JP2000008145A - Ferritic stainless steel excellent in antifungal property and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a ferritic stainless steel having antifungal property, toughness and machinability combinedly. SOLUTION: This ferritic stainless steel in the one in which phases essentially consisting of Cu are exposed to the surface of ferritic stainless steel contg., by mass, <=0.02% C, 0.01 to 0.4% Si, <=1.0% Mn, 0.06 to 0.12% S, 18 to 32% Cr, 1.5 to 4.5% Mo, 1.0 to 3.5% Cu, 0.05 to 0.3% Ti, <=0.2% Al, <=0.02% N, 0 to 0.2% Nb, and the balance substantial Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel suitable as a material for a watch side member or the like which comes into direct contact with a human body and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, ferrite stainless steel containing no Ni has been used as a material such as a watch side member that comes into direct contact with the human body. This is because if a stainless steel material containing Ni directly touches the human body, Ni allergy may occur.

[0003] In addition to the above-described prevention of Ni allergy, the following is required for stainless steel used as a material for a watch side member or the like that comes into direct contact with the human body.

[0004] That is, it has a cutting property for processing a steel material into a predetermined shape, a toughness when processed into a product, and an antibacterial property against Escherichia coli and the like.

As an example of imparting antibacterial properties to ferritic stainless steel, for example, Japanese Patent Application Laid-Open No. Hei 8-60303 discloses that Cu contains 0.01 to 3% by weight of Cu and 0.1% by atom or more of Cu in the surface layer. A concentrated ferritic stainless steel is disclosed.

Japanese Patent Application Laid-Open No. 9-170053 discloses a ferritic stainless steel containing 0.4 to 3% of Cu and having a Cu-rich phase precipitated in a matrix in an amount of 0.2% by volume or more.

[0007] However, these ferritic stainless steels do not take into consideration the machinability and toughness, and cannot be used as a material such as a watch side member. further,
Since Cu is concentrated in the surface layer, there is a disadvantage that the surface layer-concentrated Cu is shaved during processing or polishing, and the antibacterial property is reduced.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an unprecedented ferritic stainless steel having antibacterial properties, toughness and machinability, and a method for producing the same.

[0009]

The gist of the present invention relating to a ferritic stainless steel excellent in antibacterial properties, toughness and machinability is as follows.

(1) In mass%, C: 0.02% or less, S
i: 0.01 to 0.4%, Mn: 1% or less, S: 0.0
6-0.12%, Cr: 18-32%, Mo: 1.5-
4.5%, Cu: 1 to 3.5%, Al: 0.2% or less,
Ferritic stainless steel containing one or two of N: 0.02% or less, Ti: 0.05 to 0.3% and Nb: 0.01 to 0.2%, with the balance being Fe and unavoidable impurities A ferritic stainless steel having excellent antibacterial properties, wherein a phase mainly composed of Cu is exposed on the surface of the steel.

(2) The phase mainly composed of Cu has a maximum major axis of 0.1
The ferritic stainless steel having excellent antibacterial properties according to the above (1), which is a phase in which Cu particles of up to ² μm are precipitated on the steel surface at a density of 1 / μm ² or more.

(3) For a watch side member excellent in antibacterial property, the ferritic stainless steel described in (1) above is heated to a temperature in the range of 600 ° C. to 900 ° C. and subjected to heat treatment for 1 to 20 hours. Manufacturing method of ferritic stainless steel.

The present inventors have conducted various experiments and studies to develop a ferritic stainless steel having both antibacterial properties, toughness and machinability, and have obtained the following findings.

A) Elements that affect the machinability are S, Mn, C
R and Ti are S, and S forms a sulfide with Mn, Cr and Ti to form a composite compound, which has an effect of lowering cutting resistance, and has a fragmentary chip shape to improve cutting workability.

B) an amount exceeding the solid solubility limit;
By containing 5% of Cu and performing a predetermined heat treatment to expose a phase mainly composed of Cu on the steel surface, an excellent antibacterial action can be imparted.

C) When the maximum length of the Cu particles of the Cu main phase is as small as less than 0.1, the antibacterial effect is reduced even if there are many particles. The antibacterial effect becomes more remarkable when a Cu particle having a maximum major diameter of 0.1 to ² μm is formed into a phase precipitated on the steel surface at a density of 1 particle / μm ² or more.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a ferritic stainless steel of the present invention and a method for producing the same will be described in detail.
In addition,% display used for description of a chemical composition shall mean the mass%.

1) Chemical composition C, N: 0.02% or less for each C and N, if contained in a large amount exceeding 0.02%, respectively, deteriorates corrosion resistance and toughness. Therefore, the upper limit is made 0.02% or less. . It is desirable to reduce as much as possible.

Si: 0.01 to 0.4% Si is an element used as a deoxidizing agent, and it is necessary to contain it in an amount of 0.01% or more in order to obtain the effect. But,
Since an excessive content lowers the toughness, the upper limit of the content is set to 0.4%.

Mn: 1% or less Mn is an element effective as a deoxidizing element. Also, Mn
Reacts with S in the steel to form a sulfide to improve the machinability of the steel. The Mn content may be at an ordinary impurity level, but in order to reliably obtain the above-mentioned deoxidizing effect and machinability, the Mn content is set to 0.1%.
It is desirable to contain 1% or more. However, if the content exceeds 1%, the corrosion resistance, toughness and hot workability deteriorate, so the upper limit is 1%.

S: 0.06 to 0.12% S is an element that forms sulfides to improve the machinability of steel. In order to obtain the effect, it is necessary to contain 0.06% or more. On the other hand, if S is excessively contained in excess of 0.12%, problems of hot brittleness and deterioration of corrosion resistance occur, so the upper limit of the content is set to 0.12%. A desirable content is 0.06 to 0.08%.

Cr: 18-32% Cr is the most important element for ensuring the corrosion resistance of the ferritic stainless steel of the present invention. In order to obtain corrosion resistance comparable to that of austenitic stainless steel, the content was made 18% or more. However, if the content exceeds 32%, the toughness of the steel decreases. Therefore, the upper limit of the content is 3
2%.

Mo: 1.5 to 4.5% Mo is an element effective for improving corrosion resistance. In order to obtain the effect, it is necessary to contain 1.5% or more. However, if the content exceeds 4.5%, the toughness decreases. Therefore, the upper limit of the content is set to 4.5%.

Cu: 1 to 3.5% Cu is the most important alloying element for exhibiting the antibacterial effect, and enhances the antibacterial property by exposing a phase mainly composed of Cu to the steel surface. Cu ions slightly eluted from Cu on the surface exert antibacterial effects such as suppressing growth and proliferation of fungi such as Escherichia coli and Staphylococcus aureus attached to the surface.

In order to exhibit these antibacterial effects, Cu
Requires a content of more than 1%. On the other hand, when the content exceeds 3.5%, problems such as an increase in manufacturing cost, cracking during hot rolling, and a decrease in cold workability due to the formation of a duplex stainless steel depending on the Cr content occur. Therefore, the content of Cu is set to 1 to 3.5%.

Ti: 0.05 to 0.3%, Nb: 0 to 0
0.2% Ti and Nb are elements that combine with C and N in steel to form carbonitrides, refine crystal grains, and improve the toughness of steel. This carbonitride is generated preferentially over Cr carbonitride and has an effect of preventing deterioration of intergranular corrosion resistance due to precipitation of Cr carbonitride at the grain boundary. Therefore, Ti, Nb
One or two types are contained.

Ti is preferentially combined with S to form a hard sulfide. TiS is an important sulfide that becomes a complex compound with sulfides of Mn and Cr and prevents the presence of ductile inclusions such as MnS and CrS that cause hot embrittlement alone. Composite inclusions made of TiS, MnS, CrS, etc.
Since it is not a ductile inclusion, it is an inclusion that breaks during hot working, so it does not cause hot embrittlement. To obtain these effects, 0.05% or more of Ti is required. However, if it is contained excessively, TiS precipitates excessively and the machinability and the abrasion deteriorate. Therefore, the upper limit of the content is set to 0.3%. A desirable content is 0.05 to 0.12%.

Nb is 0.01 to obtain the above effect.
% Of Nb must be contained. However, 0.2
%, The toughness of the steel is rather reduced, and the abrasion is also deteriorated. Nb not used for fixing C and N
However, during heat treatment for forming a Cu phase, an intermetallic compound such as a Laves phase is formed, and the toughness is deteriorated. Therefore, the upper limit of the content is set to 0.2%.

Al: 0.2% or less Al is an element effective as a deoxidizing element. O in steel,
By fixing N, the toughness of the steel is improved. However, if the content exceeds 0.2%, the toughness is rather deteriorated. Therefore, the Al content is limited to 0.2%. Note that the lower limit of the content may be an amount that is mixed as inevitable impurities.

Nb: 0 to 0.2% Nb is an element to be contained if necessary. Like Nb, Nb combines with C and N in steel to form a carbonitride, refines crystal grains, and refines the steel. Improve toughness. This carbonitride is Cr
Is produced preferentially over carbonitrides of Cr, and has an effect of preventing deterioration of intergranular corrosion resistance due to precipitation of Cr carbonitrides at the grain boundaries.

2) Cu-based phase By performing a predetermined heat treatment to be described later, Cu atoms contained in the steel in excess of the solid solubility limit are precipitated randomly in the steel or on the steel surface. Grows significantly over time. In the present invention, this Cu precipitated phase is referred to as a “Cu-based phase”.
Is defined.

Some of the Cu-based phases are present in the steel, while others are exposed on the surface of the steel. Cu atoms in the Cu phase exposed on the surface of the steel are ionized to exhibit antibacterial properties. When the steel of the present invention is put into practical use, the Cu phase that is exposed on
The ions elute. When microorganisms come into contact with the steel according to the invention, C
The u ions diffuse and reach the cell membrane of the microorganism, where they are adsorbed by the protein.

The Cu ion adsorbed on the protein exerts an antibacterial action that destroys the SH group radicals and the like of the constituents and disables the energy metabolism function of the microorganism.

The size and shape of the Cu grains of the Cu-based phase are as follows:
Depending on the heat treatment temperature or heat treatment time, if heat treatment is carried out at a low temperature for a short time, the particles will precipitate in the form of particles of about 10 nm or more. When heat treatment is performed at a high temperature for a long time, the precipitate is formed into a rod shape having a maximum major axis of about 2 μm or less.

The phase mainly composed of Cu has a maximum major axis of 0.1 to 2 μm.
In order to obtain a further antibacterial property, it is desirable that the particles have a size of m and are uniformly deposited on the steel surface at a density of 1 piece / μm ² or more. If the maximum major axis is less than 0.1 μm, sufficient antibacterial properties cannot be obtained, while if it exceeds 1.5 μm, the corrosion resistance deteriorates. Further, in order to obtain large grains having a maximum major axis exceeding 1.5 μm, it is necessary to perform a long-time heat treatment at a high temperature, which increases the production cost. Therefore, it is preferable that the maximum major axis of the Cu grains be 0.5 to 1.5 μm.

The phase mainly composed of Cu may be composed of only Cu, but may be a phase containing Ni, Ti, Fe, Si, P or S which forms an intermetallic compound in addition to Cu. Good.

The maximum major axis of the Cu grain is the maximum diameter obtained by collecting a thin film sample of about 50 μm thickness from a phase mainly composed of Cu and observing it with a transmission electron microscope.

3) Manufacturing Method In the ferritic stainless steel of the present invention, a phase mainly composed of Cu is exposed on the surface of the steel in a finished state in a product shape. This phase mainly composed of Cu is obtained by heating the ferritic stainless steel having the chemical composition of the present invention to a temperature in the range of 600 ° C. to 950 ° C. and holding it for 1 to 20 hours.

When the heat treatment temperature is lower than 600 ° C., even if Cu precipitates, it is difficult to grow and become a Cu phase. Further, the steel is hardened by the extremely fine precipitated Cu, and the machinability is deteriorated. On the other hand, at a temperature exceeding 950 ° C., the once precipitated Cu will be dissolved again. Preferably, 750 ° C
The heat treatment is performed at a temperature of 900 ° C. or less. If the heat treatment is performed for less than one hour, even if Cu is precipitated, it is difficult to grow and become a Cu phase. Even if the heat treatment is performed for more than 20 hours, the growth of the Cu phase hardly occurs, so that the heating energy is wasted.

The size and density of Cu grains in the phase mainly composed of Cu are as follows:
It can be changed by adjusting the heat treatment temperature and the holding time.

The ferritic stainless steel having the chemical composition specified in the present invention is melted by a usual method, forged,
After hot working such as rolling or extrusion, and hot working, cold working is performed to form a steel pipe or a steel sheet, and then finished into a final product by cutting or press forming. Further, after being melted, the product may be cast as it is.

In order to expose the phase mainly composed of Cu on the surface in the state of the product, the heat treatment described above may be performed at the stage of the raw material, and the product may be cut and formed into a product shape. Heat treatment may be performed.

The cooling after the heating may be performed by air cooling, water cooling, or the like, but it is preferable that the cooling rate is high.

The furnace used for the heat treatment may be an ordinary heating furnace. The atmosphere during the heat treatment may be either an oxidizing atmosphere such as the air or a non-oxidizing atmosphere such as an argon gas. However, in order to expose the Cu phase on the surface of the stainless steel, it is preferable to perform the heat treatment in a non-oxidizing atmosphere that does not generate an oxide scale.

[0045]

EXAMPLE In a vacuum high-frequency furnace, steels having 22 chemical compositions shown in Table 1 were melted to form 17 kg ingots, each of which was forged, hot-rolled, finished into a hot-rolled steel sheet, and annealed at 950 ° C.
Thereafter, a heat treatment for precipitating a phase mainly composed of Cu was performed, and the sample was subjected to each test of antibacterial property, toughness, machinability and corrosion resistance.

Symbols 1 to 7 in Table 1 are steels of the present invention, and symbols A to O are comparative steels.

[0047]

[Table 1]

(A) Antibacterial test From the above 22 kinds of annealed hot-rolled steel sheets, a length of 50 mm and a width of 5
Two pieces each of 0 mm test pieces were cut out, and 800 ° C. in the air to expose a phase mainly composed of Cu on the surface of the test pieces.
For 12 hours.

A large number of test pieces were sampled from the steel sheet 3 and heat-treated under various heat-treatment conditions to expose a phase mainly composed of Cu on the surface.

With respect to the test piece subjected to the heat treatment at 800 ° C. for 12 hours, a thin film sample was taken from the surface layer of the test piece in order to measure the density of the Cu-based phase precipitated by the heat treatment and the size of the Cu grains. And observation with a transmission electron microscope. Table 2 shows the measurement results. On the surfaces of the symbols 1 to 7 of the steel of the present invention, 0.1 to 1 μm of Cu phase was precipitated at a density of 1 / μm ² or more. On the other hand, although the phase of Cu was formed in the symbol J of the comparative steel in which the amount of Cu was smaller than the prescribed value, the density was less than 1 piece / μm ² , and the symbols A to C and F
No u phase was detected.

[0051]

[Table 2]

The following antibacterial tests were carried out on test pieces that had been subjected to heat treatment at 800 ° C. for 15 hours and test pieces that had been subjected to heat treatment at various heat treatment temperatures and times.

The bacteria used for the test were Escherichia coli I
FO 3301 (Escherichia coli) and Staphylococcus aureus (Staphylococcus aureus). After the two kinds of bacteria were enriched in a peptone medium, they were diluted with sterile phosphate buffered saline to prepare a bacterial solution. 1 ml of each of the above two bacterial solutions was applied to the surface of each test piece, and allowed to stand at 25 ° C. for 24 hours. Then, the bacterial solutions were wiped off and sprinkled into the diluent. A predetermined amount of the shake-out solution was mixed with a measurement medium, cultured at 37 ° C. for 48 hours, and the number of generated colonies was counted.

Table 3 shows the measurement results.

[0055]

[Table 3]

As is clear from the table, the steel of the present invention shows excellent antibacterial properties when the number of colonies after culturing is 40 or less. However, the symbols A to C, F, and J of the comparative steels in which the amount of Cu was less than the specified amount and no Cu phase was formed, had 1.0 × 10
^There was no antibacterial effect at ⁶ or more.

In the test in which the heat treatment conditions of symbol 3 were changed, the test pieces that did not satisfy the heat treatment conditions specified by the production method of the present invention were many of Escherichia coli and Staphylococcus aureus, and no antibacterial effect was observed.

(B) Toughness and machinability Hot-rolled steel sheets annealed with symbols 1 to 7 and A to O
And heat treatment for 12 hours, then length 55mm x width 1
JIS Z with a V notch of 0 mm x 10 mm thickness
A specified Charpy impact test specimen was obtained at 2242, and the toughness was evaluated.

The test results are shown in Table 4. The test piece having an impact value of 50 J / cm ² or more at normal temperature was evaluated as having excellent toughness.

[0060]

[Table 4]

The symbols 1 to 7 of the steel of the present invention have an impact value of 50 J
/ Cm ² or more indicates excellent toughness. However, the symbols A to C, I, and M to O of the comparative steels in which the elements deteriorating the toughness are larger than the range specified in the present invention have low impact values,
Poor toughness.

The machinability test was conducted using a drill having a diameter of 4 mm under the conditions of 1000 rpm and a load of 6 kgf. The time was evaluated. Those having a cutting time of 20 minutes or less were evaluated as having good cutting properties. The results are shown in Table 4.

The symbols 1 to 7 of the steel of the present invention were excellent in machinability with a cutting time of 20 minutes or less. However, Ti, which is an element that deteriorates machinability, or S, Mn that is an element that improves machinability, the symbols A, E to K of comparative steels whose departures from the range specified in the present invention have long cutting times, It turns out that it is inferior in machinability.

(C) Corrosion resistance test After annealing of the symbols 1 to 7 of the steel of the present invention and the symbols A to N of the comparative steels, salt water spray test pieces were collected from a hot-rolled steel sheet which was heat-treated at 800 ° C. for 12 hours. A salt spray test was performed according to 2371 to evaluate corrosion resistance.

Table 5 shows the evaluation results.

[0066]

[Table 5]

Although all of the symbols 1 to 7 of the steel of the present invention show good corrosion resistance, C, an element which deteriorates the corrosion resistance,
S, Mn, Cr which is an element for improving corrosion resistance,
Rust was observed in the symbols A, C, D, G, H, and K of the comparative steels in which Mo and Nb were out of the ranges specified in the present invention. It can be seen that the steel of the present invention has the same or higher corrosion resistance than the comparative steel which is a conventional steel.

[0068]

According to the production method of the present invention, a ferritic stainless steel excellent in various properties such as antibacterial property, toughness and machinability can be obtained. In addition, since it does not contain Ni, there is no problem of Ni allergy even if it is used for the purpose of directly touching the human body.

Claims (3)

[Claims] (1) In mass%, C: 0.02% or less, Si:
0.01 to 0.4%, Mn: 1% or less, S: 0.06 to
0.12%, Cr: 18-32%, Mo: 1.5-4.
5%, Cu: 1 to 3.5%, Al: 0.2% or less, N:
0.02% or less and Ti: 0.05 to 0.3% and N
b: containing one or two kinds of 0.01 to 0.2%, the balance being a ferritic stainless steel composed of Fe and unavoidable impurities, and having a phase mainly composed of Cu exposed on the surface thereof. Ferritic stainless steel with excellent antibacterial properties. 2. A phase mainly composed of Cu having a maximum major axis of 0.1 to 2 μm.
^{2. The} ferritic stainless steel having excellent antibacterial properties according to claim 1, wherein the m particles are a phase precipitated on the steel surface at a density of 1 / μm ² or more. 3. The ferritic stainless steel according to claim 1 is heated to a temperature in the range of 600 ° C. to 900 ° C.
A method for producing a ferrite stainless steel material for watch-side materials having excellent antibacterial properties, which comprises performing heat treatment for holding for 0 hours.