JPS6156236A - Manufacture of two phase stainless steel hot rolled steel strip for working - Google Patents

Abstract

PURPOSE:To obtain stably a steel strip having both of superior corrosion resistance and high ductility even in a hot rolled state, by hot rolling a steel having a specified compsn. in which C, P, S, etc. are restricted, then cooling rapidly to remarkably low temp. range and winding said plate, further specifying the temp. of hot rolling completion and cooling time thereafter. CONSTITUTION:The steel consisting of, by wt%, <=0.05 C, <=2.0 Si, <=2.0 Mn, <=0.03 P, <=0.015 S, 16.0-30.0 Cr, 3.0-9.0 Ni, 0.2-5.0 Mo, <=0.45 N, if necessary further >=one kind among <=0.05 solAl, <=0.0100 Ca, <=0.10 rare earth element, <=0.10 Zr, <=2.0 Cu, <=0.05 Nb, <=0.05 V, <=0.05 Ti and the balance Fe is hot rolled. The hot rolling is finished at >=825 deg.C, then said plate is air cooled in 5- 20sec, successively cooled rapidly at >=5 deg.C/sec rate, and wound at <=550 deg.C temp. range.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for obtaining a soft working duplex stainless steel hot-rolled steel strip having both high ductility and excellent corrosion resistance as hot-rolled. It is something.

In recent years, there has been a growing demand for metal materials that are stable against seawater and have corrosion resistance ↑n for use in seawater heat exchangers and seawater structures, mainly in the chemical industry, ships, and power generation fields. It has shown a tendency to increase. Against this backdrop, duplex stainless steel, which has attracted attention for its excellent corrosion resistance in a severely corrosive chloride environment, has become widely used in various fields.

<Prior Art> By the way, a two-phase stainless steel sheet exhibiting two phases, a ferrite phase and an austenite phase, at around room temperature has conventionally been produced by hot rolling steel with a predetermined composition and rolling at a coiling temperature of 600°C.
Generally, hot rolled steel strips wound up in the above manner are heat treated (solution annealing on an AP line, etc.), or cold rolled and subsequently heat treated.

This is because the desired corrosion resistance, toughness, and
This is because it did not exhibit ductility, etc.

<Problems to be Solved by the Invention> However, the heat treatment and cold rolling processes essential to the conventional method for manufacturing duplex stainless steel sheets require extremely expensive equipment and are difficult to manufacture when manufacturing steel sheets. If the efficiency is significantly impaired, it becomes an inconvenience.Furthermore, these processes also make it difficult to impart sufficient workability to the product steel sheet, and the processing in the next process is extremely difficult. At present, it has not been possible to break through the preconceived notions regarding duplex stainless steel, which is a difficult material.

<Means for Solving the Problems> Therefore, the first aspect of the present invention is to omit cold rolling and heat treatment during the duplex stainless steel sheet manufacturing process, which are major obstacles to improving manufacturing efficiency and reducing manufacturing costs. A soft, highly ductile two-phase stainless steel sheet that not only exhibits high corrosion resistance through hot rolling, but also has excellent workability that completely dispels the image of duplex stainless steel sheets obtained by conventional methods. In order to manufacture stainless steel hot-rolled steel strips, we investigated the composition of steel, hot rolling conditions, etc. from various viewpoints, and as a result, we came to the following knowledge. In other words, (a) We use steel that has been adjusted to a specific chemical composition using four wheels that limit C, P, S, etc., and after hot rolling it, we roll it into a significantly lower temperature range that was previously unimaginable. If rapid cooling is performed to a temperature of To obtain a soft duplex stainless steel hot-rolled steel strip having excellent corrosion resistance and high ductility even as it is.

(b) Furthermore, at this time, if hot rolling is finished at a high temperature of 825°C or higher, and 5 to 20 seconds of air cooling time is secured, then rapid cooling to the coiling temperature range is performed, which may cause problems. The remaining warm-worked structure can be effectively prevented, and the softening and high ductility of product steel sheets can be achieved more stably and reliably.

This invention was made based on the above findings, and includes: C: 0.05% or less (hereinafter, component proportions are based on weight), 3i: 2.0% or less, Mn: 2.
Less than 0%.

p: o, 0.03% or less, 3: 0.015% or less.

Cr: 16.0-30.0%, Ni:
3.0-9.0%.

Mo: 0.2-5.0%, N: 0.
Contains 45% or less, and if necessary, further contains so
l, Affi: 0.05% or less.

Ca: 0.0100% or less.

Rare earth elements: 0.10% or less.

Zr: 0.10% or less, Cu: 2.0%
below.

Nb: 0.05% or less, V: 0.05% or less.

A steel containing at least one type of Ti: 0.05% or less, with the remainder substantially consisting of Fe, is hot rolled, and after the hot rolling is finished at 825°C or higher, it is air cooled for 5 to 20 seconds. followed by 5℃/Se
By rapidly cooling at a cooling rate of C or higher and coiling at a temperature range of 55Q or lower, a hot-rolled duplex stainless steel strip with excellent corrosion resistance and ductility can be stably produced at a low cost. It has the following characteristics.

Next, in the method of the present invention, the reason for numerically limiting the amount of the steel composition and the hot rolling/coiling conditions as described above will be explained.

Δ1 group composition ■ C C precipitates in the form of carbide in steel, and not only deteriorates the ductility of the steel material, but also deteriorates the corrosion resistance. be. If the content exceeds 0.05%, the precipitation of the carbides cannot be sufficiently suppressed even if rapid cooling is performed after hot rolling and low-temperature winding is performed, resulting in poor toughness, ductility, and corrosion resistance. Since the desired value cannot be obtained, the C content should be set to 0.
It was set at 0.05% or less.

■ 5i S1 component is a preferable element that has a deoxidizing effect on steel and also improves corrosion resistance, but its content is 2.0%.
If the Si content exceeds 2.0%, weldability and workability will be impaired, so the Si content was set at 2.0% or less. Note that the 3i component exhibits the above characteristics even in a very small amount, but in order to ensure a significant effect, it must be present at least 0.
．． It is preferable to contain 10% or more of -F.

Mn The Mn component is also a useful element that has n+>acid action in steel, and is also a preferable component that also has the action of improving the hot workability of steel, but if its content exceeds 2.0%. Since this deteriorates the corrosion resistance of steel, the Mn content is set at 2.0% or less. Although the desired effect can be obtained by adding a small amount of the Mn component, it is preferable to ensure a content of 0.50% or more.

@ PP P is an impurity element that is preferably as small as possible because it impairs the weldability and hot working (4+) of steel.In particular, when its content exceeds 0.03%, the above-mentioned adverse effects become noticeable. Therefore, the P content was set at 0.03% or less.

SS is an impurity element that is preferably as small as possible because it deteriorates the corrosion resistance, ductility, and toughness of steel.

In particular, if the S content exceeds 0.015%, the above-mentioned adverse effects become significant, so the S content was set at 0.015% or less.

■ CrC is an extremely important element that improves the corrosion resistance of steel, and it is essential to add 16.0% or more in order to impart satisfactory corrosion resistance to seawater-resistant steel. If the Ni content exceeds 30.0%, the workability and weldability of the steel will deteriorate, and a two-phase structure will be obtained, resulting in an expensive Ni content.

Since it is necessary to increase the Cr content, the Cr content is 16
．． 0 to 30.0%.

(2) The Nt Ni component is an essential component for imparting high toughness to steel and increasing corrosion resistance, and if its content is less than 3.0%, desired toughness and corrosion resistance cannot be ensured. On the other hand, if the Ni content exceeds 9.0%, the above-mentioned effect will be saturated, causing an economic disadvantage, so the Ni content should be set at 3.0%.
It was set at 0 to 9.0%.

MO The MO acid component has the effect of greatly improving the seawater resistance of steel, but if its content is less than 0.2%, the desired effect cannot be obtained. 〇 - Even if the content is avoided, no further improvement effect can be obtained, so the MOC content is set at 0.2 to 50%.

N The N component has a remarkable effect of improving the corrosion resistance of steel even when added in minute amounts such as o and oiooo%, which are close to impurities.
Since it is difficult to form a solid solution of N exceeding %,
The N content was set at 0.45% or less.

■ 5offi, Al, ca, rare earth elements, 7r.

CI, Nb, V, and Ti These components are used as deoxidizing agents for steel, and one or more of them can be added as necessary to further improve hot workability and corrosion resistance. , the reason for limiting the amount of each element added will be explained.

I) sot, Δ2 Sol, Al components are added as deoxidizing agents for steel, but even if their content exceeds 0.05%, the deoxidizing effect is saturated, so SO2, The content of Δ is 0.
It was set at 0.05% or less.

IT) Ca, rare earth elements, and 7r These components are used to improve the hot workability of steel.
Ca content is 0.01.
00%, rare earth element content is 0.10%, and Z
If the r content exceeds 0.10%, the toughness will deteriorate, so the Ca content should be 0.0100%.
, rare earth element content is 0.10% or less, 7r content is 0
．． Each was set at 10% or less.

I[[) Cu, Nb, V, and T1 These components are added in one weight or more to further improve the corrosion resistance of steel, but when the Cu content is 2.0% and the Nb , V and T1 contents are respectively 0.05
Since even if it exceeds 2.0%, no further corrosion resistance improvement effect can be obtained.
．． The Ti content was determined to be 0.05% or less, the Ti content was determined to be 0.05% or less, and the Ti content was determined to be 0.05% or less.

B. Hot rolling/winding conditions ■ Hot rolling end temperature When the end temperature of hot rolling is lower than 825°C, low humidity winding is performed after predetermined rapid cooling to suppress precipitation of Cr carbide and σ phase. Since there is a possibility that the hot rolling ffi fj remains and it becomes impossible to obtain a sufficiently soft and highly ductile copper strip, the finishing temperature of hot rolling was determined to be 825° C. or higher.

■ Air-cooling time If the air-cooling time is less than 5 seconds immediately after the end of hot rolling, even if the end temperature of hot rolling is 825°C or higher and winding is performed after rapid cooling, warm processing will not be possible. There is a risk that the structure may remain, making it difficult to stably obtain a sufficiently soft and highly ductile copper strip.On the other hand, if the air cooling time exceeds 20 seconds, even if the weave is fully recovered, Even if Cr
Precipitation of carbides and σ phase occurs, which hardens the steel strip and causes deterioration of ℃ ductility and corrosion resistance.
It was set as 20 seconds.

■ Cooling rate of quenching carried out after air cooling ■ If the cooling rate at the culm is slower than 5℃/Sec, Cr carbide and σ phase will precipitate during cooling, resulting in deterioration of the ductility and corrosion resistance of the copper strip. Therefore, the cooling rate in the quenching step was set at 5°C/Sec or more.

■ Winding temperature If the winding temperature is higher than 550°C, Cr carbide or σ phase will precipitate during slow cooling after winding, even if the air cooling time and cooling rate in the rapid cooling process are appropriate. Since the ductility and corrosion resistance of the copper strip also deteriorated, the winding temperature was set at 550°C.

In addition, if hot rolling is done at a high temperature of 870°C or higher,
The warm working structure of Ferrai I~ is further reduced and the ductility of the steel strip is improved more stably, and if the coiling temperature after air cooling and quenching is adjusted to 500~200℃, Cr carbide and Not only is the precipitation of the σ phase almost completely suppressed, but even if the warm-worked structure is carried over, the opportunity for tempering during slow cooling after winding can be secured, making it even more effective. It becomes possible to stably achieve even better workability. Therefore, it is recommended that hot rolling be completed at a high temperature of 870°C or higher, and after air cooling and rapid cooling, winding at 500 to 200°C.

The attached drawings show the method of the present invention for @0.008% (0.008%) steel.
,0,52%5i-1,60%M11-0.008%P
-0.0005%5-21.9%Cr-5,5%Ni
-2,78%MO-0,138%Net is a graph showing the influence of hot rolling end temperature, air cooling time, and coiling temperature on the strength, cross-sectional reduction ratio, and corrosion resistance, and the hot rolling heating temperature is The temperature is 1200℃, and the thickness of the second plate is 5.5mm.
That is. The cooling rate during air cooling is approximately 2℃/
The cooling rate from the end of air cooling to the winding was all 5 to 15 seconds, and the corrosion weight loss was the value measured by conducting a seawater immersion practical test (6 months).

The drawing also shows that adjusting conditions such as the humidity at the end of hot rolling, air cooling time, and coiling temperature within the range specified by the method of the present invention is an extremely important factor in improving the corrosion resistance and workability of steel. That is clear.

Next, the present invention will be specifically explained using examples and comparing with comparative examples.

<Example> First, steels A to N having the compositions shown in Table 1 were melted by a conventional method.

Next, these gold steels were hot rolled under the conditions shown in Table 2 and wound up to a thickness of +6. omm hot rolled steel strip was manufactured.

Test pieces were cut out from each of the hot rolled steel strips thus obtained, and their mechanical properties and corrosion resistance were examined, and the same results as shown in Table 2 were obtained.

As is clear from the results shown in Table 2, all the hot rolled copper strips obtained by the method satisfying the conditions of the method of the present invention are soft as hot rolled, have high ductility, In contrast to those showing even better corrosion resistance, those that deviate from the conditions in the method of the present invention, that is, those with a low hot rolling finishing temperature (Test No. 4) and those with a short air cooling time (Test No. 1) was found to be hard and have poor ductility, and those with a slow cooling rate (test number 2) and those with high winding dryness (test number 3) were not only hard and had poor ductility, but also had poor ductility. It can be seen that the corrosion resistance is also poor.

As can be understood from the results in the examples above, the hot rolled steel strip produced by the method of the present invention is soft and has high ductility, so it is extremely easy to handle in the next process. It has excellent corrosion resistance even when hot-rolled, so it can be used as is, making it an extremely economical duplex stainless steel strip with a high rating of ↑(1).

As an additional note, it is important to note that there is a need for the production of hot-rolled steel strips to reduce the load during cold pressing, as well as for the production of hot-rolled cladding made of duplex stainless steel, which has been in the spotlight in recent years.
1. Application of the method of the present invention is effective for the front part.

<Overall Effects> As described above, according to the present invention, the hot rolled
A duplex stainless steel plate material for machining that has excellent corrosion resistance and ductility and is soft! ~ It can be produced cheaply and stably, and has excellent industrial performance when used as a material for heat exchangers using seawater, chemical manufacturing equipment, salt manufacturing equipment, etc. This brings about extremely useful effects.

[Brief explanation of the drawing]

The accompanying drawing is a graph showing the effects of hot rolling end temperature, air cooling time, and coiling temperature on the strength, reduction of area, and corrosion resistance of duplex stainless steel. Applicant Sumitomo Metal Industries Co., Ltd. Agent Kazuo Toyama and 1 other RT 1002003004005QO600700 5-star rating ('CJ

Claims (2)

[Claims] (1) Weight percentage: C: 0.05% or less, Si: 2.0% or less, Mn: 2.
0% or less, P: 0.03% or less, S: 0.015% or less, Cr: 16.0-30.0%, Ni: 3.0-9.0
%, Mo: 0.2 to 5.0%, N: 0.45% or less, and further contains, if necessary, Sol, Al: 0.05% or less, Ca: 0.0100% or less, rare earth elements: One of the following: 0.10% or less, Zr: 0.10% or less, Cu: 2.0% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.05% or less A steel including the above and the remainder substantially consisting of Fe is hot rolled, and after finishing the hot rolling at 825°C or higher, air cooling is performed for 5 to 20 seconds, and then the steel is heated at 5°C/sec.
A method for producing a hot-rolled duplex stainless steel strip for processing, characterized by rapidly cooling at a cooling rate of c or more and winding in a temperature range of 550° C. or less. (2) Production of the hot-rolled duplex stainless steel strip for processing according to claim 1, wherein the hot rolling end temperature is 870°C or higher and the coiling temperature is 500 to 200°C. Method.