Non-quenched and tempered steel for low-Si-content large-specification expansion breaking connecting rod, production method of non-quenched and tempered steel, expansion breaking connecting rod and controlled forging and controlled cooling process of expansion breaking connecting rod

Abstract

The invention provides non-quenched and tempered steel for a low-Si-content large-specification expansion breaking connecting rod, a production method of the non-quenched and tempered steel, the expansion breaking connecting rod and a controlled forging and controlled cooling process of the expansion breaking connecting rod. The non-quenched and tempered steel comprises the components of 0.43%-0.47% of C, smaller than or equal to 0.10% of Si, 1.60%-1.80% of Mn, 0.015%-0.025% of P, 0.040%-0.070% of S, 0.15%-0.25% of Cr, 0.15%-0.25% of Ni, 0.020%-0.035% of Al, 0.05%-0.10% of Mo, 0.45%-0.95% of V, 0.030%-0.050% of Nb, 0.0240%-0.057% of [N] and the balance Fe and inevitable impurity elements. A controlled-forging and controlled-cooling process is adopted for production, the silicon content is reduced, the expansion fracture qualification rate is increased under the condition that the plasticity and toughness of the product are guaranteed, and the comprehensive performance of the product is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloy structural steel, and particularly relates to a non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content, a production method thereof, a fracture-split connecting rod, and a controlled forging and controlled cooling process thereof. Background Art

[0002] The connecting rod is a key part of an automobile engine. Its function is to transmit the gas force on the piston to the crankshaft and convert the reciprocating motion of the piston into the rotary motion of the crankshaft. To ensure the reliability of the engine, the connecting rod should have sufficiently high fatigue strength and stiffness. The machining process of the connecting rod is complex and has high precision requirements, especially for the big end hole of the connecting rod. The ovality of the big end hole of the connecting rod caused by machining precision error is an important factor affecting the performance reliability of the engine. In recent years, the fracture-split connecting rod technology developed abroad has solved this problem. This technology has advantages that cannot be compared with the traditional connecting rod processing technology. It has fewer processing procedures, saves finishing equipment, saves materials and energy, and significantly reduces production costs. The connecting rod materials used for fracture-split processing are mainly powder metallurgy materials, forged steel, ductile iron, and malleable cast iron. Forged steel connecting rods have high dimensional accuracy, good organizational structure and mechanical properties, and are most widely used in the traditional connecting rod manufacturing industry, especially for engines with high load and high speed, and for occasions where the connecting rod is required to have high fatigue performance and reliability.

[0003] For the 46MnVS5 material, the big end position of the connecting rod requires a lower ferrite content and impact energy (KV2≤20J) to ensure that the material has higher brittleness and avoid unqualified phenomena such as incomplete fracture and uneven fracture surface during fracture-split processing. For the connecting rod journal position, a higher ferrite content and impact energy (KV2≥30J) are required to ensure that the material has higher tensile strength and fatigue strength. Traditional 46MnVS5 is mainly in small sizes (diameter ≤ 40mm) and is mainly used in the field of passenger car engines. For commercial vehicle and large internal combustion engine fracture-split connecting rods, their sizes are mainly ≥ 60mm. Under the constraints of size effect and controlled forging and controlled cooling equipment conditions, traditional 46MnVS5 materials can no longer meet the performance requirements. Summary of the Invention

[0004] The purpose of the present invention is to provide a non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content and a production method thereof. Through composition design and process optimization, the silicon content is reduced, and under the condition of ensuring the plastic toughness of the product, the fracture-split qualification rate is improved, and the comprehensive performance of the product is enhanced.

[0005] Another purpose of the present invention is to provide a fracture-split connecting rod and a controlled forging and controlled cooling process thereof. By using the above-mentioned non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content, a large-sized fracture-split connecting rod is produced through a controlled forging and controlled cooling process, the tensile strength and the fracture-split qualification rate are improved, and the development requirements of high-performance engine fracture-split connecting rods are met.

[0006] The specific technical solution of the present invention is as follows: A non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content, comprising the following components by mass percentage: C 0.43~0.47%, Si ≤0.10%, Mn 1.60~1.80%, P 0.015~0.025%, S 0.040~0.070%, Cr 0.15~0.25%, Ni 0.15~0.25%, Al 0.020~0.035%, Mo 0.05~0.10%, V 0.45~0.95%, Nb 0.030 - 0.050%, [N] 0.0140~0.0170%, and the balance being Fe and inevitable impurity elements.

[0007] The composition of the non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content also satisfies: X = (10×Si + 15×Mn + 20×P + 50×Mo) / (Al + V + Nb + [N] / 1000) ≤ 100, where the unit of [N] is ppm and the units of other elements are wt%. When calculating, substitute the numerical values before the units into the formula for calculation.

[0008] A production method of a non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content provided by the present invention includes smelting and rolling; For the smelting, an electric furnace is used for smelting. During the LF refining process, an appropriate amount of ferrophosphorus is added to ensure that the P content in the steel is 0.015~0.025%, so as to achieve the requirement of the P content in the steel reaching the component design. After LF refining + RH vacuum degassing, it is directly continuously cast into a square billet of 380×450 mm; For the rolling, it is heated and rolled into round steel with a diameter of Φ60~100 mm. At the same time, during the rolling process of the round steel, the soaking section temperature ≥ 1200°C and the heating time ≥ 300 min; after the RH breaks the vacuum, an appropriate amount of Ca wire is fed in to achieve sulfide modification treatment, converting long strip sulfides into spindle-shaped sulfides to avoid the deterioration of the fatigue performance of the material by sulfides; The round steel with a diameter of Φ60~100 mm produced by the present invention is used to produce large-sized fracture-split connecting rods. The larger the round steel specification, the larger the part specification.

[0009] A controlled forging and controlled cooling process for a fracture-split connecting rod provided by the present invention includes: The forging induction heating temperature ≥ 1220°C and the holding time ≥ 5 min; The starting forging temperature is 1100°C~1230°C; After the starting forging heating, local strong air cooling treatment is carried out on the position of the connecting rod neck, so that the die forging temperature of the large end position of the connecting rod is 1100~1200°C, and the die forging temperature of the connecting rod neck position is 850~1030°C; After forging the connecting rod, it is treated with strong air cooling, and its cooling rate is 8 - 12 °C / s. After cooling to below 200 °C, it is treated with stacking cooling.

[0010] A split connecting rod provided by the present invention is obtained by using the above non-quenched and tempered steel for large-sized split connecting rods with low Si content.

[0011] The split connecting rod has a tensile strength of 1100 - 1250 MPa, a yield strength of 750 - 870 MPa, and an elongation after fracture of ≥10%; the room temperature impact energy (KV2) at the big end position is ≤15 J, the ferrite area ratio content is ≤20%, and the pearlite lamellar spacing is 0.35 - 0.55 μm; the room temperature impact energy (KV2) at the I-beam neck position is ≥35 J, and the ferrite area ratio content is ≥30%; the decarburized layer depth on the surface of the connecting rod is ≤100 μm, and the slag removal rate of the part during splitting is ≤5 ppm.

[0012] The design idea of the present invention is as follows: C: Element C is necessary to obtain high strength and hardness. With the increase of C content, the plasticity and toughness of the steel can be significantly reduced, thus obtaining good splitting performance. Excessive carbon content will lead to too poor toughness and too high notch sensitivity at the connecting rod neck position, resulting in low fatigue strength; too low C content is likely to lead to low strength and too good toughness, resulting in problems such as the big end position of the connecting rod not being able to split or the splitting deformation being too large. In summary, the C content is controlled at 0.43 - 0.47%.

[0013] Si: Si is a strong decarburizing element. For split connecting rods, a decarburized layer will appear on the surface during the heating and forging process of the material. During the splitting process at the big end position of the connecting rod, when the crack propagates and encounters the decarburized layer, the propagation is hindered, resulting in slag removal or uneven fracture edges at the crack edge, thus affecting the quality of the connecting rod parts. The traditional control process lies in the control of the forging heating temperature. In this application, through the design of low Si content, the lower decarburization tendency of the steel is realized, so as to control the decarburized layer and avoid the influence of the decarburized layer on the fracture quality of the split connecting rod. Therefore, the Si content ≤0.10%.

[0014] Mn: On the one hand, Mn can combine with S to form MnS, improving the cutting performance. On the other hand, it can significantly delay the pearlite-ferrite phase transformation, reduce the ferrite content, refine the pearlite spheroid and reduce the pearlite lamellar spacing, improving the strength of the steel. At the same time, the present invention adopts an ultra-low Si design. To avoid the influence of the lack of Si in the material on the strength and toughness, hardenability, etc. of the material, therefore Mn ≥1.60%. However, the Mn content should not be too high. A higher Mn content will also lead to a more serious decarburization tendency of the material. At the same time, Mn is also a strong segregation element, which will lead to more serious segregation of the material and also cause the quality of the parts to decline. Therefore, the Mn content ≤1.80%. In summary, the Mn content is controlled at 1.60% - 1.80%.

[0015] Cr: Cr can effectively improve the hardenability of steel and delay the pearlite-ferrite phase transformation to obtain the required high strength. Through solid solution strengthening, it can also significantly improve the yield ratio. At the same time, Cr can reduce the activity of C, lower the decarburization tendency on the surface of steel during heating, rolling, and forging, and is beneficial to obtaining high fatigue resistance. Therefore, Cr ≥ 0.15%. However, too high a content will deteriorate the toughness of steel. Therefore, Cr ≤ 0.25%. In summary, the Cr content is controlled at 0.15 - 0.25%.

[0016] P: Microsegregation occurs during the solidification of molten steel, and then it segregates to the grain boundaries during heating at austenite temperature, significantly increasing the brittleness of the steel, thereby reducing the plasticity and toughness of the steel. Although it can significantly improve the performance of the expanded fracture connecting rod, therefore, the P content ≥ 0.015%. However, a relatively high P content will lead to a reduction in fatigue performance, which is not conducive to the differential control of the connecting rod performance. Therefore, the P content ≤ 0.025%. In summary, the P content is controlled at 0.015 - 0.025%.

[0017] S: S forms MnS with Mn, significantly improving the cutting performance of steel. Due to the relatively high strength of the steel in this invention, to improve the cutting performance, the S content should be controlled at 0.040 - 0.070%.

[0018] Ni: It has the effect of improving the fatigue strength of steel materials. However, once the Ni content is too high, it will reduce the machinability after hot working. Therefore, the Ni content should be controlled at 0.15 - 0.25%.

[0019] Al: As an effective deoxidizing element, AlN formed by combining with N can effectively refine the austenite grain size. However, if the Al content is too high, spinel-type A1203 and AIN-type inclusions that are easy to form belong to hard and brittle inclusions, causing the decline of tundish retention and fatigue performance. Therefore, the Al content ≤ 0.035%. In summary, the Al content is controlled at 0.020 - 0.035%.

[0020] V: V forms V(C,N) precipitation phases with N and C elements in steel, which can play a role in grain refinement and precipitation strengthening, thereby improving the strength and yield ratio of steel. For this invention, through local air cooling treatment after initial forging, the performance differential treatment of the big end and the neck of the connecting rod is realized. The V-containing precipitation phase precipitates in the proeutectoid ferrite at the big end position, making the strength at the big end position high but the toughness low, which is beneficial to the expanded fracture processing. The V-containing precipitation phase precipitates in the austenite at the neck position of the connecting rod, making the strength and toughness at the neck position well-matched, which is beneficial to improving the fatigue performance and reducing the notch sensitivity. However, if the V content is too high, it is still easy to cause too much ferrite content at the big end position, shortening the forging process window and reducing the expanded fracture qualification rate. Therefore, considering both improving strength and expanded fracture performance, the V content is controlled at 0.45 - 0.95%.

[0021] Nb: The role of Nb in steel is similar to that of V. Nb forms Nb(C,N) precipitation phases with N and C elements in steel, which can refine grains and play a role in precipitation strengthening. Different from the role of V, solid-solution Nb can also significantly delay the pearlite-ferrite phase transformation. Through hot forging at the large-end position, the precipitation of Nb in austenite is avoided, and the grain refinement effect of Nb is inhibited. The strength of the large-end position is mainly improved through the precipitation strengthening effect of Nb during the pearlite-ferrite phase transformation, while the toughness is significantly reduced. For the web neck and small-end positions, through a relatively low forging temperature, the precipitation of Nb in austenite is promoted. The strength and yield ratio of the web neck position are improved through grain refinement strengthening and partial precipitation strengthening, while the toughness is improved. Therefore, considering both improving strength and fracture-expansion performance, the Nb content is controlled at 0.030 - 0.050%.

[0022] Mo: For large-sized parts, an appropriate amount of Mo can improve the hardenability of the material, promote the phase transformation of the material, cooperate with elements such as C, Mn, Nb, and V, refine the pearlite lamellar spacing, and improve the yield strength of the material. Therefore, the Mo content ≥ 0.05%. However, a relatively high Mo content is likely to lead to the formation of bainite, narrowing the differential controlled cooling process window of the parts, which is not conducive to the control of the fracture-expansion qualification rate of the parts. Therefore, the Mo content ≤ 0.010%. In summary, the Mo content is controlled at 0.05 - 0.10%.

[0023] N: N is easy to combine with Al, V, etc. to form AlN, VN, which can inhibit the growth of austenite grains in steel at high temperatures, thereby improving the fatigue performance of steel materials. However, when the N content exceeds 0.016%, especially in the steelmaking process, metallurgical defects such as bubbles are likely to occur, and a too high N content is also not conducive to mass production. Therefore, the [N] content is controlled at 0.0140 - 0.0170%.

[0024] X: To ensure that the material has good fracture-expansion performance, Si, Mn, P, and Mo are elements to improve the fracture-expansion slag-breaking performance of large-sized connecting rods, and Al, V, Nb, and [N] are elements to improve toughness, which are not conducive to fracture-expansion performance. To improve product performance, the relationship between the elements should meet the following relationship: X = (10Si + 15Mn + 20P + 50Mo) / (Al + V + Nb + [N] / 1000) ≤ 100, where the unit of [N] is ppm, and the others are wt%. Designed according to the balance of the strength and toughness and fracture-expansion performance of each element, the influence of each element on the performance is different, and the corresponding design coefficients.

[0025] Compared with the prior art, the present invention: (1) retains part of the P content by rapidly heating up during the electric furnace smelting process; (2) improves the grain boundary brittleness through the grain boundary segregation of P, thereby improving the flatness of the fracture surface of the connecting rod during the expansion breakage and reducing the slagging rate during the expansion breakage; (3) appropriately increases the Mn content on the basis of the traditional 46MnVS5, delays the pearlite-ferrite transformation temperature, refines the diameter and lamellar spacing of the pearlite spheroid, and improves the strength of the large head position of the connecting rod material; (4) adds an appropriate amount of Mo on the basis of the traditional 46MnVS5 to increase the phase transformation driving force and improve the qualified rate of the expansion breakage of the parts; (5) during the production process of the round steel, the soaking section temperature ≥ 1200 °C and the heating time ≥ 300 min to improve the homogeneity of the round steel material; (6) during the production process of the parts, the induction heating temperature ≥ 1200 °C and the heating time ≥ 5 min, thereby improving the microsegregation of the parts, enhancing the material uniformity, and avoiding the instability of cracks caused by the hard particles of the microsegregation during the expansion breakage, resulting in slagging during the expansion breakage; (7) after the parts are initially forged, local strong air cooling is adopted at the connecting rod neck position, so that the die forging temperature of the large head position of the connecting rod is 1100 - 1200 °C, and the die forging temperature of the connecting rod neck position is 850 - 1030 °C. During the forging process, the microalloying elements precipitate in the proeutectoid ferrite at the large head position of the connecting rod, thereby increasing the hardness of the ferrite and facilitating the flatness of the fracture surface of the connecting rod during the expansion breakage; at the connecting rod neck and the small head position of the connecting rod, the microalloying elements precipitate in the austenite, thereby refining the austenite grains and facilitating the improvement of the toughness of the connecting rod; (8) after the forging process of the connecting rod, strong air cooling treatment is adopted, and the cooling rate is 8 - 12 °C / s. After cooling to below 200 °C, stacking cooling treatment is adopted to eliminate the residual stress through self-tempering and avoid uneven fracture surfaces during the expansion breakage.

[0026] The present invention can produce high-performance expansion breakage connecting rods under the condition of large dimensions. The tensile strength of the product is 1100 - 1250 MPa, the yield strength is 750 - 870 MPa, and the elongation after fracture ≥ 10%; the room temperature impact energy (KV2) at the large head position ≤ 15 J, and the ferrite area ratio content ≤ 20%; the room temperature impact energy (KV2) at the I-beam neck position ≥ 35 J, and the ferrite area ratio content ≥ 30%; the decarburized layer depth on the surface of the connecting rod ≤ 100 μm, and the slagging rate of the part during the expansion breakage ≤ 5 ppm, significantly improving the product performance and meeting the development needs. Brief Description of the Drawings

[0027] Figure 1 is the microstructure of the steel in Example 1; Figure 2 is the microstructure of the steel in Example 2; Figure 3 is the microstructure of the steel in Example 3; Figure 4 is the microstructure of the steel in Comparative Example 1; Figure 5 is the microstructure of the steel in Comparative Example 2; Figure 6 The microstructure of the steel for Comparative Example 3. Specific Embodiments

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0029] Examples 1 - 3

[0030] A non - quenched and tempered steel for large - sized fracture - splitting connecting rods with low Si content, comprising the following components by mass percentage: As shown in Table 1, the balance not shown in Table 1 is Fe and inevitable impurities.

[0031] Comparative Example 4: Traditional 46MnVS5

[0032] A non - quenched and tempered steel, comprising the following components by mass percentage: As shown in Table 1, the balance not shown in Table 1 is Fe and inevitable impurities.

[0033]

[0034] The production methods of the non - quenched and tempered steels in each example are as follows: Electric furnace smelting is used for all, and rapid heating is adopted during the smelting and heating process to avoid phosphorus removal from the molten steel; an appropriate amount of ferrophosphorus is added during the LF refining process to ensure that the P content in the steel is 0.015 - 0.025%. After LF refining + RH vacuum degassing, it is directly continuously cast into a square billet of 380×450 mm, and then hot - rolled into round steel with a diameter of Φ60 - Φ100 mm. At the same time, during the round steel rolling process, the soaking section temperature ≥ 1200 °C and the heating time ≥ 300 min.

[0035] Using the non - quenched and tempered steels produced in each example to produce fracture - splitting connecting rods, including: The round steel is cut, heated in an intermediate - frequency induction furnace, and then undergoes initial forging, die forging, and trimming, and then is sent to the controlled - cooling line for controlled cooling. The controlled forging and controlled cooling process includes: The forging induction heating temperature ≥ 1220 °C and the holding time ≥ 5 min; The initial forging (starting forging) temperature is 1100 °C - 1230 °C; After initial forging heating, local strong air cooling treatment is carried out on the connecting rod neck position, so that the die - forging temperature of the large head position of the connecting rod is 1100 - 1200 °C, and the die - forging temperature of the connecting rod neck position is 850 - 1030 °C; After the connecting rod is forged and processed, strong air cooling treatment is adopted, and its cooling rate is 8 - 12 °C / s. After cooling to below 200 °C, stacking cooling treatment is adopted.

[0036] The specific controlled forging and controlled cooling processes of each example and comparative example are shown in Table 2. Comparative example 4, the traditional steel 46MnVS5, is produced and controlled forging and controlled cooling according to the traditional method of the prior art. In Table 2, comparative examples 1 - 3 are all produced according to the steel composition of Example 1, only the controlled forging and controlled cooling processes are different.

[0037]

[0038] Standard tensile, impact specimens and metallographic specimens are taken on the finished parts for mechanical property analysis and microstructural analysis. Mechanical property tests are carried out in accordance with the standards of GB / T 228.1 and GB / T 229, and microstructure tests and content statistics are carried out in accordance with the standard of GB / T 13299. The results are shown in Table 3.

[0039]

[0040] In Examples 1 - 3, under the condition that the composition and controlled forging and controlled cooling processes designed in the present invention meet the requirements of the present invention, the tensile strength is 1100 - 1250 MPa, the yield strength is 750 - 870 MPa, and the elongation after fracture is ≥10%; the room temperature impact energy (KV2) at the big head position is ≤15 J, and the ferrite area ratio content is ≤20%; the room temperature impact energy (KV2) at the I-beam neck position is ≥35 J, and the ferrite area ratio content is ≥30%; the decarburized layer depth on the surface of the connecting rod is ≤100 μm, and the part bulging and breaking slag rate is ≤5 ppm. For comparative examples 1 - 3, according to the quenched and tempered steel composition required by the present invention (the steel composition of Example 1), but the controlled forging and controlled cooling do not meet the requirements of the present invention, resulting in the product performance not meeting the requirements of the present invention.

[0041] The description of the above examples is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these examples and apply the general principles described herein to other examples without creative labor. Therefore, the present invention is not limited to the above examples, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content, characterized in that, The non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content comprises the following components by mass percentage: C 0.43~0.47%, Si ≤0.10%, Mn 1.60~1.80%, P 0.015~0.025%, S 0.040~0.070%, Cr 0.15~0.25%, Ni 0.15~0.25%, Al 0.020~0.035%, Mo 0.05~0.10%, V 0.45~0.95%, Nb 0.030 - 0.050%, [N] 0.0140~0.0170%, and the balance is Fe and inevitable impurity elements; The composition of the non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content further satisfies: X = (10×Si + 15×Mn + 20×P + 50×Mo) / (Al + V + Nb + [N] / 1000) ≤ 100, where the unit of [N] is ppm and the units of other elements are wt%; For the fracture-split connecting rod produced from the non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content, the ferrite area ratio content at the big end position ≤ 20%, and the pearlite lamellar spacing is 0.35~0.55μm; the ferrite area ratio content at the I-beam neck position ≥ 30%.

2. A production method of a non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content as described in claim 1, characterized in that, The production method of the non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content includes: smelting and rolling.

3. The production method according to claim 2, characterized in that, For the smelting, an electric furnace is used for smelting, and P iron is added during the LF refining process to ensure that the P content in the steel is 0.015~0.025%.

4. The production method according to claim 2 or 3, characterized in that, For the rolling, it is hot-rolled into a round steel with a diameter of Φ60~100mm, and meanwhile, during the round steel rolling process, the soaking section temperature ≥ 1200°C and the heating time ≥ 300min.

5. A split fracture connecting rod, characterized in that, Obtained by using the non-quenched and tempered steel for large-sized fracture-split connecting rods with low Si content as described in claim 1, the fracture-split connecting rod has a tensile strength of 1100~1250MPa, a yield strength of 750~870MPa, and an elongation after fracture ≥ 10%; the room temperature impact energy KV2 at the big end position ≤ 15J, and the room temperature impact energy KV2 at the I-beam neck position ≥ 35J; the decarburized layer depth on the surface of the connecting rod ≤ 100μm, and the slag removal rate of the part during fracture-splitting ≤ 5ppm.

6. The controlled forging and controlled cooling process of the split connecting rod according to claim 5, characterized in that, The controlled forging and controlled cooling process includes: The forging induction heating temperature ≥ 1220°C and the holding time ≥ 5min.

7. The controlled forging and controlled cooling process according to claim 6, characterized in that, The controlled forging and controlled cooling process further includes: the initial forging temperature is 1100°C~1230°C.

8. The controlled forging and controlled cooling process according to claim 6 or 7, characterized in that, The controlled forging and controlled cooling process further includes: after the initial forging heating, local strong air cooling treatment is carried out on the connecting rod neck position, so that the die forging temperature at the big end position of the connecting rod is 1100~1200°C, and the die forging temperature at the connecting rod neck position is 850~1030°C.

9. The controlled forging and controlled cooling process according to claim 8, wherein The controlled forging and controlled cooling process further includes: after the connecting rod forging process, strong air cooling treatment is adopted, and its cooling speed is 8~12°C / s. After cooling to below 200°C, stacking cooling treatment is adopted.

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