 describes a martensitic stainless steel sheet that is excellent in manufacturability and corrosion resistance, in which sensitization during quenching is avoided and oxides of 10 μm or larger are reduced to 0.2 particles/cm2 or ^less . In Patent Document 4, Al is reduced to 0.02% or less and O to 0.001 to 0.01% in order to reduce the amount of inclusions formed.

Japanese Patent Application Laid-Open No. 2001-107178 Japanese Patent Application Publication No. 5-339620 JP 2014-162948 A Japanese Patent Application Laid-Open No. 2018-9231

Patent Document 1 does not take into consideration the CaS that forms when the slab is heated before hot rolling, which can result in a deterioration in corrosion resistance.

The technologies in Patent Documents 2 and 3 have many restrictions regarding the slag composition and the concentrations of Ca and S in the molten steel. Furthermore, in martensitic stainless steel, solid solution strengthening impairs toughness, so the Al concentration must be reduced as much as possible. Therefore, there is room for further improvement.

Furthermore, the technology described in Patent Document 4 reduces coarse inclusions, making rust itself less noticeable. However, it is not possible to control the composition of sulfides formed after the refining process, so there is room for further improvement in corrosion resistance.

The present invention was made in consideration of the above problems, and its objective is to provide a martensitic stainless steel sheet with excellent rust resistance.

The inventors conducted a detailed investigation into the factors that affect rust resistance in high-C martensitic stainless steel sheets manufactured using various methods. As a result, they discovered that the composition and size of sulfides affect rusting in steel sheets that have undergone conventional quenching. Note that, in this specification, sulfides are often formed around or inside oxides, making them difficult to distinguish, so inclusions containing 5% or more S are referred to as oxysulfides.

That is, they found that rust resistance is improved when the number of oxysulfides on the steel sheet surface, each having an equivalent circle diameter of 5 μm or more and containing 5% or more of S, is 0.50 particles/mm2 or ^less , and the number ratio of (Mn, Cr)S-based inclusions having an equivalent circle diameter of 1 μm or more to the number of CaS-based inclusions among the oxysulfides is 70% or more.

The present invention was made based on the above findings to solve the above problems, and its gist is as follows:

[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%,
A martensitic stainless steel sheet, characterized in that the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm2 or ^less on the sheet surface of the steel sheet.
[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%,
A martensitic stainless steel sheet, characterized in that the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm2 or ^less on the sheet surface of the steel sheet.
[3] The martensitic stainless steel sheet according to [1] or [2], further containing, 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.

The present invention provides martensitic stainless steel sheets with excellent rust resistance.

FIG. 1 is a graph showing the rust area ratio relative to the number ratio of (Mn, Cr)S-based oxysulfides.

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 C content include the presence of a Cr-deficient layer (sensitization) that forms around carbides during quenching and the presence of CaS, which is a sulfide exposed on the steel surface.

Because sensitization in stainless steel is more accelerated in steels with a higher carbon content, it was thought that martensitic stainless steels with a high carbon content were more susceptible to sensitization. Generally, during quenching, in order to prevent austenite grain size from coarsening, the steel is heated at a temperature at which some chromium carbides do not dissolve, leaving undissolved chromium carbides that inhibit the growth of austenite grains. As a result, undissolved carbides are finely dispersed in the steel after quenching. However, the inventors have confirmed that these carbides are relatively fine, and that no chromium-depleted zones form around them during quenching and cooling, thereby not impairing corrosion resistance.

CaS is one example of a non-metallic inclusion in steel that can be the starting point for rust. Even if CaS is not present at the cast 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 the CaO in the inclusions, and S also separates from the inclusions, and these react to form CaS, or that S in the base material reacts with CaO in the inclusions to form CaS. Therefore, if both the CaO in the inclusions and the S in the base material are reduced before the slab is heated, CaS will be suppressed.

Here, in martensitic stainless steel with a high carbon content, ferrite is formed as the primary crystal when the molten steel solidifies, causing the carbon to concentrate in the liquid phase and lowering the solidus temperature. As a result, the solidification time is longer than with other stainless steels, and the growth of inclusions during solidification is more likely to be promoted.

Therefore, it was found that in high-C martensitic stainless steel, when the contents of Ca, Al, and Mg are high, CaO-Al ₂ O ₃ -MgO-based 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.

As a result, it was discovered 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 with controlled low amounts of (Mn, Cr) S. When the Mn concentration or oxygen concentration is high, MnO.Cr ₂ O ₃ is formed first, making it impossible to fix the S in the base material.

<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.

C: 0.10-0.60%
C lowers the solidus temperature to promote the growth of inclusions, and is necessary to obtain a martensite structure after quenching to obtain high strength. Therefore, the C content is set to 0.10% or more, preferably 0.20% or more. On the other hand, if an excessive amount of C is contained, coarse carbides are formed, which become rust initiation sites, so the C content is set to 0.60% or less, preferably 0.55% or less.

Si: 0.05-1.0%
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 is set to 1.0% or less. However, a content of 0.80% or less is preferable to narrow the austenite single-phase temperature range and reduce hardening stability.

Mn: [S] x 100% to 1.0% (where [S] is the S content (mass%) in the steel)
Mn, together with Cr, is an element that fixes S and suppresses CaS, and also expands the austenite single-phase region, contributing to improved hardenability. Therefore, the Mn content is set to be equal to or greater than [S] × 100%, preferably equal to or greater than [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 be equal to or less than 1.0%. Considering the coarsening of (Mn,Cr)S, it is preferable to set the Mn content to be equal to or less than 0.8%.

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.

S: 0.008% or less S forms sulfide-based inclusions, which deteriorate the corrosion resistance of the steel material, and also limits the number density of oxysulfides of 5 μm or more to 0.50/mm2 or ^less , so the upper limit of the S content is preferably as low as possible, and the upper limit is set to 0.008%, preferably 0.004% or less.

Cr: 11 to 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%. Preferably, it is 15% or less.

Ni: 0.01~0.50%
Ni stabilizes austenite without solidification segregation. While other austenite-stabilizing elements such as C, N, and Mn may decrease from the surface layer due to decarburization, denitrification, or oxidation during quenching and heating, resulting in the formation of ferrite in the surface layer, Ni is an important element because it does not decrease 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 the Ni content is set to 0.01% or more. On the other hand, since Ni increases the solidus temperature and suppresses the growth of inclusions, the 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, the Ni content is preferably set to 0.30% or less. Considering the effect of uniforming scale formation during quenching, the lower limit is preferably set to 0.05% or more.

Cu: 0.01~0.50%
Cu suppresses active dissolution and ensures corrosion resistance, so a content of 0.01% or more is necessary. 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 Cu content reduces corrosion resistance and also deteriorates manufacturability, such as causing cracks in the cast slab, so the content is set to 0.50% or less. Cu precipitates during quenching and tempering, which can impair the integrity of the passive film and thus impair corrosion resistance, so a content of 0.20% or less is preferable.

N: 0.01~0.10%
Like C, N lowers the solidus temperature, promoting the growth of inclusions, and is necessary to increase 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 is set to 0.01% or more. However, excessive N content can cause blowholes, so N content is set to 0.10% or less. To inhibit sensitization, N content is preferably set to 0.02% or more. Furthermore, N increases the hardness of hot-rolled annealed steel sheets, reducing workability, so N content is preferably set to 0.05% or less.

Mo: 0.01~1.0%
Mo is an element necessary for improving the corrosion resistance of sensitized parts, so its content is set to 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 is set to 1.0% or less to avoid promoting sensitization associated with the formation of residual ferrite. It is preferable to set it to 0.8% or less to increase temper softening resistance, thereby lengthening the annealing time of hot-rolled sheets and thereby deteriorating manufacturability.

V:0.01~0.5%
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. In order to avoid a decrease in toughness due to coarsening of precipitates, it is desirable to set it to 0.20% or less.

O: 0.0010-0.0080%
O content is set to 0.0080% or less to suppress the formation of coarse inclusions including CaO-based inclusions and _MnO.Cr2O3 , and is preferably set to 0.0050% or less. However, since excessive deoxidation makes it easier for Ca, Al, and _Mg to be mixed from the slag into molten steel during refining, O content is set to 0.0010% or more, and is preferably set to 0.0020% or more.

Al: 0.001% to 0.025%
Al is contained for deoxidation and desulfurization. Furthermore, the presence of Al ₂ O ₃ 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.

Mg: 0.0020% or less. Mg is an element effective for deoxidation and desulfurization, but excessive Mg content forms coarse inclusions, including CaO-based inclusions. It also combines with Al ₂ O ₃ in the CaO-based inclusions to form MgO·Al ₂ O _3. As a result, the proportion of CaO inclusions increases relatively, 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 of Mg may be 0.00005% or more, 0.0001% or more, or 0.0002% or more.

Ca: 0.0020% or less Ca is an element effective for deoxidation and desulfurization, but excessive content of Ca causes the formation of coarse inclusions including CaO-based inclusions and makes it easier for CaS 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.

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

In addition to the essential elements described above, the present invention can also contain Ta.

Ta: 0.0005-0.01%
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 achieve this effect, a Ta content of 0.0005% or more is necessary, and preferably 0.0010% or more. However, excessive Ta content can lead to a decrease in room temperature ductility and toughness, so the upper limit is set to 0.01% or less, and preferably 0.005% or less.

Furthermore, in addition to the elements described above, the present invention may contain one or more of Co, Ti, Nb, B, Sn, and REM.

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

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%.

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, a content of 0.001% or more is preferable.
However, an excessive content of Mn makes the martensite phase unstable and reduces hardness, so the upper limit is set to 0.05%.

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

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. Therefore, it is effective in suppressing sensitization and improving corrosion resistance, so a content of 0.005% or more is preferred. 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 preferred.

REM: 0.002% or less REM (Rare-Earth Metal) has a high affinity with O and S, making it an effective element for deoxidation and desulfurization, and may be contained as needed. However, if added in excess, a large amount of oxides is formed before or during casting, causing casting problems such as nozzle clogging and the formation of coarse inclusions, so the upper limit is set to 0.002% or less. 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.

<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.

(Number density of oxysulfides)
In the observation surface, inclusions with an equivalent circle diameter of 5 μm or more are randomly analyzed using SEM-EDS to identify the composition of the inclusions. Inclusions containing 5% or more S are defined as oxysulfides. 10 or more oxysulfides are counted in an observation area of ^{100 mm2} or more. If the number of oxysulfides with an equivalent circle 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 an equivalent circle diameter of 5 μm or more and containing 5% or more S is measured.

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

Note that JIS G0555, a commonly used method for evaluating inclusions, may consider two or more inclusions located far apart to be a single inclusion depending on their type and distance, but in this embodiment they are considered to be individual inclusions.

Next, we will explain the reasons for limiting the number of oxysulfides, the composition of the sulfide portion, and the relationship with corrosion resistance.

<The number of oxysulfides having a circle equivalent diameter of 5 μm or more and containing 5% or more of S is 0.50 particles/mm2 ^or less>
The more sulfides formed around or inside coarse oxides, the more likely they are to be observed as rust. When the relationship between inclusions before the corrosion resistance test and rusted areas after the test was investigated, 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. Therefore, oxysulfides with a circle equivalent diameter of 5 μm or more are targeted. If a large number of oxysulfides with a circle equivalent diameter of 5 μm or more are present, the number of rust initiation points increases and rust resistance decreases, so the upper limit of the number density is set to 0.50 particles/mm2 or ^less . Preferably, it is 0.40 particles/mm2 or ^less .

<The number ratio of (Mn, Cr)S-based oxysulfides having a circle equivalent diameter of 1 μm or more is 70% or more among all oxysulfides>
Since sulfides are formed around or inside oxides, it is difficult to determine their composition. The concentrations (mass%) of MnS, CrS, and CaS are calculated from the concentrations (mass%) of S, Mn, Cr, and Ca in each oxysulfide. Then, inclusions with a (Mn, Cr)S/CaS ratio exceeding 1 and inclusions with CaS at 0 are defined as (Mn, Cr)S-based oxysulfides. All other oxysulfides are defined as CaS-based oxysulfides. Then, the number of each is 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 Fig. 1, by setting the number ratio of (Mn, Cr)S-based oxysulfides to 70% or more of the oxysulfides, the rust area ratio can be set to 10% or less, and a steel sheet with excellent rust resistance can be obtained. 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".

<The total number ratio of (Mn, Cr)S-based oxysulfides and (Mn, Cr, Ta)S-based oxysulfides having an equivalent circle diameter of 1 μm or more is 70% or more of the total 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 when Ta is contained in (Mn, Cr)S-based oxysulfides, corrosion resistance can be further improved. 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 exceeding 1 and inclusions with CaS equal to 0 are defined as (Mn, Cr, Ta)S-based oxysulfides. Inclusions that do not contain Ta are defined as (Mn, Cr)S-based oxysulfides. The rest are defined as CaS-based oxysulfides. Then, the number of each is 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.

<CaS-based oxysulfides with an equivalent circle diameter of 1 μm or more account for 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 the ratio of (Mn,Cr,Ta)S/CaS is 1 or less, are defined as CaS-based oxysulfides, and the number ratio is limited. If the CaS-based oxysulfides in the oxysulfides are 30% or more, 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.

<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. In the case of a hot-rolled steel sheet, it is manufactured through the steps of melting and casting, hot rolling, and annealing and pickling of the hot-rolled sheet, while in the case of a cold-rolled steel sheet, it is manufactured through the steps of cold rolling, and then annealing and pickling of the cold-rolled sheet.

First, steel is produced by melting the steel so that it has the above-mentioned predetermined chemical composition. By reducing the S concentration and controlling the concentration of various deoxidizing elements such as Al, Mg, and Si, the oxygen concentration, and elements that form oxysulfides such as Ca, Mn, and Cr, the number density of oxysulfides with an equivalent circle diameter of 5 μm or more can be reduced to 0.50 particles/ ^mm2 or less.

Furthermore, the temperature difference ΔT0 (freezing temperature range) between the liquidus temperature (T _LL ) and the solidus temperature (T _SL ) is 70°C or more, and (Mn, Cr)S-based oxysulfides or (Mn, Cr, Ta)S-based oxysulfides can be generated. The freezing temperature range ΔT0 can be easily calculated from the steel composition using a thermodynamic data collection or commercially available thermodynamic calculation software.

In addition to the above, casting is performed with a controlled cooling rate. By controlling the average cooling rate between 1400 and 700°C near the surface of the cast 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 1 μm or more can be increased to 70% or more. The preferred range for the average cooling rate is 30°C/min or less, and a more preferred range is 15°C/min or less.

After casting, steel plates 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 plate annealing process involves annealing at 750-900°C using standard box annealing. The plate is then pickled to remove surface scale, resulting in hot-rolled steel plate. In the case of cold-rolled steel plate, it is subsequently cold-rolled and final annealed to produce the finished product.

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

In this example, steel with the chemical composition shown in Tables 1A to 1D was melted and continuously cast into 200 mm thick slabs. The cooling rate of the slab 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 slab was heated to 1150°C to 1250°C for 2 hours, and then hot-rolled to produce a 5 mm thick hot-rolled steel sheet. The hot-rolled sheet was annealed at 850°C, pickled, and cold-rolled to a thickness of 3.0 mm. This thin coil was then subjected to final annealing and pickling to produce the cold-rolled product sheet.

Inclusions were measured as follows. First, the steel plate surface was mirror-finished to allow observation.

On the mirror-finished observation surface, inclusions with an equivalent circle diameter of 5 μm or more were randomly analyzed using SEM-EDS to identify the composition of the inclusions. Inclusions containing 5% or more S were defined as oxysulfides. 10 or more oxysulfides were counted in an observation area of ^{100 mm2} . If the number of oxysulfides with an equivalent circle diameter of 5 μm or more in the set observation area was less than 10, the observation area was expanded. In this way, the number density of oxysulfides with an equivalent circle diameter of 5 μm or more and containing 5% or more S was measured.

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). They were then classified into (Mn, Cr)S-based oxysulfides, (Mn, Cr, Ta)S-based oxysulfides, and CaS-based oxysulfides based on the criteria described below, and the number of each was counted to determine the number ratio. The "Ratio of (Mn, Cr)S-based oxysulfides" column in Tables 2 and 3 shows the total ratio of (Mn, Cr)S-based oxysulfides and (Mn, Cr, Ta)S-based oxysulfides.

Note that JIS G 0555:2015, a commonly used method for evaluating inclusions, may consider two or more inclusions located far apart to be a single inclusion depending on the type and distance between them; however, in this example, they were considered to be individual inclusions.

(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 CaS equal to 0 were defined as (Mn, Cr)S-based oxysulfides. Other oxysulfides were defined as CaS-based oxysulfides.

(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 content of 0 were defined as (Mn, Cr, Ta)S-based oxysulfides. All elements other than (Mn, Cr, Ta)S-based oxysulfides and (Mn, Cr)S-based oxysulfides were defined as CaS-based oxysulfides. For Steel No. A28, Table 2 shows the total number ratio of (Mn, Cr)S-based oxysulfides and (Mn, Cr, Ta)S-based oxysulfides.

Rust resistance was evaluated by polishing the material 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 (◎).

As shown in Tables 1A, 1B, and 2, the inventive examples Nos. A1 to A30 had steel chemical compositions and oxysulfide number densities and number ratios that satisfied the ranges set forth in the present invention, and exhibited excellent rust resistance.

On the other hand, as shown in Tables 1C, 1D, and 3, the comparative examples No. 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.

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

In No. a3, the Si content was low, and MnCr oxides were formed, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance.
In No. a4, the Si content was excessive, which resulted in excessive deoxidation, which increased the Ca content in the molten steel, increased the CaS-based oxysulfides, and reduced rust resistance.

In No. a5, the Mn content was low, and the formation of (Mn, Cr)S-based oxysulfides was not promoted, resulting in a decrease in rust resistance.
In No. a6, the Mn content was excessive, and MnCr oxides were formed, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance.

In No. a7, the P content was excessive, which reduced the corrosion resistance of the steel and the rust resistance.
In No. a8, the S content was excessive, and a large amount of oxysulfides was generated, increasing the number density and decreasing the rust resistance.

No. a9 had a low Cr content, which resulted in a decrease in the corrosion resistance of the steel and a decrease in rust resistance.
In No. a10, the Cr content was excessive, and MnCr oxides were formed, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance.

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 resulted in a decrease in the corrosion resistance of the steel and a decrease in rust resistance.

No. a13 had a low Cu content, which resulted in a decrease in the corrosion resistance of the steel and a decrease in rust resistance.
In No. a14, the Cu content was excessive, which reduced the corrosion resistance of the steel and the rusting resistance.

No. a15 had a low N content, which resulted in a decrease in the corrosion resistance of the steel and a decrease in rust resistance.
No. a16 had an excessive N content, which reduced the corrosion resistance of the steel and the rust resistance.

No. a17 had an excessive Mo content, which resulted in poor manufacturability and poor rust resistance.
No. a18 had a low Mo content, which resulted in a decrease in the corrosion resistance of the steel and a decrease in rust resistance.

No. a19 had a low V content, which resulted in a decrease in the corrosion resistance of the steel and a decrease in rust resistance.
In No. a20, the V content was excessive, which caused the precipitates to become coarse, resulting in a decrease in toughness and a decrease in rust resistance.

No. a21 had a low O content, and a large amount of CaS-based oxysulfides was produced, resulting in a decrease in rust resistance.
In No. a22, the O content was excessive, and MnCr oxides were formed, which did not promote the formation of (Mn, Cr)S-based oxysulfides, resulting in a decrease in rust resistance.

In No. a23, the Al content was excessive, and a large amount of CaS-based oxysulfides was produced, resulting in a decrease in 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 number density and a decrease in rust resistance.

No. a25 had a low Ca content, and a large amount of CaS-based oxysulfides were produced, resulting in a decrease in 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.

No. a27 had chemical compositions that satisfied the ranges specified in the present invention, but the casting cooling rate was low, resulting in the formation of large amounts of CaS-based oxysulfides and reduced rust resistance.

Claims (3)

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 (Mn, Cr)S-based oxysulfides is 70% or more and the number ratio of CaS-based oxysulfides is less than 30%,
A martensitic stainless steel sheet, characterized in that the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm2 or ^less on the sheet surface of the steel sheet. 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 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 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 of 0 are defined as (Mn, Cr, Ta)S-based oxysulfides, inclusions containing no 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%,
A martensitic stainless steel sheet, characterized in that the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm2 or ^less on the sheet surface of the steel sheet. The martensitic stainless steel sheet according to claim 1 or 2, characterized in that it further contains, in mass%, one or more elements selected from 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, in place of a portion of Fe.