JPH0860251A - Method for annealing austenitic stainless hot rolled steel strip - Google Patents

Abstract

PURPOSE: To produce a stock suitable for producing a cold rolled steel sheet excellent in gloss and grindability by prescribing the atmosphere, temp. respectively in the holding stage and cooling stage at the time of annealing an austenitic stainless hot rolled steel strip. CONSTITUTION: At the time of annealing an austenitic stainless hot rolled steel strip, this hot rolled steel strip is held to the temp. range of 1000 to 1200 deg.C, preferably of 1100 to 1170 deg.C for 1 to 120sec, preferably for 20 to 100 sec in an atmosphere having <10%, preferably <=5% oxygen content. Netxt, in the cooling stage from the same temp., it is retained to >=950 deg.C for 30 to 130sec in an atmosphere having 10 to 20% oxygen content. Thus, dechromized layer caused at the time of the annealing can be removed away, and intergranular erosion grooves on the surface of the steel sheet after the annealing-pickling are suppressed, by which the cold rolled steel sheet excellent in gloss and grindability can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention provides a material suitable for producing cold rolled steel sheets (including steel strips) having excellent gloss,
The present invention relates to a method for annealing an austenitic stainless hot rolled steel strip.

[0002]

2. Description of the Related Art Austenitic stainless cold-rolled steel sheets are usually subjected to a hot-rolling-hot-rolling-annealing-pickling-cold-rolling step and then bright annealing if gloss is required. Alternatively, it is manufactured by annealing, pickling and polishing.
Austenitic stainless cold-rolled steel sheets produced by such a process have recently come to use tandem rolling with large-diameter rolls for cold rolling in order to further improve productivity and reduce costs. Came.

However, when the above-mentioned tandem rolling is applied to the cold rolling of austenitic stainless steel, the gloss and polishing property of the steel sheet after rolling (here, the number of polishing or the polishing depth required to obtain a predetermined gloss is obtained). That is, the more the number of times, or the deeper the depth, the worse.), Which causes a new problem. The cause of such a problem has so far been considered as follows. That is, the decrease in gloss and polishability of the cold-rolled steel sheet is that surface defects formed on the hot-rolled steel strip remained without disappearing even after cold rolling, and the surface defects annealed the hot-rolled steel strip. It occurs when the de-Cr layer that sometimes occurs is preferentially dissolved during pickling to form grain boundary erosion grooves.

By the way, several methods for reducing the grain boundary erosion grooves formed after annealing-pickling of an austenitic stainless steel sheet have been proposed so far. There are two ways. One is that, as in JP-A-60-248889, it is deeply dissolved by strong pickling to a position where no grain boundary erosion groove is formed, and the other is JP-A-56-2. As disclosed in -158819, by omitting or reducing the annealing, the oxidation is suppressed and the development of the Cr-free layer is suppressed.

[0005]

However, in the method disclosed in Japanese Patent Laid-Open No. 60-248889, the amount of pickling sludge increases, which causes a disadvantage in operation, and the high corrosion resistance containing a large amount of Cr and Mo. For stainless steel, there is a problem that it is difficult to sufficiently dissolve stainless steel because an appropriate pickling solution is not known. Further, the method disclosed in Japanese Patent Laid-Open No. 56-158819 has problems that the steel sheet is hard and thus the cold-rolling property is poor, and that the mechanical properties after cold-rolling are anisotropic.

Therefore, the main object of the present invention is to have the present manufacturing technology of austenitic stainless steel,
An object of the present invention is to establish an annealing technology for an austenitic stainless hot-rolled steel strip that suppresses grain boundary erosion grooves on the surface of a steel sheet after annealing-pickling without causing the above-mentioned problems. Another object of the present invention is to establish an annealing technology for an austenitic stainless hot-rolled steel strip suitable for producing a cold-rolled steel sheet having excellent gloss and excellent polishability without causing the above-mentioned problems. is there.

[0007]

Means for Solving the Problems Now, as a result of earnest research for realizing the above-mentioned object, the inventors have found that if the annealing conditions of an austenitic stainless hot-rolled steel strip are appropriately controlled. Found that the solution of
The present invention has been completed.

That is, the present invention has the following structures. (1) Austenitic stainless hot-rolled steel strip with oxygen content of 10
%, In a temperature range of 1000 to 1200 ° C. in an atmosphere of less than 1%
Austenitic stainless hot-rolled steel characterized by holding for 120 seconds and then, during the cooling process from the above temperature, holding it for 30 to 120 seconds at a temperature of 950 ° C or higher in an atmosphere with an oxygen content of 10 to 20%. Band annealing method.

[0009]

The function of the inventors is to remove the grain boundary erosion grooves.
As a result of detailed investigation on the formation process of the Cr layer, it was found that the Cr-free layer was formed when the continuous annealing furnace was annealed in the combustion atmosphere in which the oxygen content was less than 10%. However, the annealing atmosphere is generally a combustion atmosphere from the economical viewpoint, and it is difficult to increase the oxygen amount to more than 10%. Therefore, the present inventors have made various studies on whether or not the once-generated Cr-free layer can be eliminated. as a result,
During the cooling process after holding at the annealing temperature, oxygen gas is added to the atmosphere gas to increase the oxygen content in the atmosphere gas to 10% or more, and then to retain it for 30 to 120 seconds at a temperature of 950 ° C or more. For example, it was found that the depth of the grain boundary erosion groove after pickling can be markedly reduced.

An example of the results of the basic experiment up to the above conclusion is shown in FIGS. 1 and 2. Figure 1 shows that after holding for 1150 ℃ for 30 seconds in an atmosphere with 1% oxygen content, the cooling process (950 ℃
After annealing with changing the amount of oxygen in the atmosphere for a residence time of 30 seconds at the above temperature), immerse in 200 g / l sulfuric acid for 40 seconds at 80 ° C, and further add 25 g / l hydrofluoric acid and 150 g / l nitric acid. 60 to mixed acid
It shows the grain boundary erosion depth when pickled under the condition of dipping at 40 ° C for 40 seconds. In addition, in FIG. 2, after keeping the same conditions as in FIG. 1, the amount of oxygen in the atmosphere was set to 20% in the cooling process,
The graph shows the grain boundary erosion depth when an acid pickling was performed under the same conditions as in FIG. 1 after annealing was performed while changing the temperature and time of staying in the atmosphere. From Fig. 1 and Fig. 2, pickling can be carried out by carrying out a treatment in which the amount of oxygen in the atmosphere gas is set to 10% or more and retained at a temperature of 950 ° C or higher for 30 to 120 seconds in the cooling process after holding at the annealing temperature. It was found that the depth of the subsequent grain boundary erosion groove can be extremely reduced.

As described above, the mechanism by which the depth of the grain boundary erosion groove can be extremely reduced is not necessarily clear, but the inventors consider it as follows. That is, in a low oxygen atmosphere where the amount of oxygen is 10% or less, Cr in the grain boundary in steel is oxidized preferentially over Fe, so the Cr concentration in that portion decreases, and the Cr removal layer To generate. On the other hand, in an atmosphere gas with an oxygen content of 10% or more, at a temperature of 950 ° C or more for 30 to 120 seconds
When retained for a while, Cr and Fe in the steel are oxidized at almost the same rate, and the grain boundary Cr concentration in the steel does not decrease. Therefore, in the de-Cr layer of the grain boundary having a constant thickness generated in the low oxygen atmosphere during the annealing holding process, the oxidation in the grain is preferentially oxidized over the grain boundary during the cooling process in the high oxygen atmosphere. This will reduce the thickness. Moreover, most of the oxide scale layer formed in this way was removed by thermal or mechanical impact in the cooling process, and a large amount of pickling, which was a problem when annealed only in a low oxygen atmosphere, was used. The sludge has the advantage that it is significantly suppressed.

As described above, in the present invention, when annealing the austenitic stainless steel strip, it is essential to control the atmosphere, temperature and time in the holding process and the cooling process within appropriate ranges. Next, in the present invention, the reason why each condition is limited to the above-mentioned essential constitution will be described.

(1) Annealing holding conditions: oxygen content less than 10%, temperature
1000 to 1200 ° C., time 1 to 120 sec; The annealing in the present invention is performed by an open flame heating method which is an ordinary hot rolled sheet continuous annealing method.
The atmosphere of direct flame heating is an atmosphere with a low oxygen content after the fuel and air are mixed and burned. However, it is not possible to mix an excess air so that the oxygen content exceeds 10%. It requires an extra fuel to heat the air, and requires a large blower to blow a large amount of air, which causes economic disadvantages. For this reason, the amount of oxygen during annealing holding is less than 10%. In order to sufficiently recrystallize the austenitic stainless steel by annealing, it must be maintained at 1000 ° C or higher for 1 second or longer, but if it exceeds 1200 ° C or 120 seconds,
The effect of recrystallization saturates and is economically disadvantageous. Therefore, the condition for holding the annealing is that the amount of oxygen in the atmosphere is 10%.
Less than preferably 5% or less, temperature range of 1000 to 1200 ° C, preferably 1100 to 1170 ° C, holding time of 1 to 120 sec, preferably
20 to 100 sec.

(2) Cooling conditions: oxygen content of 10 to 20%, residence temperature of 950 ° C. to holding temperature, residence time of 30 to 120 seconds; cooling conditions after annealing holding are dechromized layers produced by annealing holding. Is important for reducing If the amount of oxygen in the atmosphere during the cooling process is less than 10%, Fe in the steel is not sufficiently oxidized and the effect of reducing the Cr-free layer cannot be obtained. Also, the amount of oxygen
Even if it is 10% or more, if the retention temperature during cooling is less than 950 ° C or the retention time is less than 30 sec, the Fe
Is not sufficiently oxidized, and the effect of reducing the Cr-free layer cannot be obtained. On the other hand, creating an atmosphere in which the oxygen content exceeds 20%, which is higher than that in the atmosphere, or retaining the atmosphere at a temperature higher than the annealing holding temperature requires economical facilities, which is economically disadvantageous. Is. In addition, staying for more than 120 sec is economically disadvantageous because it reduces the line speed, lowers productivity, and requires lengthening the tropical facility. Therefore, it is necessary that the cooling after the annealing is performed in the range of 10 to 20% oxygen, the retention temperature of 950 ° C to the retention temperature, and the retention time of 30 to 120 seconds.

After annealing under the conditions described above, pickling is carried out to produce an austenitic stainless hot-rolled steel strip which is a material for cold rolling. For the pickling in the above step, the pickling method usually used in the production of austenitic stainless hot-rolled steel strip is applied as it is, for example, by shot blasting, after pre-descaling, in 200 g / l sulfuric acid, at 80 ° C. Soak for 40 seconds, then add 25g / l hydrofluoric acid.
It is recommended to immerse in a mixed acid of 150g / l nitric acid at 60 ℃ for 40sec, then rinse and dry. The present invention can be applied to any austenitic stainless steel regardless of its composition.

[0016]

[Example] SUS304, SUS304L, SUS31
6 and SUS316 continuous cast slabs were heated and hot-rolled, the hot-rolled steel strip with a thickness of 4 mm was annealed and held under the conditions shown in Table 1 and then cooled, and then with sulfuric acid at 80 ° C (concentration: 200 g / l sulfuric acid). 60
Acid pickling (concentration: 150 g / l nitric acid, 25 g / l hydrofluoric acid) at ℃ was continuously pickled (pickling time: 40 sec each). Grain boundary erosion depth of the pickled hot rolled steel strip was measured. This steel strip was cold-rolled to a plate thickness of 1.5 mm with a tandem rolling machine with a roll diameter of 250 mm, and then this cold-rolled steel sheet was annealed, neutral salt electrolyzed, washed with nitric acid / hydrofluoric acid, and then buffed. The surface of the steel sheet after polishing is polished to 2 μm and the glossiness (this glossiness is represented by G1) is measured, while this cold-rolled steel sheet is bright annealed at 1100 ° C × 30 sec in ammonia decomposition gas. The glossiness of this steel plate surface (this glossiness is represented by G2) was measured.
The obtained results are also shown in Table 1. Here, the glossiness was measured by a method based on JIS Z8741.

[0017]

[Table 1]

From Table 1, the grain boundary erosion depth of the hot rolled steel strip to which the method of the present invention is applied is 3 μm or less, and the gloss G after polishing is G.
1 is 900 or more, and the gloss G2 after bright annealing is 1200 or more, and it can be seen that it has excellent characteristics. On the other hand, the amount of oxygen in the atmosphere during the cooling process after holding the annealing is 10
%, The grain boundary erosion depth is 6 μm.
The gloss G1 after polishing is 700 or less, and the gloss G2 after bright annealing is 1000 or less, which is inferior in all properties.

[0019]

As described above, according to the method of the present invention, it is possible to remove the Cr-free layer generated during annealing, so that it is possible to suppress the grain boundary erosion grooves on the surface of the steel sheet after annealing-pickling. It will be possible. Further, by using the hot-rolled steel strip according to the method of the present invention as a cold-rolled material, it becomes possible to manufacture a cold-rolled steel sheet having excellent gloss and polishing properties. Therefore, it becomes possible to improve the luster of the cold rolled steel sheet finished with BA or 2B and the polishability of the cold rolled steel sheet finished with 2B or 2D.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the grain boundary erosion depth and the amount of oxygen in the atmosphere during the cooling process.

[Fig. 2] Grain boundary erosion depth and oxygen content 10% in the cooling process
It is a graph which shows the relationship with the temperature and time which stay in the above atmosphere.

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location C21D 6/00 102 A 9269-4K 9/52 101 (72) Inventor Susumu Sato Kawasaki-cho, Chuo-ku, Chiba-shi No. 1 Kawasaki Iron & Steel Co., Ltd.

Claims (1)

[Claims] 1. A hot rolled austenitic stainless steel strip,
In an atmosphere with an oxygen content of less than 10%, the temperature range of 1000 to 1200 ° C is maintained for 1 to 120 seconds, and subsequently, in the cooling process from the above temperature, in an atmosphere with an oxygen content of 10 to 20%, 950 ° C.
A method for annealing an austenitic stainless hot-rolled steel strip, characterized in that it is retained at the above temperature for 30 to 120 seconds.