JPH08296000A - Ferritic stainless steel excellent in workability and corrosion resistance and its production - Google Patents

Abstract

PURPOSE: To produce a ferritic stainless steel causing no season cracking of slab even if air-cooled after continuous casting, reduced in ridging, and excellent in workability and intergranular corrosion resistance. CONSTITUTION: A ferritic stainless steel, which has a composition consisting of, by weight, <=0.0030% C, <=2.0% Si, <=2.0% Mn, <=1% Ni, 9-25% Cr, 0.001-0.1% Al, >0.07-0.15% Ti, 0.001-0.007% N, and the balance Fe with inevitable impurities and satisfying 5×(C+N)<=Ti<=0.12+4×C+(24/7)×N and containing, if necessary, 3-50ppm B, 0.1-1% Cu, and <=4% Mo, is used. A slab of this steel is hot-rolled by a continuous hot rolling mill, coiled at >=750 deg.C, pickled, and then cold-rolled at 50-90%. The resulting cold rolled steel sheet is annealed at 750-850 deg.C for 0.5-5min. By this method, the necessity of annealing after hot rolling as well as the necessity of slow cooling for a slab after continuous casting can be obviated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel which can be easily manufactured without causing cracks in a continuously cast slab and has excellent workability and corrosion resistance, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Ferritic stainless steels represented by AISI409 and SUS430 are superior to surface-treated steel sheets in resistance to high temperatures, oxidation resistance and corrosion resistance. It is used as a high corrosion resistant structural member such as welded structural members and building exterior panels in the environment. In the case of a muffler for automobiles, the cold rolled annealed plate of the stainless steel is formed, and then MIG welding or TIG welding is performed to complete the process.

In the weld heat affected zone, since Cr carbides are likely to precipitate at grain boundaries, there is a risk of intergranular corrosion.
In addition, since the weld heat affected zone is exposed to a high temperature during welding, crystal grains may be coarsened, and cracks may occur due to subsequent processing or deformation. In order to suppress intergranular corrosion and coarsening in the heat-affected zone of the welding, Ti is usually added to the ferritic stainless steel for welding (for example, JP-A-49-130).
No. 817). The addition amount is Ti / (C + N)
It is defined by a conditional expression of ≧ 5 (for example, Japanese Patent Laid-Open No.
-107220 gazette).

Further, ferritic stainless steel has a drawback that after forming, a wrinkle-like defect called ridging which is parallel to the rolling direction is generated, which impairs the appearance. For this reason, in applications where appearance is required, such as automobile molding materials, after hot rolling, annealing was performed at 900 ° C or higher, followed by cold rolling and recrystallization annealing (Japanese Patent Laid-Open No. 54-7911).
7 publication). However, in this conventional technique, 900
In addition to requiring a dedicated facility for heat treatment at a temperature of ℃ or more, a large amount of energy is consumed in the heat treatment process and a large amount of labor is required.

Further, in the conventional ferritic stainless steel in which Ti has been added to improve intergranular corrosion resistance, coarse precipitates are precipitated in the cast steel ingot at macro grain boundaries of the solidification structure, and these precipitates are used as the starting points. There was a crack in the slab. In order to avoid this cracking, the slab after continuous casting is slab-rolled or hot-rolled (hot-charge hot-rolled) without being cooled to room temperature as it is at a temperature of 200 ° C. or higher, or after continuous casting. The slab was covered with a heat-retaining cover and gradually cooled to reduce residual stress. In order to carry out slabbing, hot rolling equipment exclusively for slabbing is required, and energy consumption and labor force are inevitably increased due to the addition of manufacturing processes. Even when carrying out hot charge hot rolling, it requires a hot transfer facility from the dedicated continuous casting machine to the front of the hot rolling heating furnace, and a great deal of labor is spent to match the timing of casting and hot rolling. Is the current situation. Further, in the case of covering a slab after continuous casting with a heat-retaining cover, a large number of heat-retaining covers are required, and a great deal of labor is required for the mounting work.

Therefore, a ferritic stainless steel in which the amount of Ti added is limited to 700 ppm or less and the susceptibility to slab placement cracking is reduced is disclosed in JP-A-6-287718. However, the cracks in the slab were not completely resolved, and sometimes they occurred sporadically. In addition, B is added to high-purity ferritic stainless steel containing Ti.
JP-A-61-261460 discloses a ferritic stainless steel sheet in which the secondary workability after deep drawing is improved by adding 3 to 50 ppm. But,
In recent years, it has been desired to develop a ferritic stainless steel having a higher elongation, a higher r-value, and a higher critical drawing ratio than the invention steels.

[0007]

In view of the present situation, the present invention reduces the susceptibility of continuous cast slab to cracking, and prevents Ti from cracking even if left to cool in the atmosphere after continuous casting.
Provided is an additive ferritic stainless steel and a method for producing the same. Furthermore, it enables the production of ferritic stainless steel with less ridging by the hot rolling-cold rolling-annealing process without annealing the hot-rolled sheet, further improving workability and deep drawing compared with conventional ferritic stainless steel. A ferritic stainless steel having improved secondary workability and intergranular corrosion resistance after working.

[0008]

The inventors of the present invention added Ti, which is the main factor of slab placement cracking, by reducing C and N, which are the main factors causing deterioration of workability and intergranular corrosion, as much as possible. The effect of C, N, and Ti on the slab placement cracking susceptibility and ridging, workability, intergranular corrosion resistance, etc. can be reduced by focusing on the fact that the recrystallization temperature after hot rolling can be reduced. As a result of detailed examination in the laboratory, the present invention has been accomplished.

In the Ti-added ferritic stainless steel, cracking occurs during the cooling process after continuous casting. Cracks
It occurs at 100 ° C or lower where the ductility of the slab suddenly decreases and brittle fracture occurs. FIG. 1 shows the effect of the amount of solid solution Ti in the steel on the high temperature ductility behavior of the Ti-added ferritic stainless steel continuous cast slab. As is clear from the figure, the inventors of the present invention can dramatically improve the ductility of the ferritic stainless steel slab in the temperature range of room temperature to 100 ° C. by setting the amount of solid solution Ti to 0.12% or less. Found. As a result, it was found that the susceptibility to slab cracking was significantly reduced, and even after continuous casting, the slab did not crack when placed in the atmosphere.

On the other hand, in the conventional industrial mass production technology,
The amount of (C + N) contained in ferritic stainless steel is
The lower limit was about 100 ppm. Therefore, C + N ≦ 1
The mechanical properties and intergranular corrosion resistance at 00 ppm have not been sufficiently examined. The present inventors have found that the ductility and r value are significantly improved by setting the C + N amount of ferritic stainless steel to 100 ppm or less. FIG. 2 shows (a) strength, (b) elongation,
(C) The influence of the amount of C + N on the r value (Rankford value) is shown. By setting the amount of C + N to 100 ppm or less, the yield point is lowered and the elongation is improved, and r
It became clear that the value also improved dramatically. The effect of the amount of C + N on the intergranular corrosion resistance of the heat affected zone is shown in FIG. Even in such an extremely low C and N region, the Ti content is 5
It was found that intergranular corrosion does not occur by adding more than (C + N).

The conventional ferritic stainless steel having a (C + N) content exceeding 100 ppm has a high recrystallization temperature and cannot be self-annealed at a normal hot rolling coiling temperature. But,
When the amount of (C + N) is 100 ppm or less, the recrystallization temperature is 750.
It was found that the temperature was lowered to ℃ or less, and self-annealing was possible by winding after hot rolling. Figure 4 shows C + N: 75ppm
The effect of hot rolling conditions on the ridging height of cold rolled annealed ferritic stainless steel sheets is shown. As a result, it was found that the ridging height was significantly reduced when wound at 750 ° C. or higher after hot rolling. The reason is that when rolled up at less than 750 ° C, the hot-rolled sheet does not recrystallize sufficiently up to the central portion of the sheet thickness, recrystallization occurs only once in the manufacturing process of the cold-rolled annealed sheet, and the texture in the rolling direction It is believed that it remains strong and ridging appears after tensile deformation. On the other hand, when it is wound up at 750 ° C. or higher, it is recrystallized by self-annealing, and then recrystallized again by cold rolling-annealing. That is, recrystallization is performed twice in the manufacturing process of the cold rolled annealed plate, and ridging is reduced.

From the above findings, by limiting C, N and Ti to appropriate ranges and optimizing the winding temperature in the continuous hot rolling step, the susceptibility to slab placement cracking is low and the workability with small ridging and It is clear that it is possible to manufacture ferritic stainless steel with excellent intergranular corrosion resistance.

Next, the limited scope of the present invention will be described. C: Cr carbide precipitates at grain boundaries such as in the heat affected zone of welding and is an extremely harmful element for intergranular corrosion resistance.
If the content is high, the recrystallization temperature rises, and even if wound at 750 ° C. or higher after hot rolling, it does not self-anneal, so the upper limit is 0.0.
It was set to 030%. In particular, in the case of performing a severe deep drawing such that the deep drawing ratio exceeds 2.1, the elongation and the r value are remarkably improved by making the content less than 0.0020%.

Si: If contained in excess, cold workability deteriorates, so 2.0% was made the upper limit. Mn: An element that has a desulfurizing / deoxidizing effect and is extremely effective in improving grain boundary embrittlement and hot embrittlement due to segregation of S into the crystal grain boundaries. Since it lowers the inter-workability, 2.0% was made the upper limit.

Ni: Although added as an extremely effective element for improving low temperature brittleness and toughness, since it is an austenite forming element, if added in a large amount, an austenite phase precipitates during heating and after cooling. Since it transforms to the martensite phase and significantly impairs cold workability,
The upper limit was 1.0%. Cr: An essential element for improving the oxidation resistance or corrosion resistance, and it is necessary to add 9.0% or more in order to obtain the oxidation resistance required as a material for an automobile muffler. However, addition of a large amount causes not only an increase in manufacturing cost due to alloy cost but also deterioration of cold workability due to formation of σ phase, so 25.0% is made the upper limit.

Al: Since it is an element effective as a deoxidizing agent for steel, it is necessary to add 0.001% or more. However, if added in a large amount, the workability deteriorates.
The upper limit was 1%. N: TiN or Nb (C, N) is precipitated in the steel and is an extremely effective element for suppressing the coarsening of crystal grains in the heat-affected zone of welding, so it is necessary to add 0.0010% or more. . However, since it is contained in a large amount and the amount of solid solution N increases, the cold workability, particularly elongation and r value are significantly deteriorated.
The upper limit was 0070%.

Ti: TiN is precipitated in the steel to suppress the coarsening of crystal grains in the heat-affected zone of welding, and TiC which is stable even at a high temperature is secured to secure intergranular corrosion resistance of the heat-affected zone of welding. Therefore, since it is an extremely effective element,
It is necessary to add more than 07% and 5 · (C + N) or more. However, if a large amount is added, the amount of Ti that forms a solid solution in the steel increases and the susceptibility to continuous cracking of the continuously cast slab increases, so 0.15% or 0.12+ (4 × C + (2
The lower one of 4/7) × N) was set as the upper limit. In addition, (4 × C
(24/7) × N) is the amount of Ti that is precipitated as a carbide or a nitride and is not in solid solution.

Mo: It is an extremely effective element for improving the corrosion resistance and the pitting corrosion resistance, but if added in a large amount, the effect is saturated and the alloy cost rises, so the upper limit is 4.0%. And Cu: Since it is an extremely effective element for improving the corrosion resistance, 0.1% was made the lower limit. However, if added in a large amount, it cannot form a solid solution in the ferrite phase and the hot workability is significantly impaired, so 1.0% was made the upper limit. B: In the present invention, it is the most important element for improving the secondary workability, and its effect is exhibited at 0.0003% or more. However, if a large amount is added, deep drawability deteriorates and cracks in the slab occur, so the upper limit was made 0.0050%. Preferably, 0.0004-
0.0015% is good.

For the purpose of ensuring productivity and manufacturing thin materials,
A steel casting slab having the above-described composition is hot-rolled by a continuous hot rolling mill including a rough rolling mill and a plurality of hot rolling mills. Compared with the conventional ferritic stainless steel, the steel has an extremely low content of Ti, C, or N, so that the effect of suppressing the coarsening of crystal grains due to the precipitation of Ti carbonitride is small. Therefore, in such continuous hot rolling, the lower limit of the winding temperature after the completion of hot rolling is limited to 750 ° C. to recrystallize the hot rolled sheet. When the coiling temperature is lower than 750 ° C., recrystallization does not occur up to the central part of the plate thickness even by sensible heat of the coil, and the rolling texture remains strong even after subsequent cold rolling-annealing, resulting in large ridging. Therefore, the lower limit was set to 750 ° C.

If the rolling ratio of the cold rolling to be subsequently carried out is set to less than 50%, the subsequent annealing does not introduce sufficient working strain into the steel to completely recrystallize up to the central portion of the plate thickness. Was set to 50%. However, when cold rolling exceeds 90%, work hardening becomes remarkable and rolling load becomes high, so the upper limit was made 90%.

The annealing temperature range is 750 to 850 ° C.
The reason for this is that if the temperature is lower than 750 ° C, recrystallization is not sufficiently performed up to the center of the plate thickness, so 750 ° C was set as the lower limit. But,
If the temperature exceeds 850 ° C., the crystal grains become coarse, and after the molding process, surface roughness called orange peel occurs, so 85
The upper limit was 0 ° C. The holding time of the annealing temperature is 0.5 to 5.
The value of 0 min is set to 0.5 min, because if the holding time is less than 0.5 min, recrystallization does not occur sufficiently up to the center of the plate thickness.
Was set as the lower limit. However, if the holding time exceeds 5.0 min, the crystal grains become coarse, and after the molding process, surface roughness called orange peel occurs, so 5.0 min was made the upper limit.

[0022]

[Examples] Molten ferritic stainless steel melted in a converter was subjected to ultra-low carbon reduction treatment in a secondary refining process, and then continuously cast. The melted components are shown in Table 1. After continuously casting this slab, it was left to cool in the atmosphere and examined for occurrence of cracking. Furthermore, after rolling this slab into a steel strip with a plate thickness of 3.0 mm with a continuous hot rolling mill, pickling, cold rolling rate of 80%
Cold rolling (final plate thickness 0.6 mm), final annealing at 800 to 850 ° C., and electrolytic pickling. A sample was taken from this steel strip and MIG-welded to examine whether or not intergranular corrosion occurred in the heat-affected zone of the welding. Table 1 shows the results. In the case of the A to G steels of the present invention, no cracking occurred in the slab after continuous casting, and no intergranular corrosion was observed in the heat affected zone after MIG welding. On the other hand, Free Ti in steel
For H, J, L, M, N steels with an amount exceeding 0.12%, cracks occur in the slab after continuous casting, and the Ti content is 5
In the K steel having less than (C + N), intergranular corrosion occurred in the weld heat affected zone after MIG welding.

[0023]

[Table 1]

Next, the steels A to G which are the subject steels of the present invention, the steels H to K which are comparative steels, and the M and N steels which are conventional steels are shown in Table 2.
Hot rolling-cold rolling-annealing was performed under the conditions shown in, and the elongation, r value, and ridging height were measured. No. 1 produced by the method of the present invention The steels 1 to 7 show high elongation and r value, and have a small ridging height. On the other hand, after continuous casting, the steel of the present invention in which slab placement crack did not occur was hot rolled and wound at a temperature lower than that of the present invention. The steels Nos. 8 to 13 show elongation and r value almost equal to those of the high-temperature wound material, but have high ridging height. Moreover, after continuous casting, no slab-placed cracks occurred, but the C or N was higher than the steel of the present invention. 1
The No. 4 and No. 15 steels have higher elongation and higher ridging height than the low C, N steels, with a low r value.

[0025]

[Table 2]

Next, Table 3 shows the evaluation results of mechanical properties and secondary workability. As for the secondary workability, a disk-shaped blank was taken from a stainless steel plate, and primary processing products with different drawing ratios were manufactured by a two-step cylindrical drawing process. Next, after holding the drawn component at 20 ° C., an impact load was applied to the cup head by a drop weight test, and it was evaluated by repeating three times whether brittle cracks occurred in the cup side wall. In the table, ○ indicates no secondary processing cracks, ● indicates secondary processing cracks, and × indicates that deep drawing was not possible. From the results in Table 3, it can be seen that the steel sheet A of the present invention is extremely excellent in secondary workability. In addition, the present invention steel sheets A steel and G
The mechanical properties of steel are excellent in elongations of 40% and 39%, respectively, and r values of 2.1 and 2.0, which are extremely excellent, and have excellent deep drawability. Further, the G steel to which B was not added achieved the target drawing ratio of 2.00, but secondary work cracking occurred due to more severe deep drawing with a drawing ratio of 2.15 or more.

[0027]

[Table 3]

From the above examples, according to the present invention, the contents of C, N and Ti contained in the ferritic stainless steel are closely related to each other, the susceptibility of the slab to cracking, the heat-affected zone intergranular corrosion resistance, Ferrite series that is extremely effective in terms of workability and ridging characteristics, has little ridging and does not cause slab placement cracks even if left to cool in the atmosphere after continuous casting, and has excellent workability and intergranular corrosion resistance. It has become possible to manufacture stainless steel.

[0029]

Industrial Applicability According to the present invention, it is possible to produce a ferritic stainless steel which is small in ridging and excellent in workability and intergranular corrosion resistance without causing cracks in the continuously cast slab. This eliminates the need for a slab heat-retaining cover after continuous casting, a facility for hot transfer between the continuous casting process and the hot rolling process, and a slabbing process, thus simplifying the manufacturing process. Furthermore, even if the annealing after hot rolling is omitted, it is possible to manufacture ferritic stainless steel with small ridging. As a result, productivity has improved dramatically, and labor has been greatly reduced. The industrial significance of these improvements is extremely great.

[Brief description of drawings]

Fig. 1: Solid solution Ti (Free in steel) on hot ductility behavior
The figure which shows the influence of Ti) amount.

2A is a diagram showing strength (TS, YS), FIG. 2B is elongation, and FIG. 2C is a diagram showing the influence of the amount of C + N on the r value.

FIG. 3 Ti amount and C affecting intergranular corrosion resistance of heat-affected zone of welding
The figure which shows the relationship of + N amount.

FIG. 4 is a diagram showing the effect of hot rolling winding temperature on the ridging height.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takuji Shindo 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Co., Ltd.

Claims (6)

[Claims] 1. By weight%, C: 0.0030% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 9 to 25%, Al: 0.001 to 0.1%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, Cu: 0.1 to 1.0%, and 5 × ( C + N) ≦ Ti ≦ 0.12 + (4 × C + (24/7) ×
A ferritic stainless steel which satisfies N) and has the balance of Fe and unavoidable impurities, which does not cause a crack in the continuous cast slab and has excellent workability and corrosion resistance. 2. By weight%, C: 0.0030% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 9 to 25%, Al: 0.001 to 0.1%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, Mo: 4.0% or less, Cu: 0.1 to 1.0% And 5 × (C + N) ≦ Ti ≦ 0.12 + (4 × C + (24/7) ×
A ferritic stainless steel which satisfies N) and has the balance of Fe and unavoidable impurities, which does not cause a crack in the continuous cast slab and has excellent workability and corrosion resistance. 3. By weight%, C: 0.0030% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 9 to 25%, Al: 0.001 to 0.1%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, B: 0.0003 to 0.0050%, and 5 × ( C + N) ≦ Ti ≦ 0.12 + (4 × C + (24/7) ×
A ferritic stainless steel which satisfies N) and has the balance of Fe and unavoidable impurities, which does not cause a crack in the continuous cast slab and has excellent workability and corrosion resistance. 4. In% by weight, C: 0.0030% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 9 to 25%, Al: 0.001 to 0.1%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, B: 0.0003 to 0.0050%, and 5 × ( C + N) ≦ Ti ≦ 0.12 + (4 × C + (24/7) ×
N), and further contains one or two of Mo: 4.0% or less and Cu: 0.1 to 1.0%, the balance being Fe and inevitable impurities. A ferritic stainless steel with excellent workability and corrosion resistance that does not cause cracks in the continuous cast slab. 5. By weight%, C: 0.0030% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 9 to 25%, Al: 0.001 to 0.1%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, and 5 × (C + N) ≦ Ti ≦ 0.12 + (4 × C + (24/7) ×
N) is satisfied, and the rest of the steel continuously cast slab consisting of Fe and unavoidable impurities is hot-rolled by a continuous hot rolling machine consisting of a rough rolling mill and a plurality of hot rolling mills. The taking temperature is 750 ° C. or higher, followed by pickling and cold rolling with a rolling rate of 50 to 90%, and then 750 to 85.
A ferritic stainless steel with small ridging and excellent workability and corrosion resistance, which is characterized by performing softening annealing for 0.5 to 5.0 min in the temperature range of 0 ° C. Production method. 6. In wt%, C: 0.0030% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 1.0% or less, Cr: 9 to 25%, Al: 0.001 to 0.1%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, and further B: 0.0003 to 0.0050%, Mo: 4 0.0% or less, Cu: containing one or more of 0.1 to 1.0%, and 5 × (C + N) ≦ Ti ≦ 0.12 + (4 × C + (24/7) ×
N) is satisfied, and the rest of the steel continuously cast slab consisting of Fe and unavoidable impurities is hot-rolled by a continuous hot rolling machine consisting of a rough rolling mill and a plurality of hot rolling mills. The taking temperature is 750 ° C. or higher, followed by pickling and cold rolling with a rolling rate of 50 to 90%, and then 750 to 85.
A ferritic stainless steel with small ridging and excellent workability and corrosion resistance, which is characterized by performing softening annealing for 0.5 to 5.0 min in the temperature range of 0 ° C. Production method.