JPH07284804A - Method for producing ferritic stainless steel - Google Patents

Abstract

(57) [Abstract] [Purpose] An object of the present invention is to provide a manufacturing method which does not generate surface flaws involving a scale after hot rolling a CC slab slab of a ferritic stainless steel containing iron, carbon and Cr. . [Structure] Before charging a slab heating furnace for hot rolling a ferritic stainless steel slab, at least one method of grinding the slab surface with a grinder or shot blast, or cutting with a cutting tool or a milling cutter is used to obtain the surface roughness. By setting Rmax to 20 μm or more, a uniform and thick scale can be easily formed during slab heating, and then hot rolling is performed.

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 hot rolled sheet of Cr (ferrite) stainless steel in a slab heating and hot rolling process.

[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 significant reduction in yield and production efficiency. Further, when scale flaws are generated, the hot rolling rolls are also flawed, which adversely affects the rolling cost due to roll replacement and the like.

Conventionally, in order to prevent such surface defects from occurring, the rolling temperature of the material to be rolled is raised to reduce the deformation resistance during rolling, or the number of hot rolling passes is increased to reduce the rolling reduction in one pass. Measures have been taken to reduce the above. However, it has not completely prevented surface scale flaws,
Moreover, there was a problem in terms of production efficiency and production cost. From such a viewpoint, in Japanese Patent Publication No. 4-57402, oxidation is performed 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 is peeled off and the metal part is exposed during rolling. A method of promoting the is disclosed. In Japanese Patent Publication No. 4-42082, similarly, rolling is performed by blowing air, oxygen gas, or steam to the metal portion where the oxide scale is peeled during rolling to promote oxidation and regenerate the scale to prevent surface defects. Offering the law.

However, these prior arts require the installation of special spraying equipment in the hot rolling process,
A large modification is required to install the hot rolling mill in a narrow space. Further, these prior art is a countermeasure when the surface flaw of the hot-rolled sheet is caused by seizure with the rolling roll, or the crack or flaw of the slab surface, but the present inventors have detailed the hot-rolled sheet surface flaw. As a result of analysis, the cause 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.

[0005]

SUMMARY OF THE INVENTION In view of 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 after hot rolling and the surface. A method for producing a ferritic stainless steel sheet capable of effectively solving the problem of increased work load such as grinding and producing a ferritic stainless steel sheet having good surface properties without installing special equipment in the hot rolling process. The purpose is to provide.

[0006]

Means for Solving the Problems That is, the gist of the present invention is to remove surface defects and flaws from the prior art before charging a slab heating furnace for hot rolling a ferritic stainless steel slab. The purpose is not to grind the surface of the slab by grinding or shot blasting, which is used for the purpose of smoothing the surface of the slab, but to form irregularities on the surface of the slab with Rmax of 20 μm or more by these methods. Abnormal oxidation, that is, by acting as a breakaway nucleus to promote the oxidation, a uniform and thick scale can be easily formed, and then hot rolling is performed, and there are few surface scale flaws. It is in the method for manufacturing ferritic stainless steel.
Furthermore, since a uniform thick scale can be easily formed at this temperature by this method, it is possible to obtain a hot-rolled steel strip without scale flaws after hot rolling that suppresses the occurrence of ridging and roping due to the coarsening of crystals. There is a feature that can be. The present invention will be described in detail below.

[0007]

In order to elucidate the cause of 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 step, and after finishing rolling and after pickling. As a result of thoroughly investigating the material surface at the stage and conducting some reproduction experiments, we came to discover the following facts. 1) In the analysis of the surface scale flaw occurrence tendency, flaws are likely to occur in steel types having a high Cr concentration or containing Nb, Ti, Mo, etc. 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 found in steel types that are prone to flaws, or the scale thickness is 0.4 to 1.
The surface is a mixture of extremely thick areas of about 5 mm and thin areas of about 10 μm. That is, in the bump-shaped scale or the extremely thick scale, the oxidation progresses significantly more than the surrounding area, and as a result, the metal surface is dented at that portion and the base metal surface is uneven. On the other hand, in the case of a steel type that does not easily generate a flaw, the entire surface is covered with a uniform scale having a thickness of about 1.0 mm, and the underlying 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) The dent on the metal surface caused by the oxide existing on the surface of the slab on the exit side of the slab heating furnace has its edge extended in a rough hot rolling step to form a "scab" shape, and covers the oxide underneath. Be in shape. Further, in the final hot rolling, the morphology thereof spreads and becomes shallow in the plate thickness direction, resulting in a minute whitish surface flaw containing an 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 described in 4) above, and are in the form of micro-shavings, in which oxide is eroded below. .

From the above, the inventors of the present invention have found that the scale flaw generated on the surface of the hot-rolled sheet is generated by rolling the unevenness of the metal surface caused by the oxidation in the slab heating stage. I came to the conclusion. That is, FIG.
A scale-like scale 3 is formed on the surface of the slab 1 as shown in (a).
In the case where the metal oxide is formed, since the metal surface is highly uneven, scale flaws frequently occur on a scale in which such a material is scattered. As shown in FIG. 1B, the upper layer is Fe oxide 4, and the lower layer is (Fe, It was clarified that a thick double scale of Cr) oxide 5 covers the surface, and that the scale flaw is remarkably small when the metal surface is smooth. Furthermore, the inventors of the present invention controlled the bump-shaped oxidation form in the slab heating stage, and performed hot rolling in a state where the metal surface on which the uniform scale 6 was formed had few irregularities, the composition of the steel and the hot rolling. It was concluded that the occurrence of scale flaws can be significantly reduced regardless of the deformation resistance in the.

Further, the present inventors have found from the study of the generation process of the nodular scale and the thick scale that the ferritic stainless steel scale is oxidized by the following mechanism. (1) First, Cr ₂ O is formed on the metal surface 1 as shown in FIG.
Dense and thin oxide film 12 of ₃ is formed. (2) Next, as the heating temperature and the heating time increase, the thickness of the Cr ₂ O ₃ coating increases, and the Cr-deficient layer 13 with a small amount of Cr at the metal interface with the oxide coating 12 as shown in FIG. 2B.
Is formed. Therefore, the supply of Cr to the oxide film becomes insufficient, and Fe is easily oxidized. (3) Further, the increase in the oxide film thickness of Cr ₂ O ₃ is shown in FIG.
As shown in (c), the surface of the Cr-deficient metal surface is directly exposed to oxygen through cracks, Fe oxidation starts, and Fe is locally oxidized to increase the thickness of the film. Region and a region where Fe oxidation does not proceed. It is the lump-shaped scale 15 where the local oxidation has progressed. In this case, the origin of the film destruction may be caused by 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 concentrated at the oxide / metal interface. Was clarified by various analytical instruments such as EPMA. (4) When the heating temperature and time are further increased, FIG.
As shown in Fig. 2 (e), the bump-shaped scale 15 grows large and is finally connected to form a thick double scale 18.

From the above knowledge, a Cr-deficient layer is formed on the interface between the oxide film and the underlayer, and the number of cracks is increased to increase the density of bump-shaped scale formation. It was concluded that a uniform double scale with less can be formed. Table 1 shows Cr: 1
9%, Mo: 2.0%, Nb: 0.4%, Ti: 0.2
%, C: 0.003%, N: 0.003%, the surface of the ferritic stainless steel is cut with a cutting tool to have a surface roughness Rmax of 3 μm, 18 μm, 20 μm,
The results of observing the oxidation state formed by heating at 1230 ° C. for 1 hour in an LPG combustion atmosphere using samples of 50 μm, 100 μm, and 140 μm are shown. Under any of the surface roughness conditions, the bump-shaped scale is formed on the convex portion of the cutting mark, and the size of the bump-shaped scale is increased as the surface roughness Rmax becomes rough from 3 μm to 140 μm. Is less than 18 μm, the bump-shaped scales are small in size, so that they are not connected to each other, and a thick portion and a thin portion of the scale are mixed. On the other hand, when the surface roughness is 20 μm or more, the bump-shaped scales formed on the protrusions are connected to each other, and the scale becomes uniform and thick.

[0013]

[Table 1]

The above results are due to the difference in the supply process to the oxide film and the easiness of cracking of the film at the convex portion and the smooth portion. That is, when a Cr oxide layer having the same thickness is formed on the projection and the smooth surface, as shown in FIG. 3, at the point A, Cr is supplied only directly to the surface, but the projection B
At the point, it is necessary to supply not only directly to both sides but also to both sides, and thus the deficiency state of Cr at the point B becomes larger than that at the point A. Further, when an oxide is formed on the metal surface, stress is generated at the interface due to the difference in expansion coefficient, but the size thereof becomes large at the convex portion and becomes a starting point of cracks. That is, as described above, since the bump-shaped scale has a Cr-deficient layer and is formed in a place where a crack is likely to occur in the coating, the bump-shaped scale is easily formed starting from the convex portion. This was confirmed by observing the initial formation process of the nodular scale with an optical microscope, EPMA or the like, using samples prepared under various conditions in which the heating temperature and the heating time were changed.

Next, the reasons for limiting the surface roughness in the present invention will be described. The scale flaw of the present invention is caused by the surface oxidization morphology in the slab heating furnace, which is supplied to the hot rolling step while controlling the slab metal surface in a smoother state. As a method, means for forming a Cr-deficient layer and further forming a structure in which cracks are likely to occur on the surface to generate a high-density bump-shaped scale and form a uniform double scale on the surface where they are connected To adopt.

For that purpose, it is preferable that the supply of Cr from the underlayer is insufficient before the slab is heated, and the convex portions where stress is likely to be concentrated on the interface between the formed oxide film and the underlayer are formed at a high density on the surface. However, in the case where the roughness of the convex portion is less than 20 μm in Rmax, the generation density of the nodular scale is higher than that of the smooth surface, but the nodular scale to be formed has a small shape and the mutual connection ability of the nodular scales is large. In order to form a small and uniform double scale, high temperature and long time heating are required, so the lower limit was made 20 μm. The upper limit is not limited because the occurrence frequency and shape of bump-like scales increase with the increase of surface roughness and it is easy to connect with each other and a uniform thick scale can be formed. However, when the surface roughness exceeds 1 mm, scale-off occurs. The surface roughness is set to 1 because the amount increases and the effect of unevenness in processing and processing distortion may remain.
It is preferably not more than mm.

The condition required for the unevenness of the surface for forming a uniform scale in the present invention is not only the average height of the above-mentioned convex portions of the surface but also the interval between the convex portions. That is, if the intervals of the uneven portions are widely separated such as between the oscillation marks such as a cast material, high temperature and long time heating are required for connecting the formed bump-shaped scales, and the convex portions It is desirable that the intervals be close to each other to some extent. Therefore, as a means for forming the unevenness on the surface, cutting using a cutting tool, a milling cutter, grinding such as a grinder, shot blasting, or the like is preferable.

[0018]

EXAMPLE A, B, C, D, whose chemical composition is shown in Table 2,
Tests were performed on each of the ferritic stainless steels E and F. Table 3 shows the slab thickness 1 created by continuous casting
A CC slab having a width of 50 to 300 mm and a slab width of 950 to 1250 mm is cut with a bite and a milling machine, # 12 to # 400 grinder is ground, and a steel grid having a grain size of 0.2 to 0.5 mm is used to obtain a projection density of 7 to The surface roughness is Rma by shot blasting in the range of 100 kg / m ^2.
The cast slab finished to 6 μm to 245 μm with x is set to a target heating temperature before hot rolling of 1000 ° C. to 1270 ° C. in a subsequent slab heating furnace, and LNG (atmosphere is approximately N ₂ : 72 to 7).
4 vol%, O ₂ : 0-8 vol%, CO ₂ : 6-10 vol
_{%, H 2 O: 12~18 vol} %), LPG ( combustion atmosphere is approximately _{N 2: 73~81 vol%, O} 2: 0~8 vol
%, CO ₂ : 5 to 12 vol%, H ₂ O: 7 to 15 vol
%), COG (atmosphere is approximately N ₂ : 70 to 74 vol)
%, O ₂ : 0 to 10 vol%, CO ₂ : 4 to 8 vol%, H
₂ O: 12 to 22 vol%) Heating rate 4 in each combustion atmosphere
The heating time is 60 to 180 minutes at -20 ° C / minute, the dew point of the combustion atmosphere is 45 to 65 ° C, and the oxygen concentration is 0.5% to 8%. The results of the macro observation and the micro observation of the cross-section are shown by suspending the measurement, cooling and sampling.

According to the present invention, the surface roughness Rmax is 19
Despite a heating condition that a muddy scale is obtained when the thickness is less than μm, a uniform thick scale can be obtained in any steel type having a surface roughness Rmax of 20 or more, and the uniform thick scale forming temperature is 20 ° C to 50 ° C. It was possible to reduce the temperature by about 0 ° C., 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, and hot rolling is performed after the heating is finished to manufacture a hot coil having a thickness of 2.5 to 5 mm, and thereafter 10 to 60 in a temperature range of 800 to 1000 ° C.
50 to 10 after descaling by mechanical descaling or shot blasting in which iron sand grains are mixed and sprayed in high pressure water without annealing for a short time of 10 seconds
The number of surface scale flaws, which was obtained by dipping and pickling the surface layer for about 10 to 25 μm in a 300 g / l sulfuric acid solution at 0 ° C., and shaving the surface layer by about 10 to 25 μm, and then involving the scale of the coil surface which was smut-treated. Indicates. According to the present invention, it is possible to obtain a hot rolled pickled steel strip which has no scale flaw in any of the steel grades having a surface roughness Rmax of 20 μm or more.

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

As described above, according to the present invention, it is possible to easily generate a scale having a uniform thickness by forming and connecting bump-shaped scales of ferritic stainless steel with high density, and A hot-rolled sheet that does not cause surface scale flaws in which scales are rolled up after rolling is obtained, and the process for removing scale flaws can be omitted, which has a large industrial effect. Furthermore, the temperature for forming a uniform thick scale can be reduced, and the slab heating temperature can be lowered.
The effect is extremely large because the characteristics of ferritic stainless steel are improved by making the crystal grains finer.

[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.

FIG. 2 is a schematic diagram of the process of forming an oxide film formed on the surface of ferritic stainless steel when heated for hot rolling in a slab heating furnace. Growth of film and formation of Cr deficient layer, (c) Destruction of protective film and penetration of oxygen etc., (d) Formation of bump-like scale after breakaway, (e) Formation of thick scale by connection of bump-like scale Indicates the status.

FIG. 3 When a ferritic stainless steel is heated for hot rolling in a slab heating furnace, a smooth surface (point A) and a convex portion (B)
The difference in the replenishment state of Cr at the point) indicates that the point A is only directly above, but the point B is immediately above, and it is necessary to replenish both ends.

[Explanation of symbols]

1 ... Slab metal surface 2 ... Thin protective oxide film 3 ... Nodular scale 4 ... Outer layer scale (mainly Fe ₃ O ₄ ) 5 ... Inner layer scale (mainly (Fe, Cr) ₂ O ₃ ) 6 ... Uniform thick scale 11 ... Slab metal part 12 ... Protective oxide film 13 ... Cr deficient layer 14 ... Oxidation progressing region 15 ... Rough scale 16 ... Outer layer scale (mainly Fe ₃ O ₄ ) 17 ... Inner layer scale ((Fe, Cr) ₂ O ₃ main body) ) 18: Uniform thick scale A: Point in smooth portion (Cr diffuses only directly above) B: Point in convex portion (Cr directly diffuses on both ends)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location B24C 1/00 C (72) Inventor Keiichi Omura No. 1 Hibatacho, Tobata-ku, Kitakyushu, Fukuoka Prefecture New Nippon Steel Co., Ltd., Yawata Works

Claims (1)

[Claims] 1. The surface of the slab is treated by at least one method of grinding by a grinder or shot blasting, or cutting by a cutting tool or a milling machine before charging a slab heating furnace for hot rolling a ferritic stainless steel slab. A method for producing a ferritic stainless steel with less surface scale flaws, characterized in that a uniform and thick scale can be easily formed during slab heating by setting the roughness Rmax to 20 μm or more, and then hot rolling is performed. .