WO2023153184A1

WO2023153184A1 - Austenitic stainless steel and method for producing austenitic stainless steel

Info

Publication number: WO2023153184A1
Application number: PCT/JP2023/001835
Authority: WO
Inventors: 太一朗溝口
Original assignee: 日鉄ステンレス株式会社
Priority date: 2022-02-10
Filing date: 2023-01-23
Publication date: 2023-08-17
Also published as: KR20240118837A; JPWO2023153184A1; CN118541502A

Abstract

The present invention achieves an austenitic stainless steel having high productivity while reducing processing load during production and increasing the strength of the final product. The austenitic stainless steel contains, in mass%, 0.005-0.03% of C, 0.1-2.0% of Si, 0.3-2.5% of Mn, 0.04% or less of P, 0.015% or less of S, 3.0-6.0% of Ni, 16.0-18.5% of Cr, 1.5-4.0% of Cu, and 0.08-0.25% of N, with the remainder comprising Fe and inevitable impurities, includes 20 vol% or more of an austenite phase, and a Cu-rich phase having a number density of 1.0×10³·μm^-3 or more and a major axis of 30 nm or less, with the remainder comprising deformation-induced martensite phases and inevitable formation phases, and has an Md₃₀ value of 0.0-80.0.

Description

Austenitic stainless steel and method for producing austenitic stainless steel

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

　Metastable austenitic stainless steel represented by SUS301 is known as an austenitic stainless steel used for applications requiring corrosion resistance and strength. Such austenitic stainless steels are used, for example, as materials for spring products such as cylinder head gaskets for automobile engines or structural members such as frame materials for automotive batteries.

Such stainless steel generally increases its strength by increasing the rolling rate of cold rolling, etc., so there is a tendency for the processing load such as rolling in the manufacturing process to be large. In order to reduce such a load, for example, Patent Document 1 discloses a method for producing a spring material having a martensite phase in which precipitates composed of a Cu-rich phase are dispersed. A method of subjecting a spring steel plate to aging treatment has been proposed.

Japanese Patent Application Laid-Open No. 2008-195976

　Cu-rich phase precipitation is effective in increasing the strength of stainless steel. Therefore, according to the method described in Patent Document 1, the steel plate for spring is subjected to aging treatment to precipitate a Cu-rich phase, thereby reducing the processing load in the manufacturing process of the steel plate for spring, and the spring, which is the final product. It is possible to increase the strength of the material. However, since an aging treatment process is required, there is a problem in the productivity of the spring material.

An object of one aspect of the present invention is to realize an austenitic stainless steel with high productivity while simultaneously reducing the processing load during manufacturing and increasing the strength of the final product.

In addition, since the precipitation temperature of the Cu-rich phase and the precipitation temperature of Cr carbide are relatively close, it is preferable to use N without excessively increasing the amount of C in order to reduce the deterioration of corrosion resistance due to precipitation of Cr carbide. I focused on that. Reducing the amount of C in the austenitic stainless steel is also preferable for achieving the desired reduction in processing load.

In order to solve the above problems, the austenitic stainless steel according to one aspect of the present invention has C: 0.005% or more and 0.03% or less and Si: 0.1% or more and 2.0% by mass. Below, 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% 18.5% or less, Cu: 1.5% or more and 4.0% or less, N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, 20% by volume or more and a Cu-rich phase with a number density of 1.0×10 ³ μm ⁻³ or more and a major axis of 30 nm or less, and the balance consists of a deformation-induced martensite phase and an unavoidable formation phase, and the following ( 1) The value of Md ₃₀ represented by the formula 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)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.

In order to solve the above problems, a method for producing an austenitic stainless steel according to one aspect of the present invention comprises: C: 0.005% or more and 0.03% or less; Si: 0.1% or more 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, the balance being Fe and unavoidable impurities. (1) A method for producing an austenitic stainless steel having an _Md30 value of 0.0 or more and 80.0 or less represented by the formula, 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 final annealing step is 850 ° C. or higher, the time to heat at 850 ° C. or higher is within 30 seconds, and in the final annealing step, the average temperature from 700 ° C. to 500 ° C. after the final annealing A cooling rate of 1°C/sec or more:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.

According to one aspect of the present invention, it is possible to realize an austenitic stainless steel with high productivity while achieving both reduction in processing load during production and high strength of the final product.

FIG. 2 shows an EBSD grain boundary map and a TEM image of an austenitic stainless steel according to one embodiment; FIG. 2 is a diagram showing the relationship between 0.2% proof stress (YS18%) and reference strength (HV60%) of austenitic stainless steel according to an example and a comparative example;

The austenitic stainless steel according to one embodiment of the present invention will be described in detail below. The following description provides a better understanding of the spirit of the invention and is not intended to limit the invention unless otherwise specified.

[Organizational structure]
Austenitic stainless steel according to one embodiment of the present invention is stainless steel containing 20% by volume or more of an austenitic phase. Hereinafter, "austenitic stainless steel" refers to austenitic stainless steel according to one embodiment of the present invention unless otherwise specified. Austenitic stainless steels may be, for example, steel sheets or strips.

Austenitic stainless steel contains a deformation-induced martensite phase, which is part of the austenite phase transformed by the deformation-induced transformation plasticity (TRIP) phenomenon. From the viewpoint of increasing the strength of the austenitic stainless steel, the proportion of the deformation-induced martensite phase is preferably 5% by volume or more, more preferably 10% by volume or more, and 15% by volume or more. is more preferable, and 20% by volume or more is more preferable. In addition, the austenitic stainless steel preferably has a deformation-induced martensitic phase ratio of less than 80% by volume, more preferably 75% by volume or less. As long as the austenitic stainless steel contains at least 20% by volume of the austenite phase, the proportion of the deformation-induced martensite phase may decrease as the proportion of the deformation-induced martensite phase increases.

Austenitic stainless steels also contain Cu-rich phases. The Cu-rich phase is a phase containing 60 atom % or more of Cu (copper), such as an ε-Cu phase. Austenitic stainless steel contains at least a Cu-rich phase with a number density of 1.0×10 ³ μm ⁻³ or more and a major axis of 30 nm or less. The major diameter means the diameter of the maximum length among the diameters of the Cu-rich phase precipitated in the form of particles. Note that the austenitic stainless steel may contain a Cu-rich phase with a major axis of more than 30 nm. The Cu-rich phase may be dispersed in the austenite phase, may be dispersed in the deformation-induced martensite phase, or may be dispersed in the inevitable formation phase described later.

The Cu-rich phase may be determined by structural observation using a transmission electron microscope (TEM). For example, a TEM sample including an arbitrary cross section of austenitic stainless steel is prepared, and a predetermined range of the cross section is observed using a TEM. The number of rich phases can be measured. Further, the number density per volume can be calculated by calculating the volume based on the thickness of the TEM sample used for the number measurement and the area of the range where the number measurement was performed. For the thickness of the TEM sample, for example, a measured thickness of the TEM sample may be used, or an estimate of the thickness based on the method by which the TEM sample was made may be used. Examples of the method for producing a TEM sample include, but are not limited to, electropolishing.

The strength of austenitic stainless steel increases as the precipitated Cu-rich phase becomes finer and more abundant. The amount and size of the Cu-rich phase as described above are effective in increasing the strength of the austenitic stainless steel. Austenitic stainless steel does not precipitate a Cu-rich phase during production such as cold rolling before final annealing, and keeps the strength low to reduce the working load. By precipitating a Cu-rich phase in the final annealing step, the strength of the manufactured austenitic stainless steel is increased. Manufacturing processes such as the finish annealing process will be described later.

In addition, the austenitic stainless steel may contain unavoidably formed phases other than the austenite phase, the strain-induced martensite phase and the Cu-rich phase. Inevitably formed phases are not particularly limited, but include, for example, delta ferrite phases and phases containing carbides, nitrides and/or oxides. Phases containing carbides, nitrides and/or oxides include, for example, phases containing carbides and/or nitrides of Cr, Ti and/or Nb, and oxidation of Si, Ti, Al, Mg and/or Ca. phase containing substances.

Austenitic stainless steel preferably has an average crystal grain size of 10.0 μm or less. The strength of austenitic stainless steel increases as the crystal grains become finer. Further, in austenitic stainless steel, it is common that ductility decreases as strength increases. However, by refining grains, it is possible to achieve both strength improvement and ductility improvement in austenitic stainless steel.

The average grain size may be measured using the EBSD (Electron Back Scattering Diffraction) method. For example, for an arbitrary cross section of austenitic stainless steel, grain sizes in multiple fields of view may be calculated by the EBSD method, and the average value of the grain sizes calculated in the multiple fields of view may be used as the average grain size. Also, the average crystal grain size may be measured using a method other than the EBSD method. As a method other than the EBSD method, for example, a method of exposing grain boundaries by nitric acid electrolytic treatment as shown in JIS G0551 and measuring by an intercept method or the like may be used.

[Component composition]
Austenitic stainless steel is, 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 less than 2.5%, 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 more3. 8% or less and N: 0.08% or more and 0.25% or less. 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 described below.

(C)
C (carbon) is an austenite forming element that facilitates the formation of an austenite phase, has a high solid-solution strengthening action, 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. When the C content is 0.005% by mass or more, an austenitic stainless steel having a sufficient solid-solution strengthening effect and good strength can be obtained.

Excessive addition of C causes the precipitation of Cr carbides due to relatively low temperature annealing, which leads to a decrease in the corrosion resistance of the austenitic stainless steel, especially the weld zone, so the C content is 0.03% by mass or less. and If the C content is 0.03% by mass or less, an austenitic stainless steel having good corrosion resistance even at the weld zone can be obtained.

(Si)
Si (silicon) is an element that is effective as a deoxidizing agent and has a solid-solution strengthening action. The austenitic stainless steel contains 0.1% by mass or more and 2.0% by mass or less of Si, preferably 0.2% by mass or more and 1.0% by mass or less of Si. When the Si content is 0.1% by mass or more, deoxidizing action and solid solution strengthening action are effectively exhibited in the austenitic stainless steel. More preferably, the Si content is 0.2% by mass or more.

In addition, Si is a ferrite-forming element that facilitates the formation of a ferrite phase. The δ ferrite phase causes edge splitting or splitting in hot rolling. From the viewpoint of reducing the formation of the δ ferrite phase, the Si content is 2.0% by mass or less, preferably 1.0% by mass or less.

(Mn)
Mn (manganese) is an austenite-forming element and an element effective for maintaining the austenite phase. Also, Mn is an element that has the effect of promoting the precipitation of the Cu-rich phase. The austenitic stainless steel contains 0.3 mass % or more and 2.5 mass % or less of Mn, preferably 0.5 mass % or more and 2.0 mass % or less of Mn. If the content of Mn is 0.3% by mass or more, it is easy to secure the amount of precipitation of the Cu-rich phase, and if the content of Mn is 0.5% by mass or more, it is more preferable. Moreover, excessive addition of Mn causes deterioration of the hot workability of the austenitic stainless steel. Therefore, the content of Mn is set to 2.5% by mass or less, preferably 2.0% by mass or less.

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

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

(Ni)
Ni (nickel) is an austenite-generating element and an element effective for maintaining the austenite phase. The austenitic stainless steel contains 3.0% by mass or more and less than 6.0% by mass of Ni, preferably 3.5% by mass or more and 5.5% by mass or less of Ni, and 4.0% by mass or more and 5 More preferably, it contains less than 0.0% by mass of Ni. When the Ni content is 3.0% by mass or more, the austenite phase is well formed and maintained. More preferably, the Ni content is 4.5% by mass or more.

On the other hand, Ni is an expensive element, and when added in excess, it stabilizes the austenite phase and reduces the amount of deformation-induced martensite phase produced. Therefore, the Ni content is 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 effective element for ensuring the corrosion resistance of austenitic stainless steel. The austenitic stainless steel contains 16.0% by mass or more and 18.5% by mass or less of Cr, preferably 16.5% by mass or more and 18.0% by mass or less of Cr. If the Cr content is 16.0% by mass or more, good corrosion resistance of the austenitic stainless steel can be ensured. More preferably, the Cr content is 16.5% by mass or more.

On the other hand, since Cr is also a ferrite forming element like Si, excessive addition of Cr results in excessive formation of the δ ferrite phase. Therefore, the Cr content is 18.5% by mass or less, preferably 18.0% by mass or less.

(Cu)
Cu is an austenite-forming element and an element effective for maintaining the austenite phase. It is also effective in increasing the strength of austenitic stainless steel by precipitation of Cu-rich phases. Cu is an element that effectively acts also for crystal grain refinement. This is probably because the Cu-rich phase exhibits an inhibitory effect on grain growth. In addition, Cu reduces the work hardening of the austenite phase in a solid solution state, so that the rolling load in the manufacturing process of the austenitic stainless steel can be reduced.

The austenitic stainless steel contains 1.5% by mass or more and 4.0% by mass or less of Cu, preferably 2.0% by mass or more and 3.5% by mass or less of Cu, and more than 2.0% by mass. It is more preferable to contain 0.5% by mass or less of Cu. When the Cu content is 1.5% by mass or more, the austenite phase is well formed and maintained, and the Cu-rich phase is well precipitated. More preferably, the Cu content is 2.0% by mass or more, and even more preferably more than 2.0% by mass.

On the other hand, if Cu is excessively added, a CuMn phase is generated at the center of the slab during the solidification process of the slab, which reduces the hot workability of the slab. Therefore, the Cu content is 4.0% by mass or less, preferably 3.5% by mass or less.

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

Also, excessive addition of N increases the rolling load of the austenitic stainless steel. Therefore, the N content is 0.25% by mass or less, preferably 0.20% by mass or less.

(other elements)
In addition to the above elements, the austenitic stainless steel contains, by mass%, Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, and B: 0.0001% or more and 0.0001% or less. 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% Below, 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 metal): 0.5% or less. It may further contain one or more selected from 2% or less.

(Mo, W, V)
Mo (molybdenum), W (tungsten) and V (vanadium) are elements effective in improving corrosion resistance. On the other hand, Mo, W and V are ferrite-forming elements and are also expensive elements, so excessive addition is not preferable. Therefore, the austenitic stainless steel preferably contains one or more selected from Mo of 1.0% by mass or less, W of 1.0% by mass or less, and V of 0.5% by mass or less.

(B)
B (boron) is an element that improves hot workability, and is an element that is effective in reducing the occurrence of edge splitting and double cracking in hot rolling. The austenitic stainless steel preferably contains 0.0001% by mass or more and 0.01% by mass or less of B. If the B content is 0.0001% by mass or more, it is effective in improving hot workability and reducing edge splitting and split cracking in hot rolling. However, excessive addition of B to an austenitic stainless steel containing Cr causes deterioration of corrosion resistance due to precipitation of Cr ₂ B. Therefore, the content of B is preferably 0.01% by mass or less.

(Co)
Co (cobalt) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. It also contributes to reducing the coarsening of the Cu-rich phase and maintaining it fine. In order to obtain such an effect, it is preferable to contain 0.10% by mass or more of Co. However, Co is an expensive element, and from the viewpoint of cost reduction, the Co content is preferably 0.8% by mass or less.

(Sn)
Sn (tin) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. However, since excessive addition of Sn causes deterioration of the hot workability of the austenitic stainless steel, the Sn content is preferably 0.1% by mass or less.

(Al, Ca, Mg, Ti)
Al (aluminum), Ca (calcium), Mg (magnesium) and Ti (titanium) are all deoxidizing elements. The austenitic stainless steel is selected from 0.3 wt% or less Al, 0.03 wt% or less Ca, 0.03 wt% or less Mg, and 0.5 wt% or less Ti as a deoxidizing agent. It is preferable to include one or more of

(Nb)
Nb (niobium) is an element effective in reducing sensitization of austenitic stainless steel. It is also effective for making the structure fine and uniform. The austenitic stainless steel preferably contains 0.5% by mass or less of Nb.

(Sb, Zr, Ta, Hf, REM)
Sb (antimony), Zr (zirconium), Ta (tantalum), Hf (hafnium) and REM (rare earth metal) are all elements that improve hot workability and are also effective in oxidation resistance. The austenitic stainless steel contains up to 0.5% by weight Sb, up to 0.5% by weight Zr, up to 0.03% by weight Ta, up to 0.03% by weight Hf, and up to 0.2% by weight It preferably contains one or more selected from REM.

[Value of Md ₃₀ ]
The austenitic stainless steel has an _Md30 value of 0.0 or more and 80.0 or less, preferably 20.0 or more and 70.0 or less, as indicated by the following formula (1).

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.

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

The value of _Md30 of the austenitic stainless steel according to one embodiment of the present invention is preferably 0.0 or more and 80.0 or less. The higher the value of Md ₃₀ , the easier the transformation from the austenite phase to the deformation-induced martensite phase occurs, and the application of mild cold-rolling strain can provide high strength and ensure excellent ductility. Further, even when the austenitic stainless steel is subjected to forming processing, the portion to which processing strain is imparted, such as a bent portion, tends to obtain higher strength due to the TRIP phenomenon.

In the process of manufacturing austenitic stainless steel, the presence of deformation-induced martensite phase in the rolled material before final annealing works effectively to refine grains by final annealing. Such an effect appears remarkably when the value of _Md30 is 0.0 or more. Moreover, when the value of Md ₃₀ exceeds 80.0, the TRIP phenomenon tends to occur excessively, and the properties of the austenitic stainless steel are difficult to stabilize.

Therefore, if the value of _Md30 , which is an index of 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.

Incidentally, in the conventionally known component regression equation of Md ₃₀ , it is common to use the same value for the coefficients of Ni and Cu. On the other hand, in one embodiment of the present invention, the coefficient of Cu is set smaller than the coefficient of Ni in the component regression equation of Md ₃₀ . Many of the component regression equations of Md ₃₀ based on conventional knowledge are based on the results of austenitic stainless steels that are not Ni-saving types. On the other hand, in the Ni-saving type composition of the present invention, it has been found that the influence of Cu on the stabilization of the austenite phase is clearly smaller than the conventional knowledge. This is a novel finding obtained as a result of diligent studies by the present inventors, and based on this finding, the coefficient of Cu in the component regression equation for Md ₃₀ is set. This facilitates adjustment of the Cu content and increases the degree of freedom in manufacturing the austenitic stainless steel.

〔Production method〕
A method for producing an austenitic stainless steel according to an embodiment of the present invention includes, in mass %, C: 0.005% to 0.03%, Si: 0.1% to 2.0%, 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% Below, Cu: 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, Md ₃₀ represented by the above formula (1) is 0.0 or more and 80.0 or less. Also, the method for producing austenitic stainless steel includes a finish annealing step.

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

In the method for producing austenitic stainless steel according to one embodiment of the present invention, for example, a slab is produced by continuously casting molten steel whose composition is adjusted. Then, the slab produced by continuous casting is heated to 1100° C. or higher and 1300° C. or lower, and then hot rolled to produce a hot rolled steel strip. The precipitation rate of the 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 may be the same conditions as in the general method for producing austenitic stainless steel. From the viewpoint of minimizing the precipitation of the Cu-rich phase until the final annealing, the coiling temperature of the hot-rolled steel strip after hot rolling is preferably 850° C. or lower, more preferably 650° C. or lower.

The hot-rolled steel strip that has been hot-rolled may be pickled. The hot-rolled steel strip may be annealed before pickling, or may be pickled without annealing. When the hot-rolled steel strip is annealed before pickling, the annealing temperature is preferably in the range of 900° C. or higher and 1150° C. or lower. It is more preferable to operate at a temperature within the range, but is not limited to the above range. Then, the pickled hot-rolled steel strip is cold-rolled to a predetermined thickness to obtain a cold-rolled steel strip.

　In the method of manufacturing austenitic stainless steel, recrystallization and precipitation of the Cu-rich phase proceed simultaneously in the finish annealing process after the cold rolling process. Since the Cu-rich phase is particularly likely to precipitate from the strain-induced martensite phase, the cold rolling process should be performed at a rolling reduction and a rate such that the strain-induced martensite phase in the cold-rolled steel strip accounts for 20% by volume or more of the total. It is preferable to carry out by rolling temperature. By performing such a cold rolling process, a Cu-rich phase can be effectively precipitated in the steel strip in the subsequent finish annealing process.

In the austenitic stainless steel, the value of _Md30 is adjusted to 0.0 or more and 80.0 or less. An austenitic stainless steel having such a value of Md ₃₀ precipitates a Cu-rich phase in the amount specified in one embodiment of the present invention, regardless of the amount of deformation-induced martensitic phase in the cold-rolled steel strip. However, increasing the rolling reduction in the cold-rolling process, controlling the temperature in the cold-rolling process to be low, etc. as necessary are more effective for the precipitation of the Cu-rich phase.

In the cold-rolled steel strip, from the viewpoint of making the deformation-induced martensite phase 20% by volume or more, for example, the rolling reduction in the cold rolling step is preferably 40% or more, more preferably 50% or more, It is more preferably 60% or more. Also, the temperature in the cold rolling step is preferably 90° C. or lower, more preferably 60° C. or lower.

(Finish annealing process)
The cold-rolled steel strip is subjected to finish annealing. The finish annealing step is carried out under conditions that promote the precipitation of the Cu-rich phase. A Cu-rich phase is effective in increasing the strength of austenitic stainless steel. Therefore, the strength of the hot-rolled steel strip and the cold-rolled steel strip before precipitation of the Cu-rich phase is rather low, and the rolling load in the cold rolling process can be reduced. Then, the Cu-rich phase is precipitated in the final annealing step, so that the austenitic stainless steel after the final annealing has high strength.

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

Thus, according to the method for producing austenitic stainless steel according to one embodiment of the present invention, it is possible to achieve both a reduction in the processing load during production and an increase in the strength of the final product at a high level. In addition, the productivity of the austenitic stainless steel is also good because the additional step of the aging treatment is not required for the precipitation of the Cu-rich phase unlike the conventional method.

The finish annealing temperature in the finish annealing step is preferably 750°C or higher and 980°C or lower, and preferably 800°C or higher and 925°C or lower so that the Cu-rich phase is effectively precipitated in the austenitic stainless steel. If the final annealing temperature is less than 750°C, recrystallization of the structure will be insufficient. Moreover, when the temperature of the final annealing exceeds 980° C., the Cu-rich phase dissolves, so the amount of the Cu-rich phase remaining after the final annealing becomes insufficient.

In addition, the Cu-rich phase that precipitates from the deformation-induced martensite phase is particularly likely to dissolve in the austenite phase if it is held at a temperature of 850°C or higher for a long time in the final annealing. Therefore, when the maximum temperature reached in the final annealing step 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 final annealing step is 850° C. or higher, the heating time at 850° C. or higher is set to 30 seconds or less, preferably 15 seconds or less. The term "heating time at 850° C. or higher" refers to the total time of the plurality of heating times when the final annealing step is divided into multiple times for heating to 850° C. or higher.

　Austenitic stainless steel has a C content of 0.03% by mass or less, so precipitation of Cr carbide during cooling is unlikely to occur. Therefore, the cooling rate after finish annealing may be the same as in a general stainless steel manufacturing method. From the viewpoint of productivity, it can be said that a faster cooling rate is preferable, but the average cooling rate from 700 ° C. to 500 ° C. may be a relatively slow speed of 1 ° C./sec or more, for example. Taking this into consideration, 5° C./second or more is preferable. Also, considering the flatness of the steel sheet, the cooling rate is preferably less than 75° C./second, more preferably 50° C./second or less.

In the cold rolling process, intermediate annealing and intermediate rolling may be performed as necessary. Further, in order to further increase the strength of the steel strip after finish annealing, skin pass rolling may be performed as necessary. The temperature of the intermediate annealing is preferably 980° C. or higher and 1150° C. or lower in order to avoid precipitation of the Cu-rich phase when priority is given to reducing the rolling load. In order to increase the strength by repeating the precipitation treatment, it is preferable that the temperature of the intermediate annealing is the same as that of the finish annealing. In addition, the temperature of the intermediate annealing is not limited to the above range.

[Strength evaluation]
An austenitic stainless steel according to an embodiment of the present invention has a relatively low strength in the manufacturing process to reduce the rolling load and achieves high strength after manufacturing. Such properties of austenitic stainless steel can be expressed, for example, by the relationship between 0.2% proof stress (YS18%, MPa) and reference strength (HV60%).

　0.2% proof stress (YS18%) is an indicator of the strength of austenitic stainless steel. The 0.2% yield strength (YS18%) indicates the 0.2% yield strength when the austenitic stainless steel is further subjected to temper rolling with an elongation of 18% after finish annealing. The 0.2% yield strength can be evaluated using a method conforming to JIS Z2241.

The reference strength (HV60%) is an index that hypothetically indicates the strength of the austenitic stainless steel before precipitation of the Cu-rich phase in the final annealing process. For the reference strength (HV60%), although the chemical composition of the austenitic stainless steel is the same, the manufacturing method is partially changed from the manufacturing method according to the embodiment of the present invention, and after hot rolling, annealing at 1050 ° C. is performed. It shows the Vickers hardness when cold rolling is applied at a rolling reduction of 60%. That is, the reference strength (HV60%) 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 produced for evaluation, for example. Vickers hardness can be measured based on the Vickers hardness test method conforming to JIS Z2244.

The present inventors have found that the austenitic stainless steel, which achieves both a reduction in the processing load during manufacturing and a high strength of the final product, has a relationship between the 0.2% proof stress (YS18%) and the reference strength (HV60%). satisfies the following formula (2).

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

[Preferred uses]
Austenitic stainless steel has very high strength and corrosion resistance. Therefore, austenitic stainless steel is used for spring products that require high strength and corrosion resistance, such as cylinder head gaskets, spiral springs, springs for electronic device parts, train vehicle members, automotive battery frame materials, structural materials, and metal packings. It is suitable as a material for In particular, austenitic stainless steel is excellent in corrosion resistance (weldability) even when welded. Therefore, the austenitic stainless steel according to one embodiment of the present invention is suitable even for applications in which a relatively large number of welded structures are used, such as train vehicle members or automotive battery frame materials manufactured for welding. Available.

〔summary〕
The austenitic stainless steel according to aspect 1 of the present invention has, 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 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: Contains 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, 20% by volume or more of the austenitic phase, and a number density of 1 .0×10 ³ μm ⁻³ or more and a Cu-rich phase with a major axis of 30 nm or less, and the balance consists of a deformation-induced martensite phase and an unavoidable formation phase, and the value of Md ₃₀ shown 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)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.

The austenitic stainless steel according to aspect 2 of the present invention, in aspect 1 above, contains Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, and B: 0.5% by mass. 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 metal): may further contain one or more selected from 0.2% or less.

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

The method for producing an austenitic stainless steel according to aspect 4 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: 0.1% or more, and 0.03% or less; 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, the balance being Fe and unavoidable impurities, Md ₃₀ represented by the following formula (1) A method for producing austenitic stainless steel having a value of 0.0 or more and 80.0 or less, comprising a finish annealing step of performing finish annealing at a temperature of 750° C. or more and 980° C. or less, wherein the final annealing step achieves the maximum When the temperature is 850° C. or higher, the heating time at 850° C. or higher is 30 seconds or less, and the average cooling rate from 700° C. to 500° C. after the finish annealing is 1° C./second or more in the final annealing step. do:
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.

The present invention is not limited to the above-described embodiments, but can be modified in various ways within the scope of the claims, and can be obtained by appropriately combining technical means disclosed in different embodiments. is also included in the technical scope of the present invention.

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

[Evaluation conditions]
<Component composition>
The chemical compositions (% by mass) and Md ₃₀ values of the austenitic stainless steels according to the examples of the present invention (invention steels A1 to A15) and the austenitic stainless steels according to the comparative examples (comparative steels B1 to B5) are shown below. Table 1 shows. The value of Md ₃₀ was calculated by the above formula (1). The underlined values in Table 1 below are out of the specified range of the present invention. The same applies to Table 2 below.

<Manufacturing method>
Austenitic stainless steels according to each of the examples and comparative examples of the present invention were produced by the following method. Austenitic stainless steel having the chemical composition shown in Table 1 was melted, and the production method according to one example of the present invention (invention examples C1 to C8) or the production method according to the comparative example (comparative examples D1 and D2), From hot rolling to finish annealing, a cold-rolled annealed material was obtained. The conditions of each manufacturing method are shown in Table 2 below.

In the finish annealing process, when the finish annealing temperature was 850°C or higher, the time for reaching 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 when the final annealing temperature reached 850 ° C., but for convenience, in Table 2, the time to reach 850 ° C. or higher is described as "1 second". are doing.

<Evaluation method>
Various indices of the austenitic stainless steel according to each of the examples and comparative examples of the present invention were evaluated as follows.

(Number density of Cu-rich phase)
A TEM sample was produced by an electropolishing method from the cold-rolled annealed material produced under each condition. A plane parallel to the rolling direction of the cold-rolled and annealed material in the TEM sample was observed in three fields of view in a range of 400 nm×400 nm. Cu-rich phases were determined from the contrast of the TEM image, and the number of Cu-rich phases was counted. The thickness of the TEM sample was assumed to be 150 nm, and the number density per unit volume was determined. When the Cu-rich phase coarsened, it became observed in a clear shape instead of contrast. Cu-rich phases with longer diameters greater than 30 nm were excluded from the measurements.

(Crystal grain size)
The average grain size was evaluated using the EBSD method. A cross section parallel to the rolling direction and perpendicular to the rolling surface of the cold-rolled annealed material manufactured under each condition was mechanically polished and then electrolytically polished. After that, EBSD analysis was performed on a 40 μm×40 μm range of the cross section with a step interval of 0.2 μm in a field of view with a magnification of 2000×. With regard to the misorientation in the orientation relationship satisfying the Σ3 correspondence grain boundary, except for annealing twins with misorientation of 1° or less, boundaries with misorientation of 2° or more are regarded as grain boundaries, and the area of each grain is S (μm ² ), and the diameter of a circle having the same area as the crystal grain was defined as D (μm), and the crystal grain size was calculated by the following formula (3). This was performed for 5 fields of view, and the average of the grain sizes obtained in the 5 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 (% by volume) is as it is when the plate thickness is 1.5 mm or more, and when the plate thickness is less than 1.5 mm, the material after cold rolling or temper rolling is adjusted so that the total is 1.5 mm or more. Later materials were layered. These materials were measured with a ferrite scope (Fischer FMP30, electromagnetic induction method), and the value obtained by dividing the measured value by 0.7475 was taken as the amount of martensite phase. The amount (% by volume) of the austenite phase was regarded as a value obtained by subtracting the amount of the martensite phase from 100% by volume of the entire matrix of the austenitic stainless steel. The amounts of Cu-rich phases and unavoidably formed phases in austenitic stainless steel may be calculated as extraneous numbers because their ratios are small and accurate measurement is difficult.

(tensile properties)
As an index of tensile properties, the 0.2% proof stress (YS18%) when subjected to temper rolling with an elongation of 18% was evaluated. The 0.2% proof stress (YS18%) was measured by a tensile test according to JIS Z2241 by preparing a JIS No. 13B test piece. 0.2% proof stress (YS18%) was measured at a crosshead speed of 3 mm/min.

(Strength)
60% rolling in which the manufacturing conditions were partially changed from the conditions of the examples and comparative examples of the present invention, and the hot rolled steel strip was annealed at 1050 ° C. and cold rolled at a rolling reduction of 60%. The Vickers hardness of the material was measured as a reference strength (HV60%). The Vickers hardness was measured by performing a Vickers hardness test (JIS Z2244) with a Vickers hardness tester on the surface of the 60% rolled material. The load at the time of measurement in the Vickers hardness test was 10 kg.

(Corrosion resistance of weld zone)
A cold-rolled annealed material having a plate thickness of 1.5 mm was subjected to TIG tanning welding at an electrode diameter of 1.6 mm, a welding speed of 70 cm/min, and a welding current of 90 A under the conditions of an Ar gas seal. A 10 mm × 10 mm area including the weld was used as an evaluation surface, and #600 polishing was applied to remove the influence of the film, and the corrosion resistance of the evaluation surface was evaluated using the electrochemical reactivation rate as an index.

The reactivation rate was measured according to JIS G0580. Specifically, in a 0.5 mol/L sulfuric acid and 0.01 mol/L potassium thiocyanate aqueous solution at a liquid temperature of 30°C, polarization was performed from the spontaneous potential to 0.3 V (vs SCE) at a sweep rate of 100 mV/min ( hereinafter referred to as the “outward route”). After reaching 0.3 V (vs SCE), the potential was swept in the direction opposite to the forward trip, and after reactivation of the hot-rolled material, the sweep was terminated at a potential at which the anode current became 0 again (hereinafter, "return trip"). .

The ratio (ir/ia) of the maximum current density ia on the outward trip and the maximum current density ir on the return trip was calculated as the reactivation rate. Such an evaluation method is strict as a sensitization determination method for evaluating corrosion resistance, so even if the reactivation rate is, for example, about 1.5%, it is considered that there is no problem in the actual environment. be done. However, since the cold-rolled annealed material according to one example of the present invention may have fine crystal grains, it is difficult to evaluate corrosion resistance. It can be said that it has Therefore, the corrosion resistance of the weld zone was evaluated as "O" (good) when the reactivation rate was 1% or less, and as "x" (poor) when the reactivation rate exceeded 1%.

〔result〕
Table 3 below shows the amount of precipitated Cu-rich phase and the grain size of the cold-rolled annealed material obtained under the conditions shown in Table 2 for the invention steel A2. In addition, regarding the 0.2% yield strength (YS18%) under each condition, and the amount of martensite phase after cold rolling (before finish annealing) and after temper rolling at which the elongation is 18% after finish annealing. are also shown in Table 3 below.

The underlines in Table 3 below indicate that the precipitation amount of the Cu-rich phase is outside the specified range of the present invention.

For the invention steel A2, the cold-rolled annealed materials produced under the respective conditions of invention examples C1 to C8 have a Cu-rich phase precipitation amount within the specified range of the present invention, and have a fine average grain size of 10.0 μm or less. Indicated. On the other hand, precipitation of the Cu-rich phase was not observed in any of the cold-rolled annealed materials produced under the conditions of Comparative Examples D1 and D2.

Regarding the invention steel A2, the EBSD grain boundary map is shown on the left side of FIG. 1, and the TEM imaged image is shown on the right side of FIG. As shown in the TEM image on the right side of FIG. 1, precipitation of a Cu-rich phase (indicated as “Cu” in FIG. 1) was observed in the austenitic stainless steel according to one embodiment of the present invention.

Also, since the reference strength (HV60%) of Inventive Steel A2 was 445, the 0.2% yield strength (YS18%) is preferably 1094 MPa or more based on the above formula (2). The 0.2% proof stress (YS18%) of the cold-rolled annealed materials of the invention steel A2 manufactured under the respective conditions of invention examples C1 to C8 was 1094 MPa or more. On the other hand, the 0.2% proof stress (YS18%) of the cold-rolled annealed materials manufactured under the respective conditions of Comparative Examples D1 and D2 was lower than 1094 MPa. Thus, under the conditions of Comparative Examples D1 and D2, Cu-rich phases do not precipitate, so it is difficult to obtain an austenitic stainless steel with a good balance between workability before final annealing and high strength after final annealing. was shown to be

Next, Table 4 below shows the Cu-rich phase precipitation amount and grain size after finish annealing of the cold-rolled and annealed materials manufactured from the invention steels A1 to A15 or the comparative steels B1 to B5 under the manufacturing conditions shown in invention example C2. show. The 0.2% proof stress (YS18%), reference strength (HV60%), and corrosion resistance of the weld under these conditions are also shown in Table 4 below.

The underlined portion in Table 4 below indicates that the precipitation amount of the Cu-rich phase is outside the specified range of the present invention, and the 0.2% proof stress (YS18%) is a value that does not satisfy the above formula (2). or that the corrosion resistance of the weld is poor.

The cold-rolled and annealed materials of the invention steels A1 to A15 had a precipitation amount of the Cu-rich phase within the specified range of the present invention, and exhibited a fine average crystal grain size of 10.0 μm or less. In addition, the 0.2% proof stress (YS18%) showed good values satisfying the above formula (2).

On the other hand, the cold-rolled and annealed material of comparative steel B1 had poor corrosion resistance at the weld. The cold-rolled annealed materials of comparative steels B2 to B5 do not satisfy the above formula (2) in terms of 0.2% proof stress (YS18%), and have a good balance between workability before finish annealing and high strength after finish annealing. Austenitic stainless steel was not obtained.

Fig. 2 shows a plot of the relationship between the 0.2% proof stress (YS18%) and the reference strength (HV60%) under each condition in Table 4. In FIG. 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 FIG. 2, the higher the upper left plot, the better the balance between the workability before the finish annealing and the high strength after the finish annealing.

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

At a cooling rate of 5° C./second or more, there was no change in the amount of Cu precipitation in the cold-rolled annealed material. If the cooling rate is sufficiently high, it can be said that coarsening of the Cu-rich phase during cooling and accompanying disappearance of the Cu-rich phase do not occur. When the cooling rate was 2° C./second, the amount of Cu precipitation decreased slightly. It is considered that this is because the Cu-rich phase coarsened during cooling and disappeared along with the Cu-rich phase. This is a phenomenon commonly called Ostwald growth. When the cooling rate is less than 1° C./sec, the Cu-rich phase coarsens during cooling and disappears accordingly, and the precipitation amount becomes less than 1.0×10 ³ pieces·μm ⁻³ . Ta.

As shown in Table 4 and FIG. 2, the cold-rolled annealed material manufactured by the manufacturing method according to one embodiment of the present invention using the austenitic stainless steel having the composition according to one embodiment of the present invention is It was shown that both the reduction of the processing load and the increase in strength of the product are compatible. It was also shown that such a cold-rolled annealed material is excellent in corrosion resistance of welded parts and suitable for applications in which many weldings are performed.

Claims

% 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: containing 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities,
20% by volume or more of an austenite phase, and a Cu-rich phase with a number density of 1.0×10 3 μm −3 or more and a major axis of 30 nm or less, and the balance being a deformation-induced martensite phase and an unavoidably formed phase. become,
Austenitic stainless steel having an Md 30 value of 0.0 or more and 80.0 or less, represented by the following formula (1):
Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
% by mass, 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 metal): 0.2% or less The austenitic stainless steel according to claim 1, wherein
The austenitic stainless steel according to claim 1 or 2, having an average grain size of 10.0 µm or less.
% 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: Austenitic stainless steel containing 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, and having a value of Md 30 represented by the following formula (1) of 0.0 or more and 80.0 or less A manufacturing method of
Including a finish annealing step of performing finish annealing at a temperature of 750 ° C or higher and 980 ° C or lower,
When the maximum temperature reached in the final 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 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)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.