WO2024057705A1

WO2024057705A1 - Stainless steel and manufacturing method therefor, and stainless steel product and manufacturing method therefor

Info

Publication number: WO2024057705A1
Application number: PCT/JP2023/026330
Authority: WO
Inventors: 賢太郎福本
Original assignee: 株式会社プロテリアル
Priority date: 2022-09-12
Filing date: 2023-07-18
Publication date: 2024-03-21
Also published as: CN118510930A; JP7501802B1; JPWO2024057705A1

Abstract

Provided are: a stainless steel having excellent cold workability and a manufacturing method therefor; and a stainless steel product for which surface hardness is sufficiently high and for which corrosion resistance against acids, particularly non-oxidative acids, is excellent and a manufacturing method therefor. Provided are the stainless steel and the manufacturing method therefor, said steel having a component composition, in terms of % by mass, of 0.18-0.25% of C, 0.1-1.5% of Si, 0.35-1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 0.05-0.20% of Ni, 12.5-14.6% of Cr, 1.5-3.0% of one or both of Mo and W according to the relational expression (Mo + 1/2W), 1.0-3.0% of Cu, and 0.03-0.10% of N, the remainder being Fe and impurities. Further provided are the stainless steel product and the manufacturing method therefor, said stainless steel product being obtained by quenching and tempering the foregoing stainless steel.

Description

Stainless steel and its manufacturing method, and stainless steel products and its manufacturing method

The present invention relates to a stainless steel suitable for producing stainless steel products that can be used in corrosive environments, such as various sliding products and high-strength products, and a method for producing the same. The present invention also relates to the above stainless steel product and its manufacturing method.

Conventionally, in order to improve properties such as corrosion resistance, hardness, and fatigue strength of stainless steel products, methods of optimizing the content of constituent components, high-frequency induction heating, and forming a nitrided layer on the surface at a relatively low temperature of about 500°C have been used. Surface treatment methods have been proposed, such as nitriding treatment to form a carbonaceous substance, and solid-phase nitrogen absorption treatment to form a solid solution of nitrogen in the surface layer at a high temperature of about 1000°C.

For example, in Patent Document 1, ``The components are approximately 0.10 to 0.40% C, approximately 0.01 to 2.0% Mn, approximately 2.0% maximum Si, and P approximately 2.0% by weight. Maximum approximately 0.2%, S maximum approximately 0.030%, Cr 10 to 15%, Ni approximately 0.5% maximum, Mo approximately 0.75 to 4.0%, N 0.02 to 0.15%, Ti up to about 0.01%, Al up to about 0.01%, Nb+Ta up to about 0.10%, V up to about 0.20%, Zr less than 0.001%, Ca A "corrosion-resistant martensitic stainless steel alloy containing less than 0.001% of iron, the balance being substantially iron, and a Ni/Cu ratio of less than 0.2" has been proposed.

Furthermore, in Patent Document 2, "in mass %, C: 0.15 to 0.75%, Si: 0.05 to 2.00%, Mn: 0.01 to 2.00%, Ni: 0.01 ~2.00%, Cr: 10.00~15.00%, Mo: 0.50~3.00%, Cu: 0.01~2.00%, N: 0.010~0.150%. contains, satisfies 0.20≦C+N≦0.80, the remainder is Fe and impurities, has a surface hardening layer on part or all of the surface, and the carbonitride size in the surface hardening layer is 1.0 μm "Stainless steel for fuel system products for engines" is proposed, which has a carbonitride content of 0.50 mass% or less, an HV hardness of 500 HV or more, and a bulk HV hardness of 350 HV or less. . Furthermore, as a manufacturing method of this stainless steel, "a manufacturing method in which a steel having the above chemical composition is hot rolled, and then controlled annealing, forging with a reduction ratio of 60% or more, and surface hardening treatment are performed." is proposed.

Special Publication No. 2009-503257 Japanese Patent Application Publication No. 2021-80544

Patent Document 1 is an effective method for increasing the corrosion resistance of martensitic stainless steel products. However, in the case of Patent Document 1, when increasing the hardness using normal quenching and tempering, for example, if the C content is high, the surface hardness is high, but the corrosion resistance is reduced; If it is low, the corrosion resistance is good, but the surface hardness is low, and it is difficult to improve the surface hardness and corrosion resistance of the product at the same time. In addition, since the annealing temperature after hot rolling is as high as 788 to 843°C, there is concern that cold workability will deteriorate due to precipitation hardening due to the Cu phase if Cu is contained in the range of 1.5 to 4.0%. Ru. Furthermore, in the case of Patent Document 2, induction hardening is essentially performed to harden and temper only the surface layer of the forged product, and although the surface hardness of the product is high, there are concerns about corrosion resistance, and furthermore, the internal hardness is 350 HV or less. There are concerns about the internal strength of the product.

The object of the present invention is to provide a stainless steel with excellent cold workability, a method for producing the same, a sufficiently high surface hardness of 630 HV or more, a high internal hardness of 480 HV or more and less than 630 HV, and corrosion resistance against non-oxidizing acids. Our goal is to provide superior stainless steel products and their manufacturing methods.

In order to further improve the cold workability of stainless steel, the present inventor investigated the annealing temperature after hot working in order to further improve the cold workability of stainless steel. Furthermore, solid-phase nitrogen absorption treatment was investigated in order to simultaneously increase the hardness and corrosion resistance of stainless steel products. As a result, we found that the cold workability of stainless steel can be improved by lowering the annealing temperature after hot working and coarsely dispersing the Cu phase, and that improving the composition of stainless steel can improve the hardness and corrosion resistance of stainless steel products. It was found to be effective. By finely dispersing the "Cu phase" in the surface layer of stainless steel products after quenching and tempering, and forming martensite containing a large amount of nitrogen, we can simultaneously impart excellent hardness and corrosion resistance to the product. I found out. Furthermore, in order to achieve hardness and corrosion resistance in the surface layer of such stainless steel products, we have found that it is effective to lower the tempering temperature after quenching and sub-zero treatment, and have arrived at the present invention.

That is, in the present invention, C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, P: 0.04% or less , S: 0.01% or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W) This stainless steel has a composition of seeds: 1.5 to 3.0%, Cu: 1.0 to 3.0%, N: 0.03 to 0.10%, and the balance is Fe and impurities. Preferably, in the present invention, the area ratio of the Cu phase having an equivalent circular diameter of 0.03 μm or more in the cross-sectional structure is 2.4% or more.

Further, the present invention provides C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, and P: 0.04% or less in mass %. , S: 0.01% or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W) Seed: 1.5-3.0%, Cu: 1.0-3.0%, N: 0.03-0.10%, balance Fe and impurities. This is a method for producing stainless steel in which heat treatment is performed at 60 minutes or more at ℃, followed by hot working, and then annealing is performed at 760 to 780 ℃ for 4 hours or more.

Furthermore, the present invention provides C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, and P: 0.04% or less. , S: 0.01% or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W) A nitride layer is formed on the surface of stainless steel with a composition of seeds: 1.5 to 3.0%, Cu: 1.0 to 3.0%, N: 0.03 to 0.10%, and the balance Fe and impurities. It is a stainless steel product with
The stainless steel product has a center hardness of 480 HV or more and less than 630 HV, and a surface hardness of 630 HV or more.

Furthermore, the present invention provides C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, and P: 0.04% or less. , S: 0.01% or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W) Seed: 1.5 to 3.0%, Cu: 1.0 to 3.0%, N: 0.03 to 0.10%, balance Fe and impurities to stainless steel in a nitrogen atmosphere. Stainless steel products that are hardened at a quenching temperature of 1000 to 1090°C, followed by sub-zero treatment at a temperature of -50°C or lower, and then tempered at a tempering temperature of 150 to 470°C. This is the manufacturing method. Preferably, the above tempering temperature is 150 to 400°C.

According to the present invention, the cold workability of stainless steel can be improved, and high hardness and excellent corrosion resistance can be simultaneously imparted to stainless steel products.

FIG. 2 is a mapping diagram of Cu obtained by FE-EPMA (field emission type microscopic X-ray analyzer) showing the cross-sectional structure of the stainless steel of the example of the present invention. FIG. 2 is a mapping diagram of Cu obtained by FE-EPMA, showing a cross-sectional structure of a stainless steel of a comparative example. FIG. 3 is a mapping diagram of Cu obtained by FE-EPMA, showing the cross-sectional structure of stainless steel of another comparative example.

Conventionally, when producing various stainless steel products, first, a steel ingot or a steel billet obtained by blooming the same is used as a material, and then hot processing such as hot rolling is performed on the material, and generally annealing is performed. State stainless steel (intermediate material) is prepared. Then, this stainless steel is cold-worked into a shape suitable for various product shapes, and then, for example, if it is a plate-shaped product, it is cold-rolled to the product thickness, and finally, it is subjected to various machining processes. The material is shaped into the product shape by quenching and tempering, and the material is hardened to the desired hardness.

In such a series of manufacturing processes, the feature of the present invention is that in order to further improve the cold workability of stainless steel, it has been discovered that there is a suitable composition of stainless steel and a structure form of the stainless steel. It is at the point. Furthermore, the present invention is characterized by the discovery that there is a combination of the composition and surface morphology of stainless steel that is suitable for achieving both high hardness and excellent corrosion resistance in stainless steel products. Stainless steel products with this combination of composition and surface morphology can simultaneously achieve high hardness and excellent corrosion resistance, especially in corrosive environments containing non-oxidizing acids such as formic acid, sulfuric acid, and hydrochloric acid. . Such stainless steel products can be used for various sliding products and high-strength products, such as cutlery, plastic molding tools, and punches.
Hereinafter, the stainless steel and stainless steel products of the present invention will be described, together with a manufacturing method preferable for achieving the same.

(1) The stainless steel of the present invention has C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, and P: 0 in mass %. .04% or less, S: 0.01% or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, of Mo and W according to the relational expression (Mo + 1/2W) One or two types: 1.5 to 3.0%, Cu: 1.0 to 3.0%, N: 0.03 to 0.10%, and the balance is Fe and impurities.
In the case of the present invention, in order to impart excellent wear resistance (that is, high hardness) to the product, this material is made of "stainless steel" whose composition has been adjusted to develop a martensitic structure by quenching, sub-zero treatment, and tempering. ” is used. Note that the content of less than 0.1% Al or less than 0.1% Nb in mass % is acceptable as an impurity.

・C: 0.18 to 0.25% by mass (hereinafter simply referred to as "%")
C is an element effective in increasing the hardness of the martensitic structure after quenching and tempering. However, if there is too much C, the amount of Cr and Mo dissolved in the matrix will be relatively reduced, and the corrosion resistance of the product will deteriorate. Even if C is reduced, in the case of the present invention, the hardness of, for example, 630 HV or more is imparted to the surface layer of the product by the combination of Mo inclusion and solid phase nitrogen absorption treatment (quenching heating), which will be described later. can do.
Therefore, the C content is set to 0.18 to 0.25%. Preferably it is 0.20% or more. Moreover, it is preferably 0.23% or less.

・Si: 0.1-1.5%
Si is an element that is used as a deoxidizing agent and the like during the melting process and may be included unavoidably. If there is too much Si, the hardness of the annealed material (that is, the stainless steel of the present invention before quenching and tempering (before adjustment to product hardness)) increases and cold workability decreases.
Therefore, the Si content is set to 0.1 to 1.5%. Preferably it is 1.0% or less. More preferably it is 0.8% or less. More preferably, it is 0.5% or less. Moreover, it is preferably 0.2% or more, more preferably 0.3% or more.

・Mn: 0.35-1.5%
Mn is an element that is used as a deoxidizing agent etc. during the melting process and may be included unavoidably. In particular, in the present invention, it is an element that has the effect of promoting solid solution of nitrogen into tissues in the solid-phase nitrogen absorption treatment described below. However, if there is too much Mn, the austenitic structure will be stabilized, making it difficult to obtain a martensitic structure and making it difficult to obtain high surface hardness.
Therefore, the Mn content is set to 0.35 to 1.5%. Preferably it is 0.5% or more, more preferably 0.6% or more, and still more preferably 0.7% or more. Moreover, it is preferably 1.3% or less, more preferably 1.1% or less, and still more preferably 1.0% or less.

- P: 0.04% or less P is an element that deteriorates the toughness of the product, so it should be 0.04% or less. Preferably it is 0.03% or less.

- S: 0.01% or less S is an element that forms MnS in the coexistence of Mn and improves the machinability of the product. However, if it is too large, hot workability deteriorates, so the content of S is set to 0.01% or less. Preferably it is 0.005% or less, more preferably 0.003% or less.

・Ni: 0.05-0.20%
Ni is an effective element for improving corrosion resistance to non-oxidizing acids such as formic acid, sulfuric acid, and hydrochloric acid. However, if there is too much Ni, the Cu phase becomes fine in the stainless steel (the area ratio of the coarse Cu phase becomes small), and cold workability deteriorates. Furthermore, in stainless steel products after quenching and tempering, the austenite structure is stabilized, making it difficult to obtain a martensitic structure and making it difficult to obtain high surface hardness. In particular, in the solid phase nitrogen absorption treatment described below, a large amount of N is dissolved in the surface layer, so that only the surface layer becomes an austenitic structure, and the hardness is significantly reduced. Therefore, the Ni content is set to 0.05 to 0.20%. Preferably it is 0.07% or more, more preferably 0.10% or more. Moreover, it is preferably 0.18% or less, more preferably 0.15% or less.

・Cr: 12.5-14.6%
Cr is an element that forms an amorphous passive film on the surface of stainless steel and imparts corrosion resistance to the product. It also has the effect of increasing the amount of nitrogen that can be dissolved in stainless steel, and in the present invention, it is an element that promotes the solid solution of nitrogen in the solid phase nitrogen absorption treatment described below. However, if there is too much Cr, the ferrite structure will be stabilized, making it difficult to obtain a martensitic structure and making it difficult to obtain high surface hardness.
Therefore, the Cr content is set to 12.5 to 14.6%. Preferably it is 14.0% or less, more preferably 13.5% or less.

・One or two of Mo and W according to the relational expression (Mo+1/2W): 1.5 to 3.0%
Mo and W are similar elements and can be treated equally using the relational expression (Mo+1/2W). These elements have the effect of increasing the amount of nitrogen absorbed during solid phase nitrogen absorption treatment. Mo and W are elements that have the effect of stabilizing the passive film of stainless steel in a solid solution state, and also contribute to increasing the corrosion resistance of the product surface. Moreover, when the passive film made of Cr is damaged, Mo and W have the function of increasing the amount of Cr in the damaged area and strengthening the repairing power of the passive film. However, if Mo and W are too large, the ferrite structure will be stabilized, similar to the above-mentioned Cr, making it difficult to obtain a martensitic structure. Therefore, the content of Mo and W is set to 1.5 to 3.0% using the relational expression (Mo+1/2W). Preferably it is 1.7% or more, more preferably 2.0% or more. Moreover, it is preferably 2.5% or less, more preferably 2.3% or less.

・Cu: 1.0-3.0%
Cu is an essential element for simultaneously achieving the high hardness and excellent corrosion resistance of the present invention. In particular, Cu is an effective element for improving corrosion resistance against non-oxidizing acids such as formic acid, sulfuric acid, and hydrochloric acid. However, if there is too much Cu, hot workability will be extremely deteriorated. Therefore, the Cu content is set to 1.0 to 3.0%. Preferably it is 1.5% or more, more preferably 2.0% or more. Moreover, it is preferably 2.5% or less, more preferably 2.3% or less.

・N: 0.03~0.10%
N is an element that can suppress the precipitation of delta ferrite, which is harmful to the microstructure, in stainless steel before quenching and tempering. It is an element that dissolves solidly in the martensitic structure of stainless steel products after quenching and tempering and improves hardness and corrosion resistance. However, if there is too much N, bubbles are generated during casting, which not only significantly deteriorates manufacturability but also may cause coarse nitrides to crystallize after solidification. Furthermore, when stainless steel before quenching is finished into a product shape, it tends to be work hardened during cold working, requires repeated intermediate annealing, and machinability also deteriorates. Therefore, the N content is set to 0.03 to 0.10%. Preferably it is 0.04% or more, more preferably 0.05% or more. Moreover, it is preferably 0.08% or less, more preferably 0.06% or less.

(2) The stainless steel of the present invention preferably has a coarse Cu phase in its matrix. Specifically, the area ratio of the Cu phase having an equivalent circle diameter of 0.03 μm or more in the cross-sectional structure is 2.4% or more.
By annealing a stainless steel material that satisfies the above component composition (1), a Cu phase can be formed in the matrix. By performing this annealing at a temperature of 760 to 780°C as described in (3) below, the Cu phase in the matrix tends to remain coarse, which contributes to a decrease in the hardness of stainless steel (for example, at 270 HV or lower). Vickers hardness), and can impart excellent cold workability to stainless steel. In the case of the present invention, it is preferable to increase the area ratio of this coarse Cu phase in order to maintain cold workability. Specifically, it is preferable to increase the area ratio of this coarse Cu phase. It is preferable that the area ratio is 2.4% or more. More preferably, the area ratio is 3.5% or more and 4.5% or more. At this time, it is not necessary to particularly set the upper limit of the above-mentioned area ratio. However, assuming that cold workability deteriorates due to precipitation hardening of the Cu phase, it is realistic to set the upper limit to 10.0%.

In addition, in the above, the "cross-sectional structure" for measuring the distribution state of the coarse Cu phase can be the cross-sectional structure of the central part of the stainless steel. This cross-sectional structure is then confirmed by elemental mapping using an FE-EPMA (field emission microscopic X-ray analyzer) attached to the scanning electron microscope, and the 80 μm ² minute field of view area of approximately 9 μm square is image analyzed. By doing so, it is possible to measure the area ratio of the coarse Cu phase having an equivalent circle diameter (area circle equivalent diameter) of 0.03 μm or more.

(3) The method for manufacturing stainless steel of the present invention is to perform hot working on a stainless steel material having the above-mentioned composition (1) after heat treatment at 850 to 1090°C for 60 minutes or more. be. Then, annealing is further performed at a relatively low temperature of 760 to 780° C. for 4 hours or more.
According to Patent Document 1, "the alloy ingot is hot worked from a furnace temperature of 1093 to 1260°C, preferably 1149 to 1232°C, with any reheating required after intermediate processing." However, when an alloy ingot having the composition of Patent Document 1 is heated to the above temperature, delta ferrite is generated. Delta ferrite is a stable phase that forms at high temperatures, cannot be easily decomposed even by long-term heating, and is a harmful structure that deteriorates hardness and corrosion resistance.

At this time, also in the present invention, when hot working the above-mentioned stainless steel material, a heat treatment that also has the effect of diffusion annealing is performed as a pretreatment. In the case of the component composition of the present invention, delta ferrite begins to precipitate at about 1100°C, so the above heat treatment temperature needs to be 1090°C or lower. Preferably it is 1080°C or less, more preferably 1060°C or less. By setting the holding time at this heat treatment temperature to 60 minutes or more, the effects of the above-mentioned diffusion annealing and the like can be sufficiently obtained. In order to completely dissolve the Cu phase in solid solution, the above holding time is preferably 80 minutes or more, more preferably 100 minutes or more, and still more preferably 120 minutes or more. Furthermore, since the starting temperature of hot working subsequent to this heat treatment can be within the above heat treatment temperature range, precipitation of delta ferrite during hot working can also be suppressed.

Regarding the lower limit temperature of the heat treatment, it is desirable to completely dissolve Cu in solid solution in order to make the precipitation of the Cu phase uniform after this heat treatment or hot working. Therefore, in the present invention, the temperature is set to 850°C or higher. The temperature is preferably 880°C or higher, more preferably 900°C or higher, even more preferably 950°C or higher. Further, there is no particular specification regarding the upper limit of the holding time of the heat treatment. If consideration is given to the work time required for the heat treatment, it is realistic to set it to 180 minutes or less, for example.

Annealing performed after hot working is performed to reduce the hardness of stainless steel so that it can be easily cold worked into the desired shape. According to Patent Document 1, annealing is performed at 788 to 843° C. after hot rolling. However, when annealing is performed at this temperature, the coarse Cu phase partially becomes a solid solution in the matrix and finely re-precipitates during cooling after annealing, resulting in poor cold workability. On the other hand, when annealing is performed at a low temperature of 750° C. or lower, the growth of the Cu phase is slow and remains fine, resulting in high hardness and poor cold workability. Therefore, in the present invention, by holding the temperature at a relatively low temperature of 760 to 780°C, solid solution of the Cu phase and fine reprecipitation are suppressed, and the Cu phase is allowed to grow coarsely. ) Stainless steel can be used. By holding the stainless steel at this annealing temperature for 4 hours or more, the hardness of the stainless steel can be lowered to a value sufficient to impart cold workability.

Note that the lower limit temperature of the annealing is preferably 770° C. or higher, since it is desirable to coarsen the Cu phase in order to improve cold workability. Moreover, the upper limit of the holding time of the annealing is not particularly limited. In consideration of shortening the treatment time, the treatment time is preferably 10 hours or less, more preferably 6 hours or less, and still more preferably 5 hours or less.

(4) The stainless steel product of the present invention has a center hardness of 480 HV or more and less than 630 HV, and a surface hardness of 630 HV or more.
The stainless steel product of the present invention does not improve its overall hardness in order to maintain the toughness of the product. If the hardness of the center of the product is 480HV or more, even if it is less than 630HV (that is, the hardness is lower than the surface), the hardness of the surface layer of the product is 630HV or more. It is possible to impart excellent wear resistance (that is, high hardness) and high toughness to the steel. The hardness of the center of the product is preferably 500 HV or more, more preferably 510 HV or more, and still more preferably 520 HV or more. Moreover, it is preferably 610 HV or less, more preferably 590 HV or less, and still more preferably 570 HV or less. Note that the hardness of the center of a stainless steel product can be measured at a position that is not affected by the nitrogen absorption treatment described below (in other words, a position that does not include a nitrided layer). For example, the hardness at a position 1 mm from the surface of a stainless steel product can be measured.

The surface hardness is preferably 650 HV or more. More preferably, it is 670 HV or more, and still more preferably 690 HV or more. It is also possible to increase the voltage to 700HV or more. Although it is not necessary to specify the upper limit of hardness, approximately 900HV or 800HV is realistic. This hardness can be measured on the surface of the stainless steel product, including its nitrided layer.

As a result, the stainless steel product of the present invention can achieve high hardness and excellent corrosion resistance even in large products that are difficult to harden to the inside. At this time, since the penetration depth of nitrogen by the solid phase nitrogen absorption method is approximately 0.05 mm, the lower limit of the product thickness is, for example, in the direction from the surface of the product with the nitrided layer to the inside. , 0.20 mm, 0.30 mm, and 0.40 mm, which do not harden to the inside. At this time, the stainless steel product can have a nitrided layer on only one surface or both surfaces. Furthermore, the thickness of the stainless steel product can be greater than 1 mm or greater than 3 mm. Preferably, the product may have a thickness of 5 mm or more, 7 mm or more, or 9 mm or more. Although it is not necessary to set an upper limit for the thickness of the product, an upper limit of 50 mm is realistic.

(5) The method for manufacturing stainless steel products of the present invention includes quenching the stainless steel of (1) above by heating it to a temperature of 1000 to 1090°C in a nitrogen atmosphere and cooling it, followed by After performing sub-zero treatment in which the temperature is -50°C or lower, tempering is performed at a tempering temperature of 150 to 470°C.
Quenching, sub-zero treatment and tempering are performed to adjust the mechanical properties of stainless steel to suit its application. Regarding quenching, in the present invention, the above-mentioned stainless steel is subjected to quenching accompanied by solid phase nitrogen absorption treatment. By performing this solid phase nitrogen absorption treatment at a temperature of 1000 to 1090°C, precipitation of delta ferrite, which is harmful to corrosion resistance, can be suppressed. Then, after the subsequent sub-zero treatment and tempering, the stainless steel product of the present invention whose surface layer portion satisfies the above-mentioned condition (4) can be obtained.

According to US Pat. No. 5,002,300, the final product form is austenitized at 954-1093°C, preferably at least 1038°C, followed by hardening, preferably heating in vacuum for 1 hour to prevent oxidation. Hardened by rapid gas cooling. However, by hardening in a vacuum, it is difficult to simultaneously increase surface hardness and corrosion resistance.
On the other hand, in the case of the present invention, the solid phase nitrogen absorption treatment is performed in conjunction with the quenching. Since stainless steel has the above-mentioned composition (1) and contains Cr and Mo, firstly, the amount of nitrogen that can be dissolved in the alloy is increased. Furthermore, due to the above-mentioned component composition (1), the structure of this stainless steel is sufficiently austenitized at a heating temperature of 1000° C. or higher, so that the amount of nitrogen that can be dissolved in solid solution is further increased. Therefore, if the above-mentioned stainless steel is maintained at this heating temperature in a nitrogen atmosphere, a sufficient amount of nitrogen can be solid-dissolved in the structure of the surface layer which is in an austenitic state at that time. Preferably, the heating temperature is 1050°C or higher. By setting the upper limit of this heating temperature to 1090° C., precipitation of harmful delta ferrite can be suppressed, and excellent corrosion resistance of the product can be maintained.

For example, nitrogen gas can be used as the nitrogen atmosphere. A specific example is an atmosphere containing 90% by volume or more of this nitrogen gas. Preferably, by making this nitrogen atmosphere a "pressurized atmosphere" (including atmospheric pressure), absorption of nitrogen from the surface of the stainless steel is promoted, which is effective in reducing processing time and processing costs. It is. Regarding this, generating plasma in a nitrogen atmosphere and using more active radical nitrogen is also effective in reducing processing time and processing cost.
By using the above solid phase nitrogen absorption treatment conditions, the quenching according to the present invention can be performed in a standard quenching pattern for martensitic stainless steel, so while suppressing delta ferrite, the surface layer after quenching can be It can be a martensitic structure.

By applying sub-zero treatment to the stainless steel that has been quenched, high hardness and excellent corrosion resistance can be obtained. Unless sub-zero treatment is performed, it is difficult to obtain high hardness in the surface layer portion as shown in the examples described later. The processing temperature can be, for example, −50° C. or lower. The lower limit temperature is not particularly limited, but −200° C., which is the temperature of general cryo processing, is realistic. The holding time at the treatment temperature can be, for example, 10 seconds or more. In the case of the present invention, it is important to increase the hardness and corrosion resistance of the surface layer, and even if the treatment time is short, the surface layer undergoes sufficient martensitic transformation and is hardened. The holding time at the processing temperature is not particularly limited, but since the longer the holding time, the higher the processing cost, a realistic upper limit of the holding time is 1 hour.

Then, the stainless steel that has undergone the above sub-zero treatment is tempered to adjust mechanical properties such as hardness. The tempering temperature can be 150-470°C. Fine carbides are precipitated in the structure by tempering. The tempering temperature is set to 150° C. or higher because the fine carbide precipitation reduces the carbon content of the martensite base and provides the product with appropriate toughness. The temperature is preferably 250°C or higher, more preferably 350°C or higher. However, if the tempering temperature becomes too high, the Cu phase will grow and form a local battery with the martensite base, deteriorating the corrosion resistance, so the upper limit temperature is set at 470°C. The temperature is preferably 450°C, more preferably 440°C, even more preferably 410°C, and even more preferably 400°C. The holding time at the tempering temperature can be, for example, 30 seconds to 3 hours. By these things, it is possible to impart appropriate toughness to the product, and furthermore, it is possible to increase the hardness of the product surface to 630 HV or more.

The stainless steel product of the present invention can be produced by subjecting the surface of the stainless steel (1) to, for example, the nitrogen absorption treatment (5) described above (that is, quenching heating that also serves as solid-phase nitrogen absorption treatment). It is preferable to have a nitride layer containing a Cu phase. As a result, high hardness and excellent corrosion resistance can be simultaneously imparted to the surface layer (nitrided layer) of the stainless steel product of the present invention.
Furthermore, the stainless steel product of the present invention has a fine Cu phase in its base by subjecting the stainless steel of (1) to, for example, the solid phase nitrogen absorption treatment, subzero treatment, and tempering of (5) above. It is preferable that By satisfying the tempering temperature in (5) above, the Cu phase in the matrix becomes fine, and excellent corrosion resistance can be imparted to the stainless steel product.

10 kg of molten metal melted in a high-frequency induction melting furnace was cast to produce stainless steel ingots having multiple component compositions shown in Table 1. Next, these ingots were used as materials A to Z and AA, and were subjected to heat treatment at a holding temperature of 950 to 1150° C. for 60 minutes. This holding temperature is used as the forging starting temperature, and after cooling by performing hot forging with a forging ratio (cross-sectional area before forging / cross-sectional area after forging) of about 10, at a holding temperature of 700 to 860 ° C. Annealing was performed for a period of time or longer to obtain stainless steel having a thickness of approximately 20 mm.

<Evaluation of stainless steel>
First, stainless steel before quenching and tempering was evaluated. Among the above materials, the heat treatment and annealing conditions (holding temperature x holding time) for the materials V, Z, and AA of the invention examples and the materials T and U of the comparative examples are shown in Table 2. Note that air cooling was used for cooling after hot forging, and furnace cooling was used for cooling after annealing. As a result, annealed stainless steels V-1 to 3, Z-1 to 3, AA-1, T-1 to 3, and U-1 were produced.

The hardness of stainless steels V-1 to 3, Z-1 to 3, AA-1, T-1 to 3, and U-1, the presence or absence of delta ferrite phase, and the equivalent circle diameter of 0.03 μm in the cross-sectional structure. The area ratio of the above coarse Cu phase and cold workability were investigated. Regarding stainless steel U-1, only the hardness, presence or absence of delta ferrite phase, and area ratio of coarse Cu phase were investigated.
The hardness of the stainless steel was determined by measuring the Vickers hardness at the center in the thickness direction of a cross section parallel to the forging (stretching) direction. The load at the time of measurement was 300 g, and the surface was polished to a mirror surface before measurement.
The presence or absence of delta ferrite phase in stainless steel can be determined by examining the structure at the center (approximately 10 mm depth from the surface) in the thickness direction (perpendicular to the direction of elongation) using an optical microscope (100x magnification) in the above cross section. I observed it and did it.

To observe the coarse Cu phase of stainless steel, the structure at the center in the thickness direction (approximately 10 mm depth from the surface) in the above cross section was observed using an FE-EPMA (field emission microscopic X-ray analyzer). Observation was made using Cu element mapping (10,000x magnification). Elemental mapping was performed using JEOL JXA-8530F, acceleration voltage 15.0 kV, irradiation current 0.05 μA, analysis time per point 20 ms, spectroscopic crystal LIFH, characteristic X-ray intensity of Cu. was measured. FIG. 1 is a binary map of the Cu element of stainless steel V-1 according to the present invention, in which characteristic X-ray intensity of more than 125 is shown as white, and that of less than 125 is shown as black. 2 and 3 show the elemental mapping of Cu in stainless steels V-3 and T-1 of comparative examples, which were binarized using the same procedure. In FIGS. 1 to 3, the distribution substance confirmed as a granular white contrast phase (for example, the arrow in the figure) is a Cu phase. Then, the area ratio of the coarse Cu phase having an equivalent circle diameter of 0.03 μm or more was measured by performing ^{a 2-} minute image analysis of this visual field area of about 80 μm. For the above image analysis, image processing software "ImageJ (http://imageJ.gov/ij/)" provided by the National Institutes of Health (NIH) was used.

The cold workability of stainless steel was evaluated by the presence or absence of "cracks" when a 20 x 45 x 3.5 mm test piece was cut from each stainless steel and rolled at room temperature to a rolling reduction of 80%. The evaluation criteria are "◎ (Excellent)" for those with no cracks up to 80% reduction, "○ (Good)" for those with slight cracks only at the edges, and a comparison of approximately 1 mm at the edges. Those in which large cracks occurred were rated "△ (fair)," and those in which large cracks of 5 mm or more occurred were rated "x (poor)." The above results are also shown in Table 2.

Among the stainless steels made from material V, stainless steel V-1 of the present invention example has a large area ratio of coarse Cu phase due to the moderately low holding temperature during annealing after hot forging. Since the precipitation of fine Cu phase was suppressed, it could be processed without cracking and had excellent cold workability. Further, no harmful delta ferrite precipitation was observed in the microstructure. On the other hand, no precipitation of delta ferrite was observed in the microstructures of stainless steels V-2 and V-3 as comparative examples. However, due to the high holding temperature during annealing after hot forging, the Cu phase re-dissolved in the austenite, so the area ratio of the coarse Cu phase was relatively small, and cracks occurred in both cases.
Among the stainless steels made from material Z, stainless steel Z-3 of the present invention example has a large area ratio of coarse Cu phase due to the moderately low holding temperature during annealing after hot forging. Since the precipitation of fine Cu phase was suppressed, it could be processed without cracking and had excellent cold workability. On the other hand, in stainless steels Z-1 and Z-2 of comparative examples, the holding temperature during annealing after hot forging was as low as 750°C or less, and the growth of the Cu phase was slow and remained fine. , the hardness increased, and cracks occurred in both cases.
In addition, the stainless steel AA-1 of the present invention has a very large area ratio of coarse Cu phase due to the low hot forging temperature, and the precipitation of fine Cu phase is suppressed, which prevents cracking. It was possible to process the material without any complication, and it had very good cold workability.

Furthermore, in the stainless steel made from material T, no precipitation of delta ferrite was observed in any of the microstructures. However, stainless steels T-2 and T-3, which had a high holding temperature during annealing after hot forging, had a small area ratio of coarse Cu phase, and large cracks of 5 mm or more occurred at the ends. Even with stainless steel T-1, which has been kept at an appropriate holding temperature during annealing after hot forging, coarse Cu phase is formed due to the high Ni content in the material T. The area ratio was small, and relatively large cracks of about 1 mm occurred.

In addition, even though the area ratio of the coarse Cu phase in stainless steel U-1 made from material U is appropriate, in addition to the low N content in the component composition of material U, the Because the holding temperature during heat treatment was also high, delta ferrite, which is harmful to the microstructure, precipitated. As a result, stainless steel products after quenching and tempering may lack hardness and corrosion resistance.

<Evaluation of stainless steel products>
Next, stainless steel products 1 to 53 were produced by quenching and tempering the stainless steels produced from materials A to Z and AA. Note that the heat treatment and annealing conditions when producing stainless steel from the raw material were in accordance with those for stainless steel V-1 in Table 2.

The stainless steel was subjected to three different types of quenching and tempering. That is, in the examples of the present invention and some of the comparative examples, quenching heating is carried out by heating and holding in a nitrogen atmosphere made of nitrogen gas (purity 99%) at atmospheric pressure. The heating temperature in the above-mentioned quenching heating and the holding time at that heating temperature are shown in Table 4. The quenching was performed by quenching to room temperature using nitrogen gas pressurized to 2 atmospheres, and after the quenching, sub-zero treatment was immediately performed except for product 25. The conditions for the sub-zero treatment were to use liquefied carbon dioxide at -75°C and hold it there for 30 minutes. After this, tempering was performed at the tempering temperature shown in Table 4 for 1 to 2 hours.

As another comparative example of the present invention, two different types of quenching and tempering were performed. That is, a quenching method was performed in which the material was heated and held in the air for quenching, and then rapidly cooled in oil. The heating temperature in the above-mentioned quenching heating and the holding time at the heating temperature are as shown in Table 3. Immediately after quenching, sub-zero treatment was performed. The conditions for the sub-zero treatment were to use liquefied carbon dioxide at -75°C and hold it there for 30 minutes. After this, tempering was performed at the tempering temperature shown in Table 3 for 1 to 3 hours. Since an oxide generated by atmospheric heating was formed on the surface of the test piece, this oxide was removed by grinding to a depth of 1 mm or more. As another comparative example, this ground stainless steel product, which had been subjected to atmospheric heating, sub-zero treatment, and tempering, was further subjected to gas nitriding treatment in an ammonia gas atmosphere at 500°C.

The hardness of the surface layer of the stainless steel products was determined by measuring the Vickers hardness of the surfaces of Products 1 to 53. The load at the time of measurement was 50 g, and the surface of the product was polished by about 0.002 mm with #1500 emery paper.
The hardness of the center of stainless steel products can be determined by cutting the products 1 to 53 in half along a cross section perpendicular to the surface of the block with the nitrided layer, and measuring the Vickers hardness at a position 1 mm from the surface of the cross section. did. The load at the time of measurement was 300 g, and the cross section was mirror polished.

A constant temperature corrosion test was conducted in which Products 1 to 53 were immersed in a 5% formic acid solution and a 20% sulfuric acid solution held at 50°C for 5 hours to evaluate their corrosion resistance. The corrosion resistance of the product was evaluated by calculating the corrosion rate (mg/hcm ² ) from the amount of weight loss before and after the corrosion test. The evaluation criteria for the formic acid corrosion resistance test is that those smaller than 0.1 mg/hcm ² are rated "◎ (excellent)", those that are relatively small from 0.1 to 0.5 mg/hcm ^{2 are also rated "○ (good)", and those that are relatively small (0.1 to 0.5 mg/hcm 2} ) are rated "○ (good)". Those larger than 5 mg/ ^hcm2 were rated "× (poor)". In addition, the evaluation criteria for the sulfuric acid corrosion resistance test are: ``◎ (Excellent)'' for less than 30 mg/ ^hcm2 , ``Good'' for relatively small amounts of 30 to 40 mg/ ^hcm2 , and ``Good'' for less than 40 mg/ ^hcm2 . The items were marked as "x" (poor). These results are also shown in Tables 3 and 4.

Products 1, 4 to 6 are comparative examples made using stainless steel with a high carbon content of 0.67 to 1.01%. Under standard quenching conditions, high hardness of 660 HV or higher was obtained for both the surface and center hardness of the product. However, due to the large amount of carbon, many coarse carbides remained that did not dissolve into solid solution during quenching, which became the starting point for corrosion and had a negative impact on corrosion resistance.

Products 2 and 3 are comparative examples made using a general martensitic stainless steel material containing 0.33 to 0.38% carbon. Under standard quenching conditions, some products had relatively good corrosion resistance, but the surface hardness of the product was less than 600 HV, and the targeted high hardness could not be obtained.

Products 7 to 16 are examples of the present invention or comparative examples manufactured using stainless steel materials containing 0.19 to 0.24% carbon and having similar compositions. Under standard quenching conditions, although good corrosion resistance was obtained for Products 9 to 11, the surface hardness of the products was 607 HV or less, and high hardness was not obtained. Note that corrosion resistance was not evaluated for Products 7, 8, and 14 to 16 because high surface hardness could not be obtained.

Products 17 to 20 are stainless steel products with different compositions, including the composition of the stainless steel of the present invention example (Material V), which are held in ammonia gas at 500° C. for 3 hours after quenching and tempering, and then subjected to gas nitriding treatment. This is a comparative example. Under standard gas nitriding treatment conditions, extremely high hardness was obtained due to the formation of hard nitrides on the surface of the product. However, hard nitrides have low toughness and are therefore undesirable because they may peel off when subjected to an impact load.

Products 21 to 24 are comparative examples manufactured using a stainless steel material that does not contain Cu and quenched using solid phase nitrogen absorption treatment. Since Product 21 contained 0.67% carbon, a large amount of nitrogen dissolved in the solid phase nitrogen absorption treatment left austenite in the surface layer of the product during quenching, and the surface hardness significantly deteriorated. Products 22 to 24 had high hardness in the product surface layer, but did not have excellent corrosion resistance because they did not contain Cu.

Product 25 is a comparative example produced by applying the solid phase nitrogen absorption treatment to the stainless steel composition (Material V) of the present invention example and hardening it, but without performing the sub-zero treatment. Since sub-zero treatment was not performed, austenite remained and although high corrosion resistance was obtained, the hardness of the product surface was very low.

Products 26 to 39 are comparative examples manufactured by quenching using solid-phase nitrogen absorption treatment using stainless steel materials with compositions similar to those of the present invention examples. Since Product 26 contained 0.29% Al, a large amount of nitrogen dissolved in solid solution during the solid-phase nitrogen absorption treatment formed Al nitride, which adversely affected hardness and corrosion resistance. Products 27 to 30 and 34 to 38 contain 0.99 to 4.03% of Ni, an austenite stabilizing element, so austenite remains during quenching due to a large amount of nitrogen dissolved in the solid phase nitrogen absorption treatment, and the product The hardness of the surface layer was significantly reduced. Since Products 31 to 33 contained 15.95 to 16.02% Cr, delta ferrite precipitated, significantly reducing the hardness of the product center. On the other hand, in the surface layer of the product, almost all delta ferrite disappeared due to a large amount of nitrogen dissolved in the solid phase nitrogen absorption treatment, resulting in high surface hardness. Since Product 39 contained only 0.01% nitrogen, delta ferrite was formed in the center of the product, resulting in insufficient hardness in the center.

Products 40 to 53 are examples manufactured using stainless steel materials having the compositions of the examples of the present invention. In all cases, the surface hardness of the products was moderately high at 631 HV or higher, and the hardness at the center was 498 to 588 HV, which was an appropriate level of hardness. It also had excellent corrosion resistance.

Claims

In mass%, C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, P: 0.04% or less, S: 0.01 % or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W): 1.5 to 3.0%, Cu: 1.0 to 3.0%, N: 0.03 to 0.10%, and the balance is Fe and impurities.
The stainless steel according to claim 1, wherein the area ratio of the Cu phase having an equivalent circle diameter of 0.03 μm or more in the cross-sectional structure is 2.4% or more.
In mass%, C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, P: 0.04% or less, S: 0.01 % or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W): 1.5 to 3.0%, Cu: 1.0 to 3.0%, N: 0.03 to 0.10%, balance Fe and impurities in a stainless steel material kept at 850 to 1090°C for 60 minutes or more. A method for producing stainless steel, which comprises performing heat treatment, hot working, and then annealing at 760 to 780°C for 4 hours or more.
In mass%, C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, P: 0.04% or less, S: 0.01 % or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W): 1.5 to 3.0%, Cu: 1.0~3.0%, N: 0.03~0.10%, balance Fe and impurities.It is a stainless steel product with a nitride layer on the surface. can be,
A stainless steel product characterized in that the center hardness of the stainless steel product is 480 HV or more and less than 630 HV, and the surface hardness of the stainless steel product is 630 HV or more.
In mass%, C: 0.18 to 0.25%, Si: 0.1 to 1.5%, Mn: 0.35 to 1.5%, P: 0.04% or less, S: 0.01 % or less, Ni: 0.05 to 0.20%, Cr: 12.5 to 14.6%, one or two of Mo and W according to the relational expression (Mo+1/2W): 1.5 to 3.0%, Cu: 1.0~3.0%, N: 0.03~0.10%, balance Fe and impurities. ℃ quenching, followed by sub-zero treatment at a treatment temperature of -50 ℃ or less, and then tempering at a tempering temperature of 150 to 470 ℃. Method.
The method for manufacturing a stainless steel product according to claim 5, wherein the tempering temperature is 150 to 400°C.