JP2000144258A - Production of titanium-containing ferritic stainless steel sheet excellent in ridging resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a Ti-contg. ferritic stainless steel sheet in which the behavoir of recrystallized grains in hot rolling is controlled and excellent in ridging resistance. SOLUTION: A slab contg., by weight, <=0.010% C, <=0.010% N also so as to satisfy <=0.015% C+N, 6 to 35% Cr, Ti: 6×(C+N)% to 0.5%, and the balance Fe with inevitable impurities is heated to <=1160 deg.C, is subjected to rough rolling in which the cumulative draft is controlled to >=85%, and the final pass finishing temp. is controlled to >=950 deg.C, is next subjected to finish rolling in which the cumulative draft is controlled to >=90%, and the final pass finishing temp. is controlled to >=900 deg.C and is thereafter subjected to pickling, cold rolling and annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a technique for producing a Ti-containing ferritic stainless steel sheet having excellent ridging resistance.

[0002]

2. Description of the Related Art A ferritic stainless steel sheet containing Ti has both corrosion resistance and weldability and is relatively inexpensive. Therefore, it has recently been used for automobile exhaust system members. In general, this Ti-containing ferritic stainless steel sheet is subjected to hot rolling consisting of rough rolling and finish rolling on a continuously cast slab, wound up into a coil, and softened and homogenized by annealing (batch type or continuous type). After that, it is manufactured by performing cold rolling and finish annealing. In the hot rolling of a Ti-containing ferritic stainless steel, the rolling method in SUS430, which is a general-purpose steel type, has conventionally been followed from the viewpoint of operability. This SUS430 has a higher high-temperature strength and a higher rolling load because it has a higher content of C and N in a solid solution state than a Ti-containing ferritic stainless steel sheet. So, SUS
In the hot rolling of 430, rolling is performed at a high temperature by increasing the slab heating temperature to reduce the rolling load, and rolling is performed by increasing the number of passes and reducing the rolling reduction per pass. Has been deemed vital. Therefore, such rolling conditions for reducing the rolling load have also been adopted for Ti-containing ferritic stainless steels.

[0003] When rolling under such conditions, a problem with the Ti-containing ferritic stainless steel is that the strip structure at the center of the sheet thickness is not sufficiently divided because the rolling reduction per pass is small. , Cold rolling and finish annealing did not provide sufficient ridging resistance of the steel sheet (cold rolled annealed sheet). Here, ridging refers to a phenomenon in which fine streak-like wrinkles are generated along the rolling direction when a deformation such as tension or deep drawing is applied to a ferritic stainless steel sheet.
By the way, there have been several proposals for a method for improving the ridging resistance of a Ti-containing ferritic stainless steel sheet. For example, JP-A-10-17937 discloses that Cr
A stainless steel slab containing 11 wt%
By increasing the rolling reduction in the subsequent stage of rough rolling, lowering the start temperature of finish rolling, and raising the end temperature,
A method for improving ridging resistance after cold rolling annealing is disclosed.

[0004]

However, in the conventional method, since the starting temperature of the finish rolling is low, the rolling reduction cannot be increased due to the restriction of the rolling load, and the refinement of the crystal structure becomes insufficient. However, there is a problem that the band-like tissue, which is considered to be the cause of ridging, cannot be sufficiently separated. In addition, JP-A-10-60543 discloses that 0.04 to 0.30 wt.
% Ferritic stainless steel slab containing
Heating to the temperature determined by the content of Mn and Ti, cumulative reduction rate 90%
A method of rough rolling at an end temperature of 1000 ° C. or higher is disclosed. However, this method also has a similar problem in that the strong reduction in the finish rolling is insufficient and the crystal grains cannot be sufficiently refined. Therefore,
The present invention is intended to solve the above problems of the prior art, and proposes a technique for producing a Ti-containing ferritic stainless steel sheet having excellent ridging resistance by refining crystal grains. It is the purpose.

[0005]

Means for Solving the Problems The present inventors have studied in detail the hot rolling conditions of a Ti-containing ferritic stainless steel in order to achieve the above object. As a result, they have found that the problem can be solved by controlling the rough rolling and the finish rolling within an appropriate range, and have completed the present invention. The summary configuration is as follows.

(1) C: 0.010 wt% or less, N: 0.010 wt% or less, C + N: 0.015 wt% or less, Cr: 6 wt% or more, 35 wt% or less, Ti: 6 × (C + N) wt% or more, 0.5 The slab containing less than wt%, the balance being Fe and unavoidable impurities, is heated to 1160 ° C or less, and the cumulative draft is 85% or more.
In addition, rough rolling is performed so that the final pass end temperature is 950 ° C. or more, and then finish rolling is performed in which the cumulative draft is 90% or more and the final pass end temperature is 900 ° C. or more.
A method for producing a Ti-containing ferritic stainless steel sheet having excellent ridging resistance, comprising performing pickling, cold rolling, and annealing.

(2) In the production method described in the above (1), the composition of the slab is such that, in addition to the above components, Si: 2.0 wt% or less, Ni: 1.0 wt% or less, Mo: 2.0 wt% or less : 1.0 wt% or less, Co: 0.5 wt% or less, V: 0.5 wt% or less, characterized by containing one or more selected from the group consisting of: Ti-containing ferritic stainless steel excellent in ridging resistance. Steel plate manufacturing method.

(3) In the production method described in the above (1) or (2), the composition of the slab is such that, in addition to the above components, B: 0.01 wt% or less, Ca: 0.01 wt% or less, Nb: 0.05 A method for producing a Ti-containing ferritic stainless steel sheet excellent in ridging resistance, comprising one or more selected from wt% or less.

(4) The method according to any one of (1) to (3) above, wherein annealing is performed after finish rolling and before pickling, and excellent in ridging resistance. Manufacturing method of Ti-containing ferritic stainless steel sheet.

(5) The method according to any one of the above (1) to (4), wherein the rolling is performed at a rolling reduction of at least one pass of the rough rolling of 35% or more. Method for producing Ti-containing ferritic stainless steel sheet with excellent resistance.

(6) The method according to any one of (1) to (5) above, wherein the final two passes of finish rolling are each performed at a rolling reduction of 20% or more. Method for producing Ti-containing ferritic stainless steel sheet with excellent ridging properties.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first reexamined the prior art, and in the conventional hot rolling method, the band structure was not divided due to insufficient strength under rough rolling in the rough rolling. In addition, as a result, in the finish rolling, the load increases, the strong reduction is restricted from the upper limit of the mill power, the band structure remains until after rolling, and as a result, the ridging resistance of the steel sheet after cold rolling annealing Turned out to be worse.

In view of these circumstances, the present inventors have comprehensively studied rolling conditions in rough rolling and finish rolling. As a result of detailed experiments and studies, it was concluded that the following points had to be noted in order to improve ridging resistance. (1) First, by reducing the slab heating temperature as much as possible, the coarsening of crystal grains during heating is suppressed, and the initial grain size is reduced. (2) In the rough rolling, the crystal grains should be made finer by applying a low temperature and high pressure. (3) After the rough rolling and before the start of the finish rolling, a recrystallized structure must be once maintained at a recrystallization temperature or higher for a sufficient time. (4) To further reduce the band structure generated by rolling by further reducing the pressure in the subsequent finish rolling, and to terminate the hot rolling at a high temperature to promote self-annealing after the hot rolling.

When all of these conditions are satisfied, the ridging resistance of the Ti-containing ferritic stainless steel sheet can be improved. In particular, the requirements of strong rolling and high-temperature termination in the finish rolling described in (4) above are important, and thereby excellent ridging resistance can be obtained even if hot-rolled sheet annealing is omitted. In the case where even better ridging resistance is required depending on the use, it is desirable to perform hot-rolled sheet annealing by an ordinary method.

Next, specific manufacturing conditions for realizing the above items will be described.・ Heating temperature The slab heating temperature shall be 1160 ° C or less. Because 11
If the temperature exceeds 60 ° C., the crystal grains become coarse, and the refinement of the structure after rough rolling is hindered, and as a result, the ridging resistance of the cold-rolled and annealed steel sheet is not improved. If the end temperatures in the rough rolling and finish rolling described below can be satisfied, the lower the slab heating temperature, the greater the improvement in ridging resistance. From this point, the slab heating temperature is 1120 ° C
It is desirable to make the following.

Rough rolling It is effective to perform rough rolling under strong pressure in order to sufficiently refine crystal grains. If the cumulative rolling reduction from the slab to the end of rough rolling is 85% or more, ridging resistance is high. The effect of improvement appears. Further, at least one of the rolling passes of the rough rolling is rolled with a rolling reduction of 35% or more to obtain a higher effect. Further, in order to maintain the sheet bar at or above the recrystallization temperature after rough rolling and to promote recrystallization before rolling, the recrystallization temperature of the Ti-containing ferritic stainless steel according to the present invention must be 900 ° C or more. In consideration of the above, the final pass end temperature of the rough rolling needs to be 950 ° C. or more. In order to further promote the recrystallization, it is desirable that the sheet bar obtained by the rough rolling is held at 900 ° C. or more for 20 seconds or more before the finish rolling under the rough rolling end temperature. When the slab heating temperature is low,
To increase the rough rolling end temperature, it is necessary to reduce the number of rough rolling passes or increase the rough rolling speed to shorten the rolling time.Today, however, the capacity of hot rolling mills has been improved. Rolling with such a high rolling load, which was impossible in the past, is possible.

FIG. 1 shows that in the laboratory 0.004 wt% C-0.005 wt%
% N-18wt% Cr-0.3wt% Ti-based ferritic stainless steel, slab-heated in a temperature range of 1100-1200 ° C, and rough-rolled at an end temperature of 850 ° C-1000 ° C. The relationship between the recrystallization rate in the structure and the heating temperature of a steel sheet obtained by holding at a temperature for 20 seconds and immediately cooling with water is examined. From Figure 1, the slab heating temperature is 1160 ° C
It can be seen that a recrystallization structure of 50% or more is obtained when the rough rolling end temperature is set at 950 ° C. or higher.

Finish Rolling After such rough rolling, it is also important to control the rolling conditions and the winding temperature in the finish rolling within appropriate ranges in order to exhibit the excellent ridging resistance of the cold-rolled annealed sheet. is there. That is, in the finish rolling, the higher the cumulative draft is, the more the band-shaped structure is divided. This effect is due to the cumulative rolling reduction
Less than 90% is not expected. At this time, if the rolling reduction in the last two passes is set to 20% or more, the band-shaped tissue is more effectively divided. The higher the finish temperature of finish rolling, the better the ridging resistance of the cold-rolled annealed sheet,
Since the self-annealing action immediately after hot rolling is promoted, it is possible to omit annealing which is usually performed as the next step. The effect is remarkable when the finish rolling end temperature is 900 ° C or more.
Above. Therefore, the finish rolling has a 90% cumulative draft.
In addition to the above, it is necessary that the rolling end temperature be 900 ° C. or higher. Further, it is desirable that the winding temperature of the steel sheet after the finish rolling is 800 ° C or less. In the present invention, since recrystallization is performed after rough rolling, if the finish rolling end temperature is set to 900 ° C. or more, the cumulative rolling reduction is 90%.
% Or more.

FIG. 2 shows the effects of the rough rolling finish temperature and the finish rolling finish temperature on the ridging resistance. That is, 0.004 wt% C-0.005 wt% N-
After heating a ferritic stainless steel slab based on 18wt% Cr-0.3wt% Ti at 1140 ° C, the cumulative draft was 88%,
Rough rolling is performed at an end temperature of 880 to 1050 ° C., and then finish rolling is performed at a cumulative draft of 90% and an end temperature of 770 to 970 ° C. to obtain a hot-rolled sheet having a thickness of 3 mm. , And cold-rolled to 1.0 mm, and further annealed at 900 ° C. for 30 seconds. From the cold-rolled annealed sheet thus obtained,
A JIS No. 5 tensile test piece was sampled, 20% strain was applied in the rolling direction, the undulation height of the generated ridging was measured using a surface roughness meter, and the measured value was evaluated according to the criteria shown in Table 4. From FIG. 2, the end temperature of the rough rolling is 950 ° C. or higher, and
It can be seen that a cold rolled annealed sheet having excellent ridging resistance can be obtained by setting the finishing temperature of the finish rolling to 900 ° C. or higher.

The conditions of the cold rolling and the finish annealing to be performed following the above steps may be performed according to a conventional method.
Although not required, the following conditions are particularly recommended. For cold rolling, the reduction rate is 65% or more, and for finish annealing, 85
It is preferable to keep the temperature at 0 ° C or more for 30 seconds or more. After the hot rolling annealing and the finish annealing, if necessary, descaling by pickling is performed. For descaling, electrolytic pickling in a nitrate is suitable. In the present invention, it is not necessary to perform annealing for recrystallization after hot rolling, but if higher ridging resistance is required, annealing may be performed. The annealing of the hot rolled sheet is preferably carried out at a temperature of 800 ° C. or more for 1 minute or more.

The reasons for limiting the composition of the components will be described below. C: 0.010 wt% or less C is an element that has an adverse effect on ridging resistance.
If the content exceeds 010 wt%, the effect is remarkable, and
Limited to 010 wt% or less. In order to obtain better ridging resistance, the C content is preferably limited to 0.006 wt% or less.

N: 0.010 wt% or less, and C + N: 0.01
5 wt% or less N, like C, is an element that has an adverse effect on ridging resistance. If it exceeds 0.010 wt%, the effect becomes remarkable, so it is limited to 0.010 wt% or less. In order to obtain better ridging resistance, the content is preferably limited to 0.007 wt% or less. Further, from the viewpoint of improving workability, the amount of C and N
The total amount (C + N) is limited to 0.015 wt% or less.

Ti: 6 × (C + N) wt% or more and 0.5 wt% or less Ti is an element that fixes C and N in steel and improves ridging resistance and weldability. These effects are exhibited by including Ti in an amount of 6 × (wt% C + wt% N) or more. However, even if it is added in excess of 0.5 wt%, the effect is not only saturated, but also the solid solution Ti raises the recrystallization temperature of the steel, preventing the steel from softening after the completion of rough rolling.
Therefore, Ti is added in a range of 6 × (C + N) wt% or more and 0.5 wt% or less. In order to further improve the ridging resistance by recrystallization after rough rolling, the Ti content is desirably 0.3 wt% or less.

Cr: not less than 6 wt% and not more than 35 wt% Cr is an element for improving corrosion resistance. The effect is 6
If the content is less than 35 wt%, the content is not sufficient. On the other hand, if the content is more than 35 wt%, embrittlement occurs and becomes a practical obstacle. Therefore, the Cr content is limited to the range of 6 to 35 wt%.

In addition to the above components, Si, Ni, Mo, and Cu are further added to improve corrosion resistance, Co and V are added to improve secondary work brittleness, and deep drawability is improved. B, Ca,
Nb can be added. Hereinafter, these components will be described. Si: 2.0 wt% or less Si is a useful element for improving corrosion resistance and oxidation resistance. However, even if added in excess of 2.0 wt%, the effect is not only saturated, but also impairs the manufacturability and economic efficiency.

Ni: 1.0 wt% or less, Mo: 2.0 wt% or less, C
u: 1.0 wt% or less Ni, Mo and Cu are all useful elements for improving corrosion resistance. However, Ni: 1.0 wt%, Mo: 2.0 wt%
%, Cu: If added in excess of 1.0 wt%, the effects will be saturated and the productivity and economic efficiency will be impaired.

Co: 0.5 wt% or less, V: 0.5 wt% or less Co and V are both effective elements for improving the brittleness in secondary working. However, if any of the elements exceeds 0.5 wt%, the effect is saturated and the productivity and economic efficiency are impaired. Therefore, 0.5 wt% is added as an upper limit.

B: 0.01 wt% or less, Ca: 0.01 wt% or less, N
b: 0.05 wt% or less B, Ca and Nb are effective elements for refining the recrystallized fiber and improving the deep drawability of steel by adding a trace amount. However, B: 0.01 wt%, Ca: 0.01 wt%, N
b: Even if added in excess of 0.05 wt%, the effect is saturated,
Since the productivity and economic efficiency are impaired, these values are used as the upper limits.

Components other than those described above are inevitably contained in Fe and the material. this house
Mn and Al are elements necessary for deoxidation in the steelmaking process, and are usually contained in the steel in a range of 1.0 wt% or less and 0.1 wt% or less, respectively.

[0030]

EXAMPLE A ferritic stainless steel having a chemical composition shown in Table 1 was continuously cast into a 200 mm thick slab, and after heating this slab, various pass schedules shown in (a) to (e) in Table 2 were adopted. Rough rolling was performed at the rolling end temperature. Subsequently, using a finishing rolling mill consisting of seven stages, rolling was performed while changing the rolling end temperature and the cumulative rolling reduction, and after water cooling to 500 ° C,
Wound on a coil. Table 3 summarizes these rolling conditions. Here, all the inventive examples were kept at 900 ° C. or higher for 30 seconds before the transition from rough rolling to finish rolling. The hot-rolled sheet thus obtained was subjected to pickling, cold rolling, finish annealing, and pickling to obtain a cold-rolled annealed sheet having a thickness of 0.6 mm. Here, as shown in Table 3, some of them were annealed before pickling the hot rolled sheet.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

Each steel sheet produced under the above conditions was evaluated for ridging resistance. Evaluation of ridging resistance was performed by taking a JIS No. 5 tensile test specimen from a cold-rolled annealed plate, applying a 20% strain in the rolling direction, and measuring the undulation height of the generated ridging using a surface roughness meter. The measured values were measured according to the standards shown in Table 4 (in this standard, if the ridging grade is 2 or less, there is no problem in practical use). The results obtained are shown in Table 3. As is clear from Table 3,
It can be seen that according to the combination of components and steps according to the present invention, a steel sheet having excellent ridging resistance can be manufactured.

[0036]

As described above, according to the present invention,
It is possible to manufacture a Ti-containing ferritic stainless steel sheet having excellent ridging resistance, which cannot be obtained by the conventional technology. Therefore, according to the present invention, it is possible to easily produce a ferritic stainless steel sheet suitable for use in which the surface properties after cold working are problematic, and it is expected to have excellent industrial effects. .

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of slab heating temperature on the recrystallization rate after rough rolling in ferritic stainless steel containing 0.3 wt% Ti.

FIG. 2 is a graph showing the influence of a rough rolling end temperature and a finish rolling end temperature on ridging characteristics of a 0.3 wt% Ti-containing ferritic stainless steel sheet.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/54 C22C 38/54 (72) Inventor Kazuhide Ishii 1 Kawasakicho, Chuo-ku, Chiba-shi, Chiba Kawasaki (72) Inventor Susumu Sato 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture F-term (reference) 4K032 AA02 AA04 AA08 AA09 AA12 AA13 AA14 AA19 AA20 AA22 AA31 AA32 AA35 AA36 BA01 CA01 CA02 CB02 CC04 4K037 EA02 EA04 EA09 EA10 EA12 EA13 EA17 EA18 EA19 EA20 EA27 EA28 EA31 EA32 EB03 EB07 EB09 FA01 FA02 FC04 FC05

Claims (6)

[Claims] C: 0.010 wt% or less, N: 0.010 wt% or less, C + N: 0.015 wt% or less, Cr: 6 wt% or more, 35 wt% or less, Ti: 6 × (C + N) wt% or more, 0.5 wt %, And the remainder is heated to 1160 ° C. or less, and the cumulative draft is 85% or more.
In addition, rough rolling is performed so that the final pass end temperature is 950 ° C. or more, and then finish rolling is performed in which the cumulative draft is 90% or more and the final pass end temperature is 900 ° C. or more.
A method for producing a Ti-containing ferritic stainless steel sheet having excellent ridging resistance, comprising performing pickling, cold rolling, and annealing. 2. The manufacturing method according to claim 1, wherein the composition of the slab is such that, in addition to the above components, Si: 2.0 wt% or less, Ni: 1.0 wt% or less, Mo: 2.0 wt% or less Cu: 1.0 or less wt. or less, Co: 0.5 wt.% or less, and V: 0.5 wt.% or less selected from the group consisting of Ti-containing ferritic stainless steel sheets having excellent ridging resistance. Production method. 3. The method according to claim 1, wherein the composition of the slab further comprises, in addition to the above components, B: 0.01 wt% or less, Ca: 0.01 wt% or less, and Nb: 0.05 wt% or less. A method for producing a Ti-containing ferritic stainless steel sheet having excellent ridging resistance, comprising one or more selected ones. 4. The Ti-containing ferrite system excellent in ridging resistance according to claim 1, wherein annealing is performed after finish rolling and before pickling. Manufacturing method of stainless steel sheet. 5. The method according to claim 1, wherein the rolling reduction of at least one pass of the rough rolling is 35%.
%. A method for producing a Ti-containing ferritic stainless steel sheet having excellent ridging resistance, characterized in that rolling is performed at a rate of at least%. 6. The method according to claim 1, wherein the final two passes of the finish rolling are rolled at a rolling reduction of 20% or more, and the ridging resistance is excellent. Of producing a ferritic stainless steel sheet containing Ti.