JP2003268454A - Method for manufacturing ferritic or martensitic stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a ferritic or martensitic stainless steel, which shortens the lead time and reduces the energy in annealing in a bell type furnace. <P>SOLUTION: In a process for manufacturing a ferritic or martensitic stainless steel, which includes a step of annealing a hot-rolled and wound coil in the bell type furnace, this manufacturing method is characterized by directly sending the coil to the bell type annealing furnace after winding it up, and annealing it in the furnace. The annealing for the coil in the bell type furnace is preferably started within 11 hours after completion of the winding of the coil. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a ferritic or martensitic stainless steel, and more particularly to a ferritic or martensitic stainless steel including a step of bell-annealing a coil formed by hot rolling and winding. Manufacturing method.

[0002]

2. Description of the Related Art A hot-rolled steel strip of stainless steel (including a steel plate, both of which will be hereinafter abbreviated as a plate) is generally prepared by hot rolling (hot rolling) a slab heated to 1000 ° C. or higher. )
Then, the obtained steel strip is wound up at 500 ° C or higher, and the obtained coil is air-cooled. After that, for ferritic and martensitic stainless steels, a batch furnace usually called a bell-type annealing furnace (hereinafter appropriately referred to as BAF) for the purpose of improving the material quality by uniformly dispersing carbides and nitrides. Is annealed. This annealing temperature is, for example, SU
In the case of S430, the temperature is 780 to 850 ° C (edited by Japan Iron and Steel Institute, 3rd edition Iron and Steel Handbook IV, p. 159). Regarding the winding temperature after hot rolling and the heat treatment pattern in BAF, conditions other than the above have been proposed. For example, Japanese Patent Laid-Open No. 55-134133 discloses winding in order to prevent nitrogen absorption during bell annealing. Take temperature 55
It is shown that the temperature is 0 ° C. or less, and in JP 2001-262234 A, hot rolled sheet annealing (continuous annealing or box annealing even in continuous annealing is performed in order to make ferritic stainless steel high in ductility and deep drawability. However, the annealing temperature of () is set to 850 to 980 ° C.

[0003]

By the way, the coil after hot rolling and winding is conventionally cooled by natural air cooling, forced air cooling or water cooling to a temperature lower than the heat resistant temperature of the transfer facility (for example, about 150 ° C.), It was being transported to BAF. The above-mentioned JP-A-55-134133, JP-A-2001-262234, and other documents do not particularly describe the temperature control of the coil from the hot rolling winding to the conveyance to the BAF. It seems that the process of hot rolling, coil cooling, and BAF is adopted.

However, hot rolling / winding → coil cooling → BA
The step F has a problem that a waiting time for cooling is generated to extend the lead time, and that the coil is reheated after being cooled, so that the cooling cost and the fuel for the reheating are wasted. An object of the present invention is to solve the above problems and to provide a method for producing a ferritic or martensitic stainless steel capable of shortening the lead time in bell annealing and saving energy.

[0005]

SUMMARY OF THE INVENTION The present invention is a method for producing a ferritic or martensitic stainless steel which comprises a step of bell-annealing a coil which is hot-rolled and wound. It is a method for producing a ferritic or martensitic stainless steel, which is characterized in that it is directly sent to a die annealing furnace and subjected to bell annealing. In the present invention, it is preferable to start bell annealing of the coil within 11 hours after the completion of winding the coil.

[0006]

According to the present invention, in the method for producing a ferritic or martensitic stainless steel, the coil after hot rolling and winding is conveyed (directly sent) to the BAF as it is at a high temperature and reheated. Therefore, it is possible to save water and power for cooling the coil and fuel for reheating in the BAF. Incidentally, the ferrite-based in the present invention,
The definition of martensitic stainless steel is in accordance with JIS G4304.

As the transfer equipment for directly sending the coil, it is necessary to use one having a heat resistance temperature higher than the hot rolling temperature of the ferritic and martensitic stainless steels. The hot rolling temperatures for these steels are usually
Since the temperature is 500 ° C or higher, use a transfer facility with a heat resistant temperature of 500 ° C or higher. Since the upper limit of the hot rolling coiling temperature is approximately 900 ° C, it is preferable that the heat-resistant temperature of the transfer equipment is 900 ° C or higher. As such a transportation facility, a trailer, a truck, a railway freight car, etc. are preferable, and a plurality of coils are made as close to each other as possible, and a heat insulating box, a hood, etc. are appropriately used for these to prevent heat diffusion, and measures such as securing heat resistance of the cargo bed. It is better to take

Further, according to a preferred embodiment of the present invention, the coil starts its bell annealing within 11 hours after completion of its hot rolling and winding. The reason for limiting the time from the completion of hot rolling to the start of bell annealing within 11 hours is as follows. That is, from the viewpoint of preventing embrittlement at 475 ° C, ferritic stainless steel has 47% during direct delivery to BAF (before the start of bell annealing).
It is recommended not to hold at around 5 ℃ for a long time. However,
As mentioned earlier, it is more effective to reduce the energy loss by preventing heat diffusion by transporting multiple coils densely, but it is kept for a long time due to some reason such as trouble, and it is kept at around 475 ° C for a long time. There are cases where it will end up. Then, the application of the coil embrittled at 475 ° C. may be changed, and the continuous annealing line may be passed through the plate, but in this case, there is a problem that the plate may be broken due to embrittlement.

Therefore, ferritic stainless steel (representative
SUS430: Temperature just before embrittlement of JIS G4304 (approx. 475
The target condition was to start the bell annealing before reaching the temperature (° C), and in order to satisfy this, the time from hot rolling of the coil to the start of the bell annealing was limited to within 11 hours. When the coils are densely packed, the temperature reaches 475 ° C in 11 hours. If it exceeds 11 hours, it will not affect the majority if it is a short time, for example, about 5 hours, but if not, the coil itself will be placed in a state of There is a remedy to prevent air from cooling as it is, or to spread it by passing it through some line to allow it to cool, or to soak the coil in a water tank to quench it so that it will not be held at about 475 ° C for a long time. However, it takes time and handling, and the economy is lost. Therefore, it is desirable to start BAF at a temperature higher than 475 ° C as much as possible. On the other hand, 475 ° C for martensitic stainless steel
Although embrittlement does not occur, both ferritic and martensitic stainless steels are often mixedly loaded in the transportation equipment, so the ferritic stainless steels, which have severe restrictions, are controlled accordingly.

[0010]

[Example] When a ferritic stainless steel equivalent to SUS430 specified in JIS G4304 is manufactured, until the coil is bell-annealed after hot rolling and winding, the comparative example (coils separated from each other) , Was following the following process.
The temperature history of the coil (single weight 15 tons) of this comparative example is shown by a dotted line 2 in FIG.

Completion of hot rolling and winding (point A in Fig. 1; 750 ° C)
→ Natural air cooling to 150 ℃ (1 day or more) → Loading coil to trailer (2.5 hours) → Transfer to BAF (2.5 hours) → Loading coil to BAF (3.0 hours) (This loading start is shown in Fig. 1) C point) → BAF furnace atmosphere replacement (3.0 hours) → BAF start (D point in Fig. 1; 42 hours, 70 ° C) →
Heat treatment with a predetermined heat pattern (heating rate 30 ° C./hour) On the other hand, in the embodiment of the present invention, the heat-resistant temperature is 1500 ° C. by applying refractory bricks to the transportation facility (trailer).
The temperature was raised to 150 ° C. in the past and the above steps were omitted. A coil of high temperature (750 ° C.) was loaded onto the trailer immediately after the hot rolling and winding was completed and conveyed to the BAF. The temperature history of the coil (single weight: 15 tons, the coils are separated) of this example is shown by the solid line 1 in FIG. In the preferred embodiment of the present invention, the time from the completion of hot rolling and winding to the start of BAF (start of bell annealing) is set within 11 hours, but when this time is set to just 11 hours as in the embodiment, the start of bell annealing ( The coil temperature at point B in FIG. 1) is 410 ° C. The temperature is below 475 ° C, but
Within 5 hours. If it is annealed without expanding, it can be restored to a non-embrittled structure by annealing without being affected by embrittlement. In the comparative example, it takes about 11 hours from the point D (70 ° C) to start annealing and reach 410 ° C. That is, in the example, as compared with the comparative example, the lead time was shortened by 42 hours (= 42 + 11-11) as shown by the arrow range 3, and the heating fuel from 70 ° C to 410 ° C as shown by the arrow range 4 ( Energy savings). Although this example shows the case of exactly 11 hours, it goes without saying that it is better to start BAF earlier and start BAF at a temperature higher than 475 ° C.

[0012]

According to the present invention, in the production of ferritic and martensitic stainless steels, it is possible to simultaneously achieve reduction of lead time and energy saving in the bell annealing step.

[Brief description of drawings]

FIG. 1 is a graph illustrating a coil temperature history of the present invention and a comparative example.

[Explanation of symbols]

1 Solid line (Example) 2 dotted line (comparative example) 3 Arrow range (shortening lead time) 4 Arrow range (reheating energy saving)

Claims (2)

[Claims] 1. A method for producing a ferritic or martensitic stainless steel comprising a step of bell-annealing a coil obtained by hot rolling and winding, and the coil is directly sent to a bell-type annealing furnace after being wound, A method for producing a ferritic or martensitic stainless steel characterized by annealing. 2. The method for producing a ferritic or martensitic stainless steel according to claim 1, wherein the bell annealing of the coil is started within 11 hours after the winding of the coil is completed.