JPS6077934A - Method for cooling steel sheet in continuous annealing - Google Patents

Abstract

PURPOSE:To manufacture a steel sheet with high suitability to chemical conversion treatment at a low cost by successively carrying out cooling with cooling rolls and direct cooling with a cooling liq. under specified conditions when a steel sheet cooled to a prescribed temp. is further cooled. CONSTITUTION:When a cold rolled steel sheet cooled to 300-500 deg.C is further cooled to a lower temp. in continuous annealing, the sheet is cooled from 300- 500 deg.C to 100-200 deg.C by winding around plural cooling rolls. A refrigerant is being circulated through the rolls along the insides. Direct cooling is then carried out with a cooling liq. to cool the sheet to a lower temp.

Description

[Detailed Description of the Invention] 'i3! : During the gap between making a copper plate (hereinafter simply referred to as a lead plate) that has been annealed using NTT,
The technical contents described in this specification in connection with the Mg cooler of the steel plate are related to measures that do not cause deterioration of chemical conversion treatment properties, and the technology related to continuous furnace annealing of this type of steel plate. occupies the field.

Continuous furnace annealing furnace is becoming popular as one of the recrystallization annealing methods for this type of steel plate.When treating CQ plate with such a continuous furnace annealing furnace, first 14 plates are processed. is passed through a heating zone and a soaking zone and heated to a recrystallization annealing temperature or a predetermined workability from 650'C to A1 transformation point (910°C), and then cooled to 300 to 500°C, It is then cooled to a temperature near room temperature. At this time, in order to precipitate the carbon present in the steel, it is heated to 300 to 500°C for a few seconds to 1
A so-called over-aging treatment, in which the carbon content is held for 0 minutes, is sometimes carried out as necessary.
In the case of steel sheets containing 0.005 or more carbon, the carbon content is 0.005 or more.
Since there are originally few carbon furnaces for steel plates of less than wt ratio, it is not necessarily essential.

Conventional technology and its problems Now, in a continuous annealing line, regardless of whether or not the above-mentioned overaging treatment is performed, a steel plate that has been cooled to 800 to 500°C after heating and soaking is cooled to a temperature close to room temperature ( This process is hereinafter referred to as final cooling).

As the final cooling method, the gas jet cooling method is known as the most practical method.

This method uses the symbol A for the heat cycle shown in Figure 1.
As shown in Figure 2, the cooling rate at the final activation is 10"C/
SeC can only be obtained in a moderately good manner, and therefore has the drawback that the power J of the fringe cooling zone becomes long.
% hydrogen, residual nitrogen, dew point approximately -40°C), etc., so that no temper color or oxide film so thin that it cannot be seen with the naked eye is generated on the board surface during cooling. Therefore, during the phosphate film formation treatment (hereinafter simply referred to as chemical conversion treatment) that is applied before painting steel plates for mountain vehicles, a dense crystalline film of phosphate is easily formed, resulting in extremely high corrosion resistance after painting. ing.

On the other hand, in order to reduce the high cost associated with force jet cooling, l'=J, a method of cooling by immersion in a cooling liquid such as water immediately after the overaging is completed (see symbol B in FIG. 1) can be considered. However, in this method, although the cost required for cooling is extremely low, a very thin oxide film or temper color is formed during cooling, and the corrosion resistance of the tone plate is inevitably deteriorated.

A method that is attracting attention as a method to compensate for the shortcomings of both methods is, for example, the rotary cooling body shown in Japanese Patent Application Laid-Open No. 118815/1982, that is, cooling while winding a steel plate around a cooling roll with fluid passing through it. (referred to as roll cooling). This method does not suffer from any poor chemical conversion properties due to cooling 1 water immersion, and also provides a higher cooling rate than cass jet cooling, but the cooling rate in the low temperature range of 150'C or less is Since the amount decreases rapidly as shown in O in the figure, it is difficult to say that the cost is sufficiently low.

Origin of the idea In order to solve these drawbacks, combinations of cooling systems were considered, such as gas jet + cooling water immersion as shown in the symbol in Figure 1, and roll cooler as shown in Figure E. Immersion in cooling water is considered, and the latter in particular is considered to be the most efficient and low-cost cooling method unless there is a problem with chemical conversion treatment.

Regarding the combination of roll cooling and cooling water immersion, see JP-A-5
As proposed in Japanese Patent No. 7-23081, in this invention, for example, after over-aging treatment at 400 to 450°C, the cooling efficiency is high at about 200 to 800°C. While cooling (roll cooling), the material is cooled down to 50.degree. C. or lower by spraying or immersion in water or a water-soluble rolling agent. However, as will be described later, it was found that even under such conditions, the chemical conversion treatment properties of the steel sheet were insufficient.

Purpose of the Invention As a result of numerous experiments and studies, the inventors have discovered a final cooling procedure that fully satisfies the requirements for 1) chemical conversion treatment properties, and based on this, they have developed a new continuous process with good chemical conversion treatment properties. It is an object of this invention to provide a method for cooling the w1 band during annealing.

Structure of the Invention The above object is advantageously achieved by a procedure consisting of the following matters.

In other words, the continuous heating and annealing of cold-rolled steel sheets,
After soaking, the steel plate has been cooled to 800-500°C, is subjected to over-aging treatment if necessary, and then further cooled to a lower temperature from 300-500°C to 100°C or more and less than 200°C. In the above temperature range, the steel plate is cooled by being rolled around multiple cooling rolls with a coolant flowing through the inner circumferential surface, and then the steel plate is directly cooled with a cooling liquid until it reaches a temperature below that temperature range, allowing chemical conversion treatment to be performed at low cost. Manufactures steel and plates with good properties.

The inventors conducted an experiment under the conditions shown below in order to examine the relationship between the cooling method in the final cooling of a continuous annealing cycle and chemical conversion treatment properties.

[Experimental conditions]

O material Low carbon At killed W4 0 limit/ (wt%) C/ 0.046, Si/
0.01, In/0.27, P/0.011
, S/0.008, AI 10.041, N
/ 0.0019.0 / 0.00810 Hot rolling conditions: Finishing excitation, 870°C 1 coiling temperature: 670°CO Cold rolling conditions: Reduction rate: 75, Finished plate thickness: 0.80mO Continuous annealing heat cycle ( Figure 2) Cooling method after O over-aging treatment Measure the crystal size of phosphate crystals formed during chemical conversion treatment as a bond value of corrosion resistance after painting of steel plate after 0.8 choga compression did. Here, Bt is used as a chemical conversion treatment liquid.
8030 (manufactured by Nippon Barker), and added 120 to the liquid.
A method of immersion for 20 minutes was used. At the same time, the state of occurrence of temper color after annealing was observed with the naked eye. Figure 8 shows the results.

If the water immersion start temperature TV after cooling the roll is 230°C or higher, a temper color that is clearly visible to the naked eye will occur.
As a result, the diameter of the phosphate crystal grains during the chemical conversion treatment is extremely large, and therefore the corrosion resistance after painting is not good.

Next, as Tw decreases, there is a temperature range where the occurrence of temper color is no longer observed but the phosphoric acid crystal grain size is still large. Furthermore, it can be seen that when the TV is lowered to less than 200'C, the phosphate crystal grain size becomes significantly smaller, and therefore the corrosion resistance becomes better.

Even though no temper color is observed on TVs in the temperature range of 200°C to 230°C, the chemical conversion properties are not good because the temperature is probably around 100 A, which is not observed with the naked eye.
It is presumed that the chemical conversion treatment properties were not very good due to the formation of the following thin m-conversion film.2 For the above reasons, the water immersion starting temperature TV is preferably less than 200°C. One limit of the immersion starting temperature is determined by chemical formation.

That is, if a sheet is cooled to 100"C or less using roll cooling: tll, this will cause an increase in cost such as an increase in the number of rolls. Therefore, the lower limit of the water immersion M starting temperature is 100"C.

In this experiment, water was used as the cooling liquid, but it is also possible to immerse the solid fine powder in a flowing/moving state with hot water or dregs. A similar effect can be expected by using a method such as attachment.

As described above, the present invention provides a method for producing a cold (Ah1+) band with good chemical conversion treatment properties at low cost, and its effects are significant.

EXAMPLE One plate 1 and 2 with the components shown below and the hot rolling conditions were subjected to each separate cycle (symbol α
and β).

-Plate 1 Component 7 00.051%, SiO, O2%, Mn
0.21%.

PO, 011%, SO0 old] 8, AI 0
．． 031%, N O,0037%, Q O,005
1% steel plate 2 Ingredients: CO, 0017%, si, o, olcs, M
n O, 17%, P O, 010%, S O, 006%
, flo, 047%, Nb O, 017%, N O, 0
021chi, OO, 0027% hot rolling conditions; finishing temperature 860
°C1 winding fFA degree 700°C cold rolling strip I4: Cold rolling reduction rate 75%, cold rolled plate thickness 0.8 After continuous roasting and annealing, 0.6
After applying a W1 stress test of ~0.8, a chemical conversion treatment was performed in an il + normal process, and the particle size of phosphate crystals was measured. The conditions for the chemical conversion treatment were the same as in the experiment described above.

The results are shown in Table 1.

FIG. 5 shows a scanning electron micrograph showing the state of precipitation of phosphorus ρ crystals after the chemical conversion treatment.

Table 1 When the starting temperature of water immersion was within the range of this invention, the phosphate crystal grain size was extremely small, and therefore the corrosion resistance after painting was good; however, outside the range, the phosphate grain size was large and the corrosion resistance was extremely poor. Inferior.

[Brief explanation of drawings]

Figure 1 is a diagram showing the heat cycle of continuous annealing and the cooling rate by the final cooling method of the gods. Figure 2 is a diagram showing the heat cycle of continuous annealing used in Experiment 1. Figure 8 is a diagram showing the heat cycle of continuous annealing. FIG. 4 is a diagram showing the relationship between phosphate salt crystal grain size and water immersion start temperature; FIG. 4 is a diagram showing the heat cycle of continuous annealing used in Examples; FIG. This is a scanning electron micrograph showing the area where crystals are deposited. Patent Applicant: Kawasaki Steel Corporation Applicant: Mitsubishi Heavy Industries, Ltd. Figure 1 Figure 2 Figure 3 Figure 4 Continued from page 1 0 Inventor Takeo Fukushima Kannon Shinmachi, Nishi-ku, Hiroshima City. Inventor Hyodo Kin Akira Kannon Shinmachi, Nishi-ku, Hiroshima City
L-6-22 Mitsubishi Heavy Industries, Ltd. Hiroshima L-6-22 Mitsubishi Heavy Industries, Ltd. Hiroshima

Claims (1)

[Claims] L In continuous furnace annealing treatment of cold-rolled steel sheets, 800 to 5
When cooling a steel plate that has been cooled to 00°C to an even lower temperature, 1. For temperatures above OO''C and up to 200°C, the steel plate is cooled by winding it around multiple cooling rolls with coolant flowing through the inner circumferential surface, and then for temperatures below that, the steel plate is directly cooled. A method for cooling a steel plate in continuous furnace annealing, characterized by cooling with a liquid.