JP7758973B2 - Austenitic stainless steel and method for producing austenitic stainless steel - Google Patents

Description

The present invention relates to austenitic stainless steel and a method for producing austenitic stainless steel.

Metastable austenitic stainless steels, such as SUS301, are known as austenitic stainless steels used in applications requiring corrosion resistance and strength. These austenitic stainless steels are used, for example, in spring products such as cylinder head gaskets for automobile engines, or as materials for structural components such as automotive battery frames.

Such stainless steels are generally strengthened by increasing the reduction ratio in cold rolling, etc., which tends to result in a large processing load during the manufacturing process, such as rolling. To reduce this load, Patent Document 1, for example, proposes a method for manufacturing spring material having a martensite phase in which precipitates consisting of a Cu-rich phase are dispersed, in which spring steel sheet exhibiting a multi-phase structure without Cu-rich phase precipitation is subjected to aging treatment.

Japanese Patent Application Publication No. 2008-195976

Precipitation of Cu-rich phases is effective in increasing the strength of stainless steel. Therefore, according to the method described in Patent Document 1, by subjecting spring steel sheet to aging treatment to precipitate Cu-rich phases, it is possible to reduce the processing load in the manufacturing process of spring steel sheet while achieving high strength in the final product, spring material. However, the need for an aging treatment poses challenges in the productivity of spring material.

One aspect of the present invention aims to realize an austenitic stainless steel that is highly productive and achieves both reduced processing load during manufacturing and high strength in the final product.

In addition, because the precipitation temperature of the Cu-rich phase and the precipitation temperature of Cr carbide are relatively close, we noticed that it is preferable to utilize N without excessively increasing the C content in order to reduce the decrease in corrosion resistance caused by the precipitation of Cr carbide. Lowering the C content in austenitic stainless steel is also preferable for reducing the desired processing load.

In order to solve the above problems, an austenitic stainless steel according to one aspect of the present invention contains, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and includes 20% or more by volume of an austenite phase and a Cu-rich phase having a number density of 1.0 × ¹⁰³ particles μm ^-3 or more and a major axis of 30 nm or less, with the balance being a deformation-induced martensite phase and an unavoidably formed phase, and the value of _Md30 represented by the following formula (1) is 0.0 or more and 80.0 or less:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

In order to solve the above problems, a method for producing an austenitic stainless steel according to one aspect of the present invention comprises, in mass %, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and Md A method for producing an austenitic stainless steel having a JIS _{K 30} value of 0.0 or more and 80.0 or less, the method comprising: a finish annealing step of performing finish annealing at a temperature of 750°C or more and 980°C or less; when the maximum temperature reached in the finish annealing step is 850°C or more, the heating time at 850°C or more is set to 30 seconds or less; and in the finish annealing step, the average cooling rate from 700°C to 500°C after the finish annealing is set to 1°C/second or more:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

According to one aspect of the present invention, it is possible to realize an austenitic stainless steel that is highly productive and achieves both reduced processing load during manufacturing and high strength in the final product.

FIG. 1 illustrates an EBSD grain boundary map and TEM image of an austenitic stainless steel according to an embodiment. FIG. 1 is a diagram showing the relationship between 0.2% yield strength (YS 18%) and reference strength (HV 60%) of austenitic stainless steels according to an example and a comparative example.

The following provides a detailed description of an austenitic stainless steel according to one embodiment of the present invention. The following description is intended to provide a better understanding of the spirit of the invention, and does not limit the present invention unless otherwise specified.

[Organizational structure]
An austenitic stainless steel according to one embodiment of the present invention is a stainless steel containing 20 volume % or more of an austenite phase. Hereinafter, "austenitic stainless steel" refers to an austenitic stainless steel according to one embodiment of the present invention unless otherwise specified. The austenitic stainless steel may be, for example, a steel plate or a steel strip.

Austenitic stainless steel contains a stress-induced martensite phase, which is formed when a portion of the austenite phase is transformed by the stress-induced transformation plasticity (TRIP) phenomenon. From the perspective of increasing strength, the proportion of stress-induced martensite in austenitic stainless steel is preferably 5% by volume or more, more preferably 10% by volume or more, more preferably 15% by volume or more, and even more preferably 20% by volume or more. Furthermore, the proportion of stress-induced martensite in austenitic stainless steel is preferably less than 80% by volume, more preferably 75% by volume or less. The proportion of austenite in austenitic stainless steel may decrease as the proportion of stress-induced martensite increases, as long as it is 20% by volume or more.

The austenitic stainless steel further contains a Cu-rich phase. The Cu-rich phase is a phase containing 60 atomic % or more of Cu (copper), such as an ε-Cu phase. The austenitic stainless steel at least contains a Cu-rich phase having a number density of 1.0 × 10 ³ particles μm ⁻³ or more and a major axis of 30 nm or less. The major axis refers to the longest diameter of the Cu-rich phase precipitated in the form of particles. Note that the austenitic stainless steel may contain a Cu-rich phase having a major axis of more than 30 nm. The Cu-rich phase may be dispersed in the austenite phase, in a deformation-induced martensite phase, or in an unavoidably formed phase, which will be described later.

Cu-rich phases may be identified by structural observation using a transmission electron microscope (TEM). For example, a TEM sample containing an arbitrary cross section of austenitic stainless steel is prepared, and a predetermined range of the cross section is observed using a TEM. This allows the number of Cu-rich phases within that range whose cross section has a major axis of 30 nm or less to be counted. Furthermore, the number density per volume can be calculated by calculating the volume based on the thickness of the TEM sample used for the counting and the area of the range where the counting was performed. The thickness of the TEM sample may be, for example, an actual measured value of the thickness of the TEM sample, or an estimated value based on the method used to prepare the TEM sample. Methods for preparing TEM samples include, but are not limited to, electrolytic polishing.

The finer the precipitated Cu-rich phase and the greater the amount present, the higher the strength of austenitic stainless steel. Cu-rich phases of the above-mentioned amounts and sizes are effective in increasing the strength of austenitic stainless steel. During the manufacturing process of austenitic stainless steel, such as cold rolling before finish annealing, the Cu-rich phase is not precipitated, keeping the strength low and reducing the processing load. Then, by precipitating the Cu-rich phase during the finish annealing process, high strength is achieved in the austenitic stainless steel after manufacturing. Manufacturing processes such as the finish annealing process are described below.

Austenitic stainless steel may also contain unavoidably formed phases other than the austenite phase, the deformation-induced martensite phase, and the Cu-rich phase. The unavoidably formed phases are not particularly limited, but examples include a delta-ferrite phase and a phase containing carbides, nitrides, and/or oxides. Examples of phases containing carbides, nitrides, and/or oxides include phases containing carbides and/or nitrides of Cr, Ti, and/or Nb, and phases containing oxides of Si, Ti, Al, Mg, and/or Ca.

Austenitic stainless steel preferably has an average crystal grain size of 10.0 μm or less. The finer the crystal grains, the stronger the austenitic stainless steel. Furthermore, in austenitic stainless steel, improving strength generally results in a decrease in ductility. However, by reducing the crystal grain size, it is possible to achieve both improved strength and improved ductility in austenitic stainless steel.

The average grain size may be measured using the EBSD (Electron Backscattering Diffraction) method. For example, for any cross section of austenitic stainless steel, the grain size in multiple fields of view may be calculated using the EBSD method, and the average value of the grain size calculated in those multiple fields may be used as the average grain size. The average grain size may also be measured using methods other than the EBSD method. Examples of methods other than the EBSD method include a method in which grain boundaries are revealed by nitric acid electrolysis and then measured by the intercept method, as specified in JIS G0551.

[Component composition]
The austenitic stainless steel contains, by mass%, C: 0.005% to 0.03%, Si: 0.1% to 2.0%, Mn: 0.3% to less than 2.5%, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% to less than 6.0%, Cr: 16.0% to 18.5%, Cu: 1.5% to 3.8%, and N: 0.08% to 0.25%. The balance of the austenitic stainless steel may consist of Fe (iron) and unavoidable impurities. The significance of the content of each element contained in the austenitic stainless steel will be explained below.

(C)
Carbon (C) is an austenite-forming element that facilitates the formation of the austenite phase, has a high solid-solution strengthening effect, and is also an effective element for obtaining strength. Austenitic stainless steel contains 0.005% by mass or more and 0.03% by mass or less of C. If the C content is 0.005% by mass or more, austenitic stainless steel exhibits sufficient solid-solution strengthening and has good strength.

Excessive addition of C causes the precipitation of Cr carbides during annealing at relatively low temperatures, resulting in a decrease in the corrosion resistance of austenitic stainless steel, particularly in welds. Therefore, the C content is set to 0.03 mass% or less. If the C content is 0.03 mass% or less, an austenitic stainless steel with good corrosion resistance can be obtained, even in welds.

(Si)
Silicon (Si) is an element that is effective as a deoxidizer and also has a solid-solution strengthening effect. Austenitic stainless steel contains 0.1 mass% to 2.0 mass% Si, and preferably 0.2 mass% to 1.0 mass% Si. If the Si content is 0.1 mass% or more, the austenitic stainless steel effectively exhibits the deoxidizing effect and solid-solution strengthening effect. It is more preferable that the Si content be 0.2 mass% or more.

Si is also a ferrite-forming element that facilitates the formation of ferrite phase. The delta-ferrite phase causes edge breaks or splitting during hot rolling. From the perspective of reducing the formation of delta-ferrite phase, the Si content should be 2.0 mass% or less, and preferably 1.0 mass% or less.

(Mn)
Manganese (Mn) is an austenite-forming element and is effective in maintaining the austenite phase. Furthermore, Mn promotes the precipitation of a Cu-rich phase. Austenitic stainless steel contains 0.3 mass% to 2.5 mass%, preferably 0.5 mass% to 2.0 mass%. A Mn content of 0.3 mass% or more facilitates the precipitation of a Cu-rich phase, and a Mn content of 0.5 mass% or more is more preferable. Excessive addition of Mn reduces the hot workability of austenitic stainless steel. Therefore, the Mn content is set to 2.5 mass% or less, preferably 2.0 mass% or less.

(P)
P (phosphorus) is an element that is mixed in as an unavoidable impurity, and the lower the P content, the better. From the viewpoint of manufacturability, austenitic stainless steel may contain 0.04 mass% or less of P. If the P content is 0.04 mass% or less, adverse effects on material properties such as ductility can be reduced in austenitic stainless steel.

(S)
S (sulfur) is an element that is mixed in as an unavoidable impurity, and the lower the S content, the better. From the viewpoint of manufacturability, austenitic stainless steel may contain 0.015 mass% or less of S. If the S content is 0.015 mass% or less, adverse effects on material properties such as ductility can be reduced in austenitic stainless steel.

(Ni)
Ni (nickel) is an austenite-forming element and is also effective for maintaining the austenite phase. Austenitic stainless steel contains 3.0 mass% or more and less than 6.0 mass% Ni, preferably 3.5 mass% or more and less than 5.5 mass% Ni, and more preferably 4.0 mass% or more and less than 5.0 mass% Ni. If the Ni content is 3.0 mass% or more, the generation and maintenance of the austenite phase are favorable. It is more preferable that the Ni content is 4.5 mass% or more.

On the other hand, Ni is an expensive element, and if added in excess, it stabilizes the austenite phase, reducing the amount of stress-induced martensite phase that forms. Therefore, the Ni content should be less than 6.0% by mass, preferably 5.5% by mass or less, and more preferably less than 5.0% by mass.

(Cr)
Cr (chromium) is an element effective for ensuring the corrosion resistance of austenitic stainless steel. Austenitic stainless steel contains 16.0 mass% to 18.5 mass% of Cr, and preferably 16.5 mass% to 18.0 mass% of Cr. If the Cr content is 16.0 mass% or more, the corrosion resistance of the austenitic stainless steel can be well ensured. It is more preferable if the Cr content is 16.5 mass% or more.

However, because Cr, like Si, is a ferrite-forming element, excessive Cr addition will result in excessive formation of the delta ferrite phase. Therefore, the Cr content should be 18.5% by mass or less, and preferably 18.0% by mass or less.

(Cu)
Cu is an austenite-forming element and is effective in maintaining the austenite phase. It is also effective in increasing the strength of austenitic stainless steels by precipitation of a Cu-rich phase. Cu is also an element that effectively acts to refine crystal grains. This is thought to be because the Cu-rich phase has an inhibitory effect on grain growth. Furthermore, Cu reduces the work hardening of the austenite phase in its solid solution state, thereby reducing the rolling load in the manufacturing process of austenitic stainless steels.

Austenitic stainless steel contains 1.5% to 4.0% by mass of Cu, preferably 2.0% to 3.5% by mass of Cu, and more preferably more than 2.0% to 3.5% by mass of Cu. A Cu content of 1.5% by mass or more improves the generation and maintenance of the austenite phase, and also improves the precipitation of a Cu-rich phase. A Cu content of 2.0% by mass or more is more preferable, and a Cu content of more than 2.0% by mass is even more preferable.

On the other hand, if excessive Cu is added, a CuMn phase will form in the center of the slab during solidification, reducing the hot workability of the slab. Therefore, the Cu content should be 4.0% by mass or less, and preferably 3.5% by mass or less.

(N)
N (nitrogen) is an austenite-forming element and also has the effect of solid solution strengthening and improving corrosion resistance. Since the C content of austenitic stainless steel is set to 0.03 mass% or less to ensure the corrosion resistance of welds, the N content is set to 0.08 mass% or more, preferably 0.10 mass% or more, more preferably 0.11 mass% or more, and even more preferably 0.12 mass% or more. This N content is effective in ensuring the strength and corrosion resistance required of austenitic stainless steel.

Furthermore, excessive addition of N increases the rolling load of austenitic stainless steel. Therefore, the N content should be 0.25 mass% or less, and preferably 0.20 mass% or less.

(Other elements)
In addition to the above-mentioned elements, the austenitic stainless steel may further contain, by mass%, one or more selected from Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more and 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metals): 0.2% or less.

(Mo, W, V)
Mo (molybdenum), W (tungsten), and V (vanadium) are elements effective in improving corrosion resistance. However, because Mo, W, and V are ferrite-forming elements and expensive elements, excessive addition of these elements is undesirable. Therefore, it is preferable that austenitic stainless steel contain one or more elements selected from 1.0 mass% or less of Mo, 1.0 mass% or less of W, and 0.5 mass% or less of V.

(B)
B (boron) is an element that improves hot workability and is effective in reducing the occurrence of edge breaks and lamination during hot rolling. Austenitic stainless steel preferably contains 0.0001% by mass or more and 0.01% by mass or less of B. A B content of 0.0001% by mass or more is effective in improving hot workability and reducing the occurrence of edge breaks and lamination during hot rolling. However, excessive addition of B to austenitic stainless steel containing Cr leads to a decrease in corrosion resistance due to the precipitation of _Cr2B . Therefore, the B content is preferably 0.01% by mass or less.

(Co)
Co (cobalt) is an element effective in ensuring the corrosion resistance of austenitic stainless steel. It also contributes to reducing the coarsening of the Cu-rich phase and maintaining it fine. To achieve this effect, it is preferable to contain 0.10 mass% or more of Co. However, since Co is an expensive element, from the viewpoint of cost reduction, it is preferable to limit the Co content to 0.8 mass% or less.

(Sn)
Sn (tin) is an element effective in ensuring the corrosion resistance of austenitic stainless steel, but since excessive addition of Sn leads to a decrease in the hot workability of austenitic stainless steel, the Sn content is preferably 0.1 mass% or less.

(Al, Ca, Mg, Ti)
Al (aluminum), Ca (calcium), Mg (magnesium), and Ti (titanium) are all elements that have a deoxidizing effect. The austenitic stainless steel preferably contains one or more deoxidizers selected from 0.3 mass% or less of Al, 0.03 mass% or less of Ca, 0.03 mass% or less of Mg, and 0.5 mass% or less of Ti.

(Nb)
Niobium (Nb) is an element effective in reducing sensitization of austenitic stainless steel. It is also effective in making the structure finer and more uniform. Austenitic stainless steel preferably contains 0.5 mass % or less of Nb.

(Sb, Zr, Ta, Hf, REM)
Sb (antimony), Zr (zirconium), Ta (tantalum), Hf (hafnium), and REMs (rare earth metals) are all elements that improve hot workability and are also effective in improving oxidation resistance. Austenitic stainless steel preferably contains one or more elements selected from 0.5 mass% or less of Sb, 0.5 mass% or less of Zr, 0.03 mass% or less of Ta, 0.03 mass% or less of Hf, and 0.2 mass% or less of REMs.

[ _Md30 value]
The austenitic stainless steel has an _Md30 value represented by the following formula (1) of 0.0 or more and 80.0 or less, and preferably 20.0 or more and 70.0 or less.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

In austenitic stainless steel, the _Md30 value indicates the temperature (°C) at which 50% of the structure of austenitic stainless steel transforms to martensite when a 30% tensile strain is applied to a single-phase austenitic stainless steel. Therefore, the _Md30 value can be used as an index of the stability of the austenitic phase. The _Md30 value can also be used as an index that influences the likelihood of the TRIP phenomenon occurring in austenitic stainless steel.

The _{Md30 value of an austenitic stainless steel according to one embodiment of the present invention is preferably 0.0 or more and 80.0 or less. The larger the Md30 value} , the more easily the transformation from the austenite phase to the strain-induced martensite phase occurs, and high strength can be obtained by imparting slight cold-rolling strain, while excellent ductility can be ensured. Furthermore, when austenitic stainless steel is subjected to forming, portions imparted with processing strain, such as bent portions, tend to achieve even higher strength due to the TRIP phenomenon.

Furthermore, in the manufacturing process of austenitic stainless steel, the presence of a strain-induced martensite phase in the rolled material before finish annealing is effective in refining crystal grains by finish annealing. This effect is particularly pronounced when the _Md30 value is 0.0 or greater. Furthermore, when the _Md30 value exceeds 80.0, the TRIP phenomenon is likely to occur excessively, making it difficult to stabilize the properties of the austenitic stainless steel.

Therefore, if the value of _Md30, which is an index of the stability of the austenite phase, is 0.0 or more and 80.0 or less, austenitic stainless steel having high strength and good ductility can be stably produced.

In conventionally known component regression equations for Md ₃₀ , the same value is generally used for the Ni and Cu coefficients. In one embodiment of the present invention, the Cu coefficient is set smaller than the Ni coefficient in the component regression equation for Md _30. Many of the component regression equations for Md ₃₀ based on conventional knowledge are based on the results of non-Ni-reduced austenitic stainless steels. In contrast, in Ni-reduced components such as those of the present invention, it has been found that the effect of Cu on the stabilization of the austenite phase is significantly smaller than that of conventional knowledge. This is a novel finding obtained as a result of extensive research by the present inventors, and the Cu coefficient in the component regression equation for Md ₃₀ is set based on this finding. This facilitates adjustment of the Cu content and increases the manufacturing flexibility of austenitic stainless steels.

[Manufacturing method]
A method for producing an austenitic stainless steel according to one embodiment of the present invention is a method for producing an austenitic stainless steel containing, by mass%, C: 0.005% to 0.03%, Si: 0.1% to 2.0%, Mn: 0.3% to 2.5%, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% to less than 6.0%, Cr: 16.0% to 18.5%, Cu: 1.5% to 4.0%, and N: 0.08% to 0.25% or less, with the balance being Fe and unavoidable impurities, and having an _Md30 value of 0.0 to 80.0 inclusive, as shown in formula (1). The method for producing an austenitic stainless steel also includes a finish annealing step.

The method for producing austenitic stainless steel may include general austenitic stainless steel production processes other than the finish annealing process. Below is an example of a method for producing austenitic stainless steel according to one embodiment of the present invention, but the method is not limited to this.

In one embodiment of the present invention, a method for producing austenitic stainless steel involves, for example, continuously casting molten steel with adjusted composition to produce a slab. The slab produced by continuous casting is then heated to 1100°C or higher and 1300°C or lower, and then hot-rolled to produce a hot-rolled steel strip. The rate of precipitation of Cu-rich phase from the less-strained austenite phase after hot rolling is slow. Therefore, the finishing temperature and coiling temperature of the hot-rolled steel strip after hot rolling can be similar to those used in general austenitic stainless steel production. From the perspective of minimizing precipitation of Cu-rich phase before finish annealing, the coiling temperature of the hot-rolled steel strip after hot rolling is preferably 850°C or lower, and more preferably 650°C or lower.

The hot-rolled steel strip may be pickled after hot rolling. The hot-rolled steel strip may be annealed before pickling, or pickled without annealing. When annealing the hot-rolled steel strip before pickling, the annealing temperature is preferably in the range of 900°C to 1150°C. To completely dissolve Cu, the annealing temperature is preferably in the range of 980°C to 1150°C, but is not limited to the above range. The hot-rolled steel strip after pickling is then cold-rolled to the specified thickness to obtain a cold-rolled steel strip.

In the manufacturing method of austenitic stainless steel, recrystallization and precipitation of Cu-rich phases proceed simultaneously in the finish annealing process after the cold rolling process. Because Cu-rich phases are particularly likely to precipitate from the deformation-induced martensite phase, it is preferable to perform the cold rolling process at a rolling reduction rate and rolling temperature that results in the deformation-induced martensite phase accounting for 20% or more by volume of the cold-rolled steel strip. By performing this type of cold rolling process, Cu-rich phases can be effectively precipitated in the steel strip during the subsequent finish annealing process.

The austenitic stainless steel has an _Md30 value adjusted to 0.0 or more and 80.0 or less. In an austenitic stainless steel having such an _Md30 value, a Cu-rich phase is precipitated in the amount specified in one embodiment of the present invention, regardless of the amount of strain-induced martensite phase in the cold-rolled steel strip. However, increasing the rolling ratio in the cold-rolling step, controlling the temperature in the cold-rolling step to a low value, etc., as necessary, is more effective in precipitating the Cu-rich phase.

To ensure that the strain-induced martensite phase in the cold-rolled steel strip is 20% by volume or more, the rolling ratio in the cold-rolling process is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. Furthermore, the temperature in the cold-rolling process is preferably 90°C or less, and more preferably 60°C or less.

(Finishing annealing process)
The cold-rolled steel strip is subjected to finish annealing. The finish annealing process is carried out under conditions that promote the precipitation of a Cu-rich phase. The Cu-rich phase is effective in increasing the strength of austenitic stainless steel. Therefore, the strength of the hot-rolled steel strip and cold-rolled steel strip before the precipitation of the Cu-rich phase is low, which reduces the rolling load in the cold rolling process. Furthermore, the precipitation of the Cu-rich phase by the finish annealing process results in high strength of the austenitic stainless steel after finish annealing.

In addition, the precipitation of Cu-rich phases is also effective in refining the recrystallized grains of the austenite phase. Therefore, by utilizing the precipitation of Cu-rich phases, the average crystal grain size can be controlled to 10.0 μm or less.

As such, the manufacturing method for austenitic stainless steel according to one embodiment of the present invention can achieve both a reduction in the processing load during manufacturing and high strength in the final product at a high level. Furthermore, since an additional aging treatment step is not required to precipitate the Cu-rich phase, as was previously required, the productivity of austenitic stainless steel is also good.

The temperature of the final annealing process is set to 750°C or higher and 980°C or lower, preferably 800°C or higher and 925°C or lower, so that the Cu-rich phase effectively precipitates in the austenitic stainless steel. If the final annealing temperature is below 750°C, the recrystallization of the structure will be insufficient. Furthermore, if the final annealing temperature exceeds 980°C, the Cu-rich phase will dissolve, resulting in an insufficient amount of Cu-rich phase remaining after final annealing.

In addition, the Cu-rich phase that precipitates from the deformation-induced martensite phase is particularly likely to dissolve into the austenite phase when held at a temperature of 850°C or higher for a long period of time during finish annealing. Therefore, when the maximum temperature reached in the finish annealing process is 850°C or higher, it is preferable to shorten the heating time at 850°C or higher. Specifically, when the maximum temperature reached in the finish annealing process is 850°C or higher, the heating time at 850°C or higher should be 30 seconds or less, and preferably 15 seconds or less. "The heating time at 850°C or higher" refers to the total time for multiple heating periods at 850°C or higher during the finish annealing process, if the heating period is divided into multiple periods.

Austenitic stainless steels have a carbon content of 0.03% by mass or less, making it difficult for chromium carbides to precipitate during cooling. Therefore, the cooling rate after finish annealing can be the same as that used in general stainless steel manufacturing processes. While a faster cooling rate is preferable from a productivity standpoint, a relatively slow average cooling rate of 1°C/sec or more from 700°C to 500°C is acceptable, with a cooling rate of 5°C/sec or more being preferable from a productivity standpoint. Furthermore, considering the flatness of the steel sheet, a cooling rate of less than 75°C/sec is preferable, with 50°C/sec or less being more preferable.

In the cold rolling process, intermediate annealing and intermediate rolling may be performed as necessary. Temper rolling may also be performed as necessary to further increase the strength of the steel strip after final annealing. When prioritizing reduction of rolling load, the intermediate annealing temperature is preferably 980°C or higher and 1150°C or lower to avoid the precipitation of Cu-rich phases. In order to achieve high strength by repeating the precipitation treatment, the intermediate annealing temperature is preferably the same as that of the final annealing. The intermediate annealing temperature is not limited to the above-mentioned range.

[Strength evaluation]
An austenitic stainless steel according to one embodiment of the present invention has a lower strength during the manufacturing process to reduce the rolling load, and yet achieves high strength after manufacturing. Such properties of austenitic stainless steel can be expressed, for example, by the relationship between 0.2% yield strength (YS 18%, MPa) and reference strength (HV 60%).

0.2% yield strength (YS 18%) is an index of the strength of austenitic stainless steel. 0.2% yield strength (YS 18%) indicates the 0.2% yield strength when austenitic stainless steel is subjected to further temper rolling to an elongation of 18% after finish annealing. 0.2% yield strength can be evaluated using a method in accordance with JIS Z2241.

The reference strength (HV 60%) is a hypothetical index showing the strength of austenitic stainless steel before the precipitation of Cu-rich phases during the final annealing process. The reference strength (HV 60%) indicates the Vickers hardness of an austenitic stainless steel with the same chemical composition but manufactured using a manufacturing method partially modified from that of one embodiment of the present invention, where hot rolling is followed by annealing at 1050°C and cold rolling at a rolling reduction of 60%. In other words, the reference strength (HV 60%) does not indicate the strength of the austenitic stainless steel according to one embodiment of the present invention, but may be the strength of a steel strip prepared for evaluation, for example. Vickers hardness can be measured based on the Vickers hardness test method in accordance with JIS Z2244.

The inventors have discovered that for austenitic stainless steels that combine reduced processing load during manufacturing with high strength in the final product, the relationship between 0.2% yield strength (YS 18%) and reference strength (HV 60%) satisfies the following formula (2):

YS18%≧3.75HV60%-575 (2)
According to a manufacturing method according to one embodiment of the present invention, it is possible to manufacture an austenitic stainless steel that satisfies the above formula (2) and achieves both a reduction in the processing load during manufacturing and high strength in the final product.

[Suitable uses]
Austenitic stainless steel has extremely high strength and corrosion resistance. Therefore, austenitic stainless steel is suitable as a material for spring products that require high strength and corrosion resistance, such as cylinder head gaskets, spiral springs, springs for electronic components, electric train car components, on-board battery frame materials, structural materials, and metal packing. In particular, austenitic stainless steel has excellent corrosion resistance (weldability) at welded joints, even when welded. Therefore, the austenitic stainless steel according to one embodiment of the present invention can be suitably used even in applications where a relatively large number of welded structures are used, such as electric train car components or on-board battery frame materials manufactured for welding.

〔summary〕
An austenitic stainless steel according to a first aspect of the present invention contains, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and inevitable impurities. The stainless steel includes 20% or more by volume of an austenite phase and a Cu-rich phase having a number density of ^1.0 × 10 μm ^or more and a major axis of 30 nm or less, with the balance being a deformation-induced martensite phase and an inevitable formed phase, and the value of _Md30 represented by the following formula (1) is 0.0 or more and 80.0 or less:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

The austenitic stainless steel according to aspect 2 of the present invention may further contain, in mass %, one or more selected from Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more and 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metals): 0.2% or less.

The austenitic stainless steel according to aspect 3 of the present invention may have an average grain size of 10.0 μm or less in aspect 1 or 2 above.

A method for producing an austenitic stainless steel according to a fourth aspect of the present invention comprises, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and Md A method for producing an austenitic stainless steel having a JIS _{K 30} value of 0.0 or more and 80.0 or less, the method comprising: a finish annealing step of performing finish annealing at a temperature of 750°C or more and 980°C or less; when the maximum temperature reached in the finish annealing step is 850°C or more, the heating time at 850°C or more is set to 30 seconds or less; and in the finish annealing step, the average cooling rate from 700°C to 500°C after the finish annealing is set to 1°C/second or more:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

The results of evaluating the austenitic stainless steels according to the invention examples and comparative examples of the present invention are described below.

[Evaluation conditions]
<Component composition>
The chemical compositions (mass%) and _Md30 values of austenitic stainless steels according to examples of the present invention (invention steels A1 to A15) and austenitic stainless steels according to comparative examples (comparison steels B1 to B5) are shown in Table 1 below. The _Md30 values were calculated using formula (1) above. Note that underlined values in Table 1 below indicate values that are outside the range specified in the present invention. The same applies to Table 2 below.

<Manufacturing method>
The austenitic stainless steels according to the examples and comparative examples of the present invention were produced by the following methods. Austenitic stainless steels having the chemical compositions shown in Table 1 were melted and then subjected to processes from hot rolling to finish annealing by a production method according to an example of the present invention (Invention Examples C1 to C8) or a production method according to a comparative example (Comparative Examples D1 and D2), to obtain cold-rolled and annealed materials. The conditions for each production method are shown in Table 2 below.

In the final annealing process, when the final annealing temperature was 850°C or higher, the time for which the temperature was maintained at 850°C or higher was adjusted as shown in Table 2. In invention example C3, the heating was adjusted so that the temperature began to decrease once the final annealing temperature reached 850°C, but for convenience, the time for which the temperature was maintained at 850°C or higher is listed in Table 2 as "1 second."

<Evaluation method>
The austenitic stainless steels according to the examples of the present invention and the comparative examples were evaluated for various indices as shown below.

(Number density of Cu-rich phase)
TEM samples were prepared by electropolishing from the cold-rolled annealed materials produced under each condition. Three 400 nm × 400 nm areas were observed on the surface of the TEM sample parallel to the rolling direction of the cold-rolled annealed material. The Cu-rich phase was identified from the contrast of the TEM image, and the number of Cu-rich phases was counted. The thickness of the TEM sample was considered to be 150 nm, and the number density per unit volume was calculated. When the Cu-rich phase coarsened, it was observed as a clear shape rather than a contrast. Cu-rich phases with a major axis exceeding 30 nm were excluded from the measurement.

(crystal grain size)
The average grain size was evaluated using the EBSD method. The cross section of the cold-rolled annealed material produced under each condition, parallel to the rolling direction and perpendicular to the rolling surface, was mechanically polished and then electropolished. Then, EBSD analysis was performed at a magnification of 2000 times over a 40 μm × 40 μm area in the cross section at a step interval of 0.2 μm. Regarding the misorientation in the orientation relationship satisfying the Σ3 correspondence grain boundary, annealing twins with a misorientation of 1° or less were excluded, and boundaries with a misorientation of 2° or more were considered as grain boundaries. The area of each grain was defined as S (μm ² ), and the diameter of a circle having the same area as the grain was defined as D (μm). The grain size was calculated using the following formula (3). This was performed for five fields of view, and the average of the grain sizes obtained in the five fields of view was calculated as the average grain size.

Crystal grain size = {Σ(D×S)}/40×40 (3)
(amount of martensite phase)
The amount of martensite phase (volume %) was measured as is when the plate thickness was 1.5 mm or more, and when it was less than 1.5 mm, the cold-rolled material or the temper-rolled material was layered so that the total thickness was 1.5 mm or more. These materials were measured using a ferrite scope (Fischer FMP30, electromagnetic induction method), and the measured value was divided by 0.7475 to obtain the amount of martensite phase. The amount of austenite phase (volume %) was considered to be the value obtained by subtracting the amount of martensite phase from the entire austenitic stainless steel matrix, which was set to 100 volume %. The amounts of Cu-rich phase and unavoidably formed phases in austenitic stainless steel may be calculated as external quantities because their proportions are small and accurate measurement is difficult.

(Tensile properties)
As an index of tensile properties, the 0.2% yield strength (YS18%) was evaluated when temper rolling was performed to obtain an elongation of 18%. The 0.2% yield strength (YS18%) was measured by preparing a JIS No. 13B test piece and conducting a tensile test in accordance with JIS Z2241. The 0.2% yield strength (YS18%) was measured at a crosshead speed of 3 mm/min.

(strength)
In each example and comparative example, the manufacturing conditions were partially changed, and the hot-rolled steel strip was annealed at 1050°C. The rolling reduction ratio was 60%, and the resulting cold-rolled material was subjected to Vickers hardness measurement as a reference strength (HV60%). The Vickers hardness was measured by subjecting the surface of the 60% rolled material to a Vickers hardness test (JIS Z2244) using a Vickers hardness tester. The load during the Vickers hardness test was 10 kg.

(Corrosion resistance of welded parts)
TIG fillet welding was performed on a 1.5 mm thick cold-rolled annealed material under Ar gas sealing conditions with an electrode diameter of 1.6 mm, a welding speed of 70 cm/min, and a welding current of 90 A. The evaluation surface was a 10 mm x 10 mm area including the weld, and was polished with #600 to remove any coating effects. The corrosion resistance of the evaluation surface was evaluated using the electrochemical reactivation rate as an index.

The reactivation rate was measured in accordance with JIS G0580. Specifically, the specimen was polarized in a 30°C aqueous solution of 0.5 mol/L sulfuric acid and 0.01 mol/L potassium thiocyanate from the natural potential to 0.3 V (vs. SCE) at a sweep rate of 100 mV/min (hereinafter referred to as the "forward run"). After reaching 0.3 V (vs. SCE), the potential was swept in the opposite direction to the forward run. After reactivation of the hot-rolled specimen, the sweep was terminated at a potential where the anodic current again became zero (hereinafter referred to as the "return run").

The ratio of the maximum current density (ia) in the forward pass to the maximum current density (ir) in the return pass was calculated as the reactivation rate (ir/ia). Because this evaluation method is strict as a sensitization assessment method for evaluating corrosion resistance, a reactivation rate of, for example, 1.5% is considered acceptable in practical environments. However, considering that the cold-rolled annealed material according to one embodiment of the present invention may have fine crystal grains, making it difficult to evaluate corrosion resistance, a reactivation rate of 1% or less can be considered to have favorable corrosion resistance. Therefore, for the corrosion resistance of the weld, a reactivation rate of 1% or less was evaluated as "O" (good), and a reactivation rate of more than 1% was evaluated as "X" (poor).

〔result〕
For invention steel A2, the amount of Cu-rich phase precipitation and the grain size of the cold-rolled and annealed material obtained under each of the conditions shown in Table 2 are shown in Table 3 below. Table 3 also shows the 0.2% yield strength (YS 18%) under each condition, and the amount of martensite phase after cold rolling (before finish annealing) and after temper rolling to give an elongation of 18% after finish annealing.

In Table 3 below, underlines indicate that the amount of Cu-rich phase precipitation is outside the specified range of this invention.

For the invention steel A2, the cold-rolled and annealed materials produced under the conditions of invention examples C1 to C8 had a Cu-rich phase precipitation amount within the range specified by the present invention and exhibited a fine average crystal grain size of 10.0 μm or less. On the other hand, the cold-rolled and annealed materials produced under the conditions of comparison examples D1 and D2 did not exhibit any Cu-rich phase precipitation.

For the cold-rolled and annealed steel A2 manufactured under the conditions of Example C2, the left side of Figure 1 shows an EBSD grain boundary map, and the right side of Figure 1 shows a TEM image. As shown in the TEM image on the right side of Figure 1, precipitation of a Cu-rich phase (shown as "Cu" in Figure 1) was observed in the austenitic stainless steel according to one embodiment of the present invention.

Furthermore, since the reference strength (HV60%) of the invention steel A2 was 445, the 0.2% yield strength (YS18%) is preferably 1094 MPa or more based on the above formula (2). The cold-rolled and annealed steels of the invention steel A2 manufactured under the conditions of invention examples C1 to C8 all had a 0.2% yield strength (YS18%) of 1094 MPa or more. On the other hand, the cold-rolled and annealed steels manufactured under the conditions of comparative examples D1 and D2 all had a 0.2% yield strength (YS18%) lower than 1094 MPa. Thus, because the Cu-rich phase does not precipitate under the conditions of comparative examples D1 and D2, it was demonstrated that it is difficult to obtain an austenitic stainless steel that has a good balance between workability before final annealing and high strength after final annealing.

Next, the amount of Cu-rich phase precipitation and grain size after final annealing for cold-rolled and annealed materials manufactured from invention steels A1 to A15 or comparative steels B1 to B5 under the manufacturing conditions shown in invention example C2 are shown in Table 4 below. Table 4 also shows the 0.2% yield strength (YS 18%), reference strength (HV 60%), and corrosion resistance of welds under each of these conditions.

In addition, the underlined parts in Table 4 below indicate that the amount of Cu-rich phase precipitation is outside the specified range of this invention, the 0.2% yield strength (YS 18%) is a value that does not satisfy the above formula (2), or the corrosion resistance of the weld is poor.

The cold-rolled and annealed steels A1 to A15 of the invention exhibited Cu-rich phase precipitation within the range specified by the present invention and a fine average grain size of 10.0 μm or less. Furthermore, all of them exhibited good 0.2% yield strength (YS 18%) values that satisfied the above formula (2).

On the other hand, the cold-rolled and annealed comparative steel B1 had poor corrosion resistance at the weld. The cold-rolled and annealed comparative steels B2 to B5 did not satisfy the above formula (2) in terms of 0.2% yield strength (YS 18%), and it was not possible to obtain austenitic stainless steels with a good balance between workability before final annealing and high strength after final annealing.

Figure 2 shows a plot of the relationship between 0.2% yield strength (YS 18%) and reference strength (HV 60%) for each condition in Table 4. In Figure 2, an example of the present invention is indicated by a white circle, and a comparative example is indicated by a black arrowhead. In the graph shown in Figure 2, the plotted value toward the top left indicates a better balance between workability before finish annealing and high strength after finish annealing.

All of the above results are for cold-rolled and annealed materials obtained under conditions where the cooling rate after final annealing was 25°C/sec. Here, using invention steels A1, A2, and A5, cold-rolled and annealed materials were produced under the conditions of invention example C2 shown in Table 2, with the cooling rate from 700°C to 500°C after final annealing varied within the range of 0.3 to 100°C/sec. The amount of Cu-rich phase precipitation in the obtained cold-rolled and annealed materials is shown in Table 5 below.

When the cooling rate was 5°C/s or more, there was no change in the amount of Cu precipitation in the cold-rolled annealed material. It can be said that if the cooling rate was sufficiently fast, coarsening of the Cu-rich phase during cooling and the accompanying disappearance of the Cu-rich phase did not occur. When the cooling rate was 2°C/s, the amount of Cu precipitation decreased slightly. This is thought to be due to coarsening of the Cu-rich phase during cooling and the accompanying disappearance of the Cu-rich phase. This is a phenomenon generally known as Ostwald ripening. When the cooling rate was less than 1°C/s, coarsening of the Cu-rich phase during cooling and the accompanying disappearance of the Cu-rich phase progressed further, and the amount of precipitation was less than 1.0 × 10 ³ μm ³ .

As shown in Table 4 and Figure 2, cold-rolled annealed material produced by a manufacturing method according to one embodiment of the present invention using an austenitic stainless steel having a composition according to one embodiment of the present invention was shown to achieve both a reduction in the processing load during manufacturing and high product strength. Furthermore, such cold-rolled annealed material also exhibited excellent corrosion resistance in welds, making it suitable for applications where frequent welding is performed.

Claims (4)

The steel sheet contains, in mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 2.0 % or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities,
The alloy comprises an austenite phase of 20 volume % or more, a Cu-rich phase having a number density of 1.0 × 10 ³ particles μm ⁻³ or more and a major axis of 30 nm or less, and the remainder being a deformation-induced martensite phase and an unavoidably formed phase,
An austenitic stainless steel having an _Md30 value represented by the following formula (1) of 0.0 or more and 80.0 or less:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added. The austenitic stainless steel of claim 1 further contains, by mass%, one or more selected from Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more but 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metals): 0.2% or less. The austenitic stainless steel according to claim 1 or 2, having an average grain size of 10.0 μm or less. A method for producing an austenitic stainless steel containing, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 2.0 % or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and having a value of _Md30 represented by the following formula (1) of 0.0 or more and 80.0 or less,
A final annealing step of performing final annealing at a temperature of 750°C or higher and 980°C or lower,
When the maximum temperature reached in the finish annealing step is 850 ° C or higher, the heating time at 850 ° C or higher is set to 30 seconds or less,
A method for producing an austenitic stainless steel, wherein in the finish annealing step, the average cooling rate from 700°C to 500°C after the finish annealing is 1°C/second or more:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.