High-strength and high-toughness bainite non-quenched and tempered steel and manufacturing method thereof

Abstract

The invention discloses high-strength and high-toughness bainite type non-quenched and tempered steel and a manufacturing method thereof. The high-strength and high-toughness bainite type non-quenched and tempered steel comprises the following components in percentage by mass: 0.22 to 0.30 percent of C, 0.35 to 0.45 percent of Si, 1.80 to 2.00 percent of Mn, 0.005 to 0.02 percent of P, 0.015 to 0.030 percent of S, 0.030 to 0.045 percent of Al, 0.40 to 0.60 percent of Cr, 0.03 to 0.20 percent of Mo, 0.10 to 0.20 percent of Ni, 0.012 to 0.017 percent of N, 0.12 to 0.18 percent of V and the balance of Fe and other inevitable impurities. In addition, the bainite transformation temperature TB of the non-quenched and tempered steel is 500-565 DEG C, and TB is equal to 830-270 [C]-90 [Mn]-37 [Ni]-70 [Cr]-83 [Mo]; the microalloy element coefficient gamma M / N is 3.0-4.0, and gamma M / N = ([Al] / 2 + [V] / 4) / [N]. The tensile strength of the non-quenched and tempered steel is larger than or equal to 1100 MPa, the percentage elongation after fracture is larger than or equal to 15%, and the room-temperature impact energy KU2 is larger than or equal to 50 J.

Description

Technical Field

[0001] The invention belongs to the field of non-quenched and tempered steel, and specifically relates to a high-strength and high-toughness bainite type non-quenched and tempered steel and a manufacturing method thereof. Background Art

[0002] The application of non-quenched and tempered steel in the automotive field can effectively save energy and reduce emissions, and reduce costs. It has been widely used in the manufacture of automotive crankshafts, connecting rods, camshafts, hubs, steering knuckles, shift levers and spline shafts. Taking the steering knuckle of an automobile as an example, the weight of the original ductile iron is 5.05kg. If non-quenched and tempered steel 38MnNS6 is used, the weight can be reduced by 29% to 3.61kg; and the front cantilever uses high-strength 38MnVS6 non-quenched and tempered steel, which can reduce the weight from the original 9.25kg to 8kg, a weight reduction of about 14%. At the same time, the application of non-quenched and tempered steel can also reduce the deformation and cracking caused by quenching during the quenching and tempering process. Automobile security parts, structural parts, automobile chassis, some important power systems, transmission systems, and major parts of exhaust systems have begun to use non-quenched and tempered steel. The goal of the automobile industry is to use non-quenched and tempered steel for all automotive forgings in the future.

[0003] Non-quenched and tempered steels include ferrite-pearlite, bainite and martensite. Currently, ferrite-pearlite non-quenched and tempered steel is the most widely used, such as 38MnVS6, which is widely processed into crankshafts. Although there are invention patents that report that the tensile strength of ferrite-pearlite non-quenched and tempered steel can reach 1000MPa, the toughness is still relatively low. In addition, the operating environment of the crankshaft is harsh, with both torsional stress and bending stress. Therefore, the steel for the crankshaft needs to have high tensile strength and fatigue strength, as well as good toughness. At the same time, with the increase in automobile engine power, the requirements for engine crankshaft steel are also getting higher and higher. The performance of traditional ferrite + pearlite non-quenched and tempered steels such as 38MnVS6 is gradually unable to meet the performance requirements of engine crankshaft steel.

[0004] Bainite can combine the plasticity and toughness of high-temperature transformation products and the strength of low-temperature transformation products. Therefore, bainitic non-quenched and tempered steel with good strength and toughness combination has attracted the attention of more and more companies and researchers. Bainitic non-quenched and tempered steel has the characteristics of high strength and good plasticity, which can meet the performance requirements of crankshaft steel. For example, Chinese patent publication number CN114032463A discloses "a high-strength and tough bainitic non-quenched and tempered steel and its manufacturing method", which increases the hardenability of the material, promotes the formation of bainite, reduces the cooling rate required for bainite formation, refines the grains, and obtains a tensile strength ≥1000MPa and a room temperature impact energy KU 2≥55J. However, the B element is not easy to control. The form of B in steel has a very important influence on the performance of steel, and the form of boron in iron has not yet been determined. B will exist in several forms such as solid solution boron, boron nitride, boron oxide, iron boron cementite, and iron boric acid compounds. Only in the form of solid solution boron can the hardenability of steel be improved. Therefore, adding B will cause large fluctuations in material properties. At the same time, materials with added B often need to add Ti to form titanium nitride and inhibit the formation of boron nitride. Titanium nitride has square edges and corners, which will reduce the fatigue performance of the material.

[0005] Chinese patent publication number CN112522610A discloses "Controlling V-Ti composite bainite non-quenched and tempered steel structure and its production method", which improves the strength and toughness of bainite non-quenched and tempered steel by controlling the V-Ti content matching value, with Ti content of 0.030% to 0.050%, V content of 0.10% to 0.20%, tensile strength of 1100 to 1420 MPa, yield strength of 710 to 1040 MPa, and shrinkage of 44% to 55%. However, the impact performance of the invented steel is not mentioned, and TiN formed by Ti and N will reduce the fatigue life of the invented steel.

[0006] In summary, with the increase in automobile engine power, the non-quenched and tempered steel for engine crankshafts is developing towards bainitic non-quenched and tempered steel. However, the non-quenched and tempered steel in the prior art still has the problem of having more strength and hardness but insufficient toughness. Therefore, it is necessary to develop higher strength and toughness bainitic non-quenched and tempered steel to meet the demand for high-strength and toughness steel. Summary of the invention

[0007] The object of the present invention is to provide a high-strength and high-toughness bainite non-quenched and tempered steel and a manufacturing method thereof, wherein the non-quenched and tempered steel has a tensile strength of ≥1100MPa, an elongation after fracture of ≥15%, and an impact energy KU 2 ≥50J; can be effectively used in occasions such as automobiles and engineering machinery that require high-strength and toughness steel.

[0008] To achieve the above object, the technical solution of the present invention is:

[0009] The present invention optimizes the ratio of C, Mn and Cr elements and adds Mo element, so that the material can undergo medium-temperature bainite phase transformation under air cooling conditions, and refines the grains by adding a certain amount of V. At the same time, dispersed vanadium carbonitrides are formed to play a precipitation strengthening role, and finally a high-strength and high-toughness bainitic non-quenched and tempered steel is obtained.

[0010] Specifically, the high-strength and high-toughness bainite non-quenched and tempered steel of the present invention has the following components by mass percentage: C: 0.22-0.30%, Si: 0.35-0.45%, Mn: 1.80-2.00%, P: 0.005-0.02%, S: 0.015-0.030%, Al: 0.030-0.045%, Cr: 0.40-0.60%, Mo: 0.03-0.20%, Ni: 0.10-0.20%, N: 0.012-0.017%, V: 0.12-0.18%, and the rest includes Fe and other inevitable impurities; and, it must also meet the following requirements:

[0011] Bainite transformation temperature T of non-quenched and tempered steel B 500~565℃,

[0012] T B =830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo];

[0013] Microalloying element coefficient γ M / N 3.0~4.0,γ M / N =([Al] / 2+[V] / 4) / [N].

[0014] Each chemical element in the formula is substituted by the numerical value before the percentage sign of the mass percentage content of the chemical element.

[0015] Furthermore, the remainder is Fe and other inevitable impurities.

[0016] Preferably, among the impurities, Ti≤0.003%, and / or, O≤0.002%, and / or, Ca≤0.005%.

[0017] The microstructure of the non-quenched and tempered steel of the present invention is bainite+ferrite and / or pearlite+retained austenite, wherein the area content of bainite is greater than 88%.

[0018] The non-quenched and tempered steel of the present invention has a tensile strength Rm ≥ 1100 MPa, an elongation after fracture ≥ 15%, and a room temperature impact energy KU 2 ≥50J.

[0019] In the composition design of the non-quenched and tempered steel of the present invention:

[0020] C: C is necessary to obtain high strength and hardness. Although high C content is beneficial to the strength and hardness of steel, it is extremely detrimental to the plasticity and toughness of steel, and reduces the yield strength ratio of bainitic non-quenched and tempered steel, increases the decarburization sensitivity, and deteriorates the fatigue resistance and processing performance of steel. Taking comprehensive considerations, the C content in the present invention is designed to be 0.22% to 0.30%.

[0021] Mn: Mn is an effective element for deoxidation and desulfurization, and has a strong solid solution strengthening effect, but when the content of Mn is less than 1.5%, its strengthening effect is not obvious. Mn can significantly reduce the chemical free energy of austenite, increase the chemical free energy of the α phase, form a medium-temperature stable zone of austenite, and reduce the driving force of bainite transformation. When the content of Mn exceeds 1.5%, it will promote the formation of bainite structure; but when the content of Mn exceeds 2.0%, it will form overly stable residual austenite, which does not transform into martensite during plastic deformation and does not contribute to the improvement of plasticity. Therefore, the Mn content in the present invention is designed to be 1.80% to 2.00%.

[0022] Si is the main deoxidizing element in steel, and has a strong solid solution strengthening effect, which improves the hardness and strength of steel. At the same time, Si in steel can inhibit the precipitation of brittle carbides during bainite transformation during the cooling process, so that a certain amount of residual austenite is retained in the steel, which is beneficial to improving the toughness of the steel. Si can also increase the elastic modulus of steel and improve the rigidity of the crankshaft. However, too high Si content will reduce the plastic toughness of the steel, increase the activity of C, promote the decarburization and graphitization tendency of the steel during rolling and forging heating, and make smelting difficult and easy to form inclusions, deteriorating the fatigue resistance of the steel. Therefore, in the present invention, the content of Si is designed to be 0.35% to 0.45%.

[0023] Cr: Cr can effectively improve the hardenability of steel and delay the pearlite transformation, so that the steel can obtain bainite structure to obtain the required high strength, and can also significantly improve the hardness of bainite ferrite through solid solution strengthening; at the same time, Cr can also reduce the activity of C, which can reduce the decarburization tendency of the steel surface during heating, rolling and forging, which is conducive to obtaining high fatigue resistance. However, the increase in chromium content also increases the sensitivity of the second type of temper brittleness, so the Cr content in the present invention is designed to be 0.40% to 0.60%.

[0024] Ni can improve the hardenability and corrosion resistance of steel and ensure the toughness of steel at low temperatures. It can effectively improve the toughness of the core of steel, reduce the tough-brittle transition temperature, and improve the tensile strength and impact toughness of bainitic steel. When the Ni content in the present invention is designed to be 0.10% to 0.20%, the performance of bainitic non-quenched and tempered steel is optimal.

[0025] Mo: Mo has a solid solution strengthening effect in steel and can improve the hardenability of steel; at the same time, Mo can significantly delay the transformation of proeutectoid ferrite, reduce the starting temperature of bainite transformation, refine the bainite structure, and expand the bainite transformation zone. Mo can form strong carbides, hinder the diffusion of atoms, the movement of dislocations and the migration of grain boundaries, and effectively prevent the coarsening of austenite grains, but the price of Mo element is relatively high, so the Mo content in the present invention is designed to be 0.03% to 0.20%.

[0026] V: V is a strengthening element in steel. V has a strong affinity with C and N. It mainly exists in the form of VC and V (C, N) in steel, which plays a role in precipitation strengthening and grain refinement. V enables non-quenched and tempered steel to obtain fine bainite structure in a wider cooling rate range. V dissolved in the matrix has the effect of inhibiting ferrite nucleation on the grain boundary and promoting bainite transformation. Increasing the V content can further refine the bainite structure. However, when the V content exceeds a threshold value, coarse vanadium carbonitrides will be formed, which cannot play a role in pinning the grain boundary and refining the grains. Therefore, the V content in the present invention is designed to be 0.12% to 0.18%.

[0027] Al: The role of Al in bainitic non-quenched and tempered steel is similar to that of Si, which can inhibit the precipitation of cementite, accelerate bainite transformation and reduce the hydrogen embrittlement sensitivity of steel. Al is the main deoxidizing element in steel, and forms AlN precipitation phase with N element in steel, which has the effect of inhibiting grain growth. Too low Al content leads to insufficient AlN precipitation, which cannot inhibit grain growth. However, if the Al content is too high, nozzle blockage is likely to occur during casting. In the present invention, the Al content is designed to be 0.03% to 0.045%.

[0028] N: N can promote the precipitation of vanadium in non-quenched and tempered steel and improve the precipitation strengthening effect. Increasing N in steel can save 20% to 40% of vanadium usage. Increasing N promotes the precipitation of vanadium carbonitride at the austenite-ferrite phase interface, effectively preventing the growth of ferrite grains. Secondly, the precipitation of VN or V(C,N) promotes the formation of intracrystalline ferrite IGF and refines the ferrite structure. At the same time, AlN formed by the combination of N and Al can also effectively pin the grain boundary and inhibit the growth of grains. The N content designed in the present invention is 0.012% to 0.017%.

[0029] S: S element combines with Mn in non-quenched and tempered steel to form MnS inclusions, which improves the cutting performance of non-quenched and tempered steel and improves the finish of workpieces. At the same time, during the austenite transformation process, ferrite precipitates at the original austenite grain boundary and also precipitates with MnS as the core. Since the number of ferrite nucleation cores increases, the ferrite-pearlite structure is refined. Therefore, the S content in the present invention is controlled to be 0.015% to 0.030%.

[0030] P: can improve the strength and hardness of steel, facilitate the expansion and breaking process of connecting rods, and make the fracture smoother. However, excessive phosphorus will reduce the toughness and plasticity of steel and accelerate the intergranular corrosion rate of steel. Therefore, the present invention requires the P content to be 0.005% to 0.02%.

[0031] In particular: the present invention controls the bainite transformation temperature T of non-quenched and tempered steel B 500~565℃, T B=830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo]. By rationally designing the composition ratio of C, Mn, Cr, and Ni, the ferrite pearlite transformation curve and the bainite transformation curve in the TTT curve are separated. By adding 0.03-0.20% Mo, the ferrite pearlite transformation curve is further shifted to the right, and the ferrite pearlite transformation curve and the bainite transformation curve are further separated, thereby reducing the critical cooling rate of bainite precipitation. Controlling the bainite transformation temperature T B The temperature is 500-565°C, so that the final forging temperature or final rolling temperature of the steel of the present invention can quickly enter the medium-temperature bainite transformation from 900-960°C, so that the bainite content in the final organization meets the requirement of more than 88%.

[0032] The present invention adds V, Al and N, while satisfying the micro-alloying element coefficient γ M / N 3.0~4.0,γ M / N =([Al] / 2+[V] / 4) / [N], Al will preferentially combine with N, and the solid solution temperature of V nitride is lower than that of Al nitride. The ratios of Al to N and V to N are optimized to promote the formation of dispersed and fine vanadium nitride precipitation phase, further refine the grains, and improve the strength and toughness of the steel of the present invention.

[0033] The present invention also provides a method for producing high-strength and high-toughness bainite non-quenched and tempered steel, which comprises the following steps:

[0034] 1) Smelting and casting

[0035] Smelting and casting into ingots according to the above ingredients;

[0036] 2) Heating

[0037] The ambulation temperature of the steel billet in the heating furnace is controlled at 1170-1220°C, and the total time of preheating, heating and ambulation is controlled at 3-9h;

[0038] 3) Rolling or forging

[0039] The starting rolling temperature or the initial forging temperature is controlled at 1100-1200°C, and the final rolling temperature or the final forging temperature is controlled at 900-960°C;

[0040] 4) Cooling: Cooling to below 300°C.

[0041] Preferably, in step 1), the smelting is performed by electric furnace smelting or converter smelting, and is refined and vacuum treated; and the casting is performed by die casting or continuous casting.

[0042] Preferably, in step 2), preheating + one-stage or two-stage heating + soaking are adopted; wherein, preheating temperature: 800-880°C; preheating time: 0.5-2h; heating temperature: 1000-1180°C; heating time: 0.5-2h; soaking temperature: 1170-1220°C; soaking time: 2-5h.

[0043] Preferably, the cooling is performed by air cooling or wind cooling on a cooling bed, and the cooling rate is ≤2°C / s.

[0044] In the manufacturing method of the present invention:

[0045] In step 1), the smelting can be performed by electric furnace smelting or converter smelting, and then refined and vacuum treated. Of course, in some other embodiments, a vacuum induction furnace can also be used for smelting. Accordingly, casting is required after the smelting is completed. In step 1) of the present invention, casting can be performed by die casting or continuous casting.

[0046] In step 2), when forging is performed, the steel can be directly forged to the final product size; when rolling is performed, the steel billet can be directly rolled to the final product size, or the steel billet can be first rolled to the specified intermediate billet size, and then intermediately heated and rolled to the final product size. The soaking temperature of the intermediate billet can be controlled between 1170 and 1220°C, so that the high-strength and high-toughness non-quenched and tempered steel can be completely austenitized during the heating process, so that the alloy is fully dissolved and evenly distributed.

[0047] The size of the billet is relatively large in large-scale production on site. Considering the danger of temperature stress in the initial stage of heating, the uniformity of the temperature of the billet section in the middle stage and the uniformity of the temperature of the billet section in the later stage, and taking into account both the output and quality, preheating + one-stage or two-stage heating + soaking are adopted; among which, the preheating temperature is 800-880℃; the preheating time is 0.5-2h; the heating temperature is 1000-1180℃; the heating time is 0.5-2h; the soaking temperature is 1170-1220℃; the soaking time is 2-5h. The total heating time is controlled between 3-9h.

[0048] Step 3) During the rolling or forging process, the starting temperature of forging, i.e., the austenitizing temperature, has a great influence on the performance of non-quenched and tempered steel. When the initial forging temperature is high, the alloy elements can be fully dissolved in the austenite, and the stability of the residual austenite is increased during subsequent cooling; however, too high an initial forging temperature makes the grains of the steel coarse and reduces the toughness of the steel; when the initial forging temperature is low, the alloy elements cannot be fully dissolved in the austenite, which reduces the hardenability of the steel, and the obtained structure cannot meet the design requirements. The initial forging temperature or the start rolling temperature corresponding to the design of the component system of the present invention is controlled at 1100-1200°C, and the final forging temperature or the final rolling temperature is controlled at 900-960°C.

[0049] Step 4) After forging or rolling, cooling can be performed by air cooling or wind cooling. The cooling rate of wind cooling is ≤2°C / s. When the cooling rate is greater than 2°C / s, martensite will precipitate, reducing the toughness of the material.

[0050] According to the present invention, a cooling curve of supercooled austenite corresponding to the composition is designed. When the cooling rate is different, the organizational transformation and organizational transformation amount of the supercooled austenite are different. When the cooling rate of the steel is greater than the critical cooling rate, the faster the cooling rate, the greater the degree of supercooling, the finer the steel organization, and the better the strength and toughness of the steel. When the cooling rate is lower than the critical cooling rate, the organization will coarsen, and too much proeutectoid ferrite will precipitate, the plasticity of the steel will increase, but the strength and toughness will decrease.

[0051] The present invention separates the ferrite pearlite transformation curve and the bainite transformation curve in the TTT curve by rationally designing the composition ratio of C, Mn, Cr, and Ni. By adding 0.03% to 0.20% of Mo, the ferrite pearlite transformation curve is further shifted to the right, and the ferrite pearlite transformation curve and the bainite transformation curve are further separated, thereby reducing the critical cooling rate of precipitating bainite. The component system involved can obtain bainite structure under general air cooling conditions. Too fast cooling rate will obtain martensite. Therefore, the steel designed by the present invention requires air cooling or wind cooling after rolling or forging, and the cooling rate is controlled at ≤2°C / s. At the same time, by adding 0.12% to 0.18% of V, 0.030% to 0.045% of Al, and 0.012% to 0.017% of N, the microalloying element coefficient γ is satisfied. M / N =([Al] / 2+[V] / 4) / [N] is in the range of 3.0 to 4.0, which optimizes the ratios of Al to N and V to N, promotes the formation of dispersed and fine V nitride precipitation phase, further refines the grains, and improves the strength and toughness of the steel of the present invention.

[0052] Compared with the prior art, the beneficial effects of the present invention are:

[0053] Compared with Chinese patent CN114032463A, the present invention optimizes the ratio of C, Mn and Cr elements, adds a certain amount of Mo and V, increases the hardenability of steel, promotes the formation of bainite, and reduces the cooling rate required for the formation of bainite. Under general air cooling conditions, non-quenched and tempered steel with a matrix structure mainly composed of bainite can be obtained, with a wide process window, lower cost, and good plasticity and toughness; at the same time, the present invention avoids the addition of B and Ti, reduces the difficulty of production control, avoids the formation of boron nitride and titanium nitride in the material, and reduces the risk of large fluctuations in material properties and reduced fatigue life.

[0054] Compared with Chinese patent CN112522610A, the high-strength and high-toughness bainitic non-quenched and tempered steel described in the present invention overcomes the defects of high strength but insufficient toughness of non-quenched and tempered steel, and has a tensile strength ≥1100MPa, an elongation after fracture ≥15%, and a room temperature impact energy KU2 ≥50J.

[0055] The non-quenched and tempered steel of the present invention improves the toughness of the material while controlling the strength of the material not to be reduced, and can be effectively applied to occasions such as automobiles and engineering machinery that require high-strength and toughness steel. It is green, low-carbon, energy-saving and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a metallographic photograph of Example 1 of the high-strength and high-toughness bainite non-quenched and tempered steel of the present invention;

[0057] Figure 2 This is a metallographic photograph of Example 2 of the high-strength and high-toughness bainite non-quenched and tempered steel of the present invention;

[0058] Figure 3 This is a metallographic photograph of Example 5 of the high-strength and high-toughness bainite non-quenched and tempered steel of the present invention;

[0059] Figure 4 This is a metallographic photograph of Example 6 of the high-strength and high-toughness bainitic non-quenched and tempered steel of the present invention. DETAILED DESCRIPTION

[0060] The present invention will be further described below in conjunction with the embodiments and drawings.

[0061] The composition of the embodiment of the present invention is shown in Table 1, and the remainder includes Fe and other inevitable impurities. The manufacturing process parameters of the embodiment of the present invention are shown in Table 2, and the performance parameters of the steel of the embodiment of the present invention are shown in Table 3.

[0062] The manufacturing method of the embodiment of the present invention comprises the following steps:

[0063] 1) Smelting and casting

[0064] Smelting and casting are carried out according to the chemical composition shown in Table 1. The smelting can be carried out in a 50kg vacuum induction furnace or a 150kg vacuum induction furnace, or in an electric furnace smelting + refining outside the furnace + vacuum degassing method;

[0065] 2) Heating: Control the heating temperature to 1170-1220°C and the insulation time to 3-9h;

[0066] 3) Rolling or forging: Control the starting rolling temperature or the initial forging temperature at 1100-1200°C, and the final rolling temperature or the final forging temperature at 900-960°C;

[0067] 4) Cooling: the cooling method is air cooling or wind cooling, the cooling rate is ≤2℃ / s, and it can be cooled to below 300℃ or stacked.

[0068] Example 1

[0069] According to the chemical composition shown in Table 1, smelting was carried out in a 50kg vacuum induction furnace. The molten steel was cast into an ingot, heated and forged to form a blank. The heating temperature was 1170°C. The total heating and heat preservation time in the heating furnace was 3h. Forging was carried out, and the initial forging temperature was controlled to be 1100°C, the final forging temperature was 900°C, and air cooling was performed after forging at a cooling rate of 0.1°C / s.

[0070] Example 2

[0071] According to the chemical composition in Table 1, smelting was carried out in a 150kg vacuum induction furnace. The molten steel was cast into an ingot, heated and forged into a blank, the heating temperature was 1180℃, the total heating and holding time in the heating furnace was 5h, forging was carried out, the initial forging temperature was controlled to be 1120℃, the final forging temperature was 920℃, and air cooling was carried out after forging, and the cooling rate was 0.2℃ / s.

[0072] Example 3

[0073] According to the chemical composition shown in Table 1, electric furnace smelting, LF refining and VD vacuum treatment are carried out, and then cast into 320mm×425mm continuous casting billets. The continuous casting billets are controlled to be first heated to 850℃ in the preheating section, and then continue to be heated to 1000℃ in the first heating section after 1h. After keeping warm for 0.5h, they are continued to be heated to 1160℃ in the second heating section. After keeping warm for 1h, they enter the soaking section with a temperature of 1200℃. After keeping warm for 2h, subsequent rolling is carried out; the billets are removed from the heating furnace and descaled by high-pressure water before rolling begins. The start rolling temperature is controlled to be 1160℃, the final rolling temperature is 955℃, and they are air-cooled after rolling with a cooling rate of 0.3℃ / s; they pass ultrasonic flaw detection and magnetic particle flaw detection.

[0074] Example 4

[0075] According to the chemical composition shown in Table 1, the steel was smelted in an electric furnace, and LF refining and VD vacuum treatment were performed, and then cast into 320mm×425mm continuous casting billets. The continuous casting billets were first heated to 800℃ in the preheating section, and then continued to be heated to 1120℃ in the first heating section after 1.5h. After being kept warm for 0.5h, they were continued to be heated to 1170℃ in the second heating section. After being kept warm for 1h, they entered the soaking section with a temperature of 1190℃. After being kept warm for 3h, subsequent rolling was performed. The billets were descaled by high-pressure water after leaving the heating furnace and then began to be rolled. The starting rolling temperature was controlled to be 1130℃, the final rolling temperature to be 940℃, and air-cooled after rolling with a cooling rate controlled at 0.5℃ / s. They were inspected by ultrasonic flaw detection and magnetic particle flaw detection.

[0076] Example 5

[0077] According to the chemical composition shown in Table 1, the steel was smelted in an electric furnace, and LF refining and VD vacuum treatment were performed, and then cast into 220mm×220mm continuous casting billets. The continuous casting billets were first heated to 860℃ in the preheating section, and then continued to be heated to 1020℃ in the first heating section after 1h. After keeping warm for 1h, they were continued to be heated to 1180℃ in the second heating section. After keeping warm for 0.5h, they entered the soaking section with a temperature of 1215℃. After keeping warm for 1.5h, subsequent rolling was performed. The billets were descaled by high-pressure water after leaving the heating furnace and then started to be rolled. The starting rolling temperature was controlled to be 1180℃, the final rolling temperature to be 960℃, and they were air-cooled after rolling with a cooling rate controlled at 1℃ / s. They were inspected by ultrasonic flaw detection and magnetic particle flaw detection.

[0078] Example 6

[0079] According to the chemical composition shown in Table 1, electric furnace smelting, LF refining and VD vacuum treatment are carried out, and then cast into 220mm×220mm continuous casting billets. The continuous casting billets are controlled to be first heated to 830℃ in the preheating section, and then continue to be heated to 1100℃ in the first heating section after 2h. After keeping warm for 0.5h, they are continued to be heated to 1180℃ in the second heating section. After keeping warm for 1h, they enter the soaking section with a temperature of 1220℃. After keeping warm for 2h, subsequent rolling is carried out; the billets are descaled by high-pressure water after leaving the heating furnace and then rolling begins. The starting rolling temperature is controlled to be 1200℃, the final rolling temperature is 950℃, and they are air-cooled after rolling. The cooling rate is controlled to be 2℃ / s, and they are inspected by ultrasonic flaw detection and magnetic particle flaw detection.

[0080] The smelting chemical composition of the comparative example is shown in Table 1, and its chemical composition is quite different from the composition range designed by the present invention, and 0.0021% B is added. Electric furnace smelting-LF refining-RH vacuum treatment-continuous casting is adopted, and the continuous casting billet is kept at 1242℃, the total heating time is 7.6h, the starting rolling temperature is 1121℃, the final rolling temperature is 921℃, and after rolling, it is cooled to 600-650℃ on the cooling bed and enters the pit, the pit temperature is 503℃, and the slow cooling time is 49h; the round steel is heated to 1220-1240℃ by medium frequency induction heating, the initial forging temperature is 1150-1200℃, the final forging temperature is 850-950℃, and air cooling is performed after forging.

[0081] The tensile and impact methods are determined according to GB / T 228.1-2010 and GB / T 229-2020. The tensile specimen and impact specimen are respectively taken from the 1 / 4 diameter and the center of the forged crankshaft round steel. The experimental results are averaged. The impact specimen is a U-shaped impact specimen with a size of 10×10×55mm, and the tensile specimen is a threaded tensile specimen with a size of M16×10×128mm. It can be seen from the mechanical properties of the bainitic non-quenched and tempered steels of Examples 1-5 listed in Table 3 that the tensile strength of the steel of the present invention is ≥1100MPa, the elongation after fracture is ≥15%, and the room temperature impact energy KU 2 ≥50J.

[0082] Compared with the comparative example, the bainite in the bainite non-quenched and tempered steel prepared by the present invention exists more in the form of granular bainite, the Al, V, and N contents are optimized, and the precipitated fine dispersed second phase inhibits the growth of grains, making the bainite structure finer than that of the comparative example, see Figure 1 to Figure 4 Therefore, the toughness of the bainite non-quenched and tempered steel of the present invention is better than that of the comparative steel. Figure 1 to Figure 4 The area percentage of bainite is ≥88%, and the microstructure also contains residual austenite and a small amount of ferrite + pearlite.

[0083] At the same time, the preparation process of the bainitic non-quenched and tempered steel of the present invention is simple, and a bainitic structure with a content of more than 88% can be obtained by air cooling after rolling or forging; while the comparative example uses Ti and V microalloying, and adds B elements to improve the hardenability of the material, and a certain controlled cooling process is required after rolling, and the generation control is relatively difficult and cumbersome.

[0084] In summary, the present invention obtains high-strength and high-toughness bainite non-quenched and tempered steel through alloy composition design and optimization of production process control, and its mechanical properties meet the requirements of tensile strength ≥ 1100MPa, elongation after fracture ≥ 15%, and room temperature impact energy KU 2 ≥50J, meeting the performance requirements of high-power and large-size crankshafts.

[0085] It should be noted that the combination of various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0086] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

[0087]

[0088]

[0089]

Claims

1. A high-strength and high-toughness bainitic non-quenched and tempered steel, whose components by mass percentage are: C: 0.22-0.30%, Si: 0.35-0.45%, Mn: 1.80-2.00%, P: 0.005-0.02%, S: 0.015-0.030%, Al: 0.030-0.045%, Cr: 0.40-0.60%, Mo: 0.03-0.20%, Ni: 0.10-0.20%, N: 0.012-0.017%, V: 0.12-0.18%, and the rest includes Fe and other inevitable impurities; and, it must also meet the following requirements: Bainite transformation temperature T of non-quenched and tempered steel B : 500~565℃, T B =830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo]; Microalloying element coefficient γ M / N 3.0~4.0,γ M / N =([Al] / 2+[V] / 4) / [N].

2. The high-strength and high-toughness bainite non-quenched and tempered steel according to claim 1, It is characterized in that The rest is Fe and other inevitable impurities.

3. The high-strength and high-toughness bainite non-quenched and tempered steel according to claim 1 or 2, It is characterized in that The impurities include Ti≤0.003%, and / or O≤0.002%, and / or Ca≤0.005%.

4. The high-strength and high-toughness bainite non-quenched and tempered steel according to claim 1, 2 or 3, It is characterized in that The microstructure of the non-quenched and tempered steel is bainite+ferrite and / or pearlite+retained austenite; Among them, the area content of bainite is greater than 88%.

5. The high-strength and high-toughness bainite non-quenched and tempered steel according to claim 1, 2, 3 or 4, It is characterized in that The tensile strength of the non-quenched and tempered steel is Rm≥1100MPa, the elongation after fracture is ≥15%, and the room temperature impact energy KU 2 ≥50J.

6. The method for producing high-strength and high-toughness bainite non-quenched and tempered steel according to any one of claims 1 to 5, Its characteristics are: The following steps are involved: 1) Smelting and casting Smelting and casting into ingots according to the composition of claim 1, 2 or 3; 2) Heating The ambulation temperature of the steel billet in the heating furnace is controlled at 1170-1220°C, and the total time of preheating, heating and ambulation is controlled at 3-9h; 3) Rolling or forging The starting rolling temperature or the initial forging temperature is controlled at 1100-1200°C, and the final rolling temperature or the final forging temperature is controlled at 900-960°C; 4) Cooling: Cooling to below 300°C.

7. The manufacturing method according to claim 6, Its characteristics are: In step 1), the smelting is performed by electric furnace smelting or converter smelting, and the smelting is performed by refining and vacuum treatment; and the casting is performed by die casting or continuous casting.

8. The manufacturing method according to claim 6, Its characteristics are: In step 2), preheating + one-stage or two-stage heating + soaking are adopted; Among them, preheating temperature: 800 ~ 880 ° C; preheating time: 0.5 ~ 2h; Heating temperature: 1000~1180℃; Heating time: 0.5~2h; Soaking temperature: 1170~1220℃; Soaking time: 2 to 5 hours.

9. The manufacturing method according to claim 6, Its characteristics are: Step 4) The cooling is carried out by air cooling or wind cooling on a cooling bed, and the cooling rate is ≤2°C / s.

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