JP3364322B2 - Stainless steel for automotive exhaust manifolds with excellent manufacturability, workability and high-temperature strength after long-term aging at high temperatures - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to steel for use in automobile exhaust manifolds. More specifically, the steel used for an exhaust manifold of an automobile is required to have high strength at high temperature and at the same time have good workability. However, the present invention relates to a stainless steel excellent in these properties and manufacturability.

[0002]

2. Description of the Related Art Exhaust manifolds for automobiles are 800-
It is a component used at a high temperature of 900 ° C. In addition to the conventional cast iron type, this exhaust manifold has SUH
Although 409L (11Cr-Ti) is used, SUH409L has insufficient heat resistance when the exhaust temperature is relatively high. It is effective to add Nb to improve the high temperature strength of ferritic stainless steel.
The publication describes a ferritic stainless steel in which Nb or Ta is added to improve the creep strength. And, SUS430J1L (18
Cr-Nb-Cu) and the like made of ferritic stainless steel tubes have come to be used. Further, it is known that addition of Mo is also effective for improving the high temperature strength of ferritic stainless steel, and in JP-A-5-33104, a ferritic heat resistant stainless steel containing Nb and Mo added together has been developed. It is described that it was done.

[0003]

However, it is expected that the heat resistance of the currently used stainless steel will become insufficient if the exhaust temperature becomes higher with the improvement of the performance of the engine. For the exhaust manifold material that responds to the rise in exhaust temperature, first of all, use high-temperature strength SUS430J1L (1
8Cr-Nb-Cu) is required. In addition, the exhaust manifold material has high temperature fatigue resistance,
Since thermal fatigue resistance is required, these characteristics are important, and a material that can maintain high high-temperature strength even after heating at high temperature for a long time is required. High temperature strength can be improved by increasing the amount of Nb added. On the other hand, if the amount of Nb added is greater than the amount added to the current steel, the toughness of the steel decreases and the manufacturability deteriorates. There is a problem.
As described above, JP-A-2-2441 describes a ferritic stainless steel in which Nb or Ta is added to improve the creep strength, but here, the manufacturability and workability of the steel are taken into consideration. It has not been. Further, JP-A-5-33104 describes a stainless steel to which Mo is added in addition to Nb in order to improve high temperature strength, but addition of Mo improves high temperature strength and strength at room temperature. Also increases, and the workability decreases. Since the exhaust manifold has a very complicated shape, if the workability deteriorates, the shape of the manifold is restricted, which is not preferable. Therefore, the present invention further improves the high temperature strength compared to the ferritic stainless steel currently used for automobile exhaust manifolds, has a small decrease in the high temperature strength even after aging at high temperature for a long time, and is the same as the conventional steel (SUS430J1L). It is an object of the present invention to provide a stainless steel which has a workability and a manufacturability above a level.

[0004]

[Means for Solving the Problems] Among JIS G 4304 hot rolled stainless steel sheets and JIS G 4305 cold rolled stainless steel sheets, there is a standard for stainless steel containing stabilizing elements such as Ti, Nb and Zr. However, the effects of these stabilizing elements are considered to be equivalent, and the amount of each element added is not specified. The inventors of the present invention investigated the influence of various stabilizing element additions on the high temperature strength, hot rolled sheet toughness, and workability of ferritic stainless steels, and as a result, the addition of Ta increased the high temperature strength in the same manner as Nb addition. Can be improved, and
It was found that the Ta-added steel can maintain high high-temperature strength even after being held at high temperature for a long time. Further, if Nb is added excessively, the toughness of the steel decreases and the manufacturability deteriorates.
It was found that the decrease in toughness was small when the alloy was added. Further, it was found that the toughness is remarkably improved by adding an appropriate amount of Zr to Nb and Ta-added steel.

In FIG. 1, 14Cr-0.01C-0.01
The effect of addition of Nb or Ta on the 0.2% proof stress at 900 ° C of a 2 mm thick cold rolled annealed sheet of N-0.06Zr steel is shown. Moreover, in FIG. 2, 14Cr-0.01C-0.01N
-2 mm thick cold rolled annealed sheet of 0.06 Zr steel at 900 ° C for 2
The effect of Nb or Ta and their composite addition on the 0.2% proof stress at 900 ° C. after 00 hour aging heat treatment is shown. From FIG. 1, in Nb and Ta, the increase in the high temperature strength of the cold rolled annealed plate with the addition amount is at the same level. Also, 0.4%
It can be seen that the high temperature strength increases even if Ta is added to the steel containing Nb. From FIG. 2, after aging heat treatment at 900 ° C. for 200 hours, in the case of adding Nb, the high temperature strength increases with an increase in the addition amount, but if it is added more than that, the high temperature strength increasing tendency decreases. In the case of adding Ta, 1.0% even after aging heat treatment at 900 ° C for 200 hours
In the range up to, the high temperature strength increases with an increase in the added amount, and the high temperature strength after aging heat treatment does not decrease much. Also,
Even if Ta is added to steel containing 0.4% Nb,
The high temperature strength after aging increases with the addition amount of a.

Further, the present inventors have found that Nb and Ta together with Z
It has been found that when an appropriate amount of r is added, the toughness of the hot-rolled sheet is improved and the manufacturability is improved, and the added amounts of Nb and Ta can be increased. It was also found that the addition of Ta causes less decrease in toughness than the addition of Nb even if the addition amount is large.

In FIG. 3, 14Cr-0.01C-0.01
Steel with Nb added to N steel, Steel with Ta added, and Nb
-The change of toughness at room temperature of a 5 mm-thick hot rolled annealed plate when Zr is added to the steel to which Ta is compounded is shown. In any steel, the Zr addition amount is 0.02 to 0.1%
Shows a high absorbed energy value, and addition of more than that results in poor toughness. Although the reason for this is not clearly understood, Zr strongly stabilizes C and N and improves toughness, but Zr is brittle when Zr is excessively added.
It is considered that the intermetallic compound of r precipitates and the toughness decreases. Therefore, in order to avoid adding more than the necessary amount for fixing C and N, it is necessary to limit the range to satisfy the following formula (1). Zr% / 91 ≦ C% / 12 + N% / 14 Formula (1)

In FIG. 4, 14Cr-0.01C-0.0.
The influence of addition of Nb and Ta on the toughness at room temperature of a 5 mm-thick hot rolled annealed plate of 1N-0.06Zr steel is shown. In the case of adding Nb, the maximum absorbed energy value is exhibited when the amount of Nb added is about 0.3%, and the value of the absorbed energy drops sharply when the amount is 0.7% or more. When Ta is added, the addition amount is 0.6
%, The maximum absorbed energy value is shown, and even with an addition amount of 1.0%, it is as high as about 500 J / cm ² . 0.4%
When Ta is added to Nb steel, the toughness decreases as the amount of Ta added increases, but a high absorbed energy value is exhibited even when the amounts of Nb and Ta added together are 1.1%.

[0009] Further, Ta-Zr composite addition steel, Ta-Nb
It was found that the -Zr composite addition steel has a workability equal to or higher than that of the current steel with Nb added alone. Steels 1 to 24 of the present invention having the chemical compositions shown in Table 1 and Table 2 and the current steel (SUS430J
For Comparative Steels 1 to 13 including IL and SUH409L), a 50 kg steel ingot was melted in a vacuum melting furnace, hot rolled into a 5 mm thick plate, and annealed. 2 mm V notch) was sampled perpendicularly to and subjected to a Charpy impact test to evaluate the manufacturability. Further, the workability of the cold rolled annealed sheet obtained by cold rolling the hot rolled annealed sheet to a thickness of 2 mm and then annealing was evaluated. As for the evaluation of workability, the lower the value of YS (breaking strength) at room temperature, the better the workability. Therefore, a tensile test was performed at room temperature to measure the value of YS. These cold rolled annealed sheets were subjected to an aging heat treatment at 900 ° C. for 200 hours and then subjected to a high temperature tensile test. The results of these tests are also shown in Tables 1 and 2.

From Tables 1 and 2, the steels 1 to 24 of the present invention are
YS value at room temperature is 300 MPa or less, and 90
0.2% at 900 ° C after aging heat treatment at 0 ° C for 200 hours
The yield strength is 15 MPa or more, and the hot rolled annealed plate toughness is 200 J / cm ² or more. On the other hand, the comparative steels 1 to 13 are inferior to the inventive steels 1 to 24 in any of the items. The steels 1 to 24 of the present invention are steels having improved workability and toughness of a hot-rolled annealed plate and improved high-temperature strength after being heated at a high temperature for a long time. By finding the above, the inventors of the present invention,
Ta is added in an amount equal to or more than the Nb addition amount of the current steel, or the Nb amount is further added as the current steel (18Cr-0.4Nb) level, and in any case, Zr is expressed by the formula (1). By adding a sufficient amount, an automobile exhaust manifold that can maintain hot-rolled sheet toughness to the extent that it can be manufactured in the current process, further improve high-temperature strength compared to current steel, and maintain high high-temperature strength even after high-temperature long-time heating It came to invent stainless steel for use.

That is, the first aspect of the present invention is the weight%
, C: 0.02% or less, Si: 0.4% or more and less than 1.0%, Mn: 1.0% or less, P
: 0.04% or less, S: 0.03% or less,
Al: 0.015% or less, Cr: 10% or more and 20% or less, N: 0.02% or less, Nb: 0.5% or less,
Ta: 0.02% or more and 1.5% or less, and contains Nb and Ta in a total amount of 0.5% or more and 1.5% or less, Zr: 0.01% or more and 0.15% or less, In addition, the content of C, N, and Zr is represented by the following formula (1), and the balance is iron and unavoidable impurities, and the remainder is excellent in manufacturability, workability, and high-temperature strength after high-temperature long-term aging. Is. Zr% / 91 ≦ C% / 12 + N% / 14 Formula (1)

The second aspect of the present invention is, by weight%, C:
0.02% or less, Si: 0.4% or more and 1.0%
Less than, Mn: 1.0% or less, P: 0.04
% Or less, S: 0.03% or less, Al: 0.0
15% or less, Cr: 10% or more and 20% or less, N:
0.02% or less, Nb: 0.5% or less, T
a: 0.02% or more and 1.5% or less, and Nb, T
a is contained in a total amount of 0.5% or more and 1.5% or less, and Zr:
0.01% or more and 0.15% or less, and C, N,
The content of Zr is the above formula (1), and further Ca,
Manufacturability, workability, and high-temperature length containing one or more of Y, La, Ce, Pr, and Nd in a total amount of 0.01% or more and 0.2% or less, and the balance being iron and inevitable impurities. A stainless steel for automobile exhaust manifolds that has excellent high-temperature strength after time aging.

[0013]

The reason for limiting the range of each component in the above composition of the present invention will be explained below. The content of C is preferably as low as possible in order to enhance the toughness and from the viewpoint of manufacturability, and is made 0.02% or less, preferably 0.01% or less. Mn may be added for deoxidation and desulfurization, but excessive addition deteriorates the oxidation resistance and workability of steel. Further, in order to maintain a stable ferrite structure, it is 1.0% or less, preferably 0.
5% or less.

Si has the effect of improving the oxidation resistance of steel, but excessive addition thereof is not preferable because it lowers the workability of steel. Therefore, the range of addition amount is 0.4% or more 1.
It is less than 0%. The range of 0.40% or more and 0.85% or less is more preferable.

Cr is an element necessary for ensuring the oxidation resistance of steel. However, Cr is an expensive element, and excessive addition causes an increase in cost. Therefore, the Cr content is 1
It is set to 0% or more and 20% or less. Desirably 14% or more 1
The range is 9% or less.

Al may be added as a deoxidizing agent. However, excessive addition leads to a decrease in the toughness of steel and a decrease in manufacturability. Further, solid solution Al reduces workability. Therefore, the content of Al is set to 0.015% or less. 0.0
It is more preferable to set it to not more than 06%.

P is an element which adversely affects the hot workability of steel. Therefore, the P content is 0.04% or less. It is more preferably set to 0.01% or less.

S, like P, is an element that adversely affects the hot workability of steel. Therefore, the content of S is 0.0
3% or less. It is more preferably 0.005% or less.

As with C, N is preferably as small as possible in order to improve toughness and from the viewpoint of manufacturability. The N content is 0.02% or less, preferably 0.01% or less.

Nb has the effect of improving high temperature strength.
With respect to the high temperature strength after heating at high temperature for a long time, the effect of increasing the Nb addition amount is saturated at an addition amount of about 0.5%, and if Nb is added excessively, the toughness of the steel is significantly reduced. Let Therefore, the upper limit of Nb addition is 0.5%.
And preferably in the range of 0.3 to 0.5%.

Ta is a characteristic element of the steel of the present invention,
Addition of Ta can improve the high temperature strength without lowering the workability of steel, and can maintain its effect even after heating at high temperature for a long time. It is necessary to add Ta in an amount of 0.5% or more in order to improve the high temperature strength more than that of the current steel, but if it is added in an amount of more than 1.5%, the toughness of the steel decreases. Therefore, the amount of Ta added when Ta and Nb are not added together is in the range of 0.5% to 1.5%. Particularly, it is preferable to set it in the range of 0.5% or more and 1.0% or less. When Nb is added, the addition amount of Ta is 0.02% or more and 1.5% or less, preferably 0.05
% Or more and 0.4% or less, and the total amount of Nb and Ta added is preferably 0.5% or more and 1.5% or less in order to secure both high temperature strength and toughness. Particularly preferably, 0.5
% Or more and 1.0% or less.

Zr is effective in improving the toughness of steel, and since the addition amount of Nb and Ta is large, the toughness of the hot-rolled sheet is low and the manufacturability is poor. Can be improved. However, if added in a range that does not satisfy the above formula (1), the toughness is adversely affected. Therefore, the amount of Zr added is 0.01% or more and 0.15% or less, and is in a range that satisfies the above formula (1). Particularly, the range is preferably 0.05% or more and 0.10% or less.

Ca and Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), and Nd (neodymium) are effective in improving the oxidation resistance of steel. Therefore, 1 type or 2 types are added as needed. In order to improve the oxidation resistance of steel, it is necessary to add 0.01% or more of these elements, but if added excessively, the toughness of the steel is lowered and the hot workability is lowered. Therefore, the addition amount range is 0.01% or more and 0.2% or less, preferably 0.0% or less in total of one kind or two kinds.
4% or more and 0.1% or less.

[0024]

【Example】

Example 1 Stainless steels having the chemical compositions shown in Tables 1 and 2 were melted in a vacuum melting furnace to obtain a 50 kg steel ingot. In the following, a well-known method was hot-rolled into a 5 mm thick plate, annealed, cold-rolled into a 2 mm thick plate, and finish-annealed. Charpy test pieces (2 mm V notches in the vertical direction of rolling) of the 5 mm thick hot rolled annealed sheets of the invention steels 1 to 24 and the comparative steels 1 to 13 obtained as described above were respectively taken and subjected to a Charpy impact test. . The invention steel 1
-24 and comparative steels 1 to 13 were hot-rolled and annealed, and cold-rolled to a thickness of 2 mm and then annealed. The evaluation of each physical property was performed as follows, and each evaluation was performed. The results are shown in Table 1 and Table 2.
It was shown to. In Tables 1 and 2, REM represents Y, La, Ce,
It means Pr and Nd.

(Evaluation of Physical Properties) The workability was evaluated by the value of YS obtained by the tensile test at room temperature. As described above, the lower the value of YS, the better the workability, but the steels of the present invention shown in Table 1 were all 300 MPa, which was superior to the current steel SUS430J1L shown in Table 2. 900
Regarding the 0.2% proof stress at 900 ° C. after the aging heat treatment at 200 ° C. for 200 hours, the steels of the present invention shown in Table 1 are all 15 MP.
a or more is shown, which is also the current steel SUS430J1L
Was better than. Further, regarding the toughness of the hot-rolled annealed sheet, all showed 200 J / cm ² or more at room temperature, which was a level that could be produced in the current production process.

As shown in Table 2, Comparative Steel 1 (SUS43
0J1L), Comparative Steel 4 and Comparative Steel 6 are Ta-free,
Further, since the total amount of Nb and Ta added was smaller than that of the steel of the present invention, it was shown that the high temperature strength after high temperature aging for a long time was inferior to that of the steel of the present invention. Comparative Steel 2 (SUH409L) is N
Since no b or Ta was added, the high temperature strength was low. In Comparative Steel 3 and Comparative Steel 10, the high temperature strength was improved by adding Nb and Mo in combination, but the addition of Mo also increased the strength at room temperature and was inferior in hardenability. Since the comparative steel 5 has a large amount of Nb added, the toughness of the hot rolled annealed plate was poor. Comparative Steel 7 has a total amount of Nb and Ta within the range of the present invention, and has both high temperature strength and workability.
Since the additive was not added, the toughness of the hot rolled annealed plate was low and the productivity was poor. Comparative Steel 8 contained Zr within the range of the present invention, but the amount of Nb added was large, and since Ta was not added, the toughness and workability of the hot rolled annealed sheet were inferior to those of the present invention. . Comparative Steel 9 was inferior in toughness of the hot-rolled annealed plate because the Zr amount did not satisfy the above formula (1) and the addition amount was excessive. In Comparative Steels 11 to 13, the amounts of Nb, Ta, and Zr were within the range of the present invention, but any one of Ca, La, and Ce was excessively added, so that the toughness of the hot rolled annealed plate was poor. As described above, it was shown that the steels 1 to 24 of the present invention are materials having both high temperature strength after high temperature long time heating, workability, and hot rolled sheet toughness.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

As described above, according to the present invention, Ta,
By optimizing the addition amounts of Nb and Zr, stainless steel for the current exhaust manifold (SUS430J1
L, SUH409L) is higher in high temperature strength, and the high temperature strength can be maintained even after being heated at high temperature for a long time, and the stainless steel for exhaust manifold having the same workability and manufacturability as conventional steel can be obtained.

[Brief description of drawings]

FIG. 1 14Cr-0.01C-0.01N-0.0
0.2% at 900 ° C of 2mm thick cold rolled annealed 6Zr steel
It is a figure which shows the influence of Nb or Ta addition on yield strength.

FIG. 2 14Cr-0.01C-0.01N-0.0
N of the 2 mm thick cold rolled annealed sheet of 6Zr steel, which is 0.2% proof stress at 900 ° C. after aging heat treatment at 900 ° C. for 200 hours
It is a figure which shows the influence of b or Ta addition.

FIG. 3: Nb in 14Cr-0.01C-0.01N steel
It is a figure which shows the change of the toughness at room temperature of a 5 mm-thick hot-rolled annealed board at the time of adding Zr to the steel which added [which added], the steel which added Ta, and the steel which added Nb-Zr combined.

FIG. 4 14Cr-0.01C-0.01N-0.0
It is a figure which shows the influence of Nb and Ta addition on the toughness at room temperature of a 5 mm thick hot rolled annealing board of 6Zr steel.

Claims (2)

(57) [Claims] 1. By weight%, C: 0.02% or less, Si: 0.4% or more and less than 1.0%, Mn: 1.0% or less, P: 0.04% or less, S: 0.0. 03% or less, Al: 0.015% or less, Cr: 10% or more and 20% or less, N: 0.02% or less, Nb: 0.5% or less, Ta: 0.02% or more and 1.5% or less Yes, and Nb and Ta total 0.5% or more and 1.5%
Zr: 0.01% or more and 0.15% or less, and the content of C, N, and Zr is represented by the following formula (1), and the balance is iron and unavoidable impurities. Stainless steel for automobile exhaust manifolds that has excellent workability and high-temperature strength after high-temperature long-term aging. Zr% / 91 ≦ C% / 12 + N% / 14 Formula (1) 2. By weight%, C: 0.02% or less, Si: 0.4% or more and less than 1.0%, Mn: 1.0% or less, P: 0.04% or less, S: 0.0. 03% or less, Al: 0.015% or less, Cr: 10% or more and 20% or less, N: 0.02% or less, Nb: 0.5% or less, Ta: 0.02% or more and 1.5% or less Yes, and Nb and Ta total 0.5% or more and 1.5%
Zr: 0.01% or more and 0.15% or less, the content of C, N and Zr is represented by the following formula (1), and Ca, Y, La, Ce, Pr and Nd are contained. 0.01% or more and 0.2% or more of one or more of them in total
Stainless steel for automobile exhaust manifolds, which contains the following and is excellent in manufacturability, processability, and high-temperature strength after high-temperature long-term aging, with the balance consisting of iron and inevitable impurities. Zr% / 91 ≦ C% / 12 + N% / 14 Formula (1)