JPH06269841A - Manufacture of hot rolled steel sheet excellent in surface properties - Google Patents

Abstract

PURPOSE:To provide a manufacture of hot rolled Si-containing steel sheet excellent in surface properties. CONSTITUTION:In manufacture of a hot rolled Si-containing steel sheet, (1) a descaling stage wherein high pressure water is infected, (2) a reheating stage for the surface layer part at least at 1170-1300 deg.C within 30min, (3) a second descaling stage by infection of high pressure water are carried out to a slab heated by a heating furnace at 1170-1300 deg.C. In this way, since Si-containing high strength hot rolled steel sheet is little in generation of scale flaws, the steel sheet excellent in surface properties can be manufactured. This steel sheet is suitable for an automobile.

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 Si-containing steel sheet which has few surface defects during hot rolling and has good surface properties.

[0002]

2. Description of the Related Art The weight reduction of a vehicle body is one of the means for improving the fuel efficiency of an automobile. The high-strength steel sheet has high strength and can satisfy the strength required for the vehicle body with a thinner thickness than conventional ones, and thus is suitable for weight reduction of the vehicle body. Since these high-tensile steel sheets are required to have high strength and good formability, it is preferable to manufacture them by solid solution strengthening the ferrite structure.

To strengthen the solid solution of the ferrite structure,
It suffices to add P, Si, Mn, etc., but P cannot be added so much because it causes embrittlement of the steel, and Mn is expensive and reduces the ductility of the steel sheet. Si does not deteriorate ductility so much and the raw material is inexpensive, but it has the following problems.

1) Occurrence of scale flaw.

FIG. 3 is a schematic cross-sectional view for explaining how the scale generated in the Si-containing steel material changes before and after descaling and rolling. As shown in Fig. 3 (a), when steel containing Si is heated, FeO (wustite)
A FeO-Fe ₂ SiO ₄ (firelite) eutectic compound layer is formed in the vicinity of the interface between and the base iron. Since this layer and the base iron are intricately meshed with each other, they cannot be completely removed by descaling, and the eutectic compound and FeO complicated in the eutectic compound remain on the steel surface (Fig. Three
(b)). In the subsequent rolling process, FeO is crushed and comes into contact with air to become Fe ₂ O ₃ (hematite), which is pushed into the surface of the steel material. In this way, what is called red scale is generated on the surface of the steel material (Fig. 3 (c)). This red scale is also called island scale because it occurs unevenly (island-like) on the surface of the steel sheet, but it damages the appearance of the steel material and remains as scale marks with recessed parts even after pickling. It is easy to become.

2) Plating failure of hot dip galvanizing.

Even in the case of a steel material having scale defects as described above, it is possible to eliminate the dents by cold rolling, but it is possible to eliminate the black scale (FeO, Fe ₃ O ₄ There is a difference in the chemical composition near the surface of the steel between the part where (scale) was generated and the part where red scale was generated, so alloying during hot dip galvanization progressed unevenly,
Uneven plating occurs.

To add Si as a method for increasing the strength of steel, these problems must be solved.

Japanese Unexamined Patent Publication No. 3-72031 and Japanese Unexamined Patent Publication No. 3-72031
The invention of 79718 gazette, in order to make the surface texture uniform,
Although the amount of Si in steel is increased to generate red scale on the entire surface of the steel sheet, it is not a fundamental solution because the plating failure during hot dip galvanizing becomes severe. Further, the back surface of the slab is likely to have a non-uniform surface state due to contact of skids during heating, dropping of the scale, and the like, and it is difficult to make the entire back surface into a red scale.

In Japanese Patent Laid-Open No. 1-246318, an oxide such as CaO or MgO is applied to the surface of the slab before heating,
By changing the surface oxidation state in the heating furnace,
A method for improving the condition of the interface with Fe is described. However, it is extremely difficult and time-consuming to apply the oxide uniformly on the surface of the slab. Moreover, this method is also difficult to apply to the back surface.

The invention of Japanese Examined Patent Publication No. 60-15682 is to control the air-fuel ratio of the heating furnace atmosphere to change the oxidation state of the slab surface, but the range in which the air-fuel ratio can be controlled is limited by the limitation of combustion efficiency and the like. Limited and not generally available.

As a countermeasure against the generation of the island scale, there is another method of heating the steel material at a temperature higher than usual, or conversely, at a low temperature, but in the former case, the life of the heating furnace is shortened and the heating energy is wasted. However, in the latter case, it is difficult to actually carry out the method because the desired amount of reduction cannot be obtained. Also, there is a method of using a mechanical brush for descaling and an invention of injecting high-pressure water after rolling to some extent (Japanese Patent Publication No. Sho 60-1085), but a fundamental solution to prevent the generation of red scale is I won't.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a hot-rolled steel sheet containing Si which has few scale defects and excellent surface properties.

[0014]

[Means for Solving the Problems] The present inventors have found that when a slab (steel slab) that produces a scale that is difficult to peel, particularly a slab of Si-containing steel, is heated to a temperature at which the aforementioned red scale is likely to occur. However, it was found that if the surface layer was heated again after descaling with high-pressure water, the scale releasability was improved.

The present invention has been made on the basis of such knowledge, and its gist is to "descale by injecting high-pressure water onto a steel slab containing Si heated to 1170 to 1300 ° C in a heating furnace. After that, at least the surface layer part is reheated at 1170 to 1300 ° C. for 30 minutes or less, followed by descaling by high-pressure water injection, and then hot rolling, followed by hot rolling. It is in.

[0016] The term "Si-containing steel slab" as used in the present invention means a steel slab containing 0.1% by weight or more of Si.

[0017]

FIG. 4 shows the state of descaling of the surface of the Si-containing steel material in the method of the present invention in the same manner as in FIG. The surface of the steel material immediately after the first descaling is the same as that shown in FIG. If reheating is performed after descaling, the oxygen concentration in the vicinity of the eutectic compound increases due to the diffusion of oxygen in the atmosphere, and the base iron at the intricate interface between the steel and the eutectic compound is oxidized to become FeO. .
Further, since the priority is the generation of FeO in this short period of heating, FeO-Fe ₂ SiO ₄ (fayalite) is the product amount generated at the interface does not become the state intricate and less iron metal, earth The interface between iron and scale is smooth as shown in Fig. 4 (C). In this state, the steel and the newly generated scale layer are relatively easily separated. Therefore, after this, by performing the second descaling, the descaling property is significantly improved (FIG. 4 (d)).

[Heating Conditions] The slab is heated to the temperature required for rolling. In order to make the temperature of the entire slab uniform, it is preferable to maintain the temperature in the soaking zone of the heating furnace for 30 minutes or more. The heating temperature is 1170 to 1300 ° C. It should be noted that rolling is possible even if this temperature range is slightly shifted up and down. However, when heating is performed in such a temperature range, a scale with extremely bad descaling is not generated, and thus the method of the present invention is inadequate.

That is, when the heating temperature is less than 1170 ° C., FeO-Fe ₂ SiO ₄ does not form an intricate interface with the base iron, and the scale is removed by ordinary descaling before rough rolling. You can Also, when heated to a temperature exceeding 1300 ° C, FeO-Fe ₂ SiO ₄ existing at the interface between the base metal and the scale is melted at the time of descaling before rough rolling, so the scale can be adjusted by normal descaling. Although it can be removed, it is not preferable because it causes a large scale loss and a loss of heat energy.

[First Descaling Condition] Of the thick scales formed on the surface of the steel plate during the heating,
In order to remove at least most of the FeO layer above the eutectic compound layer, descaling is performed by spraying high-pressure water with a water pressure of 100 kgf / cm ² or more at the nozzle outlet at a nozzle height of 350 mm or less. Is good.

[Second heating condition] For the purpose of increasing the temperature of the slab lowered during the descaling and forming a new FeO layer at the interface between the intricately complex eutectic compound and the base iron. To do.

The heating is performed in an oxidizing atmosphere. Therefore, although it may be carried out by a burner or the like on the hot rolling transfer line, it is desirable to use a heating furnace for slab heating in order to uniformly heat the upper and lower surfaces.

In the heating of the slab in the normal operation, the temperature of the heating furnace is adjusted to sequentially bring it into a temperature range called pre-tropical zone, heating zone, and soaking zone. However, the second heating in the method of the present invention heats the slab which has already been heated once and is close to the predetermined temperature, and only the surface layer portion needs to be heated to the predetermined temperature.

Therefore, after the first descaling,
Heating within 30 minutes should be carried out directly in the soaking zone.

Since the second heating is also premised on the temperature required for rolling, the temperature must be 1170 to 1300 ° C. In this case, if the heating temperature is less than 1170 ° C, the rate at which the internal ferrous iron inside is oxidized is slower than the intricate interface between the residual ferrous iron and the eutectic compound, and as shown in Fig. 4 (c), iron and scale The effect of smoothing the interface with is not sufficient.
When heated to 1300 ° C. or higher, the eutectic compound melts, but it is not preferable for the above reason.

Further, if the amount of oxidation by the second heating is too large, a large amount of FeO-Fe ₂ SiO ₄ is again generated near the interface between the scale and the base iron, and the interface also becomes complicated and complicated. There is no point in performing heating in stages. Therefore, it is necessary to appropriately oxidize the FeO—Fe ₂ SiO ₄ eutectic compound at a temperature and time such that the eutectic compound does not become complicated.

The heating time is set to 30 minutes or less so that the generation of FeO-Fe ₂ SiO ₄ is reduced and the interface between the generated FeO-Fe ₂ SiO ₄ and the base iron is not complicated. The heating time may be adjusted, but it is particularly preferably within 15 minutes.

Hereinafter, description will be made based on examples.

[0029]

EXAMPLE FIG. 1 is a diagram schematically showing a hot rolling line for heating and rolling a slab.

A slab having the chemical composition shown in Table 1 was heated to 1170 to 1300 ° C. in the atmosphere and held for 1 hour, then extracted from the furnace and dewatered with high pressure water having a nozzle outlet water pressure of 120 kgf / cm ^2. Scaled.

The slab subjected to the high-pressure water descaling in: was returned to the heating furnace, charged again, heated and maintained at various temperatures to generate scales, and the same descaling was performed, and three-pass rolling was performed. . The influence of the reheating temperature and time on the surface properties is shown in FIG. This result is a visual observation of the amount of red scale on the surface of the steel sheet after the step. The symbols in the figure define the ratio of the area of the portion where the red scale is generated on the surface of the steel sheet to the total surface area (hereinafter referred to as the generation rate) as follows.

○: When the red scale occurrence rate is 20% or less, △: 〃 exceeds 20% and 40% or less, ▲: 〃 exceeds 40% and 70% or less, ×: 〃 70% When exceeds.

In the case of rolling without reheating, red scale was generated over 80% or more of the surface of the steel sheet.

The hot-rolled steel sheet produced by the method of the present invention has less red scale than when it is rolled without reheating, especially when the reheating time is within 15 minutes. 20% red scale generation rate in all reheating temperature range
You can see that

In the present example, the reheating temperature was the same as that of the first heating. In the figure, the temperature when the reheating time is 0 represents the first heating temperature, and each temperature is the surface temperature of the slab measured.

Table 2 shows the first and second heating conditions, their times, and the red scale generation rate in some examples of the hot rolling. Manufactured by the method of the present invention
The hot-rolled steel sheets of Nos. 1 to 6 had a good red scale generation rate of 0 to 25% of the steel sheet surface. A large amount of red scale remained on the No. 7 steel plate with a long holding time for the second heating and the No. 8 steel plate with a low second heating temperature.
The remaining amount was reduced as compared with 0.9.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

When the hot rolled steel sheet containing Si is produced by applying the method of the present invention, since the scale flaws are few, it is possible to produce the one having excellent surface properties. This steel sheet is suitable as a steel sheet for automobiles.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an example of a hot rolling line for producing a hot rolled steel sheet by the method of the present invention.

FIG. 2 is a diagram showing the influence of the second heating temperature and its holding time on the surface texture after rolling in the method of the present invention.

FIG. 3 is a schematic cross-sectional view illustrating how the scale generated in a Si-containing steel material changes before and after descaling and rolling.

FIG. 4 is a view showing the situation of scale adhesion on the surface of a steel material in the method of the present invention in the same manner as in FIG.

Claims (1)

[Claims] 1. After descaling by injecting high-pressure water to a Si-containing steel slab heated to 1170 to 1300 ° C. in a heating furnace, at least the surface layer is reheated at 1170 to 1300 ° C. for 30 minutes or less. A method for producing a hot-rolled steel sheet having a good surface property, which comprises performing heating and subsequent descaling by high-pressure water injection, followed by hot rolling.