JP2602699B2 - Method for preventing blistering flaws on ultra-low carbon Ti-killed cold rolled steel sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing blistering defects when annealing a cold-rolled steel sheet in the field related to the production of an ultra-low carbon Ti-killed cold-rolled steel sheet. is there.

2. Description of the Related Art In recent years, needs for thin steel sheets having excellent workability and formability have been increasing. To respond to this demand, ultra-low carbon Ti
Killed steel production is increasing.

However, this ultra-low carbon Ti-killed steel is one of the steel types whose production is not always easy for the following reasons. In other words, in the steelmaking stage, the viscosity of the molten steel increases due to the low carbon content and the presence of titanium, and nonmetallic inclusions mixed in the molten steel are difficult to float and separate. Further, in the cold rolling stage, the difference between the soft matrix and the hard non-metallic inclusions during cold rolling from the place where the C is low and the matrix is soft causes a gap between the two. When annealing is performed with such voids, hydrogen enters the cold-rolled steel sheet from the gas in the annealing atmosphere. Then, H ₂ partial pressure in the gap between inclusions and the matrix is increased by melting of the hydrogen is reduced at the time of cooling, expanding the soft matrix portion to appear swelling flaws. As a technique for preventing such flaws, there are JP-A-61-276753 and JP-A-61-276757, but even if a misch metal is added or the material of the immersion nozzle is changed, extremely low carbon Ti The swelling of the steel was not impeded.

Problems to be Solved by the Invention As described above, voids are formed in the blister due to the difference in elongation between the matrix and the nonmetallic inclusion. Accordingly, it is an object of the present invention to provide a rolling method that does not generate this void.

Means for Solving the Problems In order to solve the above problems, the present inventors,
As a result of elucidating the cause of blistering flaws, it was found that the index of blistering flaws differs depending on the primary cold rolling rate.

That is, it has been found that when cold rolling a hot rolled coil made of extremely low carbon Ti-killed steel, the occurrence of blistering defects is suppressed by performing primary cold rolling at a cold rolling reduction of 80% or more. Here, primary cold rolling refers to cold rolling in which a hot-rolled coil is rolled by a cold-rolling machine and before the coil is supplied to an annealing step.

The occurrence of blistering flaws is caused by the difference in elongation between the soft matrix and the hard non-metallic inclusion during cold rolling as described above, and the generation of voids therebetween. However, by increasing the rolling reduction during the cold rolling and increasing the force applied to the inclusions, the inclusions are finely broken and dispersed to prevent the formation of voids between the matrix and the nonmetallic inclusions. . This prevents swelling from appearing on the surface of the cold-rolled steel sheet.

EXAMPLES Example 1 In order to investigate the relationship between blistering flaws and the rate of cold rolling, extremely low carbon was used.
A laboratory test was conducted using a 50 kg high-frequency melting furnace for Ti-killed steel.

The steel ingot melted in the furnace was hot-rolled to a thickness of 3.2 mm and 3.5 mm,
Cold rolling was performed by setting the former cold rolling ratio to 69% and the latter to 83% to produce cold rolled steel sheets.

This steel sheet was annealed at an annealing temperature of 750 ° C. Thereafter, the surfaces of both steel sheets were examined, and burrs were found at a cold rolling reduction of 69%. A section of the cold-rolled flaw at which the blistering flaw was generated was cut at 69%, and the cross section was observed with a microscope. As shown in FIG.

No blistering flaw was observed in the test material with a cold rolling rate of 83%, but as a result of examining the cross section, it was found that the inclusions were broken and extended thinly.

Example 2 The results of investigating the defect index of the extremely low carbon Ti-killed steel manufactured by the ordinary method, in which the primary cold reduction rate was changed from 55% to 80% or more, in which a swelling defect occurred in a cold-rolled material was obtained. As shown in Fig. 2, the index is large when the cold rolling reduction is 55 to 59%, and it is 80%.
When the rolling is performed at the above rolling reduction, the number of defects becomes very small.

Therefore, an ultra-low carbon Ti killed steel in which 0.01% or more of Ti is added to molten steel having a C content of 0.01% or less by a normal method,
After being pickled through a casting and hot rolling process, it was subjected to cold rolling.

As an example of the present invention, as shown in Table 1, cold rolling was performed in a rolling reduction range of 80 to 90%, and thereafter, annealing was performed in the continuous cold rolling coil annealing line. As a result of examining the surface of the annealed coil, the defect index of blistering flaw was able to be significantly reduced as compared with the conventional case.

Effect As described above, the swelling defect can be surely reduced by setting the cold rolling rate to 80% or more when cold rolling ultra-low carbon Ti-killed steel.

[Brief description of the drawings]

FIG. 1 is a view for explaining the location of blistering flaws and the distribution of inclusions at those locations. FIG. 2 shows the occurrence index of blistering flaw and 1
It is a figure explaining the relation with the next cold rolling reduction.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Katsuhiro Ichiba 11-1 Showa-cho, Kure City, Hiroshima Prefecture Examiner, Takeshi Ogawa, Nisshin Steel Corporation Kure Works (56) References JP-A-2-70354 (JP) JP-A-62-107026 (JP, A) JP-A-63-53220 (JP, A) JP-A-61-291922 (JP, A)

Claims (1)

(57) [Claims] (1) The content of carbon is 0.010 (wt)% or less, and the content of Ti is
t) Cold-rolling of ultra-low carbon Ti-killed cold rolled steel sheet, characterized in that primary cold rolling is performed at a cold rolling rate of 80% or more in cold rolling of ultra-low carbon Ti-killed steel containing coil. Prevention method.