A steel shell component with gradient distribution of microstructure and mechanical properties and forming method

Abstract

The present invention discloses a steel shell component with a gradient distribution of microstructure and mechanical properties and a forming method. The steel blank base material is preformed into a steel blank with a semi-solid structure, placed in a preheated concave die, and the punch is driven to press the steel blank down to achieve thixotropic plastic deformation composite forming of the steel blank. The temperature of the blank in the mold is controlled to form it. After the forming is completed, the cooling process of the component is controlled to cause an alloy phase change, and after cooling, a steel shell component with a gradient distribution of microstructure and mechanical properties is obtained. The microstructure and mechanical properties such as strength and hardness of the steel shell component prepared by the present invention are gradiently distributed along the length direction, which meets the use requirements of the steel shell component, and the equipment used in the preparation process is conventional equipment and is easy to control.

Description

Technical Field

[0001] The invention relates to the technical field of material forming, and in particular to a steel shell component with a gradient distribution of microstructure and mechanical properties and a forming method. Background Art

[0002] The high recrystallization temperature of steel materials and the complex metal phase transformation behavior during the large-scale heating and cooling process limit the industrial application of new material processing and forming technologies such as additive manufacturing and semi-solid forming in the forming and manufacturing of steel components. Today, when the world advocates and implements the goals of "carbon peak" and "carbon neutrality", the processing, forming and manufacturing of steel components are still mainly based on traditional machining, casting and forging. The subtractive manufacturing process represented by traditional machining is not only limited to low material utilization, but also leads to poor mechanical properties of components due to discontinuous streamlines. The traditional equal material manufacturing process represented by casting and forging faces the dilemma of losing one thing while manufacturing advanced steel components that have specific requirements for macroscopic geometric features, microstructure morphology, and mechanical performance. For example, the existing forming methods of large aspect ratio steel shell components include: first undergoing multi-fire multi-stage forging and block forging, and then welding the parts to form a large aspect ratio shell component. The former has a long production cycle and high energy consumption, and the latter is prone to structural defects in the weld and heat-affected zone of the component, which seriously affects the mechanical properties.

[0003] In order to process and form steel components with gradient distribution of microstructure and mechanical properties, researchers at home and abroad have mainly tried to use methods such as laser cladding additive manufacturing and arc fuse additive manufacturing. However, the above two methods have a long production cycle and are highly dependent on special equipment. They are more suitable for the forming and processing of personalized, individual steel components with complex geometric features, but are not suitable for the rapid production of large quantities of steel components. In addition, split forming + welding / riveting and regional heat treatment of one-piece formed steel components can also achieve gradient distribution of microstructure and mechanical properties. However, the hidden mechanical properties of the former welding / riveting and the design and manufacture of special heat treatment equipment for the latter hinder the widespread application of the above two processes. This brings greater challenges to the integrated forming research of steel components with gradient distribution of microstructure and mechanical properties. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to solve the problems of long production cycle, high equipment requirements and difficulty in large-scale rapid production of steel shell components with gradient distribution of microstructure and mechanical properties, and to provide a steel shell component and forming method with gradient distribution of microstructure and mechanical properties.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A steel shell component with a gradient distribution of microstructure and mechanical properties is prepared by prefabricating a steel blank base material into a steel blank with a semi-solid structure, placing the steel blank into a preheated concave die, driving a punch to press the steel blank downward to achieve thixotropic-plastic composite forming of the steel blank, controlling the temperature of the blank in the die to form the blank, and controlling the cooling process of the component after forming to cause an alloy phase change, so that a steel shell component with a gradient distribution of microstructure and mechanical properties is obtained after cooling.

[0007] The present invention also provides a method for forming a steel shell component with a gradient distribution of microstructure and mechanical properties, comprising the following steps:

[0008] Prefabrication of steel billets: The temperature change of the steel billets is measured by a K-type thermocouple, and the steel billet base material is heated to 1380-1425°C at a heating rate of 10-20°C / s in a CNC induction heating furnace, and kept warm for 0.5-1.0 minutes to obtain a prefabricated semi-solid steel billet with a liquid phase volume fraction increasing gradually from the inside to the outside in the radial direction; wherein the liquid phase volume ratio of the radial outermost edge area of ​​the steel billet is 35%, and the liquid phase volume ratio of the centralmost area is 0%;

[0009] Preheating of the male and female molds: preheat the male mold to 500-600℃, preheat the female mold to 950-1100℃, and keep them warm for 15-20 minutes;

[0010] Steel billet placement: Place the prefabricated semi-solid steel billet into the preheated die;

[0011] Thixotropic plastic deformation composite forming: put the punch into the die, drive the punch to press down according to the predetermined time-pressure stroke trajectory until the punch and the die are completely closed; in this process, the temperature of the die is measured by a K-type thermocouple, and the power of the induction coil is adjusted according to the measured real-time temperature to maintain the working temperature of the die within the temperature range of 900-1050℃ until the blank completely fills the cavity formed by the punch and the die, and the composite forming is completed;

[0012] Temperature-controlled cooling of components: After forming, the computer system measures and obtains the temperature of the die through a K-type thermocouple, compares the measured real-time temperature data with the preset cooling curve of the cooling rate and the holding time in the computer system, and uses the difference between the measured temperature and the preset temperature as the basis for adjusting the induction coil and the cooling water pipe around the die, so as to achieve precise regulation of the alloy phase transformation from γ-Fe to α-Fe of the steel component during the cooling process according to the preset cooling curve; that is, the phase transformation is achieved by adjusting the induction coil and the cooling water pipe so that the die and the blank in the die are cooled from the forming temperature to the room temperature along the preset cooling curve; after the formed component is cooled to the room temperature, the punch is separated from the die, the steel component is taken out from the die, and the steel component and the mold surface are cleaned to obtain the steel shell component with the microstructure and mechanical property gradient distribution.

[0013] Furthermore, 4-8 blind holes with a diameter of 2-5 mm and a depth of 10-800 mm are processed on the steel billet base material, and a K-type thermocouple is inserted into each blind hole to measure the temperature change of the steel billet base material.

[0014] Furthermore, nitrogen is used as a protective gas during the prefabrication process of the steel billet parent material.

[0015] Furthermore, the punch and the die are fixedly mounted on the working table of the CNC servo hydraulic press, so that the CNC servo hydraulic press can drive the punch to move downward according to a predetermined stroke trajectory, thereby realizing the thixotropic-plastic composite forming of the steel billet.

[0016] Furthermore, the cavity parts of the punch and die are made of nickel-based high-temperature alloy, and the parts of the punch and die connected and assembled with the servo hydraulic press are made of hot working die steel; the shape and size of the inner cavity after the punch and die are closed are the shape and size of the steel shell component to be prepared.

[0017] Furthermore, 8-10 holes with a diameter of 10-15 mm are opened around the concave mold cavity for placing cooling water pipes; a hook for loading and unloading the armored heating sleeve is provided on the non-working surface of the punch.

[0018] Furthermore, during the heating and heat preservation process of the male and female dies, the temperatures of the male and female dies are measured and recorded by an infrared thermometer and a paperless recorder.

[0019] Furthermore, in the process of the punch being pressed down until it is completely closed with the die, the stroke, speed, and deformation resistance of the semi-solid steel billet of the punch are accurately recorded and controlled by the numerical control servo system.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. The steel shell component provided by the present invention is formed by five steps of steel blank prefabrication, convex and concave mold preheating, steel blank placement, thixotropic plastic deformation composite forming and component temperature control cooling. The microstructure and mechanical properties such as strength and hardness of the steel shell component are gradiently distributed along the length direction, meeting the use requirements of the steel shell component.

[0022] 2. The forming method provided by the present invention adopts a commonly used CNC servo hydraulic press as a forming device, and uses a general nickel-based high-temperature alloy and hot-working tool steel to make a forming male and female die, which can accurately control the changes of process parameters such as forming temperature and pressing amount in the thixotropic-plastic composite forming process over time, and control the microstructure evolution of the semi-solid steel billet in the time dimension before, during and after forming and the space dimension of the billet from the bottom to the top, so as to obtain a steel component suitable for a gradient distribution of microstructure and mechanical properties, with high material quality utilization and material performance utilization, which is in line with the development direction of modern green manufacturing technology. The forming equipment used in the present invention has low cost and strong versatility, a simple mold structure and an easy manufacturing method. The preparation, placement, forming and cooling of the steel billet in the entire forming process all adopt relatively simple conventional technologies and devices, and the induction heating rate and cooling temperature can be controlled by adjusting the current of the induction coil and the cooling water flow rate of the cooling water pipe, respectively. The equipment used in the preparation process is conventional equipment and is easy to control.

[0023] 3. The semi-solid billet used in the present invention is not a traditional uniform semi-solid billet, but a gradient distribution according to the uneven distribution of temperature. This semi-solid billet with non-uniform microstructure and gradient distribution, on the one hand, can quickly fill the deep cavity and thin-walled cavity under a lower forming load due to the good fluidity and formability of the semi-solid billet with a higher liquid phase in the edge area; on the other hand, due to the plastic deformation of the solid billet in the center area causing the microstructure to be refined, better mechanical properties can be obtained, which is conducive to the processing and forming of steel components with gradient distribution of microstructure and mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The figure is a schematic diagram of forming a steel shell component with a gradient distribution of microstructure and mechanical properties according to the present invention.

[0025] Figure 2 The present invention is a schematic diagram of blank heating and assembly during the forming process of a steel shell component with a gradient distribution of microstructure and mechanical properties.

[0026] Figure 3 The figure is a schematic diagram of mold preheating during the forming process of a steel shell component with a gradient distribution of microstructure and mechanical properties according to the present invention.

[0027] Figure 4The present invention is a schematic diagram of a cooling mechanism of a mold during the forming process of a steel shell component with a gradient distribution of microstructure and mechanical properties.

[0028] Figure 5 It is a schematic diagram of the variation of the punch reduction amount and the forming rate with time during the forming process of a steel shell component with a gradient distribution of microstructure and mechanical properties according to the present invention.

[0029] Figure 6 This is the microstructure distribution diagram of the steel shell component prepared in Example 1 of the present invention.

[0030] Figure 7 This is a distribution diagram of the mechanical properties of the steel shell component prepared in Example 1 of the present invention.

[0031] In the figure: component 1, steel billet 2, die 3, punch 4, induction coil 5, cooling water pipe 6, armored heating sleeve 7. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0033] Example 1

[0034] This embodiment provides a steel shell component with a gradient distribution of microstructure and mechanical properties, which is a 300M ultra-high strength steel shell component with a large aspect ratio. The steel shell component is prepared by prefabricating a steel blank 2 with a semi-solid structure from a steel blank base material, placing it in a preheated die 3, driving a punch 4 to press down the steel blank 2 to achieve thixotropic plastic deformation composite forming of the steel blank 2, controlling the temperature of the blank in the mold to form it, and controlling the cooling process of the component 1 after forming to cause an alloy phase change, and obtaining a steel shell component 1 with a gradient distribution of microstructure and mechanical properties after cooling.

[0035] Example 2

[0036] This embodiment provides a method for forming a steel shell component with a gradient distribution of microstructure and mechanical properties, which is used to prepare the 300M ultra-high strength steel shell component with a large aspect ratio in Example 1.

[0037] See also Figure 1 , including the following steps:

[0038] (1) Prefabrication of steel billets. The geometric shape and size of the billet required for the steel component 1 are simulated and calculated using the finite element method. In this embodiment, a cylinder with a diameter of 800 mm and a height of 1200 mm is used to ensure that the billet can be easily demolded. M300 ultra-high strength steel in a plastic processed state is selected as the billet base material, and the optimized steel billet 2 is cut by mechanical processing means, and 8 blind holes with a diameter of 5 mm and a depth of 50 mm are processed on the edge of the steel billet 2 using a drilling machine. The steel billet base material is placed in a CNC induction heating furnace, and 8 K-type thermocouples inserted into each blind hole of the steel billet 2 are used to measure its temperature change. The steel billet 2 is heated to 1395°C at a heating rate of 10°C / s using a CNC induction heating furnace, and kept warm for 1.0 minute to obtain a semi-solid steel billet 2 with a liquid phase volume fraction that increases gradually from the inside to the outside in the radial direction, see. Figure 2 The liquid phase volume ratio of the radial outermost edge region of the steel billet 2 is 35%, and the liquid phase volume ratio of the centralmost region is 0%. During the heating and heat preservation process, the temperature of the initial steel material is measured and recorded by a K-type thermocouple and a paperless recorder. In order to inhibit oxidation of the steel material during the heating and heat preservation process, nitrogen is used as a protective gas.

[0039] (2) Preheating of male and female molds. According to the finite element method, the geometric shape and size of the punch 4 and the die 3 required for the forming of the steel component 1 are optimized and simulated, and then further optimized according to the actual situation to ensure that the formed component 1 is easy to demold. Subsequently, according to the designed shape and size of the punch 4 and the die 3, the cavity part of the punch 4 and the die 3 is made of nickel-based high-temperature alloy, and the connection and assembly part of the punch 4 and the die 3 and the servo hydraulic press are made of hot working die steel. The shape and size of the inner cavity after the male and female molds are closed are the designed shape and size of the blank. In addition to placing the induction coil 5 on the outer edge of the die 3, 8 holes with a diameter of 10 mm are processed around the die cavity by mechanical processing means to facilitate the placement of the cooling water pipe 6, and a hook for loading and unloading the armor heating sleeve 7 is processed on the non-working surface of the punch. The punch 4 and the die 3 are flanged and fixed on the work table of the CNC servo hydraulic press used for the forming of the steel component 1. Before forming, refer to Figure 3 and Figure 4 The armored heating jacket 7 and the induction coil 5 are used to preheat the male and female molds to 600°C and 950°C respectively, and keep them warm for 15 minutes. During the heating and warming process, the temperatures of the male mold 4 and the female mold 3 are measured and recorded by an infrared thermometer and a paperless recorder.

[0040] (3) Steel billet placement. Graphite suspension is evenly sprayed as a lubricant on the working surfaces of the cavities of the punch 4 and the die 3. The robot arm wrapped with thermal insulation cotton is precisely controlled to place the prefabricated semi-solid steel billet 2 that has been heated to the forming temperature and kept warm into the die 3 that has been heated to a predetermined temperature and kept warm. In order to inhibit the oxidation of the steel material during the transmission process, thermal insulation cotton is used to isolate the semi-solid steel billet 2 from the outside air in terms of space, and in terms of time, the movement and placement of the semi-solid steel billet 2 are completed quickly in a short time.

[0041] (4) Thixotropic plastic deformation composite forming. Use a computer interface to control the CNC servo hydraulic press to drive the punch 4 to press down according to the predetermined time-pressure stroke trajectory until the punch 4 and the die 3 are completely closed. During the forming process, the stroke and speed of the punch 4 are accurately recorded and controlled by the CNC servo system. The deformation resistance of the semi-solid steel billet 2 is measured and recorded by the module connected to the forming punch 4 through the CNC servo. The temperature of the forming die 3 is measured by the K-type thermocouple welded around the forming die 3. The power of the induction coil 5 is adjusted according to the real-time temperature data measured by the paperless recorder to ensure that the working temperature of the die 3 is maintained at 900°C. See Figure 5 In the initial stage of forming, a lower strain rate is used. Due to the low temperature of the punch 4, the liquid phase in the contact area between the semi-solid steel billet 2 and the punch 4 solidifies into a solid phase first due to cooling. The solid phase in this area undergoes plastic deformation, and the strain type is compression strain and shear strain. The semi-solid billet undergoes semi-solid thixoforming at the part that is not in contact with the punch 4, and quickly fills the mold cavity under the driving of the forming load; in the middle stage of forming, a higher strain rate is used. On the one hand, as the contact area between the billet and the punch 4 increases, more liquid phase solidifies into a solid phase. On the other hand, because the liquid phase flow velocity is greater than the solid phase under the forming load, liquid phase segregation occurs in the blank, and a large amount of liquid phase quickly fills the unfilled parts of the mold cavity; at the end of the forming, a lower strain rate is used, and the die 3 is continuously pressed down. The plastic deformation at the bottom of the steel blank 2 where the solid phase is more concentrated induces dynamic recrystallization of austenite grains, thereby refining the microstructure. The plastic deformation in other areas of the steel blank 2 with more liquid phase crushes the continuous eutectic compounds and welds the shrinkage and looseness caused by the liquid phase solidification, thereby eliminating the organizational defects. After the forming is completed, the punch 4 and the die 3 are kept closed.

[0042] (5) Component temperature control cooling. After forming is completed, the computer system measures the temperature of the forming die 3 through the K-type thermocouples welded around the forming die 3, and compares the real-time temperature data measured by the paperless recorder with the cooling curve of the cooling rate and holding time set in advance in the computer system. Based on the difference between the real-time temperature data and the established cooling curve, the induction coil 5 and the cooling water pipe 6 around the die 3 are adjusted to cool the steel component 1 according to the established cooling curve, so as to achieve precise control of the alloy phase transformation from γ-Fe to α-Fe of the steel component 1 during the cooling process according to the established cooling curve. After the formed component 1 is cooled to room temperature, the computer interface is used to control the CNC servo hydraulic press to separate the punch 4 from the die 3, remove the steel component 1 from the die 3, and clean the steel component 1 and the mold surface. The microstructure of the steel component 300M steel large aspect ratio shell component 1 is as follows: Figure 6 As shown in Figure 7 shown.

[0043] from Figure 6 and Figure 7 It can be seen that the 300M steel shell component with a large aspect ratio prepared by the present invention has an increasing amount of eutectic compounds, a decreasing amount of martensite, and an increasing grain size from the bottom to the top. This is because, on the one hand, a large amount of liquid phase rapidly gathers to the top of the component under the driving force of the forming load during the forming process and solidifies into eutectic compounds and coarse grains during the subsequent cooling process. On the other hand, a large amount of solid phase is retained at the bottom of the shell component during the thixoforming process and the grains are refined due to recrystallization during the subsequent plastic forming process, thereby obtaining finer grains. At the same time, a large amount of alloying elements move to the top of the component with the liquid phase. Since the martensitic transformation starting temperature is closely related to the alloying element content, the martensitic transformation starting temperature in the area with more alloying elements is lower. Therefore, the martensitic transformation at the top of the component is not sufficient during the cooling process to room temperature, resulting in less martensite at the top of the component. The tensile yield strength and elongation gradually decrease from bottom to top, and the hardness gradually increases, indicating that the eutectic compounds gradually increasing from the bottom to the top of the component gradually increase the hardness of this part on the one hand, and on the other hand, because the eutectic compounds are relatively hardened but lack toughness and are distributed on the grain boundaries of the solid phase particles, the strength and elongation of the area with a higher eutectic compound content are lower than those of the area with a lower eutectic compound content.

[0044] It can be seen that the microstructure and mechanical properties such as strength and hardness of the steel shell component provided by the present invention are distributed in a gradient along the length direction, which can meet the use requirements of the steel shell component. In addition, the commonly used CNC servo hydraulic press is used as the forming equipment, and the forming male and female dies are made of general nickel-based high-temperature alloys and hot-working tool steels, which can accurately control the changes of process parameters such as forming temperature and pressing amount over time in the thixotropic-plastic composite forming process, and control the microstructure evolution of the semi-solid steel billet in the time dimension before, during and after forming and the space dimension of the billet from the bottom to the top, so as to obtain a steel component suitable for the gradient distribution of microstructure and mechanical properties, with high material quality utilization and material performance utilization, which is in line with the development direction of modern green manufacturing technology.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A method for forming a steel shell component with a gradient distribution of microstructure and mechanical properties, characterized in that: The following steps are involved: Prefabrication of steel billets: The temperature change of the steel billets is measured by a K-type thermocouple, and the steel billet base material is heated to 1380-1425°C at a heating rate of 10-20°C / s in a CNC induction heating furnace, and kept warm for 0.5-1.0 minutes to obtain a prefabricated semi-solid steel billet with a liquid phase volume fraction increasing gradually from the inside to the outside in the radial direction; wherein the liquid phase volume ratio of the radial outermost edge area of ​​the steel billet is 35%, and the liquid phase volume ratio of the centralmost area is 0%; Preheating of the male and female molds: preheat the male mold to 500-600℃, preheat the female mold to 950-1100℃, and keep them warm for 15-20 minutes; Steel billet placement: Place the prefabricated semi-solid steel billet into the preheated die; Thixotropic plastic deformation composite forming: the punch is placed in the die, and the punch is driven to press down according to the predetermined time-pressure stroke trajectory until the punch and the die are completely closed; wherein, the strain rate in the middle section of the forming is greater than the strain rate in the initial and final sections of the forming; during this process, the die temperature is measured by a K-type thermocouple, and the power of the induction coil is adjusted according to the measured real-time temperature, so that the working temperature of the die is maintained in the temperature range of 900-1050°C, until the blank completely fills the cavity formed by the punch and the die, and the composite forming is completed; Component temperature-controlled cooling: After forming, the computer system measures and obtains the temperature of the die through a K-type thermocouple, compares the measured real-time temperature data with the cooling curve of the cooling rate and holding time preset in the computer system, and uses the difference between the measured temperature and the set temperature as the basis for adjusting the induction coil and cooling water pipe around the die, so as to achieve precise control of the alloy phase transformation from γ-Fe to α-Fe of the steel component during the cooling process according to the set cooling curve; after the formed component is cooled to room temperature, the punch and the die are separated, the steel component is taken out of the die, and the steel component and the mold surface are cleaned to obtain the steel shell component with the microstructure and mechanical property gradient distribution.

2. The forming method of the steel shell component with gradient distribution of microstructure and mechanical properties according to claim 1 is characterized in that: 4-8 blind holes with a diameter of 2-5mm and a depth of 10-800mm are processed on the steel billet base material, and a K-type thermocouple is inserted into each blind hole to measure the temperature change of the steel billet base material.

3. The forming method of the steel shell component with gradient distribution of microstructure and mechanical properties according to claim 1, characterized in that: Nitrogen is used as protective gas in the prefabrication process of steel billet parent material.

4. The forming method of the steel shell component with gradient distribution of microstructure and mechanical properties according to claim 1 is characterized in that: The punch and the die are fixedly mounted on the working table of the CNC servo hydraulic press, so that the CNC servo hydraulic press can drive the punch to move downward according to a predetermined stroke trajectory, thereby realizing the thixotropic and plastic deformation composite forming of the steel billet.

5. The forming method of the steel shell component with gradient distribution of microstructure and mechanical properties according to claim 4, characterized in that: The cavity parts of the punch and die are made of nickel-based high-temperature alloy, and the parts of the punch and die connected and assembled with the servo hydraulic press are made of hot working die steel; the shape and size of the inner cavity after the punch and die are closed are the shape and size of the steel shell component to be prepared.

6. The forming method of the steel shell component with gradient distribution of microstructure and mechanical properties according to claim 1, characterized in that: There are 8-10 holes with a diameter of 10-15 mm around the concave mold cavity for placing cooling water pipes; a hook for loading and unloading the armored heating sleeve is provided on the non-working surface of the punch.

7. The forming method of the steel shell component with gradient distribution of microstructure and mechanical properties according to claim 1, characterized in that: During the heating and heat preservation process of the punch and die, the temperatures of the punch and die are measured and recorded by an infrared thermometer and a paperless recorder.

8. The forming method of a steel shell component with a gradient distribution of microstructure and mechanical properties according to claim 1, characterized in that: During the process of the punch pressing down until it is completely closed with the die, the CNC servo system accurately records and controls the punch pressing stroke, speed, and deformation resistance of the semi-solid steel billet.

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