Patent Application
20250075302
The invention relates to a stainless steel suitable for manufacturing a stainless steel product that can be used in a corrosive environment, such as various sliding products or high-strength products, and a manufacturing method therefor. In addition, the invention relates to the stainless steel product and a manufacturing method therefor.
Conventionally, in order to facilitate the properties of stainless steel products, such as corrosion resistance, hardness, fatigue strength, etc., means for suitably providing contents of configuration components, a nitriding process of forming a nitride layer on the surface by using high-frequency induction heating or at a relatively low temperature, such as about 500° C., a solid-phase nitrogen absorption process in which nitrogen is dissolved on the surface at a high temperature of approximately 1000° C., have been proposed.
For example, Patent Document 1 proposes a corrosion-resistant martensitic stainless steel alloy, containing, in terms of % by weight, components as follows: about 0.10% to 0.40% of Cu, about 0.01% to 2.0% of Mn, at maximum about 2.0% of Si, at maximum about 0.2% of P, at maximum about 0.030% of S, about 10% to 15% of C, at maximum about 0.5% of Ni, about 0.75% to 4.0% of Mo, 0.02% to 0.15% of N, at maximum about 0.01% of Ti, at maximum about 0.01% of Al, at maximum about 0.10% of Nb+Ta, at maximum about 0.20% of V, less than 0.001% of Zr, less than 0.001% of Ca, the balance is substantially iron, and Ni/Cu less than 0.2.
In addition, Patent Document 2 proposes stainless steel for an engine fuel system product, characterized in that the stainless steel contains, by mass %, C: 0.15% to 0.75%, Si: 0.05% to 2.00%, Mn: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cr: 10.00% to 15.00%, Mo: 0.50% to 3.00%, Cu: 0.01% to 2.00%, N: 0.010% to % 0.150%, and satisfies 0.20≤C+N≤0.80, the balance is Fe and impurities, a surface hardened layer is formed in a portion or over the entirety of the surface, the carbonitride size in the surface hardened layer is 1.0 μm or less and the carbonitride amount is 0.50 mass % or less, the HV hardness is 500 HV or more, and the bulk HV hardness is 350 HV or less. In addition, as a manufacturing method for the stainless steel, “a manufacturing method for performing controlled annealing, forging with a reduction rate of 60% or more after hot-rolling steel having the above chemical composition and performing a surface hardening process” is proposed.
Patent Document 1 discloses an effective method for facilitating the corrosion resistance of a martensitic stainless steel product. However, in the case of Patent Document 1, if the conventional quenching and tempering is used to increase the hardness, for example, when the content of C is high, even though the surface hardness is high the corrosion resistance is low, and on the contrary, even though the corrosion resistance is favorable when the content of C is low, the surface hardness decreases. It is difficult to increase the surface hardness and corrosion resistance at the same time. In addition, since the annealing temperature after hot working reaches as high as 788° C. to 843° C., in the case where the content of Cu is 1.5% to 4.0%, the cold workability may deteriorate due to precipitation hardening caused by the Cu phase. In addition, in the case of Patent Document 2, it substantially discloses high-frequency annealing in which quenching and tempering are performed only on the surface of the forged product. Although the surface hardness of the product is high, the corrosion resistance is a concern. In addition, the internal hardness drops to or below 350 HV, and the product internal strength becomes a concern.
An objective of the invention is to provide a stainless steel with excellent cold workability and a manufacturing method therefor, as well as a stainless steel product with a surface hardness that is sufficiently high at 630 HV or more, an internal hardness that is increased to 480 HV or more and less than 630 HV and further excellent corrosion resistance against non-oxidative acids and a manufacturing method therefor.
In response to the issues, the inventors have investigated the annealing temperature after hot working in order to further improve the cold workability of stainless steel. Furthermore, the solid-phase nitrogen absorption process is investigated to increase the hardness and corrosion resistance of stainless steel products at the same time. As a result, it is found that, by decreasing the annealing temperature after hot working and coarsely dispersing the Cu phase, the cold workability of stainless steel can be improved, and the improvement to the component composition of the stainless steel is effective in terms of the hardness and corrosion resistance of the stainless steel product. In addition, it is found that, by finely dispersing “Cu phase” to the surface part of a stainless steel product after quenching and tempering and further forming a martensite containing a large amount of nitrogen, excellent hardness and corrosion resistance can also be exhibited at the same time. It is found that decreasing the tempering temperature after quenching and sub-zero treatment is effective in order to exhibit such hardness of the surface part and the corrosion resistance of the stainless steel product. Accordingly, the invention is achieved.
That is, an aspect of the invention provides a stainless steel having a component composition: in terms of % by mass, 0.18% to 0.25% of C, 0.1% to 1.5% of Si, 0.35% to 1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 0.05% to 0.20% of Ni, 12.5% to 14.6% of Cr, 1.5% to 3.0% of one or both of Mo and W according to a relational expression of (Mo+½W), 1.0% to 3.0% of Cu, and 0.03% to 0.10% of N, remainders being Fe and impurities. Preferably, in the invention, an area ratio of a Cu phase having an equivalent circle diameter of 0.03 μm or more in a cross-sectional structure is 2.4% or more.
In addition, another aspect of the invention provides a manufacturing method for a stainless steel product, including: performing, on a stainless steel having a component composition: in terms of % by mass, 0.18% to 0.25% of C, 0.1% to 1.5% of Si, 0.35% to 1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 0.05% to 0.20% of Ni, 12.5% to 14.6% of Cr, 1.5% to 3.0% of one or both of Mo and W according to a relational expression of (Mo+½W), 1.0% to 3.0% of Cu, and 0.03% to 0.10% of N, remainders being Fe and impurities, hot working after performing a heat treatment that holds at 850° C. to 1090° C. for 60 minutes or more, and then performing annealing at 760° C. to 780° C. for four hours or more.
In addition, another aspect of the invention provides a stainless steel product having a nitride layer on a surface of a stainless steel having a component composition: in terms of % by mass, 0.18% to 0.25% of C, 0.1% to 1.5% of Si, 0.35% to 1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 0.05% to 0.20% of Ni, 12.5% to 14.6% of Cr, 1.5% to 3.0% of one or both of Mo and W according to a relational expression of (Mo+½W), 1.0% to 3.0% of Cu, and 0.03% to 0.10% of N, remainders being Fe and impurities. A hardness of a center of the stainless steel product is 480 HV or more and less than 630 HV, and a hardness of a surface of the stainless steel product is 630 HV or more.
In addition, another aspect of the invention provides a manufacturing method for a stainless steel product, including: performing, on a stainless steel having a component composition: in terms of % by mass, 0.18% to 0.25% of C, 0.1% to 1.5% of Si, 0.35% to 1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 0.05% to 0.20% of Ni, 12.5% to 14.6% of Cr, 1.5% to 3.0% of one or both of Mo and W according to a relational expression of (Mo+½W), 1.0% to 3.0% of Cu, and 0.03% to 0.10% of N, remainders being Fe and impurities, quenching that sets a quenching temperature at 1000° C. to 1090° C. in a nitrogen atmosphere, performing, following the quenching, a sub-zero treatment that sets a processing temperature at −50° C. or lower, and then performing tempering that sets a tempering temperature at 150° C. to 470° C. Preferably, the tempering temperature is set at 150° C. to 400° C.
According to the invention, the cold workability of the stainless steel can be increased, and the stainless steel product is imparted with high hardness and excellent corrosion resistance at the same time.
Conventionally, at the time of manufacturing various stainless steel products, firstly, hot working, such as hot rolling, is performed on a steel block or steel sheets obtained by performing a block-dividing process on the steel block as materials, and stainless steel (intermediate material) in an annealed state is prepared. Then, the stainless steel is subjected to cold working into shapes suitable for various product shapes. For example, in the case of a plate-shaped product, the stainless steel is cold-rolled to be imparted with the product thickness, and then the stainless steel is shaped into a product shape through various machining and arranged to exhibit a product thickness. And hardness is arranged through quenching and tempering.
In such series of manufacturing processes, the invention is characterized in the point of finding a component composition or a structure of suitable stainless steel suitable for further facilitating the cold workability of stainless steel. In addition, the invention is characterized in finding a combination of a component composition and a surface configuration of suitable stainless steel in order to exhibit high hardness as well as excellent corrosion resistance of a stainless steel product. With the stainless steel product having the combination of the component composition and the surface configuration, high hardness and excellent corrosion resistance can be realized at the same time in a corrosion environment containing a non-oxidative acid in particular, such as formic acid, sulfuric acid, and hydrochloric acid. In addition, such stainless steel product can be used in various sliding products, high-strength products, such as blades, plastic molding tools, punches, etc.
In the following, the stainless steel and the stainless steel product of the invention will now be described, along with exemplary manufacturing methods for exhibiting the same.
(1) The stainless steel of the invention has a component composition as follows: in terms of % by mass, 0.18% to 0.25% of C, 0.1% to 1.5% of Si, 0.35% to 1.5% of Mn, 0.04% or less of P, 0.01% or less of S, 0.05% to 0.20% of Ni, 12.5% to 14.6% of Cr, 1.5% to 3.0% of one or both of Mo and W according to the relational expression (Mo+½W), 1.0% to 3.0% of Cu, and 0.03% to 0.10% of N, the remainder being Fe and impurities.
In the case of the invention, in order to allow the product to exhibit excellent abrasion resistance (that is, high hardness), in the material, “stainless steel” whose component composition is adjusted to produce a martensite structure through quenching, sub-zero treatment, and tempering is used. In mass %, contents of less than 0.1% of Al and less than 0.1% of Nb are permissible as impurities.
C is an element effective for increasing the hardness of the martensite structure after quenching and tempering. However, if the content of C is too high, the amounts of Cr and Mo dissolved in the matrix are relatively reduced, and the corrosion resistance of the product deteriorates. In addition, even if the content of C is reduced, in the case of the invention, through the combination of containing Mo to be described in the following and a solid-phase nitrogen absorption process (heating of quenching), the surface part of the product can be imparted with a hardness of 630 HV or more, for example.
Accordingly, the content of C is set to be 0.18% to 0.25%. Preferably, the content is 0.20% or more. In addition, preferably, the content is 0.23% or less.
Si is used as a deoxidizer in a melting process, and is an element that can be unavoidably contained. In addition, when the content of Si is excessive, the hardness of an annealed material (i.e., the stainless steel of the invention before quenching and tempering (before the product hardness is adjusted)) increases, and the cold workability is reduced.
Accordingly, the content of Si is set to be 0.1% to 1.5%. Preferably, the content is 1.0% or less. More preferably, the content is 0.8% or less. Even more preferably, the content is 0.5% or less. In addition, the content is preferably 0.2% or more, and more preferably 0.3% or more.
Mn is used as a deoxidizer in a melting process, and is an element that can be unavoidably contained. In addition, Mn is an element having an effect of facilitating the dissolving of nitrogen into the structure in the solid-phase nitrogen absorption process to be described afterwards in the invention, in particular. However, if the content of Mn is too high, the austenite structure becomes stable, it is difficult to attain the martensite structure, and it is difficult to attain a high surface hardness.
Accordingly, the content of Mn is set to be 0.35% to 1.5%. Preferably, the content is 0.5% or more. More preferably, the content is 0.6% or more. Even more preferably, the content is 0.7% or more. In addition, preferably, the content is 1.3% or less. More preferably, the content is 1.1% or less. Even more preferably, the content is 1.0% or less.
Since P is an element that deteriorates the toughness of the product, P is set to be 0.04% or less. Preferably, the content is 0.03% or less.
S is an element that forms MnS in the presence of Mn and facilitates the machinability of the product. However, when then content of S is excessive, the hot workability deteriorates, so the content of S is set as 0.01 or less. Preferably, the content is 0.005% or less. More preferably, the content is 0.003% or less.
Ni is an element that is effective in facilitating the corrosion resistance against a non-oxidative acid, such as formic acid, sulfuric acid, and hydrochloric acid. However, when the content of Ni is excessive, in the stainless, the Cu phase becomes fine (the area ratio of the coarse Cu phase becomes small), and the cold workability deteriorates. In addition, in the stainless steel product after quenching and tempering, the austenite structure becomes stable, the martensite structure is difficult to attain, and it is difficult to attain a high surface hardness. In particular, a large amount of N on the surface is dissolved in the solid-phase nitrogen absorption process, only the surface becomes the austenite structure and the hardness decreases significantly. Accordingly, the content of Ni is set to be 0.05% to 0.20%. Preferably, the content is 0.07% or more. More preferably, the content is 0.10% or more. In addition, preferably, the content is 0.18% or less. More preferably, the content is 0.15% or less.
Cr is an element that forms an amorphous passive film on the surface of the stainless steel, imparting corrosion resistance to the product. In addition, Cr also has an effect of increasing the amount of nitrogen that can be dissolved in the stainless steel. In the invention, it is an element that works to facilitate the dissolution of nitrogen in the solid-phase nitrogen absorption process to be described in the following. However, if the content of Cr is too high, the ferrite structure becomes stable, the martensite structure is difficult to attain, and it is difficult to attain a high surface hardness.
Accordingly, the content of Cr is set to be 12.5% to 14.6%. Preferably, the content is 14.0% or less. More preferably, the content is 13.5% or less.
Mo and W are similar elements and can be treated equally according to the relational expression (Mo+½W). Such elements have the effect of increasing the amount of nitrogen absorbed during the solid-phase nitrogen absorption process. Mo and W are elements that have an effect of stabilizing the passive film of the stainless steel in a dissolution state and contribute to facilitating the corrosion resistance of the surface of the product. In addition, Mo and W serve to increase the amount of Cr at a damaged area when the passive film is damaged due to Cr, thereby strengthening the restoration ability of the passive film. However, if the contents of Mo and W are too high, like Cr, the ferrite structure becomes stable, and the martensite structure is difficult to attain. Accordingly, the contents of Mo and W are set to be 1.5% to 3.0% by using the relational expression of (Mo+½W). Preferably, the content is 1.7% or more. More preferably, the content is 2.0% or more. In addition, preferably, the content is 2.5% or less. More preferably, the content is 2.3% or less.
Cu is an essential element for exhibiting high hardness and excellent corrosion resistance at the same time according to the invention. In particular, Cu is an element that is effective in facilitating the corrosion resistance against a non-oxidative acid, such as formic acid, sulfuric acid, and hydrochloric acid. However, when the content of Cu is excessive, the hot workability deteriorates extremely. Accordingly, the content of Cu is set to be 1.0% to 3.0%. Preferably, the content is 1.5% or more. More preferably, the content is 2.0% or more. In addition, preferably, the content is 2.5% or less. More preferably, the content is 2.3% or less.
Nis an element that can suppress of delta ferrite from being precipitated. Delta ferrite is harmful to the microstructure in the stainless steel before quenching and tempering. In addition, in the stainless steel product after quenching and tempering, N is an element dissolved in the martensite structure and facilitating the hardness and the corrosion resistance. However, if the amount of N is excessive, bubbles are generated during forging, which not only significantly deteriorate manufacturability but may also be a factor in the crystallization of coarse nitrides after solidification. In addition, when finishing is performed on the stainless steel to form a product shape before the stainless steel is quenched, work hardening may occur easily during cold working, it is necessary to perform working while repeatedly performing intermediate annealing, and machinability also deteriorates. Accordingly, the content of N is set to be 0.03% to 0.10%. Preferably, the content is 0.04% or more. More preferably, the content is 0.05% or more. Preferably, the content is 0.08% or less. More preferably, the content is 0.06 or less.
(2) The stainless steel of the invention may have 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 component composition of (1), the Cu phase can be formed in the matrix. In addition, by setting annealing at 760° C. to 780° C., as in (3) to be described in the following, the Cu phase in the matrix is easily held to be coarse, which helps decrease the hardness of the stainless steel (e.g., the Vickers hardness can be adjusted to 270 HV or less) and helps impart excellent cold workability to the stainless steel. In addition, in the case of the invention, increasing the area ratio of the coarse Cu phase is preferable for holding cold workability. Specifically, it is preferable to have the area ratio of the Cu phase having an equivalent circle diameter of 0.03 μm or more in the cross-sectional structure to be 2.4% or more. More preferably, the area ratio is 3.5% or more. More preferably, the area ratio is 4.5% or more. At this time, it is not necessary to particularly determine the upper limit of the area ratio.
However, considering the deterioration of the cold workability due to precipitation hardening of the Cu phase, it is practical to set the upper limit at 10.0%.
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. In addition, the cross-sectional structure is verified through element mapping by using a field emission type micro X-ray analysis device (FE-EPMA), and a visual field area of approximately a 9 μm square and 80 μm2 is subjected to image analysis. Accordingly, the area ratio of the coarse Cu phase having an equivalent circle diameter (area equivalent circle diameter) of 0.03 μm or more can be measured.
(3) A manufacturing method for stainless steel according to the invention performs, on a stainless steel material having the component composition of (1), hot working after a heat treatment that holds at 850° C. to 1090° C. for 60 minutes or more. Then, annealing is performed to hold at a relatively low temperature of 760° C. to 780° C. for 4 hours or more. According to Patent Document 1, it is arranged as “the alloy ingot is subjected to hot working from a furnace temperature of 1093° C.-1260° C., preferably 1149° C.-1232° C., with reheating as required after intermediate working.”. However, in the case where the alloy ingot of the component composition of Patent Document 1 is heated to the above temperature, delta ferrite is produced. Delta ferrite is in a stable phase that is generated at a high temperature and cannot be easily decomposed even when heated for a long period of time. It is a harmful structure that deteriorates the hardness and the corrosion resistance.
At this time, in the invention, when the stainless steel material is subjected to hot working, a heat treatment with also the effect of diffusion annealing, etc., is performed as a pre-process. In addition, in the case of the component composition of the invention, since delta ferrite begins to precipitate at about 1100° C., the heat treatment temperature must be 1090° C. or lower. Preferably, the temperature is 1080° C. or lower, and more preferably, the temperature is 1060° C. or lower. In addition, by setting the holding time at the heat treatment temperature to 60 minutes or more, the effect of the diffusion annealing can be sufficiently attained. In addition, to completely dissolve the Cu phase, the holding time is preferably 80 minutes or more, more preferably 100 minutes or more, and even more preferably 120 minutes or more. In addition, in the hot working following the heat treatment as well, the initial temperature can be set within the heat treatment temperature range, so the precipitation of delta ferrite during hot working can also be suppressed.
Regarding the lower temperature limit of the heat treatment, in order to unify the precipitation of the Cu phase after the heat treatment or after the hot working, Cu may be completely dissolved. Therefore, in the invention, the lower temperature limit is set to 850° C. or higher. Preferably, the lower temperature limit is 880° C. or higher. More preferably, the lower temperature limit is 900° C. or higher. Even more preferably, the lower temperature limit is 950° C. or higher. In addition, the upper limit of the holding time of the heat treatment is not particularly specified. Considering the operation time required for the heat treatment, it is practical to set the time to, for example, 180 minutes or less.
The annealing performed after hot working is performed to reduce the hardness, so as to easily perform cold working to form the stainless steel into a target shape. According to Patent Document 1, annealing is set at 788° C. to 843° C. after hot rolling. However, when annealing is performed at the temperature, the coarse Cu phase is partially dissolved in the matrix, and fine re-precipitation occurs during cooling after the annealing, so the cold workability deteriorates. Meanwhile, when annealing is performed at a temperature equal to or lower than 750° C., since the Cu phase grows slowly and remains in fine state, the hardness is high and the cold workability is poor. Accordingly, by holding at a relatively low temperature of 760° C. to 780° C. in the invention, the dissolution and the fine re-precipitation of the Cu phase can be suppressed, the Cu phase can grow to be coarse, and, preferably, the stainless steel satisfying the state of (2) can be attained. In addition, by making the holding time at the annealing temperature to be 4 hours or more, the hardness of the stainless steel can drop to a value sufficient to impart cold workability.
Regarding the lower temperature limit, since the Cu phase may be coarse to facilitate the cold workability, the lower temperature limit is preferably 770° C. or more. In addition, the upper limit of the holding time of the annealing is not particularly specified. Considering reducing the processing time, the holding time is preferably 10 hours or less, more preferably 6 hours or less, and even more preferably 5 hours or less.
(4) In the stainless steel product of the invention, the hardness of the center is 480 HV or more and less than 630 HV, and the surface hardness is 630 HV or more.
In the stainless steel product of the invention, the overall hardness is not facilitated to secure the toughness of the product. If the hardness of the center of the product is equal to or higher than 480 HV, even if the hardness is less than 630 HV (that is, less hard than the surface), with the surface part of the product being 630 HV or more, the product can be imparted with excellent abrasion resistance (that is, high hardness) and high toughness. In addition, the hardness of the center of the product is preferably 500 HV, more preferably 510 HV, and even more preferably 520 HV. In addition, the hardness is preferably equal to or less than 610 HV, more preferably equal to or less than 590 HV, and even more preferably equal to or less than 570 HV. The hardness of the center of the stainless steel product can be measured at a position not affected by the nitrogen absorption process to be described afterwards (in other words, a position not provided with a nitride layer). For example, the hardness at a position that is 1 mm away from the surface of the stainless steel product can be measured.
The hardness of the surface is preferably 650 HV or more. More preferably, the hardness of the surface is 670 HV or more, and more preferably 690 H or more. The hardness of the surface may also be 700 HV or more. Although it is not necessary to specify the upper hardness limit, it is practical to set the hardness of the surface to be about 900 HV or 800 HV. The hardness can be measured at the surface that includes the nitride layer in the stainless steel product.
Accordingly, even if the stainless steel product of the invention is a large product which is difficult to harden till the inside, high hardness and excellent corrosion resistance can be exhibited. At this time, considering that the depth of entry of nitrogen by using the solid-phase nitrogen absorption method is approximately 0.05 mm, the lower limit of the thickness of the product can be set to a value such as 0.20 mm, 0.30 mm, or 0.40 mm, where the product does not harden to the inside, in the direction from the surface of the product having the nitride layer toward the inside. At this time, the stainless steel product may have a nitride layer on the surface of a single side or on the surface of both sides. Furthermore, the thickness of the stainless steel product can also be set to exceed 1 mm or be 3 mm or more. Preferably, the product can be set with a thickness of 5 mm or more, 7 mm or more, 9 mm or more, etc. Although the upper limit of the thickness of the product is not particularly limited, it is practical to set an upper limit of 50 mm.
(5) The method for manufacturing the stainless steel product according to the invention includes: performing quenching in which the stainless steel as described in (1) is heated to a temperature of 1000° C. to 1090° C. in a nitrogen atmosphere and then cooled. Subsequently, a sub-zero treatment is performed at a processing temperature of −50° C. or lower. Then, tempering at a tempering temperature of 150° C. to 470° C. is performed.
The quenching, sub-zero treatment, and tempering are performed to adjust the mechanical properties of the stainless steel to a state suitable for its intended application. Among the above, regarding the quenching, in the invention, quenching is performed on the stainless steel together with the solid-phase nitrogen absorption process. Then, by setting the solid-phase nitrogen absorption process between 1000° C. and 1090° C., the precipitation of delta ferrite, which adversely affects the corrosion resistance, can be suppressed. After the subsequent sub-zero treatment and tempering, the stainless steel product in accordance with the invention in which the surface part satisfies the state of (4) can be manufactured.
According to Patent Document 1, the final product form is austenitized at a temperature ranging from 954° C. to 1093° C., preferably at least 1038° C., followed by hardening. Preferably, the product is heated for one hour in a vacuum environment and then quenched by rapid gas cooling to avoid oxidation. However, in quenching in a vacuum environment, it is difficult toy enhance surface hardness and corrosion resistance at the same time.
Comparatively, in the case of the invention, the solid-phase nitrogen absorption process is performed in conjunction with the quenching process. In addition, with the stainless steel having the component composition as in (1) and containing Cr, Mo, firstly, the amount of nitrogen that can be dissolved as an alloy thereof is increased. Moreover, according to the component composition described in (1) above, the structure of the stainless steel undergoes sufficient austenitization at a heating temperature of 1000° C. or higher. Therefore, the amount of nitrogen that can be dissolved is increased. Therefore, if the stainless steel is held at the heating temperature in the nitrogen atmosphere, a sufficient amount of nitrogen can be dissolved into the structure of the surface part in the austenitic state at the time. Preferably, the heating temperature is 1050° C. or more. Furthermore, by setting the upper limit of the heating temperature at 1090° C., the precipitation of harmful delta ferrite can be suppressed, thereby holding the excellent corrosion resistance of the product.
Regarding the aforementioned nitrogen atmosphere, nitrogen gas may be used. As a specific example, the atmosphere contains 90% or more by volume of nitrogen gas. Furthermore, it is preferable to arrange the nitrogen atmosphere as a “pressurized atmosphere” (including atmospheric pressure), as such atmosphere facilitates the absorption of nitrogen from the surface of the stainless steel. As a result, the processing time and cost can be effectively reduced. Regarding this, generating plasma in the nitrogen atmosphere and utilizing more active radical nitrogen are also effective in reducing processing time and cost.
According to the condition of the solid-phase nitrogen absorption process, the quenching according to the invention can be performed by using a standard quenching pattern for martensitic stainless steel. Therefore, delta ferrite can be suppressed, and the surface part after quenching can form a high-nitrogen martensitic structure.
In addition, by performing the sub-zero treatment on the stainless steel that has undergone the quenching process, high hardness and excellent corrosion resistance can be attained. If the sub-zero treatment is not performed, it will be difficult to exhibit high hardness in the surface part as described in the following examples. The processing temperature can be set, for instance, at −50° C. or lower. Although the lower temperature limit is not particularly limited, it is practical to set the lower temperature limit to −200° C., which is typical for a cryogenic process. Furthermore, the holding time at the processing temperature can be, for example, set to 10 seconds or longer. In the case of the invention, it is important to facilitate the hardness and corrosion resistance of the surface part. Even with a short processing time, the surface part still undergoes sufficient martensitic transformation and becomes hard. Regarding the holding time at the processing temperature, there are no specific limitations. However, as the retention time increases, the processing cost also increases. Therefore, it is practical to set the retention time upper limit to be one hour.
In addition, the stainless steel that has undergone the sub-zero treatment is subjected to tempering to adjust mechanical properties, such as hardness. The tempering temperature may be set between 150° C. and 470° C. Fine carbides within the structure precipitate through tempering. Through the precipitation of the fine carbides, the carbon content in the martensitic matrix is reduced, thereby imparting adequate toughness to the product. Therefore, the tempering temperature is set at 150° C. or higher. Preferably, the tempering temperature is 250° C. or more. More preferably, the tempering temperature is 350° C. or more. However, when the tempering temperature becomes too high, the Cu phase grows, a local cell and a martensitic matrix are formed, and the corrosion resistance deteriorates. Therefore, the upper temperature limit is set at 470° C. Preferably, the upper temperature limit is set at 450° C. More preferably, the upper temperature limit is set at 410° C. Even more preferably, the upper temperature limit is set at 400° C. Furthermore, the holding time at the tempering temperature can be, for example, set between 30 seconds and 3 hours. Accordingly, it is possible to impart adequate toughness to the product and further increase the surface hardness of the product to a high hardness of 630 HV or more.
The stainless steel product of the invention preferably has a nitride layer containing a fine Cu phase on the surface of the stainless steel (1) by performing, for example, the nitrogen absorption process (i.e., heating of quenching combined with solid-phase nitrogen absorption process) as described in (5) on the surface of the stainless steel of (1). Accordingly, it is possible to impart high hardness and excellent corrosion resistance to the surface part (nitride layer) of the stainless steel product of the invention at the same time.
Furthermore, it is preferable that, by performing the solid-phase nitrogen absorption process, the sub-zero treatment, and tempering as described in (5), for example, on the stainless steel of (1), the stainless steel product of the invention contains a fine Cu phase in the matrix thereof. Moreover, by satisfying the tempering temperature specified in (5), the Cu phase within the matrix becomes fine, and the stainless steel product is thus imparted with excellent corrosion resistance.
A 10 kg molten metal melted by using a high-frequency induction melting furnace was forged to manufacture ingots of stainless steel with various component compositions as shown in
Table 1. Then, the ingots were designated as materials A to Z and AA. Subsequently, a heat treatment was conducted to hold at a holding temperature of 950° C. to 1150° C. for 60 minutes. The holding temperature was set as a forging start temperature. Then, hot forging with a forging ratio (cross-sectional area before forging/cross-sectional area after forging) of approximately 10 was subsequently performed, and cooling was performed. Afterwards, annealing holding a holding temperature of 700° C. to 860° C. was performed for four hours or more, resulting in stainless steel with a thickness of approximately 20 mm.
Firstly, the stainless steel prior to quenching and tempering was evaluated. Regarding the materials V, Z, and AA of the invention examples and the materials T and U of the comparative examples among the materials, the heat treatment and the annealing conditions (holding temperature×holding time) are as shown in Table 2. Furthermore, the cooling following hot forging was set as air cooling, while the cooling after annealing was set as furnace cooling. Accordingly, annealed stainless steel V-1 to V-3, Z-1 to Z-3, AA-1, T-1 to T-3, and U-1 were manufactured.
In addition, the hardness, the presence/absence of the delta ferrite phase, the area ratio of the coarse Cu phase with an equivalent circle diameter of 0.03 μm or more occupying the cross-sectional microstructure, and the cold workability of the stainless steel V-1 to V-3, Z-1 to Z-3, AA-1, T-1 to T-3, and U-1 were examined. Regarding the stainless steel U1, only the hardness, the presence/absence of the delta ferrite phase, and the area ratio of the coarse CU phase were examined.
The hardness of the stainless steel was measured by determining the Vickers hardness at the central part in the thickness direction of the cross-section parallel to the forging (extending) direction. The load during measurement was set at 300 g, and the surface was polished to form a mirror surface before measurement.
The presence/absence of the delta ferrite phase in stainless steel was examined by observing the structure at the position of the central part (at a depth of approximately 10 mm from the surface) in the cross-section in the thickness direction (a direction perpendicular to the extending direction) by using an optical microscope (with a magnification rate of 100×).
The observation of the coarse Cu phase in the stainless steel was performed by observing the structure at a position of the central part (approximately 10 mm depth from the surface) in the thickness direction through elemental mapping of Cu at a magnification rate of 10,000×) by using a field emission electron probe micro-analyzer (FE-EPMA). Additionally, regarding element mapping, the characteristic X-ray intensity of Cu was measured by using JXA-8530F manufactured by JEOL, with an accelerating voltage of 15.0 kV, an irradiation current of 0.05 μA, and an analysis time of 20 ms per point and an LIFH spectrometer crystal.
The cold workability of stainless steel was evaluated by cutting test pieces of 20×45×3.5 mm from each stainless steel, rolling the test pieces at room temperature to a reduction ratio of 80%, and evaluating the presence/absence of “cracks”. Regarding the evaluation criteria, a test piece without cracks when the reduction rate reached 80% was classified as “(Excellent)”, a test piece only with a minor crack occurring at an end was classified as “∘ (Good)”, a test piece with a relatively large crack of about 1 mm at an end was classified as “Δ (Acceptable)”, and a test piece with a large crack of 5 mm or more was classified as “x (Bad)”. The above results are also presented in Table 2.
Among the stainless steel manufactured from the material V, in the stainless steel V-1 of the invention example, due to the appropriately low holding temperature during the annealing after hot forging, the area ratio of the coarse Cu phase is large, and the precipitation of the fine Cu phase was suppressed. As a result, processing can be performed without cracks, and excellent cold workability was exhibited. Furthermore, no precipitation of harmful delta ferrite was observed in the microstructure. Meanwhile, in the stainless steels V2, V3 of the comparative examples, no evidence of delta ferrite precipitation was observed in the microstructure, either. However, since the holding temperature during annealing after hot forging was high, the Cu phase re-dissolved into the austenite. As a result, the area ratio of the coarse Cu phase is relatively small, so cracks occur in these cases.
Among the stainless steel manufactured from the material Z, in the stainless steel Z-3 of the invention example, due to the appropriately low holding temperature during the annealing after hot forging, the area ratio of the coarse Cu phase is large, and the precipitation of the fine CU phase was suppressed. As a result, processing can be performed without cracks, and excellent cold workability was exhibited. Meanwhile, the stainless steels Z-1 and Z-2 of the comparative examples had a holding temperature of 750° C. or lower the during annealing after hot forging, so the growth of the Cu phase is slow, and the Cu phase is held to be fine. Accordingly, the hardness increases, and cracks occur in both cases.
In the stainless steel AA-1 of the invention example, due to the low hot forging temperature, the area ratio of the coarse Cu phase is large, and the precipitation of the fine CU phase was suppressed. As a result, processing can be performed without cracks, and very excellent cold workability was exhibited.
In addition, in the stainless steel manufactured from the material T, no precipitation of delta ferrite was verified in any microstructure in the stainless steels manufactured from the material T, either. However, in the stainless steels T-2, T-3 with a high holding temperature in the annealing after hot forging, the area ratio of the coarse Cu phase is small, and cracks as large as 5 mm or more occurred at an end. Even in the case of stainless steel T-1, which had an appropriate holding temperature during the annealing after hot forging, due to a high Ni content in the component composition of the material T, the area ratio of the coarse Cu phase was small, and relatively large cracks of approximately 1 mm occurred.
Furthermore, although the stainless steel U-1 manufactured from the material U had an appropriate area ratio of the coarse Cu phase, the holding temperature in the heat treatment before hot forging was high in addition to the content of N of the component composition of the material U being low. Therefore, delta ferrite harmful to the microstructure was precipitated. Accordingly, the stainless steel product after quenching and tempering may have insufficient hardness and corrosion resistance.
Next, stainless steel products 1 to 53 were manufactured by performing quenching and tempering on the stainless steel made from the materials A to Z and AA. Further, the heat treatment and annealing conditions for manufacturing stainless steel from the materials followed those specified for the stainless steel V-1 in Table 2.
Three different types of quenching and tempering processes were performed on the stainless steel. That is, in the invention examples and some comparative examples, heating of quenching was performed by heating and holding in a nitrogen atmosphere containing nitrogen gas at atmospheric pressure (purity 99%). The heating temperature for the heating of quenching and the holding time at that heating temperature are as set forth in Table 4. Quenching was performed by rapid cooling to room temperature using nitrogen gas pressurized to 2 atmospheres. After quenching, with the exception of Product 25, the subzero process was conducted immediately. Regarding the condition for the sub-zero treatment, liquid carbon dioxide at −75° C. was used, and the product was held therein for 30 minutes. Subsequently, tempering was performed by holding the tempering temperature set forth in Table 4 for 1 to 2 hours.
As other comparative examples of the invention, two types of different quenching and tempering processes were performed. That is, a method in which heating of quenching that heats and holds under the atmosphere was performed, and then a quenching process of rapid cooling in oil was performed. The heating temperature for the heating of quenching and the holding time at that heating temperature are as set forth in Table 3. After quenching, the sub-zero treatment was performed immediately. Regarding the condition for the sub-zero treatment, liquid carbon dioxide at −75° C. was used, and the product was held therein for 30 minutes. Subsequently, tempering was performed by holding the tempering temperature set forth in Table 3 for 1 to 3 hours. Since oxides were formed on the surface of the test pieces due to atmospheric heating, the oxides were removed by grinding to a depth of 1 mm or more. In addition, as another comparative example, following the atmospheric heating, the sub-zero treatment, and the tempering, a gas nitriding process in an ammonia gas atmosphere at 500° C. was performed on the stainless steel product after grinding.
Regarding the hardness of the surface part of the stainless steel product, the hardness was measured by using the Vickers hardness test on the surfaces of Products 1 to 53. The load during measurement was set at 50 g, and the surfaces of the products were polished through polishing using a #1500 emery paper for approximately 0.002 mm.
Regarding the hardness of the center of the stainless steel product, each of Products 1 to 53 was cut into half along a cross-section perpendicular to a surface of the block containing the nitride layer. On the cross-section, the Vickers hardness at a position that is 1 mm from the surface was measured. The load during measurement was set at 300 g, and the cross-section was polished to form a mirror surface.
Corrosion resistance was evaluated by performing a constant temperature corrosion test, where Products 1 to 53 were immersed in a 5% formic acid solution and a 20% sulfuric acid solution, both held at 50° C., for 5 hours. The evaluation of the corrosion resistance of the product was performed by calculating a corrosion rate (mg/h·cm2) based on the weight loss measured before and after the corrosion test. The evaluation criteria for the formic acid corrosion resistance test were as follows: a corrosion rate of less than 0.1 mg/h·cm2 was rated as “⊚ (Excellent)”; a corrosion rate between 0.1 and 0.5 mg/h·cm2 are rated as “∘(Good)”; a corrosion rate greater than 0.5 mg/h·cm2 was rated as “x (Poor).” The evaluation criteria for the sulfuric acid corrosion resistance test were as follows: a corrosion rate of less than 30 mg/h·cm2 was rated as “⊚ (Excellent)”; a corrosion rate between 30 and 40 mg/h·cm2 was rated as “∘ (Good)”; a corrosion rate greater than 40 mg/h·cm2 was rated as “x (Poor).” The results are presented together in Tables 3 and 4.
Products 1, 4 to 6 were comparative examples manufactured by using stainless steel with a high carbon content of 0.67% to 1.01%. Under standard quenching conditions, a high hardness of 660 HV or more was attained for both the surface hardness and the core hardness of the product. However, due to the high carbon content, a substantial amount of coarse carbides remained undissolved during quenching, served as the start point for corrosion, and adversely affected the corrosion resistance.
Products 2 and 3 were comparative examples manufactured by using a general martensitic stainless steel material containing a carbon content of 0.33% to 0.38%. Under standard quenching conditions, while some products exhibited relatively favorable corrosion resistance, the surface hardness of the product fell below 600 HV, failing to impart the targeted high hardness.
Products 7 to 16 were the invention examples or comparative examples manufactured by using stainless steel materials containing a carbon content of 0.19% to 0.24%, which either embody the present invention examples or have a similar component composition. Under standard quenching conditions, although Products 9 to 11 exhibited good corrosion resistance, the products did not attain a hardness as high as 607 HV or less. For Products 7, 8, and 14 through 16, due to the inability to exhibit a high surface hardness, the evaluation of corrosion resistance was not performed.
Products 17 to 20 were comparative examples that included different component compositions (material V) of stainless steel. Quenching and tempering were performed on the stainless steel products of different component compositions and held for three hours in ammonia gas of 500° C. for three hours, and a gas nitriding process was performed. Under standard gas nitriding conditions, a significantly high hardness was exhibited due to the formation of hard nitrides on the surface of the product. However, due to the low toughness of hard nitrides, there is a concern that the nitrides may be peeled off under impact loads, making the products undesirable.
Products 21 to 24 were comparative examples manufactured by using stainless steel materials without Cu and manufactured through quenching in which a solid-phase nitrogen absorption process was applied. Product 21 contained a carbon content of 0.67%. Therefore, due to a large amount of nitrogen dissolved in the solid-phase nitrogen absorption process, austenite remains on the product surface during quenching, and the surface hardness significantly deteriorated. Products 22 to 24 had high hardness on the product surface. However, due to the absence of Cu, Products 22 to 24 did not exhibit superior corrosion resistance.
Product 25 was a comparative example in which the component composition (material V) of the stainless steel of the invention examples was quenched by applying the solid-phase nitrogen absorption process, but for which the sub-zero treatment was not performed. Since the sub-zero treatment was not performed, even austenite remained. Even though high corrosion resistance was attained, the surface hardness of the product was very low.
Products 26 to 39 were comparative examples manufactured by using stainless steel materials with component compositions similar to that of the invention examples, on which quenching applied with the solid-phase nitrogen absorption process was performed. Product 26 contained 0.29% of Al. Therefore, a large amount of nitrogen dissolved in the solid-phase nitrogen absorption process formed aluminum nitrides and adversely affected the hardness and corrosion resistance. Products 27 to 30 and 34 to 38 contained 0.99% to 4.03% of Ni that is an austenite-stabilizing element. Therefore, during the solid-phase nitrogen absorption process, due to the large amount of dissolved nitrogen, austenite remained during quenching, and the hardness of the product surface decreased significantly. Products 31 to 33 contain 15.95% to 16.02% of Cr. Therefore, delta ferrite precipitated which significantly reduced the hardness at the central part of the product. Meanwhile, in the surface layer of the product, almost all of the delta ferrite disappeared due to the large amount of nitrogen dissolved through the solid-phase nitrogen absorption process. As a result, the surface hardness was high. Since Product 39 only contained 0.01% of nitrogen, delta ferrite was generated at the central part of the product, resulting in insufficient hardness at the central part.
Products 40 to 53 were examples manufactured by using stainless steel materials with the component composition of the invention examples. Among these products, the surface hardness was suitably high, measuring 631 HV or more, and the hardness of the central part was appropriate within the range of 498 HV to 588 HV. In addition, corrosion resistance was also excellent.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-144924 | Sep 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/026330 | 7/18/2023 | WO |
