JPH08144021A - Method for producing ferritic stainless steel and cold rolled steel sheet thereof - Google Patents

Abstract

(57) [Summary] (Corrected) [Purpose] Ferritic stainless steel with excellent corrosion resistance and workability. [Structure] Cr: 14-19, Ti: 0.02 by weight%
.About.0.40, and in each of the following, Si: 0.5, Mn:
0.5, Al: 0.1, Nb: 0.5, Cu: 1.0,
V: 1.0, Ni: 1.0, Ca: 0.005 or less,
The balance consists of Fe and unavoidable impurities, and as impurities, C: 0.015, N: 0.015, P: 0.0
4 or less and C + N ≦ 0.025; Ti / (C
+ N) ≧ 5; 0.5 × C ≦ S ≦ 0.010; Ti / Mn
Ferritic stainless steel with excellent workability and corrosion resistance, with ≧ 0.5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel having excellent corrosion resistance and workability, and a method for producing a cold rolled steel sheet thereof.

[0002]

2. Description of the Related Art Ferritic stainless steel is less expensive than austenitic stainless steel, has small thermal expansion, and is hard to work and harden. Therefore, it is widely used as a material for building materials such as roofing materials, exhaust manifolds that are repeatedly heated repeatedly, and exhaust gas system parts for automobiles such as mufflers where corrosion resistance is important. In particular, in recent years, the demand for ferritic stainless steel as an automobile exhaust system material has been increasing in place of the conventionally used Al-plated steel sheet.

Corrosion of an automobile muffler includes outer surface corrosion which is promoted by chlorides adhering to the outer surface and inner surface corrosion which occurs when a corrosive liquid is condensed on the inner surface. The inner surface is exposed to a severe corrosive environment where NH ₄ ⁺ , CO ₃ ⁻ , SO ₄ ²⁻ , SO ₃ ^2−, etc. are contained in the condensate and the temperature rises due to high temperature exhaust gas. Therefore, for example, in the case of SUH409L steel, which is a low Cr steel, there are some cases in which the metal thinning due to corrosion is severe and the holes become through holes in a short period of time. Has become.

However, even SUS436 steel containing 17% or more of Cr has a problem of sensitization at the welded portion. Therefore, as can be seen in JP-A-3-219055 and JP-A-4-228547, a measure to add Ti or Nb sufficiently can be considered. Addition of Nb and Ti reduces solid solution C and solid solution N to improve toughness and workability, but addition of a large amount adversely deteriorates workability and causes a problem in formability, particularly pipe forming property. Occurs.

On the other hand, in the production of cold-rolled steel sheet of ferritic stainless steel, the hot-rolled steel sheet is usually annealed by box annealing or continuous annealing, and then cold-rolled. Similar to the production of steel cold-rolled steel sheet, attempts have been made to carry out cold rolling without annealing the hot-rolled steel sheet (so-called hot-rolled sheet annealing). The problem when omitting hot-rolled sheet annealing is the reduction of cold rolling efficiency due to poor workability of the material before cold rolling (hot rolled steel sheet) and the product after cold rolling (cold rolled steel sheet). Is inferior in workability.

[0006] In the case of steel grades having a two-phase mixed structure of ferrite austenite during hot rolling, for example, Japanese Patent Laid-Open No.
52-95527 discloses a method of lowering the yield strength and increasing the elongation by winding at a temperature of 850 ° C or higher, and JP-A-57-70231 discloses the addition of Al and 700 ° C or lower. The method by low temperature winding is shown. However, these methods are not applicable to ferrite single phase Ti such as the steel of the present invention.
It cannot be directly applied to the manufacturing method of the Cr-containing ferritic stainless steel.

[0007]

Ferrite stainless steel containing 14 to 19% of Cr is suitable as a weather resistant material or an automobile material because it has excellent corrosion resistance and workability. However, since the environment in which it is used often contains sulfate ions, chloride ions, etc., which are corrosion factors, rusting due to pitting corrosion,
Intergranular corrosion that occurs in the Cr-deficient region near the weld is a problem.

As described above, it is effective to fix C and N by adding Ti and Nb in order to secure the corrosion resistance of the welded portion, but a sufficient amount of stabilizing element is required to suppress the intergranular corrosion. Must be added. However, addition of a certain amount or more of these elements causes deterioration of workability such as reduction of elongation and Rankford value (r value) of steel.

The inventor of the present invention conducted an intergranular corrosion test, a pitting corrosion test and an automobile muffler inner surface corrosion simulation test, which are specified in JIS, using a highly purified steel in which the contents of C and N were extremely low. went. As a result, although an improvement effect of corrosion resistance due to high purification was observed, it was confirmed that pitting corrosion and erosion along grain boundaries started from the vicinity of the welded portion as in the case of conventional steel. From this, it was found that the effect of improving the corrosion resistance is limited only by reducing C and N.

It is an object of the present invention to provide a ferritic stainless steel having excellent intergranular corrosion resistance and pitting corrosion resistance even in a welded portion and having good workability, and a method for producing a cold rolled steel sheet thereof. is there.

[0011]

The gist of the present invention is a method for producing a ferritic stainless steel of the following (1) and a cold rolled steel sheet thereof (2).

(1) In weight%, Cr: 14 to 19%, Si: 0.5% or less, Mn: 0.5% or less, Al: 0.1% or less, Ti: 0.02 to 0.40%,
Nb: 0-0.5%, Cu: 0-1.0%, V: 0-1.0%, Ni: 0-
1.0%, Ca: 0-0.005%, the balance Fe and unavoidable impurities, C: 0.015% or less as impurities, N: 0.015
%, P: 0.04% or less, and ferritic stainless steel excellent in workability and corrosion resistance satisfying the following formulas.

C (%) + N (%) ≦ 0.025 (%) ・ ・ ・ Ti (%) / (C (%) + N (%)) ≧ 5 ・ ・ ・ 0.5 × C (%) ≦ S (%) ≦ 0.010 (%) ・ ・ ・ Ti (%) / Mn (%) ≧ 0.5 ・ ・ ・ (2) Ferrite stainless steel with the above chemical composition is hot-rolled and wound at 600 ℃ or higher, and hot-rolled sheet annealed. A method for producing a ferritic stainless cold-rolled steel sheet having excellent workability and corrosion resistance, which is characterized by cold rolling without applying heat treatment.

[0014]

[Advantages] With recent advances in refining technology, it has become possible to lower the C and N in steel, enabling the industrial production of high-purity steel with a total C and N content of 100 ppm or less. Became. In the above-mentioned Japanese Patent Laid-Open No. 3-219055, a steel sheet excellent in corrosion resistance and workability can be obtained by suppressing the amount of C to a small amount and adding Ti and Nb in amounts that do not deteriorate workability and secondary workability. It has been shown to be obtained. However, as a result of the same study conducted by the present inventor, even if the highly purified steel with C and N reduced to 50 ppm or less was subjected to a corrosion test, Ti was 10 or more (C + N) and 0.15%. Unless it is above, the corrosion resistance was not improved. Since these corrosion forms were intergranular corrosion or pitting corrosion starting from the intergranular boundary, it seems that there is a limit to the reduction of C and N only to suppress the formation of Cr carbonitride.

Furthermore, as a result of investigating the influence of trace elements on high-purity steel, the contents of C and N were reduced, and
It was confirmed that adding S in a specific range under the condition of containing Ti is effective for improving the intergranular corrosion resistance. S
The reason why the addition of Al is effective in improving the intergranular corrosion resistance of ferritic stainless steel is not clear yet, but it is caused by the decrease of solid solution C due to the formation of stable titanium carbosulfide (Ti ₄ C ₂ S ₂ ). It is estimated that

It is generally said that S is a cause of local corrosion of stainless steel, and the content thereof should be as low as possible. Contrary to this common sense, it was also confirmed that the addition of S under the conditions specified in the present invention does not affect the local corrosion resistance. It is considered that the reason is that since S is fixed as Ti sulfide or Ti carbosulfide, the precipitation of Mn sulfide that most deteriorates the local corrosion resistance is suppressed.

Further, Nb up to 0.5%, respectively
It was also confirmed that the corrosion resistance can be further improved by adding one or more of Cu, V and Ni up to 1.0% and Mo up to 2%.

The ferritic stainless steel of the present invention made on the basis of the above findings is easy to recrystallize due to the coarsening of precipitated carbonitride and the decrease of solute C and N, and hot rolling after hot rolling. By setting the coiling temperature of the plate to 600 ° C or higher, excellent workability can be obtained even if the cold-rolled steel sheet is manufactured by omitting the hot-rolled sheet annealing.

The effects of each component of the ferritic stainless steel of the present invention and the reasons for limiting the content will be described below. The% relating to the content means% by weight.

Cr: Cr is a main element responsible for the corrosion resistance of stainless steel, and 11% or more is required to improve pitting corrosion resistance and intergranular corrosion resistance. On the other hand, if it exceeds 19%, the strength will remarkably increase and the toughness and workability will deteriorate, so the upper limit is 19%.

Si, Mn: Si and Mn are added as deoxidizing agents for steel. However, if the content is excessive, the workability and corrosion resistance may be impaired, so the respective contents were suppressed to 0.5% or less. The lower limit may be substantially zero.

Al: Al is an element having a very large deoxidizing ability of steel, and is effective in increasing the deoxidizing and adding yield of Ti, and also has the effect of increasing the oxidation resistance. On the other hand, excessive addition may lead to hardening of the steel and deteriorate workability, so its content should be kept below 0.1%. The Al content may be substantially 0, but in order to obtain the above effect, it is desirable to contain 0.005% or more.

Ti: Ti is added to improve workability and corrosion resistance. In the steel of the present invention, it is less due to the addition of S described later.
The effect of fixing C and N is exhibited even with the Ti amount. But,
Even if S in the range described below is added, the Ti content is less than 0.02%,
Or if Ti (%) / (C (%) + N (%)) is less than 5,
The effect of fixing C and N in steel as Ti (C, S), TiC, and TiN is not sufficient, and the workability improving effect is not clearly seen. On the other hand, when the Ti content exceeds 0.40%, the steel is hardened, and the high workability aimed at by the present invention cannot be obtained.

In addition, Ti (%) / Mn (%) must be 0.5 or more in order to suppress precipitation of Mn-based sulfide by adding Ti. The precipitation density is increased and the local corrosion resistance is reduced.

For the above reasons, the Ti content is 0.02 to 0.40%.
And satisfying the above equations and.

S: S is generally considered to be an element that deteriorates corrosion resistance, but in the present invention, it is positively used to improve workability and corrosion resistance. As described above, the principle of the action and effect of S in the steel of the present invention has not been fully clarified yet, but it is estimated as follows.

S forms a stable titanium carbosulfide Ti (C, S) together with C and Ti to reduce the solid solution C. As a result, the grain size of the precipitates increases and the precipitation density decreases, resulting in a steel material that is easily recrystallized, and deep drawability and workability such as elongation are improved. Also, the intergranular corrosion resistance in the welded portion is improved.

As shown in Examples described later, when S is contained in an amount of 0.5 times or more of the content of C, the workability and corrosion resistance are significantly improved. However, when the total content of S exceeds 0.010%, TiS alone comes to be precipitated and the pitting corrosion resistance decreases.

Therefore, the content of S was defined so as to satisfy the above formula.

Nb, Cu, V, Ni, Mo: These are elements added as necessary in order to further enhance the corrosion resistance of the ferritic stainless steel. When Nb exceeds 0.5%, C
u, V and Ni exceed 1.0% respectively, and M
When o exceeds 2%, the degree of improvement in corrosion resistance corresponding to the increase in the content decreases, and there is no effect comparable to the cost increase. In addition, workability may be impaired.

Ca: Ca is also an element that can be added if necessary. Ca prevents nozzle clogging due to oxides and nitrides adhering to the nozzle during continuous casting. This eliminates the accumulation of inclusions in the slab and improves workability and corrosion resistance. In order to ensure such effects, the content of 0.0005% or more is desirable. On the other hand, if the Ca content exceeds 0.005%,
The precipitates in the steel undergo a morphological change from Al-Ca-based complex oxides to Ca oxides, and the amount of precipitation also increases depending on the amount added. Ca oxide is a starting point of initial corrosion and also has poor ductility, which causes deterioration of corrosion resistance and workability. Therefore, if Ca is added, its content should be 0.005% or less.

C, N, P: These are impurities which should be as small as possible.

C and N increase the strength of the steel and improve the workability,
Deteriorates toughness. Further, the solid solubility limit in ferritic stainless steel is small, and it precipitates as Cr carbonitride and causes intergranular corrosion. Therefore, the contents of C and N are
It was set to 0.015% or less and the total content satisfies the above formula. Since P is an element harmful to corrosion resistance, it is 0.
Limited to 04% or less.

Next, a method for manufacturing a cold rolled steel sheet of ferritic stainless steel according to the present invention will be described.

Steel having the above-mentioned chemical composition is melted by a conventional method, and slabs are formed by ingot ingot or continuous casting. Then, hot rolling is performed. The conditions may be the usual hot rolling conditions for ferritic stainless steel.

The coiling temperature after hot rolling must be 600 ° C. or higher. It is known that recrystallization of ferritic stainless steel is promoted by accelerating grain growth, but as in the steel of the present invention, the number of precipitates is reduced by reducing C and N and adding S. When the reduced steel is wound at a temperature of 600 ° C or higher (upper limit is about 800 ° C), the growth of crystal grains is further promoted, and recrystallization becomes easier. The hot-rolled steel sheet after winding can be cold-rolled without annealing, and the cold-rolled steel sheet thus produced has high elongation and r-value and is excellent in workability.

[0037]

[Example 1] Steels having chemical compositions shown in Tables 1 and 2 were vacuum-melted, cast into a 25 kg ingot, forged to a thickness of 40 mm, heated to 1150 ° C, and hot-rolled to a thickness of 4.5 mm in 7 passes. Rolled.

The hot rolling finishing temperature was 900 ° C. After hot rolling, it was air-cooled to a predetermined coiling temperature. After reaching the coiling temperature, it was gradually cooled at a cooling rate of 50 ° C / hr. For some steels, hot-rolled sheet annealing was carried out at 950 ° C for 5 minutes.

The hot-rolled steel sheet obtained as described above was cold-rolled to a thickness of 1.0 mm by a cold rolling machine with a work roll diameter of 500 mm, and after finish annealing was carried out at 950 ° C. for 2 minutes, workability and corrosion resistance were improved. investigated.

[Evaluation of Workability] The workability is investigated according to JIS Z
It was carried out on a tensile test piece having a shape of No. 13B of 2201. For elongation, the value in the direction of 90 ° to the rolling direction is used, and the r value, which is an index of deep drawability, is the average value of the values in the directions of 0 °, 45 °, and 90 ° [(r ₀ + 2r ₄₅ + r ₉₀ ) / 4] was used.

[Evaluation of Corrosion Resistance] Regarding the corrosion resistance, an intergranular corrosion test and a pitting corrosion test were performed. After pickling the steel sheet after the finish annealing, test pieces were taken from the tanned material by the following TIG method. Specimen is wet # 320 polished
The thickness was 0.8 mm and the test was performed.

TIG licking method: flat plate bead (to the extent that a back wave appears). No filler.

Clamped to the base plate to prevent buckling deformation.

TIG licking conditions: current 30 mA, voltage 7
V, speed 15 cm / min.

Shielding gas (Ar) flow rate 15 l / min.

Intergranular Corrosion A sulfuric acid-copper sulfate corrosion test (Strauss test) specified in JIS-G-0575 was carried out. Evaluation was carried out by measuring the corrosion weight loss after boiling and soaking for 16 hours and observing the cracking condition after 0.5R bending. Further, steel having a Cr content of 15% or more was further boiled and soaked for 72 hours in a sulfuric acid-ferric sulfate corrosion test (striker test) specified in JIS-G-0572. When corrosion is significant, ferric sulfate may be insufficient and may be actively dissolved, so the amount of ferric sulfate added to sulfuric acid was set to twice the JIS standard for the purpose of preventing this. The evaluation was performed by measuring the corrosion weight loss and visually observing cracks after contact bending.

Pitting corrosion As a pitting corrosion test, a complex corrosion test under the following conditions was carried out in a test apparatus according to JIS D 0201 Annex 2, and evaluated against a rating number standard chart. Test piece thickness 1.0
mm × width 60mm × length 90mm, wet surface and end
# 600 Polished one was used. Pitting depth was measured after descaling with boiling diammonium citrate.

(Compound Corrosion Test Conditions) Salt Spray (35 ° C., 5
% NaCl aqueous solution, 4 hours) → Drying (60 ℃, 2 hours, humidity 50)
%) → wet (50 ° C., 2 hours, humidity 98%) as one cycle, and 50 cycles were performed.

The above test results are summarized in Tables 3 and 4.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

[Table 4]

As is apparent from Tables 3 and 4, the steels of the present invention are excellent in workability represented by elongation and R value, and are also extremely excellent in corrosion resistance under all conditions. And the characteristic is not influenced by the presence or absence of hot-rolled sheet annealing. That is, the steel of the present invention is a cold-rolled steel sheet having excellent properties even when cold rolling is performed without the step of hot-rolled sheet annealing.

[0055]

Example 2 Steel sheets having the chemical compositions shown in Table 5 were manufactured by an actual manufacturing process, and workability and corrosion resistance were evaluated.

In the manufacturing process, molten steel was smelted by the VOD method, and then a slab having a thickness of 200 mm and a width of 1200 mm was formed by a continuous casting facility. This slab was cut, and cold-rolled steel sheets were manufactured by the process of the present invention without hot-rolled sheet annealing (a process similar to the production of ordinary steel) shown in Table 6 and a process including conventional hot-rolled sheet annealing.

[0057]

[Table 5]

[0058]

[Table 6]

[Evaluation of Workability] The workability of the steel sheet manufactured by the above process was evaluated by the same method as in Example 1 by performing a tensile test on a test piece having a shape of JIS Z 2201 No. 13B.

[Evaluation of Corrosion Resistance] Intergranular Corrosion The intergranular corrosion resistance was evaluated by JIS G 0572 (striker test) as in Example 1.

Pitting corrosion As a pitting corrosion test, the pitting corrosion potential specified in JIS G 0577 was measured.

Test solution: 0.1 M sodium chloride aqueous solution Test piece: Plate thickness × width 25 mm × length 30 mm, spot welded in the center Welding conditions: current 7000 A, energization time 20 cycles, pressure 2
For the evaluation of 50 kg, the electric potential = Vc'10 when a current of 10 µA / cm ² flows was used.

Car Muffler Inner Surface Corrosion Simulation Test The car muffler inner surface corrosion resistance was evaluated by a long-term half-immersion test in a solution simulating the sampled corrosive liquid. The outline of this test is shown in FIG. The test piece 3 was placed on the supporting jig 2 in the semi-enclosed test container 1 having the cooling water circulation function shown in the figure, and was semi-immersed in the test solution 4, and the test was conducted under the following conditions. The corrosion resistance was evaluated by measuring the maximum pitting depth.

Test liquid composition: As shown in Table 7 Test piece: Plate thickness × width 50 mm × length 100 mm Immersion temperature: 80 ° C. Immersion time: 500 hours Consider the temperature rise when used as a muffler before immersion. Preheat treatment was carried out at 300 ° C for 10 hours. Table 8 shows the results of the above tests.

[0065]

[Table 7]

[0066]

[Table 8]

As is clear from the results shown in Table 8, the steel of the present invention has the same properties as those obtained by performing the annealing without hot rolling. In addition, the corrosion resistance of the welded portion, especially the corrosion resistance in the automobile muffler simulation test, is dramatically improved compared to the comparative steel.

[0068]

INDUSTRIAL APPLICABILITY The ferritic stainless steel of the present invention has excellent workability and corrosion resistance, especially the corrosion resistance of the welded portion, and is therefore suitable as a welded structural material such as an automobile muffler material. Moreover, since the cold-rolled steel sheet of this steel can be manufactured in the same process as the cold-rolled steel sheet manufacturing of ordinary steel without hot-rolled sheet annealing, it has a great advantage of being inexpensive.

[Brief description of drawings]

FIG. 1 is a diagram showing a procedure of a corrosion test simulating an internal environment of an automobile muffler.

[Explanation of symbols]

1 ... Test container, 2 ... Support jig, 3 ... Test piece, 4 ...
Test solution

Claims (2)

[Claims] 1. By weight%, Cr: 14-19%, Si: 0.5% or less,
Mn: 0.5% or less, Al: 0.1% or less, Ti: 0.02 to 0.40%, Nb: 0
~ 0.5%, Cu: 0 ~ 1.0%, V: 0 ~ 1.0%, Ni: 0
〜1.0%, Ca: 0〜0.005%, balance of Fe and unavoidable impurities, C: 0.015% or less as impurities, N: 0.01
Ferritic stainless steel with 5% or less and P: 0.04% or less and excellent workability and corrosion resistance satisfying the following formulas. C (%) + N (%) ≤ 0.025 (%) ・ ・ ・ Ti (%) / (C (%) + N (%)) ≥ 5 ・ ・ ・ 0.5 × C (%) ≤ S (%) ≤ 0.010 ( %) ・ ・ ・ Ti (%) / Mn (%) ≧ 0.5 ・ ・ ・ 2. A workability characterized in that the ferritic stainless steel having the chemical composition according to claim 1 is hot-rolled, wound at 600 ° C. or higher, and cold-rolled without hot-rolled sheet annealing. A method for manufacturing a ferritic stainless cold-rolled steel sheet having excellent corrosion resistance.