JPH11131191A - Ferritic stainless steel excellent in ridging resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the ridging resistance of a ferritic stainless steel without deteriorating surface characteristic. SOLUTION: With respect to oxide inclusions contained in the ferritic stainless steel, their average diameter in a shape approximating to sphere is regulated to <=2 μm and Al2 O3 concentration is regulated to <=30 wt.%, and further, the average density of these fine oxide inclusions in the plane one-fourth the thickness of a plate is controlled to <=0.01 pieces/μm<2> . It is desirable to regulate the amount of 0 in the steel to <=40 ppm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel having excellent ridging resistance, and more particularly, to appropriately controlling the formation state of oxide inclusions formed in the process of producing a ferritic stainless steel. Thus, the ridging resistance is advantageously improved without deterioration of the surface properties.

[0002]

2. Description of the Related Art Ferritic stainless steels are generally
Since it is not only excellent in stress corrosion cracking resistance but also inexpensive, it is widely used in various fields such as various kitchen appliances and automobile exhaust system parts. However, this ferritic stainless steel has a problem in that it is inferior in press formability (such as stretch formability and ridging resistance).

Therefore, many attempts have been made to improve the press formability of ferritic stainless steel. For example, JP-A-49-41227 and JP-A-2-
JP 250925 proposes a method for improving ridging resistance by making the solidification structure finer and equiaxed. However, these methods have problems that crystal grains cannot be sufficiently refined or columnar crystals, which are considered to be the main cause of ridging, cannot be sufficiently reduced.

Japanese Patent Application Laid-Open No. 5-179358 discloses a method for improving ridging resistance by regulating the respective rolling reductions in rough rolling and finish rolling in hot rolling. The gazette proposes a technique for improving the ridging resistance by controlling the relationship between the strain rate and the coefficient of friction in the hot rolling process of a ferritic stainless steel while increasing the strain rate to 150 s ^-1 or more. ing. However, in these techniques, although the ridging resistance is improved, there is a problem that hot rolling flaws are generated due to seizure between the steel sheet and the roll, and the surface properties of the steel sheet are significantly deteriorated.

[0005]

As described above, in the prior art, sufficient improvement in ridging resistance cannot be expected, or even if ridging resistance is improved, the surface properties of the steel sheet are significantly impaired. In addition, various problems to be solved remain, such as poor biting and poor shape during hot rolling. The present invention advantageously solves the above-described problems, and not only improves the ridging resistance, but also provides an advantage of a ferritic stainless steel that does not need to be concerned with the various problems of the known art as described above. The purpose is to propose a simple manufacturing method.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies for realizing the above-mentioned object, and as a result, have found that oxide-based inclusions inevitably generated in the process of producing ferritic stainless steel. It has been found that properly controlling the state of formation is extremely effective in improving ridging resistance. The present invention is based on the above findings.

That is, according to the present invention, the oxide inclusions contained in ferritic stainless steel are limited to an average diameter d of not more than 2 μm and an Al ₂ O ₃ concentration C of not more than 30 wt% when approximated to a sphere. A ferritic stainless steel excellent in ridging resistance, characterized in that the average density D of the fine oxide-based inclusions in a 1/4 plane with a thickness of 1/4 is suppressed to 0.01 / μm ² or less. .

In the present invention, the O content in the ferritic stainless steel is preferably suppressed to 40 ppm or less.
In the present invention, the oxide-based inclusions include not only mere oxides but also composite forms in which sulfides and nitrides are mixed. Here, as main oxides, in addition to Al ₂ O ₃ , MnO,
Cr ₂ O ₃ , SiO _2, FeO and the like. In addition, the present invention is applicable to all steel types as long as it is a so-called ferritic stainless steel. Particularly preferred steel types include SUS430, SUS430LX, SUH409, SUS436, SUS44.
7 mag.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following is a description of experimental results that led to the present invention. C: 0.06 wt% and Cr:
A steel containing 16.4 wt% and having a composition equivalent to SUS 430 with Al varied in the range of 0.0006 to 0.05 wt% was melted in a vacuum high-frequency melting furnace to obtain a small steel ingot (50 kg). At that time, the generation state of the oxide-based inclusions was variously changed by changing the melting conditions and the cooling conditions. Then, a 120 mm thick test piece was cut out from these steel ingots, heated to 1170 ° C,
A hot-rolled sheet having a thickness of 4.0 mm was formed by hot rolling. Then, after annealing at 860 ° C. for 7 hours, these hot-rolled sheets were subjected to slow cooling treatment, and then cold-rolled into 0.6 mm-thick cold-rolled sheets. further,
These cold-rolled sheets were subjected to finish annealing at 810 ° C for 30 seconds to obtain product sheets.

[0010] From the obtained product plate, J parallel to the rolling direction
FIG. 1 shows the results of examining the occurrence of ridging after 15% tensile test of IS5 test pieces in relation to the average diameter d of oxide-based inclusions and the ridging grade. The ridging grades A to D represent cases where the ridging height is A: 10 μm or less, B: 10 to 15 μm, C: 15 to 20 μm, and D: 20 μm or more. It can be said that it has excellent ridging resistance.
As is clear from the figure, the average diameter d of the oxide-based inclusions
It can be seen that the ridging resistance is significantly improved when is less than 2 μm.

FIG. 2 shows an Al ₂ O ₃ concentration C (= Al ₂ O ₃ / oxide-based inclusions × 100) for an oxide-based inclusion having a sphere approximate average diameter d of 2 μm or less. 2 shows the results obtained by examining the relationship between and ridging grades. As is clear from the figure, the Al ₂ O ₃ concentration C in the oxide-based inclusions was 30 wt.
%, Ridging resistance could be improved.

Further, FIG. 3 shows the average density D in a 1/4 plane of the thickness of oxide inclusions having a sphere approximate average diameter d of 2 μm or less and an Al ₂ O ₃ concentration C of 30 wt% or less. The result of examining the relationship with the ridging grade is shown.
As shown in FIG.
It can be seen that when the value is 0.01 particles / μm ² or less, the ridging resistance is remarkably improved.

Therefore, in the present invention, the approximate inclusion diameter d of the sphere is set to 2 μm or less and the concentration C of Al ₂ O _{3 is set} to 30 wt% or less for the oxide inclusions generated in the process of producing the ferritic stainless steel. The average density D of the fine oxide-based inclusions on a 1/4 plane with a thickness of 1/4 plane is suppressed to a range of 0.01 / μm ² or less.

In the present invention, the formation state of the oxide inclusions as described above can be controlled as follows. First, as slag at the time of smelting, CaO-SiO ₂
-Al ₂ O ₃ using ternary slag. Then, the amount of Al ₂ O ₃ in the slag is suppressed to 20 wt% or less, and the basicity (CaO / SiO ₂ ) is adjusted to a range of 1.0 to 2.0. In addition,
It is desirable to reduce the amount of O in molten steel to 40 ppm or less. By the above treatment, the amount of oxide inclusions themselves is reduced, and Al ₂ in oxide inclusions is reduced.
The O ₃ concentration is controlled within a predetermined range.

As for the particle size of such oxide-based inclusions, the casting speed in continuous casting is higher than in the past.
m / min or more can be miniaturized,
As a result, the average density of the fine grains in the 1/4 plane thickness can be suppressed to 0.01 particles / μm ² or less, in combination with the reduction in the amount of oxide-based inclusions formed by the above-described slag component adjustment. . In addition, the measurement position of the average density of the fine oxide-based inclusions was set to a plate thickness 1/4 plane.
This is because the columnar crystal part of the slab, which is the main cause of the ridging, is not only a rolled part, but also a part where oxide-based inclusions easily aggregate.

The subsequent treatment may be carried out in accordance with a conventional method. For example, after hot rolling at 1300 to 700 ° C., ordinary winding, annealing and pickling are performed, and a reduction of 50 to 90% is carried out. Cold rolling and then finish annealing at a temperature of about 700 to 1100 ° C. In addition, this invention steel is applicable not only to a normal cold-rolled steel plate but also to a bright annealed plate.

[0017]

EXAMPLES Steel having the composition shown in Table 1 was melted and slabs having a thickness of 200 mm were formed by continuous casting. At that time, the state of formation of oxide-based inclusions in the slab was variously changed by changing the deoxidation method and casting conditions. That is, the slag used for deoxidation, using those CaO-SiO ₂ -Al ₂ O ₃ system, to adjust the composition of the slag in order to control the morphology of inclusions. Those with a basicity of about 1.7 and a concentration of Al ₂ O ₃ in the slag of 10 wt% are MnO-SiO ₂ inclusions, whereas those with a basicity of more than 2.0 and a concentration of Al ₂ O ₃ in the slag of 20
Those containing wt% or more were Al ₂ O ₃ inclusions. In this experiment, the time required for deoxidation was about 30 minutes, and the amount of O was reduced to 40 ppm. Thereafter, molten steel was poured into a mold and cast at a speed of 1.2 m / min to obtain a slab.

Next, these continuously cast slabs were hot-rolled into hot-rolled sheets having a thickness of 4.0 mm. Thereafter, among these hot-rolled sheets, A was subjected to annealing at 860 ° C. for 8 hours and then subjected to annealing treatment, B and C were subjected to annealing at 1000 ° C. and 60 s, and D were subjected to annealing at 960 ° C. and 60 s. Air-cooling treatment was performed to obtain a hot-rolled annealed sheet. Next, these hot-rolled annealed sheets were pickled and then cold-rolled to form cold-rolled sheets having a thickness of 0.5 mm. Further, among these cold rolled sheets, finish annealing was performed at 810 ° C., 30 s for A, 950 ° C., 30 s for B and C, and 880 ° C., 30 s for D, to obtain cold rolled annealed sheets. From each of the thus obtained cold-rolled annealed sheets, a JIS No. 5 test piece was sampled in parallel with the rolling direction, and the occurrence of ridging after 15% tension was examined. The structure of each cold-rolled annealed sheet was examined with a scanning electron microscope.
Observation and identification were performed for 200 visual fields, and the size, composition, and distribution of oxide inclusions were investigated.
Table 2 shows the obtained results.

[0019]

[Table 1]

[0020]

[Table 2]

As is apparent from Table 2, according to the present invention, the oxide-based inclusions have a sphere approximate average diameter of 2 μm or less,
The concentration of Al ₂ O ₃ is limited to 30 wt% or less, and the average density of such fine oxide-based inclusions in a plane having a thickness of 1/4 plane is 0.1%.
In all of the samples controlled to 01 / μm ² or less, the ridging resistance was extremely good, and hardly any scuffing was generated.

[0022]

As described above, according to the present invention, a ferritic stainless steel excellent in ridging resistance can be stably obtained without deterioration of surface properties, and its industrial value is extremely large.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the average diameter d of oxide-based inclusions and ridging grades.

FIG. 2 is a graph showing the relationship between the Al ₂ O ₃ concentration C in oxide-based inclusions and ridging grades.

FIG. 3 is a graph showing a relationship between an average density D and a ridging grade of an oxide-based inclusion on a plane having a thickness of 1/4.

Continuing from the front page (72) Inventor Takumi Uki 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corp. (72) Inventor Susumu Suto 1 Kawasaki-cho, Chuo-ku Chiba City, Chiba Prefecture Inside the company technology laboratory

Claims (2)

[Claims] An oxide inclusion contained in a ferritic stainless steel is limited to an average diameter d of not more than 2 μm and an Al ₂ O ₃ concentration C of not more than 30 wt% when approximated to a sphere. 1/4 thickness of fine oxide inclusions
A ferritic stainless steel excellent in ridging resistance, wherein an average density D in a plane is suppressed to 0.01 / μm ² or less. 2. The steel according to claim 1, wherein the O content in the steel is 40 ppm.
The following ferritic stainless steels with excellent ridging resistance.