JP3247287B2 - Ferritic free-cutting stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a free-cutting ferritic stainless steel in which the machinability and corrosion resistance of a small-diameter steel bar and a wire rod having a diameter of 30 mm or less are significantly improved as compared with the conventional SUS430F (containing 17Cr-S). It is about.

[0002]

2. Description of the Related Art Ferritic free-cutting stainless steels have S
US430F (added with 17Cr-S) is widely used, but improvement of its machinability and corrosion resistance has recently been desired. Among them, as a material having better machinability and corrosion resistance than SUS430F, a material obtained by adding S, Te and Pb to a 20Cr-2Mo base in a complex manner is known. this is,
Since it contains a large amount of Mo which is a rare metal, there is a problem that the corrosion resistance is excessively high as compared with the SUS303 level, and it is expensive in view of recent demand for cost reduction. Therefore, conventionally, the cost is SUS
There has been a demand for the development of a free-cutting ferritic stainless steel that satisfies such requirements as 430F, SUS430F or higher in machinability and SUS430F or higher in corrosion resistance.

[0003]

According to the present invention, the cost is equal to SUS430F, the machinability is SUS430F or more,
An object of the present invention is to provide a ferritic free-cutting stainless steel satisfying such items as corrosion resistance of SUS430F or more.

[0004]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors focused on the form and composition of sulfide-based inclusions in steel and conducted intensive research. It has been found that when the material is rolled into a small-diameter bar or wire rod having a diameter of 30 mm or less, some of the material is extended and some of the material are not extended very much. Compared to non-extended sulfide-based inclusions, the extended sulfide-based inclusions have much better machinability than the non-extended sulfide-based inclusions. Shown below). Therefore, in order to improve machinability, as a result of various investigations for the purpose of leaving sulfide-based inclusions that do not spread even when rolled to a small diameter, as a result, S
It has been discovered that the objective can be achieved by adding 0.05% by weight or more of e. Further, it has been found that when 1.5 <S / Se <5.0, sulfide-based inclusions that are more effective for improving machinability can be provided.

[0005] Regarding this, if the S / Se is 1.5 or less, the Se content in the sulfide-based inclusions increases and the machinability decreases, and the S / Se is 5.0 or more. Then, the sulfide-based inclusions spread and the machinability decreases.
Further, it has been found that when the ratio of Mn / (S + Se)> 3.5, the Cr content in the sulfide-based inclusion is reduced, and a sulfide-based inclusion effective for machinability can be provided. When Pb is further added, machinability is improved. Further, the stainless steel of the present invention increases the amount of Cr in the matrix as compared with the conventional SUS430F (with 17Cr-S added), and by adding Se, the sulfide-based inclusion contains Se. Corrosion resistance is greatly improved.

That is, in order to achieve the above object, according to the first free-cutting ferritic stainless steel of the present invention, C: 0.10% by weight or less Si: 0.05 to 2.00% by weight Mn : 2.00 wt% or less P: 0.20 wt% or less Ni: 2.00 wt% or less Cr: 20.0 to 30.0 wt% Mo: 1.0 wt% or less S: 0.05 to 0.45 wt% Se: 0 0.055% by weight or more: O: 0.020% by weight or less, the balance being Fe and the relatively large sulfide-based inclusions having a major axis of 5 μm or more having at least 80% of the major axis / minor axis ratio Is 8 or less, and the Mn / (S + Se) ratio> 3.5,
1.5 <S / Se <5.0.

According to the ferritic free-cutting stainless steel according to the second aspect of the present invention, C: 0.10 wt% or less Si: 0.05 to 2.00 wt% Mn: 2.00 wt% or less P: 0.20 wt% %: Ni: 2.00 wt% or less Cr: 20.0-30.0 wt% Mo: 1.0 wt% or less S: 0.05-0.45 wt% Se: 0.05 wt% or more O: 0.020 wt% or less Pb: 0.03 to 0.30 wt%, the balance being Fe, and the major diameter of the sulfide-based inclusion being 5 μm.
At least 80% of the relatively large ones having m or more have a ratio of major axis to minor axis of 8 or less, and Mn / (S + Se) ratio> 3.
5, 1.5 <S / Se <5.0.

The reasons for limiting the content of each component element in the ferritic free-cutting stainless steel of the present invention will be described below. C: C is an element that forms a solid solution in the substrate and increases the hardness. If it exceeds 0.10 wt%, the precipitation amount of carbides increases and the corrosion resistance deteriorates, so the upper limit was made 0.10 wt%. Si: Si is added in order to prevent peroxidation during steelmaking. However, if it is too much, the toughness is reduced, and if it is too small, the effect cannot be obtained. Mn: Mn is necessary for generation of S-based inclusions.
If it exceeds 00 wt%, the corrosion resistance is reduced, so the upper limit was made 2.00 wt%. P: If a large amount of P is contained, the hot workability deteriorates, so the upper limit was made 0.20 wt%.

Ni: Ni may be added to improve the hardenability, but if it exceeds 2.0 wt%, the annealing hardness will increase significantly, so the upper limit was made 2.0 wt%. Cr: Cr is an element necessary for securing the corrosion resistance and oxidation resistance required for stainless steel. 20.0w
No improvement in corrosion resistance was observed below 3% by weight.
If it is more than 20.0 to 30.0 w
t%. Mo: Mo is an element that enhances corrosion resistance, but is an expensive element, so that the content was set to 1.0 wt% or less.

S: S is extremely effective as an element for improving machinability. When the content is 0.05 wt% or less, the effect of improving machinability is not sufficiently exhibited. If it is added in excess of 0.45 wt%, the hot workability deteriorates.
t%. Se: Se is an indispensable element for preventing the sulfide-based inclusions of small-diameter steel bars and wires from spreading, and therefore, it is necessary to add 0.05 wt% or more. O: If O exceeds 0.020 wt%, the amount of oxides becomes extremely large, and the machinability and hot workability are reduced, so the upper limit is made 0.020 wt%. Pb: Pb is an element that improves machinability without deteriorating corrosion resistance, but its effect starts to be exhibited at 0.03% or more. Since the workability deteriorates, the upper limit is set to 0.30 wt%.

An embodiment will be described below. The steel of the present invention and the comparative steel shown in Table 1 were melted in a vacuum melting furnace, and then forged into a φ15 mm steel bar. 800 of these steel bars
After annealing at ℃, test specimens were sampled and investigated for sulfide inclusions and subjected to machinability tests. (1) Investigation of sulfide-based inclusions The obtained steel bar is cut in the forging direction, the cut surface is polished, and then microscopically examined with a microscope to measure the length of sulfide-based inclusions of 5 μm or more and to determine the inclusion composition. A survey was conducted. (2) Machinability test The machinability was evaluated by a drilling test. Table 2 shows the drill piercing test conditions. The evaluation was made based on the time required for perforating 10 mm. (3) Corrosion resistance The corrosion resistance was evaluated by the weight loss (g / m ² · h) after immersion in 1% hydrochloric acid at 25 ° C. for 6 hours. Evaluation A: Corrosion weight loss 10 to 100, Evaluation B: Corrosion weight loss 100 to 200

[0012]

[Table 1]

[0013]

[Table 2]

As shown in Table 1, when the Se content is 0.03% and 0.04% as in Comparative Steel 1 and Comparative Steel 2,
In the sulfide-based inclusions having a major axis of 5 μm or more, the ratio of major axis to minor axis ratio of 8 or less is as low as 25% and 40%, and S / Se is also 5.0 or more. And the machinability is poor. Further, when the Mn / (S + Se) ratio was as low as 2.14 as in Comparative steel 3, the Cr content in the sulfide-based inclusions was as high as 6.2%, and the drilling index was accordingly high. And the machinability has worsened. Furthermore, S / Se is 1.
When it is as low as 36, the Se content in the sulfide-based inclusions increases, and accordingly, the drilling index increases, and the machinability deteriorates. Since the comparative steel 5 is based on 17Cr, the corrosion resistance is poor.

[0015]

As described above, according to the present invention, the machinability and corrosion resistance of small-diameter steel bars and wire rods are reduced by the conventional SUS.
A free-cutting ferritic stainless steel significantly improved from 430F (17Cr-S added) was obtained.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-122857 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 C22C 38/60

Claims (2)

(57) [Claims] C: 0.10 wt% or less Si: 0.05 to 2.00 wt% Mn: 2.00 wt% or less P: 0.20 wt% or less Ni: 2.00 wt% or less Cr: 20.0 to 30 wt% 0.0 wt% Mo: 1.0 wt% or less S: 0.05 to 0.45 wt% Se: 0.05 wt% or more O: 0.020 wt% or less, the balance being Fe and the sulfide present therein At least 80% of relatively large-sized inclusions having a major axis of 5 μm or more have a major / minor axis ratio of 8 or less, and a Mn / (S + Se) ratio> 3.5,
A free-cutting ferritic stainless steel, wherein 1.5 <S / Se <5.0. 2. C: 0.10 wt% or less Si: 0.05 to 2.00 wt% Mn: 2.00 wt% or less P: 0.20 wt% or less Ni: 2.00 wt% or less Cr: 20.0 to 30 wt% 0.0 wt% Mo: 1.0 wt% or less S: 0.05 to 0.45 wt% Se: 0.05 wt% or more O: 0.020 wt% or less Pb: 0.03 to 0.30 wt%, the balance Fe Consisting of sulfide-based inclusions having a major axis of 5μ
At least 80% of the relatively large ones having m or more have a ratio of major axis to minor axis of 8 or less, and Mn / (S + Se) ratio> 3.
5, a ferritic free-cutting stainless steel, wherein 1.5 <S / Se <5.0.