KR20150140423A - Heat-resistant ferrite-type stainless steel plate having excellent oxidation resistance - Google Patents

Abstract

C: not more than 0.015%, N: not less than 0% and not more than 0.020%, P: not more than 0.04%, S: 0.001 to 0.001%, Si: 0.3 to 1.5%, Mn: 0.3 to 0.7%, Cr: 11.0 to 17.0%, Cu: 0.8 to 1.5%, Ni: 0.05 to 1.0% The heat resistance and the heat resistance of the steel sheet containing the elements adjusted in the amounts of the elements so that the? Value defined by the following formula (1) is 35 or less within a range of Al: more than 0.01 to 0.1% and Ti: 10 (C + A ferritic stainless steel sheet excellent in oxidation resistance.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant ferritic stainless steel sheet having excellent oxidation resistance,

TECHNICAL FIELD The present invention relates to a heat-resistant ferritic stainless steel sheet excellent in oxidation resistance, which is most suitable for use in an exhaust system member and the like which require particularly high temperature strength and oxidation resistance.

Since exhaust gas components such as exhaust manifolds, front pipes, and center pipes of an automobile pass high temperature exhaust gas discharged from the engine, various characteristics such as oxidation resistance, high temperature strength, and thermal fatigue characteristics Is required.

Conventionally, cast iron is generally used for an exhaust manifold among automotive exhaust members, but an exhaust manifold made of stainless steel is used from the viewpoints of strengthening exhaust gas regulation, improving engine performance, and reducing the weight of a vehicle body . Although the exhaust gas temperature differs depending on the vehicle type and the engine structure, a material having a high temperature strength and oxidation resistance is desired in an environment in which the temperature is 600 to 800 DEG C or so and is used for a long time in such a temperature range.

Among austenitic stainless steels, austenitic stainless steels are excellent in heat resistance and workability, but have a large coefficient of thermal expansion. Therefore, austenitic stainless steels are prone to thermal fatigue failure when they are applied to members subjected to repeated heating and cooling like exhaust manifolds.

On the other hand, ferritic stainless steels are superior in thermal fatigue characteristics and scaling resistance because they have a smaller thermal expansion coefficient than those of austenitic stainless steels. Further, as compared with the austenitic stainless steel, since it does not contain Ni, the material cost is also low, and it is widely used. However, since ferritic stainless steels have lower high temperature strength than those of austenitic stainless steels, techniques for improving high temperature strength have been developed. For example, SUS430J1L (Nb-added steel), Nb-Si-added steel, and SUS444 (Nb-Mo added steel). This was to increase the high temperature strength by solid solution strengthening or precipitation strengthening by Nb.

However, there is another problem that the Nb-added steel has a low r value which is an index of hardening of the product plate, deterioration of elongation, and deep drawability. This is because the presence of solid Nb or precipitated Nb suppresses the development of hardened or recrystallized texture at room temperature, thereby impairing the pressability and the degree of freedom in shaping the exhaust part. Further, since Nb has a high raw material cost and a high manufacturing cost, it is possible to suppress the addition amount of Nb if the high temperature characteristics can be ensured by the addition elements other than Nb, thereby providing a heat resistant ferritic stainless steel plate excellent in workability at low cost . Since Mo added to SUS444 has a high alloy cost, there is a problem that the cost of parts is remarkably increased.

Patent Documents 1 to 6 disclose techniques relating to Cu addition. In Patent Document 1, for the purpose of improving the low-temperature toughness, the addition of Cu is considered to be added in an amount of 0.5% or less and is not added from the viewpoint of heat resistance. Patent Document 2 is a technique which utilizes the action of enhancing the corrosion resistance and weather resistance of steel and is not added from the viewpoint of heat resistance. Patent Documents 3 to 6 disclose techniques for improving the high temperature strength at 600 ° C or 700 to 800 ° C by using precipitation hardening by Cu precipitates.

Japanese Patent Application Laid-Open No. 2006-37176 Patent No. 3446667 International Publication WO2003 / 004714 Patent No. 3468156 Patent No. 3397167 Japanese Patent Application Laid-Open No. 2008-240143

The inventors have conducted investigations to improve the high temperature strength due to the fine dispersion of Cu precipitates by adding Cu in a steel component in which Nb is not added. Further, in the heat resistant steel sheet, significant oxidation resistance has also been studied in detail. As a result, in the case of a steel to which Cu is added in a large amount, there is an example in which the oxidation resistance is extremely lowered in a region exceeding 900 캜, as compared with a steel to which no Cu is added. Particularly, such a tendency appears in low Cr steel.

In the exhaust system member, there is a possibility that the exhaust gas temperature rises while being in an abnormal state, and it is preferable that the exhaust gas system can have stable oxidation resistance even when the temperature exceeds 900 deg. Further, it can be used as a member which is not required to have such a strength.

From the above, it is an object of the present invention to provide a heat-resistant ferritic stainless steel sheet having improved oxidation resistance of Cu-added steel and excellent oxidation resistance.

In the present invention, a new ferritic stainless steel sheet which can be used for exhaust parts suitably for exhaust parts by suppressing the addition of expensive Nb and Mo as much as possible and using relatively inexpensive Cu for the purpose of providing a heat- . As a result, they have already filed a patent application (Japanese Patent Application No. 2010-055944, Japanese Patent Application No. 2010-072889) to invent a Cu-added ferritic stainless steel excellent in heat resistance.

The present invention further investigates the oxidation resistance. As a result, it has been found that, in the case of a low-Cr steel containing Cu, the oxidation resistance is rapidly deteriorated in a temperature range exceeding 900 캜. It has also been found that this phenomenon is correlated with the formation of the? Phase immediately below the oxidation scale, and the oxidation resistance tends to decrease due to the generation of? Phase. However, it has also been found that sufficient oxidation resistance can be maintained if a small amount of γ phase is generated. On the basis of these new knowledge, the inventors of the present invention have examined the addition of various alloying elements and found that there is a correlation between the γ value defined by the following equation (1) and the oxidation resistance.

The above Equation 1 is based on the expression of Castro (Equation 2 below) which is an expression for evaluating the stability of the? Phase. In Equation (2), carbon and nitrogen directly affect the stabilization of the? Phase. On the other hand, in the high-purity ferritic stainless steel to which the present invention is applied, since carbon and nitrogen are almost fixed as carbonitride by Ti at 1000 ° C or lower, they do not directly contribute to the? Stability. The influence of Ti is limited to the portion of the Ti that is not fixed as the carbonitride. Therefore, based on the above-described thinking method, the equation (2) is modified to derive the equation (1).

The above formula (1) is an index indicating the ease of formation of the? Phase at 900 to 1000 占 폚 of the high purity ferritic stainless steel. The larger the number, the easier the formation of the? Phase. According to this formula (1), when the value of? Is not more than the constant value (35), abnormal oxidation and scale separation do not occur even at 930 占 폚, and oxidation resistance is remarkably improved. That is, it is possible to obtain a heat-resistant ferritic stainless steel excellent in oxidation resistance while maintaining the improvement of high-temperature strength by Cu addition by adjusting the alloy components in accordance with this formula.

The present invention has been made on the basis of the above knowledge, and its gist is as follows.

P: 0.04% or less, S: 0.01% or less, Si: 0.3-1.5%, Mn: 0.3-0.7% or less, V: more than 0% to 0.5%, Al: 0.01 to 0.1%, Ti: 10 (C + N) to 0.3% of Cr, Within the scope,

An element which has been subjected to adjustment of the amounts of the elements so that the? Value defined by the following formula (1) is 35 or less,

And the balance being Fe and unavoidable impurities. The ferritic stainless steel sheet of the present invention is excellent in heat resistance and oxidation resistance.

[Equation 1]

(2) The steel sheet according to any one of (1) to (3), further comprising at least one of Nb in an amount of 0.001 to 0.3%, Mo in an amount of 0.01 to 0.5% and B in an amount of 0.0003 to 0.0050% Ferritic stainless steel plate.

(3) The steel sheet according to any one of (1) to (2), further comprising at least one of Zr in an amount of not more than 1.0%, Sn in an amount of not more than 1.0%, and Co in an amount of not more than 0.5% Based ferritic stainless steel sheet.

According to the present invention, a heat-resistant ferritic stainless steel sheet excellent in oxidation resistance can be obtained without adding expensive Nb and Mo, and particularly to a exhaust system member such as an automobile or a boiler, Loses.

Here, those having no lower limit are shown to include inevitable impurity levels. The reason for limiting the present invention will be described below. % Means mass%.

C deteriorates the moldability and corrosion resistance and causes deterioration of high-temperature strength. Therefore, the content of C is preferably less than 0% and not more than 0.015%. In addition, excessive reduction is preferable in the range of 0.002 to 0.010% in view of the increase in refining cost and oxidation resistance.

N, like C, deteriorates the moldability and corrosion resistance and causes deterioration of the high-temperature strength. Therefore, the content of N is preferably as small as possible, 0.020% or less. In addition, excessive reduction is preferable in the range of 0.002 to 0.015% in view of the increase in refining cost and oxidation resistance.

P is a component inevitably contained in the steel, but if it exceeds 0.04%, the toughness decreases, so the upper limit is 0.04%.

S is a component inevitably contained in the steel, but in the present invention, when it is contained in an amount exceeding 0.01%, CaS tends to be formed, so that the upper limit is 0.01%. In addition, when S is less than 0.0005%, the steelmaking cost is considerably increased, so it is preferable to set the lower limit to 0.0005%.

Si is an element that improves oxidation resistance and is a ferrite stabilizing element and therefore is essential in the present invention and is added positively. 0.3% or more. On the other hand, if it exceeds 1.5%, the workability remarkably decreases and the scale peeling is promoted, so that the upper limit is set to 1.5%. Considering the balance between workability and oxidation resistance, 0.4% to 1.0% is more preferable.

Mn is an element that improves oxidation resistance, particularly, an element that improves scale peelability, and is an essential element in the present invention. However, since it has an effect of increasing the amount of oxidation, excessive addition makes it easy to cause abnormal oxidation. In addition, since it is an austenite forming element, in the present invention, the appropriate range is 0.3 to 0.7%. Considering workability, 0.3 to 0.6% is more preferable.

In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. When the content is less than 11.0%, the effect does not appear, so the lower limit is set at 11.0%. Further, Cr is a ferrite stabilizing element. On the other hand, if the Cr content exceeds 17.0%, the α phase is stabilized by the amount of Cr, so that there is no need for mutual adjustment of each element, and the upper limit of the Cr amount of the present invention is set to 17.0%. That is, the present invention exhibits the effect as the lower the Cr steel. The preferred range is from 12.0% to 15.0%.

Cu is an element effective for improving high-temperature strength at high temperature strength, particularly at an intermediate temperature range of about 600 to 800 占 폚. This is mainly due to precipitation strengthening by the formation of Cu precipitates in the temperature region. In addition, even at a temperature exceeding 900 캜, it has an effect of improving the strength to some extent. Since this effect occurs at 0.8% or more, the lower limit was set at 0.8%. On the other hand, if it is added in an amount exceeding 1.5%, the oxidation resistance and workability are deteriorated, so the upper limit is set to 1.5%. Considering the balance between the high-temperature strength, the oxidation resistance and the workability, 1.0 to 1.4% is preferable.

Ni is an element which improves corrosion resistance and high-temperature salting resistance, and the effect is exhibited by addition of 0.05% or more. However, since it is an austenite stabilizing element, excessive addition decreases the oxidation resistance, so 1.0% is the upper limit. In view of processability, a trace amount is preferable, and 0.05 to 0.50% is more preferable.

V is added because it is a ferrite stabilizing element. However, if it exceeds 0.5%, the toughness of the hot rolled steel sheet is lowered, so that the upper limit is 0.5%. From the viewpoint of steelmaking cost and processability, 0.03% to 0.5% is preferable.

Al is not only added as a deoxidizing element but also added as needed in order to improve oxidation resistance. It is also a ferrite stabilizing element and improves oxidation resistance. Excessive addition leads to hardening to remarkably decrease the uniform elongation, and further to decrease the toughness remarkably, so that the upper limit is set to 0.1%. In consideration of generation of surface scratches, weldability, and composition, it is preferably 0.01 to 0.05%.

Ti is an element which bonds with C and N to improve corrosion resistance, intercalation corrosion resistance, room temperature ductility and deep drawability. Particularly, in an exhaust system member or the like in which the steel sheet of the present invention is used, since it is usually a welded structure, the intergranular corrosion resistance is essential, and the Ti addition amount is important. Since these effects are expressed at 10 (C + N)% or more, the lower limit of 10 (C + N)% is set. On the other hand, if it is added in an amount of more than 0.3%, the oxidation resistance is lowered, so that the upper limit is set to 0.3%. From the viewpoint of workability and manufacturability, 10 (C + N) to 0.25% is preferable.

In order to improve the oxidation resistance within the range of these alloying elements, it is necessary to mutually adjust the respective elements so that the value of? Represented by the following formula (1) is 35 or less. If it exceeds 35, a? Phase is easily formed under a scale in a high temperature region exceeding 900 占 폚, and abnormal oxidation tends to occur easily, which is not preferable. In addition, the effect of unavoidable impurities is set at zero. The reason for deriving Equation (1) is as described above.

[Equation 1]

In the present invention, the following elements may be added depending on the application and characteristics.

Although Nb is expensive, it is an element that improves high-temperature strength and is a ferrite stabilizing element, so that even if a trace amount of Nb is added, heat resistance and oxidation resistance can be improved. The effect is expressed at 0.001% or more. If it is added in an amount of more than 0.3%, Fe ₂ Nb is formed coarsely and the effect of improving the high-temperature strength is reduced. Therefore, the upper limit is set to 0.3%.

Although Mo is expensive, it is an element that improves high-temperature strength and is a ferrite stabilizing element. Therefore, if a trace amount is added, heat resistance and oxidation resistance can be improved. The effect is expressed at 0.01% or more. If it is added in an amount of more than 0.5%, the effect of improving high-temperature strength becomes small, so the upper limit is set to 0.5%.

B is an element for improving the secondary workability at the time of press working of a product, and since the effect acts from 0.0003%, the lower limit is set at 0.0003%. Excessive addition causes problems of hardening and intergranular corrosion due to formation of Cr and B precipitates. In addition, welding cracks become a problem, so the upper limit is 0.0050%. Further, when considering the production, it is preferably 0.0003 to 0.0015%.

Zr is a carbonitride-forming element stronger than Ti. The carbonitride can be fixed to a higher temperature, so that the effect of lowering the austenite phase stability can be expected. However, excessive addition causes deterioration in manufacturability, so the upper limit is set to 1.0%.

Sn is an element which is effective for strengthening solubility at a high temperature and has a large atomic radius, but is a element to be added as needed because the decrease in the mechanical properties at room temperature is small. However, if it is added in an excess amount, the composition and weldability are lowered, so the upper limit is set to 1.0%.

Co is an element which improves high-temperature strength, but when it is added in excess, the composition is lowered. Therefore, the upper limit is set at 0.5%.

Next, the manufacturing method will be described. The method for producing a steel sheet according to the present invention comprises the steps of steelmaking - hot rolling - pickling - cold rolling - annealing and pickling. In steelmaking, it is preferable to use a method in which a steel containing the above-mentioned essential components and a component to be added as required is passed through a converter furnace followed by secondary refining. The molten steel to be melted is made into a slab in accordance with a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness in continuous rolling. For the cold rolling conditions, the cold rolling of the stainless steel sheet is usually performed by reverse rolling in a Zenjimier mill or by one-direction rolling in a tandem mill. In the present invention, any rolling method may be employed. However, since tandem rolling is superior in productivity to zen zymed rolling as well as r value, which is an index of workability, is increased in a tandem type rolling machine having a roll diameter of 400 mm or more It is preferable to perform cold rolling.

From the viewpoint of productivity, it is preferable to omit the hot-rolled sheet annealing usually performed in the production of the ferritic stainless steel sheet, but the hot-rolled sheet may be annealed.

The production method of other steps is not particularly specified, but the hot rolling conditions, the hot rolled sheet thickness, the cold rolled sheet annealing temperature, the atmosphere, and the like may be appropriately selected. In addition, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the thickness of the product plate may be selected in accordance with the thickness of the required member.

[Example]

The steel having the composition shown in Table 1 was melted and cast into a slab, and the slab was hot-rolled to obtain a hot-rolled coil having a thickness of 5 mm. Thereafter, the hot-rolled coil was pickled, cold-rolled to a thickness of 2 mm, and subjected to annealing and pickling to obtain a product plate. The annealing temperature of the cold-rolled sheet was set at 850 to 1000 占 폚 in order to make the grain size number 6 to 8 or so. The annealing time is 120 seconds. Nos. 1 to 15 in the tables are the inventive steels, and Nos. 16 to 39 are comparative steels. The No. 1A steel and the No.2A steel were subjected to hot-rolled sheet annealing at 850 to 1000 ° C for 120 seconds, respectively, after the hot rolling with the steel having the No. 1 steel and the No. 2 steel and copper components, Followed by pickling, cold rolling, annealing and pickling. From the product plate thus obtained, a high-temperature tensile test piece was taken and subjected to a tensile test at 800 ° C and 900 ° C to measure a 0.2% proof stress (in accordance with JIS G0567). Here, it is almost equivalent to the 0.4Nb-1Si steel which is the most commonly used as the exhaust manifold steel at present, and the acceptance standard is 25 MPa at 800 ° C and 15 MPa at 900 ° C.

As a test for oxidation resistance, a continuous oxidation test was performed in air at 900 캜 and 930 캜 for 200 hours to evaluate the occurrence of abnormal oxidation (in accordance with JIS Z 2281). In addition, JIS No. 13 B test piece was produced as a workability at room temperature, and a tensile test in the rolling direction was carried out, and fracture elongation was measured. Here too, the passing standard of 32%, which is almost the same level as the existing 0.4Nb-1Si steel, was adopted.

Further, in order to clarify the intergranular corrosion resistance of the welded portion, after performing the tig welding by the TIG welding method, the Strauss test was carried out to examine whether intergranular corrosion occurred.

The test results are shown in Table 1.

As apparent from Table 1, it is found that the steel having the component composition specified in the present invention has no problems at high temperature strength, oxidation resistance, room temperature elongation and intercalation corrosion, and exhibits excellent properties.

On the other hand, in the comparative steels Nos. 16 and 17, the respective element elements are in the scope of the present invention, and the γ value exceeds 35, so that the abnormal oxidation occurs at 930 ° C. and the oxidation resistance deteriorates. In No.18 and 19 steel, C and N are out of the upper limit, respectively, and high temperature strength, oxidation resistance and workability are poor. No. 20 steel lacks Si, and oxidation resistance is poor. No.21 steel is excessively added with Si, resulting in poor workability. In No. 22, the amount of Mn added is small and oxidation resistance is poor. No.23 steel is excessively added with Mn, and oxidation resistance and processability are poor. In No. 24, P was excessively added and the toughness was inferior, and micro-cracks were observed in the hot-rolled steel sheet at the steel sheet producing step. In the No.25 steel, S was excessively added, and generation of CaS, which is the cause of deterioration of corrosion resistance, was confirmed. No.26 steel has a low high-temperature strength and low oxidation resistance due to a small amount of Cr. In No.27 steel, Cu content is small and high-temperature strength is low. No, 28 steel is excessively added with Cu, resulting in poor workability. In No.29 steel, Ni is added excessively, resulting in poor workability. No. 30 steel is excessively added with V, resulting in poor workability. In No.31 steel, Al is added excessively, resulting in poor workability. In No.32 steel, the amount of Ti added is small, and the intercalation corrosion is poor. No.33 Steel is excessively added with Ti, resulting in poor workability. In No.34 steel, Nb is added excessively, resulting in poor workability. In No. 35 steel, Mo is added excessively, resulting in poor workability. In No. 36 steel, B is excessively added, resulting in poor processability and low intercalation corrosion resistance. Thirty-seven, thirty-eight, and thirty-nine steels are excessively added with Zr, Sn, and Co, respectively. However, these steels have poor workability and micro cracks are observed on the hot- .

INDUSTRIAL APPLICABILITY As apparent from the above description, according to the present invention, it is possible to provide a heat-resistant stainless steel sheet excellent in oxidation resistance even when a large amount of expensive alloying elements such as Nb and Mo is not added. Social contribution such as environmental measures by reduction and weight reduction is remarkable.

Claims (3)

In terms of% by mass,
C: more than 0% and not more than 0.015%
N: more than 0% and not more than 0.020%
P: not more than 0.04%
S: not more than 0.01%
Si: 0.3 to 1.5%
Mn: 0.3 to 0.7%
Cr: 11.0 to 17.0%
Cu: 0.8 to 1.5%
Ni: 0.05 to 1.0%
V: more than 0% and not more than 0.5%
Al: 0.01 to 0.1%
Ti: 10 (C + N) to 0.3%
In which the amount of the elements is adjusted so that the value of? Defined by the following formula (1) becomes 35 or less,
And the balance being Fe and unavoidable impurities. The ferritic stainless steel sheet according to claim 1, wherein the ferritic stainless steel sheet has excellent heat resistance and oxidation resistance.
[Equation 1]

The method according to claim 1,
In terms of% by mass,
Mo: 0.01 to 0.5%
B: 0.0003 to 0.0050%
Wherein the ferrite-based stainless steel sheet further contains one or more of the following components: The method according to claim 1,
In terms of% by mass,
Zr: not more than 1.0%
Sn: not more than 1.0%
Co: 0.5% or less
Wherein the ferrite-based stainless steel sheet further contains one or more of the following components: