EP3034642B1

EP3034642B1 - Martensitic stainless steel having excellent wear resistance and corrosion resistance, and method for producing same

Info

Publication number: EP3034642B1
Application number: EP14835747.8A
Authority: EP
Inventors: Shinichi Teraoka; Shunji Sakamoto; Eiichiro Ishimaru; Keiichi Oomura
Original assignee: Nippon Steel and Sumikin Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2013-08-12
Filing date: 2014-08-11
Publication date: 2018-12-19
Anticipated expiration: 2034-08-11
Also published as: EP3034642A4; WO2015022932A1; EP3034642A1; JPWO2015022932A1; JP6353839B2

Description

TECHNICAL FIELD

The present invention relates to martensitic stainless steel having excellent corrosion resistance after quenching or after quenching and tempering, and a method for producing the same. In more detail, the present invention relates to martensitic stainless steel whch is used to produce edged tools such as knives and scissors, loom components, tools, and the like, and has excellent corrosion resistance when having predetermined hardness, and to a method for producing the same.

BACKGROUND ART

According to the classification of ordinary uses of martensitic stainless steel and the types of steel used for each uses, SUS420J1 steel and SUS420J2 steel are generally used for western tableware knives (table knives), scissors, loom components, and tools such as calipers, and SUS440A steel is used for western kitchen knives, fruit knives, and the like requiring higher hardness. In addition, SUS410 steel is generally used for structural members such as disc brakes for a motorcycle and reinforcing bars. In the above-described uses, it is difficult to use plating, paint, or rustproofing oil for rustproofing, and steel needs to have strong abrasion resistance and high hardness, and thus martensitic stainless steel is used. According to the standards of martensitic stainless steel, martensitic stainless steel is classified depending on the amount of C and the amount of Cr into SUS410 containing 0.15% or less of C and 11.5% to 13.5% of Cr, SUS420J1 containing 0.16% to 0.25% of C and 12% to 14% of Cr, SUS420J2 containing 0.26% to 0.40% of C and 12% to 14% of Cr, and SUS440A containing 0.60% to 0.75% of C and 16% to 18% of Cr. As the amount of C increases, higher as-quenched hardness can be obtained. On the other hand, productivity or toughness after quenching degrades. Therefore, generally, SUS410 steel is used in a quenched state, and SUS420 steel is used in a state in which the steel is tempered after being quenched and thus has improved toughness.
Generally, the corrosion resistance of stainless steel is evaluated on the basis of components, and it is known that the corrosion resistance is improved by addition of Cr, Mo, and N. There have been a number of studies regarding the effects of individual elements, and it has been reported that, in martensitic stainless steel as well, the corrosion resistance can be evaluated using the pitting resistance equivalent (PRE=Cr+3.3Mo+16N) and the corrosion resistance improves as this value increases. Furthermore, in some cases, since this steel is polished after being quenched, it is necessary to suppress generation of large-sized inclusions by decreasing the amount of Al or the like, and thus improve abradability.
These findings will be described below with reference to the patent literatures. PTL 1 describes martensitic stainless steel having excellent corrosion resistance and high hardness which contains less than 0.15% of C, 12.0% to 18.5% of Cr, and 0.40% to 0.80% of N.
Nitrogen is an inexpensive element that is not only effective for improving corrosion resistance but also broadens an austenite range, but causes problems in that nitrogen which exceeds the solid solubility limit during melting and casting produces gas bubbles and sound ingot cannot be obtained. The solid solubility limit of nitrogen varies depending on components other than nitrogen or atmospheric pressure of an atmosphere. Cr and C are components having a large influence on the solid solubility limit of nitrogen. In a case in which martensitic stainless steel such as SUS420J1 or SUS420J2 is cast at atmospheric pressure, the amount of nitrogen solved therein is generally reported to be approximately 0.1%. Therefore, in PTL 1, 0.40% or more of nitrogen is solution heat-treated using a pressure casting method. However, the pressure casting method cannot be easily applied to continuous casting and exhibits poor productivity, and thus this method is not suitable for mass production. In addition, regarding pressure casting, there has been a problem of generation of a nitrogen-blowing.
PTL 2 discloses martensitic stainless steel containing 0.15% to 0.50% of C, 0.05% to 3.0% of Cu, 0.05% to 3.0% of Ni, 13.0% to 20.0% of Cr, 0.2% to 4.0% of Mo, 0.30% to 0.80% of N, or the like. In PTL 2, in the method for solution heat-treating nitrogen using the pressure casting method, the amount of N solved therein is increased and the nitrogen-blowing is suppressed by actively adding Mo, Ni, or the like to martensitic stainless steel. This method is considered to improve blowholes during pressure casting, but pressure casting is essential in this method. Thus, continuous casting is difficult, and the problem of poor productivity is not solved. Furthermore, there has been another problem in that addition of Ni, Mo, or the like causes an increase in raw material costs.
A technique for improving the corrosion resistance of martensitic stainless steel without performing the pressure casting method or adding a large amount of Mo, Ni, or the like is disclosed in PTL 3. In PTL 3, 0.03% to 0.25% of C, 0.03% to 0.15% of Sn, and 0.01% to 0.08% of N are added to martensitic stainless steel, and quenched and tempered hardness (hardness after quenching and tempering is performed) is set in a range of 300 HV to 600 HV, thereby obtaining an effect of Sn improving corrosion resistance.
As techniques for obtaining high hardness, there are EN1.4034 steel, EN 1.4110, and the like which are disclosed in NPL 1. EN1.4034 contains 0.43% to 0.50% of C, 12.5% to 14.5% of Cr, 1% or less of Si, 1% or less of Mn, 0.04% or less of P, and 0.015% or less of S. In addition, EN1.411 contains 0.48% to 0.60% of C, 13.0% to 15.0% of Cr, 0.50% to 0.80% of Mo, 0.15% or less of V, 1% or less of Si, 1% or less of Mn, 0.04% or less of P, and 0.015% or less of S. However, even when the amount of C is increased, heating at a high temperature for a long period of time is required to solution-treat carbides. Thus, there is a problem in that productivity of a quenching step is decreased. In addition, in a case in which the cooling rate during quenching is slow, there has been a problem in that precipitation of Cr carbides sensitizes the steel and corrosion resistance degrades.

CITATION LIST

PATENT LITERATURE

[PTL 1] Japanese Unexamined Patent Application, First Publication No. 2002-256397
[PTL 2] Japanese Unexamined Patent Application, First Publication No. 2005-344184
[PTL 3] Japanese Unexamined Patent Application, First Publication No. 2010-215995

NON-PATENT LITERATURE

[NPL 1] European Standards for stainless steel EN10088-2

SUMMARY OF INVENTION

TECHNICAL PROBLEM

As described above, there have been various proposals regarding techniques for improving the corrosion resistance of martensitic stainless steel. However, according to studies by the present inventors, in PTL 1 and 2 described above, the pressure casting method becomes necessary to add N which improves rust resistance. Thus, there have been problems in that these techniques cannot be easily applied to continuous casting and have disadvantages in terms of productivity. In addition, regarding pressure casting as well, a nitrogen-blowing is easily generated, and it becomes necessary to increase the solid solubility limit of nitrogen by adding Mo, Ni, or the like. Thus, there has been a problem of an increase in alloy costs.
Furthermore, in the method described in PTL 3, since the amount of C is within the range of SUS420J1, the range of as-quenched hardness increased by C is small. Therefore, particularly, when quenching is performed with a slow cooling rate, there has been a problem in that it is difficult to obtain quenched and tempered hardness exceeding 550 HV. In addition, when it is attempted to solution heat-treat a relatively small amount of C fully so as to increase hardness, heating at a high temperature for a long period of time becomes necessary to solution-treat carbonitrides, and consequently, there has been another problem in that γ grains are coarsened and quenched and tempered toughness (toughness after quenching and tempering is performed) is decreased. Therefore, the method described in PTL 3 has not been suitable for uses requiring higher hardness.
In high-carbon martensitic stainless steel as described in NPL 1, it is difficult to solution-treat carbides fully (solution heat-treat the carbides in the steel), and there are carbides remaining without being solution heat-treated even after heating at a high temperature for a long period of time is performed. Therefore, a decrease in quenched and tempered toughness attributed to γ grains being coarsened does not easily occur. There has been another problem in that carbonitrides coarsened during annealing of a hot-rolled plate are not capable of rapidly being solution-treated during hardening heating and as-quenched hardness commensurate with the amount of C cannot be easily obtained, or another problem in that steel is easily sensitized in a hardening cooling step and consequently corrosion resistance degrades.
Generally, the corrosion resistance of stainless steel is significantly affected by components thereof, and the corrosion resistance of stainless steel is evaluated using the pitting resistance equivalent (PRE=Cr+3.3Mo+16N) or the like. Stainless steel having a higher value of the pitting resistance equivalent has more favorable corrosion resistance. At this time, corrosion resistance refers to corrosion resistance to an environment of a neutral aqueous solution of a chloride, and as an evaluation method, for example, methods of pitting potential measurement for stainless steels regulated by JIS G 0577:2014, methods of salt spray testing regulated by JIS Z 2371:2000, or the like, can be used. However, in daily indoor environments excluding chemical and food plants, water storage tanks such as water heaters, and a seashore environment, stainless steel, is barely exposed to a high concentration of an aqueous solution of a chloride, and sufficient corrosion resistance can be obtained with an amount of Cr of approximately 13% as in SUS420J1 steel used for table knives. In addition, in disc brakes for a motorcycle, sufficient corrosion resistance can be obtained with 12% of Cr.
However, in some cases, corrosion resistance commensurate with components of a base material cannot be obtained. A typical cause for the deterioration of corrosion resistance is sensitization. This phenomenon refers to a phenomenon in which, in a case in which a stainless steel material is welded or the like, Cr carbides are precipitated due to the welding temperature history of the stainless steel material, Cr-depleted zones are generated in the matrix around the carbides, and consequently, corrosion resistance is impaired. It is known that, in a welded portion of SUS430 or in a case in which SUS304 is used for a long period of time at a temperature in a range of 650°C to 700°C, sensitization occurs.
While the sensitization phenomenon in martensitic stainless steel is not well known, it is assumed that martensitic stainless steel is also sensitized from the fact that, when a commercially available knife is subjected to a salt spray test, significant rust is observed. Martensitic stainless steel has property of self-hardening, and, even with air hardening, as-quenched hardness comparable with that obtainable through water quenching can be obtained. Thus, in many cases, martensitic stainless steel is quenched with a slow cooling rate. Accordingly, it is assumed that Cr carbides are precipitated in a cooling step with a slow cooling rate and sensitization occurs. In stainless steel, sensitization is further accelerated as the amount of C increases, and thus, in EN1.4034 steel, EN1.411 steel, and SUS440 steel, sensitization easily occurs. Therefore, there has been a desire for a technique for suppressing sensitization of high-carbon martensitic stainless steel.
In a step for producing martensitic stainless steel, in order to enhance workability before quenching, it is necessary to soften steel by precipitating carbonitrides through sufficient annealing. During hardening heating, it becomes necessary to accelerate the solution treatment of the carbonitrides. In EN1.4034 steel, EN1.411 steel, and SUS440 steel in which the amount of C is increased in order to obtain high hardness, the carbonitrides are coarsened during annealing, and a high temperature and a long period of time are required to solution-treat the carbonitrides. Therefore, suppression of coarsening of the carbonitrides and acceleration of the solution treatment are desired.
The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide martensitic stainless steel having excellent corrosion resistance at low cost.

SOLUTION TO PROBLEM

In order to achieve the above-described object, regarding the sensitization phenomenon in high-carbon martensitic stainless steel, the present inventors investigated precipitation of carbonitrides or a solution treatment phenomenon. As a result, it was found that addition of a small amount of Sn and addition of an appropriate amount of N to the amount of C suppress the sensitization phenomenon in martensitic stainless steel and improve corrosion resistance. In addition, it was also found that the solution treatment during hardening heating is accelerated, higher as-quenched hardness can be obtained with heating at a relatively lower temperature for a shorter period of time than those for steel of the related art, and tempered toughness is also improved.
Summaries of the investigation are as described below.

(1) Martensitic stainless steel having excellent corrosion resistance consisting of: by mass%, C: 0.40% to 0.50%; Si: 0.25% to 0.60%; Mn: 2.0% or less; P: 0.035% or less; S: 0.010% or less; Cr: 11.0% to 15.5%; Ni: 0.01% to 0.60%; Cu: 0.50% or less; Mo: 0.10% or less; Sn: 0.005% to 0.10%; V: 0.10% or less; Al: 0.03% or less; N: 0.01% to 0.05%; and optionally one or more of Nb: 0.005% to 0.05%; Ti: 0.005% to 0.05%; Zr: 0.005% to 0.05%; and B: 0.0005% to 0.0030%, and a remainder including Fe and inevitable impurities, in which amounts of C, N, and Sn satisfy Expression (1): $S value = 16 \times Sn / C + 2 \times N / C \geq 0.40$
in which C, N, and Sn in the above expression represent amounts thereof (by mass%), respectively.
(2) A method for producing martensitic stainless steel including: casting a steel having a composition of the martensitic stainless steel according to (1) to obtain an ingot; heating the obtained ingot at a temperature in a range of 1140°C to 1240°C and then hot-rolling the heated ingot to obtain a hot-rolled plate; coiling the obtained hot-rolled plate; tempering the coiled hot-rolled plate at a temperature in a range of 700°C to 900°C for four hours; and holding the tempered hot-rolled plate in a temperature range of 950°C to 1100°C for 5 seconds to 10 minutes in a nitrogen atmosphere and then quenching the hot-rolled plate,
wherein the quenching is air hardening, and
a finishing temperature of the hot rolling is 800°C or higher, and a coiling temperature of the hot-rolled plate is in a range of 700°C to 900°C.

ADVANTAGEOUS EFFECTS OF INVENTION

In the present invention, 0.005% to 0.10% of Sn is added to high-carbon martensitic stainless steel, and the amount of N is balanced according to the amounts of C and Sn. Therefore, it is possible to prevent sensitization at a slow hardening cooling rate (slow cooling) as in air hardening. In addition, solution treatment during hardening heating is accelerated, and thus it becomes possible to improve the productivity of quenching. According to the present invention, there is no need for a special casting facility for pressure casting or the like for producing. In addition, corrosion resistance can be improved only by addition of a small amount of Sn without adding expensive elements such as Mo, Ni, Cu, and the like. Thus, alloy costs are also relatively inexpensive. As described above, according to the present invention, it is possible to provide martensitic stainless steel having excellent corrosion resistance at low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating the influence of the amounts of Sn and N added according to the amount of C on corrosion resistance.
FIG. 2 is a view illustrating the influence of the amounts of Sn and N added according to the amount of C on as-quenched hardness.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described in detail.
The present inventors performed a number of studies regarding corrosion resistance of high-carbon martensitic stainless steel after quenching. As a result, the present inventors found that corrosion resistance after quenching is significantly poorer than the corrosion resistance of ordinary stainless steel commensurate with the amount of Cr and performed various studies regarding a method for improving corrosion resistance. In addition, the present inventors performed detailed studies regarding precipitation of carbonitrides during annealing of high-carbon martensitic stainless steel, the growth process of the carbonitrides, and a process of solution treatment of carbonitrides during hardening heating. As a result, it was found that addition of a small amount of Sn had a significant influence on behavior regarding the precipitation, growth, and solution treatment of carbonitrides. For these phenomena, a common action mechanism was considered. That is, Sn is an element that easily segregates in crystal grain boundaries or interfaces between precipitates and a matrix. In a material in which carbonitrides are precipitated in a cooling process of quenching and which is easily sensitized such as high-carbon martensitic stainless steel, when Sn is added thereto, Sn segregates in the interface between the carbonitrides and the matrix in a hardening cooling process. In addition, since the segregated Sn inhibits the precipitation and growth of the carbonitrides, formation of Cr-depleted zones is delayed, and sensitization is suppressed, and thus corrosion resistance is improved. However, Sn is an element that deteriorates the hot workability of the base material and also degrades high-temperature aging embrittlement characteristics (makes steel easily embrittled when the steel is used at a high temperature for a long period of time). Thus, there is an optimal range for the amount of Sn added. An effect of suppressing sensitization is obtained when the amount of Sn added is 0.005% or higher. On the other hand, addition of more than 0.10% of Sn degrades the hot workability of high-carbon martensitic stainless steel and causes not only cracks during hot-rolling but also aging embrittlement. Therefore, it is necessary to set the additive amount of Sn to 0.1% or less. Similar to the effect of Sn suppressing sensitization, addition of Sn inhibits the growth of the carbonitrides in an annealing step and thus makes the carbonitrides finer. Therefore, solution treatment during hardening heating is accelerated, and, compared with steel to which no Sn is added, high hardness can be obtained with heating at a relatively low temperature for a short period of time. This effect inhibits softening in an annealing process. Since the annealing process is generally performed over a long period of time using a box annealing furnace including a coil, the effect of Sn making the carbonitrde finer is produced, but the amount of the carbonitrides precipitated does not change, and thus hardness after quenching is barely affected. That is, addition of Sn barely decreases hardness after annealing.
On the basis of the above-described findings, the present embodiment describes the optical component balance of martensitic stainless steel in the above-described uses. The reasons for limiting individual components will be described below. In the following description, the unit "%" for the amounts of individual elements indicates "mass%" unless particularly otherwise described.

C: 0.40% to 0.50%

C is an element that dominates as-quenched hardness (hardness after quenching). In order to stably obtain a Vickers hardness of 550 Hv or higher required for high-carbon martensitic stainless steel, it is necessary to set the amount of C to 0.40% or higher. Addition of excess C accelerates sensitization during quenching and thus impairs corrosion resistance, and also degrades toughness after quenching due to carbonitrides that is not solution heat-treated. Therefore, the amount of C is set to 0.50% or lower. When degradation of hardness or toughness due to changes in hardening heating conditions is taken into account, the amount of C is desirably set in a range of 0.42% to 0.48%.

Si: 0.25% to 0.60%

Si is required for deoxidization during melting and refining and is also effective for suppressing generation of oxide scales during a hardening thermal treatment (hardening heating). Therefore, the amount of Si is set to 0.25% or higher. However, Si narrows the austenite single-phase region temperature and impairs quenching stability. Therefore, the amount of Si is set to 0.60% or lower. In order to reduce the proportion of defects generated by oxide-based inclusions, the amount of Si is desirably set to 0.30% or higher. In addition, since Si narrows the austenite single-phase region temperature and impairs quenching stability, the amount of Si is desirably set to 0.50% or lower.

Mn: 2.0% or lower

Mn is an austenite-stabilizing element, while Mn accelerates generation of oxide scales during a hardening thermal treatment (hardening heating) and increases the subsequent polishing load. Therefore, the upper limit of the amount of Mn is set to 2.0%. When degradation of corrosion resistance due to coarsening of sulfide-based inclusions such as MnS is taken into account, the amount of Mn is desirably set to 1.0% or lower. In addition, since Mn is also included in other alloy raw materials, and it is difficult to further reduce the amount of Mn, the amount of Mn is preferably set to 0.10% or higher.

P: 0.035% or lower

P is an element included as an impurity in hot metal or an alloy such as ferrochromium which is a raw material. Since P is a harmful element to the toughness of a hot-rolled and annealed plate and to the toughness thereof after quenching, the amount of P is set to 0.035% or lower. P is also an element that degrades workability, and thus the amount of P is desirably set to 0.030% or lower. In addition, an excess decrease in the amount of P creates a necessity of a high-purity raw material for producing steel, which leads to an increase in costs. Therefore, the lower limit of the amount of P is preferably set to 0.010%.

S: 0.010% or lower

S is an element only a small amount of which is solution heat-treated in an austenite phase, and which segregates in grain boundaries and thus accelerates degradation of hot workability. When the amount of S exceeds 0.010%, the influence of the above-described action becomes significant, and thus the amount of S is set to 0.010% or lower. A decrease in the amount of S decreases the amount of sulfide-based inclusions generated and improves corrosion resistance, while a desulfurization load for decreasing the amount of S is increased (a step and a facility for desulfurization become necessary), and production costs are increased. Therefore, the lower limit of the amount of S is preferably set to 0.001%. The amount of S is preferably in a range of 0.001% to 0.008%.

Cr: 11.0% to 15.5%

The amount of Cr needs to be at least 11.0% in order to maintain corrosion resistance required for principal uses of martensitic stainless steel. In order to prevent generation of retained austenite after quenching, the upper limit of the amount of Cr is set to 15.5%. In order to make the above-described characteristics more effective, the range of the amount of Cr is preferably set in a range of 12.0% to 14.0%.

Ni: 0.01% to 0.60%

Ni is an austenite-stabilizing element like Mn. During hardening heating, C, N, Mn, and the like are removed from the surface layer portion through decarburization, denitrification, or oxidization, and there are cases in which ferrite is generated in the surface layer. Since Ni is highly resistant to oxidization, there are no cases in which C, N, Mn, and the like are removed from the surface layer. Thus, Ni is highly effective for stabilizing an austenite phase. This effect begins to appear at an amount of Ni of 0.01%, and thus the amount of Ni is set to 0.01% or higher. However, since Ni is an expensive raw material, the amount of Ni is set to 0.60% or lower. On the other hand, since addition of a large amount of Ni creates a concern that press formability obtained by solid solution strengthening in a hot-rolled and annealed plate may degrade, the upper limit of the amount of Ni is desirably set to 0.30%. In addition, when an effect of Ni homogenizing formation of scales during quenching is also taken into account, the lower limit of the amount of Ni is desirably set to 0.05%.

Cu: 0.50% or less

In many cases, Cu is inevitably contained in steel by being mixed into the steel from scraps during melting. In addition, there are also cases in which Cu is intentionally added to steel in order to increase austenite stability. However, inclusion of excess Cu degrades hot workability or corrosion resistance, and thus the amount of Cu is set to 0.50% or lower. In some cases, Cu precipitates during quenching and tempering, impairs the soundness of a passivation film, and thus degrades corrosion resistance. Therefore, the amount of Cu is preferably set to 0.20% or lower. In order to decrease the amount of Cu inevitably mixed into steel, a high-purity raw material is essentially required to produce steel, which leads to an increase in raw material costs. Therefore, the amount of Cu is preferably set to 0.01% or higher.

V: 0.10% or lower

In many cases, V is inevitably mixed into steel from ferrochromium which is an alloy raw material. Since V has a strong action of narrowing the austenite single-phase region temperature, the amount of V is set to 0.10% or lower. In addition, V is an element highly capable of forming a carbide, and in Cr carbonitrides including a V-based carbide as a nucleus, there is a tendency that solution treatment of the Cr carbonitrides is delayed. Therefore, the amount of V is preferably set to 0.08% or lower. In addition, since it is difficult to decrease the amount of V mixed into steel as an inevitable impurity, the lower limit of the amount of V is preferably set to 0.01%. When productivity or the production cost is collectively taken into account, the amount of V is preferably set in a range of 0.03% to 0.07%.

Mo: 0.10% or lower

Mo is an element effective for improving corrosion resistance. However, similar to Cr and Si, Mo is an element stabilizing a ferrite phase, and there are problems in that addition of Mo narrows the hardening heating temperature range and non-transformed ferrite is generated after the quenching. Furthermore, since Mo enhances tempering softening resistance (suppresses softening by means of tempering), addition of Mo deteriorates productivity. For example, the annealing time of a hot-rolled plate is extended. Therefore, the upper limit of the amount of Mo is set to 0.10%. While being an expensive element, Mo is not effective for suppressing sensitization and for common uses, does not easily produce an effect of improving corrosion resistance commensurate with costs. Therefore, the amount of Mo is preferably set to 0.05% or lower. In addition, since it is difficult to avoid Mo mixed into steel from a raw material, the amount of Mo is preferably set to 0.01% or higher.

Al: 0.03% or lower

Al is an effective element for deoxidization. However, in some cases, Al increases the basicity of slags, precipitates CaS as water-soluble inclusions in steel, and degrades corrosion resistance. Therefore, the upper limit of the amount of Al is set to 0.03%. In addition, when degradation of polishing properties due to alumina-based non-metallic inclusions is taken into account, the amount of Al is preferably set to 0.01% or lower. However, in order to obtain a deoxidization effect of a combination of Si and Mn, the amount of Al is preferably set to 0.003% or higher.

N: 0.01% to 0.05%

N has an effect of increasing as-quenched hardness like C. In addition, as another effect that C does not have, N improves corrosion resistance by means of the following two actions. The first one is an action of strengthening a passivation film and the second one is an action of suppressing the precipitation of Cr carbides (suppressing the generation and growth of Cr-depleted zones). In order to obtain the above-described effects, the amount of N is set to 0.01% or higher. However, addition of excess N generates blowholes during casting at atmospheric pressure, and thus the amount of N is set to 0.05% or lower. Regarding an effect of N suppressing sensitization, the optical range of the amount of N varies depending on the amount of Sn added. Since Sn is an expensive element, it is preferable to set the amount of Sn added to the lowest level, and thereby to suppress an increase in the raw material cost. Therefore, in order to suppress sensitization together with a small amount of Sn, the amount of N is preferably set to 0.025% or higher. In addition, since N increases the hardness of a hot-rolled and annealed plate and thus degrades workability, the amount of N is preferably set to 0.035% or lower.

Sn: 0.005% to 0.10%

Sn is a segregated element which is concentrated in not only crystal grain boundaries in the matrix but also interfaces between precipitates and the matrix. Thus, Sn suppresses the growth and coarsening of the precipitates. Therefore, addition of Sn suppresses sensitization in a hardening cooling step, and thus an effect of improving corrosion resistance is obtained. Since this effect can be reliably obtained by setting the amount of Sn to 0.005%, the lower limit of the amount of Sn is set to 0.005%. However, it is known that the solid solubility limit of Sn in an austenite phase is low, and, in plain carbon steel, Sn causes cracks during hot-rolling or defects. In addition, when steel is aged at a temperature in a range of 400°C to 700°C for a long period of time, there are cases in which the toughness of steel degrades. Thus, the amount of Sn is desirably decreased as much as possible. In ferritic stainless steel, Sn has a relatively large solid solubility limit, and thus, in certain types of ferritic stainless steel, similar to Cr or Mo, 0.1% or more of Sn is added thereto in order to strengthen a passivation film by actively adding Sn to steel. However, martensitic stainless steel is austenite in a producing process thereof or during hardening heating. In addition, addition of Sn degrades hot workability, and, when steel is used in a high-temperature environment, aging embrittlement occurs. Therefore, there is an optimal range for the amount of Sn added. The limit amount of Sn at which hot workability and high-temperature aging embrittlement characteristics are not deteriorated varies depending on the types of steel. In high-carbon martensitic stainless steel, the upper limit of the amount of Sn is 0.1%.
In order to suppress sensitization by optimizing the balance between N and Sn and stably obtain favorable corrosion resistance, the amount of Sn is preferably set to 0.01% or higher. In addition, in order to prevent high-temperature aging embrittlement without being affected by tempering conditions, the amount of Sn is preferably set to 0.05% or lower. $S value = 16 \times Sn / C + 2 \times N / C \geq 0.40 %$
Sn and N have an effect of suppressing sensitization caused by precipitation of Cr carbides in a hardening cooling process. However, this effect varies depending on the amount of C and thus is not consistent. The present inventors studied the optimal balance between Sn, C, and N in high-carbon martensitic stainless steel having an as-quenched hardness of higher than 550 HV. That is, hot-rolled plates having a plate thickness of 6 mm were produced in a laboratory using steels in which 13.3% Cr-0.4% Si-0.5% Mn-0.027% P-0.001% S-0.005% Al-0.05% V-0.02% Mo-0.02% Cu steel was used as a base composition, the amount of C was changed in a range of 0.40% to 0.50%, the amount of N was changed in a range of 0.01% to 0.05%, and the amount of Sn was changed in a range of 0.000% to 0.20%, respectively. Specifically, hot-rolled plates were produced by heating ingots having a thickness of 100 mm at 1240°C and then hot-rolling the ingots to a plate thickness of 6 mm. The hot-rolled plates were box-annealed at 850°C for four hours, thereby obtaining hot-rolled and annealed plates. These hot-rolled and annealed plates were held at 1050°C for 10 minutes, and then were air-hardened (air cooling, slow cooling), and the surfaces thereof were polished using a grain size #600 (JIS R 6001:1998 (corresponding to ISO 8486-1:1996 and ISO 8486-2:1996)). On each of the samples obtained as described above, a salt spray test regulated by JIS Z 2371:2000 (based on ISO 9227:1990) was performed for 24 hours, and the degree of rust was visually evaluated. Samples having no rust were evaluated as A (PASS), samples having rust spots were evaluated as B (FAIL), and samples having a number of rust flows were evaluated as C (FAIL). That is, samples in which rust was generated were evaluated as FAIL. The results are illustrated in FIG. 1. In FIG. 1, the horizontal axis indicates the S value of Expression (1), and the vertical axis sequentially indicates A (PASS), B (FAIL), and C (FAIL) from the bottom. It was found that samples satisfying Expression (1) were evaluated as PASS in terms of corrosion resistance. Here, individual element names in the expression such as C, N, and Sn represent amounts (mass%) of the respective elements.
Furthermore, after the same hot-rolled and annealed plates were held at 1050°C for one minute and were air-hardened, hardness (as-quenched hardness) thereof was measured. The relationship between the hardness and the S value is illustrated in FIG. 2. According to FIG. 2, it was found that the as-quenched hardness increases as the S value increases and, when the S value is set to 0.40% or higher, the as-quenched hardness reaches 550 HV or higher. From these results, it was found that, in high-carbon martensitic stainless steel, an as-quenched hardness of 550 HV or higher can be obtained with a relatively short high-temperature holding time. In addition, it was found that a component range in which degradation of corrosion resistance after quenching caused by slow cooling (air hardening) does not occur and an as-quenched hardness of 550 HV can be obtained can be regulated using the S value. In order to develop the above-described effects, the S value may be lower than 2.0, and the effects are saturated even when the S value exceeds 4.25.
In addition to the above-mentioned elements, the high-carbon martensitic stainless steel according to the present embodiment preferably includes one or more of Nb: 0.005% to 0.05%, Ti: 0.005% to 0.05%, Zr: 0.005% to 0.05%, and B: 0.0005% to 0.0030%. Alternatively, it is preferable to control the upper limit of the amount of one or more of these elements to the above-described value by using a high-purity raw material. The reasons for limiting these components will be described below.

Nb: 0.005% to 0.05%

Nb has an action of making Cr carbonitrides finer and accelerating solution treatment during hardening heating by being precipitated during hot rolling in a form of fine Nb(C, N) and acting as a precipitation nucleus of the Cr carbonitrides. Therefore, Nb is preferably added to steel as necessary. Since this effect is developed when 0.005% or more of Nb is added to steel, the lower limit of the amount of Nb is preferably set to 0.005%. However, when excess Nb is added to steel, there are cases in which, in a temperature region higher than heating temperature for hot rolling, coarse Nb(C, N) is precipitated, and defects resulting from inclusions are generated. Therefore, the upper limit of the amount of Nb is preferably set to 0.05%. The amount of Nb is more preferably set in a range of 0.01% to 0.03%.

Ti: 0.005% to 0.05%

Ti has an action of making Cr carbonitrides finer and accelerating solution treatment during hardening heating by being precipitated during hot rolling in a form of fine Ti(C, N) and acting as a precipitation nucleus of the Cr carbonitrides. Therefore, Ti is preferably added to steel as necessary. Since this effect is developed when 0.005% or more of Ti is added to steel, the lower limit of the amount of Ti is preferably set to 0.005%. However, when excess Ti is added to steel, there are cases in which, in a temperature region higher than heating temperature for hot rolling, coarse TiN is precipitated, and defects resulting from inclusions are generated. Therefore, the upper limit of the amount of Ti is preferably set to 0.05%. The amount of Ti is more preferably set in a range of 0.01% to 0.03%.

Zr: 0.005% to 0.05%

Zr has an action of making Cr carbonitrides finer and accelerating solution treatment during hardening heating by being precipitated during hot rolling in a form of fine Zr(C, N) and acting as a precipitation nucleus of the Cr carbonitrides. Therefore, Zr is preferably added to steel as necessary. Since this effect is developed when 0.005% or more of Zr is added to steel, the lower limit of the amount of Zr is preferably set to 0.005%. However, when excess Zr is added to steel, there are cases in which, in a temperature region higher than heating temperature for hot rolling, coarse Zr(C, N) is precipitated, and defects resulting from inclusions are generated. Therefore, the upper limit of the amount of Zr is preferably set to 0.05%. The amount of Zr is more preferably set in a range of 0.01% to 0.03%.

B: 0.0005% to 0.0030%

Since B improves high-temperature ductility during hot rolling and suppresses a decrease in yield caused by edge cracks in a hot-rolled plate, B may be added to steel as necessary. In order to develop the above-described effect, the lower limit of the amount of B is desirably set to 0.0005%. However, when excess B is added to steel, toughness or corrosion resistance is impaired due to precipitation of Cr₂B and (Cr, Fe)₂₃(C, B)₆. Therefore, the upper limit of the amount of B is set to 0.0030%. When workability or production cost is taken into account, the amount of B is more desirably set in a range of 0.0008% to 0.0015%.
The martensitic stainless steel according to the present embodiment is preferably produced by casting steel having the above-described composition, hot-rolling the obtained ingot so as to obtain a hot-rolled plate, coiling the hot-rolled plate, tempering (annealing) the coiled hot-rolled plate, and quenching the tempered hot-rolled plate. In this producing method, it is desirable that the heating temperature during the hot rolling is set in a range of 1140°C to 1240°C, the coiling temperature is set in a range of 700°C to 840°C, and the hot-rolled plate is annealed using a batch furnace at a temperature in a range of 700°C to 900°C for four hours or longer.
That is, when the heating temperature for the hot rolling is higher than 1240°C, a γ single phase turns into a two-phase region of γ+δ. Cr, Si, and the like are concentrated in the δ phase, C, N, Ni, and the like negatively segregate in the δ phase, and the δ phase inhibits formation of the γ single phase during quenching. Therefore, hardenability is impaired. Conversely, when the heating temperature for the hot rolling is lower than 1 140°C, a soaking time as a diffusion time for eliminating the segregation (solidification segregation) needs to be two hours or longer, and thus the productivity of hot rolling is significantly impaired, which is not preferable. The finishing temperature (temperature during finishing rolling) of the hot rolling is set to 800°C or higher. At a temperature lower than 800°C, cracks during hot-rolling are likely to be generated. When the finishing temperature of the hot rolling is lower than 800°C, the coiling temperature is lowered, and thus the subsequent annealing time of the hot-rolled plate is extended, and productivity degrades.
In addition, during the coiling of a steel strip (the hot-rolled plate obtained through hot rolling) after the hot rolling, the coiling temperature is set in a range of 700°C to 900°C. When the steel strip is coiled at a temperature lower than 700°C, the metallographic structures become significantly different between the coolest portion and the hottest portion in a coil, this structural difference is not eliminated even after the coiled hot-rolled plate is annealed, and the material qualities become inconsistent in the coil, which is not preferable. When the coiling temperature is set to 700°C or higher, during the cooling of the coil, the carbide is precipitated and coarsened, and the hot-rolled plate is softened. In addition, when coiling temperature is higher than 900°C, thick oxide scales are formed on the surface, and problems such as degradation of corrosion resistance due to formation of a decarbonized phase or poor polishing properties after quenching are caused, which is not desirable.
Next, regarding the annealing conditions of the hot-rolled plate, in order to improve workability before quenching, it is necessary to soften the hot-rolled plate by means of annealing. In a continuous annealing furnace, it is not possible to ensure a sufficient annealing time for softening. Thus, it is desirable to hold the hot-rolled plate in a temperature region of 700°C to 900°C for four hours or longer using a batch furnace. When the temperature is lower than 700°C or higher than 900°C, the hot-rolled plate is not sufficiently softened. That is, when the hot-rolled plate is annealed at a temperature higher than 900°C for a long period of time, due to the influence of an atmosphere gas, nitrification or decarburization of the surface layer causes an inconsistent metallographic structure of the surface layer or a change in material qualities, which is not preferable. In addition, when the hot-rolled plate is annealed for shorter than four hours, the inconsistent temperature in the coil causes a change in material qualities in the coil.
The hot-rolled plate turns into a hot-rolled product by being pickled after the annealing, but a part of the annealed hot-rolled plate turns into a cold-rolled product by being cold-rolled and annealed.
As a hardening thermal treatment (hardening heating) of the product, the product is held at a temperature region of 950°C to 1100°C for 5 seconds to 10 minutes in a nitrogen atmosphere. After that, the product is quenched by air hardening. At a heating temperature of lower than 950°C, solution treatment of the carbonitrides becomes insufficient, and thus the target as-quenched hardness cannot be obtained. When the heating temperature is set to 950°C or higher, it becomes possible to solution-treat the carbonitrides, and a metallographic structure mainly including austenite can be obtained. In addition, when the heating temperature increases, delta ferrite is precipitated in an austenite matrix, and corrosion resistance or hardenability is impaired. Therefore, the heating time is desirably set to 1100°C or lower. Regarding the heating time (holding time) at this time as well, the time needs to be five seconds or longer in order to accelerate the solution treatment. When the time is shorter than five seconds, the amount of the solution heat-treated C and N is small, and sufficient hardness cannot be obtained. On the other hand, when the time is 10 minutes or longer, the oxidization of the surface progresses, and decarburization of the surface layer causes degradations of corrosion resistance and hardness after quenching, which is not preferable. In addition, the cooling rate of quenching is preferably in a range of 3°C/sec to 100°C/sec.

EXAMPLES

Steels having chemical compositions (mass%) shown in Tables 1 and 2 were melted in a vacuum melting furnace and then were cast in an inert gas atmosphere, in detail, in a nitrogen atmosphere at atmospheric pressure, thereby obtaining ingots having a thickness of 100 mm and a weight of 50 kg. The ingots were self-hardened and could not be easily processed, and thus the ingots were tempered by being held at 850°C for four hours and then being cooled in a furnace. After casting skins of the ingots were removed by means of polishing, the ingots were heated at 1220°C and held for one hour. After that, the ingots were hot-rolled to a plate thickness of 6 mm, thereby obtaining hot-rolled plates. In this hot rolling, the finishing temperature was set to 900°C, and the hot-rolled plates were coiled at 800°C. The coiled hot-rolled plates were, subsequently, held at 850°C for four hours and then were cooled in the furnace, thereby being tempered. Hot-rolled plates having a crack with a deepness of 1 mm or more on the end surface were determined as FAIL since edge cracks were generated therein. The results are shown in the note column of Tables 3 and 4. Edge cracks having a deepness of less than1 mm were determined as slightly edge cracks. In addition, the hardness after annealing (after tempering) was measured using a method described in JIS Z 2245:2011 (based on ISO 6508-1:2005). Hot-rolled plates having a hardness after annealing of higher than 92 HRB were determined as FAIL since the plates were hard. The results are shown in the note column of Tables 3 and 4.

The tempered hot-rolled plates were, subsequently, held at 1050°C in a thermal treatment furnace having a nitrogen atmosphere for 10 minutes, were ejected from the furnace, and then were air-hardened, thereby obtaining quenched steel plates. Using the obtained quenched steel plates as test specimens, the as-quenched hardness and the corrosion resistance were evaluated by the following method. The results are shown in Table 3 and 4. A test specimen (No. 40) was oil-quenched, thereby obtaining a quenched steel plate. In Table 1 to 4, numerical values outside the ranges regulated by the present embodiment were underlined.

[Table 1]

Classification	Steel No.	Chemical composition (unit: mass%, remainder: Fe and inevitable impurities)															S value
Classification	Steel No.	C	Si	Mn	P	S	Cr	Ni	Cu	Sn	Mo	V	Al	N	Nb, Ti, Zr	B	S value
Steels of the present invention	1	0.400	0.30	1.45	0.017	0.008	12.2	0.06	0.48	0.010	0.02	0.04	0.003	0.030			0.55
	2	0.500	0.32	0.44	0.027	0.001	12.3	0.12	0.01	0.010	0.01	0.05	0.002	0.020			0.40
	3	0.430	0.36	1.52	0.012	0.003	12.4	0.05	0.06	0.040	0.03	0.03	0.004	0.025			1.60
	4	0.500	0.39	1.50	0.018	0.002	12.1	0.09	0.00	0.060	0.02	0.02	0.002	0.025			2.02
	5	0.430	0.25	0.85	0.010	0.008	13.2	0.15	0.03	0.050	0.03	0.01	0.003	0.027			1.99
	6	0.420	0.60	0.90	0.027	0.002	12.5	0.10	0.25	0.040	0.02	0.01	0.010	0.025			1.64
	7	0.410	0.45	2.00	0.007	0.008	13.2	0.06	0.25	0.060	0.02	0.00	0.003	0.024			2.46
	8	0.440	0.25	0.65	0.028	0.009	11.0	0.25	0.20	0.080	0.02	0.05	0.020	0.023			3.01
	9	0.450	0.60	0.78	0.027	0.007	15.5	0.06	0.25	0.040	0.00	0.04	0.008	0.025			1.53
	10	0.450	0.56	0.10	0.026	0.006	13.8	0.60	0.05	0.050	0.02	0.07	0.003	0.032			1.92
	11	0.452	0.32	0.55	0.010	0.005	12.8	0.20	0.50	0.050	0.03	0.03	0.004	0.035			1.92
	12	0.400	0.30	0.53	0.017	0.003	12.7	0.21	0.02	0.005	0.04	0.02	0.003	0.050			0.45
	13	0.450	0.36	0.58	0.012	0.008	13.2	0.01	0.10	0.100	0.05	0.01	0.003	0.037			3.72
	14	0.480	0.32	1.15	0.025	0.007	13.5	0.01	0.03	0.080	0.10	0.02	0.006	0.035			2.81
	15	0.420	0.29	1.80	0.035	0.006	14.5	0.30	0.02	0.060	0.02	0.10	0.030	0.010			2.33
	16	0.410	0.36	0.42	0.027	0.005	13.2	0.07	0.05	0.070	0.03	0.05	0.030	0.020			2.83
	17	0.430	0.36	1.52	0.029	0.003	12.4	0.05	0.06	0.040	0.03	0.03	0.004	0.025	0.05Nb		1.60
	18	0.430	0.36	1.52	0.029	0.003	12.4	0.05	0.06	0.040	0.03	0.03	0.004	0.025		0.0005	1.60
	19	0.430	0.36	1.52	0.029	0.003	12.4	0.05	0.06	0.040	0.03	0.03	0.004	0.025	0.05Zr, 0.005Nb		1.60
	20	0.430	0.36	1.52	0.029	0.003	12.4	0.05	0.06	0.040	0.03	0.03	0.004	0.025	0.05Ti	0.0010	1.60
	21	0.430	0.36	1.52	0.029	0.003	12.4	0.05	0.06	0.040	0.03	0.03	0.004	0.025	0.05Nb	0.0030	1.60

[Table 2]

Classification	Steel No.	Chemical composition (unit: mass%, remainder: Fe and inevitable impurities)															S value
Classification	Steel No.	C	Si	Mn	P	S	Cr	Ni	Cu	Sn	Mo	V	Al	N	Nb, Ti, Zr	B	S value
	24	0.450	0.10	0.84	0.010	0.009	12.9	0.25	0.30	0.003	0.02	0.05	0.008	0.022			0.20
	25	0.450	1.00	0.84	0.030	0.007	13.1	0.23	0.33	0.002	0.02	0.02	0.003	0.023			0.17
	26	0.460	0.31	2.50	0.025	0.005	11.6	0.15	0.01	0.001	0.04	0.01	0.003	0.035			0.19
	27	0.440	0.32	0.84	0.027	0.006	10.7	0.22	0.05	0.002	0.02	0.01	0.006	0.025			0.19
	28	0.420	0.31	0.84	0.028	0.006	15.6	0.20	0.08	0.050	0.05	0.00	0.003	0.025			2.02
	29	0.410	0.25	0.74	0.027	0.005	11.8	1.00	0.10	0.050	0.02	0.05	0.009	0.029			2.09
	30	0.480	0.55	1.45	0.025	0.007	12.1	0.07	1.00	0.050	0.03	0.04	0.004	0.008			1.70
	31	0.470	0.30	0.80	0.029	0.008	11.5	0.11	0.20	0.200	0.03	0.07	0.009	0.025			6.91
Comparative steels	32	0.420	0.58	0.80	0.025	0.009	11.5	0.20	0.21	0.002	0.02	0.08	0.003	0.025			0.20
	33	0.410	0.45	1.45	0.025	0.001	12.5	0.07	0.06	0.050	0.50	0.15	0.003	0.025			2.07
	34	0.458	0.60	1.45	0.026	0.005	12.4	0.15	0.08	0.002	0.02	0.02	0.045	0.035			0.22
	35	0.480	0.52	1.45	0.028	0.009	12.3	0.29	0.10	0.050	0.03	0.03	0.003	0.006			1.69
	36	0.410	0.60	1.45	0.029	0.007	12.1	0.30	0.12	0.050	0.02	0.04	0.003	0.055			2.22
	37	0.390	0.31	1.43	0.016	0.007	12.2	0.06	0.47	0.010	0.02	0.04	0.002	0.030			0.56
	38	0.530	0.33	0.43	0.026	0.001	12.3	0.12	0.02	0.010	0.01	0.05	0.003	0.030			0.42
	39	0.480	0.32	0.44	0.025	0.001	12.2	0.12	0.02	0.009	0.01	0.04	0.003	0.020			0.38
	40	0.452	0.32	0.55	0.010	0.005	12.8	0.20	0.50	0.050	0.03	0.03	0.004	0.035			1.92

Hardness

On a section along the thickness of each of the plates, hardness was measured using an applied load (testing force) of 49 N on the basis of the Vickers hardness test regulated by JIS Z 2244:2009 (based on ISO 6507-1:2005 and ISO 6507-4:2005). Hardness of 550 HV or higher were determined as PASS.

Corrosion resistance

The surface of each of the quenched specimens (quenched steel plates) was polished using a milling machine so as to be flattened, was polished using sandpaper, and then was buffed, thereby being mirror-finished. On each of the specimens, the salt spray test regulated by JIS Z 2371:2000 was performed, and the presence or absence of rust was evaluated. Specimens having no rust were determined as PASS. Specimens having defects on the finished surface were determined as FAIL.

Toughness (DBTT)

On each of the materials before quenching (tempered hot-rolled plates), a Charpy impact test regulated by JIS Z 2242:2005 (based on ISO/DIS 148-1:2003) was performed, and the ductile-brittle transition temperature (DBTT) was measured. In each test, using a sub-size test specimen having a V-notch and its original plate thickness (approximately 6 mm) as a test specimen, DBTT was evaluated. Test specimens having a DBTT of 50°C or lower were determined as PASS.

[Table 3]

Classification	Steel No.	After quenching		Toughness before quenching (DBTT [°C])	Note
Classification	Steel No.	Corrosion resistance	Hardness HV5	Toughness before quenching (DBTT [°C])	Note
	1	Favorable	555	10
	2	Favorable	615	0
	3	Favorable	595	-10
	4	Favorable	610	5
	5	Favorable	555	-5
	6	Favorable	560	-10
	7	Favorable	565	0
	8	Favorable	562	-5
	9	Favorable	574	0
Steels of the present invention	10	Favorable	582	0
	11	Favorable	565	-5
	12	Favorable	571	-10
	13	Favorable	580	-10
	14	Favorable	590	0
	15	Favorable	595	5
	16	Favorable	620	0
	17	Favorable	610	-20
	18	Favorable	590	0	No slightly edge cracks
	19	Favorable	609	10
	20	Favorable	603	20	No slightly edge cracks
	21	Favorable	605	5	No slightly edge cracks

[Table 4]

Classification	Steel No.	After quenching		Toughness before quenching (DBTT [°C])	Note
Classification	Steel No.	Corrosion resistance	Hardness HV5	Toughness before quenching (DBTT [°C])	Note
	24	Poor	540	0	Due to insufficient deoxidization, a number of inclusions and poor polishing properties
	25	Poor	525	-5	Due to retained ferrite, poor hardenability
	26	Poor	520	5	Due to an increase in the amount of quenched scales, poor polishing properties
	27	Poor	500	10
	28	Favorable	526	-10	Due to retained ferrite, poor hardenability
	29	Favorable	558	10	Poor due to the high hardness of the hot-rolled and annealed plate
Comparative steels	30	Favorable	565	5	Generation of edge cracks while being hot
	31	Favorable	582	100
	32	Poor	540	10
	33	Favorable	556	15	Not employable due to an increase in raw material costs
	34	Poor	530	10	Due to hard inclusions, poor polishing properties
	35	Poor	567	-10
	36	Favorable	582	-20	Poor due to defects due to blowhole
	37	Favorable	530	5
	38	Poor	620	5
	39	Poor	605	5
	40	Favorable	590	-5	The same components as in Steel No. 11, Oil-quenched, poor shape

As is clear from the results shown in Table 3, in the steels of the present invention, the hardness after quenching was 550 Hv or higher, and rust was not generated due to addition of Sn in the salt spray test after the air hardening. This fact shows that the steel of the present invention has excellent corrosion resistance in a practical environment. In contrast, in the comparative steels outside the ranges of the present embodiment, as is clear from the results in Table 4, the corrosion resistance, the as-quenched hardness, and the toughness before quenching were insufficient or other characteristics (the raw material costs and the hot workability) were poor. As described above, the comparative steels were determined as FAIL in terms of productivity, qualities, and/or costs. That is, for Nos. 24 to 27, 32, and 34, the S values were low, and the corrosion resistance and the as-quenched hardness were poor. In addition, in No. 24, the amount of Si was small, and the polishing properties were poor due to insufficient deoxidization. In No. 25, since the amount of Si was large, retained ferrite was generated. In No. 26, since the amount of Mn was large, quenched scales became thick, and the polishing properties were poor. In No. 27, the amount of Cr was small, and the corrosion resistance was poor. In No. 34, since the amount of Al was large, the polishing properties were poor. In addition, in No. 28, the amount of Cr was large, and the hardness was low due to retained ferrite. In No. 29, the amount of Ni was large, the hardness after hot rolling and annealing was 92 HRB, and the steel was hard. In No. 30, the amount of Cu was large, and edge cracks were generated on the edge surface of the hot-rolled plate. In No. 31, since the amount of Sn was large, the toughness of the hot-rolled and annealed plate was decreased. In No. 35, since the amount of N was small, the corrosion resistance was poor. In No. 36, since the amount of N was large, defects due to blowhole were observed on the polished surface, and the steel was determined as a poor material. In No. 37, the amount of C was below the lower limit, and the as-quenched hardness was low. In No. 38, the amount of C exceeded the upper limit, and the corrosion resistance was poor. In No. 39, the S value was below the lower limit, and the corrosion resistance was poor. In No. 40, the steel having components identical to those in No. 11 was oil-quenched, and thus the as-quenched hardness was low.
In addition, regarding the steels of the present invention as well, in Nos. 17, 19 and 20, the as-quenched hardness was slightly increased by adding Nb, Zr, and Ti compared with the invention steel No. 3 having the same amounts of C, N, and Sn and the same S value. In addition, in Nos. 18, 20, and 21, the hot workability was improved by adding B, and even edge cracks having a deepness of less than1 mm were not observed.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to produce martensitic stainless steel having high hardness and excellent corrosion resistance at low cost with favorable productivity without adding a large amount of an expensive element such as Mo. Therefore, the present invention contributes to significant improvement of the producing costs and qualities of stainless steel for table knives, stainless kitchen knives, tools, and disc brakes for a motorcycle.

Claims

Martensitic stainless steel having excellent corrosion resistance consisting of: by mass%,
C: 0.40% to 0.50%;

Si: 0.25% to 0.60%;

Mn: 2.0% or less;

P: 0.035% or less;

S: 0.010% or less;

Cr: 11.0% to 15.5%;

Ni: 0.01% to 0.60%;

Cu: 0.50% or less;

Mo: 0.10% or less;

Sn: 0.005% to 0.10%;

V: 0.10% or less;

Al: 0.03% or less;

N: 0.01% to 0.05%; and

optionally one or more of:
Nb: 0.005% to 0.05%;

Ti: 0.005% to 0.05%;

Zr: 0.005% to 0.05%; and

B: 0.0005% to 0.0030%, and

a remainder including Fe and inevitable impurities,

wherein amounts of C, N, and Sn satisfy Expression (1): $S value = 16 \times Sn / C + 2 \times N / C \geq 0.40$
in which C, N, and Sn in the above expression represent amounts thereof (by mass%), respectively.
A method for producing martensitic stainless steel, comprising:
casting a steel having a composition of the martensitic stainless steel according to Claim 1 to obtain an ingot;

heating the obtained ingot at a temperature in a range of 1140°C to 1240°C and then hot-rolling the heated ingot to obtain a hot-rolled plate;

coiling the obtained hot-rolled plate;

tempering the coiled hot-rolled plate at a temperature in a range of 700°C to 900°C for four hours; and

holding the tempered hot-rolled plate in a temperature range of 950°C to 1100°C for 5 seconds to 10 minutes in a nitrogen atmosphere and then quenching the hot-rolled plate, wherein the quenching is air hardening, a finishing temperature of the hot rolling is 800°C or higher, and a coiling temperature of the hot-rolled plate is in a range of 700°C to 900°C.