JP5082389B2 - Austenitic free-cutting stainless steel - Google Patents

Description

  The present invention relates to a free-cutting stainless steel suitable for use as a material for equipment or parts that require excellent machinability, corrosion resistance, cold and hot workability.

  Conventionally, stainless steel, for example, 300 stainless steels such as JIS SUS304, SUS316, SUS303 or similar steels, or SUS836L, SUS890L, SUSXM7, SUSXM15J1, etc. are widely used as materials for equipment or parts that require corrosion resistance. It has been.

  The present invention relates to stainless steel that can be used in applications where these stainless steels are used, and in particular, shafts for parts, industrial equipment, OA equipment, pumps, ships, etc. that are subjected to cold forging in addition to machinability. The present invention relates to stainless steel that can be suitably used for various types.

  In order to improve the productivity of equipment, parts, etc. where cutting is performed, free-cutting stainless steel to which elements that improve machinability such as S, Pb, Se, Bi are added is used for the stainless steel materials used in these. Widely used in the past.

Usually, in order to impart machinability to stainless steel, S is generally added.
The added S forms MnS in the steel, and this MnS serves to enhance the machinability of the steel.
When higher machinability is required, such as when precise finishing is performed, it is common to increase the amount of S added, but this has a limit, and in this case other than S alone A composite addition of a machinability improving element is performed.

  MnS formed in steel is not very effective in improving machinability when its size is small, so the amount of S added as a straightforward method for forming MnS larger than a certain level in steel is limited. Many techniques have been used.

However, the addition of a large amount of S simultaneously causes deterioration of corrosion resistance and mechanical properties.
In addition, since the MnS inclusions formed in the steel are soft inclusions themselves, they are deformed during the rolling process and become elongated in the direction of rolling in a string shape, causing anisotropy in the mechanical properties. Cause.
Specifically, the impact characteristics of the MnS in the direction perpendicular to the extending direction are deteriorated and workability such as cold working is also lowered.

In addition, the following Patent Document 1 discloses an invention relating to a free-cutting stainless steel excellent in outgas characteristics, and the following Patent Document 2 discloses a highly corrosion-resistant free-cutting stainless steel excellent in surface finish, and further described in Patent Document 3 below. Each show inventions on free-cutting stainless steel.
However, these patent documents do not disclose characteristic configuration control of MnS in the present invention, and are different from the present invention.

JP 2001-11581 A JP 2001-98352 A JP 2002-38241 A

  The present invention is based on the above circumstances, and can suppress the addition amount of S while reducing the machinability of steel by forming MnS of the required size in steel, and at the time of rolling. In addition, the present invention has been made for the purpose of providing an austenitic free-cutting stainless steel that can satisfactorily solve the problem of causing anisotropy in the properties of the steel by stretching MnS in the rolling direction.

Thus, the content of claim 1 is mass%, C: 0.01 to 0.20%, Si: 0.10 to 2.00%, Mn: 0.80 to 2.50%, P: ≤0.10%, S: 0.10 to 0.40%, Ni: 5.0 to 15.0%, Cr: 15.0 to 25.0%, Te: 0.01 to 0.10%, B: 0.003 to 0.010%, O: 0.006 to 0.030%, N: ≤ 0.050%, balance Fe and inevitable impurities And the following expressions (1) to (3) are satisfied.
3.0 ≦ [Mn] / [S] ≦ 15.0 ・ ・ ・ Formula (1)
0.10 ≦ [Te] / [S] ≦ 0.50 ・ ・ ・ Formula (2)
10 ≦ [S] / [O] ≦ 40 ・ ・ ・ Formula (3)

  A second aspect of the present invention is characterized in that, in the first aspect, Cu is contained in mass% and Cu: 0.01 to 4.0%.

  A third aspect of the present invention is characterized in that in any one of the first and second aspects, Mo is further contained in mass% and Mo: 0.01 to 3.0%.

Claim 4 contains any one or two of Pb and Bi in mass%, Pb: 0.03 to 0.30%, Bi: 0.01 to 0.30% in any one of claims 1 to 3. It is characterized by.

  Claim 5 is any one of claims 1 to 4, and any one or more of Ca, Mg, and REM in mass%, Ca: 0.0001 to 0.05%, Mg: 0.0001 to 0.02% , REM: 0.0001 to 0.02%.

  A sixth aspect of the present invention is characterized in that in any one of the first to fifth aspects, W is further contained in mass% and W: 0.01 to 2.0%.

  In a seventh aspect of the present invention, according to any one of the first to sixth aspects, any one or more of Nb, Ta, and V is in mass%, Nb: 0.01 to 0.50%, Ta: 0.01 to 0.50% , V: 0.01 to 0.50% is contained.

Effects and effects of the invention

The present invention makes an austenitic stainless steel having the above component composition, and in particular, by adding S, O, Mn, Te and optimizing the relative proportions of these additive elements, MnS generated in the steel. The point of controlling the form of inclusions is the gist of the invention.
In the case of conventional austenitic free-cutting stainless steel, it has been dealt with by increasing the amount of S added as described above in order to enhance machinability.

This is based on the following reason.
If the amount of S added to the steel is small, it will be dispersed in the steel as fine inclusions when forming MnS inclusions in the steel.
Such small MnS inclusions do not work effectively to improve machinability.
Therefore, by increasing the amount of S added, the MnS inclusions generated in the steel were increased beyond a certain level, and the machinability was enhanced by the large MnS inclusions.

On the other hand, in the present invention, by containing a predetermined amount of O as a component of steel, MnS inclusions are formed in this O as a core, and this is grown and grown to increase the machinability. MnS.
Therefore, in the present invention, MnS having a size effective for machinability can be effectively formed in steel with a small amount of S added.
In the present invention, Te added to the steel binds around the MnS inclusions, which exerts a pinning effect, and when rolling the steel, the MnS inclusions extend long in the rolling direction, It works to prevent becoming.
That is, according to the present invention, due to the pinning effect of the added Te, when the steel is rolled, the MnS inclusions are not stretched long in the shape of a string, but are retained in the shape of a spindle. It is possible to effectively suppress the occurrence of anisotropy. That is, the occurrence of a characteristic difference between the rolling direction and the direction perpendicular thereto is effectively suppressed.

  Therefore, according to the present invention, an austenitic stainless steel having good machinability, toughness, cold workability, hot workability and corrosion resistance can be obtained.

[Reason for limiting ingredients]
Next, the reasons for limiting the chemical components and the like in the present invention will be described in detail below.
C: 0.01-0.20%
C is an element that dissolves in the substrate and increases the hardness. Therefore, 0.01% or more is contained in order to obtain sufficient hardness.
However, if it exceeds 0.20%, a large amount of a single carbide which is not effective for improving the machinability is generated, or corrosion resistance is deteriorated, so the content is limited to 0.20% or less. A desirable range is 0.05 to 0.10%.

Si: 0.10 to 2.00%
Si is added as a steel deoxidizer. Its content is 0.10% or more. However, if the content is excessive, the amount of δ-ferrite increases and the hot workability of the steel deteriorates, so the upper limit is made 2.00%. In the case where importance is attached to hot workability, a desirable range is 0.50% or less.

Mn: 0.80-2.50%
In addition to acting as a deoxidizer for steel, Mn must be added in an amount of 0.80% or more in order to produce a compound effective for machinability when coexisting with S. On the other hand, especially MnS greatly deteriorates the corrosion resistance and inhibits the cold workability, so the upper limit is made 2.50%. In particular, when emphasizing hot workability and corrosion resistance, it is 1.50 to 2.00%.

P: ≤0.10%
P is segregated at the grain boundaries to increase the intergranular corrosion susceptibility and lower toughness. However, it is desirable that the P content be lower. Preferably it is 0.050% or less.

S: 0.10 to 0.40%
S is a constituent element of a compound effective for improving machinability, and the lower limit is 0.10% at which the effect becomes clear. Excessive addition reduces hot workability, so 0.40% is made the upper limit. Desirably, the content is 0.13 to 0.20% in balance with a decrease in hot workability.

Ni: 5.0-15.0%
It is an element necessary to compensate for insufficient corrosion resistance only by containing Cr. However, excessive addition increases the cost, so the upper limit is 15.0%. However, when the effect of addition to sufficient corrosion resistance is obtained, the content is desirably 12.0% or less in view of the blending cost.

Cr: 15.0-25.0%
Cr is an element that improves corrosion resistance, and its content is 15.0 to 25.0%. If it is less than 15.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if it exceeds 25.0%, not only the cost is increased but also hot workability is lowered. A desirable range is 17.0 to 20.0% where sufficient corrosion resistance is obtained and the cost can be suppressed.

Te: 0.01-0.10%
Te is an element effective for improving machinability, and the lower limit is 0.01% at which the effect becomes clear. Excessive addition reduces hot workability, so the upper limit is made 0.10%. It is preferably 0.05% or less due to the balance between machinability and reduction in hot workability.

B: 0.003-0.010%
B is an element effective for improving the hot workability, and the lower limit is 0.003% at which the effect becomes clear.
Excessive addition causes an increase in cost, so the upper limit is made 0.010%. Desirably, the balance between hot workability and cost is considered to be 0.004 to 0.008%.

O: 0.006 to 0.030%
Add 0.006% or more because it is an element involved in the formation of sulfides required to improve machinability. However, excessive addition forms an oxide that is not effective in improving machinability, so 0.030% is made the upper limit. This is a balance with the manufacturing cost, but is preferably 0.020% or less.

N: ≤ 0.050%
N forms nitrides that are not effective for improving machinability, so N should be suppressed as low as possible, and the upper limit is 0.050%. This is a balance with the manufacturing cost, but is preferably 0.030% or less.

Cu: 0.01-4.0%
Cu is effective for improving the corrosion resistance, particularly the corrosion resistance in a reducing acid environment, and may improve the cold workability. However, excessive addition degrades hot workability, so 4.0% is made the upper limit. Desirably, it is 3.0% or less.

Mo: 0.01-3.0%
Since Mo can further improve the corrosion resistance and strength, it may be added as necessary. However, excessive addition impairs hot workability and increases costs, so the upper limit is made Mo: 3.0%. Considering the increase in cost, it is desirable to set it to 1.0% or less.

Pb: 0.03-0.30%
Pb is an element effective for improving the machinability and may be added as necessary. The lower limit is 0.03% where the effect becomes clear. However, excessive addition reduces hot workability, so 0.30% is made the upper limit. The amount is preferably 0.10 to 0.25% due to the balance between the amount effective for improving the machinability and the decrease in hot workability.

Bi: 0.01-0.30%
Since Bi can further improve the machinability, Bi may be added as necessary. The lower limit is set to 0.01% at which the machinability improving effect is recognized. However, excessive addition reduces hot workability, so 0.30% is made the upper limit.

Ca: 0.0001 to 0.05%
Mg: 0.0001 to 0.02%
REM: 0.0001 to 0.02%
Since Ca, Mg, and REM are effective elements for improving the hot workability of steel, 0.0001% or more may be added as necessary. However, excessive addition will saturate the effect and conversely reduce hot workability, so the upper limit is set to Ca: 0.05%, Mg: 0.02%, and REM: 0.02%, respectively.

W: 0.01-2.0%
Since W can further improve the corrosion resistance and strength, 0.01% or more may be added as necessary. However, excessive addition harms hot workability and increases costs, so the upper limit is made 2.0%.

Nb: 0.01-0.50%
Ta: 0.01-0.50%
V: 0.01 to 0.50%
Addition of Nb, V, and Ta has the effect of forming carbonitrides to refine the crystal grains of the steel and increasing the toughness, so it may be added in the range of 0.01 to 0.50%.

3.0 ≦ [Mn] / [S] ≦ 15.0 ・ ・ ・ Formula (1)
In the present invention, Mn and S need to be in an appropriate ratio. When Mn is less than S and [Mn] / [S] is smaller than 3.0, an excess of S relative to Mn is combined with Cr to produce CrS.
MnS is itself a soft inclusion and has deformability, so it can be deformed together with its surrounding matrix, but CrS is hard and only the periphery is deformed without deformation of CrS during processing. As a result, the manufacturability is deteriorated, and the site where CrS is formed tends to be the starting point of cracking, resulting in a decrease in hot workability.

On the other hand, if the amount of Mn is too large, S is insufficient and good machinability cannot be obtained, and corrosion resistance deteriorates due to excessive Mn.
Therefore, in the present invention, [Mn] / [S] is set to 15.0 or less.
In addition, the ratio of Mn and S is a ratio in mass%. The same applies to the ratio of Te to S and the ratio of S to O below.

0.10 ≦ [Te] / [S] ≦ 0.50 ・ ・ ・ Formula (2)
In order to secure the pinning effect by Te, the amount of Te must be a certain amount or more in relation to S. Here, [Te] / [S] must be 0.10 or higher.
On the other hand, if the amount of Te is too large in relation to S, excess Te and Mn will be combined, and if the inclusions that Te and Mn are combined increase, the melting point will be low and the hot workability will deteriorate. I will let you.
Therefore, in the present invention, [Te] / [S] needs to be 0.50 or less.

10 ≦ [S] / [O] ≦ 40 ・ ・ ・ Formula (3)
O also needs a certain amount or more in relation to S. For example, if the amount of O is small in relation to S, MnS cannot be formed satisfactorily with O as a nucleus, and desired machinability cannot be obtained. Therefore, in the present invention, [S] / [O] needs to be 40 or less.
On the other hand, if the amount of O becomes excessive in relation to S, O forms a hard oxide and adversely affects the machinability.
Therefore, in the present invention, [S] / [O] needs to be 10 or more.

  Ingots were prepared by melting 50 kg steel ingots of the steel types having the composition shown in Table 1 in a high frequency induction furnace and then cooling. Each ingot was heated to 1000 to 1200 ° C., and was processed into 60 mm and 20 mm round bars and 60 mm × 30 mm square bars by hot forging. These steel bars were further heated at 1050 ° C. for 1 hour, then cooled with water (solution heat treatment) and subjected to the following tests.

1) Hot workability As shown in Table 2, hot workability was evaluated based on the degree of occurrence of flaws during forging. The case where there was no wrinkle was rated as “◯”, the case where slight wrinkles that could be cut with a grinder were generated, and the case where large wrinkles were generated were marked as “X”.
Further, a tensile test was performed at 1000 ° C. in a hot high-speed tensile test, and the drawing value was measured.

2) Machinability (turnability, drill drillability)
In the machinability evaluation, the turning performance was evaluated by the amount of tool wear after machining and the chip shape, and the drill drillability was evaluated by the cutting speed and chip shape at which the tool life (unable to drill) was 5000 mm.
Turning was performed in a dry manner using a carbide tool (Uti20T) at a peripheral speed of 150 mm / min, a cutting depth of 1.0 mm per rotation, and a feed rate of 0.2 mm / rev per rotation.

The amount of tool wear in turning is the average side flank wear amount, the chip shape is visually observed, the one with good crushability is `` good '', the one crushed to about several turns is `` medium '', Those with poor crushability and connected chips are indicated as “poor”.
As shown in Table 3, the amount of tool wear in this turning process was judged as large, medium, or small (smaller is better).

On the other hand, in drilling, using a high-speed drill SKH51 (Φ5.0 mm), the cutting depth is fluctuated at a hole depth of 15 mm (blur hole) and the feed rate per rotation is 0.07 mm / rev, and the tool life distance ( The cutting distance until breakage) was measured.
In this drilling process, the above square bar was used as a test material. For other tests, the above round bar is used.

In this drilling process, the chip shape is visually observed for initial chips at a cutting speed of 20 m / min. “Good” indicates that the crushability is good, “Medium” indicates that the crushed material is several turns, and crushes. Those with poor quality and connected chips are indicated as “poor”.
Here, as shown in Table 4, the cutting speed was determined at a low speed, a medium speed, and a high speed (the higher the speed, the better).

3) Corrosion resistance The corrosion resistance evaluation test was conducted by a salt spray test in accordance with JIS Z 2371.
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 held for 96 hours in salt spray. In the evaluation, the presence or absence of rusting was observed by visual inspection after 96 hours.

  Inventive steels 1 to 15 were compared with the comparative steel 2 in which “Excellent” is equivalent to “◎”, and inferior one was “△”.

4) Cold workability The cold workability was evaluated by using a cylindrical test piece of φ12 × 18 mm, conducting a uniaxial compression test with a 600 t hydraulic press, and evaluating with a limit compression ratio at which no cracks occurred.

5) Toughness (anisotropic)
Toughness shows the impact value measured by the Charpy impact test of each direction of L (rolling direction) and T (perpendicular to the rolling direction) after processing the specimen after the heat treatment.

These results are shown in Table 2.
In Tables 1 and 2, comparative steel 1 is conventional SUS304, and comparative steel 2 is SUS303.
SUS304 to which S is not added has poor machinability. SUS303 of Comparative Steel 2 to which S is added has lower toughness in both the L direction and the T direction than SUS304 of Comparative Steel 1, and is particularly low in toughness in the T direction, and has a remarkable impact property. Showing sex.

Comparative steel 3 has a high content of O, and neither turning ability nor drilling ability is sufficient.
In comparison steel 4, the amount of S is small and the turning ability and drilling ability are also insufficient.

The comparative steel 5 has a small amount of Te added, has a large difference in impact value between the L direction and the T direction, and exhibits anisotropy.
Comparative steel 6 has a small amount of O, and is insufficient in both turning and drilling performance.

The comparative steel 7 has a large amount of S exceeding the upper limit of the present invention, and both the machinability and the drillability are good, but both the critical compression ratio and the impact value are low.
Further, the comparative steel 8 has an insufficient amount of Mn, and as a result, the turning and drilling properties are insufficient.

On the other hand, each of the inventive steels has good results in all of hot workability, machinability, that is, machinability, that is, drillability, corrosion resistance, critical compressibility, and toughness.
Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

Claims (7)

% By mass
C: 0.01-0.20%
Si: 0.10 to 2.00%
Mn: 0.80-2.50%
P: ≤0.10%
S: 0.10 to 0.40%
Ni: 5.0-15.0%
Cr: 15.0-25.0%
Te: 0.01-0.10%
B: 0.003-0.010%
O: 0.006 to 0.030%
N: ≤ 0.050%
An austenitic free-cutting stainless steel having a composition of remaining Fe and inevitable impurities and satisfying the following formulas (1) to (3).
3.0 ≦ [Mn] / [S] ≦ 15.0 ・ ・ ・ Formula (1)
0.10 ≦ [Te] / [S] ≦ 0.50 ・ ・ ・ Formula (2)
10 ≦ [S] / [O] ≦ 40 ・ ・ ・ Formula (3) In Claim 1, Cu is mass%.
Cu: 0.01-4.0%
An austenitic free-cutting stainless steel characterized by comprising In any one of Claims 1 and 2, Mo is further contained in mass%.
Mo: 0.01-3.0%
An austenitic free-cutting stainless steel characterized by comprising Either of Claims 1-3 WHEREIN: Furthermore, either 1 type or 2 types of Pb and Bi is mass%.
Pb: 0.03-0.30%
Bi: 0.01-0.30%
An austenitic free-cutting stainless steel characterized by comprising In any one of Claims 1-4, any 1 type (s) or 2 or more types of Ca, Mg, and REM are further in the mass%.
Ca: 0.0001 to 0.05%
Mg: 0.0001 to 0.02%
REM: 0.0001 to 0.02%
An austenitic free-cutting stainless steel characterized by comprising In any one of Claims 1-5, W is further in the mass%.
W: 0.01-2.0%
An austenitic free-cutting stainless steel characterized by comprising In any one of Claims 1-6, 1 type (s) or 2 or more types of Nb, Ta, and V are further further in the mass%.
Nb: 0.01-0.50%
Ta: 0.01-0.50%
V: 0.01 to 0.50%
An austenitic free-cutting stainless steel characterized by comprising