JP2001262234A - Method for producing ferritic stainless steel sheet for automotive exhaust system excellent in deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a ferritic stainless steel sheet having high strength, excellent in ductility and also excellent in deep drawability. SOLUTION: A slab having a composition containing 11.0 to 23.0% Cr, <=3.0% Mo and 0.005 to 1.0% Al, in which the contents of C, N, Si, Mn, P and S are controlled to the proper ones, containing one or more kinds selected from Ti, B, Ta, V and Zr, and the balance Fe with inevitable impurities is heated and is subjected to hot rolling into a hot rolled sheet, and after that, the hot rolled sheet is subjected to hot rolled sheet annealing at a temperature in the range of 850 to 980 deg.C and is then subjected to cold rolling and annealing. Moreover, one or more kinds selected from Cu, Ni, W and Sn or, further, Ca may be incorporated therein.

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 sheet, and more particularly to a method for producing a ferritic stainless steel sheet having excellent deep drawability suitable for automobile exhaust system parts. The steel sheet in the present invention includes a steel strip in addition to the steel sheet.

[0002]

2. Description of the Related Art In recent years, exhaust gas regulations for automobiles have become stricter in relation to the preservation of the global environment. For this reason, reducing the body weight of automobiles has become an extremely important issue. In recent years, demand for compact automobiles has been increasing, and components having complicated shapes have been adopted.

[0003] Against this background, for example, components such as end plates of mufflers are also required to have a conventional 1.0.
A ferrite stainless steel sheet with a thickness of ~ 1.5 mm has been used, but recently a thinner ferrite stainless steel sheet with a thickness of 0.5 to 0.6 mm has been considered for application.
Also, in order to be able to process into a complicated shape corresponding to the miniaturization of automobiles, it is required that the conventional steel sheet having a thickness of 1.0 to 1.5 mm has excellent workability.

Various methods have been proposed for improving the workability of such a ferritic stainless steel sheet. For example, Japanese Patent Laying-Open No. 57-76127 proposes a method for producing a ferritic stainless steel sheet containing Ti. In this method, C: 0.10% or less, Cr: 10 to 15%, Ti:
A ferrite single-phase stainless steel strip containing 0.6% or less and N: 0.025% or less is continuously annealed in a temperature range of recrystallization temperature to recrystallization temperature + 100 ° C, and is subjected to cold rolling and annealing. However, steel sheets manufactured by the technology described in JP-A-57-76127 are said to be excellent in in-plane anisotropy, ridging, and earring after cylindrical drawing. However, the required characteristics cannot be sufficiently satisfied.

Japanese Patent Application Laid-Open No. 58-61258 discloses that Cr: 1
1.0 to 16.0%, C and N are reduced to a predetermined amount or less, and furthermore, an appropriate amount of Ti that binds to C and N is added to purify the matrix, and Cr, Si, Mn, Ti, P, Al Ferrite-based stainless steels with excellent overhangability and secondary workability in which the respective contents are limited to 0.8 or less as defined by the relational expression of Cr, Si, Mn, Ti, P, and Al have been proposed. . But,
The technique described in Japanese Patent Application Laid-Open No. 58-61258 requires twice cold rolling and twice annealing, and furthermore, the manufactured steel sheet has an r value,
Although the Erichsen value and the like are high and the workability is excellent, the tensile strength and the like are low and it does not sufficiently satisfy the current required properties as an automobile exhaust system material.

Japanese Patent Application Laid-Open No. 8-333639 discloses that, with Ti as an essential additive element, C: 0.001 to 0.030% and Si: 1.0%.
Mn: 1.0% or less, Cr: 15.0 to 22.0%, Al: 0.002
-0.150%, Ti: 0.02-0.70% and Ti / (C + N) ≧
6, Mo: 0.4 to 2.0%, Cu: 0.10 to 0.60%, B: 0.0003
After hot-rolling a ferritic stainless steel sheet containing up to 0.0050%, heat it at 800-900 ° C for 3-15 hours, or
Heating at 950 to 1050 ° C for 30 to 90 seconds, or holding in a cover maintained at 750 to 850 ° C for 60 to 90 minutes, then cold rolling and soft annealing to produce ferritic stainless steel sheets with good workability A method has been proposed. JP 8-
Steel plate manufactured by the technology described in 333639, deep drawing is possible, does not crack even in secondary processing, does not generate ridging on the processing surface, excellent in ridging and secondary workability Have been.

[0007]

SUMMARY OF THE INVENTION
Steel plate manufactured by the technology described in 8-333639,
As the current automotive exhaust system material, there is also a problem that the strength typified by tensile strength is somewhat low, and furthermore, wall cracking is observed during deep drawing in automotive exhaust system components, and as the current automotive exhaust system material Had a problem that the deep drawability was insufficient.

The present invention advantageously solves the above-mentioned problems of the prior art, and has a yield strength of YS: 24 as an automobile exhaust system material.
0 MPa or more, tensile strength TS: 440 MPa or more, elongation: 28%
It is an object of the present invention to propose a method for producing a ferritic stainless steel sheet having the above and having high strength, excellent ductility, and excellent deep drawability.

[0009]

Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied the improvement of the deep drawability of a ferritic stainless steel sheet. As a result, the slab composition is limited to an appropriate range, and the annealing conditions after hot rolling, that is, the temperature of hot-rolled sheet annealing is set to an appropriate range, so that the yield strength and the tensile strength are high. However, it was found that the r value became higher as compared with the same elongation value.

The present inventors have conducted a hot-rolled sheet annealing while changing the annealing temperature, and further subjected to cold rolling-annealing.
The r value and the tensile properties of the ferritic stainless steel sheet were determined. FIG. 1 shows the results in terms of the relationship between r value and elongation value.
Shown in ○ indicates hot-rolled sheet annealing temperature of 850 to 980 ° C, △ indicates
It is a thing of 1000-1050 degreeC. From FIG. 1, the hot-rolled sheet annealing was
It can be seen that the r-value is significantly higher than that of a steel sheet marked with a triangle (同一) having a high yield strength and tensile strength (not shown) and having the same El by being carried out within the range of 0 to 980 ° C. Further, it has been found that a steel plate having a high yield strength and a high tensile strength and a high r value does not cause wall cracking during the conical cup test and can be drawn out.

The present invention has been completed based on the above findings and further studies. That is, in the present invention, C: 0.05% or less, N: 0.05% or less, Si:
Less than 0.2%, Mn: 2.0% or less, P: 0.05% or less, S: 0.0
3% or less, Cr: 11.0 to 23.0%, Mo: 3.0% or less, Al: 0.0
05-1.0%, and Ti: 0.05-1.0%, B:
0.0002 to 0.005%, Ta: 0.01 to 1.0%, V: 0.05 to 1.0
%, Zr: one or two selected from 0.01 to 1.0%
A slab having a composition comprising at least one seed and the balance of Fe and unavoidable impurities is heated, and then hot-rolled to form a hot-rolled sheet.Then, the hot-rolled sheet is subjected to hot-rolled sheet annealing, and then cold-rolled and annealed. Producing a ferritic stainless steel sheet for automobile exhaust system excellent in deep drawability, wherein the hot-rolled sheet annealing is performed at a temperature in the range of 850 to 980 ° C. Method, and
In the present invention, in addition to the above-mentioned composition, Cu:
1.0% or less, Ni: 1.0% or less, W: 1.0% or less, Sn: 1.
It is preferable to contain one or more selected from 0% or less, and in the present invention, in addition to each of the above-mentioned compositions, it is preferable that Ca: 0.010% or less by mass% is further contained. preferable.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the composition of a slab used in the present invention will be described. C: 0.05% or less C is preferably reduced as much as possible for improving ductility. In ferritic stainless steel, the solid solubility limit of C is low, and C precipitates mainly at the grain boundaries as Cr carbide,
Since it causes intergranular corrosion, it is preferable to reduce as much as possible. If it is contained in excess of 0.05%, ductility is reduced, and a Cr-free layer, coarse precipitates, and inclusions that cause rust increase. For these reasons, C was limited to 0.05% or less. In addition, it is preferably 0.01% or less.

N: 0.05% or less N is, like C, N in ferritic stainless steel.
Has a low solid solubility limit, and N precipitates mainly at the grain boundaries as Cr nitrides, which causes intergranular corrosion. As in the case of C, if the content exceeds 0.05%, the ductility is reduced and the de-Cr layer, which causes rust,
Coarse precipitates and inclusions increase. For these reasons, N is limited to 0.05% or less. Incidentally, preferably 0.02
% Or less.

Si: less than 0.2% Si is an element that acts as a deoxidizing agent, but when contained in a large amount, ductility and deep drawability deteriorate. Therefore, Si is 0.2
%. In addition, it is preferably 0.01 to 0.17%. Mn: 2.0% or less Mn acts as a deoxidizing agent and further combines with S in the steel to prevent cracking during hot rolling. , Lowers cold workability and corrosion resistance. For this reason, Mn was limited to 2.0% or less. In addition, it is preferably 0.05 to 0.50%.

P: 0.05% or less P is an element that deteriorates hot workability, and is preferably reduced as much as possible. Since the adverse effect is not significant up to 0.05%, P is limited to 0.05% or less. In addition,
Preferably it is 0.04% or less. S: 0.03% or less S is an element that forms a sulfide to lower the cleanliness of steel, and also serves as a starting point of rust as MnS and lowers corrosion resistance. It is preferable to reduce as much as possible. Since the adverse effect is not significant up to 0.03%, S is limited to 0.03% or less. In addition, it is preferably 0.01% or less.

Cr: 11.0 to 23.0% Cr is an element effective in improving corrosion resistance, and has an effect of remarkably improving corrosion resistance in concentrated water of an automobile exhaust pipe (muffler). At least 11%
Is required. On the other hand, if the content exceeds 23.0%, it becomes economically expensive. Therefore, Cr is 11.0-23.0%
Limited to.

Mo: 3.0% or less Mo is an element effective in improving corrosion resistance, and has an effect of remarkably improving corrosion resistance in concentrated water of automobile exhaust pipes (mufflers). However, an excessive content reduces the mechanical properties, especially the ductility. For this reason, Mo is 3.
Limited to 0% or less. Incidentally, the content is preferably 0.3 to 1.5%.

Al: 0.005% to 1.0% Al is an element which acts as a deoxidizing agent and also has a function of detoxifying N which binds to N, causes intergranular corrosion and lowers corrosion resistance, and is at least 0.005%. Is required. On the other hand, if the content exceeds 1.0%, processability is impaired. For this reason, Al was limited to the range of 0.005 to 1.0%. In addition, it is preferably 0.01 to 0.10%.

Ti: 0.05-1.0%, B: 0.0002-0.005
%, Ta: 0.01 to 1.0%, V: 0.05 to 1.0%, Zr: 0.01 to
One or more of Ti, B, Ta, V, and Zr selected from 1.0% have the effect of binding C and N and detoxifying C and N, which lower the corrosion resistance. Yes,
One or two or more may be selected as necessary. Such an effect is as follows: Ti: 0.05% or more, B: 0.0002% or more,
Ta: 0.01% or more, V: 0.05%, Zr: 0.01% or more. On the other hand, Ti: 1.0%, B: 0.005%, Ta: 1.
If the content exceeds 0%, V: 1.0%, and Zr: 1.0%, the workability deteriorates. Therefore, if contained, Ti: 0.
05-1.0%, B: 0.0002-0.005%, Ta: 0.01-1.0
%, V: 0.05 to 1.0%, and Zr: 0.01 to 1.0%.

Cu: 1.0% or less, Ni: 1.0% or less, W: 1.
One or more selected from 0% or less and Sn: 1.0% or less Cu, Ni, W and Sn are elements that improve corrosion resistance and are preferably contained as necessary. Cu is an element that improves pitting corrosion resistance, but when contained in large amounts,
Produces martensite and reduces corrosion resistance. Therefore, when it is contained, it is preferable to limit Cu to 1.0% or less. The content is more preferably 0.1% or less.

Ni has the effect of improving the corrosion resistance of concentrated exhaust water of automobile exhaust pipes (mufflers), but when contained in large amounts, it forms martensite and lowers the corrosion resistance. Therefore, when it is contained, it is preferable to limit Ni to 1.0% or less. The content is more preferably 0.5% or less. If W is contained in a large amount, the workability is reduced. Therefore, when W is contained, it is preferably limited to 1.0% or less.

If Sn is contained in a large amount, the workability is deteriorated. Therefore, when Sn is contained, it is preferably limited to 1.0% or less. Ca: 0.010% or less Ca has the effect of preventing nozzle clogging during continuous casting and preventing the occurrence of blistering defects in steel sheets. As a result, corrosion resistance is improved, but a large amount reduces the surface quality. If contained, the content is preferably limited to 0.010% or less.

The balance Fe and inevitable impurities The balance other than the above components is Fe and inevitable impurities. As inevitable impurities, O: 0.01% or less is acceptable. The slab having the above composition is heated and hot-rolled to form a hot-rolled sheet having a predetermined size. The heating temperature and the rolling conditions of the hot rolling need not be particularly limited. Generally, all known conditions are suitable.

The hot rolled sheet is then subjected to hot rolled sheet annealing.
In the present invention, hot-rolled sheet annealing is performed in a temperature range of 850 to 980 ° C. When the temperature of the hot-rolled sheet annealing is lower than 850 ° C., recrystallization is insufficient and a hot-rolled structure remains. For this reason, the ratio of the crystal grains having the (110) orientation vector oriented in the thickness direction increases, which adversely affects the structure after the cold-rolled sheet annealing. Here, when the ratio of crystal grains oriented in the (110) azimuth vector in the thickness direction increases, the deep drawability generally decreases. On the other hand, 980 ℃
Exceeds, the recrystallized grains become coarse and then cold-rolled,
The number of recrystallized grain nucleation sites during cold-rolled sheet annealing is reduced, and a uniform and fine recrystallized structure advantageous for deep drawability cannot be obtained. For this reason, the annealing temperature in hot-rolled sheet annealing was set to a temperature range of 850 to 980 ° C. Note that it is preferable that the temperature exceeds 900 ° C and 960 ° C
It is as follows.

The hot-rolled sheet annealing may be either continuous annealing or box annealing, but continuous annealing has many advantages such as saving of heat energy and improvement of productivity. The holding time at the above-mentioned annealing temperature in hot-rolled sheet annealing is 0.5 in continuous annealing.
1010 min, and preferably 300-1200 min for box annealing. If the holding time is short, recrystallization is not completed, while if too long, the grain growth becomes remarkable. More preferably, it is 1 to 6 min for continuous annealing, and 400 to 900 min for box annealing.

After the hot-rolled sheet annealing, the hot-rolled sheet is further subjected to cold rolling to form a cold-rolled sheet, and then subjected to cold-rolled sheet annealing to obtain a product sheet. It is needless to say that intermediate annealing may be performed during cold rolling, if necessary. However, in the present invention, cold rolling can be easily performed without requiring intermediate annealing. The cold rolling does not need to be particularly limited, and may be performed under ordinary conditions. There is no problem as long as a steel sheet having a predetermined thickness can be obtained.

After the cold rolling, cold-rolled sheet annealing is further performed as finish annealing. The cold rolled sheet annealing is preferably performed at a temperature equal to or higher than the recrystallization temperature. The steel sheet that has been subjected to cold-rolled sheet annealing is regarded as a product sheet. Due to the slab composition and the manufacturing process described above, as a vehicle exhaust system material, it has a yield strength of YS: 240 MPa or more, a tensile strength of TS: 440 MPa or more, and an elongation of 28% or more, high strength, excellent ductility, and Ferritic stainless steel sheet with excellent deep drawability can be manufactured at low cost.

[0028]

EXAMPLE A slab (200 mm thick) having the composition shown in Table 1 was used in Table 2
Hot rolling, hot rolled sheet annealing, cold rolling,
The cold rolled sheet was annealed to obtain a product sheet having a sheet thickness shown in Table 2. What
The intermediate annealing in the cold rolling was not performed. these
Conducted tensile tests and conical cup tests on product boards
, Tensile properties, r-value, wall cracking during conical cup test
The presence or absence of life was determined. (1) Tensile properties Rolling direction (L direction) of each product plate, 45 ° to rolling direction
Direction (D direction), 90 ° to rolling direction (C direction)
JIS 13B specimens were collected and subjected to a tensile test.
The yield strength YS, tensile strength TS, and elongation El were measured. (2) r value Rolling direction (L direction) of each product plate, 45 ° to rolling direction
Direction (D direction), 90 ° to rolling direction (C direction)
JIS 13B test pieces were collected. 15% for these specimens
Strain and thickness strain of each test piece when uniaxial tensile prestrain is applied
And the ratio of width strain to thickness strain, r = ln (w / w₀) / Ln (t / t₀(Where w₀, T₀Is the width and thickness of the specimen before the test
And w and t are the width and thickness of the test piece after the test.
You. ) Is determined in each direction from the following equation._mean= (R_L+2 r_D+ R_c) / 4 gives the average r value r_meanI asked. Where r_LRolled
R value in the direction (L direction), and r_DIs the rolling direction (L direction
Direction) is the r value in the 45 ° direction (D direction) with respect to _cIs
The r value in the 90 ° direction (C direction) with respect to the rolling direction (L direction)
is there. (3) Conical cup test The conical cup test is based on JIS Z 2249.
Collect test specimens of specified dimensions from each product plate, and
Deep drawing using a predetermined die and ball punch
And molded into a cup shape. Test piece molded into a cup
The presence or absence of cracks generated on the walls of the specimens was observed.

The results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

[Table 5]

[0035]

[Table 6]

In the example of the present invention, YS: 240 MPa or more, T
S: 440 MPa or more, El: 28% or more, high strength and excellent ductility, and r _mean is 1.20 or more. Further, the steel sheet is drawn out without occurrence of wall cracks in the conical cup test and has excellent deep drawability. On the other hand, in the comparative examples outside the range of the present invention, the strength was low, the elongation was low, or the r-value was low. Therefore, in the conical cup test, generation of wall cracks was observed before complete drawing out.

[0037]

According to the present invention, high strength, excellent ductility,
Ferritic stainless steel sheet with excellent deep drawability
It can be manufactured at low cost and has a remarkable industrial effect. Further, the steel sheet according to the present invention is not limited to automotive exhaust materials, but can be used for home appliances, building materials, and the like, and has the effect of expanding the use of ferritic stainless steel sheets.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an average r value of a steel sheet and elongation El.

Continuing from the front page (72) Inventor Hideaki Yamashita 1-term Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K037 EA01 EA02 EA04 EA05 EA09 EA12 EA13 EA15 EA17 EA18 EA20 EA23 EA27 EA27 EA31 EA32 EA33 EA35 EB08 FF03 FG00 FH00 HA04 HA06

Claims (3)

[Claims] 1. In mass%, C: 0.05% or less, N: 0.05% or less, Si: less than 0.2%, Mn: 2.0% or less, P: 0.05% or less, S: 0.03% or less, Cr: 11.0 to 23.0 %, Mo: 3.0% or less, Al: 0.005 to 1.0%, and Ti: 0.05 to 1.0%, B: 0.0002 to 0.005%, Ta: 0.01 to
1.0%, V: 0.05 to 1.0%, Zr: 0.01 to 1.0% selected from the group consisting of 0.01 or 1.0%, and a slab having a composition comprising the balance of Fe and unavoidable impurities is heated and hot rolled. Hot-rolled sheet, and then subjected to hot-rolled sheet annealing, then cold-rolled and annealed ferrite stainless steel sheet manufacturing method, wherein the hot-rolled sheet annealing 850
A method for producing a ferritic stainless steel sheet for an automobile exhaust system, which is excellent in deep drawability, which is carried out at a temperature in the range of -980 ° C. 2. The composition according to claim 1, further comprising:
u: 1.0% or less, Ni: 1.0% or less, W: 1.0% or less, S
The method for producing a ferritic stainless steel sheet for an automobile exhaust system according to claim 1, wherein one or more kinds selected from n: 1.0% or less are contained. 3. The composition according to claim 1, further comprising:
2. The composition according to claim 1, wherein the content of a: 0.010% or less.
Or the method for producing a ferritic stainless steel sheet for an automobile exhaust system according to item 2.