JPH06280049A - Steel excellent in corrosion resistance and workability - Google Patents

Abstract

PURPOSE:To provide steel excellent in corrosion resistance at a low cost by using steel contg. specified wt.% of Si, Mn, Cr, Al, C, N, Nb, V or the like, and the balance Fe as a base material and forming the layer of a specified metal on the outer face. CONSTITUTION:The componental compsn. of steel is by weight, >=0.01% Si, 5.5 to 9.9% Cr, 0.3 to 3% Al, <=0.02% C, <=0.03% P, <=0.01% S, <=0.02% N, total <=0.03% of C+N and total 0.01 to 1% of one or more kinds among Nb, V, Ti, Zr, Ta and Hf, which satisfies the inequality I, and the balance Fe with inevitable impurities. This steel is used as a base material, and at least on the upper face on the side exposed to the outer face, the layer of a metal having a potential baser than that of the base metal in an aq. soln. environment is formed to make up into 0.5 to 50mum thickness. In this way, the steel excellent in workability can be provided at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having excellent corrosion resistance and workability, and more specifically, it is a severe corrosive environment such as an exhaust system of an internal combustion engine of an automobile or a ship or an environment in which dry and wet conditions are repeated. Relates to a steel having excellent corrosion resistance and excellent workability.

[0002]

2. Description of the Related Art Conventionally, in the exhaust system of an internal combustion engine mainly for automobiles, in order to suppress corrosion from the inner surface or the outer surface, steel obtained by plating ordinary steel with aluminum or zinc has been used. Since the exhaust system is equipped with a catalyst etc. for the purpose of purifying exhaust gas to suppress environmental pollution, the corrosion resistance of such plated steel material becomes insufficient, and 5-10% Cr is contained for the purpose of improving the corrosion resistance of the steel base material. The made steel is the one disclosed in JP-A-63-143240 or JP-A-63-143240
It is proposed in Japanese Patent Publication No. 143241.

In recent years, with the extension of the service life and warranty period of vehicles, Cr has been further added up to about 18%,
Alternatively, high-grade stainless steel to which Mo is further added is often used in the exhaust system. However, even such high-grade stainless steel may not always have sufficient corrosion resistance, since local corrosion like pitting may occur. Further, since such high-grade stainless steel contains a large amount of Cr and Mo, it has poor workability, and is extremely difficult to manufacture in order to form a complicated shape such as an exhaust system member. Also,
Since the manufacturing process is significantly complicated, the processing cost is high and the material cost is high.

As a representative of such an exhaust system, in a steel containing a certain amount of Cr, local corrosion is likely to occur when the environment becomes harsh, and as a means against this, in order to improve resistance to corrosion, Cr or Mo is further added.
Increasing the content of was a very common technical means. In addition, when Cr and Mo are used to retain corrosion resistance, even if they have sufficient corrosion resistance to the exhaust gas environment, they are for the purpose of preventing freezing of the road surface in winter, such as in cold regions of the United States and Canada. When a large amount of salt is sprayed, it is also a problem that the exhaust system member is eroded from the outer surface by the salt content.

[0005]

In view of the above situation, the present invention has a large resistance to a corrosive environment such as an exhaust system of an internal combustion engine, and a strong resistance to a salt damage corrosion from the outer surface or a corrosion in a dry-wet repeated environment. The purpose of the present invention is to provide a low-cost steel that is excellent in workability when manufacturing parts and the like.

[0006]

The present inventors have studied from various points of view to develop a steel having excellent corrosion resistance in a corrosive environment including an exhaust system in order to achieve the above-mentioned object. . First, the inventors of the present invention examined the corrosive environment of the exhaust system, and the corrosion of the internal combustion engine exhaust system was caused by chlorides, sulfate ions, etc. contained in the exhaust gas of 80 to 15
It was found to occur in an environment heated to 0 ° C.

Further, as a result of various studies on means for improving the corrosion resistance in the corrosive environment, contrary to the conventional stainless steel, Cr is reduced to 5.5 to 9.9% and Al to 0.3 to. It has been found that the steel added with 3.0% exhibits extremely excellent corrosion resistance in a corrosive environment including an exhaust system.
Further, the inventors of the present invention continued to study in order to obtain more excellent steel, and as a result, Cr was 5.5 to 9.9% and Al was 0.3 to
After reducing C and N of steel containing 3.0%, Nb,
Addition of V, Ti, Zr, Ta, and Hf so as to satisfy specific conditions has the effect of improving corrosion resistance and workability, and Si and Mn as deoxidizing and strengthening elements.
Is appropriate, Cu, Mo, Sb, Ni in the above steel
It has been found that more excellent corrosion resistance can be obtained by adding these alone or in combination.

On the other hand, the inventors of the present invention have also investigated means for obtaining a steel material having further improved resistance to salt corrosion and repeated salt water wet and dry, and as a result, the content of Cr is 5.5 to 9.9% and Al is 0%. It has also been found that particularly excellent salt damage corrosion resistance can be obtained by forming on the surface of a steel containing 0.3% or more of a metal, a metal layer having a base potential lower than that of the base material in an aqueous solution environment. Furthermore, the present inventors continued to study, and as the metal whose potential in an aqueous solution environment is baser than the base material, aluminum, an alloy mainly containing aluminum, zinc, an alloy mainly containing zinc, chromium, and mainly chromium. It was also found that the above alloys, manganese, and alloys mainly containing manganese are metals suitable for the purpose of the present invention.

The present invention has been made on the basis of these findings, and has as its gist the following matters. % By weight, Si 0.01% or more and less than 1.2%,
Mn 0.02-2.0%, Cr 5.5-9.9%,
Al: 0.3% to 3.0%, C: 0.02%
Hereinafter, P is reduced to 0.03% or less, S to 0.01% or less, N to 0.02% or less, and C + N to 0.03% or less,
1 selected from Nb, V, Ti, Zr, Ta and Hf
0.01 or more in total of the content of one kind or two or more kinds of elements
Containing 1.0% and satisfying the following formula, Using the steel consisting of the balance Fe and unavoidable impurities as a base material,
At least on the upper surface exposed to the outer surface, a metal layer whose electric potential in an aqueous solution environment is baser than that of the base material is 0.5 to 50 μm.
Steel with excellent corrosion resistance and workability, characterized by being formed to a thickness of m.

In the above steel, the base material is further added as an additional component in a weight percentage of Cu 0.05 to 3.0% and Mo 0.05 to 3.0.
%, W 0.05 to 3.0%, Sb 0.01 to 0.5%
And steel containing one or more of Ni 0.01 to 2.0%. In the steels of 1 and 2, the base material is further added as an additional component in a weight percentage of 0.001 to 0.1 of the rare earth element.
% And Ca 0.0005 to 0.03% Steel containing 1 or 2 kinds. In the steels of the above 1 to 4, steels having Al plating layers coated on both sides of the steel sheet.

In the steels 1 to 3, the metal whose electric potential in an aqueous solution environment is baser than that of the base material is either aluminum or an alloy mainly containing aluminum. In the steels 1 to 3, the metal whose electric potential in an aqueous solution environment is baser than that of the base material is either zinc or an alloy mainly containing zinc. In the steels 1 to 3, the metal whose electric potential in the aqueous solution environment is baser than that of the base material is either chromium or an alloy mainly containing chromium. In the steels 1 to 3, the metal whose electric potential in an aqueous solution environment is lower than that of the base material is either manganese or an alloy mainly containing manganese.

The present invention will be described in detail below. First,
The reason for limiting the range of each component of the base material will be described. S
i is effective when added as a deoxidizing agent and a strengthening element to steel containing Cr at 5.5% or more. Content is 0.01
%, The deoxidizing effect is not sufficient, and when 1.2% or more is contained, the effect is saturated and the workability is deteriorated. Therefore, the content range is 0.01% or more and less than 1.2%. Limited to

Mn must be contained as a deoxidizing agent for steel in an amount of 0.02% or more. However, even if it is contained in an amount of more than 2.0%, the effect is not only saturated, but excessively. If Mn is contained, the workability decreases, so the upper limit content is made 2.0%. It is necessary that Cr is contained in an amount of 5.5% or more in order to ensure corrosion resistance against exhaust gas environment and the like. However, if the content exceeds 9.9%, not only the cost is unnecessarily increased but also the workability is deteriorated, so the upper limit content is made 9.9%. Al is an important element along with Cr in order to secure corrosion resistance. As described above, if the Al content is less than 0.3%, the effect of suppressing the occurrence of pitting corrosion is not sufficient, while if added over 3.0%, the effect saturates. Therefore, the content is limited to 0.3% or more and 3.0% or less.

C and N deteriorate the workability of the steel sheet, and C forms Cr and carbide to deteriorate the corrosion resistance.
Since N lowers the toughness, it is desirable that the amounts of C and N be small. The upper limit content is 0.02%,
It is preferable that both are smaller. Further, in order to secure excellent workability, it is necessary to reduce the total amount of C + N, and as the steel to which the present invention is applied, C + N is 0.03%.
% Or less. If P is present in a large amount, it lowers the toughness, so it is desirable that the amount be small, and the upper limit content is 0.03%. If a large amount of S is present, it lowers the pitting corrosion resistance, so it is preferable that the amount of S is small, and the upper limit content is 0.01%.

Nb, V, Ti, Zr, Ta and Hf
Has a remarkable effect in improving corrosion resistance and workability by fixing C and N in high Cr steel as carbides. Although it is possible to add each element alone or to add two or more kinds of elements in combination, the effect is not obtained if the addition amount of each element or the total addition amount of compound addition is less than 0.01%. Further, if added in excess of 1.0%, it will unnecessarily increase the cost and cause rolling defects, etc., so the upper limit content is made 1.0%. Further, in order to effectively improve the workability, it is necessary that the total addition amount satisfies the following formula.

The above are the basic components of the base material of the steel excellent in corrosion resistance targeted by the present invention. However, if necessary, the following elements are further added to improve the properties of the steel as a base material. It also covers steel products. Cu has the effect of improving resistance to general corrosion when added to steel containing Cr 5.5% or more and Al 0.3% or more in an amount of 0.05% or more. However, Cu is added in an amount of more than 3.0%. Not only the effect is saturated, but also the hot workability is deteriorated, so the upper limit content is 3.
0%

Mo is composed of Cr 5.5% or more and Al 0.
Addition of 0.05% or more to steel containing 3% or more has the effect of suppressing the occurrence and growth of pitting corrosion, but addition of more than 3.0% will not only saturate the effect but also reduce workability. Therefore, the upper limit content is made 3.0%. W is Cr
0.0 for steel containing 5.5% or more and Al 0.3% or more
Addition of 5% or more has the effect of suppressing the occurrence and growth of pitting corrosion, but addition of more than 3.0% not only saturates the effect but also reduces the workability, so the upper limit content is 3. 0
%.

Sb is Cr 5.5% or more, Al 0.
Addition of 0.01% or more to steel containing 3% or more has the effect of improving resistance to pitting corrosion and general corrosion, but addition of more than 0.5% reduces hot workability. The amount is 0.5%. Ni is Cr
0.0 for steel containing 5.5% or more and Al 0.3% or more
Addition of 1% or more has the effect of suppressing pitting corrosion,
Even if added over 2.0%, not only the effect is saturated but also the hot workability is deteriorated, so the upper limit content is made 2.0%.

Rare earth elements (REM) and Ca are elements that are effective in improving hot workability and pitting corrosion resistance, but the addition amount of rare earth elements is less than 0.001% and Ca is 0.0005. If it is less than%, the effect is not sufficient. In addition, it exceeds 0.1% for rare earth elements and 0.0 for Ca.
If added in excess of 3%, coarse non-metallic inclusions are generated and conversely deteriorate hot workability and pitting corrosion resistance. Therefore, the upper limit content is 0.1% for rare earth elements and 0.1% for Ca. It was set to 03%. In the present invention, the rare earth elements refer to elements having atomic numbers 57 to 71 and 89 to 103 and Y.

When the steel according to the present invention is used as an exhaust system member or the like, at least the surface exposed to the outer surface is coated with a metal having a base potential lower than that of the base material. This does not necessarily need to be coated when the inner surface is, for example, in the exhaust gas environment of an internal combustion engine, but it is coated with a metal whose potential is lower than that of the base material against salt damage from the outer surface and corrosion due to repeated dry and wet conditions. The reason is that it is effective to blame. If you form a metal whose electric potential is lower than the base material,
Even if there is a defect in the metal layer or the metal layer partially disappears during use, the base material is effectively protected, and high corrosion resistance is exerted by the synergistic action with the corrosion resistance of the base material. . Therefore, if at least one surface corresponding to the outer surface in the finally used state is coated with Al, it is sufficient for the purpose of corrosion resistance to use that surface as the outer surface. That is, the side used as the inner surface need not be coated. However, in the process of forming a metal layer having a base potential, even if the inner surface side is coated at the same time, it is not against the spirit of the present invention.

On the side used as the outer surface, if the thickness of the Al plating is 0.5 μm or less, the corrosion resistance for a long period of 2 years or 3 years or more is not sufficient, and even if it is coated to a thickness of more than 50 μm, Since the effect is already saturated, the productivity is lowered and the cost is increased, so that the coating thickness of at least one surface is 0.5 to 50 μm. For the same reason as described above, the other surface may or may not be coated with a base metal, so that the thickness is not limited. It is desirable that the thickness be considered to be optimum in terms of cost and manufacturing. Also,
In the case where only one side needs to be coated for the purpose of use, even if steel coated on both sides is used from the process of coating a base metal, there is no deviation from the purpose and effect of the present invention. Not something to do. In such a case, whether to use the coating on only one side or the steel coated on both sides may be selected in consideration of other characteristics such as cost and weldability.

Aluminum, zinc, chromium, manganese, and alloys containing these as a main component can be used as an embodiment of the metal that is used for coating and has a lower electric potential than the base material in an aqueous environment. Further, the coating process does not limit the process as long as these metals are sufficiently fixed to the base material, and the application, cost,
Etc. should be taken into consideration when selecting, hot-dip plating, electrodeposition plating, molten salt electrolytic plating, vacuum deposition, sputtering,
Ion plating, etc. can be used.

When the steel of the present invention is used in the exhaust system of an internal combustion engine, it is first manufactured as a steel plate, then formed into a predetermined shape by a press or the like, and further processed / welded to be manufactured as a product. The steel plate may be formed into a steel pipe shape, for example, as an electric resistance welded steel pipe, and then used in a product by secondary processing, welding, and the like. Any steel having a combination is the subject of the present invention.
Further, the optimum product manufacturing process can be selected depending on the cost, restrictions of the existing manufacturing equipment, etc., and which manufacturing process is selected does not depart from the scope of the present invention. Further, the steel proposed in the present invention is applied to various corrosive environments, such as an exhaust system of an internal combustion engine, an environment in which an aqueous solution containing chloride, sulfate ions, etc. is exposed to high temperature, and heating and cooling are repeated. can do.

[0024]

EXAMPLES The present invention will be further described below based on examples. The steel having the components shown in Table 1 is melted and made into a steel plate having a thickness of 1 mm by a normal steel plate manufacturing process such as hot rolling and cold rolling.
After annealing at 40 ° C., each side was coated with 15 ± 2 μm per side under the conditions shown in Table 2. The X value in Table 1 indicates a value calculated by the following formula.

Further, coatings 1 to 4 in Table 2 represent coating 1: aluminum coating, coating 2: zinc coating, coating 3: chromium coating and coating 4: manganese coating, respectively. Next, from these steel plates, width 50 mm, length 70 m
A test piece of m was sampled and subjected to a corrosion test. Sample No. 47 of the comparative steels is SUS4 which is a representative of conventional materials.
It is steel equivalent to 10L.

As a corrosion test corresponding to the internal environment of the automobile exhaust system, sulfate ion 100 ppm, chloride ion 1
The test piece shall be completely evaporated and volatilized by immersing the test piece in half in 50 cm ^{3 of} an aqueous solution containing 00 ppm and bicarbonate ion of 500 ppm in the form of ammonium salt and keeping the test container in an atmosphere of 130 ° C. Was repeated 20 times. The maximum pitting depth of the test piece after the test was measured and used as the test result. Maximum pitting depth is 0.2 mm
The following are ⊚, and the maximum pitting depth exceeds 0.2 mm and is less than 0.
4 mm is ◯, maximum pitting depth is more than 0.4 mm and 0.8 mm or less is x, maximum pitting depth is 0.8 mm
Those exceeding the above are indicated by XX.

Further, as a test assuming the atmospheric environment and the salt damage corrosion of the outer surface of the automobile exhaust system, after spraying salt water at 50 ° C. for 1 hour, after maintaining at 60 ° C. in an environment of 96% humidity for 5 hours,
Further, it was set as a test in which freezing and holding for 1 hour was repeated 100 times. The maximum pitting depth of the test piece after the test was measured and used as the test result. ◎ for maximum pitting depth of 0.2 mm or less, 0.4 m for maximum pitting depth exceeding 0.2 mm
m or less, ○, maximum pitting depth exceeding 0.4 mm and 0.8 mm or less, x, maximum pitting depth 0.8 mm
Those exceeding the above are indicated by XX.

As a workability test, test pieces were taken from the steels shown in Table 1 and subjected to a cylindrical drawing test with a drawing ratio of 1.8 to judge whether or not there was a crack.

[0029]

[Table 1]

The test results of corrosion resistance and workability are shown in Table 2. Regarding the workability in Table 2, ◯ indicates that the result of the cylindrical drawing test was good, and x indicates that cracking occurred in the cylindrical drawing test.

[0031]

[Table 2] As is clear from Table 2, sample numbers 1 to 1 of the present invention steel
No. 18 shows good corrosion resistance even in a very severe corrosive environment such as exhaust environment and salt corrosive environment, and shows excellent workability even in a severe cylindrical drawing test, whereas it is a comparative steel. It can be seen that Sample Nos. 19 to 23 have poor corrosion resistance and insufficient workability.

[0032]

As described above, according to the present invention, it is possible to provide a steel having excellent corrosion resistance in an exhaust system of an internal combustion engine such as an automobile at low cost.

[Table 3]

[Table 4]

Claims (8)

[Claims] 1. Si 0.01% or more by weight%, 1.2
%, Mn 0.02% to 2.0%, Cr 5.5%
.About.9.9% and Al 0.3% to 3.0%, C 0.02% or less, P 0.03% or less, S
0.01% or less, N 0.02% or less and the total amount of C + N 0.03% or less, and selected from Nb, V, Ti, Zr, Ta, and Hf 1
The total content of one kind or two or more kinds of elements is 0.01 to
1.0% and satisfy the following formula, A steel layer consisting of the balance Fe and unavoidable impurities is used as a base material, and a metal layer whose electric potential in an aqueous solution environment is lower than that of the base material is 0.5 to at least on the upper surface exposed to the outer surface.
Steel with excellent corrosion resistance and workability, characterized by being formed to a thickness of 50 μm. 2. The base material as an additional component further comprises, by weight%, Cu 0.05% to 3.0% and Mo 0.05% to 3.%.
0%, W 0.05% to 3.0%, Sb 0.01% to
The steel excellent in corrosion resistance and workability according to claim 1, containing one or more of 0.5% and Ni 0.01% to 2.0%. 3. A base material as an additional component, further comprising, by weight%, rare earth elements 0.001% to 0.1% and Ca 0.
The steel excellent in corrosion resistance and workability according to claim 1 or 2, containing 1% or 2% of 0005% to 0.03%. 4. A steel excellent in corrosion resistance and workability according to claim 1, 2 or 3, wherein a metal layer whose electric potential in an aqueous environment is lower than that of the base material is formed on both surfaces of the steel sheet. 5. The metal whose base potential in an aqueous environment is lower than that of the base material is either aluminum or an alloy mainly containing aluminum.
Steel with excellent corrosion resistance and workability as described in. 6. The corrosion resistance and workability according to claim 1, 2, 3 or 4, wherein the metal whose potential in an aqueous environment is lower than that of the base material is either zinc or an alloy mainly containing zinc. Excellent steel. 7. The corrosion resistance and workability according to claim 1, 2, 3 or 4, wherein the metal whose potential in an aqueous environment is lower than that of the base material is either chromium or an alloy mainly containing chromium. Excellent steel. 8. The corrosion resistance and workability according to claim 1, 2, 3 or 4, wherein the metal whose potential in an aqueous environment is lower than that of the substrate is manganese or an alloy mainly containing manganese. Excellent steel.