JPH03219055A - Stainless steel for engine exhaust gas system material excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a stainless steel having superior corrosion resistance and high workability by combinedly adding Cr and Mo as principal components and incorporating specific amounts of Nb and Ti in correlation with C+N. CONSTITUTION:A stainless steel has a composition consisting of, by weight, <=0.010% C, <=0.2% Si, 0.05-1.5% Mn, 12-20.0% Cr, 0.2-3.0% Mo, 0.005-0.1% Al, <=0.015% N, <=0.025% P, <=0.010% S, 10X(C%+N%) to 0.5% of Ti and/or 5X (C%+N%) to 0.5% of Nb, and the balance Fe with inevitable impurities. Further, small amounts of one or more elements among W, V, and Zr and either or both of Ca and Ce are incorporated, if necessary. This stainless steel shows excellent corrosion resistance over a long period in a severe corrosive condensate environment, such as engine exhaust gas environment.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to wet gases and exhaust gases generated in exhaust gas mufflers of automobile and motorcycle engines, and in exhaust pipes before and after the mufflers (hereinafter collectively referred to as mufflers). The present invention relates to stainless steel that has excellent corrosion resistance in condensate and the ability to stop the occurrence of local corrosion.

[Conventional technology]

Conventionally, in the case of automobiles and motorcycles, high-temperature exhaust gas generated by the engine passes through an exhaust manifold, a converter, etc., and is then discharged from the exhaust system via a center pipe, muffler, and tail pipe. When the engine is started, the temperature of the muffler is low, so condensed liquid, which is water in the exhaust gas, adheres to the inner wall of the muffler and remains at the bottom of the muffler. The condensate thus generated contains CO3-, NH4'', 5O4-, N(h
- Contains other C1" and trace amounts of organic matter, which affect the corrosion resistance of the muffler. This process occurs because as the exhaust gas temperature rises, the muffler temperature also rises, and ammonia etc. contained in the generated condensate evaporates. , the liquid property of the condensate changes from alkaline to acidic.

In order to make a muffler suitable for such environments, Affi, which has corrosion resistance in environments ranging from alkaline to acidic, must be used.
Plated steel plates and Cr-containing steel plates have been used.

[Problems to be Solved by the Invention] However, with the tightening of exhaust gas regulations in recent years, the usage conditions of mufflers have become more diverse. Under such usage conditions, even a muffler made of the above-mentioned steel plate cannot provide sufficient corrosion resistance, and its service life is reached in a relatively short period of time. From the viewpoint of longer life, materials with better corrosion resistance have been desired. In response to these demands, steels containing 5% to 10% Cr (Japanese Unexamined Patent Application Publications No. 63-143240, No. 63-143241)
However, the current situation is that it has not been able to meet recent demands for longer life.

In light of these circumstances, the present invention has analyzed in detail the actual state of mufflers actually used (corrosion morphology, environmental analysis, etc.), and based on the results, we have analyzed the main characteristics of stainless steel mufflers under corrosive environmental conditions. The components Cr, Mo,
The influence of Ti and Nb components, as well as Ni, Cu, W,
By examining the effects of V, Zr, (:a, and Ce) on corrosion resistance (local corrosion initiation/stopping characteristics; hereinafter referred to as corrosion resistance) when added alone or in combination, and by clarifying the effects of each element, It shows excellent corrosion resistance even when applied to actual mufflers, making it possible to ensure long-term lifespan, safety, and prevention of environmental pollution of the equipment, and the pipe making equipment also uses a common steel manufacturing process. ,
Lankford (ti!(F)
The object of the present invention is to provide a stainless steel for use in engine exhaust gas from automobiles, motorcycles, etc., which has excellent workability into plate materials and pipe materials by obtaining 1.

[Means to solve the problem]

From the above-mentioned perspective, we conducted research to develop a muffler that exhibits better corrosion resistance and workability than conventional mufflers, and as a result, we discovered that this can be achieved through the following knowledge.

■The form of corrosion in muffler condensate in the actual environment is localized corrosion on Cr-containing steel, and in order to evaluate it in the laboratory, an evaluation method that takes this point into consideration is required. need to be considered.

■In order to develop a muffler material that can be used for a long period of time even in harsh environments, we first investigated mufflers in each area where the vehicle was actually driven. As a result, there are two types of pitting caused by corrosion: one is due to local corrosion of the base material of the material, and the other is corrosion of grain boundaries leading to pitting. 5UH409, 5US43
0LX.

5US436L is local corrosion where the base material is corroded, and 5
UH409 (7) welded parts and 5US430LX and 5US4
It has been found that intergranular corrosion occurs in welded parts for 36L materials in which Ti/C+N is less than 10 and Nb/C+N is less than 5. It was also found that the cause of intergranular corrosion was a Cr-deficient layer near Cr, which precipitated at grain boundaries during welding cooling.

As a result of such investigations and corrosion resistance studies of various alloys, the present invention found that the amount of Cr in the base material needs to be 12.0% or more, and that Ti/C and N are required to prevent Cr23C from precipitating in the weld. 10 or more, and Nb/C+N is required to be 5 or more.Also, as a result of searching for a method to improve the workability during pipe making and the workability after pipe making, which is a feature of the present invention, we found that the Si content was By suppressing the hardening caused by solid solution St as much as possible and cold rolling using large diameter rolls in the ordinary steel process, it is possible to manufacture thin sheets with a high Lankford value (F) that cannot be obtained with the conventional Sendzimer process. I discovered something. In addition, the Ti and Nb contents are kept to the amount necessary to prevent intergranular corrosion, and do not deteriorate workability or secondary workability.
In addition, consideration was given to suppressing the recrystallization temperature as much as possible and making it possible to manufacture the product on an ordinary steel line.

From this point of view, the present invention basically provides the following Cr;
Based on the combined addition of 12 to 20.0%, Mo; 0.2 to 3.0%, Nb; 5X (C% + N%) to 0.5%, T
i; l0X (C% + N%) ~ 0.5% stainless steel containing one or two types, with the remainder consisting essentially of Fe and unavoidable impurities, and has excellent corrosion resistance in an engine exhaust gas environment. Indicates processability. In order to provide further corrosion resistance to this, Ni; 0.1 to 1.0%.

Cu: 0.03~1.0%, W: 0.05~
0.5%, ■; 0.05-0.5%, Zr; 0.0
This is a stainless steel for use in an engine exhaust gas environment containing one or more types of stainless steel in an amount of 5 to 1.0%.

Furthermore, the present invention incorporates Ca and Ce in an amount of 0.001% or more and 0.03% or less, respectively, to suppress the formation of MnS-based inclusions that can cause local corrosion and improve corrosion resistance. This is a stainless steel for use in engine exhaust gas environments.

[For production]

In order to analyze the corrosion behavior of stainless steel in the muffler environment and develop suitable materials, it is necessary to chemically analyze the environment inside the muffler and clarify the environmental factors that affect the corrosion behavior of stainless steel. In order to achieve this, it is necessary to evaluate corrosion resistance in a test environment that simulates the actual environment.

Therefore, the present inventors chemically analyzed the inner surface deposits (corrosion products) and condensate collected from various parts inside the muffler, and created a simulated condensate based on the analysis results. Also, the corrosion behavior inside the muffler is 8! In the case of plated steel plate,
Although it is generally corrosive, it has become clear that in the case of steel sheets with a high Cr content, it becomes locally corrosive. As the Cr content in steel increases, the form of corrosion becomes more localized, so it is important to evaluate this localized corrosion.

Therefore, based on the analysis results, the present inventors changed the condensate environment to sulfate ions (5000 ppm) and carbonate ions (300 ppm).
A condensate environment simulation liquid was prepared by adding and adjusting predetermined amounts of chloride ions (1000 ppm), chloride ions (1000 ppm), nitrate ions (100 ppm), and formic acid (100 ppm).

The electrochemical local corrosion evaluation described below in this environment was carried out in 12
% to 20.0% Cr-containing steel, and steel types with Mo content varied from 0.2% to 3%, as well as Ti, Nb, Ni, Cu, W, V, Zr, Ca.
Tests were conducted using steel types with different +Ce contents. By performing multiple regression analysis on the dependence of the obtained local corrosion characteristic value (local corrosion occurrence characteristic value: El) on each alloy component (local corrosion occurrence characteristic value: EI is in the form of alloy component dependence, El = As a new index showing the dependence of each alloy component on local corrosion inhibition (organized as A+B-CI), a relationship of C1 value=Cr+2.0Mo was obtained.

From this relationship, the comparative steels (k22, 23.2
4) As an alloy system with a higher local corrosion potential than
The C5 value was set at 12.4 or more and 24.5 or less.

Another characteristic required of stainless steel muffler materials is that not only is local corrosion difficult to occur, but even if local corrosion does occur, the rate of progress of local corrosion in the plate thickness direction is slow. In order to evaluate this, Cr
, steel with varying amounts of Mo, and in addition to these, Ti, Nb, Ni, Cu, and w.

Regarding the materials to which V, Zr, Ca, and Ce were added, each sample was immersed in the above-mentioned simulated condensate for a certain period of time, and evaluated based on the depth of local corrosion that occurred on the sample surface. The results are shown in Table 1.

The reasons for limiting the components of the muffler material are described below.

CTC precipitates as CrziCa at the grain boundaries of the weld zone and causes intergranular corrosion, so the lower the CTC is, the better.

In addition, from the viewpoint of the strength, workability, and toughness of the base metal, a lower value is preferable, but steelmaking takes time and costs increase. However, in order to improve the corrosion resistance and processability, which are the characteristics of the present invention, the content was limited to an extremely low value of 0.010% or less.

Si: Si has a deoxidizing effect, but if it is less than 0.01%, no effect can be expected. Moreover, when it exceeds 0.8%, workability deteriorates significantly. Considering surface treatment properties and processability, it is desirable that the content be 0.2% or less, so it was set to 0.2% or less.

Mn: Mn does not particularly affect corrosion resistance in an exhaust gas condensate environment, but the normal component content is 0.
05% or more and 1.5% or less.

Cr; Cr is a basic component of the present invention. In an environment where high corrosion resistance is required, such as an environment containing condensate, it is necessary to add 12% or more of Mo and, if necessary, Ni, Cu, etc. together. Corrosion resistance and oxidation resistance improve as the amount increases, but even if it exceeds 20.0%, the corrosion resistance is saturated. Moreover, it is difficult to manufacture and is economically expensive.

Mo; Mo is Cr, and if necessary, Ni, C
It is added in coexistence with u, etc., and is an essential element for suppressing the occurrence and progression of local corrosion in a condensate environment.

Addition of 0.2% or more and 3.0% or less becomes extremely effective in coexistence with Cr and other components described in the claims (hereinafter referred to as other elements).

If it is less than 0.2%, the corrosion resistance will be insufficient, but if it is less than 0.2%, corrosion resistance will be insufficient.
%, it does not contribute much to improving corrosion resistance and becomes expensive.

^IHAI is added as a deoxidizing agent in a range of 0.1% or less. If it exceeds 0.1%, corrosion resistance and hot workability will deteriorate. Further, if it is less than 0.005%, there is no effect.

NUN is an element that deteriorates the corrosion resistance of stainless steel.
Since the less the better, the content was set to 0.015% or less.

FDP affects corrosion resistance in condensate environments, so
The less the better. If it exceeds 0.025%, corrosion resistance will deteriorate.

SO3 is also an element that affects corrosion resistance in a condensate environment, and the lower the better, the upper limit was set at 0.010%.

Tt: Ti fixes C or N and prevents deterioration of the corrosion resistance of stainless steel. Coexists with Ca to fix O, Si,
Suppresses the formation of Mn oxides and improves hot workability and corrosion resistance. If it exceeds 0.5%, hot workability deteriorates. As a result of actual running muffler investigation and grain boundary corrosion resistance evaluation, it became clear that the Ti amount was required to be 10X (C%+N%) or more, so this was set as the lower limit.

Nb, Nb fixes C or N and prevents deterioration of the corrosion resistance of stainless steel. If it exceeds 0.015%, hot workability deteriorates. From the grain boundary corrosion evaluation results, the amount of Nb is 5
Since it became clear that more than ×(C%+N%) was required, this was set as the lower limit.

Note that one or both of the above Ti and Nb are added.

Ni: Ni is a selectively added component of the stainless steel of the present invention. In environments that require high corrosion resistance, such as environments containing condensate, it is used in coexistence with Cr, Mo, and other elements. It is effective in suppressing the progress of local corrosion, but it is ineffective if it is less than 0.1%, and the effect is saturated if it exceeds 1.0%.
It is also economically expensive.

Cu; Cu is an additive element that is added in the form of a component system based on Cr and Mo, as well as Ni and other elements in coexistence, and provides corrosion resistance in an environment containing condensate.

At 0.03% or more, the coexistence effect is significant, and at more than 1.0%, corrosion resistance is saturated and hot workability is deteriorated.

W: Since the co-addition of W improves the corrosion resistance and local corrosion resistance of stainless steel, it is added in an amount of 0.5% or less as necessary. If it exceeds 0.5%, the effect is saturated. 0.0
There is no effect if it is less than 5%.

■: The co-addition of ■ improves the corrosion resistance and local corrosion resistance of stainless steel, so it may be added in an amount of 0.5% or less as necessary. If it exceeds 0.5%, the effect is saturated. 0.0
There is no effect if it is less than 5%.

Zr; The co-addition of Zr improves the corrosion resistance of stainless steel,
Since it improves local corrosion resistance, it is added in an amount of 1.0% or less as necessary. If it exceeds 1.0%, the effect is saturated. There is no effect if it is less than 0.05%.

Ca, Ce; Ca, Ce coexists with Al in low sulfur steel and fixes O, suppresses the formation of MnS-based inclusions that can be the starting point of local corrosion in condensate, and improves corrosion resistance. do. Ca and Ce are each 0.001 to 0.
One or two types may be added as necessary within a range of 0.3%.

[Example]-1 The characteristics of the muffler material for engine exhaust gas of the present invention will be explained using an example.

Table 1 shows the chemical composition of the invention steel and comparative steel.

The steel of the present invention was produced by melting steel having the composition shown in Table 1 using an ordinary vacuum melting furnace. After casting into an ingot, it was hot rolled under normal heating conditions and subjected to appropriate heat treatment tests.

The local corrosion occurrence potential in Table 1 is a value obtained by an electrochemical local corrosion occurrence evaluation test (A), and the larger this value is, the more difficult it is for local corrosion to occur.

For the local corrosion evaluation test (A), the test piece for local corrosion evaluation test shown in FIG. 1 was used. In Figure 1, 1 is a lead wire, 2
3 indicates a sealed portion other than the test surface, 3 indicates a test surface, and 4 indicates a polycarbonate bolt/nut. This created an artificial gap on the test surface, making it possible to accelerate the occurrence of localized corrosion. Using this test piece, in a simulated condensate environment, from the natural potential (Ecorr) as shown in Figure 2,
When the potential was swept in the anode direction at 20 mV/+min, the potential at the point where the current density reached 100 μA/cd was defined as the local corrosion occurrence potential. The larger this potential value is, the less likely local corrosion is to occur.

In addition, the local corrosion depth test (B) was conducted in a condensate environment with a radius (W) of 50 mm and a length (f) of 60 mm.
+u+.

Plate (t): Using a test piece in the shape of 1.2 ms+, the test piece surface was polished with #320 polishing and degreased, and then the test was conducted.

The test environment consisted of sulfate ions (5000ppm), carbonate ions (3000pp+m), and chloride ions (1000ppm).
ppm), nitrate ion (100ppm), formic acid (1
A condensate prepared by adding and adjusting a predetermined amount of 00 popm) was used. The test method is a glass beaker (200cc)
A test piece was placed in the stand, and 100 cc of condensate was poured into the test piece so that it was immersed up to half of the test piece. Thereafter, the beaker was heated under boiling conditions for 2 hours and left at rest for 24 hours, which was repeated for 30 days. After the test, all the depths of local corrosion observed on the test piece were measured, and the maximum depth was evaluated.

[Example]-2 Furthermore, in order to evaluate the mechanical properties of the material manufactured by the actual manufacturing process, stainless steel having the chemical composition shown in Table 2 was extracted from a converter using the same equipment used for manufacturing ordinary steel. ,
Hot rolling, pickling, cold rolling, annealing, pickling, and temper rolling were performed in sequence to obtain a product with a plate thickness of 0.61. In order to evaluate the characteristics of the product, ■ Corrosion test method A (JIS Z2371 salt water spray test 6 hr-+70°C warm air 4 hr-+ 49
'C1C108% 4hr → -20°C freezing for 4hr was repeated for 28 days to determine the corrosion depth, and the target was 0.10mm or less in order to have corrosion resistance as a muffler for about 5 years. ), ■ Corrosion test method B (0.5% NaC1
+ Using a 0.2% H20□ solution, JISZ2371
The rusting was conducted for 4 days in accordance with the above, and the rusting ranks were A (good) to F (poor), with A-C being the target for the same reason as (2). )
, ■Corrosion test C method (after TIG welding, JIS 005
75 stainless steel sulfuric acid/sulfuric acid wAl11 food test 1
After 6 hours, the inner surface was bent at a radius of 0.3++uw, and the presence or absence of intergranular corrosion cracking in the welded part and heat affected zone on the outer surface was observed, and for the same reason as (2), the goal was to have no cracking. )
, ■Tensile test (0.2% yield strength and elongation are determined, and from the viewpoint of pipe forming property on the ordinary steel production line and workability as plates and pipes, 0.2% proof stress and elongation are determined.
．． 2% yield strength is 30 kgf/mm2 or less, elongation is 30%
The above was the goal. ), ■Lankford value (The Lankford value (F) is used as an evaluation standard for the workability of pipes and plate materials. By using the ordinary steel process, the F (direction) can be significantly improved, so the 1.70 or higher), ■Secondary processing test (from plate thickness 0.6mm to 0.4
After processing by cold rolling to 2s+s, closely target the bending twill line to be parallel to the rolling direction, and rank it from 1 (no cracking) to 6 (severe cracking) depending on the degree of cracking. The target was 1 from the viewpoint of workability of plates and tubes. ) tests were conducted.

〔Effect of the invention〕

All of the steels of the present invention exhibited superior corrosion resistance compared to the comparative steels based on the local corrosion occurrence characteristics and local corrosion depth shown in Table 1. Furthermore, the results of the actual manufacturing examples shown in Table 2 also showed that the steel of the present invention had excellent characteristics in corrosion resistance and workability.

Therefore, the steel of the present invention is a material that exhibits excellent corrosion resistance over a long period of time in a highly corrosive condensate environment such as an engine exhaust gas environment, and has high workability.
This shows that it is extremely effective in practical terms.

[Brief explanation of drawings]

FIG. 1(a) is a side view showing the shape of a test piece used in an electrochemical local corrosion occurrence evaluation test, and FIG. 1(b) is a front view thereof. FIG. 2 is a diagram for explaining an electrochemical local corrosion evaluation test method. 1... Lead wire, 2... Seal part, 3... Test surface, 4... Bolt/nut.

Claims (3)

[Claims] (1) In weight%, C: 0.010% or less, Si: 0.2% or less, Mn: 0.05% or more and 1.5% or less, Cr: 12% or more and 20.0% or less, Mo: 0. 2% or more and 3.0% or less, Al: 0.005% or more and 0.1% or less, N: 0.015% or less, P: 0.025% or less, S: 0.010% or less, and Ti: 10×(C%+N%) or more and 0.5% or less,
Or an engine exhaust gas material with excellent corrosion resistance, containing one or two types of Nb: 5 x (C% + N%) or more and 0.5% or less, with the remainder consisting of Fe and inevitable impurities. For stainless steel. (2) Furthermore, Ni: 0.1% or more and 1.0% or less, Cu:
0.03% or more and 1.0% or less, W: 0.05% or more and 0.
5% or less, V: 0.05% or more and 0.5% or less, Zr: 0
．． 2. The stainless steel for engine exhaust gas material having excellent corrosion resistance according to claim 1, characterized in that it contains one or more types of stainless steel of 0.05% or more and 1.0% or less. (3) Furthermore, 0.001% each of Ca or Ce
The stainless steel for engine exhaust gas material having excellent corrosion resistance according to claim 1 or 2, characterized in that the stainless steel contains one or two types in an amount of 0.03% or less.