KR20030082124A - Ferritic stainless steel for exhaust manifold with improved oxidation resistance - Google Patents

Abstract

PURPOSE: A ferritic stainless steel alloy for exhaust manifold with improved oxidation resistance even at high temperature compared to existing ferritic stainless steel alloy is provided. CONSTITUTION: The ferritic stainless steel alloy for exhaust manifold comprises a principal constituent of iron (Fe), 14.0 to 16.0 wt.% of chromium (Cr), 0.5 to 4.0 wt.% of silicon (Si), 0.30 to 1.0 wt.% of niobium (Nb), 0 to 0.8 wt.% of copper (Cu), 0.05 to 0.06 wt.% of manganese (Mn), 0 to 0.2 wt.% of titanium (Ti), 0 to 0.03 wt.% of phosphorus (P), 0 to 0.02 wt.% of sulfur (S), 0 to 0.015 wt.% of carbon (C) and 0.05 to 0.5 wt.% of nitrogen (N).

Description

Ferritic stainless steel for exhaust manifold with improved oxidation resistance

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless alloy for automobile exhaust manifolds having excellent oxidation resistance, and more specifically, 14.0 to 16.0 wt% of chromium (Cr) based on iron (Fe) and silicon (Si). ) 0.5 to 4.0 wt%, niobium (Nb) 0.30 to 1.0 wt%, copper (Cu) 0 to 0.8 wt%, manganese (Mn) 0.05 to 0.06 wt%, titanium (Ti) 0 to 0.2 wt%, phosphorus (P ) 0 to 0.03 wt%, sulfur (S) 0 to 0.02 wt%, carbon (C) 0 to 0.015 wt%, nitrogen (N) 0.05 to 0.5 wt%, resistant to high temperatures compared to conventional ferritic stainless steel A ferritic stainless steel for automobile exhaust manifolds having excellent oxidation resistance.

In general, the exhaust gas combusted in the cylinder of the engine is discharged to the outside through an exhaust manifold attached to the engine. Since the exhaust gas after combustion is a gas of high temperature, the exhaust manifold which receives it is essentially required oxidation resistance at high temperature. Exhaust manifold material used in conventional engines is cast iron, which maintains oxidation resistance at high temperatures below about 800 ° C.

However, in the case of turbocharged vehicles, which have increased the combustion temperature of fuel for fuel efficiency and the recent increase in the temperature of the exhaust manifold such that the temperature of exhaust gas is about 1000 ° C, existing cast iron becomes more severe. As a material, it became impossible to secure oxidation resistance at high temperatures.

In order to cope with this, an exhaust manifold made of a stainless steel material having a high oxidation resistance by forming a Cr oxide layer at a high temperature has recently been manufactured, and this stainless steel has a high temperature due to a Cr oxide layer formed on the surface during the high temperature oxidation. Although it shows excellent oxidation resistance even in the atmosphere of exhaust gas, there is a problem that economic efficiency is lower than that of cast iron due to the price increase due to high Cr content.

In addition, the conventional stainless steel (alloy) for the exhaust manifold is a Fe-Cr-based ferritic stainless steel containing Cr of about 16 to 20 wt%, and about 16 wt% for an alloy having a reduced content of C. Cr is added and additionally less than 1 wt% of Al or Si [SUS 430J1L, POSCO]. Since the existing stainless steel is depleted from the alloy as Cr is oxidized, there is a problem in that the amount of Cr added in the alloy cannot be lowered below a predetermined level (about 16 wt%).

Accordingly, the inventors and applicants of the present application have applied for a patent for a ferritic stainless steel containing an alloying element that produces a protective oxide layer similar to Cr in a manner that can suppress the addition amount of Cr and maintain oxidation resistance (Patent application number 2001-0081947). This is to improve the oxidation resistance by adding 0.5 to 2 wt% of Al and 0.5 to 4 wt% of Si, which forms a protective oxide layer at a high temperature while reducing the amount of Cr to 14 to 16 wt%. However, Al of these alloying elements are excellent in oxidizing properties, so that the yield is poor at the time of alloy production, there is a problem that it is difficult to control the amount, such as difficulty in adding to the desired level.

Therefore, the present invention was invented to solve the above problems, while reducing the amount of Cr to 14 to 16 wt%, an alloying element, which produces a protective oxide layer similar to Cr, is added but Al is removed. It is an object to provide ferritic stainless steel for automobile exhaust manifolds having excellent oxidation resistance even at high temperatures by adding 0.05 to 0.5 wt% of N, which suppresses Cr nitride formation by forming nitride with Si.

1 is a graph showing the results of the oxidation resistance test of Examples and Comparative Examples according to the present invention.

The ferritic stainless steel for exhaust manifold having improved oxidation resistance according to the present invention is a ferritic stainless steel for exhaust manifold, which is composed of iron (Fe) 14.0 to 16.0 wt% and silicon (Si). ) 0.5 to 4.0 wt%, niobium (Nb) 0.30 to 1.0 wt%, copper (Cu) 0 to 0.8 wt%, manganese (Mn) 0.05 to 0.06 wt%, titanium (Ti) 0 to 0.2 wt%, phosphorus (P ) 0 to 0.03 wt%, sulfur (S) 0 to 0.02 wt%, carbon (C) 0 to 0.015 wt%, nitrogen (N) 0.05 to 0.5 wt%.

Ferritic stainless steel with improved oxidation resistance of the present invention was prepared by ingot using the arc melting furnace (arc melting furnace) to produce an ingot (ingot) and homogenized the ingot for 8 hours at 1050 ℃ after hot rolling Quenching with water produces super corrosion-resistant ferritic stainless steel. Ferritic stainless steel does not undergo solution treatment because there is no strength improvement by heat treatment.

In the composition of the ferritic stainless steel according to the present invention, the following effects can be obtained by adding 0.5 to 4.0 wt% of Si and 0.05 to 0.5 wt% of Si while reducing the amount of Cr to 14.0 to 16.0 wt%.

1) Increased oxidation resistance compared to cast iron

2) Prevents deterioration of oxidation resistance by reducing Cr content by adding Si, which forms a protective oxide layer similar to Cr.

3) Material cost savings by reducing Cr content

4) Suppression of Cr nitride formation by nitride formation of N and Si, suppressing grain boundary chromium deficiency and supplying silicon during high temperature oxidation

5) Improvement of oxidation resistance and inhibition of chromium oxidation by secondary oxidation of Si after initial Cr oxidation

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

Example

Iron (Fe) is used as the base metal, 13.5 wt% of chromium (Cr), 4 wt% of silicon (Si), and 0.5 wt% of chromium nitride (CrN) are dissolved in an arc melting furnace to weigh 5 kg of ingot. After the preparation, it was homogenized at 1050 ° C. for 8 hours, hot rolled to a thickness of 4 mm, and then heat treated at 450 ° C. for 2 hours to observe nitride formation inhibition to prepare ferritic stainless steel specimens.

Comparative Example 1

Iron (Fe) was used as the base metal, and 18 wt% of chromium (Cr) and 4 wt% of nickel (Ni) were used to prepare stainless steel specimens having the same specifications in the same manner as in the above example (SUS304).

Comparative Example 2

Using the 430J1L ferritic stainless steel manufactured by POSCO to prepare a specimen of the same specifications as in the above embodiment (SUS430J1L).

Test Example: Oxidation Resistance Experiment

Using the alloys of Examples and Comparative Examples 1 and 2, the results of the oxidation resistance test for 200 hours (HR) in an air atmosphere of 900 ℃ is shown in Figure 1 and the following Table 1.

First, as shown in the accompanying Figure 1, it was found that the amount of oxidized in the alloy of Example and Comparative Example 2 is much less than the amount of oxidized in the alloy of Comparative Example 1 in the initial oxidation within 100 hours, In particular, it could be confirmed that sufficient oxidation resistance was ensured even in the alloy of the example in which the amount of Cr was reduced relative to Comparative Example 2. On the other hand, the amount of oxidation in the alloy of the Example and Comparative Example 2 was found to be a distinct difference over time. That is, as shown in Table 1, the amount of oxidation after 100 hours is shown to be similar to each other in the alloy of Example and Comparative Example 2, but the amount of oxidation in the alloy of Example compared to the alloy of Comparative Example 2 over time About 15 to 20% less (after 200 hours), it was confirmed that even in the alloy with the reduced amount of Cr can be secured longer oxidation resistance than Example 2. In addition, it was confirmed that the oxidation resistance of the grain boundary due to the addition of N did not appear in the Example.

As described above, an alloying element which produces a protective oxide layer similar to Cr by adding N at 0.05 to 0.5 wt% of N, which inhibits Cr nitride formation by forming nitride with Si while reducing the amount of Cr to 14 to 16 wt%. Phosphorus Si alone can provide ferritic stainless steel for exhaust manifolds with superior oxidation resistance compared to existing cast iron and ferritic stainless steels (SUS 430J1L), and material costs of the exhaust manifolds can be reduced by reducing the amount of Cr used. Further increase in oxidation resistance is expected through the inhibition of Cr nitride formation by N-Si compound formation, thereby reducing grain boundary oxidation resistance and Si supplying role. In addition, due to the rapid oxidation of Al during dissolution and the reduction of the recovery rate and the change in composition, the addition of a large amount of Al, which was regarded as a difficulty in manufacturing the alloy, is eliminated, thereby facilitating control of the alloy composition.

Claims (1)

In ferritic stainless steel for exhaust manifolds, chromium (Cr) 14.0 to 16.0 wt%, silicon (Si) 0.5 to 4.0 wt%, niobium (Nb) 0.30 to 1.0 wt%, mainly composed of iron (Fe) (Cu) 0 to 0.8 wt%, manganese (Mn) 0.05 to 0.06 wt%, titanium (Ti) 0 to 0.2 wt%, phosphorus (P) 0 to 0.03 wt%, sulfur (S) 0 to 0.02 wt%, carbon (C) 0 to 0.015 wt% and nitrogen (N) 0.05 to 0.5 wt%, ferritic stainless steel for exhaust manifold having improved oxidation resistance.