JP3216532B2 - Method for producing hot rolled steel sheet with excellent pickling properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet used for an automobile member, a building material, a cold-rolled steel sheet material, etc., in particular, a hot-rolled steel sheet having excellent pickling properties.

[0002]

2. Description of the Related Art In order to reduce the manufacturing cost of hot-rolled steel sheets used for automobile parts, building materials, cold-rolled steel sheet materials, etc., it is necessary to increase production efficiency and to minimize the occurrence of surface defects that lead to a reduction in yield. There is a need. Above all, speeding up the pickling descaling step is an important technique that is key to improving the production efficiency, and various methods for improving pickling properties have been studied.

[0003] Further, from the viewpoint of material properties, it is desirable that the hot-rolled steel sheet has solid solution C and N, which degrade ductility and aging properties, and therefore contains Al, B, Ti, Nb and the like. In many cases, steel is hot-rolled at a high temperature after hot rolling. If the winding temperature is high, the amount of scale generated increases, and the pickling properties deteriorate. Therefore, in order to improve the pickling property of the hot-rolled steel sheet, it is necessary to obtain excellent pickling property even at a high winding temperature, and it is also necessary to consider the material at the same time. is there.

[0004] As main methods for improving pickling properties, there are 1) a method of suppressing scale formation, and 2) a method of making the composition easy to dissolve the scale.

[0005] As a method of suppressing the scale generation of 1), for example, Japanese Patent Application Laid-Open No. 4-71722 discloses a method in which, after a hot-rolled coil is wound, within one hour in an atmosphere having an oxygen concentration of 4% or less. A method has been proposed in which the formation of scale is suppressed by cooling and then water cooling to 400 ° C. or lower.

As a method of making the scale easy to dissolve in 2), a high proportion of FeO in the scale improves the pickling property, and a high proportion of Fe ₃ O ₄ deteriorates the pickling property. Focusing on the scale of the hot-rolled steel sheet,
It has been proposed to increase the composition ratio of O 2. For example, Japanese Patent Application Laid-Open No. 2-11720 discloses that a material to be rolled after rough rolling is used.
Bending is performed in the temperature range of 1000 ° C or lower and the Ar ₃ transformation point or higher, and is maintained in this temperature range for 10 seconds or more to sufficiently precipitate AlN, and then finish rolling is performed, and then the winding is performed at a temperature of 600 ° C or lower. A method has been proposed in which the removed FeO is left to prevent material deterioration due to aging and to improve pickling properties.

Japanese Patent Application Laid-Open No. 8-1229 discloses that a hot-rolled coil is wound at a temperature of 560 ° C. or more, and then heat-retained in a heat retaining device, and then water-cooled from a high temperature of 560 ° C. or more to 200 ° C. or less. Accordingly, a technique for controlling the scale composition to FeO 2 having excellent pickling properties has been proposed.

[0008]

However, the method proposed in the above publication has the following problems.

In the method proposed in Japanese Patent Application Laid-Open No. 4-71722,
Considering material limitations, cooling is performed slowly within 1 hour after winding, but if the coil weight is small, the cooling rate will increase, and it will be difficult to secure the required material with slow cooling of about 1 hour. Become. If the cooling rate to 400 ° C or lower is low,
The transformation of the scale from FeO to Fe ₃ O ₄ cannot be suppressed. Further, when the cooling end temperature is as high as about 300 to 400 ° C., the transformation of the scale from FeO to Fe ₃ O ₄ is completed during the subsequent air cooling, and the pickling property is significantly reduced. Also,
Since an inert gas such as N ₂ gas is used to reduce the oxygen concentration of the atmosphere to 4% or less, the production cost increases.

In the method proposed in Japanese Patent Application Laid-Open No. Hei 2-11720,
Since the bending process is performed by winding the sheet bar into a coil shape, the bending radius increases toward the outer periphery of the coil, and the distortion amount decreases. As a result, the form of precipitation of AlN becomes coarser toward the outer periphery of the coil and becomes finer toward the inner periphery, resulting in a difference in grain growth. Further, a device for winding the sheet bar into a coil is required, which increases the equipment cost and lowers the production efficiency.

In the method proposed in JP-A-8-1229, the coil must be stored in a heat retaining cover immediately after winding, and the temperature of the entire coil must be maintained at 560 ° C. or higher. In the rolling process, the cover for keeping a large number of coils manufactured continuously becomes long, and the capital investment becomes enormous. In addition, when the inside of the heat retention device is in an oxidizing atmosphere, scale generation during heat retention is remarkable, and the pickling property is reduced.

The present invention has been made in view of such circumstances, and even when the winding temperature is high, the cost is low, the productivity is high, and the material is not deteriorated while hot rolling.
An object of the present invention is to provide a method for producing a hot-rolled steel sheet having excellent pickling properties.

[0013]

Means for Solving the Problems In view of the above circumstances, the present inventors have investigated in detail the precipitation behavior of carbonitride and the transformation behavior of the scale composition during cooling of a hot-rolled coil. As a result, while controlling the air cooling time of the coil after hot rolling according to the winding temperature and coil weight, and by specifying the subsequent cooling conditions, the scale composition of the hot-rolled steel sheet can be controlled without deteriorating the material. It has been found that the acid washability can be significantly improved. The present invention has been made based on this finding, and the gist is as follows.

The invention according to claim 1 is characterized in that hot rolling is performed to obtain
Wind the coil at 0 ° C or higher, and cool the coil
Air cooling is performed so that (t) satisfies the following equation (2).
This is a method for producing a hot-rolled steel sheet having excellent pickling properties, which is cooled at a cooling rate of 2 ° C / min or more to 0 ° C or less.

18.1exp ((148-W) /0.176CT) ≦ t (min) ≦ 8.313W + 24.152 (2) where CT: winding temperature (° C.), W: coil weight (ton) and The invention according to claim 2 is a method for producing a hot-rolled steel sheet excellent in pickling properties that gives a plastic elongation of 0.3% or more before pickling in the invention according to claim 1.

The present inventors have studied in detail a method for producing a hot-rolled steel sheet having excellent pickling properties without deterioration of the material as it is hot-rolled. The operation and effect of the present invention and the reasons for limiting the numerical values will be described. Winding temperature: Soft hot-rolled steel sheet using ultra-low carbon steel or low-carbon aluminum killed steel containing Ti, Nb, B, Al, etc. is hot-rolled to fix solid solution C and N. Often wound up. When the winding temperature is 640 ° C. or higher, the pickling property is remarkably deteriorated with an increase in the scale generation amount, and the efficiency in the pickling step is greatly reduced. In the present invention, 640
Hot-rolled steel sheets rolled at a temperature of 640 ° C. or higher are particularly suitable for hot-rolled steel sheets that are rolled at a temperature of 640 ° C. or higher because of their great effect of improving pickling properties.

When the winding temperature exceeds 750 ° C., the amount of scale formation becomes extremely large, and the effect of improving pickling properties is relatively small. Therefore, in the present invention, when the winding temperature is 750 ° C. or lower, the effect of improving pickling properties is more excellent.

Air cooling time: The air cooling time was limited to the range defined by the above formula (2) for the following reason.

The hot rolled steel sheet is made of AlN, BN during air cooling after winding.
Alternatively, it is necessary to precipitate nitrides and carbides such as TiC and NbC to secure a stable material. However, if the air cooling time is short, precipitation is not sufficiently performed, and an excellent material cannot be obtained. On the other hand, if the air cooling time is long, the problem of the material is eliminated, but the scale composition changes to Fe ₃ O ₄ , which is inferior in pickling properties, and the efficiency of pickling-descaling is significantly reduced.

By the way, the higher the winding temperature, the more the precipitation of nitrides and carbides is promoted, and the shorter the air cooling time of the coil becomes. In addition, the larger the coil weight, the lower the cooling rate and the higher the temperature, the more the precipitation of nitrides and carbides occurs, and the shorter the air cooling time of the coil.
That is, the air cooling time of the coil largely depends on the coil weight and the winding temperature.If the air cooling time is shorter than the range defined by the above equation (2), the precipitation of carbides or nitrides becomes insufficient and the material Is caused. When the air cooling time is longer than the range defined by the above formula (2), the scale composition is changed from FeO-based composition having excellent pickling property to Fe ₃ O having poor pickling property.
_Since the composition changes to a _{four-component} composition, the pickling descaling property is significantly deteriorated, and the efficiency of the pickling step is significantly reduced.

Cooling rate after air cooling: in weight%, C: 0.030%, S
i: 0.01%, Mn: 0.20%, P: 0.010%, S: 0.009%, sol.Al: 0.05
Hot rolled coil made of aluminum-killed steel slab consisting of 0%, N: 0.0032%, balance Fe and unavoidable impurities, finished hot rolling at a finishing temperature (FT) of 870 ° C and winding at a winding temperature (CT) of 680 ° C After cooling with air for 60 minutes, the cooling rate was varied and the change in the scale composition when cooled was examined by X-ray diffraction. The cooling rate is controlled by spray water cooling.
The amount of water was varied and varied. The coil weight is 16ton. Figure 1 shows the survey results.

Usually, the coil after winding is air-cooled to room temperature, so that the cooling rate is extremely slow, generally 0.6 ° C./min or less. In this case, according to FIG. 1, most of the FeO has been transformed into Fe ₃ O ₄ , which has poor pickling properties. In addition, FeO with excellent pickling properties
In FIG. 1, the cooling rate was 2 ° C. /
When the cooling rate is 5 ° C./min or more, most of the FeO can be retained, and when the cooling rate is 10 ° C./min or more, the amount of FeO exceeding the amount of Fe ₃ O ₄ remains at min. , Mostly FeO.

Therefore, in order to obtain excellent pickling properties, the cooling rate must be 2 ° C./min or more, more preferably 5 ° C./min or more, and still more preferably 10 ° C./min or more. It is. It should be noted that even if the cooling rate exceeds 50 ° C./min, the pickling property is not impaired, but in order to obtain such a cooling rate, the equipment becomes large and the equipment cost increases, so that it is economical. It is desirable to set the upper limit to about 50 ° C./min.

Cooling end temperature: A hot rolled coil obtained by finishing hot rolling at a finishing temperature (FT) of 870 ° C. and winding at a winding temperature (CT) of 680 ° C. for an aluminum killed steel slab having the same composition as described above. After air cooling for 60 minutes, cooling was performed by spray water cooling to various temperatures at a cooling rate of 10 ° C./min, and further, air cooling was performed to room temperature, and the scale composition was examined by X-ray diffraction. Here, the coil weight is 16ton. Figure 2 shows the survey results.

FIG. 2 shows that when the coil cooling end temperature exceeds 250 ° C., most of the FeO is transformed into Fe ₃ O _4, which is inferior in pickling properties, during the subsequent air cooling to room temperature. Can't improve. When the coil water cooling end temperature becomes 250 ° C or less, the transformation to Fe ₃ O ₄ during subsequent air cooling decreases, so most of the FeO remains. FeO remains.

Therefore, in order to obtain excellent pickling properties, the coil cooling end temperature must be 250 ° C. or less, and more preferably 200 ° C. or less. Further, the temperature is more preferably set to 100 ° C. or less in consideration of the charging temperature to the pickling line in the next step.

Plastic elongation: When the scale composition is Fe ₃ O ₄ ,
Since the scale strength is high and the adhesion is high, even if a steel sheet before pickling is given a surface strain of 2%, improvement in pickling property is not recognized. However, in the present invention, since the ratio of FeO 2 having a low scale strength is high, when strain is applied to a steel sheet before pickling, cracks are introduced into the surface scale, and dissolution and peeling by the pickling solution are promoted, and Washability can be further improved.

When a plastic elongation of 0.3% or more is imparted, the effect of improving pickling properties can be sufficiently obtained even under more severe pickling conditions, for example, when the concentration and temperature of the pickling solution are low.

However, when a plastic elongation of more than 5% is given, the ductility when used as hot rolled is reduced.
The plastic elongation is preferably 5% or less.

The hot-rolled steel sheet manufactured as described above has excellent pickling properties even when the winding temperature is high, without deterioration of the material as it is when hot-rolled. Further, since no equipment for storing the hot-rolled coil is required, the productivity is low and the cost is high.

The effects of the present invention are not affected by manufacturing conditions other than those described above. For example, after continuous casting,
The above-described effects can be obtained by directly feeding, or by performing hot rolling after reheating after cooling to room temperature or reheating without cooling to room temperature.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION The hot-rolled steel sheet of the present invention is prepared by casting a hot-rolled steel sheet by a conventional method and hot-rolling the steel sheet.
It can be manufactured by winding at a temperature of 0 ° C. or higher, and then cooling the hot-rolled coil under the predetermined conditions as described above.

The hot rolling may be performed by directly feeding the steel slab after continuous casting as it is, or after reheating after cooling to room temperature or after reheating without cooling to room temperature.

The plastic elongation applied to the steel sheet before pickling may be a single strain caused by bending, stretching, skin pass or the like, or a combination of these strains. Plastic elongation can be provided by a commonly used scale breaker, and the plastic elongation is the plastic elongation of a commonly used scale breaker, for example, elongation by a temper rolling mill, elongation of a roller leveler or a tension leveler. It is.

The pickling solution used for pickling the hot-rolled steel sheet produced according to the present invention is not particularly limited, and various pickling solutions exhibit excellent pickling properties.

[0036]

【Example】

[Example 1] Steel slabs having different component compositions shown in Table 1 and the balance consisting of Fe and unavoidable impurities were hot-rolled, wound at different winding temperatures, air-cooled time, air-cooled cooling rate and cooling. After cooling by changing the termination temperature, the hot-rolled steel sheet obtained by air-cooling to room temperature was further examined for pickling properties and mechanical properties.

The pickling property was evaluated by performing a descaling test in a hydrochloric acid pickling line and by a limit line speed (maximum speed 300 mpm) at which no scale residue was generated. The pickling liquid in the hydrochloric acid pickling line was 10% HCl, and the liquid temperature was 85 ° C. The elongation (plastic elongation) of the scale breaker (roller leveler) was set to zero.

The mechanical properties were evaluated by performing a tensile test on a test piece taken from a hot-rolled steel sheet before passing it through a hydrochloric acid pickling line and evaluating the elongation.

[0039]

[Table 1]

The results of the investigation are shown in Tables 2 to 5. Table 2, Table 3, Table 4, and Table 5 correspond to steel A, steel B,
This is the result of a survey on steel C and steel D.

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

[0044]

[Table 5]

From Table 2, Table 3, Table 4, and Table 5, the steel sheet of the present invention satisfying the scope of the invention described in claim 1 of the present invention is:
As compared with the steel sheet of the comparative example in which the coil weight and the winding temperature do not satisfy the range of the present invention under the same conditions, all have higher elongation and higher pickling line speed.

Example 2 For the hot-rolled steel sheets manufactured under the conditions of Nos. 1, 8, and 22 in Tables 2, 3, 4, and 5, respectively, various plastic elongations were measured by a scale breaker (roller leveler). After the application, a descaling test was performed in a hydrochloric acid pickling line to investigate the pickling property. The pickling solution in the hydrochloric acid pickling line was 7% HCl and the solution temperature was 60 ° C. The pickling property was evaluated by the critical line speed (maximum speed 300 mpm) at which no scale residue occurs, as in Example 1. Tables 6 and 7 show the survey results.

[0047]

[Table 6]

[0048]

[Table 7]

From Tables 6 and 7, it can be seen that Example 2 of the present invention satisfying the scope of the present invention described in claim 2 having a plastic elongation of 0.3% or more.
The steel sheet of the present invention has a higher pickling line speed than the steel sheet of Example 1 of the present invention under the same conditions except that the plastic elongation is less than 0.3%. Despite the low concentration and low temperature, it was possible to operate at the maximum pickling line speed without generating scale residue.

[0050]

According to the present invention, even when the winding temperature is high, a hot-rolled steel sheet excellent in pickling properties can be produced without deterioration of the material while hot rolling. Further, since no equipment for storing the hot-rolled coil is required, the productivity is low and the cost is high.

When the hot-rolled steel sheet produced according to the present invention is pickled, it can be descaled at a high speed, so that the pickling descaling step can be sped up.

The hot rolled steel sheet produced according to the present invention is suitable for use in applications such as automobile parts, building materials and cold rolled steel sheet materials.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a cooling rate of a hot-rolled coil after winding and a scale composition.

FIG. 2 is a diagram showing a relationship between a cooling end temperature of a hot-rolled coil after winding and a scale composition.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyoshi Sakai 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yoshiyuki Tanabe 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan (56) References JP-A-10-43810 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21C 47/26 C21D 9/52

Claims (2)

(57) [Claims] 1. A hot-rolled coil which is hot-rolled and wound at 640 ° C. or higher, and air-cooled so that the air-cooling time (t) satisfies the following expression (1), and then 250 ° C. or lower Up to 2 ° C / min
A method for producing a hot-rolled steel sheet having excellent pickling properties, characterized by cooling at the above cooling rate. 18.1exp ((148-W) /0.176CT) ≤t (min) ≤8.313W + 24.152 ・ ・ ・ (1) where, CT: winding temperature (° C), W: coil weight (ton) 2. The method for producing a hot-rolled steel sheet excellent in pickling properties according to claim 1, wherein a plastic elongation of 0.3% or more is given before pickling.