An E+ grade steel material and its heat treatment process and application

Abstract

The invention discloses an E+ grade steel material and a heat treatment process and application thereof, and relates to the technical field of heat treatment of alloy castings. The heat treatment process of the E+ grade steel material comprises the following steps: taking an E+ grade steel casting, heating it to 880-900°C and keeping it warm, and then water cooling it; the carbon equivalent CE of the E+ grade steel casting is 0.64-0.88%; heating the casting to a primary tempering temperature of 500-600°C and keeping it warm, then water cooling it, heating it to a secondary tempering temperature of 500-600°C, keeping it warm, and then air cooling or wind cooling it. The hardness of the E+ grade steel casting is 260-330 HBW, the tensile strength is ≥950Mpa, the yield strength is ≥800Mpa, the elongation after fracture is ≥14%, the section shrinkage is ≥30%, and the impact absorption energy at -40°C is ≥27J.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment of alloy castings, and more specifically, to an E+ grade steel material and a heat treatment process and application thereof. Background Art

[0002] With the development of heavy-duty railway locomotives, it is increasingly important to improve the safety and reliability of heavy-duty transport coupling and buffer systems. At present, the cast steel used in heavy-duty transport coupling and buffer systems is mainly E-grade steel. E-grade steel implements the TB / T 2942.1-2020 standard, and the chemical composition is controlled according to the upper limit of the carbon equivalent (CE) of E-grade steel: CE≤0.88%; its mechanical properties are required as follows: tensile strength 830Mpa, yield strength ≥690Mpa, elongation after fracture ≥14%, cross-sectional shrinkage ≥30%, impact absorption energy (-40℃) ≥27J, hardness (HBW) 241~311.

[0003] However, with the development of heavy-duty locomotives and vehicles and increasingly stringent operating conditions, higher requirements are placed on cast steel for locomotives and vehicles in terms of strength, welding performance, low-temperature performance and cost.

[0004] Patent CN101748338A proposes a high-strength cast steel for railway vehicle couplers and a manufacturing method thereof. On the one hand, the chemical composition of the cast steel is controlled, and on the other hand, the temperature of the molten steel out of the furnace is controlled at 1590-1610°C, and the casting temperature is controlled at 1560-1580°C; the quenching temperature of the casting tempering heat treatment is controlled at 850-890°C, and the tempering temperature is controlled at 530-590°C. The yield strength of the high-strength cast steel obtained is 895-950MPa, the tensile strength is 980-1010Mpa, and the impact energy at -40°C is 22-23J. It improves the yield strength and tensile strength to a certain extent, but the impact absorption energy at low temperature fails to meet the requirements.

[0005] Patent CN105671429A discloses a high-performance E+ steel containing rare earths, the components of which include Fe, C, Si, Mn, Cr, Ni, Mo, and RE. By adjusting alloy components such as Mn, Ni, and Cr, and then adding specific rare earth elements, a low-cost and high-strength E+ grade cast steel is obtained, which is then quenched at 900-920°C and tempered at 550-590°C to obtain a steel grade with excellent strength and impact toughness. However, it focuses on improving the proportion of raw material components, but fails to find the influence of the interaction between the carbon equivalent of the raw material components and the heat treatment process on the material properties and metallographic structure, and cannot guarantee the performance of the material in the subsequent use process, such as resistance to fatigue fracture, static load tension, and dynamic tearing absorption work. In addition, it uses the amount near the median value within the composition ratio range for experiments, and cannot guarantee that materials with different carbon equivalents can simultaneously have high strength and elongation, cross-sectional shrinkage, and low-temperature impact absorption energy. Summary of the invention

[0006] The purpose of the present invention is to overcome the defects and shortcomings of the low mechanical properties of existing E+ grade steel, and to provide an E+ grade steel material which can have both high strength and elongation, cross-sectional shrinkage and low-temperature impact energy absorption, and has good static load tension and dynamic tear absorption performance.

[0007] Another object of the present invention is to provide a heat treatment process for E+ grade steel material.

[0008] Another object of the present invention is to provide an application of the above-mentioned E+ grade steel material in the preparation of a heavy-load transport hook buffer system.

[0009] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0010] The present invention protects an E+ grade steel material, which is made of an E+ grade steel casting through a heat treatment process, wherein the carbon equivalent CE of the E+ grade steel casting is 0.62-0.88%, and the heat treatment process comprises the following steps:

[0011] S1, take the E+ grade steel casting and heat it to 880-900℃ and keep it at this temperature for 3.5-4h, the heating rate is ≤180℃ / h, and then quench it by water cooling;

[0012] S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4-5 hours, with a heating rate of ≤180°C / h, the primary tempering temperature is 500-600°C, and then water-cooling quenching;

[0013] S3, heating the casting treated in step S2 to a secondary tempering temperature and keeping the temperature for 4-5 hours, with a heating rate of ≤180°C / h, the secondary tempering temperature is 500-600°C, and then air cooling or wind cooling to obtain the E+ grade steel material;

[0014] Wherein, when CE<0.7%, the primary tempering temperature is 500-540℃, and the secondary tempering temperature is 500-540℃; or,

[0015] When 0.7%≤CE<0.78%, the primary tempering temperature is 540-580°C, and the secondary tempering temperature is 540-580°C; or,

[0016] When CE≥0.78%, the primary tempering temperature is 560-600°C, and the secondary tempering temperature is 560-600°C.

[0017] The E+ grade steel material of the present invention is made of E+ grade steel castings with appropriate carbon equivalent through a heat treatment process. In the heat treatment process, high-temperature quenching and two tempering treatments are performed, and the tempering temperature parameters and cooling methods are further optimized to obtain E+ grade steel with appropriate hardness, higher tensile strength, yield strength, elongation, cross-sectional shrinkage and low-temperature impact absorption energy. The E+ grade steel material meets the TB / T 2942.1-2020 standard and can be used in the preparation of heavy-duty transport hook buffer systems. At the same time, it has more excellent low-temperature impact resistance, static load tension and dynamic tear absorption work.

[0018] The E+ grade steel material of the present invention has a hardness of 260-330 HBW, a tensile strength of ≥950 MPa, a yield strength of ≥800 MPa, an elongation after fracture of ≥14%, a cross-sectional shrinkage of ≥30%, and an impact absorption energy of ≥27 J at -40°C.

[0019] In some embodiments, the chemical composition of the E+ grade steel material includes, by mass percentage:

[0020] C 0.22-0.28%, Si 0.20-0.45%, Mn 1.1-1.50%, S≤0.02%, P≤0.025%, Cr 0.40-0.50%, Ni 0.85-1.10%, Mo 0.20-0.30%, Cu≤0.30%, Al 0.03-0.08%, and the balance is Fe.

[0021] In E+ grade steel materials, carbon is the most important strengthening element in the alloy, but when the yield strength increases by 10Mpa, the plastic-brittle transition temperature of the steel increases by about 6.5℃, because it moves the series AKV curve of the steel toward the high temperature direction, so the carbon content in this steel should not be too high. Increasing the manganese content can significantly improve the low-temperature toughness of the steel, but when the manganese content is too high, it can promote the coarsening of austenite grains and cause temper brittleness. Nickel can be completely dissolved in ferrite, which can improve its strength without significantly reducing the plasticity of the steel. Although its strengthening effect is weak, it is outstanding in improving the low-temperature toughness and hardenability of the steel. Both chromium and molybdenum can be dispersed and precipitated as alloy carbides to improve the strength of steel; and both can improve the hardenability of steel and improve the comprehensive mechanical properties of steel; they can refine the grains of steel and improve its thermal strength performance; however, chromium can reduce the plasticity and toughness of steel, improve the wear resistance, oxidation resistance and corrosion resistance of steel, so it should not be too high; while molybdenum can improve the ductility, toughness and wear resistance of steel, and can partially eliminate or inhibit the temper brittleness caused by chromium, manganese, etc. The present invention optimizes each chemical composition, controls and increases Ni and Mo and reduces Cr, so that E+ grade steel has high strength, and at the same time has good plasticity and impact toughness at low temperature.

[0022] Preferably, the chemical composition of the E+ grade steel includes: C 0.22-0.28%, Si 0.20-0.40%, Mn 1.30-1.50%, S≤0.02%, P≤0.02%, Cr 0.40-0.50%, Ni 0.90-1.05%, Mo 0.20-0.30%, Cu≤0.10%, Al 0.03-0.07%, and the balance is Fe.

[0023] More preferably, the chemical composition of the E+ grade steel includes: C 0.22-0.26%, Si 0.20-0.40%, Mn 1.30-1.40%, S≤0.02%, P≤0.02%, Cr 0.40-0.50%, Ni 0.95-1.05%, Mo 0.25-0.30%, Cu≤0.10%, Al 0.03-0.07%, and the balance is Fe. Within the above chemical composition range, the C and Mn contents are further controlled at a medium-low level, and the Ni and Mo contents are controlled at a medium-high level, so that the E+ grade steel can have high strength, good plasticity and impact toughness at low temperatures, and ensure the stability of mechanical properties.

[0024] In some embodiments, the E+ grade steel material is further doped with rare earth elements, and the added amount of the rare earth elements is 0.05-0.10 wt %.

[0025] In some embodiments, the carbon equivalent of the E+ grade steel material is 0.64≤CE≤0.82%.

[0026] The present invention protects a heat treatment process for an E+ grade steel material, comprising the following steps:

[0027] S1, heating an E+ grade steel casting to 880-900°C and keeping the temperature for 3.5-4h, with a heating rate of ≤180°C / h, and then water-quenching; the carbon equivalent CE of the E+ grade steel casting is 0.62-0.88%;

[0028] S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4-5 hours, with a heating rate of ≤180°C / h, the primary tempering temperature is 500-600°C, and then water-cooling quenching;

[0029] S3, heating the casting treated in step S2 to a secondary tempering temperature and keeping the temperature for 4-5 hours, with a heating rate of ≤180°C / h, the secondary tempering temperature is 500-600°C, and then air cooling or wind cooling to obtain the E+ grade steel material;

[0030] Wherein, when CE<0.7%, the primary tempering temperature is 500-540℃, and the secondary tempering temperature is 500-540℃; or,

[0031] When 0.7%≤CE<0.78%, the primary tempering temperature is 540-580°C, and the secondary tempering temperature is 540-580°C; or,

[0032] When CE≥0.78%, the primary tempering temperature is 560-600°C, and the secondary tempering temperature is 560-600°C.

[0033] In some embodiments, the temperature of the E+ grade steel casting after water cooling and quenching is not higher than 100° C.; the temperature of the E+ grade steel casting after air cooling or wind cooling is room temperature.

[0034] In some embodiments, the carbon equivalent CE of the E+ grade steel casting is 0.64-0.78%, the primary tempering temperature is 520-580° C., and the secondary tempering temperature is 520-580° C. Controlling the carbon equivalent CE at a medium-low level and optimizing the tempering temperature can make the E+ grade steel have high strength, good plasticity and impact toughness at low temperatures, and ensure the stability of mechanical properties.

[0035] In some embodiments, when CE is 0.64-0.68%, the primary tempering temperature is 500-530°C, and the secondary tempering temperature is 500-530°C; preferably, the primary tempering temperature and the secondary tempering temperature are 500-520°C; or,

[0036] When CE is 0.70-0.76%, the primary tempering temperature is 540-580°C, and the secondary tempering temperature is 540-580°C; or,

[0037] When CE is 0.79-0.83%, the primary tempering temperature is 560-590°C, and the secondary tempering temperature is 560-590°C.

[0038] In some embodiments, the absolute value of the temperature difference between the primary tempering temperature and the secondary tempering temperature is ≤15°C.

[0039] In some embodiments, the absolute value of the temperature difference between the primary tempering temperature and the secondary tempering temperature is 0-10°C.

[0040] In some embodiments, the secondary tempering temperature is greater than the primary tempering temperature.

[0041] The present invention protects an E+ grade steel material, which is formed by the heat treatment process of the E+ grade steel material. After the E+ grade steel material is quenched and tempered by the heat treatment process, an E+ grade steel with suitable hardness, higher tensile strength, yield strength, elongation, cross-sectional shrinkage and low-temperature impact absorption energy can be obtained.

[0042] The present invention protects an application of the E+ grade steel material in the preparation of a heavy-load transport hook buffer system. The E+ grade steel material meets the TB / T 2942.1-2020 standard and has better plasticity and low-temperature impact resistance.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] The present invention provides an E+ grade steel material, which is made of E+ grade steel castings with appropriate carbon equivalent through a heat treatment process. In the heat treatment process, high-temperature quenching and two tempering treatments are performed, and the tempering temperature parameters and cooling methods are further optimized to obtain E+ grade steel with appropriate hardness, higher tensile strength, yield strength, elongation, cross-sectional shrinkage and low-temperature impact absorption energy. Its hardness is 260-330HBW, tensile strength ≥950Mpa, yield strength ≥800Mpa, elongation after fracture ≥14%, cross-sectional shrinkage ≥30%, and impact absorption energy ≥27J at -40°C, which meets the application requirements in heavy-duty transport hook buffer systems and has more excellent plasticity and low-temperature impact resistance.

[0045] The present invention provides a heat treatment process for an E+ grade steel material. By adopting an E+ grade steel casting with a suitable carbon equivalent, high-temperature quenching and two tempering treatments are performed, and the tempering temperature parameters and the cooling method are further optimized, the E+ grade steel material with suitable hardness, high tensile strength, elongation after fracture, cross-sectional shrinkage and low-temperature impact absorption energy can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a process flow chart of the heat treatment process of an E+ grade steel material according to an embodiment 1-7 of the present invention.

[0047] Figure 2 This is a process flow chart of the heat treatment process of E+ grade steel material according to embodiments 8 and 9 of the present invention.

[0048] Figure 3 This is a process flow chart of the heat treatment process of an E+ grade steel material of Comparative Example 1 of the present invention.

[0049] Figure 4 This is a metallographic structure diagram of Example 3 of the present invention.

[0050] Figure 5 This is a metallographic structure diagram of embodiment 7 of the present invention.

[0051] Figure 6 This is a metallographic structure diagram of Example 8 of the present invention.

[0052] Figure 7This is a metallographic structure diagram of comparative example 1 of the present invention. DETAILED DESCRIPTION

[0053] The present invention will be further described below in conjunction with specific embodiments, but the embodiments do not limit the present invention in any form.

[0054] The preparation method of E+ grade steel in the following examples or comparative examples:

[0055] Smelting process: 5T electric arc furnace is used to melt and cast E+ grade steel test bar castings, and the chemical composition is controlled as shown in Table 1. Before tapping, 0.07% (0.7KG for 1 ton of molten steel) of rare earth alloy (main element is Ce) is added to the baked ladle to modify the molten steel, improve the shape of inclusions, and reduce the harm of slag inclusions. The tapping time is controlled within 3 minutes to minimize the secondary oxidation and temperature drop of the molten steel.

[0056] The chemical composition is shown in Table 1:

[0057] Table 1

[0058]

[0059] Examples 1-7

[0060] A heat treatment process for E+ grade steel materials, such as Figure 1 As shown, the following steps are included:

[0061] S1, take the E+ grade steel casting and heat it to the quenching temperature and keep it at this temperature for 4 hours, the heating rate is ≤180℃ / h, and then water-quench it to below 100℃;

[0062] S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then water-quenching to below 100°C;

[0063] S3, heating the casting treated in step S2 to a secondary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then air-cooling to room temperature.

[0064] Example 8

[0065] A heat treatment process for E+ grade steel materials, such as Figure 2 As shown, the following steps are included:

[0066] S1, take the E+ grade steel casting and heat it to the quenching temperature and keep it at this temperature for 4 hours, the heating rate is ≤180℃ / h, and then water-quench it to below 100℃;

[0067] S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then water-quenching to below 100°C;

[0068] S3, heating the casting treated in step S2 to the secondary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then cooling the casting to room temperature by air.

[0069] Example 9

[0070] A heat treatment process for E+ grade steel hook tongue castings, such as Figure 2 As shown, the following steps are performed to cast E+ grade steel into a hook knuckle casting:

[0071] S1, take the E+ grade steel hook tongue casting and heat it to the quenching temperature and keep it at this temperature for 4 hours, the heating rate is ≤180℃ / h, and then water-quench it to below 100℃;

[0072] S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then water-quenching to below 100°C;

[0073] S3, heating the casting treated in step S2 to the secondary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then cooling the casting to room temperature by air.

[0074] Comparative Example 1

[0075] A heat treatment process for E+ grade steel materials, such as Figure 3 As shown, the following steps are included:

[0076] S1, heating an E+ grade steel casting to a quenching temperature and keeping the temperature for 4 hours, with a heating rate of ≤180°C / h, and then water cooling to below 100°C; the E+ grade steel material is the above-mentioned E+ grade steel A4;

[0077] S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4.5 hours, with a heating rate of ≤180°C / h, and then water cooling to below 100°C.

[0078] The specific process parameters of the above embodiments and comparative examples are shown in Table 2.

[0079] Comparative Examples 2-3

[0080] A heat treatment process for an E+ grade steel material is different from that of Example 1 in that the material and tempering temperature of the E+ grade steel in this comparative example are shown in Table 2.

[0081] Table 2

[0082]

[0083] Performance Testing

[0084] 1. Mechanical properties test

[0085] The products treated in Examples 1-9 and Comparative Example 1 were tested for mechanical properties, and the results are shown in Table 3.

[0086] The test method for tensile test (tensile strength, yield strength, elongation after fracture, and reduction of area) is as follows: "Tensile test for metal materials" in standard GB / T 228.1, and the test is conducted at room temperature.

[0087] Impact absorption energy test method: Standard GB / T 229 "Charpy pendulum impact test method for metallic materials".

[0088] The test method for hardness is: "Brinell hardness test for metal materials" of standard GB / T 231.1.

[0089] Table 3

[0090]

[0091] As can be seen from Table 3, the tensile strength of the E+ grade steel casting of the present invention reaches more than 955 MPa, the yield strength reaches more than 880 MPa, the elongation after fracture reaches more than 14%, the cross-sectional shrinkage rate reaches more than 40%, the impact absorption energy at -40°C is more than 27 J, and the hardness is 280-307 HBW.

[0092] Compared with Example 8, in Comparative Example 1, no secondary tempering treatment was used, and its elongation after fracture and cross-sectional shrinkage were significantly reduced.

[0093] Compared with Example 1, the tempering temperature in Comparative Example 2 is too high, resulting in lower tensile strength of the material.

[0094] Compared with Example 3, the tempering temperature in Comparative Example 3 is too low, and the impact absorption energy at -40°C is low.

[0095] 2. Characterization of non-metallic inclusions and metallographic structure

[0096] Metallographic structure characterization Figure 4 , Figure 5 , Figure 6 , Figure 7 And as shown in Table 4.

[0097] Test method for non-metallic inclusion types: refer to standard TB / T 2942.1-2020;

[0098] Tempering rating method for metallographic structure: refer to the E-grade tempered structure in standard TB / T 2942.2-2018.

[0099] Table 4

[0100]

[0101] The E+ grade steel casting of the present invention has less non-metallic inclusions, uniform structure, and the metallographic grade is controlled within the range of 4-7 grades. It has good plasticity and toughness and high fatigue strength.

[0102] Reference Figure 4 and Figure 5 Its metallographic structure is quenched and tempered level 7, which is mainly tempered troostite. The austenite grains are slightly coarse, but it does not cause a significant decrease in strength or plasticity, and the fatigue strength is relatively high.

[0103] Reference Figure 6 and Figure 7 The overall metallographic structures of Example 8 and Comparative Example 1 are basically the same, both of which belong to the quenching and tempering level 5, with uniform structure and fine grains; but there are also subtle differences between the two. The metallographic structure of Example 8 is more uniform. The reason is that the secondary tempering can make the quenched martensite transform into tempered troostite more fully, promote the transformation of carbides, and is beneficial to improve the plasticity, toughness and fatigue strength of the material.

[0104] 3. Static load tensile test:

[0105] The hook knuckle product made of E+ grade steel in Example 9 was subjected to static load tensile test. The results are shown in Table 5.

[0106] Table 5

[0107]

[0108] The results show that the E+ grade steel product of the present invention has a deformation of 0.22 mm after being subjected to a load of 2500 kN, a minimum breaking load of ≥3800 kN, and has a relatively high static load tensile force.

[0109] 4. Dynamic tearing absorption

[0110] Three products made of E+ grade steel in Example 9 were tested according to the requirements of standards: TB / T 2942.1-2020 and GB / T 5482-2007 "Dynamic tear test method for metallic materials". The results are shown in Table 6.

[0111] Table 6

[0112]

[0113] The results show that the low-temperature dynamic tearing absorption energy of the E+ grade steel product of the present invention reaches more than 170J, and has a relatively high static toughness.

[0114] The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A heat treatment process for E+ grade steel material, characterized in that: The following steps are involved: S1, heating an E+ grade steel casting to 880-900°C and keeping the temperature for 3.5-4h, with a heating rate of ≤180°C / h, and then water-quenching; the carbon equivalent CE of the E+ grade steel casting is 0.62-0.88%; S2, heating the casting treated in step S1 to a primary tempering temperature and keeping the temperature for 4-5 hours, with a heating rate of ≤180°C / h, the primary tempering temperature is 500-600°C, and then water-cooling quenching; S3, heating the casting treated in step S2 to a secondary tempering temperature and keeping the temperature for 4-5 hours, with a heating rate of ≤180°C / h, the secondary tempering temperature is 500-600°C, and then air cooling or wind cooling to obtain the E+ grade steel material; When CE<0.7%, the primary tempering temperature is 500-530℃, and the secondary tempering temperature is 500-530℃; or, When 0.7%≤CE<0.78%, the primary tempering temperature is 540-580°C, and the secondary tempering temperature is 540-580°C; or, When CE≥0.78%, the primary tempering temperature is 560-600°C, and the secondary tempering temperature is 560-600°C; The chemical composition of the E+ grade steel material is: C 0.22-0.28%, Si 0.20-0.45%, Mn 1.1-1.50%, S≤0.02%, P≤0.025%, Cr 0.40-0.50%, Ni 0.85-1.10%, Mo 0.20-0.30%, Cu≤0.30%, Al 0.03-0.08%, and the balance is Fe; the E+ grade steel material meets the TB / T 2942.1-2020 standard.

2. The heat treatment process of E+ grade steel material according to claim 1, characterized in that: When CE is 0.64-0.68%, the primary tempering temperature is 500-530°C, and the secondary tempering temperature is 500-530°C; or, When CE is 0.70-0.76%, the primary tempering temperature is 540-580°C, and the secondary tempering temperature is 540-580°C; or, When CE is 0.79-0.83%, the primary tempering temperature is 560-590°C, and the secondary tempering temperature is 560-590°C.

3. The heat treatment process of E+ grade steel material according to any one of claims 1-2, characterized in that: The absolute value of the temperature difference between the primary tempering temperature and the secondary tempering temperature is ≤15°C.

4. The heat treatment process of E+ grade steel material according to claim 3, characterized in that: The absolute value of the temperature difference between the primary tempering temperature and the secondary tempering temperature is 0-10°C.

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