JP2001262280A - Pb-FREE-TYPE FERRITIC FREE-CUTTING STAINLESS STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a Pb-free-type ferritic stainless steel which is suitable for use as a stock for equipments or parts requiring excellent machinability, corrosion resistance and cold or hot workability and is remarkably reduced in Pb content. SOLUTION: The steel has a composition containing 0-2.0 mass% Ni, 12-35 mass% Cr and 0.005-0.2 mass% C and also containing at least either of Ti and Zr in amounts satisfying WTi+0.52 WZr=0.03 to 1.2 mass% (where WTi (mass%) and WZr (mass%) are Ti content and Zr content, respectively). Moreover, a compound, containing at least either of 0.01-0.5 mass% S and 0.01-0.4 mass% Se, further containing at least either of Ti and Zr as metallic element components, and also containing, as binder component with respect to the metallic element components, C as an essential component and further at least either of S and Se, is formed in the structure. By this method, the Pb-free-type ferritic stainless steel excellent in machinability can be attained with the least sacrifice of hot workability, cold workability and corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel, particularly to a ferritic stainless steel having a free-cutting property.

[0002]

2. Description of the Related Art Free-cutting stainless steel containing S, Pb, Se, Bi or the like as a machinability improving element is used as a raw material in order to improve the productivity of equipment and parts requiring cutting. May be used. Among them, ferritic free-cutting stainless steels that can contain a relatively large amount of these machinability improving elements, have high corrosion resistance, and do not contain a large amount of expensive Ni are widely used. In particular, when machinability is particularly required, such as when precision finishing is performed, to further improve machinability,
Increasing the content of the machinability improving element or adding a plurality of types of machinability improving elements in combination is also performed.

[0003]

Here, S as a machinability improving element is often added in the form of MnS or the like. However, if the added amount is excessively increased, not only does the corrosion resistance deteriorate significantly, Hot workability and cold workability are also impaired. For this reason, it has been proposed to limit the Mn content and increase the Cr content in the sulfide to suppress a decrease in corrosion resistance. However, increasing the amount of Cr in the sulfide tends to reduce machinability and hot workability, and its use is limited.

On the other hand, Pb as a machinability improving element is S
It is a very useful element because it reduces the degree of corrosion resistance and cold workability and reduces machinability compared to
Used alone or in combination with S. But,
In recent years, with increasing concerns about environmental protection on a global scale, steels containing Pb have been gradually shunned, and the number of devices and parts that restrict their use has been increasing. Therefore, stainless steel which does not substantially contain Pb and has high machinability has been required.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Pb-free material which is suitable for materials of equipment or parts requiring excellent machinability, corrosion resistance and cold or hot workability, and which has a greatly reduced Pb content. It is an object of the present invention to provide a type ferritic stainless steel.

[0006]

In order to solve the above-mentioned problems, the Pb-free ferritic free-cutting stainless steel of the present invention (hereinafter, simply referred to as the stainless steel of the present invention) contains Ni. Not contained, or even if contained, the content is set to 2.0% by mass or less, contains 12 to 35% by mass of Cr and 0.005 to 0.2% by mass of C, and contains Ti To WTi (mass%) and the Zr content to W
As Zr (% by mass), WTi + 0.52WZr is 0.03 to
At least one of Ti and Zr is contained so as to be 1.2 mass%, and 0.01 to 0.5 mass of S;
At least one of Se in an amount of 0.1 to 0.4% by mass, and further contains at least one of Ti and Zr as a metal element component, and as a binding component with the metal element component, A compound containing C and at least one of S and Se is formed in the tissue.

[0007] By containing C, Zr and / or Ti, S and / or Se in the above composition range, it becomes possible to produce a (Ti, Zr) -based compound in the steel structure, and By forming this compound, good machinability can be imparted to ferritic stainless steel without adding Pb. MnS or (Mn,
It is also possible to prevent or suppress the formation of a compound, such as Cr) S, which tends to lower the corrosion resistance and the hot workability, and thus it is possible to maintain good corrosion resistance, cold workability or hot workability of the steel material. That is, according to the present invention, a Pb-free ferritic stainless steel excellent in machinability can be realized without sacrificing hot workability, cold workability, and corrosion resistance. The Pb content is preferably 0.1% by weight or less, and desirably, Pb is not contained except for inevitable impurities.

[0008] The (Ti, Zr) -based compound formed in the stainless steel of the present invention can be dispersed and formed in a ferrite-based matrix phase of the steel. Particularly, by dispersing the compound finely in the matrix, the machinability of steel can be further enhanced. In order to enhance the effect, it is observed in the polished sectional structure of the steel material (T
The average value of the dimensions of the (i, Zr) -based compound (expressed by the maximum interval between the circumscribed parallel lines when the circumscribed parallel lines are drawn while changing the position to the observed outline of the compound particles) is, for example, 0. The area ratio in the tissue is preferably about 1 to 10 μm, and the area ratio in the tissue is preferably about 0.1 to 10%.

The (Ti, Zr) -based compound has a composition formula (T
It can contain at least a compound represented by (i, Zr) ₄ (S, Se) ₂ C ₂ (hereinafter, referred to as carbonic acid / selenide). In this compound, Ti
And Zr may contain only one of them or both of them. Also, as for S and Se, either one may be contained, or both may be contained. By containing the compound represented by the composition formula, not only the machinability of the steel can be further improved, but also the corrosion resistance of the steel is improved.

The identification of (Ti, Zr) -based compounds in steel can be performed by X-ray diffraction (for example, diffractometer method) or electron probe microanalysis (EPMA). For example, (Ti, Zr) ₄ (S, Se) ₂ C ₂
Can be confirmed by whether or not a peak of the corresponding compound appears in the measurement profile by the X-ray diffractometer method. The area where the compound is formed in the microstructure is analyzed by EPMA for the cross-sectional microstructure of the steel material, and Ti, Zr, S, Se
Alternatively, it can be specified by comparing the two-dimensional mapping result of the characteristic X-ray intensity of C.

Hereinafter, the reason for limiting the content range of each component in the stainless steel of the present invention will be described. (1) C: 0.005 to 0.2% by mass C is an important element that is constituted as a compound for improving machinability. However, if the content is less than 0.005% by mass, a sufficient machinability-imparting effect is not exhibited, and if the content exceeds 0.2% by mass, a large amount of a single carbide which is not effective in improving machinability is generated. I do. The addition amount of C is more desirably 0.01 to 0.1% by mass. Note that (T
The addition amount of C may be appropriately adjusted according to the amount of the constituent element of the compound for improving machinability, such as an (i, Zr) -based compound, so that the effect of imparting machinability is optimized. The Pb-free type ferritic free-cutting stainless steel of the present invention is a general term for a material whose main phase becomes ferritic by annealing, and may be, for example, one that generates a martensitic phase by quenching (for example, cutting). Hardening treatment may be used after processing).

(2) Ni: 2.0% by mass or less Ni can be added as necessary because it is effective for improving corrosion resistance, particularly in a reducing acid environment. However, excessive addition reduces the stability of the ferrite phase and increases the cost.
The upper limit is 0% by mass. Note that the addition amount of Ni may be zero.

(3) Cr: 12.0-35.0% by mass Cr is an essential element for securing corrosion resistance.
2.0% by mass or more is added. On the other hand, excessive addition impairs hot workability and lowers toughness.
The upper limit is 0% by mass.

(4) The content of Ti is represented by WTi (mass%), Z
r content as WZr (mass%), WTi + 0.52W
Zr is 0.03 to 1.2% by mass: Ti and Zr are essential constituent elements of a (Ti, Zr) -based compound that plays a central role in the development of the machinability improving effect in the stainless steel of the present invention. If WTi + 0.52WZr is less than 0.03% by mass, (T
The formation amount of the (i, Zr) -based compound becomes insufficient, and a sufficient machinability improving effect cannot be expected. On the other hand, WTi + 0.52
Even when WZr becomes excessive, the machinability is rather lowered.

(5) 0.01 to 0.5% by mass of S;
At least one of 0.01 to 0.4% by mass of Se: S and Se are constituent elements of a compound effective for improving machinability, and both S and Se have clear effects. 0.01% by mass as the lower limit. On the other hand, excessive addition of these elements lowers hot workability, so the upper limit of S is 0.50% by mass and the upper limit of Se is 0.40% by mass.
When S and Se are co-added, the total content thereof is 0.1.
It is good to be 75 mass% or less. Also, S and S
As for e, it is desirable to add a necessary and sufficient amount to constitute a compound effective for improving machinability (for example, the above-mentioned carbosulfur / selenide). From this viewpoint, S and Se are preferred. Is preferably twice or more the C content (% by mass).

Next, the composition of the stainless steel of the present invention is S
i: 1% by mass or less, Mn: 2% by mass or less, P: 0.05
It is preferable that at least one of the following is satisfied: at most, at most: at most 2% by mass of Cu, at most 2% by mass of N, at most 0.05% by mass of N, and at most 0.03% by mass of O :. Hereinafter, the reason will be described.

(6) Si: 1% by mass or less Si can be added as a steel deoxidizing agent. However, if the content is excessively high, the hardness after solution heat treatment becomes high, not only disadvantageous to cold workability, but also to increase the formation amount of δ-ferrite and deteriorate hot workability of steel. The upper limit is set to 1.0% by mass. However, when a more remarkable effect is expected, the content is desirably 0.1% by mass or more. When the cold workability is particularly important, the content is preferably 0.5% by mass or less.

(7) Mn: 2% by mass or less Mn acts not only as a deoxidizing agent for steel but also has an effect of suppressing the formation of a δ-ferrite phase. Further, since a compound effective for machinability is generated by coexistence with S or Se, it is effective to add the compound when machinability is particularly important. However, if a more remarkable effect is expected, the content should be set to 0.1.
It is desirable that the content be 1% by mass or more. On the other hand, especially Mn
S significantly deteriorates corrosion resistance and impairs cold workability, so the upper limit is 2% by mass. In particular, when importance is attached to corrosion resistance and cold workability, it is desirable to limit the content to 0.4% by mass or less.

(8) P: 0.05% by mass or less P segregates at the grain boundary to increase the intergranular corrosion susceptibility and may cause a decrease in toughness. It is desirable to keep it. The P content is preferably as low as possible, and more preferably 0.03% by mass or less. However, excessive reduction may lead to an increase in cost.

(9) Cu: 2% by mass or less Cu can be added as necessary, since it is effective for improving corrosion resistance, particularly in an environment in contact with a reducing acid such as hydrochloric acid. However, when a more remarkable effect is expected, the content is desirably 0.3% by mass or more. Further, excessive addition deteriorates hot workability, so the upper limit is 2% by mass.

(10) N: 0.05% by mass or less N bonds to Ti and Zr, which are constituent elements of a compound effective for improving machinability, and is not effective for improving machinability. Since the nitride is formed, it should be suppressed as low as possible, and the upper limit is 0.05% by mass. Although it is a trade-off with the manufacturing cost, it is preferably 0.025% by mass or less, and more preferably 0.01% by mass or less.

(11) O: 0.03% by mass or less also combines with Ti and Zr, which are constituent elements of a compound effective for improving machinability, and is effective for improving machinability. Should be suppressed as low as possible to form an oxide which is not 0.03% by mass as the upper limit. Although it is a trade-off with the manufacturing cost, it is preferably set to 0.01% by mass or less.

Next, Mo or W can be added to the stainless steel of the present invention as needed in order to obtain higher corrosion resistance and strength. In this case, it is desirable that Mo be 0.1% by mass or more and W be 0.1% by mass or more so that those effects become clear. On the other hand, excessive addition impairs hot workability and raises the cost. Therefore, the upper limits are set to 4% by mass of Mo and 3% by mass of W.

In order to obtain higher machinability as necessary, it is possible to contain Te or Bi. In this case, in order to make those effects clear, Te is set to 0.1.
005 mass% or more, and Bi is desirably 0.01 mass% or more. On the other hand, excessive addition lowers hot workability, so that Te: 0.1% by mass and Bi: 0.2% by mass.
Is the upper limit.

In order to further enhance hot workability and machinability, Ca, Mg, B and REM (REM
May contain one or more metal elements classified as Group 3A in the periodic table of the elements) in a total amount of up to 0.01% by mass.
In order to make these effects clear, the total content is desirably 0.005% by mass or more.
Further, an excessive addition saturates the effect and conversely lowers the hot workability, so the upper limit is made 0.01% by mass.
It should be noted that it is easy to handle mainly using an element with low radioactivity as REM, and from this viewpoint, Sc, Y, La, Ce, Pr, Nd, Sm, Eu,
It is effective to use one or more selected from Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. In particular, it is desirable to use a light rare earth element, particularly La or Ce, from the viewpoints of more remarkable manifestation of the above effects and cost. However, trace amounts of radioactive rare earth elements (for example, Th and U) inevitably remaining in the rare earth separation process, etc.
May be contained. In addition, non-separable rare earths such as misch metal and dymium can be used from the viewpoint of reducing raw material costs.

On the other hand, the stainless steel of the present invention includes Nb,
It is also possible to add one or more of V, Ta and Hf. Nb, V, Ta, and Hf have an effect of forming carbonitrides to refine the crystal grains of the steel and increase the toughness. Therefore, each of Nb, V, Ta, and Hf can be added in a range up to 0.5% by mass.
In addition, in order to clarify the effect, the amount of each additive is 0.1.
It is desirably at least 01% by mass.

[0027]

EXAMPLES The following experiments were conducted to confirm the effects of the present invention. First, according to the composition shown in Table 1, ingots of 50 kg each were melted in a high-frequency induction furnace, and
Heat to 1050 to 1100 ° C and hot forge to 20
It was processed into a round bar of mm. At 800 ° C
After heating for 1 hour, it was air-cooled (annealing treatment) and subjected to each test.

[0028]

[Table 1]

In Table 1, the contents of X and Ti are represented by WTi
(% By mass) and the content of Zr as WZr (% by mass)
It shows the value of Ti + 0.52WZr. Y is a symbol representing a main compound found in the steel. “1” represents (Ti, Zr) ₄ (S, Se) ₂ C ₂ ,
"2" is (Ti, Zr) S, "3" is Ti (C, N)
And "4" means (Mn, Cr) S, respectively, and those with parentheses indicate that the confirmed amount is relatively smaller than that without parentheses (in terms of X-ray peak ratio). , １ ／ or less as compared to those with parentheses). In addition, identification of each compound is performed as follows.
That is, an appropriate amount of a test piece is taken out from each round bar, and the metal matrix portion is electrolyzed by using a methanol solution containing tetramethylammonium chloride and 10% acetylacetone as an electrolytic solution. Then, the insoluble compound contained in the steel was extracted and dried by filtering the dissolved electrolytic solution, and then analyzed by an X-ray diffractometer. The peak position of the diffraction profile appeared. The compound is specified from. The composition of the compound particles in the steel structure was separately analyzed by EPMA. From the two-dimensional mapping, it was confirmed that a compound having a composition corresponding to the compound confirmed by X-ray diffraction was formed. ing.

In Table 1, Nos. 1 to 17 are steel types of Examples corresponding to the present invention, and Nos. 18 to 22 are Comparative Examples. The number 18 is SUS430, and the number 19 is SUS4
30F respectively. Number 20 is WTi + 0.5
The value of 2WZr, and the number 21 is a steel type whose S content deviates from the scope of the present patent. In addition, the number 2
No. 2 is a steel type to which Pb is added.

The following tests were performed on each of the above test items. Hot workability test: Whether or not a defect such as a crack has occurred in the above hot forging is evaluated by visual observation. “O” indicates that no cracks due to processing were observed or only slight cracks were present, and “X” indicates that large cracks were observed during processing.

Evaluation of machinability: cutting force during turning,
Evaluated by the finished surface roughness and chip shape. Using a cermet as a tool, peripheral speed 150 m / min, depth of cut per rotation 0.1 mm, feed amount per rotation 0.05
The turning process is carried out in a dry manner in mm, and the main component force generated in the tool during the process is measured as cutting force (unit: N). The finished surface roughness is the arithmetic average roughness (Ra; μm) of the surface of the test material after processing, measured by a method specified in JIS: B0601. Furthermore, the shape of the chips during cutting was visually observed, and those with good friability were evaluated as “good”, and those with poor friability and connected chips were evaluated as “poor”.

Evaluation of cold workability: Based on critical compression strain at the time of compression test. Using a cylindrical test piece of φ6 × 11.5 mm, compression processing was carried out by a 600 t hydraulic press, and the critical compression strain (ln (H0 / H); H0 was the height of the initial test piece, and H was the limit at which no crack occurred. And In (X) represents the natural logarithm of X).

Evaluation of corrosion resistance: by salt spray test. As a test piece, a cylindrical shape having a size of φ10 × 50 mm was used, and after finishing the surface to # 400 with emery paper,
It is degreased and washed, and is exposed to a 5% by mass aqueous solution of sodium chloride at 35 ° C. for 96 hours. The evaluation was performed by visual inspection. If no rusting was observed, "A"
“B” indicates that spot-like spots were observed in several places, “C” indicates that red rust was observed in an area ratio of 5% or less, and “Red” was observed in an area exceeding 5%. Is "D". Table 2 shows the above results.

[0035]

[Table 2]

As is clear from Table 2, the products of Examples Nos. 1 to 17 have hot workability, machinability and coldness equal to or higher than those of Pb-containing steel No. 22 even if they do not contain Pb. Has workability and corrosion resistance. On the other hand, compared to the example product,
No. 18 equivalent to SUS430 has poor machinability and SUS4
30F No. 19 has low cold workability and corrosion resistance, and has WTi +
The number 20 with 0.52 WZr deviating from the scope of the present invention has poor machinability. In the case of No. 21 in which S deviated from the range of the present invention, the hot workability was remarkably low, and a sufficient amount of a sound test material was not obtained, so that the machinability, cold workability, and corrosion resistance were poor. Could not be evaluated.

FIG. 1 shows an X-ray diffraction profile of the extracted compound of Example No. 11 steel (using Cu: Kα ray). As shown by “○”, Ti ₄ S ₂ C ₂ which is a (Ti, Zr) ₄ (S, Se) ₂ C ₂ compound (the composition of the alloy is Zr and Se
) Are clearly formed. Also, it can be seen that a small amount of TiS is formed in addition to Ti ₄ S ₂ C ₂ . Further, FIG.
1 is an optical microscope observation image of the steel surface of No. 1 (400 magnification).
Times). It can be seen that the matrix is a ferrite phase and fine compound particles are dispersed and formed. In addition, EP
According to the MA surface analysis, most of the observed compound particles were T
it has been confirmed that i is ₄ S ₂ C _2.

The embodiment of the present invention has been described above.
This is merely an example, and the present invention can be carried out in other modified forms based on the knowledge of the parties without departing from the gist of the present invention.

[Brief description of the drawings]

FIG. 1 is a view showing an X-ray diffraction profile of steel No. 11 in an experiment performed in an example.

FIG. 2 is also a view showing an image observed by an optical microscope.

 ──────────────────────────────────────────────────の Continued on the front page (71) Applicant 000003713 Daido Steel Co., Ltd. 1-118 Nishiki, Naka-ku, Nagoya City, Aichi Prefecture (71) Applicant 000222048 Tohoku Special Steel Co., Ltd.Murata, Murata-cho, Shibata-gun, Miyagi Prefecture Nishigaoka 23 (74) The above three agents 100095751 Attorney Masanori Sugawara (72) Inventor Kiyohito Ishida 3-5-20 Uesugi, Aoba-ku, Sendai, Miyagi Prefecture (72) Inventor Katsunari Oikawa Shibata, Shibata-gun, Miyagi Prefecture 4-1-34, Nishifunako-cho (72) Inventor Tetsuya Shimizu 2-1410 Takashima, Tenpaku-ku, Nagoya City, Aichi Prefecture (72) Inventor Takayuki Inokuchi 18 Minamikamochi, Kagiya-cho, Tokai City, Aichi Prefecture 18 (72) Michio Okabe, Inventor 137 Asahi Momodai, Chita Pref., Japan

Claims (8)

[Claims] 1. Ni is not contained, or even if Ni is contained, the content is set to 2.0% by mass or less, Cr of 12 to 35% by mass, and 0.005 to 0.2% by mass.
The content of Ti is WTi (mass%), and the content of Zr is WZr.
(Mass%), WTi + 0.52WZr is 0.03-
At least one of Ti and Zr is contained so as to be 1.2% by mass, and at least one of 0.01 to 0.5% by mass of S and 0.01 to 0.4% by mass of Se. And further contains at least one of Ti and Zr as a metal element component, and as a binding component to the metal element component, C as an essential component, and at least one of S and Se. A Pb-free ferritic free-cutting stainless steel characterized in that a contained (Ti, Zr) -based compound is formed in the structure. 2. Si: 1% by mass or less, Mn: 2% by mass or less, P: 0.05% by mass or less, Cu: 2% by mass or less,
The Pb-free ferritic free-cutting stainless steel according to claim 1, wherein N: 0.05% by mass or less and O: 0.03% by mass or less. 3. Mo content of 0.1 to 4% by mass and 0.1 to 3% by mass.
2. The composition according to claim 1, which contains at least one of W and W.
Or Pb-free ferritic free-cutting stainless steel according to 2. 4. Te of 0.005 to 0.1% by mass;
The Pb-free ferritic free-cutting stainless steel according to any one of claims 1 to 3, which contains at least one of 0.01 to 0.2 mass% of Bi. 5. Ca, Mg, B and REM (where R
EM contains a total of 0.005 to 0.01% by mass of at least one selected from one or more metal elements classified as Group 3A in the periodic table of elements. 4. The Pb-free ferritic free-cutting stainless steel according to any one of 4. 6. An element selected from Nb, V, Ta and Hf
The Pb-free ferritic free-cutting stainless steel according to any one of claims 1 to 5, wherein the Pb-free type ferritic free-cutting stainless steel contains 0.01 to 0.5% by mass of at least one kind. 7. The (Ti, Zr) -based compound is dispersed and formed in a ferrite-based matrix phase.
7. A Pb-free type ferritic free-cutting stainless steel according to any one of the above-mentioned items. 8. The method according to claim 1, wherein the (Ti, Zr) -based compound contains at least a compound represented by a composition formula (Ti, Zr) ₄ (S, Se) ₂ C ₂ . The described Pb-free ferritic free-cutting stainless steel.