Ferritic stainless steel for automotive exhaust system with improved heat resistance and condensate corrosion resistance, and method for manufacturing the same

Abstract

Provided are a ferritic stainless steel for automotive exhaust systems with improved heat resistance and condensate corrosion resistance and a method for manufacturing the same. The ferritic stainless steel according to an exemplary embodiment of the present invention includes a stainless steel base material comprising, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitable impurities, and an Al-plated layer formed on the stainless steel base material, wherein the ferritic stainless steel comprises a plating compound comprising (Al.sub.19FeMnSi.sub.2).sub.5.31 (Aluminum Iron Manganese Silicide) at an interface between the stainless steel base material and the Al-plated layer.

Claims

1. A ferritic stainless steel for automotive exhaust systems, comprising: a stainless steel base material comprising, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitable impurities, and an Al-plated layer formed on the stainless steel base material, wherein the ferritic stainless steel comprises a plating compound comprising Aluminum Iron Manganese Silicide at an interface between the stainless steel base material and the Al-plated layer, wherein a chrominance (ΔE) of a surface of the stainless steel before and after a heat treatment is 10 or less, wherein a content of the Sn is concentrated on a surface of the stainless steel base material adjacent to the Al-plated layer by 4.5 to 6.1 times as compared to the stainless steel base material.

2. The ferritic stainless steel according to claim 1, wherein the plating compound further comprises at least one selected from a group consisting of Al.sub.9FeSi.sub.2 (Aluminum Iron Silicon), Al.sub.3FeSi.sub.2 (Aluminum Iron Silicon), and Al (Aluminum).

3. The ferritic stainless steel according to claim 1, comprising Sn: 0.05 to 0.5%.

4. The ferritic stainless steel according to claim 1, having a maximum pitting corrosion depth of less than 0.4 mm in the evaluation of condensate corrosion characteristics (JASO-B M611-92).

5. A method for manufacturing a ferritic stainless steel for automotive exhaust systems with improved heat resistance and condensate corrosion resistance according to claim 1, comprising: hot-rolling a ferritic stainless steel slab comprising, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitable impurities; cold-rolling the hot-rolled steel plate; and aluminum-plating the cold-rolled steel plate.

6. The method according to claim 5, wherein the ferritic stainless steel is manufactured by a conventional STS 409L manufacturing process.

7. The method according to claim 5, further comprising heat-treating the aluminum-plated ferritic stainless steel at a temperature of 300 to 500° C. within 48 hours, wherein a chrominance (ΔE) of a surface of the stainless steel before and after the heat treatment is 10 or less.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing a maximum pitting corrosion depth of a Sn-added stainless steel of the present invention and a Sn-free stainless steel in a condensate solution of an automotive exhaust system.

(2) FIG. 2 is a drawing showing the results of analysis of an interface between an Al-plated layer of the Sn-added stainless steel of the present invention and a stainless steel base material by transmission electron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy).

(3) FIG. 3 is a drawing showing the results of line analysis by transmission electron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy) from the Al-plated layer of the Sn-added stainless steel of the present invention to a base material.

(4) FIG. 4 is a graph showing the results of X-ray diffraction (XRD) analysis of the interface between the Al-plated layer of the Sn-added stainless steel of the present invention and the stainless steel base material.

(5) FIG. 5 is a drawing showing the results of the line analysis by transmission electron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy) from the Al-plated layer of the Sn-free stainless steel to the base material.

(6) FIG. 6 is a graph showing the results of the X-ray diffraction (XRD) analysis of the interface between the Al-plated layer of the Sn-free stainless steel and the stainless steel base material.

(7) FIGS. 7 and 8 are photographs showing the Sn-added stainless steel of the present invention and the Sn-free stainless steel before and after a heat treatment

MODES OF THE INVENTION

(8) A ferritic stainless steel for automotive exhaust systems with improved heat resistance and condensate corrosion resistance according to an exemplary embodiment of the present invention includes a stainless steel base material comprising, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitable impurities, and an Al-plated layer formed on the stainless steel base material, wherein the ferritic stainless steel comprises a plating compound comprising (Al.sub.19FeMnSi.sub.2).sub.5.31 (Aluminum Iron Manganese Silicide) at an interface between the stainless steel base material and the Al-plated layer.

(9) Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to accompanying drawings. The following examples are provided to fully deliver the spirit of the present invention to those of ordinary skill in the art. The present invention may be specified in different forms without limitation to examples, which will not be described herein. To clarify the present invention, illustration of parts that are not associated with the explanation will be omitted, and to help in understanding, the sizes of components will be slightly exaggerated.

(10) The ferritic stainless steel for automotive exhaust systems according to an exemplary embodiment of the present invention comprises, in % by weight, C: 0.01% or less, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or less, S: 0.01% or less, Cr: 11 to 18%, N: 0.013% or less, Ti: 0.15 to 0.5%, Sn: 0.03 to 0.5%, and the remainder of Fe and other inevitable impurities.

(11) In the case of C and N being present in an interstitial form as Ti(C, N) carbonitride-forming elements, Ti(C, N) carbonitride is not formed when C and N contents are high, and C and N present at a high concentration deteriorate elongation and low-temperature impact properties of the material. When the material is used at 600° C. or below for a long period of time after welding, intergranular corrosion occurs due to generation of Cr.sub.23C.sub.6 carbide, and therefore the content of C is preferably controlled to be 0.01 wt % or less, and the content of N is preferably controlled to be 0.01 wt % or less.

(12) Moreover, when the C+N content is high, a number of surface defects such as scabs may occur due to an increase in steelmaking inclusions caused by adding a high content of Ti, a nozzle blocking phenomenon occurs in a continuous casting process, and elongation impact properties are degraded due to an increase in the high contents of C and N. For this reason, the C+N content is preferably controlled to be 0.02 wt % or less.

(13) Si is an element added as a deoxidizing element, and when its content is increased as a ferrite-phase forming element, ferrite-phase stability is increased. When the Si content is increased, a pitting corrosion potential is increased, and oxidation resistance is increased. In the present invention, for the purpose of improvement in pitting corrosion potential and oxidation resistance, at least 0.5 wt % or more of Si is preferably contained. If the Si content is increased to 1.0 wt % or more, steelmaking Si inclusions are increased and surface defects occur. For this reason, the Si content is preferably controlled not to exceed 1.0 wt % or more.

(14) When the Mn content is increased, pitting corrosion resistance is decreased by the formation of a precipitate such as MnS. However, excessive Mn reduction leads to an increase in refining cost, and therefore, the Mn content is preferably controlled to be 0.5 wt % or less.

(15) Since P and S form grain boundary segregation and an MnS precipitate, leading to degradation in hot workability, it is preferable that a small amount of P and S are present. However, since excessive reduction leads to an increase in refining cost, P is preferably controlled to be 0.035 wt % or less, and S is preferably controlled to be 0.01 wt % or less.

(16) Cr is an essential element for ensuring corrosion resistance of a stainless steel. When the Cr content is low, corrosion resistance is degraded in a condensate atmosphere, and when the Cr content is too high, corrosion resistance is improved, but due to high strength, degradation in elongation and impact properties, and an increase in production cost, the Cr content is preferably controlled to be 10 to 18 wt %.

(17) Ti is an effective element that fixes C and N to prevent intergranular corrosion. However, when a Ti/(C+N) ratio is decreased, due to intergranular corrosion occurring at welded areas, corrosion resistance is decreased, and therefore Ti is preferably controlled to be at least 0.15 wt % or more. However, when the Ti content is too high, steelmaking inclusions are increased, a number of surface defects such as scabs may occur due to an increase in steelmaking inclusions, a nozzle blocking phenomenon occurs in a continuous casting process, and elongation and impact properties are degraded. For this reason, the Ti content is preferably controlled to be 0.5 wt % or less.

(18) Sn is an essential element to ensure heat resistance and corrosion resistance in a condensate atmosphere, for which the present invention aims.

(19) In the present invention, to ensure heat resistance and condensate corrosion resistance, Sn is preferably controlled to be at least 0.03 wt % or more. However, since excessive addition of Sn leads to degradation of the manufacturing process, the Sn content is preferably controlled to be 0.5 wt %. More preferably, the Sn content is controlled to be 0.05 to 0.5 wt %. The ferritic stainless steel according to one embodiment of the present invention may be an Al-plated ferritic stainless steel, which includes a stainless steel base material and an Al-plated layer formed on the stainless steel base material.

(20) The ferritic stainless steel according to an embodiment of the present invention includes (Al.sub.19FeMnSi.sub.2).sub.5.31 (Aluminum Iron Manganese Silicide) as a plating compound at the interface between the stainless steel base material and the Al-plated layer.

(21) The ferritic stainless steel according to an embodiment of the present invention does not include Al.sub.17(Fe.sub.32Mn.sub.0.8)Si.sub.2 (Aluminum Iron Manganese Silicon) as a plating compound at the interface between the stainless steel base material and the Al-plated layer. This is a plating compound to be formed on Sn-free Al-plated stainless steel, that is, the composition of the plating compound formed on the interface between the stainless steel base material and the Al-plated layer differs depending on whether Sn is added or not.

(22) For example, the plating compound may further include at least one selected from a group consisting of Al.sub.9FeSi.sub.2 (Aluminum Iron Silicon), Al.sub.3FeSi.sub.2 (Aluminum Iron Silicon), Al (Aluminum).

(23) In the ferritic stainless steel according to the embodiment of the present invention, Sn is concentrated on a surface of the stainless steel base material adjacent to the Al-plated layer by 4.5 times or more as compared with the stainless steel base material. Sn is relatively strong in oxygen affinity compared with other elements, and may be concentrated on the surface of the stainless steel on which the oxidation scale is formed.

(24) For example, the Sn may be concentrated on the surface of the stainless steel base material by 4.5 to 6.1 times as compared to the stainless steel base material.

(25) The stainless steel not only includes a region where Sn is concentrated on the surface but also includes (Al.sub.19FeMnSi.sub.2).sub.5.31 (Aluminum Iron Manganese Silicide) as a plating compound at the interface between the stainless steel base material and the Al-plated layer, accordingly the desired improved heat resistance and condensate corrosion resistance may be obtained as compared with the ferritic stainless steels which do not contain Sn or in which Sn is not concentrated on the surface.

(26) FIG. 1 is a graph showing a maximum pitting corrosion depth of a Sn-added stainless steel of the present invention and a Sn-free stainless steel in a condensate solution of an automotive exhaust system.

(27) The embodiment of FIG. 1 is a ferritic stainless steel to which Sn is added according to an embodiment of the present invention, and a comparative example shows the maximum pitting corrosion depth after the evaluation of condensate corrosion characteristics (JASO-B M611-92) of a conventional Al-coated 409L steel.

(28) For example, the ferritic stainless steel according to one embodiment of the present invention may have the maximum pitting corrosion depth of less than 0.4 mm when evaluating the condensate corrosion characteristics (JASO-B M611-92). In contrast, conventional Al-coated 409L steels exhibit the maximum pitting corrosion depth of greater than about 0.6 mm.

(29) For example, the ferritic stainless steel according to one embodiment of the present invention may have a chrominance (ΔE) of the surface of the stainless steel before and after a heat treatment of 10 or less.

(30) The method for manufacturing the ferritic stainless steel according to one embodiment of the present invention may include subjecting a ferritic stainless steel slab containing the above composition to hot-rolling, hot annealing, hot pickling, cold-rolling and finishing annealing. This manufacturing process may be a conventional STS 409L manufacturing process. Thereafter, the cold-rolled steel sheet may be subjected to an aluminum plating process to produce an Al-coated ferrite stainless steel.

(31) For example, the method for manufacturing the ferritic stainless steel according to an embodiment of the present invention may further include heat-treating the Al-coated ferritic stainless steel at a temperature of 300 to 500° C. within 48 hours, and the chrominance (ΔE) of the surface of the stainless steel before and after the heat treatment is 10 or less.

(32) Hereinafter, the ferritic stainless steel for an automotive exhaust system according to an exemplary embodiment of the present invention is described in further detail with reference to examples.

EXAMPLES

(33) Inventive Steel 1

(34) A ferritic stainless steel slab was prepared having the composition shown in Table 1 below. The slab was hot-rolled at a temperature of 1,150° C. to prepare a hot-rolled steel plate having a thickness of 3.0 mm. The hot-rolled steel plate was annealed, pickled and cold-rolled to produce a cold-rolled steel plate having a thickness of 1.2 mm, followed by finish annealing and pickling, and then subjected to Al plating to produce a final Al-plated ferritic stainless steel product.

(35) Inventive Steel 2

(36) A ferritic stainless steel product was manufactured by the same method as that described for Inventive Steel 1, except that the composition of Inventive Steel 2 shown in Table 1 below was used.

(37) Inventive Steel 3

(38) A ferritic stainless steel product was manufactured by the same method as that described for Inventive Steel 1, except that the composition of Inventive Steel 3 shown in Table 1 below was used.

(39) Comparative Steel 1

(40) A ferritic stainless steel product was manufactured by the same method as that described for Inventive Steel 1, except that the composition of Comparative Steel 1 shown in Table 1 below was used.

(41) Comparative Steel 2

(42) A ferritic stainless steel product was manufactured by the same method as that described for Inventive Steel 1, except that the composition of Comparative Steel 2 shown in Table 1 below was used.

(43) TABLE-US-00001 TABLE 1 Classification C Si Mn P S Cr Ti Sb N Ti/(C + N) Inventive 0.005 0.597 0.30 0.021 <0.003 11.14 0.22 0.048 0.0074 17.4 Steel 1 Inventive 0.005 0.613 0.31 0.023 <0.003 11.21 0.21 0.11 0.0089 15.1 Steel 2 Inventive 0.006 0.592 0.30 0.019 <0.003 11.24 0.24 0.2 0.0072 18.2 Steel 3 Comparative 0.005 0.62 0.30 0.023 <0.003 11.24 0.22 0 0.0074 17.7 Steel 1 Comparative 0.006 0.594 0.30 0.020 <0.003 11.29 0.23 0.02 0.0072 18.8 Steel 2

(44) Table 2 shows the results of measurement of main components in the concentrated layer of the surface of the stainless steel base material adjacent to the stainless steel base material and the aluminum plating layer of Inventive Steel 3 above.

(45) TABLE-US-00002 TABLE 2 Concentrated Concentrated Concentrated Base layer layer layer material 1 (wt %) 2 (wt %) 3 (wt %) (wt %) Fe 79.11074 83.80825 82.11589 88.5022  Cr 11.32846  9.647609 10.79504 10.30857  Al  6.784723  4.033176  4.799919  0.157073 Si  1.798595  1.204887  1.276994  0.781254 Sn  0.977485  1.306278  1.012158  0.214902 Concentrated  4.5  6.1  4.7 — layer Sn/Base material Sn

(46) FIG. 2 is a drawing showing the results of analysis of an interface between an Al-plated layer of the Sn-added stainless steel of the present invention and a stainless steel base material by transmission electron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy).

(47) Referring to FIG. 2 and Table 2, in the ferritic stainless steel according to the embodiment of the present invention, Sn is concentrated on the surface of the stainless steel base material adjacent to the Al-plated layer by 4.5 to 6.1 times as compared to the stainless steel base material.

(48) FIG. 3 is a drawing showing the results of line analysis by transmission electron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy) from the Al-plated layer of the Sn-added stainless steel of the present invention to a base material. FIG. 4 is a graph showing the results of X-ray diffraction (XRD) analysis of the interface between the Al-plated layer of the Sn-added stainless steel of the present invention and the stainless steel base material.

(49) Referring to FIG. 3, when Cr and Sn were measured from the plated layer toward the base material, it was confirmed that Sn was concentrated on the surface of the stainless steel base material adjacent to the Al-plated layer.

(50) Referring to FIG. 4, the ferritic stainless steel comprises a plating compound comprising (Al.sub.19FeMnSi.sub.2).sub.5.31 (Aluminum Iron Manganese Silicide), Al.sub.9FeSi.sub.2 (Aluminum Iron Silicon), Al.sub.3FeSi.sub.2 (Aluminum Iron Silicon), and Al (Aluminum) at the interface between the stainless steel base material and the Al-plated layer.

(51) FIG. 5 is a drawing showing the results of the line analysis by transmission electron microscopy (TEM) and EDS (Energy-Dispersive Spectroscopy) from the Al-plated layer of the Sn-free stainless steel to the base material. FIG. 6 is a graph showing the results of the X-ray diffraction (XRD) analysis of the interface between the Al-plated layer of the Sn-free stainless steel and the stainless steel base material.

(52) Referring to FIG. 5, when Cr and Sn were measured from the plated layer toward the base material of the Sn-free stainless steel, it was confirmed that Sn was not concentrated on the surface of the stainless steel base material.

(53) Referring to FIG. 6, the ferritic stainless steel comprises a plating compound comprising Al.sub.17(Fe.sub.32Mn.sub.0.8)Si.sub.2 (Aluminum Iron Manganese Silicon), Al.sub.9FeSi.sub.2 (Aluminum Iron Silicon), Al.sub.3FeSi.sub.2 (Aluminum Iron Silicon), and Al (Aluminum) at the interface between the stainless steel base material and the Al-plated layer. That is, it can be confirmed that the plating compound of (Al.sub.19FeMnSi.sub.2).sub.5.31 (Aluminum Iron Manganese Silicide) formed in the Sn-added steel is not formed.

(54) Tables 3 to 5 show the results of measurement of whiteness before and after a heat treatment of the stainless steel of Inventive Steel 3 to which Sn was added and the stainless steel of Comparative Steel 1 to which no Sn was added.

Experimental Example 1

(55) The stainless steel of Inventive Steel 3 and the stainless steel of Comparative Steel 1 were heat-treated at 350° C. for 24 hours, and the whiteness before and after the heat treatment was measured and is shown in Table 3.

(56) TABLE-US-00003 TABLE 3 Whiteness Whiteness before heat after heat treatment treatment Chrominance(ΔE) Inventive Steel 3 77.84 78.04 7.11 Comparative Steel 1 78.27 71.52 10.83

Experimental Example 2

(57) The stainless steel of Inventive Steel 3 and the stainless steel of Comparative Steel 1 were heat-treated at 400° C. for 24 hours, and the whiteness before and after the heat treatment was measured and is shown in Table 4.

(58) TABLE-US-00004 TABLE 4 Whiteness Whiteness before heat after heat treatment treatment Chrominance(ΔE) Inventive Steel 3 77.93 80.04 5.70 Comparative Steel 1 77.27 68.97 13.59

Experimental Example 3

(59) The stainless steel of Inventive Steel 3 and the stainless steel of Comparative Steel 1 were heat-treated at 450° C. for 24 hours, and the whiteness before and after the heat treatment was measured and is shown in Table 5.

(60) TABLE-US-00005 TABLE 5 Whiteness Whiteness before heat after heat treatment treatment Chrominance(ΔE) Inventive Steel 3 78.95 79.50 5.38 Comparative Steel 1 77.59 65.80 15.35

(61) FIGS. 7 and 8 are photographs showing the Sn-added stainless steel of the present invention and the Sn-free stainless steel before and after a heat treatment.

(62) FIG. 7 is a photograph of the surface of the stainless steel according to Experimental Example 2, and FIG. 8 is a photograph of the surface of the stainless steel according to Experiment Example 3.

(63) Referring to the Experimental Examples, FIGS. 7 and 8, the color of the surface of the steel becomes darker and the whiteness is lowered after the heat treatment in the case of the Sn-free steel, but the whiteness does not decrease even when the heat treatment is performed in the case of Sn-added steel, for example, the difference in whiteness is 10 or less. As a result, it can be confirmed that the Sn-added steel is superior in heat resistance to the Sn-free steel.

(64) The corrosion resistance was evaluated according to Japanese standard JASO-B M611-92 for condensate corrosion resistance.

(65) That is, the maximum pitting corrosion depth was measured after repeating a cycle 4 times, and the cycle includes holding the mixture at 80° C. for 24 hours in an aqueous solution having a Cl.sup.− concentration: 100 ppm, a NO.sub.3.sup.− concentration: 20 ppm, a SO.sub.3.sup.2− concentration: 600 ppm, a SO.sub.4.sup.2− concentration: 600 ppm, a CH.sub.3COO.sup.− concentration: 800 ppm, and a pH of 8.0±0.2 for 5 times, and then holding the mixture at 250° C. for 24 hours.

(66) That is, the solution was maintained at 80° C. for 24 hours in an aqueous solution having a Cl.sup.− concentration: 100 ppm, a NO.sub.3.sup.− concentration: 20 ppm, a SO.sub.3.sup.2− concentration: 600 ppm, a SO.sub.4.sup.2− concentration: 600 ppm, a CH.sub.3COO.sup.− concentration: 800 ppm, and a pH of 8.0±0.2. The maximum formal depth was measured after repeating a total of 4 cycles with one cycle maintained at 250° C. for 24 hours.

(67) The corrosion resistance of the stainless steels according to Inventive Steel 3 and Comparative Steel 1 was evaluated according to the above method, and the maximum pitting corrosion depth was measured and is shown in Table 6.

(68) TABLE-US-00006 TABLE 6 Maximum pitting corrosion depth (mm) Inventive Steel 3 0.35 Comparative Steel 1 0.66

(69) FIG. 1 is a graph showing a maximum pitting corrosion depth of a Sn-added stainless steel of the present invention and a Sn-free stainless steel in a condensate solution of an automotive exhaust system.

(70) The embodiment of FIG. 1 is a Sn-added ferritic stainless steel of Inventive Steel 3, and a comparative example is a conventional Al-plated 409L steel of Comparative Steel 1. Referring to FIG. 1 and Table 6, Inventive Steel 3 has a maximum pitting corrosion depth of 0.35 mm and Comparative Steel 1 has 0.66 mm when evaluating condensate corrosion characteristics (JASO-B M611-92).

(71) As a result, Sn-added Al-plated ferritic stainless steels according to embodiments of the present invention have excellent condensate corrosion resistance and heat resistance.

(72) As described above, while the present invention has been described with reference to exemplary embodiments of the present invention, the present invention is not limited thereto, and it will be understood by those of ordinary skill in the art that various modifications and alternations can be made without departing from the concept and scope of the accompanying claims.

INDUSTRIAL APPLICABILITY

(73) Ferritic stainless steel according to the embodiments of the present invention is excellent in heat resistance and condensate corrosion resistance without adding expensive elements such as Cr and Mo, and may be applied to an automotive exhaust system.