JPH0158249B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention applies to continuously cast slabs (hereinafter referred to as CC slabs) of high Cr high purity ferritic stainless steel or slabs that have been once bloomed and rolled ( (Hereinafter, when the term "slab" is referred to, it includes the slab and steel slab.)
The present invention relates to a cooling method for preventing cracks occurring during or after cooling of the slab.

(Prior technology) High-purity ferritic stainless steel with a high Cr content and reduced C and N content is becoming increasingly important as an inexpensive corrosion-resistant material that does not cause the chloride stress corrosion cracking that is typical of 18Cr-8Ni. It is used in many applications to improve corrosion resistance, rust resistance, and oxidation resistance by selectively adding Mo, Ni, Cu, etc., or by extremely reducing impurities such as S and O. I'm starting. It is also known that 25Cr containing 2 to 4% Mo exhibits excellent seawater resistance, and its importance is expected to increase in the future.

However, one of the drawbacks of high-purity, high-Cr ferrite stainless steel, which has these excellent properties, is manufacturing problems. Already published in 1978-128464
Publication No. 1987-39732, Japanese Patent Publication No. 1983-39732
As disclosed in Publication No. 2628, Japanese Patent Application Laid-open No. 60-2622, etc., these steels are used during cooling of CC slabs, during surface treatment after cooling, and even during the next hot rolling process. It is known that slabs undergo transverse cracking or breakage during transport to a heating furnace (referred to as "place cracking"), and when this cracking occurs, it often becomes impossible to manufacture the slab.
It is no exaggeration to say that especially high Cr steel containing large amounts of Nb, Mo, Al, etc. will always break.

The above-mentioned patent documents suggest countermeasures against these phenomena.
It is disclosed that the temperature is not cooled below 150°C, and that it is not cooled below the transition temperature (300°C in the example). On the other hand, JP-A-60-2628, JP-A-60-
Publication No. 2622 describes a very slow cooling method for cooling slabs, that is, from 800 to 1300℃ to 300℃.
Method of slow cooling at a cooling rate of ℃/hr or less (800℃ to 300℃
(up to 12.5hr or more) or from 700℃ to 400℃
It discloses a method for slow cooling (at least 16.7 hr) at a temperature of ℃/hr or less and not lowering the temperature below 200 ℃. According to conventional knowledge, two methods are known: maintaining the temperature at 150°C or higher, or cooling extremely slowly to around 700°C to 400°C.

(Problems to be Solved by the Invention) As the advantages of high-purity ferrite stainless steel become clearer, the production volume of these stainless steels increases and the types of steels are expanded to include 25Cr-4Mo series, which have even better properties. In order to take advantage of the characteristics of these steel types, it is necessary to meet the manufacturing constraints of not allowing cold pieces (surface temperature below 100°C),
Restrictions on slow cooling conditions in the high temperature range are becoming a major obstacle in terms of cost, delivery time, and productivity, and there is a strong demand for a cooling method for slabs that can be handled as cold pieces.

(Means for Solving the Problems) As a result of researching the cracking phenomenon of slabs of high-purity ferritic stainless steel, the present inventors found that two
We found that two important factors come into play.

(1) is the embrittlement phenomenon of the material itself caused by the precipitation of intermetallic compounds (Laves phase) whose precipitation peaks in the temperature range of approximately 700-800°C. (2) is due to the non-uniformity of thermal stress caused by the difference in cooling rate in each part when cooling the slab. It has been found that these two effects interact to cause cracking during cooling. Of course, depending on the alloy composition, there are things that strongly affect (1), or (2) such as cooling of CC slabs.
Since there are some factors that strongly influence , causes (1) and (2) influence each other. The present inventors have discovered that typical high-purity ferrite stainless steel is the most susceptible to cracking.
25Cr−4Mo−4Ni−0.4Nb steel CC slab (25mm thickness)
A cooling experiment was conducted on steel slabs (180 mm thick) that had been subjected to solidification and blooming rolling, and the effect of the cooling method on the occurrence of cracking in cold slabs was investigated. We conducted tests in which CC slabs and steel slabs made by blooming CC slabs were cooled as they were, and the cooling rate was varied by placing them on a covered trolley at various temperatures during cooling. Using these results as the base point for the time when the temperature dropped below 800°C, the subsequent temperature/time changes and the cold piece (cooled to below 100°C)
FIG. 1 shows the results in relation to the occurrence of cracking when It was separately confirmed that the precipitation of intermetallic compounds, which cause embrittlement, is fastest at 800°C.
The slab cooled quickly, and cracks occurred when the slab was cooled. This is due to the temperature difference between different parts of the slab.
The temperature exceeds 200℃, which is due to nonuniform thermal stress. When it was slowly cooled with a simple cover, the temperature difference was greatly reduced and it did not crack.
, the slab was placed in a heated slab at 600°C, maintained at that temperature, taken out after about 100 or 200 minutes, and then left to cool without causing any cracks. is similarly
No cracking occurred when it was charged into a heating cart at 400℃ and left to cool after 20 days. However, when it was charged into a heated slab truck at 700°C and allowed to cool after 200 minutes, surface cracking occurred during cooling, and severe cracking occurred in the cold slab. Again, it was kept at 600°C and left to cool after 250 minutes, but cracking occurred in the same way. In the example, intermetallic compounds (Laves phase) are clearly formed while kept in the hot plate.
This is because the material itself became brittle. In this way, it is the type of steel that is most prone to cracking.
Such a relationship was observed for 25Cr−5Ni−4Mo−0.4Nb steel. Other steel types 18Cr−2Mo−0.6Nb
Almost the same tendency was observed for steel, although there were differences in the degree of cracking.

In this way, we have been able to prevent cracking of high-purity ferrite stainless steel CC slabs and steel slabs.
For the first time, we were able to clarify the existence of a cooling zone in which cracks do not occur even when cold pieces are made. That is, point A (0 min, 250°C) and point B (60°C) shown in Figure 1.
min, 250°C), point C (60 min, 100°C) and point D (150°C)
minutes, 800℃), point E (230 minutes, 550℃), point F (350℃), point E (230 minutes, 550℃),
minute, 550℃)
By cooling DEF so that it does not pass through the time-temperature curve starting at 800℃, cracks will not occur even if CC slabs or steel slabs are used as cold slabs.

If it is cooled so as to cut through the danger zone straight line AB or ABC, there will be a large temperature difference between parts, and thermal stress will cause cracks. When cooled so as to cut the straight lines DE and EF in the dangerous range, the material passes through the precipitation region of intermetallic compounds (Laves phase) whose precipitation peaks at 800℃, and the Laves phase precipitates within the material, causing the material itself to deteriorate. becomes brittle and cracks occur.

Although some alloy composition systems in which intermetallic compounds (Laves phase) tend to precipitate have already been theoretically elucidated ("Transformation and Precipitation in Iron and Steel", Japan Institute of Metals, 1968), in the Fe-based alpha phase, Ti, Zr,
When Nb, Mo, W, Ta, etc. are added to an alloy, it tends to occur and precipitates as a composition of, for example, Fe ₂ Ti or Fe ₂ Nb. Therefore, these Ti, Zr, Nb, Mo,
It is thought that it is likely to occur when containing W etc.
Facts such as 25Cr−5Ni−4Mo−0.4Nb and 19Cr−2Mo−
It was confirmed that 0.3Nb−0.1Ti etc. are extremely easy to generate.

As a result of examining the above phenomenon by expanding the component system, it was found that it holds true in the following component system.

Contains C+N: 0.08 or less, Mn: 3% or less, Si: 3% or less, Cr: 9-35%, P: 0.050% or less, S: 0.010% or less, O: 0.010% or less, and further Ni: 8% or less Mo: 6% or less Co: 10% or less Cu: 2% or less Al: 6% or less of one or more types, and Nb: 0.05-0.8% Ti: 0.01-0.8% Zr: 0.01-0.5% W: 0.03-0.5 %, and the remainder is substantially
Ferritic stainless steel consisting of Fe and unavoidable inclusions.

The reason for limiting the steel composition range of the present invention will be explained below.

C, N: The amount of C and N greatly affects toughness, and C
The greater the amount of +N, the lower the toughness of the material. It is also a harmful element in terms of corrosion resistance because it forms carbonitrides and tends to cause intergranular corrosion. Therefore, it is desirable that C and N be lower from the viewpoint of manufacturability and corrosion resistance, and the amount of C+N is set to 0.08% or less.

Mn: Usually added as a deoxidizing and desulfurizing agent,
It is an effective element as a substitute for Ni in terms of toughness. However, if it exceeds 3%, the effect will be saturated, so it was set at 3% or less.

Si: An element used as a deoxidizing agent and also effective in improving corrosion resistance. However, if it is contained in a large amount, the toughness deteriorates, so the upper limit was set at 3%.

Cr: Cr is a major element that increases the corrosion resistance of stainless steel, and the higher the content, the better the corrosion resistance. If it is less than 9%, there is no effect of improving corrosion resistance, so the lower limit was set at 9%. Further, from the viewpoint of corrosion resistance, the more the content is, the more advantageous it is, but in high Cr steel, intermetallic compounds such as σ phase are likely to occur during cooling, which deteriorates toughness, so the upper limit was set at 35%.

P: Since P generally deteriorates pitting corrosion resistance, the lower the content, the better, and the upper limit was set at 0.050%.

S: S is an element harmful to corrosion resistance, and the lower its content is, the more desirable it is, and the upper limit was set at 0.010%.

O: Residual elements exist in the form of oxides in steel and degrade toughness, so the content is desirably low, and the upper limit is set at 0.010%.

Ni: Ni is an effective element for improving the toughness of ferritic stainless steel, and is a desirable element from the standpoint of preventing cracking. However, if it is contained in a large amount, stress corrosion cracking tends to occur, so it is desirable to add it in an amount of 8% or less.

Mo: Along with Cr, Mo is a major element for improving pitting corrosion resistance, and its addition is desirable from the viewpoint of corrosion resistance.
However, if it is contained in large amounts, during cooling or heat treatment
It is added in an amount of 6% or less because it produces intermetallic compounds such as Laves phase and σ phase, which deteriorates toughness and promotes cracking.

Co: Like Cr and Mo, Co is an effective element in terms of corrosion resistance, but it promotes precipitation of the σ phase and causes embrittlement. Moreover, if it is contained in a large amount, hot workability will deteriorate, so it should be added in an amount of 10% or less.

Cu: Cu is effective in improving corrosion resistance, especially sulfuric acid resistance, and is added as a selective element in an amount of 2% or less. Even if it is added in excess of 2%, the effect is saturated.

Al: Usually added as a deoxidizing agent, but it is an effective element for improving oxidation resistance and is added as a selective element. However, if it is contained in a large amount, the toughness of the material will deteriorate, so it should be added in an amount of 6% or less.

Nb: When added, Nb is an element that fixes C and N and is effective in improving toughness and preventing intergranular corrosion. However, if it is less than 0.05%, it has no effect, and if it is added in excess, solid solution Nb increases and intermetallic compounds such as Laves phase precipitate, which significantly deteriorates toughness, so it is added in a range of 0.05 to 0.8%.

Ti: Similar to Nb, Ti is an element that fixes C and N and is effective in improving toughness and preventing intergranular corrosion. 0.01%
If it is less than this, there will be no effect, whereas if it is added in excess, intermetallic compounds such as Laves phase will precipitate and the toughness will be significantly deteriorated, so it should be added in a range of 0.01 to 0.8%.

Zr: Zr is an element that fixes C and N like Nb and Ti, and is an effective element for improving toughness and preventing intergranular corrosion, and is added in a range of 0.01 to 0.5%. If it is less than 0.01%, the effect will be weak, and if it exceeds 0.5%, intermetallic compounds will tend to precipitate and the toughness will deteriorate.

W: W is added to fix C and is an effective element for improving corrosion resistance, but if added in excess, σ
The addition range was set to 0.03 to 0.5% in order to promote the precipitation of intermetallic compounds such as phases and deteriorate toughness.

(Example) 25Cr-4Mo-2Ni-
0.3Nb steel and 19Cr−2Mo−0.3Nb−0.1Ti and
15Cr-4Al steel was produced. Later as CC slab
250 mmt, and during cooling, it was transferred to a heating cart at a temperature of 550 to 400°C, transported, and allowed to cool manually within 2 to 3 hours, and the surface was partially treated during cooling. Some are 300
Slowly cool with a cover at ~400℃, transfer to a heat insulating pit,
After a week, it was treated at 300°C and made into cold pieces. All of these were able to be heated in a heating furnace, hot-rolled, and made into hot-rolled coils without causing any cracks.

However, when CC slabs were loaded onto a heated slab truck at 600℃ and transported for 6 hours, the high Cr 25Cr steel developed cracks, and the 19Cr steel also developed small cracks.
In the 15Cr-4Al series, there were occasional cracks in the hot-rolled coil. In this way, it was confirmed that if the material entered the danger zone shown in FIG. 1, cracking would occur if it was made into a cold piece, whereas if it was made into a cold piece outside the dangerous region, no cracking would occur.

(Effect of the invention) High Cr ferritic stainless steel with low C and N has excellent corrosion resistance and oxidation resistance and is used in many fields. Effective countermeasures were desired. Conventional methods include extremely slow cooling during cooling, and
A method of heat treatment at 350°C or higher was known.

On the other hand, the present invention has significant advantages in terms of cost and productivity, since it is possible to produce cold pieces without causing cracks by controlling the cooling rate.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the surface temperature and elapsed time of a slab and the risk area of cracking due to placement.

Claims (1)

[Claims] 1. C+N: 0.08% or less, Mn: 3% or less, Si: 3% or less, Cr: 9 to 35%, P: 0.050% or less, S: 0.010% or less, O: 0.010%. Contains the following, and further contains one or more of the following: Ni: 8% or less, Mo: 6% or less, Co: 10% or less Cu: 2% or less, Al: 6% or less, and Nb: 0.05 to 0.8%. Contains one or more of Ti: 0.01-0.8%, Zr: 0.01-0.5%, W: 0.03-0.5%, and the remainder is substantially
Figure 1 shows the relationship between the surface temperature and elapsed time of a continuously cast slab of ferritic stainless steel consisting of Fe and unavoidable inclusions, or a slab obtained by blooming rolling the slab. A point (0 min, 250℃),
Straight line ABC connecting point B (60 minutes, 250℃), point C (60 minutes, 100℃), point D (150 minutes, 800℃), point E (230 minutes, 550℃), point F (350 minutes, A cooling method for ferritic stainless steel slabs that does not cause cracks due to placement, which is characterized by cooling without passing through each dangerous zone of the straight line DEF connecting the temperature range (550℃).