JP2000073147A - Chromium-containing steel excellent in high temperature strength workability and surface property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stock applicable to a wide temp. range from high temp. parts to low temp. parts of an exhaust member, excellent in high temp. strength and workability and having good surface properties. SOLUTION: This steel has a compsn. contg., by weight, <=0.02% C, <=0.10% Si, 0.4 to 0.2% Mn, <=0.04% P, <=0.02% S, 3.0 to 20% Cr, <=1.0% Ni, <=0.02% N and 0.2 to 1.0% Nb, contg., at need, one or more kinds among <=0.5% Ti, <=0.5% Zr, <=0.5% V, <=0.5% Al, <=3.0% Mo, <=1.0% Cu, <=0.3% rare earth metals, <=0.03% Ca and 0.005% B, and the balance Fe with inevitable impurities, and, in the steel before working, the content of Nb as Fe2Nb Laves phases is suppressed to low value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr-containing steel excellent in high-temperature strength, workability and surface properties suitable for use in engines of automobiles and motorcycles and exhaust system members of thermal power plants.

[0002]

2. Description of the Related Art Among members used in an exhaust system environment of an automobile, an exhaust manifold needs to have excellent high-temperature strength. Conventionally, Nb-added SUS430J1L (17Cr-0.4Si-
0.4Nb-0.5Cu) steel has been used. However, in general, Nb
Since the added steel has a high strength at room temperature, it is difficult to process the steel, and its improvement has been demanded. On the other hand, since exhaust system members used in a relatively low temperature range, such as a muffler, require a low yield strength (YS) from the viewpoint of workability, conventional Nb-added steel is not suitable, and Alternatively, Ti-added steels whose corrosion resistance has been improved by adding Mo have been mainly used. As this Ti-added steel, for example, Type432 (17Cr-T
i-0.5Mo) steel and Type436 (17Cr-Ti-1.3Mo) steel, etc., which have been selected and applied according to the required corrosion resistance. However, these Ti-added steels have low high-temperature strength, cannot be applied to high-temperature members, and have poor surface properties due to the generation of coarse TiN, requiring surface care.
There was a problem that the productivity was poor.

[0003]

As described above, conventionally, when manufacturing exhaust system members, Nb-added steel and Ti-added steel are distinguished and used depending on the operating temperature range of a high temperature portion and a low temperature portion. This has been a bottleneck for both material and product manufacturers in terms of production efficiency and management. For this reason, there has been a strong demand for the development of a steel type that can be used uniformly over a wide temperature range from a high temperature part to a low temperature part. Several proposals have been reported to respond to such a request. For example, in Japanese Patent Application Laid-Open No.
A ferritic stainless steel of 6 to 1.5% Si and 16 to 22% Cr for automotive exhaust system members is disclosed. However, this steel assumes the unification of the material in the high temperature part of the exhaust system as clearly indicated in the specification from the exhaust manifold to the center pipe, and cannot be applied to the low temperature region up to the muffler. In addition, since this steel type contains a relatively high amount of Si, the strength at room temperature was high and the workability was poor. Also, JP-A-57-60056 discloses that
A ferritic stainless steel with improved brazeability is disclosed. However, this steel also has a problem in that the Si content is 0.38% or more, the strength at room temperature is high, and processing is difficult.

Therefore, the present invention is applicable to exhaust system members such as automobile engines, motorcycle engines, and power generation plants in a wide temperature range from a high temperature part to a low temperature part. An object is to provide a material having surface properties.

[0005]

Means for Solving the Problems The present inventors have proposed that Nb be 0.2%
(Wt%, hereinafter simply referred to as%) More experiments were conducted, focusing on how to suppress the increase in strength at room temperature, based on a component system having increased high-temperature strength. FIG.
Means that the component composition is (0.005 to 0.010%) C- (0.05 to 0.20%)
%) Mn- (0.01 to 0.03%) P- (0.003 to 0.005%) S- (1
4.0-15.0%) Cr- (0.05-0.20%) Ni- (0.004-0.012
%) N- (0.45 to 0.50%) Nb is used to arrange the effect of Si on the yield strength YS using a cold-rolled annealed plate having a thickness of 2 mm. In addition, the parentheses also show the X-ray intensity ratio of Fe ₂ Nb Laves phase and Nb (C, N) by X-ray diffraction described later. From FIG. 1, a new finding has been obtained that when the Si content is lower than 0.10%, the YS at room temperature is rapidly reduced. Conventionally, since Si is a solid solution strengthening element,
It has been known that YS at room temperature decreases with the decrease. However, when Si was reduced to a certain critical amount or less, a phenomenon that YS at room temperature rapidly decreased as shown in FIG. 1 could not be expected.

Although the reason why such a phenomenon has appeared is not necessarily clear, the following mechanism is considered in consideration of the observation result of the Fe ₂ Nb Laves phase in FIG. That is, the increase in strength at room temperature due to the addition of Nb is caused not only by fine Nb (C, N) but also by the Fe ₂ Nb Laves phase. When 0.3% or more of Nb is added to ensure high-temperature strength, the Fe ₂ Nb Laves phase easily precipitates and the strength at room temperature is remarkably increased in a normal Si content range. On the other hand, the Si content is 0.1%
Below, the Fe ₂ Nb Laves phase significantly decreases,
As a result, the increase in strength is reduced, and YS at room temperature is rapidly reduced. At the same time, it was found that the surface properties were improved with low Si. Also, at high temperatures, even low Si materials
It was also found that Fe ₂ Nb easily precipitated and exhibited high high-temperature strength. To determine the degree of precipitation of the Fe ₂ Nb Laves phase, the extraction residue was subjected to X-ray diffraction, and (112) and Nb of the Fe ₂ Nb Laves phase were determined.
Comparison was made based on the intensity ratio of (C, N) to (200). As described above, even if Nb is added, by controlling the Si content, it is possible to suppress the rise in YS at room temperature,
The inventors have concluded that excellent high-temperature strength, workability, and even better surface properties can be imparted together, and have completed the present invention. The summary configuration is as follows.

(1) By weight%, C: 0.02% or less, Si: 0.10%
%, Mn: 0.4 or more and 2.0% or less, P: 0.04% or less,
S: 0.02% or less, Cr: 3.0 to 20%, Ni: 1.0% or less,
N: 0.02% or less, Nb: 0.2 to 1.0%, the balance being Fe
Cr-containing steel with excellent high-temperature strength, workability and surface properties, characterized by being composed of unavoidable impurities.

(2) In the steel according to the above (1), in addition to the above components, Ti: 0.5% or less and Zr: 0.5% by weight.
%, V: 0.5% or less, and Al: 0.5% or less. A Cr-containing steel excellent in high-temperature strength, workability and surface properties, characterized by containing one or more selected from the group consisting of: .

(3) In the steel according to the above (1) or (2), in addition to the above components, of the following, by weight, Mo: 3.0% or less, Cu: 1.0% or less, and REM: 0.3% or less. Excellent in high-temperature strength, workability and surface properties, characterized by containing one or more selected from the group consisting of
Cr-containing steel.

(4) The steel according to any one of the above (1) to (3), further comprising
And one of Ca: 0.03% or less and B: 0.005% or less
High temperature strength, characterized by containing one or two species,
Cr-containing steel with excellent workability and surface properties.

(5) In the steel according to any one of the above (1) to (4), the form of Nb in the steel is determined by X-ray diffraction of a precipitate (extraction residue) of Fe ₂ Nb. (112) Strength and Nb (C,
A Cr-containing steel excellent in high-temperature strength, workability and surface properties, wherein the ratio of (N) to (200) strength is 0.2 or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION C: 0.02% or less C is an element that deteriorates toughness and workability.
%, The adverse effect on toughness and workability becomes remarkable. Therefore, the content is limited to 0.02% or less. From the viewpoint of toughness and workability, the C content is preferably as low as possible.
It is desirable to limit it to 0.008% or less.

Si: 0.10% or less Si is one of the particularly important elements. As shown in FIG. 1, when the Si content of the Nb-added steel exceeds 0.10%, the Fe ₂ Nb Laves phase is remarkably easily precipitated, high YS, and the surface properties are deteriorated. Therefore, the Si content is limited to 0.10% or less.

Mn: 0.4% or more and 2.0% or less Mn has a role as a steel deoxidizing agent.
% Or more. When added in excess, it forms MnS, reduces workability, and reduces the corrosion resistance of the muffler weld. Therefore, the Mn content is 2.0% or less, preferably 0.75%
Or less, more preferably 0.6% or less.

P: 0.04% or less Since P is an element that deteriorates the toughness and the corrosion resistance of the muffler weld, the smaller the better, the better. Therefore, the P content is 0.
04% or less, preferably 0.03% or less, more preferably 0.1% or less.
015% or less.

S: 0.02% or less S is an element that lowers the elongation and the r value to deteriorate workability and also deteriorates corrosion resistance, which is a basic characteristic of stainless steel. Therefore, the content is set to 0.02% or less, preferably 0.01% or less. However, lowering excessively causes high cost and becomes impractical, so the lower limit is 0.00
It is desirable to keep it at about 2%.

Cr: 3.0 to 20% Cr is an element useful for improving oxidation resistance and corrosion resistance. These improvement effects appear when the addition is 3.0% or more. However, when the addition exceeds 20%, the workability is remarkably deteriorated, so the content is set in the range of 3.0 to 20%. The amount of Cr may be selected within this range according to the level of oxidation resistance and corrosion resistance required for the exhaust system member. And, especially when considering the workability, the Cr content is preferably 9.0 to 19.0.
%, More preferably 10.0 to 17.0%.

Ni: 1.0% or less Ni has the effect of improving the toughness, but since it makes the ferrite structure unstable, it is 1.0% or less, preferably 0.1% or less.
It is added in a range of from 0.5 to 0.8%, more preferably from 0.5 to 0.8%.

N: 0.02% or less N is an element that deteriorates the toughness and workability of steel.
In consideration of these adverse effects, the N content is set to 0.02% or less, preferably 0.01% or less.

Nb: 0.2-1.0% Nb is an element having an effect of enhancing high-temperature strength, workability, and intergranular corrosion resistance of a weld. In order to exert such an effect, it is necessary to add 0.2% or more.
%, Si content of 0.1% or less,
A large amount of Fe ₂ Nb Laves phase precipitates and degrades the toughness and surface properties of steel, so it is added in the range of 0.2 to 1.0%. The preferred range of addition of Nb is more than 0.4 to 0.6%, more preferably 0.45 to 0.55%.

In addition to the above basic elements, the following elements can be added according to required characteristics. Ti: 0.5% or less Ti has the effect of improving the r value, but if added in excess of 0.5%, coarse Ti (C, N) precipitates and deteriorates the surface properties. Add within 0.5%. Preferably, it is set to 8 (% C +% N) or less.

Zr: not more than 0.5% Zr has the effect of improving the r value and oxidation resistance. However, if added in excess of 0.5%, coarse Zr (C, N)
Precipitates and deteriorates the surface properties, so it is added in a range of 0.5% or less. Preferably, it is 15 (% C +% N) or less.

V: not more than 0.5% V has the effect of improving the r value, but if added excessively, coarse V (C, N) precipitates and deteriorates the surface properties. Add within the range. The preferred amount of V is in the range of 0.03% or more and 15 (% C +% N) or less.

Al: 0.5% or less Al is generally inevitably contained when Al deoxidization is performed, but does not have any adverse effect. this
Al forms a surface protection scale at the time of welding and suppresses a decrease in toughness due to penetration of C, N, and O from the atmosphere.
Addition of at least 005% is preferred. However, if it is added in excess of 0.5%, the processability is significantly reduced, so the content is limited to 0.5% or less. The preferred range of addition of Al is more than 0.05 to 0.2%.
It is. Further, it is desirable to satisfy 15 (% C +% N) or less. However, since Al deoxidation is costly,
When importance is attached to the cost, it is preferable not to perform it.

Mo: 3.0% or less Mo is an element that has a solid solution strengthening effect, is effective in improving high-temperature strength, and is also effective in improving corrosion resistance. However, since Mo is also an expensive element, it is better to keep it within the range of 3.0% or less in order to avoid excessive cost increase.

Cu: 1.0% or less Cu is an element effective for improving the corrosion resistance and workability, and particularly, the addition of 0.1% or more is effective. Excessive addition may cause embrittlement due to precipitation of ε-Cu, so the addition amount is 1.0% or less, preferably more than 0.15 to less than 0.3%.

REM: 0.3% or less REM is a general term for lanthanoid elements such as La and Ce and Y and Sc, and is industrially added as a misch metal or the like. This REM is an element effective for improving the oxidation resistance, and its effect increases as the amount of addition increases. However, if added in excess of 0.3%, the toughness decreases, so it is limited to 0.3% or less.

Ca: 0.03% or less Ca is an element having an effect of suppressing nozzle clogging during casting of a slab. However, the addition of more than 0.03% not only saturates the effect, but also causes pitting corrosion starting from Ca-containing inclusions and deteriorates the corrosion resistance.

B: 0.005% or less B is an element effective for improving the secondary workability. 0.005%
If added in excess of 0.005%, a large amount of BN is formed and the workability deteriorates. The preferred content range is 0.0004% or more at which the effect is remarkable, and 0.0015% or less at which there is almost no deterioration in workability.

Existence form of Nb in steel before working As shown in FIG. 1, the Laves phase of Fe ₂ Nb causes an increase in YS and deterioration of surface properties, so that Nb in the steel before working becomes Fe ₂ N
b Desirably not present as Laves phase. The presence of the Fe ₂ Nb Laves phase in the steel can be known by X-ray diffraction of the extraction residue. Since Nb (C, N) is preferentially precipitated, the diffraction peak of Nb (C, N) and the ratio of that of Fe ₂ Nb were examined. Thus, the lower the diffraction intensity ratio of the Fe ₂ Nb Laves phase detected by X-ray diffraction of the extracted residue, that is, the smaller the amount of Nb as the Fe ₂ Nb Laves phase, the lower the YS and the better the surface properties. Can be achieved. It is most preferable that the Fe ₂ Nb Laves phase is not detected by X-ray diffraction during the extraction residue. Note that Fe ₂ N
b The Laves phase precipitates and is detected when heated at high temperatures after processing, even if it is not detected before processing. In such a case, it is important that Fe ₂ Nb does not exist in the steel or is in a small amount before working.

In the production of the invention steel described above, a method generally adopted for Cr-containing steel such as stainless steel can be applied almost as it is. As an example of the preferred manufacturing process and its specific conditions, annealing at a temperature of 950 ° C or higher after hot rolling, a cold rolling reduction of 60% or higher, and a final finish annealing temperature of 950 ° C or higher, Deposited
It is desirable to dissolve the Fe ₂ Nb Laves phase.

[0032]

Next, the present invention will be specifically described based on examples. After ingoting steel having the composition shown in Table 1, the steel was heated to 1250 ° C., and then hot-rolled into a hot-rolled sheet having a thickness of 5 mm. Annealing, pickling, cold rolling, finish annealing, and pickling were sequentially performed on the hot-rolled sheet to obtain a cold-rolled annealed sheet having a thickness of 2 mm. Various evaluations were performed on the thus obtained cold-rolled annealed sheets by the following methods. (1) High-Temperature Strength A 0.2% proof stress at 700 ° C. was measured at a tensile rate of 0.3% / min using a plate-like test piece having a thickness of 2 mm. The value was evaluated according to the following criteria. 100 MPa or more: AA 80 MPa or more, less than 100 MPa: A 50 MPa or more, less than 80 MPa: B less than 50 MPa: C (2) Workability at room temperature From the rolling direction, a JIS 13B tensile test piece (sheet thickness) 2m
m) was sampled, the yield strength YS was measured, and the elongation was measured. It can be said that the smaller the YS and the larger the elongation, the better the workability. (3) Surface properties In the state of the hot rolled sheet, it was visually determined whether or not surface care was necessary.

[0033]

[Table 1]

Table 2 summarizes the results of these tests.
Among the comparative examples, the steel A is the conventional steel SUS430J1L, which has too much Si, so that a large amount of Fe ₂ Nb Laves phase is detected, the YS at room temperature, which indicates the degree of deterioration in workability, is high, and the surface properties and elongation are low. Inferior. Steel B also has high Si, so (14 ~
16%) YS is higher and surface properties and elongation are inferior to steels 1 to 4 and steel 9 of invention examples which are Cr-Mo-less materials. Steel C is
Although Nb is added in an appropriate amount, the surface properties are poor because Ti is added in excess. Since steel D and steel E do not contain Nb, they have low high-temperature strength and are inferior in both workability and surface properties. On the other hand, steel 1 to steel 11, which are examples of the invention, have high-temperature strength of conventional steel (SUS430J1L).
In addition to the above values, it is easy to work with low YS and has excellent surface properties.

[0035]

[Table 2]

[0036]

As described above, according to the present invention,
It is possible to provide a Cr-containing steel having excellent high-temperature strength, workability, and surface properties. Therefore, in exhaust system parts such as automobile and motorcycle engines,
Not only can it be suitably used in high-temperature sections such as exhaust manifolds, front pipes, and converter shells, but also can be applied to low-temperature sections such as mufflers and middle pipes, which are conventionally difficult to process because they are soft at room temperature. . When the steel of the present invention is used as a low-temperature member for which corrosion resistance is required, depending on the required level of corrosion resistance, Cr, Mo
Etc. may be adjusted. Further, since the exhaust path member of the thermal power plant is required to have the same characteristics as the exhaust member of the automobile engine, the steel of the present invention is applicable to such applications. Further, since the steel of the present invention has excellent surface properties, it can be sufficiently used in many other fields where surface properties are required.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of Si content on room temperature yield strength YS.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Susumu Sato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref.

Claims (5)

[Claims] [Claim 1] By weight%, C: 0.02% or less, Si: 0.10% or less, Mn: 0.4 to 2.0%, P: 0.04% or less, S: 0.02% or less, Cr: 3.0 to 20%, Ni: 1.0 %, N: 0.02% or less, Nb: 0.2 to 1.0%, the balance being Fe and unavoidable impurities, excellent in high-temperature strength, workability and surface properties
Cr-containing steel. 2. The steel according to claim 1, wherein, in addition to the above components, by weight, Ti: 0.5% or less, Zr: 0.5% or less, V: 0.5% or less, and Al: 0.5% or less. A Cr-containing steel excellent in high-temperature strength, workability and surface properties, characterized by containing one or more selected from the group consisting of: 3. The steel according to claim 1 or 2,
In addition to the above components, the composition further comprises at least one selected from the group consisting of Mo: 3.0% or less, Cu: 1.0% or less, and REM: 0.3% or less. Cr-containing steel with excellent high-temperature strength, workability and surface properties. 4. The steel according to claim 1, further comprising, in addition to the above components, by weight: 0.03% or less of Ca and 0.005% or less of B. Or characterized by containing two kinds,
Cr-containing steel with excellent high-temperature strength, workability and surface properties. 5. The steel according to claim 1, wherein the form of Nb in the steel is the (112) strength of Fe ₂ Nb by X-ray diffraction of a precipitate (extraction residue). And Nb (C, N)
A Cr-containing steel excellent in high-temperature strength, workability and surface properties, characterized in that its ratio to (200) strength is 0.2 or less.