Cr-mn-n austenitic heat-resistant steel and a method for manufacturing the same

Abstract

Cr-Mn-N austenitic heat-resistant steel, containing in mass percentages: carbon 0.30% to 0.45%, silicon 0.80% to 1.50%, manganese 3.00% to 4.80%, phosphorus less than 0.02%, sulfur less than 0.02%, chromium 23.00% to 26.00%, nickel 6.00% to 8.00%, molybdenum less than 0.20%, niobium less than 0.30%, tungsten less than 0.40%, vanadium less than 0.12%, nitrogen 0.30% to 0.60%, zirconium less than 0.08%, cobalt less than 0.08%, yttrium less than 0.08%, boron less than 0.10%, with iron for balance. The application contains 2 more patent applications.

Description

Description TECHNICAL FIELD

[0001] The present invention relates to the field of automotive steel, and in particular to Cr-Mn-N austenitic heat-resistant steel and to a subprocess for producing the same. STATE OF THE ART

[0002] With the increasing function and lightness of automobiles, the temperature of the exhaust manifold of automobiles has increased with the increase of engine speed, and the maximum operating temperature of the exhaust manifold and the turbocharger connected to the engine can reach 1050°C or even higher. Therefore, this requires that the materials used to make the turbine housing and the exhaust manifold not only have sufficient strength at high temperatures and high temperature resistance, but also have good dimensional stability and high ductility, as well as good thermal conductivity during long-term operation at elevated temperatures.

[0003] At present, the materials of turbocharger housings and exhaust manifolds are mainly "hi-sil-moly" ductile iron (ductile iron with molybdenum and high silicon content) and "Ni-resist" ductile iron (see CN 103898398A and CN 103898397A). The highest operating temperature of the material is lower than 1000 °C, and it cannot operate normally at higher temperatures. In addition, when operating at temperatures higher than 1000 °C, the materials have the problem of low thermal conductivity, power reduction at high temperatures, and high thermal expansion coefficient associated with the oxidation limit and thermal fatigue of the material. In addition, the materials also have the disadvantage of high cost due to the addition of a large amount of nickel. Accordingly, these materials cannot meet the requirements of high-performance engines.

[0004] US 5 019 332 describes a heat, corrosion and wear resistant stainless steel alloy. ESSENCE OF THE INVENTION

[0005] Accordingly, the object of the present invention is to provide a Cr-Mn-N austenitic heat-resistant steel with high strength at high temperatures, high thermal conductivity and low coefficient of thermal expansion, as well as characteristics of high stability of metallographic structure, good dimensional stability, high ductility, heat resistance, impact resistance and low production cost, thereby meeting the requirements for high-performance engines.

[0006] To achieve the above object, the present invention provides the following technical schemes.

[0007] The present invention provides a Cr-Mn-N austenitic heat-resistant steel as defined in the appended claims.

[0008] Preferably, the Cr-Mn-N austenitic heat-resistant steel contains in weight percentages carbon 0.30% to 0.45%, silicon 0.80% to 1.50%, manganese 3.00% to 4.80%, phosphorus less than 0.02%, sulfur less than 0.02%, chromium 23.00% to 26.00%, nickel 6.50% to 7.00%, molybdenum less than 0.20%, niobium less than 0.30%, tungsten less than 0.40%, vanadium less than 0.12%, nitrogen 0.40% to 0.50%, zirconium less than 0.08%, cobalt less than 0.08%, yttrium less than 0.08%, boron less than 0.10%, with iron for balance.

[0009] In the present invention, the elements manganese and nitrogen can facilitate the formation of austenite, and the element nitrogen has a 30 times greater ability to facilitate the formation of austenite than the element nickel. The element nickel is replaced by the element manganese and nitrogen to facilitate the formation of austenite. The cost of the element manganese and nitrogen is only 20% to 30% of the cost of the element nickel. Therefore, austenitic heat-resistant steel can be produced with lower production costs. In addition, the element nitrogen also has the ability to stabilize the microstructure at elevated temperatures, enhance the strength at elevated temperatures, improve the corrosion resistance and the resistance to corrosion cracking. The element manganese can act as a good desulfurizing agent and a good deoxidizer, thereby keeping the sulfur and oxygen content contained in the liquid steel at a lower level, enhancing the instantaneous strength at elevated temperatures, and improving the fracture toughness and cracking performance of the material.The Cr-Mn-N austenitic heat-resistant steel provided by the present invention has the characteristics of high temperature strength, high thermal conductivity, excellent high temperature fatigue performance, lower thermal expansion coefficient, higher stability of metallographic structure, good dimensional stability, higher ductility, heat resistance, impact resistance, low production cost, etc., thereby meeting the requirements of high-performance engines. Therefore, the steel of the present invention can be widely used as a material for automobile turbine housings and exhaust manifolds.

[0010] The present invention further provides a process for producing a Cr-Mn-N austenitic heat-resistant steel as defined in the appended claims.

[0011] Preferably, after the melt has been allowed to stand, in step (b), as defined in the appended claims, a slag removal process is further carried out.

[0012] The process for producing Cr-Mn-N austenitic heat-resistant steel is simple. The Cr-Mn-N austenitic heat-resistant steel produced by this process has the characteristics of high temperature strength, high thermal conductivity, excellent high temperature fatigue performance, lower thermal expansion coefficient, higher stability of metallographic structure, good dimensional stability, higher ductility, heat resistance, impact resistance, low production cost, etc., thus meeting the requirements of high-performance engines. DETAILED DESCRIPTION

[0013] In the present invention, the source of elements of the raw alloy material is not particularly limited, and any commodity on the market of raw alloys well known to those skilled in the art is available. In embodiments of the present invention, it is preferable that the raw alloy material is silicon-iron, manganese, ultra-low-carbon ferrochrome, ferroniobium, ferrotungsten, ferrovanadium, nickel-plated sheet, nitrided ferrochrome alloy, zirconium metal, yttrium metal, cobalt metal and ferroboron.

[0014] In the present invention, the melting temperature in step (a) is 1580 to 1700 °C, preferably 1600 to 1680 °C, and most preferably 1630 to 1650 °C.

[0015] In this invention, the preferred melting time in step (a) is 0.5 to 3.0 h, more preferably 0.6 to 2.0 h, and most preferably 0.8 to 1.5 h.

[0016] In the present invention, the heating methods for melting raw alloy materials are not particularly limited, any heating method well known to those skilled in the art is available. Melting devices for raw alloy materials are not particularly limited, any melting device well known is available to those skilled in the art. In embodiments of the present invention, the melting process is preferably carried out in a medium frequency induction furnace.

[0017] After the melt is obtained, the melt is allowed to stand for a few minutes, then it is prepared for shaping to obtain a Cr-Mn-N austenitic heat-resistant steel. The standing time is 3 to 20 minutes, more preferably 5 to 15 minutes, and most preferably 8 to 12 minutes.

[0018] After standing, it is preferable to carry out a slag removal process on the surface of the melt. The slag removal process is not particularly limited, any slag removal process is well known to those skilled in the art. In the present invention, a mechanical slag removal process is preferred.

[0019] According to the present invention, the melt, after being allowed to stand, is prepared for shaping. The temperature of the shaped casting of the Cr-Mn-N austenitic heat-resistant steel is 1550 to 1650 ° C, more preferably 1560 to 1630 ° C, and most preferably 1580 to 1620 ° C.

[0020] In the present invention, the melt preparation for molding after standing is not particularly limited, and any device well known to those skilled in the art is available. In the embodiment of the present invention, it is preferable that the melt preparation for molding is carried out in a ladle.

[0021] In the present invention, after pouring the molding melt, sandblasting, grinding, shortening and inspection are carried out. The sandblasting, grinding, shortening and inspection process is not particularly limited, and any process well known to those skilled in the art is available.

[0022] The process for producing Cr-Mn-N austenitic heat-resistant steel is simple. The Cr-Mn-N austenitic heat-resistant steel produced by this process has the characteristics of high temperature strength, high thermal conductivity, excellent high temperature fatigue performance, high temperature oxidation resistance, lower thermal expansion coefficient, higher stability of metallographic structure, good dimensional stability, higher ductility, heat resistance, impact resistance, low production cost, etc., thus meeting the requirements of high-performance engines.

[0023] The Cr-Mn-N austenitic heat-resistant steel and the method for producing the same of the present invention will be described in detail hereinafter in combination with examples, but these examples should not be construed as limiting the scope of the invention. Example 1

[0024] I. Ingredients: main raw materials in weight percentage: fuel 0.32%, steel scrap 43.39%, chromium nitrate 8.58%, ultra-low carbon ferrochrome 34.31%, electrolytic manganese 5.15%, ferrosilicon 1.25% and nickel plate 7.0%. II. Melting: A medium frequency induction furnace was used for melting. The capacity of the induction furnace can range from 0.5 tons to 3 tons. The measured raw materials are sequentially fed into the medium frequency induction furnace, which is then fed and heated. After the materials are completely melted, the temperature inside the medium frequency induction furnace is raised to 1580 ° C. Spectroscopic analysis was carried out for the melt inside the medium frequency induction furnace using a spectroscopic analysis test strip. The analysis result is shown in the following table. Element C Si Mn PS Cr Ni Mo Nb Mass.(%) 0.43 1.20 4.72 0.010 0.008 25.64 6.72 0.013 0.0076 Element WVN Zr YB Co Fe Mass.(%) 0.0141 0.1084 0.4967 0.052 0.061 0.002 0.07 60.4472 III. Melt separation and processing: after the chemical composition of the melt meets the requirements, the liquid steel inside the furnace is heated to 1630 ° C and then separated. Before separation, the furnace is turned off for 8 minutes, and then the slag on the surface of the liquid steel is removed. The casting ladle, which has been preheated, is placed at the place where the liquid steel exits the induction furnace and waits for the liquid steel to be separated. After the separation is completed, the slag on the surface of the liquid steel is removed and casting is expected. IV. Casting separation and box disposal: when the casting temperature reached 1550 ° C, the casting process was carried out. After 40 minutes from the completion of casting, the box separation process was carried out. V. Post-processing: after the box separation process, sandblasting, grinding, shortening, control, etc. processes were performed, so that Cr-Mn-N heat-resistant austenitic steel was obtained.

[0025] The austenitic heat-resistant steel produced in Example 1 was tested, and the results were as follows: the tensile strength at 1050°C was 78 MPa or more, the tensile strength was 75 MPa or more, the thermal conductivity was 28.1 W / (m2•K) or more, the elastic modulus was 105 GPa or more, and the thermal expansion coefficient at 1100°C was 20.0 (1 / K•10-6); the Cr-Mn-N austenitic heat-resistant steel had properties such as excellent high-temperature strength, high thermal conductivity, and high thermal diffusion rate; and Ni was replaced with Mn and N, thereby greatly reducing the production cost. Reference Example

[0026] I. Ingredients: main raw materials in weight percentage: fuel 0.35%, steel scrap 43.29%, chromium nitrate 8.65%, ultra-low carbon ferrochrome 33.71%, electrolytic manganese 5.35%, ferrosilicon 1.55% and nickel plate 7.1%. II. Melting: A medium frequency induction furnace was used for melting. The capacity of the induction furnace can range from 0.5 tons to 3 tons. The measured raw materials are sequentially fed into the medium frequency induction furnace, which is then fed and heated. After the materials are completely melted, the temperature inside the medium frequency induction furnace is raised to about 1600 ° C. Spectroscopic analysis was carried out for the melt inside the medium frequency induction furnace using a spectroscopic analysis test strip. The analysis result is shown in the following table. Element C Si Mn PS Cr Ni Mo Nb Mass.(%) 0.50 1.23 4.76 0.020 0.010 25.40 6.79 0.034 0.0015 Element WVN Zr YB Co Fe Mass.(%) 0.0079 0.0966 0.4395 0.043 0.055 0.0018 0.09 60.5207 III. Melt separation and processing: after the chemical composition of the melt meets the requirements, the liquid steel inside the furnace is heated to 1680 ° C and then separated. Before separation, the furnace is turned off for 3 minutes, and then the slag on the surface of the liquid steel is removed. The casting ladle, which has been preheated, is placed at the place where the liquid steel exits the induction furnace and waits for the liquid steel to be separated. After the separation is completed, the slag on the surface of the liquid steel is removed and casting is expected. IV. Casting separation and box disposal: when the casting temperature reached 1650 ° C, the casting process was carried out. After 60 minutes from the completion of casting, the box separation process was carried out. V. Post-processing: after the box separation process, sandblasting, grinding, shortening, inspection, etc. processes were performed, so that Cr-Mn-N heat-resistant austenitic steel was obtained. Comparative Example

[0027] The same raw materials were used and measured according to their quantities. Comparison between Cr-Ni austenitic heat-resistant steel designated GX40CrNiSiNb25-20 according to European standard EN 10295 and Cr-Mn-N austenitic heat-resistant steel produced in Example 2. The result of the composition analysis above is shown in the following table. Result of the analysis of the composition of Cr-Ni austenitic heat-resistant steel with the designation GX40CrNiSiNb25-20 Element C Si Mn ps Cr Ni Mo Nb Mass.(%) 0.40 1.24 1.06 0.020 0.010 24.85 19.54 0.03 1.42 Element WVN Zr YB Co Fe Mass.(%) - 0.089 - - - - - 51.341

[0028] From the comparison between the compositions of the above two materials, it can be seen that the main differences are the amounts of Mn, Ni, Nb and N elements. The cost comparison between the above two materials is based on 1000 kg of liquid steel as shown in the following table (number 1 represents the Cr-Mn-N austenitic heat-resistant steel produced in Example 2, and number 2 represents the heat-resistant steel designated as GX40CrNiSiNb25-20). Raw material Mn Ultra-low carbon Fe-Cr Ni plate Fe-Nb CrN Steel scrap Total (RMB) Price (RMB / kg) 11.1 12.55 70.3 175.5 17.4 1.8 Yield % 100% 60% 100% 60% 8.5% 100% No. 1 Add amount (Kg) 50 367 70 - 56 457 Cost of added raw material 555 4606 4921 - 974 823 11879 No.2 Add amount 13 417 200 24 - 346 Cost of added raw material 144 5233 14060 4212 - 623 24272 PS: Alloy cost for Zr, Y, Co IB for material no. 1 was 580 RMB in total

[0029] From a cost perspective, the cost of Cr-Mn-N austenitic heat-resistant steel was only 51% of the cost of the heat-resistant steel designated GX40CrNiSiNb25-20.

[0030] Compared to the comparative example, the Cr-Mn-N heat-resistant austenitic steel of this invention showed a 219 MPa increase in room temperature tensile strength, a 379 MPa increase in tensile strength, a 7.8% increase in room temperature elastic modulus, a 30.4% increase in room temperature thermal conductivity, and a 14.4% increase in thermal conductivity at 1100°C. Specific test results are listed in Table 1. Table 1 Comparison of test results between Example 2 and comparative example ''"PROPERTY Number\ RpO.2(MPa) Rm(MPa) E (GPa) A / W / (mK)-1 RT 1100°C RT 1100°C RT 1100°C RT 1100°C 1 523 58 850 59 193 105 13.3 30.9 2 304 46 471 50 179 104 10.2 27.0

[0031] From the above property comparison, it can be seen that the properties of the Cr-Mn-N austenitic heat-resistant steel of the present invention were superior in the comparative example, and the production cost was significantly reduced.

[0032] The above descriptions are only suggested embodiments of the present invention.

Claims

Patent claims 1. Cr-Mn-N austenitic heat-resistant steel, containing in mass percentages: carbon 0.30% to 0.45%, silicon 0.80% to 1.50%, manganese 3.00% to 4.80%, phosphorus less than 0.02%, sulfur less than 0.02%, chromium 23.00% to 26.00%, nickel 6.00% to 8.00%, molybdenum less than 0.20%, niobium less than 0.30%, tungsten less than 0.40%, vanadium less than 0.12%, nitrogen 0.30% to 0.60%, zirconium less than 0.08%, cobalt less than 0.08%, yttrium less than 0.08%, boron less than 0.10%, with iron for balance.

2. A method for producing Cr-Mn-N austenitic heat-resistant steel according to claim 1, comprising the following steps: (a) forming melts by melting the raw materials of alloying elements; and (b) after standing the melt formed in step (a) is ready for casting and shaping to obtain a Cr-Mn-N austenitic heat-resistant steel; wherein the melting temperature in said step (a) is 1580 to 1700 ° C; the time for which the melt is left to stand in said step (b) is 3 to 20 minutes; and the temperature for the Cr-Mn-N austenitic heat-resistant steel to be cast into the mold is 1550 to 1650 ° C.

3. The method according to claim 2, wherein, after the melt has been allowed to stand in said step (b), a slag removal process is further carried out. Published and printed by: Intellectual Property Office, Belgrade, Kneginje Ljubice 5