A preparation method of Cr8 die steel with high wear resistance and high fatigue life

Abstract

The present invention discloses a method for preparing Cr8 die steel with high wear resistance and high fatigue life, and belongs to the technical field of metal die steel. The method for preparing Cr8 die steel with high wear resistance and high fatigue life of the present invention designs deformation and heat treatment processes, including smelting, pouring, forging, rolling, cyclic heat treatment, tempering, etc., and regulates carbide precipitation of steel, refines original austenite grains by cyclic heat treatment, and performs secondary hardening by tempering, etc., so that Cr8 die steel obtains high wear resistance and high fatigue life, wear resistance is improved by about 3 times, fatigue life is improved by 10 times, and fatigue times can reach 300,000 times under 20% amplitude.

Description

Technical Field

[0001] The invention relates to a preparation method of Cr8 die steel with high wear resistance and high fatigue life, belonging to the technical field of cold working. Background Art

[0002] In the application fields of stamping and punching, the mold surface is often subjected to high cyclic mechanical stress and friction load. Fatigue failure and wear failure are serious problems encountered by mold steel. Improving the wear resistance and fatigue life of mold steel is conducive to reducing economic costs and improving production efficiency. The traditional Cr12 mold steel has poor toughness and extremely low fatigue life due to the number and size of carbides, which seriously affects production. Therefore, in order to solve the problem of insufficient toughness, the addition of elements such as Mo and V can improve the hardenability and the size of precipitated carbides, thereby improving the toughness of the material. However, large eutectic carbides and internal inclusions are inevitable and are easy to become crack sources. Although some large eutectic carbides can be broken by upsetting and drawing processes, most of the broken carbides have sharp corners, and the sharp corner effect is also easy to cause cracks and cracks, causing the mold to break and fail. Therefore, from the research of composition design to heat treatment process, it is of great significance to develop a Cr8 mold steel with high wear resistance and high fatigue life.

[0003] Chinese invention patent CN115491473A discloses a treatment method for improving the performance of large-size DC53 cold-working die steel, whose chemical element composition and mass percentage are: C: 0.9-1.2%, Si: 0.8-1.2%, Mn: 0.2-0.5%, Cr: 7.0-9.0%, Mo: 1.5-2.5%, V: 0.2-0.5%, and the balance is Fe and unavoidable impurity elements; this method can enhance the strength and wear resistance of the die steel; however, the injection molding process has material loss, and the heat treatment time is long, which increases the economic cost.

[0004] Chinese invention patent CN113355597A discloses a high-toughness and high-wear-resistant cold-working die steel and its manufacturing process, the chemical elements and their mass percentages are: C: 0.7%-0.9%, Si: 0.2%-0.4%, Mn: 0.5%-0.8%, Mo: 1.0%-1.5%, Cr: 12.0%-15.0%, V: 0.5%-0.8%, Nb: 0.6%-0.8%, Y: 0.02%-0.05%, Ti: 0.01%-0.05%, W: 1.0%-1.5%, S≤0.02%, P≤0.02%; the balance is Fe and other unavoidable impurities. This method improves the wear resistance and toughness of the material through composition design. However, excessive addition of alloying elements and poor control of the types of precipitates increase economic costs.

[0005] Chinese invention patent CN 112251580A discloses a heat treatment process for high carbon cold working die steel. This invention improves the heat treatment process, avoids heat loss during the transfer of high carbon cold working die steel during heat treatment, saves money and improves work efficiency; however, it has little significance for improving material performance.

[0006] Obtaining mold steel with high wear resistance and high fatigue life has always been a research hotspot; the biggest challenge currently faced is the dual nature of carbide strengthening the matrix and its deterioration of toughness; regulating precipitates from composition design to subsequent deformation heat treatment processes is a hot issue worthy of research. Summary of the invention

[0007] In view of the shortcomings of the above-mentioned prior art, the present invention provides a method for preparing Cr8 die steel with high wear resistance and high fatigue life, by adjusting the morphology of precipitates and original austenite grains through alloy composition design and addition of rare earth elements, as well as innovative heat treatment process, so as to improve the wear resistance and fatigue life of the material from the perspectives of precipitation strengthening and fine grain strengthening.

[0008] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing Cr8 die steel with high wear resistance and high fatigue life, comprising the following steps:

[0009] (1) The raw materials are weighed according to the weight percentage of the chemical composition of Cr8 die steel, and then smelted. When the molten metal is completely melted, rare earth La is added, and then cast into a billet.

[0010] (2) The ingot is heated to 600±20℃ in the middle temperature zone, and the holding time is controlled to 0.5h to ensure that the core of the casting is heated evenly and the internal and external structures are not affected by the temperature difference in subsequent processing. The temperature is then raised to make the ingot uniform, and it is cooled out of the furnace for forging, and then air-cooled to room temperature.

[0011] (3) The forged steel is subjected to a cyclic heat treatment, rapidly heated to 950-1030°C, then quenched to room temperature, and repeated 2 to 3 times.

[0012] (4) The heat-treated steel is heated to the tempering temperature and tempered twice, and then air-cooled to room temperature. The two temperings ensure the improvement of the material's toughness, the precipitation of secondary carbides, and the complete transformation of retained austenite.

[0013] Preferably, the high hardness and high toughness bainitic wear-resistant steel described in the present invention comprises the following chemical components in weight percentage: C: 0.9-1.1%, Si: 0.9-1.05%, Mn: 0.35-0.45%, Cr: 7.5-8.5%, P<0.001%, S<0.001%, V: 0.2-0.4%, Ni: 0.2-0.3%, Mo: 1.85-2.05%, La: 0.0001%-0.0002%, and the balance is Fe and unavoidable impurities.

[0014] Preferably, the pouring temperature of the present invention is 1510-1530°C.

[0015] Preferably, the heating rate in the medium temperature zone in step (2) of the present invention is 10-30°C / min.

[0016] Preferably, in step (2) of the present invention, the temperature for homogenizing the ingot is 1150-1250° C. and the time is 1 hour.

[0017] Preferably, the forging temperature in step (3) of the present invention is 1100-1150°C, and the forging ratio is 2-3.

[0018] Preferably, the heating rate in step (3) of the present invention is 25-40°C / min.

[0019] Preferably, the tempering heating rate in step (4) of the present invention is 10-30°C / min, and the temperature is controlled at 500-600°C.

[0020] As a preferred embodiment of the present invention, the raw materials include pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel.

[0021] The principle of the invention: from the perspective of component design, the invention increases the hardenability of the material by adding Mo and V, and provides the material with the ability of secondary hardening; a small amount of rare earth elements added before casting can purify the molten steel and avoid excessive oxygen content; more importantly, only adding a few parts per million of rare earth can significantly improve the fatigue life; there are some defects inside the cast steel ingot, the temperature is increased for forging treatment, the internal defects and small cracks are welded, and the grains are recrystallized under the premise of a large deformation amount, and the grains are refined; the internal carbides are fractured under plastic deformation; because the precipitation temperature of carbides is high, heat treatment cannot completely eliminate large pieces of carbides, and a cyclic heat treatment process is used to repeatedly heat and keep warm for a short time, trim the shape of carbides, and make the carbide sharp corners solid-dissolve into the matrix to avoid the deterioration caused by the sharp corner effect; two high-temperature temperings ensure that the quenching stress caused by the cyclic heat treatment can be completely eliminated, and the high-temperature tempering can transform the residual austenite into the tempered martensite, and fine carbides are precipitated in this process to perform secondary hardening on the matrix, thereby ensuring the surface hardness of the material. Under this process, the obtained mold steel has high hardness, high wear resistance and high fatigue life.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] On the one hand, the present invention improves the fatigue life of the material and reduces the size and number of internal inclusions through the addition of rare earth elements in the composition design; on the other hand, the preparation process is optimized, the forging process temperature is increased, part of the carbides are remelted, large pieces of carbides are broken, the recrystallized grains are increased, and the original austenite grains are refined; the introduction of cyclic heat treatment melts the sharp corners of the broken carbides after forging, and the temperature is quickly raised through the A1-A3 line, which can refine the grains under subsequent multiple quenching, and the short holding time makes the element diffusion insufficient to make the carbides and grains grow too large. The precipitation of Mo2C by high temperature tempering ensures the occurrence of secondary hardening, and in this temperature range, the addition of Mo and V promotes the uphill diffusion of Cr elements in the matrix, resulting in the transformation of M7C3→M23C6. The structural stability of M23C6 carbides and the Young's modulus of M23C6 are stronger than those of other types of carbides, which increases the toughness of the material. The Cr8 mold steel with high wear resistance and high fatigue life can achieve 300,000 fatigue times at a strain amplitude of 20%, which is 10 times that of ordinary mold steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a process flow chart for preparing the Cr8 die steel with high wear resistance and high fatigue life according to the present invention.

[0025] Figure 2 The organizational diagram of the Cr8 die steel with high wear resistance and high fatigue life described in Example 1

[0026] Figure 3This is the element distribution diagram of the organizational diagram of the Cr8 die steel with high wear resistance and high fatigue life described in Example 1.

[0027] Figure 4 This is the fatigue curve of the Cr8 die steel with high wear resistance and high fatigue life described in Example 1. DETAILED DESCRIPTION

[0028] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0029] Example 1

[0030] A method for preparing Cr8 die steel with high wear resistance and high fatigue life, specifically comprising the following steps:

[0031] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volumes will prolong the melting time. Crush the raw materials with larger volumes into particles with a diameter not exceeding 5 mm.

[0032] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1430°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0033] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1510°C. The casting time is controlled to be 150s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0034] (4) Forging: The ingot was heated to 600°C at a rate of 10°C / min and kept at that temperature for 0.5 h. It was then heated to 1150°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1100°C with a forging ratio of 3. The final forging temperature was controlled at 1000°C and air-cooled to room temperature.

[0035] (5) Cyclic heat treatment: heat to 950°C at 25°C / min and keep warm for 30 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 15 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 5 min, quench with water to room temperature.

[0036] (6) High temperature tempering: Heat to 500℃ at 10℃ / min and keep it for two hours, air cool to room temperature, and then heat to 500℃ at 10℃ / min again and keep it for two hours and air cool to room temperature to produce Cr8 mold steel with high wear resistance and high fatigue life.

[0037] The Cr8 die steel with high wear resistance and high fatigue life was observed under electron microscope. Figure 2 As shown in the figure, the matrix is ​​tempered martensite and dispersed precipitates. The composition analysis of the precipitates shows that they are Cr-rich or Mo-rich carbides. The surface hardness can be 53HRC, and the wear rate is 1.76×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 310,700 times.

[0038] Example 2

[0039] A method for preparing Cr8 die steel with high wear resistance and high fatigue life, specifically comprising the following steps:

[0040] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volumes will prolong the melting time. Crush the raw materials with larger volumes into particles with a diameter not exceeding 5 mm.

[0041] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace, heat to 1430°C, take samples for chemical composition analysis, and reasonably control the chemical composition within the specified range;

[0042] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1520°C. The casting time is controlled to be 160s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0043] (4) Forging: The ingot was heated to 600°C at 20°C / min and kept at that temperature for 0.5 h. It was then heated to 1180°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1125°C with a forging ratio of 3. The final forging temperature was controlled at 1030°C and then air-cooled to room temperature.

[0044] (5) Cyclic heat treatment: heat to 990°C at 35°C / min and keep warm for 30 min, quench with water to room temperature, heat to 990°C at 35°C / min and keep warm for 15 min, quench with water to room temperature, heat to 990°C at 35°C / min and keep warm for 5 min, quench with water to room temperature.

[0045] (6) High temperature tempering: Heat to 550℃ at 15℃ / min and keep it for two hours, air cool to room temperature, and then heat to 550℃ at 15℃ / min again and keep it for two hours to air cool to room temperature. Cr8 mold steel with high wear resistance and high fatigue life can be produced.

[0046] The Cr8 die steel with high wear resistance and high fatigue life was observed under an electron microscope. The matrix was tempered martensite and dispersed precipitates. The composition analysis of the precipitates revealed that they were Cr-rich or Mo-rich carbides. The volume fraction and type of precipitates changed, which will be statistically reported in the table below. The surface hardness can be 56HRC, and the wear rate is 2.07×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 305,900 times.

[0047] Example 3

[0048] A method for preparing Cr8 die steel with high wear resistance and high fatigue life, specifically comprising the following steps:

[0049] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volume will prolong the melting time. The raw materials with larger volume shall be crushed into particles with a diameter not exceeding 5 mm.

[0050] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1435°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0051] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1530°C. The casting time is controlled to be 165s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0052] (4) Forging: The ingot was heated to 600°C at 30°C / min and kept at that temperature for 0.5 h. It was then heated to 1200°C for homogenization and kept at that temperature for 1 h. It was then taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1150°C with a forging ratio of 3. The final forging temperature was controlled at 1050°C and air-cooled to room temperature.

[0053] (5) Cyclic heat treatment: heat to 1030°C at 40°C / min and keep warm for 30 min, quench with water to room temperature, heat to 1030°C at 40°C / min and keep warm for 15 min, quench with water to room temperature, heat to 1030°C at 40°C / min and keep warm for 5 min, quench with water to room temperature.

[0054] (6) High temperature tempering: Heat to 600℃ at 30℃ / min and keep it for two hours, air cool to room temperature, and then heat to 600℃ at 30℃ / min again and keep it for two hours and air cool to room temperature to produce Cr8 mold steel with high wear resistance and high fatigue life.

[0055] The Cr8 die steel with high wear resistance and high fatigue life was observed under electron microscope. The matrix was tempered martensite and dispersed precipitates. The composition analysis of the precipitates revealed that they were Cr-rich or Mo-rich carbides. The volume fraction and type of precipitates changed, which will be statistically reported in the table below. The surface hardness can be 54HRC, and the wear rate is 1.94×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 302,500 times.

[0056] Comparative Example 1

[0057] A method for preparing Cr8 die steel specifically comprises the following steps:

[0058] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel according to the weight percentage of the chemical composition of Cr8 die steel with high wear resistance and high fatigue life (C: 1%, Si: 1%, Mn: 0.4%, Cr: 8%, P<0.001%, S<0.001%, Ni: 0.25%, the remainder is Fe and inevitable impurities). Raw materials with larger volumes will prolong the melting time, and the raw materials with larger volumes are crushed into particles with a diameter not exceeding 5 mm.

[0059] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1430°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0060] (3) Pouring: Sand casting is used to pour the molten metal into a ladle and pour it at 1510°C. The casting time is controlled to be 150s. After pouring, the riser is covered with a heat preservation agent to cool it to room temperature to obtain a casting.

[0061] (4) Forging: The ingot was heated to 600°C at a rate of 10°C / min and kept at that temperature for 0.5 h. It was then heated to 1150°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1100°C with a forging ratio of 3. The final forging temperature was controlled at 1000°C and air-cooled to room temperature.

[0062] (5) Cyclic heat treatment: heat to 950°C at 25°C / min and keep warm for 30 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 15 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 5 min, quench with water to room temperature.

[0063] (6) High temperature tempering: heat to 500℃ at 10℃ / min and keep it for two hours, air cool to room temperature, and then heat to 500℃ at 10℃ / min again and keep it for two hours, and air cool to room temperature to produce Cr8 mold steel.

[0064] The mold steel with changed composition design was observed under electron microscope. The matrix was tempered martensite and precipitates. The composition analysis of the precipitates revealed that they were Cr-rich carbides. The surface hardness was only 41HRC, and the wear rate was 9.73×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 72,100 times.

[0065] Comparative Example 2

[0066] A method for preparing Cr8 die steel specifically comprises the following steps:

[0067] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volumes will prolong the melting time. Crush the raw materials with larger volumes into particles with a diameter not exceeding 5 mm.

[0068] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1430°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0069] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1510°C. The casting time is controlled to be 150s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0070] (4) Forging: The ingot was heated to 600°C at a rate of 10°C / min and kept at that temperature for 0.5 h. It was then heated to 1150°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1100°C with a forging ratio of 3. The final forging temperature was controlled at 1000°C and air-cooled to room temperature.

[0071] (5) Ordinary quenching heat treatment: heat to 950℃ at 25℃ / min, keep warm for 60min, and quench to room temperature.

[0072] (6) High temperature tempering: heat to 500℃ at 10℃ / min and keep it for two hours, air cool to room temperature, and then heat to 500℃ at 10℃ / min again and keep it for two hours, and air cool to room temperature to produce Cr8 mold steel.

[0073] The Cr8 die steel with changed heat treatment process was observed under electron microscope. The matrix was tempered martensite and dispersed precipitates. The composition analysis of the precipitates revealed that they were Cr-rich or Mo-rich carbides. The surface hardness was 49HRC and the wear rate was 3.21×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude are 27,100 times.

[0074] Comparative Example 3

[0075] A method for preparing Cr8 die steel specifically comprises the following steps:

[0076] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volumes will prolong the melting time. Crush the raw materials with larger volumes into particles with a diameter not exceeding 5 mm.

[0077] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace, heat to 1430°C, take samples for chemical composition analysis, and reasonably control the chemical composition within the specified range;

[0078] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1510°C. The casting time is controlled to be 150s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0079] (4) Cyclic heat treatment: heat to 950°C at 25°C / min and keep warm for 30 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 15 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 5 min, quench with water to room temperature.

[0080] (5) High temperature tempering: heat to 500℃ at 10℃ / min and keep it for two hours, air cool to room temperature, and then heat to 500℃ at 10℃ / min again and keep it for two hours, and air cool to room temperature to produce Cr8 mold steel.

[0081] The Cr8 die steel with changed blanking process was observed by electron microscope. The matrix was tempered martensite and large precipitates. The composition analysis of the precipitates revealed that they were Cr-rich or Mo-rich carbides. The surface hardness was 51HRC and the wear rate was 7.34×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 20,300 times.

[0082] Comparative Example 4

[0083] A method for preparing Cr8 die steel specifically comprises the following steps:

[0084] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volumes will prolong the melting time. Crush the raw materials with larger volumes into particles with a diameter not exceeding 5 mm.

[0085] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1430°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0086] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1510°C. The casting time is controlled to be 150s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0087] (4) Forging: The ingot was heated to 600°C at a rate of 10°C / min and kept at that temperature for 0.5 h. It was then heated to 1150°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1100°C with a forging ratio of 3. The final forging temperature was controlled at 1000°C and air-cooled to room temperature.

[0088] (5) Cyclic heat treatment: heat to 950°C at 25°C / min and keep warm for 30 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 15 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 5 min, quench with water to room temperature.

[0089] (6) High temperature tempering: Heat to 500℃ at 10℃ / min and keep it for two hours, air cool to room temperature, and then heat to 500℃ at 10℃ / min again and keep it for two hours and air cool to room temperature to produce Cr8 mold steel with high wear resistance and high fatigue life.

[0090] The Cr8 die steel with changed La composition was observed under electron microscope. The matrix was tempered martensite and large precipitates. The composition analysis of the precipitates revealed that they were Cr-rich or Mo-rich carbides. The surface hardness was 43HRC and the wear rate was 10.26×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 10,900 times.

[0091] Comparative Example 5

[0092] A method for preparing Cr8 die steel specifically comprises the following steps:

[0093] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel according to the weight percentage of the chemical composition of Cr8 die steel with high wear resistance and high fatigue life (C: 1%, Si: 1%, Mn: 0.4%, Cr: 8%, P<0.001%, S<0.001%, V: 0.3%, Ni: 0.25%, Mo: 1.95%, the remainder is Fe and inevitable impurities). Raw materials with larger volumes will prolong the melting time, and the raw materials with larger volumes are crushed into particles with a diameter not exceeding 5 mm.

[0094] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1430°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0095] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1510°C. The casting time is controlled to be 150s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0096] (4) Forging: The ingot was heated to 600°C at a rate of 10°C / min and kept at that temperature for 0.5 h. It was then heated to 1150°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1100°C with a forging ratio of 3. The final forging temperature was controlled at 1000°C and air-cooled to room temperature.

[0097] (5) Cyclic heat treatment: heat to 950°C at 25°C / min and keep warm for 30 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 15 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 5 min, quench with water to room temperature.

[0098] (6) High temperature tempering: Heat to 500℃ at 10℃ / min and keep it for two hours, air cool to room temperature, and then heat to 500℃ at 10℃ / min again and keep it for two hours and air cool to room temperature to produce Cr8 mold steel with high wear resistance and high fatigue life.

[0099] The Cr8 die steel with changed La composition was observed under electron microscope. The matrix was tempered martensite and large precipitates. The composition analysis of the precipitates revealed that they were Cr-rich or Mo-rich carbides. The surface hardness was 51HRC and the wear rate was 9.61×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude is 11,300 times.

[0100] Comparative Example 6

[0101] A method for preparing Cr8 die steel specifically comprises the following steps:

[0102] (1) Ingredients: Calculate and weigh the raw materials such as pig iron, ferrosilicon, ferromanganese, ferrochrome and scrap steel. Raw materials with larger volumes will prolong the melting time. Crush the raw materials with larger volumes into particles with a diameter not exceeding 5 mm.

[0103] (2) Melting: Use a medium frequency induction furnace with an acid lining for smelting. Add the prepared raw materials into the furnace and heat them to 1430°C before taking samples for chemical composition analysis. The chemical compositions are then reasonably regulated within the specified range.

[0104] (3) Pouring: When the molten metal is completely melted, rare earth La is added and then casting is prepared. Sand casting is adopted. The molten metal is taken out of the furnace and loaded into a ladle and poured at 1510°C. The casting time is controlled to be 150s. After pouring, the heat preservation agent is covered on the riser and cooled to room temperature to obtain a casting.

[0105] (4) Forging: The ingot was heated to 600°C at a rate of 10°C / min and kept at that temperature for 0.5 h. It was then heated to 1150°C for homogenization and kept at that temperature for 1 h. The ingot was taken out of the furnace and sent to the forging machine for forging. The forging process was started at 1100°C with a forging ratio of 3. The final forging temperature was controlled at 1000°C and air-cooled to room temperature.

[0106] (5) Cyclic heat treatment: heat to 950°C at 25°C / min and keep warm for 30 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 15 min, quench with water to room temperature, heat to 950°C at 25°C / min and keep warm for 5 min, quench with water to room temperature.

[0107] The Cr8 die steel without high temperature tempering was observed under electron microscope. Figure 2 As shown in the figure, the matrix is ​​tempered martensite and dispersed precipitates. The composition analysis of the precipitates shows that they are Cr-rich or Mo-rich carbides. The surface hardness can be 61HRC, and the wear rate is 8.44×10 -6 mm 3 ·N -1 mm -1 , the fatigue times under 20% strain amplitude are 13,900 times.

[0108] Table 1 Statistics of total volume fraction of precipitates and volume fraction of each type of precipitates

[0109] Case Overall points <![CDATA[Cr7C3]]> <![CDATA[Cr 23 C6]]> <![CDATA[Mo2C]]> Maximum size Example 1 21％ 41％ 39％ 20％ 33μm Example 2 19％ 44％ 41％ 15％ 31μm Example 3 21％ 37％ 38％ 25％ 29μm Comparative Example 1 14％ 53％ 47％ 0 44μm Comparative Example 2 17％ 41％ 35％ 24％ 38μm Comparative Example 3 16％ 37％ 44％ 19％ 117μm Comparative Example 4 16％ 31％ 47％ 22％ 198μm Comparative Example 5 18％ 33％ 51％ 16％ 277μm Comparative Example 6 17％ 71％ 26％ 3％ 47μm

[0110] Table 2 Comparison of wear rate and fatigue life

[0111] Case <![CDATA[Wear rate / mm 3 ·N -1 ·mm -1 > Fatigue life / 10,000 times Example 1 <![CDATA[1.76×10 -6 ]]> 31.07 Example 2 <![CDATA[2.07×10 -6 ]]> 30.59 Example 3 <![CDATA[1.94×10 -6 ]]> 30.25 Comparative Example 1 <![CDATA[9.73×10 -6 ]]> 7.21 Comparative Example 2 <![CDATA[3.21×10 -6 ]]> 2.71 Comparative Example 3 <![CDATA[7.34×10 -6 ]]> 2.03 Comparative Example 4 <![CDATA[10.26×10 -6 ]]> 1.09 Comparative Example 6 <![CDATA[9.61×10 -6 ]]> 1.13 Comparative Example 6 8.44×10-6 1.39

[0112] According to Examples 1-3 and Comparative Example 1, it can be seen that after removing Mo, V and rare earth La elements from Comparative Example 1, no Mo2C is precipitated during high temperature tempering, and the secondary hardening ability is lost; under the same process conditions, the fatigue life drops sharply, which is related to the rare earth La purifying the molten steel, reducing the internal oxygen content and the number of inclusions, and improving the fatigue life. The composition of the present invention can obtain ideal wear resistance and fatigue life.

[0113] According to the comparison between Example 1 and Comparative Example 2, the grain size of the sample in Comparative Example 2 after not undergoing cyclic heat treatment becomes relatively larger. Although the difference in hardness and wear resistance is relatively small, and the size difference is also small, most of the broken carbides after plastic deformation have sharp corners, and stress concentration exists when subjected to force, becoming the source of cracks. A large number of cracks merge, crack instability accelerates expansion, and fatigue life is deteriorated.

[0114] According to the comparison between Example 1 and Comparative Example 3, Comparative Example 3 is not forged, and the large eutectic carbides deteriorate the toughness of the material; and there are no recrystallized grains, the original austenite grain size is large, and it cannot contribute to fine grain strengthening. Although the presence of carbides does not reduce the surface hardness, the large carbides and large-sized grains deteriorate the fatigue performance of the die steel. In addition, the shedding of large carbides during wear will also aggravate the wear on the surface, causing the wear resistance to decrease.

[0115] According to Example 1, compared with Comparative Examples 4 and 5, the addition of excessive rare earth element La leads to the generation of rare earth oxides in the melt, which become heterogeneous nucleation points and increase the size of carbides. The dual effects of rare earth oxides and large-sized carbides lead to a sharp deterioration in the performance of the mold steel. The samples without La have lost the purification link of the molten steel, and the number and size of internal inclusions are greater than those in Example 1. The presence of large carbides deteriorates the performance.

[0116] Compared with Example 1 and Comparative Example 6, the surface hardness of Comparative Example 6 in the quenched state is higher, which is attributed to the higher quenching stress. However, the toughness of the material deteriorates under high stress, especially the fatigue resistance. In addition, without high temperature tempering, the transformation of carbides is limited, and the ability to improve wear resistance during wear is insufficient.

[0117] Figure 2This is the organizational diagram of the Cr8 die steel with high wear resistance and high fatigue life described in Example 1. It can be seen from the figure that the carbides do not have obvious sharp corners, and their sizes are relatively uniform and are all around 2 microns, which can enhance the surface hardness.

[0118] Figure 3 This is the organizational diagram of the Cr8 die steel with high wear resistance and high fatigue life described in Example 1. It can be seen from the figure that the carbide elements are mainly Cr-rich and Mo-rich, which proves that the strategy of adding elements is correct, especially the addition of Mo, which precipitates Mo-rich carbides during tempering to increase the surface hardness.

[0119] 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 scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing Cr8 die steel with high wear resistance and high fatigue life, characterized in that: Through alloy composition design, addition of rare earth elements, and heat treatment process, the morphology of precipitates and original grains are regulated to improve the wear resistance and fatigue life of the material. The specific steps include: (1) Weighing raw materials according to the weight percentage of the chemical composition of Cr8 die steel, then smelting, adding rare earth La when the molten metal is completely melted, and then pouring into a casting billet; (2) The ingot is heated to 600±20°C in the medium temperature zone, and the holding time is controlled to be 0.5h. Then the temperature is raised to homogenize the ingot, and the ingot is cooled out of the furnace for forging, and then air-cooled to room temperature; (3) subjecting the forged steel to a cyclic heat treatment, rapidly heating the temperature to 950-1030°C, then quenching to room temperature, and repeating this process 2-3 times; (4) The heat-treated steel is heated to the tempering temperature and tempered twice, and then air-cooled to room temperature.

2. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: The Cr8 die steel includes the following chemical components in weight percentage: C: 0.9-1.1%, Si: 0.9-1.05%, Mn: 0.35-0.45%, Cr: 7.5-8.5%, P<0.001%, S<0.001%, V: 0.2-0.4%, Ni: 0.2-0.3%, Mo: 1.85-2.05%, La: 0.0001%-0.0002%, and the balance is Fe and unavoidable impurities.

3. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: The pouring temperature is 1510-1530°C.

4. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: The heating rate in the medium temperature zone in step (2) is 10-30°C / min.

5. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: In step (2), the temperature of the ingot homogenization treatment is 1150-1250° C. and the time is 1 hour.

6. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: The forging temperature in step (3) is 1100-1150° C., and the forging ratio is 2-3.

7. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: The heating rate in step (3) is 25-40°C / min.

8. The method for preparing the Cr8 die steel with high wear resistance and high fatigue life according to claim 1, characterized in that: The tempering heating rate in step (4) is 10-30°C / min, and the temperature is controlled at 500-600°C.

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