JP2000061524A - Manufacture of ferritic stainless steel hot rolled strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing ferritic stainless steel hot rolled strip capable of manufacturing with excellent productivity a ferritic stainless steel hot strip excellent in forming-workability and uniformity in quality of material even as hot rolled or as hot rolled sheet annealed. SOLUTION: In manufacturing of a ferritic stainless steel hot rolled strip in which a ferritic stainless steel slab is subjected to rough rolling after heated and is formed into a sheet bar and then the sheet bar is subjected to finish rolling to be formed into a hot rolled steel strip, the sheet bar at a temperature To: 850 deg.C-1150 deg.C after the rough rolling is heated as high as a temperature rise amount ΔT>= (-0.1T0+125) deg.C by a sheet bar heating device located between the rough roping equipment and a finish rolling equipment before finish rolling.

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 hot rolled steel strip, and more particularly to a ferritic stainless steel which can produce a ferritic stainless hot rolled steel strip excellent in formability and material uniformity with high productivity. The present invention relates to a method for manufacturing a hot rolled steel strip.

[0002]

2. Description of the Related Art A ferritic stainless steel sheet is usually produced by heating a continuously cast slab (slab) and then hot rolling (rough rolling and finish rolling) → hot rolled sheet annealing / pickling → cold rolling (cold rolling). → Manufactured through each step of finish annealing (cold rolled sheet annealing). If some of these manufacturing steps can be omitted, equipment costs and operating costs can be significantly reduced, and the cost of the ferritic stainless steel sheet can be further reduced. However, among the above-mentioned steps, especially in the steps after cold rolling, the surface texture of the steel sheet is adjusted and the plastic strain necessary for improving the workability by promoting the recrystallization structure formation of the (111) orientation by finish annealing is performed. It also plays an important role in giving. Therefore, omitting this step generally inevitably causes deterioration of the workability of the steel sheet. This was the main factor making it difficult to omit the steps after cold rolling.

[0003]

In recent years, there have been proposed some techniques for overcoming this difficulty and enabling the omission of steps after cold rolling. For example, in Japanese Unexamined Patent Publication No. 9-287021, high purity ferritic stainless steel having a limited amount of C and N in the material is subjected to rough rolling at a temperature of 1100 to 1000 ° C and a reduction rate of 80% or more, and then at 950 ° C. A technique is disclosed in which after heat-retaining at 1050 ° C. for 5 minutes or more, finish rolling is performed to omit steps after hot-rolled sheet annealing or cold-rolling.

However, in this method, since it is necessary to reduce C and N in the raw material, the refining time in the steelmaking process becomes long, the steelmaking productivity is impaired and the cost is increased, and further, finish rolling is started after the rough rolling is finished. Since a heat retention time of 5 minutes or more is required, it is difficult to avoid deterioration of hot rolling productivity. Further, in JP-A-9-287060, a temperature of 1100 ° C.
A rough rolling is disclosed in which a rolling reduction at 1000 ° C. per pass of 30% or more and then a time between passes to the next pass is 15 seconds or more.

However, in this method, since the time between passes is provided, a temperature distribution is generated in the longitudinal direction of the material during rolling, and particularly in the portion near the leading and trailing ends of the sheet bar, heat is removed because it comes into contact with a cold roll during rolling. Is large and the temperature drops drastically. That is, since the material temperature differs in the longitudinal direction and the degree of progress of recrystallization during subsequent holding varies, the material in the longitudinal direction of the product (hot-rolled steel strip) becomes uneven.

The object of the present invention is to solve the above-mentioned problems of the prior art and to produce with high productivity a ferritic stainless hot-rolled steel strip which is excellent in formability and material uniformity even when hot-rolled or annealed. It is to provide a method for producing a ferritic stainless hot rolled steel strip that can be performed.

[0007]

Means for Solving the Problems The present invention, which was conceived to achieve the above-mentioned object, is to subject a ferritic stainless steel slab to heating, rough rolling to a sheet bar, and finish rolling the sheet bar to hot rolling. In the method for producing a ferritic stainless hot-rolled steel strip to be a steel strip, the temperature T ₀ : 85 after rough rolling
Before the finish rolling of the 0 ° C to 1150 ° C sheet bar, the sheet bar heating device provided between the roughing and finishing mills heats the sheet bar by the temperature rise amount ΔT (° C) that satisfies the following formula (1). Is a method for producing a ferritic stainless hot rolled steel strip.

Description ΔT ≧ −0.1 T ₀ +125 (1) In the present invention, the sheet bar is once wound into a coil shape between the rough rolling and finish rolling, and the winding start end of the preceding coil and the winding end of the succeeding coil are joined. When performing finish rolling continuously over multiple coils, heating of the sheet bar by the sheet bar heating device may be performed after rough rolling, before winding, after winding, before joining, or after joining and before finish rolling. Good.

The sheet bar heating device is preferably an induction heating device.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION First, reasons for limiting steel types will be described. The ferritic stainless steel which is a steel type of the present invention exhibits a single ferrite phase or a structure in which a ferrite phase partially contains an austenite phase in the slab heating temperature range before hot rolling. In the hot rolling temperature range where the diffusion rate is higher than that of the austenite phase, the ferrite phase, which occupies the majority, only spreads the crystal grains even when hot rolling strain is introduced, and also causes phase transformation during cooling. Therefore, coarse crystal grains tend to remain during slab casting. Therefore, hot rolled sheet (hot rolled steel strip product)
The workability of is easily affected by the coarse ferrite crystal grains generated during slab casting.

Therefore, as a result of diligent research into means for refining the coarse ferrite crystal grains generated during slab casting in the hot rolling process, the present inventors have found that the sheet bar after rough rolling has a specific temperature before finish rolling. It was found that by heat-treating the slab, it is possible to induce recrystallization even in the coarse ferrite crystal grains generated during slab casting to make it finer, and to greatly improve the formability of the hot-rolled sheet. Got

The experiment conducted to obtain this finding will be disclosed below. Chemical composition is 0.011C-0.24Si-0.31Mn-11.2.
A sample with a thickness of 60 mm was cut out from the columnar crystal part of the continuously cast ferritic stainless steel slab that becomes Cr-0.23Ti-0.009 N (the numerical value is the wt% value, the same applies below), and after heating to 1150 ° C,
Two-pass rolling (25% reduction in each pass) is performed to make a sheet bar sample, which is subsequently heated by an induction heating device (device that heats by an induction coil), heated to a predetermined temperature within 3 seconds, and then water-cooled. FIG. 1 shows the results of observing the recrystallization rate by observing the water-cooled microstructure with an optical microscope.

[0013] From FIG. 1, with respect to the sheet bar samples, recrystallization proceeds in a short time by the heating before the temperature T ₀ is applied a relatively small temperature Yutakaryou ΔT in response to T ₀ when above 850 ° C. However, when T ₀ is 800 ° C. or lower, recrystallization does not proceed even if ΔT is increased to 60 ° C. and the temperature is raised. Furthermore, an experiment similar to the above was conducted by using SUS430 steel (0.06C-0.33Si-0.60Mn-16.2), which is a typical ferritic stainless steel.
Cr-0.05N) and SUS430LN steel containing Nb (0.014C-0.42)
It was also conducted for Si-0.30Mn-17.5Cr-0.40Nb-0.011 N), and it was confirmed that the recrystallization behavior similar to that shown in FIG.

Therefore, the present invention can be preferably applied to ferritic stainless steel, and also to low Cr (Cr ≧ 9 wt%) heat-resistant steel (SUH409 steel etc.) used for automobile exhaust gas system members and the like. be able to. Further, in a subsequent experiment, in the case of a steel to which elements such as Ti, Nb and Zr are added for the purpose of precipitating and fixing C and N and improving the formability,
It was also found to be more effective.

Next, regarding the rolling conditions, the steel slab is heated and
The conditions for heating the slab from the rough rolling to the sheet bar are not particularly limited, and may be within the usual range. For example, the slab heating temperature is preferably 1050-1250 ° C. The rolling reduction of the rough rolling is not particularly limited, and it may be carried out within the usual range. The finish temperature of the rough rolling is preferably 850 ° C to 1180 ° C in consideration of the sheet bar temperature rising start temperature.

The gist of the present invention is the following requirements. Temperature before starting temperature rise of the sheet bar T ₀ : 850 to 1150 ° C. From FIG. 1, when T ₀ is less than 850 ° C., the recrystallization rate is too low and the ferrite crystal grains are fine even if the heating amount ΔT is increased to 60 ° C. Does not turn into Further, if an attempt is made to obtain ΔT of more than 60 ° C., the input energy becomes excessively large, and it is necessary to reduce the strip running speed and lengthen the sheet bar heating device, which is economically disadvantageous. Therefore, T ₀
Lower limit is 850 ℃.

On the other hand, the upper limit of T ₀ is 1150 ° C. This is because if T ₀ is more than 1150 ° C., rough rolling also needs to be performed at more than 1150 ° C. However, in the rough rolling in this temperature range, crystal strain is hard to be accumulated in the ferrite phase in the sheet bar, The amount of recrystallization due to temperature rise decreases, and the effect of grain refinement cannot be expected. Further, in order to respond to the rough rolling of more than 1150 ° C., the slab heating temperature must be raised, which leads to coarsening of ferrite grains, deteriorating the hot workability, and is also disadvantageous in terms of energy.

Heated amount of seat bar: ΔT ≧ −0.1 T ₀ +125 (° C.) (1) When heating the seat bar so that the heated amount ΔT is less than the right side of the formula (1), the result shown in FIG. As described above, the recrystallization of the ferrite phase cannot be sufficiently advanced. From the viewpoint of promoting recrystallization, there is no particular upper limit to ΔT, but since excessive temperature rise causes energy loss, it is desirable to set ΔT to about 60 ° C. from the viewpoint of energy saving. The heating time required for raising the temperature of the sheet bar is not particularly limited, but the longer this is, the more burdensome in terms of equipment and efficiency such as lengthening of the heating device and slowing of the conveying speed. It will be. The sheet bar heating time is preferably 5 seconds or less in consideration of the economical efficiency and the productivity related to the production of ferritic stainless steel.

Sheet bar heating device: The type of heating device is not particularly limited, and a direct-fire gas heating device, an induction heating device, or the like can be appropriately used. Among them, rapid heating is possible and the sheet bar thickness is high. It is desirable to use an induction heating device that has excellent temperature uniformity in the depth direction and that can easily perform temperature increase control according to the seat bar temperature by applying electric power. By the way, in normal hot rolling,
It is performed by a so-called batch rolling method of finishing rolling one by one, but in recent years, a continuous rolling method has been proposed in which a preceding material and a trailing material of a sheet bar are joined and continuously finish-rolled,
According to this continuous rolling method, finish rolling conditions such as the reduction amount, rolling speed, rolling temperature and the like can be adjusted more accurately than the batch rolling method, and there is an advantage that the operation can be further stabilized and the characteristics in the longitudinal direction can be made uniform.

The sheet bar heating according to the present invention can be applied to both the batch rolling method and the continuous rolling method as long as the above requirements are satisfied. Regarding the application to the continuous rolling method, for example, the sheet bar is once wound into a coil between the rough and finish rollings, the winding start end of the preceding coil and the winding end of the succeeding coil are joined, and finish rolling is performed over a plurality of coils. When continuously performed, heating of the sheet bar by the sheet bar heating device may be performed after rough rolling, before winding, after winding, before joining, or after joining and before finish rolling.

The finish rolling may be carried out within the usual range, and the rolling reduction, the finish plate thickness, etc. are set according to the product specifications. When the hot-rolled sheet is not annealed in the as-rolled state, the finish rolling end temperature (FDT) is 800 ° C or higher, preferably 900 ° C, in order to mitigate the effect of work hardening by finish rolling.
The coil winding temperature (CT) is 650 ° C. or higher, preferably 750 ° C. or higher. On the other hand, when the hot-rolled sheet is annealed as necessary, the FDT and CT are not limited to the above, but the hot-rolled sheet is preferably annealed in the temperature range of 800 ° C to 1000 ° C.

[0022]

Example 1 A slab (thickness: 200 mm) of stainless steels A to D having the composition shown in Table 1 was heated to 1150 ° C. and subjected to rough rolling for 7 passes to form a sheet bar having a plate thickness of 30 mm,
Using an induction heating device, the temperature was raised under various conditions shown in Table 2 (including no temperature rise). In Nos. 2, 11, and 16, the sheet bar was once wound into a coil between the rough and finish rollings and joined to the preceding sheet bar. The temperature rising time was No. 2 after rough rolling and before winding, No. 11 after winding and joining, and No. 16 after joining and before finish rolling. Then, finish rolling is performed and wound to obtain a hot-rolled coil with a plate thickness of 2 mm. A sample is cut out at a position 10 m from the winding start end of this hot-rolled coil, and some samples are further annealed in a laboratory to obtain each sample. From JIS No. 5 tensile test piece in parallel with the rolling direction from
The Lankford value (r value) was determined by tensile, and the yield strength (YS) and elongation (EL) were determined by ultimate breaking tensile.

As a result, as shown in Table 2, in the examples having the requirements of the present invention, the r value was high and the elongation was good in any of the steel types, as compared with the comparative example lacking the requirements of the present invention. Workability is improved.

[0024]

[Table 1]

[0025]

[Table 2]

Example 2 A slab (thickness 200 mm) of stainless steel A having the composition shown in Table 1 was heated to 1150 ° C.
A sheet bar having a plate thickness of 30 mm was obtained by rough rolling the pass. The temperature was raised using an induction heating device under various conditions shown in Table 3 (including no heating). No. 18, 19 and 21 perform rough rolling with the waiting time between passes as short as possible in terms of equipment.
For No. 20, the sixth pass was performed at a temperature of 1050 ° C. and a reduction rate of 40%, and after waiting for 60 seconds, the seventh pass was performed. In No. 21, the sheet bar was once wound into a coil after rough rolling, joined to the preceding sheet bar, and heated before finish rolling.

Subsequently, each sheet bar was finish-rolled to a plate thickness of 2 mm (coil length of about 520 m) and wound to obtain a hot rolled coil. 10m, 50m in the longitudinal direction from the winding start end of this hot rolled coil
From the sample collected at the position of m, 250m, 470m, 510m,
A JIS No. 5 test piece was cut out parallel to the rolling direction and the r value was obtained by pulling 15%. As a result, as shown in Table 3,
Compared to the comparative example, the example was remarkably good, and the moldability was uniform in the longitudinal direction of the coil.

[0028]

[Table 3]

[0029]

As described above, according to the present invention, a ferritic stainless steel which is excellent in formability and material uniformity in the coil longitudinal direction even in the as-hot-rolled state without cold rolling or cold-rolling annealing or in the as-annealed state of hot-rolled sheet. The steel strip can be manufactured without lowering the productivity. As a result, hot-rolled stainless steel sheets for automobile exhaust system materials and building materials, which had previously been forced to use cold-rolled annealed stainless steel due to lack of workability, can be supplied at low cost in a short period of time by omitting the process. It has become possible to make industrial contributions. Naturally, the steel sheet manufactured according to the present invention can also be applied to a cold rolling material.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between a sheet bar temperature rising condition and recrystallization behavior.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Susumu Sato             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. (72) Inventor Taichi Kukizaki             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Chiba Steel Works, Ltd. F-term (reference) 4K037 EA05 EA12 EA15 EA18 EA27                       EA31 FB04 FB10

Claims (3)

[Claims] 1. A method for producing a ferritic stainless hot rolled steel strip, comprising heating a ferritic stainless steel slab and then roughly rolling it into a sheet bar, and finishing rolling the sheet bar into a hot rolled steel strip. After that, temperature T ₀ : 850 ℃ ~
Before finish rolling, a 1150 ° C sheet bar is heated by a sheet bar heating device provided between the rough and finish rolling mills by a heating amount ΔT (° C) that satisfies the following formula (1), and is a ferrite type. Manufacturing method of stainless hot-rolled steel strip. Note △ T ≧ -0.1 T ₀ +125 (1) 2. A sheet bar is temporarily wound into a coil between rough and finish rolling, and a winding start end of a preceding coil and a winding end of a succeeding coil are joined to continuously perform finish rolling over a plurality of coils. Heating the sheet bar with a heating device, after rough rolling, before winding, after winding and before joining,
The method according to claim 1, which is carried out either after joining or before finish rolling. 3. The method according to claim 1, wherein the sheet bar heating device is an induction heating device.