JP2549018B2 - Method for producing hot rolled steel strip of ferritic stainless steel with excellent heat resistance and corrosion resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel hot rolled steel strip having excellent heat resistance or corrosion resistance.

[0002]

2. Description of the Related Art In recent years, attention has been paid to ferritic stainless steel containing Al or Si as a heat resistant material and ferritic stainless steel containing Mo as a corrosion resistant material. Such ferritic stainless steel is
It has become clear that it has much better oxidation resistance than austenitic stainless steel in heat resistance, and exhibits strong resistance to stress corrosion cracking which is a problem in austenitic stainless steel in corrosion resistance. is there. These materials are used for exhaust gas parts for automobiles, stove parts, furnace walls of heating furnaces, etc., but in recent years, further heat resistance or corrosion resistance has been required due to the severer usage environment. The content of Si or Mo is increasing.

However, since the hot rolled steel strip of ferritic stainless steel containing Al, Si and Mo has remarkably low toughness, cracking occurs when the coil of the steel strip is rewound at room temperature or further cold rolled. In some cases, cold rolling may not be possible due to breakage, plate breakage, or the like. To avoid this, it is sufficient to heat the steel strip above the transition temperature and pass it through the strip, but this causes cost increase due to heating, and if the heating temperature is high, workability and efficiency are poor and safety is concerned. Is also not preferable. Also, bend and cut hot-rolled steel strip products,
There is a problem of cracking when performing punching or other processing.

Therefore, a method for producing a high heat-resistant corrosion-resistant ferritic stainless steel hot-rolled steel strip having excellent toughness, which does not cause cracks in the subsequent manufacturing steps such as unwinding or cold rolling of the hot-rolled steel strip coil or product processing. Was strongly requested. As a conventional technique for preventing the embrittlement phenomenon of the ferritic stainless steel hot-rolled steel strip containing Al, Si, Mo as described above,
For example, as disclosed in JP-A-60-22816, there is a method of reducing C and N, quenching after hot rolling at a cooling rate of 10 ° C./sec or more, and winding at a low temperature of 450 ° C. or less. . However, with ferritic stainless steel containing more than 5.0% of one or more elements selected from Al, Si and Mo, the toughness is not sufficiently improved even by this method.

[0005]

The present invention is based on Al, S
By improving the toughness of a high heat-resistant, corrosion-resistant ferritic stainless steel hot-rolled steel strip containing i, Mo, the workability is improved and the workability of the product is further improved.

[0006]

The present invention has been completed as a result of examining the components, hot rolling conditions, cold rolling conditions and winding conditions for this purpose, and its gist is as follows. Is. Pressure of 1 Al, Si, 1 kind of element selected from Mo or material component containing two or more low-carbon low nitrogen ferritic stainless steel billet, recrystallization temperature below the recovery temperature region in hot rolling The sum of the rates is 2
Ferritic stainless steel hot rolled steel strip excellent in heat resistance and corrosion resistance, characterized by performing 0% or more and 60% or less strong reduction rolling to finish hot rolling, followed by rapid cooling and low temperature winding. Manufacturing method.

At 2% by weight, C: 0.03% or less,
N: 0.03% or less, C (%) + N (%): 0.03%
Hereinafter, Cr: 10 to 40%, and one or more elements selected from Al, Si and Mo in total of more than 5.0 to 15.
The ferritic stainless steel slab containing 0% of the material component is hot-rolled by performing strong reduction rolling with a total reduction ratio R of 20% or more in a recovery temperature region of the recrystallization temperature T _S or less represented by the first formula. A method for producing a hot rolled steel strip of ferritic stainless steel excellent in heat resistance and corrosion resistance, characterized by finishing rolling, cooling at a cooling rate of 10 ° C./sec or more, and then winding at 500 ° C. or less. .

[0008] T _S [℃] = 700 + 20 (Al + Si + Mo) + 4R ... first equation Al, Si, Mo: weight of the material component% R: rolling reduction of total% 3 material component of the recovery temperature range, further rare-earth element (REM 2) is contained in a total amount of 0.01 to 0.50%.
A method for producing a ferritic stainless steel hot-rolled steel strip having excellent heat resistance and corrosion resistance as described.

4 The material components are Ti, Nb, V,
One or two of the elements selected from Zr, Ta, Hf and B
0.005 to 0.50% in total, and the sum total R of the rolling reductions in the recovery temperature range is the limit rolling reduction R shown by the second formula.
_The method for producing a ferritic stainless steel hot-rolled steel strip excellent in heat resistance and corrosion resistance according to the above item 2, characterized in that strong reduction rolling of _S or more is performed and hot rolling is finished.

R _S [%] = 20− (5Ti + 10Nb + 2V + 2Zr + 2Ta + 2Hf + 10B) Second formula Ti, Nb, V, Zr, Ta, Hf, B:% by weight of material component 5 The material component further contains a rare earth element (REM). The above-mentioned item 4, characterized by containing 0.01 to 0.50% in total.
A method for producing a ferritic stainless steel hot-rolled steel strip having excellent heat resistance and corrosion resistance as described.

[0011]

The reason for limiting the present invention will be described in detail below.
Incidentally, the steel slab in the present invention includes a continuously cast slab, a steel slab obtained by breaking down the slab, an ingot slab, and a slab subjected to slab rolling. Since the ferritic stainless steel of the present invention has a composition that forms a ferrite single phase in the hot rolling temperature range, martensitic transformation does not occur due to cooling after completion of hot rolling. Although there is no martensite structure that serves as a nucleus for microcracking, which is one of the causes of deterioration of toughness, it is not possible to obtain sufficient toughness in ferritic stainless steel containing Al, Si, Mo only by this effect. .

The inventors of the present invention have studied the effects of components, hot rolling conditions, cooling conditions and winding conditions on the toughness of hot-rolled steel strips, and as a result, have carried out strong reduction rolling in the recovery temperature range below the recrystallization temperature. It was found that the toughness is remarkably improved by performing hot rolling to finish hot rolling, followed by rapid cooling and low temperature winding. In the final stage of hot rolling, the recrystallization temperature T
By performing in a recovery temperature range of _S [° C] or less, the dislocations introduced in the path form sub-grain boundaries as an energy-stable rearrangement structure, and the hot-rolled structure forms sub-grains in the crystal grains. It is believed to have. Therefore, Al,
In order to improve the toughness of a ferritic stainless steel hot-rolled steel strip containing one or more of Si and Mo, stress concentration due to dislocation accumulation at grain boundaries is relaxed by a fine subgrain structure. Is effective.

Any one of Al, Si and Mo or 2
In the hot-rolled ferritic stainless steel strip containing more than 5.0% of seeds or more, in order to introduce dislocations necessary for forming such a fine subgrain structure, the above-mentioned T _S or less in hot rolling is required. It is necessary to set the total sum R of the rolling reductions in the recovery temperature range of 20% or more. On the other hand, when R exceeds 60%, dislocations introduced remain in the subgrains to promote stress concentration in the subgrain boundaries, so R is preferably 6 or less.
0% or less is good.

Further, the recrystallization temperature T_SFor [℃]
Al, Si, Mo total content [wt%] and recovery temperature
The results of a detailed examination of the effect of the total R [%] of the rolling reduction in the region
Fruit, T _SHas been found to be given by the first equation below. T_S[° C.] = 700 + 20 (Al + Si + Mo) + 4R ... 1st formula Furthermore, Ti, Nb, V, Zr, Ta, Hf, B are added.
Then, the nitride or charcoal deposited by these elements
The compound facilitates the formation of a subgrain structure,
When 0.005 to 0.50% is added in total, the above recovery
Sum of rolling reductions required in temperature range (called limit rolling reduction) R_S
Has been found to be given by the second equation below.

R _S [%] = 20− (5Ti + 10Nb + 2V + 2Zr + 2Ta + 2Hf + 10B) Formula 2 Further, the cooling rate after completion of hot rolling is rapid cooling to 500 ° C. or less at 10 ° C./sec or more. Cooling rate is 10 ℃ / s
If it is less than ec, the σ phase and the embrittlement phase such as the intermetallic compound tend to precipitate, so that the toughness of the hot rolled material is deteriorated and the fine subgrain structure after the hot rolling cannot be frozen.

The winding temperature is 500 ° C. or lower. If this is higher than 500 ° C, the cooling rate after hot rolling is 10 ° C /
Even if it is not less than sec, the toughness of the hot rolled material is deteriorated because the σ phase and the embrittlement phase such as the intermetallic compound are easily precipitated by the heat cycle during the slow cooling after winding. In the hot rolling, the strong reduction rolling performed at a temperature lower than the recrystallization temperature is not performed only in the final rolling pass, but is preferably performed in two or more passes including the final rolling pass.

Next, the reasons for limiting the components will be described.
If C and N are present in excess of 0.03%, the toughness of the hot-rolled steel strip will be reduced.
Of 0.03% or less. Cr is the most basic element that secures the heat resistance or oxidation resistance of stainless steel. In the present invention, if the content is less than 10%, these properties are not sufficiently secured, whereas if the content exceeds 40%, the toughness and ductility of the hot-rolled steel strip are remarkably reduced. Therefore, the Cr component range is set to 10 to 40%.

Al and Si are elements that improve the oxidation resistance of ferritic stainless steel, and Mo is an element that improves the corrosion resistance. In the present invention, these 3
Among the elements, one kind or two or more kinds are contained depending on desired characteristics. Withstands less than 5.0% in total. It is not sufficient to improve the chemical resistance or the corrosion resistance, and if the content exceeds 15.0%, the toughness of the hot-rolled steel strip decreases. Therefore, A
The total composition range of l, Si, and Mo exceeds 5.0 to 15.0.
%.

In claims 3 and 5, in addition to the above components, RE
It contains M (rare earth metal element). These elements are L
lanthanoids such as a, Ce, Pr and Nd,
It is added to remarkably improve the oxidation resistance, and if this effect is less than 0.01%, it is not sufficient. However, 0.5
If added in excess of 0%, the REM-based oxide will be coarsened, so that the hot workability will be significantly reduced and cracks will occur during hot rolling. Therefore, the total range of REM components is 0.01
Was set to 0.50%.

In claims 4 and 5, in addition to the components of claim 2, Ti, Nb, V, Zr, Ta, Hf and B are further contained. Ti, Nb, V, Zr, Ta, Hf, and B respectively form nitrides or carbides to reduce the solute C and N, and precipitate on dislocations introduced by large reduction during hot rolling. To refine the structure and further improve the toughness of the hot rolled steel strip. This effect is not sufficient if less than 0.005% of one kind or a total of two kinds or more is sufficient, and if it exceeds 0.50%, cold workability is significantly deteriorated. Therefore,
The component range of Ti, Nb, V, Zr, Ta, Hf, B is
The total was 0.005 to 0.50%.

When the hot-rolled steel strip is used as a hot-rolled product, the hot-rolled steel strip may be hot-rolled as it is, but may be annealed as necessary for the purpose of performing particularly strong working in the next cold working. As a preferable annealing condition, from the viewpoint of ensuring toughness, an annealing temperature of 850 to
10 at a cooling rate of 800-500 ° C at 1100 ° C
℃ / sec or more.

[0022]

Example A ferritic stainless steel shown in Table 1 was melted by a converter-AOD method or a vacuum melting method. REM
Was added with a misch metal wire immediately before casting.
These steels were manufactured according to the conditions shown in Table 2, and the plate thickness:
A hot rolled steel strip of 3.5 to 6.0 mm was used. Here, the rolling reduction of the rolling pass in the recovery temperature range is R _f ,
R _{f- 1} and R _f-2 (%) were used. Toughness evaluation subsize (thickness: 2.5 mm) in accordance with JIS standards do parallel sampled impact test and the rolling direction of V-notch Charpy test piece, the temperature at which the impact value becomes 2kgm / cm _² (vT ² : ° C). When vT ₂ is 20 ° C. or lower, cold rolling of the hot rolled steel strip is possible without heating. If the temperature exceeds 20 ° C, cold rolling without heating increases the risk of plate breakage due to impact or the like.

It can be seen that the hot-rolled steel strip produced according to the present invention has greatly improved toughness, and troubles such as sheet breakage do not occur during cold rolling.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

EFFECTS OF THE INVENTION As is apparent from the above, when a high heat resistant corrosion resistant ferritic stainless steel hot rolled steel strip containing a large amount of Al, Si and Mo is manufactured according to the present invention, the unwinding of the hot rolled steel strip is achieved. Also, it prevents cracking and plate breakage during cold rolling, and when using these steel strips as products, bending,
When cutting or punching, cracking is eliminated and workability is greatly improved.

Claims (5)

(57) [Claims] 1. An element selected from Al, Si and Mo.
Of low carbon low nitrogen ferritic stainless steel slabs of material components containing two or more species, the total reduction rate in the recovery temperature range below the recrystallization temperature in hot rolling is 20% or more.
A method for producing a ferritic stainless steel hot-rolled steel strip excellent in heat resistance and corrosion resistance, which comprises performing hot rolling at 60% or less to finish hot rolling, followed by rapid cooling and low-temperature winding. . 2. C: 0.03% or less by weight%, N:
0.03% or less, C (%) + N (%): 0.03% or less, Cr: 10 to 40%, and one or more elements selected from Al, Si and Mo in total of more than 5.0. ~ 15.0
%, The ferritic stainless steel slab containing the material component is hot-rolled by performing a strong reduction rolling with a total reduction ratio R of 20% or more in a recovery temperature region of the recrystallization temperature T _S or less represented by the first formula. Is completed, followed by cooling at a cooling rate of 10 ° C./sec or more, and then winding at 500 ° C. or less, a method for producing a hot rolled ferritic stainless steel strip having excellent heat resistance and corrosion resistance. T _S [° C.] = 700 + 20 (Al + Si + Mo) + 4R ... First formula Al, Si, Mo: weight% of material components R: total reduction rate in recovery temperature range% 3. The rare earth element (RE)
A total of 0.01 to 0.50% of M) is contained, The manufacturing method of the ferritic stainless steel hot rolled steel strip excellent in heat resistance and corrosion resistance according to claim 2. 4. The material composition further comprises Ti, Nb, V and Z.
It contains one or more elements selected from r, Ta, Hf and B in a total amount of 0.005 to 0.50%, and the sum R of the rolling reductions in the recovery temperature range is the critical rolling reduction represented by the second formula. Rate R _S
The method for producing a hot rolled steel strip of a ferritic stainless steel having excellent heat resistance and corrosion resistance according to claim 2, wherein the hot rolling is completed by performing the above-mentioned hot rolling. R _S [%] = 20− (5Ti + 10Nb + 2V + 2Zr + 2Ta + 2Hf + 10B) ... Second formula Ti, Nb, V, Zr, Ta, Hf, B:% by weight of material components 5. The material component further comprises a rare earth element (RE
A total of 0.01 to 0.50% of M) is contained, The manufacturing method of the ferritic stainless steel hot-rolled steel strip excellent in heat resistance and corrosion resistance of Claim 4.