JPH0633146A - Manufacture of high corrosion resistant cr-based stainless steel sheet - Google Patents

Abstract

PURPOSE:To manufacture a Cr-based stainless steel hot rolled sheet excellent in the appearance of the surface and corrosion resistance by subjecting a high corrosion resistant Cr-based stainless steel slab to heating and soaking in a specified atmosphere under specified temp. conditions and executing hot rolling. CONSTITUTION:The Cr-based stainless steel slab having a compsn. of, by weight (C+N)<0.1% and Cr>13% as well as Ti, Mo or the like so as to satisfy (5XTi%+Mo%)>1% is subjected to heating and soaking under the conditions satisfying t<=8Cr%-0.38 (T-1100)+100 (heating temp. is T deg.C and soaking time is t min) in an atmosphere under the conditions in which the residual oxygen concn. (O2%) is regulated to <4.5vol% and the dew point (DP) satisfies the following inequality: DP ( deg.C)<-10logO2%+60), and hot rolling is executed. Scale flaws are not occurred on the surface of the Cr series stainless steel hot rolled sheet contg. Ti, Mo or the like and having more excellent corrosion resistance, and the hot rolled sheet excellent in surface properties can stably be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for effectively preventing so-called scale flaws which may occur on the surface of a Cr-type stainless steel hot-rolled sheet. More specifically, Cr
An object of the present invention is to provide a slab heating method for suppressing hot-rolled scale flaws of so-called highly corrosion-resistant Cr-based stainless steels containing a relatively large amount of Ti and / or Mo among the system-based stainless steels. .

[0002]

2. Description of the Related Art When a so-called scale flaw is generated on the surface of a Cr-based hot rolled stainless steel sheet, the flaw remains even after pickling of the hot rolled sheet and is succeeded to the material surface after cold rolling. Since the appearance and corrosion resistance of the surface, which are important characteristics of the stainless steel thin plate of the above, are significantly impaired, if scale flaws occur on the surface of the hot-rolled sheet, the number of surface conditioning steps such as pickling or grinding should be increased to remove them. Was forced to. In particular, in so-called high corrosion resistant stainless steel containing Ti and / or Mo, the dissolution rate in acid is slow, so the work load of such a manufacturing process becomes remarkably large, and the production efficiency is extremely reduced. The current situation is the cause.

Some morphologies are found in the scale flaws formed on the surface of the hot-rolled sheet, but the cross-section has a minute whiskers shape, and the oxide is eroded under the "cover" of the metal. The majority are doing. Thus, there have been disclosed some techniques for suppressing the occurrence of such defects on the surface of the hot rolled sheet.

For example, in JP-A-58-29897, scale flaws and crack flaws affect the mechanical action between the roll and the material during hot rolling and the electrochemical action dominated by the environment there. From the idea that it is performed, an appropriate amount of an aqueous solution containing one or more of phosphate, acetate, carbonate, benzoate, metal soap and phosphate ester in a proper range is used on the side of the rough heat rolling. A technique is disclosed in which the surface of the hot-rolled sheet is coated to suppress the generation of microscopic flaws. Further, in JP-A-58-116903, Fe is
_A method of taking the above oxide (scale) formation process prior to finish rolling is disclosed in order to suppress the occurrence of hot-rolling defects in martensitic stainless steel by utilizing the lubricating action of ₂ O ₃ and Fe ₃ O _4. ing.

Further, JP-A-60-170503, JP-A-60-174201, and JP-A-60-1
74202, JP-A-60-184405,
JP-A-60-187404 and JP-A-63-25
Japanese Patent No. 4195 discloses that the occurrence of hot-rolling defects is caused by the seizure of the material and the surface of the rolling roll, and the scale is completely peeled off to expose the metal surface. To regenerate the material, heat the material between the passes, spray one or two of steam and oxygen gas when heating the material between the passes to promote the regeneration of the scale, and further add iron hydroxide to the water. Or apply a suspension in rolling oil to the material surface to promote regeneration of the scale, or apply a suspension of carboxylic acid in water or rolling oil instead of iron hydroxide to the material surface. Disclosed is to promote scale regeneration and to apply a composition in which iron oxide powder is mixed with a non-water-soluble polymer compound to the material surface as a hot rolling lubricant that utilizes the lubricating action of iron oxide. It To have.

From the standpoint that surface flaws of hot-rolled sheets are caused by intergranular cracks, Japanese Patent Laid-Open No. 61-44120 discloses that an appropriate amount of Mo compound is attached to the surface of a slab in order to suppress intergranular cracks. After that, a process of heating and hot rolling is disclosed. However, in all of these prior arts, a foreign material is mixed in the middle of the rolling process, or a special heating device or a spray of a suspension or the like is required, and a coating process is required, and an increase in the work load in the hot rolling process is avoided. Absent. In addition, when the coating agent is attached to the slab, there is a problem that the effect disappears due to the heating step, especially peeling of the coating agent on the lower surface of the slab. Further, although these prior arts are countermeasures when the micro defects on the surface of the hot rolled sheet are caused by the seizure of the rolling roll and the material or the intergranular cracks, the micro defects on the surface of the hot rolled sheet are However, according to the detailed observation by the present inventors, the majority occurs regardless of the presence or absence of the hot rolling scale on the material surface,
It was strongly suggested that the cause was other than the above.

[0007]

DISCLOSURE OF THE INVENTION The present invention suppresses the occurrence of scale flaws on the surface of hot-rolled sheet of so-called highly corrosion-resistant Cr-based stainless steel containing Ti and / or Mo in an amount exceeding a specific amount, and heat treatment Effectively solves the problem of increased work load such as re-pickling and surface grinding after pickling following hot rolling,
An object of the present invention is to provide a manufacturing method capable of manufacturing a Cr-based stainless steel sheet having good surface properties at a cost as low as possible.

[0008]

The gist of the present invention is that, in weight%, at least C + N: 0.1% or less, Cr: 13% or more, and further Ti and / or M is used.
Cr containing o of 5 × Ti% + Mo% in an amount exceeding 1%
Of stainless steel slabs for residual oxygen concentration (O ₂ %)
In an atmosphere of 4.5 vol.% Or less and a dew point (DP) of DP (° C) <-10 · log O ₂ % +60, heating and soaking are performed at a heating temperature and a soaking time that satisfy the following formula. It is a method for producing a highly corrosion-resistant Cr-based stainless steel sheet characterized by having a process.

T ≦ 8 Cr% −0.38 (T-1100) +10 where T: slab heating temperature (° C.) t: slab soaking time (minutes) Cr%: slab Cr concentration (%) The invention will be described in detail.

In order to elucidate the cause of scale defects on the surface of a hot rolled sheet of Cr-based stainless steel, the present inventors have examined the heating furnace outlet side, the rough rolling mill outlet side in the hot rolling step, after finish rolling and acid rolling. Thoroughly investigate the material surface at each stage after washing,
As a result of trying some reproduction experiments, the following findings were obtained. 1) On the slab surface on the slab heating furnace outlet side, in the case of a steel type that is susceptible to flaws, a large number of nodular oxide scales are recognized, or where the scale is extremely thick and thin Under a bumpy scale or an extremely thick scale, oxidation progresses deeper than the surroundings, and as a result, the metal surface is dented at that portion. On the other hand, in the case of steel types that are not prone to flaws, the entire surface varies in thickness depending on the oxidation resistance of the material, but it is several hundred μm to several tens of μm.
It is covered by a scale with a uniform thickness of approximately m, and as a result, the metal surface has few irregularities.

2) Each of the bump-shaped scale and the extremely thick scale has a two-layer structure, the outer layer is an oxide rich in Fe, and the inner layer is an oxide rich in Cr with good adhesion. . The boundary between the outer layer oxide and the inner layer oxide substantially corresponds to the original metal surface position. 3) The dent on the metal surface caused by the oxide existing on the surface of the slab on the outlet side of the slab heating furnace has its edge extended by rough hot rolling to form a "scab", and the oxide below the slab is removed. It becomes a covering shape, and further this shape spreads in finish rolling, and becomes shallow in the plate thickness direction, and it becomes a surface defect in the form of a minute hedging shape in which oxide is invaded.
This situation was confirmed again in the laboratory.

4) Most of the scale flaws observed on the surface of the material after pickling are very similar to the surface flaws described in the above item 3), and the scale flaws are in the form of micro-heavy and oxides exist below them. ing. 5) The non-uniform oxidization morphology described in 1) above, which causes severe irregularities on the metal surface, is likely to occur in steel grades with good laboratory oxidation resistance, and the surface flaws of the actual hot-rolled sheet also occur. This type of steel has a high frequency of occurrence.

From the above, the present inventors have found that the scale flaws generated on the surface of the hot-rolled sheet are generated by rolling the surface unevenness caused by oxidation in the slab heating stage. In the slab heating step, when such an oxide form is controlled to form an oxide (scale) having a uniform thickness, the slab surface shape becomes more smooth in Cr-based stainless steel, and heat treatment is performed in such a state. When hot rolling was started, it was concluded that scale defects can be significantly suppressed regardless of the composition of the steel and the resistance to hot deformation.

Therefore, based on the above conclusions, further examination was made on various Cr-based stainless steels for slab heating conditions such that the metal surface had a smooth oxidized form in the heating stage, and the following findings were obtained. It was The surface oxidation form is affected by the heating temperature and the heating time as well as the heating atmosphere. Especially, the balance between the residual oxygen concentration and the water vapor content in the heating atmosphere most significantly affects the oxidation form.

In the case of steel grades that have good laboratory oxidation resistance and are prone to flaws, in a combustion exhaust gas such as a slab heating furnace,
When the oxygen concentration in the atmosphere is high, there is a tendency that the above-mentioned oxidized form in which the surface unevenness is severe is formed as a bump. Similarly, the water vapor content (dew point) also has a large influence on the surface oxidation form, but the direction of the influence differs depending on the steel composition, and more specifically, the Ti and / or Mo content is 5 × Ti% + Mo%. When it is 1% or less, the hump-like oxidation form tends to occur on the low dew point side, whereas 5 × Ti% + Mo% is 1
If it exceeds 0.1%, nodular oxide tends to be generated on the high dew point side.

When 5 × Ti% + Mo% is 1% or less, when the atmosphere is low oxygen concentration and high dew point side, a large number of bump-like oxides are generated within a short time, and these grow and grow and are connected to each other. Then, since the entire surface of the slab is covered, a thick two-layer oxide scale of about 500 μm is eventually formed on the entire surface of the slab, and as a result, smoothing of the metal surface is achieved. In contrast,
In a Cr-based stainless steel slab containing 5% Ti% + Mo% in excess of 1%, when both the oxygen concentration and the dew point are simultaneously decreased, the generation of the nodular oxide itself is suppressed, which is shown in FIG. Such an oxidized form having a smooth surface is obtained. In this case, the scale is a relatively thin single layer of several tens of μm or less.

That is, in general, the oxidation of a stainless steel slab in a slab heating furnace in which combustion exhaust gas is used as an atmosphere,
Significantly faster that the speed compared to oxidation in the environment of use of the conventional heat-resistant applications, therefore becomes a problem with the use environment that allowed to exhibit the heat resistance of the stainless steel, so-called abnormal oxidation phenomenon, i.e. Cr ₂ O ₃ principal protection of The oxide film is destroyed, an oxide layer mainly composed of Fe ₂ O ₃ or Fe ₃ O ₄ is formed in the outer layer, and a thick Cr-rich oxide with reduced protective property is formed in the inner layer, resulting in remarkable resistance to oxidation. It will be lowered. In the early stage of heating of stainless steel, since they are localized, the appearance looks like a bump, so it is called abnormal oxidation nodule, and as shown in FIG. 2, the metal surface of this portion becomes a recess (1). . Such an oxidation phenomenon is dominant in the slab heating furnace, and when the slab is extracted from the heating furnace and hot-rolled with the abnormal oxidation nodules scattered on the slab surface, scale defects frequently occur. It will be.

The abnormal oxide nodules in ordinary Cr-based stainless steel grow and grow with environmental conditions such as heating time and heating temperature, and are connected to each other so that the entire surface of the slab is covered with a thick two-layer scale. The situation is as shown. However, in a so-called highly corrosion-resistant Cr-based stainless steel containing Ti and / or Mo in a specific amount, abnormal oxidation of nodule is caused in the combustion gas of hydrocarbon-based fuel such as in the slab heating furnace atmosphere. Although generation is recognized, the fact that they are connected to each other to form a thick two-layer scale does not occur within a temperature range of 1300 ° C. or lower, which is an actual operating temperature, or a heating time range of about 3 hours or less. Therefore, in order to suppress the scale flaw due to the oxidation form of the slab surface in the slab heating furnace described above, no abnormal oxidation nodules are generated, that is, as shown in FIG.
The slab heating condition for achieving a surface having a morphology such as that of C containing Ti and / or Mo in excess of a specified amount
This is important for r type stainless steel.

From these viewpoints, the present inventors have made various investigations on the slab heating conditions as a method for controlling the slab surface oxidization morphology as described above, and as a result, the actual oxygen of the oxidizing atmosphere in the heating furnace along with the heating temperature and the heating time were examined. The balance between the concentration and the water vapor content (dew point) plays an important role, and the oxidation resistance of the slab based on the steel material composition is also closely related. By controlling these factors within a certain range, the cause of scale defects can be identified. The present invention has been accomplished by clarifying that it is possible to suppress the generation of the following nodule-like oxide.

However, if the actual measured oxygen concentration at room temperature in the soaking zone where the slab is held at the maximum temperature in the heating furnace exceeds 4.5 vol%, the occurrence of abnormal oxidation nodules can be suppressed. In this case, in order to suppress the generation of abnormal oxidation nodules, depending on the heating temperature and time, it is necessary to lower the dew point to about 15 ° C or less and to the water vapor content of normal atmosphere. It is virtually impossible to realize the atmosphere of the heating furnace to be used. Therefore, in the present invention, the oxygen concentration in the heating furnace atmosphere is 4.5 vol% or less.

Further, even in the case where the oxygen concentration is 4.5% or less, in the steel in which 5% Ti% + Mo% exceeds 1%, abnormal oxidation nodules are generated when the dew point is relatively high. Dew point (DP) and oxygen concentration (O ₂
%), It is necessary to establish an atmosphere that satisfies the relationship DP (° C.) <− 10 · logO ₂ % + 60.

Further, even in an atmosphere satisfying such a relationship, the oxidation resistance based on the component system of the material largely affects the oxidation form of the slab surface. It is necessary to control within a range associated with. The above atmospheric conditions are 13%
In addition to the above Cr, Ti and / or Mo is 5 · Ti%
+ Mo% is a Cr-based stainless steel containing more than 1%. In this case, the oxidation resistance derived from the material components is arranged by the Cr amount in the present invention,
The heating temperature T (° C.) and the soaking time t (min) are in relation to the Cr concentration: t ≦ 8 × Cr% −0.38 (T-1100) +10 (Cr%: wt% of Cr in steel) Need to be satisfied.

That is, in the case of so-called highly corrosion-resistant ferritic stainless steel in which 5 × Ti% + Mo% exceeds 1%, if the slab heating is performed in the atmosphere and temperature / time range as described above, It is possible to suppress the occurrence of abnormal oxidation that causes scale defects, but even if the heating atmosphere side conditions are satisfied, the heating time,
When the soaking temperature exceeds the range of the above equation related to the oxidation resistance of the steel material, abnormal oxidation occurs, and as a result, it becomes difficult to suppress the occurrence of scale flaws.

Further, the scale flaw referred to in the present invention is caused by the surface oxidation form in the slab heating furnace, as described above, and by controlling this, the metal surface of the slab remains smooth. It is intended to supply to the hot rolling process.In this case, as a measure for preventing unevenness due to oxidation of the slab metal surface, there is one means for preventing abnormal oxidation nodules, and as another measure, abnormal There is a means for connecting the oxide nodules to each other to bring the entire surface into an abnormally oxidized state. It has been clarified by the present inventors that both of these are effective in preventing scale flaws of the hot rolled sheet, but Ti and / or Mo targeted by the present invention have been revealed.
It has been found that it is difficult to use the latter means, that is, to bring the entire surface into an abnormal oxidation state, in a Cr-based stainless steel containing 5 × Ti% + Mo% in excess of 1%. This is interpreted to be due to the effect of improving the oxidation resistance of Ti and Mo. Therefore, the present invention provides the former, that is, a means for preventing abnormal oxide nodules from being generated in the slab heating furnace as its heating condition. Therefore, there is an upper limit for the amount of C and N that facilitates the generation of abnormal oxidized nodules under the slab heating approximation condition of this type of stainless steel, and in the present invention, C% + N% is 0.1% or less. Become.

Regarding Ti and Mo, as described above,
Although it has the effect of suppressing the generation of abnormal oxidized nodules, its effect is not perfect within the fluctuation range of normal slab heating conditions. Furthermore, when 5 × Ti% + Mo% exceeds 1% and when it is less than 1%, as described above, the directions for making the metal surface smoother after heating the slab are different. That is, when 5 × Ti% + Mo% is 1% or less, abnormal oxidation nodules are likely to occur, so that oxidation is promoted under the slab heating conditions, that is, more specifically,
It is effective as a countermeasure to set the atmospheric conditions such that the oxygen concentration is reduced as much as possible and the water vapor content is increased.
When xTi% + Mo% exceeds 1%, although abnormal oxidation occurs, it is not easy to form a form in which these are entirely covered. Therefore, in this case, it is necessary to set the atmospheric conditions, heating temperature and soaking time specified in the present invention.

Regarding Cr, when the content is less than 13%, the generation of flaws is not so remarkable irrespective of the contents of Ti and Mo, and the surface by acid in the pickling step after hot rolling is Due to the relatively high dissolution rate, the occurrence of scale defects is not a problem under normal operating conditions. Therefore, steel with a Cr content of less than 13% is excluded from the scope of the present invention.

Regarding the other constituent elements, there is no problem as long as they are within the range of the constituents usually added to this type of Cr-based stainless steel or the range inevitably mixed, and the surface condition of the slab before heating is also good. As a starting material, a slab having the same surface as in a normal manufacturing process, such as an as-cast casting surface or a ground surface treated by a grinder or the like, can be used.

The hot rolling process including the heating furnace extraction other than the above may be carried out by a conventional method, and no special oxidation time or supply of an oxidation accelerator is required during hot rolling. When the Cr-based stainless steel thin plate is manufactured according to the present invention, the effect of reducing scale flaws on the surface after hot rolling is remarkable, and thus it is possible to easily manufacture a highly corrosion-resistant stainless steel plate having good surface properties. Become.

[0029]

[Example] From the surface layer of two Cr-based stainless steel slabs A and E whose chemical compositions are shown in Table 1, thickness 10 mm, width 30 m
A square piece with a length of m and a length of 80 mm was sampled, the oscillation mark part of the slab surface was ground and removed by a grinder, and then heated in gasoline combustion exhaust gas with controlled oxygen concentration and water vapor content (dew point) to oxidize the surface. The morphology was observed. In addition,
Steel E is a Cr-based stainless steel having a composition outside the scope of the present invention.

The results are shown in the "oxidized form" column of Table 2.
In this case, the heating temperature and the soaking time were all set within the range of the present invention determined by the Cr content. From the table, in the steel A which is the target steel type of the present invention, the dew point is relatively high with respect to the oxygen concentration (in other words, the oxygen concentration is high with respect to the dew point).
In this case, or when the oxygen concentration exceeds 4.5% regardless of the dew point, the oxidized form of the surface is so-called bump-like (nodule-like) oxidation, which causes the metal surface to be highly uneven. On the other hand, in the E steel containing substantially no addition of Ti and Mo, since the oxidation resistance is relatively low, the so-called abnormal oxidation state occurs even under atmospheric conditions (balance of oxygen concentration and dew point) outside the scope of the present invention. Since it covers the entire surface, the metal surface is smooth as a result. However, in this case, several hundred μm
A thick two-layer scale is formed, and the outer layer side is easily peeled off after extraction in a heating furnace. On the contrary, in this steel, even under atmospheric conditions within the scope of the present invention, the nodule-like oxidized form remains, and the metal surface becomes extremely rough. Here is an example of the grounds for determining the atmospheric conditions in the present invention.

Furthermore, thickness 50 mm, width 100 mm, length 2
In the same manner as above, 00 mm square test pieces were sampled for A and E, and two pieces were simultaneously heated under the heating conditions shown in Table 3 to change the oxidized form of the surface of the slab surface layer.
The individual pieces were allowed to cool as they were to investigate the surface oxidization morphology, and the remaining ones were immediately hot-rolled, and then the state of surface defects was investigated. Upon hot rolling, immediately after extracting from the heating furnace, the peeling scale on the surface was removed with a jig, and a polished work roll was used for every three test pieces. Further, with 11 passes, the rolling reduction was in the range of 15 to 35%, the final pass temperature was in the range of 880 to 940 ° C, and the final plate thickness was 3.2 mm. A slab surface layer obtained by subjecting the hot-rolled sheet thus obtained to a longitudinal center length of 1.5 m by rough descaling by shot blasting and then pickling for 60 seconds in a mixed solution of nitric acid and hydrofluoric acid at 70 ° C. The appearance of scale defects on the side surface was visually inspected. The results are shown in Table 3.

Also in this case, in the case of steel A, when the atmospheric conditions are A-2 to 4 within the range of the present invention, nodule-like oxidized form is not recognized on the surface after heating, and the metal surface is Since the surface remains smooth, no scale flaws are observed on the surface after hot rolling and pickling. Also,
In the E steel whose components are out of the scope of the present invention, even if the atmospheric conditions are E-3 as in the above, the nodule-like oxidized form is generated and the hot rolled sheet after pickling has There are many scale defects.

From the above examples, when so-called abnormal oxidation nodules are generated during heating before hot rolling, scale defects occur on the surface of the plate after hot rolling, and 5 × Ti% + Mo.
For Cr-based stainless steel with a% exceeding 1%, it is clear that heating under the atmospheric conditions prescribed in the present invention has a great effect of suppressing the occurrence of such scale flaws.

Next, from the slab surface layers of the A, B, C and D steels of the present invention whose chemical compositions are shown in Table 1, a thickness of 10 mm, a width of 30 mm and a length were taken in the same manner as described above. A test piece of 80 mm in length is taken, and the oxygen concentration is 0.9 to 1.3 vol.
%, In gasoline combustion exhaust gas controlled to a dew point of 54 ° C,
After heating by changing the heating temperature and time, the oxidized form of the surface of the slab surface layer was visually observed. The results are shown in Table 4 together with the heating conditions.

As can be seen from the table, the holding time (soaking time) at that temperature exceeds a certain range with respect to the heating temperature (in other words, when the soaking time is kept constant, the heating temperature exceeds a certain range). Therefore, even if the atmospheric conditions are within the range of the present invention, it becomes difficult to suppress the generation of abnormal oxidized nodules. The limit of the soaking time varies depending on the Cr content, and the higher the Cr content, the wider the soaking time range. Here is an example of the grounds for the correlation between the heating temperature, the soaking time and the Cr content specified in the present invention.

Next, an example carried out on an actual machine production line will be described. LNG was prepared by using 250 mm thick CC slabs of two Cr-based stainless steels, F and G, whose chemical compositions are shown in Table 5.
After heating under various conditions described in Table 6 by using a combustion heating furnace, a hot rolled coil having a thickness of 4 mm was manufactured by hot rolling in the same manner as in ordinary Cr-based stainless steel, and descaled. After that, the presence or absence of scale flaws was observed on the front and back surfaces. The results are also shown in Table 6. In this case, the scale flaw occurrence status is determined by dividing the total length of the hot-rolled coil into sections with a length of 1 m, and dividing the number of sections with scale flaws by the total number of sections in one hot-rolled coil. % As "defect rate"
It is displayed in units, and when this value is less than 2%, it is judged as “good” and is indicated by a circle, and when 2% or more and less than 60% is indicated by an x, and when 60% or more is “severe occurrence”. Items are indicated by XX marks. In this case, the total number of sections in one hot-rolled coil was about 700 in total on the front and back sides.

From these results, it is clear that the effect of preventing the scale flaws of Cr-based stainless steel is great when manufactured according to the method of the present invention.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

[Table 6]

[0044]

As described above, by adopting the method of the present invention in manufacturing a Cr-based stainless steel sheet, it is possible to remarkably reduce scale defects that are likely to occur on the surface of the hot rolled sheet. Thereby, the surface quality of the Cr-based stainless steel hot-rolled sheet is improved, and the step for removing the scale flaw can be omitted.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a metallographic photograph of a cross section showing an example of an oxidation state generated on the surface of a Cr-based stainless steel according to the present invention when heated for hot rolling in a slab heating furnace.

FIG. 2 is a schematic diagram of a metallographic photograph of a cross section showing an example of an oxidation state generated on the surface of Cr-based stainless steel when heated for hot rolling in a slab heating furnace.

FIG. 3 is a schematic diagram of a metallographic photograph of a cross section showing an example of an oxidation state generated on the surface of Cr-based stainless steel when heated for hot rolling in a slab heating furnace.

[Explanation of symbols]

 1 Slab metal surface 2 Metal part (slab) 3 Relatively thin protective oxide film 4 Abnormal oxidation nodule 5 Recesses on the metal surface formed by oxidation 6 Thick double-layer scale covered by abnormal oxidation 7 Outer scale 8 Inner layer scale

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naofumi Mikami 1-1 Hibahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Shin Nippon Steel Co., Ltd. Yawata Works

Claims (1)

[Claims] 1. By weight%, at least C + N: 0.1% or less, Cr: 13% or more, and further Ti and / or M.
Cr containing o of 5 × Ti% + Mo% in an amount exceeding 1%
In a slab of stainless steel with a residual oxygen concentration (O ₂ %) of 4.5 vol.% Or less and a dew point (DP) of DP (° C) <-10 · log O ₂ % +60, the following formula The method for producing a highly corrosion-resistant Cr-based stainless steel sheet having a heating temperature and a soaking time satisfying the above conditions. t ≦ 8 Cr% −0.38 (T-1100) +10 where T: slab heating temperature (° C.) t: slab soaking time (minutes) Cr%: slab Cr concentration (%)