JP3190825B2 - Ferritic stainless steel hot-rolled steel sheet excellent in corrosion resistance, formability and material uniformity, and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled ferritic stainless steel sheet having excellent corrosion resistance, formability and material uniformity, and a method for producing the same.

[0002]

2. Description of the Related Art Stainless steel, compared with ordinary steel,
Since it contains a large amount of alloying elements such as Cr and Ni, it is a material that is expensive, but has remarkably excellent corrosion resistance, and has the advantage that it can be used purely even in an environment where corrosion resistance is required. Examples of such stainless steel include a ferritic stainless steel having a ferrite single phase structure at room temperature, an austenitic stainless steel having an austenitic single phase structure, a duplex stainless steel having a two phase structure of ferrite and austenite, and further a martensitic structure. Types such as martensitic stainless steel.

[0003] Among these, ferritic stainless steel is slightly inferior in workability and corrosion resistance as compared with austenitic stainless steel, but is superior in stress corrosion cracking resistance and is inexpensive because it hardly contains Ni. It is widely used for various kitchen appliances and automotive exhaust system parts (exhaust manifolds, exhaust pipes, converter shells, mufflers). like this,
When used in applications requiring workability as well as corrosion resistance, in order to improve the workability, for example, as disclosed in Japanese Patent Application Laid-Open No. 52-31919, ferritic stainless steel
A technique has been applied in which elements such as i and Nb are added to fix impurity elements such as C and N which form a solid solution in steel.

[0004] Incidentally, such a ferritic stainless steel sheet is usually prepared by heating a continuous cast slab and then performing steps of hot rolling, hot-rolled sheet annealing and pickling, cold rolling, finish annealing and pickling. It is provided as a cold-rolled stainless steel sheet manufactured through the process. On the other hand, the hot-rolled stainless steel sheet manufactured by omitting the steps after the cold rolling in the above steps is capable of greatly reducing equipment costs and operating costs after the cold rolling, and is therefore inexpensive compared to the austenitic system. The advantages of the ferritic stainless steel sheet are further enhanced, and the ferrite stainless steel sheet can be manufactured in a short time.

However, hot-rolled ferritic stainless steel sheets generally have a problem that their corrosion resistance and formability are inferior to those of cold-rolled stainless steel sheets of the same composition.
In this regard, Japanese Patent Application Laid-Open No. 7-233449 discloses that a ferritic stainless steel hot-rolled steel sheet having both corrosion resistance and workability has a low Cr ferritic stainless steel in which C and N are suppressed to 0.015% or less. And a certain amount of S
Are disclosed. However, for the above steel sheet, the corrosion resistance was evaluated from the center of the steel sheet of a hot-rolled sheet having a thickness of 3.6 mm, and the thickness was reduced to 2 mm.
As in the actual use environment, when the corrosion resistance is evaluated on the hot rolled sheet surface without reducing the thickness, there is a problem that the corrosion resistance is insufficient. In addition, the corrosion resistance and formability of the hot-rolled ferritic stainless steel sheet vary greatly in the longitudinal direction of the coil as compared with the cold-rolled stainless steel sheet, and it is difficult to maintain uniform characteristics over the entire length of the coil. Therefore, when shipped as a hot-rolled stainless steel sheet, the portion where the target material could not be obtained had to be cut off, leaving a problem in terms of productivity and cost.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems. First, since a hot-rolled stainless steel sheet has insufficient corrosion resistance, a cold-rolled stainless steel sheet is conventionally used. It is an object of the present invention to provide an inexpensive ferritic stainless steel hot-rolled steel sheet having good corrosion resistance and being applicable to a field which had to be neglected. A second object of the present invention is to significantly improve the formability such as the deep drawability particularly required at the time of press forming as compared with a conventional hot-rolled stainless steel sheet. . Further, a third object of the present invention is to improve the material homogeneity such as corrosion resistance and forming workability in the length direction of the steel sheet as compared with a conventional hot-rolled stainless steel sheet, and to improve product yield and productivity. is there.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, have found that a ferritic stainless steel hot rolled with carbonitride precipitated and fixed sufficiently by adding Ti and Nb. In steel sheets, the average grain size of carbides precipitated in the hot-rolled sheet is 0.02μ at the surface layer (the outermost layer to 1/5 thickness).
m or more and 0.05 at the center of the sheet thickness (2/5 to 3/5 thickness)
By controlling it to at least μm, it is possible to achieve an advantageous improvement in corrosion resistance and moldability, and such carbide control is achieved by winding the sheet bar during hot rolling once into a coil under predetermined conditions. It is possible to improve the uniformity of the material in the longitudinal direction of the hot-rolled coil by reversing the rear end of the rolling end in the rough rolling of the material to be rolled in the finish rolling in the rough rolling. It has been found that by connecting the rear end of the finish rolled material and the winding end end of the succeeding sheet bar coil and performing finish rolling, the material uniformity in the longitudinal direction of the hot rolled coil is further improved. The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows. 1. C: 0.02 wt% or less, Si: 2.0 wt% or less, Mn: 0.8 wt% or less, P: 0.05 wt% or less, S: 0.03 wt% or less, Cr: 10 to 25 wt%, N: 0.02 wt% or less, Ni : 1.0 wt% or less, Al: 0.1 wt% or less, and one or two selected from Ti and Nb
Species is contained in a range satisfying the following relationship: (Ti / 48 + Nb / 93) / (C / 12 + N / 14)> 1.5 (Ti + Nb) <0.5 wt%, and the balance substantially becomes Fe, Furthermore, the average grain size of the carbide contained in the steel is 0.02 μm or more at the surface layer (outermost layer to 1/5 thickness) and 0.05 μm at the center of the sheet thickness (2/5 to 3/5 thickness). A hot-rolled ferritic stainless steel sheet excellent in corrosion resistance, formability and material uniformity, characterized by satisfying the above conditions.

[0009] 2. In the above item 1, the steel composition further contains B
A hot-rolled ferritic stainless steel sheet excellent in corrosion resistance, formability and material uniformity, containing one or two kinds selected from Ca and Ca: 0.01 wt% or less.

[0010] 3. In the above 1 or 2, the steel composition is
Further, a hot-rolled ferritic stainless steel sheet excellent in corrosion resistance, formability and material uniformity, containing one or more selected from Mo, Cu, Co, V and Zr: 5.0% or less.

[0011] 4. C: 0.02 wt% or less, Si: 2.0 wt% or less, Mn: 0.8 wt% or less, P: 0.05 wt% or less, S: 0.03 wt% or less, Cr: 10 to 25 wt%, N: 0.02 wt% or less, Ni : 1.0 wt% or less, Al: 0.1 wt% or less, and one or two selected from Ti and Nb
When hot rolling a steel slab having a composition containing a seed in a range satisfying the relationship of (Ti / 48 + Nb / 93) / (C / 12 + N / 14)> 1.5 (Ti + Nb) <0.5 wt%. The sheet bar that has undergone the rough rolling stage is once wound into a coil while the surface temperature of the sheet bar is between 800 and 5050 ° C., and then finish rolling is started from the end of winding, corrosion resistance, formability And method for producing hot rolled ferritic stainless steel sheet with excellent material uniformity.

5. In the above item 4, in the finish rolling from the end of winding, the end of winding of the succeeding sheet bar coil is connected to the rear end of the sheet bar, and the finish rolling is continuously performed. Of hot-rolled ferritic stainless steel sheet with excellent heat resistance and material uniformity.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the reason why the composition of steel in the present invention is limited to the above range will be described. C: not more than 0.02 wt% C is an element that is particularly important in the present invention together with N described later. C is the corrosion resistance and workability (r value) of the steel sheet
It is desirable to reduce as much as possible because it is an element that reduces the refining. However, unnecessarily high purification causes an increase in refining costs. Therefore, in the present invention, even a small amount of C is precipitated and fixed by adding Ti and Nb. Here, as will be described later, the precipitation form and distribution of carbides are important for improving corrosion resistance and workability. However, even with such precipitation control, if the C content exceeds 0.02 wt%, the total precipitation amount in the steel sheet increases, leading to a decrease in corrosion resistance and workability, so the content was limited to 0.02 wt% or less. .

Si: 2.0 wt% or less Si is an element that is effective not only for deoxidation of steel but also for oxidation resistance at high temperatures and salt damage characteristics at high temperatures. Therefore, particularly when it is used in applications such as automobile exhaust system members, it is desirable to contain at least about 0.6 wt%. However, when the content exceeds 2.0 wt%, the elongation characteristics are significantly deteriorated, so that the content is limited to 2.0 wt% or less.

Mn: 0.8 wt% or less Mn is an element effective for precipitating and fixing S in steel and improving hot rollability, but when contained in an excessively large amount, causes deterioration of workability. Therefore, it was limited to 0.8 wt% or less.

P: 0.05 wt% or less P is not more than 0.05 wt% because P enhances hot cracking property and decreases not only hot rollability but also hot rolled sheet toughness.

S: 0.03 wt% or less S is a harmful element that degrades hot workability, but usually combines with Mn to form MnS.
%, The adverse effect is small. However, 0.03wt
%, The precipitated MnS becomes a starting point of rust, which deteriorates corrosion resistance and segregates at crystal grain boundaries to promote grain boundary embrittlement. Therefore, the content is limited to 0.03 wt% or less.

Cr: 10 to 25 wt% Cr is an indispensable element for obtaining corrosion resistance and oxidation resistance at high temperatures, but its effect is poor when the content is less than 10 wt%. On the other hand, if the content exceeds 25 wt%, the corrosion resistance of the material itself becomes sufficiently high, so that the corrosion resistance does not depend much on the precipitation form of carbides on the surface layer. As a result, the production cost is increased due to a decrease in yield. Therefore, the Cr content is limited to the range of 10 to 25 wt%.

N: 0.02 wt% or less N, like C, is an element that lowers the workability (r value) of a steel sheet, so it is desirable to reduce it as much as possible, but unnecessarily high purification increases the refining cost. Therefore, as in the case of C, a small amount of N is fixed and rendered harmless by the addition of a nitride-forming element (Ti, Nb) as described later. However, if the content exceeds 0.02 wt%, the amount of precipitates in the steel sheet increases, leading to a decrease in corrosion resistance and workability.
%.

Ni: 1.0 wt% or less Ni not only contributes to the improvement of the toughness of the hot-rolled sheet, but is also effective in improving the corrosion resistance to acids. However, since it is an expensive element, the addition of a large amount leads to an increase in cost. In addition, if added excessively, adverse effects such as hot cracking may occur, so the content was limited to 1.0 wt% or less.

Al: 0.1 wt% or less Al is an effective element for deoxidation. However, if added excessively, not only will Al ₂ O ₃ remain in the steel sheet to cause a reduction in corrosion resistance, but also
Since the workability of the hot rolled annealed sheet also deteriorates, the content is limited to 0.1 wt% or less.

(Ti / 48 + Nb / 93) / (C / 12 + N / 14)> 1.5 (Ti + Nb) <0.5 wt% Both Ti and Nb are strong C, N stabilizing elements, and in this invention, especially Element. By adding one or two of these elements, most of the contained C and N can be deposited and fixed as carbides and nitrides. For that purpose, it is important to contain at least an amount of Ti and / or Nb exceeding 1.5 in atomic ratio with respect to (C + N). However, when the content is 0.5 wt% or more, whether used alone or in combination, not only the workability of the base material is reduced but also the workability of the welded portion is significantly reduced. In addition, since the toughness is significantly deteriorated and hinders the production process, the upper limit is set to 0.5 wt%.

The essential components have been described above. In the present invention, the following elements can be contained as required. B and / or Ca: 0.01 wt% or less B is a useful element that concentrates at the grain boundaries of the steel sheet and improves the brittleness in secondary working. Further, it promotes the accumulation of processing strain during hot rolling, enhances the accumulation of {111} planes of the recrystallized texture after annealing, and effectively contributes to the improvement of formability. However, excessive addition not only saturates the effect, but also hardens the material and consequently causes deterioration of elongation characteristics and consequently deterioration of workability. Therefore, the B content is limited to 0.01 wt% or less. Ca, in the Ti-containing steel,
Although effectively contributing to the suppression of nozzle clogging due to Ti-based inclusions during molten steel casting, the content is limited to 0.01 wt% or less because excessive addition causes deterioration of corrosion resistance.

One or more selected from Mo, Cu, Co, V and Zr: 5.0% or less Both Mo, Cu and Co have an effect of further improving the corrosion resistance, and may be added as necessary. . Both V and Zr are elements that form carbonitrides with C and N, like Ti and Nb, and have the effect of assisting Ti and Nb in the present invention and improving workability. However, if the content of these elements, alone or in combination, exceeds 5.0 wt%, the workability during hot rolling is significantly reduced, so the content was limited to 5.0 wt% or less.

Next, the form of carbide precipitation in the present invention will be described. By the way, the inventors conducted a detailed study on the precipitates of a hot-rolled steel sheet of ferritic stainless steel, and found that no difference was observed in nitrides and oxides, whereas the precipitation behavior of Ti or Nb-based carbides was In the hot rolled sheet manufactured by performing hot rolling according to the usual method, which differs in the thickness direction,
The carbide grain size near the steel sheet surface is clearly smaller (0.003 μm) than the carbide grain size near the steel sheet center (about 0.02 μm).
Degree). In addition, as a result of investigating the relationship between the precipitation state of carbides and corrosion resistance and formability, when compared with steel sheets of the same composition, when the carbide particle size increases, especially when the carbide particle size in the surface layer increases, the corrosion resistance improves. It turned out to be. Although the reason has not been clearly elucidated, (1) the number of carbides in the surface layer, which becomes the rusting point, decreases when the surface carbide precipitates and coarsens, and (2) Ti or Nb particularly near the surface layer. It is presumed that, when the carbides are fine, the fixation of C by these elements is weak or insufficient, and the precipitation of Cr-based carbides which deteriorates the corrosion resistance occurs simultaneously.

Also, there was a tendency that the formability was improved as the carbide particle size increased. In particular, the formability was strongly correlated with the carbide grain size near the center of the sheet thickness. The reason for this is not clear, but possible reasons are:
(1) The precipitation growth of carbides proceeds during hot rolling, which promotes purification of the base iron part, and the formation of {111} texture, which is known to be advantageous for formability, in the recrystallization process in subsequent annealing. And (2) increasing the carbide particle size, suppressing re-solid solution in the annealing step, and accelerating the development of {111} texture.

Therefore, the precipitation state of carbide is limited as follows. 0.02μ at the surface (the outermost layer to 1/5 thickness)
m, 0.05μm at the center of the thickness (2/5 to 3/5 thickness)
As described above, the corrosion resistance improves as the carbide particle size in the surface layer increases, but the effect is remarkable when the average particle size in the surface layer is 0.02 μm or more. Although the formability is also improved, the effect is remarkable when the average particle size of the carbide at the center of the plate thickness is 0.05 μm or more. Therefore, as the average particle size range of the carbide having both excellent corrosion resistance and moldability, the average particle size of the carbide in the surface layer portion (from the outermost layer to 1/5 thickness) is 0.02 μm or more, and the position at the center of the sheet thickness (2 / The average particle size of the carbide (5 to 3/5 thickness) was limited to 0.05 μm or more.

Next, the reasons for limiting the manufacturing method in the present invention will be described. The above-mentioned control of carbide precipitation form can be realized by the manufacturing method described below. Rolling of sheet bar in hot rolling: The sheet bar after the rough rolling is wound around a coil, so that a rapid temperature drop during rolling is avoided, and the sheet bar is soaked over the entire length of the coil. As a result, it is considered that carbide coarsening proceeds due to stabilizing elements such as Ti and Nb. In other words, when hot rolling is performed according to a conventional method, a local temperature drop occurs near the surface layer of the steel sheet due to contact with rolls and injection of cooling water, so that precipitates precipitated during hot rolling are generated at the surface layer portion. It becomes fine. In this regard, according to the method of the present invention, when the winding process is performed once in the state of the sheet bar, the surface layer portion whose temperature has dropped is regained heat, and the temperature uniformity in the thickness direction of the steel plate is achieved. . According to this winding process, the time from the end of rough rolling to the end of finish rolling is longer than that of the conventional method, and the temperature drop of the entire sheet bar coil is suppressed. As described later, the temperature range in which the winding process is performed is a temperature at which the base iron structure is recrystallized and the precipitation of carbides of Ti and Nb actively proceeds. As a result, it is presumed that carbide precipitation growth proceeds over the entire area of the sheet bar coil, and in particular, carbide precipitation growth near the surface layer portion of the sheet thickness, which has been difficult with the conventional method, easily progresses.

Further, the sheet bar after the rough rolling is wound around a coil, and then subjected to finish rolling while unwinding, so that the front and rear ends of the material to be rolled in the rough rolling are reversely rolled by finish rolling. Become. For this reason, in the rough rolling, the temperature gradient was inevitably generated from the leading end side to the trailing end side, whereas in the finish rolling, the material to be rolled is rolled from the rear end side of the rough rolling having a lower temperature. Also, in finish rolling, there is also an effect that the temperature is soaked over the entire length of the rolled material.

Sheet bar winding temperature: 800 to 1050 ° C. The gist of the present invention is to sufficiently precipitate and fix C by Ti or Nb at the sheet bar stage to grow carbide. However, when the surface temperature exceeds 1050 ° C., precipitation does not sufficiently proceed even by winding. On the other hand, 800 ℃
If it is less than, the precipitation of the carbonized portion near the center of the plate thickness proceeds, but the deposition rate decreases near the surface layer, and sufficient carbide particle growth cannot be obtained. If the temperature is lower than 800 ° C., there is also a disadvantage that the temperature of the subsequent finish rolling is lowered and the surface quality is deteriorated. Therefore, the winding temperature is limited to the range of 800 to 1,050 ° C, preferably 800 to 1,000 ° C.

Continuous finishing rolling: In the finishing rolling, the material to be rolled is rolled and wound in a predetermined temperature range during the rolling and after passing through the finishing rolling mill and wound up by the coiler. It is cooled by a method such as water cooling. Here, until the coil tip passes through the finishing mill and winds around the coiler, while the coil rear end passes through the finishing mill until it is wound on the coiler.
In any case, since the steel sheet is not constrained, water cooling causes non-uniform cooling and non-uniform shape, and satisfactory cooling cannot be performed in these portions. In this regard, when winding the sheet bar after rough rolling into a coil, and then starting finish rolling while rewinding, connecting the winding end of the succeeding sheet bar coil to the rear end of the preceding sheet bar, finishing If rolling is performed continuously, from finish rolling to coiling take-up, rolling is performed with virtually no rolling edges except for the leading end of the first rolled material and the trailing end of the final rolled material. it can. Thus, it is possible to perform rolling without causing the above-described problems of uneven cooling and uneven shape at the rolling end.

The steel sheet after hot rolling must be softened by annealing to improve workability. The annealing conditions are not particularly limited, and may be performed according to a conventional method. Is preferably performed in a temperature range of 800 to 1,050 ° C.

[0033]

【Example】

Example 1 A ferritic stainless steel slab having the composition shown in Table 1 was rolled to a thickness of 2.0 mm by a hot rolling mill having three stages of rough rolling stands and seven stages of finishing rolling stands. that time,
At a temperature shown in Table 2, the sheet bar after the rough rolling was once wound into a coil, and then finish rolling was performed while unwinding.
On the other hand, as a comparative example, finish rolling was performed according to a conventional method without performing a winding process following rough rolling. For some rolled materials, the preceding and subsequent sheet bars were joined and finish rolling was continuously performed. After that, 900 ℃ × 3
After annealing for a minute, pickling descaling was performed.

[0034]

[Table 1]

[0035]

[Table 2]

A sample was taken from almost the center in the longitudinal direction of the hot rolled coil thus obtained, and the corrosion resistance and the formability were evaluated. The corrosion resistance was evaluated by performing a salt-dry and wet composite (CCT) cycle test under the conditions shown in Table 3 and evaluating the rusting area after the test. In addition, the rusting area ratio of the test material manufactured by rolling without performing the sheet bar winding process in accordance with the ordinary method of the same composition as a reference value, the corrosion resistance improvement rate with respect to this reference value is represented by the following equation. (%) = {(Reference area ratio) − (rusting area ratio of test material)} / (reference area ratio) × 100, which was used as an evaluation index. The formability was determined by cutting out a JIS No. 13 B test piece parallel to the rolling direction,
The Rankford value in tension was determined and evaluated.
In addition, the sample taken from the vicinity of the center in the longitudinal direction of the coil was cut into a surface layer 1 / 5thickness (surface layer sample) and a center 1 / 5thickness (central portion sample), respectively. After preparing the sample, the precipitate was extracted and collected by the electrolytic method. Then, the precipitates obtained were measured by partially using observation and analysis with a centrifugal particle size distribution analyzer and a transmission electron microscope (TEM), and the average particle size of the carbide was determined.

[0037]

[Table 3]

Table 2 shows the sheet bar winding temperature, the presence or absence of sheet bar joining, the average particle size of the precipitated carbide, the rusting area ratio,
Corrosion resistance improvement rate and Rankford value in rolling direction (r
Values) are shown together. In Table 1, Steel I is a comparative steel having a low Ti, Nb content and a content ratio of Ti, Nb to C, N out of the proper range of the present invention. Other steels A, B, C,
D, E, F, G and H satisfy the component composition range of the present invention, but test materials 1, 7, 11, 14 manufactured by hot rolling by a conventional method without sheet bar winding. , 17, 2
In all of Examples 1, 25 and 29, the average grain size of the carbide near the surface layer and near the center of the plate thickness is small. On the other hand, according to the present invention, the test materials 3, 4, 5, and 6 (the above were compared based on the test material 1) and the test materials 8, 9 (the above Indicates the reference material 7), reference material 13 (the reference is based on reference material 11), reference materials 15 and 16 (the comparison is based on the reference material 14), reference material 18, 19 (compared based on specimen 17), Specimens 22, 23 (compared based on specimen 21), 27, 28 (compared based on specimen 25) In both cases, the average particle size of the carbide near the surface layer and near the center of the sheet thickness is larger than that of the reference material, and the corrosion resistance, Both moldability and processability have been improved.

However, even when the sheet bar is wound, the test material 2 (comparison with the test material 1) and the test material 12 (the test material 11 For the test material 26 (compared to the reference material) and the test material 26 (compared to the test material 25), the carbides did not grow to the extent of the present invention in both the surface layer and the central portion, and the corrosion resistance and moldability were compared with the reference material. There was no noticeable improvement. On the other hand, on the contrary, the test material whose winding temperature is lower than the range of the present invention.
10 (compared to test material 7), test material 20 (test material 17
The test material 24 (compared on the basis of the test material 21) and the test material 24 (compared on the basis of the test material 21) did not have sufficient carbide growth near the surface layer, and the corrosion resistance was almost the same as the reference material. on the other hand,
Steel I, which has a low Ti and Nb content and does not satisfy the steel composition of the present invention, shows that the growth of carbides is increased even in the case of sheet bar winding according to the manufacturing method of the present invention (sample 32). Neither of them was sufficient, and almost no improvement was observed in both the corrosion resistance and the moldability with respect to the reference material (test material 31) manufactured by the conventional method.

The results obtained using the steels A, B, C, D, E, F, G, H and the comparative steel I were evaluated as follows.
FIGS. 1 and 2 summarize the effects of the particle size of the surface layer and the central part on the values. From the figure, it is clear that the corrosion resistance depends on the carbide grain size in the surface layer, while the formability represented by the r value depends on the carbide grain size near the center of the sheet thickness.

Example 2 Samples of test materials 1, 4, and 6 made of steel A shown in Example 1 were intermittently sampled from coils other than the center in the longitudinal direction of the coil to obtain corrosion resistance and molding. The homogeneity of both properties over the entire length of the coil was investigated. FIG. 3 shows changes in the corrosion resistance improvement rate and r value in the coil longitudinal direction. Here, LE is a rolling tip in rough rolling,
TE is the trailing end in the rough rolling. Specimen 1, which is a comparative example manufactured according to the conventional method, not only has low overall properties, but also has a remarkable deterioration in the r value at the front and rear ends of the coil and particularly in the corrosion resistance on the TE side. The test material 4 on which the sheet bar was wound according to the present invention shows excellent corrosion resistance and moldability over the entire length of the coil, and the variation width thereof tends to be small. Further, with respect to the test material 6 which has been continuously rolled by joining the sheet bars after winding, the material variation width is even smaller. Therefore, it is clear that the uniformity in the longitudinal direction of the coil is also improved by the method of the present invention.

[0042]

Thus, according to the present invention, in the hot-rolled ferritic stainless steel sheet, the carbonitride is sufficiently precipitated and fixed by the addition of Ti and Nb, and the average particle size of the carbide precipitated in the hot-rolled sheet. The surface layer (the outermost layer to 1/5 thickness)
By controlling the thickness to 0.02 μm or more and the thickness at the center of the thickness (2/5 to 3/5 thickness) to 0.05 μm or more, the corrosion resistance and formability of hot-rolled ferritic stainless steel sheet can be significantly improved. It is also possible to control the precipitation of the carbide by temporarily winding the sheet bar during hot rolling under predetermined conditions, more preferably, after winding the sheet bar once, the rear end of the preceding rolled material and the sheet. By performing the finish rolling continuously by joining the winding ends of the bar coil, the material homogeneity in the longitudinal direction of the hot-rolled coil can be remarkably improved, greatly stabilizing the material and improving the yield. Contribute.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the average particle size of carbide in the surface layer portion of the steel sheet (the outermost layer and 食 thickness), the corrosion resistance improvement rate, and the r value.

FIG. 2 is a graph showing the relationship between the average particle size of carbide in the central part (2/5 to 3/5 thickness) of a steel sheet, the corrosion resistance improvement rate, and the r value.

FIG. 3 is a graph showing a change in a corrosion resistance improvement rate and a change in an r value over a longitudinal direction of a coil.

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/54 C22C 38/54 (72) Inventor Higashi Shiro Ikeda 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (58 ) Surveyed field (Int.Cl. ⁷ , DB name) C22C 1/00-45/10 C21D 8/00-8/10 C21D 9/46 B21B 3/02

Claims (5)

(57) [Claims] 1. C: 0.02% by weight or less, Si: 2.0% by weight or less, Mn: 0.8% by weight or less, P: 0.05% by weight or less, S: 0.03% by weight or less, Cr: 10 to 25% by weight, N: 0.02% 1 wt% or less, Ni: 1.0 wt% or less, Al: 0.1 wt% or less, and one or two selected from Ti and Nb
Species is contained in a range satisfying the following relationship: (Ti / 48 + Nb / 93) / (C / 12 + N / 14)> 1.5 (Ti + Nb) <0.5 wt%, and the balance substantially becomes Fe, Furthermore, the average grain size of the carbide contained in the steel is 0.02 μm or more at the surface layer (outermost layer to 1/5 thickness) and 0.05 μm at the center of the sheet thickness (2/5 to 3/5 thickness). A hot-rolled ferritic stainless steel sheet excellent in corrosion resistance, formability and material uniformity, characterized by satisfying the above conditions. 2. The steel according to claim 1, wherein the steel composition further comprises B
A hot-rolled ferritic stainless steel sheet excellent in corrosion resistance, formability and material uniformity, containing one or two kinds selected from Ca and Ca: 0.01 wt% or less. 3. The steel composition according to claim 1, wherein the steel composition is
Further, a hot-rolled ferritic stainless steel sheet excellent in corrosion resistance, formability and material uniformity, containing one or more selected from Mo, Cu, Co, V and Zr: 5.0% or less. 4. C: 0.02 wt% or less, Si: 2.0 wt% or less, Mn: 0.8 wt% or less, P: 0.05 wt% or less, S: 0.03 wt% or less, Cr: 10 to 25 wt%, N: 0.02 1 wt% or less, Ni: 1.0 wt% or less, Al: 0.1 wt% or less, and one or two selected from Ti and Nb
When hot rolling a steel slab having a composition containing a seed in a range satisfying the relationship of (Ti / 48 + Nb / 93) / (C / 12 + N / 14)> 1.5 (Ti + Nb) <0.5 wt%. The sheet bar that has undergone the rough rolling stage is once wound around a coil while the surface temperature of the sheet bar is between 800 and 5050 ° C.
A method for producing a hot-rolled ferritic stainless steel sheet having excellent corrosion resistance, formability, and material uniformity, wherein finish rolling is started from an end of winding. 5. The finish rolling according to claim 4, wherein at the time of finish rolling from the winding end end, the winding end of a subsequent sheet bar coil is connected to the rear end of the sheet bar, and the finish rolling is performed continuously. A method for producing a hot-rolled ferritic stainless steel sheet having excellent corrosion resistance, formability and material uniformity.