CN115058655A

CN115058655A - Non-quenched and tempered steel for Nb microalloying medium-carbon expansion-broken connecting rod, expansion-broken connecting rod produced by using non-quenched and tempered steel and forging-controlled cooling process

Info

Publication number: CN115058655A
Application number: CN202210759428.9A
Authority: CN
Inventors: 金国忠; 汪开忠; 许倩; 胡芳忠; 杨少朋; 杨志强; 陈世杰; 王自敏; 高振波
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2022-06-29
Filing date: 2022-06-29
Publication date: 2022-09-16
Anticipated expiration: 2042-06-29
Also published as: CN115058655B

Abstract

The invention provides non-quenched and tempered steel for a Nb microalloying medium carbon expansion-broken connecting rod, an expansion-broken connecting rod produced by the non-quenched and tempered steel and a forging and cooling control process, which comprises the following components: 0.40-0.50% of C, 0.40-0.90% of Si, 1.30-1.60% of Mn, less than or equal to 0.045% of P, 0.040-0.070% of S, 0.10-0.30% of Cr, 0.40-0.90% of V, 0.020-0.050% of Nb, less than or equal to 0.030% of Al, 0.0100-0.0160% of [ N ], and the balance of Fe and inevitable impurity elements. The forging and cooling control process is adopted for production, so that the product achieves the tensile strength of 1100-1250 MPa, the yield strength of 750-850 MPa, the elongation after fracture of more than or equal to 12%, the fatigue strength of more than or equal to 590MPa and the Brinell hardness of more than or equal to 330HB, and the performance requirement of a high-performance expansion fracture connecting rod is met.

Description

Nb microalloying medium carbon non-quenched and tempered steel for expansion-broken connecting rod, expansion-broken connecting rod produced from same and forging-controlled and cooling-controlled process

Technical Field

The invention belongs to the technical field of alloy structural steel, and particularly relates to non-quenched and tempered steel for a Nb microalloying medium carbon expansion-fracture connecting rod, an expansion-fracture connecting rod produced by the non-quenched and tempered steel and a forging-control and cooling-control process.

Background

The connecting rod is a key part of an automobile engine, the function of the connecting rod is to transmit gas acting force on a piston to a crankshaft, reciprocating motion of the piston is converted into rotary motion of the crankshaft, and in order to ensure the reliability of the engine, the connecting rod is required to have high enough fatigue strength and rigidity. The connecting rod has complex machining process and high precision requirement, and is particularly a big-end hole of the connecting rod. Out-of-roundness of a connecting rod big end hole caused by machining precision errors is an important factor influencing the performance reliability of an engine. The expansion-break (cracking) connecting rod technology developed abroad in recent years solves the problem. The technology has incomparable advantages compared with the traditional connecting rod processing technology, has less processing procedures, saves finish machining equipment, saves materials and energy, and obviously reduces the production cost. The connecting rod material for expansion fracture processing mainly comprises powder metallurgy material, forged steel, ball-milling cast iron and malleable cast iron. The forged steel connecting rod has high dimensional precision and good structure and mechanical properties, is most widely applied in the manufacturing industry of the traditional connecting rod, and is particularly used for engines with large load and high rotating speed and occasions requiring the connecting rod to have high fatigue performance and reliability.

The high-carbon non-heat-treated steel represented by C70S6 steel has a low strength (connecting rod neck position: R) _m ≥950MPa，R _p0.2 Not less than 550MPa), and after a large amount of production practices, the machinability of C70S6 is poor, the cutter is worn quickly, and the development requirements of future engines cannot be met. The performances of the medium carbon steel expansion-fracture connecting rod represented by 36MnVS4 and 46MnVS are improved in all aspects. For 46MnVS5 material, the large end position of the connecting rod needs lower ferrite content (less than or equal to 30 percent) and impact power (KV) ₂ Less than or equal to 20J) to ensure that the material has higher brittleness and avoid the unqualified phenomena of continuous expansion and uneven fracture of the material during expansion-fracture processing. While the position of the connecting rod neck requires higher ferrite content (more than or equal to 25 percent) and impact energy (KV) ₂ Not less than 30J) to ensure that the material has higher tensile strength and fatigue strength (R) _m ：≤1000MPa，R _p0.2 ≤750MPa，σ _-1 Not less than 500MPa) to meet the development requirements of high explosion and light weight of the engine.

The patent with the publication number of CN 110616363A, which is published in 2019, 12 and 27, discloses a steel for a medium-carbon non-quenched and tempered expansion-broken connecting rod and a manufacturing method thereof, and discloses a steel for a medium-carbon non-quenched and tempered expansion-broken connecting rod and a manufacturing method thereof, wherein the steel has the tensile strength of 950-1200 MPa, the yield strength of more than or equal to 750MPa, the elongation of more than or equal to 10%, the reduction of area of more than or equal to 20%, and the impact power KV ₂ Is 7 to 25J.

A patent with publication number CN 111286670A published in 6, 16 and 2020 and discloses medium carbon non-quenched and tempered steel and a manufacturing process thereof, a connecting rod and a preparation process thereof, and discloses non-quenched and tempered steel and a forging method thereof, wherein P, Cu is added to the steel on the basis of the traditional non-quenched and tempered steel to increase the brittleness value of the steel, and the medium carbon non-quenched and tempered steel connecting rod is obtained by the invention method of the patent, and has the yield strength of more than or equal to 750MPa, the tensile strength of more than or equal to 1000MPa, the elongation after fracture of more than or equal to 8 percent and the reduction of area of more than or equal to 25 percent.

A patent with publication number CN 101892424A, which is published in 11/24/2010, discloses a medium-carbon non-quenched and tempered steel for an expansion-broken connecting rod, which discloses a steel for the expansion-broken connecting rod with tensile strength more than or equal to 800MPa, and the steel is added with rare earth alloy, so that the cost is high, and the production difficulty is high.

In conclusion, the component 46MnVS5 and the forging process related in the prior patent can obtain the expansion broken connecting rod with low tensile strength and fatigue strength and low expansion broken qualification rate, and can not meet the development requirement of the expansion broken connecting rod of the future high-performance engine.

Disclosure of Invention

The invention aims to provide non-quenched and tempered steel for a Nb microalloying medium carbon expansion-broken connecting rod, which improves the obdurability of the steel through component design and meets the requirement of a high-performance engine expansion-broken connecting rod.

The invention also aims to provide the expansion-fracture connecting rod produced by using the non-quenched and tempered steel for the Nb microalloying medium carbon expansion-fracture connecting rod and the forging-controlled cooling process thereof, wherein the designed process is matched with the components of the non-quenched and tempered steel, so that the tensile strength of the product is 1100-1250 MPa, the yield strength is 750-850 MPa, the elongation after fracture is more than or equal to 12%, the fatigue strength is more than or equal to 590MPa, the Brinell hardness is more than or equal to 330HB, the connecting rod structure is ferrite and pearlite, and the ferrite area percentage content is 15-25%.

The specific technical scheme of the invention is as follows:

the non-quenched and tempered steel for the Nb microalloying medium carbon spalling connecting rod comprises the following components in percentage by mass:

0.40-0.50% of C, 0.40-0.90% of Si, 1.30-1.60% of Mn, less than or equal to 0.045% of P, 0.040-0.070% of S, 0.10-0.30% of Cr, 0.40-0.90% of V, 0.020-0.050% of Nb, less than or equal to 0.030% of Al, 0.0100-0.0160% of [ N ], and the balance of Fe and inevitable impurity elements.

Further optimizing the components: 0.45-0.50% of C, 0.60-0.90% of Si, 1.40-1.60% of Mn, less than or equal to 0.035% of P, 0.040-0.055% of S, 0.20-0.30% of Cr, 0.40-0.60% of V, 0.030-0.050% of Nb, less than or equal to 0.030% of Al, 0.0140-0.0160% of [ N ], and the balance of Fe and inevitable impurity elements.

The Nb microalloying medium carbon spalling connecting rod non-quenched and tempered steel also meets the requirement that X is 16 multiplied by Mn +3 multiplied by V +5 multiplied by Nb-3/C, and X is more than or equal to 10. When the formula is calculated, the numerical value indicated by each element is the content multiplied by 100 of the corresponding element of the non-quenched and tempered steel;

the invention provides a forging and cooling control process for producing an expansion-fracture connecting rod by using non-quenched and tempered steel for a Nb microalloying medium carbon expansion-fracture connecting rod, which specifically comprises the following steps: the forging heating temperature is 1150-1250 ℃, the initial forging temperature is 1150-1200 ℃, and the final forging temperature is more than or equal to 900 ℃; after forging, cooling by strong wind at a cooling rate of 7-15 ℃/s to be less than or equal to 400 ℃, and then adopting an air cooling process.

The expansion-breaking connecting rod provided by the invention is produced by adopting the forging-controlled and cooling-controlled process. The structure of the product is ferrite and pearlite, and the ferrite area percentage content is 15-25%.

The tensile strength of the produced expansion-fracture connecting rod is 1100-1250 MPa, the yield strength is 750-850 MPa, the elongation after fracture is more than or equal to 12%, the fatigue strength is more than or equal to 590MPa, and the Brinell hardness is more than or equal to 330 HB.

The design principle of the elements is as follows:

c: the C element is necessary for obtaining high strength and hardness, and the plasticity and toughness of the steel can be obviously reduced along with the increase of the C content, so that good expansion fracture performance is obtained. Too high carbon content can lead to too poor toughness and too high notch sensitivity of the connecting rod neck position, and lead to low fatigue strength; too low C content easily causes the problems of low strength, too good toughness and large deformation of the large head of the connecting rod due to no expansion or breakage. Therefore, the C content is preferably controlled to be 0.40 to 0.50%.

Si: si is a main deoxidizing element in steel and has strong solid solution strengthening effect, but the plasticity and toughness of the steel are reduced due to the excessively high content of Si, the activity of C is increased, the decarburization and graphitization tendency of the steel in the rolling and forging heating processes is promoted, smelting is difficult, inclusions are easily formed, and the fatigue resistance of the steel is deteriorated. Therefore, the Si content is controlled to be 0.40-0.90%.

Mn: mn can be combined with S to form MnS to improve the machinability on one hand, and can also remarkably delay pearlite-ferrite phase transformation, reduce the ferrite content, refine pearlite pellets, reduce the pearlite lamellar spacing and improve the strength of steel on the other hand. However, the Mn content is not easy to be too high, and bainite is easily generated due to too high Mn content, so that the toughness of the steel is extremely deteriorated. Therefore, compared with the traditional C70S6, the Mn content is moderately improved and controlled to be 1.30-1.60%.

Cr: cr can effectively improve the hardenability of steel and delay pearlite-ferrite phase transformation so as to obtain required high strength, and can also obviously improve the yield ratio through solid solution strengthening; meanwhile, Cr can also reduce the activity of C, can reduce the decarburization tendency of the steel surface in the heating, rolling and forging processes, and is beneficial to obtaining high fatigue resistance. However, since too high a content deteriorates the toughness of the steel, the Cr content is controlled to 0.10 to 0.30%.

P: micro segregation is formed when molten steel is solidified, and then the micro segregation is deviated to a grain boundary when the molten steel is heated at a temperature after austenite, so that the brittleness of steel is remarkably increased, the ductility and toughness of the steel are reduced, and the expansion-fracture performance of the expansion-fracture connecting rod can be remarkably improved, but fatigue, particularly the notch fatigue performance is reduced. In the invention, high carbon and forging and cooling processes are mainly used for controlling the position of the big end of the connecting rod to obtain high strength and coarse grains so as to reduce the toughness and improve the breaking performance, and if the content of P is too high, the toughness of the position of the neck of the connecting rod is lower and the fatigue performance is reduced. Therefore, the P content should be controlled to 0.045% or less.

S: and the S and Mn form MnS, so that the cutting processability of the steel is obviously improved. Because the strength of the steel is relatively high, the S content is controlled to be 0.040-0.070% in order to improve the cutting processing performance.

V: v is a strengthening element in steel, both V and C, N have strong affinity, the V exists in the steel in the form of carbide, the V realizes the improvement of the strength and toughness of the material by refining the structure and the grain size in the steel mainly due to the precipitation strengthening of VC and V (CN), so the content of V is more than or equal to 0.40; however, when the V content is too high, the toughness of the steel material is deteriorated and surface cracks are liable to occur during the cooling process of continuous casting, so that the V content should be 0.90% or less, and the V content is controlled to be 0.40 to 0.90% as a whole.

Al: since the S content in the steel of the invention is high, the addition of Al deteriorates the pourability of the steel, and therefore, the Al content should be controlled to be less than or equal to 0.030%.

Nb: the function of Nb in steel is similar to that of V, Nb forms Nb (C, N) precipitation phase with N, C elements in steel to play the roles of grain refinement and precipitation strengthening, and compared with the function of V, solid solution Nb can also remarkably delay pearlite-ferrite phase transformation. The size of the big end position is large, the cooling speed is relatively slow under the same condition, the temperature is high, the high-temperature forging is carried out on the big end position, the precipitation of Nb in austenite is avoided, the grain refining effect of Nb is inhibited, the pearlite-ferrite phase transformation is inhibited mainly through the precipitation strengthening effect of Nb in the pearlite-ferrite phase transformation process and the solid solution of Nb, the strength of the big end position is improved, and meanwhile, the toughness is obviously reduced; for the I-shaped neck and the small head position, the cooling speed is relatively high under the same condition, the temperature is relatively low, the relatively low forging temperature is realized, the Nb is promoted to be precipitated in austenite, and the strength and the yield ratio of the I-shaped neck position are improved and the toughness is improved mainly through fine grain strengthening and partial precipitation strengthening. Therefore, the Nb content is controlled to 0.030 to 0.050% from the viewpoint of both improvement of the strength and improvement of the expansion fracture property.

N: n and Nb and V in steel form a carbonitride precipitated phase, and the tiny precipitated phase is favorable for pinning austenite grain boundaries and hindering grain growth in the heating and forging processes, so that the N is more than or equal to 100 ppm; however, high N content is not more than 160ppm because it tends to form coarse precipitated phases, consumes microalloying elements in steel, prevents the solid solution action of microalloying elements such as Nb and V, and does not contribute to customization of parts. In summary, the N content should be 100-160 ppm.

The alloy of the invention has the design idea that 1) the Mn content is properly increased, the pearlite-ferrite transformation temperature is delayed, the diameters of pearlite pellets and pearlite lamellae are refined, and the strength of the fractured connecting rod is improved; 2) the grain size is refined by adopting the pinning effect of Nb-Ti microalloyed nano precipitation relative to grain boundary, the strength and the yield ratio are further improved, so that the expansion-fracture deformation is reduced, the expansion-fracture pass percentage is improved, and in order to meet the expansion-fracture processing performance of an expansion-fracture connecting rod, the connecting rod component meets the requirement that the formula X is 16 xMn +3 xV +5 xNb-3/C, and X is more than or equal to 10.

The forging process needs to be limited in order to guarantee the expansion-fracture qualified rate of the connecting rod, the higher forging temperature is beneficial to dissolving Nb and V elements, so that the hardness difference of ferrite and pearlite at the large end position of the connecting rod is improved, the toughness of the large end position of the connecting rod is reduced, the expansion-fracture qualified rate of the connecting rod is improved, the higher cooling speed is beneficial to refining the interlayer spacing of the pearlite, the toughness of the large end position of the connecting rod is further reduced, the lower cooling line temperature can avoid spheroidization of pearlite tissues caused by self tempering, and the improvement of the toughness of the large end position is avoided. Therefore, the forging process of the connecting rod is as follows: the forging heating temperature is 1150-1250 ℃, the initial forging temperature is 1150-1200 ℃, the final forging temperature is more than or equal to 900 ℃, strong wind is adopted for cooling after forging, the cooling rate is 7-15 ℃/s, and the air cooling process is adopted after cooling to below 400 ℃.

Compared with the prior art, the invention can reduce the toughness of the big end of the expansion-broken connecting rod, improve the small end and the improved strength and toughness and improve the expansion-broken qualified rate by producing the steel through the designed steel components under the controlled forging and cooling process. The tensile strength of the obtained product is 1100-1250 MPa, the yield strength is 750-850 MPa, the elongation after fracture is more than or equal to 12%, the fatigue strength is more than or equal to 590MPa, the Brinell hardness is more than or equal to 330HB, and the performance requirement of the fractured connecting rod is met.

Drawings

FIG. 1 is a microstructure of the steel of example 1;

FIG. 2 is a microstructure of the steel of example 2;

FIG. 3 is a microstructure of the steel of example 3;

FIG. 4 is a microstructure of the steel of comparative example 1;

FIG. 5 is a microstructure of a steel of comparative example 2;

FIG. 6 is a microstructure of a steel of comparative example 3;

FIG. 7 shows the microstructure of 46MnVS6 steel.

Detailed Description

The invention will be described in detail with reference to the accompanying drawings of the invention and examples 1-3, comparative examples 1-3 and comparative example 4, namely 46MnVS6, wherein the comparative examples 1-3 are steels of example 1 but do not adopt the controlled forging and cooling process of the invention, and the comparative example 4 is conventional steel 46MnVS 6. The chemical composition weight percentages of the non-heat-treated steels of examples 1 to 3 and the conventional 46MnVS5 non-heat-treated steel are shown in table 1, and the balance not shown in table 1 is Fe and inevitable impurity elements.

TABLE 1 chemical composition (wt%) of examples 1-3 and conventional steels

The chemical composition weight percentages of the non-heat-treated steels of examples 1-3 and the comparative example 4 of the conventional 46MnVS5 non-heat-treated steel are shown in Table 1, the examples and the comparative examples are smelted by an electric furnace, are directly continuously cast into a square billet with the thickness of 250 x 250mm after LF refining and RH vacuum degassing, and are rolled into a square billet by heating

The method comprises the following steps of feeding round steel, heating the round steel by a medium-frequency induction furnace, performing roll forging, die forging and trimming, and then feeding a controlled cooling line for controlled cooling, wherein the controlled forging and controlled cooling process comprises the following steps: the forging heating temperature is 1150-1250 ℃, the initial forging temperature is 1150-1200 ℃, the final forging temperature is more than or equal to 900 ℃, strong wind cooling is adopted after forging, the cooling rate is 7-15 ℃/s, and an air cooling process is adopted after cooling to below 400 ℃. The specific process parameters for each example and comparative example are shown in table 2. And (3) taking standard tensile and impact samples and metallographic samples on the finished product of the part to perform mechanical property, Brinell hardness and microstructure analysis, and detecting the overall fatigue property of the part through an MTS (methanol to sulfur) tester, wherein the results are shown in Table 3.

TABLE 2 forging and cooling control Process for examples and comparative examples

TABLE 3 statistical table of hardness, mechanical properties and ferrite area percentage for each example and comparative example

According to the formula and the forging and cooling control process, the product which meets the requirements of 1100-1250 MPa of tensile strength, 750-850 MPa of yield strength, more than or equal to 12% of elongation after fracture, more than or equal to 590MPa of fatigue strength, ferrite plus pearlite in the connecting rod structure, more than or equal to 330HB of Brinell hardness and 15-25% of ferrite area percentage content can be obtained. It can also be seen from the comparative examples and comparative examples 1 to 3 that even if the composition meets the requirements of the present invention, but the product structure which is not produced according to the controlled forging and cooling process of the present invention does not meet the requirements of the present invention, the hardness, tensile strength, yield strength, and fatigue strength properties are significantly lower than those of the present invention. Compared with the traditional expansion broken connecting rod 46MnVS5, the mechanical property of the invention is better, and the invention can meet the development requirement of the future high-performance engine.

Claims

1. The non-quenched and tempered steel for the Nb microalloying medium carbon expansion-fracture connecting rod is characterized by comprising the following components in percentage by mass:

2. The Nb microalloying medium carbon expansion-break connecting rod non quenched and tempered steel as claimed in claim 1, wherein the Nb microalloying medium carbon expansion-break connecting rod non quenched and tempered steel comprises the following components in percentage by mass:

0.45-0.50% of C, 0.60-0.90% of Si, 1.40-1.60% of Mn, less than or equal to 0.035% of P, 0.040-0.055% of S, 0.20-0.30% of Cr, 0.40-0.60% of V, 0.030-0.050% of Nb, less than or equal to 0.030% of Al, 0.0140-0.0160% of [ N ], and the balance of Fe and inevitable impurity elements.

3. The non-quenched and tempered steel for a Nb microalloyed medium carbon spalling connecting rod according to claim 1, wherein the composition of the non-quenched and tempered steel for a Nb microalloyed medium carbon spalling connecting rod satisfies:

X＝16×Mn+3×V+5×Nb-3/C，X≥10。

4. a controlled forging and cooling process for producing an expansion-fracture connecting rod by using the non-quenched and tempered steel for a Nb microalloying medium carbon expansion-fracture connecting rod, which is characterized by comprising the following steps:

the forging heating temperature is 1150-1250 ℃, the initial forging temperature is 1150-1200 ℃, and the final forging temperature is more than or equal to 900 ℃.

5. The controlled forging and cooling process according to claim 4, wherein cooling is performed by strong wind after forging.

6. The controlled forging and cooling process according to claim 4 or 5, wherein the cooling rate is 7-15 ℃/s.

7. The controlled forging and cooling process according to claim 4 or 6, wherein an air cooling process is adopted after cooling to a temperature of less than or equal to 400 ℃ after forging.

8. An expansion-broken connecting rod produced by the controlled forging and controlled cooling process according to any one of claims 4 to 7.

9. The expansion-fracture connecting rod according to claim 8, wherein the microstructure of the expansion-fracture connecting rod is ferrite plus pearlite, and the ferrite area percentage is 15-25%.

10. The expansion-fracture connecting rod according to claim 8 or 9, wherein the expansion-fracture connecting rod has a tensile strength of 1100 to 1250MPa, a yield strength of 750 to 850MPa, an elongation after fracture of not less than 12%, a fatigue strength of not less than 590MPa, and a brinell hardness of not less than 330 HB.