WO2015022932A1 - Martensitic stainless steel having excellent wear resistance and corrosion resistance, and method for producing same - Google Patents

Abstract

This martensitic stainless steel having excellent corrosion resistance has a steel composition that is composed, in mass%, of 0.40-0.50% of C, 0.25-0.60% of Si, 2.0% or less of Mn, 0.035% or less of P, 0.010% or less of S, 11.0-15.5% of Cr, 0.01-0.60% of Ni, 0.50% or less of Cu, 0.10% or less of Mo, 0.005-0.10% of Sn, 0.10% or less of V, 0.03% or less of Al, 0.01-0.05% of N and the balance made up of Fe and unavoidable impurities. In this connection, the ranges of C, N and Sn satisfy formula (1). S value = 16 × Sn/C + 2 × N/C ≥ 0.40% (1)

Description

Martensitic stainless steel excellent in wear resistance and corrosion resistance and method for producing the same

The present invention relates to a martensitic stainless steel excellent in corrosion resistance after quenching or after quenching and tempering and a method for producing the same. More specifically, the present invention relates to a martensitic stainless steel having excellent corrosion resistance when used for the manufacture of blades such as knives and scissors, loom parts, tools, and the like, and a method of manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2013-167780 filed in Japan on August 12, 2013, the contents of which are incorporated herein by reference.

The general use of martensitic stainless steel and the types of steel used in each application are simply classified. For tools such as table knife, scissors, loom parts and calipers, SUS420J1 and SUS420J2 steel are used. SUS440A steel is used in Western knives and fruit knives that require higher hardness. SUS410 steel is generally used for structural members such as two-wheel disc brakes and reinforcing bars. In such applications, martensitic stainless steel is used because it is difficult to use rust-preventive plating, coating, and rust-preventing oil, and it must be resistant to wear and have high hardness. . These martensitic stainless steel standards categorize martensitic stainless steel according to the amount of C and Cr. C: 0.15% or less for SUS410, Cr: 11.5 to 13.5%, C for SUS420J1 : 0.16 to 0.25%, Cr: 12 to 14%, C: 0.26 to 0.40% for SUS420J2, Cr: 12 to 14%, C: 0.60 to 0.75% for SUS440A, Cr: Classified as 16-18%. The higher the amount of C, the higher the quenching hardness. On the other hand, manufacturability and toughness after quenching are reduced. For this reason, in general, SUS410 steel is used in a quenched state, and SUS420 steel is used in a state in which toughness is improved by tempering after quenching.

The corrosion resistance of these stainless steels is generally evaluated based on the components, and it is known that the corrosion resistance is improved by the addition of Cr, Mo, and N. Many studies have been made on the effect of each element, and it is reported that martensitic stainless steel can also evaluate corrosion resistance with the pitting corrosion resistance index PRE = Cr + 3.3Mo + 16N, and the larger this value, the better the corrosion resistance. Moreover, since the said steel may be grind | polished after quenching and used, it is also necessary to suppress generation | occurrence | production of a large-sized inclusion and to improve polishability by reducing content, such as Al.

These findings will be explained in the patent literature. First, Patent Document 1 discloses a high-hardness martensite system excellent in corrosion resistance, containing C: less than 0.15%, Cr: 12.0 to 18.5%, and N: 0.40% to 0.80%. Stainless steel is described.

Nitrogen is effective in improving corrosion resistance, and is an inexpensive element that expands the austenite range, but nitrogen that exceeds the solid solubility limit during melting and casting creates bubbles, and a healthy steel ingot cannot be obtained. . The solid solubility limit of nitrogen varies depending on components other than nitrogen and the atmospheric pressure. Components having a great influence on the solid solubility limit of nitrogen are Cr and C. When martensitic stainless steels such as SUS420J1 and SUS420J2 are cast under atmospheric pressure, the amount of nitrogen dissolved is generally reported to be about 0.1%. Therefore, in Patent Document 1, 0.40% or more of nitrogen is solid-dissolved by the pressure casting method. However, the pressure casting method is difficult to apply to continuous casting, and the productivity is low, so it is not suitable for mass production. In addition, there is a problem that nitrogen blow occurs with pressure casting.

Therefore, in Patent Document 2, C: 0.15% to 0.50%, Cu: 0.05% to 3.0%, Ni: 0.05% to 3.0%, Cr13.0 % To 20.0%, Mo: 0.2% to 4.0%, N: 0.30% to 0.80% and the like martensitic stainless steel is disclosed. In Patent Document 2, by adding Mo, Ni or the like to martensitic stainless steel positively, the amount of N dissolved in the method of solid solution of nitrogen using the pressure casting method is increased, and nitrogen blowing is suppressed. It is supposed to be done. Although this method seems to improve the blow hole in pressure casting, since this method requires pressure casting, continuous casting is difficult and the problem of low productivity has not been solved. Furthermore, there has been a problem of an increase in raw material costs due to the addition of Ni, Mo and the like.

On the other hand, Patent Document 3 discloses a technique for improving the corrosion resistance of martensitic stainless steel without performing a pressure casting method and without adding a large amount of Mo, Ni, or the like. In Patent Document 3, martensitic stainless steel contains C: 0.03 to 0.25%, Sn: 0.03% to 0.15%, N: 0.01 to 0.08%. The effect of improving the corrosion resistance by Sn is obtained by setting the quenching and tempering hardness (hardness after quenching and tempering) to 300 to 600 HV.

Examples of techniques for obtaining high hardness include EN1.4034 steel and EN1.4110 disclosed in Non-Patent Document 1. EN1.4034: C: 0.43% to 0.50%, Cr: 12.5% to 14.5%, Si: 1%, Mn: 1%, P: 0.04% , S: 0.015% or less is contained. EN1.411 is C: 0.48% to 0.60%, Cr: 13.0% to 15.0%, Mo: 0.50% to 0.80%, V: 0.00. 15% or less, Si: 1% or less, Mn: 1% or less, P: 0.04% or less, and S: 0.015% or less. However, even if the amount of C is simply increased, heating and heating for a long time are required for solutionization of the carbide, and there is a problem that productivity of the quenching process is lowered. Moreover, when the cooling rate at the time of quenching is slow, sensitization due to precipitation of Cr carbide occurs, and there is a problem that the corrosion resistance is lowered.

Japanese Patent Laid-Open No. 2002-256397 JP 2005-344184 A JP 2010-215995 A

As described above, various techniques for improving the corrosion resistance of martensitic stainless steel have been proposed. However, in the studies by the present inventors, in Patent Documents 1 and 2 mentioned above, a pressure casting method is required for the addition of N for improving the weather resistance, so that the application to continuous casting is not possible. The problem was that it was difficult and productivity was difficult. Further, with regard to pressure casting, nitrogen blow is likely to occur, and it is necessary to increase the solid solubility limit of nitrogen by adding Mo, Ni or the like, so that an increase in alloy cost has been a problem.

Furthermore, in the method described in Patent Document 3, since the amount of C is in the range of SUS420J1 steel, the increase in quenching hardness due to C is small. Therefore, there has been a problem that it is difficult to obtain a quenching and tempering hardness exceeding 550 HV, particularly when quenching is performed under slow cooling conditions. Also, when trying to increase the hardness by completely dissolving a relatively small amount of C, high temperature and long time heating is required for solutionization of carbonitride, resulting in coarsening of γ grains and quenching and tempering toughness. There was also a problem that (toughness after quenching and tempering) was lowered. Therefore, the method described in Patent Document 3 is unsuitable for applications requiring higher hardness.

In high carbon martensitic stainless steel as described in Non-Patent Document 1, it is difficult to completely dissolve carbides (dissolve in steel), and it is not possible to perform heating at high temperature for a long time. Solid solution carbides exist. For this reason, the quenching and tempering toughness due to the coarsening of the γ grains hardly occurs. On the other hand, carbonitrides coarsened during annealing of hot-rolled sheets are slow to form during quenching heating, and it is difficult to obtain quenching hardness commensurate with the amount of C, and sensitization easily occurs during quenching and cooling. As a result, there was a problem that the corrosion resistance was lowered.

Generally, the corrosion resistance of stainless steel is greatly influenced by its components, and the corrosion resistance of stainless steel is evaluated by a pitting corrosion resistance index PRE = Cr + 3.3Mo + 16N or the like. A stainless steel having a higher numerical value of the pitting corrosion resistance index has higher corrosion resistance. The corrosion resistance at this time is corrosion resistance against a neutral chloride aqueous solution environment. As an evaluation method, for example, a method for measuring pitting corrosion potential of stainless steel specified in JIS G 0577: 2014, or specified in JIS Z 2371: 2000. The salt spray test method to be used is used. However, stainless steel is very unlikely to be exposed to high-concentration chloride aqueous solutions in applications other than those used in water storage tanks such as chemical / food plants and water heaters, and in the beach environment, that is, in everyday indoor environments. As SUS420J1 steel is used as an instrument knife, sufficient corrosion resistance can be obtained with a Cr amount of about 13%. Further, in a two-wheel disc brake, sufficient corrosion resistance can be obtained with 12% Cr.

However, the corrosion resistance assumed based on the components of the base material may not be obtained. Sensitization is a typical cause of corrosion resistance deterioration. This phenomenon is a phenomenon in which, for example, when a stainless steel material is welded, Cr carbide precipitates due to the welding temperature history, and a Cr-deficient layer is formed on the matrix around the carbide, thereby impairing corrosion resistance. It is known that sensitization occurs at the welded part of SUS430 or when SUS304 is used at 650-700 ° C. for a long time.

Although the sensitization phenomenon of martensitic stainless steel is not well known, it is speculated that sensitization has occurred because remarkable rust is observed when a commercial knife is subjected to a salt spray test. Martensitic stainless steel is self-hardening, and quenching is often performed under mild cooling conditions because quenching hardness comparable to water quenching can be obtained even with air quenching. For this reason, it is presumed that Cr carbide precipitates and is sensitized during the cooling process under slow cooling conditions. Sensitization in stainless steel is promoted with a steel type having a larger amount of C. Therefore, EN1.4034 steel, EN1.411 steel, SUS440 series steel types and the like are likely to be sensitized. Therefore, a technique for suppressing sensitization of high carbon martensitic stainless steel has been desired.

In the manufacturing process of martensitic stainless steel, in order to improve the workability before quenching, it is necessary to perform sufficient annealing to precipitate carbonitride and soften the steel. On the other hand, it is necessary to promote solutionization of carbonitride during quenching heating. In EN1.4034 steel, EN1.411 steel, SUS440 steel grades, etc. with increased C content to obtain high hardness, carbonitrides become coarse during annealing, requiring high temperature and long time for solution treatment. Become. For this reason, the technique which suppresses the coarsening of carbonitride and accelerates solution formation was also desired.
This invention is made | formed in view of the said fact, The objective is to provide the martensitic stainless steel excellent in corrosion resistance at low cost.

In order to achieve the above object, the present inventors investigated carbonitride precipitation and solution phenomenon in relation to the sensitization phenomenon of high carbon martensitic stainless steel. As a result, it was found that the addition of a small amount of Sn and the addition of N in an optimum amount relative to the amount of C suppresses the sensitization phenomenon of martensitic stainless steel and improves the corrosion resistance. Moreover, the solution solution at the time of quenching heating progressed, and the knowledge that higher quenching hardness was obtained by heating at a relatively low temperature and in a short time as compared with conventional steel, and tempering toughness was also improved was obtained.

The gist is as follows.
(1) 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 %: 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: 0.01-0.05%, balance Fe and steel composition consisting of inevitable impurities, C, N and Sn range Is a martensitic stainless steel excellent in corrosion resistance satisfying the formula (1).
S value = 16 × Sn / C + 2 × N / C ≧ 0.40% (1)
However, in said formula, each element name C, N, and Sn represents content (mass%) of each element.
(2) Further, by mass%, Nb: 0.005% to 0.05%, Ti: 0.005% to 0.05%, Zr: 0.005% to 0.05%, B: The martensitic stainless steel excellent in corrosion resistance according to the above (1), which contains one or more of 0.0005% or more and 0.0030% or less.
(3) Obtaining an ingot having the composition of martensitic stainless steel according to (1) or (2) by casting, and heating the obtained ingot to 1140 to 1240 ° C. for hot rolling. The hot-rolled sheet thus obtained is wound, the wound hot-rolled sheet is tempered at 700 to 900 ° C. for 4 hours, and the tempered hot-rolled sheet is heated to 950 to 1100 ° C. A method for producing martensitic stainless steel, which is kept in a temperature range for 5 seconds to 10 minutes and then quenched.
(4) The method for producing martensitic stainless steel according to (3), wherein the quenching is air quenching.
(5) The method for producing martensitic stainless steel according to (3) or (4), wherein a finishing temperature of the hot rolling is 800 ° C. or higher, and a winding temperature of the hot-rolled sheet is 700 to 900 ° C. .

In the present invention, 0.005 to 0.10% of Sn is added to high carbon martensitic stainless steel, and the N balance (N amount) corresponding to the amount of C and Sn is made. Thereby, sensitization at a slow quenching cooling rate (slow cooling) such as air quenching can be prevented. Moreover, it becomes possible to improve the productivity of quenching by promoting solution formation during quenching heating. According to the present invention, no special casting equipment such as pressure casting is required for production. In addition, since the corrosion resistance can be improved only by adding a small amount of Sn without adding an expensive element such as Mo, Ni, or Cu, the alloy cost is relatively low. Thus, according to the present invention, martensitic stainless steel having excellent corrosion resistance can be provided at low cost.

It is a figure which shows the influence which Sn and N addition amount according to C amount have on corrosion resistance. It is a figure which shows the influence which Sn and N addition amount according to the amount of C exert on quenching hardness.

Hereinafter, embodiments of the present invention will be described in detail.

The present inventors have made many investigations on the corrosion resistance of high-carbon martensitic stainless steel after quenching. As a result, it was found that the corrosion resistance after quenching was significantly lower than the corrosion resistance of general stainless steel assumed in terms of Cr amount, and various methods for improving it were studied. At the same time, the present inventors conducted a detailed study on carbonitride precipitation and growth process during annealing of high carbon martensitic stainless steel, and carbonitride solutionizing process during quenching heating. As a result, it has been found that the addition of a small amount of Sn greatly affects the behavior of carbonitride precipitation, growth, and solution treatment. A common mechanism of action was considered for these phenomena. That is, Sn is an element that easily segregates at the grain boundaries and the interface between the precipitate and the parent phase. When Sn is added to a material such as high carbon martensitic stainless steel, in which carbonitride precipitates during the quenching cooling process and is easily sensitized, Sn segregates at the interface between the carbonitride and the parent phase during quenching cooling. Then, segregated Sn inhibits the growth of carbonitride, thereby delaying the formation of a Cr-deficient layer, suppressing sensitization and improving corrosion resistance. However, Sn is an element that degrades the hot workability of the base material and also reduces the high temperature aging embrittlement characteristics (making steel easily embrittled when used for a long time at a high temperature), so the addition amount is in an optimal range. There is. The effect of suppressing sensitization can be obtained by adding 0.005% or more of Sn. On the other hand, the addition of Sn exceeding 0.10% reduces the hot workability of the high carbon martensitic stainless steel and causes hot-rolling cracking, and also causes aging embrittlement. For this reason, it is necessary to make the addition amount of Sn 0.1% or less. Similar to the effect of suppressing sensitization by Sn, the addition of Sn inhibits the growth of carbonitride in the annealing process and refines the carbonitride. For this reason, solution forming at the time of quenching heating is promoted, and high hardness can be obtained by heating at a relatively low temperature for a short time with respect to Sn-free steel. This effect inhibits softening during the annealing process. In general, since the annealing process is performed over a long period of time using a box annealing furnace containing a coil, the amount of carbonitride precipitation is not changed, although there is an effect of refinement of carbonitride by Sn. Almost no influence on the later hardness. That is, the hardness after annealing hardly decreases by the addition of Sn.

Based on the above knowledge, the present embodiment has found the optimum component balance of martensitic stainless steel in the above-mentioned applications. The reason for limitation of each component is demonstrated below. In the following description, “%” indicating the content of each element indicates “mass%” unless otherwise specified.

C: 0.40 to 0.50%
C is an element that controls the quenching hardness (hardness after quenching). In order to stably obtain a Vickers hardness of 550 Hv or higher, which is required as a high carbon martensitic stainless steel, the C content needs to be 0.40% or higher. On the other hand, when excessively added, sensitization at the time of quenching is promoted and the corrosion resistance is impaired, and the toughness after quenching is also lowered by the undissolved carbonitride. Therefore, the C content is 0.50% or less. In consideration of a decrease in hardness and toughness due to fluctuations in quenching heating conditions, it is desirable that the C content be 0.42% or more and 0.48% or less.

Si: 0.25 to 0.60%
In addition to being necessary for deoxidation during melting and refining, Si is also effective in suppressing oxide scale formation during quenching heat treatment (quenching heating). For this reason, content of Si shall be 0.25% or more. However, Si narrows the austenite single phase temperature range and impairs quenching stability. For this reason, content of Si shall be 0.60% or less. In order to reduce the generation rate of soot due to oxide inclusions, the Si content is desirably 0.30% or more. Moreover, since Si narrows an austenite single-phase temperature range and impairs quenching stability, it is desirable that the Si content be 0.50% or less.

Mn: 2.0% or less Although Mn is an austenite stabilizing element, it promotes the generation of oxide scale during quenching heat treatment (quenching heating) and increases the subsequent polishing load. For this reason, 2.0% is made the upper limit. Considering the decrease in corrosion resistance due to the coarsening of sulfide inclusions such as MnS, the Mn content is desirably 1.0% or less. Further, Mn is also contained in other alloy raw materials, and it is difficult to further reduce it. Therefore, the content is preferably made 0.10% or more.

P: 0.035% or less P is an element contained as an impurity in a raw material alloy such as hot metal or ferrochrome. Since P is an element harmful to the toughness of the hot-rolled annealed sheet and the toughness after quenching, the P content is set to 0.035% or less. Since P is also an element that reduces workability, it is desirable to make it 0.030% or less. Moreover, excessive reduction leads to an increase in cost, such as requiring high-purity raw materials for production. For this reason, it is preferable that the minimum of content of P shall be 0.010%.

S: 0.010% or less S is an element that has a small amount of solid solution with respect to the austenite phase and segregates at the grain boundaries to promote a decrease in hot workability. If the S content exceeds 0.010%, the effect of such action becomes significant, so the content is made 0.010% or less. As the S content is reduced, the sulfide inclusions are reduced and the corrosion resistance is improved. However, when the S content is reduced, the desulfurization load increases (requires a process and equipment for desulfurization) and the production cost increases. Therefore, the lower limit is preferably 0.001%. Preferably, the content is 0.001% to 0.008%.

Cr: 11.0 to 15.5%
Cr is required to be at least 11.0% or more in order to maintain the corrosion resistance required in the main application of martensitic stainless steel. On the other hand, in order to prevent the formation of retained austenite after quenching, the upper limit is 15.5%. In order to make these characteristics more effective, the Cr range is preferably 12.0 to 14.0%.

Ni: 0.01 to 0.60%
Ni is an austenite stabilizing element like Mn. During quenching heating, C, N, Mn, and the like may be reduced from the surface layer portion by decarburization, denitrification, or oxidation, and ferrite may be generated in the surface layer portion. Since Ni has high oxidation resistance, C, N, Mn and the like do not decrease from the surface layer, and are very effective for stabilizing the austenite phase. Since the effect appears from 0.01%, the Ni content is set to 0.01% or more. However, since Ni is an expensive material, it is made 0.60% or less. On the other hand, since a large amount of Ni may cause a decrease in press formability due to solid solution strengthening in the hot-rolled annealed sheet, the upper limit is desirably set to 0.30%. In consideration of the effect of Ni that makes the scale formation uniform during quenching, the lower limit is preferably 0.05%.

Cu: 0.50% or less Cu is often inevitably contained due to contamination from scrap during melting. Moreover, it may be added intentionally in order to increase the austenite stability. However, excessive content decreases hot workability and corrosion resistance, so the Cu content is 0.50% or less. Cu may precipitate during quenching and tempering and may deteriorate the corrosion resistance by impairing the soundness of the passive film. For this reason, it is preferable to make Cu content 0.20% or less. On the other hand, reducing the amount of Cu inevitably mixed requires a high-purity raw material for production, leading to an increase in raw material cost. For this reason, it is preferable to make it 0.01% or more.

V: 0.10% or less V is often inevitably mixed from ferrochrome or the like which is an alloy raw material. V has a strong effect of narrowing the austenite single-phase temperature range, so the V content is 0.10% or less. Further, V is an element having a high carbide forming ability, and in the case of Cr carbonitride having a V-based carbide as a nucleus, there is a tendency that solutionization is delayed. Therefore, it is preferable to make it 0.08% or less. Moreover, since it is difficult to reduce the amount mixed as an inevitable impurity, the lower limit is preferably set to 0.01%. In consideration of manufacturability and manufacturing cost, it is desirable that the content be 0.03% to 0.07%.

Mo: 0.10% or less Mo is an element effective for improving corrosion resistance. However, Mo is an element that stabilizes the ferrite phase in the same manner as Cr and Si, and there is a problem that the addition of Mo narrows the quenching heating temperature range and causes untransformed ferrite after quenching. Furthermore, since Mo increases temper softening resistance (suppresses softening due to tempering), the addition of Mo deteriorates manufacturability, such as increasing the annealing time of hot-rolled sheets. For this reason, the upper limit is made 0.10%. Although Mo is an expensive element, it is not effective in suppressing sensitization, and in general uses, it is difficult to obtain an effect of improving corrosion resistance commensurate with cost. Therefore, the Mo content is preferably 0.05% or less. Moreover, since it is difficult to avoid mixing from a raw material, it is preferable to set it as 0.01% or more.

Al: 0.03% or less Al is an effective element for deoxidation. However, Al increases the basicity of the slag, may precipitate water-soluble inclusions CaS in the steel, and may reduce the corrosion resistance. For this reason, 0.03% is made an upper limit. In consideration of a decrease in abrasiveness due to alumina-based nonmetallic inclusions, the Al content is preferably 0.01% or less. However, in order to obtain the deoxidation effect by the combination with Si and Mn, the content is preferably 0.003% or more.

N: 0.01% to 0.05%
N, like C, has the effect of increasing the quenching hardness. Further, as an effect different from C, N improves the corrosion resistance by the following two actions. The first function is to strengthen the passive film, and the other function is to suppress the precipitation of Cr carbide (to suppress the formation and growth of the Cr-deficient layer). In order to obtain these effects, the N content is set to 0.01% or more. However, since excessive addition causes blowholes during casting under atmospheric pressure, the N content is 0.05% or less. The optimum range of the sensitization suppression effect by N varies depending on the amount of Sn added. Sn is an expensive element, and it is preferable to suppress the increase in raw material cost by keeping it to a minimum. Therefore, in order to suppress sensitization on the premise of a small amount of Sn, the N content is preferably 0.025% or more. Moreover, since N raises the hardness of a hot-rolled annealing board and reduces workability, it is preferable to make N content into 0.035% or less.

Sn: 0.005 to 0.10%
Sn is a segregating element and concentrates not only at the crystal grain boundary of the matrix but also at the interface between the precipitate and the matrix, thereby suppressing the growth and coarsening of the precipitate. For this reason, since the sensitization in the quenching and cooling process is suppressed by the addition of Sn, the effect of improving the corrosion resistance can be obtained. Since the effect is reliably obtained by setting the Sn content to 0.005%, the lower limit is made 0.005%. However, Sn has a small solid solubility limit in the austenite phase and is known to cause hot rolling cracks and flaws in ordinary steel. Further, when aged for a long time at 400 to 700 ° C., the toughness of the steel may be lowered, and it is desirable that the Sn content be reduced as much as possible. In ferritic stainless steel, Sn has a relatively large solid solubility limit, so 0.1% or more is added for the purpose of strengthening the passive film similarly to Cr and Mo by actively adding Sn. There are also steel grades. On the other hand, martensitic stainless steel is austenite during the production process and quenching heating. Moreover, hot workability falls by Sn addition, and when used in a high temperature environment, aging embrittlement arises. For this reason, there is an optimum range for the amount of Sn added. The limit Sn amount that does not deteriorate the hot workability and high temperature aging embrittlement characteristics varies depending on the steel type. In the high carbon martensitic stainless steel, the upper limit of the Sn content is 0.1%. In order to suppress sensitization by optimizing the balance of N and Sn and to stably obtain good corrosion resistance, the Sn content is preferably 0.01% or more. Moreover, in order to prevent high temperature aging embrittlement without being influenced by tempering conditions, it is preferable to make it 0.05% or less.

S value = 16 × Sn / C + 2 × N / C ≧ 0.40% (1)
Sn and N have an effect of suppressing sensitization caused by the precipitation of Cr carbide during the quenching and cooling process. However, the effect is not uniform because it varies depending on the amount of C. The inventors examined the optimum balance of Sn, C, and N in a high carbon martensitic stainless steel having a quenching hardness exceeding 550 HV. That is, 13.3% Cr-0.4% Si-0.5% Mn-0.027% P-0.001% S-0.005% Al-0.05% V-0.02% Mo- Using 0.02% Cu steel as the base composition, the C content was varied from 0.40 to 0.50%, N was varied from 0.01% to 0.05%, and Sn was varied from 0.000% to 0.20%. Using steel, a hot-rolled sheet having a thickness of 6 mm was manufactured in a laboratory. Specifically, a hot-rolled sheet was manufactured by heating a steel ingot having a thickness of 100 mm to 1240 ° C. and then hot rolling to a sheet thickness of 6 mm. The hot-rolled sheet was subjected to box annealing at 850 ° C. for 4 hours to obtain a hot-rolled sheet. After this hot-rolled annealed sheet was held at 1050 ° C. for 10 minutes, it was quenched by air (air cooling, slow cooling), and the surface was subjected to grain size # 600 (JIS R 6001: 1998 (ISO 8486-1: 1996 and ISO 8486-2). : Corresponding to 1996))). The sample obtained in this manner was subjected to a salt spray test defined in JIS Z 2371: 2000 (based on ISO 9227: 1990) for 24 hours, and the degree of rust was visually evaluated. The thing which does not rust was made into A: pass, the point rust was made into B: rejection, and the thing with many flow rust was made into C: rejection. That is, the rusted product was rejected. The result is shown in FIG. In FIG. 1, the horizontal axis is the S value of the above formula (1), and the vertical axis is A: pass, B: fail, C: fail from the bottom. It can be seen that those satisfying the above expression (1) have acceptable corrosion resistance. However, in the above formula, each element name C, N, Sn is the content (mass%) of each element.

Moreover, after holding the same hot-rolled annealing plate at 1050 ° C. for 1 minute and air quenching, hardness (quenching hardness) was measured. The relationship between the hardness and the S value is shown in FIG. According to FIG. 2, it was found that the quenching hardness increased with the increase of the S value, and the S value was increased to 550 HV or higher by increasing the S value to 0.40% or higher. From these results, it is understood that a quenching hardness of 550 HV or higher can be obtained in a high carbon martensitic stainless steel with a relatively short high temperature holding time. Moreover, it turns out that the corrosion resistance fall after quenching resulting from slow cooling (air quenching) does not arise, and the component range which can make quenching hardness 550HV can be prescribed | regulated by S value. In order to exhibit the effect, the S value may be less than 2.0, and even if it exceeds 4.25, the effect is saturated.

In addition to the above elements, the high-carbon martensitic stainless steel according to the present embodiment includes Nb: 0.005% to 0.05%, Ti: 0.005% to 0.05%, Zr: It is preferable to contain one or more of 0.005% to 0.05% and B: 0.0005% to 0.0030%. Or it is preferable to restrict | limit the upper limit of content of 1 or more types of these elements to said value by using a high purity raw material. The reason for limitation of these components is demonstrated below.

Nb: 0.005 to 0.05%
Nb finely precipitates as Nb (C, N) during hot rolling and becomes a precipitation nucleus of Cr carbonitride, thereby having a function of refining Cr carbonitride and promoting solution formation during quenching heating. For this reason, it is preferable to add as needed. Since this effect is manifested at 0.005% or more, the lower limit is preferably made 0.005%. However, when excessively added, coarse Nb (C, N) may precipitate in a temperature range equal to or higher than the hot rolling heating temperature, resulting in inclusion-based soot. For this reason, it is preferable to make the upper limit 0.05%. The Nb content is more preferably 0.01 to 0.03%.

Ti: 0.005% to 0.05%
Ti finely precipitates as Ti (C, N) during hot rolling and becomes a precipitation nucleus of Cr carbonitride, thereby having the effect of refining Cr carbonitride and promoting solution formation during quenching heating. For this reason, it is preferable to add as needed. Since this effect is manifested at 0.005% or more, the lower limit is preferably made 0.005%. However, when excessively added, coarse TiN may precipitate in a temperature range higher than the hot rolling heating temperature, resulting in inclusion-based soot. For this reason, it is preferable to make the upper limit 0.05%. The Ti content is more preferably 0.01 to 0.03%.

Zr: 0.005% to 0.05%
Zr finely precipitates as Zr (C, N) during hot rolling and becomes a precipitation nucleus of Cr carbonitride, thereby having a function of refining Cr carbonitride and promoting solution formation during quenching heating. For this reason, it is preferable to add as needed. Since this effect is manifested at 0.005% or more, the lower limit is preferably made 0.005%. However, if added in excess, coarse Zr (C, N) may precipitate in the temperature range above the hot rolling heating temperature, resulting in inclusion-based soot. For this reason, it is preferable to make the upper limit 0.05%. The Zr content is more preferably 0.01 to 0.03%.

B: 0.0005% to 0.0030%
B may be added as necessary in order to improve the high temperature ductility during hot rolling and to suppress the yield reduction due to the ear cracking of the hot rolled sheet. In order to exert the effect, the lower limit is desirably set to 0.0005%. However, excessive addition impairs toughness and corrosion resistance due to precipitation of Cr ₂ B, (Cr, Fe) ₂₃ (C, B) ₆ . For this reason, the upper limit is made 0.0030%. In view of workability and manufacturing cost, 0.0008 to 0.0015% is more desirable.

On the other hand, the martensitic stainless steel according to the present embodiment casts steel having the above composition, hot-rolls the obtained ingot to obtain a hot-rolled plate, winds the hot-rolled plate, and winds it up. It is preferable that the manufactured hot-rolled sheet is tempered (annealed) and the tempered hot-rolled sheet is quenched. In this production method, the heating temperature during hot rolling (hot rolling) is set to 1140 to 1240 ° C., the winding temperature is set to 700 to 840 ° C., and the hot rolled sheet is annealed in a batch type annealing furnace at 700 to 900 ° C. It is desirable to carry out at 4 degreeC for 4 hours or more.

That is, when the heating temperature during hot rolling is higher than 1240 ° C., a two-phase region from γ single phase to γ + δ is obtained. In the δ phase, Cr, Si and the like are concentrated, C, N, Ni and the like are segregated negatively, and the γ single phase at the time of quenching is inhibited, so the hardenability is impaired. Conversely, if the hot rolling heating temperature is less than 1140 ° C., a soaking time of 2 hours or more is required as a diffusion time for eliminating segregation (solidification segregation), which is not preferable because productivity of hot rolling is greatly impaired. The hot rolling finishing temperature (temperature during finish rolling) is desirably 800 ° C. or higher. If the temperature falls below this, hot-rolling cracks are likely to occur. In addition, when the finishing temperature of hot rolling is less than 800 ° C., the coiling temperature is lowered, and thereby the productivity is lowered, for example, the subsequent hot rolling sheet annealing time is prolonged.

In addition, after the hot rolling, when winding the steel strip (hot rolled sheet obtained by hot rolling), the winding temperature is preferably set to 700 to 900 ° C. When winding at less than 700 ° C, the difference in structure between the coldest part and the hottest part of the coil becomes large, and this difference in structure is not eliminated even after hot-rolled sheet annealing after winding. It is not preferable to invite. By setting the temperature to 700 ° C. or higher, carbides are precipitated and coarsened during coil cooling, and are softened. On the other hand, when the temperature exceeds 900 ° C., a thick oxide scale is formed on the surface, which causes undesirable problems such as a decrease in corrosion resistance due to the formation of a decarburized phase and poor polishing properties after quenching.

Next, in order to improve the workability before quenching under the annealing conditions of the hot-rolled sheet, it is necessary to soften the hot-rolled sheet by annealing. In a continuous annealing furnace, a sufficient annealing time cannot be ensured due to softening, and therefore it is desirable to hold in a temperature range of 700 to 900 ° C. for 4 hours or more using a batch type annealing furnace. Softening becomes insufficient when the temperature is lower than 700 ° C or higher than 900 ° C. That is, if annealing is performed at a temperature higher than 900 ° C. for a long time, the surface structure is not uniform and the material is changed due to the nitridation or decarburization of the surface layer due to the influence of the atmospheric gas. Also, if the time is less than 4 hours, the material will vary in the coil due to temperature non-uniformity in the coil.

The hot-rolled sheet is pickled after annealing to become a hot-rolled product, but a part of the annealed hot-rolled sheet is cold-rolled and annealed to become a cold-rolled product.

As a quenching heat treatment (quenching heating) of the product, it was held for 5 seconds to 10 minutes in a temperature range of 950 to 1100 ° C. Thereafter, quenching (water quenching or air quenching) is desirable. If the heating temperature is less than 950 ° C., the solution of carbonitride is insufficient, and the desired quenching hardness cannot be obtained. By setting the temperature to 950 ° C. or higher, the carbonitride can be solutionized, and a structure mainly composed of austenite can be obtained. Further, when the heating temperature is increased, delta ferrite is precipitated in the austenite matrix, and the corrosion resistance and hardenability are impaired. The heating time (holding time) at this time also requires 5 seconds or more in order to proceed with solution treatment. If it is less than 5 seconds, there is little solid solution C and N and sufficient hardness cannot be obtained. On the other hand, if it is 10 minutes or more, oxidation on the surface proceeds, and corrosion resistance and hardness after quenching decrease due to decarburization of the surface layer, which is not preferable. The quenching cooling rate is preferably 3 to 100 ° C./sec. Preferable quenching methods include air quenching and water quenching.

Steel having the chemical composition (mass%) shown in Tables 1 and 2 was melted in a vacuum melting furnace, and then cast in an inert gas atmosphere at atmospheric pressure, specifically in a nitrogen atmosphere, and had a thickness of 100 mm and a weight. A 50 kg steel ingot was obtained. Since the steel ingot was baked and difficult to process, the steel ingot was kept at 850 ° C. for 4 hours and then tempered by furnace cooling. After removing the molten metal on the surface of the steel ingot, it was heated to 1220 ° C. and held for 1 hour. Then, it hot-rolled to plate thickness 6mm and obtained the hot rolled sheet. In this hot rolling, the finishing temperature was 900 ° C., and the hot rolled sheet was wound at 800 ° C. The wound hot-rolled sheet was continuously held at 850 ° C. for 4 hours and then tempered by furnace cooling. A crack having a depth of 1 mm or more on the end face of the hot-rolled sheet was judged to be defective as an ear crack occurred. The results are shown in the remarks column of Tables 3 and 4. When the ear crack was less than 1 mm, it was judged as a mild ear crack. In addition, the hardness after annealing (after tempering) was measured by the method described in JIS Z 2245: 2011 (based on ISO 6508-1: 2005). Those whose hardness after annealing exceeded 92HRB were judged to be defective because they were hard, and are shown in the remarks column of Tables 3 and 4.
The tempered hot-rolled sheet was subsequently held in a heat treatment furnace in a nitrogen atmosphere at 1050 ° C. for 10 minutes, then taken out and air-quenched to obtain a quenched steel sheet. The obtained hardened steel sheet was used as a test material, and the hardness and corrosion resistance were evaluated by the following methods. The results are shown in Tables 3 and 4. In addition, about some test materials (NO.40), the quenching steel plate was obtained by oil quenching. In Tables 1 to 4, numerical values that are out of the range defined in the present embodiment are underlined.

 

 

Hardness Measured with an applied load (test force) of 49 N based on the Vickers hardness test defined by JIS Z 2244: 2009 (based on ISO 6507-1: 2005, ISO 6507-4: 2005) in the plate thickness section. did. The hardness was 550 HV or higher.

Corrosion resistance After quenching, the surface of the sample (quenched steel plate) was ground by a milling machine and flattened, then polished by sandpaper, then buffed and mirror finished. A salt spray test specified in JIS Z 2371: 2000 was performed to evaluate the presence or absence of rusting, and a product without rusting was regarded as acceptable. Those with wrinkles on the finished surface were considered defective.

Toughness (DBTT)
A Charpy impact test specified in JIS Z 2242: 2005 (based on ISO / DIS 148-1: 2003) is performed on the material (tempered hot-rolled sheet) before quenching, and the ductile-brittle transition temperature (DBTT) is set. It was measured. The test piece was evaluated as a V-notch using a sub-size test piece with a plate thickness (about 6 mm). A product having a DBTT of 50 ° C. or lower was regarded as acceptable.

 

 

As can be seen from the results shown in Table 3, the steel of the present invention had a hardness after quenching of 550 Hv or more, and no rust was generated in the salt spray test after air quenching by adding Sn. As can be seen from the above, the steel of the present invention has excellent corrosion resistance in a practical environment. On the other hand, in comparative steels outside the scope of the present embodiment, as can be seen from the results shown in Table 4, the corrosion resistance, quenching hardness, toughness before quenching is insufficient, or other characteristics (raw material cost, The hot workability was inferior. Thus, the comparative steel was unacceptable in terms of manufacturability, quality, and / or cost. That is, NO. For 24-27, 32, and 34, the S value was low, and the corrosion resistance and quenching hardness were low. In addition, no. No. 24 had poor Si properties due to low Si and poor deoxidation. NO. Since No. 25 contained much Si, residual ferrite was generated. NO. No. 26 had a large amount of Mn, so the quenching scale became thick and the polishing property was poor. NO. No. 27 had little Cr and low corrosion resistance. NO. Since No. 34 had a large amount of Al, the polishing property was inferior. In addition, NO. No. 28 had a lot of Cr, and its hardness was low due to residual ferrite. NO. No. 29 had a large amount of Ni, and the hardness after hot rolling annealing was as hard as 92 HRB. NO. No. 30 had a lot of Cu, and ear cracks occurred on the end face of the hot rolled sheet. NO. Since No. 31 had a lot of Sn, the toughness of the hot-rolled annealed plate was lowered. NO. Since 35 had less N, its corrosion resistance was poor. NO. Since No. 36 had a large amount of N, blowhole defects were observed on the polished surface and were judged to be defective materials. No. In No. 37, the C content was lower than the lower limit, and the quenching hardness was low. No. In No. 38, the C content exceeded the upper limit, and the corrosion resistance was low. No. No. 39 had an S value below the lower limit and low corrosion resistance. No. 40 is No. steel component. Same as 11, but as a result of oil quenching, quenching hardness was low.

Also in the present invention steel, NO. Nos. 17 and 19 to 20 are invention steels having the same C, N, Sn and S values by adding Nb, Zr and Ti. Compared to 3, the quenching hardness was slightly higher. In addition, NO. Nos. 18, 20, and 21 improved the hot workability by adding B, and no ear cracks having a depth of 1 mm or less were observed.

According to the present invention, it is possible to produce martensitic stainless steel having high hardness and excellent corrosion resistance at low cost and high productivity without using a large amount of expensive elements such as Mo. Therefore, the present invention contributes to greatly improving the production cost and quality of stainless steel for Western tableware knives, stainless steel knives, tools, and two-wheel disc brakes.

Claims (5)

1. % 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%
   Having a steel composition consisting of the balance Fe and inevitable impurities,
   A martensitic stainless steel with excellent corrosion resistance in the range of C, N and Sn satisfying the formula (1).
   S value = 16 × Sn / C + 2 × N / C ≧ 0.40% (1)
   However, in said formula, each element name C, N, and Sn represents content (mass%) of each element.
2. Furthermore, in mass%,
   Nb: 0.005% or more and 0.05% or less,
   Ti: 0.005% or more and 0.05% or less,
   Zr: 0.005% or more and 0.05% or less,
   B: The martensitic stainless steel excellent in corrosion resistance according to claim 1, comprising one or more of 0.0005% or more and 0.0030% or less.
3. An ingot having the composition of the martensitic stainless steel according to claim 1 or 2 is obtained by casting,
   The obtained ingot is heated to 1140 to 1240 ° C. and hot rolled to obtain a hot rolled sheet,
   Winding up the obtained hot-rolled sheet,
   Tempering the rolled hot rolled sheet at 700-900 ° C. for 4 hours,
   A method for producing martensitic stainless steel, wherein the tempered hot-rolled sheet is kept in a temperature range of 950 to 1100 ° C. for 5 seconds to 10 minutes and then quenched.
4. The method for producing martensitic stainless steel according to claim 3, wherein the quenching is air quenching.
5. The hot rolling finishing temperature is 800 ° C. or higher,
   The method for producing martensitic stainless steel according to claim 3 or 4, wherein a winding temperature of the hot-rolled sheet is 700 to 900 ° C.

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