Medium-to-large-sized medium-carbon low-alloy steel castings and their heat treatment methods and applications

Abstract

The present invention relates to the technical field of heat treatment of steel castings, and specifically provides a medium-to-large-sized medium-carbon low-alloy steel casting and a heat treatment method and application thereof. The medium-to-large-sized casting blank prepared by a conventional steelmaking method is subjected to homogenization annealing, controlled-speed slow cooling-self-heating quasi-isothermal-controlled-speed rapid cooling composite quenching, and medium-to-low temperature tempering treatment in sequence, and the medium-to-large-sized steel casting obtains a complex phase structure of willow-leaf bainite, lath bainite, martensite, and a small amount of residual austenite from the surface to a certain distance in the core, and the core is a complex phase structure of willow-leaf bainite and martensite. The medium-to-large-sized steel casting obtained by the above technology has the characteristics of good matching of strength and toughness from the surface to a certain distance in the core and high strength in the core. The medium-to-large-sized steel casting with such characteristics is particularly suitable for impact wear working conditions. At the same time, quasi-isothermal treatment is added during the cooling process to improve the cooling uniformity of the medium-to-large-sized steel casting, and avoid the problem that the medium-to-large-sized steel casting is prone to cracking due to large stress generated during the cooling process.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of steel castings, and specifically provides a medium-to-large-sized medium-carbon low-alloy steel casting and a heat treatment method and application thereof. Background Art

[0002] At present, the most widely used metal wear-resistant materials include wear-resistant cast iron and wear-resistant cast steel. Wear-resistant cast iron is mainly chromium-containing cast iron, which has the advantage of high hardness, but is brittle and not suitable for use in wear conditions with impact, such as mine crusher linings. Wear-resistant cast iron materials are used to adapt to impact wear conditions.

[0003] Chinese invention patent CN 113881894 A discloses a method for preparing a precipitate-reinforced bainite-martensite multiphase wear-resistant lining plate, which adopts composition optimization and graded quenching to prepare the wear-resistant lining plate, and the prepared wear-resistant lining plate has good hardness and toughness matching. In the graded quenching process, the wear-resistant lining plate is firstly cooled to a certain temperature (about 300°C) by water cooling. Obviously, it is difficult to ensure that the casting is evenly cooled to this temperature during the water cooling process for medium and large parts, and the wear-resistant lining plate cools quickly during the immersion water quenching process to form martensite, resulting in high surface strength and poor toughness.

[0004] Chinese invention patents CN 114959498 A and CN 103602928 A both disclose a method for preparing a wear-resistant steel liner material for a ball mill. Patent CN 114959498 A prepares a high wear-resistant liner by adding rare metals niobium and hafnium to refine the grains and using conventional quenching and tempering treatment; Patent CN103602928 A prepares a high wear-resistant liner by adding rare metals tungsten, magnesium, molybdenum, and antimony to refine the grains and using heat treatment. It can be seen that the above technical solutions all contain expensive rare metal elements and are relatively costly. At the same time, conventional heat treatment methods are used, and the liner surface has a high cooling rate, high surface strength, and good subsurface toughness.

[0005] Chinese invention patent CN 113699459 A discloses a method for improving the service life of bainite / martensite low alloy wear-resistant lining. The bainite / martensite low alloy wear-resistant lining is prepared by adopting the technical scheme of intermittent immersion water cooling and air temperature uniformity. Obviously, it is difficult to make the temperature of the wear-resistant lining uniform during the immersion water quenching process, and the wear-resistant lining cools quickly during the immersion water quenching process to form martensite, resulting in high surface strength and poor toughness.

[0006] Based on the above invention patents and research findings, the existing technology uses component optimization and process control to prepare medium and large wear-resistant castings with matching properties. The common characteristics are that the surface cooling rate is fast and the core cooling rate is slow, resulting in high surface hardness and good sub-surface or core toughness. This is contrary to the large impact force on the surface of the casting. In addition, continuous rapid cooling (such as water cooling) processes are mostly used, and the castings are prone to large quenching stresses, which may cause the castings to crack.

[0007] In view of this, the present invention is proposed. Summary of the invention

[0008] In view of the problems of large stress leading to cracking and high surface hardness but poor toughness in the heat treatment process of medium-to-large medium-carbon low-alloy steel castings, the first object of the present invention is to provide a heat treatment method for medium-to-large medium-carbon low-alloy steel castings.

[0009] The second object of the present invention is to provide medium-to-large-sized medium-carbon low-alloy steel castings obtained by the above-mentioned heat treatment method.

[0010] The third object of the present invention is to provide the application of the above-mentioned heat treatment method or medium-to-large-sized medium-carbon low-alloy steel castings.

[0011] In order to achieve the above object, the present invention adopts the following technical solution:

[0012] The present invention provides a heat treatment method for medium-to-large-sized medium-carbon low-alloy steel castings, which specifically comprises the following steps:

[0013] S1: Perform homogenization annealing treatment on the steel casting.

[0014] S2: After S1, the steel casting is subjected to a composite quenching treatment, which includes:

[0015] S2-1: Heating and heat preservation treatment.

[0016] S2-2: Controlled speed slow cooling treatment, so that the surface of the steel casting and the area at a distance d from the surface are cooled to below the martensite start transformation temperature, thereby forming an appropriate amount of willow-leaf bainite and martensite structure, while the core temperature is still high and no structure transformation occurs.

[0017] S2-3: Self-heating quasi-isothermal treatment causes the carbon atoms in the martensite formed on the surface of the steel casting and the area within a distance d from the surface to be distributed into the untransformed austenite, while part of the austenite is transformed into bainite structure. At this time, the core temperature is still high and no structural transformation occurs.

[0018] And S2-4: controlled speed rapid cooling treatment, so that part of the austenite on the surface of the steel casting and the area at a distance d from the surface is transformed into martensite, while the austenite in the core is transformed into bainite and martensite.

[0019] And S3: after S2, the steel casting is subjected to medium-low temperature tempering treatment.

[0020] The inward distance d from the surface of the steel casting is specifically 1 / 10-1 / 5 of the equivalent thickness δ of the steel casting.

[0021] And, the medium-to-large medium-carbon low-alloy steel castings in the present invention, by mass percentage, include: 0.3%-0.5% C, 0.8%-3.5% Mn, 0.8%-1.7% Si, 0.5%-3.2% Cr, 0.3%-0.8% Mo and 0.2%-1.2% Ni, and at least one of 0.05%-0.2% V and 0.05%-0.2% Ti, and the rest is Fe and other inevitable impurities. At the same time, the sum of the contents of Mn and Cr is not less than 2.5% and not higher than 4%, and the sum of the contents of Mn, Si, Cr, Mo, Ni, V, and Ti is not higher than 5%.

[0022] The microstructure of the steel casting treated by the heat treatment method of the present invention on its surface and the area at a distance d from the surface is a complex phase structure of willow-leaf bainite, lath bainite, martensite and a small amount of residual austenite, and the microstructure of the core of the steel casting is a complex phase structure of willow-leaf bainite and martensite.

[0023] Compared with the prior art, the technical effects of the present invention are:

[0024] The medium and large steel castings obtained by the above technology have the characteristics of good strength and toughness matching from the surface to the core for a certain distance and high strength in the core. The medium and large steel castings with this characteristic are particularly suitable for impact and wear conditions, such as jaw crusher liners, mill roller rings, etc.

[0025] Adding quasi-isothermal treatment during the quenching and cooling process of medium and large steel castings can improve the cooling uniformity of medium and large steel castings and avoid the problem of large stress and cracking of medium and large steel castings during the cooling process.

[0026] The microstructure of the medium and large steel castings heat-treated by the scheme of the present invention contains austenite and carbide particles, which hinder the diffusion of hydrogen atoms, improve the ability of the castings to resist hydrogen-induced delayed fracture, prevent hydrogen embrittlement, and avoid sudden accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The various technical features of the present application and the relationship between them are further explained below with reference to the accompanying drawings. The accompanying drawings are exemplary, and some technical features are not shown in actual proportion. In addition, some drawings may omit technical features that are commonly used in the technical field to which the present application belongs and are not essential for understanding and implementing the present application, or additional technical features that are not essential for understanding and implementing the present application may be shown. In other words, the combination of the various technical features shown in the accompanying drawings is not used to limit the present application. In addition, throughout the present application, the same figure numbers refer to the same content. The specific description of the drawings is as follows:

[0028] Figure 1 is the isothermal transformation curve of the large steel casting material in Example 1, wherein F represents ferrite, P represents pearlite, B represents bainite, M represents martensite, Ac1 represents the austenite start transformation temperature, Ac3 represents the austenite end transformation temperature, Ms represents the martensite start transformation temperature, and Mf represents the martensite end transformation temperature;

[0029] Figure 2 is the continuous cooling transformation curve of the large steel casting material in Example 1, wherein F represents ferrite, P represents pearlite, B represents bainite, M represents martensite, Ac1 represents the austenite start transformation temperature, Ac3 represents the austenite end transformation temperature, Ms represents the martensite start transformation temperature, and Mf represents the martensite end transformation temperature;

[0030] Figure 3 This is a scanning microstructure photograph of the test block in Example 2 after being processed in the first chemical experiment by a scanning electron microscope, wherein LeafBainite represents willow-leaf bainite, Lath Bainite represents lath bainite, and Martensite represents martensite;

[0031] Figure 4 This is a scanning microstructure photograph of the test block in Example 2 after being processed in the second chemical experiment by a scanning electron microscope, wherein Leaf Bainite is represented as willow-leaf bainite, and Martensite is martensite;

[0032] Figure 5 is a structural diagram of a large steel casting in Example 3;

[0033] Figure 6 is a flow chart of the S2 process at different parts of a large steel casting in Example 3;

[0034] Figure 7 This is a scanning microstructure photograph taken by a scanning electron microscope at 5 mm below the surface of the large steel casting after treatment in Example 3, wherein LeafBainite represents willow-leaf bainite, Lath Bainite represents lath bainite, and Martensite represents martensite;

[0035] Figure 8 This is a scanning microstructure photograph of the core of the large steel casting in Example 3 taken by a scanning electron microscope, in which LeafBainite is represented by willow-leaf bainite and Martensite is martensite. DETAILED DESCRIPTION

[0036] Unless otherwise defined, all technical and scientific terms used in this application are the same as those generally understood by those skilled in the art to which this application belongs. If there is any inconsistency, the meaning described in the full text of this application or the meaning derived from the content recorded in the full text of this application shall prevail. In addition, the terms used in this description are only for the purpose of describing the embodiments of the present application and are not intended to limit the present application. Below, the specific implementation methods of the present application are described in detail.

[0037] Different from the idea of ​​component optimization in the prior art, the present invention improves the heat treatment method (avoiding continuous rapid cooling process) so that the microstructure of medium-to-large medium-carbon low-alloy steel castings on the surface and a distance inside the surface is a complex phase structure of willow-leaf bainite, lath bainite, martensite and a small amount of residual austenite, which has good strength and toughness matching. At the same time, the microstructure of the core of the steel casting is a complex phase structure of willow-leaf bainite and martensite, which has good high strength, thus meeting the requirements of impact wear working conditions.

[0038] Medium and large in the present invention means that one dimension of the workpiece is greater than 800 mm and less than 2500 mm (such as length, diameter, height, etc.), while the other two dimensions are greater than 50 mm and less than 800 mm (such as width, thickness, etc.).

[0039] The medium-carbon low-alloy steel casting in the present invention refers to a steel casting in which the mass of the C element is between 0.3% and 0.5%, and the sum of the mass fractions of alloying elements such as Mn, Si, Cr, Mo, Ni, V, and Ti is not higher than 5%; more specifically, in terms of mass percentage, the components of medium-to-large-sized medium-carbon low-alloy steel castings include: 0.3%-0.5% C, 0.8%-3.5% Mn, 0.8%-1.7% Si, 0.5%-3.2% Cr, 0.3%-0.8% Mo and 0.2%-1.2% Ni, and at least one of 0.05%-0.2% V and 0.05%-0.2% Ti, and the rest is Fe and other inevitable impurities; at the same time, the sum of the contents of Mn and Cr is not less than 2.5% and not higher than 4%, and the sum of the contents of Mn, Si, Cr, Mo, Ni, V, and Ti is not higher than 5%.

[0040] In the steel casting of the present invention, 0.3-0.5% of C is added to improve the hardness of the steel casting, which is beneficial to improving its wear resistance; by controlling the content of Si, the steel casting is prevented from forming carbides during the cooling process, which deteriorates its impact performance; by controlling the content of Mn and Cr, the high-temperature ferrite structure transformation of the steel casting material is separated from the medium-temperature bainite structure transformation, which is beneficial to obtaining the bainite structure; by controlling the content of Mo, the hardenability of the steel casting material is improved, and the formation of ferrite and pearlite structures during the cooling process is avoided; adding a certain amount of Ni can improve the impact toughness of the steel casting, but the price of Ni is relatively high and needs to be strictly controlled; by adding a certain amount of V and Ti elements, dispersed carbide particles are formed, which is helpful to refine the grains of medium and large steel castings.

[0041] The heat treatment method of the present invention mainly includes three stages: homogenization annealing treatment in S1, composite quenching treatment in S2, and medium-low temperature tempering treatment in S3.

[0042] Homogenization annealing, also known as diffusion annealing, is an annealing method applied to ingots or castings of steel and non-ferrous alloys. Specifically, the ingot or casting is heated to a higher temperature below the solidus temperature of the alloy, kept warm for a long time, and then slowly cooled down. Homogenization annealing is to diffuse the elements in the alloy in the solid state to reduce the chemical composition inhomogeneity (segregation), mainly to reduce the chemical composition inhomogeneity within the grain scale (intracrystalline segregation or dendritic segregation). The homogenization annealing temperature of alloy steel is much higher than Ac3 (austenite end transformation temperature), usually 1050-1200℃.

[0043] Quenching is a heat treatment process. The quenching of steel is a heat treatment process in which the steel is heated to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), kept at this temperature for a period of time to make it fully or partially austenitized, and then cooled rapidly to below Ms (or isothermally near Ms) at a cooling rate greater than the critical cooling rate to undergo martensite (or bainite) transformation.

[0044] Tempering is a heat treatment process in which the workpiece is heated to a temperature below Ac1 (the starting temperature of pearlite to austenite transformation during heating) after quenching, kept at a certain temperature for a certain period of time, and then cooled to room temperature. Tempering is divided into low-temperature tempering (tempering of the workpiece at 150-250℃), medium-temperature tempering (tempering of the workpiece between 350-500℃) and high-temperature tempering (tempering of the workpiece above 500-650℃).

[0045] In a preferred embodiment, the specific steps of the homogenization annealing treatment of S1 in the present invention are as follows:

[0046] S1-1: Place the steel casting into the first heating furnace, heat the steel casting to T11 at 30-60℃ / h, and keep it at temperature for t11.

[0047] T11 is 500-650℃, t11 (in min) is 1.5-2 times of the equivalent thickness δ of medium and large steel castings in mm. Assuming that the equivalent thickness δ of a steel casting in mm is 100mm, t11 is 150min-200min (time calculation method, the same below).

[0048] S1-2: Increase the heating power of the first heating furnace so that the steel casting is heated from T11 to T12 at 80-120°C / h and kept at this temperature for a time of t12.

[0049] T12 is 900-1120℃, t12 (in min) is 1-1.5 times the equivalent thickness δ of medium and large steel castings in mm. Assuming that the equivalent thickness δ of a steel casting in mm is 100mm, t12 is 100min-150min.

[0050] S1-3: Turn off the power of the first heating furnace, cool the steel castings that have completed heating and insulation in S1-1 and S1-2 to T13 with the furnace, take out the medium and large steel castings from the first heating furnace and air-cool them to room temperature, and complete the homogenization annealing treatment of the steel castings S1.

[0051] T13 is 200-400℃.

[0052] The equivalent thickness δ of the steel casting in the present invention refers to a physical quantity set in order to determine the time of each stage of heat treatment, and its unit is mm. This parameter comprehensively considers the shape factor and heat transfer characteristics of the workpiece. The specific method of obtaining this parameter refers to: Wang Xueqian and Chen Zhijuan, Further Study on the Calculation Method of Heat Treatment Heating Time, Journal of Sichuan Industrial College, Vol. 7, No. 3, 1988, pp. 11-17.

[0053] It should be noted that the heating rate in S1-1 can be but is not limited to 30℃ / h, 35℃ / h, 40℃ / h, 45℃ / h, 50℃ / h, 55℃ / h or 60℃ / h; T11 can be but is not limited to 500℃, 520℃, 540℃, 560℃, 580℃, 600℃, 620℃ or 650℃.

[0054] The heating rate in S1-2 can be but is not limited to 80℃ / h, 85℃ / h, 90℃ / h, 95℃ / h, 100℃ / h, 105℃ / h, 110℃ / h, 115℃ / h or 120℃ / h; T12 can be but is not limited to 900℃, 930℃, 960℃, 990℃, 1000℃, 1030℃, 1060℃, 1090℃, 1100℃ or 1120℃.

[0055] In S1-3, T13 may be, but is not limited to, 200°C, 230°C, 260°C, 290°C, 300°C, 330°C, 360°C, 390°C or 400°C.

[0056] In a preferred embodiment, the composite quenching treatment of S2 in the present invention includes the following steps of controlled rate slow cooling-self-heating quasi-isothermal-controlled rate rapid cooling:

[0057] S2-1: Heating and heat preservation treatment.

[0058] S2-2: Controlled speed slow cooling treatment, so that the surface of the steel casting and the area at a distance d from the surface are cooled to below the martensite start transformation temperature, thereby forming an appropriate amount of willow-leaf bainite and martensite structure, while the core temperature is still high and no structure transformation occurs.

[0059] S2-3: Self-heating quasi-isothermal treatment, so that the carbon atoms that first form martensite on the surface of the steel casting and the area within a distance d from the surface are distributed into the untransformed austenite, and part of the austenite is transformed into bainite structure. At this time, the core temperature is still high and no structural transformation occurs.

[0060] S2-4: Controlled-speed rapid cooling treatment, so that part of the austenite on the surface of the steel casting and the area at a distance d from the surface is transformed into martensite, while the austenite in the core is transformed into bainite and martensite.

[0061] In the above step S2, the distance d is 1 / 10-1 / 5 of the equivalent thickness δ of the steel casting. Assuming that the equivalent thickness of the steel casting is 100 mm, d is 10-20 mm.

[0062] In the composite quenching treatment of the present invention, the steel casting begins to implement controlled-speed slow cooling in the cooling stage, and the cooling time is appropriate, so that the surface of the steel casting is cooled to below the martensite start transformation temperature for a certain distance inward, forming an appropriate amount of willow-leaf bainite and martensite structure, while the core temperature is still high and no structure transformation occurs; then self-heating quasi-isothermal is implemented, at this time, the heat of the core of the steel casting is transferred to the surface and the part of the surface a certain distance inward, and at the same time, under the action of the latent heat of phase change, the surface of the steel casting and the part of the surface a certain distance inward appear quasi-isothermal phenomenon, at this time, the carbon atoms in the martensite first formed on the surface of the steel casting and the part of the surface a certain distance inward are distributed to the untransformed austenite, and at the same time, part of the austenite is transformed into bainite structure, while the core temperature is still high and no structure transformation occurs; finally, controlled-speed rapid cooling is implemented, that is, the steel casting is cooled to room temperature by rapid cooling, at this time, a small amount of austenite on the surface of the steel casting and the part of the surface a certain distance inward is transformed into martensite, and the austenite in the core is transformed into bainite and martensite.

[0063] In order to achieve the purpose of the above-mentioned composite quenching treatment, the specific process of S2 is preferably as follows:

[0064] S2-1: The steel casting that has completed the uniform annealing treatment in S1 is placed in a second heating furnace that has been heated to T21 and kept at this temperature for a time period of t21.

[0065] T21 is 880-1050°C, and t21 (in min) is 0.75-1 times of the equivalent thickness δ of the steel casting in mm. T21 can be, but is not limited to, 880°C, 900°C, 920°C, 940°C, 960°C, 980°C, 1000°C, 1030°C or 1050°C.

[0066] S2-2: Controlled speed slow cooling treatment, the steel casting that has completed S2-1 is taken out from the second heating furnace, so that the surface of the steel casting and a certain distance d inside the surface are forced to cool to T22.

[0067] T22 is 150-300° C. T22 may be, but is not limited to, 150° C., 170° C., 190° C., 200° C., 220° C., 240° C., 260° C., 280° C., or 300° C.

[0068] In a preferred embodiment, the cooling method may be air cooling or spray cooling, and more preferably, the cooling rate at 10 mm below the surface of the steel casting is 0.5-4°C / s during cooling, so that the surface of the steel casting and the area at a distance d from the surface undergo structural transformation, while the core does not undergo structural transformation.

[0069] S2-3: The steel casting that has completed step S2-2 is cooled for a period of time t22. At this time, the temperature of the surface of the steel casting and the portion at a certain distance d from the surface to the center rises to T23 under the action of the heat from the center and the latent heat of phase change, and a quasi-insulation stage occurs, that is, the temperature of the surface and the portion at a certain distance d from the surface to the center remains unchanged or the temperature changes within the range of ±20°C, and the temperature of the center of the steel casting is T24.

[0070] T23 is greater than the martensite transformation end temperature of the steel casting component material and less than the bainite transformation start temperature, preferably a temperature value between 150°C and 550°C; T24 is greater than the bainite transformation start temperature of the steel casting component material and less than T21, preferably a temperature between 450°C and 900°C. T23 may be, but is not limited to, 150°C, 180°C, 200°C, 230°C, 250°C, 280°C, 300°C, 330°C, 350°C, 400°C, 430°C, 450°C, 500°C, 530°C or 550°C; T24 may be, but is not limited to, 450°C, 500°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C or 900°C.

[0071] In a preferred embodiment, the cooling of S2-3 can be staying in the air or intermittent air spraying / spraying. t22 (in min) is 1 / 10-1 / 5 of the equivalent thickness δ (in mm). Assuming that the equivalent thickness of a medium-sized or large steel casting is 100 mm, t22 is 10-20 min.

[0072] S2-4: Cooling the steel casting after step S2-3 to room temperature.

[0073] In a preferred embodiment, the cooling in S2-4 can be spray cooling or water spray cooling.

[0074] In a preferred embodiment, the specific steps of the medium-low temperature tempering treatment of S3 in the present invention are as follows:

[0075] The steel castings that have completed S2-4 are placed in the third heating furnace that has been heated to T31 and kept warm for time t31. They are then taken out of the furnace and cooled to room temperature to complete the medium and low temperature tempering treatment.

[0076] Wherein, T31 is 100-350℃, and t31 in min is 2-4 times of the equivalent thickness δ of the steel casting in mm. T31 can be, but is not limited to, 100℃, 150℃, 200℃, 250℃, 300℃ or 350℃.

[0077] In a preferred embodiment, the cooling method of S3 can be natural air cooling or wind cooling.

[0078] The microstructure of the surface of the steel casting treated by the heat treatment method and the microstructure of the surface to the core at a certain distance is a complex phase structure of willow leaf bainite, lath bainite, martensite and a small amount of residual austenite, and the core is a complex phase structure of willow leaf bainite and martensite. The surface of the steel casting and the microstructure of the surface to the core at a certain distance have good strength and toughness matching, and the core has high strength.

[0079] Embodiment 1:

[0080] The composition and content of steel for medium and large medium carbon low alloy steel castings are shown in Table 1 below:

[0081] Table 1 Composition and content of steel for large steel castings in Example 1 (mass percentage)

[0082] element C Si Mn Cr Mo Ni V P S content 0.32 0.9 2.0 0.8 0.4 0.3 0.07 0.01 0.005

[0083] The conventional smelting method was used to prepare the Keel sample with reference to GB / T 26651-2011 Wear-resistant Steel Castings, and the Keel sample was subjected to homogenization annealing.

[0084] The specific homogenization annealing process is as follows: the Keel sample is heated to 550°C in a furnace at 55°C / h and kept at this temperature for 2h, then heated to 1050°C at 80°C / s and kept at this temperature for 3h, then the power of the heating furnace is turned off, and the Keel sample is cooled to 200°C, taken out of the heating furnace and air-cooled to room temperature.

[0085] Take a φ4mm, 10mm long sample from the Kiel sample, and measure the isothermal transformation curve and continuous cooling transformation curve of the above-mentioned steel material using DIL805A phase change dilatometer according to YB / T5128-1993 "Measurement Method of Continuous Cooling Transformation Curve of Steel (Expansion Method)". The results are as follows Figure 1 and Figure 2 .

[0086] Figure 1 Table 1 shows the isothermal transformation curve of the steel material used for large steel castings. Figure 1 It shows that the austenite start transformation temperature of the large casting material in Table 1 is 745℃, the austenite end transformation temperature is 857℃, the martensite start transformation temperature is 285℃, and the martensite end transformation temperature is 142℃; under the action of alloying elements, high-temperature ferrite / pearlite structure transformation (A→F+P in the figure) and medium-temperature bainite transformation (A→B in the figure), and the incubation period of high-temperature ferrite / pearlite structure transformation is significantly longer than that of medium-temperature bainite transformation, that is, compared with the medium-temperature bainite structure transformation, the high-temperature ferrite / pearlite structure transformation is significantly shifted to the right, which is conducive to obtaining bainite structure for workpieces with this composition as the material.

[0087] Figure 2 Table 1 shows the continuous cooling transformation curve of the steel material used for large steel castings. Figure 2 It shows that after the large casting material is completely austenitized, when the cooling rate is less than 0.04℃ / s, the room temperature structure obtained is a ferrite and pearlite dual phase structure, when the cooling rate is greater than 0.04℃ / s and less than 0.08℃ / s, the room temperature structure obtained is a ferrite and bainite dual phase structure, when the cooling rate is greater than 0.6℃ / s and less than 6℃ / s, the room temperature structure obtained is a bainite and martensite dual phase structure, and when the cooling rate is greater than 6℃ / s, the room temperature structure obtained is a martensite structure.

[0088] Example 2

[0089] A test piece with a length of 70 mm, a width of 50 mm and a thickness of 20 mm was taken from the kiel test bar that had completed homogenization annealing for physical and chemical experiment 1.

[0090] The heat treatment process system of physical and chemical experiment 1 is: put the test block into the first heating furnace and heat it to 900℃, keep it warm for 20 minutes, then cool it to 200℃ (cooling rate is about 1℃ / s), then quickly put it into the second heating furnace that has been heated to 280℃ and keep it warm for 20 minutes, then take it out and air cool it to room temperature, put it into the third heating furnace that has been heated to 280℃ and heat it and keep it warm for 30 minutes, finally take it out and air cool it to room temperature. The tissue photos corresponding to the sample of physical and chemical experiment 1 are as follows Figure 3 As shown, it is willow-leaf bainite + a small amount of lath bainite + martensite + a small amount of film-like retained austenite.

[0091] A test piece with a length of 70 mm, a width of 50 mm and a thickness of 20 mm was taken from the kiel test bar that had completed homogenization annealing for physical and chemical experiment 2.

[0092] The heat treatment process of physical and chemical experiment II is as follows: the test block is placed in the first heating furnace and heated to 900°C, kept at this temperature for 20 minutes, and then oil-cooled to room temperature (cooling rate is about 10°C / s), and then placed in the second heating furnace which has been heated to 280°C and heated and kept at this temperature for 45 minutes, and finally taken out and air-cooled to room temperature. The corresponding organization photos of the sample after physical and chemical experiment II are shown in the figure below. Figure 4 As shown, it is willow-leaf bainite + martensite.

[0093] The conventional mechanical properties of the test block prepared in Example 2 were tested according to the national standard GB / T 228-2002, and the impact toughness of the test block prepared in Example 2 was tested according to the national standard GB / T229-1994, as shown in Table 2.

[0094] Table 2 Conventional mechanical properties and impact toughness of different physical and chemical tests in Example 2

[0095]

[0096] It can be seen from Table 2 that after heat treatment in chemical test 1, the material has a good match between strength and toughness, and after heat treatment in chemical test 2, the material has good strength.

[0097] Example 3

[0098] The chemical composition shown in Table 1 is used to prepare steel by conventional methods and cast steel. Figure 5 As shown, its equivalent thickness is δ=147 mm, and 145 mm is taken. The steel casting is processed in the following three steps.

[0099] S1, homogenization annealing, comprising the following steps:

[0100] S1-1: Place the steel casting in the first heating furnace, heat the steel casting to 500°C at 30-60°C / h, and keep it warm for 250 minutes.

[0101] S1-2: Increase the heating power of the first heating furnace so that the steel casting is heated from 500°C to 1050°C at 80-120°C / h and kept at this temperature for 175 minutes.

[0102] S1-3: Turn off the power of the first heating furnace, cool the steel castings that have completed S1-1 and S1-2 heating and insulation to 200°C with the furnace, take the steel castings out of the first heating furnace and air-cool them to room temperature, and complete the S1 homogenization annealing treatment of the steel castings.

[0103] S2, air spray cooling - gap air spray - water spray cooling, including the following steps (flow chart as shown in Figure 6 ):

[0104] S2-1: The steel casting that has completed the S1 uniform annealing treatment is placed in a second heating furnace that has been heated to 900°C and kept warm for 120 minutes.

[0105] S2-2: The steel castings completed in S2-1 are taken out from the second heating furnace, and the surface of the steel castings is cooled to 200° C. by air jet cooling.

[0106] S2-3: The surface temperature of the steel casting that has completed step S2-2 is raised to 300°C under the action of the core heat and the latent heat of phase change, and the steel casting is cooled by gap air spray for 20 minutes in the air so that the surface temperature of the steel casting is maintained between 290°C and 310°C for quasi-insulation.

[0107] S2-4: Spray water to cool the medium and large steel castings that have completed step S2-3 to room temperature. The cooling time is about 30 minutes.

[0108] S3, tempering treatment, including the following steps:

[0109] S3-1: Place the steel castings completed in S2 into the third heating furnace which has been heated to 280°C and keep the temperature for 420 minutes.

[0110] S3-2: Take out the medium and large steel castings that have completed S3-1 from the third heating furnace, naturally air cool or wind cool to room temperature, and complete the medium and low temperature tempering treatment of the medium and large steel castings.

[0111] The scanned microstructure photos of the steel castings 5 ​​mm below the surface and at the core of the finished S1, S2 and S3 are shown in Figure 2. Figure 7 and 8 It can be seen that the structure 5 mm below the surface is a complex structure of willow-leaf bainite, lath bainite, martensite and retained austenite, while the core is a complex structure of willow-leaf bainite and martensite.

[0112] According to national standards GB / T 228-2002 and GB / T 229-1994, conventional mechanical properties and impact toughness of the large steel casting 5 mm below the surface and at the core of Example 3 were tested, as shown in Table 3. The surface has a good strength-toughness match (i.e., a large strength-toughness product), while the core has a higher strength.

[0113] Table 3 Conventional mechanical properties and impact toughness at different locations in Example 3

[0114]

[0115] It can be understood that those skilled in the art can combine the features mentioned in one or more embodiments mentioned in the entire application with the features in other embodiments in any appropriate manner to implement the application.

[0116] Note that the above are only preferred embodiments of the present application and the technical principles used. Those skilled in the art will understand that the present application is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the protection scope of the present application. Therefore, although the present application is described in more detail through the above embodiments, the present application is not limited to the above embodiments, and may also include more other equivalent embodiments without departing from the technical concept of the present application, all of which belong to the protection scope of the present application.

Claims

1. A heat treatment method for medium-to-large-sized medium-carbon low-alloy steel castings, characterized in that: The following steps are involved: S1: homogenizing annealing treatment of steel castings; S2: after S1, the steel casting is subjected to a composite quenching treatment, which includes: S2-1: Heating and heat preservation treatment; S2-2: Controlled speed slow cooling treatment, so that the surface of the steel casting and the position at a distance d from the surface are cooled to below the martensite start transformation temperature, thereby forming an appropriate amount of willow-leaf bainite and martensite structure, while the core temperature is still high and no structure transformation occurs; S2-3: self-heating quasi-isothermal treatment, so that the carbon atoms of the martensite formed on the surface of the steel casting and the part at a distance d from the surface are distributed into the untransformed austenite, and part of the austenite is transformed into bainite structure, while the core temperature is still high and no structure transformation occurs; S2-4: Controlled-speed rapid cooling treatment, so that part of the austenite on the surface of the steel casting and the area at a distance d from the surface is transformed into martensite, and at this time, the austenite in the core is transformed into bainite and martensite; and S3: performing a medium-low temperature tempering treatment on the steel casting after S2; The distance d is 1 / 10-1 / 5 of the equivalent thickness δ of the steel casting; The components of the medium-to-large medium-carbon low-alloy steel castings include, by mass percentage, 0.3%-0.5% C, 0.8%-3.5% Mn, 0.8%-1.7% Si, 0.5%-3.2% Cr, 0.3%-0.8% Mo and 0.2%-1.2% Ni, and at least one of 0.05%-0.2% V and 0.05%-0.2% Ti, and the rest is Fe and other inevitable impurities; at the same time, the sum of the contents of Mn and Cr is not less than 2.5% and not higher than 4%, and the sum of the contents of Mn, Si, Cr, Mo, Ni, V and Ti is not higher than 5%; The medium and large size refers to a steel casting with one dimension larger than 800 mm and smaller than 2500 mm, and the other two dimensions larger than 50 mm and smaller than 800 mm; After completing S3, the microstructure of the surface of the steel casting and the area at a distance d from the surface is a complex phase structure of willow-leaf bainite, lath bainite, martensite and a small amount of residual austenite, and the microstructure of the core of the steel casting is a complex phase structure of willow-leaf bainite and martensite.

2. The heat treatment method according to claim 1, characterized in that S2-2 includes: taking out the steel casting that has completed S2-1 from the heating furnace, so that the surface of the steel casting and the portion at a distance d from the surface are cooled to T22, where T22 is 150-300°C.

3. The heat treatment method according to claim 2, characterized in that: The cooling method described in S2-2 is air cooling or spray cooling.

4. The heat treatment method according to claim 2, characterized in that: In the cooling of S2-2, the cooling rate at 10 mm below the surface of the steel casting is 0.5-4°C / s.

5. The heat treatment method according to claim 1, characterized in that: S2-3 includes: cooling the steel casting after S2-2 for a time t22, at which time the surface of the steel casting and the part at a distance d from the surface rise to T23 under the action of the core heat of the steel casting and the latent heat of phase change, and a quasi-insulation stage occurs, that is, the temperature of the surface of the steel casting and the part at a distance d from the surface remains unchanged or changes within the range of ±20°C, and the core temperature of the steel casting is T24; Among them, T23 is greater than the martensite transformation end temperature of the steel casting and less than the bainite transformation start temperature; T24 is greater than the bainite transformation start temperature of the steel casting.

6. The heat treatment method according to claim 5, characterized in that: T23 is 150℃-550℃.

7. The heat treatment method according to claim 5, characterized in that: T24 is 450℃-900℃.

8. The heat treatment method according to claim 5, characterized in that: The cooling method described in S2-3 is staying in the air or intermittent air spraying / spraying.

9. The heat treatment method according to claim 5, characterized in that: The t22 in min is 1 / 10-1 / 5 of the equivalent thickness δ of the steel casting in mm.

10. The heat treatment method according to claim 1, characterized in that: S2-4 includes: cooling the steel casting completed in S2-3 to room temperature.

11. The heat treatment method according to claim 10, characterized in that: The cooling method described in S2-4 is spray cooling or water spray cooling.

12. The heat treatment method according to any one of claims 1 to 11, characterized in that: S2-1 includes: placing the steel casting into a heating furnace that has been heated to T21 and keeping it warm for a time of t21, wherein T21 is 880-1050° C., and t21 in min is 0.75-1 times of the equivalent thickness δ of the steel casting in mm.

13. The heat treatment method according to any one of claims 1 to 11, characterized in that: S1 includes: The steel casting is heated to T11 at 30-60°C / h and kept at temperature for t11; then, it is heated from T11 to T12 at 80-120°C / h and kept at temperature for t12; then, the steel casting is cooled in the furnace to T13, and then taken out of the furnace and air-cooled to room temperature; Among them, T11 is 500-650℃, and t11 in min is 1.5-2 times of the equivalent thickness δ of the steel casting in mm; T12 is 900-1120℃, and t12 in min is 1-1.5 times of the equivalent thickness δ of the steel casting in mm; T13 is 200-400℃.

14. The heat treatment method according to any one of claims 1 to 4, characterized in that: S3 includes: placing the steel casting completed in S2-4 into a heating furnace that has been heated to T31, and keeping the temperature for a time t31, and then taking it out of the furnace and cooling it to room temperature; Wherein, T31 is 100-350°C, and t31 in min is 2-4 times of the equivalent thickness δ of the steel casting in mm.

15. The heat treatment method according to claim 14, characterized in that: The cooling method described in S3 is natural air cooling or wind cooling.

16. Medium to large-sized medium-carbon low-alloy steel castings produced by the heat treatment method according to any one of claims 1 to 15.

17. Use of the medium-to-large-sized medium-carbon low-alloy steel castings as claimed in claim 16 in impact wear working conditions.

18. Use of the heat treatment method according to any one of claims 1 to 15 or the medium-to-large-sized medium-carbon low-alloy steel casting according to claim 16 in manufacturing jaw crusher liners or mill roller rings.

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