JPH0794688B2 - Manufacturing method for improving the toughness of a high Al content ferritic stainless steel hot rolled steel strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a hot rolling of Fe-Cr-Al ferritic stainless steel used for heat-resistant applications such as chimney of stoves, exhaust gas purifying devices of automobiles, and heating wires. The present invention relates to a manufacturing method for improving the toughness of a steel strip.

[Conventional technology]

Fe-Cr-Al steel has excellent high-temperature oxidation resistance and is suitable for various heat-resistant applications such as chimney of stoves.
For stove chimneys, the operating temperature is usually 850-900
Although it is about ℃, the operating temperature tends to rise to about 1000 ℃ in order to improve the combustion efficiency. In addition, the plate thickness used tends to be thin.

It has been proposed to use a metal substrate carrier in place of a conventional ceramic honeycomb carrier in a catalytic converter for an automobile exhaust gas purification device. In this case, a coating layer of γAl ₂ O ₃ , which is a catalyst carrier, is formed on the surface of a metal substrate forming the honeycomb structure, and the catalyst carrier layer is impregnated or coated with the catalyst substance. As a metal substrate material constituting such a honeycomb structure, for example, JP-B-58-1971, JP-A-56-96726, JP-A-57-71898, and JP-A-58-177437. Is Fe-C
It has been proposed to use r-Al steel.

In this application, the temperature of the exhaust gas itself and the heat of reaction between the exhaust gas and the catalyst cause a marked rise in temperature, so strict high-temperature oxidation resistance is required for the metal substrate, but Fe--Cr--Al steel is the surface of Al formed on the surface. Oxidation resistance is maintained by the ₂ O ₃ film. Therefore, the inclusion of Al is essential. Further, it is desirable that the metal substrate has a small thickness, for example, an ultrathin film of about several tens of μm, but if it is made extremely thin, Al will be depleted and abnormal oxidation will occur. Therefore, it is necessary to further increase the Al. To be done.

[Problems to be solved by the invention]

In order to meet the above requirements, it is necessary to increase the Al content of Fe-Cr-Al steel. It is difficult to manufacture high Al content steel. 3% Al in terms of manufacturability and characteristics in conventional applications
Was mainly used, but as described above, with the increasing demand for oxidation resistance, it became necessary to contain Al at a 3% Al level or higher. However, such high Al level Fe-
Since the toughness of the hot rolled steel strip of Cr-Al steel is extremely lowered, the hot rolled steel strip cannot be produced by a normal production line.

In other words, Fe-Cr-Al steel inherently has problems related to manufacturability such as 475 ° C brittleness that is more likely to occur than before, slab cracking, and poor toughness of hot-rolled sheets. Is further enhanced. In particular, cracks occur during the rewinding work of hot-rolled steel strips, causing major problems, and this tendency becomes even more pronounced in the winter.

In addition, when an appropriate amount of Ti is contained in Fe-Cr-Al steel, Fe-Cr
-The peeling of the oxide film formed on the surface of -Al steel can be prevented, and the cast structure can be refined, so that a steel suitable for the above-mentioned applications can be obtained. However, the presence of this Ti further increases the 475 ° C brittleness. There is the problem of increasing it.

The present invention is intended to solve such a problem, and it is possible to manufacture a hot-rolled steel strip of a Fe-Cr-Al steel containing Ti and having a high Al content without a large decrease in toughness. The purpose is to establish technology.

[Means for solving problems]

The present invention is C; 0.03% or less, Si; 1.5% or less, Mn; 0.5% or less, C
r; 13 to 26%, S; 0.005% or less, Al; 3 to 7%, N; 0.03% or less, T
i: 0.05% to 0.4% and excess [Ti] represented by the following formula Is 0.2% or less, and in some cases, REM, Ca and Y
0.1% or less of 1 type or 2 or more types, and the balance is Fe
When slabs of ferritic stainless steel consisting of and unavoidable impurities are hot-rolled, the slabs are heated to 1150 ℃.
The hot-rolling finishing temperature Tf (° C) in hot rolling after rough rolling and finish rolling after heating in the range of ~ 1300 ° C satisfies the following formula according to the Al content and [Ti] content in the steel. Tf ≧ 720 ℃ + 80 ・ (Al-3) +300 ・ [Ti] High Al content ferritic stainless steel hot rolled steel with excellent high temperature oxidation resistance characterized by rapid water cooling after winding It is intended to provide a manufacturing method for improving the toughness of a strip.

According to the present invention, the toughness of the hot rolled steel strip of ferritic stainless steel containing high Al content of preferably 3.5% or more, more preferably 4% or more and containing Ti and excellent in high temperature oxidation resistance is reduced. It can be manufactured by a normal steel strip manufacturing line without performing.

The contents of the present invention will be specifically described below.

[Detailed Description of the Invention]

The present invention improves the toughness of a hot rolled steel strip of a ferritic stainless steel having excellent high temperature oxidation resistance in terms of both alloy content and hot rolling conditions.

Since the heat resistance of the Fe-Cr-Al alloy is maintained mainly by the Al ₂ O ₃ film formed on the surface as described above, the oxidation resistance of this alloy depends largely on the Al content. For example, when the plate thickness is about 2 mm, the Fe-18Cr-3Al alloy has sufficient oxidation resistance up to 1200 ° C. However, as the sheet thickness decreases, the oxidation resistance rapidly deteriorates. For example, at 50 μm, abnormal oxidation occurs at 1050 ° C for 100 hours continuously.

Fig. 1 shows C ≦ 0.015%, Si ≒ 0.4%, Mn ≒ 0.4%, P ≒ 0.02%,
S ≦ 0.002%, N ≦ 0.015%, Cr; 13-23%, Al; 2-6%, Ti; 0.
Fe-Cr-Al alloy containing 1 to 0.5% of thickness 50 μm 11
The increase in oxidation at 00 ° C is organized by the amount of Cr and the amount of Al.
The black circles in the figure indicate that abnormal oxidation occurred. The white circles indicate a healthy state, and the numbers in the figure indicate the increase in oxidation.
It can be seen that the boundary line indicating the occurrence of abnormal oxidation mainly depends on the Al content and then on the Cr content. The alternate long and short dash line in FIG. 1 is 1150.
The dotted line at ° C indicates the boundary where abnormal oxidation occurs at 1000 ° C.

It can be seen from Fig. 1 that at a thickness of 50 μm, 1100
This means that it is necessary to contain more than 3% of Al, preferably 4% or more of Al in order to obtain sufficient oxidation resistance at a temperature of not less than ° C. On the other hand, from the results of the surface observation after the oxidation test, it was found that when Al alone was added, the scale peeled off from the surface during the cooling process from the temperature. That is, the peel resistance of the scale is inferior when only Al is added. In order to prevent the exfoliation of the scale, it is effective to add a small amount of Ti and further add a small amount of REM, Ca and Y as described later.

On the other hand, manufacturing tests of high Al and high Cr steels were conducted. Table 1 shows the chemical composition values of the steels used in the study. After 200 kg of these steels were melted in vacuum, forging was performed to 50 mm ^t × 200 mm ^w × L, and after hot scratch removal, hot rolling was performed by extracting at 1150 ° C. After hot rolling, rapid water cooling was performed to prevent brittleness at 475 ° C.
The finished plate thickness is 2.9 mm. The obtained hot-rolled steel strip was rewound in a coil build-up line for rewinding, but it broke brittlely in the leveler roller. This phenomenon was especially remarkable in high Al steels.

One of the inferior toughnesses of the hot rolled steel strip in the above-mentioned manufacturability test is 475
It is thought that this is due to brittleness at 0 ° C, and to prevent this, rapid water cooling was performed after winding, but it was still unsuccessful to secure the toughness of the high Al steel. As a result of various studies on the cause of this, as to the toughness inferiority of the hot-rolled steel strip, with respect to 475 ° C brittleness, (1) In the case of the same Cr content, Al promotes 475 ° C brittleness, (2) It was found that the 475 ° C brittleness is remarkably promoted by an increase in the amount of [Ti]. Furthermore, regarding the toughness of the hot-rolled steel strip, (3) the lower the hot-rolling finishing temperature, the higher the toughness of the hot-rolled steel strip. It turns out that there is a tendency for it to become lower.

The hot rolling finish temperature tends to be low in general ferrite single-phase system to prevent ridging, but for this system steel, if the hot rolling method with a lower hot rolling finish temperature is adopted as in the conventional method,
We obtained new knowledge that the toughness cannot be secured. Therefore, based on this finding, in order to improve the toughness of hot-rolled hot-rolled steel strips, systematically investigated the relationship between hot-rolling finish and bending properties of each steel using slab pieces of the above-mentioned steel types. That is, 50mm ^t × 100mm ^w
8 pass after heating and holding a small piece of × 100mm ^L at 1200 ℃ ～ 1250 ℃
The hot rolled plate was 3.2 mmt. To adjust the finishing temperature, delay rolling was performed on the low temperature side and heat retention was performed on the high temperature side. The toughness of the hot rolled sheet was evaluated by a bending test. This is to reproduce the bending operation which is the first problem such as bending by the roll of the leveler or the roll of the looper or raising the tip of the coil. If the bending angle is about 50 °, it is possible to pass the coil, but if it is more than 90 °, it is possible to pass the coil sufficiently. Therefore, the relationship between the bending angle and the hot rolling finishing temperature was obtained and Fig. 2 was obtained.

From Fig.2, first of all, the bending properties are remarkably deteriorated by the increase of Al content and [Ti] content.
It is understood that excessive addition of Ti deteriorates the toughness of the hot-rolled sheet in the region where the Al content is high, and is not preferable. Secondly,
It can be seen that increasing the finishing temperature of hot rolling is extremely advantageous for improving the toughness.

From the knowledge of this basic fact, to obtain a sufficient bending angle, Al
Content, Ti content (specifically [Ti] content that is excess Ti content), and the relationship between hot-rolling finishing temperature satisfy the relationship of Tf ≥ 720 ° C + 80 · (Al-3) + 300 · [Ti] It was found that the hot rolling finish temperature Tf should be determined as described above.

In order to improve the impact toughness of hot-rolled sheet, it has been considered that the hot-rolling finishing temperature is lowered to develop a fibrous structure and the separation phenomenon is used to develop toughness. However, in the steel to which the present invention is applied, the toughness increases as the temperature is increased. Considering this point metallurgically, the following can be considered. Ferritic stainless steel is a dynamic recovery type alloy that hardly hardens in the preceding stage of hot rolling, but shows significant work hardening even in hot rolling at temperatures below 850 ° C. It is also clear from the change in surface hardness shown in Fig. 2).
This work hardening is generally frozen by subsequent quenching and becomes a recrystallization driving force during hot-rolled sheet annealing. However, in the high Al steel targeted by the present invention, the increase in the strength level due to the remaining work hardening causes an increase in the surface stress value in the bending process, which is harmful to the toughness. Ie high Al
When work strain due to hot rolling remains in the hot rolled steel strip, the strength level of the hot rolled steel strip increases and the toughness decreases.

Similarly, solid solution Al and excess solid solution [Ti] also increase the strength level of the hot-rolled steel strip. And this Al and [Ti]
Also promotes 475 ° C brittleness. In order to suppress this 475 ° C brittleness, it is essential to cool with water after hot rolling. However, if the hot rolling finishing temperature is low, the hot-rolled steel strip will retain the work strain during hot rolling, and as described above, the strength level will increase and the bending toughness will be rather deteriorated. Therefore, in order to prevent the deterioration of toughness, it is important to raise the hot rolling finishing temperature, that is, to make the ferritic steel dynamically recover so that the working strain in hot rolling does not remain as much as possible. Rapid water cooling is used to prevent brittleness. By thus preventing work hardening in hot rolling and preventing brittleness at 475 ° C., it is possible to reduce the surface stress generated during bending in the next step.

As described above, in the present invention, in relation to the amount of Al and the excess [Ti], the hot rolling finishing temperature should satisfy the relationship of Tf ≧ 720 ° C. + 80 · (Al-3) + 300 · [Ti] It was newly found that hot rolling is a basic condition for achieving the above-mentioned purpose.

Next, an outline of the reason why the content of alloying elements in the present invention is limited as described in the claims will be described.

The effect of C and N on the oxidation resistance of C and N: Fe-Cr-Al steel is that abnormal oxidation occurs when the C and N contents increase. Further, C and N deteriorate the toughness of the steel sheet and promote cracking during cold rolling. To prevent this, Ti
Is effective, but the larger the amount of C and N, the more
Ti is required, which causes adverse effects due to the addition of a large amount of Ti. For this reason, the upper limits of the C and N contents are 0.03%.

Si: Si is effective in improving high temperature oxidation resistance. However, when added in a large amount, the steel becomes remarkably hard and cold ductility and workability deteriorate. For this reason, the upper limit of Si is set to 1.5%.

Mn: Mn deteriorates the oxidation resistance of Fe-Cr-Al steel containing a large amount, so the upper limit is made 0.5% or less.

Cr: This is a basic element that maintains the oxidation resistance, but if it is less than 13%, its characteristics cannot be obtained, and if it is added in excess of 26%, the toughness deteriorates, so it is contained in the range of 13 to 26%.

S: S becomes MnS and becomes a starting point of oxidation. In particular, from the viewpoint of abnormal oxidation, 0.005% or less is desirable.

Al: Al, together with Cr, is the main element that maintains the oxidation resistance of this steel. The intended use of the present invention is in the field of thin plates or ultra-thin plates, and it is necessary to maintain higher oxidation resistance than in the past. For that purpose, Al is 3% or more, preferably 3.5%.
More preferably, it is more than 4.0%. On the other hand, 7%
If the content exceeds the range, hot working becomes substantially difficult.

Ti: Ti is an important element in the production of the steel of the present invention.
That is, Ti refines the cast structure of the steel and improves hot workability and cold workability. Ti also improves the adhesion of scale (Al ₂ O ₃ ) on the steel surface caused by high temperature oxidation,
It also improves the formability of steel. However, if it is added excessively, the toughness of the hot-rolled steel strip is rather deteriorated, which is contrary to the purpose of the present invention. Therefore, it is necessary to limit the amount of excess solid solution [Ti] excluding Ti necessary for fixing C and N. Generally, to utilize the effect of Ti, it is necessary to add at least 0.05%. On the other hand, if added over 0.4%, the workability, especially the secondary workability, deteriorates. Ti required for fixing C, C and N
However, if the excess [Ti] excluding 0.2% exceeds 0.2%, the toughness of the hot-rolled steel strip deteriorates significantly, so the Ti content is 0.05 to 0.4%.
And the excess amount of solid solution represented by the following formula [T
i] should be 0.2% or less.

The present invention is intended to provide a steel plate excellent in high temperature oxidation resistance having the required characteristics as a chimney material for a stove or a metal substrate material for a catalyst carrier such as an automobile exhaust gas purifying device. However, in order to prepare for more severe conditions, it is effective to add one or more of REM, Ca, and Y. These increase the abnormal oxidation resistance through improving the adhesion of the scale. However, excessive addition causes blistering due to so-called inclusions, which is a major defect in thin plates. Therefore, the upper limit of the content of these elements should be 0.1%.

When manufacturing a hot-rolled steel strip of Fe-Cr-Al having the above chemical composition values, the reason why the starting temperature of rough rolling (slab heating temperature) is 1150 ° C or higher in the method of the present invention is It is impossible to secure the hot rolling finish temperature and, in the case of steels with excellent oxidation resistance such as the steel of the present invention, when hot rolling at a low temperature, metal adhesion occurs on the rolls and a large amount of linear scratches occur. Because. On the other hand, the slab heating (extraction) temperature was set to 1300 ° C or lower because if the temperature exceeds 1300 ° C, the crystal grains become coarse and recrystallization in the next step becomes difficult.

The lower limit of the hot rolling finish temperature was set according to the amount of Al and the excess [Ti], because the toughness of the hot rolled steel strip deteriorates as the amount of Al and [Ti] increases, as already explained. .
That is, increasing the Al content and [Ti] content promotes 475 ° C embrittlement, and the strength level of the steel increases with solid solution Al and Ti.
As a result, the toughness deteriorates and the toughness deteriorates, and strengthening due to strain accumulation in hot rolling further promotes brittleness.
The finish rolling temperature (T
This is because it is necessary to limit the lower limit of f).

Sufficient toughness is secured in the hot-rolled steel strip obtained by limiting Tf at the temperature defined by the following equation, and the Ti-containing high Al Fe-Cr-Al according to the present invention can be manufactured as a hot-rolled steel strip. it can.

Tf ≧ 720 ° C. + 80 · (Al-3) + 300 · [Ti] Examples of the present invention will be shown below.

〔Example〕

Thickness of the test steels having the chemical composition values shown in Table 2 and the Tf values calculated from the Al content and excess [Ti] content of each steel
Manufacture 200mm slabs, perform rough rolling and finish rolling at the slab heating temperature (slab extraction temperature) shown in Table 3,
The finishing temperature and the winding conditions were set under the conditions shown in Table 3 to produce a 3.0 mm hot rolled coil.

The toughness of the hot-rolled steel strip obtained (90 ° bending test result), and 1100 ° C × 100h of a 50 μm thin steel sheet manufactured from the hot-rolled steel strip.
The results of the continuous oxidation test of r for the atmosphere are also shown in Table 3.

From the results in Table 3, it can be seen that according to the present invention, the toughness of the hot rolled steel strip of high Al steel is good, and the high temperature oxidation resistance of these 50 μm thin sheets is very good.

On the other hand, in Comparative Example (b) where the finishing temperature is lower than the Tf value, cracking occurred in the hot-rolled sheet and ultra-thin steel sheets could not be manufactured, and as in Comparative Example (d), the slab heating temperature and finishing temperature were Even if the conditions of the present invention are satisfied, if hot water cooling is not carried out during winding, cracks will occur in the hot rolled sheet, and it will be impossible to manufacture an ultra thin steel sheet in the same manner, and as in Comparative Example (f). When the slab heating temperature is higher than the range of the present invention, the toughness of the hot-rolled sheet is inferior and ridging of the cold-rolled sheet occurs even when the finishing temperature and the rapid water cooling winding are performed. Furthermore, in No. 5 steel, the excess [Ti] is large and cracks occur in the hot-rolled sheet, making it impossible to manufacture ultra-thin steel sheets.
i] is 5 for steels with a low Al content even within the scope of the present invention.
With a 0 μm plate, abnormal oxidation occurs and the object of the present invention cannot be achieved.

[Brief description of drawings]

Fig. 1 shows a continuous oxidation test in the air for 50-μm thin steel sheet of Fe-Cr-Al steel using an Elema electric furnace.
200hr, 1100 ℃ × 100hr and 1150 ℃ × 100hr) is a diagram showing the regions where abnormal oxidation occurs when arranged by Al amount and Cr amount. Fig. 2 shows the three types of Fe-Cr-Al It is a figure which shows the relationship between the hot rolling finish temperature of steel, the surface hardness of a hot rolled sheet, and the bending angle (bending test result of tip R = t (sheet thickness)) in a bending test.

Claims (4)

[Claims] 1. C: 0.03% or less, Si; 1.5% or less, Mn; 0.5% or less, Cr; 13 to 26%, S; 0.005% or less, Al; 3 to 7%, N; 0.03% or less, Ti 0.05% to 0.4% and the excess [T
i], Of 0.2% or less and the balance of Fe and unavoidable impurities during hot rolling of a slab of ferritic stainless steel, after heating the slab in the range of 1150 ℃ to 1300 ℃,
The hot-rolling finishing temperature Tf (° C) in hot rolling for rough rolling and finish rolling is performed under the conditions that satisfy the following formula according to the amount of Al and [Ti] contained in the steel: Tf ≥ 720 ° C + 80 -(Al-3) + 300- [Ti] A manufacturing method for improving the toughness of a high Al content ferritic stainless steel hot-rolled steel strip excellent in high temperature oxidation resistance, characterized by rapid water cooling after winding. 2. The manufacturing method according to claim 1, wherein the Al content in the steel is 3.5% to 7%. 3. C: 0.03% or less, Si; 1.5% or less, Mn; 0.5% or less, Cr; 13 to 26%, S; 0.005% or less, Al; 3 to 7%, N; 0.03% or less, Ti 0.05% to 0.4% and the excess [T
i], Is 0.2% or less, and one or more of REM, Ca, and Y is 0.1% or less, and the balance is 1150 when hot-rolling a slab of ferritic stainless steel containing Fe and unavoidable impurities. The hot rolling finish temperature Tf (° C) in hot rolling after rough rolling and finish rolling after heating in the range of 1 ℃ to 1300 ℃ is determined by the amount of Al contained in the steel and [Ti].
Depending on the amount, Tf ≥ 720 ℃ + 80 · (Al-3) + 300 · [Ti] High Al with excellent high temperature oxidation resistance, characterized by rapid water cooling after winding. A manufacturing method for improving the toughness of a ferritic stainless steel containing hot rolled steel strip. 4. The manufacturing method according to claim 3, wherein the Al content in the steel is 3.5% to 7%.