JPS634023A - Method for annealing coil of steel strip - Google Patents

Abstract

PURPOSE:To obtain a steel strip having a clean surface by tight annealing without using a coating material by using gaseous NH having a specified dew point as an atmospheric gas in the heating and cooling stages of the annealing cycle. CONSTITUTION:When a coil of a cold rolled steel strip is subjected to tight annealing in an annealing furnace, the dew point of gaseous NH used as an atmospheric gas in the heating and cooling stages of the annealing cycle is regulated to <=-20 deg.C so that the conversion of carbon sticking to the inner cover into gaseous CO can be inhibited. Accordingly, a steel strip having a clean surface can be obtd. even in case where the coil is subjected to tight annealing without passing through a cleaning stage.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method of tight annealing a steel strip coil after cold rolling, and particularly relates to a tight annealing method that makes the surface of the steel strip clean after annealing. It is about the method.

(Prior Art) Generally, a steel strip after cold rolling is subjected to an annealing treatment, and this annealing includes continuous annealing and tight annealing.

Batch annealing is available and is selected depending on the required mechanical properties. In the case of tight annealing, high-grade products are annealed after going through a cleaning process to remove rolling oil, etc.
Low-grade products are directly annealed without the cleaning process.

For so-called no-clean materials, which omit this cleaning process, Milclean rolling oil is used as the rolling oil during cold rolling, or lubricity is added to the stand at the front stage of the cold rolling mill. Mill-cleanliness is achieved by using rolling oil rich in milling oil and by using a cleaning agent at the final stand. However, rolling oil adhering to the steel strip and abrasion powder generated from the steel strip and rolling rolls during cold rolling cannot be completely removed. When a steel strip is annealed with rolling oil or wear powder attached to it, the rolling oil deteriorates and polymerizes using iron powder such as wear powder as a catalyst, and carbon is generated on the surface of the steel strip during annealing. As CO gas is generated, the rolling oil is decomposed, and this CO gas undergoes the reaction shown in equation (1) to form 2CO-CO□10 (precipitated adhesion)... (1)
Carbon was generated, and a black stain called edge carbon was generated on the edge of the steel strip. As a result, the surface quality of the steel strip has significantly deteriorated, and the yield rate has been reduced due to the necessity of cutting edges of the steel strip, and it has been necessary to rewash the steel strip to remove dirt.

As a method to solve these problems, conventionally,
As proposed in Japanese Patent Application Laid-open No. 62103 and Japanese Patent Application Laid-open No. 57-35619, methods have been implemented in which the ambient gas flow rate during annealing is specified or a specific coating agent is applied to the steel strip after cold rolling. has been done.

(Problems to be Solved by the Invention) However, with the method of applying a coating agent to the steel strip, the cost of the coating agent increases, a separate coating process is required, and the coating agent must be removed after annealing. There is. Also,
As a result of studies conducted by the present inventors, it has been found that the method of regulating the atmospheric gas flow rate has no significant effect.

The above-mentioned techniques have various problems and are still insufficient to improve the surface quality of the steel sheet to a level where it is not a problem, and there is a desire for a method that can further significantly improve the surface quality.

An object of the present invention is to obtain a steel strip with a clean surface by annealing under the current annealing conditions without using a coating agent or changing the annealing conditions.

(Means for solving the problem) When a steel strip coil is immediately tight annealed after cold rolling, CO gas is generated due to the reasons described above, and carbon is generated according to the reaction formula (1), and this A large amount of carbon adheres to the steel strip end surface and the inner cover 6 of the annealing furnace shown in FIG.

Conventionally, carbon adhesion has been prevented by focusing only on the steel strip surface, but according to studies by the present inventors, if annealing is repeated in the same inner cover in an atmospheric gas containing water vapor,
Carbon accumulates on the inner cover during the cooling process of annealing,
This carbon becomes CO gas again according to formula (2), and this CO gas is precipitated as carbon at the edge of the steel strip coil according to formula (1) above, together with CO gas from rolling oil, etc. We were able to clarify that this was due to dirt on the surface of the parts.

That is, annealing is performed by heating to the annealing temperature as shown in FIG.
Although it is soaked and cooled, CO gas is generated due to the rolling oil etc. that adhered during cold rolling as described above. This CO gas is stable at high temperatures;
In the temperature range of 0°C, the reaction of formula (1) described above occurs and carbon is generated, which adheres to the edge of the steel strip coil to become edge carbon and also adheres to the inner surface of the inner cover. Carbon adheres to the inner cover. If the next annealing is performed while the carbon is in the same condition, this carbon becomes CO gas by the reaction of equation (2) mentioned above during the temperature rise in the next annealing cycle, and together with the CO gas originating from the rolling oil etc., the CO in the atmospheric gas
The gas concentration will increase significantly. In this way CO
When the cooling process of annealing begins in an atmosphere with a high gas concentration, carbon is generated at a temperature of 550°C to 630°C, and at the same time, the carbon accumulates on the inner cover, and at the same time, the end surface of the steel strip is damaged due to the high CO gas concentration. A large amount of carbon also adheres to the surface, forming edge carbon. In order to prevent this, the present inventors have found that it is effective to suppress the carbon adhering to the inner cover from becoming CO gas through the reaction of the above-mentioned formula (2).

Based on this knowledge, the present invention, when tightly annealing a steel strip coil after cold rolling in an atmospheric gas, uses HN with a dew point of 12'0°C or less as an atmospheric gas during the temperature rising period and the cooling period of the annealing cycle. The above-mentioned problems were solved by using a tight annealing method for steel strip coils, which is characterized by using gas.

In the present invention, HN gas with low water content is used as the atmospheric gas during the temperature raising and cooling processes of the annealing cycle, so carbon adhering to the inner cover is
Equation 2) can suppress the formation of CO gas. Therefore, the concentration of CO gas in the atmospheric gas within the inner cover can be reduced, and generation of edge carbon in the steel strip coil can be prevented.

In addition, after annealing the no-clean material (mil-clean material) and the cleaning material using HN gases with different dew points as the HN gases used in the temperature raising and cooling steps of the annealing cycle, the L value of the end face of the steel strip coil was measured. I did an experiment to measure it. The L value is the whiteness measured by pasting cellophane tape on the end face of a steel strip coil, peeling it off, stacking three sheets on a white paper, and measuring with a color difference meter.If the L value is 65 or more, the cleanliness is good. It is a steel strip coil.

Figure 3a shows the results of measuring the dew point of HN gas used in the heating and cooling processes of the annealing cycle and the CO gas concentration in the inner cover, and Figure 3b shows the results of annealing the mill-clean material, that is, the no-clean material. The figure shows the results of measuring the CD concentration in the inner cover and the L value of the steel strip after annealing. From both figures, the dew point at which the CO gas concentration in the inner cover falls within the range of 0.15 to 0.32% is -20℃.
If the following NH gas is used as the atmospheric gas, a clean steel strip having an L value of 65 or more after annealing can be obtained. From these experimental results, it is effective that the dew point of the HN gas used as the atmospheric gas is 120° C. or less.

Further, Fig. 3C is a diagram showing the results of measuring the CO gas concentration in the inner cover when annealing the cleaning material and the L value of the steel strip after annealing, and shows that the CO gas concentration in the inner cover Regardless, the L value is 65 or more.

Note that the solid line in FIGS. 3b and 3c indicates the case where the atmospheric gas flow rate is 0.13 Nm2/hr-T, and the broken line indicates the case where the atmospheric gas flow rate is 0.20 Nm2/hr-T.

(Example y HN gas () I2: 7%, N2: 9 as atmospheric gas
3%), C: 0.05 to 0.09% by weight, Mn
: 0.28-0.37% by weight, P: 0.014-0
．． The steel strip has a composition of 0.016% by weight and a thickness of 0.6 to 2.0% by weight. Om
Steel strips having a width of 900 to 1600 mm were annealed in the annealing furnace shown in FIG. 1 under various annealing conditions shown in Tables 1 to 3. In addition,
In this example, the dew points of the NH gas used in the temperature raising process and cooling process of the annealing cycle were 0°C, -20°C, and -35°C.
The dew point of the HN gas used as a heat equalizer was 2 to 4°C.

Mill cleaning material cold-rolled using Mill Clean rolling oil with a composition of 8% mineral oil, 12% ester, and 24% fatty acid, that is, a steel strip that is not subjected to a cleaning process.
-CL, C steel strip subjected to cleaning process after cold rolling
The L value of the end face of the steel strip after annealing is measured, and the first
- Shown in Table 3.

Table 1 Table 2 Table 3 Table 1 shows the results when HN gas with a dew point of 0°C was used in the heating process and cooling process of the annealing cycle. Table 3 shows the results using HN gas with a dew point of 120° C. in the annealing cycle, and Table 3 shows the results using HN gas with a dew point of 135° C. in the temperature raising and cooling steps of the annealing cycle.

As can be seen from each table, when HN gas with a dew point of 120°C or less is used in the temperature raising and cooling processes of the annealing cycle,
A clean steel strip with an L value of 65 or more at the end face of the steel strip was obtained, and no edge carbon was generated at all.

(Effects of the Invention) As explained above, according to the method of the present invention, when a steel strip coil that is not subjected to a cleaning process such as a mill clean material is tightly annealed, a steel strip coil with a clean surface can be obtained. No edge carbon is generated at all.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of an annealing furnace. FIG. 2 is a diagram showing an annealing cycle. Figure 3a is a diagram showing the relationship between the dew point of the HN gas used and the CO gas concentration in the inner cover, and Figure 3b is a diagram showing the relationship between the dew point of the HN gas used and the CO gas concentration in the inner cover.
It is a diagram showing the relationship between the gas concentration and the L value of the steel strip after annealing, and FIG. 3c shows the relationship between the CO gas concentration in the inner cover when the cleaning material is annealed and the L value of the steel strip after annealing. 1 is a diagram showing the relationship between: 1...Motor 2...Atmospheric gas circulation fan 3...Base 4...Spacer 5...
Coil 6... Inner cover 7...
Heating cover body

Claims (1)

[Claims] 1. A steel characterized in that when tightly annealing a steel strip coil after cold rolling in an atmospheric gas, the atmospheric gas during the temperature rising period and the cooling period of the annealing cycle is HN gas with a dew point of -20°C or less. Tight annealing method for band coils.