JP3014626B2 - Hot rolling method of ferritic stainless steel slab - 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 rolling method for a ferritic stainless steel slab, and more particularly to a hot rolling method for a ferritic stainless steel slab which hardly causes cracks called "scratch flaws".

[0002]

2. Description of the Related Art When a steel strip coil is manufactured by hot rolling a ferritic stainless steel slab, the surface of the steel strip coil after hot rolling is referred to as "scratch flaw" or simply "scraping". Cracks are likely to occur. These barge flaws remain without disappearing even after cold rolling, and may cause surface defects, or may transfer flaws to cold rolls to generate secondary flaws. The above is a big problem. For this reason, the surface of the hot-rolled steel strip coil is constantly monitored, and if a barbed flaw occurs, measures such as grinding and removing the barred portion of the coil before cold rolling or performing partial truncation are taken. However, this leads to a significant decrease in yield and an adverse effect such as a delivery time trouble due to the addition of an extra step.

[0003] Various studies have been made on the prevention of the occurrence of the scab defect, but the generation mechanism itself is unclear, and there is no way of empirical improvement. For example, Japanese Patent Application Laid-Open No. 55-33801 discloses a method in which light rolling is performed once hot, followed by cooling to repair surface defects, and then hot rolling. Also, in JP-A-4-350123,
A slab heated at a surface temperature of 1050 ° C or more and 150 minutes or less, (a) 3 to 18% of the total length of both sides of the slab immediately before hot rolling
(B) Diameter 3 or more passes at the beginning of hot rolling
It is disclosed that at least one of the following measures is taken: rolling with a small-diameter roll having a diameter of 1000 mm or less, and (c) setting the total draft of three passes in the initial stage of hot rolling to 35 to 55%.

[0004] However, these methods are two in the former case.
Since the hot rolling is required, serious problems such as waste of energy, reduction in yield due to maintenance, increase in operation cost, and extension of time required for production occur. In the latter case, (a)
Then, special equipment is required to make a depression on the side of the slab, and in (b) small diameter rolls with a diameter of 1000 mmφ or less are unusually small, so that they can be applied to general tandem hot rolling equipment. Due to problems in strength and life, it is difficult to imagine that it will withstand mass production in practical use. In (c), there remains a problem that the degree of freedom in the initial three-pass rolling is reduced and the restrictions on size and shape control are increased. .

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention is directed to a hot rolling method which does not easily generate scabs, requires extra steps, adds special equipment and equipment, or controls the size and shape. It is an object of the present invention to provide a hot rolling method for a ferritic stainless steel slab which can be realized at high efficiency and at a low cost with a tandem hot rolling mill for general steel rolling without any restriction to the slab.

[0006]

SUMMARY OF THE INVENTION In order to clarify at what stage during hot rolling and to what extent barge defects occur, the present inventors first manufactured a barge in a normal hot tandem rolling process. The length of barge flaws generated on the hot-rolled steel strip coil after annealing and pickling of ferritic stainless steel
These are the majority (for example, slab 200m
In case of mt → steel strip 4mmt, there are 8 scabs over 50mm in length.
7%). Therefore, a rolling experiment was conducted by attaching artificial protrusions of 1 mmφ to the first to fourth stages of the rough rolling mill and the first stages of the seven-stage finishing rolling mill among the rolls of the hot tandem rolling mill. It has been newly recognized that a tendency that a barb flaw having a length of 50 mm or more is generated only when a projection is formed on the first roll of No. 1 is obtained. From this, attention was paid to the first pass of the rough rolling mill, and the relationship between the rolling reduction and the generation rate of scabs there was eagerly investigated. As a result, the present invention was completed.

According to the present invention, when a ferritic stainless steel slab is continuously hot-rolled, the rolling reduction in the first pass is 12%.
Than as a hot rolling method of generating a scab defect hardly Fe <br/> write stainless steel slab, characterized by.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, in continuous hot rolling of a ferritic stainless steel slab, the rolling reduction in the first pass is regulated to less than 12%. The reason for such limitation is described below. Fig. 1 shows the continuous hot rolling
It is a graph which shows the relationship between the rolling reduction of a path | pass, and a scab defect generation rate. The data on the occurrence rate of bark flaws shown in FIG. 1 are obtained by rolling a 200 mm-thick ferritic stainless steel slab by changing the rolling reduction of the first pass of continuous hot rolling by the first roll of a rough rolling mill. To obtain a hot-rolled steel strip having a thickness of 4 mm, and observing the surface at the stage of annealing and pickling.

From FIG. 1, it can be seen that if the rolling reduction in the first pass of the continuous hot rolling is controlled to be less than 12%, the generation of barbed flaws in the annealing-pickling stage can be considerably suppressed. In order to prove that such a flaw is truly caused by the first pass of the hot rolling, a 200 mm thick ferrite which has been subjected to a penetrant inspection (PT) after care to confirm that there is no crack has been confirmed. The system stainless steel slab is rolled only one pass by changing the rolling reduction in the first roll of the roughing mill and cooled immediately,
By observing the surface, the crack occurrence rate on the slab surface in the rough rolling stage was investigated. FIG. 2 is a graph showing the relationship between the rolling reduction in hot one-pass rolling and the incidence of cracks on the slab surface obtained in this study. From FIG. 2, the crack generation rate of the slab surface after rough rolling, when rolling reduction of one pass rolling in the rough rolling mill ing more than 12% increases rapidly, this trend is very to FIG 1 above Good agreement. In other words, a causal relationship has been clarified that the barbed flaws after pickling and annealing are caused by cracks on the slab surface generated in the first pass of hot rolling. Further, just in case, the slab after one-pass rolling is continuously hot-rolled as it is without taking care of cracking of the slab generated in the hot one-pass rolling to form a hot-rolled steel strip having a thickness of 4 mm. After observing the surface of the steel strip coil after annealing and pickling, it was confirmed that a barge having a length of 50 mm or more was generated at a position corresponding to a crack position on the slab surface in a one-to-one correspondence. .

[0010] From the above, the limitation of the first pass of continuous hot rolling means that the scabs generated in the subsequent step (after annealing and pickling) are applied to the rolling pass (usually at the first stage of the rough rolling mill). Caused by cracks on the slab surface during
This is because there is almost no contribution of other rolling passes.
In addition, the reason why the reduction rate is set to less than 12% is that, as is apparent from FIGS. 1 and 2, the reduction of the first pass is performed at the reduction rate in this range, so that the generation of the bald flaws can be suitably reduced. This is because it can be reduced.

Although the lowering rate is not particularly limited, it is preferable that the lowering rate is about 5% or more from the viewpoint of the overall rolling distribution and prevention of an increase in the number of passes.

[0012]

EXAMPLE A hot-rolled steel strip having a thickness of 4 mm was produced from a 200 mm-thick ferritic stainless steel slab satisfying the component specifications shown in Table 1 using a hot tandem rolling mill. Table 2 shows the rolling schedules of the first to fourth stages of the rough rolling mills for the invention example and the conventional example. Although the rolling reduction of the first pass of the first stage of the rough rolling mill (the first pass of hot rolling) is 18% in the conventional example,
The invention example is characterized in that it is reduced to 9%. The reduction in the rolling reduction in the first pass was compensated in another rolling pass. After annealing and pickling these hot-rolled steel strips, the number of bark flaws generated on the front and back surfaces was counted. FIG. 3 is a frequency distribution diagram of barbed flaws per steel strip coil for invention examples and comparative examples in which the counting results are summarized. From FIG. 3, the average number of scabs (0.55) of the invention example is 6 compared to the conventional example (1.36).
It has been reduced by about 0%, and it can be seen that the effect of the present invention is outstanding.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

According to the present invention, since barge flaws which have been generated on the surface of a hot-rolled steel strip of ferritic stainless steel can be greatly reduced, coil grinding and cutting off after annealing and pickling can be performed. Is omitted, and there are industrially beneficial effects such as improvement in yield and improvement in delivery time.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the rolling reduction in the first pass of continuous hot rolling and the occurrence of barbed flaws.

FIG. 2 is a graph showing a relationship between a rolling reduction in hot one-pass rolling and a crack occurrence rate on a slab surface.

FIG. 3 is a frequency distribution diagram of barbed flaws per steel strip coil for the inventive example and the comparative example.

Continuation of the front page (56) References JP-A-61-163216 (JP, A) JP-A-62-199721 (JP, A) JP-A-4-138803 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46-9/48 B21B 1/00-3/02

Claims (1)

(57) [Claims] If 1. A rolled between successive ferritic stainless steel slab heat, of generating a scab defect hardly ferritic stainless steel slab, characterized in that the rolling reduction of the first pass to less than 12% heat Rolling method.