JP2001049327A - Production of ferritic stainless steel excellent in ridging resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a ferritic stainless steel containing less oxide base and being excellent in ridging resistance. SOLUTION: When the ferritic stainless steel containing 9-32 wt.% Cr and <0.005 wt.% Al is melted, quantities of CaO and Al are added so that slag composition (CaO wt.%)/(Al2O3 wt.%) becomes 0.7-2.5. The molten steel is stirred by blowing inert gas until an area ratio of the oxide base contained becomes <=0.05%, and the molten steel at 20-70 deg.C overheat degree is continuously cast to produce a cast slab halving >=60% equi-axed crystal ratio as the area ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel having an increased equiaxed crystal ratio and improved ridging resistance by controlling the form of oxide inclusions.

[0002]

2. Description of the Related Art Ferritic stainless steel represented by SUS430 has good workability and corrosion resistance and is relatively inexpensive, and is therefore used in a wide range of fields as kitchen equipment, electric products, automotive materials and the like. I have. However, when cold-working such as deep drawing and bending is applied to a steel sheet manufactured by rolling a continuous cast slab of ferritic stainless steel, striped undulations called ridging occur along the rolling direction, May be significantly impaired. Ridging is generally caused by the fact that the coarse columnar crystal structure generated during continuous casting is not sufficiently destroyed in the hot rolling process, and that the texture consisting of the coarse band-like structure remains. It is considered.

Therefore, in order to suppress the occurrence of ridging or band-like structure, a method of increasing the equiaxed crystal ratio when producing a slab by a continuous casting method (Japanese Patent Laid-Open No. 9-49010,
Japanese Unexamined Patent Publication No. 2-250925) and a method of refining the structure by repeating cold rolling and annealing a plurality of times after hot rolling to promote recrystallization have been proposed. The method of refining the structure by repeating cold rolling and annealing is effective in eliminating the band-like structure that causes ridging, but the load on the process is required because multiple cold rolling and annealing are required. As a result, the manufacturing cost is increased. Therefore, it cannot be said to be a method suitable for mass-produced steel types. On the other hand, the method of increasing the equiaxed crystal ratio at the slab production stage is
This is an industrially advantageous method because it is not necessary to repeat cold rolling and annealing a plurality of times. As a means for increasing the equiaxed crystal ratio, a method of casting molten steel at a relatively low temperature, a method of casting molten steel with electromagnetic stirring, and the like are adopted.

[0004]

In order to raise the equiaxed crystal ratio by low-temperature casting, it is necessary to lower the casting temperature to near the solidification temperature of molten steel for casting. In such a low-temperature casting, troubles such as nozzle clogging are apt to occur during the operation, and it is difficult to carry out the operation on a mass-production basis. On the other hand,
Although electromagnetic stirring of molten steel is effective for equiaxed crystallization of the solidified structure, the equiaxed crystal ratio that can be stably achieved is limited to about 40 to 50%, and a steel strip excellent in ridging resistance by ordinary rolling can be obtained. The obtained equiaxed crystal ratio does not reach the lower limit of 60%. by the way,
It is said that when Ti is added to ferritic stainless steel and TiN precipitated in molten steel is used as a solidification nucleus of ferrite, the solidified structure tends to be equiaxed. However, depending on the type of steel, the solidification structure does not always become equiaxed by simple Ti addition. Further, in order to improve the equiaxed crystal ratio, a large amount of Ti must be added, but excessive Ti addition promotes generation of defects such as nozzle clogging and surface flaws. In this regard, equiaxed crystallization of the solidified structure due to the addition of Ti cannot be simply considered as a measure.

[0005]

SUMMARY OF THE INVENTION The present invention has been devised in order to solve such a problem, and has been proposed by controlling the form of oxide inclusions dispersed and precipitated in steel. An object of the present invention is to produce a ferritic stainless steel having an increased axial crystal ratio and improved ridging resistance. In the production method of the present invention, in order to achieve the object, when melting a ferritic stainless steel containing 9 to 32% by weight of Cr and less than 0.005% by weight of Al, the slag composition is (CaO% by weight). / (SiO ₂ wt%) = CaO and Si in an amount of 0.7 to 2.5 are added, and molten steel is blown with inert gas until the area ratio of the oxide-based inclusions becomes 0.05% or less. Is stirred, and molten steel having a superheat degree of 20 to 70 ° C. is continuously cast to obtain a slab having an area ratio of 60% or more of equiaxed crystal ratio.

[0006]

The present inventors have observed the morphology of inclusions formed in a slab obtained from molten steel deoxidized with Si, and investigated the relationship between the inclusions and the equiaxed crystal ratio. Inclusions generated in the slab are
Depending on the type of steel, oxide inclusions such as SiO ₂ (hereinafter, referred to as silica), MnO.SiO ₂ (hereinafter, referred to as manganese silicate), nitride inclusions such as TiN, MnS, etc. There are sulfide inclusions. Among these inclusions, it was found that in the slab having a high equiaxed crystal ratio, the cleanliness was high and the amount of oxide-based inclusions was small. Further investigations and examinations were conducted on the quantitative relationship between the area ratio of the oxide-based inclusions and the equiaxed crystal ratio. When the area ratio of the oxide-based inclusions became 0.05% or less, electromagnetic stirring and the like were performed. Elucidated that it is possible to produce slabs with an equiaxed crystal ratio of 60% or more and to obtain ferritic stainless steel with excellent ridging resistance, without having to employ special equipment or difficult casting conditions such as low-temperature casting. .

[0007] Oxide-based inclusions are compounds produced by a refining reaction and are suspended in molten steel as reaction products.
From the results of investigations and research conducted by the present inventors, it has been found that in a system containing a large amount of oxide-based inclusions, the equiaxed crystal ratio of a slab obtained by continuous casting is reduced, and ridging resistance tends to be deteriorated. Was done. The adverse effects of oxide inclusions on ridging resistance are:
Due to the presence of oxide-based inclusions in molten steel for a long time from the time of refining, it has some effect on the solidification mechanism, such as affecting the degree of supercooling during the formation of primary ferrite. It is presumed that it has given. Therefore, oxide inclusions that exist in molten steel for a long time are separated and separated into slag as quickly as possible during refining.
Need to absorb.

The area ratio of inclusions is determined by observing a cross section perpendicular to the casting direction and calculating the ratio occupied by inclusions. For example, according to the “method for measuring inclusion cleanliness” defined in JIS G0555. Can be measured in compliance. However, it is necessary to classify the inclusions observed in the observed cross section and count the area ratio of the oxide-based inclusions. The sulfide-based inclusions and the nitride-based inclusions may be present alone, or may be present in combination with the oxide-based inclusions. In the present specification, a single oxide-based inclusion and a composite inclusion with a sulfide are treated as an oxide-based inclusion, and a composite form of a nitride and an oxide is excluded. The oxide-nitride composite inclusion exists in steel as a form in which nitride precipitates around the oxide, a form in which oxide and nitride are combined, and the like. Investigation of the effect of the area ratio of each oxide on the equiaxed crystal ratio revealed that the oxide-based inclusions alone and the composite inclusions of oxide and sulfide reduced the equiaxed crystal ratio when the area ratio was high. However, no effect on the equiaxed crystal ratio was detected in the composite inclusion of the oxide and the nitride. Therefore, even if the oxide-based inclusions are controlled while excluding the composite inclusions of the oxide and the nitride, there is no problem in improving the equiaxed crystal ratio.

In order to reduce oxide-based inclusions, Si is added as a deoxidizer in a vacuum atmosphere or an inert atmosphere to add CaO
To produce a slag mainly comprising -SiO ₂ system, the slag / metal and stirred silica, refining process to sufficiently absorb oxide inclusions, such as manganese silicate-based slag is employed. At this time, a slag containing a slag-making agent such as CaF ₂ can also be used, and in addition to Si, Mn, Ti, Ca, Mg,
One or more REMs (rare earths) can be used in combination as a deoxidizing agent. The deoxidizing agent is added to the molten steel prior to or during the stirring of the molten steel by blowing the inert gas. When the deoxidizing agent is added during the blowing of the inert gas, the deoxidizing agent placed on the inert gas can be fed into the molten steel. When the molten steel to which the deoxidizing agent is added is stirred by blowing inert gas, the generated oxide-based inclusions coalesce and float to separate from the molten steel. The stirring time is not particularly limited to the present invention, but is preferably 5 minutes or more in order to obtain a sufficient deoxidizing effect. After the stirring, the degree of superheat of the molten steel in the tundish (= the molten steel temperature during casting−the liquidus temperature of the molten steel) is maintained at 20 to 70 ° C., and the molten steel is continuously cast into slabs, billets, blooms, and the like. When the degree of superheat is adjusted in this way, a cast slab having an equiaxed crystal ratio of 60% or more can be obtained without the need for electromagnetic stirring. However, the use of electromagnetic stirring further increases the equiaxed crystal ratio. The equiaxed crystal ratio is determined by the area ratio of the equiaxed crystal zone in a cross section perpendicular to the casting direction of the slab.

Hereinafter, the manufacturing conditions specified in the present invention will be described. Slag composition: (CaO wt%) / (SiO ₂ wt%) =
During refining of 0.7 to 2.5 stainless steel, CaO (slag component) and Si (deoxidizer) are added to molten steel to deoxidize. The added Si is partially oxidized to SiO ₂ , and CaO—S
It becomes a component of iO ₂ -based slag. The oxide inclusions in the molten steel in order to efficiently absorb the slag, since absorption of oxide inclusions as CaO concentration increases is improved, (CaO wt%) / (SiO ₂ wt%) ≧ It is necessary to use a slag of 0.7. However, if the CaO concentration becomes too high, the melting point of the slag rises, and the effect of absorbing oxide-based inclusions also ceases. Therefore, set to 2.5 to the upper limit of (CaO wt%) / (SiO ₂ wt%). Ratio of (CaO wt%) / (SiO ₂ wt%),
It is controlled by adjusting the addition amounts of CaO (slag component) and Si (deoxidizer).

[0011] Superheat degree of molten steel during continuous casting: When the superheat degree of molten steel during continuous casting at 20 to 70 ° C is low, the equiaxed crystal ratio increases, but troubles such as nozzle clogging tend to occur frequently. Therefore, in order to prevent trouble and enable stable operation, the degree of superheat of the molten steel in the tundish during continuous casting is set to 20 ° C. or more. However, if the degree of superheat exceeds 70 ° C., a solidified shell is generated unevenly in the mold, and quality problems such as surface cracks are liable to occur in the slab.

Alloying components of stainless steel Increasing the equiaxed crystal ratio by controlling the morphology of oxides is applied to various ferritic stainless steels. The ferritic stainless steels targeted by the present invention have the following alloying components. Contains. Cr: 9 to 32% by weight An alloy component effective for improving corrosion resistance, and the addition effect becomes significant when the content is 9% by weight or more. Corrosion resistance improves as the Cr content increases, but the upper limit of the Cr content is set to 32% by weight from the viewpoint of economy. Al: less than 0.005% by weight In the present invention, since the steel type subjected to Si deoxidation is targeted, the Al content is suppressed to a low value of less than 0.005% by weight. When the Al content in the steel increases, the oxide-based inclusions become alumina-based, spinel-based, and the like, deviating from the conditions specified in the present invention.

C: 0.15% by weight or less Since the corrosion resistance of stainless steel is improved as the C content is lower, the upper limit of the C content is set to 0.15% by weight in the ferritic stainless steel targeted by the present invention. Is preferred. Si: 1.0% by weight or less Si is an alloy component that is used as a deoxidizing agent for molten steel and is effective for improving strength. However, if contained excessively, the workability is deteriorated. Therefore, it is preferable to set the upper limit of the Si content to 1.0% by weight. Mn: 1.0% by weight or less The effect of improving the manufacturability and fixing the harmful element S in steel as MnS is exhibited. However, excessive addition of Mn exceeding 1.0% by weight causes a reduction in corrosion resistance.
It is preferable to set the upper limit of the n content to 1.0% by weight.

The ferritic stainless steels to which the present invention is applied have, in addition to the alloy components listed above, 3.0% by weight or less of Mo effective for improving corrosion resistance and 1.0% effective for improving strength.
% Or less of Zr, 1.0% by weight or less of Ti effective for improving workability, and 1.0% by weight or less of N effective for improving workability.
b, V of 1.0% by weight or less effective for improving strength, B and / or RE of 0.05% by weight or less effective for improving hot workability.
One or more kinds of M (rare earth) may be contained. Further, one or more of Y, Ca, Mg and W may be contained as other optional components. S and P contained as impurities are 0.02% by weight or less and 0.05% by weight, respectively.
There is no adverse effect on the properties as long as the content is regulated to not more than% by weight.

[0015]

[Example] SUS430 ferritic stainless steel (7
(0 tons / charge) through an electric furnace, a converter, and a VOD process, and was continuously cast into a slab. In the vacuum refining, the degree of vacuum is maintained at 50 to 200 Pa, and (CaO weight%) / (S
CaO and Si are added so that iO ₂ wt%) becomes a predetermined value, and the flow rate is 300 to 500 N through a porous plug.
The molten steel was stirred by blowing Ar gas into the molten steel at L / min. Table 1 shows the composition of the obtained ingots, and Table 2 shows the production conditions. From each manufactured slab, a test piece was cut out in a cross section orthogonal to the casting direction. The cross section of the test piece was observed, the ratio of the thickness of the equiaxed crystal zone to the thickness of the slab was measured at several points, and the measured values were averaged to calculate the equiaxed crystal ratio. Also,
Cut out a specimen for optical microscope observation from the same slab,
The area ratio of oxide-based inclusions was measured in accordance with JIS G0555 “Method for measuring inclusion cleanliness”. Further, the slab was rolled according to a conventional method, and a JIS No. 5 tensile test piece was cut out from the obtained cold-rolled sheet. After the tensile test piece was mirror-polished, it was pulled at a deformation rate of 20%, and the ridging was visually observed. The degree of ridging was determined from the observation results, and a ridging level of 2 or less was evaluated as a ridging level having no practical problem and evaluated on a 5-point scale.

As can be seen from the investigation results in Table 2, in Test Nos. 1 to 5 in which the operating conditions satisfy the conditions specified in the present invention, the area ratio of the oxide-based inclusions becomes 0.05% or less. The equiaxed crystal ratio in the slab cross section was as high as 60% or more, and the ridging judgment after cold rolling was also good. On the other hand, in Test Nos. 6 to 10 (Comparative Example), the area ratio of the oxide-based inclusions exceeded 0.05%, and the equiaxed crystal ratio did not reach 60% in any of the slabs. Later ridging decisions were also bad. As is clear from this comparison, by setting the area ratio of the oxide-based inclusions of the continuously cast slab to 0.05% or less, the equiaxed crystal ratio at the slab stage is increased, and the ridging resistance is excellent. It was confirmed that a steel sheet was obtained.

[0017]

[0018]

[0019]

As described above, in the present invention, (CaO wt%) / (SiO ₂ wt%) =
By generating slag of 0.7 to 2.5 and stirring the molten steel by blowing inert gas, oxide-based inclusions in the molten steel are effectively absorbed by the slag, and oxides in the slab stage are removed. The system inclusion is suppressed to an area ratio of 0.05% or less. By suppressing oxide-based inclusions in this manner, a slab having an equiaxed crystal ratio of more than 60% can be obtained without the necessity of low-temperature casting or the addition of Ti, and even when cold-working such as deep drawing and bending is performed. A steel strip with excellent ridging resistance without wrinkle-like undulations is manufactured. By using the excellent appearance, materials used in a wide range of fields such as kitchen appliances, various electric appliances, and steel plates for automobiles are provided. .

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[Procedure amendment]

[Submission date] August 5, 1999 (1999.8.5)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Name of invention

[Correction method] Change

[Correction contents]

Patent application title: Method for producing ferritic stainless steel with excellent ridging resistance

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Shuhisa Hiruhama 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture F-term in Nissin Steel Engineering Co., Ltd. F-term (reference) 4K013 AA02 AA09 CB03 CC01

Claims (1)

[Claims] 1. Cr: 9 to 32% by weight and Al: 0.0
When melting the ferritic stainless steel comprises less than 05 wt%, was added in an amount of CaO and Si slag composition is (CaO wt%) / (SiO ₂ wt%) = 0.7 to 2.5, The molten steel was stirred by blowing inert gas until the area ratio of the oxide-based inclusions became 0.05% or less, and the degree of superheat was increased to 20%.
Continuous casting of molten steel at ~ 70 ° C, 60% equiaxed crystal ratio
A method for producing a ferritic stainless steel having excellent ridging resistance, characterized by obtaining the above slab.