JPH08132123A - Hot rolling method of stainless steel - Google Patents

Abstract

PURPOSE: To reduce the load for the pickling by the integrated process quality assurance in the hot rolling operation to suppress the generation of scales difficult to be peeled. CONSTITUTION: In cooling the strip on a hot run table up to the coiling operation after the finish rolling operation in the hot rolling of the stainless steel, the coiling is achieved at the prescribed coiling temperature after the forced cooling in the first stage is performed with ΔS<=250, preferably ΔS<=200 where the product ΔS of the time (sec) >=800 deg.C with the temperature difference( deg.C) from 800 deg.CΔ, and the pickling is achieved in the next process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolling method for stainless steel, and more particularly to a hot rolling method for suppressing the formation of scales that are difficult to peel off.

[0002]

2. Description of the Related Art In recent years, consumers' demands on the surface properties of stainless steel have become more and more severe. Therefore, in the next pickling process for removing surface defects and scale, the pickling conditions must be made more stringent than ever before, resulting in inconveniences such as reduced processing capacity and increased cost. It has been lost.

In the conventional hot-rolling process, the cooling conditions after finish rolling are such that the finish outlet temperature and the winding temperature are set only in consideration of the material properties such as ridging property and the rollability, and the descaling property is taken into consideration. It has not been.

For this reason, under the conventional hot rolling conditions, the scale cannot be sufficiently removed, and the disadvantages such as a reduction in efficiency due to low-speed rolling in the pickling process and a reduction in yield due to an increase in the amount of fusing are avoided. There was a problem that I could not.

Various methods have heretofore been proposed as methods for improving the surface properties of stainless steel.
As one of them, for example, Japanese Patent Laid-Open No. 63-111125.
As described in Japanese Patent Application Laid-Open No. 3-207505 and Japanese Patent Application Laid-Open No. 3-207505, it is intended to enhance the adhesiveness of the scale.

However, even if the adhesion of the scale is increased, the scale peeling is inevitable regardless of the degree of the scale, and it is desirable to be able to completely perform the scale removing operation.

[0007]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to suppress the generation of scale that is difficult to peel off and reduce the load of pickling, by forming by hot rolling.

[0008]

According to the hot rolling method for stainless steel of the present invention, in the hot rolling of stainless steel, when the strip is cooled on the hot run table after finish rolling and before winding, the temperature is 800 ° C. 80 seconds and above
The product ΔS of the temperature difference (° C.) from 0 ° C. is characterized in that after the first-stage strong cooling is performed so that ΔS ≦ 250, it is wound up at a required winding temperature and then pickled in the next step. To do. Further, it is desirable that the ΔS is ΔS ≦ 200.

[0009]

In the present invention, generation of scale that is difficult to peel off is suppressed by controlling the temperature conditions after hot rolling finish in hot rolling.

1 (a), 1 (b), 1 (c) and Table 1 show
The results of examining the scale properties of the surface of the hot-rolled sheet rolled by changing the temperature conditions after the hot-rolling finish are shown. Further, FIG. 2 shows the results of examining the scale properties of the sample in which the scale remained after the pickling.

The finishing outlet temperature is set to the same level of 900 to 930 ° C, and the winding temperature is low (500 ° C) and medium (6).
(00 ° C) and high temperature (800 ° C), the hot-rolled sheet is basically (F) near the surface layer.
e, Cr) and ₂ O ₃ (corundum oxide), consists more inner layer of FeCr ₂ O ₄ (spinel oxide), higher temperatures wound coverage, the inner layer of FeCr ₂ O ₄ (spinel oxide) Cr-rich FeCr ₂ O ₄ is generated in a thicker layer closer to the base iron, and a Cr-deficient layer is generated in the base iron in contact with this.

[0012]

[Table 1] On the other hand, when the scale properties of the plate on which the scale remained after the pickling was examined, it was found that it was Cr-rich FeCr ₂ O ₄ , as shown in FIGS. 2 (a) and 2 (b). Note that FIG.
2 shows the result of pickling the three levels of FIG. 1, and FIG. 2 (a) shows the results of FIG.
FIG. 2 shows the results of pickling FIG. 2B and FIG.
1B shows the result of pickling FIG. 1A.

Therefore, in the hot rolling, in order to control the scale formation which is easy to peel off, the production is performed while satisfying other conditions and avoiding the condition that the Cr-rich FeCr ₂ O ₄ is easily produced as much as possible. It is necessary. That is,
If the temperature range where scales are likely to be generated is avoided, or if the temperature range where scales are likely to be generated cannot be avoided, avoid this temperature range as much as possible and minimize the time to pass through that temperature range. Further, it is important to control the atmosphere by using a cooling means to obtain a desired cooling pattern.

Specific manufacturing conditions will be described below.

First, an experiment was conducted to investigate the relationship between temperature and scale formation rate. Here, the scale generation rate was measured using the thickness of the Cr-rich FeCr ₂ O ₄ layer. As a result of the experiment, as shown in FIG.
Spinel type oxide FeCr _{2 in} a temperature range of 830 ° C or higher
It was found that the thickness of the FeCr ₂ O ₄ layer increases as the generation of O ₄ progresses rapidly and the time (holding time) in that temperature range increases.

On the other hand, in the actual machine, it is necessary to consider the material build-in conditions such as ridging property or the rolling property and to set the optimum conditions for both of them together with the knowledge from the experimental results.
Table 2 lists examples of hot rolling conditions derived from each quality characteristic item.

[0017]

[Table 2] In practice, the condition that simultaneously satisfies the above A, B, and C (the above-mentioned "avoid temperature range where scale is easily generated") does not hold. Therefore, assuming water cooling, the above-mentioned condition setting ("scale is It is necessary to avoid the temperature range that is easy to generate as much as possible, and to minimize the time for passing through the temperature range. Therefore, one feature of the present invention lies in the fact that the cooling pattern that does not particularly affect the quality characteristics is focused on. FIG.
The concept of setting the conditions is displayed schematically in.

FIG. 4 shows a cooling pattern when the strip is cooled on the hot run table from the finish outlet to the winding in the hot rolling of stainless steel. The symbols A, B and C in FIG. 4 correspond to the cooling conditions shown in Table 2.

According to the present invention, based on the knowledge obtained from the above experimental results, the areas abf, acf, adf of the temperature difference region (hereinafter referred to as ΔS) in the cooling pattern above the time of 800 ° C. or more × 800 ° C. , Aef are regulated below a certain value, and the water injection pattern is set within this value based on the finish outlet temperature. For this setting, as shown in FIG. 5, the winding temperature and the cooling pattern described in FIG. 4 were changed in an actual machine test, and the rate of scale generation for each hot rolling condition was arranged. In FIG. 5, the horizontal axis is
ΔS, which is the product of the temperature difference between 800 ° C or more (seconds) and 800 ° C, is shown, and the vertical axis shows the scale generation in terms of the number of remaining scales.

As can be seen from the actual machine data shown in FIG.
Since the scale is likely to be rapidly generated when ΔS ≧ 250, it is an essential point of the present invention to control so that ΔS ≦ 250, and desirably to suppress ΔS ≦ 200, which hardly occurs.

The temperature drop curve of the cooling pattern immediately after the finishing outlet can be approximated by a substantially straight line, and therefore ΔS can also be approximated by the triangular area. Therefore, when the time of 800 ° C. or higher is Time and the temperature difference from 800 ° is ΔTemp,
ΔS is represented by ΔS = (Time × ΔTemp) / 2. Therefore, cooling time up to 800 ° Ti
me is represented by Time = (2 × ΔS) / ΔTemp.

As an example, finish outlet temperatures of 910 ° C. and 85
In the case of 0 ° C, 3.6 seconds (≈
(2 × 200) / (910-800) seconds, 8.0 seconds (≈ (2 × 200) / (850-800) seconds) If cooled to 800 ° C., a scale with poor peelability is produced. Can be suppressed. That is, within the above time, 80
The cooling header may be selected so as to cool it to 0 ° C.

Therefore, according to the present invention, the cooling condition for suppressing the scale formation is set to ΔS ≦ 2 according to the finish outlet temperature.
It is characterized in that it can be freely set on the assumption of 00.

[0024]

[Example] In hot rolling of stainless steel, when cooling the strip on the hot run table after finish rolling and before winding, as shown in FIG.
m, and water injection section 21 sections (7 headers /
The cooling equipment (section x each top and bottom) was used.

Conventionally, as shown by straight lines P1 and P2 in FIG. 4, so-called uniform cooling is carried out by selecting the sections so that the cooling pattern has a substantially constant inclination. In this case, since the area exceeding 800 ° C occupies 7 to 10 seconds, a large amount of scale is generated. For example, when performing pickling such as sulfuric acid pickling with shot treatment, the pickling rate is increased to pickle. When trying to lighten the load, there was a problem that the scale was not completely removed and the scale remained on the surface of the plate. The straight line P1 shows the cooling pattern when the winding temperature is near the maximum temperature restricted by the material, and the straight line P2 shows the cooling pattern when the winding temperature is near the lower limit when variations are taken into consideration. The pattern is shown. The lower limit of the winding temperature is limited from the winding temperature control uncontrollable range.

On the other hand, in the present invention, as described in the section of the action, the condition that the product ΔS, which is the product of the temperature difference between 800 ° C. or more and 800 ° C., is 200 or less is satisfied. As described above, strong cooling is performed in the preceding stage of the cooling path, and thereafter, cooling is performed relatively gently. Curve Q in FIG.
1 and Q2 show examples of cooling patterns according to the present invention.

In order to realize such a cooling pattern,
Priority is given to the finishing head of the water injection section, 6-8
Use zones to cool rapidly, then cool gently with air. As a result, generation of scale that is difficult to peel off was suppressed, the pickling load could be reduced, and the pickling rate could be 1.5 to 2.0 times that of the conventional one.

It should be noted that the plate edge portion is easier to cool than the plate central portion, so that edge cracking occurs during the work in the cold rolling process. Therefore, as a countermeasure against edge cracking, an edge mask is used to prevent the cooling water from splashing on the edge portion in order to secure the temperature at the edge portion of the plate. This edge mask is a device that shields only the edge portion of the plate, prevents the cooling water from directly contacting the plate, and prevents the temperature of the edge portion from decreasing. In this case, the usage ratio of the edge mask is optimized so that the temperature of the plate edge part behaves at the same level as the temperature of the plate center part (here, the above-mentioned finishing side temperature and winding temperature). It is also important to control the scale behavior in the width direction, and in the case of pre-stage strong cooling, operation with an edge mask usage rate of 30 to 40% is seen from the temperature profile in the width direction. This is the optimum condition in view of the effect of preventing ear cracks. The usage rate (%) of the edge mask is (the number of sections using the edge mask / the number of water injection sections).
It is specified by × 100.

[0029]

According to the present invention, by controlling the cooling pattern after finishing, it is possible to reduce the generation of scale that occurs during rolling of ferritic stainless steel, and to reduce the load in pickling and the amount of fusing. It can greatly contribute to yield improvement.

[Brief description of drawings]

FIG. 1 is a diagram showing the results of an examination of the scale properties of the surface of a hot-rolled sheet rolled by changing the temperature conditions after hot rolling finish.

FIG. 2 is a diagram showing the results of investigation of the scale properties of samples in which scale remained after pickling.

FIG. 3 is a graph showing the relationship between temperature and scale generation rate.

FIG. 4 is a graph showing a cooling pattern.

FIG. 5 is a graph showing the relationship between ΔS and the scale residual occurrence rate.

FIG. 6 is a schematic diagram showing a configuration example of cooling equipment.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Kiyota 1-1 Tobahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Shin Nippon Steel Co., Ltd. Yawata Works

Claims (2)

[Claims] 1. In hot rolling of stainless steel, when the strip is cooled on a hot run table after finish rolling and before winding, a time (second) of 800 ° C. or more and 800
The product ΔS of the temperature difference (° C) from ° C is ΔS ≦ 250. After the first-stage strong cooling, it is wound up to a required winding temperature and then pickled in the next step. Hot rolling method for stainless steel. 2. The hot rolling method for stainless steel according to claim 1, wherein the ΔS is ΔS ≦ 200.