JPH10251759A - Production of ferritic stainless hot rolled steel strip excellent in cold rollability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a stainless hot rolled steel strip capable of direct cold rolling with hot rolled steel strip annealing obviated and excellent in cold rollability by subjecting a continuously cast slab of a ferritic stainless steel having a specified compsn. contg. Cr, C, N, Ti and Nb to finish rolling under specified conditions. SOLUTION: The continuously cast slab contains, by weight, >=10% Cr, <=0.30% C+N and one or two kinds of Ti and Nb by >=4(C+N) in total. Furthermore, it may contain <=2.0% Mo. Then, finish rolling is executed in such a manner that the finishing temp. (FDT)( deg.C) satisfies FDT>=800+A, where A=(wt.%Cr)+10(wt.%Ti)+100(wt.%Nb)+30(wt.%Mo). Then, the average cooling rate till coiling after the completion of the finish rolling is regulated to <=10 deg.C/sec. In this way, the generation of ear cracking, roll baking flaws or the like can be evaded even in the case hot rolled steel strip annealing is evaded to improve its surface quality.

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 strip, and more particularly to a method for producing a hot-rolled ferritic stainless steel strip having excellent cold rolling properties.

[0002]

2. Description of the Related Art A ferritic stainless steel cold-rolled steel strip is formed into a hot-rolled steel strip by hot rolling using a continuously cast slab.
After being subjected to annealing and then pickling to remove oxide scale formed on the surface of the steel sheet, the steel sheet is subjected to cold rolling, annealing, and pickling to manufacture. In ferritic stainless steel cold-rolled steel strips manufactured from continuous cast slabs, ridging is said to be remarkable due to the columnar crystals formed during casting. In order to reduce the development of a band-like structure parallel to the above, appropriate hot-rolling conditions are used, and a hot-rolled sheet is annealed to obtain a uniform recrystallized structure.

However, the method for producing a cold-rolled steel strip including hot-rolled sheet annealing requires a long production step and has a problem in productivity. As a method for producing a ferritic stainless steel cold-rolled steel strip that does not require annealing of a hot-rolled steel strip, for example, Japanese Unexamined Patent Publication No. 6-17
No. 9921 discloses a stainless steel slab containing Cr: 11 to 21% by weight, C: 0.02% by weight or less, N: 0.02% by weight or less, and at least one of Ti and Nb or further added Mo. Heated to ~ 1250 ° C, hot rolled at a finishing temperature of 850 to 950 ° C, wound up at 550 ° C or less, and subjected to cold rolling at a reduction rate of 50 to 70% with a large-diameter rolling roll of 100mmφ or more.
A method for producing a ferritic stainless steel sheet excellent in high-temperature strength subjected to high-temperature annealing and excellent in formability is disclosed.

Japanese Patent Publication No. Hei 8-26436 discloses that Cr: 10
Up to 20 wt%, C: 0.10 wt% or less, N: 0.04 wt% or less, S
i: 2.0 wt% or less, Mn: 1.0 wt% or less, and Ti: 0.03
~ 0.50wt%, Nb: 0.03 ~ 0.50wt%, B: 0.0005 ~ 0.0100
When hot rolling a slab of ferritic stainless steel containing wt%, the rolling reduction at 900 ° C or less should be 50% or more, rolling should be completed at a finishing temperature of 800 ° C or less, and the coil winding temperature should be 600 ° C or more. A method for producing a ferritic stainless steel having excellent press-forming workability and surface characteristics, in which annealing is performed after coiling, or annealing is omitted, and then cold rolling and cold-rolled sheet annealing are combined.

However, if the annealing of the hot-rolled steel strip is omitted as in the methods described in JP-A-6-179921 and JP-B-8-26436, work hardening occurs during cold rolling of the hot-rolled steel strip. Significantly, there were problems such as the occurrence of edge cracks in the steel strip, the occurrence of seizure flaws in the rolling roll, and the necessity of increasing the number of rolling passes in a reverse type cold rolling mill. A method for producing a hot-rolled ferritic stainless steel strip which can omit hot-rolled steel strip annealing and has excellent cold-rolling properties has not been established.

[0006]

The method for producing a cold-rolled steel strip including hot-rolled sheet annealing requires a long production step, low productivity,
Because it is economically expensive, there has been a demand for the development of a hot-rolled steel strip that can be directly cold-rolled without performing hot-rolled steel strip annealing. An object of the present invention is to provide a method for producing a ferritic stainless steel hot-rolled steel strip which can omit hot-rolled steel strip annealing and can be directly cold-rolled and has excellent cold-rollability.

[0007]

Means for Solving the Problems In view of the above problems, the present inventors have conducted intensive studies on the cold rollability of a hot-rolled steel strip. By optimizing the cooling rate from the end of rolling to the winding of the coil, it has been newly found that there is no problem in the cold rolling property even when the hot-rolled steel strip annealing is omitted, and the present invention has been constituted.

[0008] The experimental results on which the present invention is based will be described. First, the effect of the finish rolling end temperature was examined.
A continuous cast slab having the composition shown in steel No. A1 to A6
And then a three-stand rough rolling mill and 7
Hot rolling was performed by a continuous finishing rolling mill including a stand. At this time, finish rolling was performed while changing the finish rolling end temperature, and after finishing rolling, the sample was cooled at an average cooling rate of 4 ° C./s and wound around a coil to obtain a hot-rolled steel strip. The obtained hot-rolled steel strip was pickled, and cold-rolled with a reduction ratio changed using a Sendzimer-type cold rolling mill having a work roll diameter of 100 mm φ to obtain a cold-rolled steel strip. About this cold-rolled steel strip, the degree of occurrence of ear cracks was investigated. The result is shown in FIG. The degree of occurrence of ear cracks was evaluated based on the length of the ear cracks, and classified into four ranks of large, medium, small, and no. Ear crack length is large, 30mm or more, medium is 10-30m
m, small was 10 to 3 mm, and none was less than 3 mm.

[0009]

[Table 1]

From FIG. 1, the finish rolling end temperature (FDT)
When (° C.) is equal to or more than (800 + A) (° C.), even if cold rolling is performed at 90%, generation of ear cracks is not observed, and it can be seen that generation of ear cracks can be prevented. The A value is a value determined by the steel composition, and is represented by A = (wt% Cr) +10 (wt% Ti) +100 (wt% Nb) +30 (wt% Mo).

[0011] From this, it has been found that it is important to adjust the finish rolling end temperature according to the steel composition. Next, the effect of the cooling rate after finishing rolling was examined. A continuous cast slab having the composition shown in steel No.
And then a three-stand rough rolling mill and 7
Hot rolling was performed by a continuous finishing mill comprising a stand to obtain a hot-rolled steel strip. The finish rolling finish temperature was 850 to 860 ° C. After the finish rolling, it was cooled at various average cooling rates and wound around a coil. The obtained hot-rolled steel strip was pickled and cold-rolled with a reduction ratio changed by a Sendzimer type cold rolling mill having a work roll diameter of 80 mmφ to obtain a cold-rolled steel strip. This cold-rolled steel strip was examined for occurrence of ear cracks. Figure 2 shows the result.
Shown in

FIG. 2 shows that the cooling rate after finishing rolling is 10
It is understood that generation of ear cracks can be prevented at a temperature of not more than ℃ / s. The present invention has been made based on the above findings.
That is, the gist of the present invention is a method of manufacturing a ferritic stainless steel hot-rolled steel strip to be a hot-rolled steel strip by hot rolling consisting of rough rolling and continuous finishing rolling of a continuously cast slab of ferritic stainless steel. The continuous cast slab is weight%
And contains Cr: 10% or more, C + N: 0.03% or less, and T
One or two selected from i and Nb for a total of 4
A ferritic stainless steel containing (C + N) or more, and a finish rolling end temperature (FDT) of the finish rolling.
(° C.) is the following formula (1) FDT ≧ (800 + A) (1) (where, FDT: finish rolling end temperature (° C.), A = (wt%)
(Cr) + 10 (wt% Ti) + 100 (wt% Nb) + 30 (wt% Mo)), and the average cooling rate from completion of the finish rolling to coil winding is 10 ° C / s or less. This is a method for producing a hot-rolled ferritic stainless steel strip having excellent cold rollability.

Further, in the present invention, the continuous cast slab is, by weight%, Cr: 10% or more, C + N: 0.03% or less, Mo: 2.0%.
% Or less, and one or two selected from Ti and Nb may be a ferritic stainless steel containing a total of 4 (C + N) or more.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of the continuous cast slab used in the present invention will be described. Cr: 10% or more Cr needs to be 10% or more in order to obtain a ferrite single phase structure by a synergistic effect with C, N, Ti, and Nb. Meanwhile, 30%
If it exceeds, the brittle phase is likely to be formed after hot rolling,
Preferably, it is limited to 30% or less. In addition, Cr is corrosion resistance,
It is preferably 11 to 22% from the viewpoint of oxidation resistance and the like.

C + N: 0.03% or less C and N are strong austenite stabilizing elements.
Even if Nb is added, in order to stably obtain a ferrite single phase structure, the total amount of C and N needs to be 0.03% or less. In addition, C + N is preferably 0.025% or less. Total of one or two selected from Ti and Nb: 4
(C + N) and above Both Ti and Nb are elements necessary for fixing C and N, which are strong austenite stabilizing elements, as carbonitride and stably obtaining a ferrite single phase structure. For that purpose, it is necessary to add 4 (C + N) or more in total of one or two selected from Ti and Nb. Also, on the other hand,
If the Ti content exceeds 0.5% and the Nb content exceeds 0.8%, the elongation of the cold-rolled annealed steel strip decreases.
It is better to be 8% or less.

Mo: 2.0% or less Mo is an element for improving corrosion resistance, and is added as necessary. However, if the addition exceeds 2.0%, the upper limit is 2.0% because the steel material becomes brittle. For components other than the above,
Depending on the purpose, it can be added as necessary, and the preferred range including impurities is shown below, but the present invention is not limited to this range.

Si: 1.0% or less, Mn: 1.0% or less, Al: 0.
1% or less, P: 0.05% or less, S: 0.015% or less, O: 0.
01% or less, Ca: 0.002% or less, Mg: 0.002% or less, REM
: 0.015% or less, Ni: 0.7% or less, Cu: 0.5% or less, C
o: 0.3% or less, V: 0.5% or less, Zr: 0.3% or less,
W: 0.3% or less, Ta: 0.3% or less, B: 0.003% or less.
Next, the hot rolling conditions will be described.

Heating temperature of hot rolling: 1160 ° C. or less Since the structure becomes coarse when the heating temperature is high, the heating temperature is preferably 1160 ° C. or less. On the other hand, if the heating temperature is too low, seizure with the rolls during rolling tends to occur and rough surface defects are likely to occur on the surface of the steel sheet.

Rough rolling temperature: 950 to 1100 ° C. If the rough rolling temperature exceeds 1100 ° C., crystal grains grow or a band structure parallel to the rolling direction is easily formed.
If the temperature is lower than ℃, it is difficult to form recrystallized grains. Therefore, the rough rolling temperature is preferably in the range of 950 to 1100 ° C. The temperature is more preferably in the range of 980 to 1080 ° C.

Finish rolling end temperature (FDT) (° C.): (80
0 + A) ° C or higher The A value is determined by the following equation (1): A value = (wt% Cr) +10 (wt% Ti) +100 (wt% Nb) +30 (wt% Mo) (2) If the finish rolling end temperature is lower than (800 + A) ° C., ear cracks are likely to occur during cold rolling. For this reason, in order to suppress the occurrence of edge cracks and increase the cold rolling property,
The finish rolling finish temperature is (800 + A value) ° C or more. In addition, from the viewpoint of descaling, the temperature is preferably set to (800 + A) ° C or more and 930 ° C or less.

Average cooling rate after finishing rolling to coil winding: 10 ° C./s or less If the cooling rate after finishing rolling exceeds 10 ° C./s, ear cracks occur during cold rolling. For this reason, the cooling rate after finishing rolling was limited to 10 ° C./s or less. In addition, the lower limit of the cooling rate is not particularly limited.
It is preferable to make the above.

The hot-rolled steel strip wound up into the coil after finish rolling by the method of the present invention is subjected to cold rolling-annealing-pickling one or more times by omitting annealing of the hot-rolled steel strip. It can be a rolled steel strip.

[0023]

EXAMPLE A continuously cast slab having the composition shown in steel Nos. A1 to A8 in Table 1 was heated to 1120 ° C. to 1160 ° C., and was then adjusted by a three-stand rough rolling mill and a seven-stand continuous finishing mill. Hot rolling was performed under the conditions shown in FIG. 2 to obtain a hot-rolled steel strip.
The resulting hot-rolled steel strip is pickled and then the work roll diameter is 90
Cold rolling was performed at a rolling reduction of 90% with a mmφ Sendzimer cold rolling mill. After cold rolling, the occurrence of edge cracks was examined for these cold-rolled steel strips. Table 2 shows the results. In addition,
The state of occurrence of ear cracks was evaluated by measuring the length of the ear cracks. The evaluation was as follows: ○: cracks of less than 3 mm, no roll seizure flaws, Δ: cracks of 3 mm or more and less than 10 mm, cracks of 10 mm or more, stop rolling.

[0024]

[Table 2]

Rolled materials No. 1 to No. 4, No. 10
, No.12 and No.13, no hot-rolled steel strip annealing was performed, and even if cold rolling was performed at a rolling reduction of 90%, no cracking was observed at the time of cold rolling. It is clear that the ferrite-based stainless steel strip has excellent surface appearance and surface quality, and has excellent cold rolling properties.

On the other hand, in the rolled materials No. 5, No. 6, No. 9, and No. 11, which are comparative examples in which either the finish rolling end temperature or the cooling rate until coil winding is out of the range of the present invention, ,
In each case, a crack of 2-5 mm in length occurred during cold rolling, so a trimming step was added to remove the crack. The trimming resulted in a decrease in width yield. In the rolled material No. 7, which is a comparative example in which both the finish rolling end temperature and the cooling rate until coil winding are out of the range of the present invention, 10-30 mm edge cracks occur during the cold rolling, and the rolling is performed as it is. If this continued, there was a danger of leading to serious accidents such as breakage of the inner plate of the mill and the occurrence of fire, and rolling was stopped. Also, rolled material N
In o.14 and No.15, the composition of the continuously cast slab was out of the range of the present invention, and ear cracks occurred.

As described above, according to the present invention, even if the hot rolling annealing is omitted, the occurrence of edge cracks and roll seizure flaws is not recognized.
A ferritic stainless steel cold rolled steel strip having extremely excellent surface quality can be manufactured with high efficiency.

[0028]

According to the present invention, by optimizing the hot rolling conditions, the occurrence of edge cracks, roll seizures and the like during cold rolling can be avoided even if the hot rolling annealing is omitted, and excellent surface quality can be obtained. A hot rolled ferritic stainless steel strip having excellent cold rollability can be efficiently produced, and the industrially remarkable effect is exhibited.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of finish rolling end temperature on the degree of edge cracking.

FIG. 2 is a graph showing the effect of the cooling rate until finish-rolling winding on the degree of edge cracking.

Claims (2)

[Claims] 1. The method for producing a hot-rolled ferritic stainless steel strip according to claim 1, wherein the continuous casting slab of ferritic stainless steel is hot-rolled by hot rolling comprising rough rolling and continuous finishing rolling. Ferritic stainless steel in which the slab contains 10% or more of Cr and 0.03% or less of C + N by weight, and contains one or two selected from Ti and Nb in total of 4 (C + N) or more. The finish rolling finish temperature (FDT) (° C.) of the finish rolling satisfies the following formula (1), and the average cooling rate from the finish rolling to coil winding is 10 ° C./s or less. A method for producing a hot-rolled ferritic stainless steel strip having excellent cold-rolling properties. Note FDT ≧ (800 + A) (1) where, FDT: finish rolling end temperature (° C.) A = (wt% Cr) +10 (wt% Ti) +100 (wt% Nb) +30 (wt% Mo) 2. The continuous cast slab contains 10% by weight of Cr: 10% by weight.
As described above, C + N: 0.03% or less, Mo: 2.0% or less, and one or two kinds selected from Ti and Nb in total of 4%
2. The method for producing a hot-rolled ferritic stainless steel strip according to claim 1, wherein the ferritic stainless steel strip contains (C + N) or more.