JP2772802B2 - Manufacturing method of S-based free-cutting stainless steel with excellent chip disposal method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to machinability in turning or drilling a S-based free-cutting stainless steel containing 0.15% or more of S, particularly a SUS303-based wire. In particular, the present invention relates to a method for producing an S-based free-cutting stainless steel for improving chip disposability.

(Prior Art) Recently, a demand for a stainless steel wire having a high cutting efficiency as a microshaft material for an electronic communication device represented by a VTR or the like is rapidly increasing. However, stainless steel generally has significantly poor machinability when turning or drilling compared to ordinary steel, and as a countermeasure, an element that improves machinability represented by S, Pb, Se, Te, etc. Various free-cutting stainless steels are currently in practical use, for example, by adding a small amount of a free-cutting element (hereinafter simply referred to as a free-cutting element). In this type of free-cutting stainless steel, machinability can be further improved by increasing the amount of the free-cutting element.
Conversely, various problems such as deterioration of corrosion resistance and deterioration of hot workability at the time of production occur, and the amount of addition is limited.

(Problems to be Solved by the Invention) Therefore, the present invention uses S-based free-cutting stainless steel as a basic steel, and has a higher machinability, particularly good chips, as compared with the basic steel without changing the amounts of constituent elements. This is a manufacturing technique invented for the purpose of providing an S-based free-cutting stainless steel wire having processability. Therefore, according to the present invention, the machinability can be improved without any problem such as a reduction in corrosion resistance of the product or a decrease in hot workability at the time of manufacturing.

(Means for Solving the Problems) In general, it is known that the form of sulfide, which is a nonmetallic inclusion, greatly affects the machinability, and the machinability increases as the number of large oval-type sulfides increases. Is excellent. Most of MnS of stainless steel wire is stretched into a thread shape to an aspect ratio of about 10 to 20 by hot rolling from a slab, and the machinability is deteriorated. Therefore, if it is possible to form as large an oval type MnS as possible at the stage of a slab produced by continuous casting, it is possible to bring a shape close to a relatively large oval type even in subsequent hot rolling. The size of MnS in the slab is affected by the cooling rate of the slab. For example, it has been found that, in a slab produced at a normal cooling rate, MnS is larger in a central portion having a lower cooling rate than in a surface layer portion having a higher cooling rate, and the machinability is excellent. As described above, in order to form the oval-type large MnS, the solidification process and the cooling rate after the solidification may be reduced as much as possible. However, when producing a slab by the continuous casting method, the cooling rate at the peripheral portion of the surface layer of the slab is extremely fast at 500 to 1500 ° C./min by ordinary cooling, and a sufficiently large sulfide is hardly generated. In order to solve the problem, the extreme surface layer from the surface layer to about 3 mm is usually removed before reaching the final product, so the highest free-cutting property is required when secondary processing of the final product after rolling What is necessary is just to control the sulfide form from 3 mm to about 10 mm from the surface layer of the slab to be formed. In order to achieve the above object, the present inventors solidified stainless steel SUS303 with various cooling patterns and examined the influence of the cooling rate during and after solidification on the sulfide morphology. As a result, the slower the cooling rate from the liquidus temperature to the solidus temperature, the larger the sulfide that crystallizes, and the slower the cooling rate from the solidus temperature to about 1000 ° C, the more spherical and coarse the sulfide. The present invention was found to show a tendency to change.

That is, as described below, the present invention is to make the cooling speed of a slab slower than in the past when manufacturing a slab by a continuous casting method. That is, (1) 3 m from the surface layer of the slab
A method of controlling the cooling rate from the liquidus temperature to the solidus temperature to a temperature of 120 ° C./min or less in a portion of m to 10 mm or less,
(2) The cooling rate from the solidus temperature to 1000 ° C is 20 ° C / min.
The following control method, or (3) a method that implements both of these methods.

The steel suitable for the production method of the present invention has a Cr content of 12% by weight.
%, And S-based free-cutting stainless steel containing 0.15% or more of S. Ni-based free-cutting stainless steel SUS303 is typical, but Cr-based free-cutting stainless steel SUS416, 420F, 430F, 440F, etc. may be used. When manufacturing the slab of the S-based free-cutting stainless steel by the continuous casting method, heating or keeping the temperature of the mold, which has not been conventionally performed, is specially performed, and the molten steel in the mold is super-slowly cooled. The cooling rate of the surface layer of the slab is remarkably reduced compared to the conventional method, whereby the sulfide to be crystallized can be increased, and the large sulfide close to a sphere can be retained even after hot wire rolling.

From the temperature at the end of solidification to 1000 ° C, the cooling rate of the slab is reduced to promote sulfide coarsening.

It should be noted that by combining the above two processes, the coarsening of the sulfide can be significantly increased as compared with the case of the single process.

(Example) Hereinafter, an example of the present invention will be described.

S-based free-cutting stainless steel SUS303 with the chemical components shown in Table 1
Was prepared at a normal cooling rate using a continuous casting facility, and the billet (a) was redissolved at 1500 ° C. to obtain a solidus temperature of 13 ° C.
Sample (b) prepared by gradually cooling to 50 ° C at 54 ° C / min and cooling to 1000 ° C at 500 ° C / min.
Cool at 1000 ℃ / min up to 50 ℃, 1350 ℃ to 1000 ℃
Sample (c) prepared by gradually cooling at 5.4 ° C / min, and also re-dissolved at 1500 ° C and gradually cooled at 15.4 ° C at 54 ° C / min and from 1350 ° C to 1000 ° C at 5.4 ° C / min. FIG. 1 shows a sketch of the sulfide form and chip appearance of the prepared sample (d). The cutting conditions were non-lubricated longitudinal turning with a cermet tool at a cutting speed of 70 m / min and a feed rate of 0.01 mm / re.
v, the depth of cut is 0.2 mm. FIG. 2 shows the result of image analysis of the average sulfide size. Slower cooling rate from 1500 ° C to solidus temperature than normal cooling (b)
(C) where the cooling rate from the solidus temperature to 1000 ° C. is slower, (c) has a larger sulfide size, and both (b) and (c) have been cooled. It is evident that the size increases and that the chip breaking performance is significantly improved with increasing sulfide size.

(Effects of the Invention) According to the present invention, without adding a special element harmful to the corrosion resistance of stainless steel, the element for improving machinability is S
A free-cutting stainless steel with good performance can be manufactured by controlling the cooling rate of continuous casting alone.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the form and cutting state of MnS, and FIG. 2 is a graph showing the average area of MnS.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mizuo Sakakibara 3434 Shimada, Hikari-shi, Yamaguchi Pref. Inside Nippon Steel Corporation Hikari Works (56) References ) Field surveyed (Int. Cl. ⁶ , DB name) B22D 11/00 B22D 11/124

Claims (3)

(57) [Claims] (1) Cr: 12% or more and S: 0.1% by weight
When manufacturing stainless steel slabs containing 5% or more by continuous casting, the cooling rate from the liquidus temperature to the solidus temperature is measured at a part 3 mm or more and 10 mm or less from the surface layer of the slab.
A method for producing an S-based free-cutting stainless steel having excellent chip controllability, characterized by a temperature of 120 ° C./min or less. (2) Cr: 12% or more by weight and S: 0.1
When producing a stainless steel slab containing 5% or more by the continuous casting method, the cooling rate from the solidus temperature to 1000 ° C at the part 3mm to 10mm from the surface layer of the slab is 20 ° C.
/ min or less, characterized by excellent chip controllability
Manufacturing method for free-cutting stainless steel. (3) Cr: 12% or more by weight and S: 0.1
When manufacturing stainless steel slabs containing 5% or more by continuous casting, the cooling rate from the liquidus temperature to the solidus temperature is measured at a part 3 mm or more and 10 mm or less from the surface layer of the slab.
A method for producing an S-based free-cutting stainless steel having excellent chip disposability, wherein the cooling rate from the solidus temperature to 1000 ° C is 20 ° C / min or less.