JPS5933172B2 - Continuous annealing treatment method for cold rolled steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous annealing method for cold-rolled steel sheets, and particularly provides a method for economically producing cold-rolled steel sheets for processing with excellent surface quality.

The present invention targets steel sheets with excellent workability, such as slow (non-)aging ultra-low carbon steel sheets and high-tensile, low-carbon cold-rolled steel sheets.

Conventionally, the annealing method used to produce cold-rolled steel sheets for drawing, etc., in which tightly or loosely wound cold-rolled coils are processed in a batch method such as in a box furnace, takes a long time for the entire process to take several days. The process required is extremely inefficient, and in order to solve this problem, many continuous annealing methods that can be processed in more than 10 minutes have been proposed, and some are actually in use.

The continuous annealing method can be said to be revolutionary in terms of efficiency compared to the batch-type annealing method, but recently there has been a strong demand for even greater efficiency (there is a demand for technology that can complete the annealing process in a shorter time than before).

As a measure to shorten the annealing time, there is a method of rapidly heating the steel plate in a direct-fired heating furnace, which also eliminates the electrolytic cleaning process, and a method of rapidly cooling the steel plate using water cooling or mixed cooling of air and water using gas and water for the primary cooling. is proposed.

When heating a steel plate over direct fire or cooling it with a water-based refrigerant, the formation of an oxide film on the surface of the steel plate cannot be avoided, so a method for removing the oxide film is required. Examples of these methods include the following: be.

b) As disclosed in JP-A No. 52-144331, heating and soaking are performed in a direct-fired heating furnace, then water cooling is performed, and after further heating and aging treatment, an oxide film is formed in the final step. How to remove pickling.

(1) As disclosed in JP-A-53-17518, heating and soaking are carried out in a direct-fired heating furnace, and after primary water cooling, an oxide film is reduced and removed at the same time as overaging treatment.

In both of these methods, heating and soaking are performed in a direct-fired heating furnace, which results in the formation of a fairly thick oxide film, which takes a long time to remove. There is a tendency to

Furthermore, as is well known, the air ratio is adopted as the combustion condition for fuel in a direct-fired heating furnace, but depending on the air ratio, the surface of the steel plate may be oxidized, which may cause deterioration of the appearance.

Therefore, it has been considered to use a non-oxidizing furnace that is normally operated at an air ratio of 1 or less, but strictly speaking, this non-oxidizing furnace functions as an oxidizing furnace.

If the cold-rolled steel strip subjected to a continuous annealing furnace with a direct-fired heating furnace is not subjected to any main treatment or if it is simply cleaned, the iron powder generated during cold rolling may be removed from the cold-rolled steel. It remains attached to the strip and is oxidized in a direct-fired heating furnace.

If the amount of this oxide is large, part of the oxide film will peel off in the soaking furnace, and this will be picked up and fixed to the hearth roll in the soaking furnace due to a sintering phenomenon.

This pickup causes scratches to occur in the highly humid copper strip passing through the hearth roll.

All of the above methods include an additional step called overaging treatment.

The present invention does not require over-aging treatment during continuous annealing, and also solves the problems of the conventional method seen on the surface of steel sheets as described above.It uses a radiant heating furnace instead of a direct-fired furnace to process the heating and soaking process. The method is characterized in that it prevents the formation of an oxide film during the primary water cooling, and that the oxide film formed during the primary water cooling is removed by pickling in the final step.

The gist of the present invention is to heat a cold-rolled steel sheet in a radiant heating furnace maintained in a non-oxidizing or reducing atmosphere, and then heat it in the radiant heating furnace for 10 seconds or more in a temperature range of Ac 3 points above the recrystallization temperature. After soaking, a continuous annealing treatment method for a cold rolled copper plate is characterized in that, after soaking, a gas-liquid consisting of water and gas is used as a refrigerant to perform gas-liquid spray cooling at a cooling rate of 10 to 10 b 2 , followed by pickling.

The present invention will be explained in detail below.

The steel sheets to be treated in the present invention are mainly cold-rolled or deep-drawn cold-rolled steel sheets commonly used for automobile bodies, or high-strength steel sheets, and in some cases metal-plated steel sheets. It may be a steel plate for surface treatment which is subjected to surface treatment, etc., and hereinafter, in the present invention, these will be simply referred to collectively as a steel plate.

The processes before cold rolling that process this steel plate are processed in the usual way, and the steel plate that undergoes continuous annealing treatment is either cleaned or not cleaned (including simple cleaning) of rolling oil during the cold rolling process. can.

The present invention uses a radiant heating furnace as a means of heating a steel plate to a temperature range of Ac3 points from the recrystallization temperature, and then heats the steel plate to a temperature range of Ac3 points from the recrystallization temperature in a radiant heating furnace for more than 10 seconds. Preferably, it is held for 10 seconds or more and 60 seconds or less.

The temperature to be maintained varies depending on the characteristics (composition and desired properties) of the steel plate to be treated, but in any case it is not necessary to maintain a uniform temperature, and it is necessary to use a heat cycle suitable for recrystallization and crystal grain growth. Can be adopted.

When heating in a radiant heating furnace, the heating speed is slower than direct flame heating, but the steel plate is oxidized in the heating valve vortex because the atmosphere inside the furnace is maintained in a non-oxidizing or reducing atmosphere using an inert gas atmosphere. can be prevented.

In addition, a non-oxidizing or reducing atmosphere is used to prevent the steel plate from oxidizing during soaking and holding after the temperature rise.

Note that when creating a reducing atmosphere, it is desirable that the H2 concentration be 4% or more and the dew point be 10° C. or less.

By performing temperature raising and soaking by radiation heating in this manner, it is possible to prevent the formation of an oxide film during the heating process, thereby reducing the time required for the final pickling process and preventing oxides from being picked up by the hearth roll.

Further, since it is only necessary to carry out a simple pickling treatment, a steel plate with excellent surface properties can be obtained.

The present invention uses a conventionally known radiant heating furnace as a heating means, but performs primary rapid cooling with gas and liquid after heating and soaking in the radiant heating furnace, and eliminates the heat generated during the primary cooling. This method differs from conventional continuous annealing methods in that it combines the removal of the oxidized film in the final step and the absence of over-aging treatment.

The steel plate that has been uniformly heated in the radiant heating furnace as described above is then rapidly cooled.

Although the cooling rate in this case varies depending on the steel type, the steel plate is rapidly cooled at a rate of 10° C. to 300° C./sec based on the material quality of the steel plate after annealing.

In the present invention, the cooling rate in the gas-liquid spray cooling is set to 1
0 to b.

(1) First, the reason why the upper limit was set to 300°C/sec is as follows.

(2) Alloying elements are added for anti-aging properties, strength, etc. In this case, the amount of alloying elements added can be reduced as the cooling rate is increased.

However, since the effect of the cooling rate also reaches saturation at 300°C/sec, the upper limit of the cooling rate is set at 300'C/sec.

■ If the temperature exceeds 300°C/sec, cooling will be too fast and the shape of the copper strip will become defective.

(The reason why the lower limit of iD is set to 10°C/sec is that below 10°C/sec, the following difficulties occur.

■ The cooling rate is too slow, which takes time to cool down and increases the size of the equipment.

(2) As in (i) (2) above, the effect of reducing the amount of alloying elements added cannot be obtained.

■ A large amount of oxide film is formed on the surface of the copper strip, and the surface of the copper strip after pickling becomes noticeably uneven, resulting in deterioration of quality.

In the present invention, rapid cooling is performed immediately from the soaking temperature, but if necessary, a part of the material may be gradually cooled from the holding temperature (preferably to a range of 600° C. or higher), and then rapidly cooled.

As for the cooling method, there are methods of immersion in water or boiling water, but a method of spraying a mixed fluid of gas and liquid is preferable because the shape of the steel sheet is good and the cooling rate can be easily controlled.

In this case, the gas used is an inert gas such as nitrogen gas, and the liquid most preferably is water.

As described above, when cooling is performed using water as a refrigerant, the formation of an oxide film due to steam oxidation is unavoidable, and if left as it is, the appearance will be poor and the surface treatment performance will be poor, so it is necessary to remove the oxide film in a subsequent process.

The present invention provides non-aging steel sheets such as ultra-low carbon Al-killed steel,
Alternatively, non-(slow) aging steel sheets such as ultra-low carbon steels with appropriate amounts of carbonitride-forming elements such as Ti, Nb, V, and B added are manufactured by continuous annealing, and over-aging treatment is required. Since there is no oxide film, the steel plate after cooling is subjected to pickling treatment to remove the oxide film without performing overaging treatment.

As the pickling solution, conventionally used inorganic salts such as hydrochloric acid, sulfuric acid, and phosphoric acid are preferable, but in some cases, formic acid, oxalic acid,
Organic acids such as may also be used.

Further, as the pickling method, conventionally known methods such as dipping, spraying, electrolysis, etc. may be used.

In addition, the present invention is characterized in that pickling removes only the thin oxide film generated during gas-liquid cooling, so it can be removed with a very simple acid treatment.
Since the surface roughness of the steel plate after pickling does not become coarse as in the case where a thick oxide film formed during soaking is pickled, a steel plate with excellent surface properties can be obtained.

The pickled steel plate is then washed with water, but since the pickled steel plate is active and prone to rust, discoloration, rust, etc. can be prevented by neutralizing it with an alkaline solution after washing with water.

Cold-rolled steel sheets, such as steel sheets used for automobile bodies, are used after being painted, and in this case, phosphate treatment is generally applied as a base treatment for painting.

In the present invention, the oxide film formed on the surface of the steel sheet during the manufacturing process is finally removed by pickling, but the performance of the phosphate coating can be improved by subjecting the steel sheet after pickling to the following treatment.

That is, an insoluble phosphate such as Zn3 is added to the steel plate after pickling in advance.
Spraying a suspension containing (PO4)2 or N
By forming a film of i, Zn, Mn, etc. (for example, by flash coating by electroplating), a phosphate film can be obtained as a base treatment for the above-mentioned painting.

Therefore, spraying Z n3 (PO4)2 as described above and applying coatings such as Ni to the steel plate after pickling are useful for improving paint adhesion, corrosion resistance, etc.

As explained above, the features of the present invention are that during continuous annealing of steel sheets, heating and soaking is performed in a radiant heating furnace to prevent oxidation of the steel sheet during the heating process, and rapid cooling is performed using gas and liquid. The generated oxide film is removed by pickling in the final step, and no over-aging treatment is performed, and the following effects can be obtained by manufacturing a steel plate using such a continuous annealing method.

(b) There is no need to perform overaging treatment, so the process can be omitted.

((b) No oxide film is formed during the heating process, and only the oxide film generated during gas-liquid cooling needs to be removed, so the pickling process in the post-process can be simplified.

(c) The pick-up phenomenon of oxides on the hearth roll can be prevented.

(b) A steel plate with good surface quality can be obtained.

Next, the present invention will be explained based on Examples, but the present invention is not necessarily limited to these Examples.

Table 1 shows the test results conducted under various conditions using a steel plate with a thickness of 0.8 mm.

The amount of oxide film formed in Comparative Example 1 and Comparative Example 2 in Table 1 was measured prior to the main test by heating steel plates in a direct-fired furnace under the same conditions as the main test and the test in a subsequent zone under normal humidity. A sample was collected by rapidly cooling it by blowing nitrogen gas.

Next, the sample steel plate taken was pickled in a 5th grade hydrochloric acid aqueous solution,
The amount of oxide film was converted into the thickness of FeO from the difference in weight before and after pickling.

Claims (1)

[Claims] 1. A cold-rolled steel sheet is heated in a radiant heating furnace maintained in a non-oxidizing or reducing atmosphere, and then equalized in the radiant heating furnace for 10 seconds or more in a temperature range of 3 AC points above the recrystallization temperature. 1. A continuous annealing method for cold-rolled steel sheets, which comprises heating, cooling using a gas-liquid consisting of water and gas as a refrigerant, and then pickling.