JP2009540115A - Cast iron alloy with excellent high-temperature oxidation resistance - Google Patents

Abstract

Chemical components are C: 2.8 to 3.6 mass%, Si: 2.0 to 3.0 mass%, Al: 2.5 to 4.3 mass%, Ni: 1.0 mass% or less, Mo: 0.8 mass% or less, Mn: 0.3 mass% or less, Ce : 0.002 to 0.1 mass%, Mg: 0.023 to 0.06 mass%, S: 0.01 mass% or less, balance: Fe and normal impurities, suitable for cast iron products having high oxidation stability at a surface temperature of 800 to 950 ° C Cast iron alloy.

Description

  The present invention relates to a cast iron alloy for cast iron products having high oxidation resistance at a high surface temperature.

  Automakers are required to follow new emission standards. Catalytic converters perform better as the exhaust gas temperature is higher. Palladium can be used in place of platinum as a catalyst material and the maximum exhaust gas temperature will increase from the current 850 ° C. to 950 ° C. At these temperatures, conventionally known cast iron alloys have a problem in scale resistance. In conventional ferrite alloys, the phase transformation from the ferrite lattice to the austenite lattice occurs at a temperature of about 860 ° C. or higher. The expansion behavior of the ferrite lattice is different from that of the austenite lattice. Since the thermal expansion coefficient of the austenite lattice is larger than that of the ferrite lattice and changes more drastically, the volume change occurs at the transformation temperature. This volume change causes non-uniform expansion behavior and microcracks in the cast part. Cast parts exposed to frequent temperature changes are subjected to mechanical stress due to this non-uniform expansion and cracking. As a result, the thin oxide layer (= scale) is separated from the surface of the cast part. Ideally, a thin oxide film should be formed on the surface of the turbocharger housing and / or the exhaust manifold that is well deposited over a long period of time and prevents oxygen diffusion.

  Patent Document 1 discloses a ferritic heat-resistant cast iron having spheroidal graphite. This alloy has C: 3.4% by mass or less, Si: 3.5-5.5% by mass, Mn: 0.6% by mass or less, Cr: 0.1-0.7% by mass, Mo: 0.3-0.9% by mass, and components for forming spherical graphite: 0.1 Contains up to mass%. This alloy is used in the manufacture of turbocharger housings in automobile manufacturing.

  Patent Document 2 discloses an alloy for cast iron products having high thermal stability. This alloy has C: 2.5-2.8 mass%, Si: 4.7-5.2 mass%, Mo: 0.5-0.9 mass%, Al: 0.5-0.9 mass%, Mg: 0.04 mass% or less, S: 0.02 mass% or less, Ni: 0.1 to 1.0 mass%, Zr: 0.1 to 0.4 mass%, balance: Fe and normal impurities. This alloy is used in exhaust manifolds and turbocharger housings in automobile manufacturing.

European Patent No. 076701 (B1) specification European Patent No. 1386976 (B1) Specification

  Based on this prior art, the present invention is a cast iron that can be used at as high a temperature as possible, can be manufactured as economically as possible, and ensures the longest possible service life under frequent temperature changes. The object is to provide an alloy.

  The purpose is C: 2.8 to 3.6 mass%, Si: 2.0 to 3.0 mass%, Al: 2.5 to 4.3 mass%, Ni: 1.0 mass% or less, Mo: 0.8 mass% or less, Mn: 0.3 mass% or less, Ce : 0.002 to 0.1% by mass, Mg: 0.023 to 0.06% by mass, S: 0.01% by mass or less, balance: Fe and chemical components of ordinary impurities, high oxidation resistance at a surface temperature of 800 to 950 ° C Achieved by cast iron alloy for cast iron products.

  Preferred improvements of the invention are found in the dependent claims.

  An important concept of the present invention is that it has high scaling resistance in the turbocharger housing and exhaust manifold so that the operating temperature can be as high as possible, and it is as economical and simple as possible in the casting process. It is to provide a cast iron alloy to be obtained. Conventional standard solutions for higher operating temperatures are limited to the use of expensive cast steel or austenitic cast iron, or the use of carefully manufactured sheet metal designs.

The transformation from the ferrite phase to the austenite phase of this alloy is shown as a function of temperature. Here, it can be seen that the equilibrium phase transformation occurs at about 900 ° C. It can also be seen how this alloy changes its solid state at a melting temperature of 1240-1280 ° C. The coefficient of thermal expansion of the new alloy with the designation symbol SiMo1000plus measured as a function of temperature is shown in comparison with other cast iron alloys. The thermal conductivity of this alloy, SiMo1000plus, is shown as a function of temperature compared to other cast iron alloys. Here, D5S represents a so-called Ni-resist alloy and GJV SiMo and SiMoNi represent conventionally known spheroidal graphite cast iron alloyed with about 1% Mo.

It is advantageous for the cast part to elastically expand as uniformly as possible at the operating temperature. This is achieved by the transformation temperature from the ferrite phase to the austenite phase of the alloy above 880 ° C. It is also achieved by the uniform and continuous change in the thermal expansion of the alloy sample measured with an dilatometer up to 880 ° C. Furthermore, it is achieved by an alloy having a thermal expansion coefficient of 8 × 10 ^{−6 to} 12 × 10 ⁻⁶ / K at 25 ° C. and 13.5 × 10 ^{−6 to} 15.5 × 10 ⁻⁶ / K at 900 ° C. These values are plotted against temperature and are always about 30% lower than those of so-called Ni-resist alloys with standard designation D5S or GJSA XniSiCr35-5-2.

It is further advantageous that the cast iron part is not brittle at room temperature. This is a tensile strength R _m 500~650MPa, is achieved by the yield point alloy having a strength value of 470~620MPa and elongation at break A ₅ at 2.0 to 4.0 in R _0.2. These strength values are about 1.3 to 1.5 times larger than those of so-called Ni-resist alloys. The ductility of the proposed cast iron alloy corresponds to the average value of standard industrial ferritic materials that cannot be exposed to temperatures above 860 ° C.

  It is also advantageous that the cast part can be easily processed. This is achieved with an alloy having a Brinell hardness of 220-250.

  It is also advantageous that the alloy consists of as economical elements as possible. This is achieved with alloys containing Mo: less than 0.8% by weight, Cr: less than 1% by weight and Ni: less than 1% by weight. Ni-resist alloys usually contain Ni: about 30-35% by mass and Cr: about 2-5% by mass. Spheroidal graphite cast iron alloys alloyed with molybdenum usually contain about 0.8% by weight molybdenum.

  It is also advantageous that the cast part is as dull as possible with respect to heat. This is achieved with a sample of an alloy having a thermal conductivity of 25 W / mK at 25 ° C. and a thermal conductivity of 26 W / mK at 900 ° C. Niresist alloys have a thermal conductivity 20-50% lower at 400 ° C.

  C: 3.02 mass%, Si: 2.96 mass%, Al: 2.53 mass%, Ni: 0.79 mass%, Mo: 0.65 mass%, Mn: 0.23 mass%, Cu: 0.04 mass%, P: 0.031 mass%, Cr: Exhaust manifold made of spheroidal graphite cast iron for automobile combustion engine having chemical composition of 0.026 mass%, Mg: 0.023 mass%, Ti: 0.017 mass%, S: less than 0.01 mass% and Ce: 0.002 mass% is a ferrite Has a lattice. This exhaust manifold is cast by pouring molten metal previously treated with a GF converter into a mold directly. The heat treatment which requires a long time such as subsequent solution annealing and austempering treatment is unnecessary.

  Magnesium treatment has a positive effect on the sulfur content of the alloy and also ensures graphite spheroidization or vermiculization. A sufficient amount of Mg must remain in the melt to promote the growth of spherical nuclei (spherical graphite particles), but magnesium has a desulfurization effect. An Mg content of about 0.025% by weight is ideal for an Al content of about 2.5% by weight of the alloy. A sample of this alloy has a density that is at least 5% less than the density of conventional comparable cast iron alloys.

  A carbon content of 2.8-3.6% by weight ensures a composition close to eutectic. A C content of less than 2.8% is not preferable for the raw material of cast parts. A C content exceeding 3.6% is not preferred for the high temperature properties of the alloy.

  Cerium is added as a nucleation promoter at 0.002 to 0.1% by mass. A cerium content exceeding 0.1% is not preferable because it causes the formation of so-called chunky graphite.

  A silicon content of 2-3% by weight in this alloy has a good effect on the formation of the ferrite phase, improves the fluidity of the melt, raises the yield point, and improves the heat resistance of the cast part. A Si content of less than 2% is not preferred for the chill depth. A Si content greater than 3% increases the brittleness of the cast part.

  An aluminum content of 2.5 to 4.3% by weight has a good effect on the formation of the ferrite phase as well and neutralizes nitrogen. An Al content of less than 2.5% is not preferable for the stabilization of graphite. An Al content exceeding 4.3% is not preferable for the formation of spherical graphite.

  A nickel content of 0.1 to 1% by weight raises the yield point and improves the corrosion resistance without substantially increasing the brittleness. A Ni content of less than 0.1% is not preferable for the stabilization of graphite. Ni content exceeding 1% is not preferable because of the formation of bainite and martensite in the thin part of the cast part. Nickel is a relatively expensive alloy element.

  Molybdenum content of 0.4 to 0.8% by mass has a good effect on increasing the yield point, thermal stability, creep strength and thus thermal cycle stability. A Mo content of less than 0.4% is not preferred for graphite stabilization. Mo contents exceeding 0.8% are not preferred due to the formation of carbides and bubbles. Molybdenum is an extremely expensive element.

  A manganese content of 0.3% by mass or less has a good effect on the bond with sulfur. A Mn content greater than 0.3% is undesirable due to grain boundary carbide formation and damage to the nucleation state. Excess Mn promotes the formation of pearlite in the crystal lattice. The bainite lattice becomes increasingly brittle.

  A chromium content of 1% by mass or less has a good effect on the creep strength and thermal stability of the cast product.

  In general, lower contents of alloy additives are preferred to reduce grain boundary carbide formation and brittleness at room temperature. This is the case, for example, for copper and titanium contents.

  Compared to cast steel, the melting temperature of spheroidal cast iron is about 100-200 ° C lower. This means that less energy is consumed and less alloy elements are released to the surroundings by evaporation.

Claims (12)

  C: 2.8-3.6 mass%, Si: 2.0-3.0 mass%, Al: 2.5-4.3 mass%, Ni: 1.0 mass% or less, Mo: 0.8 mass% or less, Mn: 0.3 mass% or less, Ce: 0.002-0.1 Mass%, Mg: 0.023 to 0.06 mass%, S: 0.01 mass% or less, with the balance being a chemical component consisting of Fe and normal impurities, for cast iron products having high oxidation resistance at a surface temperature of 800 to 950 ° C Cast iron alloy.   The cast iron alloy according to claim 1, wherein the alloy further contains Ni: 0.1 to 1.0 mass%, Mo: 0.4 to 0.8 mass%, and Cr: 1.0 mass% or less.   The cast iron alloy according to claim 1 or 2, wherein a transformation temperature of the alloy from a ferrite phase to an austenite phase is 880 ° C or higher.   The cast iron alloy according to any one of claims 1 to 3, wherein the thermal expansion of the alloy measured by an dilatometer changes uniformly and continuously up to a temperature of 880 ° C. The alloy has a coefficient of thermal expansion of 8 × 10 ^{−6 to} 12 × 10 ⁻⁶ / K at 25 ° C. and 13.5 × 10 ^{−6 to} 15.5 × 10 ⁻⁶ / K at 900 ° C. The cast iron alloy of any one of -4. 500~650MPa in strength R _m the alloy tensile any one of claims 1 to 5 470～620MPa, characterized by having a strength value of elongation at break A ₅ at 2.0 to 4.0 at yield R _0.2 The cast iron alloy described in 1.   The cast iron alloy according to any one of claims 1 to 6, wherein the alloy has a Brinell hardness of 220 to 250.   8. The cast iron alloy according to claim 1, wherein the alloy has a thermal conductivity of 20 to 25 W / mK at 25 ° C. and a thermal conductivity of 23 to 29 W / mK at 900 ° C. 9. .   9. The cast iron alloy according to any one of claims 1 to 8, wherein the alloy has a density that is at least 5% less than the density of a conventional similar cast iron alloy.   The cast iron alloy according to any one of claims 1 to 9, wherein the alloy is processed by a magnesium converter so as to be an extremely low sulfur alloy.   The method for producing a cast iron alloy according to any one of claims 1 to 10, wherein after the preliminary treatment with the magnesium converter, the alloy is poured into a mold and is not subjected to any heat treatment.   The cast iron alloy according to any one of claims 1 to 11, wherein the cast iron alloy is used as an exhaust manifold and / or a turbocharger housing in automobile manufacture.

Images