JPH11350089A - Austenitic stainless steel having excellent antibacterial characteristic and high workability, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic antibacterial stainless steel capable of maintaining superior antibacterial characteristic over a long period and excellent in workability. SOLUTION: The stainless steel has a composition containing, by weight, <=0.1% C, <=2% Si, <=5% Mn, 10-30% Cr, 5-15% Ni, and 1.0-5.0% Cu and also has a structure where a secondary phase composed essentially of Cu is dispersed in a proportion of >=0.2 vol.% in a matrix. Moreover, the value of ΔD, represented by ΔD=76-90(C+N)-7Ni-0.2Cr-Si-2Mn, is regulated to 0-25. The secondary phase composed essentially of Cu is dispersedly precipitated in the matrx by subjecting the austenitic stainless steel having the prescribed composition to heat treatment at 500-900 deg.C one or more times in the course between the completion of hot rolling and the final product.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates not only to applications requiring antibacterial properties but also to overhang and deep drawing in a wide range of fields such as kitchen appliances, electric appliances, building materials, mechanical appliances and chemical appliances. The present invention relates to an austenitic stainless steel suitable for high working applications and a method for producing the same.

[0002]

2. Description of the Related Art Corrosion resistance in general environments is required for various equipment used in kitchen equipment, hospitals, etc., and pipes for handrails of transportation means such as buses and trains.
Stainless steel represented by 304 is mainly used. However, in recent years, in-hospital infections caused by Staphylococcus aureus and the like have become a problem, and there is a demand for improved hygiene even in environments used by unspecified large numbers of people, such as buses and trains. Along with this, the materials used for various machines and instruments are not limited to the properties of general structural materials, and are maintenance-free with functions such as antibacterial properties that do not require regular infection prevention such as disinfection. Material is desired. As a material imparted with antibacterial properties, an antibacterial coat by an organic film or plating is generally used as disclosed in JP-A-5-22820 and JP-A-6-10191.

[0003]

However, the antibacterial coat has a drawback that the antibacterial property is reduced as the film disappears.
In addition, the appearance may be degraded due to dissolution, abrasion, chipping or the like of the film, and the antibacterial effect may be reduced. Also,
Generally, metal elements such as Ag and Cu are known to exhibit an antibacterial action, but Ag is a noble metal element, which is very expensive, and is considered to be unsuitable as an additive element for industrial production. On the other hand, Cu is an inexpensive element as compared with Ag, and can cope with the residual Cu in scrap iron as a playing card element, which has recently become a problem, and can easily recycle scrap iron as a raw material. It has been studied to add antimicrobial properties to materials such as stainless steel.

The present inventors have also studied various improvements in antibacterial properties by adding Cu, and found that the antibacterial properties can be improved by increasing the Cu concentration on the surface of stainless steel, as disclosed in JP-A-8-53738 and JP-A-8-53738. No. 8-225895. Further, prior to the present invention, Japanese Unexamined Patent Publication No. 9-176800, which has been proposed to further enhance the action of Cu, discloses an austenitic stainless steel having antibacterial properties. However, in an austenitic stainless steel to which Cu is added, a Cu-rich phase is precipitated as a second phase in order to exhibit antibacterial properties. There is a problem that the rich phase deteriorates the workability, particularly stretch forming and drawing workability. An object of the present invention is to provide an austenitic stainless steel having excellent antibacterial performance and additionally excellent workability.

[0005]

In order to achieve the object, the austenitic stainless steel of the present invention has the following features: C: 0.1% by weight or less, Si: 2% by weight or less, Mn: 5% by weight or less,
Cr: 10 to 30% by weight, Ni: 5 to 15% by weight, C
u: having a composition containing 1.0 to 5.0% by weight, wherein the second phase mainly composed of Cu is dispersed in the matrix at a rate of 0.2% by volume or more, and ΔD = 76-90. (C + N)
ΔD represented by −7Ni−0.2Cr−Si−2Mn is 0 or more,
5 or less. This austenitic stainless steel further contains Nb: 0.01 to 0.50% by weight, Ti: 0.02 to 1% by weight, Mo: 2.5% by weight or less, Al: 1% by weight or less, Zr: 1% by weight. % Or less, V: 1
1 wt% or less, B: 0.05 wt% or less, and rare earth element (REM): 0.05 wt% or less. In that case, ΔD is 76−90 (C + N)
−7Ni−0.2Cr−Si−2Mn−30Nb−0.8Mo, ΔD
Is set to satisfy 0 to 25 inclusive. The second phase containing Cu as a main component is subjected to heat treatment at least once in a temperature range of 500 to 900 ° C. from after hot rolling the austenitic stainless steel having a predetermined composition to a final product, Disperse and precipitate in the matrix.

[0006]

[Action] Stainless steel is made of Cr, which is called a passive film.
Since the surface is covered with a hydroxide mainly composed of, a high corrosion resistance is exhibited. The present inventors added Cu exhibiting effective antibacterial properties to austenitic stainless steel,
The amount of Cu contained in the passive film was measured, and the antibacterial properties of the solution containing Staphylococcus aureus by dropping were investigated.
As a result, it was found that stainless steel containing Cu to a certain degree or more had antibacterial properties. However, simply dissolving a few percent or less of Cu in steel may not always provide sufficient antibacterial properties and its durability. Then, as a result of further study, it was found that if the same Cu-rich phase was finely and uniformly precipitated, Cu was easily eluted in a use environment, and the antibacterial property was improved. Further, even if the surface is worn during processing or use, the internal Cu-rich phase appears on the new surface, so that the antibacterial durability is excellent.

As means for precipitating a Cu-rich phase,
It is conceivable to perform isothermal heating such as aging in the temperature range where the Cu-rich phase is likely to precipitate, and to extend the passage time in the precipitation temperature range as much as possible by slow cooling. It has been found that aging treatment in the range of -900 ° C promotes precipitation, and that good antibacterial properties can be obtained even when the amount of Cu added is low. Also,
When elements that easily form carbonitrides and precipitates, such as Ti, Nb and Mo, are added, C
The u-rich phase is easily dispersed uniformly in the matrix, and the antibacterial property and the productivity are improved. Further, it was found that when a part of Cu was precipitated as a Cu-rich phase, the Cu concentration on the surface was increased and the antibacterial property was also improved.

[0008] However, the material obtained based on the above-mentioned knowledge was able to provide a material having excellent antibacterial properties. However, since Cu was dispersed and precipitated in the matrix, it was compared with ordinary austenitic stainless steel. It has been found that the processability, particularly the stretch forming and drawing processability, is reduced. It is considered that the cause of this is that the precipitated Cu atoms do not trap the dislocations but slowly drag them during the movement and propagation of the dislocations generated by the processing. Therefore, various studies were conducted to improve the workability of austenitic stainless steel. In general, as a measure to improve the workability of austenitic stainless steel, component design is performed in the metastable austenite phase region, and the dislocation of the austenite phase is dispersed by forming the work-induced martensite phase generated by processing, improving workability. There is a so-called TRIP effect. Although this effect depends on the austenite stability of the steel, the present inventors have investigated the austenite stability and found that even if Cu is added in excess of 1% by weight, the workability due to the addition of Cu is impaired. The effect is not remarkably recognized, and it has been found that the workability equivalent to that of the conventional austenitic stainless steel can be obtained by regulating the additive elements other than Cu. That is, as an alternative to the above-described index Md30 of austenite stability, ΔD = 76−90 (C
+ N) -7Ni-0.2Cr-Si-2Mn-30Nb-0.8Mo It has been found that good workability can be obtained by setting the alloy components such that ΔD is 0 or more and 25 or less. Nb and Mo in the formula of ΔD are optional additive elements, and when not added, the terms of these elements in the formula may be omitted.
This is based on the finding that the formation of the work-induced martensite phase generally involves the balance of the elements forming the austenite phase and the ferrite phase, but can be controlled by the above-described derivation formula by adding Cu. Obtained.

Hereinafter, alloying elements contained in the austenitic stainless steel of the present invention and their contents will be described. C: 0.1% by weight or less C is an alloy element effective for generating a Cr carbide effective as a precipitation site of a Cu-rich phase and uniformly dispersing a fine Cu-rich phase. However, if added in excess, the manufacturability and corrosion resistance deteriorate, so the upper limit of the C content is 0.1%.
It was restricted to wt%. Si: 2% by weight or less Si is an alloy element effective for improving corrosion resistance, and also exhibits an action of improving antibacterial properties. However, excessive Si addition exceeding 2% by weight deteriorates the productivity.

Mn: not more than 5% by weight The effect of improving manufacturability and fixing harmful S in steel as MnS is exhibited. In addition, since MnS acts as a nucleus for generating a Cu-rich phase, it precipitates a fine Cu-rich phase. However, a large content of Mn exceeding 5% by weight deteriorates corrosion resistance. Cr: 10 to 30% by weight Cr is an alloy element necessary for maintaining the corrosion resistance of austenitic stainless steel, and 10% by weight or more of Cr is required from the viewpoint of ensuring the required corrosion resistance. However, when a large amount of Cr exceeding 30% by weight is contained, the manufacturability and workability deteriorate.

Ni: 5 to 15% by weight An important alloying element for stabilizing the austenite phase. However, since the addition of a large amount consumes expensive Ni and raises the cost of steel materials, the upper limit of the Ni content is restricted to 15% by weight. Cu: 1.0 to 5.0% by weight and Cu rich phase:
0.2% by volume or more It is the most important alloying element in the stainless steel of the present invention. To maintain good antibacterial properties, it is necessary that 0.2% by volume or more of a Cu-rich phase is precipitated. In order to precipitate a Cu-rich phase of 0.2% by volume or more in the present austenitic stainless steel, a Cu content of 1.0% by weight or more is required. However, when an excessive amount of Cu exceeding 5.0% by weight is contained, manufacturability, workability, and corrosion resistance deteriorate. The Cu-rich phase is not particularly limited in the size of the precipitate, but exhibits good antibacterial properties evenly on the entire product surface and maintains good antibacterial properties even when polished or the like. In order to do so, it is preferable that the precipitated phase is appropriately dispersed and distributed on the surface and inside.

The selected elements will be described below. Nb: 0.01 to 0.5% by weight It is an element added as needed. Cu rich phase is N
It shows a tendency to precipitate around the precipitate b. Therefore, in order to uniformly precipitate the Cu-rich phase, a structure in which carbides, nitrides, and carbonitrides are finely precipitated is preferable. However, if Nb is excessively added, the manufacturability and workability deteriorate. Therefore, when Nb is added, 0.01 to
It is preferable to adjust the content in the range of 0.5% by weight. Ti: 0.02 to 1% by weight An element that is added as necessary, and forms a carbonitride similarly to Nb, and has an effect of uniformly depositing a Cu-rich phase around the carbonitride. However, excessive addition of Ti deteriorates manufacturability and workability, and easily causes flaws on the product surface.
Therefore, when Ti is added, its content is set to 0.02
It is preferable to set it in the range of 1 to 1% by weight.

Mo: 3% by weight or less Mo is an alloying element that is added as required, and has an effect of improving corrosion resistance, and precipitates as an intermetallic compound such as Fe2Mo, and becomes a core site of a fine Cu-rich phase. To facilitate. In addition, Mo and a compound containing Mo exhibit an action of improving antibacterial properties by themselves.
However, an excessive addition of Mo exceeding 3% by weight deteriorates manufacturability and processability. Al: 1% by weight or less Al is an alloying element that is added as necessary, and improves the corrosion resistance as well as Mo, forms precipitates, and produces fine Cu.
It is an alloy element effective for the precipitation of a rich phase. However, Al
Manufacturability and workability deteriorate due to excessive addition of
When adding Al, the upper limit is restricted to 1% by weight. Zr: 1% by weight or less Zr is an alloy component added as needed, forms a carbonitride, and facilitates precipitation of a fine Cu-rich phase. However, if it is added excessively, the manufacturability and workability deteriorate. Therefore, when Zr is added, the upper limit is restricted to 1% by weight.

V: 1% by weight or less An alloying element added as necessary, forms a carbonitride like Zr, and facilitates precipitation of a fine Cu-rich phase. However, if it is added excessively, the manufacturability and workability deteriorate. Therefore, when V is added, the upper limit is regulated to 1% by weight. B: 0.05% by weight or less An alloy component that is added as necessary, improves hot workability, and is dispersed as a precipitate in the matrix. However, excessive workability deteriorates hot workability.
Is added, the upper limit is regulated to 0.05% by weight. Rare earth element (REM): 0.05% by weight or less It is an alloy component added as needed. Hot workability is improved as in B by adding an appropriate amount. Further, it becomes a precipitate effective for the precipitation of the Cu-rich phase and is dispersed in the mother phase. However, if added excessively, the hot workability deteriorates.
When M is added, the upper limit is regulated to 0.05% by weight.

Heat treatment temperature: 500-900 ° C. In order to precipitate a Cu-rich phase, 500-900 ° C.
Is effective. The lower the aging temperature, the lower the amount of solute Cu in the matrix and the greater the amount of Cu-rich phase deposited. However, if the aging treatment temperature is too low, the diffusion rate becomes slow, and the amount of precipitation decreases. It has been found that performing various aging treatments by changing the temperature conditions is an industrially effective temperature range. This aging treatment is effective at any stage from the end of hot rolling to the end product, and may be applied to a cold rolled sheet or a processed product.

ΔD: 0 to 25 ΔD is an index indicating the workability of the steel material, and ΔD = 76−90 (C
+ N) -7Ni-0.2Cr-Si-2Mn-30Nb-0.8Mo. In the case of a steel material in which ΔD is in the range of 0 to 25, the Erichsen value is 13.0 or more, indicating good workability.
On the other hand, when ΔD is less than 0, it is less than 13.0,
Even when D exceeds 25, it is less than 13.0, so that ΔD needs to be 0 to 25 in order to obtain good workability.

[0017]

EXAMPLES 30 kg of austenitic stainless steel having the composition shown in Table 1 was melted in a vacuum melting furnace, and subjected to forging and hot rolling, followed by annealing and aging to obtain a hot rolled annealed sheet. Then, cold rolling and annealing were repeatedly performed to finally obtain a cold rolled annealed sheet having a thickness of 0.7 mm. Aging condition is 85
At 0 ° C., set for 100 hours. The obtained test material was observed with a transmission electron microscope, and the amount of precipitated Cu-rich phase was quantified. Each test piece was subjected to the following antibacterial test. Staphy
lococusaureus IFO12732 (Staphylococcus aureus) was cultured with shaking in a normal broth medium at 35 ° C for 16 to 24 hours to prepare a culture solution. The culture is diluted with sterile phosphate buffer to 20,000.
The solution was prepared by diluting by a factor of two. 1 ml of a bacterial solution was dropped on a surface of a 5 cm × 5 cm test piece polished with # 400, and stored for 24 hours. After storage, the test pieces were washed off with 9 ml of SCDLP medium (manufactured by Nippon Pharmaceutical Co., Ltd.), and the number of viable cells was counted by a pour plate method (cultured at 35 ° C. for 2 days) using a standard agar medium with respect to the obtained liquid. . Further, as a reference, a bacterial solution was directly dropped on a petri dish, and the number of viable bacteria was similarly counted.

When the number of killed cells was 99% or more as compared with the reference viable cell count, the sample was evaluated as ○, and the sample with less than 99% was evaluated as ×. As for the evaluation of workability, the Erichsen test (J
ISZ 2247 Erichsen test B method)
1 equivalent to 5% reduction from 14.0 of US304 steel
Those with 3.3 or more were evaluated as ○, those with 13.3 to 11.9 corresponding to a reduction of 5 to 15% as Δ, and those with less than that were evaluated as ×. Table 1 shows the evaluation results of these antibacterial properties and the workability together with ΔD. As shown in Table 1, 1% or more of Cu was added, and
In Test Nos. 1 to 12, in which a u-rich phase was precipitated and ΔD was in the range of 0 to 25, good results were shown for not only antibacterial properties but also processability. On the other hand, although the Cu content was added by 1.0% by weight or more and the aging treatment was performed, the antibacterial properties were sufficiently exhibited in Test Nos. 13 to 18 in which ΔD was out of the range of 0 to 25. The workability was poor. In addition, even though ΔD was within the regulated range of 0 to 25 regardless of the presence or absence of Cu, good results were obtained with regard to workability in Test Nos. 19 and 20, which were not subjected to aging treatment. No antibacterial properties were observed. Further, the aging treatment is not performed, and ΔD is 0 to 25.
In Test Nos. 21 to 23 other than the above, no antibacterial property was observed, and it was shown that the processability was poor. From the above, in order to obtain an austenitic stainless steel having high workability while expressing the excellent antibacterial properties of the steel of the present invention, not only the aging treatment under the above conditions, but also Δ
It turns out that setting of the alloy component in which D is in the range of 0 to 25 is effective.

[0019]

[Table 1]

[0020]

As described above, the austenitic stainless steel of the present invention exhibits an excellent antibacterial property even with a solid material when the Cu content is 1.0% by weight or more and the aging treatment is performed. This antibacterial property lasts for a long time because it is derived from the material. Further, the alloy component is represented by ΔD = 76−90 (C
+ N) -7Ni-0.2Cr-Si-2Mn-30Nb-0.8Mo
When D is in the range of 0 to 25, high workability can also be obtained. For this reason, stainless steel is required to have high workability in kitchen equipment, various equipment used in hospitals, etc., equipment that comes into contact with the human body in transportation such as trains and buses,
In addition, it is used in a wide range of fields where antibacterial properties are required, and the living environment is further improved.

Claims (3)

[Claims] C: 0.1% by weight or less, Si: 2% by weight
Hereinafter, Mn: 5% by weight or less, Cr: 10 to 30% by weight,
Ni: has a composition containing 5 to 15% by weight and Cu: 1.0 to 5.0% by weight, and the second phase mainly composed of Cu is dispersed in the matrix at a rate of 0.2% by volume or more, And ΔD
ΔD defined as = 76−90 (C + N) −7Ni−0.2Cr−Si−2Mn
Is an austenitic stainless steel excellent in antibacterial property and high in workability, characterized in that it is 0 to 25. 2. Nb: 0.01 to 0.50% by weight, Ti: 0.02 to 1% by weight, Mo: 2.5% by weight or less, Al: 1% by weight or less, Zr: 1% by weight or less , V: 1
% Or less, B: 0.05% by weight or less, and rare earth element (REM): 0.05% by weight or less. Dispersed at a rate of 2% by volume or more, and ΔD = 76−90
The austenitic stainless steel according to claim 1, wherein ΔD defined by (C + N) -7Ni-0.2Cr-Si-2Mn-30Nb-0.8Mo is 0 to 25. 3. The method according to claim 1, wherein a heat treatment is performed at least once in a temperature range of 500 to 900 ° C. from a time after the hot rolling to a final product to precipitate a second phase mainly composed of Cu. Item 3. The method for producing an austenitic stainless steel according to item 1 or 2.