JP4305137B2 - Ferritic free-cutting stainless steel with excellent surface finish roughness and outgas resistance - Google Patents

Description

  The present invention relates to a ferritic free-cutting stainless steel excellent in surface finish roughness and outgas resistance.

  In recent years, ferritic stainless steel, which provides high corrosion resistance at a relatively low cost, has been widely used as a component material in order to make maintenance free of computers, peripheral devices, and other weak electrical products. Especially for parts that require precise finishing to ensure dimensional accuracy and parts with complex shapes with large machining allowances, emphasis is placed on improving machinability, so the content of free machinability imparting elements tends to increase. In addition, these elements are also used in combination without being added alone.

  As the machinability imparting element, S, Pb, Se, Bi, Te, Ca and the like are known. Among these, Pb is gradually shunned in recent years when the concern for environmental protection is increasing on a global scale, and the number of devices and parts that limit its use is increasing. Therefore, a material using S as a main component of the machinability improving element is considered as an alternative material (Patent Documents 1 to 4). These mainly produce Mn-based sulfides such as MnS, and enhance the machinability and grindability by the stress concentration effect during chip formation on the sulfides and the lubricating action between the tool and the chips.

JP-A-56-16653 JP-A-62-258955 JP 54-17567 A Japanese Patent Laid-Open No. 10-46292

  However, when MnS-based sulfides are generated, the corrosion resistance and outgas resistance of the alloy are deteriorated. Deterioration of outgas resistance means that, when exposed to the atmosphere, the S component contained in the alloy material is released as a sulfur-containing gas and is likely to cause corrosion of the peripheral circuit. Such sulfur-containing gases are particularly problematic in components such as computer peripherals that are often used in a sealed state, such as hard disk drives (HDDs).

  An object of the present invention is to provide a ferritic free-cutting stainless steel having excellent surface finish roughness, corrosion resistance and outgas resistance while having good machinability.

Means for solving the problems / effects of the invention

In order to solve the above problems, in the ferritic free-cutting stainless steel of the present invention,
In mass%, C: 0.015 % or less, Si: 0.05 to 1.0%, Mn: 2.0% or less, P: 0.050% or less, S: 0.05 to 0.50%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 9.0-25.0%, Mo: 4.0% or less, Ti: 0.065-2.0%, O: 0.0. 0150% or less, N: 0.020% or less, Al: 0.001 to 0.100%, the balance is made of Fe and inevitable impurities,
When the Ti content of the steel is [Ti], the S content is [S], and the Mn content is [Mn], the following formulas (1) and (2) are satisfied, and
When the Ti content of the sulfide produced in the steel structure is WTi, the Cr content is WCr, and the Mn content is WMn, the following formula (3) is satisfied.
[Ti] ≧ 1.3 × [S] (1) Formula [Mn] / [Ti] ≦ 3 (2) Formula (WTi + WCr)> 2 × WMn (3) Formula

  Usually, S tends to form sulfide with Mn which is a component of steel. However, as described above, Mn-based sulfides cause deterioration of the corrosion resistance and outgas resistance of the alloy. Therefore, in the present invention, by adding Ti, Ti-based sulfides such as TiS, not Mn-based sulfides, are generated in the steel structure, thereby improving corrosion resistance and outgas resistance. In addition, since the Ti-based sulfide takes a spherical shape and is finely dispersed, the ferritic free-cutting stainless steel of the present invention has good machinability, and further, the inclusions fall off, particularly precision cutting. Excellent surface finish roughness. The same effect can be obtained when the sulfide contains Cr. In the present specification, the “A” -based sulfide refers to a sulfide whose component (element) having the highest mass content among the components bonded to S is “A”. That is, in Ti-based sulfide, more Ti is bonded to S than other elements (Mn and the like).

Next, the reasons for limiting the claims of the present invention will be described.
C (carbon): 0.015% or less If the content of C is excessive, a large amount of a single carbide is generated to inhibit improvement of machinability, so the upper limit is made 0.015%.

Si (silicon): 0.05 to 1.0%
Si is added as a deoxidizer for steel. In order to obtain the effect, 0.05% or more is required. However, if the content is excessive , the hot workability of the steel is deteriorated, so the upper limit is made 1.0%. A desirable range in which hot workability is emphasized is 0.05 to 0.5%.

Mn (manganese): 2.0% or less In addition to acting as a deoxidizer for steel, Mn produces Mn-based sulfides (MnS) and has an effect of improving machinability. However, since Mn-based sulfide (MnS) deteriorates corrosion resistance, the upper limit is made 2.0%. When emphasizing corrosion resistance in particular, the content is made 1.0% or less. More preferably, it is 0.5% or less.

P (phosphorus) 0.050% or less P segregates in grain boundaries, in addition to increase the susceptibility to intergranular corrosion, rather low better is desirable for lowering the toughness, 0.050% or less. Desirably, it is good to set it as 0.030% or less.

S (sulfur): 0.05 to 0.50%
S is a constituent element of sulfide that improves machinability, and 0.05% is required to obtain the effect. However, if the content is excessive, the hot workability is lowered, so the upper limit is made 0.50%. It is preferably 0.15 to 0.40% due to the balance between improvement of machinability and reduction of hot workability.

Cu (copper): 2.0% or less Cu may be added as necessary because it is effective for improving the corrosion resistance, particularly the corrosion resistance in a reducing acid environment. However, excessive addition degrades hot workability, so the upper limit is made 2.0%. Desirably 1.0% or less

Ni (nickel): 2.0% or less An element necessary for supplementing corrosion resistance that is not sufficient only by containing Cr. However, excessive addition increases the cost, so the upper limit is made 2.0%. Further, it is preferably made 1.0% or less in view of sufficient corrosion resistance and blending cost.

Cr (chromium): 9.0 to 25.0%
Cr is an element that improves corrosion resistance, and in order to obtain the effect, 9.0% or more is added. On the other hand, if the content is excessive, the cost is not only high, but the hot workability is lowered, so the upper limit is made 25.0%. Moreover, it is desirable to set it as 13.0-21.0% from the balance of sufficient corrosion resistance and mixing | blending cost.

Mo (molybdenum): 4.0% or less Mo can further improve the corrosion resistance and strength, but excessive addition harms hot workability and increases the cost, so the upper limit is 4.0%. And Considering the increase in cost, it is desirably 1.5% or less.

Ti (titanium): 0.065 to 2.0%
Ti is an element necessary for producing a Ti-based sulfide that improves machinability, and 0.065% or more is necessary to obtain the effect. On the other hand, if the content is excessive, the cost is increased, so the upper limit is made 2.0%. Moreover, in order to obtain more sufficient machinability, it is desirably 0.075 to 2.0%.

O (oxygen): 0.0150% or less O is combined with Ti, which is a constituent element of a compound effective for improving machinability, and forms an oxide that does not contribute to improvement of machinability. The upper limit is made 0.0150%. In order to ensure the balance between the manufacturing cost and the effective Ti amount necessary for the formation of the Ti-based sulfide, the content is preferably 0.0080% or less, and more preferably 0.0050%.

N (nitrogen): 0.020% or less N combines with Ti, which is a constituent element of a compound effective for improving machinability, and forms a nitride that does not contribute to the improvement of machinability. The upper limit is 0.020%. In order to ensure the balance between the manufacturing cost and the effective Ti amount necessary for forming the Ti-based sulfide, it is preferably 0.010% or less, and more preferably 0.006% or less.

Al (aluminum): 0.001 to 0.100%
Al is added as a steel deoxidizer, but if the content is excessive, an oxide harmful to machinability is formed, so the upper limit is made 0.100%. Desirably, it is 0.050% or less.

[Ti] ≧ 1.3 × [S] (1) Formula Suppresses the generation of Mn-based sulfides (MnS) that degrade corrosion resistance and outgas resistance, and fixes all S in the steel structure to Ti Therefore, the Ti content is set to 1.3 times or more the S content. More desirably, [Ti] ≧ 1.5 × [S], that is, the Ti content should be 1.5 times or more the S content. In addition, [] points out component content in steel.

[Mn] / [Ti] ≦ 3 (2) In order to suppress the generation of Mn-based sulfides (MnS) that deteriorate the corrosion resistance and outgas resistance and generate Ti-based sulfides (in the sulfides) In order to lower the Mn content and raise the Ti content), the Mn content is set to not more than three times the Ti content.

(WTi + WCr)> 2 × WMn (3) Formula In order to improve the corrosion resistance and outgas resistance of steel, the sum of Ti content and Cr content is 2 in the sulfide. It is preferable to exceed twice. “W” refers to the component content in the sulfide.

Next, in the ferritic free-cutting stainless steel of the present invention, in addition to the components defined in claim 1, in terms of mass%, Se: 0.01 to 0.30%, Bi: 0.01 to 0.30% or less Any one of them can be contained.

Since Se (selenium) and Bi (bismuth) can further improve machinability, they can be added as necessary. However, excessive addition reduces the hot workability, so the upper limits of Se are 0.30% and Bi are 0.30% . In order to sufficiently obtain the machinability improving effect, each component is desirably added in an amount of 0.01% or more.

Next, in the ferritic stainless steel of the present invention, in addition to the components of claim 1 or claim 2, by mass%, Mg: 0.02% or less, B: 0.02% or less, REM: 0.02% Hereinafter, any one or more of V: 0.50% or less, Nb: 0.50% or less, Ta: 0.50% or less can be contained.

Mg (magnesium), B (boron), and REM (one or more of rare earth elements) can improve the hot workability of steel, and can be added as necessary. . However, excessive addition will saturate the effect and conversely reduce hot workability. Therefore, the upper limits of Mg are 0.02%, B is 0.02%, and REM is 0.02% .

  Nb (niobium), V (vanadium), and Ta (tantalum) are added in the range of 0.50% or less because they have the effect of forming carbonitrides to refine the crystal grains of the steel and increasing the toughness. be able to.

In order to confirm the effect of the present invention, the following experiment was conducted.
First, 50 kg steel ingots having the component compositions shown in Table 1 were melted in a high frequency induction furnace and then cooled to prepare ingots. Each ingot was heated to 1050 to 1100 ° C. and processed into a 20 mm round bar by hot forging. These round bars were further heated at 800 ° C. for 1 hour, then air-cooled (annealing treatment), and subjected to each test. The test results are shown in Table 1.

(1) Machinability evaluation The machinability evaluation was evaluated based on the workpiece outer diameter change amount, the finished surface roughness, and the chip shape after cutting.
Cutting is performed by wet treatment with water-insoluble oil at a peripheral speed of 100 mm / min, a cutting amount of 0.10 mm per revolution, and a feed rate of 0.01 mm / rev per revolution using a carbide coating tool under the following conditions. Cutting was performed. Cutting was performed on 50 samples, and the outer diameter and surface tool edge wear after cutting of the test piece were measured.
The outer diameter change amount is the change amount from the initial workpiece. The criteria for determining the amount of change were “small” when the lateral flank wear was less than 50 μm, “medium” when 50 μm or more and 100 μm or less, and “large” when exceeding 100 μm.
The finished surface roughness is an arithmetic average roughness (Ra: μm) of the workpiece surface after processing measured by a method defined in JIS-B0601.
Further, the shape of the chips was visually observed, and those having good crushability where the chips were about 10 mm or less were represented as “good”, and those other than those having poor crushability were represented as “poor”.

(2) Corrosion resistance The corrosion resistance evaluation test was conducted by a wet test. As a test piece, a cylindrical shape having a diameter of 10 mm and a height of 50 mm was used. The surface was polished to count # 400 with emery paper, degreased and washed, and each sample was heated to a temperature of 50 ° C. and a humidity of 98% RH. It preserve | saved for 98 hours in a humid atmosphere, and the presence or absence of rusting was seen by visual appearance determination.

(3) Outgas resistance,
The outgas resistance was evaluated by defining the amount of S generated. Specifically, a test piece having a rectangular parallelepiped shape with dimensions of 15 mm in length, 3 mm in width, and 25 mm in thickness, and whose entire surface is polished with emery paper with count # 400 is used. Then, put the test piece, silver foil (dimensions: vertical 0.1 mm, horizontal 5 mm, thickness 10 mm, purity: 99.9% or more) and 0.5 cc pure water in a sealed container having a volume of 250 cc, The temperature in the container was maintained at 85 ° C. for 20 hours. The silver foil functions as a getter when a gas containing S is generated, and when the adsorbed S component increases, the surface of the silver foil turns black due to the formation of silver sulfide. Therefore, the discoloration on the surface of the silver foil was visually confirmed, and the evaluation was made in three stages, with “A” indicating no discoloration, “B” indicating a slight discoloration, and “C” indicating a clear discoloration. A sample having a determination result of B was considered good in outgas resistance.

  According to the test results of Table 1, it can be seen that all the inventive steels according to the present invention have excellent machinability and surface roughness, and are excellent in corrosion resistance and outgas resistance.

Next, the composition analysis of the sulfide produced | generated in the steel structure was performed with respect to a part of invention steel and comparative steel by the electron beam probe microanalysis (EPMA) method. The analysis results are shown in Table 2. According to this, in the inventive steel, the sulfide composition satisfies the formula (3), and it can be seen that the invention steel is a sulfide having a high Ti content. On the other hand, in comparative steel 4 in which the composition of the steel does not satisfy the formulas (1) and (2), and in comparative steel 7 that does not satisfy the formula (1), Ti and Mn are contained in the sulfide to the same extent. The composition of sulfide does not satisfy the formula (3). As is clear from Table 1, the comparative steels 4 and 7 having such sulfides are inferior in corrosion resistance and outgas resistance.
[Ti] ≧ 1.3 × [S] (1) Formula [Mn] / [Ti] ≦ 3 (2) Formula (WTi + WCr)> 2 × WMn (3) Formula

Claims (3)

In mass%, C: 0.015 % or less, Si: 0.05 to 1.0%, Mn: 2.0% or less, P: 0.050% or less, S: 0.05 to 0.50%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 9.0-25.0%, Mo: 4.0% or less, Ti: 0.065-2.0%, O: 0.0. 0150% or less, N: 0.020% or less, Al: 0.001 to 0.100%, the balance is made of Fe and inevitable impurities,
When the Ti content of the steel is [Ti], the S content is [S], and the Mn content is [Mn], the following formulas (1) and (2) are satisfied, and
Surface finish roughness and outgas resistance characterized by satisfying the following formula (3) when Ti content of the sulfide formed in the steel structure is WTi, Cr content is WCr, and Mn content is WMn. Excellent ferritic free-cutting stainless steel.
[Ti] ≧ 1.3 × [S] (1) Formula [Mn] / [Ti] ≦ 3 (2) Formula (WTi + WCr)> 2 × WMn (3) Formula In addition to the components defined in claim 1, the composition contains any one of Se: 0.01 to 0.30% and Bi: 0.01 to 0.30% or less in mass%. Ferritic free-cutting stainless steel with excellent surface finish roughness and outgas resistance. In addition to the components of Claim 1 or Claim 2, in mass%, Mg: 0.02% or less, B: 0.02% or less, REM: 0.02% or less, V: 0.50% or less, Nb: A ferritic free-cutting stainless steel excellent in surface finish roughness and outgas resistance, characterized by containing one or more of 0.50% or less and Ta: 0.50% or less .