Novel non-quenched and tempered steel for expansion breaking connecting rod and manufacturing method and controlled forging and controlled cooling process of novel non-quenched and tempered steel

Abstract

The invention discloses novel non-quenched and tempered steel for an expansion breaking connecting rod and a manufacturing method and a controlled forging and controlled cooling process of the novel non-quenched and tempered steel. According to the technical scheme, the novel non-quenched and tempered steel is characterized by comprising, by weight, 0.45%-0.49% of C, 0.20%-0.40% of Si, 1.35%-1.75% of Mn, 0.015%-0.025% of P, 0.050%-0.070% of S, 0.15%-0.25% of Cr, 0.15%-0.25% of Ni, 0.05%-0.10% of Mo, 0.15%-0.25% of V, 0.030%-0.050% of Nb, 0.015%-0.040% of Al, 0.040%-0.060% of N and the balance Fe and inevitable impurity elements. A novel controlled forging and controlled cooling process of non-quenched and tempered steel for an expansion breaking connecting rod comprises the steps that the forging induction heating temperature is controlled to be larger than or equal to 1220 DEG C, the heat preservation time is controlled to be larger than or equal to 5 min, local strong wind cold treatment is conducted on the connecting rod neck position after roll forging heating, so that the die forging temperature of the connecting rod large head position is 1100-1200 DEG C, and the die forging temperature of the connecting rod neck position is 900-1050 DEG C; the connecting rod is subjected to strong wind cold treatment after being forged, the cooling speed is 7-12 DEG C / s, and stack cold treatment is adopted after the connecting rod is cooled to 200 DEG C. The expansion breaking connecting rod can meet the development requirements of future high-performance engine expansion breaking connecting rods.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a novel non-quenched and tempered steel for a breaking connecting rod and a manufacturing method and a controlled forging and controlled cooling process thereof. Background Art

[0002] The connecting rod is a key part of the automobile engine. Its function is to transmit the gas force on the piston to the crankshaft and convert the reciprocating motion of the piston into the rotational motion of the crankshaft. In order to ensure the reliability of the engine, the connecting rod is required to have sufficiently high fatigue strength and stiffness. The machining process of the connecting rod is complex and requires high precision, especially the big end hole of the connecting rod. The out-of-roundness of the big end hole of the connecting rod caused by the machining precision error is an important factor affecting the performance and reliability of the engine. In recent years, the expansion (cracking) connecting rod technology developed abroad has solved this problem. This technology has unparalleled advantages over traditional connecting rod processing technology. It has fewer processing steps, saves finishing equipment, saves materials and energy, and significantly reduces production costs. The connecting rod materials used for expansion processing are mainly powder metallurgy materials, forged steel, ductile cast iron and malleable cast iron. Forged steel connecting rods have high dimensional accuracy, good organizational structure and mechanical properties. They are most widely used in the traditional connecting rod manufacturing industry, especially for engines with large loads and high speeds, and occasions where connecting rods are required to have high fatigue performance and reliability.

[0003] For 46MnVS5 material, the connecting rod big end position requires lower ferrite content (≤30%) and impact energy (KV2≤20J) to ensure that the material has high brittleness and avoid the unqualified phenomenon of material expansion and uneven fracture during expansion and breaking. For the connecting rod neck position, higher ferrite content (≥25%) and impact energy (KV2≥30J) are required to ensure that the material has high tensile strength and fatigue strength (Rm: ≤1000MPa, Rp0.2≤750MPa, σ-1≥500MPa). Traditional 46MnVS5 is mainly small in size (diameter ≤40mm), mainly used in the field of passenger car engines. For commercial vehicles and large internal combustion engine expansion and breaking connecting rods, the specifications are mainly ≥60mm. Under the constraints of size effect and controlled forging and controlled cooling equipment conditions, traditional 46MnVS5 materials can no longer meet performance requirements.

[0004] The patent "A medium-carbon non-quenched and tempered steel for expanding connecting rods and its manufacturing method" (CN 110616363A) discloses a medium-carbon non-quenched and tempered steel for expanding connecting rods and its manufacturing method. The steel has a tensile strength of 950-1200MPa, a yield strength of ≥750MPa, an elongation of greater than or equal to 10%, a cross-sectional shrinkage of ≥20%, and an impact energy KV2 of 7-25J.

[0005] The patent "Medium carbon non-quenched and tempered steel and its manufacturing process and connecting rod and its preparation process" (CN 111286670A) discloses a non-quenched and tempered steel and its forging method. This type of steel adds P and Cu to the traditional non-quenched and tempered steel to increase the brittleness value of the steel. Through the patented invention method, a medium carbon non-quenched and tempered steel connecting rod is obtained, whose yield strength is ≥750MPa, tensile strength is ≥1000MPa, elongation after fracture is ≥8%, and cross-sectional shrinkage is ≥25%.

[0006] The patent "A medium carbon non-quenched and tempered steel for expansion-fracture connecting rods" (CN 101892424 B) discloses a steel for expansion-fracture connecting rods with a tensile strength of ≥800 MPa.

[0007] In summary, the 46MnVS5 composition, manufacturing method and forging process involved in the existing patents have low tensile strength and fatigue strength under large-scale conditions, and a low breaking qualification rate, which cannot meet the development needs of future high-performance engine breaking connecting rods. Summary of the invention

[0008] In view of the problems mentioned in the background technology, the purpose of the present invention is to provide a roll-resistance segmented film cutting device and a film cutting process to solve the problems mentioned in the background technology.

[0009] The above technical objectives of the present invention are achieved through the following technical solutions:

[0010] A novel non-quenched and tempered steel for an expansion-breaking connecting rod comprises the following components in weight percentage: C 0.45-0.49%, Si 0.20-0.40%, Mn 1.35-1.75%, P 0.015-0.025%, S 0.050-0.070%, Cr 0.15-0.25%, Ni 0.15-0.25%, Mo 0.05-0.10, V 0.15-0.25%, Nb 0.030-0.050%, Al 0.015-0.040%, [N] 0.040-0.060%, and the rest are Fe and unavoidable impurity elements.

[0011] The invention also discloses a method for preparing a novel non-quenched and tempered steel for a novel expansion-breaking connecting rod, comprising the following steps: using rapid heating during the smelting and heating process to avoid dephosphorization of molten steel, and at the same time, during the round steel rolling process, the temperature of the soaking section is ≥1200°C and the heating time is ≥300min.

[0012] Preferably, the tapping temperature of the steel after smelting is 1580-1620°C.

[0013] Preferably, the heating rate of the rapid heating is 400-800°C / h.

[0014] The present invention also discloses a novel controlled forging and controlled cooling process for non-quenched and tempered steel for expansion-breaking connecting rods, wherein the forging induction heating temperature is controlled to be ≥1220°C, the holding time is ≥5min, and the connecting rod neck position is subjected to local strong wind cooling treatment after rolling forging heating, so that the die forging temperature at the connecting rod big end position is 1100-1200°C, and the die forging temperature at the connecting rod neck position is 900-1050°C. After the connecting rod forging process, strong wind cooling treatment is adopted, and the cooling rate is 7-12°C / s. After cooling to 200°C, stack cooling treatment is adopted.

[0015] Preferably, the forging induction heating temperature is 1230-1245°C.

[0016] Preferably, the insulation time is 8-15 minutes.

[0017] Preferably, the die forging temperature at the connecting rod big end position is 1160-1200°C, and the die forging temperature at the connecting rod neck position is 1020-1050°C.

[0018] Preferably, the connecting rod is subjected to strong air cooling after forging, and the cooling rate is 10-12°C / s.

[0019] Preferably, the tensile strength, yield strength, elongation after fracture and reduction of area of ​​the cold-treated steel are randomly inspected and statistically analyzed.

[0020] In summary, the present invention mainly has the following beneficial effects:

[0021] The invention provides a non-quenched and tempered steel for a 60-80mm round steel for a connecting rod for expansion and fracture, and a manufacturing process thereof and a controlled forging and controlled cooling process. After the round steel is forged, the tensile strength at the big end of the connecting rod part is 1150-1250MPa, the yield strength is 830-900MPa, the elongation after fracture is 12.5%-14.5%, and the cross-sectional shrinkage is 35-40%; the tensile strength at the connecting rod neck is 1200-1300MPa, the yield strength is 850-950MPa, the elongation after fracture is 14-17%, and the cross-sectional shrinkage is 35-40%; the depth of the decarburized layer on the connecting rod surface is ≤0.2μm, and the slag rate of the part for expansion and fracture is ≤8ppm.

[0022] Compared with the traditional medium carbon 46MnVS5 expansion-break connecting rod, this steel type has a round steel specification of φ60-φ100mm. After forging, the connecting rod parts have a tensile strength of 1050-1100MPa at the big end, a yield strength of 740-780MPa, an elongation of 13%-15%, and a cross-sectional shrinkage of 35-40%; the tensile strength of the connecting rod neck is 1100-1150MPa, the yield strength is 760-800MPa, the elongation is 15-18%, and the cross-sectional shrinkage is 35-40%; the depth of the decarburized layer on the connecting rod surface is ≤0.2μm, and the slag rate of the parts is ≤10ppm. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a crystal phase comparison diagram of Examples 1-7 of the present invention. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Combined with Figure 1 The present invention is described in detail with reference to Examples 1-6 and conventional steels, wherein Examples 4-6 use the steel of Example 1 but do not use the controlled forging and controlled cooling process of the present invention, and Example 7 (conventional steel) is 46MnVS5.

[0026] The chemical composition weight percentages of the non-quenched and tempered steels of Examples 1-3 and the conventional 46MnVS5 non-quenched and tempered steels are shown in Table 1. Examples 1-3 and the conventional steels are all smelted in an electric furnace, and are directly continuously cast into 250×250 mm square billets after LF refining + RH vacuum degassing, and are heated and rolled into φ16-95 mm round steels. After the round steels are cut, heated in a medium frequency induction furnace, rolled, die forged, and trimmed, they are controlled cooled on a controlled cooling line. The specific controlled forging and controlled cooling processes and conventional steel heat treatment processes are shown in Table 2. Standard tensile and impact specimens and metallographic specimens are taken from the finished parts for mechanical property analysis and microstructure analysis.

[0027] Table 1 Chemical composition of Examples 1-3 and conventional steel ([N] is ppm, others are wt%)

[0028]

[0029]

[0030] The object of the present invention is to provide a novel non-quenched and tempered steel for expansion-breaking connecting rods, which has a tensile strength of 1150-1300MPa, a yield strength of 800-900MPa, an elongation after fracture ≥8%; a room temperature impact energy (KV2) of the big head position ≤10J, and a ferrite content ≤20%; a room temperature impact energy (KV2) of the I-neck position ≥45J, and a ferrite content ≥35%, and a controlled forging and controlled cooling process thereof.

[0031] The present invention also provides a controlled forging and controlled cooling process for non-quenched and tempered steel for automobile expansion-breaking connecting rods. The heat treatment process is characterized by: a forging heating temperature of 1220-1260°C, an initial forging (roll forging) temperature of 1200°C-1250°C, a relatively high final forging (die forging) temperature (1150°C-1200°C) is adopted at the big end position, and local air cooling is adopted at the I-neck and small end position after roll forging to obtain a relatively low final forging temperature (950°C-1050°C). After forging, strong wind cooling is adopted, and the cooling rate is 5-7°C / s. After cooling to 400°C, air cooling is performed to a surface temperature of the part ≤150°C.

[0032] Table 2 Summary of heat treatment process, mechanical properties and ferrite content of Examples 1-6 and conventional steel

[0033]

[0034]

[0035]

[0036] like Figure 1 Table 2 is a summary of the metallographic structures of Examples 1-6 and conventional non-quenched and tempered steels, and Table 2 is a summary of Examples 1-6 and mechanical properties and ferrite content. It can be seen that the steel of the present invention adopts the controlled forging and controlled cooling process provided by the present invention to fully trial-produce a tensile strength of 1150-1300MPa, a yield strength of 800-900MPa, and an elongation after fracture ≥8%; the room temperature impact energy (KV2) of the big end position is ≤10J, and the ferrite content is ≤20%; the room temperature impact energy (KV2) of the I-neck position is ≥45J, and the ferrite content is ≥35%. The new type of non-quenched and tempered steel for expansion-breaking connecting rods and its controlled forging and controlled cooling process. Although the steel of the present invention is used but the controlled forging and controlled cooling process of the present invention is not used, the impact energy at the big end of the connecting rod is significantly lower. Although the impact toughness of the steel of the present invention is slightly lower than that of conventional quenched and tempered steel, its comprehensive mechanical properties fully meet the requirements of safety parts.

[0037] Embodiment 8, the purpose of the present invention is to provide a non-quenched and tempered steel for a round steel specification of φ60-φ80mm for a connecting rod for expansion and breaking, and its manufacturing and controlled forging and controlled cooling process. After the round steel is forged, the tensile strength of the connecting rod part at the big end is 1150-1250MPa, the yield strength is 830-900MPa, the elongation after fracture is 12.5%-14.5%, and the cross-sectional shrinkage is 35-40%; the tensile strength of the connecting rod neck is 1200-1300MPa, the yield strength is 850-950MPa, the elongation after fracture is 14-17%, and the cross-sectional shrinkage is 35-40%; the depth of the decarburized layer on the connecting rod surface is ≤0.2μm, and the slag rate of the part after expansion and breaking is ≤8ppm.

[0038] According to the above objectives, the technical scheme adopted by the present invention is: (1) retaining part of the P content by rapidly heating up during the electric furnace smelting process; (2) improving the grain boundary brittleness through the grain boundary segregation effect of P, thereby improving the flatness of the connecting rod fracture and reducing the slag rate of fracture; (3) appropriately increasing the Mn content on the basis of traditional 46MnVS5, delaying the pearlite-ferrite transformation temperature, refining the pearlite pellet diameter and lamellar spacing, and improving the strength of the connecting rod material at the big end position; (4) adding an appropriate amount of Mo on the basis of traditional 46MnVS5 to increase the phase change driving force and improve the fracture qualification rate of parts; (5) During the production of round steel, the temperature of the equalizing section is ≥1200℃ and the heating time is ≥300min to improve the homogeneity of the round steel material; (6) During the production of parts, the induction heating temperature is ≥1220℃ and the heating time is ≥5min, so as to improve the micro-segregation of parts, improve the uniformity of materials, and avoid crack instability caused by hard points of micro-segregation during the expansion and breaking process, resulting in expansion and breaking and slag falling; (7) After the parts are roll forged, local strong air cooling is used at the connecting rod neck position, so that the die forging temperature at the connecting rod big end position is 1100-1200℃, and the die forging temperature at the connecting rod neck position is 900-1050℃. During the forging process, microalloying elements precipitate in the proeutectoid ferrite at the connecting rod big end, thereby increasing the hardness of the ferrite and facilitating a smooth fracture of the connecting rod; at the connecting rod neck and the connecting rod small end, microalloying elements precipitate in the austenite, thereby refining the austenite grains and facilitating the improvement of the toughness of the connecting rod; (8) After forging, the connecting rod is subjected to strong air cooling at a cooling rate of 7-12°C / s. After cooling to 200°C, it is subjected to stack cooling to eliminate residual stress through self-tempering and avoid uneven fracture.

[0039] The invention provides a novel non-quenched and tempered steel for a breaking connecting rod, characterized in that the chemical composition weight percentage is: C 0.45-0.49%, Si 0.20-0.40%, Mn 1.35-1.75%, P0.015-0.025%, S 0.050-0.070%, Cr 0.15-0.25%, Ni 0.15-0.25%, Mo0.05-0.10, V 0.15-0.25%, Nb 0.030-0.050%, Al 0.015-0.040%, [N] 0.040-0.060%, and the rest is Fe and inevitable impurity elements.

[0040] The present invention also provides a novel non-quenched and tempered round steel smelting and rolling process for expansion-breaking connecting rods. The process is characterized in that rapid heating is used during the smelting heating process to avoid dephosphorization of molten steel. At the same time, during the round steel rolling process, the temperature of the soaking section is ≥1200°C and the heating time is ≥300min.

[0041] The present invention also provides a novel controlled forging and controlled cooling process for non-quenched and tempered steel parts for expansion-breaking connecting rods. The process is characterized in that: the forging induction heating temperature is ≥1220°C, the insulation time is ≥5min, and after rolling forging heating, the connecting rod neck position is subjected to local strong air cooling treatment, so that the die forging temperature at the connecting rod big end position is 1100-1200°C, and the die forging temperature at the connecting rod neck position is 900-1050°C. After the connecting rod forging process, strong air cooling treatment is adopted, and the cooling rate is 7-12°C / s. After cooling to 200°C, stack cooling treatment is adopted.

[0042] The functions and proportions of the above elements and the basis of controlled forging and controlled cooling process are as follows:

[0043] C: C is necessary to obtain high strength and hardness, and with the increase of C content, the plasticity and toughness of steel can be significantly reduced, thus obtaining good expansion and fracture performance. Too high carbon content will lead to poor toughness at the connecting rod neck position and high notch sensitivity, resulting in low fatigue strength; too low C content will easily lead to low strength and good toughness, resulting in the problem that the connecting rod big end position cannot expand or the expansion and fracture deformation is too large. In summary, the C content is controlled to be 0.45-0.49%.

[0044] Si: Si is the main deoxidizing element in steel and has a strong solid solution strengthening effect, so the Si content should be ≥0.20%. However, too high Si content will reduce the plasticity and toughness of the steel, increase the activity of C, promote the decarburization and graphitization tendency of the steel during rolling and forging heating, which is not conducive to the control of the decarburization layer of the connecting rod parts, resulting in uneven fracture during the expansion and breaking process, and reducing the qualified rate of expansion and breaking. Therefore, the Si content is ≤0.40%. In summary, the Si content is controlled to be 0.20~0.40%.

[0045] Mn: On the one hand, Mn can combine with S to form MnS, improving cutting performance. On the other hand, it can significantly delay the pearlite-ferrite phase transformation, reduce the ferrite content, refine the pearlite spheroids, reduce the pearlite lamellar spacing, and improve the strength of the steel. Therefore, Mn ≥ 1.35%. However, the Mn content should not be too high. A higher Mn content is not conducive to improving the process window of the material and deteriorates the strength and toughness of the material. In addition, a higher Mn content is not conducive to improving the decarburization layer control of the connecting rod parts. Therefore, the Mn content is ≤ 1.75%. In summary, the Mn content is controlled to be 1.35%-1.75%.

[0046] Cr: Cr can effectively improve the hardenability of steel and delay the pearlite-ferrite phase transformation to obtain the required high strength, and can also significantly improve the yield strength ratio through solid solution strengthening; at the same time, Cr can also reduce the activity of C, reduce the decarburization tendency of the steel surface during heating, rolling and forging, and obtain high fatigue resistance, so Cr≥0.10%. However, too high a content will deteriorate the toughness of the steel, so Cr≤0.25%. In summary, the Cr content is controlled to be 0.10-0.30%.

[0047] P: forms micro segregation when the molten steel solidifies, and then segregates to the grain boundary when heated at the austenite post-temperature, which significantly increases the brittleness of the steel, thereby reducing the plastic toughness of the steel. Although it can significantly improve the expansion and fracture performance of the expansion and fracture connecting rod, the P content is ≥0.015%. However, a higher P content will lead to a decrease in fatigue performance, which is not conducive to the differential control of the connecting rod performance. Therefore, the P content is ≤0.035%. In summary, the P content is controlled to be 0.015-0.025%.

[0048] S: S and Mn form MnS, which significantly improves the cutting performance of the steel. Since the strength of the steel of the present invention is relatively high, in order to improve the cutting performance, the S content should be controlled at 0.050-0.070%.

[0049] Ni: It has the effect of improving the fatigue strength of steel materials. However, if the Ni content is too high, the machinability after hot working will be reduced, so the Ni content is controlled to 0.15-0.25%.

[0050] Al: Al is an effective deoxidizing element. It can effectively refine the austenite grain size by combining with N to form AlN. However, if the Al content is too high, A1203 spinel and AIN will be easily formed, which are hard and brittle inclusions, causing sprue accumulation and fatigue performance degradation. Therefore, the Al content is ≤0.040%. In summary, the Al content is controlled to be 0.015-0.045%.

[0051] V: V forms V (C, N) precipitation phase with N and C elements in steel, which can play a role in grain refinement and precipitation strengthening, thereby improving the strength and yield strength ratio of steel. For the present invention, local air cooling treatment is performed after roll forging to achieve differentiated performance treatment of the connecting rod big end and the connecting rod neck position. The V-containing precipitation phase is precipitated in the proeutectoid ferrite at the big end position, so that the big end position has high strength but low toughness, which is conducive to expansion and breaking processing. The V-containing precipitation phase is precipitated in the austenite at the connecting rod neck position, so that the strength and toughness of the connecting rod neck position are well matched, which is conducive to improving fatigue performance and reducing notch sensitivity. Therefore, the V content is ≥0.15%. However, if the V content is too high, it is still easy to cause excessive ferrite content at the big end position, shorten the forging process window, and reduce the expansion and breaking qualification rate. Therefore, considering the two aspects of improving strength and improving expansion and breaking performance, the V content is controlled to be 0.15-0.25%.

[0052] Nb: The role of Nb in steel is similar to that of V. Nb forms Nb(C,N) precipitation phase with N and C elements in steel, which can refine grains and strengthen precipitation. Unlike V, solid solution Nb can also significantly delay the pearlite-ferrite phase transformation. High temperature forging is used at the large head to avoid Nb precipitation in austenite and inhibit the grain refinement of Nb. The precipitation strengthening effect of Nb in the pearlite-ferrite phase transformation process can significantly reduce the toughness while improving the strength of the large head. For the I-neck and small head, relatively low forging temperature is used to promote the precipitation of Nb in austenite. The strength and yield strength ratio of the I-neck are improved while improving the toughness. Therefore, considering the two aspects of improving strength and improving expansion and fracture properties, the Nb content is controlled at 0.030-0.050%.

[0053] Mo: For large-sized parts, an appropriate amount of Mo can improve the hardenability of the material, promote phase transformation of the material, and cooperate with elements such as C, Mn, Nb, and V to refine the spacing between pearlite lamellae and improve the yield strength of the material. Therefore, the Mo content is ≥0.05%. However, higher Mo is prone to cause bainite formation, which narrows the window of differential controlled cooling process for parts, which is not conducive to the control of the qualified rate of part expansion and fracture. Therefore, the Mo content is ≤0.010%. In summary, the Mo content is controlled at 0.05-0.10%

[0054] N: N is easy to combine with Al, V, etc. to form AIN, VN, which can inhibit the growth of austenite grains at high temperatures, thereby improving the fatigue performance of steel materials. However, when steel contains Ti, N is very easy to combine with Ti to form TiN, which deteriorates the fatigue performance of the material, so the N content is ≤60ppm. In summary, the [N] content is controlled at 0.040-0.060%.

[0055] Example 9, a new type of non-quenched and tempered steel for expansion-breaking connecting rods, comprising the following components in weight percentage: C 0.45-0.49%, Si 0.20-0.40%, Mn 1.35-1.75%, P 0.015-0.025%, S0.050-0.070%, Cr 0.15-0.25%, Ni 0.15-0.25%, Mo 0.05-0.10, V0.15-0.25%, Nb 0.030-0.050%, Al 0.015-0.040%, [N] 0.040-0.060%, and the rest are Fe and unavoidable impurity elements.

[0056] This embodiment also discloses a method for preparing a novel non-quenched and tempered steel for a breakaway connecting rod, comprising the following steps: rapid heating is used during the smelting and heating process to avoid dephosphorization of molten steel, and the temperature of the soaking section is ≥1200°C and the heating time is ≥300min during the round steel rolling process. The tapping temperature after smelting is 1580-1620°C. The heating rate of the rapid heating is 400-800°C / h.

[0057] This embodiment also discloses a controlled forging and controlled cooling process for a novel non-quenched and tempered steel for a new type of expansion-breaking connecting rod, wherein the forging induction heating temperature is controlled to be ≥1220°C, the holding time is ≥5min, and the connecting rod neck position is subjected to local strong wind cooling treatment after rolling forging heating, so that the die forging temperature at the connecting rod big end position is 1100-1200°C, and the die forging temperature at the connecting rod neck position is 900-1050°C. After the connecting rod forging process, strong wind cooling treatment is adopted, and the cooling rate is 7-12°C / s. After cooling to 200°C, stack cooling treatment is adopted. Among them, the forging induction heating temperature is 1230-1245°C. Among them, the holding time is 8-15min.

[0058] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A new type of non-quenched and tempered steel for expansion-breaking connecting rods, characterized in that: The invention comprises the following components in weight percentage: C 0.45-0.49%, Si 0.20-0.40%, Mn 1.35-1.75%, P 0.015-0.025%, S0.050-0.070%, Cr 0.15-0.25%, Ni 0.15-0.25%, Mo 0.05-0.10, V0.15-0.25%, Nb 0.030-0.050%, Al 0.015-0.040%, [N] 0.040-0.060%, and the rest are Fe and unavoidable impurity elements.

2. A method for preparing a novel non-quenched and tempered steel for a novel expansion-breaking connecting rod, characterized in that: The following steps are involved: Rapid heating is used during the smelting heating process to avoid dephosphorization of molten steel. At the same time, during the round steel rolling process, the temperature of the soaking section is ≥1200℃ and the heating time is ≥300min.

3. The method for preparing a novel non-quenched and tempered steel for a breaking connecting rod according to claim 2, characterized in that: The tapping temperature of the steel after smelting is 1580-1620°C.

4. The method for preparing a novel non-quenched and tempered steel for a breaking connecting rod according to claim 2, characterized in that: The heating rate of the rapid heating is 400-800° C. / h.

5. A new controlled forging and cooling process for non-quenched and tempered steel for expansion-breaking connecting rods, characterized in that: Control the forging induction heating temperature ≥1220℃, and the holding time ≥5min. After rolling forging heating, perform local strong air cooling treatment on the connecting rod neck position, so that the die forging temperature of the connecting rod big end position is 1100-1200℃, and the die forging temperature of the connecting rod neck position is 900-1050℃. After the connecting rod forging process, strong air cooling treatment is adopted, and the cooling rate is 7-12℃ / s. After cooling to 200℃, pile cooling treatment is adopted.

6. The controlled forging and controlled cooling process of a novel non-quenched and tempered steel for a breaking connecting rod according to claim 5 is characterized in that: The forging induction heating temperature is 1230-1245°C.

7. The controlled forging and controlled cooling process of a novel non-quenched and tempered steel for a breaking connecting rod according to claim 5 is characterized in that: The insulation time is 8-15 minutes.

8. The controlled forging and cooling process of a novel non-quenched and tempered steel for a breaking connecting rod according to claim 5 is characterized in that: The die forging temperature of the connecting rod big end position is 1160-1200°C, and the die forging temperature of the connecting rod neck position is 1020-1050°C.

9. The controlled forging and cooling process of a novel non-quenched and tempered steel for a breaking connecting rod according to claim 5 is characterized in that: After the connecting rod is forged, it is subjected to strong air cooling treatment, and the cooling rate is 10-12°C / s.

10. The controlled forging and cooling process of a novel non-quenched and tempered steel for a breaking connecting rod according to claim 5, characterized in that: The tensile strength, yield strength, elongation after fracture and reduction of area of ​​the cold-treated steel are randomly inspected and statistically analyzed.

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