A method for preparing non-quenched and tempered steel for medium-carbon high-strength and high-toughness connecting rod

Abstract

The invention discloses a method for preparing a medium-carbon high-strength and high-toughness non-quenched and tempered steel for a connecting rod, which belongs to the process field of quenched and tempered steel. The chemical composition of the non-quenched and tempered steel includes: C: 0.44-0.48%, Si: 0.55-0.70%, Mn: 1.00-1.20%, S: 0.055-0.070%, P: ≤0.025%, Cr: 0.10-0.40%, Mo: 0.02-0.25%, Cu: ≤0.20%, Ni: 0.10-0.40%, V: 0.05-0.35%, Al: 0.005-0.025%, N: 0.010-0.020%, H≤2.0ppm; the rest are Fe and other inevitable impurities. The preparation steps are as follows: electric furnace smelting, LF refining, VD refining, continuous casting and rolling process. The invention achieves the synergistic effect of precipitation strengthening and fine grain strengthening through microalloying of Nb, V and Mo, thereby improving the strength and toughness of steel; the preparation method obtains high-clean steel through refining outside the furnace and obtains fine and uniform structure through controlled rolling and controlled cooling technology, which can further improve the strength.

Description

Technical Field

[0001] The invention belongs to the technical field of quenched and tempered steel, and relates to a preparation method of non-quenched and tempered steel for medium-carbon high-strength and high-toughness connecting rods. Background Art

[0002] At present, general non-quenched and tempered connecting rod steel adopts Nb, V, Ti micro-alloy refined composition design. The tensile strength of non-quenched and tempered expansion-break connecting rod steel can reach 900-1000Mpa, and the yield strength is 600-700Mpa. However, it is difficult to meet the tensile strength of non-quenched and tempered expansion-break connecting rod steel for heavy trucks, which is above 1000MPa, and the yield strength is ≥750Mpa.

[0003] The existing patent application CN 110343954A is a steel for automobile engine connecting rod and a manufacturing method thereof. The patent introduces a steel for automobile engine connecting rod with a tensile strength greater than 1000MPa and a yield strength greater than 700MPa and a manufacturing method thereof. The steel mainly uses Nb and Ti to react with Nb and Ti in the steel to generate TiN, Ti(C, N), and Nb(C, N), further refining the grains and improving the strength of the steel. However, the yield strength still does not reach more than 750MPa, which cannot meet the requirement of a yield strength ≥750MPa for connecting rods for heavy trucks.

[0004] Therefore, it is urgent to solve the above problems. Summary of the invention

[0005] In view of the above problems, the first purpose of the present invention is to provide a non-quenched and tempered steel for connecting rods with medium carbon, high strength and high toughness, which achieves the synergistic effect of precipitation strengthening and fine grain strengthening through microalloying of Nb, V and Mo, thereby improving the strength and toughness of the steel.

[0006] The second object of the present invention is to provide a method for preparing the non-quenched and tempered steel for medium carbon high-strength and high-toughness connecting rod. The preparation method obtains high-cleanliness steel through refining outside the furnace and obtains a fine and uniform structure through controlled rolling and controlled cooling technology, which can further improve the strength.

[0007] The technical solution of the present invention is: the present invention provides a medium-carbon high-strength, high-toughness non-quenched and tempered steel for connecting rods, characterized in that: the chemical components of the non-quenched and tempered steel are calculated by weight percentage: C: 0.44-0.48%, Si: 0.55-0.70%, Mn: 1.00-1.20%, S: 0.055-0.070%, P: ≤0.025%, Cr: 0.10-0.40%, Mo: 0.02-0.25%, Cu: ≤0.20%, Ni: 0.10-0.40%, V: 0.05-0.35%, Al: 0.005-0.025%, N: 0.010-0.020%, H≤2.0ppm; the rest are Fe and other inevitable impurities.

[0008] Furthermore, the microstructure of the non-quenched and tempered steel is fine lamellar pearlite and ferrite, and the metallographic microstructure grain size of the round steel can reach level 7 to 8.

[0009] Furthermore, the tensile strength of the connecting rod made of non-quenched and tempered steel reaches ≥1100Mpa, the yield strength ≥780Mpa, the elongation ≥12%, and the cross-sectional shrinkage ≥25%; the tensile strength of the big head of the connecting rod reaches ≥1080Mpa, the yield strength ≥730Mpa, the elongation ≥10%, and the cross-sectional shrinkage ≥20%.

[0010] Furthermore, a method for preparing a medium carbon high strength and high toughness non-quenched and tempered steel for a connecting rod comprises the following steps:

[0011] Electric furnace smelting, LF refining, VD refining, continuous casting, heating and rolling steps.

[0012] Furthermore, nickel-iron alloy is added during electric furnace smelting to reduce the use of pure nickel plate after steelmaking and increase Ni;

[0013] After the steel is smelted in an electric arc furnace, Alt is adjusted to 0.010-0.030%; high-purity graphite carbon materials, ferromanganese, ferrosilicon, ferromolybdenum, nickel plates, ferroniobium, ferrochrome and ferrovanadium are used for alloying.

[0014] Furthermore, N2 gas is blown during the whole LF refining process, which can not only make the inclusions float fully, but also increase N through N2 gas to reduce the use of N alloy. SiC deoxidation is used in the early stage, the white slag time is ≥30min, and the Alt content is above 0.015%.

[0015] Furthermore, the slag basicity is increased to 2.5-4.0 in the early stage of LF to ensure that the distribution ratio of sulfur in the slag is increased after sufficient desulfurization. FeS is used to adjust S to 0.050-0.065% before entering the vacuum at the end of LF refining. The basicity of the slag is lowered to 2.0-3.0 in the later stage to increase the S recovery rate.

[0016] Furthermore, N2 gas was blown throughout the VD vacuum degassing process, and the treatment lasted for ≥15 min at a vacuum degree ≤1 mbar. After breaking the vacuum, static stirring lasted for ≥10 min to ensure that the inclusions fully floated and the H was reduced to below 2 ppm.

[0017] Furthermore, the continuous casting process adopts full-protection casting, electromagnetic stirring is used in the continuous casting process, the electromagnetic stirring intensity of the crystallizer is 150-300A, the end electromagnetic stirring is 500-700A, and the light pressure reduction technology is used, with a reduction of 8-13mm.

[0018] Furthermore, the rolling process includes heating, start rolling and final rolling; the rolling heating temperature is 1200-1250°C, the start rolling temperature is 1050-1150°C, and the final rolling temperature is 850-930°C.

[0019] The present invention discloses a connecting rod steel, wherein the connecting rod is made of the above-mentioned high-strength and high-toughness non-quenched and tempered steel.

[0020] The beneficial effects of the present invention are as follows: 1. In the process scheme of the present invention, N2 gas is blown during the whole refining process to make the inclusions fully float, and the use of N alloy can be reduced by increasing N through N2 gas, thereby saving costs; high-purity steel and a higher alloy yield can be obtained by controlling the refining outside the furnace and changing the slag system; 2. In the present invention, by using Al, Nb, V and combining the controlled rolling and controlled cooling process, the structure of the non-quenched and tempered steel is fine lamellar pearlite and ferrite, AlN, Nb (C, N), V (C, N) carbonitrides are precipitated, and the grains are refined. The metallographic grain size of the round steel can reach level 7 to 8; 3. The present invention precipitates MoC, Nb (C, N), V (C, N) carbide precipitation strengthening effect through the combination of Mo, Nb, V and C, N, so that the tensile strength of the connecting rod reaches ≥1100Mpa, the yield strength is ≥780Mpa, the elongation is ≥12%, and the cross-sectional shrinkage is ≥25%; the tensile strength of the big head of the connecting rod reaches ≥1080Mpa, the yield strength is ≥730Mpa, the elongation is ≥10%, and the cross-sectional shrinkage is ≥20%. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the metallographic structure of round steel in an embodiment of the present invention; wherein (a) is a schematic diagram of the edge, and (b) is a schematic diagram of the core;

[0022] Figure 2 It is a preparation flow chart of the present invention. DETAILED DESCRIPTION

[0023] The specific technical solutions of the present invention are further described in detail below in conjunction with the accompanying drawings and specific examples.

[0024] The present invention discloses a non-quenched and tempered steel for a medium-carbon, high-strength and high-toughness connecting rod. The components of the non-quenched and tempered steel are, by mass percentage, C: 0.44-0.48%, Si: 0.55-0.70%, Mn: 1.00-1.20%, S: 0.055-0.070%, P: ≤0.025%, Cr: 0.10-0.40%, Mo: 0.02-0.25%, Cu: ≤0.20%, Ni: 0.10-0.40%, V: 0.05-0.35%, Al: 0.005-0.025%, N: 0.010-0.020%, H≤2.0ppm; the rest are Fe and other inevitable impurities.

[0025] The present invention provides a non-quenched and tempered steel for an engine connecting rod. By using Al, Nb, and V and combining a controlled rolling and controlled cooling process, the non-quenched and tempered steel has a fine lamellar pearlite and ferrite structure, precipitates AlN, Nb (C, N), and V (C, N) carbonitrides, and refines the grains. The grain size of the metallographic structure of the round steel can reach level 7 to 8.

[0026] The present invention combines Mo, Nb, V with C and N to precipitate MoC, Nb (C, N) and V (C, N) carbides to strengthen the precipitation, so that the tensile strength of the connecting rod reaches ≥1100Mpa, the yield strength reaches ≥780Mpa, the elongation reaches ≥12%, and the cross-sectional shrinkage reaches ≥25%; the tensile strength of the big head of the connecting rod reaches ≥1080Mpa, the yield strength reaches ≥730Mpa, the elongation reaches ≥10%, and the cross-sectional shrinkage reaches ≥20%.

[0027] The method for preparing a medium-carbon high-strength and high-toughness non-quenched and tempered steel for a connecting rod described in the present invention comprises the following steps: electric furnace smelting, LF refining, VD refining, continuous casting, heating and rolling processes.

[0028] Furthermore, nickel-iron alloy is added during electric furnace smelting to reduce the use of pure nickel plate after steelmaking and increase Ni;

[0029] After the steel is smelted in an electric arc furnace, Alt is adjusted to 0.010-0.030%; high-purity graphite carbon materials, ferromanganese, ferrosilicon, ferromolybdenum, nickel plates, ferroniobium, ferrochrome and ferrovanadium are used for alloying.

[0030] Furthermore, N2 gas is blown throughout the LF refining process, which can not only make the inclusions fully float, but also increase the N through N2 gas to reduce the use of N alloy; SiC deoxidation is used in the early stage, the white slag time is ≥30min, and the Alt content is above 0.015%;

[0031] In the early stage of LF, the slag basicity is increased to 2.5-4.0 to ensure that the distribution ratio of sulfur in the slag is increased after sufficient desulfurization. Before entering the vacuum at the end of LF refining, FeS is used to adjust S to 0.050-0.065%. In the later stage, the slag basicity is lowered to 2.0-3.0 to increase the S recovery rate.

[0032] Furthermore, N2 gas was blown throughout the VD vacuum degassing process, and the treatment lasted for ≥15 min at a vacuum degree ≤1 mbar. After breaking the vacuum, static stirring lasted for ≥10 min to ensure that the inclusions fully floated and the H was reduced to below 2 ppm.

[0033] Furthermore, the continuous casting process adopts full protection casting, electromagnetic stirring is used in the continuous casting process, the electromagnetic stirring intensity of the crystallizer is 150-300A, the end electromagnetic stirring is 500-700A, and the light pressure reduction technology is used, with a reduction of 8-13mm.

[0034] Furthermore, the rolling process includes heating, start rolling and final rolling; the rolling heating temperature is 1200-1250°C, the start rolling temperature is 1050-1150°C, and the final rolling temperature is 850-930°C.

[0035] The present invention discloses a connecting rod steel, wherein the connecting rod is made of the above-mentioned high-strength and high-toughness non-quenched and tempered steel.

[0036] Example 1

[0037] This embodiment 1 provides a high-strength and high-toughness non-quenched and tempered connecting rod steel. The component contents of the non-quenched and tempered steel are measured in percentage by mass and include: C: 0.47%, Si: 0.65%, Mn: 1.15%, S: 0.065%, P: 0.013%, Cr: 0.25%, Mo: 0.06%, Cu: 0.05%, Ni: 0.18%, Nb: 0.036%, V: 0.16%, Al: 0.015%, N: 0.0155%, H: 1.4ppm; the rest are Fe and other inevitable impurities.

[0038] The preparation method thereof comprises the following steps:

[0039] Electric furnace smelting, LF refining, VD refining, continuous casting, heating and rolling processes.

[0040] Add nickel-iron alloy during electric furnace smelting to reduce the use of pure nickel plate after steelmaking and increase Ni;

[0041] After the steel is smelted in an electric arc furnace, the Alt is adjusted to 0.010-0.030%; high-purity graphite carbon materials, ferromanganese, ferrosilicon, ferromolybdenum, nickel plates, ferroniobium, ferrochrome, and ferrovanadium are used for alloying;

[0042] Blowing N2 gas during the whole LF refining process can not only make the inclusions float fully, but also increase N through N2 gas to reduce the use of N alloy. SiC deoxidation is used in the early stage, the white slag time is ≥30min, and the Alt content is above 0.015%;

[0043] In the early stage of LF, the slag basicity is increased to 2.5-4.0 to ensure that the distribution ratio of sulfur in the slag is increased after sufficient desulfurization. Before entering the vacuum after the LF refining, FeS is used to adjust the S to 0.050-0.065%. In the later stage, the slag basicity is lowered to 2.0-3.0 to increase the S recovery rate;

[0044] During the VD vacuum degassing process, N2 gas is blown throughout the process, and the treatment is carried out at a vacuum degree of ≤1mbar for ≥15min. After breaking the vacuum, static stirring is carried out for ≥10min to ensure that the inclusions are fully floated and the H is reduced to below 2ppm.

[0045] The continuous casting process adopts full protection casting, and electromagnetic stirring is used in the continuous casting process. The electromagnetic stirring intensity of the crystallizer is 150~300A, the end electromagnetic stirring is 500~700A, and the light pressure reduction technology is used, with a reduction of 8~13mm.

[0046] The rolling process includes heating, start rolling and final rolling; the rolling heating temperature is 1200-1250°C, the start rolling temperature is 1050-1150°C, and the final rolling temperature is 850-930°C.

[0047] Table 1 shows the chemical composition of the non-quenched and tempered steel of Examples 1-3 and Comparative Examples 1-3, and Table 2 shows the comparison of connecting rod performance. The contents not mentioned in the table are the same as those in Example 1.

[0048] Table 1 Chemical composition of non-quenched and tempered steel (wt, %)

[0049]

[0050]

[0051] Table 2 Connecting rod performance

[0052]

Claims

1. A method for preparing non-quenched and tempered steel for medium carbon high strength and high toughness connecting rod, characterized in that: The components of the non-quenched and tempered steel are measured in percentage by mass and include: C: 0.47%, Si: 0.65%, S: 0.065%, P: 0.013%, Mn: 1.15%, Ni: 0.18%, Cr: 0.25%, Mo: 0.06%, V: 0.16%, Cu: 0.05%, Al: 0.015%, Nb: 0.036%, N: 0.0155%, H: 1.4ppm; the rest is Fe and other inevitable impurities; The preparation method thereof comprises the following steps: Electric furnace smelting, LF refining, VD refining, continuous casting, heating and rolling processes; Adding nickel-iron alloy during electric furnace smelting can reduce the use of pure nickel plates after steelmaking; After the steel is smelted in an electric furnace, the Alt is adjusted to 0.010-0.030%; high-purity graphite, ferromanganese, ferrosilicon, ferromolybdenum, nickel plate, ferroniobium, ferrochrome and ferrovanadium are used for alloying; N2 gas is blown throughout the LF refining process; SiC deoxidation is used in the early stage, the white slag time is ≥30min, and the Alt content is above 0.015%; In the early stage of LF, the slag basicity is increased to 2.5-4.

0. At the end of LF refining, FeS is used to adjust the S to 0.050-0.065% before entering the vacuum. In the later stage, the slag basicity is lowered to 2.0-3.

0. During the VD vacuum degassing process, N2 gas is blown throughout the process, and the vacuum degree is ≤1mbar for ≥15min. After breaking the vacuum, the mixture is stirred for ≥10min to ensure that the inclusions float fully and the H is reduced to below 2ppm. The continuous casting process adopts full protection casting, and electromagnetic stirring is used in the continuous casting process. The crystallizer electromagnetic stirring intensity is 150-300A, the end electromagnetic stirring is 500-700A, and the light pressure reduction technology is used, with a pressure reduction of 8-13mm. The rolling process includes heating, start rolling and final rolling; the rolling heating temperature is 1200-1250°C, the start rolling temperature is 1050-1150°C, and the final rolling temperature is 850-930°C; The properties of the connecting rod made of non-quenched and tempered steel are as follows: rod body: yield strength: 800 Mpa, tensile strength: 1130 Mpa, elongation after fracture: 14.5%, section shrinkage: 29%; Big head: yield strength: 750Mpa, tensile strength: 1105 Mpa, elongation after break: 13%, section shrinkage: 27%.

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