Low-cost and free-cutting bainite non-quenched and tempered steel for front axle of heavy truck and preparation method of bainite non-quenched and tempered steel

Abstract

The invention discloses bainite non-quenched and tempered steel for a front axle of a low-cost free-cutting heavy truck and a preparation method of the bainite non-quenched and tempered steel. The bainite non-quenched and tempered steel comprises the following chemical components in percentage by weight: 0.20 to 0.28 percent of C, 1.7 to 2.6 percent of Mn, 0.3 to 0.6 percent of Si, 0.4 to 0.8 percent of Cr, 0.003 to 0.005 percent of B, 0.02 to 0.03 percent of Ti, 0.02 to 0.05 percent of Nb, 0.008 to 0.012 percent of N, 0.030 to 0.065 percent of S, less than or equal to 0.025 percent of P, less than or equal to 0.0015 percent of O and the balance of Fe and inevitable impurity elements. The preparation method comprises the steps of electric arc furnace / converter smelting, LF refining, RH vacuum treatment, continuous casting, rolling, slow cooling, induction heating, controlled forging, trimming and finishing and air cooling, the induction heating temperature is smaller than or equal to 1200 DEG C, the final forging temperature is larger than or equal to 850 DEG C, the trimming temperature is larger than or equal to 800 DEG C, and the cooling speed is larger than or equal to 3 DEG C / min when the workpiece temperature is larger than 550 DEG C in the air cooling stage. According to the prepared bainite non-quenched and tempered steel front axle, the yield strength Rp0.2 is larger than or equal to 760 MPa, and the tensile strength Rm is larger than or equal to 960 MPa; the percentage elongation after fracture A is greater than or equal to 14%, the percentage reduction of area Z is greater than or equal to 40%, and the normal-temperature impact KU2 is greater than or equal to 35J.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy structural steel and automobile parts manufacturing, and in particular is a bainite non-quenched and tempered steel for low-cost easy-to-cut machining of heavy truck front axles and a preparation method thereof. Background Art

[0002] In the steel industry, non-quenched and tempered steel is widely used in shafts, rods and other parts in the automotive industry. Non-quenched and tempered steel mainly includes three types of microstructures: (1) ferrite + pearlite non-quenched and tempered steel; (2) bainite non-quenched and tempered steel; (3) low carbon martensite non-quenched and tempered steel.

[0003] Ferrite + pearlite type non-quenched and tempered steel is generally made by adding micro-alloying elements such as V, Nb, Ti, and Al to medium carbon steel (0.2-0.5wt.%), and controlling the dispersion and precipitation of carbonitrides in the non-quenched and tempered steel by controlling rolling (forging) and cooling, so as to achieve the goal of strengthening the material and obtain mechanical properties equivalent to those of quenched and tempered treatment. However, ferrite + pearlite type non-quenched and tempered steel has certain limitations in strength. Its maximum tensile strength can only reach nearly 900MPa, its yield strength is also low, and its toughness and fatigue strength are low. It is only used in automobile and tractor parts with medium and low strength and small impact load, and its application in parts requiring high fatigue strength, yield strength and toughness is limited.

[0004] Low-carbon martensitic non-quenched and tempered steel can usually obtain higher strength and toughness, and is a type of material with relatively high development potential. However, when such materials are used to make large-section and variable-section workpieces, the workpieces will have large thermal stress and quenching stress during the quenching process, which will cause the workpieces to deform; on the other hand, due to the high strength of low-carbon martensitic non-quenched and tempered steel, it is difficult to machine (especially drilling). Therefore, such materials have not yet been applied in batch engineering and are still in the experimental research and development stage.

[0005] In comparison, bainitic non-quenched and tempered steel usually adopts air cooling or natural cooling process. The thermal stress and phase transformation stress of the workpiece are small during the heat treatment cooling process, and the deformation of the workpiece can be effectively controlled; at the same time, bainite can have both the plasticity and toughness of high-temperature transformation products and the strength of low-temperature transformation products, and can obtain better comprehensive mechanical properties. Therefore, bainitic non-quenched and tempered steel has been paid attention to by scholars at home and abroad in recent years, and it is also a type of non-quenched and tempered steel material that has developed rapidly in engineering applications. However, there are still the following problems when using bainitic non-quenched and tempered steel to manufacture large-size heavy-duty truck front axles:

[0006] (1) There is a certain amount of soft phase austenite in the front axle structure made of bainitic non-quenched and tempered steel, and its phase transformation internal stress is also large, resulting in a low yield strength. In order to improve the yield strength of bainitic non-quenched and tempered steel, a certain amount of V element is usually added to bainitic non-quenched and tempered steel to promote the precipitation of V carbonitrides, thereby improving its yield strength. For example, Japanese brand FAS2225 non-quenched and tempered steel (corresponding domestic brand 25Mn2CrVS) will add 0.15-0.30wt.% of V element to the alloy, which increases the material cost and limits its engineering application.

[0007] (2) During the die forging heating process, the heating temperature of the bainitic non-quenched and tempered steel front axle usually reaches about 1200℃, or even higher, to reduce the flow stress of the workpiece during the forming process and improve the filling property of the metal. However, higher heating temperatures will lead to coarsening of the workpiece grains, resulting in a significant deterioration in its toughness. In addition, if the final forging temperature and trimming temperature of the workpiece are not properly controlled, deformation-induced ferrite structure will appear at the trimming position, significantly reducing the fatigue performance of the workpiece. How to inhibit the grain coarsening of the non-quenched and tempered steel front axle and improve its toughness and fatigue performance are technical difficulties that must be overcome to achieve the industrial application of bainitic non-quenched and tempered steel front axles.

[0008] (3) The non-quenched and tempered steel front axle still needs to be machined after die forging. Due to the high strength of bainitic non-quenched and tempered steel, the cutting difficulty is increased, especially the turning processability of the axle pin hole will be deteriorated, resulting in an increase in the machining cost of the non-quenched and tempered steel front axle, which restricts its engineering application.

[0009] Therefore, how to prepare high-strength and toughness bainitic non-quenched and tempered steel and its front axle products through alloy design and process control, while improving its machining performance and reducing costs, is the key to realizing the large-scale engineering application of non-quenched and tempered steel front axles for heavy trucks. Summary of the invention

[0010] In order to solve the above technical problems, the present invention provides a low-cost, easy-to-cut bainite non-quenched and tempered steel for heavy truck front axles and a front axle controlled forging process. The main technical ideas are:

[0011] (1) Microalloying design using Ti, Nb, and N combined with controlled rolling technology to achieve grain refinement, while improving alloy hardenability through B microalloying, thereby improving material strength and toughness, ensuring that the non-quenched and tempered steel front axle has sufficient yield strength;

[0012] (2) Microalloying design is used to replace the large amount of V element added in bainitic non-quenched and tempered steel to reduce costs. At the same time, the use of a small amount of Ti for alloying design can avoid the formation of liquid TiN and reduce the toughness and fatigue properties of the material, and can also reduce the addition of the more expensive Nb element, further reducing material costs;

[0013] (3) By controlling the S content in the alloy and the controlled rolling process, a certain amount of MnS is introduced into the bainitic non-quenched and tempered steel and its aspect ratio and distribution are controlled to improve the machinability of the bainitic non-quenched and tempered steel front axle. Compared with the front axle made of quenched and tempered steel and the currently used 25Mn2CrVS non-quenched and tempered steel, this technical solution significantly reduces the cost and significantly improves the comprehensive performance of the front axle.

[0014] To achieve the above purpose, the technical solution adopted by the present invention is:

[0015] A low-cost, easy-to-cut bainitic non-quenched and tempered steel for heavy truck front axles comprises the following chemical components in weight percentage: C 0.20-0.28%, Mn 1.7-2.6%, Si 0.3-0.6%, Cr 0.4-0.8%, B 0.003-0.005%, Ti 0.02-0.03%, Nb 0.02-0.05%, N 0.008-0.012%, S 0.030-0.065%, P≤0.025%, O≤0.0015%, and the rest are Fe and unavoidable impurity elements.

[0016] In alloy design, Ti, Nb, and N are used for microalloying design and combined with controlled rolling process to achieve grain refinement. At the same time, B microalloying is used to improve the alloy hardenability, thereby improving the strength and toughness of the material; it replaces the technology of adding a large amount of V elements to improve the yield strength of bainitic non-adjustable steel, reducing material costs. In the alloy design that uses microalloying to achieve grain refinement, Nb and Ti elements are added in combination. Using a trace amount of Ti for alloying design can avoid the formation of liquid TiN and reduce the toughness and fatigue performance of the material, and can also reduce the addition of the more expensive Nb element, further reducing material costs.

[0017] A low-cost, easy-to-cut controlled forging method for a bainite non-quenched and tempered steel front axle for a heavy truck front axle, the method comprising the following steps:

[0018] Step (1): Alloy smelting and rolling

[0019] The present invention provides a production process for the non-quenched and tempered steel for the front axle of a heavy truck with low cost and easy cutting, comprising the following steps: smelting in an electric arc furnace / converter, and then continuously casting a round billet or a square billet after LF refining and RH vacuum degassing treatment, and then performing controlled rolling and slow cooling treatment on the continuous casting billet. The controlled rolling and slow cooling process comprises the following process steps:

[0020] Step 1) Heating of continuous casting slab

[0021] After preheating, the continuous casting billet is heated to 1200-1250℃ and kept warm. The total time of preheating, heating and soaking is controlled to be 6.0h-10.0h. The higher heating temperature (≥1200℃) is mainly to promote the full dissolution of Ti and Nb carbonitrides in the billet, so that Ti and Nb carbonitrides can be dispersed and precipitated in the subsequent controlled rolling process, thereby providing a guarantee for inhibiting grain coarsening and achieving microstructure refinement in the subsequent forging heating process.

[0022] Step 2) Controlled rolling process

[0023] The heated continuous casting billet is rolled, with the starting rolling temperature of 1100-1160℃, the final rolling temperature of 850-920℃, and the rolling ratio ≥3, of which the rolling ratio in the temperature range of 930-980℃ is ≥1.5. Based on thermodynamic calculations, the nose temperature of the precipitation of Nb and Ti carbonitrides is around 950℃. The rolling with a large deformation in the temperature range of 930-980℃ is mainly to promote the dispersion and precipitation of Nb and Ti carbonitrides; the total deformation is controlled to achieve grain refinement.

[0024] Step 3) Slow cooling after rolling

[0025] After hot rolling, the bar is cooled to 650-720℃ on a cooling bed and then slowly cooled in a pit. The slow cooling time is ≥24h. The CCT curve test results show that 650-720℃ is in the ferrite / pearlite transformation temperature zone of the austenite cooling transformation of the alloy material. At the same time, due to the slow ferrite / pearlite transformation kinetics of the alloy, slow cooling in the pit at 650-720℃ is to allow austenite to recrystallize, preparing for grain refinement and organization refinement in the subsequent product processing; on the other hand, it is to avoid the appearance of bainite and martensite in the bar structure, which is convenient for blanking in the subsequent die forging process.

[0026] Step (2): Front axle controlled forging

[0027] The present invention provides a low-cost, easy-to-cut controlled forging process for a heavy-duty truck non-quenched and tempered steel front axle, comprising the following steps: induction heating after the bar is cut, then die forging, hot trimming and finishing the die forged workpiece, and finally air cooling. The process mainly includes the following steps:

[0028] Step 1) Induction Heating

[0029] The blanked bar is induction heated, and the heating temperature is controlled below 1200°C to avoid excessively high heating temperature, which may cause a large amount of Nb and Ti carbonitrides to dissolve and lose the function of pinning the grain boundaries, thereby causing coarse grains.

[0030] Step 2) Controlled Forging

[0031] After induction heating, the workpiece is roll forged and die forged. The final forging temperature should be controlled above 850°C and the trimming temperature should be controlled above 800°C to prevent the formation of deformation-induced ferrite at the trimming position, which would lead to a decrease in the fatigue resistance of the front axle.

[0032] Step 3) Post-forging cooling control

[0033] The forged front shaft is blow-cooled. When the workpiece temperature is greater than 550°C, the cooling rate should be controlled above 3°C / min to avoid the formation of ferrite and pearlite in the structure, thereby reducing the yield strength, toughness and fatigue properties of the workpiece.

[0034] The yield strength R of the front axle of the low-cost, easy-to-cut heavy-duty truck front axle after controlled forging of bainitic non-quenched and tempered steel p0.2 ≥760MPa, tensile strength R m ≥960MPa; elongation after fracture A≥14%, section shrinkage Z≥40%, normal temperature impact A KU2 ≥35J.

[0035] Compared with the prior art, the present invention replaces the addition of V elements based on microalloying design, thereby improving the hardenability and toughness of the alloy and reducing the alloy cost; by increasing the S content in the alloy and controlling the aspect ratio and distribution of MnS, the mechanical processing performance of the bainitic non-quenched and tempered steel front axle is improved, the cutting processing performance is good, and the part manufacturing cost is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The metallographic structure of the front axle of Example 1;

[0037] Figure 2 The metallographic structure of the front axle of Example 2;

[0038] Figure 3 The metallographic structure of the front axle of Example 3;

[0039] Figure 4 The metallographic structure of the front axle of Example 4;

[0040] Figure 5 This is the metallographic structure of the front axle of Example 1. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0042] The chemical composition percentages of the low-cost, easy-to-cut bainitic non-quenched and tempered steel, 42CrMo quenched and tempered steel, and 25Mn2CrVS non-quenched and tempered steel for heavy truck front axles of Examples 1-4 are shown in Table 1. The alloys of Examples 1-4, 42CrMo quenched and tempered steel, and 25Mn2CrVS non-quenched and tempered steel are all smelted in electric furnaces / converters, and continuously cast into round billets or square billets after LF refining + RH vacuum degassing. The continuous cast billets are heated and rolled into round steel and then slowly cooled to obtain the raw materials for manufacturing the front axle; the round steel after slow cooling is cut and medium-frequency induction heated, and then rolled, die forged, trimmed, and finished, and then the workpiece is controlled cooled, i.e., controlled forging and controlled cooling. The controlled rolling and controlled cooling process of the alloys of Examples 1-4 and 25Mn2CrVS non-quenched and tempered steel and the controlled forging and controlled cooling process of the front axle of the corresponding material are shown in Table 2, and the controlled rolling and controlled cooling process of 42CrMo quenched and tempered steel and the controlled forging and quenching and tempering process of the front axle of the corresponding material are also listed in Table 2. Standard tensile test specimens, impact test specimens and metallographic test specimens were taken from the neck of the front axle of the heavy truck (the part with the largest cross-sectional size) for mechanical property testing and microstructure analysis; at the same time, the front axle was drilled for the axle pin hole to evaluate the mechanical processing performance of the material, and the performance indicators are listed in Table 3. Among them, Comparative Example 1 is a front axle made of 42CrMo quenched and tempered steel, and Comparative Example 2 is a front axle made of 25Mn2CrVS non-quenched and tempered steel. It can be seen that the front axle prepared by the steel composition and controlled forging process of the present invention has a tensile strength of ≥960MPa, a yield strength of ≥760MPa, an elongation after fracture of ≥14%, and an impact energy of ≥35J at room temperature, and its comprehensive mechanical properties fully meet the requirements of safety parts.

[0043] Figure 1-4 They are the metallographic structures of the front axles of Examples 1-4, Figure 5 This is the metallographic structure of the front axle of comparative example 1. The front axle structures of embodiments 1-4 are all bainite structures, and no proeutectoid ferrite structure appears. A polygonal ferrite group exists in the front axle structure of comparative example 1, indicating that the embodiment alloy has better hardenability.

[0044] Table 1 Chemical composition (wt%) of Examples 1-4 and 42CrMo quenched and tempered steel and 25Mn2CrVS non-quenched and tempered steel

[0045]

[0046]

[0047] Table 2: Front axle controlled forging process of steel materials of Examples 1-4 and Comparative Examples and corresponding materials

[0048]

[0049]

[0050] Table 3 Mechanical properties of front axle of materials of Examples 1-4 and Comparative Examples

[0051]

[0052] The process parameters (such as temperature, time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed one by one here.

[0053] Any contents not described in detail in the present invention can be based on the conventional technical knowledge in the art.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A low-cost, easy-to-cut bainite non-quenched and tempered steel for heavy truck front axles, characterized in that: The bainitic non-quenched and tempered steel includes the following chemical components in weight percentage: C 0.20-0.28%, Mn 1.7-2.6%, Si0.3-0.6%, Cr 0.4-0.8%, B 0.001-0.005%, Ti 0.02-0.03%, Nb 0.02-0.05%, N 0.008-0.012%, S0.030-0.065%, P≤0.025%, O≤0.0015%, and the rest are Fe and unavoidable impurity elements.

2. A method for preparing low-cost, easy-to-cut bainite non-quenched and tempered steel for heavy truck front axles, the preparation method comprising the following steps: Electric arc furnace / converter smelting-LF refining-RH vacuum treatment-continuous casting-rolling-slow cooling-induction heating-controlled forging-trimming and finishing-air cooling.

3. The preparation method according to claim 2, characterized in that: During the rolling, the soaking temperature of the steel billet in the heating furnace is controlled at 1200-1250°C, and the total time of preheating, heating and soaking is controlled at 6.0h-10.0h; the starting rolling temperature is 1100-1160°C, the final rolling temperature is 850-920°C, the rolling ratio is ≥3, and the rolling ratio in the temperature range of 930-980°C is ≥1.

5.

4. The preparation method according to claim 2, characterized in that: After rolling, the steel is cooled to 650-720℃ on a cooling bed and then taken offline into a pit for slow cooling. The slow cooling time is ≥24h.

5. The preparation method according to claim 2, characterized in that: Induction heating temperature ≤1200℃.

6. The preparation method according to claim 2, characterized in that: Final forging temperature ≥850℃.

7. The preparation method according to claim 2, characterized in that: Cutting edge temperature ≥800℃.

8. The preparation method according to claim 2, characterized in that: During the air cooling stage, when the workpiece temperature is greater than 550°C, the cooling rate is ≥3°C / min.

9. The preparation method according to claim 2, characterized in that: Yield strength R of the prepared bainite non-quenched and tempered steel front axle p0.2 ≥760MPa, tensile strength R m ≥960MPa; elongation after fracture A≥14%, section shrinkage Z≥40%, normal temperature impact A KU2 ≥35J.

10. The preparation method according to claim 2, characterized in that: The metallographic structure of the prepared bainitic non-quenched and tempered steel front axle is bainite structure, and no proeutectoid ferrite structure appears.

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