JPS61243118A - Production of hot-rolled two-phase stainless steel - Google Patents

Abstract

PURPOSE:To make possible the production of a hot rolled steel strip free from the generation of an edge crack, etc. at a good yield by subjecting a two-phase stainless steel billet to hot rough rolling and cooling under specific conditions then reheating the same and hot rolling the reheated steel. CONSTITUTION:The continuously cast slab of the stainless steel having two phases; ferrite phase and austenite phase has poor hot workability and even if a steel strip is produced therefrom by any means, the edge crack arises at both side ends and the product yield is extremely low. The continuously cast slab of the two-phase stainless steel is once roughly rolled at the finishing temp. of >=980 deg.C and 10-50% draft in this case. The slap is then cooled to <=600 deg.C at a cooling rate of >=0.4 deg.C/sec down to an ordinary temp. and thereafter the slab is reheated to about 1,200-1,300 deg.C and is hot rolled in the temp. region of >=900 deg.C to form the steel strip. The production of the hot rolled strip of the two-phase stainless steel at the high yield without the generation of the edge crack which is the defect of the conventional practice at the steel strip is thus made possible.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention stably produces hot rolled steel strip from continuously cast slabs of duplex stainless steel without deterioration of quality such as edge cracks. It is about the method.

Duplex stainless steel is a stainless steel that has a mixed structure of austenite and ferrite by adjusting the amount of Cr and Ni, and has good corrosion resistance, especially acid resistance of 5US316. Because it is superior to austenitic stainless steel and has excellent stress corrosion cracking resistance, it is widely used in various chemical industrial equipment and equipment such as condensers that use seawater as cooling water. It has become.

Although its usage is not limited to pipes and plates, it can be used in a variety of ways, but hot-rolled steel strips have occupied an extremely large position.

<Prior art and its problems> Incidentally, even in the midst of the recent situation in steel mills where the continuous casting rate has become extremely high, most of the hot rolled steel strips of duplex stainless steel are produced in the conventional manner. Usually, they are manufactured from slabs obtained through the ingot-forming method, which has been a major obstacle in improving the equipment arrangement within the factory.

This is because duplex stainless steel slabs obtained by continuous casting have extremely poor hot workability, and when hot rolling is attempted to produce copper strips, edge cracks inevitably occur on both ends of the slabs, making them sound unsound. This is because it was extremely difficult to obtain a product with a good yield.

Of course, under these circumstances, research has been carried out to produce duplex stainless steel hot-rolled copper strips from continuously cast slabs at a high yield without causing inconveniences such as edge cracking, and recently: hand.

``After hot working to a continuous casting slab of duplex stainless steel with a total thickness reduction of 5% or more,
There has also been a proposal to prevent cracking of the copper strip during hot rolling by performing a pretreatment called "slab soaking at 300°C for 2 to 30 hours" (Japanese Patent Laid-Open No. 59-35620). In other words, this method homogenizes and disperses the α phase by hot working with a total thickness reduction rate of 5% or more and soaking the slab at 1000 to 1300°C for 2 to 30 hours. This is to prevent cracking by utilizing the characteristics of the hardened structure.

However, this method requires high temperature
1-slap soaking process held at ℃ for 2-30 hours”
It has the disadvantage that it requires a lot of energy and the processing process is very complicated, and furthermore, the conclusion is that the effect of preventing copper strips from cracking is still not fully satisfactory. I had no choice but to give up.

Means for Solving the Problems> The present inventors have solved the above-mentioned problems in the conventional technology, and have succeeded in producing a continuously cast slab of duplex stainless steel without deterioration of quality such as edge cracks. In order to find a means to produce hot rolled copper strips stably and at low cost, we conducted repeated research, particularly on the hot workability of duplex stainless steel continuously cast slabs, and found the following (a).
) and (bl).

In other words, (a) various factors are considered to be involved in the occurrence of cracks during the production of hot rolled steel strip from duplex continuously cast slabs of duplex stainless steel. 1. This type of steel has a temperature of 980 to 700℃.
The second reason is that the occurrence of the α phase is remarkable in the bottom of the base, and that it is an extremely important cause of cracking that cannot be overlooked.

That is, if the slab has a cast structure, the metal structure during hot rolling becomes a coarse dendrite structure, which significantly impairs hot workability. Since alpha phase, which makes phase stainless steel brittle, inevitably occurs, cracking cannot be prevented unless these two phases are removed.

(b) Therefore, when manufacturing a hot rolled steel strip from a continuously cast slab of duplex stainless steel, rough reduction is once applied to the continuous cast slab to crush the cast structure, and the temperature of 1 rough reduction and 1 rough reduction are By regulating the subsequent cooling rate within a range that does not precipitate the alpha phase, and further regulating the slab heating temperature during hot rolling and the hot rolling temperature, the above-mentioned problems can be solved all at once. to be resolved.

This invention was made based on the above findings.

Duplex stainless steel slab obtained by continuous casting method,
Rough rolling is performed using a blooming mill or plate rolling mill at a finishing temperature of 980°C or higher and a rolling reduction of 10 to 50 g5, followed by 60 g.
After cooling to a temperature below 0℃ at a cooling rate of 0.4'C/sea or higher, it is reheated to 1200-1300℃ and then hot rolled in a temperature range of 900℃ or higher. It is characterized by being able to produce duplex hot-rolled stainless steel strip stably and at low cost.

Here, the reason why the manufacturing conditions for hot-rolled copper strips of duplex stainless steel are limited to specific numerical values will be explained.

■ Finishing temperature in rough rolling Rough rolling is carried out to crush the cast structure and improve the hot workability of duplex stainless steel tubes, but if the finishing temperature is below 980°C, α Since the phase precipitates and cracks appear in the slab due to α-phase embrittlement, the finishing temperature in rough rolling was set at 980° C. or higher. Furthermore, in order to obtain the effect of suppressing the σ phase by rapid cooling such as water cooling, it is important to set the finishing temperature of rough rolling to 980° C. or higher, which is the temperature before the start of σ phase precipitation. This also prevents the inconvenience that cleaning becomes difficult due to slab hardening due to σ phase precipitation.

■ Reduction ratio during rough rolling If the reduction ratio during rough rolling is less than 10%, the cast structure will not be completely crushed and coarse crystal grains will remain, resulting in insufficient hot workability improvement effect. on the other hand.

If the reduction rate exceeds 50%, the destruction of the cast structure will not proceed even if further reduction is applied.

The reduction ratio in rough rolling was set at 10 to 50%.

■ Rapid cooling temperature range and cooling rate after rough rolling Rapid cooling to a specific temperature range after rough rolling
This is extremely important in suppressing phase precipitation and improving the hot workability of the slab. And the cooling rate at this time is 0
．． It's slower than 4℃/see.

If the quenching temperature range does not reach 600℃, the σ phase will precipitate and cause the slab to become brittle.
The cooling rate in the quenching treatment was set at 0.4°C/sec or higher, and the required quenching temperature range was set at 600°C or lower.

■ Reheating temperature during hot rolling If the slab reheating temperature during hot rolling to obtain a copper strip is less than 1200°C, hot workability during the subsequent hot rolling will deteriorate, and Hot rolling finishing temperature C
On the other hand, if the temperature exceeds 1300°C, grain boundary oxidation will occur and the hot workability will deteriorate. The reheating temperature was set at 1200-1300°C.

By the way, FIG. 1 is a diagram showing the relationship between reheating temperature and hot workability (hot deformability index) of a continuously cast duplex stainless steel slab.

The results shown in Fig. 111 are obtained by converting duplex stainless steel having the chemical composition shown in Table 1 into slabs by continuous casting, and then converting some of them into slabs without applying rough reduction. raw material), and the rest is rough reduction (reduction rate:
20 m) was added to form a slab, and test pieces were taken from each slab and subjected to a hot torsion test. The hot torsion test results were expressed as "r hot deformability index" determined from the number of twists.

It can also be seen from the diagram that the hot workability of the crudely reduced material is significantly improved compared to the as-cast material, and this hot workability reaches its maximum value at approximately 1250°C. is 12
It is also clear that preferred values are no longer exhibited outside the temperature range of 00 to 1300°C.

0 Finishing temperature during hot rolling The finishing temperature during hot rolling is extremely important to obtain a sound copper strip, but if the temperature falls below 900°C,
Since the ductility of the phase stainless steel decreases and cracks occur, the finishing temperature during hot rolling was set at 900° C. or higher. As mentioned earlier, the σ phase tends to precipitate in a temperature range below 980°C, and in order to completely eliminate this problem, the finishing temperature is set as high as possible, above 900°C. is desirable.

FIG. 2 is a diagram showing the relationship between hot rolling finishing temperature, tensile strength, and elongation of duplex stainless steel 1).

Duplex stainless steel with the chemical composition shown in Table 2 was hot rolled (heating temperature: 1250°C, coiling temperature = 400°C) to form a hot rolled steel strip, and test pieces taken from it were subjected to a tensile test. It was obtained by subjecting it to

It is clear from FIG. 2 that the elongation of the duplex stainless steel sharply decreases when the hot rolling finishing temperature falls below 900°C.

Next, the present invention will be explained by examples and in comparison with comparative examples.

<Example> First, two-phase stainless steels 1 to 0 having the chemical composition shown in Table 3 are melted by a conventional method, and then made into a slab by a continuous casting method or a steel ingot method. By performing blooming rolling or rough rolling and hot rolling under the conditions shown in the table (
However, rough rolling was not performed for test numbers 3 and 5)
A hot rolled copper strip coil was manufactured.

At this time, we investigated the occurrence of flaws in the slab after single block rolling or rough rolling, whether the flaws could be taken care of, and the occurrence of coil re-cracking after hot rolling.

The results are also shown in Table 4.

As is clear from the results shown in @4, when a duplex stainless steel hot rolled copper strip was manufactured according to the conditions of the present invention, no cracks were generated from the continuously cast slab.
Or, even if cracks occur, a healthy product can be stably obtained with only light maintenance.
It can be seen that if the steel strip manufacturing conditions are outside the scope of the present invention, a sound duplex stainless steel hot rolled copper strip cannot be expected from a continuous cast slab.

Overall results> As explained above, according to the present invention, hot-rolled steel strip can be stably produced from continuously cast slabs of duplex stainless steel without causing quality deterioration such as edge cracking. Moreover, it has extremely useful effects industrially, such as being able to be mass-produced at low cost.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between reheating temperature and hot workability C hot deformability index of continuously cast duplex stainless steel slabs; Figure 2 is a diagram showing the relationship between hot rolling of duplex stainless steel It is a diagram showing the relationship between finishing temperature, tensile strength, and elongation. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo Haka 2 persons 1st figure form U printing (re, to Kenn Wenbin) [0C] Figure 2

Claims (1)

[Claims] Continuously cast slabs of duplex stainless steel have a finishing temperature of 980℃.
Rough rolling is performed at a rolling reduction rate of 10 to 50% at a temperature of 1200 to 130 degrees Celsius or above, followed by cooling to a temperature of 600 degrees C or less at a cooling rate of 0.4 degrees C/sec or more to form a cold piece.
A method for producing a hot-rolled duplex stainless steel strip, the method comprising heating to 0°C and hot rolling in a temperature range of 900°C or higher.