JPH079036B2 - Method for producing ferritic stainless hot rolled steel sheet with excellent heat resistance, workability and weldability - Google Patents

Description

The present invention relates to a method for producing a hot rolled steel sheet having excellent heat resistance, workability and weldability, which is suitable for producing parts such as an industrial furnace and a combustion device. Regarding

[Conventional technology]

Stainless steel is often used in parts used at high temperatures. This is because Cr contained in stainless steel forms a dense protective oxide film on the surface at high temperature and exhibits excellent oxidation resistance. In addition, since the addition of Si further improves the oxidation resistance, stainless steel containing Si is widely used for heat resistance applications. Furthermore, the ferritic stainless steel added with Si is an atmosphere with low oxygen and high water content,
In other words, since so-called abnormal oxidation that produces reddish-brown iron-based oxides in the combustion atmosphere is unlikely to occur, it is attracting attention as a material for combustion devices or appliances. However, when Si is added to ferritic stainless steel, the toughness at room temperature is reduced, and brittle cracking is likely to occur during bending or welding. Since this is remarkable in hot-rolled medium-thickness steel plates with a large thickness, heat-resistant ferritic stainless steel containing Si is used in cold-rolled thin steel sheets, but the hot-rolled medium-strength steel sheet is bent and welded due to insufficient ductility. At present, it cannot be used as a heat-resistant structural material that requires Therefore, austenitic heat-resisting steel has been used for the parts that require plate thickness. However, austenitic heat-resisting steels are superior in ductility or high temperature strength to ferritic materials, but they tend to deform due to thermal expansion due to their high coefficient of thermal expansion. Therefore, there has been a demand for a hot-rolled steel sheet that is ferritic and has excellent heat resistance, workability, and weldability.

Japanese Patent Publication No. 57-2267 discloses that C or N in steel is reduced as a method of improving the workability of ferritic stainless steel with improved heat resistance by adding Si. It is also possible to add Ti, Nb or the like which fixes C and N, in Japanese Patent Laid-Open Nos. 55-161049 and 57-164967,
It is disclosed in JP-A-57-164968 and JP-B-62-10. The purpose of all of these techniques is to suppress the precipitation of carbides and nitrides, which are the starting points of cracks during processing and welding, and are effective in improving the ductility of the cold-rolled thin sheet and improving the workability. However, by reducing C and N, ferrite is stabilized, austenite does not precipitate at high temperature, and a ferrite single phase structure is formed. For this reason, ferrite grains are coarsened during heating before hot rolling, and since the ferrite is in a single phase during hot rolling, strain introduced during hot rolling is not accumulated and recrystallization does not proceed. Therefore, the structure after hot rolling becomes coarse spread grains elongated in the rolling direction. Due to this coarsening of the hot rolled structure, the toughness of the hot rolled sheet is not sufficiently improved, roping occurs during bending, brittle fracture occurs, and cold cracking occurs from the weld metal part to the weld heat affected zone, The hot rolled steel sheet could not be used as a heat resistant structural member.

[Problems to be Solved by the Invention]

It is an object of the present invention to improve the workability and weldability of an Si hot-rolled hot rolled sheet in order to produce a ferritic stainless hot rolled sheet that can be used as a heat resistant structural member.

[Means for Solving the Problems]

The structure of the present invention will be described below. In order to achieve the above object, the present invention focused on the refinement of the microstructure of the hot-rolled steel sheet to which Si was added, and pursued the optimum components and manufacturing method therefor. As a result, the steel composition is
0.05 to 0.10%, Si1 to 3%, Cr12 to 18%, Al0.05 to 0.20
%, And the heating temperature before hot rolling is 1100-1250 ℃.
We found a hot rolling method characterized by rolling at a cumulative reduction of 70% or more in a temperature range of 1050 ° C or higher, and succeeded in making the hot-rolled sheet into fine particles. Also, the annealing temperature after hot rolling is set to 850 to 1000.
It has been found that the toughness of the hot-rolled sheet is further improved and the hot-rolled intermediate sheet excellent in workability and weldability can be produced by rapidly cooling the hot-rolled sheet to 0 ° C and then rapidly cooling it at 0.5 ° C / sec or more.

[Action]

The reason for defining the range of each component and the details of the production method will be further described below.

C is the most important element for refining the structure of the hot rolled sheet. By adding more than 0.05% of this element, 1050
The austenite phase necessary for fine graining can be precipitated at ℃ ~ 1250 ℃. If it is less than 0.05%, the austenite phase does not precipitate, or even if it does precipitate, it is not an amount sufficient to fine-grain the structure of the hot-rolled sheet. C in conventional technology
Was in the direction of reducing as much as possible a harmful element that forms a carbide that deteriorates the toughness of the material, but C, which was conventionally considered only as a harmful element in the present invention, is the most effective in securing an austenite phase that precipitates at high temperatures. I'm sure it's an element. However, if the amount of C added exceeds 0.10%, the carbides that precipitate will increase the hardness at room temperature, making it difficult to process, and excess C will have no effect.

Si is an element for maintaining oxidation resistance at high temperatures, and a content of 1% or more is necessary to have a sufficient effect. However, Si is an element that embrittles the ferrite phase, so it is desirable to avoid excessive addition. Even if the structure of the hot-rolled sheet is made finer by the manufacturing method described in the present invention, 3
If added in excess of 10%, embrittlement becomes significant, so the upper limit is 3
%.

Cr is also an essential element for maintaining the oxidation resistance at high temperature, and the content of 12% or more is necessary to have sufficient effect. However, since Cr is an element that stabilizes the ferrite phase, if the content is too large, precipitation of the austenite phase that refines the structure does not occur, or it is not possible to secure a necessary and sufficient precipitation amount. Therefore, the range of the Cr content is set to 12 to 18%.

Al is an important additional element necessary for performing annealing after hot rolling at 850 ° C or higher. 850-100 when Al is not added
An austenite phase precipitates during annealing of a hot-rolled sheet at 0 ° C, and the austenite phase precipitated by quenching after annealing transforms to martensite. This martensite significantly embrittles the hot-rolled sheet, making bending extremely difficult. In order to carry out annealing after hot rolling, it is necessary to raise the ferrite-austenite transformation point to the annealing temperature or higher, and for that purpose, the amount of Al added is required to be 0.05% or more. However, if added over 0.20%, the transformation point will rise further, the austenite phase will not precipitate during hot rolling, or even if it precipitates it will not be possible to secure the amount necessary for grain refinement, so the addition amount of Al is 0.05 ~
It was 0.20%.

Next, the manufacturing method will be described.

First, the heating temperature before hot rolling is set to 1100 to 1250 ° C at which the austenite phase is sufficiently precipitated. The precipitated austenite phase suppresses the growth of ferrite grains and prevents the structure from becoming coarse. However, when heated above 1250 ° C, the austenite phase that precipitates on the contrary decreases, the ferrite phase rapidly grows, and the structure becomes coarse. If the heating temperature is less than 1100 ° C, it is difficult to secure a cumulative rolling reduction of 70% or more by 1050 ° C. In order to perform rolling in a temperature range where the amount of austenite is large and to suppress the growth of ferrite grains as much as possible, it is desirable that the heating temperature before hot rolling is 1150 to 1220 ° C.

Further, in order to recrystallize the ferrite grains during rolling, a sufficient amount of austenite phase and a reduction amount are necessary. Since the austenite phase is harder at a higher temperature than the ferrite phase, strain is concentrated on the ferrite phase during rolling to promote recrystallization. In order to perform sufficient recrystallization, a rolling reduction of 70% or more is required at a rolling temperature of 1050 ° C or more. 1050
When the temperature falls below ℃, the austenite phase is rapidly reduced, and the effect of promoting recrystallization by the austenite phase is lost. Therefore, reduction below 1050 ° C has no effect on refining ferrite grains.

Annealing after hot rolling can soften the material of the hot rolled sheet and further improve the toughness by subsequent rapid cooling. As mentioned above, Al
By adding 0.05 to 0.20% and increasing the transformation point from ferrite to austenite, it becomes possible to anneal while suppressing precipitation of the austenite phase in the temperature range of 850 to 1000 ° C, and the toughness improvement effect by subsequent quenching is improved. Remarkable. 850
If the annealing temperature is lower than ℃, the toughness improving effect due to subsequent rapid cooling is small, and if the temperature exceeds 1000 ° C., the austenite phase precipitates, the rapid transformation causes the austenite phase to transform into martensite, and the toughness significantly deteriorates. 100
Set to 0 ° C. Further, the cooling rate after annealing is required to be 0.5 ° C./sec or more, and at a cooling rate lower than this, the toughness is decreased.
This is due to the fact that solid solution C and N are precipitated as carbides or nitrides during annealing. Furthermore, the annealing temperature is 90
Since the annealing time can be shortened at 0 ° C. or higher, a continuous annealing apparatus can be used, and further rapid cooling after annealing is easy, and therefore, it is desirable to use the continuous annealing method in actual production.

When both the composition range according to the present invention and the hot rolling or annealing method are satisfied, a hot rolled steel sheet having excellent workability and weldability can be produced. FIG. 1 schematically shows the features of the present invention. That is, Si1 to 3 according to the present invention
%, Cr12-18% steel, it is necessary to precipitate a sufficient amount of austenite phase in order to refine the hot rolled structure by the hot rolling method according to the present invention.
Must be added at 0.05% or more. Further, it is necessary to add Al in an amount of 0.05% or more in order to suppress the precipitation of an austenite phase at 1000 ° C or lower and enable annealing.
Next, in the hot rolling, the heating temperature must be 1100 to 1250 ° C. within the component range according to the present invention, and a cumulative rolling reduction of 70% or more must be secured at 1050 ° C. or more where the austenite phase is sufficiently precipitated. Further, the toughness of the hot-rolled sheet can be further improved by performing the annealing at 850 to 1000 ° C. at which the austenite phase does not precipitate and then cooling at preferably 0.5 ° C./sec or more.

〔Example〕

 Examples of the present invention will be described below.

Table 1 shows the chemical composition (% by weight) of the test materials. The steel ingots A, B, C, D, F, and G shown in Table 1 are small steel ingots melted in the laboratory. The steel ingots E and H are steel ingots produced by the actual converter and continuous casting used in production.

Example-1 Using the ingots A, B, C, D, F, and G shown in Table 1,
The heating temperature before hot rolling is 1130 ℃, 1150 ℃, 1200 ℃, 1250 ℃.
℃, 1300 ℃, and rolling was performed by controlling the cumulative rolling reduction above 1050 ℃ to 60%, 70%, 80%, and finally hot rolled to a thickness of 7 mm. A 5 mm thick sub-size Charpy test piece was cut out from a hot-rolled sheet and the impact value was 4 kgm / cm ²
The toughness was evaluated at a temperature of over 10. The results are shown in Table 2. From this table, in order to obtain a hot-rolled sheet with good toughness exceeding 4 kgm / cm ² at room temperature or less, it is necessary to set the heating temperature to 1250 ° C or less and to secure the cumulative rolling reduction of 70% or more up to 1050 ° C or more. I find it necessary. Next, No. 5 and No. 8 to 12 hot-rolled sheets were heated at 800 to 1050 ° C. for 10 minutes and air-cooled. The average cooling rate at this time was 5 ° C./sec. 5mm from this annealed plate
A thick subsize Charpy test piece, a 6 mm thick bending test piece and a welding test piece were cut out and evaluated. Toughness is impact value 4k
At a temperature exceeding gm / cm ² , bending workability was evaluated by performing 90 ° bending with a radius of 12 mm at room temperature and checking for cracks, and weldability by conducting a constraint welding test at room temperature and checking for cracks. .
A schematic diagram of the restraint welding test is shown in FIG. 2 and evaluation results are shown in Table 3. From this evaluation result, precipitation of austenite phase
If it exceeds 1000 ° C, the toughness is remarkably lowered, the workability and the weldability are deteriorated, and if the annealing temperature is less than 850 ° C, there is no difference from the toughness of the hot rolled sheet, and the toughness improving effect is not seen. Furthermore, using No. 5 and No. 10 hot rolled sheets, after annealing at 930 ° C. for 10 minutes,
After cooling at 0.1 to 10 ° C / sec, a Charpy test piece having a thickness of 5 mm was cut out, and toughness was evaluated at a temperature exceeding an impact value of 4 kgm / cm ² . The results are shown in Table 4. From this result, it is understood that the cooling rate of 0.5 ° C./sec or more is required to improve the toughness.

Example-2 E and H steel ingots shown in Table 1 were rolled by a hot rolling mill in a factory. The heating temperature at this time is 1180 ℃, 1050 ℃
A 91% cumulative reduction was performed by the above rough rolling, and finally by final hot rolling to a thickness of 6 mm, which was wound into a coil at 850 ° C. The rolling reduction per pass at this time was 5 to 30%. Next, this hot rolled coil was annealed by a continuous annealing device. The heating temperature at this time is 20 ° C / se after holding for 2 minutes at 930 ° C.
Cooled in c. A 5 mm thick sub-size Charpy test piece was cut out from this coil, and toughness was evaluated at a temperature exceeding an impact value of 4 kgm / cm ² . Bendability is a 6 mm thick test piece with a radius of 12 m
Bending at 90 ° at m was performed at room temperature, and the presence or absence of cracking was evaluated. The weldability was evaluated by using a 6 mm thick test piece and performing a restraint welding test at room temperature to determine whether cracking occurred. The results are shown in Table 5. From these results, steel in the composition range of the present invention, when manufactured by the method of the present invention, heat resistance, workability,
It can be seen that a hot rolled steel sheet with excellent weldability can be produced.

[Effects of the Invention] As described above, the present invention limits the components of the ferritic stainless steel hot rolled steel sheet to a certain range, defines the heating conditions in hot rolling, the hot rolling conditions, and, if necessary, after hot rolling. The heat treatment provides a material which is excellent not only in heat resistance but also in workability and weldability, which have been problems in the past.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a manufacturing method according to the present invention, and FIG.
The figure is a diagram schematically showing the welding conditions and the restraint state of the restraint welding test.

Front page continuation (72) Inventor Toshiyuki Suehiro 3434 Shimada, Hikari City, Yamaguchi Pref., Nippon Steel Co., Ltd., Komatsu Ltd. (56) Reference JP-A-63-162818 (JP, A) JP-B 57- 2267 (JP, B2) JP-B-58-19725 (JP, B2)

Claims (3)

[Claims] 1. In weight percent, C: 0.05-0.10%, Si: 1
~ 3%, Cr: 12-18%, Al: 0.05-0.20%, the balance Fe and unavoidable impurities ferritic stainless steel is heated to a temperature of 1100-1250 ℃ before hot rolling to 1050 ℃.
A method for producing a ferritic stainless steel hot rolled steel sheet having excellent heat resistance, workability and weldability, which comprises rolling at a cumulative reduction of 70% or more in the above temperature range. 2. In weight percent, C: 0.05-0.10%, Si: 1
~ 3%, Cr: 12-18%, Al: 0.05-0.20%, the balance Fe and unavoidable impurities ferritic stainless steel is heated to a temperature of 1100-1250 ℃ before hot rolling to 1050 ℃.
Excellent heat resistance, workability, and weldability, which are characterized by rolling 70% or more with a cumulative reduction in the above temperature range, heating the hot-rolled sheet thus obtained to 850 to 1000 ° C, and then rapidly cooling. Method for producing ferritic stainless hot rolled steel sheet. 3. The cooling rate after heating the hot-rolled sheet to 850 to 1000 ° C. is set to 0.5 ° C./sec or more.
A method for producing a ferritic stainless hot rolled steel sheet having excellent heat resistance, workability and weldability as described.