Medium-carbon non-quenched and tempered steel expansion breaking connecting rod and manufacturing method thereof

Abstract

The invention relates to a medium-carbon non-quenched and tempered steel expansion breaking connecting rod and a manufacturing method thereof. The connecting rod comprises the following chemical components in percentage by weight: 0.50 to 0.56 percent of C, 0.75 to 0.85 percent of Si, 0.75 to 0.90 percent of Mn, 0.045 to 0.080 percent of P, 0.030 to 0.045 percent of S, 0.10 to 0.20 percent of Cr, less than or equal to 0.08 percent of Ni, 0.15 to 0.20 percent of Cu, less than or equal to 0.06 percent of Mo, less than or equal to 0.010 percent of Al, 0.15 to 0.25 percent of V, 0.015 to 0.022 percent of N and the balance of Fe and impurity elements. Low plasticity of the connecting rod is guaranteed by controlling C, S, P and Cu to meet the condition that 3C + 150P + 5Cu + 80S is larger than or equal to 11 and smaller than or equal to 15, high dimensional precision and accurate control over length tolerance of bar steel are achieved by adopting two-heating forming, high-temperature cogging, low-temperature rolling and rolling whole-process technological management and control, the steel utilization rate can be increased by about 1.5%, the optimal balance of high strength, low plasticity and high fatigue performance is achieved, and the production cost is reduced. And by controlling 1.15% < = M < = 1.36%, high strength and stable mechanical properties of the connecting rod are ensured, and the structure is ferrite and pearlite.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and in particular relates to a medium-carbon non-quenched and tempered steel expansion-fracture connecting rod and a manufacturing method thereof. Background Art

[0002] With the continuous progress of the automobile industry, the requirements for engine performance are increasing day by day. The engine needs to achieve a balance between higher speed, greater power output and better fuel economy. The emergence of the expansion connecting rod technology is to meet this demand. The traditional connecting rod processing method is gradually unable to meet the requirements of modern high-performance engines in terms of precision and performance. For example, when running at high speed, the traditional connecting rod may produce greater vibration and energy loss due to the lack of precision in the connection part.

[0003] The lightweighting of automobiles is also an important factor in promoting the development of the expansion-break connecting rod technology. The use of expansion-break connecting rods can appropriately reduce the use of materials while ensuring the strength of the connecting rod, thereby reducing the overall weight of the engine. This helps to improve the fuel economy of the automobile and reduce exhaust emissions.

[0004] Modern engines have extremely high requirements for the quality and reliability of parts. The expansion-break connecting rod technology can make the connection between the connecting rod big end and the connecting rod cap tighter and more precise. Compared with the traditional connecting rod processing method, the fracture surface of the expansion-break connecting rod is irregular, which can better fit after assembly, thereby reducing the gap between the connecting rod big end and the connecting rod cap. This tight fit helps to improve the operating stability of the engine and reduce the risk of engine failure caused by loose connecting rod connections.

[0005] The expansion-fracture connecting rod also has advantages in fatigue strength. Its unique processing method enables the connecting rod to better disperse stress when it is subjected to alternating loads, prolonging the service life of the connecting rod and meeting the quality requirements of long-term and high-intensity operation of the engine.

[0006] The patent for "Non-quenched and tempered steel and non-quenched and tempered parts" disclosed in application number 201280024294.0, through the design of chemical composition, can be hot forged into a specified shape and then fractured and split. It is suitable for use as a connecting rod for automobile engines that requires high fatigue strength to reach Rel ≥450Mpa.

[0007] Application No. 201010248516.X discloses a medium-carbon non-quenched and tempered steel for expansion-fracture connecting rods. This patent designs the chemical composition so that the C, P, and B contents satisfy the brittleness relationship, and is used to manufacture expansion-fracture connecting rods with a tensile strength of 900 MPa or above.

[0008] The invention of "Steel for Automobile Engine Breakaway Connecting Rods" disclosed with application number 201510332645.X, through scientific design of the alloy system, can produce steel for automobile engine breakaway connecting rods with high strength, appropriate brittleness and good cutting performance without making major adjustments to the existing production process.

[0009] As cars develop towards energy saving and lightweight, higher requirements are placed on the engine connecting rod, a power transmission component of the car, requiring the connecting rod to have higher strength. Summary of the invention

[0010] The technical problem to be solved by the present invention is to provide a medium carbon non-quenched and tempered steel expansion connecting rod and a manufacturing method thereof in view of the above-mentioned prior art. The manufactured connecting rod forging has Rel≥800Mpa, 1100≤Rm≤1200MPa, A≤25%, Z≤35%, a hardness of 300-360HBW, and a metallographic structure of the forging of fine ferrite plus pearlite, which is beneficial to the lightweight of automobile engines.

[0011] The technical solution adopted by the present invention to solve the above problems is: a medium carbon non-quenched and tempered steel expansion connecting rod, the chemical composition of the connecting rod includes C: 0.50-0.56%, Si: 0.75-0.85%, Mn: 0.75-0.90%, P: 0.045-0.080%, S: 0.030-0.045%, Cr: 0.10-0.20%, Ni: ≤0.08%, Cu: 0.15-0.20%, Mo: ≤0.06%, Al: ≤0.010%, V: 0.15-0.25%, N: 0.015-0.022%, the remainder is Fe and impurity elements, and the product form is hot-rolled round steel bar.

[0012] The chemical composition design of the present invention is based on the following: Determination of C content C is the most economical and basic strengthening element in steel, which can improve the strength of steel. As the carbon content in steel increases, the yield and tensile strengths increase, while the plasticity and impact resistance decrease. The range of C content in the present invention is determined to be 0.50-0.56%. The steel material to which the present invention relates belongs to the category of medium carbon steel; Determination of Si content When Si is dissolved in ferrite in steel, it can significantly improve the elastic limit, yield strength and tensile strength of the steel, and reduce its plasticity and toughness. Si has a strong effect of increasing the cold deformation hardening rate of ferrite, making cold processing of steel difficult. In order to achieve the purpose of improving strength while ensuring the processing and expansion fracture performance of the engine connecting rod, the range of Si content in the present invention is determined to be 0.75-0.85%.

[0013] Determination of Mn content Mn is an important alloying element that increases the strength of steel and improves toughness. Mn is a weak carbide-forming element. It mainly exists in the form of solid solution in steel. It is solid-dissolved in ferrite at room temperature. The solid-dissolved Mn has a solid solution strengthening effect. In ferrite, the solid solution strengthening effect of Mn is second only to P and Si. The range of the Mn content in the present invention is determined to be 0.75-0.90%.

[0014] Determination of P content P improves the strength and hardness of steel: Phosphorus can solid-solution strengthen ferrite, increase the strength and hardness of steel, and improve the cutting performance of steel: an appropriate content of phosphorus can make the steel easy to break chips during cutting and improve cutting efficiency; when the phosphorus content is too high, the plasticity and toughness of the steel are reduced, especially at low temperatures. The range of the P content in the present invention is determined to be 0.045-0.080%.

[0015] Determination of S content The S element is added to form MnS inclusions in the steel, especially to make the oxide inclusions that are not easily deformed in the steel be wrapped by sulfides, thereby improving the cutting performance of the steel. At the same time, in addition to precipitating at the original austenite grain boundaries, ferrite is also precipitated with MnS as the core. Since the core formed by ferrite is increased, the number of ferrite blocks is increased, thereby refining the ferrite-pearlite structure. However, adding too much S will not only lead to the deterioration of the mechanical properties of the steel, but also cause more serious segregation in the steel, thereby having a harmful effect on the continuous casting and rolling of the steel. The range of S content in the present invention is determined to be 0.030-0.045%.

[0016] Determination of Cr content Cr is a carbide-forming element. When heated, Cr dissolved in austenite can significantly shift the C curve to the right, significantly improve hardenability, and thus improve the strength and hardness of steel. However, the content should not be too much, otherwise it is easy to produce bainite structure and reduce toughness. The range of Cr content in the present invention is determined to be 0.10-0.20%.

[0017] Determination of Ni content In order to keep the steel with good toughness, the breaking connecting rod requires lower toughness, and the range of Ni content in the present invention is determined to be ≤0.08%.

[0018] Determination of Cu content Cu can improve the stability of austenite in steel and strengthen ferrite. Cu is concentrated at the grain boundary, reducing the grain boundary strength and increasing brittleness. A high Cu content will cause hot brittleness of the steel, making hot forging and rolling difficult. The range of Cu content in the present invention is determined to be 0.15-0.20%.

[0019] Determination of Mo content Mo exists in the solid solution and carbide of steel and has a certain strengthening effect; however, Mo will significantly improve the hardenability of steel, and Mo has a significant delaying effect on pearlite transformation, while having a smaller effect on bainite transformation, so steel containing Mo can easily obtain bainite structure, which has an adverse effect on the toughness of steel; the steel of the present invention belongs to ferrite-pearlite type non-quenched and tempered steel, so the steel of the present invention has strict control over the Mo content, and the range of the Mo content of the present invention is determined to be ≤0.06%.

[0020] Determination of Al content Al is an effective deoxidation element. If the Al content is too high, brittle inclusions such as Al2O3 are easily formed during the molten steel smelting process, reducing the purity of the molten steel. The range of the Al content in the present invention is determined to be ≤0.010%.

[0021] Determination of V content V, as the main added alloying element, can reduce the phase transition temperature from the γ phase to the α phase and form more dispersed pearlite. The solid solution temperature of vanadium in austenite is low, but the diffusion rate is quite fast. During the heating and soaking stages, the precipitate dissolves and remains dissolved during hot working. During cooling, the dispersed precipitate precipitates to increase the strength of the steel. As the vanadium content in the steel increases, the strength of the steel increases linearly, but more vanadium content does not have a significant effect on further strengthening the steel. The range of the V content in the invention is determined to be 0.15~0.25% Determination of N content N in steel mainly enhances the precipitation strengthening effect and refines the grains. N and V have a strong affinity and can form a very stable interstitial phase. Nitrides and carbides can dissolve each other to form carbonitrides, and nitrides can also dissolve each other to form composite nitrides. These compounds often exist as fine particles, producing a dispersion strengthening effect and improving the strength of steel. When nitrogen is added to vanadium-containing steel, its precipitation strengthening effect is significantly improved. When the N content increases from 50×10-6 to 250×10-6, the yield strength of the steel increases by 100-150MPa, that is, 10×10-6 N increases the yield strength of the steel by about 5MPa. VN in steel is not only a strengthening phase, but also can inhibit the migration of austenite grain boundaries, refine austenite grains, thereby refining ferrite grains and pearlite groups. During phase transformation, it plays a core role and further refines ferrite grains. However, too high N content in steel is not good for continuous casting production and is prone to form cracks in the continuous casting billet. The Mn content of the present invention is determined to be in the range of 0.015% to 0.022%.

[0022] The present invention achieves the best balance of high strength, low plasticity and high fatigue performance. If M is low, the mechanical properties of the connecting rod do not meet the requirements, and if M is high, the connecting rod structure will be abnormal (martensite or bainite). Therefore, M is controlled to be 1.15%~1.36% to ensure the high strength and stable mechanical properties of the connecting rod, and the structure is pearlite+ferrite. C, Si, Mn, P, Cu, Cr, and V are mass percentages. M=[C]+0.16[Si]+0.20[Mn]+0.80[P]+0.25[Cu]+0.25[Cr]+1.54[V]. By controlling C, S, P, and Cu to meet 11≤3C+150P+5Cu+80S≤15, K=3C+150P+5Cu+80S, low plasticity is guaranteed.

[0023] The manufacturing process of the above-mentioned medium carbon non-quenched and tempered steel expansion-breaking connecting rod is as follows: KR hot metal pretreatment - electric furnace or converter - refining outside the furnace - VD or RH vacuum degassing - continuous casting - continuous rolling. The specific manufacturing method is as follows: Raw materials: Blast furnace molten iron undergoes KR molten iron pretreatment to control its P at 0.030~0.080% to provide high-quality molten iron for converters or electric furnaces.

[0024] Electric furnace or converter smelting KR hot metal pretreatment accounts for 90%, and the rest is scrap steel. The steel type belongs to high P steel. A small amount of lime is added during smelting. The low alkalinity is 2.5~3.0 to reduce the oxidizability for smelting, retaining most of the P elements in the molten steel. The P content of steel is controlled at 0.030~0.055%, C≥0.15%. Deoxidizer is added during steel tapping, and eccentric bottom steel tapping reduces the slag entering the ladle. Ferrophosphorus is added according to the P content during steel tapping to control its P target value of 0.055%. LF refining and smelting Argon stirring is used in the refining process. The chemical argon stirring intensity is adjusted so as not to expose the molten steel, so that the silicon carbide particles and carbon powder can be diffused and deoxidized, and the white slag time is guaranteed to be ≥15 minutes. According to the analysis results, the ingredients are adjusted to meet the specified content requirements. A small amount of alloy is added for fine-tuning in LF refining to significantly reduce the non-metallic inclusion content in the molten steel. The ladle temperature is 1580℃~1600℃.

[0025] VD or RH vacuum degassing smelting: This type of steel is a high-N steel. It uses nitrogen as the lifting gas and argon to form bubbles. The gas in the steel, such as H2, diffuses into the N bubbles. At the same time, the surface of the bubbles adsorbs inclusions in the molten steel and is brought out of the molten steel as the bubbles float up, thus dehydrogenating, removing inclusions, and increasing N. The whole process is kept at a low vacuum degree for more than 30 minutes.

[0026] The continuous casting process is specially controlled. This type of steel is a high-Si, high-P, high-S non-quenched and tempered steel with very strong crack sensitivity. This solves the problem that phosphorus- and sulfur-containing steels are prone to internal and surface cracks. A brand-new process design is used for this continuous casting process: first, the continuous casting light reduction process is used, and then the pulling speed is reduced by 0.40m / min. The continuous casting process adopts M-EMS (head end electromagnetic stirring) and F-EMS (inter-stream electromagnetic stirring) in pairs, and the electromagnetic stirring at the head end and inter-stream is increased to ensure the uniformity of the cross-sectional composition. The temperature gradient inside the ingot is reduced through a weak cooling process (the amount of water for water mist cooling in the second cooling zone is 0.12 L / kg), thereby avoiding the generation of internal cracks. When going offline, a high-temperature offline slow cooling process is implemented, and the offline temperature is ≥600℃ to reduce the temperature gradient and avoid the generation of internal and surface cracks. It effectively improves and reduces the composition segregation of continuous casting billets, especially after adding electromagnetic stirring between streams. The density of the solidification structure of the billet is improved, the looseness and shrinkage holes in the center of the billet are effectively controlled, the spacing of the secondary dendrite arms is significantly improved, the central equiaxed crystal rate is significantly improved, and the grains are refined, thereby significantly improving the quality of the billet and reducing the composition segregation.

[0027] Steel rolling The steel is made by two-stage heating. Inert gas such as argon is used in the furnace for protection heating. The billet is opened at high temperature. The heating temperature is 1250-1300℃ and the heating time is ≥4h. The precipitation of steel is improved. Flame cleaning is performed during the opening of the billet to improve the surface quality of the intermediate billet. There is no complete decarburization on the surface of the steel, and the decarburization does not exceed the standard. The billet is opened into a 150mm×150mm square.

[0028] Low temperature rolling: The temperature of the heating and soaking section is controlled at 1000-1100℃, the heating time is ≥2.5h, the holding time is ≥0.75min / mm, and the billet is taken out of the furnace after being held and descaled by high-pressure water; it enters the continuous rolling unit and is rolled into the required specifications, with a rolling compression ratio of ≥25. After rolling, it is air-cooled, and the steel is put into storage after finishing and inspection.

[0029] Through the refinement of the whole process of steel rolling and the refinement of tooling management, high dimensional accuracy and precise control of length tolerance of steel bars can be achieved. Figure 2 The company has developed 10 batches of steel with dimensional accuracy and length precision control. The precise length control enables customers to cut steel into equal parts without generating waste ends. High dimensional accuracy can improve the accuracy of customers’ cutting and forging, reduce forging flash, and avoid “under-forging”. Through the above two measures, the utilization rate of steel can be increased by about 1.5%.

[0030] Compared with the prior art, the advantages of the present invention are: The present invention achieves an optimal balance among high strength, low plasticity and high fatigue performance. The high strength and stable mechanical properties of the connecting rod are ensured by controlling 1.15%≤M≤1.36%. The microstructure is ferrite and pearlite. The low plasticity of the connecting rod is ensured by controlling C, S, P, and Cu to meet 11≤3C+150P+5Cu+80S≤15. The whole process of double-fired material forming, high-temperature blanking, low-temperature rolling, and rolling is controlled to achieve high dimensional accuracy of the steel bar and precise control of the length tolerance. The steel utilization rate can be increased by about 1.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the finished product structure of the hot-rolled round steel bar in the embodiment of the present invention.

[0032] Figure 2 , Figure 3 In the embodiment of the present invention, the hot-rolled round steel bar has high dimensional accuracy and length tolerance is precisely controlled and counted. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0034] The manufacturing process of the medium carbon non-quenched and tempered steel expansion-breaking connecting rod of Examples 1-4 is KR molten iron pretreatment - electric furnace or converter - refining outside the furnace - VD or RH vacuum degassing - continuous casting - continuous rolling.

[0035] KR hot metal pretreatment P content is shown in Table 1 Table 1

[0036] KR molten iron pretreatment accounts for 95%, and the rest is scrap steel. The converter tapping parameters are shown in Table 2.

[0037] Table 2

[0038] According to the composition design requirements: the order of adding alloys into the ladle is ferrotitanium, ferrosilicon, silicomanganese, ferromanganese, ferrosulfur, ferrophosphorus, 200kg of lime, etc.; the alloys are added according to the lower limit of the controlled composition Argon stirring is used in the refining process, and the chemical argon stirring intensity is adjusted so as not to expose the molten steel, so that the silicon carbide particles and carbon powder can be diffused and deoxidized, and the white slag time is guaranteed to be ≥20 minutes. According to the analysis results, the components are adjusted to meet the specified content requirements, and a small amount of alloy is added for fine-tuning in LF refining.

[0039] Vacuum degassing uses nitrogen as the lifting gas, and the low vacuum is strictly controlled for 32-35 minutes. The temperature of the upper argon blowing table is 1555℃. The upper argon blowing table is a place for soft argon blowing, and the soft argon blowing lasts more than 10 minutes.

[0040] The continuous casting controls the tundish superheat to be 10-20°C. The continuous casting adopts the crystallizer electromagnetic stirring parameter 4A / 300HZ, the terminal electromagnetic stirring parameter 20A / 300HZ, and the tundish induction heating. Heating: The billet is heated in a walking beam heating furnace, including a preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the soaking section is controlled at 1250-1230°C, and the total heating time is more than 4-4.5 hours.

[0041] The starting rolling temperature is 980-1050°C, the final rolling temperature is 800-850°C, the rolling is carried out in the single-phase region, the maximum reduction of a single pass is controlled between 25 and 30 mm, and the rolling is slowly cooled after completion.

[0042] The chemical composition of the steel involved in the above embodiments is shown in Table 3. Table 3. (wt.%, the remainder is Fe and other unavoidable impurity elements)

[0043] The performance test of the connecting rods in the above embodiments is shown in Table 4. Table 4

[0044] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A medium carbon non-quenched and tempered steel expansion connecting rod, characterized in that: The chemical composition of the connecting rod is C: 0.50-0.56%, Si: 0.75-0.85%, Mn: 0.75-0.90%, P: 0.045-0.080%, S: 0.030-0.045%, Cr: 0.10-0.20%, Ni: ≤0.08%, Cu: 0.15-0.20%, Mo: ≤0.06%, Al: ≤0.010%, V: 0.15-0.25%, N: 0.015-0.022%, and the balance is Fe and impurity elements.

2. According to claim 1, a medium carbon non-quenched and tempered steel expansion connecting rod is characterized in that: The optimal balance value of high strength, low plasticity and high fatigue performance of the connecting rod is M, M is controlled at 1.15%~1.36%, and M=[C]+0.16[Si]+0.20[Mn]+0.80[P]+0.25[Cu]+0.25[Cr]+1.54[V]. Low plasticity is ensured by controlling C, S, P, and Cu to meet 11≤3C+150P+5Cu+80S≤15.

3. The medium carbon non-quenched and tempered steel expansion-breaking connecting rod according to claim 1, characterized in that: The mechanical properties of the connecting rod meet Rel≥800Mpa, 1100≤Rm≤1200MPa, A≤25%, Z≤35%, the hardness is 300-360HBW, and the metallographic structure is fine ferrite plus pearlite.

4. A method for manufacturing a medium carbon non-quenched and tempered steel expansion-breaking connecting rod as claimed in claim 1, characterized in that: The method comprises the following steps: 1) Raw materials The blast furnace molten iron is pretreated with KR molten iron to control its P at 0.030~0.080% to provide high-quality molten iron for the converter or electric furnace; 2) Electric furnace or converter smelting KR hot metal pretreatment accounts for 90%, and the rest is scrap steel. The P content is controlled at 0.030~0.055% and C≥0.15% when tapping. 3) LF refining and smelting During the refining process, argon gas stirring is used to ensure that the white slag time is ≥15 minutes. The components are adjusted according to the analysis results to meet the specified content requirements. A small amount of alloy is added for fine-tuning in LF refining to significantly reduce the non-metallic inclusion content in the molten steel; 4) VD or RH vacuum degassing smelting: Using nitrogen as the lifting gas, the gas in the steel diffuses to the bubbles formed by argon blowing. At the same time, the surface of the bubbles adsorbs the inclusions in the molten steel and is taken out of the molten steel as the bubbles float up, thus playing the role of dehydrogenation, removing inclusions and increasing nitrogen. The whole process is kept under low vacuum for more than 30 minutes. 5) Continuous casting First, the continuous casting light pressure process is used, and then the casting speed is reduced by 0.40m / min. The continuous casting process adopts M-EMS and F-EMS double connection, and the electromagnetic stirring at the head end and between streams is increased to ensure the uniformity of the cross-sectional composition. Then, the temperature gradient inside the ingot is reduced through the weak cooling process, thereby avoiding the generation of internal cracks. When the ingot is offline, the high-temperature offline slow cooling process is implemented, and the offline temperature is ≥600℃ to reduce the temperature gradient and avoid the generation of internal cracks and surface cracks. 6) Steel rolling The steel is made by two-stage heating. The furnace is heated by inert gas protection. The steel is opened at high temperature. The heating temperature is 1250-1300℃ and the heating time is ≥4h. The precipitation of steel is improved. Flame cleaning is performed during opening to prevent the steel surface from being completely decarburized. At the same time, the decarburization is ensured not to exceed the standard. The steel is opened into 150mm×150mm square. 7) Low temperature rolling: the heating temperature is controlled at 1000-1100℃, the heating time is ≥2.5h, the holding time is ≥0.75min / mm, the billet is kept warm and then removed from the furnace and descaled by high-pressure water; it enters the continuous rolling unit and is rolled into the required specifications, with a rolling reduction ratio of ≥25. After rolling, it is air-cooled, and the steel is put into storage after finishing and inspection.

5. The method for manufacturing a medium carbon non-quenched and tempered steel expansion-breaking connecting rod according to claim 4, characterized in that: In step 2), a small amount of lime is added during smelting, and the basicity is low at 2.5-3.0 to reduce the oxidizing property and retain most of the P element in the molten steel. A deoxidizer is added during steelmaking to deoxidize the steel. Ferrophosphorus is added according to the P content to control the P target value to 0.055%.

6. The method for manufacturing a medium carbon non-quenched and tempered steel expansion-break connecting rod according to claim 4, characterized in that: Step 3) The temperature of the hanging bag is 1580℃~1600℃.

7. The method for manufacturing a medium carbon non-quenched and tempered steel expansion-break connecting rod according to claim 4, characterized in that: In step 4), the low vacuum is controlled for 32-35 minutes, the temperature of the upper argon blowing table is 1555°C, and the soft argon blowing is more than 10 minutes.

8. The method for manufacturing a medium carbon non-quenched and tempered steel expansion-break connecting rod according to claim 4, characterized in that: In step 5), the superheat of the tundish during continuous casting is controlled to be 10-20°C, and the electromagnetic stirring parameters of the crystallizer are 4A / 300HZ, and the electromagnetic stirring parameters of the end are 20A / 300HZ.

9. The method for manufacturing a medium carbon non-quenched and tempered steel expansion-break connecting rod according to claim 4, characterized in that: In step 7), the starting rolling temperature is 980-1050°C, the final rolling temperature is 800-850°C, the rolling is carried out in the single-phase region, the maximum reduction of a single pass is controlled between 25 and 30 mm, and the rolling is slowly cooled after completion.

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