JP6798907B2 - Low magnetic austenitic stainless steel and cold-rolled steel sheet - Google Patents

Description

The present invention relates to austenitic stainless steel suitable for applications requiring low magnetism, high strength, and springiness, and a cold-rolled steel sheet using the austenitic stainless steel.

Stainless steel materials used for housings and parts of electronic devices are often required to have properties that make them difficult to magnetize. Further, in recent years, due to the demand for miniaturization and weight reduction of electronic device members, there is an increasing need for thinning of the stainless steel material used for the electronic device member. In order to fully meet the needs, it is desired to have excellent strength, ductility, and springiness in addition to the property of being hard to be magnetized. Also, depending on the application, it is expected to be used at high temperatures. Therefore, it is also desired that the material has a property that softening does not easily occur when the temperature rises (aging softening resistance).

The required level for the property of being hard to be magnetized can be roughly divided into non-magnetic (μ ≦ 1.01) and low magnetic (μ ≦ 1.10) depending on the application. The present invention is directed to steel, which is applicable to low magnetic applications.

For non-magnetic applications, austenitic steel grades (such as SUS305 series) whose components are generally designed so that the Ni content is increased and a work-induced martensite phase is not formed are applied. Existing steel grades that can be applied to non-magnetic applications are also applicable to low magnetic applications in terms of magnetic properties. However, in general, non-magnetic austenitic steel grades have a high raw material cost, and it is difficult to obtain excellent spring properties.

On the other hand, as austenitic steel grades that can be applied to low magnetic applications, steels in which the amount of process-induced martensite phase formation is suppressed by increasing the N content or adding Cu in the SUS304 series, and a large amount of Mn are used. The added austenitic steel grade is known. However, with existing low magnetic austenitic steel grades, it is difficult to obtain a steel sheet that simultaneously satisfies strength, ductility, springiness, and aging softening resistance.

Patent Documents 1 to 3 describe austenitic stainless steels that are considered to be applicable to non-magnetic or low-magnetic applications because the strength and spring limit values are less likely to decrease in a high-temperature use environment. However, according to the research by the inventors, in the component compositions disclosed in these documents, the "0.01% proof stress" that can severely evaluate the elastic limit stress is at the same level as that of general non-magnetic stainless steel grades. .. In order to fully meet the needs for thinning, further improvement of "spring property" evaluated by 0.01% proof stress is desired.

JP-A-2007-197806 JP-A-2007-197807 Japanese Unexamined Patent Publication No. 2008-38191

The present invention discloses an austenitic stainless steel capable of obtaining a cold-rolled material having "low magnetism", excellent "strength-ductility balance", "aging softening resistance", and excellent "springiness". To do. Further, a cold-rolled steel sheet using the steel is provided.

The above purpose is mass%, C: more than 0.010% and less than 0.160%, Si: 0.1-10.20%, Mn: 3.00-6.00%, Ni: 3.00-6. .00%, Cr: 15.00 to 19.00%, N: 0.090 to 0.210%, Cu: 0.50 to 3.50%, Mo: 0.1 to 1.50%, V: 0-0.35%, Nb: 0-0.35%, Ti: 0-0.35%, the balance is Fe and unavoidable impurities, and the MD value defined by the following equation (1) is -70. It is achieved by austenitic stainless steel having a δcal value of 0 or less and defined by the following equation (2) of 3.20 or less.
MD = 551-462 (C + N) -9.2Si-19.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo ... (1)
δcal = -15-44.91C-0.88Mn-2.31Ni + 2.2Cr-1.08Cu-28.8N ... (2)
Here, the content value of the element represented by mass% is substituted in place of the element symbol of the equations (1) and (2).

Further, the cold-rolled steel sheet made of the above steel was obtained by performing a tensile test in the rolling direction using a JIS13B test piece with a specific magnetic permeability μ of 1.10 or less at a tensile speed of 5 mm / min. Provided is a low magnetic austenitic stainless steel cold-rolled steel sheet having a 0.01% proof stress of 500 N / mm ² or more obtained by an offset method from a nominal stress-strain curve. The hardness of the plate surface (rolled surface) of this steel plate is, for example, 345 HV or more. The plate thickness is, for example, 0.02 to 1.5 mm.

According to the present invention, in a low magnetic austenitic stainless steel sheet, excellent strength-ductility balance, aging softening resistance, and excellent springiness can be stably realized at the same time. Since this steel sheet exhibits high strength and springiness in a thinned low-magnetic steel sheet, it is useful for reducing the size and weight of housings and parts of electronic devices that require low magnetism.

The graph which illustrated the relationship between the process-induced martensite amount and the aging softening temperature for the tempered rolled material with a plate thickness of 0.2 mm.

[Chemical composition]
Hereinafter, "%" regarding the chemical composition of steel means "mass%" unless otherwise specified.
C is an extremely important element related to low magnetism, strength, ductility, and springiness. According to the study by the inventors, in order to stably realize a remarkable ductility improving effect by the final cold rolling (hereinafter referred to as "tempering rolling"), C content exceeding 0.1100% is contained. It is very effective to secure the amount. In addition, in steels that meet the requirements for each component described below, by setting the C content to a range higher than 0.1100%, excellent "strength-ductility balance" and excellent "springiness" are realized after temper rolling. can do. However, if the C content is too high, the steel will be excessively hardened and the workability will be impaired. As a result of detailed examination, the present invention limits the C content to the range of 0.160% or less.

Si is effective for increasing the strength. It is also used as an antacid. In order to obtain these effects, the Si content needs to be 0.10% or more, and more preferably 0.40% or more. On the other hand, in order to maintain low magnetism after temper rolling, the Si content is limited to the range of 1.20% or less. It may be managed to 0.80% or less.

Mn is a stabilizing element for the austenite phase, and it is extremely effective to set the Mn content to 3.00% or more in order to stably maintain low magnetism in those whose strength has been increased by temper rolling. Yes, it is more effective to set it to 4.00% or more. However, as the Mn content increases, hot workability and low temperature toughness decrease. The Mn content should be 6.00% or less. When importance is attached to hot workability and low temperature toughness, it is effective to adjust the components within the Mn content range of 5.50% or less.

Ni is a basic component of austenitic stainless steel. In order to maintain low magnetism after temper rolling, it is necessary to secure a Ni content of 3.00% or more, and it is more effective to set it to 4.00% or more. However, when a large amount of Ni is added, the effect of increasing the strength due to temper rolling becomes small. The Ni content is limited to 6.00% or less and may be controlled within the Ni content range of less than 5.00%.

Cr is a basic component responsible for the corrosion resistance of stainless steel. However, if a large amount of Cr is contained, a δ ferrite phase is likely to be formed, and low magnetism cannot be maintained. In the present invention, the Cr content is defined in the range of 15.00% or more and 19.00% or less in order to achieve both excellent corrosion resistance required for electronic device members and low magnetism.

N is a stabilizing element of the austenite phase, and is an important element for increasing the strength while maintaining low magnetism. In order to fully exert its action, the present invention secures an N content of 0.090% or more. The N content may be controlled to 0.120% or more or 0.150%. However, a large amount of N content causes a decrease in ductility after temper rolling. The N content is limited to the range of 0.210% or less.

Cu is a stabilizing element of the austenite phase, and secures a content of 0.50% or more in order to maintain low magnetism after temper rolling. However, since a large amount of Cu content causes a decrease in hot workability, the Cu content is set in the range of 3.50% or less. It may be managed in the range of 2.50% or less.

Mo is effective in improving corrosion resistance and work hardening ability, and secures a content of 0.10% or more. When a large amount of Mo is added, a δ ferrite phase is easily formed, and low magnetism cannot be maintained. The Mo content shall be in the range of 1.50% or less.

Since V, Nb, and Ti have an action of enhancing work hardening ability, one or more of them can be added as needed. At that time, it is more effective to secure a content of V of 0.10 or more, Nb of 0.10% or more, and Ti of 0.10% or more. However, if a large amount of these elements is contained, a δ ferrite phase is likely to be formed, and it becomes difficult to maintain low magnetism. When one or more of these elements are added, the content range of V is 0.35% or less, Nb is 0.35% or less, and Ti is 0.35% or less.

In addition, it is desirable that P, which is an unavoidable impurity, is 0.060% or less, and S is 0.010% or less. According to the usual melting method of austenitic stainless steel in a mass production site, P and S can be sufficiently suppressed to the above desired range. Further, ordinary scrap raw materials, Al, Ca, rare earth elements, B and the like that can be mixed from the electric furnace and the ladle used in the previous melting charge do not hinder the achievement of the subject of the present invention.

According to the studies by the inventors, it is extremely effective to sufficiently secure the total content of C and N in order to improve the "0.01% proof stress" which is an evaluation index of the elastic limit stress. In the present invention, as described above, the C content of more than 0.010% and the N content of 0.090% or more are indispensable, so that the total C + N content is more than 0.190%, which is temper rolling. It works effectively to improve the 0.01% proof stress (spring property) of the material. In particular, when aiming for a very excellent spring property such that the 0.01% proof stress is 600 N / mm ² or more, it is extremely effective to control the total content of C and N to 0.250% or more.

The MD value defined by the following equation (1) is an index showing the ease of formation of the work-induced martensite phase. The lower the MD value, the more difficult it is to form a work-induced martensite phase, which is advantageous in achieving stable low magnetism after temper rolling. It was also found that reducing the amount of process-induced martensite is extremely effective in improving the aging softening resistance. In the present invention, it is necessary to adopt a steel composition in which the MD value is adjusted to −70.0 or less. In particular, in a steel composition in which the MD value is adjusted to -85.0 or less, the amount of work-induced martensite after temper rolling can be controlled to be extremely low, for example, 2.0% by volume or less, and has low magnetism and aging softening resistance. A remarkable improvement effect of sex can be obtained. Therefore, when the aging softening resistance is emphasized, it is more preferable to adjust the MD value to −85.0 or less.
MD = 551-462 (C + N) -9.2Si-19.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo ... (1)
Here, the content value of the element represented by mass% is substituted in place of the element symbol in the formula (1).

The δcal value defined by the following equation (2) is an index showing the amount of δ ferrite (volume%) in the central part of the thickness of the slab after heating the slab produced by continuous casting at 1230 ° C. for 2 hours. is there. If the amount of the δ ferrite phase present in the slab before hot rolling is large, it may be difficult to completely eliminate it in the subsequent steps, which may hinder the realization of low magnetism. Further, the δ ferrite phase present in the slab before hot rolling greatly affects the hot workability. In the present invention, Mn and Cu are contained as described above, but when the content of these elements increases, the Cu—Mn phase tends to precipitate when the slab is heated before hot rolling. Since the Cu-Mn phase has a low melting point and is fragile in the hot rolling temperature range, it causes "double cracking" of the plate during hot rolling. The amount of precipitation of the Cu—Mn phase correlates with the amount of the abundance of the δ ferrite phase, and as the amount of δ ferrite increases, the amount of precipitation of the Cu—Mn phase increases. As a result of various studies, in the present invention, it is necessary to adjust the δcal value to 3.20 or less, and it is more preferable to adopt the steel composition adjusted to 0.50 or less.
δcal = -15-44.91C-0.88Mn-2.31Ni + 2.2Cr-1.08Cu-28.8N ... (2)
Here, the content value of the element represented by mass% is substituted in place of the element symbol in the formula (2).

When a steel having the above chemical composition is used, for example, when a cold-rolled steel sheet having a cold rolling ratio of 40% is produced, the hardness of the plate surface (rolled surface) of the cold-rolled steel sheet is 345 HV or more, JIS Z2241: It is possible to obtain the characteristics that the breaking elongation in the rolling direction by the tensile test of 2011 is 10.0% or more and the specific magnetic permeability is 1.10 or less.

〔Production method〕
The low magnetic austenitic stainless steel cold-rolled steel sheet according to the present invention can be manufactured by using a general mass production facility for stainless steel sheets. Specifically, it can be produced by subjecting an intermediate product plate material produced through continuous casting, hot rolling, and cold rolling to annealing and temper rolling. In temper rolling, it is desirable to set the rolling ratio in the range of 20 to 50%. With the temper rolling ratio in this range, steel plates having various thicknesses can be obtained, for example, from a foil material having a final plate thickness of 0.02 mm to a plate material having a final plate thickness of 1.5 mm.

Austenite-based stainless steel with the chemical composition labeled "Invention Steel" in Table 1 is melted, and a cold-rolled steel sheet, which is an intermediate product, is manufactured by a conventional method in a process including hot rolling and cold rolling at 1060 ° C. After quenching, pickling and temper rolling were carried out to obtain a cold-rolled steel sheet having a thickness of 0.2 mm. The tempered rolling ratio is shown in Table 2. The rolling ratio is determined by the following equation (3).
Rolling rate (%) = (h ₀ −h ₁ ) / h ₀ × 100… (3)
Here, h ₀ is the plate thickness (mm) before rolling, and h ₁ is the plate thickness (mm) after rolling.

In addition, a 0.2 mm thick cold-rolled steel sheet (commercially available material equivalent to 3/4 H finish) having a chemical composition labeled as "comparative steel" in Table 1 was prepared.
These cold-rolled steel sheets with a thickness of 0.2 mm are called test materials. The following surveys were conducted on each test material.

(Hardness measurement)
Hardness measurement (HV20) according to JIS Z2244: 2009 was performed on the polished surface of the plate surface (rolled surface).

(Tensile test)
A JIS 13B test piece was taken from the test material and subjected to a tensile test according to JIS Z2241: 2011 to determine 0.2% proof stress, tensile strength, and elongation at break (EL) in the rolling direction.
Further, 0.01% proof stress was obtained by the offset method from the nominal stress-strain curve obtained by performing a tensile test in the rolling direction using the JIS13B test piece at a tensile speed of 5 mm / min.

(Measurement of processing-induced martensite amount)
Five 25 mm × 25 mm test pieces are taken from the test material, and the amount of martensite phase, which is a magnetic phase, is measured using a ferrite scope (manufactured by Fisher) in a state where these five test pieces are laminated. By doing so, the amount of process-induced martensite was determined.

(Magnetic measurement)
For a sample with a diameter of 5 mm and a plate thickness of 2 mm collected from the test material, a sample vibration type magnetic field meter (BHV525 manufactured by RIKEN Electronics Co., Ltd.) was used to sweep 1 kOe (79.58 kA / m) / minute and 1 kOe (79. A magnetic field of 58 kA / m) was applied for magnetization, and the magnetic permeability was obtained from the slope of the magnetic field-magnetization curve obtained there, and the magnetic permeability was divided by the vacuum magnetic permeability of 4π × 10 ^-7 H / m to obtain the relative magnetic permeability. In each case, the number of tests was n = 5, and the example No. in which the relative permeability was 1.10 or less in all five samples was ○ (low magnetism; passed), and the others were × (low magnetism; Failed).

(Measurement of aging softening temperature)
The test piece collected from the test material was held at various temperatures in the temperature range of 300 to 800 ° C. in 5 ° C increments for 120 minutes, and then aged under the condition that it was taken out of the furnace and air-cooled to room temperature. The hardness of the subsequent plate surface (rolled surface) was measured by the above method. Then, the minimum temperature in increments of 5 ° C. at which the hardness after the aging treatment becomes lower than the hardness of the test material (hardness before the aging treatment) was defined as the aging softening temperature of the test material.

(Evaluation of low magnetic maintenance performance and strength-ductility balance)
For each steel, a cold-rolled steel sheet having the characteristics that the hardness of the plate surface (rolled surface) is 345 HV or more, the breaking elongation EL in the rolling direction is 10.0% or more, and the relative magnetic permeability is 1.100 or less can be obtained. It was evaluated whether or not, that is, whether or not a tempered rolled finish having each of the above characteristics was possible. Austenitic stainless steel with a chemical composition that can obtain a cold-rolled steel sheet with a thickness of 0.20 mm having each of the above characteristics is extremely useful for thin-walled high-strength members such as electronic devices that require low magnetism, and has excellent "strength". -It is evaluated as a steel that can achieve "ductility balance". Therefore, it is possible to obtain a cold-rolled steel sheet having the characteristics that the hardness of the plate surface (rolled surface) is 345 HV or more, the breaking elongation EL in the rolling direction is 10.0% or more, and the specific ductility is 1.100 or less. The confirmed steels were evaluated as ◯ (low magnetic and strength-ductility balance; pass), and the others were evaluated as × (low magnetism and strength-ductility balance; fail). Regarding the comparative steel, the hardness of the plate surface (rolled surface) of the test material (3/4H finish equivalent material) is 345 HV or more, the breaking elongation EL in the rolling direction is 10.0% or more, and the relative magnetic permeability is high. Those satisfying 1.100 or less are indicated by ○, and those satisfying 100 or less are indicated by ×.
These results are shown in Table 2.

In the examples of A1 to A17 having the steel composition specified in the present invention, it was confirmed that a cold-rolled steel sheet having an excellent "low magnetism and strength-ductility balance" can be obtained. In particular, invented steels (other than A2) having a total content of C and N of 0.250% or more have a 0.01% proof stress of 600 N / mm ² or more when they are tempered and rolled to a strength level of 345 HV or more. The excellent springiness is realized. Further, in the invention steel (other than A2 and A7) having an MD value of −85.0 or less, the amount of work-induced martensite is 2.0% by volume or less in the cold-rolled steel sheet tempered to a hardness of 350 HV or more. As a result, a high aging softening resistance with a aging softening temperature of 530 ° C. or higher was obtained.

For reference, FIG. 1 illustrates the relationship between the amount of process-induced martensite and the aging softening temperature.

Claims (4)

In terms of mass%, C: more than 0.010% and less than 0.160%, Si: 0.1 to 1.20%, Mn: 3.0 to 6.00%, Ni: 3.0 to 6.00%, Cr: 15.00 to 19.00%, N: 0.090 to 0.210%, Cu: 0.50 to 3.50%, Mo: 0.1 to 1.50%, V: 0 to 0. 35%, Nb: 0 to 0.35%, Ti: 0 to 0.35%, the balance is Fe and unavoidable impurities, and the MD value defined by the following equation (1) is -70.0 or less, the following Austenitic stainless steel having a δcal value of 3.20 or less as defined by equation (2).
MD = 551-462 (C + N) -9.2Si-19.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo ... (1)
δcal = -15-44.91C-0.88Mn-2.31Ni + 2.2Cr-1.08Cu-28.8N ... (2)
Here, the content value of the element represented by mass% is substituted in place of the element symbol of the equations (1) and (2). A cold-rolled steel sheet made of steel according to claim 1, obtained by performing a tensile test in the rolling direction using a JIS13B test piece with a specific magnetic permeability μ of 1.10 or less at a tensile speed of 5 mm / min. A low-magnetic austenitic stainless steel cold-rolled steel sheet having a 0.01% proof stress of 500 N / mm ² or more obtained by the offset method from the obtained nominal stress-strain curve. The low magnetic austenitic stainless steel cold-rolled steel sheet according to claim 2, wherein the hardness of the plate surface (rolled surface) is 345 HV or more. The low magnetic austenitic stainless steel cold-rolled steel sheet according to claim 2 or 3, wherein the sheet thickness is 0.02 to 1.5 mm.

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