JP3101411B2 - Ferritic stainless steel excellent in workability and manufacturing method - Google Patents

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 excellent in workability such as drawing and bending, particularly excellent in bending workability, and a method for producing the same.

[0002]

2. Description of the Related Art SUS430 ferritic stainless steel is an inexpensive material as compared with austenitic stainless steel containing a large amount of Ni, and is therefore used in a wide range of fields including thin plates. However, ferritic stainless steel is inferior in workability and generates processing defects such as ridging and roping when rolled into a thin plate or the like.

[0003] In order to improve the workability of ferritic stainless steel, studies have conventionally been made from the viewpoints of component adjustment, rolling conditions, heat treatment and the like. As an example of component adjustment,
A method of adding Al, Ti, or the like to molten steel at the time of smelting so as to contain 0.1 to 1.0% by weight of Al or Ti at the product stage is adopted.

[0004]

If Al, Ti or the like is added to ferritic stainless steel, the dipping nozzle may be closed during continuous casting. In addition, Al added to molten steel,
In order to improve the yield of Ti and the like, it is necessary to sufficiently remove slag containing Cr oxide and the like from molten steel before adding Al and Ti and the like. As a result, valuable metals such as Cr are discarded together with the slag.

[0005] When hot rolling a ferritic stainless steel having a high Ti content, defects such as titanium streaks tend to occur on the surface of the hot rolled sheet. When defects occur, not only the yield is reduced but also a polishing step for removing surface defects becomes necessary after hot rolling. Moreover, expensive Ti or T
Since the i-containing material is consumed, the cost of the obtained ferritic stainless steel increases.

In order to avoid the occurrence of defects caused by Ti,
By adding Al instead of Ti, SUS4
Improvement of bending workability of ferritic stainless steels such as 30 series has also been performed. However, in order to contain an appropriate amount of Al, it is necessary to adjust components for a long time during melting. Further, the addition of Al increases non-metallic inclusions which are not preferable for improving the workability, often causes slab cracks, clogging of immersion nozzles, and the like, thereby lowering productivity.

The present invention is provided to solve such a problem, and improves the yield of the addition of Ti by adding a predetermined amount of Ti under conditions where the S and O contents are regulated. In addition, an object of the present invention is to obtain a ferritic stainless steel excellent in workability such as drawing and bending, particularly excellent in bending workability, and a method for producing the same.

[0008]

In order to achieve the above object, the ferritic stainless steel of the present invention has a C content of 0.12.
Wt% or less, Si: 0.75 wt% or less, Mn: 1.0
% By weight, P: 0.04% by weight or less, Ni: 0.6% by weight or less, Cr: 16.0 to 18.0% by weight, N: 0.
04% by weight or less, Al: 0.01% by weight or less and Ti:
0.01 to 0.05% by weight, the balance being substantially F
e, the content of S and O mixed as impurities is regulated to S: 0.005% by weight or less and O: 50 ppm or less, respectively, and the A-based inclusion is replaced by the C-based inclusion. I do.

Further, the ferritic stainless steel of the present invention is obtained by refining chromium-containing hot metal, and has a C content of 0.12% by weight or less.
Si: 0.75% by weight or less, Mn: 1.0% by weight or less,
P: 0.04% by weight or less, Ni: 0.6% by weight or less, C
r: 16.0 to 18.0% by weight, N: 0.04% by weight or less, Al: 0.01% by weight or less and Ti: 0.01 to
S and O, each containing 0.05% by weight, and the balance substantially having a composition of Fe,
When the stainless steel in which the A-based inclusions are replaced with the C-based inclusions is controlled to 0.005% by weight or less and O: 50ppm or less, when the molten steel component is adjusted at the final stage of the refining in the refining process, Is produced by adding Ti or a Ti-containing material to the molten steel under the condition that Si contains 0.40% by weight or more. Ti or Ti-containing
It is preferable that Ti is added to molten steel at a rate of 1.5 to 2.0 kg / ton in terms of Ti.

[0010]

[Operation] The present inventors have investigated and studied from various aspects the factors affecting particularly the bending workability of cold rolled SUS430 ferritic stainless steel.

[Effects of Components on Bending Workability] SUS430 ferritic stainless steel in which the Al content and the Ti content were changed in the range of 0 to 1.0% by weight was experimentally melted. Cold-rolled steel coil (final sheet thickness 2.0mm) of BA material which is considered to be inferior in bending workability to the annealed and pickled 2D material and the temper-rolled 2B material from the adjusted steel ) Manufactured. A test piece was cut out from the cold-rolled steel sheet coil and subjected to a 180-degree close contact bending test.

As a result of the close contact bending test, a cold rolled steel sheet coil that does not crack even if it is bent 180 degrees and brought into close contact with each other is used in Group A.
The cold-rolled steel sheet coil in which cracks occurred was classified as Group B. As shown in FIG. 1, among ferritic stainless steels containing no Al, Ti, etc., 10% belonged to group A and 90% belonged to group B. On the other hand, among the ferritic stainless steels containing Al or Ti, 95% of
5% belonged to group B. As is clear from this comparison, it can be seen that the bending workability of ferritic stainless steel is significantly improved by the addition of Al or Ti.

With respect to ferritic stainless steel containing no Al and Ti, the component difference between those belonging to Group A and Group B was examined. As a result, all of the ferritic stainless steels belonging to Group A have an S content of 0.0
The content of O was 0.05% by weight or less and the O content was 0.005% by weight or less. On the other hand, the ferritic stainless steel belonging to Group B has a large variation between the S content of 0.002 to 0.008% by weight and the O content of 30 to 70 ppm, and belongs to Group A. Many steels had higher S and O contents than ferritic stainless steels.

[Effect of Inclusions on Bendability] Inclusions in steel of a cold-rolled steel sheet coil were investigated in accordance with a test method specified in JIS G0566. JIS G05
According to the provisions of 66, the types of inclusions are classified into the following three types.

(1) A-based inclusions: Inclusions that have been viscously deformed by processing. Are further classified based on the main component of the inclusions, the inclusions consisting mainly of sulfides A _1-based inclusions consisting mainly of silicates such as the A ₂ system called.

(2) B-based inclusions: B-type inclusions are those which are grouped in the processing direction and in which granular inclusions are arranged discontinuously. If there is a need in the steel containing Ti is further classified according to the main component of the inclusions, mainly containing inclusions B ₁ system, carbonitride of Ti as a main component oxide such as alumina inclusions to the called B ₂ system.

(3) C-based inclusion: without viscous deformation
Inclusions such as particulate oxides that are randomly dispersed in steel.
When it is necessary to use Ti-containing steel, it is further classified according to the main components of inclusions. Inclusions mainly composed of oxides such as TiO ₂ are mainly C ₁ -based, and Ti carbonitrides are mainly used. Inclusions as components are referred to as C ₂ -based.

As a result of examination according to a microscopic test method specified in JIS G0566, ferritic stainless steel containing Al or Ti and belonging to Group A is:
All inclusions were C-based. Further, in the ferritic stainless steel containing Al or Ti but belonging to Group B, C-based inclusions and A-based inclusions were mixed, and 0.01% or more of A-based inclusions were present. On the other hand, in the ferritic stainless steels that do not contain Al and Ti and belong to Group A, the content of A-based inclusions was 0.01% or less, and some C-based inclusions were also present. In the ferritic stainless steel that does not contain Al and Ti and belongs to Group B, it contains only A-based inclusions.
030%.

From the above results, it can be seen that inclusions have the following effects on the workability of ferritic stainless steel.

[0020] If there are many A-based inclusions, processing cracks are very likely to occur when bending stainless steel. Therefore, in order to prevent bending cracks, the content of the A-based inclusions must be 0.010% or less, preferably 0%.

In order to reduce A-based inclusions in stainless steel containing no Al and Ti, it is necessary to use S and O
Needs to be reduced. However, simply reducing the S content and the O content does not eliminate the A-based inclusions.

In stainless steel containing Al or Ti, C-based inclusions are mainly contained, and A-based inclusions are reduced. And, it shows excellent bending workability as compared with stainless steel not containing Al and Ti.

As described above, the bending workability of ferritic stainless steel is improved by adding Al or Ti.
However, in order to obtain the effect of improving the bending workability, it is necessary to contain 0.10 to 1.0% by weight of Al or Ti. As a result, as described above, problems such as an increase in production cost, a decrease in productivity, slab cracking, blockage of the immersion nozzle, generation of surface flaws, and the like become remarkable.

Therefore, the present inventors have determined that the Al content and T
The relationship between the i content and the A-based inclusion was investigated in further detail.
The ferritic stainless steel to be investigated has an S content of 0.005% by weight or less and an O content of 50p in the steel.
pm or less, and the other components had components specified in SUS430.

[Relationship between Al content and A-based inclusions]
When the morphology of inclusions was investigated by changing the content, the results shown in FIG. 2 were obtained. FIG. 2 also shows the components and manufacturing conditions of the test cold-rolled steel sheet. From FIG. 2, it can be seen that the form of the inclusion changes depending on the Al content. That is, when the Al content is less than 0.04% by weight, only the A-based inclusion or the A-based inclusion and the C-based inclusion coexist.
When the Al content is 0.04% by weight or more, there is no A-based inclusion, and there is a C-based inclusion mainly composed of Al ₂ O ₃ .

[Relationship between Ti content and A-based inclusions] Ti
When the morphology of inclusions was investigated by changing the content, the results shown in FIG. 3 were obtained. FIG. 3 also shows the components and manufacturing conditions of the test cold-rolled steel sheet. From FIG. 3, it can be seen that the form of the inclusion changes according to the Ti content. That is, when the Ti content is less than 0.01% by weight, only the A-based inclusions or the A-based inclusions and the C-based inclusions coexist. On the other hand, at a Ti content of 0.01% by weight or more, no A-based inclusion was detected, and C-based inclusions such as TiO ₂ and TiN were present.

As described with reference to FIGS. 2 and 3, S
Content is 0.005% by weight or less and O content is 50pp
Under the condition of m or less, even if the Al content is at least 0.04% by weight in the case of adding Al and the Ti content is at least 0.01% by weight in the case of adding Ti, each A-based inclusion is eliminated. It was found that all the remaining inclusions could be changed to C-based inclusions.

Thus, the conditions regarding the lower limit of the Al content or the Ti content for eliminating the A-based inclusions have been clarified. However, the above-mentioned problem in the conventional method caused by adding a large amount of Al or Ti has not been solved. Therefore, in order to investigate the relationship between the problem in actual operation and Al and Ti, the Al content and the Ti content were changed and investigated.

As a result, when the Al content is less than 0.04% by weight, although the consumption of Al can be reduced, the clogging of the immersion nozzle frequently occurs in the continuous casting process, and the productivity is greatly reduced. . Further, since slag must be removed, valuable metals such as Cr are wasted. In particular, when the Al content was 0.2% by weight, a linear surface flaw occurred in the cold-rolled product. When the Al content is 0.04% by weight or more,
Since a large amount of Al is consumed, there are problems such as an increase in cost, a decrease in productivity due to blockage of the immersion nozzle, and a loss of valuable metals such as Cr. In particular, by increasing the Al content, slab cracking due to 475 ° C. brittleness is likely to occur more frequently, and handling of the warm slab including the crack countermeasures becomes extremely difficult, causing a fatal problem of slab cracking. I was
Therefore, by setting the Al content to 0.04% by weight or more, all the residual inclusions are converted into C-based inclusions,
Although the bendability can be improved, various problems occur in terms of manufacturability to improve the bendability by adding Al.

When test production was performed by changing the Ti content, no clogging occurred in the immersion nozzle when the Ti content was 0.05% by weight or less. Further, since the Ti content is small, it is not necessary to remove the slag before adding Ti. The valuable metal contained as an oxide in the slag is reduced and recovered, and the valuable metal such as Cr is effectively used. We were able to. Furthermore, no surface defects such as titanium streaks were generated on the hot-rolled steel sheet and the cold-rolled steel sheet. However, when the Ti content exceeds 0.05% by weight, the cost increases due to the consumption of a large amount of expensive Ti, slag removal is required, and valuable metals such as Cr are not recovered. The problem of wasteful consumption occurs. In particular, when the N content exceeds 100 ppm at a Ti content of 0.2% by weight or more,
Surface defects such as clogging of the immersion nozzle, slab cracks, titanium streaks, etc. occurred, and had problems in cost, productivity, quality, yield, resource saving, and the like.

The above results are summarized in Table 1. As is clear from Table 1, when performing inclusion control from the A system to the C system by adding Al or titanium to improve the bending workability of the SUS430 stainless cold-rolled steel sheet, the most advantageous method is to use Ti. It is a method of adding 0.01 to 0.05% by weight. At this time, the contents of S and O mixed as impurities are set to 0.005% by weight or less and 50 pp, respectively.
It is important to regulate to m or less. Also, the method of improving the bending workability by adding Al causes various problems in manufacturing, but when Ti is contained in the above-described range, Al is not positively added and is not contained in steel. However, in order to avoid that expensive Ti actively added to contain Ti reacts with oxygen in the steel to form slag and is wasted and leads to an increase in cost, Al is added in advance before adding Ti. The amount of oxygen in steel can be reduced as much as possible, and Al can be coexisted in steel at a content of 0.01% by weight or less.

[0032]

[Table 1]

When adding Ti to molten steel, SUS430 stainless steel molten steel having an S content of 0.005% by weight or less and an O content of 50 ppm or less is scoured by degassing or the like, and the scouring in this scouring step is performed. In the final smelting component adjustment step, the amount of Ti in the steel is 0.01 to 0.1.
A Ti source is added so as to be 05% by weight. Next, the molten steel after the component adjustment is continuously cast and subjected to hot rolling, cold rolling and the like according to a usual method to produce a cold-rolled steel sheet.

The ferritic stainless steel excellent in bending workability obtained according to the present invention is SUS430 type, and specifically, C: 0.
12% by weight or less, Si: 0.75% by weight or less, Mn:
1.0% by weight or less, P: 0.04% by weight or less, Ni:
0.6% by weight or less and Cr: 16.0 to 18.0% by weight
Having a basic composition containing In addition to this basic composition, N: 0.40% by weight or less, A
l: 0.01% by weight or less, S: 0.005% by weight or less,
O: contains 50 ppm or less and Ti: 0.01 to 0.05% by weight. The function of each alloy component is as follows.
Further, in order to improve properties such as corrosion resistance and strength, Cu, M
Known alloy components such as o, Nb, and Zr can also be added.

C: an austenite-forming element, similar to Mn, Ni, and N, which will be described later. In order to obtain a ferritic stainless steel having particularly excellent bending workability according to the present invention, it is preferable to use as little as possible. When the amount of C is increased, the hardness increases, the elongation deteriorates, and the workability deteriorates. Therefore, the upper limit is set to 0.12% by weight as specified. The range of 0.04 to 0.10% by weight is more preferable from a comprehensive viewpoint such as performance and economy.

Si: Si increases the yield point and the tensile strength to increase the strength, but lowers the ductility and impairs the workability. Therefore, the upper limit is 0.75% by weight. And since it is an element mainly used as a deoxidizing agent at the time of smelting together with Mn to be described later, if the allowable range is narrow, the productivity is deteriorated. From the comprehensive viewpoint, it is more preferably in the range of 0.35 to 0.75% by weight, for example, because it is also an element that forms A ₂ -based inclusions, which is a problem such as

Mn: When Mn increases, hardness increases, elongation decreases, and workability is impaired. Therefore, the upper limit is 1.0% by weight as specified. Since it is MnS (sulfide) together with S described later and forms an A ₁ -based inclusion,
The smaller the amount, the better, and more preferably 0.50% by weight or less through experience.

P: The content of P is preferably small because it degrades hot workability and toughness, but the upper limit is set at 0.04% by weight as specified in consideration of the economics of removing P.

Ni: Ni enhances the tensile strength,
Since it is easy to generate processing defects such as ridging and roping of cold-rolled steel sheet products, it is preferable that the number be small, and 0.6% by weight be used.
Is the upper limit.

Cr: Cr is an important element for improving the general corrosion resistance of steel, and 16.0% by weight or more is required to sufficiently obtain this corrosion resistance. On the other hand, if it is contained in a large amount, the elongation and the impact value are deteriorated, and the workability is also deteriorated.

N: N has the same behavior as C as described above,
In order to increase the ductility and improve the workability, the lower the better. Since it is an element forming a C ₂ -based inclusion in a carbonitride system together with C, a lower one is preferable. Further, when the N content exceeds 0.04% by weight, TiN increases, and even when the Ti content is 0.05% by weight or less, surface defects such as titanium streaks tend to occur. So N
The upper limit of the content was limited to 0.04% by weight.

Al: As described above, the total upper limit of the content of Al is 0.01% by weight, taking into consideration various problems due to addition and content, various findings, and various problems during actual operation. Limited to the following. Moreover, as described above, Al is not an element that is positively added to and contained in steel, but is used and added as necessary to reduce the consumption of expensive Ti.

S: S is an element that easily forms MnS (sulfide) together with Mn to form problematic A ₁ -based inclusions as described above, and particularly deteriorates bending workability. Then, as described above, A together with O described later
The amount of inclusions and the A system-
In order to greatly affect the generation form of the C-based inclusions, it is desirable that the content is low, and the content is set to 0.005% by weight or less through various knowledge and experience of actual operation.

O: O is A together with the Si,
_{A two-} system inclusion is formed, and in any case, an A-based inclusion is easily formed together with the S, and an oxide-based B- or C-based inclusion is also easily formed and generated. As in the case of S, it has a great influence on the amount and form of inclusions generated, and the generated inclusions cause not only bending workability but also surface defects (flaws) generated on the surface of the cold-rolled steel sheet. Therefore, the upper limit of the content is 5
Restrict to 0 ppm.

Ti: As described above, due to various problems caused by addition and content of Ti, various findings, and various problems during actual operation, the content of Ti may be too large in connection with S and O. Too little can cause problems. Ti
Is an element that is positively added and contained unlike Al, but S: 0.005% by weight or less and O: 50 ppm
At a content of 0.01% by weight under the following conditions, A-based inclusions can be completely eliminated and replaced by C-based inclusions,
On the other hand, if the content exceeds 0.05% by weight, various problems such as increased costs, uncollected valuable metals, and reduced productivity will occur. Therefore, the total content is limited to 0.01 to 0.05% by weight. I do.

In this limited range, substantially all of the inclusions remaining in the stainless steel are JIS G05.
It is a C-based inclusion defined by 56.

Metal Ti or Ti added as a Ti source
The inclusions react with O, S, etc. in the steel and partially migrate to slag. Then, when the relationship between the addition amount of the Ti source and the Ti content in the steel was examined, it was found that there was a relationship shown in FIG. From this, the Ti content in steel was reduced to 0.01
In order to make the content 0.05 to 0.05% by weight, 1.5 to 1.5% in terms of Ti
It can be seen that 2.0 kg / ton of Ti or a Ti-containing material may be added.

[0048]

EXAMPLE A ferritic stainless steel having five components shown in Table 2 was melted by a converter-degassing method, and Al was added in advance in the final component adjustment step to add oxygen [O] in the molten steel.
After the content was reduced, Ti was added. Then, the slab obtained by the continuous casting was subjected to hot rolling and cold rolling, and then subjected to annealing to finally produce a 2.0 mm thick BA-finished cold-rolled steel sheet coil.

[0049]

[Table 2]

When continuously casting the ferritic stainless steel molten steel shown in Table 2, no immersion nozzle was clogged. In addition, even if slag cutting is not performed during converter tapping, since the Ti content is low, Ti
It was easy to hit the content, and valuable metals such as Cr were not wasted. Further, since the amount of Ti added was as small as 1.5 kg / ton, a significant increase in cost was avoided.

For each of the hot-rolled steel sheet and the cold-rolled steel sheet,
The surface properties were investigated. As a result, there were no surface flaws and no problem. When the inclusions in the final cold-rolled steel sheet were measured, no A-based inclusions harmful to the bending workability were detected, and 0.010 to 0.025% of C-based inclusions were present. Further, a test piece was cut out from this final cold-rolled steel sheet and subjected to a bending test in which the test piece was bent by 180 ° to make close contact. As a result, no crack occurred in any of the test pieces.

[0052]

As described above, the ferritic stainless steel of the present invention does not contain harmful A-based inclusions and has excellent bending workability. Moreover, since the Ti content can be set low, the added Ti is efficiently contained in the steel without increasing the production cost and without performing slag cutting or the like accompanied by loss of valuable metals such as Cr. be able to. Therefore, the Ti content can be accurately matched with the target value, and a ferritic stainless steel excellent in workability and quality stability can be manufactured with high productivity.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of Al and Ti on crack initiation.

FIG. 2 is a graph showing the effect of Al content on the morphology of inclusions.

FIG. 3 is a graph showing the effect of Ti content on the morphology of inclusions.

FIG. 4 is a graph showing the relationship between the added Ti source and the amount of Ti contained in steel.

────────────────────────────────────────────────── (5) References JP-A-60-155648 (JP, A) JP-A-59-166655 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 302 C22C 33/04 C22C 38/28

Claims (3)

(57) [Claims] 1. C: 0.12% by weight or less, Si: 0.7
5% by weight or less, Mn: 1.0% by weight or less, P: 0.04
% By weight, Ni: 0.6% by weight or less , Cr: 16.0%
１８18.0% by weight, N: 0.04% by weight or less, Al:
It contains 0.01% by weight or less and Ti: 0.01 to 0.05% by weight, and the balance substantially has a composition of Fe, and S and O mixed as impurities are each S: 0.005% by weight or less. And O: a ferritic stainless steel excellent in workability, characterized in that A-based inclusions are replaced by C-based inclusions, which are regulated to 50 ppm or less. 2. A chromium-containing hot metal is refined, and C: 0.12% by weight or less, Si: 0.75% by weight or less, Mn: 1.0% by weight or less, P: 0.04% by weight or less, Ni: 0.6% by weight or less , Cr: 16.0 to 18.0% by weight, N: 0.0
4% by weight or less, Al: 0.01% by weight or less and Ti:
0.01 to 0.05% by weight, the balance being substantially F
e, the content of S and O mixed as impurities is regulated to 0.005% by weight or less of S and 50 ppm or less of O, respectively, and the stainless steel in which the A type inclusion is replaced by the C type inclusion is melted. In the production, at the time of adjusting the molten steel component in the final stage of the refining in the refining process, Ti or a Ti-containing material is added to the molten steel under the condition that the molten steel contains Si: 0.40% by weight or more. A method for producing ferritic stainless steel with excellent workability. 3. The method according to claim 1, wherein under the condition that the molten steel contains Si: 0.40% by weight or more, 1.5 to 2.0 Kg / ton in terms of Ti or Ti-containing material is added to the molten steel. 3. The method according to claim 2, wherein: