JP3486899B2 - Method for producing hot rolled steel strip with excellent pickling properties - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a hot-rolled steel strip having excellent pickling properties in hot rolling.

[0002]

2. Description of the Related Art When a hot rolled steel strip is manufactured by hot rolling,
Scale is generated on the surface of the hot-rolled steel strip. The scale is
Usually, it is removed by pickling in the next step. Most of the scale generated during hot rolling is wustite (FeO), but during the cooling process on the hot run table and the gradual cooling process after winding, wustite is magnetite (FeO) due to the progress of oxidation and transformation. ₃ O ₄ ) and hematite (Fe ₂
O ₃ ). Magnetite and hematite are less soluble in acid than wustite. Therefore, in order to shorten the pickling time and reduce the consumption of the acid solution, it is important to prevent this reaction from progressing as much as possible and leave a large amount of wustite.

A conventional technique for solving this problem is disclosed in Japanese Patent Laid-Open No.
It is described in JP-A 1-221621 and JP-A 9-267120. The technique described in Japanese Patent Application Laid-Open No. 11-21621 is to supply a calcium compound such as a highly basic calcium sulfonate to the surface of the rolled material after the descaling and before the first finishing rolling mill exit side, so that calcium functions. Is a technique for suppressing the above reaction. The technique described in Japanese Patent Application Laid-Open No. 9-267120 discloses a method of producing magnetite or hematite by bringing a combustion flame into contact with the surface of a hot-rolled steel strip after finishing rolling and before winding the coiler. Is to prevent.

[0004]

However, the above-mentioned conventional techniques have the following problems, respectively. That is, with the technique described in Japanese Patent Application Laid-Open No. 11-21621, it is difficult to apply the calcium compound uniformly to the surface of the rolled material. When mixed with cooling water and sprayed on the rolled material, the aqueous solution slips on the surface of the rolled material due to the Leidenfrost phenomenon, and almost no calcium compound remains on the surface of the rolled material. If the undiluted solution of the compound is applied to increase the residual amount, a large amount of the compound will be consumed more than necessary. When the calcium compound is put into the roll bite, the time required for the rolled material to pass through the roll bite is extremely short, and it is difficult to leave a necessary amount on the surface of the rolled material. Further, since the rolling mill is soiled by the calcium compound, there is a problem in maintainability. further,
The cost of the calcium compound itself is also a problem.

The technique described in Japanese Patent Application Laid-Open No. 9-267120 only produces a barrier that prevents the supply of oxygen on the surface of the scale. Wustite decomposes after winding and transforms into magnetite or hematite. I can't prevent that. Further, since the combustion flame is used, the energy consumption rate deteriorates. Also from the viewpoint of global environmental protection, the technology that generates carbon dioxide should not be used.

The present invention has been made in view of the above circumstances, and prevents the transformation of wustite into magnetite or hematite without using a technique that is difficult to realize.
An object of the present invention is to provide a method for producing a hot-rolled steel strip having excellent pickling properties.

[0007]

[Means for Solving the Problems] A first means for solving the above problems is to form a steel slab into a hot-rolled steel strip by rough rolling and finish rolling, and to form the hot-rolled steel strip on a hot run table. 3 seconds in the temperature range of 600 ℃ or less and 300 ℃ or more
A method for producing a hot-rolled steel strip having excellent pickling property, which comprises cooling to 300 ° C. or lower by a cooling method as described below, and then winding up with a coiler (claim 1). The second means for solving the above-mentioned problems is to use a steel slab with a rough pressure.
Formed into hot rolled steel strip by hot rolling and finish rolling, hot run
The hot-rolled steel strip shall be placed in a temperature range of 600 ° C or less and 300 ° C or more
The enclosure is cooled at a cooling rate of 100 ° C / s or more, and up to 300 ° C or less.
An acid characterized by being cooled and then wound up with a coiler
It is a method for manufacturing a hot-rolled steel strip having excellent washability (claim 2).

The inventors have found that the transformation reaction from wustite to magnetite or hematite occurs when the steel strip temperature is 600 ° C. or lower, but it is almost the same when the steel strip temperature is 300 ° C. or lower, more ideally 200 ° C. or lower. It was found that it would not occur, and was confirmed by experiments as will be shown later in Examples. According to this knowledge, the temperature range below 600 ° C at which transformation occurs is passed as quickly as possible, and the transformation is no longer occurring.
If cooled to a low temperature range of 0 ° C or lower, wustite can remain.

In the production of hot-rolled steel strip by a general hot-strip mill, the temperature of the rolled material falls to 600 ° C. or less when the hot-rolled steel strip is on the hot run table and is gradually wound up. When it is cold. Comparing the time on the hot run table with the time after winding, the time for slow cooling after winding is overwhelmingly longer. Therefore, even if the coiling temperature is set to 300 ° C. or less, a considerable part of the above object can be achieved.

[0010]

Further, even on the hot run table, the time when transformation occurs, that is, the steel strip temperature is 600 ° C. or less 3
It is better to shorten the time when the temperature is over 00 ℃ as much as possible. To this end, it is important to cool the hot-rolled steel strip between 600 ° C and 300 ° C as quickly as possible. If the cooling rate in this temperature range is 100 ° C./s or more, the transformation takes only 3 seconds or less, which is effective in preventing the formation of magnetite or hematite.

A third means for solving the above problems is
The first means or the second means, wherein at least part of the cooling of the hot-rolled steel strip on the hot run table is performed by a multi-hole jet cooling method (claim 3). is there.

In a normal hot strip mill, a cooling device for a hot run table is designed on the assumption of a winding temperature of 450 to 700 ° C.
It is not easy to lower the temperature below ℃. In order to lower the coiling temperature, additional cooling equipment may be added. However, there is almost no room on the already installed hot run table for installing new cooling equipment. The length of the hot run table can be increased to increase the number of cooling devices, but a large amount of capital investment is required to install a new hot run table.

Therefore, in the present means, a perforated jet cooling system as described in JP-A-53-127313 is adopted as the cooling system. With the multi-hole jet cooling method, a high cooling rate of 100 ° C / s or more can be easily obtained, and a short cooling stand is newly installed, or a part of the pipe laminar flow method or spray nozzle method cooling device is replaced. A sufficient temperature drop can be achieved.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a first embodiment of the present invention.
It is a figure which shows an example of the layout of the hot rolling equipment row which implements the manufacturing method of the hot rolled steel strip which is an example. In FIG. 1, 1
Is a finish rolling mill, 2 is a hot run table, 3 is a primary cooling zone, 4 is a secondary cooling zone inlet side thermometer, 5 is a secondary cooling zone, 6 is a winding thermometer, 7 is hot rolled steel A belt, 8 is a pinch roller, and 9 is a coiler.

The hot rolled steel strip 7 rolled by the finish rolling mill 1 is
While being conveyed on the hot run table 2, it is cooled by cooling water in the primary cooling zone 3 and the secondary cooling zone 5, and is wound up by the coiler 9 via the pinch roller 8. The primary cooling zone 3 is an ordinary cooling stand in which a pipe laminar and a spray are combined, and the secondary cooling zone 5 is a cooling stand capable of high-speed cooling by using a porous jet cooling system. Secondary cooling zone inlet side thermometer 4 and winding thermometer 6
Is used for cooling control by detecting the temperature of the hot rolled steel strip 7.

A method of controlling cooling on the hot run table will be described below. The control of the primary cooling zone is performed so that the temperature detected by the secondary cooling zone inlet side thermometer 4 becomes the target temperature of 600 ° C. or higher. In the cooling by the secondary cooling zone, the winding temperature detected by the winding thermometer 6 is 30
The target temperature is controlled to 0 ° C. or lower, and the cooling rate is controlled to 100 ° C./s or higher. In this way, the rolling method of the present invention can be realized.

The technique of the present invention does not specify the cooling rate from the Ar ₃ transformation point temperature to 600 ° C.
By gradually cooling this temperature range, ferrite, pearlite, etc. can be generated, and bainite and martensite in the low temperature transformation forming phase do not occur, so that the workability of the hot-rolled steel strip may be hindered. There is no.

[0019]

[Examples] Steel ingots having the components shown in Table 1 (the balance being Fe and unavoidable impurities) were melted by lab melting and hot-rolled to make steel plates having a thickness of 30 mm, a width of 150 mm, and a length of 300 mm. , As the test material.

[0020]

[Table 1] Table 1 (Unit: wt%)

The test material was subjected to thermomechanical treatment under the conditions shown in FIG. That is, after heating at 1250 ° C for 1 hour in an atmospheric furnace, rolling it from 30 mm to 3 mm with a laboratory rolling mill,
After air cooling to 0 ° C, forced cooling was performed. The forced cooling here is mist cooling or perforated jet cooling, and the cooling rate is changed depending on the method and the amount of cooling water. After being forcibly cooled to T _C ° _C corresponding to the winding temperature, it was kept for 24 hours in a heating furnace heated to the same temperature. The temperature of the test material was measured with a thermocouple embedded in the center of the plate thickness. In this experiment, descaling with high-pressure water as in the actual machine was not performed, but the primary scale produced by the first heating was almost completely exfoliated by rolling.

From the test material thus prepared, 30 mm
A 50 mm pickled sample was cut out. The scale thickness of each test material was measured by SEM observation before pickling. The pickling solution was prepared by adding 5% by weight of ferrous chloride and inhibitor Hibiron A5 to 10% hydrochloric acid kept at 85 ° C in a constant temperature bath.
0.1% was added. A number of pickling samples were prepared for one test material, and tests were performed with different pickling times. After the pickling test, the surface of the sample was observed with an optical microscope to determine the pickling end time. In this experiment, the thickness of the scale generated on the sample surface was 8 to 12 μm, and no dependence on the thermomechanical treatment conditions was observed.

FIG. 3 shows the experimental result of this embodiment. The pickling completion time depends on T _C , and when T _C is 300 ° C. or lower, the pickling property is remarkably improved. From the experimental results, in the rolling method of the present invention, the winding temperature was set to 300 ° C or lower. Further, it can be seen from FIG. 3 that further effects can be obtained when the cooling rate is 100 ° C./s.

[0024]

As described above, according to the inventions according to claims 1 and 2 of the present invention, from wustite
The time when the transformation reaction to magnetite or hematite occurs
Since it is short , the transformation reaction from wustite to magnetite or hematite can be suppressed, and a hot-rolled steel strip excellent in pickling property can be manufactured.

[0025]

According to the third aspect of the present invention, by adopting a cooling system having a large cooling capacity, the first means or the second means can be implemented without making a major facility modification.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a layout of a hot rolling equipment row for carrying out a method for manufacturing a hot-rolled steel strip, which is an example of an embodiment of the present invention.

FIG. 2 is a view showing thermomechanical treatment conditions in one example of the present invention and a comparative example.

FIG. 3 is a diagram showing a relationship between a winding temperature T _C , a forced cooling rate, and an end time of pickling in Examples and Comparative Examples of the present invention.

[Explanation of symbols]

1 ... Finishing mill 2 ... Hot run table 3 ... Primary cooling zone 4 ... Secondary cooling zone inlet side thermometer 5 ... Secondary cooling zone 6 ... Winding thermometer 7 ... Hot rolled steel strip 8 ... Pinch roller 9 ... coiler

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 59-7414 (JP, A) JP 61-195702 (JP, A) JP 11-129001 (JP, A) JP 5- 105952 (JP, A) JP-A-58-122105 (JP, A) JP-A-53-127313 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 45/02 B21B 1 / 00-11/00

Claims (3)

(57) [Claims] 1. A steel slab is formed into a hot-rolled steel strip by rough rolling and finish rolling, and the hot-rolled steel strip is placed on a hot run table in a temperature range of 600 ° C. or less and 300 ° C. or more for 3 seconds or less.
A method for producing a hot-rolled steel strip having excellent pickling properties, which comprises cooling to 300 ° C. or lower by a cooling method as shown below, and then winding with a coiler. 2. A steel slab is subjected to rough rolling and finish rolling.
No hot rolled steel strip, hot rolled steel strip on hot run table
And the temperature range of 600 ° C. or less 300 ° C. or higher, 100 ° C. / s or more
Cool at a cooling rate, cool to below 300 ° C, then
Hot-rolled steel with excellent pickling characteristics, characterized by being wound with a wire
Method of manufacturing obi. 3. An excellent pickling property according to claim 1 or 2, wherein at least part of the cooling of the hot-rolled steel strip on the hot run table is performed by a multi-hole jet cooling method. Method for manufacturing hot rolled steel strip.