Non-quenched and tempered steel for fractured connecting rod and preparation method of non-quenched and tempered steel

Abstract

The invention discloses non-quenched and tempered steel for an expansion breaking connecting rod and a preparation method of the non-quenched and tempered steel, and relates to the technical field of steel production. The carbon equivalent Ceq of the non-quenched and tempered steel for the fractured connecting rod is larger than or equal to 0.85 and smaller than or equal to 1.03. The strength of the steel can be improved by controlling the content of C and Mn in the steel; by controlling a certain content of Si, the plasticity of the steel can be reduced; by controlling a certain content of Mn and S, the cutting performance of the steel can be improved; by controlling a certain content of P and Cu, the austenite grain boundary can be embrittled, and the expansion fracture performance of the connecting rod is improved; certain contents of V, N and C are controlled, so that the overall toughness of the steel can be improved; cr can increase the hardenability and improve the mechanical property of the steel, so that the non-quenched and tempered steel for the expansion breaking connecting rod provided by the invention is relatively high in strength, improved in yield ratio, enhanced in fatigue property and enhanced in machinability, and the non-quenched and tempered steel has relatively good mechanical property and also has good expansion breaking capacity.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a non-quenched and tempered steel for an expansion-breaking connecting rod and a preparation method thereof. Background Art

[0002] The machining process of traditional automobile engine connecting rods is complex and requires high precision. The reliability of the engine is often affected by machining precision problems. In recent years, with the rise and widespread application of expansion-break connecting rod technology, expansion-break connecting rods have gradually replaced traditional automobile engine connecting rods. An expansion-break connecting rod is a kind of connecting rod that is designed and prefabricated with a notch (crack groove) at an appropriate position on the big end of the connecting rod to form a stress concentration, and then a load perpendicular to the predetermined fracture surface is applied to induce cracking. When the conditions for brittle fracture are met, it fractures regularly at the notch with almost no plastic deformation, achieving chipless fracture separation of the connecting rod body and the connecting rod cover. Since the fracture surface between the connecting rod body and the connecting rod cover is in a natural state of jagged teeth, it has extremely high positioning and matching accuracy, and no need to process the joint surface. In addition, it can simplify the structural design and overall processing technology of the connecting rod bolt hole, and has a series of advantages such as fewer processing steps, saving finishing equipment, saving materials and energy, high product quality, and low production cost.

[0003] However, the existing expansion-breaking connecting rods still have the defect of poor expansion-breaking ability. For example, although some expansion-breaking connecting rod steels are prone to brittle cracking, they have low yield strength ratio, low fatigue performance, or poor machinability, which are not conducive to the application of expansion-breaking connecting rods.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] The object of the present invention is to provide a non-quenched and tempered steel for an expansion-breaking connecting rod and a preparation method thereof.

[0006] The present invention is achieved in that:

[0007] In the first aspect, the present invention provides a non-quenched and tempered steel for a breaking connecting rod, wherein the element composition, calculated by mass fraction, includes C: 0.34-0.41%, Si: 0.50-0.75%, Mn: 0.95-1.05%, P: 0.032-0.050%, S: 0.030-0.045%, Cr: 0.10-0.30%, V: 0.25-0.35%, Cu: 0.05-0.25%, Al≤0.010%, N: 0.015-0.025%, and the remainder is Fe and other inevitable impurities.

[0008] The carbon equivalent Ceq of non-quenched and tempered steel for expansion-breaking connecting rods satisfies: 0.85≤Ceq≤1.03.

[0009] The calculation formula for carbon equivalent is: Ceq=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.20Cr+V.

[0010] In a second aspect, the present invention provides a method for preparing non-quenched and tempered steel for a fractured connecting rod as in any of the aforementioned embodiments, comprising smelting and casting the steel into round bars according to the elemental composition, then heating the round bars and forging them to obtain a connecting rod blank, and then cooling the connecting rod blank, wherein the cooling rate of the connecting rod blank is 1.9 to 3.0°C / s.

[0011] The present invention has the following beneficial effects:

[0012] The invention provides a non-quenched and tempered steel for a fracture-breaking connecting rod and a preparation method thereof. By controlling the element composition in the steel, the carbon equivalent range of the steel is further controlled. Specifically, the steel contains a certain content of C and Mn, which can improve the strength of the steel; the steel plasticity can be reduced by controlling a certain content of Si; the steel cutting performance can be improved by controlling a certain content of Mn and S; the steel austenite grain boundaries can be embrittled by controlling a certain content of P and Cu, and the fracture-breaking performance of the connecting rod can be improved; the steel overall strength and toughness can be improved by controlling a certain content of V, N and C; and Cr can increase the hardenability and improve the mechanical properties of the steel, so that the non-quenched and tempered steel for a fracture-breaking connecting rod provided by the invention has high strength, improved yield strength ratio, enhanced fatigue performance and enhanced cutting performance, and has good fracture-breaking ability while having better mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0014] Figure 1 This is an electron microscope image of the non-quenched and tempered steel for the expansion-breaking connecting rod provided in Example 1 of the present invention;

[0015] Figure 2 This is an electron microscope image of the non-quenched and tempered steel for the expansion-breaking connecting rod provided in Example 3 of the present invention;

[0016] Figure 3 This is an electron microscope image of the non-quenched and tempered steel for the expansion-breaking connecting rod provided in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0018] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0019] In the first aspect, the present invention provides a non-quenched and tempered steel for a breaking connecting rod, wherein the element composition, calculated by mass fraction, includes C: 0.34-0.41%, Si: 0.50-0.75%, Mn: 0.95-1.05%, P: 0.032-0.050%, S: 0.030-0.045%, Cr: 0.10-0.30%, V: 0.25-0.35%, Cu: 0.05-0.25%, Al≤0.010%, N: 0.015-0.025%, and the remainder is Fe and other inevitable impurities.

[0020] The carbon equivalent Ceq of non-quenched and tempered steel for expansion-breaking connecting rods satisfies: 0.85≤Ceq≤1.03.

[0021] The calculation formula for carbon equivalent is: Ceq=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.20Cr+V.

[0022] In the calculation formula of carbon equivalent Ceq, each element refers to the mass fraction of each element in the non-quenched and tempered steel for the expansion-breaking connecting rod directly minus %. For example, when the mass fraction of C in the non-quenched and tempered steel for the expansion-breaking connecting rod is 0.4%, C=0.4 is substituted into the calculation formula of Ceq.

[0023] In some embodiments of the present invention, the above elements and their value ranges have the following functions:

[0024] C: 0.34-0.41%. C is one of the components necessary to ensure the service strength of the expansion-fracture connecting rod. If the C content is too low, the content of pearlite in the steel will be reduced, thereby reducing the strength, increasing the toughness, and affecting the expansion-fracture performance of the connecting rod; if the C content is too high, it will have an adverse effect on the fatigue performance and cutting performance of the steel.

[0025] Si: 0.50-0.75%. Si does not form carbides in steel, but exists in ferrite in the form of solid solution, which has a significant effect of solid solution strengthening ferrite and reducing its plasticity. In addition, Si can also promote the grain boundary segregation of elements P and S, making the grain boundaries appropriately embrittled. In order to achieve this goal, the Si content needs to be above 0.50%, but too high Si content will deteriorate the hot workability of steel and the surface quality of forgings, so its content is controlled not to exceed 0.75%.

[0026] Mn: 0.95-1.05%. Mn can dissolve in ferrite and play a role in solid solution strengthening. It is one of the necessary components to ensure the service strength of the expansion-breaking connecting rod. At the same time, Mn and S form MnS inclusions to improve the machinability of steel. When the Mn content is too low, the effect on the machinability of steel is small; but when the Mn content is too high, bainite is likely to appear in the metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod, resulting in unqualified structure and deterioration of the fatigue performance of the expansion-breaking connecting rod.

[0027] P: 0.032-0.050%. P element can form micro segregation when the molten steel solidifies, and then segregate at the grain boundary when heated at high temperature, which significantly increases the brittleness of the steel. Most steels need to strictly control the P content in the steel at a low level, and the present invention adopts the idea of ​​increasing the P content of impurity elements in the steel to reduce the austenite grain boundary strength (embrittlement), and strengthen the ferrite phase solid solution, reduce its plasticity, and further improve the machinability of the steel. However, too high a P content will significantly deteriorate the hot workability of the steel and increase the difficulty of smelting. Therefore, it is more appropriate to control the P content within the above range.

[0028] S: 0.030-0.045%. S and Mn can form fine and dispersed MnS, thereby improving the machinability of steel. When the S content is too low, the effect on the machinability of steel is weak, but when the S content is too high, the hot workability of steel will be deteriorated.

[0029] Cu: 0.05-0.25%. Cu is easily segregated at the grain boundary when heated at high temperature, which significantly increases the brittleness of the steel. Therefore, most steels need to strictly control the Cu content in the steel to a low level, and the present invention adopts the idea of ​​increasing the Cu content of the impurity element in the steel to reduce the austenite grain boundary strength (embrittlement). However, too high a Cu content will significantly deteriorate the hot workability of the steel, so it is more appropriate to control the Cu content within the above range.

[0030] V: 0.25-0.35%. V is a strong carbide-forming element, which mainly inhibits the deformation and recrystallization of austenite and prevents the growth of its grains during hot working. As the austenitizing temperature of steel increases, the amount of V (C, N) dissolved in austenite increases. In the subsequent controlled forging and controlled cooling process, the amount of fine V (C, N) precipitation also increases. The price of V is lower than that of Ni and Mo. Therefore, the present invention mainly improves the material strength by adding V. However, if the V content is too high, the cost of steel will increase. Therefore, the present invention determines the V content to be 0.25-0.35%.

[0031] Cr: 0.10-0.30%. Cr in steel can increase hardenability and improve the mechanical properties of steel, but too high a Cr content will deteriorate the cutting performance of the steel.

[0032] N: 0.015-0.025%. The N content of the present invention is relatively high, and the purpose of increasing the N content is to ensure that the V element in the non-quenched and tempered steel for the expansion-breaking connecting rod obtains the best strength and toughness after precipitation strengthening and forging.

[0033] In an optional embodiment, the metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod mainly includes pearlite and ferrite, and the mass fraction of ferrite is ≤35wt%, preferably, the mass fraction of ferrite is ≤33wt%.

[0034] In an optional embodiment, the metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod includes ferrite with a mass fraction of 25-33wt%, and the remainder is pearlite. That is, the metallographic structure of some non-quenched and tempered steels for the expansion-breaking connecting rod provided by the present invention only has pearlite and ferrite, so as to ensure that the non-quenched and tempered steel for the expansion-breaking connecting rod has better mechanical properties and good expansion-breaking ability.

[0035] In an optional embodiment, the elemental composition, by mass fraction, includes C: 0.36-0.41%, Si: 0.68-0.75%, Mn: 0.97-1.05%, P: 0.035-0.045%, S: 0.030-0.045%, Cr: 0.22-0.30%, V: 0.26-0.33%, Cu: 0.05-0.15%, Ni≤0.20%, Mo≤0.05%, Al≤0.010%, N: 0.017-0.024%, and 0.95≤Ceq≤1.02.

[0036] The calculation formula for carbon equivalent is: Ceq=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.20Cr+0.4Mo+V.

[0037] Mo≤0.05%. Mo can increase the hardenability and improve the mechanical properties of steel in steel, but the presence of Mo in steel can easily lead to the appearance of bainite in crankshaft forgings. Therefore, if Mo is to be added, the Mo content must be strictly controlled to a low level, which can not only reduce the cost of steel, but also avoid the appearance of unfavorable structures in steel.

[0038] Ni≤0.20%. Ni can improve the strength of steel, but if the Ni content is too high, the cost of steel will increase. Therefore, the present invention mainly uses V, which is relatively low in price, as the main strengthening element. When the strengthening ability of V is insufficient, a small amount of Ni and / or Mo can be added to assist. If the strengthening ability of V is sufficient, Ni and / or Mo do not need to be added.

[0039] In an optional embodiment, the elemental composition, by mass fraction, includes C: 0.37-0.40%, Si: 0.68-0.73%, Mn: 1.00-1.05%, P: 0.035-0.040%, S: 0.035-0.040%, Cr: 0.24-0.26%, V: 0.28-0.32%, Cu: 0.08-0.12%, Ni≤0.15%, Mo≤0.05%, Al≤0.010%, N: 0.019-0.022%, and 0.95≤Ceq≤1.00.

[0040] In an optional embodiment, the tensile strength of the non-quenched and tempered steel for the expansion-fracture connecting rod is ≥1000MPa, the yield strength is ≥800MPa, the elongation after forging is ≥10%, the cross-sectional shrinkage is ≥20%, the grain size is ≥4, and the Brinell hardness of the connecting rod big end and the connecting rod small end is 280-325HBW.

[0041] In an optional embodiment, the tensile strength of the non-quenched and tempered steel for the expansion-fracture connecting rod is ≥1020MPa, the yield strength is ≥830MPa, the elongation after forging is ≥16%, the cross-sectional shrinkage is ≥40%, the grain size is ≥grade 5, and the Brinell hardness of the connecting rod big end and the connecting rod small end is both 290-320HBW.

[0042] In a second aspect, the present invention provides a method for preparing non-quenched and tempered steel for a fractured connecting rod as in any of the aforementioned embodiments, comprising smelting and casting the steel into round bars according to the elemental composition, then heating the round bars and forging them to obtain a connecting rod blank, and then cooling the connecting rod blank, wherein the cooling rate of the connecting rod blank is 1.9 to 3.0°C / s.

[0043] For example, the cooling rate of the connecting rod blank can be any value among 1.9℃ / s, 2.0℃ / s, 2.3℃ / s, 2.5℃ / s, 2.8℃ / s or 3.0℃ / s, or a range value between any two values, or any value in the above range values.

[0044] In addition to the significant influence of the element composition of steel on the mechanical properties of the expansion-fracture connecting rod, the cooling process of the connecting rod blank also has a key influence on its mechanical properties. Since the cooling process of the connecting rod blank is a process of rapid transformation and distribution of the structure in the steel, after the element composition of the steel is accurately controlled, it is necessary to control the cooling process of the connecting rod blank to ensure that each element can eventually form the target organizational morphology (a combination of ferrite + pearlite) to ensure the mechanical properties of the steel.

[0045] In an optional embodiment, the temperature of the connecting rod blank before cooling is ≥900°C, the temperature after cooling is 510-650°C, and the cooling is controlled according to the above cooling rate. The metallographic structure of the obtained non-quenched and tempered steel for the expansion-breaking connecting rod consists of ferrite and pearlite.

[0046] The temperature of the connecting rod blank before cooling is the final forging temperature when the connecting rod blank is obtained after the round steel forging is completed. For example, the temperature of the connecting rod blank before cooling is any value among 900°C, 950°C, or 1000°C, or a range value between any two values, or any value in the above range values.

[0047] The temperature of the connecting rod blank after cooling is the temperature of the connecting rod blank after the cooling is completed. For example, the temperature of the connecting rod blank after cooling is any value among 510°C, 520°C, 550°C, 580°C, 600°C, 620°C or 650°C, or a range value between any two values, or any value in the above range values.

[0048] Preferably, in order to achieve the above-mentioned control of the cooling speed, during the cooling process of the connecting rod blank, 2 to 4 fans are turned on, the fan rotation frequency is 30 to 50 Hz, and the conveyor chain rotation frequency is 10 to 25 Hz.

[0049] Among them, the number of fans turned on and the fan rotation frequency are both the air volume of the cooling wind for the connecting rod blank. The present invention controls the fan rotation frequency within a relatively high range, so that the connecting rod blank can be quickly cooled down, and the performance and hardness of the connecting rod are improved; in addition, the conveyor chain rotation frequency can control the moving speed of the connecting rod blank. The lower the conveyor chain rotation frequency, the slower the moving speed of the connecting rod blank, the longer the air cooling time of the connecting rod blank, and the higher the performance and hardness of the connecting rod. Therefore, in order to ensure that the connecting rod blank obtains sufficient strength and hardness, it is also necessary to control the conveyor chain rotation frequency within a relatively low range.

[0050] Since the steel smelting process generally involves melting various raw materials in proportion to obtain molten steel with a specified element content, then casting the molten steel into a solid ingot, and then performing subsequent shaping operations, etc. Therefore, in the early stage of steel production, cylindrical or prismatic prefabricated steel samples are generally prepared, and then plasticized through corresponding processes to obtain products with corresponding shape requirements.

[0051] In the present invention, the prefabricated steel sample obtained in the early stage of steel production is a cylindrical round bar. To produce the round bar into a structure of an expansion-breaking connecting rod, the round bar needs to be further forged. In order to facilitate the shaping process of the round bar, the round bar needs to be heated before forging. In an optional embodiment, the heating temperature of the round bar is 1180-1260°C, preferably 1205-1220°C.

[0052] For example, the heating temperature of the round rod is any value among 1180° C., 1200° C., 1220° C., 1240° C. or 1260° C., or a range value between any two values, or any value in the above range values.

[0053] It should be noted that the production process steps not mentioned in the present invention can be implemented by using existing process methods, as long as it can ensure the materials required by the present invention.

[0054] Example 1

[0055] This embodiment provides a non-quenched and tempered steel for a breakaway connecting rod, the element composition of which is shown in Table 1, and the preparation method is as follows:

[0056] The raw materials are melted in proportion to obtain refined molten steel, and then the refined molten steel is cast, rolled and other processes to obtain round bars. The round bars are heated to 1205℃ and then roll forged, pre-forged, final forged, punched, trimmed and thermally corrected. The final forging temperature of the connecting rod blank obtained after thermal correction is 932℃. The connecting rod blank is sent to the air-cooled roller for controlled cooling, the cooling rate is 2.6℃ / s, the number of fans is turned on, the rotation frequency of each fan is 40Hz, the rotation frequency of the conveyor chain is 20Hz, and the connecting rod blank is put into the frame after 4 minutes, and the temperature of the connecting rod blank entering the frame is 510~550℃.

[0057] The elemental compositions of the remaining embodiments and comparative examples are also shown in Table 1. The preparation method is similar to that of Example 1, except that the parameters are slightly different. For details, see Table 2.

[0058] In the following tables, S refers to Examples and D refers to Comparative Examples.

[0059] Table 1 Element composition of non-quenched and tempered steel for expansion-break connecting rod (wt%)

[0060]

[0061]

[0062] Table 2 Process parameters of non-quenched and tempered steel for expansion-break connecting rod

[0063]

[0064] Test Example 1

[0065] The performance of the non-quenched and tempered steel used for the expansion-breaking connecting rods provided in Examples 1 to 3 and Comparative Examples 1 to 4 was tested, and the results were shown in Tables 3, 4 and Figures 1 to 3 Results shown.

[0066] Among them, the yield strength is tested by a tensile testing machine, the tensile strength is tested by a tensile testing machine, the elongation is tested by a tensile testing machine, the cross-sectional shrinkage is tested by a tensile testing machine, the Brinell hardness is tested by a Brinell hardness tester, the metallographic structure is tested by an electron microscope, and the expansion and fracture condition is tested by a expansion and fracture machine.

[0067] Table 3 Mechanical properties of non-quenched and tempered steel for expansion-break connecting rods

[0068]

[0069]

[0070] Table 4 Metallographic structure and expansion and fracture properties of non-quenched and tempered steel for expansion and fracture connecting rod

[0071] Metallographic structure Ferrite content / wt% Bulging and breaking S1 Pearlite + Ferrite 30～35 The fracture is flat and the expansion is qualified S2 Pearlite + Ferrite 28～35 The fracture is flat and the expansion is qualified S3 Pearlite + Ferrite 25～33 The fracture is flat and the expansion is qualified D1 Pearlite + Ferrite 40～45 The fracture is not flat and the connecting rod is torn D2 Pearlite + Ferrite + Bainite 20～30 The fracture is not flat and the connecting rod is torn D3 Pearlite + Ferrite 42～50 The fracture is not flat and the connecting rod is torn D4 Pearlite + Ferrite + Bainite 20-30 The fracture is not flat and the connecting rod is torn

[0072] As shown in Tables 3 and 4, the carbon equivalent value of Comparative Example 1 is relatively small, and the carbon equivalent value of Comparative Example 2 is relatively large. When the preparation method and parameters are similar to those of the embodiment, the yield strength and tensile strength of Comparative Example 1 are significantly reduced, and the Brinell hardness is reduced, indicating that the overall strength of the non-quenched and tempered steel for the expansion-breaking connecting rod of Comparative Example 1 is insufficient. After metallographic detection, it is found that the ferrite content in its structure increases, and after the expansion-breaking detection, due to its weak strength, the fracture is uneven, the connecting rod is torn, and the expansion-breaking ability is poor. The yield strength and tensile strength of Comparative Example 2 are significantly increased, and the Brinell hardness is increased, indicating that the overall strength of the non-quenched and tempered steel for the expansion-breaking connecting rod of Comparative Example 2 is relatively large. Although the mechanical properties are good, due to the presence of bainite structure in the metallographic structure of Comparative Example 2, the non-quenched and tempered steel for the expansion-breaking connecting rod is uneven after expansion, the connecting rod is torn, and the expansion-breaking ability is poor.

[0073] Figure 1 and Figure 2 The metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod of Examples 1 and 3 is shown, which is composed of pearlite and ferrite. Figure 3 In the metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod in the comparative example 2 shown, in addition to pearlite and ferrite, there is also a certain amount of bainite, which is not conducive to the expansion-breaking process of the expansion-breaking connecting rod.

[0074] The components of Comparative Examples 3 and 4 are basically the same as those of Example 1, but the expansion and breaking effects are poor compared to Example 1. The main reason for Comparative Example 3 is that the cooling rate is too slow during the cooling process of the connecting rod, the ferrite content in the structure is high, the hardness is low, and the expansion and breaking effect of the connecting rod is poor. The main reason for Comparative Example 4 is that the cooling rate is too fast, and bainite structure appears in the structure, resulting in poor expansion and breaking effect of the connecting rod.

[0075] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, 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 non-quenched and tempered steel for a breaking connecting rod, characterized in that: The element composition includes C: 0.34-0.41%, Si: 0.50-0.75%, Mn: 0.95-1.05%, P: 0.032-0.050%, S: 0.030-0.045%, Cr: 0.10-0.30%, V: 0.25-0.35%, Cu: 0.05-0.25%, Al≤0.010%, N: 0.015-0.025%, and the balance is Fe and other inevitable impurities; The carbon equivalent Ceq of non-quenched and tempered steel for expansion-breaking connecting rods meets the following requirements: 0.85≤Ceq≤1.03; The calculation formula of the carbon equivalent is: Ceq=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.20Cr+V.

2. The non-quenched and tempered steel for the expansion-breaking connecting rod according to claim 1, characterized in that: The metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod includes pearlite and ferrite, and the mass fraction of the ferrite is ≤35wt%, preferably, the mass fraction of the ferrite is ≤33wt%.

3. The non-quenched and tempered steel for the expansion-breaking connecting rod according to claim 2, characterized in that: The metallographic structure of the non-quenched and tempered steel for the expansion-breaking connecting rod includes ferrite with a mass fraction of 25-33wt%, and the remainder is pearlite.

4. The non-quenched and tempered steel for the expansion-breaking connecting rod according to claim 1, characterized in that: By mass fraction, the element composition includes C: 0.36-0.41%, Si: 0.68-0.75%, Mn: 0.97-1.05%, P: 0.035-0.045%, S: 0.030-0.045%, Cr: 0.22-0.30%, V: 0.26-0.33%, Cu: 0.05~0.15%, Ni≤0.20%, Mo≤0.05%, Al≤0.010%, N: 0.017~0.024%, 0.95≤Ceq≤1.02; Preferably, the calculation formula of the carbon equivalent is: Ceq=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.20Cr+0.4Mo+V.

5. The non-quenched and tempered steel for the expansion-breaking connecting rod according to claim 4, characterized in that: By mass fraction, the element composition includes C: 0.37-0.40%, Si: 0.68-0.73%, Mn: 1.00-1.05%, P: 0.035-0.040%, S: 0.035-0.040%, Cr: 0.24-0.26%, V: 0.28-0.32%, Cu: 0.08~0.12%, Ni≤0.15%, Mo≤0.05%, Al≤0.010%, N: 0.019~0.022%, 0.95≤Ceq≤1.

00.

6. The non-quenched and tempered steel for the expansion-breaking connecting rod according to claim 1, characterized in that: The tensile strength of the non-quenched and tempered steel for the expansion-fracture connecting rod is ≥1000MPa, the yield strength is ≥800MPa, the elongation after fracture is ≥10%, the cross-sectional shrinkage is ≥20%, the grain size is ≥4, and the Brinell hardness of the connecting rod big end and the connecting rod small end is 280-325HBW.

7. The non-quenched and tempered steel for the expansion-breaking connecting rod according to claim 1, characterized in that: The tensile strength of the non-quenched and tempered steel for the expansion-fracture connecting rod is ≥1020MPa, the yield strength is ≥830MPa, the elongation after fracture is ≥16%, the cross-sectional shrinkage is ≥40%, the grain size is ≥5 levels, and the Brinell hardness of the connecting rod big end and the connecting rod small end are both 290-320HBW.

8. A method for preparing non-quenched and tempered steel for expansion-breaking connecting rods according to any one of claims 1 to 7, characterized in that: The method comprises smelting and casting into round bars according to element composition, heating the round bars and then forging to obtain connecting rod blanks, and then cooling the connecting rod blanks, wherein the cooling rate of the connecting rod blanks is 1.9-3.0°C / s.

9. The preparation method according to claim 8, characterized in that: The temperature of the connecting rod blank before cooling is ≥900°C, and the temperature after cooling is 510-650°C; Preferably, during the cooling process of the connecting rod blank, 2 to 4 fans are turned on, the fan rotation frequency is 30 to 50 Hz, and the conveyor chain rotation frequency is 10 to 25 Hz.

10. The preparation method according to claim 8 or 9, characterized in that: The heating temperature of the round rod is 1180-1260°C, preferably 1205-1220°C.

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