Hot-rolled hot-formed steel for 1500MPa-grade commercial vehicle rim and preparation method of hot-rolled hot-formed steel

Abstract

The invention belongs to the technical field of production and manufacturing of steel for automobiles, and particularly relates to hot-rolled hot-formed steel for 1500MPa-grade commercial vehicle rims and a preparation method of the hot-rolled hot-formed steel. The steel comprises the following chemical components in percentage by weight: 0.21 to 0.26 percent of C, 0.05 to 0.15 percent of Si, 1.40 to 1.60 percent of Mn, less than or equal to 0.010 percent of P, less than or equal to 0.006 percent of S, 0.02 to 0.05 percent of Al, 0.001 to 0.005 percent of B, 0.01 to 0.03 percent of Ti, 0.15 to 0.25 percent of Cr, 0.02 to 0.06 percent of Nb, less than or equal to 0.005 percent of N, 0.005 to 0.03 percent of Ce, less than 0.01 percent of Ca and the balance of Fe and inevitable impurities. The edge and core structures of the hot-formed steel plate are martensite, the yield strength is larger than or equal to 1000 MPa, the tensile strength is larger than or equal to 1500 MPa, and the requirements for high strength and light weight of commercial vehicles are met.

Description

Technical Field

[0001] The invention belongs to the technical field of automobile steel production and manufacturing, and specifically relates to a 1500MPa grade hot-rolled hot-formed steel for commercial vehicle wheel rims and a preparation method thereof. Background Art

[0002] With the introduction of the concept of lightweight automobiles, the demand for advanced high-strength steel plates in automobiles has increased year by year. As one of the representatives of advanced high-strength steel, hot-formed steel is in line with the development direction of lightweight automobiles and is a product with continuous incremental applications. There are more than 200 production lines in China, and they are still showing an increasing trend. At present, hot-formed steel is mainly used in passenger cars, with a thickness of 1.0mm to 2.5mm. With the introduction of the concept of lightweight commercial vehicles, hot-formed steel is increasingly used in commercial vehicles, mainly supplied in hot-rolled state, such as the rims in commercial vehicle wheels, with a thickness of 3.0mm to 6.0mm. Applying hot-formed steel to commercial vehicles can achieve a weight reduction of 20% to 50%, with significant lightweighting effects. The application and promotion of hot-formed steel in new areas of commercial vehicles and further lightweighting of commercial vehicles through weight reduction are effective ways to reduce carbon emissions. Major domestic advanced steel companies have also developed related products. The main problem is that the increase in thickness leads to an increase in the amount of heat that needs to be removed during the stamping process, and the problem of insufficient hardenability in the core becomes prominent. Due to the inconsistent structure of the edge and core of the hot-formed parts, the strength and toughness are reduced, resulting in the failure of the stamped parts. At the same time, commercial vehicle wheel rim products require an indirect hot forming process to manufacture, that is, 90% to 95% of the final parts are pre-formed, and then hot stamping is performed. Therefore, there are certain requirements for the cold forming performance indicators of steel.

[0003] Patent CN 109136759 discloses a thick specification 1300MPa grade hot-formed steel for spokes and a preparation method thereof, wherein the chemical composition is calculated by mass percentage as follows: C: 0.2-0.24%, Si: 0.20-0.30%, Mn: 1.1-1.5%, P: ≤0.01%, S: ≤0.004%, Ti: 0.02-0.04%, B: 0.003-0.004%, and the remainder is Fe and unavoidable impurities. The hot rolling + hood annealing process can reduce the strength of the steel plate and increase the elongation, so that the steel has good forming performance and welding performance. The tensile strength of the hot-rolled steel plate is ≤520MPa, the yield strength is ≤400MPa, and the elongation is ≥30%. After the hot-formed steel is treated by conventional hot forming process (the die quenching cooling rate is greater than 50℃ / s), the tensile strength is ≥1300MPa, the yield strength is ≤400MPa, and the elongation is ≥30%.

[0004] ≥1000MPa, elongation ≥10%. Patent CN 109023092 A discloses a 1300MPa grade hot-formed steel for wheel rims and a preparation method thereof, wherein the chemical composition is as follows by mass percentage: C: 0.19-0.24%, Si: 0.10-0.30%, Mn: 1.00-1.40%, P: ≤0.01%, S: ≤0.004%, Ti: 0.020-0.040%, B: 0.003-0.004%, the remainder being Fe and unavoidable impurities, and adopting hot rolling + hood annealing treatment process, the tensile strength of the hot-rolled steel plate is ≤520MPa, the yield strength is ≤400MPa, and the elongation is ≥30%. After the hot-formed steel is treated by conventional hot forming process (the die quenching cooling rate is greater than 50℃ / s), the tensile strength is ≥1300MPa, the yield strength is ≥1000MPa, and the elongation is ≥10%. Both of the above patents require hood annealing treatment, which increases production costs, and 0.02-0.04% Ti is added in the composition design to refine the grains, but the degree of grain refinement is limited, and the tensile strength after hot forming is only 1300MPa. At the same time, the stamping die quenching cooling rate is required to be greater than 50℃ / s, which places high requirements on the cooling capacity of the die, increasing the difficulty of die design.

[0005] Patent CN 111534760 A discloses a hot-rolled hot-formed steel and a preparation method thereof, wherein the chemical composition is as follows by mass percentage: C: 0.18-0.50%, Si: 0.3-1.7%, Mn: 1.0-4.0%, P: ≤0.015%, S: ≤0.015%, Alt: 0.02-0.09%, Cr: 0.8-2.5%, Nb: 0.02-0.12%, Ti: 0.01-0.07%, B: 0.001-0.012%, Mo :0.03~0.2%, the balance is Fe and inevitable impurities, and the hot-rolled steel plate is obtained by steelmaking, continuous casting, primary dephosphorization, rough rolling, secondary dephosphorization, finishing rolling, laminar cooling, coiling and leveling. After the hot-rolled steel plate is processed by hot stamping forming process (heating temperature 850~880℃, holding time 3~6min, quenching cooling rate 45-50℃ / s), the yield strength is 900~1100MPa, the tensile strength is 1500~2200MPa, and the elongation is 6~12%. The hot-rolled steel plate prepared by this patent needs to be leveled after coiling, which increases the production cost. At the same time, the expensive Mo element is added to the alloy component system, and the added Cr element is 0.8~2.5%, which is relatively high, increasing the cost from the source. Summary of the invention

[0006] In order to solve the technical problems in the background technology and the above-mentioned patents, the purpose of the present invention is to provide a 1500MPa grade hot-rolled hot-formed steel for commercial vehicle rims and a preparation method thereof. Through reasonable component design and optimized process flow, a large reduction rate in rough rolling, appropriate proportional convexity parameter setting in the finishing rolling process, low temperature final rolling, medium temperature coiling and slow cooling after rolling are adopted, so that the hot-rolled substrate has excellent plate shape, the unevenness of the steel plate is ≤10.0mm / m, and no flattening process is required after coiling. The yield strength of the hot-rolled substrate is 340-410MPa, the tensile strength is 500-570MPa, and the elongation A is 2.5-3.0mm / m. 50 ≥28%, the average aspect ratio of inclusions ≤1.50, it has the advantages of excellent comprehensive mechanical properties and good formability, no need for hood annealing treatment, saving production costs; through reasonable composition design combined with hot stamping process design, the hot-formed steel plate has good hardenability and organizational uniformity, the edge and core are both martensitic structures, the qualified rate of stamping parts is improved, the yield strength is ≥1000MPa, the tensile strength is ≥1500MPa, the elongation is ≥10%, and the strength-plasticity product is ≥15.9GPa·%, which can meet the high-strength and lightweight requirements and safety goals of commercial vehicle steel.

[0007] The present invention adopts the following technical solutions:

[0008] The invention provides a hot-rolled hot-formed steel for a 1500MPa grade commercial vehicle wheel rim. The chemical composition of the steel comprises, by weight percentage, the following: C: 0.21-0.26%, Si: 0.05-0.15%, Mn: 1.40-1.60%, P≤0.010%, S≤0.006%, Al: 0.02-0.05%, B: 0.001-0.005%, Ti: 0.01-0.03%, Cr: 0.15-0.25%, Nb: 0.02-0.06%, N≤0.005%, Ce: 0.005-0.03%, Ca<0.01%, and the remainder is Fe and unavoidable impurities. The thickness of the steel is 3.0-6.0 mm.

[0009] The reasons for the composition design in the present invention are as follows:

[0010] C: C is a necessary element to ensure high hardenability and strength, and is an austenite stabilizing element. If the carbon content is too low, the strength will be reduced and the performance will not meet the target requirements; if the carbon content is too high, the carbon equivalent will increase and the welding performance will be affected, so its content is limited to between 0.21% and 0.26%.

[0011] Si: Si is a necessary alloying element and has the effect of solid solution strengthening. It can also inhibit the precipitation of cementite and improve the plasticity of the material. However, excessive Si will cause brittleness and produce oxides during hot rolling, reducing the surface quality of the steel. Therefore, its content is limited to 0.05-0.15%.

[0012] Mn: Mn has the function of improving hardenability and can delay the transformation of pearlite and bainite, but too high Mn content will cause band defects in the structure, so its content is limited to between 1.40 and 1.60%.

[0013] P, S: P and S elements are usually harmful elements in steel. P element easily causes center segregation of the ingot. S element combines with Mn to form MnS, which reduces the performance of the steel. The P content is limited to less than 0.010%, and the S content is limited to less than 0.006%.

[0014] Al: Al element is a deoxidizer in steel. A certain amount of acid-soluble aluminum should be present in the steel. However, too high an aluminum content will cause aluminum inclusions in the steel. The content should be limited to 0.02-0.05%.

[0015] B: The B element can improve the hardenability of steel. At the same time, a small amount of B added to hot-formed steel will gather at the austenite grain boundary, delaying the formation of ferrite, thereby providing favorable conditions for the subsequent hot stamping process without ferrite transformation. However, too much B element will destroy the toughness of the material, and its content is limited to 0.001-0.005%.

[0016] Ti: A small amount of Ti added to steel can refine the grains and improve strength and toughness, but too much Ti will combine with N to form TiN, which is larger in size and is not conducive to the toughness of hot stamping parts. Its content is limited to 0.01-0.03%.

[0017] Cr: Cr can delay pearlite transformation, improve the hardenability of steel plates, and refine the quenched martensite structure. Too high Cr content will reduce the welding performance of hot-formed steel, and its content is limited to 0.15-0.25%.

[0018] Nb: Nb can hinder the growth of original austenite grain size, refine the original austenite grain size, and refine the martensite laths. At the same time, the precipitated carbides can improve the strength, plasticity and hydrogen embrittlement resistance of the material. If the amount is small, the grain refining effect is not obvious. If the amount is large, the surface quality of the continuous casting billet is reduced and the alloy cost is increased. Its content is limited to 0.02-0.06%.

[0019] N: N can improve the toughness and welding performance of steel. If added in large amounts, it will form non-metallic inclusion TiN with the alloy, affecting the comprehensive performance of the material. Its content is limited to less than 0.005%.

[0020] Ce: Ce can change the shape and distribution of inclusions in steel, improve the plasticity of steel plates, and at the same time improve the cold forming performance of steel plates, and can improve the fatigue performance and welding performance of steel plates. If the addition amount is too low, the effect of rare earth elements is not obvious. If the addition amount is too high, inclusions will aggregate and reduce the plasticity of the material. Its content is limited to 0.005-0.03%.

[0021] The present invention provides a method for preparing 1500MPa grade hot-rolled hot-formed steel for commercial vehicles, which mainly includes smelting, refining, casting, heating, rough rolling, finish rolling, laminar cooling and coiling, and hot stamping.

[0022] In the above technical scheme, further, the smelting, refining and casting process: the molten steel is refined by LF+RH dual paths after converter smelting, and the refining process adopts an optimized slag making method, and the basicity R of the slag is controlled at 2.5-2.8. The LF refining process maintains a slight positive pressure, controls the bottom blowing argon intensity, and tries to avoid the exposure of the molten steel to achieve LF N increase ≤ 10ppm. The RH refining is degassing, and the RH refining cycle time is controlled to be 35-45min. The order of alloy addition is strictly controlled to add titanium first and then boron. After boron alloying, rare earth alloy is added. The rare earth alloy consists of Ce: 20%, Fe and other unavoidable impurities: 80%, and the addition amount is 120-220kg. The molten steel is treated with calcium using a silicon calcium wire to ensure the cleanliness of the molten steel and smooth production. Each furnace is fed with 300-400m of silicon calcium wire. During the continuous casting process, the overheat of the ladle is controlled at 15-25℃, and the pulling speed is kept constant during the steel pouring process, with the pulling speed control target of 1.0-1.3m / min. After production, the ingot is off the line and stored in the middle of the ingot stack (avoiding the tuyere and cooling as slowly as possible). The low-power inspection of the ingot shows that the center segregation is less than 1.0 level and the center porosity is less than 1.0 level.

[0023] In the above technical solution, further, the heating process: during heating, the atmosphere in the furnace is controlled to be weakly oxidizing, the heating time is 150-210 minutes, and the furnace temperature is 1250-1270°C to avoid the occurrence of surface defects of the slab and the hot-rolled coil.

[0024] In the above technical solution, further, in the rough rolling process: the rough rolling passes select the 3+3 mode, that is, R1 adopts 3 rolling passes, R2 adopts 3 rolling passes, the cumulative reduction rate in the rough rolling stage is controlled to be 80-85%, and the R2 final rolling temperature is controlled to be 1020-1070°C.

[0025] In the above technical solution, further, in the finishing rolling process: the starting rolling temperature of the finishing rolling F1 is controlled to be 1000-1040° C., and the final rolling temperature is 870-890° C. In order to obtain a good plate shape, the present invention adopts a load adjustment method to increase the load of the finishing rolling stands F1-F4, reduce the load of the finishing rolling stands F5-F7, and especially reduce the load of the finishing rolling stand F7, which is beneficial to the plate shape control of the final stand. The finishing process is strictly carried out in accordance with the proportional crown control mode. Specifically, the proportional crown parameters of the F1 finishing stand are controlled at -0.35 to -0.3, the proportional crown parameters of the F2-F4 finishing stands are controlled at -0.2 to -0.1, the proportional crown parameters of the F5-F6 finishing stands are controlled at 0.05 to 0.15, and the proportional crown parameters of the F7 finishing stand are controlled at 0.3 to 0.35. After this parameter configuration, the crown changes greatly in F1-F4, and changes slightly in F5 and F6, and basically no crown changes in F7. This method amplifies the control and adjustment of the plate crown by the upstream stand, and also provides a powerful condition for the downstream stand to reduce the plate crown fluctuation and control the flatness.

[0026] In the above technical scheme, further, the laminar cooling and coiling process: the laminar cooling is performed by the front-stage cooling method to a coiling temperature of 650-680°C. The reason for adopting medium-temperature coiling is that low-temperature coiling will make the steel plate too strong, which is not conducive to plate shape control, and it is not easy to control during rolling. High-temperature coiling leads to a large release of phase change stress during cooling, and a small thermal stress. The thermal stress cannot offset the structural stress, resulting in poor plate shape. On the other hand, the use of this coiling temperature can promote the precipitation of niobium-containing carbonitrides and improve the strength, toughness and hydrogen embrittlement resistance of the final product. The coiled plate is hoisted into the slow cooling pit as soon as possible for slow cooling, and the insulation pit is preheated by hoisting hot coils in advance. The plate is placed in the slow cooling pit at a temperature of 390-500°C, the slow cooling time is 48-60 hours, and the temperature out of the slow cooling pit is 80-150°C to ensure the release of plate shape and internal stress.

[0027] The hot-rolled base plate of the hot-formed steel obtained by the present invention has an unevenness of ≤10.0 mm / m, has an excellent plate shape, does not need to be flattened, and has a metallographic structure of ferrite and pearlite, wherein the volume fraction of ferrite is 70-80%, the volume fraction of pearlite is 20-30%, the yield strength is 360-430 MPa, the tensile strength is 500-570 MPa, and the elongation A 50 ≥28%, average aspect ratio of inclusions ≤1.50, excellent comprehensive mechanical properties, good formability, no need for hood annealing treatment.

[0028] In the above technical solution, further, the hot stamping forming process: the hot rolled coil is hot stamped after uncoiling and blanking, the heating temperature is 890-930°C, and the holding time is 260-420s. The stamping die temperature is controlled to be greater than 750°C, the die holding time is 10-20s, the die holding pressure is 10-25MPa, the die cooling rate is greater than 25°C / s, and the furnace should have a protective atmosphere such as inert gas nitrogen to reduce the formation of iron oxide scale.

[0029] The original austenite grain size grade of the hot-formed steel obtained by the present invention is ≥10, the edge and core structures are both martensite, the yield strength is ≥1000MPa, the tensile strength is ≥1500MPa, the elongation is ≥10%, and the strength-ductility product is ≥15.9GPa·%.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The hot-formed steel composition design of the present invention does not add expensive alloy elements such as Mo and Ni, which reduces costs from the source, and the preparation process is simple. Through reasonable composition design and optimized process flow, the rough rolling large reduction rate is used to reduce the thickness of the intermediate billet, and the finishing process adopts appropriate proportional convexity parameter setting, low-temperature final rolling, medium-temperature coiling and post-rolling slow cooling. The hot-rolled substrate has excellent plate shape and unevenness ≤10.0mm / m, and no subsequent flattening process is required, which realizes the free flattening of the hot-rolled substrate. Compared with the process that requires flattening, the cost of 30 to 50 yuan per ton of steel can be saved, which is conducive to stable batch production. At the same time, the yield strength of the hot-rolled substrate is 360 to 430MPa, the tensile strength is 500 to 570MPa, and the elongation A 50 ≥28%, excellent comprehensive mechanical properties, no need for hood annealing and subsequent cold rolling process, saving production energy in the intermediate process, with significant economic benefits, easy to promote in industry.

[0032] 2. The traditional Mn-B hot-formed steel will have the problem of poor collision energy absorption during use. The present invention introduces an appropriate amount of Nb element and rare earth Ce element on the basis of the traditional Mn-B hot-formed steel composition, wherein the Nb element plays a role in refining the original austenite grains in the hot-formed steel, making the lath martensite bundles finer and improving the strength-ductility product of the steel plate. The prepared hot-formed steel has a strength-ductility product of ≥15.9GPa·%, which can absorb more energy during the collision process and can achieve good safety goals when used in the field of commercial vehicle wheel rims; the Ce element can modify the shape of inclusions in the steel, so that the average aspect ratio of the hot-rolled substrate inclusions is ≤1.5, and the Ce element can improve the heat The toughness of the rolled base plate, that is, the strength-plastic product, further improves the cold forming performance of the hot-rolled base plate, which is convenient for providing good material conditions for pre-forming in the commercial vehicle rim manufacturing process. In addition, the Ce element can significantly reduce the critical cooling rate of martensite, promote martensite transformation, and improve the hardenability of the steel plate. Through the composition design combined with the hot stamping process design, the hot-formed steel plate has excellent organizational uniformity and high hardenability. The organization of the edge and the core is martensite, which improves the qualified rate of stamping parts without the need for hot stamping equipment modification. In terms of performance, the yield strength is ≥1000MPa, the tensile strength is ≥1500MPa, and the elongation is ≥10%, which can meet the high-strength and lightweight requirements of commercial vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the metallographic structure diagram (ferrite + pearlite) of the hot-rolled substrate of the hot-formed steel according to Example 1 of the present invention.

[0034] Figure 2 This is the original austenite grain diagram (grain size grade 10.5) of the hot-formed steel plate after hot stamping in Example 1 of the present invention.

[0035] Figure 3 This is the metallographic structure diagram (martensite) of the edge of the hot-formed steel plate after hot stamping in Example 1 of the present invention.

[0036] Figure 4 This is the metallographic structure diagram (martensite) of the core of the hot-formed steel plate after hot stamping in Example 1 of the present invention.

[0037] Figure 5 This is the original austenite grain diagram (grain size grade 8.5) of the hot-formed steel plate of comparative example 4 of the present invention after hot stamping. DETAILED DESCRIPTION

[0038] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0039] Example 1

[0040] The process steps for preparing 1500MPa grade hot-rolled hot-formed steel for commercial vehicle rims with a thickness of 3.4mm are as follows:

[0041] (1) Smelting, refining and casting process: After the molten steel is smelted in the converter, it is refined by LF+RH dual paths. The refining process adopts an optimized slag-making method, and the basicity R of the slag is controlled at 2.8. During the LF refining process, a slight positive pressure is maintained, the bottom blowing argon intensity is controlled, and the exposure of the molten steel is avoided as much as possible to achieve LF N increase of 10ppm. RH refining is degassing treatment, and the RH refining cycle time is controlled to 40min. The order of alloy addition is strictly controlled to add titanium first and then boron. After boron alloying, rare earth alloy is added. The rare earth alloy consists of Ce: 20%, Fe and other unavoidable impurities: 80%, and the addition amount is 150kg. Silicon calcium wire is used to treat the molten steel with calcium, and the silicon calcium wire is fed for 330m. The overheating degree of the package is 15℃ during the continuous casting process, and the casting speed is kept constant during the steel casting process. The pulling speed control target is 1.0m / min. After production, the ingot is off the line and stored in the middle of the ingot stack (avoiding the tuyere and cooling as slowly as possible). The ingot has a low-power inspection and a central segregation of 0.5 and a central porosity of 0.5. The chemical composition of the final sulfur-printed ingot sampling components by mass percentage is: C: 0.23%, Si: 0.08%, Mn: 1.50%, P: 0.007%, S: 0.006%, Al: 0.039, Nb: 0.060, Cr: 0.24, B: 0.001%, Ti: 0.020%, N: 0.005, Ce: 0.0083%, Ca: 0.0006%, and the balance is Fe and unavoidable impurities.

[0042] (2) Heating process: During heating, the atmosphere in the furnace is strictly controlled to be weakly oxidizing, the heating time is 168 minutes, and the furnace temperature is 1270°C.

[0043] (3) Rough rolling process: The rough rolling process adopts the 3+3 mode, that is, R1 is rolled in 3 passes, R2 is rolled in 3 passes, the cumulative reduction rate in the rough rolling stage is 83.3%, and the final rolling temperature of R2 is 1057°C.

[0044] (4) Finishing rolling process: The starting rolling temperature of finishing rolling F1 is 1035°C. The proportional crown control method is used to control the plate shape. The crown parameter of F1 finishing rolling stand is -0.33, the crown parameters of F2-F4 finishing rolling stands are -0.15, -0.15, -0.15, the crown parameters of F5-F6 finishing rolling stands are 0.15, 0.15, the crown parameter of F7 finishing rolling final stand is 0.33, and the final rolling temperature is 881°C.

[0045] (5) Laminar cooling and coiling process: The laminar cooling is carried out by front-stage cooling to a coiling temperature of 669°C. The coiled sheet is placed in a slow cooling pit for slow cooling as soon as possible. The temperature of the slow cooling pit is 430°C, the slow cooling time is 48 hours, and the temperature out of the slow cooling pit is 142°C. The roughness of the obtained hot-rolled substrate is 8.5 mm / m, and the metallographic structure is ferrite and pearlite, the volume fraction of ferrite is 74%, the volume fraction of pearlite is 26%, the yield strength is 385 MPa, the tensile strength is 551 MPa, the elongation is 29.5%, and the average aspect ratio of inclusions is 1.48.

[0046] (6) Hot stamping process: After the hot rolled coil is unrolled and blanked, hot stamping is performed, the heating temperature is 890°C, and the holding time is 260s. The stamping die temperature is 755°C, the die holding time is 10s, and the die holding pressure is 15MPa. The original austenite grain size of the obtained hot-formed steel is 10.5, the edge and core structures are both martensite, the yield strength is 1185MPa, the tensile strength is 1625MPa, the elongation is 10.5%, and the strength-ductility product is 17.06GPa·%.

[0047] Example 2

[0048] A hot-rolled hot-formed steel with a thickness of 3.3 mm and a grade of 1500 MPa for commercial vehicle wheel rims was prepared in the same steps as in Example 1, except that the amount of rare earth alloy added was 120 kg, the silicon-calcium wire was fed 300 m, the overheating degree of the continuous casting process was 25° C., the casting speed was 1.2 m / min, the center segregation of the ingot at low magnification inspection was 0.5 level, and the center porosity was 0.5 level. The temperature of the slow cooling pit was 500° C., the slow cooling time was 60 hours, and the temperature out of the slow cooling pit was 80° C.

[0049] Example 3

[0050] A hot-rolled hot-formed steel with a thickness of 4.5 mm and a grade of 1500 MPa for commercial vehicle wheel rims was prepared in the same steps as in Example 1, except that the amount of rare earth alloy added was 180 kg, the silicon-calcium wire was fed at 350 m, the overheating degree of the continuous casting process was 20° C., the casting speed was 1.3 m / min, the center segregation of the ingot at low magnification inspection was 0.5 level, and the center porosity was 0.5 level. The temperature of the slow cooling pit was 460° C., the slow cooling time was 50 hours, and the temperature out of the slow cooling pit was 112° C.

[0051] Example 4

[0052] A hot-rolled hot-formed steel with a thickness of 4.0 mm and a 1500 MPa grade for commercial vehicle wheel rim is prepared by the same steps as in Example 1, except that: 200 kg of rare earth alloy is added, 380 m of silicon-calcium wire is fed, the temperature of the slow cooling pit is 420°C, the slow cooling time is 48 hours, and the temperature out of the slow cooling pit is 126°C.

[0053] Example 5

[0054] A hot-rolled hot-formed steel with a thickness of 5.5 mm and a 1500 MPa grade for commercial vehicle wheel rim is prepared, and the steps are the same as those in Example 1, except that: the amount of rare earth alloy added is 220 kg, 400 m of silicon-calcium wire is fed, the temperature of the slow cooling pit is 440°C, the slow cooling time is 55 hours, and the temperature out of the slow cooling pit is 95°C.

[0055] Comparative Example 1

[0056] The hot-rolled hot-formed steel for commercial vehicle rims with a thickness of 5.5 mm and a grade of 1500 MPa is prepared in the same manner as in Example 5, except for the hot rolling process parameters. The specific parameters are shown in Table 2.

[0057] Comparative Example 2

[0058] A hot-rolled hot-formed steel with a thickness of 4.0 mm and a grade of 1500 MPa for commercial vehicle wheel rims is prepared in the same steps as in Example 1, except that the proportional crown target values ​​of the F1-F7 stands in the finishing rolling process are all set to 0.015, and the proportional crown parameters are not adjusted.

[0059] Comparative Example 3

[0060] The steps for preparing hot-rolled hot-formed steel with a thickness of 3.5 mm and a grade of 1500 MPa for commercial vehicle wheel rims are the same as those in Example 1, except that there is no slow cooling process after the hot-rolled steel plate is coiled.

[0061] Comparative Example 4

[0062] The hot-rolled hot-formed steel for commercial vehicle wheel rims with a thickness of 4.0 mm and a grade of 1500 MPa is prepared in the same manner as in Example 1, except that the Nb element is not added to the alloy smelting billet.

[0063] Comparative Example 5

[0064] A 1500 MPa grade hot-rolled hot-formed steel for commercial vehicle rims with a thickness of 6.0 mm was prepared in the same steps as in Example 5, except that no rare earth alloy was added during the smelting process, no silicon-calcium wire feeding treatment was performed, the continuous casting tundish was overheated at 40°C, and the casting speed target during the steel casting process was 1.4 m / min. The low-power inspection of the ingot showed a center segregation of level 1 and a center porosity of level 1.

[0065] Comparative Example 6

[0066] The hot-rolled hot-formed steel with a thickness of 3.5 mm and a grade of 1500 MPa for commercial vehicle wheel rims is prepared by the same steps as in Example 1, except that 240 kg of rare earth alloy is added during the smelting process and 420 m of silicon-calcium wire is fed.

[0067] Comparative Example 7

[0068] The steel plate described in Example 3 disclosed in application publication number CN 109023092 A was compared as Comparative Example 7.

[0069] The chemical composition, process parameters and performance of the above-mentioned embodiments and comparative examples are specifically shown in Tables 1 to 5. Table 1 is a list of chemical composition values ​​of the embodiments and comparative examples of the present invention; Table 2 is a list of main process parameter values ​​of hot rolling of the embodiments and comparative examples of the present invention; Table 3 is a list of the organization, performance and unevenness of the hot-rolled substrate of the embodiments and comparative examples of the present invention; Table 4 is a list of main process parameter values ​​of hot stamping of the embodiments and comparative examples of the present invention; Table 5 is a list of hot forming organization and performance test values ​​of the embodiments and comparative examples of the present invention;

[0070] Table 1 Composition of steel in the embodiments of the present invention and comparative examples

[0071] C Si Mn P S Al Nb Cr B Ti N Ce Ca Example 1 0.23 0.08 1.60 0.007 0.006 0.039 0.060 0.24 0.001 0.010 0.005 0.0083 0.0006 Example 2 0.26 0.05 1.43 0.010 0.003 0.020 0.020 0.19 0.002 0.019 0.004 0.0058 0.0005 Example 3 0.25 0.13 1.55 0.008 0.002 0.042 0.049 0.21 0.001 0.026 0.003 0.0147 0.0007 Example 4 0.21 0.15 1.47 0.007 0.004 0.047 0.053 0.25 0.003 0.030 0.004 0.0180 0.0008 Example 5 0.24 0.06 1.50 0.010 0.003 0.036 0.036 0.15 0.002 0.015 0.002 0.0263 0.0009 Comparative Example 1 0.23 0.08 1.45 0.008 0.005 0.023 0.035 0.24 0.003 0.025 0.004 0.0082 0.0009 Comparative Example 2 0.25 0.11 1.40 0.005 0.002 0.034 0.043 0.16 0.004 0.012 0.005 0.0078 0.0006 Comparative Example 3 0.24 0.12 1.52 0.007 0.003 0.041 0.052 0.22 0.003 0.025 0.003 0.0074 0.0005 Comparative Example 4 0.22 0.11 1.44 0.006 0.004 0.028 -- 0.24 0.003 0.028 0.004 0.0081 0.0005 Comparative Example 5 0.21 0.09 1.60 0.008 0.002 0.036 0.039 0.18 0.004 0.017 0.003 -- 0.0009 Comparative Example 6 0.23 0.14 1.48 0.005 0.003 0.021 0.023 0.15 0.002 0.014 0.002 0.0350 0.0009 Comparative Example 7 0.24 0.22 1.30 0.005 0.002 -- -- -- 0.0036 0.033 -- -- --

[0072] Table 2 Main process parameters of hot rolling of the steel of the embodiment of the present invention and the comparative example

[0073]

[0074] Table 3 Performance and flatness of hot-rolled substrates of the examples of the present invention and the comparative examples

[0075]

[0076] Table 4 Main process parameters of hot stamping of the steel of the embodiment of the present invention and the comparative example

[0077]

[0078] Table 5: Microstructure and properties of the steels of the embodiments of the present invention and the comparative examples after hot forming

[0079]

[0080] From the performance and unevenness of the hot-rolled substrates of the embodiments and comparative examples in Table 3, it can be seen that the unevenness of the hot-rolled substrates of the embodiments is better than that of comparative examples 1, 2, and 3, the strength and toughness of the embodiments are higher than those of comparative example 5 (no rare earth alloy added) and comparative example 6 (rare earth alloy added in an amount of 240 kg), and the average aspect ratio of inclusions of the hot-rolled substrates of the embodiments is lower than that of comparative example 5, and the embodiments have excellent cold forming properties.

[0081] It can be clearly seen from the microstructure and mechanical properties of the hot-formed steel of the embodiment and comparative example in Table 5 that the edge and core microstructures of the final hot-formed steel of the embodiment are both martensite, and the strength-ductility product is significantly higher than that of the hot-formed steel of comparative example 4 without adding Nb element. It can be used in the manufacture of commercial vehicle wheel rims to achieve good safety goals. The comprehensive performance of the final hot-formed steel of the embodiment is significantly higher than that of comparative example 7, which can meet the high-strength and lightweight requirements of commercial vehicles.

Claims

1. A 1500MPa grade hot-rolled hot-formed steel for commercial vehicle wheel rim, characterized in that: The chemical composition of the steel includes, by weight percentage, C: 0.21-0.26%, Si: 0.05-0.15%, Mn: 1.40-1.60%, P≤0.010%, S≤0.006%, Al: 0.02-0.05%, B: 0.001-0.005%, Ti: 0.01-0.03%, Cr: 0.15-0.25%, Nb: 0.02-0.06%, N≤0.005%, Ce: 0.005-0.03%, Ca<0.01%, and the balance is Fe and unavoidable impurities.

2. The 1500MPa grade hot-rolled hot-formed steel for commercial vehicle wheel rim according to claim 1, characterized in that: The thickness of the steel is 3.0-6.0 mm.

3. The 1500MPa grade hot-rolled hot-formed steel for commercial vehicle wheel rim according to claim 1, characterized in that: The hot-rolled base plate of the hot-formed steel has an unevenness of ≤10.0 mm / m, a yield strength of 340-410 MPa, a tensile strength of 500-570 MPa, and an elongation of A 50 ≥28%, average aspect ratio of inclusions ≤1.50; The hot-formed steel has a martensite structure at the edge and the core, a yield strength of ≥1000MPa, a tensile strength of ≥1500MPa, an elongation of ≥10%, a strength-ductility product of ≥15.9GPa·%, and a grain size grade of the original austenite of ≥10.

4. The method for preparing the 1500MPa grade hot-rolled hot-formed steel for commercial vehicle wheel rim according to claim 1, characterized in that: The method comprises smelting, refining, casting, heating, rough rolling, finish rolling, laminar cooling and coiling, and hot stamping forming; In the smelting, refining and casting process, the molten steel is smelted in a converter and then refined by LF+RH dual paths, the basicity R of the slag is controlled at 2.5-2.8, the order of alloy addition in the RH refining process is first adding titanium and then adding boron, rare earth alloy is added after boron alloying, and calcium treatment of the molten steel is performed by using silicon calcium wire after re-pressing; In the finishing process, the entire finishing mill group F1-F7 stands control the proportional convexity parameters of the F1 finishing stand to be -0.35 to -0.3, the proportional convexity parameters of the F2-F4 finishing stands to be -0.2 to -0.1, the proportional convexity parameters of the F5-F6 finishing stands to be 0.05 to 0.15, and the proportional convexity parameters of the F7 finishing final stand to be 0.3 to 0.

35.

5. The preparation method according to claim 4, characterized in that: In the smelting, refining and casting process, the rare earth alloy is composed of Ce: 20%, Fe and other inevitable impurities: 80%, and the added amount is 120-220kg; 300-400m of silicon-calcium wire is fed to each furnace; the overheat of the package is controlled at 15-25°C during the continuous casting process, and a constant pulling speed is maintained during the steel pouring process, and the pulling speed control target is 1.0-1.3m / min.

6. The preparation method according to claim 4, characterized in that: The center segregation of the obtained ingot is less than 1.0 level, and the center porosity is less than 1.0 level.

7. The preparation method according to claim 4, characterized in that: In the heating process, the atmosphere in the furnace is controlled to be weakly oxidizing during heating, the heating time is 150 to 210 minutes, and the furnace outlet temperature is 1250 to 1270°C.

8. The preparation method according to claim 4, characterized in that: In the rough rolling process, the rough rolling passes are selected in 3+3 mode, that is, R1 is rolled in 3 passes, R2 is rolled in 3 passes, the cumulative reduction rate in the rough rolling stage is controlled to be 80-85%, and the final rolling temperature of R2 is controlled to be 1020-1070°C.

9. The preparation method according to claim 4, characterized in that: In the laminar cooling and coiling process, the front-stage cooling method is adopted, and the laminar cooling is performed to a coiling temperature of 650-680°C; after coiling, the slow cooling is performed, the temperature of the slow cooling pit is 390-500°C, the slow cooling time is 48-60 hours, and the temperature out of the slow cooling pit is 80-150°C.

10. The preparation method according to claim 4, characterized in that: The heating temperature of hot stamping is 890-930°C, and the holding time is 260-420s; the stamping die temperature is greater than 750°C, the die holding time is 10-20s, and the die holding pressure is 10-25Mpa; the die cooling rate is >25°C / s.

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