JPH0753897B2 - Ferrite Free Cutting Stainless Steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a ferritic free-cutting stainless steel, and particularly to a 17Cr stainless steel excellent in hot workability and machinability.

(Conventional technology and its problems) As a ferritic free-cutting stainless steel, SUS430F (17% C
r, 0.18% S) etc. are known, but in recent years, those with improved machinability have been demanded, and S, Se, Te, Pb are added to these steels.
Studies have been conducted to meet the above requirements by adding machinability improving elements such as. A free-cutting steel (0.01% C-20% Cr-2% Mo-S-Pb-Te) proposed by the applicant of the present invention is used as a stainless steel whose machinability is improved by adding S, Pb and the like. 55
-152158. This free-cutting steel has excellent machinability and cold forgeability, but since it contains a large amount of rare metals such as Cr and Mo, it has a problem in terms of cost reduction.

Therefore, if the ferrite stabilizing elements such as Cr and Mo are reduced, the phase balance becomes unstable, and the austenite phase appears in the ferrite phase, which deteriorates the hot workability.
Among 17Cr-based stainless steels, there is no known stainless steel in which machinability is improved by adding S, Pb, Se or the like while ensuring hot workability.

The present invention has been made to solve such problems, and an object thereof is to provide a free-cutting stainless steel which is inexpensive and has excellent machinability without impairing hot workability.

(Means for Solving the Problems) As a result of various studies conducted by the present inventors in order to achieve the above-mentioned object, one or two kinds of B and REM, preferably, when B is added together with Zr, conventional cutting Of S, Se, Te, Pb, etc., which is effective for heat resistance but impairs hot workability, can be increased without impairing hot workability, S alone,
Alternatively, by adding one or two from Se, Te, Pb, etc., S
It was found that the hot workable range can be expanded by adding one or two of Se, Te and Pb in combination. The present invention is based on such findings, and according to the present invention, C:% by weight:
0.1% or less, Si: 2.0% or less, Cu: 1.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 15 to 20%, Mo: 0.5% or less, S: 0.0
5 ~ 0.4%, Se: 0.05 ~ 0.3%, Te: 0.01 ~ 0.2%, Pb: 0.05
~ 1% or more selected from the group consisting of 0.3%,
B: 0.001 to 0.010% and REM (rare earth metal): 0.01 to 0.1%
A ferritic free-cutting stainless steel, which is excellent in hot workability and is composed of one or two of the following, and the balance being substantially Fe, is provided.

It is preferable to add Zr: 0.01 to 0.1% together with B addition.

(Operation) Next, the operation of each component of the steel of the present invention and the reason for limiting the composition will be described.

C: 0.1% or less When the amount of C increases, precipitation of carbides increases and corrosion resistance is significantly impaired, and austenite precipitates during hot working, which also impairs hot workability. Therefore, an upper limit is set to 0.1% or less. .

Si: 2.0% or less It is generally added as a deoxidizing agent during the melting of steel, but if it is added in excess of the specified value of 2.0%, the ductility-brittleness transition temperature will be significantly increased, so the center of the ingot during heating during hot working It is not preferable because cracks tend to occur on the surface.

Mn: 2.0% or less Improves machinability by forming a compound with S, Se and Te. However, if the specified value exceeds 2.0%, the corrosion resistance, which is the original mission of stainless steel, is impaired.

Cu, Ni: 1.0% or less Cu and Ni are impurity elements, and the content of these elements is
If it exceeds 1.0%, the ferrite phase becomes unstable at high temperatures and the hot workability is impaired.

Cr: 15-20% Cr is a basic additive element and imparts extremely strong corrosion resistance to an oxidizing acidic solution. It is necessary to add 15% or more to make the matrix a ferrite phase.
%, The ductile-brittle transition temperature is increased like Si,
Cracks are likely to occur at the center of the steel ingot during heating during hot working.

Mo: 0.5% or less Mo is an element that is very effective for strengthening the passive film and corrosion resistance, but it is a high temperature element and was set to 0.5% or less for economic reasons.

S: 0.4% or less S is a basic free-machining property-imparting element, and as described above, it combines with Mn to form MnS and improves the machinability. If S is added excessively, hot workability and corrosion resistance are impaired, so the upper limit was restricted to 0.4%.

Se: 0.05-0.3% Se forms Mn and MnSe and enhances machinability, but if it is less than 0.05%, it has no effect, and if it exceeds 0.3%, it is harmful to hot workability and corrosion resistance.

Te: 0.01 to 0.2% Te alone improves the machinability, but improves the machinability by granulating sulfide and selenide. If it is less than 0.01%, there is no effect, and if it exceeds 0.2%, large segregation tends to occur.

Pb: 0.05-0.3% Pb alone is finely and uniformly distributed in the steel, and improves the machinability due to the lubricating effect of the cutting tool during cutting. If it is less than 0.05%, it has no effect, and if it exceeds 0.3%, it is harmful to hot workability.

The above machinability-improving elements S, Se, Te, and Pb contribute to the improvement of machinability when added to steel alone, but S, Se, Te, and Pb
If one or more of the above are added in combination, the range in which the hot workability can be added can be expanded without impairing the hot workability.

B: 0.001 to 0.010% When the above machinability improving elements are added to steel, the hot workability deteriorates. However, when B is added to this, B itself precipitates at the grain boundaries and the hot workability is significantly improved. To be done. If it is less than 0.001%, there is no effect, and if it exceeds 0.010%, B bonds with N and BN
Precipitates as a compound and significantly deteriorates corrosion resistance.

Zr: 0.01-0.1% Zr fixes N. As a result, B is prevented from being bound to N during solidification to form BN, which lowers the hot workability improving effect. Therefore, there is no effect by itself,
Added with B. The amount of Zr added depends on the amount of [N] in the steel, but 0.01 to 0.1% is the preferred range.

REM: 0.01 to 0.1% REM is an element for improving hot workability, and 0.01 to 0.1% is a preferable range.

(Examples) Examples of the present invention will be described below.

Table 1 shows each chemical composition of the steel of the present invention and the comparative steel, and after the test steel of each composition was melted, a 6φ round bar test piece and a plate thickness of 20 mm
A thick plate test piece was prepared and subjected to a hot high-speed load test and a cutting test, respectively.

In the hot high-speed load test, after heating the round bar test piece to the test temperature, pull the test piece at a predetermined speed (2 inch / sec),
The draw ratio of the fracture surface (= (A ₀ −A) × 100 / A ₀ (%), A is the fracture area, A ₀ is the original cross-sectional area of the test piece), and the hot workability is calculated from this draw ratio. Evaluate. The results of the hot high-speed load test of each sample steel are shown in FIG.

For the cutting test, a SKH9 taper shank drill (10φ, tip angle 118 °) was used for the thick plate test piece, and the drill feed rate was 0.15.
Under the conditions of mm / rev, hole depth: 20 mm, and cutting oil: none, the cutting speed was changed, and the cumulative length (mm) of the depth of holes drilled until the drill became uncuttable was obtained. FIG. 2 shows the relationship between the tool life (cumulative hole depth) and cutting speed of each sample steel.

As shown in FIGS. 1 and 2, the comparative steels C1 and C2 are steels equivalent to SUS430 and SUS430F, respectively. As a result of increasing the S content of the comparative steel C2 as compared with the comparative steel C1, machinability is improved. Is remarkably improved, but it is still insufficient compared with the steels of the present invention. Therefore, the comparative steel C3 further containing the machinability improving element Te has improved machinability, but does not contain the hot workability improving element, and thus the hot workability is deteriorated. Further, the comparative steel C4 containing the hot workability improving element, but having added the machinability improving element in excess beyond the specified range of the present invention, the effect of containing the hot workability improving element is thin, The sex has deteriorated significantly.

On the other hand, the steels S1 to S8 of the present invention show remarkable improvement in machinability due to the addition of the machinability improving element, and further, any of the test steels due to the addition of the hot workability improving element has substantially the entire test temperature range. A good hot workability of 60% or more was exhibited.

(Effects of the invention) As described in detail above, according to the ferritic free-cutting stainless steel of the present invention, S: 0.05 to 0.4% as a machinability improving element,
And Se: 0.05-0.3%, Te: 0.01-0.2%, Pb: 0.05-0.3%
One or two or more selected from the group consisting of and are added, and B: 0.001 to 0.010% as a hot workability improving element.
Zr: 0.01 to 0.1% and REM (rare earth metal): 0.01 to 0.1% selected from the group consisting of one or more added, so machinability is improved without impairing hot workability. It has an excellent effect that it can be done.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the drawing temperature and the test temperature obtained by the hot high-speed load test of the steels of the present invention and comparative steels.
The figure is a graph showing the relationship between the cutting speed and the tool life obtained by the cutting test of the present invention steel and the comparative steel.

Claims (2)

[Claims] 1. In weight%, C: 0.1% or less, Si: 2.0% or less, Cu:
1.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 15-20
%, Mo: 0.5% or less, S: 0.05 to 0.4%, Se: 0.05 to 0.3% and T
One or more selected from the group consisting of e: 0.01 to 0.2% and Pb: 0.05 to 0.3%, one selected from B: 0.001 to 0.010% and REM (rare earth metal): 0.01 to 0.1%, or Ferrite-based free-cutting stainless steel with excellent hot workability consisting of 2 types and the balance consisting essentially of Fe. 2. The ferritic free-cutting stainless steel according to claim 1, wherein Zr: 0.01 to 0.1% is added together with B addition.