JPH07278650A - Production of ferritic stainless steel - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel strip of reduced scale flaw by applying mineral oil or carbonic fine particle on the surface of ferritic stainless slab, heating, forming uniform scale and then subjecting to hot rolling. CONSTITUTION:Before a ferritic stainless steel slab is charged in heating furnace, after one kind at least among mineral oil, fat, esther group, ether group, surfactant, carbon fine particle dispersed liquid is applied on the surface of slab, the slab is charged in heating furnace. By suppressing Cr2O3 film forming during heating, Fe oxide is formed at low temp. Successively, the slab is subjected to hot rolling. In heating step of slab, a lumpy oxidizing form is controlled so as to form uniform scale, by executing hot rolling at the condition of reduced ruggedness of metal surface, regardless of the condition of steel composition and hot deforming resistance, the occurrence of scale can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for effectively preventing surface scale flaws generated on the surface of a ferritic (Cr) stainless steel hot rolled sheet during slab heating and hot rolling.

[0002]

2. Description of the Related Art When scale flaws occur on the surface of a ferritic stainless hot rolled steel sheet, the flaws remain after mechanical descaling of the hot rolled sheet and after pickling, and also as fine surface flaws even after cold rolling. It remains and significantly impairs the surface aesthetics and corrosion resistance, which are important properties of stainless steel. Therefore, in the conventional hot rolling of stainless steel, the process is strictly controlled, and if scale flaws occur on the surface of the hot-rolled sheet, the number of surface conditioning steps such as pickling and grinding must be increased to remove them. Was not obtained, which was a cause of greatly reducing yield and production efficiency. Also,
When a scale flaw occurs, the hot rolling roll is also flawed, which adversely affects the rolling cost due to roll replacement or the like.

Conventionally, in order to prevent the occurrence of such surface defects, the rolling temperature of the material to be rolled is raised to reduce the deformation resistance during rolling, or the number of hot rolling rolling passes is increased in one pass. Measures have been taken such as reducing the rolling reduction of the.
However, even these methods have not yet completely prevented surface scale flaws, and have problems in terms of production efficiency and production cost.

From this point of view, Japanese Patent Publication No. 4-5740
Japanese Patent Publication No. 2 discloses a method of promoting oxidation by supplying an aqueous solution containing a carboxylic acid or a rolling lubricating oil containing a carboxylic acid to the surface where the oxide scale peels off during rolling and the metal portion is exposed. In addition, Japanese Patent Publication No. 4208
Japanese Patent Publication No. 2 similarly provides a rolling method in which air, oxygen gas, and steam are blown to a metal portion where the oxide scale is peeled off during rolling to promote oxidation and regenerate the scale to prevent surface defects. Is. Furthermore, JP-A-5
Japanese Patent Laid-Open No. 8-138501 discloses a method of adhering oxides, hydroxides, sulfides, salts and the like in a molten state to a flaw portion on the surface of a steel ingot and accelerating surface flaws on the attached portion.

However, these prior arts require the installation of a special spraying device in the hot rolling process, which is disadvantageous in terms of cost and increase of work load. Further, in the method of adhering oxides and the like in a molten state, Na and V, which easily form a molten salt with a steel ingot, are used, so that there is a problem in that they evaporate during heating and deteriorate the heating furnace. Further, these prior arts are countermeasures against seizure of a hot rolled sheet surface flaw with a rolling roll, or cracks or flaws on the slab surface, but the present inventors have detailed the hot rolled sheet surface flaw. As a result of the analysis, the cause of the majority of defects is related to the state of scale generated during slab heating.
It was strongly suggested that the cause was other than the above-mentioned causes.

[0006]

SUMMARY OF THE INVENTION From the above problems, the present invention suppresses the occurrence of scale flaws that may occur on the surface of a ferritic stainless steel hot-rolled sheet, and repickling after pickling following hot rolling and An object of the present invention is to provide a method for producing a ferritic stainless steel sheet, which can effectively solve the problem of increased work load such as surface grinding and can produce a ferritic stainless steel sheet having good surface properties at low cost.

[0007]

Means for Solving the Problems The gist of the present invention is that mineral oil, fats, esters, ethers are applied to the surface of a slab before charging a slab heating furnace for hot rolling a ferritic stainless steel slab. A surface scale flaw characterized in that a uniform and thick scale can be easily formed during the heating of the slab by applying at least one of the above, a surfactant, and a carbon fine particle dispersion, and then hot rolling is performed. There is little ferritic stainless steel manufacturing method.

According to the present invention, since a uniform thick scale can be easily formed at a low temperature by applying mineral oil or carbon-based fine particles on the surface, it is possible to suppress the occurrence of ridging or roping due to the coarsening of crystals. Further, there is a feature that a hot rolled steel strip having no scale flaw can be obtained after hot rolling without requiring a special device. The present invention will be described in detail below.

[0009]

In order to elucidate the cause of the scale defects on the surface of the hot rolled sheet, the inventors of the present invention used the outlet side of the heating furnace, the outlet side of the rough rolling mill in the hot rolling process, and after finishing rolling and pickling. As a result of thoroughly investigating the surface of the material at each stage and conducting some reproduction experiments, the following facts were discovered.

(1) Analysis of the tendency of surface scale flaws to occur in the factory shows that the Cr concentration is high, or Nb, Ti,
Defects are likely to occur in steel types containing Mo or the like and having good oxidation resistance. (2) From the observation of the slab surface on the outlet side of the heating furnace, a large number of nodular scales are recognized in the steel type where defects are likely to occur, or the scale thickness is extremely 0.4-1.5 mm. The surface is a mixture of thick parts and thin parts of about 10 μm. That is, since the oxidation in the bump-shaped scale or the extremely thick scale is much more advanced than in the surrounding area, the metal surface is dented at that portion, and the metal surface is uneven. On the other hand, in the case of a steel type that does not easily get scratched, the entire surface is uniform,
It is covered with a scale having a thickness of about mm, and the metal surface under the scale is smooth.

(3) Both the bump-shaped scale and the extremely thick scale portion have a two-layer structure, the outer layer is a Fe-rich oxide that is easily peeled off, and the inner layer is Cr, which has good adhesion.
It is a rich oxide. (4) On the slab heating furnace exit side, the dent on the metal surface caused by the oxide existing on the surface of the slab is extended in the rough hot rolling step to form a “scab” shape, and the oxide below it is removed. It will be covered. Further, in the finish hot rolling, the morphology thereof spreads and becomes shallower in the plate thickness direction, resulting in minute barbed surface flaws containing oxide inside.

(5) Most of the scale flaws observed on the surface of the steel sheet after pickling are very similar to the surface flaws of the above item (4), and the scale flaws are in the form of micro-shavings, and oxides dig under them. It is in a state. From these, the present inventors conclude that the scale flaw generated on the surface of the hot-rolled sheet is generated by rolling the surface unevenness caused by the oxidation in the slab heating stage. It was That is, in the bump-shaped scale as shown in FIG. 1A, the metal surface is highly uneven, and therefore, in the scale in which such things are scattered, scale flaws frequently occur and the upper layer as shown in FIG. It was clarified that a thick double scale of Fe oxide 5 and a lower layer of (Fe, Cr) oxide 6 covers the surface, and that when the metal surface is smooth, the scale flaw is remarkably small. Furthermore, the inventors of the present invention controlled such a bump-shaped oxidation morphology in the slab heating stage, and in hot rolling in a state where there are few irregularities on the metal surface forming a uniform scale, the composition of the steel and the hot rolling It was concluded that the occurrence of scale defects can be significantly reduced regardless of the deformation resistance of the.

Further, the inventors of the present invention have found from the study of the generation process of thick scale that the scale of ferritic stainless steel changes its structure with the increase of heating temperature and time as follows. (1) First, a dense and thin oxide film of Cr ₂ O ₃ is formed. (2) Then, as the heating temperature and the heating time increase, the Cr ₂ O ₃ film thickness increases, and a Cr-deficient region called a Cr-deficient layer is formed at the oxide film-metal interface.
Therefore, the supply of Cr to the oxide film becomes insufficient, and F
e is easily oxidized. Furthermore, an increase in the film thickness of Cr ₂ O ₃ causes destruction of the film, the metal surface depleted in Cr is directly exposed to oxygen, Fe oxidation is started, and Fe is locally oxidized. A region where the film becomes thick and a region where the oxidation of Fe does not proceed occur. It is the nodular scale that the local oxidation has progressed. The origin of the film destruction at that time may be due to random cracks due to the difference in mechanical properties such as the thermal expansion coefficient of the oxide / metal, and inclusions or internal oxide layers that concentrate at the oxide / metal interface. This was clarified by various analytical instruments such as EPMA.

(3) When the heating temperature and time are further increased, the density of the scaly scale portion increases, and finally a thick double scale is formed. From such knowledge, Fe oxidation is simultaneously advanced without forming Cr ₂ O ₃ , or the film thickness is increased so that the Cr ₂ O ₃ film is easily broken, or nuclei are generated inside. It was concluded that by increasing the number of fractured portions of the film, the density of scale-shaped scale formation can be increased to form a uniform double scale with few irregularities on the metal surface.

Table 1 shows Cr: 19%, Mo: 2.0%,
Nb: 0.4%, Ti: 0.2%, C: 0.003%,
N: Ferritic stainless steel containing 0.003%
LPG burned with or without mineral oil applied to the surface of
Formed by heating for 1 hour at each temperature in a baking atmosphere
Shows the results of identifying the oxides by X-ray diffraction.
It If not coated, identified by X-ray diffraction from 1000 ° C
Cr as possible film thickness₂O₃Starts to be detected, but 1
Since it has excellent oxidation resistance even at 250 ° C, Cr ₂O₃of
And is Fe at 1280 ° C.₃O_FourIs detected
Like However, its oxidized form is nodular scale
These are scattered, and the metal surface has severe irregularities. on the other hand,
When applied with mineral oil, at 800 ° C
(Fe, Cr)₂O₃Has already been detected, Cr₂O₃Shape
Not formed, most of it is Fe above 1100 ℃₃O_FourTona
It becomes a double scale that evenly covers the surface.

[0016]

[Table 1]

That is, by applying mineral oil to the surface,
Cr₂O₃Suppresses film formation and forms Fe oxide at low temperature
It can be achieved. The cause is the low temperature region
C in mineral oil reacts with Cr to form Cr carbide,
r₂O₃The growth of the oil and the H in the mineral oil is oxidized.
H₂O and H near the surface₂O partial pressure rises to H ₂
This is because abnormal oxidation due to O occurred. About this
For thin film X-ray diffraction and X-ray photoelectrons in the process of oxide film formation.
Presence of Cr carbide is confirmed by spectroscopy and further heating
H by atmosphere extraction gas analysis₂Check even if the amount of O increases
It was

Next, the reasons for limitation of the coating material targeted by the present invention will be explained. Scale defects of ferritic stainless steel are caused by the surface oxidation form in the slab heating furnace,
The present invention controls this and supplies the slab metal surface to the hot rolling step in a more smooth state. As a measure for preventing unevenness due to oxidation of the slab metal surface, C
Suppresses the formation of r ₂ O ₃ film to promote Fe oxidation and raises the partial pressure of H ₂ O near the surface to cause abnormal oxidation, and further destroys the Cr ₂ O ₃ film to form a high-density nodular scale. And a uniform double scale is formed on the surface to which they are connected.

To this end, it is necessary to select a substance that easily reacts with Cr and does not adversely affect the base stainless steel. As substances that easily react with Cr and suppress the formation of Cr ₂ O _3, there are chloride-based, sulfide-based, and carbide-based substances, but for chloride-based and sulfide-based substances, the inside of the slab heating furnace is It is contaminated and shortens the life of the heating furnace, and also has an adverse effect on subsequent heating steel grades. On the other hand, the carbide-based substance reacts with oxygen in the heating furnace at high temperature and is finally released as carbon dioxide gas, and has no influence on the heating furnace and the base stainless steel at all.

Further, even in the case of a carbide type substance, when a low boiling point organic solvent such as ethanol evaporates in a moment when it is necessary to maintain a temperature piece or before or immediately after introduction into a heating furnace, C
It has no r-carbide forming ability and needs to remain on the surface to some extent even in the range of at least about 100 to 300 ° C., and mineral oil used as rolling oil or the like is preferable. Further, even a high boiling point carbide-based substance preferably contains H and O to serve as an H ₂ O source and does not contain a large amount of harmful Cl, S, etc. in the furnace. Ester, ether, surfactant, etc. are preferable. Further, carbon fine particles such as coal powder and coke powder are also suitable as a carbon source, and these fine particles may be dispersed in a solvent such as mineral oil and applied on the surface. However, if the size of the fine particles is 50 μm or more, the density of formation of bump-shaped scales decreases, and the fine particles remain even after the heating in the heating furnace is completed.

[0021]

EXAMPLE A, B, C, D, whose chemical composition is shown in Table 2,
Tests were carried out on each of E, F, and G ferritic stainless steels. Table 3 shows a CC slab having a slab thickness of 150 to 300 mm and a slab width of 950 to 1250 mm produced by continuous casting, a slab whose surface layer was adjusted by grinding with a grinder, and a slab whose surface was adjusted by shot blasting as it was and with mineral oil. , Vacuum oil, tar naphthene, carbon fine particle dispersion, beef tallow, ester, vegetable oil, etc., and then set a target heating temperature of 1000 to 1270 ° C. in the subsequent slab heating furnace before hot rolling, and LNG (atmosphere is approximately N
₂ : 72-74 vol%, O ₂ : 0-8 vol%, CO
₂ : 6 to 10 vol%, H ₂ O: 12 to 18 vol
%), LPG (combustion atmosphere is approximately N ₂ : 73-81)
vol%, O ₂ : 0 to 8 vol%, CO ₂ : 5 to 12 v
_{ol%, H 2 O: 7~15vol} %), COG ( atmosphere approximately _{N 2: 70~74vol%, O 2} : 0~10
_{vol%, CO 2: 4~8vol%} , H 2 O: 12~2
2 vol%) in each annealing atmosphere at a temperature rising rate of 4 to 20 ° C./minute, a heating time of 60 to 180 minutes, and a dew point of the combustion atmosphere of 4
The results of heating at 5 to 65 ° C. and an oxygen concentration of 0.5 to 8%, interrupting hot rolling immediately after the end of heating, sampling after cooling, and performing macro observation and cross section micro observation are shown. It is a thing.

According to the present invention, a uniform and thick scale can be obtained in any steel type, and the uniform and thick scale forming temperature is 30 to 50 ° C., regardless of the heating conditions in which the scale becomes a bump-like scale without coating. It was possible to reduce the degree, and it was possible to reduce the unevenness of the metal surface under the scale. In Table 4, heating is performed in the same manner as in Table 3, hot rolling is performed after the heating is finished, a hot coil having a thickness of 2.5 to 5 mm is manufactured, and then short-time annealing is performed in a temperature range of 800 to 1000 ° C for 10 to 60 seconds. Or descaling by mechanical descaling or shot blasting, in which iron sand grains are mixed in high-pressure water and sprayed without annealing.
30-120 in 300g / l sulfuric acid solution at ~ 100 ° C
It shows the number of surface scale flaws in which the scale of the coil surface, which is smut-treated as it is or after smut-processed, is subjected to immersion pickling for 2 seconds and the surface layer is ablated by about 10 to 25 μm.

According to the present invention, a hot-rolled pickled steel strip having no scale flaw was obtained in any of the steel types.

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

[0027]

As described above, according to the present invention, C
Surface scale flaws that suppress the formation of bump-like scales that occur in ferritic stainless steels with an r content of 17% or more and are excellent in oxidation resistance, and that generate scales of uniform thickness and involve the scales after hot rolling. It is possible to obtain a hot-rolled sheet that does not generate a crack and to eliminate the step for removing the scale flaw, which is a great industrial effect. Furthermore, the Cr content is 17%
Even in ferritic stainless steel of less than, according to the present invention, the occurrence of surface scale flaws can be suppressed even when the slab heating temperature is lowered, so that the characteristics of the ferritic stainless steel can be improved, such as grain refinement. , Its effect is extremely large.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a metallurgical micrograph showing an example of an oxidation state generated on a surface of ferritic stainless steel when heated for hot rolling in a slab heating furnace, in which (a) is locally formed. A nodular scale is shown, and (b) shows a uniform thick scale.

[Explanation of symbols]

1 Slab metal surface 2 Thin protective oxide film 3 Nodular scale 4 Uniform thick scale 5 Upper scale (mainly Fe ₃ O ₄ ) 6 Lower scale ((Fe, Cr) ₂ O ₃ main)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Keiichi Omura, Inventor Keiichi Omura 1-1 Tobata-cho, Tobata-ku, Kitakyushu, Fukuoka Inside Nippon Steel Co., Ltd. Yawata Works (72) Kiyoshi Nishida 20-Shintomi, Futtsu, Chiba Prefecture 1 Nippon Steel Co., Ltd. Technology Development Division

Claims (1)

[Claims] 1. Prior to charging a slab heating furnace for hot rolling a ferritic stainless steel slab, at least one of mineral oil, fats and oils, esters, ethers, surfactants, and carbon fine particle dispersion liquid is applied to the surface of the slab. A method for producing a ferritic stainless steel with less surface scale flaws, characterized in that a uniform and thick scale is easily formed during heating of the slab by applying one kind, and then hot rolling is performed.