JP5033584B2 - Martensitic stainless steel with excellent corrosion resistance - Google Patents

Description

  The present invention relates to a martensitic stainless steel having excellent corrosion resistance after quenching. More specifically, the present invention relates to a martensitic stainless steel that is used in the manufacture of tableware knives, looms, tools, two-wheel disc brakes, etc., and has excellent corrosion resistance even when air-cooled and quenched.

  SUS420J1 and SUS420J2 steel are generally used for tools such as tableware knives (table knives), scissors, looms, and vernier calipers, and SUS440A steel is used for Western knives and fruit knives that require higher hardness. . This is because in such applications, it is difficult to use rust-preventive plating, coating, and rust-preventing oil, and high hardness that is resistant to wear is required. The standards of these martensitic stainless steels are defined by the amount of C, SUS410 is C: 0.15% or less, Cr: 11.5 to 13.5%, SUS420J1 is C: 0.16 to 0.25% Cr: 12-14%, SUS420J2 is C: 0.26-0.40%, Cr: 12-14%, SUS440A is C: 0.60-0.75% and Cr: 16-18% ing. SUS420J1 grade is generally used for Western tableware knives and the like because higher quenching hardness is obtained as the amount of C is higher, while manufacturability and toughness after quenching decrease.

  Regarding the manufacturing process of a Western-style kitchen knife made of martensitic stainless steel, it has been introduced in several literatures, but after cutting from a steel plate, heating and quenching, it is finished into a knife through various polishing processes. As for quenching, oil quenching is introduced as a representative example. However, martensitic stainless steel has good hardenability and can be hardened even at a cooling rate of about air cooling. That is, it is known that sufficient hardness as a blade can be obtained even by air cooling. For this reason, air-cooled quenching is generally performed in the manufacturing process of western tableware knives in order to save processes.

  In addition, the corrosion resistance of these stainless steels is generally organized by component and is known to be improved by the addition of Cr, Mo, and N. Many studies have been made on the effect of each element. In martensitic stainless steel, there is a report that it can be arranged by Cr + 3.3Mo + 30N, and that the larger this value, the better the corrosion resistance. In addition, since stainless steel is used after being quenched, it is also necessary to improve polishing properties by avoiding large inclusions by lowering Al or the like.

  These findings are explained in the patent literature. First, in Patent Document 1, a high-strength martensite system excellent in weather resistance containing Cr: 12-16%, Mo: 1.3-3.5%, N: 0.06% -0.13% Stainless steel wire is described.

  Cr and Mo, which improve the weather resistance, are expensive elements and have a problem of reducing the hardenability by narrowing the austenite temperature range. If austenite cannot be converted to a single phase during quenching heat treatment, quenching starts from the two-phase region of austenite and ferrite, but the temperature at which austenite begins martensitic transformation is as low as about 300 ° C, which is a slow cooling rate like air-cooled quenching. In this case, carbide precipitates from ferrite having a high C diffusion rate, and the resulting Cr-deficient layer significantly reduces the corrosion resistance. Of course, even in the austenite phase, if the cooling rate is slow, carbides are precipitated and the corrosion resistance is lowered. However, since the diffusion rate is smaller than in the ferrite, the influence of the cooling rate is relatively small. Such a decrease in corrosion resistance due to the precipitation of carbide is a problem that becomes apparent during air-cooling quenching.

  On the other hand, nitrogen expands the austenite region and is an inexpensive element, but nitrogen exceeding the solid solubility limit at the time of melting and casting creates bubbles, and a problem is that a healthy steel ingot cannot be obtained. The solid solubility limit of nitrogen varies depending on the components and atmospheric pressure. The effects of Cr and C as components are large, and when martensitic stainless steels such as SUS420J1 and SUS420J2 are cast under atmospheric pressure, the amount of dissolved nitrogen is generally reported to be about 0.1%. Patent Document 2 also describes that N: 0.06 to 0.10% as martensitic stainless steel having no pinhole defect.

  As an attempt to obtain higher weather resistance, a technique for casting under high pressure exceeding atmospheric pressure has also been developed. For example, in Patent Document 3, N: 0.40% to 0.80%, Cr: 13.0% or more and 20.0%, Mo: 0.2% -4. It describes a martensitic stainless steel containing 0%.

  As described above, various martensitic stainless steels having both weather resistance and manufacturability have been proposed.

  However, in the study of the present inventors, the above-mentioned Patent Document 1 focuses on improving the corrosion resistance by adding Cr and Mo, so that the alloy cost is increased, and Cr and Mo are caused by solidification segregation. It has been found that the concentrated portion becomes a stable δ ferrite phase, and it takes a long time to make austenite single phase during quenching heat treatment.

  In addition, the method described in Patent Document 2, that is, adding 0.06% to 0.10% of nitrogen for improving corrosion resistance without causing pinhole defects was also performed in Patent Document 1. However, it has been clarified that although the corrosion resistance is improved by adding nitrogen, the precipitation of carbides is excessively suppressed and the wear resistance is impaired as the amount of dissolved nitrogen increases.

  Furthermore, in the method described in Patent Document 3, a dedicated facility is required to pressurize the casting atmosphere, and it is not suitable for mass production.

JP-A-5-287456 JP 2005-163176 A JP 2005-248263 A

  In view of the present situation, the present invention has an object to provide a martensitic stainless steel sheet having good corrosion resistance during air-cooling and quenching and excellent wear resistance at low cost.

  In order to achieve the above object, the present inventors investigated the influence of the components and the cooling rate on the corrosion resistance of martensitic stainless steel, and examined the influence on the situation and wear resistance of the precipitates after quenching, In order to obtain good corrosion resistance, it is important to increase the stability of austenite in the heating temperature range during quenching and to adjust the amount of carbonitride deposited during air-cooling quenching. It has been found that it is very important to optimize the component balance of N, Cr and Ni and to control dispersion of insoluble carbides.

The gist is as follows.
(1) By mass%, C: 0.13-0.18%, Si: 0.30-0.60%, Mn: 0.10-0.60%, P: 0.035% or less, S: 0.015% or less, Cr: 11.0 to 15.0%, Ni: 0.10 to 0.60%, Cu: 0.50% or less, V: 0.10% or less, Al: 0.05% Hereafter, it has a steel composition consisting of N: 0.03% or more and less than 0.06%, the balance Fe and inevitable impurities, C + N: 0.19 to 0.23%, C / N: 2.3 to 4. 3, γmax = 420C + 470N + 30Ni + 7Mn + 9Cu-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al + 189 martensitic stainless steel excellent in corrosion resistance during air-cooling and quenching, characterized by being 120 or more.
(2) Furthermore, by mass%, Mo: 0.01 to 1.0%, Ti: 0.005 to 0.050%, Nb: 0.005 to 0.050%, or one or more of them The martensitic stainless steel having excellent corrosion resistance during air-cooling and quenching as described in (1).

  By optimizing the balance of each component of martensitic stainless steel, it prevents corrosion deterioration due to sensitization during air cooling and quenching, and is ideal for applications such as western tableware knives that combine wear resistance and manufacturability New stainless steel can be provided at low cost.

  The present invention is described in detail below.

  In general, the corrosion resistance of stainless steel is arranged by its components and arranged by an index such as Cr + 3.3Mo + 16N, and the higher this value, the higher the corrosion resistance. The corrosion resistance at this time refers to a neutral chloride aqueous solution environment. As an evaluation method, for example, a method for measuring the pitting potential of stainless steel defined in JIS G0577 or a salt spray test method defined in JISZ2371. Etc. are raised. However, it is extremely unlikely to be exposed to high-concentration chloride aqueous solutions in water tanks such as chemical / food plants and water heaters, in beach environments, that is, in everyday indoor environments. As SUS420J1 steel is used, sufficient corrosion resistance is obtained with a Cr amount of about 13%.

  However, when SUS420J1 or SUS420J2 steel, which is a general martensitic stainless steel, is air-cooled and quenched, Cr carbide precipitates at the grain boundary and a chromium-deficient layer is formed in the vicinity of the grain boundary (sensitization). The corrosion resistance is not obtained. Still, it does not corrode in a daily indoor dry environment, but rust may occur when exposed to a high-temperature chlorine ion environment such as when washing in a dishwasher. That is, in general martensitic stainless steel applications, it is important to reliably maintain the corrosion resistance corresponding to Cr by preventing sensitization.

  There are several points in preventing sensitization. The first is to reduce the carbon content, but there is an appropriate range to reduce the quenching hardness. The second is to austenite single phase during quenching heat treatment, but it should be noted that austenite stabilizing elements such as C and N are reduced at the steel surface during steel manufacturing and quenching heat treatment. In some cases, ferrite is generated only in the surface layer portion. The reduction of carbon is called decarburization, and the reduction of nitrogen is called denitrification, but oxygen and hydrogen in the atmosphere react with carbon and nitrogen in the steel at high temperatures, and the carbon and nitrogen move from the steel to the atmosphere. It is a phenomenon. Since decarburization and denitrification at the time of hot rolling and heating before quenching are unavoidable, it is necessary to add excessively to some extent.

  As a third effective method for preventing martensitic stainless steel from being sensitized, there is a balance adjustment between nitrogen and carbon. Even when the amount of carbon is constant, when the amount of nitrogen increases, precipitation of Cr carbide is suppressed, and sensitization is suppressed. Equilibriumly, if the amount of carbon is the same and nitrogen is increased, Cr nitride is precipitated in addition to Cr carbide, which may promote sensitization. However, at a cooling rate comparable to air cooling, Cr carbide, Cr The precipitation of both nitrides is delayed and sensitization is suppressed. However, there is an optimum range for the amount of carbon and nitrogen. If the amount is outside the optimum range, quality problems such as sensitization and deterioration of hardness and wear resistance occur.

  Based on the above knowledge, this invention discovered the optimal component balance as a martensitic stainless steel in the said use. 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: 0.13-0.18%
C is an element that controls the quenching hardness, and is required to be 0.13% or more in order to obtain the hardness necessary for a western tableware knife. On the other hand, excessive addition increases the quenching hardness more than necessary, increases the load during polishing, and decreases toughness. In addition, Cr carbide precipitates during air-cooling and quenching, resulting in a problem that the corrosion resistance is impaired. Preferably it is 0.17% or less.

Si: 0.30 to 0.60%
Si is necessary for deoxidation at the time of melting and refining, and is also effective in suppressing the formation of oxide scale during the quenching heat treatment, so it was set to 0.30% or more. However, Si is made 0.60% or less in order to narrow the austenite single phase temperature range and impair quenching stability.

Mn: 0.60% or less Mn is an austenite stabilizing element, is balanced with other elements, and at least 0.10% or more is added to make austenite single phase in the surface layer part during quenching heat treatment. . On the other hand, 0.60% was made the upper limit in order to promote the generation of oxide scale during quenching heat treatment and increase the subsequent polishing load.

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 it is an element harmful to hot-rolled annealed plate and toughness after quenching, it was set to 0.035% or less.

S: 0.015% or less S is an element that has a small amount of solid solution in the austenite phase and segregates at the grain boundary to promote a decrease in hot workability. If the content exceeds 0.015%, the effect becomes significant. Therefore, it was made 0.015% or less.

Cr: 11.0-15.0%
Cr needs to be at least 11.0% in order to maintain the corrosion resistance as a western tableware knife. On the other hand, since it also has an effect of narrowing the austenite stable temperature range, the upper limit was made 15.0%. In order to make these characteristics more effective, the Cr range is preferably 12 to 15%, more preferably 13 to 14%.

Ni: 0.10 to 0.60%
Ni is an austenite stabilizing element like Mn. During quenching heat treatment, C, N, Mn, etc. may decrease from the surface layer portion to generate ferrite in the surface layer portion, but Ni does not decrease and is very effective for the effect of stabilizing the austenite phase. In order to balance with other elements and to make austenite single phase in the surface layer part at the time of quenching heat treatment, it was decided to add at least 0.10% or more. On the other hand, Ni is more expensive than other alloy elements, and is set to 0.60% or less in order to increase the cost.

Cu: 0.50% or less Cu is often inevitably contained, such as mixing from scrap during melting, but excessive content reduces hot workability and corrosion resistance, so 0.50% or less It was.

V: 0.10% or less V is often inevitably mixed from ferrochromium or the like, which is an alloy raw material, but is 0.10% or less because of its strong effect of narrowing the austenite single-phase temperature range.

Al: 0.05% or less Al is an element effective for deoxidation, but raises the basicity of slag, precipitates soluble inclusions CaS in steel, and lowers corrosion resistance. In addition, the lowering of abrasiveness due to alumina-based non-metallic inclusions is caused, so 0.05% was made the upper limit.

N: 0.03% or more and less than 0.06% N, like C, has the effect of increasing the quenching hardness. Further, as an effect different from C, the corrosion resistance is improved in the following two points. The first is to strengthen the passive film, and the other is to suppress the precipitation of Cr carbide (suppression of the Cr-deficient layer). In order to obtain these effects, N is set to 0.03% or more. However, excessive addition drastically reduces the amount of Cr carbide precipitated, impairs wear resistance, and impairs manufacturability, so it was made less than 0.06%.

  In the martensitic stainless steel described above, addition of Mo, Ti or Nb to the steel works effectively to further improve the corrosion resistance.

  In order to improve the corrosion resistance, it is necessary to add at least 0.01% by mass of Mo. However, if it exceeds 1.0%, the effect of improving the corrosion resistance is saturated and the workability is deteriorated. Therefore, 1.0% is the upper limit.

  Ti is an element effective for suppressing precipitation of water-soluble sulfides that are harmful to corrosion resistance. A content of 0.005% or more is necessary, but if added over 0.050%, quenching hardness is caused by precipitation of carbides. It will cause the decrease. Therefore, the lower limit of the Ti content is 0.005% from the viewpoint of corrosion resistance, and the upper limit is 0.050% from the viewpoint of hardenability.

  Nb is a carbonitride-forming element like Ti. A content of 0.005% or more is necessary to suppress precipitation of water-soluble sulfides that are harmful to corrosion resistance. However, if added over 0.050%, it causes quenching hardness to decrease due to precipitation of carbides. Become. Therefore, the lower limit of Nb content is 0.005% from the viewpoint of corrosion resistance, and the upper limit is 0.050% from the viewpoint of hardenability.

  A desirable range of components can be obtained by satisfying the component conditions defined by the formula. The reason for the regulation will be explained below.

C + N: 0.19 to 0.23%
In order to obtain the quenching hardness required for Western tableware knives and to prevent quenching defects due to the decarburized layer and denitrified layer of the surface layer during quenching heat treatment, the total amount of C + N is 0.19% or more is necessary. However, when the amount of C + N is excessive, the quenching hardness is excessively increased, and the upper limit is set to 0.23% in order to impair the toughness of intermediate materials (slabs and the like) in products and manufacturing processes.

C / N: 2.3 to 4.3
Regarding quenching hardness, C and N act almost equivalently, but the effect differs with respect to corrosion resistance and wear resistance, so it is necessary to control the ratio of C and N. The lower limit of C / N is 2.3 from the viewpoint of improving corrosion resistance. The upper limit was set to 4.3 in order to maintain the corrosion resistance by improving the wear resistance and preventing the decarburized layer.

γmax = 420C + 470N + 30Ni + 7Mn + 9Cu-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al + 189 is 120 or more It is an indispensable condition in the steel that it becomes austenite single phase at the time of heat treatment before quenching. It is necessary to ensure a sufficient temperature range. If γmax represented by the above formula is 120 or more for the main element, a sufficient austenite single-phase temperature range can be secured. Therefore, γmax is set to 120 or more. Here, the element symbol in the γmax formula represents the content (% by mass) of each element.

  FIG. 1 shows the relationship between the optimal range of C + N and C / N and the range defined for each of C and N, and FIG. 2 shows the relationship between Cr, Ni and γmax. In the calculation of γmax in FIG. 2, the components other than Cr and Ni are set to 0.15C-0.05N-0.35Si-0.1Mn-0.05V (both mass%). Only when all the conditions that are the constituent requirements of the present invention are satisfied, it becomes possible to satisfy all of the corrosion resistance, wear resistance, and quenching hardness during air-cooling quenching.

  In the production of the steel, the heating temperature at the time of hot rolling is set to 1140 to 1240 ° C, the coiling temperature is set to 700 to 840 ° C, and hot-rolled sheet annealing is performed at 700 to 900 ° C in a batch type annealing furnace for 4 hours or more. It is desirable to do so.

  That is, when the hot rolling heating temperature is higher than 1240 ° C., the two-phase region from γ single phase to γ + δ is obtained. In the δ phase, Cr, Si and the like are concentrated, and C, N, Ni and the like are segregated negatively, hindering the γ single phase at the time of quenching and impairing the hardenability. On the other hand, when the temperature is lower than 1140 ° C., the soaking time is required for 2 hours or more as the diffusion time for eliminating the solidification segregation, which is not preferable because the productivity of hot rolling is greatly impaired.

  In addition, when the steel strip is wound after hot rolling, it is desirable that the winding temperature is 700 to 840 ° C. By setting the temperature to 700 ° C. or higher, the coarsening of the carbide effective for improving the wear resistance proceeds when the coil is cooled. On the other hand, when the temperature exceeds 840 ° 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, although it is the annealing conditions of a hot-rolled sheet, in order to improve the workability before quenching, it is necessary to make it soft. For this purpose, since a continuous annealing furnace cannot secure an annealing time for sufficient softening, a heat treatment is preferably performed in a batch annealing furnace in a temperature range of 700 to 900 ° C. for 4 hours or more. Softening becomes insufficient when the temperature is lower than 700 ° C or higher than 900 ° C. If the time is less than 4 hours, material variation in the coil due to temperature non-uniformity in the coil occurs.

  In the quenching heat treatment, it is desirable to hold for 5 to 30 minutes in a temperature range of 950 to 1100 ° C. and perform air cooling and quenching. The cooling rate at this time is preferably 1 to 5 ° C./second at an average cooling rate up to 400 ° C. when the thickness is 6 mm.

  Steel having a chemical composition value (% by mass) shown in Table 1 was melted in a vacuum melting furnace, and then cast in an inert gas nitrogen atmosphere at atmospheric pressure to obtain a steel ingot having a thickness of 100 mm. Then, after heating to 1220 ° C., it was hot-rolled to a plate thickness of 6 mm and wound up at 700 ° C. Subsequently, after heat treatment at 850 ° C. for 4 hours, the furnace was cooled and tempered. Subsequently, after being held at 1050 ° C. for 10 minutes in a heat treatment furnace in a nitrogen atmosphere, it was taken out and allowed to cool naturally, and was air-cooled and quenched. Using the obtained hardened steel sheet as a test material, the quenching hardness, corrosion resistance, and wear resistance were evaluated by the following methods.

Hardness In the plate thickness section, it was measured at a load of 50 N based on the Vickers hardness test specified in JISZ2244.

Corrosion resistance After quenching, the sample surface was ground by a milling machine and flattened, and then sanded paper was used for No. 600 polishing finish. The salt spray test prescribed in JISZ2371 was conducted for 4 hours, and the number of glazing points was scored.

For steel types 2 and 21, the hot-rolled annealed plate was heated at 1050 ° C. for 10 minutes with an induction heating device, then cooled to 300 ° C. or less at a constant cooling rate (1 to 100 ° C./second), and soaking A test piece was cut out from the part and the entire surface was mirror-polished, and then a sulfuric acid-copper sulfate corrosion test was performed to measure the corrosion weight loss. The boiling liquid composition at that time was 0.5 mass% H ₂ SO ₄ +6 mass% CuSO ₄ , and Cu ions were saturated by putting small pieces of copper. The sample was immersed and held for 20 hours.

Abrasion resistance The specimens were processed into the shape of a table knife edge, subjected to wear resistance tests on cow bones and porcelain, and compared with reference steel.

  As can be seen from the results shown in Table 2, the steel of the present invention satisfies the hardness range desired for Western dishes knives: Hv 460 to 580 when subjected to air cooling and quenching, and the occurrence of rust in the SST test is extremely high. Less wear resistance and excellent sharpness over a long period of time.

  On the other hand, components outside the scope of the present invention have poor quenching hardness, poor corrosion resistance after quenching, or other properties (hot rolled annealed sheet toughness, hot workability, raw material cost, abrasiveness) However, it is inferior by 10% or more with respect to steel type No. 20, which is a typical component of SUS420J1 steel, and is unacceptable in terms of manufacturability, quality, and cost.

  FIG. 3 shows the results of detailed examination of the effect of quenching and cooling rate on the corrosion resistance using steel type number 2 as a representative of the steel of the present invention and steel type number 20 as a representative of the comparative steel. It can be seen that when the cooling rate of the comparative steel is slow, the corrosion weight loss increases rapidly, but the steel according to the present invention is less dependent on the cooling rate. In the case of a thick steel plate such as a western tableware knife, the cooling rate in the temperature range where chromium carbide precipitates is about 0.5 ° C / s, but the difference in corrosion weight loss between the inventive steel and the comparative steel at the cooling rate is clear. .

  In the case of a thick steel plate such as a western tableware knife, for example, when the plate thickness is 6 mm, the cooling rate in air cooling is about 3 ° C./s.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to manufacture the martensitic stainless steel excellent in corrosion resistance and wear resistance after air-cooling and quenching at low cost and with high productivity. 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.

The figure which shows the optimal component range of C and N The figure which shows the optimal component range of Cr and Ni Diagram showing the effect of steel grade and quenching cooling rate on corrosion weight loss in sulfuric acid-copper sulfate corrosion test

Claims (2)

  In mass%, C: 0.13-0.18%, Si: 0.30-0.60%, Mn: 0.10-0.60%, P: 0.035% or less, S: 0.015 %: Cr: 11.0-15.0%, Ni: 0.10-0.60%, Cu: 0.50% or less, V: 0.10% or less, Al: 0.05% or less, N : 0.03% or more and less than 0.06%, having a steel composition consisting of the balance Fe and inevitable impurities, C + N: 0.19 to 0.23%, C / N: 2.3 to 4.3, γmax = 420C + 470N + 30Ni + 7Mn + 9Cu-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al + 189 is a martensitic stainless steel excellent in corrosion resistance during air-cooling and quenching, characterized by being 120 or more.   Furthermore, it contains one or more of Mo: 0.01 to 1.0%, Ti: 0.005 to 0.050%, and Nb: 0.005 to 0.050% by mass%. The martensitic stainless steel having excellent corrosion resistance during air-cooling and quenching according to claim 1.

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