JPH09235621A - Productionof titanium-containing ferritic stainless steel excellent in formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cold rolled ferritic stainless steel sheet having excellent formability equal to or higher than that of a steel sheet prepared by being subjected to hot rolled plate annealing, even if hot rolled plate annealing prior to cold rolling is omitted. SOLUTION: A slab of a ferritic stainless steel, which has a composition consisting of, by weight, <=0.015% C, <=0.015% N, 10-14% Cr, 5×(C+N) to 0.6% Ti, <=0.03% P, and the balance essentially Fe and satisfying C+N<=0.02% and [Ti-5×(C+N)]/P=3 to 20, is used. This slab is hot-rolled, coiled at =600'C, and, after pickling without annealing, cold rolling and finish annealing are applied.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has excellent moldability.
The present invention relates to a method for manufacturing a Ti-containing ferritic stainless steel, and in particular, the manufacturing process and the manufacturing cost are reduced by omitting hot-rolled sheet annealing after hot rolling during the manufacturing process. .

[0002]

2. Description of the Related Art Ti-containing low Cr ferritic stainless steel has a relatively low Cr content, so it has the excellent corrosion resistance of stainless steel, and since it contains Ti, it has good formability and weldability. It has been used for automobile exhaust system materials, which have been conventionally made of ordinary steel, galvanized steel, or aluminized steel.
In the production of ordinary Ti-containing low-Cr ferritic stainless steel sheets, a continuously cast slab or a slab obtained by slabbing an ingot is heated to 1100 ° C to 1300 ° C, hot-rolled, and then at a temperature of 600 ° C or less. After winding and cooling, the product is obtained by performing batch annealing or continuous annealing for the purpose of softening and homogenizing the hot rolled sheet, followed by cold rolling and finish annealing.

However, since there is a strong demand for reduction in manufacturing cost today, a method of reducing the manufacturing cost by omitting the annealing of the hot rolled sheet is widely used. The most problematic point in this case is that by omitting the annealing of the hot-rolled sheet, the formability of the steel sheet after cold-rolling annealing is deteriorated compared to the case where the hot-rolled sheet annealing is performed, such as elongation and r-value. It is to be. Therefore, among the automobile exhaust system parts, when the processing is strict and excellent formability is required, it is still necessary to use a high-cost material that has been subjected to hot-rolled sheet annealing.

As a method for improving the deterioration of formability when omitting such hot-rolled sheet annealing, Japanese Patent Publication No. 3-49973 discloses a composition of C: 0.06 wt% and Cr: 13-16 wt%. For steel types that have a ferrite-austenite two-phase mixed structure during hot rolling, the winding temperature is 750 to 1000 ° C,
Then, a method has been proposed in which hot-rolled sheet annealing is omitted by gradually cooling to a temperature of at least 700 ° C or lower to soften it. However, the above method does not contain Ti,
Further, the present invention relates to a steel type which has a two-phase mixed structure of ferrite-austenite during hot rolling. Therefore, even during hot rolling, it is a ferrite single phase and the precipitation of Ti compound affects the recrystallization behavior of the steel. Is essentially different from steel.

Further, in Japanese Patent Application Laid-Open No. 50-15722, 17 wt%
Cr steel and 15 wt% Cr steel containing Ti and ferritic stainless steel containing only a ferrite single phase or at most 5% austenite phase at hot working temperature are annealed after hot rolling. A method has been proposed in which a steel material having excellent formability is obtained by cold rolling without performing the annealing and then performing annealing. High-Cr steels such as those mentioned above, after being hot-rolled and coiled at a high temperature of 600 ° C or higher, will embrittle the steel during subsequent cooling. It is winding up. However, at a coiling temperature of 600 ° C. or lower, steel is not sufficiently softened during gradual cooling after winding, so that a steel sheet having sufficient workability after cold rolling annealing cannot be obtained.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above problems by omitting hot-rolled sheet annealing performed before cold rolling for Ti-containing low Cr ferritic stainless steel. Even so, it is an object to propose an advantageous manufacturing method of a ferritic stainless cold-rolled steel sheet having excellent formability equal to or higher than that of the hot-rolled sheet annealed.

[0007]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors have conducted a detailed study on the composition and hot rolling conditions of the Ti-containing low Cr ferritic stainless steel. As a result, in order to produce a steel sheet with excellent formability, the content ratio of Ti and P should be regulated within an appropriate range, and the coil should be wound into a coil at a high temperature of 600 ° C or higher after hot rolling. Has been found to be important.

[0008] That is, according to the study by the inventors, in Ti-containing low Cr ferritic stainless steel, particularly for Ti and P contents, 3 ≦ {Ti-5 × (C + N)} / P ≦
When the conditions of 20 are satisfied, even if the annealing of the hot-rolled sheet is omitted, the r value and elongation do not deteriorate, and the excellent formability equal to or better than that of the hot-rolled sheet annealed is obtained. It was determined to show.

FIG. 1 shows the effects of Ti and P on the r value of the cold rolled annealed sheet. The results shown in the figure are Fe-
P, C and N for the basic composition of 11% Cr-0.2% Ti
For steel grades with various contents of slabs, 200 mm thick slabs were heated to 1080 ° C and the final rolling final pass was rolled down:
30%, friction coefficient: 0.25, and the previous pass was hot-rolled under the conditions of rolling reduction: 25%, friction coefficient: 0.25 to make a 3.5 mm thick hot-rolled sheet, and then wound. Temperature: 55
Winding under 4 conditions of 0, 600, 700 and 800 ℃, then omitting hot-rolled sheet annealing, cold-rolling to 1.2 mm thickness, then finish annealing at 880 ℃, 1 minute The r value was measured in the test. The r-value is calculated by the following formula, which was measured with a JIS 13 B-shaped tensile test piece in the directions of 0 °, 45 ° and 90 ° with respect to the rolling direction. r = (r ₀ ° + 2 × r ₄₅ ° + r ₉₀ °) / 4

As shown in FIG. 1, when the winding temperature is constant, a high r value is obtained when the value of {Ti-5 × (C + N)} / P is in the range of 3 to 20. Further, the higher the winding temperature, the higher the r-value, but the effect is remarkable especially in the range of 600 ° C or higher. That is, in order to improve the r value, the content of Ti, P, C and N in the components should be 3 ≦.
It has been found that it is effective to set {Ti-5 × (C + N)} / P ≦ 20 and to raise the winding temperature as high as possible within the range where ordinary hot rolling can be performed at 600 ° C. or higher. .

Further, the slab heating temperature, the rolling reduction in the latter stage of finish rolling and the friction coefficient were also examined in detail. As a result, it was confirmed that at least two rear stages were subjected to lubrication rolling having a low friction coefficient at an appropriate slab heating temperature. It was newly discovered that by doing so, a Ti-containing ferritic stainless steel excellent in formability can be obtained.

The present invention is based on the above findings. That is, the gist configuration of the present invention is as follows. 1. C: 0.015 wt% or less, N: 0.015 wt% or less, C + N: 0.02 wt% or less, Cr: 10 wt% or more, 14 wt% or less, Ti: 5 × (C + N) or more, 0.6 wt% or less, P: 0.03
wt% or less, but {Ti-5 × (C + N)} / P: 3 or more and 20 or less, and the balance is Fe and unavoidable impurities. A method for producing a Ti-containing ferritic stainless steel with excellent formability, which is characterized in that it is wound on a coil at a temperature of 600 ° C or higher, then pickled without annealing, then cold rolled and then finish annealed. 1 invention).

2. In the first invention, the steel composition is such that Cu: 0.5 wt% or less, V: 0.5 wt% or less, Ni: 1.0 wt%.
% Or less, Mo: 3.0 wt% or less, and a method for producing a Ti-containing ferritic stainless steel excellent in formability, which contains one or more selected from Mo (3.0 wt% or less) (second invention).

3. In the first invention or the second invention, when hot rolling the ferritic stainless steel slab,
Prior to rolling, the slab is heated to 980 to 1100 ° C, and the final pass of the finish rolling and the preceding pass are performed under the lubrication conditions of reduction rate: 20% or more and friction coefficient: 0.25 or less, and then wind up. A method for producing a Ti-containing ferritic stainless steel excellent in formability, which is characterized by being subjected to a process (third invention).

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the component composition of the raw material is limited to the above range in the present invention will be described below. C: 0.015 wt% or less C is an element that adversely affects the formability, and its content is
If it exceeds 0.015wt%, its effect becomes remarkable, so 0.015wt%
Limited to wt% or less. From the gist of the present invention, the lower the C content, the better, and it is particularly preferable that the C content be 0.006 wt% or less.

N: 0.015 wt% or less N, like C, is an element that adversely affects the formability.
If it exceeds 0.015 wt%, the effect becomes remarkable, so 0.01
Limited to 5 wt% or less. It is preferably 0.008 wt% or less.

C + N: 0.02 wt% or less However, even if both C and N satisfy the above range, if the total amount exceeds 0.02 wt%, the formability deteriorates. Therefore, the C + N amount is 0.02 wt% or less. Limited

Cr: 10 wt% or more and 14 wt% or less Cr is an element that improves corrosion resistance, but its content is 10 wt%.
%, No significant improvement in corrosion resistance was observed, while 14
When a large amount of Cr is contained in excess of wt%, embrittlement occurs during winding at high temperature during hot rolling, so Cr was limited to 10-14 wt%.

Ti: 5 × (C + N) or more and 0.6 wt% or less Ti is an element that fixes C and N in steel and improves formability and weldability. The effect is that Ti is 5% by weight. ×
It is exhibited by containing more than (C + N). However, addition of more than 0.6 wt% not only saturates the effect, but solid solution Ti raises the recrystallization temperature of the steel and prevents softening of the steel during high temperature winding. And

P: 0.03 wt% or less P is an element that deteriorates the corrosion resistance and r value, so it is preferable to reduce it as much as possible, but 0.03 wt% or less is allowed. Desirably, it is 0.015 wt% or less.

{Ti-5 × (C + N)} / P: 3 or more, 20
Furthermore, according to the findings of the present inventors, the content of Ti, P, and further C and N is 3 ≦ {Ti-5 × (C + N)} /
Since excellent formability is obtained when the relational expression P ≦ 20 is satisfied, the contents of Ti, P, C and N are limited by this formula.

Although the essential components have been described above, the present invention may further contain one or more selected from Cu, V, Ni and Mo as the corrosion resistance improving component. Cu: 0.5 wt% or less, V: 0.5 wt% or less, Ni: 1.0 wt% or less, Mo: 3.0 wt% or less Cu, V, Ni and Mo are all elements useful for improving corrosion resistance. Cu: 0.5 wt%, V: 0.5 wt%,
Not only the addition of Ni: 1.0 wt% and Mo: 3.0 wt% saturates the effect but also impairs manufacturability and economic efficiency. Therefore, each element is added within the above range.

Next, the manufacturing conditions according to the method of the present invention will be described. The coiling temperature is particularly important in the production of this type of steel sheet, and the coil coiled at a coiling temperature of 600 ℃ or higher is softened and homogenized during the cooling process.
Since the formability of the steel sheet after cold rolling and annealing is superior to that when wound up at less than 600 ° C, the winding temperature was set to 600 ° C or higher. It should be noted that the winding temperature is preferably 700 ° C. or higher, as the higher the winding temperature is, the more the moldability is remarkably improved in the range where normal hot rolling is possible.

The hot rolling conditions are not particularly limited, but it is advantageous to carry out the hot rolling conditions. That is, if the slab heating temperature before rolling is less than 980 ° C, the rolling load increases and rolling becomes difficult.
If the temperature exceeds ℃, it is difficult to obtain the structure necessary for obtaining good formability, so the slab heating temperature before rolling is preferably 980 to 1100 ° C.

Further, in the finish rolling, the rolling conditions of the latter stage are particularly important, and if the rolling reduction of either the final pass of the finishing rolling or the preceding pass thereof is less than 20%,
Even if the slab heating temperature before rolling and the friction coefficient to be described later are within the proper ranges, the improvement in formability is insufficient, so it is preferable that the rolling reduction in each of the latter two passes of finish rolling be 20% or more.

Further, the use of the lubrication rolling in the two-pass rolling after the finishing rolling is further effective in improving the formability. Here, if the friction coefficient exceeds 0.25, the effect of improving formability becomes poor, so the friction coefficient during lubrication rolling is 0.25.
It is preferable to set the following.

[0027]

Example A ferritic stainless steel having the composition shown in Table 1 was continuously cast into a slab having a thickness of 200 mm, and the slab was roughly rolled to a thickness of 30 mm by hot rolling, followed by a finish rolling mill having seven stages. Was used to obtain a 3.5 mm thick hot rolled steel sheet. In the above hot rolling, the slab heating temperature before rolling, the sixth pass (F6) of finish rolling and the seventh pass (F7) which is the final pass, respectively, the rolling reduction and the friction coefficient, and further the winding temperature after rolling. Was changed as shown in Table 2. In Table 2, No. 1 to No. 6 and No.
The slab heating temperature, finish rolling reduction, and friction coefficient during finish rolling from No. 11 to No. 15 were in accordance with normal hot rolling conditions. After being wound around the coil, it was cooled to 200 ° C at a cooling rate of less than 500 ° C / h and then left to cool. Thereafter, according to the method of the present invention, annealing was omitted, or as a comparative example, batch type annealing (800 ° C., 8 h-furnace cooling) was performed, followed by pickling, cold rolling and finish annealing to obtain a product. Finished board thickness is 1.
2 mm, the final annealing temperature was 900 ° C., and the holding time was 1 minute. The test material thus obtained is subjected to a tensile test, and the results of examining the r value and the elongation are also shown in Table 2. The r value and the elongation were 0 ° and 45 with respect to the rolling direction.
Measured with a JIS No. B shape tensile test piece in the directions of 90 ° and 90 °, r = (r ₀ ° + 2 × r ₄₅ ° + r ₉₀ °) / 4, E1 =
(E1 ₀ ° + 2 × E1 ₄₅ ° + E1 ₉₀ °) / 4.

[0028]

[Table 1]

[0029]

[Table 2]

As is clear from Table 2, the steel sheet obtained according to the present invention has better r value and elongation than the comparative examples, and has excellent formability. Especially,
No. 7 to No. 1 in Table 2 in which the slab heating temperature and the rolling reduction and friction coefficient of the second pass on the latter stage side of finish rolling are within the range of claim 3.
When 0, more excellent formability is obtained as compared with the case where normal hot rolling is performed.

[0031]

As described above, according to the present invention, a Ti-containing ferritic stainless steel sheet having excellent formability can be obtained at low cost without hot-rolled sheet annealing.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the ratio of Ti-5 × (C + N) and P content and the r value of a cold rolled annealed sheet for each winding temperature.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuhide Ishii 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Susumu Sato Susumu 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corp.

Claims (3)

[Claims] 1. C: 0.015 wt% or less, N: 0.015 wt% or less, provided that C + N: 0.02 wt% or less, Cr: 10 wt% or more, 14 wt% or less, Ti: 5 × (C + N) or more, 0.6 wt% Hereinafter, P: 0.03 wt% or less, where {Ti-5 × (C + N)} / P: 3 or more and 20 or less, and the balance is a ferritic stainless steel slab having a composition of Fe and inevitable impurities, 600 after hot rolling
A method for producing a Ti-containing ferritic stainless steel excellent in formability, which comprises coiling at a temperature of ℃ or more, pickling without annealing, cold rolling, and finish annealing. 2. The steel composition according to claim 1, further comprising C
u: 0.5 wt% or less, V: 0.5 wt% or less, Ni: 1.0 wt% or less, Mo: 3.0 wt% or less. One or more selected from Ti, which has excellent formability. Method of manufacturing ferritic stainless steel containing. 3. The hot rolling of a ferritic stainless steel slab according to claim 1, wherein the slab is heated to 980 to 1100 ° C. prior to rolling, and the final pass of the finish rolling and its preceding pass are respectively performed. A method for producing a Ti-containing ferritic stainless steel having excellent formability, which comprises performing the rolling step under a lubricating condition of a rolling reduction of 20% or more and a friction coefficient of 0.25 or less, and then subjecting it to a winding step.