JP3067943B2 - Manufacturing method of ferritic stainless steel - Google Patents

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 Cr-based (ferritic) stainless steel sheet in a slab heating and hot rolling process.

[0002]

2. Description of the Related Art When scale flaws are generated on the surface of a hot-rolled ferritic stainless steel sheet, the flaws remain after mechanical descaling and pickling of the hot-rolled sheet, and as fine surface flaws even after cold rolling. It remains and significantly impairs the aesthetics and corrosion resistance of the surface, which are important properties of stainless steel. For this reason, in the conventional hot rolling of stainless steel, the process is strictly controlled, and when scale flaws occur on the surface of the hot-rolled sheet, the number of surface adjustment steps such as pickling and grinding must be increased to remove the scale flaws. And the yield and production efficiency were greatly reduced. Further, when scale flaws occur, flaws also occur on the hot rolling rolls, which adversely affects rolling costs due to roll replacement and the like.

Conventionally, in order to prevent the occurrence of such surface defects, the rolling temperature of the material to be rolled is increased to reduce the deformation resistance during rolling, or the number of hot rolling is increased to increase the rolling reduction in one pass. Countermeasures have been taken, such as reducing emissions. However, surface scale flaws have not been completely prevented,
Moreover, there were problems in terms of production efficiency and production cost. From this viewpoint, Japanese Patent Publication No. 4-57402 discloses that an oxidized scale is separated by supplying an aqueous solution containing a carboxylic acid or a rolling lubricant containing a carboxylic acid to a surface where a metal part is exposed due to peeling of an oxide scale during rolling. Is disclosed. In Japanese Patent Publication No. 4-42082, air, oxygen gas, and water vapor are sprayed on a metal portion from which oxide scale has been peeled off during rolling to promote oxidation and regenerate scale to prevent surface flaws. Offers the law.

[0004] However, these prior arts require special spraying equipment installation in the hot rolling process,
In order to install it in a narrow space of a hot rolling machine, a large modification is required. Furthermore, these prior arts are countermeasures when the surface flaw of the hot rolled sheet is caused by seizure with a rolling roll or cracks or flaws on the slab surface. As a result of the analysis, the cause of the flaw is related to the state of the scale generated during slab heating,
It was strongly suggested that the cause was other than the above.

[0005]

SUMMARY OF THE INVENTION In view of the above problems, the present invention suppresses the occurrence of scale flaws which may occur on the surface of a hot-rolled ferritic stainless steel sheet, and performs re-pickling or pickling after pickling following hot rolling. A method of manufacturing a ferritic stainless steel sheet that can effectively solve the problem of increased work load such as grinding and produce a ferritic stainless steel sheet with good surface properties without installing special equipment in the hot rolling process. The purpose is to provide.

[0006]

That is, the gist of the present invention is to remove defects and flaws on the surface of a ferritic stainless steel slab before the slab heating furnace for hot rolling is used. The purpose of these methods is not to remove the surface of the slab and to smooth the surface of the slab by grinding or shot blasting, but to form irregularities such that the slab surface has an Rmax of 20 μm or more. An abnormal oxidation, that is, a uniform and thick scale is easily formed by promoting the oxidation by acting as a nucleus of the breakaway to promote the oxidation, and thereafter hot rolling is performed. It is in the manufacturing method of ferritic stainless steel.
Furthermore, since a uniform thick scale can be easily formed at a low temperature by this method, a hot-rolled steel strip without scale flaws after hot rolling in which the occurrence of ridging, roping, etc. due to crystal coarsening, etc. is suppressed is obtained. There is a feature that can be. Hereinafter, the present invention will be described in detail.

[0007]

In order to elucidate the cause of the occurrence of scale flaws on the surface of the hot-rolled sheet, the inventors of the present invention have conducted a test on the exit side of the heating furnace, on the exit side of the rough rolling mill in the hot rolling process, after finish rolling and after pickling. A thorough investigation of the material surface at this stage and some reproducibility experiments have led to the following facts. 1) In the analysis of the tendency of the occurrence of surface scale flaws, flaws are likely to be generated in a steel type having a high Cr concentration or a good oxidation resistance containing Nb, Ti, Mo or the like.

2) From observation of the slab surface on the exit side of the heating furnace,
In a steel type in which flaws are easily generated, a large number of scales in the form of a knob are recognized, or the thickness of the scale is from 0.4 to 1.
The surface has an extremely thick part of about 5 mm and a thin part of about 10 μm. That is, since the oxidation of the knob-like scale or the extremely thick scale is more remarkable than that of the surrounding area, the metal surface is depressed at that part, and the surface of the underlying metal becomes uneven. On the other hand, in the case of a steel type that is hard to produce flaws, 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 knob-like scale and the extremely thick scale portion have a two-layer structure, the outer layer is an Fe-rich oxide which is easily peeled, and the inner layer is Cr, which has good adhesion.
It is an oxide rich in iron. 4) The dent of the metal surface caused by the oxide existing on the slab surface on the exit side of the slab heating furnace has its edge extended in the rough hot rolling step to form a "scab" and covers the oxide underneath. It takes shape. Further, in the finish hot rolling, the form is widened and becomes shallow in the thickness direction, resulting in minute barbed surface flaws containing an oxide inside. 5) Most of the scale flaws observed on the steel sheet surface after pickling are very similar to the surface flaws of the above item 4), and are in the form of fine barbs, with oxides biting below them. .

[0010] From these facts, the inventors of the present invention conclude that scale flaws generated on the surface of a hot-rolled sheet are generated by rolling irregularities on a metal surface caused by oxidation in a slab heating step by rolling. I got the conclusion. That is, FIG.
Nodule scale 3 on the surface of slab 1 as shown in FIG.
Is formed, the unevenness of the metal surface is severe, so that scale flaws frequently occur on a scale in which such a thing is scattered. As shown in FIG. 1B, the upper layer is Fe oxide 4 and the lower layer is (Fe, It was clarified that when the double scale having a large thickness of Cr) oxide 5 covers the surface and the metal surface is smooth, scale flaws are extremely small. In addition, the present inventors controlled such a knob-like oxidized form in the slab heating step, and performed hot rolling in a state in which the metal surface on which the uniform scale 6 was formed with little unevenness was used, in which the steel components and hot work were reduced. It was concluded that the occurrence of scale flaws could be significantly reduced irrespective of the deformation resistance in the above.

Further, the present inventors and others have found from the study of the generation process of the knob-like scale and the thick scale that the oxidation of ferritic stainless steel scale proceeds by the following mechanism. (1) First, the metal surface 1 as shown in FIG. 2 (a) Cr ₂ O
_3, a dense and thin oxide film 12 is formed. (2) Then, as the heating temperature and the heating time are increased, the thickness of the Cr ₂ O ₃ film is increased, and the Cr-deficient layer 13 with less Cr is formed on the oxide film 12 and the metal interface as shown in FIG.
Is formed. Therefore, the supply of Cr to the oxide film becomes insufficient, and Fe is easily oxidized. (3) Further, the increase in the thickness of the oxide film of Cr ₂ O ₃ is shown in FIG.
(C) causes the destruction of the coating, the Cr-deficient metal surface is directly exposed to oxygen through cracks, and the oxidation of Fe is started, and Fe is locally oxidized 14 to make the coating thick. And a region where the oxidation of Fe does not progress occurs. The above-mentioned knob-like scale 15 is where the oxidation has progressed locally. In this case, the starting point of film destruction may be caused by random cracks due to differences in mechanical properties such as thermal expansion coefficient of oxide / metal, and inclusions or internal oxide layer concentrated at oxide / metal interface. Was clarified by various analytical instruments such as EPMA. (4) When the heating temperature and time further increase, FIG.
As shown in FIG. 2 (e), the scale 15 having a nodular shape grows to a large extent, and finally becomes a thick double scale 18 as shown in FIG.

[0012] Based on such findings, a Cr-deficient layer is formed at the interface between the oxide film and the underlayer, and the number of cracks is increased to increase the density of scale-like formation of bumps. It can be 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 ferritic stainless steel was subjected to a sawer cutting with a cutting tool, and the surface roughness was 3 μm, 18 μm, 20 μm, and Rmax.
This shows the results of observing the oxidation state formed by heating samples at 50 μm, 100 μm, and 140 μm at 1230 ° C. for 1 hour in an LPG combustion atmosphere. Under any condition of the surface roughness, the knob-like scale is formed on the convex portion of the cutting mark, and the size of the knob-like scale to be formed increases as the surface roughness Rmax becomes rougher from 3 μm to 140 μm. When the diameter is 18 μm or less, the scales are not connected to each other due to the small size of the knob-like scales, and a thick portion and a thin portion are mixed. On the other hand, when the surface roughness is 20 μm or more, the knob-like scales formed on the projections are connected to each other, so that a uniform thick scale is obtained.

[0013]

[Table 1]

The above results are attributable to the difference in the process of supplying the oxide film to the convex portions and the smooth portions and the tendency of the film to crack. That is, when a Cr oxide layer having the same thickness is formed between the protrusion and the smooth surface, as shown in FIG. 3, at the point A, the supply of Cr is only directly above the surface. Of B
At the point, it is necessary to supply not only directly above but also both sides, and the Cr deficiency at the point B is inevitably greater than that at the point A. Further, when an oxide is formed on the metal surface, stress is generated at the interface due to a difference in expansion rate, but the size of the oxide becomes large at the convex portion, so that it becomes a starting point of a crack. That is, as described above, the knob-like scale has a Cr-deficient layer and is formed in a place where the film is easily cracked, so that the knob-like scale is easily formed starting from the convex portion. This was confirmed by observing the initial formation process of the knob-like scale by using an optical microscope, EPMA, or the like, using samples prepared under various conditions at different heating temperatures and heating times.

Next, the reason for limiting the surface roughness in the present invention will be described. The scale flaw of the present invention is caused by the form of surface oxidation in the slab heating furnace, is to control this and supply the slab metal surface to the hot rolling step while maintaining a smoother state. As a method, a Cr-deficient layer is formed, and a crack-prone structure is formed on the surface, so that a knob-like scale is generated at a high density, and a uniform double scale is formed on the surface where they are connected. Is adopted.

[0016] For this purpose, it is preferable to form a high-density convex portion on the surface where the supply of Cr from the underlayer becomes insufficient before the slab is heated and stress is easily concentrated on the interface between the formed oxide film and the underlayer. However, when the roughness of the convex portion is less than 20 μm in Rmax, although the density of the swelling scale increases, compared to the smooth surface, the shape of the swelling scale to be formed is small, and the interconnecting ability of the swelling scale is low. In order to form a small and uniform double scale, high-temperature and long-time heating is required, so the lower limit was set to 20 μm. The upper limit is not limited because the frequency and shape of the knob-like scale increases with the increase in surface roughness, and it is easy to connect with each other to form a uniform thick scale. However, if the surface roughness exceeds 1 mm or more, the scale is turned off. In addition, the surface roughness may be increased by 1 due to the increase in the amount of processing and the effect of processing irregularities and processing distortion.
mm or less is preferable.

In the present invention, the conditions required for the unevenness of the surface to form a uniform scale in the present invention are important in addition to the average height of the projections on the surface and the interval between the projections. In other words, if the interval between the concave and convex portions is so large as between the oscillation marks such as a cast material, a high temperature and a long time heating are necessary to connect the formed knob-like scales, and between the convex portions. It is desirable that the intervals be close to each other to some extent. Therefore, as a means for forming the above irregularities on the surface, cutting using a cutting tool or milling, grinding using a grinder, shot blasting, or the like is preferable.

[0018]

EXAMPLES Table 2 shows the chemical compositions of A, B, C, D,
A test was performed on each of ferrite stainless steels E and F. Table 3 shows the slab thickness 1 produced by continuous casting.
A CC density of 50 to 300 mm and a slab width of 950 to 1250 mm is machined with a bite and a mill, a grinder of # 12 to # 400, and a steel grid having a particle size of 0.2 to 0.5 mm. The surface roughness is reduced to Rma by shot blasting in the range of 100 kg / m ^2.
The slab finished to 6 μm to 245 μm with x was heated to 1000 ° C. to 1270 ° C. in a subsequent slab heating furnace as a target heating temperature before hot rolling, and LNG (atmosphere was 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: 0~10 vol%} , CO 2: 4~8 vol%, H
₂ O: 12 to 22 vol%) in each combustion atmosphere.
Heating is performed at a heating time of 60 to 180 minutes at a heating temperature of 20 to 180C, a dew point of a combustion atmosphere of 45 to 65C, and an oxygen concentration of 0.5 to 8%. FIG. 9 shows the results of macro observation and cross-sectional micro observation performed by sampling after cooling and sampling.

According to the present invention, the surface roughness is Rmax of 19
A uniform thick scale can be obtained in any steel type having a surface roughness Rmax of 20 or more regardless of the heating conditions of forming a knob-like scale below μm, and the uniform thick scale forming temperature is 20 ° C. to 50 ° C. The temperature could be reduced by about ° C, and the irregularities on the metal surface under the scale could be reduced.

In Table 4, heating was performed in the same manner as in Table 3, and after completion of the heating, hot rolling was performed to produce a hot coil having a thickness of 2.5 to 5 mm.
50 to 10 seconds after mechanical annealing or shot blasting, in which annealing is performed for a short time in seconds or by mixing iron sand grains in high-pressure water without annealing and spraying.
The number of surface scale flaws involving the scale of the coil surface as it is or smut-treated after immersion pickling in a 300 g / l sulfuric acid solution at 0 ° C. for 30 to 120 seconds and abrading the surface layer by about 10 to 25 μm. Is shown. According to the present invention, a hot-rolled pickled steel strip free of scale flaws can be obtained with any of the irregularities 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, a scale having a uniform thickness is easily formed by forming and connecting high-density scales in a ferritic stainless steel, and a scale having a uniform thickness is obtained. A hot-rolled sheet free from surface scale flaws in which scale is involved after rolling is obtained, and a step for removing scale flaws can be omitted, so that its industrial effect is large. Furthermore, since the formation temperature of uniform thick scale can be reduced and the slab heating temperature can be reduced,
The effect is extremely large because the characteristics of ferritic stainless steel are improved by grain refinement and the like.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a metal micrograph showing an example of an oxidation state generated on a surface of a ferritic stainless steel when heated for hot rolling in a slab heating furnace, where (a) is locally formed; It shows a knob-like scale, and (b) shows a uniform thick scale state.

FIG. 2 is a schematic view showing a process of forming an oxide film formed on a surface of a ferritic stainless steel when heated for hot rolling in a slab heating furnace, where (a) shows an initial oxidation stage and (b) shows a protective property. Film growth and formation of Cr-deficient layer; (c) destruction of protective film and penetration of oxygen, etc .; (d) formation of knob-like scale after breakaway; and (e) formation of thick scale by connection of knob-like scale. Indicates the status.

FIG. 3 shows that 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) are formed.
The point (A) indicates the difference in the supply state of Cr, and point (A) indicates that the supply is directly above, but point (B) indicates that supply to both ends is necessary in addition to the position directly above.

[Description of Reference Numerals] 1 ... slab metal surface 2 ... thin protective oxide film 3 ... nodular scale 4 ... outer scale (Fe ₃ O ₄ mainly) 5 ... inner layer scale _{((Fe, Cr) 2 O} 3 principal) 6 ... uniform thick scale 11 ... slab metal portion 12 ... protective oxide film 13 ... Cr-depleted zone 14 ... oxidation progress region 15 ... bump-like scale 16 ... outer scale (Fe ₃ O ₄ mainly) 17 ... inner layer scale ((Fe, Cr ) ₂ O ₃ main) 18 ... diffusion only directly above the point (Cr is a uniform thick scales a ... in smooth portion) B ... immediately above the point (Cr in the convex portion, spreading across)

──────────────────────────────────────────────────続 き Continuation of the front page (72) Keiichi Omura Inventor 1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (56) References JP-A-1-321012 (JP, A) Japanese Patent Publication No. 5-21642 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 3/02

Claims (1)

(57) [Claims] 1. A slab surface is ground by at least one method of grinding with a grinder or a shot blast, or cutting with a cutting tool or a milling mill before charging a slab heating furnace for hot rolling a ferritic stainless steel slab. A method for producing a ferritic stainless steel having a small number of surface scale defects, characterized in that a uniform and thick scale is easily formed during slab heating by setting the roughness to 20 μm or more in Rmax, followed by hot rolling. .