JP3728828B2 - Manufacturing method of ferritic stainless steel with excellent surface quality and deep drawability - Google Patents

Description

【００１９】
【表２】

【００２０】
表２から明らかなように、本発明に従い製造した場合には、表面品質および深絞り性に優れたフェライト系ステンレス鋼を得ることができた。これに対し、本発明の構成要件が１つでも欠けた場合には、両者を同時に満足させることはできなかった。また、従来、深絞り性の面で良好とされた低ＦＤＴでは、表面品質の著しい劣化を招いた。
【００２１】
【発明の効果】
以上説明したように、本発明に従い、フェライト系ステンレス鋼の製造に際して、(1) 連続鋳造時における鋳造速度、(2) 連鋳後、加熱炉へスラブを装入する時のスラブ表面温度および(3) 熱延時の加熱温度を、それぞれ適正化することにより、ＤＨＣＲ(ＣＣ−ＤＲ)化を利用して表面品質および深絞り性に優れたフェライト系ステンレス鋼を安定して得ることができる。
【図面の簡単な説明】
【図１】焼鈍・酸洗板の表面品質に及ぼす、スラブの加熱炉装入温度およびスラブ加熱温度の影響を示したグラフである。
【図２】スラブ加熱温度と平均ｒ値との関係を示したグラフである。
【図３】連続鋳造における鋳込み速度と平均ｒ値との関係を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a ferritic stainless steel that is more economical and efficient than conventional methods, and that is superior in surface quality and deep drawability .
[0002]
[Prior art]
In a stainless steel manufacturing process manufactured by continuous casting and hot rolling, the continuous cast slab is once cooled to room temperature, slab-cleaned, and then heated to a temperature of 1150 to 1250 ° C in a heating furnace. Was subjected to hot rolling.
Accordingly, such a continuous casting-hot rolling process requires a great deal of heat energy and processing time.
[0003]
On the other hand, in Japanese Patent Application Laid-Open No. 57-152420, as a technique related to stainless steel CC-DR (Direct Rolling) or DHCR (Direct Hot Charge Rolling), “ferritic stainless steel containing 10 to 20% Cr is used. After continuous casting, when hot rolling is performed directly or hot rolling is performed without keeping the temperature below 400 ° C, it takes 3 hours for the continuous casting slab to be placed in the (α + γ) region. There has been proposed a process for producing a ferritic stainless steel sheet having excellent workability, which includes heat holding or heating so as not to exceed, and then hot rolling.
[0004]
Ferritic stainless steel is not only cheaper than austenitic stainless steel represented by SUS 304, but also has excellent stress corrosion cracking resistance (SCC resistance), so it is widely used. However, it is inferior in terms of workability, particularly deep drawability and ridging resistance.
Therefore, various techniques for improving processability and ridging resistance have been proposed.
[0005]
For example, JP-A-4-99151 specifies the amount of Al, Ti, Nb, B, C and N for ferritic stainless steel and further reduces the rolling reduction at 900 ° C. or lower during hot rolling to 50%. As described above, a method for producing a ferritic stainless steel having both press formability and surface quality has been proposed by setting the finish rolling temperature to 800 ° C. or lower and the coil winding temperature to 600 ° C. or higher.
[0006]
[Problems to be solved by the invention]
However, each of the conventional techniques described above has left the following problems.
That is, in Japanese Patent Laid-Open No. 57-152420, a significant improvement in the average r value, which is an index of forming processability, cannot be expected, and in Japanese Patent Laid-Open No. 4-99151, deformation of the material itself during hot rolling. Since it must be rolled down in a low temperature region where the resistance is large, there has been a problem that hot-rolled rough surface defects frequently occur on the surface.
[0007]
As mentioned above, in the conventional technology,
1) Economically and efficiently,
2) What has excellent surface quality and workability
The two purposes of manufacturing cannot be achieved at the same time, and the solution has been strongly desired.
Accordingly, an object of the present invention is to propose a novel manufacturing method of ferritic stainless steel that can simultaneously achieve the above-described two objects.
[0008]
[Means for Solving the Problems]
In producing ferritic stainless steel containing 11.0 wt% or more of Cr by continuous casting-hot rolling, the present invention sets the continuous casting speed to 1.2 m / min or more, and the continuous casting slab with its surface temperature. Ferritic stainless steel with excellent surface quality and deep drawability, which is charged in a heating furnace before being cooled to 400 ° C or lower, heated to a temperature of 1200 ° C or lower, and then subjected to hot rolling. This is a manufacturing method (first invention).
[0009]
In the first invention, the ferritic stainless steel is Cr: 11.0 wt% or more, and one or more selected from Ti, Nb, V, Zr, Ta and W. A method for producing ferritic stainless steel excellent in surface quality and deep drawability (second invention), characterized by containing in a range of {4 × (C + N)} wt% or more and 0.5 wt% or less.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the experimental results that led to the completion of the present invention will be described.
Experiment 1
C: 0.058 wt%, Si: 0.37 wt%, Mn: 0.46 wt%, Cr: 16.2 wt%, S: 0.005 wt%, P: 0.026 wt% and N: 0.031 wt%, the balance being substantially Fe SUS430 having the following composition was melted and continuously cast into a continuous casting mold having a thickness of 200 mm and a width of 1260 mm. At this time, the continuous casting speed was 1.3 m / min.
Subsequently, the obtained continuous cast slab was cooled to various temperatures and then charged into a hot rolling furnace. At this time, the heating temperature was changed variously, and the slab extraction temperature during hot rolling was changed. The slab soaking time in the heating furnace was set to be within 2 hours.
After the heat treatment, hot rolling was performed to a plate thickness of 4 mm using a rough rolling mill having three stages and a continuous finish rolling mill having seven stages.
The result of investigating the surface quality of the annealed / pickled plate after removing the oxidized scale on the surface after recrystallization annealing of the obtained hot-rolled sheet is shown in FIG.
[0011]
As can be seen from the figure, when the temperature of the continuous cast slab into the furnace is 400 ° C or higher at the surface temperature of the slab and the slab heating temperature during hot rolling is 1200 ° C or lower, the bald defects A hot-rolled sheet with no surface roughness defects and excellent surface quality was obtained.
On the other hand, when the charging temperature is less than 400 ° C. or the slab heating temperature is higher than 1200 ° C., it has been found that heavier defects occur and the surface quality of the hot rolled sheet deteriorates.
[0012]
About this cause, when the bald defect part was investigated, it turned out that it originates in local abnormal oxidation occurring on the slab surface at the time of slab heating.
That is, it is considered that when the scale formed in the continuous cast slab is cooled to 400 ° C. or less, cracks are generated due to thermal stress, and abnormal oxidation occurs locally during subsequent hot rolling. Further, if the hot rolling heating temperature exceeds 1200 ° C., it is considered that even if the scale is not cracked, the oxidation is remarkably promoted and local abnormal oxidation is likely to occur.
Therefore, in the present invention, the slab charging temperature to the heating furnace is limited to the range of 400 ° C. or more at the slab surface temperature, and the heating temperature is limited to 1200 ° C. or less. Note that when the heating temperature is lower than 1050 ° C., the hot rolling temperature is lowered, and as a result, rough skin is likely to occur. Therefore, the heating temperature is preferably 1050 ° C. or higher.
[0013]
Experiment 2
C: 0.007 wt%, Si: 0.19 wt%, Mn: 0.30 wt%, Cr: 16.4 wt%, S: 0.003 wt%, P: 0.029 wt%, N: 0.008 wt% and Ti: 0.17 wt%, The balance was made by melting SUS430LX having a substantially Fe composition and continuously cast in a continuous casting mold having a thickness of 200 mm and a width of 1260 mm. At this time, the continuous casting speed was 1.3 m / min.
Next, when the obtained continuous cast slab was charged at a surface temperature of 500 to 600 ° C., the slab was heated by variously changing the temperature of the heating furnace, and then extracted from the heating furnace. Thereafter, hot rolling was performed to a plate thickness of 4 mm by using a rough rolling mill having three stages and a continuous finish rolling mill having seven stages.
The obtained hot-rolled sheet is continuously annealed at 930 ° C, pickled and finished to a thickness of 0.7mm by cold rolling, and then subjected to continuous annealing at 910 ° C and pickling treatment to achieve 2B finish cold rolling. -Annealed plate.
[0014]
FIG. 2 shows the values obtained by measuring and averaging the r values of samples taken from the L, D, and C directions. As is apparent from the figure, it can be seen that it is effective to increase the average r value and improve the deep drawability by setting the slab heating temperature during hot rolling to 1200 ° C. or lower.
[0015]
Similarly, the average r value of cold-rolled / annealed plates manufactured by changing the casting speed during continuous casting from 0.7 m / min to 1.4 m / min and maintaining the heating temperature during hot rolling at a constant 1170 ° C was obtained. As shown in FIG.
As is clear from the figure, it can be seen that the average r value is remarkably improved when the casting speed during continuous casting is 1.2 m / min or more.
Therefore, in the present invention, the casting speed during continuous casting is 1.2 m / min or more, and the slab heating temperature during hot rolling is limited to 1200 ° C. or less.
The reason why the r value is improved by setting the casting speed to 1.2 m / min or more is that the cast structure is refined or the columnar crystal ratio is reduced, and the structure after hot rolling and annealing is made uniform. It seems that this is due to the fact that
[0016]
The ferritic stainless steel to be used in the present invention is not particularly limited, and any ferritic stainless steel is suitable as long as it contains 11.0 wt% or more of Cr. In addition, for elements such as Ti, Nb, V, Zr, Ta and W, which are added for the purpose of improving deep drawability and weldability, the content is { 4 × (C + N)} wt% or more and 0.5 wt% or less are necessary. This is because if the content is less than {4 × (C + N)} wt%, the effect of improving deep drawability and weldability is not sufficiently observed. On the other hand , if the content exceeds 0.5 wt%, the carbonitride produced This is because there is a disadvantage that it is easy to coarsen and toughness and pitting corrosion resistance are liable to occur.
[0017]
【Example】
Steel having the composition shown in Table 1 was melted in a converter, and after VOD degassing, continuous casting was performed using a mold having a short side of 200 mm and a long side of 1260 mm. The continuous cast slab thus obtained was charged into a heating furnace for hot rolling and heated to various temperatures shown in Table 2, followed by a rough rolling mill having three stages and a continuous finishing rolling mill having seven stages. Then, finish rolling was performed to obtain a hot rolled sheet having a thickness of 4 mm. The finish rolling temperature (FDT) at this time was 820 to 860 ° C. Some slabs have low FDT (750 ° C., 740 ° C.), which is conventionally considered to improve the deep drawability of cold-rolled / annealed plates. Next, the obtained hot-rolled sheet was subjected to recrystallization annealing, shot blasting and pickling, and then cold-rolled to a thickness of 0.7 mm. After recrystallization annealing and pickling, the surface finish was cold rolled 2D. -An annealed plate was produced. Table 2 shows the results obtained by examining (1) hot rolled-annealed-shot blasting and steel plate surface quality after pickling and (2) average r-value of cold-rolled / annealed plates. It is written together.
[0018]
[Table 1]

[0019]
[Table 2]

[0020]
As apparent from Table 2, when manufactured according to the present invention, a ferritic stainless steel excellent in surface quality and deep drawability could be obtained. On the other hand, when even one of the constituent requirements of the present invention is lacking, both cannot be satisfied at the same time. Conventionally, the low FDT, which is good in terms of deep drawability, has caused significant deterioration of the surface quality.
[0021]
【The invention's effect】
As described above, according to the present invention, in the production of ferritic stainless steel, (1) the casting speed during continuous casting, (2) the slab surface temperature when the slab is charged into the heating furnace after continuous casting, and ( 3) By optimizing the heating temperature at the time of hot rolling, ferritic stainless steel excellent in surface quality and deep drawability can be stably obtained using DHCR (CC-DR).
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of the slab heating furnace charging temperature and the slab heating temperature on the surface quality of annealed and pickled plates.
FIG. 2 is a graph showing the relationship between slab heating temperature and average r value.
FIG. 3 is a graph showing the relationship between casting speed and average r value in continuous casting.

Claims (2)

In producing ferritic stainless steel containing 11.0 wt% or more of Cr by continuous casting-hot rolling, the continuous casting speed is set to 1.2 m / min or more and the surface temperature of the continuous casting slab is 400 ° C or less. A method for producing a ferritic stainless steel having excellent surface quality and deep drawability, wherein the steel is charged into a heating furnace before being cooled down, heated to a temperature of 1200 ° C. or less, and then subjected to hot rolling. 2. The ferritic stainless steel according to claim 1, wherein Cr: 11.0 wt% or more, and one or more selected from Ti, Nb, V, Zr, Ta and W are {4 × (C + N)}. A method for producing a ferritic stainless steel excellent in surface quality and deep drawability , characterized by containing in a range of wt% or more and 0.5 wt% or less.

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