JP2000226700A - Continuous annealing and descaling method of stainless steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous annealing and descaling method of a stainless steel strip which is capable of preventing outbreak of defective surface quality without excessive charge of the power in an electrolytic descaling process. SOLUTION: In continuously annealing a cold-rolled stainless steel strip M, and then, achieving the electrolytic descaling of the stainless steel strip, the surface temperature of the stainless steel strip during the annealing is measured, the scale thickness after the annealing is estimated from the measured value (the measured temperature T), and the electrolytic current value (the appropriate electrolytic current value ic) in the electrolytic descaling process is established based on the estimated value (the estimated scale thickness dsc).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous annealing descaling method for a stainless steel strip.

[0002]

2. Description of the Related Art Conventionally, a cold-rolled stainless steel strip (hereinafter simply referred to as "steel strip") is adjusted to a predetermined material by annealing, and then pickled to remove scale generated during the annealing. Processed. The pickling line for performing this pickling treatment usually includes a pickling tank for performing the pickling treatment and a neutral salt electrolytic tank for performing electrolytic descaling before the pickling. However, the surface of the stainless steel strip that has been subjected to the pickling treatment following the neutral salt electrolysis treatment is in a state of being eroded by acid, and a post-treatment step such as temper rolling or polishing is required to obtain good surface properties. Was indispensable.

[0003] Since such a post-processing step results in an increase in cost and a decrease in productivity, a technique has been proposed which can omit this. For example, Japanese Patent Application Laid-Open No. 9-184012 discloses that the content of Si in stainless steel is limited to 0.3 wt% or less, the holding temperature for annealing is controlled at 950 to 1200 ° C., and the pH of the neutral salt electrolyte is set at 0%. A method for manufacturing a stainless steel plate which eliminates the need for polishing treatment after descaling by controlling it to 2.5 is disclosed. In these techniques, the electrolytic current value in the electrolytic descaling step is uniquely determined according to the passing speed.

[0004]

On the other hand, the surface temperature of the steel strip in the annealing furnace should be controlled to be constant. However, in actual operation, the deviation in the thickness of the steel strip in the longitudinal direction, the atmosphere in the furnace, the annealing temperature, The steel strip surface temperature often deviates from the control range and inevitably fluctuates due to variations in the plate speed, changes in the furnace temperature setting when passing through the steel strip joint, and the like. When the steel strip surface temperature fluctuates, the scale layer generated on the steel strip surface fluctuates accordingly.

However, in the above-mentioned conventional technology, the electrolytic current in the electrolytic descaling step is uniquely determined according to the plate speed, and therefore, it is impossible to follow such a variation. As a result, for example, after pickling, scale remains partially on the surface of the steel strip, and quality mismatch occurs. There is no effective prescription for this. For example, if the electrolytic current value is set to a value corresponding to the upper limit of the actual result of the variation in the surface temperature of the steel strip, the influence of this variation is eliminated, but excessive power is forced to be applied and a large cost is required.

[0006] An object of the present invention is to solve the problems of the prior art and to provide a continuous annealing descaling method for a stainless steel strip capable of preventing the occurrence of surface quality defects without excessively supplying electric power in the electrolytic descaling step. To provide.

[0007]

The premise that the conventional method of setting the electrolytic current value according to the passing speed may be effective is that the amount of scale generated during annealing is controlled to be constant. It is almost impossible because of the unavoidable steel strip surface temperature fluctuation. Therefore, the present inventors do not stick to the idea of controlling the amount of scale generation to a constant level, but based on the idea of dynamically controlling the electrolytic current value according to the thickness of the generated scale. Was considered.

As a result, (1) According to the method of dynamically controlling the electrolytic current value according to the generated scale thickness, it is possible to efficiently descaling without applying excessive electric power, and (2) the scale thickness depends on the steel type. (3) It can be estimated from the (component) and the steel strip surface temperature at the time of annealing. (3) By controlling the electrolytic current value according to the estimated value of the scale thickness,
The inventors have found that good descaling can be performed stably and economically, and have led to the present invention.

That is, according to the present invention, when the stainless steel strip after cold rolling is continuously annealed and then electrolytically descaled, the surface temperature of the stainless steel strip during annealing is measured,
A continuous annealing descaling method for a stainless steel strip, characterized by estimating the scale thickness after annealing from the measured values and the steel type, and setting the electrolytic current value in the electrolytic descaling step based on the estimated value.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The success or failure of the present invention depends on how accurately the thickness of the scale formed on the steel strip surface after annealing and before electrolysis can be determined. Usually, in annealing performed after cold rolling, a scale of about several μm is generated. It is sufficient if the thickness of this scale can be measured off-line, but it is difficult to obtain accurate measured values with current analytical instruments.

Therefore, the relationship between the temperature of the steel strip during continuous annealing and the scale thickness after annealing was intensively investigated, and it was found that there was a substantially linear relationship between the two. This straight line differs slightly depending on the steel type. Here, the steel strip temperature data is collected by installing a steel strip temperature measuring instrument on the heating strip exit side in the continuous annealing furnace where the steel strip temperature is the highest, and online measuring the temperature of the steel strip passing therethrough. did. The steel strip temperature measuring instrument was composed of a radiation thermometer. On the other hand, the scale thickness data was measured by cutting out a sample plate and conducting off-line observation with a glow discharge analyzer and a transmission electron microscope of a thin film including scale near the surface layer of a steel strip.

Next, for various stainless steels, the scale thickness data was collected while changing the annealing temperature, and the results were arranged to create a database I. This database I
Table summarizes the relationship between steel strip temperature and scale thickness in a table format for each steel type. By using the database I obtained in this manner, the scale thickness after annealing can be estimated with sufficient accuracy from the actually measured values of the steel strip surface temperature for each steel type.

Further, neutral salt electrolysis is performed with various scale thicknesses, and an appropriate electrolytic current value for good descaling is examined for each scale thickness, and the relationship between the scale thickness and the appropriate electrolytic current value is investigated. Into a table format to create Database II. Here, the quality of descaling is determined by visually observing the steel strip surface after pickling and determining that there is no scale residue as good, and determining that there is scale residue as bad, and the lower limit of the electrolytic current domain that resulted in good determination. The value obtained by adding a margin to the above was defined as an appropriate electrolytic current value. By using the database II obtained in this manner, an appropriate electrolytic current value corresponding to the designated scale thickness can be immediately extracted. This proper electrolysis current value corresponds to the electrolysis current value necessary and sufficient to remove the scale having the thickness according to the thickness of each scale without excess or deficiency. The surface quality after pickling can be maintained in a good state while avoiding the problem.

In the present invention, the following steps (1) and (2) are performed using the databases I and II created in advance as described above.
Steps (5) to (5) are repeatedly executed at a predetermined sampling cycle to set the electrolytic current value in the electrolytic descaling step (neutral salt electrolytic cell). (1) Check the steel type of the steel strip by referring to the information from the host computer. (2) Measure the steel strip surface temperature during annealing. The method of measuring the temperature is not particularly limited, but it is preferable to install a radiation thermometer on the heating orifice side in the continuous annealing furnace. (3) With reference to Database I, extract the scale thickness corresponding to the confirmed value of the steel type and the actually measured value of the steel strip surface temperature, and use this as the estimated value. (4) Referring to Database II, extract an appropriate electrolytic current value corresponding to the estimated value of the scale thickness. (5) Measure the transport speed of the steel strip with a speed detector, and
(2) Predict when the longitudinal section of the steel strip whose surface temperature has been measured reaches the neutral salt electrolyzer (using ordinary tracking means), and synchronize the electrolysis current value with the current To the appropriate electrolytic current value extracted in step (4).

As a result, it is possible to suppress the occurrence of partial scale residue in the steel strip, and to prevent the occurrence of quality mismatch. In addition, electrolysis can be performed with a necessary and sufficient optimal current value for each scale thickness, and economical continuous annealing descaling operation can be performed. By the way, according to the present invention, descaling is completed only by the neutral salt electrolytic treatment,
Sufficient luster can be obtained without pickling or polishing with a mixed acid or the like. In addition, the present invention does not particularly require additional treatments such as nitric acid electrolysis after neutral salt electrolysis and temper rolling, but appropriately employs these additional treatments as occasion demands. It does not place any restrictions on the matter.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a continuous annealing descaling line whose layout is shown in FIG. 2 will be described. In this line, a steel strip (stainless steel strip) M wound in a coil shape is developed and paid out by a payoff reel 1, and the preceding and succeeding coils are successively welded by a welding machine 2 and conveyed in the direction of arrow A. After degreasing with the degreasing device 3, while continuously passing through the heating zone 5A and the cooling zone 5B of the annealing furnace 5 for continuous annealing, and subsequently passing through the neutral salt electrolytic cell 7 for electrolytic de-scaling, a predetermined length is set by the shear 9 as needed. While being cut into (predetermined coil single weight), it is wound up again in a coil shape on the tension reel 10. FIG. 2
, 4 and 8 are entrance-side and exit-side loopers, which accumulate an amount of steel strip M that can absorb a difference in transport speed generated in different sections of the line.

FIG. 1 is a block diagram of an electrolysis control system in the embodiment. FIG. 1 (a) shows the overall image, and FIG. 1 (b) shows details of the electrolysis controller. The electrolysis control device 15 includes an electrode potential setting means 153 for setting the potential of the electrode 13 in the neutral salt electrolyzer 7, and first and second storage means 151 and 152 storing the databases I and II, respectively. It is mounted for communication. In FIG. 1, the same or corresponding parts as those in FIG. 2 are denoted by the same reference numerals, and the description thereof will be omitted.

The first storage means 151 receives the measured temperature T from the radiation thermometer 12 for measuring the surface temperature of the steel strip installed on the exit side of the heating zone 5A, and receives the steel type C from the host computer. Is transmitted to the second storage means 152. The second storage means 152 stores an appropriate electrolytic current value ic corresponding to the received estimated scale thickness dsc.
Is transmitted to the electrode potential setting means 153.

The electrode potential setting means 153 receives the steel type C, the steel strip transport speed (sheet passing speed) v, and the steel strip thickness (plate thickness) dM from the host computer, and receives the corresponding reference electrolytic current value io, and The reference potential .PHI.o at which the reference electrolytic current value io is obtained is calculated, and the value of the potential .PHI. Of the electrode 13 is set to the reference potential .PHI.o at the signal of the tracking signal TRC generated as needed from the tracking function. The electrode potential setting means 153 further receives the current deviation Δi in response to receiving the appropriate electrolytic current value ic.
(= Io-ic) and a potential change amount .DELTA..PHI. (= K..DELTA.i; K is a control gain) corresponding to the current deviation .DELTA.i, and the value of the potential .PHI.
+ ΔΦ). Thus, the electrolysis current value is reset from the current value to a new appropriate electrolysis current value.

The present invention was implemented by operating this electrolysis control system at a constant control cycle. Note that the electrode potential setting means before the embodiment of the present invention did not have the first and second storage devices. As a result, the rate of occurrence of quality nonconformity due to scale residue, which was about 0.6% before the implementation of the present invention, became 0% after the implementation, and the basic unit of power for the neutral salt electrolytic cell was reduced to about 80% before the implementation. did.

[0021]

As described above, according to the present invention, it is possible to prevent the generation of residual scale without excessively applying electric power in the electrolytic descaling step, thereby producing a stainless steel strip with a beautiful surface stably and economically. It has an excellent effect of being able to do so.

[Brief description of the drawings]

FIG. 1 is a block diagram of an electrolysis control system in an embodiment.

FIG. 2 is a layout diagram of a continuous annealing descaling line used in Examples.

[Explanation of symbols]

 M Steel strip (stainless steel strip) 1 Payoff reel 2 Welding machine 3 Degreasing device 4 Inlet looper 5 Annealing furnace 5A Heating zone 5B Cooling zone 7 Neutral salt electrolysis tank 8 Outlet looper 9 Shear 10 Tension reel 13 Electrode 15 Electrolysis control Apparatus 151 First storage means 152 Second storage means 153 Electrode potential setting means

Continued on the front page (72) Inventor Genichi Ishibashi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside Kawasaki Steel Works Chiba Works (72) Inventor Satoshi Kasai 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel F term in Chiba Works (reference) 4K032 BA01 4K037 FH01 GA08 HA05 JA10

Claims (1)

[Claims] In order to continuously anneal a stainless steel strip after cold rolling and to perform electrolytic descaling, the surface temperature of the stainless steel strip at the time of annealing is measured. A continuous annealing descaling method for a stainless steel strip, comprising estimating a thickness and setting an electrolytic current value in an electrolytic descaling step based on the estimated value.