A bainite type non-quenched and tempered steel for a stretch-broken connecting rod, a stretch-broken connecting rod, and a method for manufacturing the same

Abstract

The application discloses a bainite type non-quenched and tempered steel for an expansion fracture connecting rod, the expansion fracture connecting rod and a preparation method thereof, and the chemical components of the bainite type non-quenched and tempered steel for the expansion fracture connecting rod are as follows: C, Si, Mn, S, Cr, Ti, Al, Nb, V, T.O and [N], and (10Si+15Mn+20P+50Ti) / (Al+V+Nb+3[N] / 1000) is less than or equal to 140. On the basis of medium-carbon manganese steel, the Ti-Nb-V composite micro-alloying component design is adopted, and the corresponding controlled forging and controlled cooling processes are matched, so that the preparation of the large-specification bainite structure expansion fracture connecting rod is realized, and the traditional ferrite-pearlite structure is replaced. The tensile strength of the prepared expansion fracture connecting rod is greater than or equal to 1350 MPa, and the high-strength, long-service-life and light-weight development requirements of the diesel engine expansion fracture connecting rod of the national VII emission standard are met.

Description

Technical Field

[0001] This invention belongs to the field of alloy structural steel technology, specifically relating to a bainitic non-quenched and tempered steel for fracture-expanding connecting rods, fracture-expanding connecting rods and their preparation methods. Background Technology

[0002] Connecting rods are crucial components in automotive engines. They connect the piston and crankshaft, transmitting gas pressure from the piston to the crankshaft, converting the piston's reciprocating motion into the crankshaft's rotational motion. Therefore, connecting rods continuously bear alternating loads of compression and tension. To ensure high engine reliability and low vibration and noise, connecting rods require sufficient strength, rigidity, and high concentricity. Fracture machining involves machining fracture grooves inside the connecting rod's large end bore. Utilizing notch sensitivity, pressure is applied to open and rapidly expand the fracture grooves, achieving separation of the connecting rod body from the connecting rod cap. This reduces processing steps and significantly lowers production costs. Furthermore, compared to tempering and cutting connecting rods, it greatly improves connecting rod concentricity and reduces vibration during engine operation.

[0003] Fracture-extension processing technology requires that the material undergoes almost no plastic deformation after fracture. This means that while ensuring overall performance, the toughness of the connecting rod's big end is limited, resulting in a brittle fracture characteristic. This ensures the parallelism of the parts and further guarantees low vibration and low noise during engine operation. For fracture-extension connecting rods, high strength and toughness are required from a product use perspective, while easy fracture is required from a manufacturing perspective. Non-quenched and tempered steel, with its adjustable properties in different parts of the part, is widely used in fracture-extension connecting rods.

[0004] Traditional non-quenched and tempered steels used for fracture-extension connecting rods are ferritic-pearlitic type, with high mechanical properties achieved by adding certain amounts of microalloying elements (Nb, V). However, for diesel engines meeting the China VII emission standard, large-diameter fracture-extension connecting rods (≥20kg) require a strength of ≥1250MPa. Ferritic-pearlitic non-quenched and tempered steels can generally only achieve 1000MPa. Further increasing the strength leads to a sharp decrease in ductility and toughness at the connecting rod diameter and a sharp increase in notch sensitivity, resulting in the connecting rod's fatigue performance failing to meet requirements.

[0005] Chinese patent CN 117259662 A discloses a controlled forging and controlled cooling process for a medium-carbon microalloyed non-quenched and tempered fracture-resistant connecting rod. The steel has a tensile strength of 1080~1163MPa, a yield strength of 770~840MPa, an elongation after fracture ≥14%, a reduction of area ≤42%, and a Brinell hardness ≥313HB. Its microstructure is ferrite + pearlite. Its strength is relatively low, and when manufacturing large-size fracture-resistant connecting rods, the strength cannot meet the ≥1250MPa requirement of diesel engines that meet the China VII emission standard.

[0006] Chinese patent CN 111286670A discloses medium-carbon non-quenched and tempered steel and its manufacturing process, as well as connecting rods and their preparation process. This steel grade, based on traditional non-quenched and tempered steel, has added phosphorus (P) and copper (Cu) to increase its brittleness. Using this patented method, a medium-carbon non-quenched and tempered steel connecting rod is obtained with a yield strength ≥750MPa, tensile strength ≥1000MPa, elongation after fracture ≥8%, and reduction of area ≥25%. This patent improves fracture resistance by increasing the brittleness of the steel through the addition of P and Cu. However, similar to the aforementioned patent, its microstructure is ferrite + pearlite. When manufacturing large-size fracture-resistant connecting rods, the strength is relatively low and cannot meet the ≥1250MPa requirement of diesel engines meeting the China VII emission standard.

[0007] Chinese patent CN 115058655 A discloses a non-quenched and tempered steel for Nb microalloyed medium-carbon fracture-resistant connecting rods, the resulting fracture-resistant connecting rods, and the controlled forging and cooling process. This product achieves a tensile strength of 1100~1320MPa, a yield strength of 750~850MPa, an elongation at fracture ≥12%, a fatigue strength ≥590MPa, and a Brinell hardness ≥330HB. Similar to the aforementioned patent, its microstructure is ferrite + pearlite. However, when manufacturing large-size fracture-resistant connecting rods, its strength is relatively low and cannot meet the ≥1250MPa requirement of diesel engines meeting the China VII emission standard.

[0008] Bainitic non-quenched and tempered steel has a good strength-toughness match because it combines the toughness of medium-temperature transformation and the strength of low-temperature transformation. However, due to its relatively good toughness, it is difficult to fracture the fractured link. There are no published patents or reports of its mass application in fractured link. Summary of the Invention

[0009] To solve the above-mentioned technical problems, the present invention provides a bainitic non-quenched and tempered steel for fracture-extension connecting rods, which can be used to manufacture large-size fracture-extension connecting rods with round steel specifications of φ60~φ100mm and a strength of ≥1350MPa, thereby meeting the development requirements of high strength, long service life and lightweight fracture-extension connecting rods for diesel engines that meet the China VII emission standards.

[0010] The present invention also aims to provide a fracture-resistant connecting rod produced using bainitic non-quenched and tempered steel, which has excellent strength, toughness and fatigue resistance.

[0011] The present invention also aims to provide a method for preparing a fracture-resistant connecting rod, which can produce a fracture-resistant connecting rod with high strength and toughness.

[0012] The technical solution adopted in this invention is as follows:

[0013] This invention provides a bainitic non-quenched and tempered steel for fracture-resistant connecting rods. The chemical composition and weight percentage of the bainitic non-quenched and tempered steel for fracture-resistant connecting rods are as follows: C: 0.36~0.40%, Si: 0.50~0.90%, Mn: 1.35~1.75%, P: ≤0.010%, S: 0.030~0.040%, Cr: 0.45~0.55%, Ti: 0.030~0.050%, Al: 0.015~0.050%, Nb: 0.035~0.045%, V: 0.10~0.15%, TO: ≤15ppm, [N]: 30~60ppm, with the remainder being Fe and unavoidable impurity elements.

[0014] The chemical composition of the bainitic non-quenched and tempered steel used for the fracture-breaking connecting rod satisfies: X = (10Si + 15Mn + 20P + 50Ti) / (Al + V + Nb + 3[N] / 1000), X ≤ 140, where [N] is in ppm and the units of other chemical components are in ppm.

[0015] The present invention also provides a fracture-resistant connecting rod manufactured using bainitic non-quenched and tempered steel, wherein the metallographic structure of the fracture-resistant connecting rod is bainite and retained austenite; wherein, the grain size at the I-neck position of the fracture-resistant connecting rod is ≥ grade 8, the bainite lath bundle size is ≤ 5 μm, the volume fraction of retained austenite is 5%-10%, and the weight percentage of carbon in the retained austenite is ≥ 1.2%; the grain size at the big end position of the fracture-resistant connecting rod is ≤ grade 7, the bainite lath bundle size is ≥ 8 μm, and the weight percentage of carbon in the retained austenite is ≤ 5%.

[0016] The tensile strength of the fracture-resistant connecting rod is 1350~1450MPa, the yield strength is 950~1150MPa, the elongation after fracture is ≥8%, the reduction of area is ≥20%, the room temperature impact energy KU2 at the big end is ≤25J, the room temperature impact energy KU2 at the I-neck position is ≥50J, and the fatigue strength is ≥700MPa.

[0017] The present invention also provides a method for preparing the fracture-expanding connecting rod, the method comprising the following steps: hot-rolled steel billet heating -- descaling -- roll forging -- die forging -- edge trimming -- controlled cooling -- offline stacking cooling -- laser grooving -- fracture expansion.

[0018] In the hot-rolled steel billet heating step, the forging heating temperature is ≥1220℃, and after descaling, the surface temperature is ≥1120℃.

[0019] In the roll forging step, the ratio of the cross-sectional area at the large end to the cross-sectional area of ​​the raw material is controlled to be ≥0.8, and the ratio of the cross-sectional area at the I-neck and small end to the cross-sectional area of ​​the raw material is ≤0.5.

[0020] On the conveyor belt from roll forging to die forging, strong air cooling is used, with the center of the air blower aligned with 1 / 2 of the connecting rod diameter, so that the cooling rate at the big end is ≤5℃ / s and the cooling rate at the connecting rod I-neck is ≥10℃ / s, ensuring that during die forging, the temperature at the big end is ≥1100℃ and the temperature at the connecting rod I-neck is ≤1000℃.

[0021] In the controlled cooling step, the upper cooling line temperature is ≥850℃, the cooling rate is 5~7℃ / s, the large end is cooled to 530~580℃, the H-neck is cooled to 500-550℃ and then air-cooled to 200℃ before being removed from the cooling line for stacking cooling. This allows for self-tempering through residual heat under the condition of fully obtaining bainite, eliminating stress in the parts and obtaining more stable residual austenite.

[0022] In the laser grooving step, the laser cutting process power is 1000~1200W. After grooving, water spray cooling is used. Through high laser power and rapid cooling, the depth of the heat-affected zone of the laser grooving is strictly controlled to be ≤0.5mm to avoid the problem of not being able to expand due to the good toughness of the heat-affected zone.

[0023] The production method of the hot-rolled steel billet is as follows: molten steel with the chemical composition is obtained by smelting, and then continuously cast and hot-rolled; in the hot rolling step, the soaking temperature is ≥1220℃, the heating time is ≥1.5×H min, where H is the diameter of the continuously cast billet in mm; the initial rolling temperature is ≥1100℃, the final rolling temperature is 850-900℃, and slow cooling is performed after rolling.

[0024] The bainitic non-quenched and tempered steel for fracture-resistant connecting rods provided by this invention has the following functions and controls for each component:

[0025] Carbon (C): Carbon is essential for achieving high strength and hardness. As the C content increases, it significantly reduces the plasticity and toughness of the steel, resulting in good fracture resistance. Excessive carbon content leads to poor toughness and high notch sensitivity at the connecting rod's I-neck, resulting in low fatigue strength. Conversely, insufficient C content leads to low strength and excessive toughness, causing problems such as incomplete expansion at the connecting rod's big end or excessive fracture deformation. Therefore, the C content should be controlled between 0.36% and 0.40%.

[0026] Si (Si): Si is a strong oxidizing element in steel, which can increase the activity of carbon (C) and effectively inhibit the coarsening of carbides in steel, thus improving the yield strength and yield strength ratio. Furthermore, Si can improve the hardenability of steel and lower the bainitic transformation initiation temperature, thereby increasing the forging process window of parts and improving the consistency of part performance. Therefore, the Si content should be ≥0.50%; however, a higher Si content can easily lower the bainitic transformation termination temperature of steel, deteriorating the impact energy and toughness. Therefore, the Si content should be ≤0.90%. Thus, the Si content range is determined to be 0.50~0.90%.

[0027] Mn: Mn is a bainite-forming element, which can expand the bainite transformation region. Mn is also an austenite-forming element, which is beneficial for improving the strength and toughness of steel. Therefore, the Mn content should be ≥1.35%. However, Mn is a segregating element, and higher Mn content can easily lead to the formation of coarse, blocky austenite in the steel. This not only hinders the consistency of part performance but also reduces the yield strength ratio and worsens the fatigue performance of the steel. Therefore, the Mn content should be ≤1.75%. Thus, the Mn content range is determined to be 1.35~1.75%.

[0028] Cr: Cr effectively improves the hardenability of steel and delays the pearlite-ferrite phase transformation to obtain the required high strength, and can also significantly improve the yield strength ratio through solid solution strengthening. Simultaneously, Cr can reduce the activity of C, thereby reducing the tendency of steel surface decarburization during heating, rolling, and forging, which is beneficial for obtaining high fatigue resistance. Therefore, Cr ≥ 0.45%. However, excessive content will deteriorate the toughness of steel; therefore, Cr ≤ 0.55%. In summary, the Cr content should be controlled between 0.45% and 0.55%.

[0029] Sulfur (S) readily combines with manganese (MnS) in steel to form MnS inclusions, causing hot brittleness. However, adding a small amount of S, without affecting product performance, allows the appropriate amount of MnS inclusions to liquefy during cutting, resulting in fragmented cutting and preventing iron filings from scratching the tool surface, thus improving the material's machinability. Therefore, the S content should be ≥0.030%. However, higher S content can easily lead to the deterioration of sulfide morphology in steel, which in turn worsens the steel's machinability. Therefore, the S content should be ≤0.040%. Thus, the S content range is determined to be 0.030~0.040%.

[0030] P: P is an element with a strong tendency to segregate, increasing the cold brittleness of steel, reducing plasticity, and being detrimental to the uniformity of product microstructure and properties. P should be controlled to ≤0.010%.

[0031] Al: As an effective deoxidizing element, Al, through its combination with N to form AlN, can effectively refine the austenite grain size. However, excessive Al content easily leads to the formation of Al2O3 spinel and AlN inclusions, which are hard and brittle, causing a decrease in the sprue's storage and fatigue performance. Therefore, the Al content should be ≤0.050%. In summary, the Al content should be controlled between 0.015% and 0.050%.

[0032] Ti readily combines with C and N in steel to form Ti(C,N) precipitates, which are beneficial for improving the strength and toughness of the material. In this patent, during the post-forging air-cooling process, at the big end of the connecting rod, due to the higher temperature, the kinetics for second-phase particle precipitation are insufficient. At this time, the strengthening of the part mainly relies on microstructure strengthening and grain strengthening, with lower impact energy, resulting in better fracture resistance. At the I-neck of the connecting rod, due to the lower temperature and larger deformation, deformation-induced precipitation of second-phase particles is promoted. Furthermore, the cooling rate is faster at this time, and the bainitic phase transformation kinetics are more sufficient. At this time, the strengthening of the part relies on microstructure strengthening, grain strengthening, and second-phase strengthening, with higher impact energy, thus achieving better fatigue and mechanical properties. Therefore, the Ti content is ≥0.030%. However, a high Ti content in the steel will form TiN inclusions, resulting in large, sharp inclusions in the steel, leading to a decrease in the fatigue performance of the part. Therefore, the Ti content is ≤0.050%. In summary, the Ti content range is determined to be 0.030~0.050%.

[0033] Nb: Nb combines with N and C elements in steel to form Nb(C,N) precipitates, which can refine grains and strengthen the structure. In this patent, during the post-forging air cooling process, at the large end, the dissolved Nb can significantly inhibit carbon diffusion, thus refining the bainitic structure. At the connecting rod I-neck, Nb and Ti combine to precipitate (Nb,Ti)(C,N) precipitates, increasing the precipitate content and appropriately increasing the precipitate size, thereby improving the crack propagation resistance and fatigue performance of the connecting rod parts. Therefore, the Nb content is ≥0.035%. However, a high Nb content can lead to Nb-containing solid inclusions in the steel, resulting in early fatigue failure. Therefore, the Nb content is ≤0.045%. In summary, the Nb content range is determined to be 0.035~0.045%.

[0034] V: V forms V(C,N) precipitates with N and C elements in steel, which can refine grains and strengthen the steel, thereby improving its strength and yield strength ratio. In this invention, air cooling after forging achieves differentiated performance between the connecting rod big end and the connecting rod I-neck. Due to the large size of the connecting rod big end, the cooling rate is relatively slow under certain conditions, and the forging temperature is relatively high. V(C,N) is almost solid-dissolved, losing its grain boundary pinning effect. This results in high strength but low toughness at the big end, which is beneficial for fracture processing. For the connecting rod I-neck and small end, due to their smaller size, the cooling rate is relatively fast, and the forging temperature is relatively low. Second-phase V(C,N) particles precipitate, effectively pinning grain boundaries and preventing grain growth, thus refining the grains. This results in a good strength-toughness match at the connecting rod I-neck, which is beneficial for improving fatigue performance and reducing notch sensitivity. Therefore, the V content is ≥0.10%. However, a higher V content leads to higher costs and a greater tendency for corner cracking in the cast billet; therefore, the V content is ≤0.15%. In summary, the V content should be controlled at 0.10~0.15%.

[0035] [N]: [N] can form compounds with Nb, V, Ti, Al, etc., refining the grain size. A reasonable Al / [N] ratio has a significant effect on grain refinement, while excessive [N] can lead to defects such as bubbles and TiN inclusions. Therefore, the [N] content should be controlled between 30-60 ppm.

[0036] TO: TO forms oxide inclusions in steel, and the TO content should be controlled to be ≤15ppm.

[0037] To ensure good fracture resistance, Si, Mn, P, and Ti are elements that improve the slag shedding performance of large-size connecting rods during fracture, while Al, V, Nb, and [N] are elements that improve toughness but are detrimental to fracture resistance. To improve product performance, the relationship between these elements should conform to the following: X = (10Si + 15Mn + 20P + 50Ti) / (Al + V + Nb + 3[N] / 1000) ≤ 140. The physical basis for this is that fracture requires the connecting rod to exhibit brittle dissociation as the primary fracture surface during disintegration, avoiding slag shedding caused by plastic tearing. In the formula, Si, Mn, P, and Ti all reduce material toughness and promote brittle fracture within the range of the invention's composition, hence their relatively large coefficients. Among them, P is extremely sensitive to toughness impairment, with a coefficient of 20. Al, V, Nb, and N mainly form nitrides or carbonitrides to refine grains and improve toughness, which is not conducive to expansion fracture, so they are placed in the denominator and have relatively small coefficients. The coefficients for Al, V, and Nb are 1, while N is included as 3[N] / 1000 due to unit conversion and consideration of its nitride-forming ability. In the above formula, Nb and V are both placed in the denominator with a coefficient of 1. The deeper physical meaning of this is that both improve material toughness through precipitation strengthening and grain refinement strengthening, thereby controlling expansion fracture behavior from the "toughness end". However, the synergy between the two is not simply a superposition of toughness, but a complementary strengthening based on the spatial-temperature-time dimensions. Specifically, it manifests as complementary precipitation timing, forming full-process pinning. V is almost solid-dissolved during the high-temperature stage of forging, but during the subsequent air cooling process, due to its relatively low precipitation temperature, it mainly precipitates in the ferrite region. In thin-walled, rapidly cooling areas such as the connecting rod I-neck and small end, V(C,N) can disperse and precipitate in time, effectively pinning grain boundaries and refining grains, ensuring that these high-stress service areas achieve a good strength-toughness match and improving fatigue life. The precipitation temperature of Nb is much higher than that of V. It precipitates in large quantities as Nb(C,N) in the early stage of post-forging air cooling (high-temperature stage). Especially in the thick cross-section of the large end, solid-dissolved Nb significantly inhibits high-temperature carbon diffusion and forcibly refines the bainite lath bundles, thereby avoiding the formation of coarse and excessively tough microstructures in the large end due to slow cooling rate. The "high-low combination" of the two in the precipitation temperature sequence allows for microstructural regulation of the entire connecting rod at different cooling rates: V ensures toughness reserves in the rapid cooling zone, while Nb forces brittleness in the slow cooling zone, jointly achieving differentiated performance goals such as easy fracture at the large end and fatigue resistance at the I-neck and fillet. In key fatigue regions such as the I-neck of the connecting rod, Nb and Ti combine to form (Nb, Ti)(C, N), while V precipitates independently or in combination. Both increase the number of precipitated phases and optimize their size distribution, improving the material's resistance to crack propagation and fatigue limit. The coefficients are relative influence weights derived from extensive fracture tests and regression analysis of connecting rod steel. X≤140 indicates that the embrittlement effect is sufficient to suppress toughness, resulting in a smooth fracture surface without slag shedding; conversely, if X>140, the toughening element is relatively too strong, and large-diameter connecting rods are more prone to ductile tearing and slag shedding defects due to segregation and slow cooling.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] Based on medium-carbon manganese steel, this invention adopts a Ti-Nb-V composite microalloying composition design and matches it with a suitable controlled forging and cooling process to realize the preparation of large-size bainitic fracture-resistant connecting rods, replacing the traditional ferrite-pearlite structure. The tensile strength of the prepared fracture-resistant connecting rod is ≥1350MPa, which meets the high strength, long service life and lightweight development requirements of diesel engine fracture-resistant connecting rods in the National VII emission standard.

[0040] In the method for preparing the fracture-resistant connecting rod provided by this invention, differentiated controlled forging and cooling processes at different locations significantly refine the grains and microstructure at the I-neck of the connecting rod. Specifically, the grain size is ≥8, the bainite lath bundle size is ≤5μm, and 5%-10% of highly stable retained austenite is introduced into the microstructure, with a carbon content ≥1.2%. This achieves high strength, high toughness, and high microstructure stability at the connecting rod diameter, preventing deformation due to retained austenite transformation during subsequent use and reducing crack sensitivity. At the big end of the connecting rod, the grains and microstructure are significantly coarsened, with a grain size ≤7, a bainite lath bundle size ≥8μm, and the retained austenite content strictly limited to ≤5%, thus limiting the toughness at the big end. Attached Figure Description

[0041] Figure 1 The image shows the metallographic structure of the fractured connecting rod in Example 1. The left image shows the metallographic structure at the large end position, and the right image shows the metallographic structure at the I-neck position.

[0042] Figure 2 The image shows the metallographic structure of the fractured connecting rod in Example 2. The left image shows the metallographic structure at the large end position, and the right image shows the metallographic structure at the I-neck position.

[0043] Figure 3 The image shows the metallographic structure of the fractured connecting rod in Example 3. The left image shows the metallographic structure at the large end position, and the right image shows the metallographic structure at the I-neck position.

[0044] Figure 4 The image shows the metallographic structure of the fractured connecting rod in Comparative Example 1. The left image shows the metallographic structure at the large end position, and the right image shows the metallographic structure at the I-neck position.

[0045] Figure 5 The following is a metallographic diagram of the fractured connecting rod in Comparative Example 2. The left diagram shows the metallographic structure at the large end position, and the right diagram shows the metallographic structure at the I-neck position.

[0046] Figure 6 The following is a metallographic diagram of the fractured connecting rod in Comparative Example 3. The left diagram shows the metallographic structure at the large end position, and the right diagram shows the metallographic structure at the I-neck position.

[0047] Figure 7 The image shows the metallographic structure of the fractured connecting rod in Comparative Example 4. The left image shows the metallographic structure at the large end position, and the right image shows the metallographic structure at the I-neck position. Detailed Implementation

[0048] This invention provides a bainitic non-quenched and tempered steel for fracture-resistant connecting rods. The chemical composition and weight percentage of the bainitic non-quenched and tempered steel for fracture-resistant connecting rods are as follows: C: 0.36~0.40%, Si: 0.50~0.90%, Mn: 1.35~1.75%, P: ≤0.010%, S: 0.030~0.040%, Cr: 0.45~0.55%, Ti: 0.030~0.050%, Al: 0.015~0.050%, Nb: 0.035~0.045%, V: 0.10~0.15%, TO: 8~12ppm, [N]: 30~60ppm, with the remainder being Fe and unavoidable impurity elements.

[0049] The chemical composition of the bainitic non-quenched and tempered steel used for the fracture-breaking connecting rod satisfies: X = (10Si + 15Mn + 20P + 50Ti) / (Al + V + Nb + 3[N] / 1000), X ≤ 140, where [N] is in ppm and the units of other chemical components are in ppm.

[0050] The preparation method of the bainitic non-quenched and tempered steel for the fracture-resistant connecting rod includes the following steps: electric furnace / converter smelting -- LF ladle refining -- RH / VD vacuum degassing -- continuous casting -- heating -- rolling -- finishing and flaw detection -- packaging and delivery.

[0051] 1) During the electric furnace or converter smelting process, the decarburization operation is not carried out. Instead, the temperature is directly raised to carry out the decarburization operation. The carbon content of the tapped steel is ≥0.06%-0.35% to avoid over-oxidation of the molten steel. The tapped steel should be kept as high as possible with a final carbon content of ≤0.030%.

[0052] 2) The basicity of the refining slag in the LF furnace is controlled at 1.5~3, with medium to low basicity. The white slag is maintained for ≥30 minutes. Except for necessary component adjustments, the entire process is carried out with medium to weak stirring and slight positive pressure operation. The diameter of the bright slag surface is maintained at 300-500 mm. Excessive stirring should be avoided to prevent excessive desulfurization and oxidation of molten steel. Soft blowing is performed for 8-15 minutes before the LF furnace leaves the station, with slight movement of the slag surface being appropriate.

[0053] 3) RH / VD degassing time ≥20min, wherein, in the last 2min of RH, the vacuum degree is reduced to 200Pa and the boosting gas is changed to nitrogen. Argon is used for stirring throughout the VD process. After RH or VD is completed, Ca treatment is performed. After RH / VD is broken, the S is first tempered to the target value. After the S content is uniform and stable, Ca treatment is performed. After Ca treatment, the Ca content in the molten steel is 6-15ppm.

[0054] 4) The superheat of continuous casting is 25-50℃. The continuous casting billet adopts slow cooling in the pit. The temperature in the pit is ≥500℃ and the temperature when it comes out of the pit is ≤150℃ to ensure that hydrogen is fully released and stress is fully removed, so as to avoid cracking.

[0055] 5) The high-temperature diffusion process is adopted for steel rolling heating to improve the uniformity of the material and reduce the slag shedding rate during the expansion and fracture process. The temperature of the soaking zone is ≥1220℃ and the heating time is ≥1.5Hmin, where H is the diameter of the continuous casting billet in mm. The initial rolling temperature is ≥1100℃ and the final rolling temperature is 850-900℃. After rolling, the steel is cooled with an insulation cover on the cooling bed. After the temperature of the lower cooling bed is ≥300℃, the steel is slowly cooled in the pit or directly in the pit after rolling. The temperature of the steel exiting the pit is ≤150℃ to ensure that hydrogen in the steel is fully released.

[0056] The method for producing fracture-resistant connecting rods using bainitic non-quenched and tempered steel includes the following steps: hot-rolled steel billet medium-frequency induction heating -- high-pressure water descaling -- roll forging -- die forging -- edge trimming -- controlled cooling line -- offline stacking cooling -- laser grooving -- fracture, specifically:

[0057] 1) Forging heating temperature ≥1220℃, and surface temperature ≥1120℃ after descaling;

[0058] 2) Through roll forging, the cross-sectional area of ​​the large end position / the cross-sectional area of ​​the raw material is ≥0.8, and the cross-sectional area of ​​the connecting rod I-neck and the small end position / the cross-sectional area of ​​the raw material is ≤0.5;

[0059] 3) On the conveyor belt from roll forging to die forging, strong air cooling is used, with the center of the air blower aligned with 1 / 2 of the connecting rod diameter, so that the cooling rate at the big end is ≤5℃ / s and the cooling rate at the connecting rod I-neck is ≥10℃ / s, ensuring that during die forging, the temperature at the big end is ≥1100℃ and the temperature at the connecting rod I-neck is ≤1000℃.

[0060] 4) After the die forging and trimming are completed, the upper controlled cooling line is subjected to strong air cooling. The temperature of the upper cooling line is ≥850℃, and the cooling rate is 5~7℃ / s. The big end is cooled to 530~580℃, and the connecting rod I-neck is cooled to 500-550℃ and then air-cooled to 200℃ before being removed from the line for stacking cooling. This allows for self-tempering through residual heat under the condition of fully obtaining bainite, eliminating stress in the parts and obtaining more stable retained austenite.

[0061] 5) When laser grooving is performed before the connecting rod breaks, the laser cutting power is 1000~1200W. After grooving, water spray cooling is used. By using high laser power and rapid cooling, the depth of the heat-affected zone of the laser grooving is strictly controlled to be ≤0.5mm to avoid the problem of not expanding due to the good toughness of the heat-affected zone.

[0062] The present invention will now be described in detail with reference to the embodiments.

[0063] The chemical composition and weight percentage of the non-quenched and tempered steel in each embodiment and comparative example are shown in Table 1. The unit of [N] is ppm, and the units of other chemical components are ppm.

[0064] Table 1

[0065]

[0066] The hot rolling process parameters of the non-quenched and tempered steel in each embodiment and comparative example are shown in Table 2. In each embodiment and comparative example, the diameter of the continuous casting billet is 250 mm.

[0067] Table 2

[0068]

[0069] The non-quenched and tempered steel in each embodiment and comparative example was processed into fracture-expanding connecting rods according to the following process: medium frequency induction heating -- high pressure water descaling -- roll forging -- die forging -- edge trimming -- controlled cooling line -- offline stacking cooling -- laser grooving -- fracture expansion. The main process parameters are shown in Table 3.

[0070] Table 3

[0071]

[0072] The metallographic structure of the fractured connecting rods in each embodiment and comparative example is shown in Table 4.

[0073] Table 4

[0074]

[0075] The performance of the fracture-resistant connecting rods in each embodiment and comparative example is shown in Table 5.

[0076] Table 5

[0077]

[0078] Figures 1-6 Metallographic images of the connecting rod big end and I-neck positions in Examples 1-3 and Comparative Examples 1-4 are shown below. Figures 1-3As can be seen from the examples, through differentiated controlled forging and cooling processes, significant coarsening microstructure was obtained at the big end of the connecting rod, with grain size ≤ 7, bainite lath bundle size ≥ 8 μm, and retained austenite content strictly controlled to within 5%. The microstructure was dominated by coarse bainite, effectively limiting the toughness of the big end. At the same time, a significantly refined microstructure was obtained at the I-neck, with grain size ≥ 8, bainite lath bundle size ≤ 5 μm, and 5%~10% of highly stable retained austenite (carbon content in retained austenite ≥ 1.2%) evenly distributed in the microstructure. The lath bainite was finely interwoven, and the microstructure was uniform and dense, thus ensuring the synergistic matching of high strength, high toughness and microstructure stability at this location.

[0079] from Figures 4-6 As can be seen from the data, Comparative Examples 1-3, which did not employ the controlled forging and cooling process of this invention, showed little difference in microstructure between the large end and the I-neck, resulting in poor overall microstructure uniformity. Specifically, the I-neck exhibited coarse grains (generally ≤7 grain size), bainite lath bundle size ≥8μm, and low retained austenite content (≤3%) with insufficient stability, failing to meet the fine-grain strengthening and phase transformation strengthening effects required for high fatigue strength. Conversely, the large end showed excessively fine microstructure (locally ≥8 grain size) or excessive retained austenite content (>5%), leading to higher impact toughness at the large end, which was detrimental to the controllability of brittle fracture during the expansion fracture process. The metallographic images clearly show significant microstructure inhomogeneity and a mixture of coarse and fine bainite lath bundles, indicating that the overall microstructure failed to achieve the differentiated design goal of low toughness at the large end and high toughness at the I-neck.

[0080] As can be seen from Table 5, the fracture-resistant connecting rods in Examples 1 to 3 meet the following performance requirements: tensile strength 1350~1450MPa, yield strength 950~1150MPa, elongation after fracture ≥8%; reduction of area ≥20%, room temperature impact energy (KU2) at the big end ≤25J, room temperature impact energy (KU2) at the I-neck ≥50J, and fatigue strength ≥700MPa.

[0081] Although the steel composition of Comparative Examples 1 to 3 is the same as that of Example 1, the controlled forging and cooling process of the present invention was not used in the preparation of the expansion fracture connecting rod, resulting in a significantly higher impact energy at the big end of the connecting rod and a significantly lower impact energy at the I-neck, which is not conducive to the controllability of brittle fracture in the expansion fracture process, and the bending fatigue strength of the connecting rod is also poor.

[0082] In Comparative Example 4, the traditional 46MnVS5 steel composition was used. The controlled forging and cooling process of this invention was not employed in the fabrication of the fracture-resistant connecting rod. The impact energy at the big end and the neck of the connecting rod was almost identical, and the overall process could not meet the performance requirements of large-size fracture-resistant connecting rods. In summary, the bainitic non-quenched and tempered steel obtained by this invention through reasonable composition design and its controlled forging and cooling process not only meets the high-performance requirements of engine fracture-resistant connecting rods but also fills the market gap for large-size fracture-resistant connecting rods required for diesel engines meeting the China VII emission standard.

[0083] The above detailed description of a bainitic non-quenched and tempered steel for a fracture-breaking connecting rod, the fracture-breaking connecting rod and its preparation method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, variations and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A bainitic non-quenched and tempered steel for fracture-resistant connecting rods, characterized in that, The chemical composition and weight percentage of the bainitic non-quenched and tempered steel used for the fracture-resistant connecting rod are as follows: C: 0.36~0.40%, Si: 0.50~0.90%, Mn: 1.35~1.75%, P: ≤0.010%, S: 0.030~0.040%, Cr: 0.45~0.55%, Ti: 0.030~0.050%, Al: 0.015~0.050%, Nb: 0.035~0.045%, V: 0.10~0.15%, TO: ≤15ppm, [N]: 30~60ppm, with the remainder being Fe and unavoidable impurity elements; The chemical composition of the bainitic non-quenched and tempered steel used for the fracture-breaking connecting rod satisfies: X = (10Si + 15Mn + 20P + 50Ti) / (Al + V + Nb + 3[N] / 1000), X ≤ 140, where [N] is in ppm and the units of other chemical components are in ppm.

2. A fracture-expanding connecting rod manufactured using the bainitic non-quenched and tempered steel for fracture-expanding connecting rods as described in claim 1, characterized in that, The metallographic structure of the fracture-resistant connecting rod consists of bainite and retained austenite; wherein, the grain size at the I-neck position of the fracture-resistant connecting rod is ≥8, the bainite lath bundle size is ≤5μm, the volume fraction of retained austenite is 5%-10%, and the weight percentage of carbon in the retained austenite is ≥1.2%; the grain size at the big end position of the fracture-resistant connecting rod is ≤7, the bainite lath bundle size is ≥8μm, and the weight percentage of carbon in the retained austenite is ≤5%.

3. The fracture-resistant connecting rod according to claim 2, characterized in that, The tensile strength of the fracture-resistant connecting rod is 1350~1450MPa, the yield strength is 950~1150MPa, the elongation after fracture is ≥8%, the reduction of area is ≥20%, the room temperature impact energy KU2 at the big end is ≤25J, the room temperature impact energy KU2 at the I-neck position is ≥50J, and the fatigue strength is ≥700MPa.

4. The method for preparing the fracture-resistant connecting rod as described in claim 2 or 3, characterized in that, The preparation method includes the following steps: hot-rolled steel billet heating -- descaling -- roll forging -- die forging -- edge trimming -- controlled cooling -- offline stacking cooling -- laser grooving -- expansion fracture.

5. The preparation method according to claim 4, characterized in that, In the hot-rolled steel billet heating step, the forging heating temperature is ≥1220℃, and after descaling, the surface temperature is ≥1120℃.

6. The preparation method according to claim 4, characterized in that, In the roll forging step, the ratio of the cross-sectional area at the large end to the cross-sectional area of ​​the raw material is controlled to be ≥0.8, and the ratio of the cross-sectional area at the I-neck and small end to the cross-sectional area of ​​the raw material is ≤0.

5.

7. The preparation method according to claim 4, characterized in that, On the conveyor belt from roll forging to die forging, strong air cooling is used, with the center of the air blower aligned with 1 / 2 of the connecting rod diameter, so that the cooling rate at the big end is ≤5℃ / s and the cooling rate at the connecting rod neck is ≥10℃ / s. During die forging, the temperature at the big end is ≥1100℃ and the temperature at the connecting rod neck is ≤1000℃.

8. The preparation method according to claim 4, characterized in that, In the controlled cooling step, the upper cooling line temperature is ≥850℃, the cooling rate is 5~7℃ / s, the large end is cooled to 530~580℃, the I-neck is cooled to 500-550℃ and then air-cooled to 200℃ before being removed from the line for stack cooling.

9. The preparation method according to claim 4, characterized in that, In the laser grooving step, the laser cutting process power is 1000~1200W, and water spray cooling is used after grooving is completed.

10. The preparation method according to claim 4, characterized in that, The production method of the hot-rolled steel billet is as follows: molten steel with the chemical composition is obtained by smelting, and then continuously cast and hot-rolled; in the hot rolling step, the soaking temperature is ≥1220℃, the heating time is ≥1.5×H min, where H is the diameter of the continuously cast billet in mm; the initial rolling temperature is ≥1100℃, the final rolling temperature is 850-900℃, and slow cooling is performed after rolling.

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