KR20240119113A - Austenitic stainless steel and method for manufacturing austenitic stainless steel - Google Patents

Abstract

It achieves both reduction of processing load during manufacturing and high strength of the final product, and also realizes highly productive austenitic stainless steel. Austenitic stainless steel, in mass%, C: 0.03% to 0.15%, 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 3.0% Contains 6.0%, Cr: 16.0 to 18.5%, Cu: 1.5 to 4.0%, and N: 0.005 to 0.25%, the balance consists of Fe and inevitable impurities, an austenite phase of 20% by volume or more, and a number density of 1.0. It contains a Cu _- rich phase with ^{more than}

Description

Austenitic stainless steel and method for manufacturing austenitic stainless steel

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

As an austenitic stainless steel used in applications requiring corrosion resistance and strength, metastable austenitic stainless steel, represented by SUS301, is known. Such austenitic stainless steel is used, for example, as a material for spring products such as cylinder head gaskets for automobile engines or structural members such as vehicle-mounted battery frame materials.

Such stainless steels generally have high strength by increasing the rolling reduction rate, such as cold rolling, and therefore tend to have a large processing load, such as rolling, in the manufacturing process. In order to reduce such a load, for example, Patent Document 1 discloses a method for manufacturing a spring material having a martensite phase in which precipitates consisting of a Cu-rich phase are dispersed, and a double-phase structure without precipitation of the Cu-rich phase. A method of performing aging treatment on a spring steel sheet that represents has been proposed.

Patent Document 1: Japanese Patent Publication No. 2008-195976

Precipitation of Cu-rich phase is effective in increasing the strength of stainless steel. Therefore, according to the method described in Patent Document 1, by subjecting the spring steel sheet to aging treatment to precipitate the Cu rich phase, it is possible to reduce the processing load in the spring steel sheet manufacturing process and achieve high strength of the spring material as the final product. However, since an aging treatment process is required, there is a problem with the productivity of the spring material.

One form of the present invention aims to achieve both reduction of processing load during manufacturing and high strength of the final product, and to realize highly productive austenitic stainless steel. Although it is desirable to have a low C content to reduce processing load, we focused on utilizing a certain amount of C to obtain high strength in the final product.

In order to solve the above problems, the austenitic stainless steel according to one embodiment of the present invention has, in mass%, C: 0.03% to 0.15%, Si: 0.1% to 2.0%, Mn: 0.3% to 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: 0.005% or more and 0.25% or less. Contains, the balance consists of Fe and inevitable impurities, an austenite phase of 20% by volume ^or more, and a Cu-rich phase with a number density of ^1.0 It consists of a strain-induced martensite phase and an unavoidable formation phase, and the value of Md ₃₀ according to the following formula (1) is 10.0 or more and 80.0 or less.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)

The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition.

In order to solve the above problems, a method for manufacturing austenitic stainless steel according to one embodiment of the present invention is, in terms of mass%, C: more than 0.03% and less than 0.15%, Si: 0.1% and more than 2.0%, Mn: more than 0.3%. 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more but less than 6.0%, Cr: 16.0% or more but 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.005% or more and 0.25% A method for producing an austenitic stainless steel containing the following, the balance being Fe and inevitable impurities, and having an Md ₃₀ value of 10.0 or more and 80.0 or less according to the formula (1) below, at a temperature of 750°C or more and 980°C or less. A final annealing process is included, and when the maximum temperature achieved in the final annealing process is 850°C or higher, the heating time at 850°C or higher is set to 30 seconds or less.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)

The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition.

According to one embodiment of the present invention, it is possible to achieve both reduction of processing load during manufacturing and high strength of the final product, and also to realize highly productive austenitic stainless steel.

1 is a diagram showing an EBSD grain boundary map and a TEM image of an austenitic stainless steel according to an embodiment.
Figure 2 is a diagram showing the relationship between 0.2% proof strength (YS 18%) and reference strength (HV 60%) of austenitic stainless steel according to an example and a comparative example.

Hereinafter, austenitic stainless steel according to an embodiment of the present invention will be described in detail. The following description is intended to better understand the spirit of the invention and does not limit the invention unless otherwise specified.

[Organization]

The austenitic stainless steel according to one embodiment of the present invention is a stainless steel containing an austenite phase of 20 volume% or more. Hereinafter, in this specification, 'austenitic stainless steel' refers to austenitic stainless steel according to one embodiment of the present invention, unless otherwise specified. Austenitic stainless steel may be, for example, a steel strip.

Austenitic stainless steel contains a processed martensite phase in which a part of the austenite phase is transformed by a TRansformation Induced Plasticity (TRIP) phenomenon. In austenitic stainless steel, from the viewpoint of high strength, the ratio of the deformed martensite phase is preferably 5 volume% or more, more preferably 10 volume% or more, more preferably 15 volume% or more, and 20 volume% or more. It is more desirable. In addition, in austenitic stainless steel, it is preferable that the proportion of the deformed martensite phase is less than 80 volume%, and more preferably, it is 75 volume% or less. The proportion of the austenite phase contained in austenitic stainless steel may be lowered as the proportion of the deformation-induced martensite phase increases, as long as it is 20 volume% or more.

Austenitic stainless steel contains more Cu rich phase. The Cu-rich phase is a phase containing 60 atomic% or more of Cu (copper), for example, an ε-Cu phase. Austenitic stainless steel contains at least a Cu-rich phase with a number density of 1.0×10 ³ pieces·μm ^-3 or more and a major axis of 30 nm or less. Long diameter means the diameter of the largest length among the diameters of the Cu-rich phase precipitated in particle form. On the other hand, austenitic stainless steel may contain a Cu-rich phase with a major axis exceeding 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 can be identified by tissue observation using a transmission electron microscope (TEM). For example, by producing a TEM sample containing an arbitrary cross section of austenitic stainless steel and observing a predetermined range of the cross section using TEM, the major diameter of the cross section of the Cu rich phase within that range is 30 nm or less. The number of reach phases can be measured. Additionally, 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 in which the number measurement was performed. Regarding the thickness of the TEM sample, for example, the actual measured value of the thickness of the TEM sample may be used, or an estimate of the thickness based on the method of producing the TEM sample may be used. Methods for producing TEM samples include, for example, electrolytic polishing, but are not limited to this.

The strength of austenitic stainless steel increases as the precipitated Cu-rich phase becomes finer and more abundant. The Cu rich phase with the content and size described above is effective in increasing the strength of austenitic stainless steel. Austenitic stainless steel does not precipitate a Cu-rich phase during manufacturing, such as during cold rolling before final annealing, and suppresses the strength to a low level, thereby reducing the processing load. And, by precipitating the Cu-rich phase in the final annealing process, high strength is realized for the austenitic stainless steel after production. Manufacturing processes such as the final annealing process will be described later.

Additionally, austenitic stainless steel may contain unavoidable formation phases other than the austenite phase, the deformed martensite phase, and the Cu-rich phase. The inevitable formation phase is not particularly limited, but examples include a δ ferrite phase and a phase containing carbide, nitride, and/or oxide. Phases containing carbides, nitrides and/or oxides include, for example, phases containing carbides and/or nitrides of Cr, Ti and/or Nb, and oxides of Si, Ti, Al, Mg and/or Ca. There are awards that can be given.

Austenitic stainless steel preferably has an average grain size of 10.0 μm or less. The strength of austenitic stainless steel improves as the crystal grains become finer. Additionally, in austenitic stainless steel, it is common for ductility to decrease when strength is improved. However, by refining the grains, both strength improvement and ductility improvement can be achieved in austenitic stainless steel.

The average grain size can be measured using the EBSD (Electron Back Scattering Diffraction) method. For example, for an arbitrary cross section of austenitic stainless steel, the grain sizes of multiple views can be calculated by the EBSD method, and the average value of the grain sizes calculated from the multiple views can be used as the average grain size. In addition, the average grain size may be measured using a method other than the EBSD method. A method other than the EBSD method may be, for example, a method as shown in JIS G0551 in which grain boundaries are made to appear by nitric acid electrolytic treatment and measured by a sectioning method or the like.

[Ingredients Composition]

Austenitic stainless steel is, in mass percentage, C: 0.03% to 0.15%, Si: 0.1% to 2.0%, Mn: 0.3% to 2.5%, P: 0.04% or less, S: 0.015% or less, Contains 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 3.8% or less, and N: 0.005% or more and 0.25% or less. The remainder of the austenitic stainless steel may be composed of Fe (iron) and inevitable impurities. Hereinafter, the significance of the content of each element contained in austenitic stainless steel will be explained.

(C)

C (carbon) is an austenite forming element that facilitates the formation of an austenite phase, has a high solid solution strengthening effect, and is also an effective element for obtaining strength. Austenitic stainless steel contains more than 0.03% by mass and less than 0.15% by mass of C. If the C content is more than 0.03% by mass, austenitic stainless steel can be obtained that not only exhibits sufficient solid solution strengthening effect but also has good strength.

Excessive addition of C causes Cr carbide to precipitate during annealing at a relatively low temperature, resulting in a decrease in corrosion resistance of austenitic stainless steel, so the C content is set to 0.15% by mass or less. If the C content is 0.15% by mass or less, austenitic stainless steel with good corrosion resistance can be obtained.

(Si)

Si (silicon) is an element that is effective as a deoxidizer and also has a solid solution strengthening effect. Austenitic stainless steel preferably contains 0.1 mass% or more and 2.0 mass% or less of Si, and preferably contains 0.2 mass% or more and 1.0 mass% or less of Si. When the Si content is 0.1% by mass or more, the deoxidation action and solid solution strengthening action are effectively exhibited in austenitic stainless steel. It is more preferable that the Si content is 0.2% by mass or more.

Additionally, Si is a ferrite forming element that facilitates the formation of a ferrite phase. The δ ferrite phase causes edge cracks or double cracks to occur during hot rolling. From the viewpoint of reducing the formation of δ ferrite phase, the Si content is preferably 2.0 mass% or less, preferably less than 1.5 mass%, and preferably 1.0 mass% or less.

(Mn)

Mn (manganese) is an austenite forming element and is also an effective element for maintaining the austenite phase. Additionally, Mn is an element that has the effect of promoting precipitation of the Cu rich phase. Austenitic stainless steel contains 0.3 mass% or more and 2.5 mass% or less of Mn, and preferably contains 0.5 mass% or more and 2.0 mass% or less of Mn. When the Mn content is 0.3% by mass or more, it is easy to secure the precipitation amount of the Cu rich phase, and it is more preferable when the Mn content is 0.5% by mass or more. Additionally, excessive addition of Mn causes a decrease in the hot workability of austenitic stainless steel. Therefore, the Mn content is preferably 2.5% by mass or less, and preferably 2.0% by mass or less.

(P)

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

(S)

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

(Ni)

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

On the other hand, Ni is an expensive element, and if added excessively, it stabilizes the austenite phase and reduces the amount of formation of the deformed martensite phase. Therefore, the Ni content is preferably less than 6.0 mass%, preferably 5.5 mass% or less, and more preferably less than 5.0 mass%.

(Cr)

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

On the other hand, since Cr is also a ferrite forming element like Si, if Cr is added excessively, δ ferrite phase is excessively produced. Therefore, the Cr content is preferably 18.5% by mass or less, and preferably 18.0% by mass or less.

(Cu)

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

Austenitic stainless steel contains 1.5 mass% or more and 4.0 mass% or less of Cu, preferably contains 2.0 mass% or more and 3.5 mass% or less of Cu, and contains more than 2.0 mass% and 3.5 mass% or less of Cu. It is more preferable. When the Cu content is 1.5% by mass or more, not only the formation and maintenance of the austenite phase becomes good, but also the precipitation of the Cu-rich phase becomes good. It is more preferable that the Cu content is 2.0 mass% or more, and it is further preferable that it is more than 2.0 mass%.

On the other hand, if Cu is added excessively, a CuMn phase is created in the center of the slab during the solidification process, and the hot workability of the slab deteriorates. Therefore, the Cu content is preferably 4.0 mass% or less and 3.5 mass% or less.

(N)

N (nitrogen) is an austenite forming element and is also an element that has a solid solution strengthening effect and a corrosion resistance improving effect. In austenitic stainless steel, the N content is set to 0.005% by mass or more in order to generate an austenite phase. With this N content, the austenite phase is effectively formed.

Additionally, if N is added excessively, the rolling load of austenitic stainless steel increases. Therefore, the N content is preferably 0.25 mass% or less, and preferably 0.20 mass% or less.

(Other elements)

Austenitic stainless steel contains, in mass%, in addition to the above-mentioned elements, 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.

(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 expensive elements, so excessive addition is undesirable. Therefore, the austenitic stainless steel preferably contains one or more types 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 cracks and double cracks in hot rolling. Austenitic stainless steel preferably contains 0.0001 mass% or more and 0.01 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 the occurrence of edge cracks and double cracks in hot rolling. However, if B is added excessively to austenitic stainless steel containing Cr, corrosion resistance is reduced due to precipitation of Cr ₂ B. Therefore, it is preferable that the B content is 0.01% by mass or less.

(Co)

Co (cobalt) is an effective element to ensure corrosion resistance of austenitic stainless steel. In addition, it also contributes to maintaining the fineness by reducing the coarsening of the Cu rich phase. In order to obtain this effect, it is preferable to contain 0.10% by mass or more of Co. However, since Co is an expensive element, from the viewpoint of cost reduction, the Co content is preferably set to 0.8% by mass or less.

(Sn)

Sn (tin) is an effective element to ensure corrosion resistance of austenitic stainless steel. However, since excessive addition of Sn causes a decrease in the hot workability of austenitic stainless steel, it is preferable that the Sn content is 0.1% by 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 at least 0.3% by mass of Al, 0.03% by mass or less of Ca, 0.03% by mass or less of Mg, and 0.5% by mass of Ti as a deoxidizing agent.

(Nb)

Nb (niobium) is an element effective in reducing sensitization of austenitic stainless steel. Additionally, it is effective in refining and homogenizing the tissue. 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 not only improve hot workability but are also effective in oxidation resistance. Austenitic stainless steel contains one or more types 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 REM. It is desirable.

[Value of Md ₃₀ ]

The austenitic stainless steel has an Md ₃₀ value of 10.0 to 80.0, preferably 20.0 to 70.0, according to the following formula (1).

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)

The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition.

In austenitic stainless steel, the value of Md ₃₀ is 50% of the structure of austenitic stainless steel when a tensile strain of 30% is applied to an austenitic single-phase austenitic stainless steel. % represents the temperature (°C) at which it transforms into martensite phase. Therefore, the value of Md ₃₀ can be used as an index of the stability of the austenite phase. Additionally, the value of Md ₃₀ can also be used as an indicator that influences the ease of occurrence of the TRIP phenomenon in austenitic stainless steel.

The Md ₃₀ value of the austenitic stainless steel according to one embodiment of the present invention is preferably 10.0 or more and 80.0 or less. As for the value of Md ₃₀ , the larger the value, the easier it is for transformation from the austenite phase to the deformed martensite phase, and not only can high strength be obtained by applying hard cold rolling deformation, but also excellent ductility can be secured. . Additionally, even when forming processing is performed on austenitic stainless steel, parts subjected to processing strain, such as bending parts, are likely to obtain higher strength due to the TRIP phenomenon.

Additionally, in the manufacturing process of austenitic stainless steel, the presence of a deformed martensite phase in the rolled material before finish annealing acts effectively to refine grains by finish annealing. This effect appears significantly when the value of Md ₃₀ is 10.0 or more. Additionally, when the value of Md ₃₀ exceeds 80.0, the TRIP phenomenon tends to occur excessively, making it difficult for the properties of austenitic stainless steel to be stabilized.

Therefore, if the value of Md ₃₀ , which is an indicator of the stability of the austenite phase, is 10.0 or more and 80.0 or less, austenitic stainless steel with high strength and good ductility can be stably manufactured.

On the other hand, in the conventionally known component regression equation for Md ₃₀ , the same value is generally used for the coefficients of Ni and Cu. On the other hand, in one embodiment of the present invention, the coefficient of Cu is set to be smaller than the coefficient of Ni in the component regression equation for Md ₃₀ . The component regression equation for Md ₃₀ according to conventional knowledge is often based on performance in austenitic stainless steels that are not Ni-saving types. In contrast, in Ni-saving components such as the present invention, it was found that the influence of Cu on the stabilization of the austenite phase was clearly smaller than conventional knowledge. This is new knowledge obtained as a result of intensive examination by the present inventor, and the coefficient of Cu in the component regression equation for Md ₃₀ is set based on this knowledge. Accordingly, adjustment of the Cu content becomes easier, and the degree of freedom in manufacturing austenitic stainless steel increases.

[Manufacturing method]

The method for manufacturing austenitic stainless steel according to an embodiment of the present invention is, in mass%, C: more than 0.03% and less than 0.15%, Si: more than 0.1% and less than 2.0%, Mn: more than 0.3% and less than 2.5%, P: Contains 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: 0.005% or more and 0.25% or less. This is a method of manufacturing austenitic stainless steel, which is composed of Fe and inevitable impurities, and has an Md ₃₀ value of 10.0 or more and 80.0 or less according to the above formula (1). Additionally, the manufacturing method of austenitic stainless steel includes a final annealing process.

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

In the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, for example, a slab is manufactured by continuously casting molten steel whose composition has been adjusted. Then, the slab manufactured 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 rate at which the Cu-rich phase precipitates from the austenite phase with little strain after hot rolling is slow. Therefore, the finishing temperature and coiling temperature of the hot rolled steel strip after hot rolling may be set to the same conditions as the general austenitic stainless steel manufacturing method. From the viewpoint of reducing the precipitation of Cu rich phase until final annealing as much as possible, 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.

Pickling may be performed on the hot-rolled steel strip that has been hot-rolled. On the other hand, annealing may be performed before pickling of the hot rolled steel strip, or pickling may be performed without annealing. When annealing is performed before pickling hot-rolled steel strips, the annealing temperature is preferably in the range of 900℃ to 1150℃. In order to completely bring Cu into solid solution, the annealing temperature is preferably in the range of 980℃ to 1150℃. This is more preferred, but is not limited to the above-mentioned range. Then, cold rolling is performed on the hot-rolled steel strip after pickling until it reaches a predetermined thickness to obtain a cold-rolled steel strip.

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

On the other hand, for austenitic stainless steel, the value of Md ₃₀ is adjusted to 10.0 or more and 80.0 or less. In the austenitic stainless steel having such a value of Md ₃₀ , a Cu-rich phase is precipitated in the amount specified in one embodiment of the present invention, regardless of the amount of the deformed martensite phase in the cold rolled steel strip. However, control to increase the rolling rate of the cold rolling process, such as lowering the temperature of the cold rolling process as needed, is more effective for precipitation of the Cu rich phase.

In cold rolled steel strips, from the viewpoint of making the deformed martensite phase 20% by volume or more, for example, the rolling reduction in the cold rolling process is preferably 40% or more, more preferably 50% or more, and 60% or more. It is more desirable. Additionally, the temperature in the cold rolling process is preferably 90°C or lower, and more preferably 60°C or lower.

(Finish annealing process)

Finish annealing is performed on cold rolled steel strips. The final annealing process is performed under conditions under which precipitation of the Cu rich phase progresses. Cu rich phase is effective in increasing the strength of austenitic stainless steel. The strength of hot-rolled steel strips and cold-rolled steel strips before precipitation of the Cu-rich phase is low, and the rolling load in the cold rolling process can be reduced. And, because the Cu-rich phase is precipitated through the final annealing process, high strength can be obtained in the austenitic stainless steel after the final annealing.

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

In this way, according to the method for manufacturing austenitic stainless steel according to one embodiment of the present invention, it is possible to achieve both reduction of processing load during manufacturing and high strength of the final product at a high level. In addition, as in the past, since precipitation of the Cu-rich phase does not require an additional process such as an aging treatment process, the productivity of austenitic stainless steel is also good.

The finish annealing temperature in the finish annealing process is preferably set to 750°C or more and 980°C or less, and 800°C or more and 925°C or less so that the Cu rich phase is effectively precipitated in the austenitic stainless steel. If the final annealing temperature is lower than 750°C, recrystallization of the structure becomes insufficient. Additionally, 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 precipitated from the deformed martensite phase tends to dissolve into the austenite phase, especially when maintained at a temperature of 850°C or higher for a long time during final annealing. Therefore, when the maximum temperature achieved in the final annealing process is 850°C or higher, it is desirable to shorten the heating time above 850°C. Specifically, when the maximum temperature achieved in the final annealing process is 850°C or higher, the heating time at 850°C or higher is preferably 30 seconds or less and 15 seconds or less. 'Heating time above 850°C' refers to the total time of several times when the time to heat above 850°C is divided into several times in the final annealing process.

Since the C content of austenitic stainless steel is less than 0.15%, Cr carbide does not necessarily precipitate during cooling. However, if the grain size is small and there are many grain boundaries, and the cooling rate after final annealing is slow, there is concern that corrosion resistance will decrease due to precipitation of Cr carbide. Therefore, regarding the cooling rate after final annealing, the average cooling rate from 700°C to 500°C is preferably 20°C/sec or more, more preferably 50°C/sec or more, and 75°C/sec or more. It is more desirable to do so.

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

[Strength Evaluation]

The austenitic stainless steel according to one embodiment of the present invention reduces the rolling load by lowering the strength in the manufacturing process and also realizes high strength after manufacturing. Such characteristics in austenitic stainless steel can be expressed, for example, by the relationship between 0.2% proof strength (YS 18%, MPa) and reference strength (HV 60%).

0.2% proof strength (YS 18%) is an indicator of the strength of austenitic stainless steel. The 0.2% proof strength (YS 18%) represents the 0.2% proof strength when temper rolling with an elongation of 18% is further performed after final annealing of austenitic stainless steel. The 0.2% proof strength can be evaluated using a method based on JIS Z2241.

Reference strength (HV 60%) is an index that hypothetically represents the strength of austenitic stainless steel before precipitation of the Cu-rich phase in the final annealing process. The reference strength (HV 60%) is the same as that of the austenitic stainless steel, but the manufacturing method is partially changed from the manufacturing method according to one embodiment of the present invention, annealing is performed at 1050°C after hot rolling, and the manufacturing method is 60 Indicates the Vickers hardness when cold rolling is performed at a rolling rate of %. That is, the reference strength (HV 60%) does not represent the strength of the austenitic stainless steel according to one embodiment of the present invention, but may be, for example, the strength of a steel strip manufactured for evaluation. Vickers hardness can be measured based on the Vickers hardness test method based on JIS Z2244.

The present inventor believes that for austenitic stainless steel that achieves both reduction of processing load during manufacturing and high strength of the final product, the relationship between 0.2% proof strength (YS 18%) and reference strength (HV 60%) is expressed in the following equation (2) was found to satisfy.

YS 18%≥1.25 HV 60%+525 … (2)

According to the manufacturing method according to one embodiment of the present invention, it is possible to manufacture austenitic stainless steel that satisfies the above equation (2) and achieves both reduction of processing load during manufacturing and high strength of the final product.

[Preferred use]

Austenitic stainless steel has very high strength and corrosion resistance. Therefore, austenitic stainless steel is required to have high strength and corrosion resistance in, for example, cylinder head gaskets, flat spiral springs, springs for electronic device parts, vehicle-mounted battery frame materials, structural materials, and metal packing. It is desirable as a material for spring products.

[organize]

The austenitic stainless steel according to Form 1 of the present invention has, in mass%, C: 0.03% to 0.15%, 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% 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: 0.005% or more and 0.25% or less, the balance being Fe and It consists of unavoidable impurities, contains an austenite phase of 20% by volume or ^more , and a Cu-rich phase with a number density of ^1.0 It consists of a formed phase, and the value of Md ₃₀ according to the following formula (1) is 10.0 or more and 80.0 or less.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)

The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition.

The austenitic stainless steel according to form 2 of the present invention is, in terms of form 1, 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: It may further contain one or more selected from the group consisting of 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 Form 3 of the present invention, in Form 1 or Form 2, may have an average grain size of 10.0 μm or less.

The method for producing austenitic stainless steel according to Form 4 of the present invention is, in mass%, C: more than 0.03% and less than 0.15%, Si: more than 0.1% and less than 2.0%, Mn: more than 0.3% 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 more and 4.0% or less, and N: 0.005% or more and 0.25% or less, the balance A method for manufacturing austenitic stainless steel consisting of Fe and inevitable impurities and having an Md ₃₀ value of 10.0 or more and 80.0 or less according to the following formula (1), wherein finish annealing is performed at a temperature of 750°C or more and 980°C or less. process, and when the maximum temperature achieved in the final annealing process is 850°C or higher, the heating time at 850°C or higher is set to 30 seconds or less.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)

The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition.

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

《Example》

Hereinafter, the results of evaluating austenitic stainless steel according to the invention examples and comparative examples of the present invention will be described.

[Conditions of evaluation]

＜Ingredients Composition＞

The composition (% by mass) and the value of Md ₃₀ of the austenitic stainless steel (invention steel A1 to A15) according to an embodiment of the present invention and the austenitic stainless steel (comparative steel B1 to B4) according to the comparative example It is shown in Table 1 below. The value of Md ₃₀ was calculated using the above formula (1). Meanwhile, the underlined values in Table 1 below indicate that they are outside the specified range of the present invention. The same applies to Table 2 below.

＜Manufacturing method＞

Austenitic stainless steel according to each example and comparative example of the present invention was manufactured by the method shown below. Austenitic stainless steel having the component composition shown in Table 1 was melted, and a manufacturing method according to an embodiment of the present invention (invention examples C1 to C8) or a manufacturing method according to a comparative example (comparative examples D1 and D2) was prepared. ), from hot rolling to final annealing, to obtain a cold-rolled annealed material. The conditions for each manufacturing 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 reaching 850°C or higher was adjusted as shown in Table 2. On the other hand, in Invention Example C3, heating was adjusted so that the temperature began to decrease when the final annealing temperature reached 850°C. However, for convenience, in Table 2, the time to reach 850°C or higher was written as '1 second'.

＜Evaluation method＞

Various indicators of austenitic stainless steel according to each Example and Comparative Example of the present invention were evaluated as shown below.

(Number density of Cu rich phase)

TEM samples were produced from the cold-rolled annealed materials produced under each condition by electrolytic polishing. In the TEM sample, three views were observed in a range of 400 nm x 400 nm on a surface parallel to the rolling direction of the cold rolled annealed material. Cu-rich phases were determined from the contrast of the TEM image, and the number of Cu-rich phases was measured. The thickness of the TEM sample was assumed to be 150 nm, and the number density per unit volume was obtained. When the Cu-rich image became coarse, it was observed as a clear shape instead of contrast. Cu-rich phases with a major diameter exceeding 30 nm were excluded from the measurement.

(Number density of Cr carbide)

In the field of view of the TEM sample used to measure the number density of the Cu rich phase described above, precipitates with {precipitate Cr concentration in TEM-EDS analysis}/{base material Cr concentration} exceeding 1.2 were regarded as Cr carbides. The base material Cr concentration is the average value of the Cr concentration of the entire cold-rolled annealed material and represents the Cr content (% by mass) contained in the cold-rolled annealed material. If the number density of the Cr carbide was 50 pieces·㎛ ^-3 or more, it was evaluated as '△', and if it was less than 50 pieces·㎛ ^-3 , it was evaluated as '○'.

(grain size)

The average grain size was evaluated using the EBSD method. For the cold rolled annealed material produced under each condition, mechanical polishing and then electrolytic polishing were performed on the cross section parallel to the rolling direction and perpendicular to the rolling surface. After that, EBSD analysis was performed at a step interval of 0.2 μm in the 40 μm × 40 μm area of the cross section in a field of view with a magnification of 2000 times. Regarding the orientation difference in the orientation relationship that satisfies the Σ3 corresponding grain boundary, excluding annealed twins with an orientation difference of 1° or less, boundaries with an orientation difference of 2° or more are considered grain boundaries, and the area of each individual grain is S (㎛ ² ), The diameter of a circle having the same area as the crystal grain was set to D (μm), and the grain size was calculated using the following formula (3). This was performed for five views, and the average of the grain sizes obtained in the five views was calculated as the average grain size.

Grain size={Σ(D×S)}/40×40… (3)

(Amount of martensite phase)

The amount (volume %) of the martensite phase was the same when the sheet thickness was 1.5 mm or more, and the material after cold rolling or temper rolling was overlapped so that the total was 1.5 mm or more when the sheet thickness was less than 1.5 mm. These materials were measured using a ferrite scope (Fischer FMP30, electromagnetic induction method), and the measured value was divided by 0.7475 to determine the amount of martensite phase. The amount (volume%) of the austenite phase was considered to be the value obtained by subtracting the amount of martensite phase from the total matrix of austenitic stainless steel as 100 volume%. The amounts of the Cu-rich phase and the unavoidable formed phase in austenitic stainless steel may be excluded and calculated because the ratio is small and accurate measurement is difficult.

(tensile properties)

As an indicator of the tensile properties, the 0.2% yield strength (YS 18%) when temper rolling was performed with an elongation of 18% was evaluated. The 0.2% yield strength (YS 18%) was measured by producing a JIS No. 13 B test piece and performing a tensile test based on JIS Z2241. The 0.2% proof strength (YS 18%) was measured with a crosshead speed of 3 mm/min.

(robbery)

Refer to the Vickers hardness of the 60% rolled material obtained by partially changing the manufacturing conditions in each of the embodiments and comparative examples of the present invention, annealing the hot rolled steel strip at 1050°C, and performing cold rolling with a rolling reduction of 60%. Measured as strength (HV 60%). Vickers hardness was measured by performing a Vickers hardness test (JIS Z2244) on the surface of 60% of the rolled material using a Vickers hardness tester. The load during measurement in the Vickers hardness test was 10 kg.

[result]

For invention steel A2, the Cu rich phase precipitation amount and crystal grain size of the cold rolled annealed material obtained under each condition shown in Table 2 are shown in Table 3 below. In addition, Table 3 below shows the amount of martensite phase after temper rolling with an elongation of 18% at 0.2% proof strength (YS 18%), after cold rolling (before finish annealing) and after finish annealing, under each condition.

Meanwhile, in Table 3 below, the underline indicates that the precipitation amount of the Cu rich phase is outside the specified range of the present invention.

With respect to invention steel A2, the cold-rolled annealed material produced under the conditions of invention examples C1 to C8 had a Cu-rich phase precipitation amount within the specified range of the present invention and exhibited a fine average grain size of 10.0 μm or less. 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.

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

In addition, since the reference strength (HV 60%) of invention steel A2 is 487, the 0.2% proof strength (YS 18%) is preferably 1134 MPa or more according to the above equation (2). All of the cold-rolled annealed materials of invention steel A2 manufactured under the conditions of invention examples C1 to C8 had a 0.2% yield strength (YS 18%) of 1134 MPa or more. On the other hand, the 0.2% proof strength (YS 18%) of the cold rolled annealed materials manufactured under the conditions of Comparative Examples D1 and D2 was lower than 1134 MPa. In this way, since the Cu-rich phase did not precipitate under the conditions of Comparative Examples D1 and D2, it was shown that it was difficult to obtain austenitic stainless steel with a good balance between workability before finish annealing and high strength after finish annealing.

Next, for the cold-rolled annealed material manufactured from invention steels A1 to A15 or comparative steels B1 to B4 under the manufacturing conditions shown in invention example C2, the Cu rich phase precipitation amount and crystal grain size after final annealing are shown in Table 4 below. In addition, the 0.2% proof strength (YS 18%) and reference strength (HV 60%) under each of these conditions are also shown in Table 4 below.

Meanwhile, 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 or that the 0.2% yield strength (YS 18%) is a value that does not satisfy the above equation (2).

The cold-rolled annealed materials of invention steels A1 to A15 had a Cu-rich phase precipitation amount within the specified range of the present invention and exhibited a fine average grain size of 10 μm or less. In addition, for 0.2% yield strength (YS 18%), all showed good values satisfying the above equation (2).

On the other hand, the cold-rolled annealed materials of comparative steels B1 to B4 are austenitic stainless steels whose 0.2% proof strength (YS 18%) does not satisfy the above equation (2) and have a good balance between workability before finish annealing and high strength after finish annealing. couldn't get it.

Figure 2 shows a diagram plotting the relationship between 0.2% proof strength (YS 18%) and reference strength (HV 60%) under 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 triangle. In the graph shown in FIG. 2, the closer it is plotted to the upper left, the better the balance between workability before final annealing and high strength after final annealing.

Meanwhile, all of the above-mentioned results are the results of a cold-rolled annealed material obtained under the condition that the cooling rate after final annealing is 25°C/sec. Here, cold-rolled annealed materials were manufactured using invention steels A1 and A2 under the conditions of invention example C2 shown in Table 2, changing the cooling rate from 700°C to 500°C after final annealing within the range of 5 to 100°C/sec. . The precipitation amount of Cr carbide in the obtained cold rolled annealed material is shown in Table 5 below.

At a cooling rate of 20°C/sec or higher, the precipitation amount of Cr carbide in the cold-rolled annealed material was in a preferable range of less than 50 pieces·μm ^-3 . When the cooling rate is 10°C/sec or less, the amount of Cr carbide precipitated becomes more than 50 ㎛ ^-3 , which is not a problem for the corrosion resistance of the cold-rolled annealed material, but an amount of Cr that may have a slight effect. Precipitation of carbides could be seen.

As shown in Table 4 and FIG. 2, the cold-rolled annealed material manufactured by using the austenitic stainless steel having the composition according to one embodiment of the present invention and the manufacturing method according to one embodiment of the present invention has the It was shown to achieve both reduction of processing load and high strength of the product.

Claims (4)

In mass%, C: more than 0.03% and less than 0.15%, Si: more than 0.1% and less than 2.0%, Mn: more than 0.3% and less than 2.5%, P: less than 0.04%, S: less than 0.015%, Ni: more than 3.0% and less than 6.0%. less, Cr: 16.0% to 18.5%, Cu: 1.5% to 4.0%, and N: 0.005% to 0.25%, the balance being Fe and inevitable impurities,
Contains an austenite phase of 20% by volume or ^more and a Cu-rich phase with a number density of ^1.0
Austenitic stainless steel having an Md ₃₀ value of 10.0 or more and 80.0 or less according to the following formula (1).
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)
The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition. According to paragraph 1,
In 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 elements) Metal): Austenitic stainless steel further containing one or more types selected from 0.2% or less. According to claim 1 or 2,
Austenitic stainless steel with an average grain size of 10.0 ㎛ or less. In mass%, C: more than 0.03% and less than 0.15%, Si: more than 0.1% and less than 2.0%, Mn: more than 0.3% and less than 2.5%, P: less than 0.04%, S: less than 0.015%, Ni: more than 3.0% and less than 6.0%. less, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.005% or more and 0.25% or less, the balance being made of Fe and inevitable impurities, and Md according to the formula (1) below. A method for manufacturing austenitic stainless steel with a value of ₃₀ of 10.0 or more and 80.0 or less,
It includes a final annealing process of performing final annealing at a temperature of 750°C or higher and 980°C or lower,
A method for manufacturing austenitic stainless steel, wherein when the maximum temperature achieved in the finish annealing process is 850°C or higher, the heating time at 850°C or higher is within 30 seconds.
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo... (One)
The content (mass %) of each element contained in the austenitic stainless steel is substituted in the place of the element symbol in the formula (1), and 0 is substituted for elements without addition.