JPH0925516A - Method for preventing surface flaw of hot rolled ferritic stainless steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provent the occurrence of fine surface flaw at the surface of a hot rolled strip of ferritic stainless steel and to obtain a ferritic stainless steel minimal in surface flaw. SOLUTION: After 2.5-15% working strain is applied to a slab of ferritic stainless steel at 300-900 deg.C, the slab is heated to 1100-1300 deg.C to regulate the crystalline grain size in the surface layer part of the slab before hot rolling to <=5mm, by which the occurrence of surface flaw at the surface of hot rolled ferritic stainless steel strip can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing surface defects from occurring in a hot rolled steel strip of ferritic stainless steel, and effectively prevents wrinkle-like dents from occurring on the surface of a steel sheet after hot rolling. Regarding the method.

[0002]

2. Description of the Related Art A cold-rolled sheet of ferritic stainless steel is manufactured by heating a continuous cast slab that is partially ground or not ground in a temperature range of 1100-1300 ° C. and then hot-rolled. 6mm thick hot rolled steel sheet, further annealed,
It is cold-rolled after the pickling or annealing is omitted.
Ferritic stainless steel has better hot working than austenitic stainless steel, and there is no particular problem other than the small edge seam flaws that occur within 15 to 20 mm from the outermost edge of the steel strip during hot rolling. It had been.

With respect to the edge seam flaws generated in the selvages of the steel strip, a method of performing lubrication rolling to control the amount of bulging at the ends to reduce the width of the edge seam flaw generation (Japanese Patent Laid-Open No. 4-279202), or A method for suppressing the occurrence of edge seam flaws by defining the horizontal reduction rate and the width reduction rate of rough rolling for horizontal rolling and width rolling (Japanese Patent Publication No. 6-
No. 241) is disclosed.

However, when the surface of the hot-rolled steel sheet after pickling of ferritic stainless steel is examined in detail, it is 4 from the outermost edge.
In some cases, a small dented flaw having a shallow depth was present on the surface at a position of 0 to 70 mm that could not be discriminated by visual observation. The small flaw on the surface of the steel plate has a width of 0.3.
~ 0.5mm, 10 ~ 30mm long spindle shape, depth 7
The size is 0 to 100 μm and 40 to 7 near the edge.
It was found that it tends to occur at the position of 0 mm.

[0007] This flaw is directly omitted because it does not require annealing of the hot rolled sheet, or remains as a flaw containing scale after pickling after mechanical descaling such as shot blasting after high temperature short time annealing at 800 to 1000 ° C. Since it cannot be sent to the cold rolling process, either repickling must be repeated or the entire surface of the steel strip must be ground with a coil grinder, which causes problems such as reduced material yield and extended delivery time. It was a major cause of high cost in steel production. Under such circumstances, there has been a strong demand for the development of an effective method for producing a ferritic stainless steel hot-rolled steel strip that is free from defects caused by hot-rolling.

[0006]

DISCLOSURE OF THE INVENTION The present invention prevents the generation of minute surface defects of hot-rolled steel strip that occurs in the above-described ferritic stainless steel hot-rolled steel strip, and is a ferritic stainless steel with few surface flaws. It is an object of the present invention to provide a manufacturing method of.

[0007]

Means for Solving the Problems The outline of the present invention is that, when a slab of ferritic stainless steel is heated to a predetermined temperature and then hot-rolled, the grain size of a slab surface layer portion produced during the heating of the slab. Is controlled to 5 mm or less. The summary is as follows. (1) 300 to 9 slabs of ferritic stainless steel
Ferrite characterized by applying a working strain of 2.5 to 15% in a temperature range of 00 ° C. and then heating to 1100 to 1300 ° C. to make the crystal grain size of the slab surface layer portion before hot rolling 5 mm or less. Method for Preventing Surface Defects of Hot Rolled Stainless Steel Series. The control for suppressing the crystal grain size of the slab surface layer portion, which is generated when the slab of ferritic stainless steel is heated, to 5 mm or less is carried out in advance at 300 to 900 at the lower part of the continuous casting device.
Add a pressure reduction of 2.5 to 15% in the temperature range of ℃ or 300
After keeping the temperature above ℃ or cooling to room temperature, raise the temperature to 300-
By manufacturing a slab for hot rolling to which a processing strain of 2.5 to 15% is applied by heating to a temperature range of 900 ° C, and further, controlling the slab heating temperature to 1100 to 1300 ° C,
It can be realized effectively.

(2) A slab of ferritic stainless steel is heated to a temperature range of 1100 to 1300 ° C. and satisfying the following formula (1), and the grain size of the surface layer of the slab before hot rolling is set to 5 mm or less. A method for preventing surface flaws in a ferritic stainless hot-rolled steel strip, comprising: T (° C) ≤ (1150 + 1.67γ _p ) (° C) (1) where γ _p = 240C + 470N + 23Ni + 9Cu + 7Mn
-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti
-52Al + 189 Here, the unit of γ _p and each component is%, and T (° C) is the slab heating temperature.

(3) A slab of ferritic stainless steel is heated to a temperature range of 1100 to 1300 ° C. and satisfying the following expression (2), and the grain size of the surface layer of the slab before hot rolling is set to 5 mm or less. A method for preventing surface flaws in a ferritic stainless hot-rolled steel strip, comprising: T (° C.) ≦ (550 + 7.5 Ci (%)) (° C.) (2) where Ci is the occupancy rate (%) of the chill crystal in the surface structure of the chill crystal and the columnar crystal of the cast slab.

In particular, immediately after the heating of the slab, that is, before the hot rolling, when the surface layer grain size exceeds 5 mm and large crystal grains are densely present, the difference in the crystal orientation of each crystal grain causes the hot rolling. Since the amount of change in plastic working for each crystal grain due to the processing is different, when the crystal grain in contact with the free surface is large, it appears as a large unevenness and causes a large wrinkle-like dent defect after hot rolling. By reducing the grain size of this crystal grain to prevent the formation of surface irregularities due to the difference in crystal orientation anisotropy during hot rolling, the wrinkled dents containing scale in the hot rolled steel strip after hot rolling It is possible to prevent defects.

[0011]

The present invention will be described below. In order to clarify the cause of wrinkle-shaped defects found in hot-rolled steel sheets, the rolling was interrupted during the hot-rolling process, and the changes in defects were traced. As a slab to be surveyed, γ of 16.2% Cr with remarkable defects
SUS430 having a _{p of} 15 to 25% was selected, and several 12 ton slabs having a plate thickness of 250 mm, a plate width of 1000 mm and a length of 6000 mm were prepared. 125 for hot rolling with this slab
250mm thick slab immediately after heating at 0 ° C for 2 hours, 240mm thick after rough rolling, 180mm thick, 125mm thick, 80mm
Samples with thickness of 50 mm, 30 mm, 20 mm and 3 mm of hot rolled steel strip after finish rolling were taken,
A survey was conducted.

It is not possible to determine the presence or absence of wrinkle-shaped depressions in a 3 mm-thick hot-rolled steel strip with the scale still attached, but after mechanically descaling the steel strip surface with high water pressure containing sand iron, 300 g / liter H ₂ SO ₄ , 60 seconds at 90 ° C
After being subjected to 10 to 15 μm pickling and shaving by sulfuric acid pickling, it appears for the first time as wrinkle-like pits containing scale. Moreover, there is a feature that it appears at a position of 40 to 70 mm near the coil edge and does not appear in the width center portion within that.

When the hot-rolled steel strip is annealed at a high temperature of 800 to 1000 ° C. for a short time, the wrinkle-like dents after pickling similarly after mechanical descaling tend to be considerably mild. Further, the width reduction for making the width of the hot rolled steel strip constant during the rough rolling is performed by using the longitudinal rolling rolls during the hot rolling to
It was clarified that when the total width reduction amount of 40 to 90 mm was added in four times, the degree of the wrinkle-like dent defects was further promoted corresponding to the reduction amount of the width.

Next, when comparing the defect-occurring portions during hot rolling with the change in the plate thickness, there is a plate thickness in which the existence of the defect becomes clear when the scale remains adhered. That is, although the defects are not clear up to the thickness of 250 to 240 mm,
It becomes clear at a plate thickness of 180 to 80 mm, and further rolling makes it difficult to see at a thickness of 50 to 20 mm. This 180-80
After mechanical descaling of a mm-thick hot rolled material with scale under high water pressure containing sand iron, 300 g /
After pickling for 60 seconds at 90 ° C. in 1 liter of H ₂ SO ₄ , it was found that large crystal grains were present corresponding to the flaws. Further, as a result of cutting and polishing the cross section of the flaw portion and performing a microinvestigation, it was found that unevenness was generated corresponding to the coarse grains and no crack was generated in any of the flaw portions.

Such a flaw that does not cause cracking is a flaw whose cause is different from the cracking flaw that occurs in a conventional material having poor hot workability. When the same position where the flaw is generated is examined immediately after heating the 240 mm thick material and the 250 mm thick slab, a depth of 4 to 7 having a width of 10 to 40 mm is obtained.
It was clarified that coarse particles of about mm especially occurred frequently on the surface layer of the edge part, and that these coarse particles were the cause of the wrinkle-shaped dents on the hot-rolled steel strip. Further, only fine grains having a width of about 2 to 4 mm were observed in the center of the width, and it was found that the surface layer structure of the center of the width immediately after heating the slab and the surface layer structure were significantly different.

Next, it was investigated how the size of the coarse particles existing in the surface layer of the heating edge contributes to the occurrence of wrinkle-shaped dent defects. The same plate thickness 25 used to investigate wrinkle-shaped dents
Using a slab of 0 mm, a plate width of 1000 mm, and a length of 6000 mm, the heating temperature is changed from 1100 to 1350 ° C. and the heating time is from 30 minutes to 3 hours. (The grain size immediately after the heating was measured by polishing and etching the cross section of the slab that was the same as the heat-rolled slab and immediately cooled after heating.), And the rough rolling start temperature was kept constant at 1070 ° C. under the same conditions. Hot-rolled steel strip having a thickness of 4.5 mm was manufactured.

After mechanically descaling the surface of the steel sheet with high-pressure water containing iron sand without annealing this hot-rolled steel strip, it was pickled with H ₂ SO _{4 at} 300 g / liter for 60 seconds at 90 ° C. After pickling and shaving, the number of wrinkle-shaped recesses containing scale is the vertical axis, and the number of wrinkled recesses on the front and back surfaces per 100 mm length of hot-rolled steel strip is the vertical axis, and the particle size immediately after heating The maximum particle size of the distribution is plotted on the horizontal axis, and the relationship between the two is plotted in FIG.

As the maximum grain size of the grain size distribution of the surface grain of the heated slab became smaller, the number of wrinkle-shaped pits decreased, and when the grain size was 5 mm or less, no wrinkled pits occurred. The reason why such results were obtained is that when adjacent crystals with different crystal orientations undergo large plastic working in the lengthwise direction such as hot rolling, grains that show large deformation due to the crystal orientation. If there are grains that do not show deformation, and if they are composed of large grains, the difference between them becomes large unevenness. That is, it is presumed that irregularities due to the anisotropy of crystal grains are generated, the grain size is further increased, and the greater the number, the more likely irregularities are generated.

Next, in order to clarify the cause of the difference in the surface layer structure after heating in the width direction of the slab described above, a portion having a large amount of columnar crystal structure at a position 40 to 70 mm from the corner of the CC slab and the width After observing the structure by cutting out a portion having a large number of columnar crystal structures at the center, the structure was heated at 1200 to 1230 ° C. for 2 hours in a non-oxidizing atmosphere and then water-quenched. The change in organization before and after was investigated. As a result, the structure of the columnar crystals at the edge portion did not change at all, but in the sample cut out from the width center portion, 1
Recrystallized grains having a grain size of ˜2 mm were produced.

Further, when the difference between the edge portion and the width center portion of the macro shape of the slab is examined, the oscillation mark generated during continuous casting is left at a position 40 to 70 mm from the corner where the flaw is generated, which is large. Although it is a dent, it was found that the oscillation mark was crushed in the central part and it was in the form of being rolled. Such a deformation of the surface of the CC slab is a phenomenon peculiar to ferritic stainless steels not seen in austenitic stainless steels, and the material strength of ferritic stainless steels at a high temperature of 700 ° C. or higher is 1/100 of that of austenitic stainless steels. It is caused by the fact that it is about 3-1 / 4. That is, it is presumed that strain was accumulated in the bulging portion at the center of the width of the CC slab, and this strain was used as a driving force to generate recrystallized grains having a small grain size during the subsequent heating.

In order to clarify the effect of such distortion, the same plate thickness 250 mm and plate width 10 as those used to investigate the wrinkle-like dents and flaws were investigated.
Experiment with a slab of 100 mm in length and 6000 mm in length that was cast without crushing the oscillation marks, and that part of the slab was preheated to confirm that recrystallization did not occur in the surface layer. I went.

As a pretreatment before hot rolling, 300
After heating at ˜900 ° C. for 1 hour, it was rolled into a CC slab to give a strain of 1 to 20%. This slab is heated for hot rolling and the heating temperature is 1100 to 1350.
The heating time is 30 minutes to 3 hours (the particle size immediately after heating is the same as the hot rolling slab, the same slab as the hot rolled slab is heated, and immediately after heating, the slab is cooled and the cross section is polished and etched. The maximum grain size of the slab surface grain size distribution immediately after heating was determined.), And hot rolling was performed to manufacture a hot rolled steel strip having a thickness of 4.5 mm.

Without hot-annealing the hot-rolled steel strip, the surface of the steel sheet was mechanically descaled with high-pressure water containing iron sand, and then 300 g / liter of H ₂ SO ₄ at 90 ° C.
Fig. 2 shows the result of arranging the relationship between the strain application rate to the CC slab and the heating temperature for the presence or absence of scale-containing wrinkle-like dents after 60 seconds of pickling and 11-14 µm pickling and lapping. . As is clear from FIG. 2, wrinkle-shaped dents are generated in all of the particles having a maximum particle size distribution of the heating slab surface layer particles larger than 5 mm, but the maximum particle size distribution is 5 mm.
In the following cases, no wrinkle-shaped dent defect occurs.

In this experiment, a slab having a γ _p of 18% was used. When no strain was applied, heating at a high temperature of 1180 ° C. or higher caused the maximum grain size of the grain size distribution to grow to a grain size exceeding 5 mm. Will end up. When a processing strain is applied to this, no change is seen at a processing strain of less than 2.5%, but when a processing strain of 2.5% or more is applied, the processing strain is driven in combination with a heating temperature of 1300 ° C or less. Recrystallization with a grain size of 5 mm or less as a force occurs.
When the temperature exceeds 1300 ° C, the recrystallized grain size exceeds 5 mm,
After hot rolling, wrinkle-shaped dent defects occur. Also, at a temperature below 1180 ° C, the maximum grain size of the grain size distribution is 5 mm even without processing strain.
It is not necessary to apply strain to the CC cast, but it is a region (118) where remarkable fine grains are obtained as the amount of reduction is increased.
It was found that 0 ° C. ≧ T (° C.) ≧ 1190-4 × R (° C.)). The mark * in Fig. 1 is 300 at the bottom of the continuous casting machine.
It is the data that the strain is applied between 900 ° C and 300 ° C.
It shows the same effect as hot rolling to 900 ° C.

In addition, the effect of such working strain can be obtained by applying an apparatus capable of imparting working strain only to a portion where a wrinkle-shaped dent defect in the vicinity of the edge is usually produced by heating for hot rolling. After that, the maximum particle size of the particle size distribution is 5 mm
You can control the following: Further, by making the grain size after heating 5 mm or less by acting on the short side of the slab, the edge seam flaws generated in the edges of the hot-rolled sheet of 15 to 20 mm could be reduced to 3 mm or less. The growth of crystal grains and the change of recrystallization due to heating proceed in a short time and stop changing after the structure changes to a structure according to each condition in 30 minutes. In actual operation, it takes 1 to 2 hours to raise the temperature, and the heating time for soaking is about 1 to 3 hours. Therefore, the structure was judged in 30 minutes to 3 hours.

Next, the reasons for limitation in the present invention will be described. Regarding the degree of processing strain imparted to CC slab, there is a lower limit that works effectively, and the upper limit requires a large capacity in the rolling reduction device directly under the continuous casting device, and if excessive strain is applied, the slab itself will crack. Occurs, 2.5 to 15
%. Regarding the temperature at which strain is applied to the CC slab, if the temperature is less than 300 ° C, cracks easily occur from the grain boundaries of the columnar crystals, so the lower limit was made 300 ° C or higher. Lower temperature is desirable for effective strain, but 900 ℃
If it exceeds, the effect becomes small, so the range is limited to 300 to 900 ° C. The temperature before heating depends on the embrittlement temperature of the slab, and a material that is cracked at room temperature requires heat treatment at 300 ° C. or higher, and a material that does not crack is cooled to room temperature.

The heating temperature T before hot rolling is 11
If it is less than 00 ° C., the deformation resistance during hot rolling increases, and the rolling load of the hot rolling machine becomes excessive, which makes rolling difficult.
The temperature is preferably 00 ° C. or higher. Further, when the temperature exceeds 1300 ° C, the slab is deformed during heating to a shape that cannot be hot-rolled, and the grain size after heating cannot be controlled to 5 mm or less even if processing strain is applied, so the temperature was set to 1300 ° C or less.

Next, factors for changing the surface grain size of the heating slab were examined. When the state in which the maximum particle size of the particle size distribution of the heating slab surface particles is changed when the slab heating temperature and the heating time are changed is shown in FIG. 3, as shown in FIG. Is remarkable, but grain growth almost disappears after 30 minutes. Further, the maximum particle size of the particle size distribution varies greatly depending on the heating temperature, and the higher the temperature is, the more coarse particles are generated. In actual operation, it takes 1 to 2 hours to raise the temperature, and heating for soaking takes about 1 to 3 hours. Therefore, it is important to control the heating temperature in order to control the particle size.

By further analyzing the operation data,
Γ _p , which represents the γ layer that changes depending on the composition, and the heating temperature (30
FIG. 4 shows the result of arranging the relationship between (minutes to 3 hours heating) and the occurrence of wrinkle-shaped dent defects. As is clear from FIG. 4, when the maximum particle size distribution of the heating slab surface particle size is larger than 5 mm, wrinkle-shaped dents and scratches are generated, but the maximum particle size distribution particle size is 5 mm or less. In the case of the above, no wrinkle-shaped dent defect occurs. It can be seen that there exists a slab manufacturing condition in which wrinkle-shaped dents and defects do not occur.

The manufacturing conditions are, as shown in FIG. 4, the heating temperature of the slab is T (° C.), and γ is calculated from the components of the slab.
_{If p} (%), then T (° C.) ≦ 1150 + 1.67γ _p (%). By setting the particle size after heating to 5 mm or less under these conditions, the edge seam flaws generated in the edges of the hot-rolled sheet of 15 to 20 mm could be reduced to 3 mm or less. Thus, γ _p
For the mechanism that the grain growth of the heating slab surface layer can be suppressed by increasing the heating temperature even if the heating temperature rises, there are more γ layers in the slab surface layer during heating of the slab,
It is considered to suppress the grain growth of ferrite grains.

The heating temperature T is limited to 1100 ° C.
If it is less than 1, the deformation resistance during hot rolling increases, and the rolling load of the hot rolling machine becomes excessive, making rolling difficult.
It is preferable to make the above. Further, when the temperature exceeds 1300 ° C, the slab is deformed during heating and cannot be hot-rolled.

Further, in order to clarify the cause of the difference in the surface layer structure after heating in the slab width direction described above, the difference in the surface layer structure at the position of 40 to 70 mm from the width center part of the CC slab and the slab corner part is described. , Another slab was used for comparison. As a result, it was found that the CC slab had a large amount of chill crystals and a small amount of columnar crystals in the center of the width, and the width edges of 40 to 70 mm tended to have a small amount of chill crystals and a large amount of columnar crystals.
Especially for the width edge part, the difference between CC casts is severe,
The proportion of chill crystals and columnar crystals changed significantly.

Thus, regarding CC casts having different chill crystals and columnar crystals, the occupancy ratio of chill crystals and columnar crystals per 120 mm of the C cross section of the CC slab was determined, and the occupancy ratio of chill crystals and the heating ratio were calculated. FIG. 5 shows the results in which the relationship between the temperature and the coarsening after heating and the occurrence of wrinkle-shaped dents is arranged. Here, the columnar crystal means that the surface of the slab has a grain size of 0.
It has a width of 3 to 2 mm, and the definition is a grain grown from the surface in the depth direction by 7 mm or more. By subtracting the total length occupied by the surface of the columnar crystal from 120 mm, the length occupied by the chill crystal can be calculated, and the value obtained by dividing this by 120 mm is the chill crystal ratio.

As is apparent from FIG. 5, wrinkle-shaped dents and defects are generated in any of the heating slabs having a maximum particle size distribution of the surface layer particles larger than 5 mm. If the thickness is 5 mm or less, no wrinkle-shaped dent defect is generated. Further, it can be seen that a large number of coarse particles are generated by heating at a high temperature, and wrinkle-shaped concave flaws are generated.

As the chill crystal ratio increases, the critical heating temperature for coarsening increases, and with 100% chill crystals, the maximum particle size distribution becomes 5 mm or less even when heated to 1300 ° C., resulting in wrinkle-like depressions. No flaws occur. Then, it can be seen that there exists a slab manufacturing condition in which such a dent defect does not occur. As shown in FIG. 5, the manufacturing condition is that the slab heating temperature is T (° C.),
Chill crystal ratio Ci (%) per 120 mm of slab surface edge
Then, the condition is T (° C.) ≦ 550 + 7.5 × Ci (%). This condition is effective even if width reduction is performed to make the width of the hot-rolled steel strip constant during rough rolling.

As described above, the chill crystal ratio and the heating temperature are regulated, and the particle size after heating is set to 5 mm or less, whereby the hot rolled sheet edge 15 to
Edge seam flaws that occur on 20 mm can also be reduced to 3 mm or less. In this way, the chill crystals increase, and the clear mechanism by which the grain growth of the heating slab surface layer can be suppressed despite the increase of the heating temperature is unknown, but the grain boundary segregation element that makes grain growth difficult to occur during heating of the slab. It is considered that the chill crystals are less likely to cause grain growth than the columnar crystals when the slab is heated because there are more chill crystals at the grain boundary portions where they are rapidly solidified.

From this cast structure, the growth of crystal grains by heating proceeds in a relatively short time, and after changing to a structure according to each condition in 30 minutes, it does not change. 1 in heating in actual operation
~ 2 hours are required, and the heating time for soaking is 1 to 3
Since it is about time, the tissue was judged in 30 minutes to 3 hours. The chill crystallinity can be controlled by controlling the ΔT obtained from the difference between the molten steel temperature and the solidification liquid layer line in the CC mold during casting of the CC slab within a range of 20 to 40 deg C. , ΔT and the chill crystal ratio of the surface layer of the CC slab are shown in FIG.

The heating temperature T is limited to 1100 ° C.
If it is less than 1, the deformation resistance during hot rolling increases, and the rolling load of the hot rolling machine becomes excessive, making rolling difficult.
It is preferable to make the above. Further, when the temperature exceeds 1300 ° C, the slab is deformed during heating and cannot be hot-rolled.

Regarding the heating time before hot rolling, the point is the time of staying (passing) in the soaking zone of the slab heating furnace, but a minimum of 30 minutes is necessary in actual operation, and heating for a long time exceeding 3 hours is required. Then, there is a concern that the slab may be deformed during heating.

Focusing on the crystal grain size of the surface layer portion of the slab immediately after heating the slab, it is clear that the correspondence with the wrinkle-shaped concave flaws is stable and that the structure after heating the slab is stable and is unlikely to change in structure. is there. That is, in general, when observing the structure immediately after heating, it is necessary to perform water cooling immediately after heating to perform rapid cooling treatment, but it is not easy to perform water cooling with a 12 ton scale slab. Simulating a 12 ton scale slab with a small sample and examining the maximum grain size of the grain size distribution of the heated structure in the slowly cooled sample and the water cooled sample, it was found that the cooling conditions had little effect. .

The ferritic stainless steel which is the subject of the present invention is preferably selected from the following composition ranges. The basis of the constituent requirements will be described below. C is an element that greatly affects the corrosion resistance. If C is large, Cr carbides are easily formed and intergranular corrosion is easily caused, so C is made 0.1% or less. Si is an element contained in the deoxidizing agent remaining during steel making, but if contained in a large amount, it impairs workability, so the upper limit was made 1.0%. Mn contains deoxidizing and desulfurizing agents remaining during steel making, but if contained in a large amount, it impairs corrosion resistance, so the upper limit is made 1.0%.

From the viewpoint of hot workability, it is desirable that P is not an element to be added, and it is preferably 0.04% or less. S is also not an element to be added intentionally, and it is desirable that it is small in terms of hot workability and corrosion resistance, and is made 0.02% or less. Ni is an element that is essentially unnecessary except when improving the toughness, but Ni is inevitable in the manufacturing process, so its allowable limit was set to 0.3%.

If the Cr content is less than 11%, the corrosion resistance as stainless steel cannot be maintained, and if it exceeds 23%, the hot workability deteriorates, so the Cr content is made 11 to 23%. Ti is an element that improves deep drawability and corrosion resistance, but addition of a large amount deteriorates manufacturability, so the upper limit is made 0.6%. Al is necessary as a strong deoxidizing agent during steel making, and is necessary for omitting high-temperature short-time annealing or annealing of the hot-rolled steel strip, but inclusion of a large amount increases inclusions, The upper limit was 0.2%.

Like N, N forms Cr nitrides, impairs corrosion resistance, and deteriorates formability. Therefore, 0.0
Reduce to 5% or less. Cu has the effect of improving corrosion resistance,
Addition of a large amount deteriorates hot workability, so the content is made 0.1% or less. Nb is an element that improves deep drawability,
Since addition of a large amount deteriorates hot workability, the upper limit is set to 0.
1%.

Mo is an element that significantly improves the corrosion resistance, but it is an expensive element, and its content is set to 0.05% or less. V
Has significantly improved the corrosion resistance, but deteriorates the hot workability, so it was made 0.1% or less. O is not preferable in terms of corrosion resistance and moldability, and 0.01% or less is desirable.

The above components are γ _p = 240C + 470N
+ 23Ni + 9Cu + 7Mn-11.5Cr-11.5
Si-12Mo-23V-47Nb-49Ti-52A
The target is ferritic stainless steel having a calculated γ _p value of 1 + 189 (%) of 5 to 60%. This is because γ _p is the minimum necessary for omitting high temperature short time annealing or annealing of the hot rolled steel strip, and when it exceeds 60%, hot workability deteriorates. Next, examples of the present invention will be described.

[0047]

【Example】

[Example 1] Two types of γ _p 5% and 29% ferritic stainless steels of A and B whose chemical components are shown in Table 1 were continuously cast into continuous cast slabs, and vertical type CCs were used.
Rolling strain was applied in a temperature range of 300 to 900 ° C. by using a pinch roller installed below the continuous casting device. In order to hot roll this slab, 1100 ~
After heating to 1300 ° C., hot rolling was performed with a hot strip mill.

The dimensions of the slab are 250 mm in thickness and 1030 in width.
The unit weight in mm is 10 to 12 tons. Hot rolling is 23
After rolling to mm, finish rolling was performed to obtain a hot coil having a thickness of 4.0 mm. Table 2 shows the rough rolling pass schedule and Table 3 shows the finishing rolling pass schedule.
It was shown to. Regarding rough rolling, rolling was performed according to pass schedule A in which width reduction was not performed and pass schedule B in which width reduction was performed.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

After the mechanical descaling of the steel sheet surface with high-pressure water containing iron sand without annealing, 3
The surface of the sample was inspected for wrinkle-shaped pits and scratches by inspecting the surface of the sample after sulfuric acid pickling with H ₂ SO ₄ of 00 g / liter at 90 ° C. for 60 seconds and pickling with 10-15 μm pickling.

When heating each slab, a part of the slab was simultaneously charged as a sample into a heating furnace and heated under the same conditions as those used in the hot rolling experiment. It was cooled without rolling, and the maximum grain size of the grain size distribution of ferrite grains generated in the surface layer portion was measured for the C cross section. The results are collectively shown in Table 4. The heating temperature shown in Table 4 is the time during which the slab surface was held in the heating furnace in a state where the slab surface almost reached the extraction temperature, and corresponds to the so-called soaking time.

[0054]

[Table 4]

As shown in the results of Table 4, those in which the maximum grain size of the grain size distribution of the surface layer immediately after heating the slab was controlled to 5 mm or less did not cause wrinkle-like dent defects. And the distortion is 2.5
~ 15%, and the heating temperature for hot rolling is 1300 ° C.
When heated below, the maximum particle size in the surface particle size distribution immediately after heating the slab becomes 5 mm or less, and wrinkle-shaped dents and scratches do not occur, but the strain is less than 2.5% and 1180 ° C or more. In the case of, when strain exceeds 1300 ° C even if strain is applied,
The maximum grain size in the grain size distribution of the surface layer immediately after heating the slab exceeded 5 mm and became large, and wrinkle-shaped dents and flaws occurred. From the above results, it is possible to prevent the occurrence of wrinkle-shaped depressions appearing on the surface of the hot rolled steel strip of ferritic stainless steel by controlling the maximum grain size of the surface grain size distribution immediately after the slab heating to 5 mm or less. Is clear.

[Example 2] A showing the chemical components in Table 1
~ E of five types of γ _p 5 to 60% ferritic stainless steel, each of which is continuously cast into a continuous cast slab,
This was heated to 1100 to 1300 ° C. in a heating furnace and then hot-rolled by a hot strip mill. The slab has a thickness of 250 mm, a width of 1030 mm and a unit weight of 10 to 12 tons. In hot rolling, after rolling to a thickness of 23 mm, finish rolling was performed to finish a hot coil having a thickness of 4.0 mm. The rough rolling pass schedule is shown in Table 2 and the finish rolling pass schedule is shown in Table 3. For rough rolling,
Rolling was carried out with a pass schedule A without width reduction and a pass schedule B with width reduction.

After mechanical descaling of the steel sheet surface with high-pressure water containing iron sand without annealing, 3
The surface of the sample was inspected for wrinkle-shaped pits and scratches by inspecting the surface of the sample after sulfuric acid pickling with H ₂ SO ₄ of 00 g / liter at 90 ° C. for 60 seconds and pickling with 10-15 μm pickling.

When heating each slab, a part of the slab was simultaneously charged as a sample into a heating furnace and heated under the same conditions as those used in the hot rolling experiment. It was cooled without rolling, and the maximum grain size of the grain size distribution of ferrite grains generated in the surface layer portion was measured for the C cross section. The results are collectively shown in Table 5. The heating temperature shown in Table 5 is the time during which the slab surface was kept in the heating furnace in a state where the slab surface almost reached the extraction temperature, and corresponds to the so-called soaking time.

[0059]

[Table 5]

As shown in the results of Table 5, those in which the maximum grain size in the grain size distribution of the surface layer immediately after heating the slab was controlled to 5 mm or less did not cause wrinkle-like dent defects. Then, the heating conditions are slab heating temperature (° C) ≤ 1150 + 1.67γ _p (%)
In the case of, the maximum grain size in the surface layer grain size distribution immediately after heating the slab is 5 mm or less, and wrinkle-shaped dents and flaws do not occur, whereas the slab heating temperature is 1150 + 1.67γ _p (%)
If the value exceeds the value of, the maximum particle size of the surface layer particle size distribution immediately after heating the slab exceeds 5 mm and becomes large, and wrinkle-shaped dents and defects are generated. From the above results, it is possible to prevent the occurrence of wrinkle-shaped depressions appearing on the surface of the hot rolled steel strip of ferritic stainless steel by controlling the maximum grain size of the surface grain size distribution immediately after the slab heating to 5 mm or less. Is clear.

[Example 3] Two kinds of γ _p 5% and 29% of ferritic stainless steels of A and B whose chemical components are shown in Table 1 are continuously cast into continuous cast slabs, which are cast during continuous casting. Change ΔT to 15 to 4 by changing the condition
The chill crystallinity of the surface layer structure of the CC cast was changed to 5 deg C. This was heated to 1100 to 1300 ° C. in a heating furnace and then hot-rolled by a hot strip mill. The slab has a thickness of 250 mm, a width of 1030 mm, and a unit weight of 10 to 12 to
n. In hot rolling, after rolling to a thickness of 23 mm, finish rolling was performed to finish a hot coil having a thickness of 4.0 mm. The rough rolling pass schedule is shown in Table 2 and the finish rolling pass schedule is shown in Table 3. Regarding rough rolling, rolling was performed according to pass schedule A in which width reduction was not performed and pass schedule B in which width reduction was performed.

After the mechanical descaling of the steel sheet surface with high-pressure water containing iron sand without annealing, 3
The surface of the sample was inspected on the surface of the sample after sulfuric acid pickling with H ₂ SO _{4 at} 00 g / liter, 90 ° C. for 60 seconds and pickling with 11 to 16 μm to examine the occurrence of wrinkle-like pits.

In addition, when heating each slab, a part of the slab was simultaneously charged as a sample into a heating furnace and heated under the same conditions as those used in the hot rolling experiment. It was cooled without rolling, and the maximum grain size of the grain size distribution of ferrite grains generated in the surface layer portion was measured for the C cross section. The results are collectively shown in Table 6. The heating temperature shown in Table 6 is the time during which the slab surface was kept in the heating furnace in a state where the slab surface almost reached the extraction temperature, and corresponds to the so-called soaking time.

[0064]

[Table 6]

As shown in the results of Table 6, those in which the maximum grain size in the grain size distribution of the surface layer immediately after heating the slab was controlled to 5 mm or less did not cause wrinkle-like dent defects. When the heating conditions are slab heating temperature (° C.) ≦ 550 + 7.5 × Ci (%), the maximum particle size of the surface layer particle size distribution immediately after slab heating is 5 mm or less, and wrinkle-shaped dent defects occur. In contrast, when the slab heating temperature exceeds the value of 550 + 7.5 x Ci (%), the maximum grain size of the surface layer grain size distribution immediately after slab heating exceeds 5 mm and becomes large, causing wrinkle-like dents. A flaw has occurred. From the above results, it is possible to prevent the occurrence of wrinkle-shaped depressions appearing on the surface of the hot rolled steel strip of ferritic stainless steel by controlling the maximum grain size of the surface grain size distribution immediately after the slab heating to 5 mm or less. Is clear.

[0066]

As described above, according to the present invention, the wrinkle-shaped dent defect containing scale is not generated in the hot-rolled steel strip, and the re-pickling rate of the pickling after mechanical descaling is significantly increased. Moreover, it is not necessary to grind the entire surface of the steel strip by the coil grinder, and the problem can be solved from the viewpoint of improving the yield and the production planning. Further, even if the coil width is rolled to produce a constant coil width and the amount of strain at the edge portion is increased, wrinkle-shaped recessed flaws do not occur, so hot rolling with a higher yield becomes possible.

Further, conventionally, from the viewpoint of not increasing the particle size immediately after heating, the upper heating temperature is set to a relatively low temperature of about 1200 ° C. Therefore, it takes a long time to uniformly heat the slab. However, by applying the strain according to the present invention, the operation can be performed without coarsening even when heated up to 1300 ° C., so that high temperature short time heating becomes possible. Since the productivity can be remarkably increased in this way, its industrial effect is great.

[Brief description of drawings]

[Fig.1] Slab thickness 250 mm, width 1000 mm, length 6
A 000 mm slab is hot rolled to a thickness of 4.5 mm, mechanically descaled and then pickled with sulfuric acid, and the length of the hot rolled steel strip front and back is 1
Under the same conditions as the number of wrinkle-shaped dents generated per 00 mm and a part of the slab at the time of heating of each slab were simultaneously put into the heating furnace as a sample and subjected to the hot rolling experiment. FIG. 4 is a diagram showing the relationship between the maximum grain size of the grain size distribution of the ferrite grain size generated in the surface layer portion after heating and cooling without hot rolling.

FIG. 2 is a diagram showing a relationship between a slab heating temperature and a strain application rate to a CC slab, on a grain size after heating and wrinkle-shaped dent defects.

FIG. 3 is a diagram showing a relationship between a slab heating temperature and a maximum grain size of a surface layer grain size distribution immediately after the slab heating is finished depending on a heating time.

FIG. 4 is a diagram showing a relationship between a slab heating temperature and γ _p on a tissue grain size after heating and generation of wrinkle-like pits.

FIG. 5 is a diagram showing the relationship between the slab heating temperature and the chill crystal ratio of the surface layer of the CC slab on the grain size of the structure after heating and the occurrence of wrinkle-shaped dent defects.

FIG. 6 is a diagram showing the relationship between the chill crystallinity of the surface layer of CC slab, the temperature difference obtained from the difference between the molten steel temperature in the CC mold and the solidification liquid layer line.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] December 27, 1995

[Procedure amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0024

[Correction method] Change

[Correction contents]

In this experiment, a slab having a γ _p of 18% was used. When no strain was applied, heating at a high temperature of 1180 ° C. or higher caused the maximum grain size of the grain size distribution to grow to a grain size exceeding 5 mm. Will end up. When a processing strain is applied to this, no change is seen at a processing strain of less than 2.5%, but when a processing strain of 2.5% or more is applied, the processing strain is driven in combination with a heating temperature of 1300 ° C or less. Recrystallization with a grain size of 5 mm or less as a force occurs.
When the temperature exceeds 1300 ° C, the recrystallized grain size exceeds 5 mm,
After hot rolling, wrinkle-shaped dent defects occur. Also, at a temperature below 1180 ° C, the maximum grain size of the grain size distribution is 5 mm even without processing strain.
It is not necessary to apply strain to the CC cast, but it is a region (118) where remarkable fine grains are obtained as the amount of reduction is increased.
It was found that 0 ° C. ≧ T (° C.) ≧ 1190-4 × R (° C.)). The * mark in Fig. 2 indicates 300 at the bottom of the continuous casting machine.
It is the data that the strain is applied between 900 ° C and 300 ° C.
It shows the same effect as hot rolling to 900 ° C.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction item name] 0053

[Correction method] Change

[Correction contents]

When heating each slab, a part of the slab was simultaneously charged as a sample into a heating furnace and heated under the same conditions as those used in the hot rolling experiment. It was cooled without rolling, and the maximum grain size of the grain size distribution of ferrite grains generated in the surface layer portion was measured for the C cross section. The results are collectively shown in Table 4. The heating time shown in Table 4 is the time in which the slab surface was kept in the heating furnace in a state where the slab surface almost reached the extraction temperature, and corresponds to the so-called soaking time.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0058

[Correction method] Change

[Correction contents]

When heating each slab, a part of the slab was simultaneously charged as a sample into a heating furnace and heated under the same conditions as those used in the hot rolling experiment. It was cooled without rolling, and the maximum grain size of the grain size distribution of ferrite grains generated in the surface layer portion was measured for the C cross section. The results are collectively shown in Table 5. The heating time shown in Table 5 is the time for which the slab surface was kept in the heating furnace in a state where the slab surface almost reached the extraction temperature, and corresponds to the so-called soaking time.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0063

[Correction method] Change

[Correction contents]

In addition, when heating each slab, a part of the slab was simultaneously charged as a sample into a heating furnace and heated under the same conditions as those used in the hot rolling experiment. It was cooled without rolling, and the maximum grain size of the grain size distribution of ferrite grains generated in the surface layer portion was measured for the C cross section. The results are collectively shown in Table 6. The heating time shown in Table 6 is the time for which the slab surface was held in the heating furnace in a state where the slab surface almost reached the extraction temperature, and corresponds to the so-called soaking time.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukihiro Kure No. 1-1 Tobata-cho, Tobata-ku, Kitakyushu, Fukuoka Prefecture (72) Inside the Yawata Works, Nippon Steel Co., Ltd. (72) Kiyoshi Yamaji Tobata-ku, Kitakyushu, Fukuoka Prefecture No. 1 Tobitacho New Steel Works Yawata Works

Claims (3)

[Claims] 1. A slab of ferritic stainless steel,
It is characterized in that after processing strain of 2.5 to 15% is applied in the temperature range of 300 to 900 ° C., it is heated to 1100 to 1300 ° C. and the crystal grain size of the slab surface layer portion before hot rolling is 5 mm or less. Method for preventing surface flaws of ferritic stainless hot rolled steel strip. 2. A slab of ferritic stainless steel,
A ferritic stainless hot-rolled steel strip, characterized in that it is heated to a temperature range of 1100 to 1300 ° C. and satisfies the following formula (1), and the grain size of the slab surface layer portion before hot rolling is set to 5 mm or less. Surface flaw prevention method. T (° C) ≤ (1150 + 1.67γ _p ) (° C) (1) where γ _p = 240C + 470N + 23Ni + 9Cu + 7Mn
-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti
-52Al + 189 Here, the unit of γ _p and each component is%, and T (° C) is the slab heating temperature. 3. A slab of ferritic stainless steel,
A ferritic stainless hot-rolled steel strip, characterized in that it is heated to a temperature range of 1100 to 1300 ° C. and satisfies the following formula (2), and the grain size of the slab surface layer portion before hot rolling is set to 5 mm or less. Surface flaw prevention method. T (° C.) ≦ (550 + 7.5 Ci (%)) (° C.) (2) where Ci is the occupancy rate (%) of the chill crystal in the surface structure of the chill crystal and the columnar crystal of the cast slab.