Hot-rolled steel strip for 1100MPa-grade thin-specification light-weight square tube and manufacturing method of hot-rolled steel strip

Abstract

The invention discloses a method for manufacturing a hot-rolled steel strip for a 1100MPa-grade thin-specification light-weight square tube. The method comprises the following steps: continuously casting molten steel with optimally designed components into a plate blank; the continuous casting plate blank is heated to 1280-1320 DEG C, rough rolling is conducted on the continuous casting plate blank to obtain an intermediate blank with the thickness smaller than or equal to 35 mm, finish rolling is conducted on the intermediate blank to obtain a steel strip with the target thickness, and the finish rolling temperature ranges from 850 DEG C to 900 DEG C; after-rolling laminar cooling is conducted on the steel strip, the water ratio of an upper water group to a lower water group in laminar cooling is 10: 12, and the steel strip is cooled to 150 DEG C or below at the cooling speed larger than or equal to 60 DEG C / s; the steel coil is subjected to bell-type furnace tempering, the tempering temperature ranges from 300 DEG C to 400 DEG C, and the heat preservation time is larger than or equal to 15 h; and after the steel coil is leveled and tempered, the leveling pressure is 600-800 tons, and the leveling ductility is smaller than or equal to 2%. The strength of the steel for the square tube is improved to 1100 MPa, the thickness is reduced to 2 mm or below, the steel is high in strength, good in forming performance, low in cost and easy to weld, the requirement for manufacturing the thin-specification and ultra-high-strength steel for the lightweight square tube is met, the good bending performance is considered while the material is highly strengthened, and the requirement of the tube manufacturing technology is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy engineering, and particularly relates to a hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes and a manufacturing method thereof. Background Art

[0002] Lightweight is the development trend of the automotive industry. Thinning and high-strengthening of materials are important ways for lightweight design. At present, the tensile strength of the steel used for square tubes is generally below 700MPa, and the material thickness is generally above 3mm. Higher-strength square tubes are generally made of aluminum alloy, but the cost is relatively high. There is an urgent market demand for steel for square tubes with a tensile strength above 900MPa and even 1100MPa.

[0003] To increase the tensile strength of the steel for square tubes to 1100MPa grade, while also taking into account excellent bending performance, stable strip shape of thin-specification steel strips with a thickness ≤ 2.0mm, good welding performance, and low cost, it poses high requirements for the production process. At present, there is no mature manufacturing method for 1100MPa grade thin-specification lightweight square tube steel. Summary of the Invention

[0004] In view of the above technical problems in the prior art, the present invention provides a hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes and a manufacturing method thereof.

[0005] In one aspect of the present invention, the manufacturing method of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes includes the following steps:

[0006] (1) Composition optimization design

[0007] The chemical composition of the hot-rolled steel for 1100MPa grade lightweight square tubes is optimized and controlled by mass percentage as follows: C: 0.08 - 0.18%, Si: 0.8 - 1.5%, Mn: 0.8 - 1.4%, P ≤ 0.02%, S ≤ 0.025%, Ti: 0.01 - 0.04%, Al: 0.02 - 0.06%, and the rest is Fe and unavoidable impurities;

[0008] (2) Obtaining molten steel that meets the composition optimization design through converter smelting and secondary refining LF + RH, and continuously casting the molten steel into slab billets;

[0009] (3) Heating the continuously cast slab billets to 1280 - 1320°C, and using a single-stand reciprocating roughing mill to rough-roll the continuously cast slab billets into intermediate billets with a thickness ≤ 35mm;

[0010] (4) Using a multi-stand hot continuous rolling mill to finish-roll the intermediate billets into steel strips with the target thickness, and controlling the finishing rolling temperature to 850 - 900°C;

[0011] (5) The post-rolling laminar cooling of the steel strip is carried out by using a strong cooling process. The water ratio of the upper and lower water groups in the laminar cooling is controlled at 10:12, and the steel strip is cooled to below 150°C at a cooling rate of ≥60°C / s. After cooling, it is coiled into a steel coil.

[0012] (6) The steel coil is subjected to tempering in a bell-type furnace. The tempering temperature is controlled at 300 - 400°C, and the holding time is controlled at ≥15 h.

[0013] (7) The steel coil after tempering in the bell-type furnace is leveled. The leveling pressure is controlled at 600 - 800 tons, and the leveling elongation is controlled at ≤2%.

[0014] Further, in the manufacturing method of the hot-rolled steel strip for the 1100 MPa grade thin-specification lightweight square tube, in the composition optimization design step, the C content is optimally controlled at 0.10 - 0.15%, the Si content is optimally controlled at 1.0 - 1.3%, the Mn content is optimally controlled at 1.0 - 1.2%, the Ti content is optimally controlled at 0.01 - 0.025%, and the Al content is optimally controlled at 0.02 - 0.04%.

[0015] Further, in the manufacturing method of the hot-rolled steel strip for the 1100 MPa grade thin-specification lightweight square tube, in the bell-type furnace tempering step, a nitrogen protection atmosphere is adopted during the tempering process.

[0016] In another aspect of the present invention, the hot-rolled steel strip for the 1100 MPa grade thin-specification lightweight square tube is prepared by the above-mentioned manufacturing method of the hot-rolled steel strip for the 1100 MPa grade thin-specification lightweight square tube. The thickness of the steel strip is 1.5 - 2.0 mm, the yield strength ReL ≥ 700 MPa, the tensile strength Rm ≥ 1100 MPa, the elongation A50 ≥ 8%, the cold bending is qualified, the carbon equivalent CEV ≤ 0.4%, and the metallographic structure of the steel strip is mainly martensite.

[0017] The hot-rolled steel strip for the 1100 MPa grade thin-specification lightweight square tube and its manufacturing method of the present invention have the following advantages and beneficial effects:

[0018] The present invention improves the strength of the steel for square tubes from the conventional 700 MPa grade to 1100 MPa grade. It has high strength, good forming performance, low cost, and is easy to weld, meeting the requirements for making thin-specification and ultra-high-strength steel for lightweight square tubes.

[0019] While the present invention achieves high strengthening of the material, it also takes into account good bending performance, and the cold bending (D = 4a, 180°) is qualified, meeting the requirements of the pipe-making process.

[0020] The thickness of the steel strip for square tubes of the present invention is reduced from more than 3 mm in the conventional case to less than 2 mm, with a weight reduction of more than 30%. It is beneficial to the lightweight development of the automotive industry, and at the same time, it has a significant cost advantage compared with the existing method of using aluminum alloy materials to make square tubes. Brief Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. In the drawings:

[0022] Figure 1 is the metallographic structure diagram of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes of the present invention;

[0023] Figure 2 is the actual product diagram of the square tube made of the hot-rolled steel strip for 1100MPa grade lightweight square tubes of the present invention. Detailed Embodiments

[0024] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions of the present invention in combination with specific embodiments of the present invention and the corresponding drawings. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0025] In the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes of the present invention and its manufacturing method, the "thin specification" means that the thickness of the product is ≤ 2.0mm, and the "1100MPa grade" means that the tensile strength Rm of the product is ≥ 1100MPa.

[0026] Generally, the manufacturing method of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes of the present invention uses molten steel mainly composed of C, Si, Mn, and Ti through processes such as smelting, continuous casting, hot continuous rolling, laminar cooling after rolling, batch annealing in a bell-type furnace, and temper rolling to obtain a hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes with high strength, good bending performance, excellent flatness, and easy weldability, and its metallographic structure is mainly martensite.

[0027] Specifically, the manufacturing method of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes of the present invention includes the following steps:

[0028] (1) Composition Optimization Design

[0029] The chemical composition of the hot-rolled steel for lightweight square tubes at the 1100 MPa level is optimized and controlled by mass percentage as follows: C: 0.08 - 0.18%, Si: 0.8 - 1.5%, Mn: 0.8 - 1.4%, P ≤ 0.02%, S ≤ 0.025%, Ti: 0.01 - 0.04%, Al: 0.02 - 0.06%, and the rest is Fe and unavoidable impurities.

[0030] The reasons for the above optimization and control of the chemical composition of the hot-rolled steel for lightweight square tubes at the 1100 MPa level in the present invention are as follows:

[0031] C is the most economical strengthening element in steel and also a strong hardenability element. The C content significantly affects the strength of the martensite structure. If the C content is too low, it will lead to insufficient strength, but if the C content is too high, it will deteriorate the cold bending performance of the material. Therefore, in the present invention, the C content is optimized and controlled to be 0.08 - 0.18%, and more preferably controlled to be 0.10 - 0.15%.

[0032] Si has a strong solid solution strengthening effect, can improve the material strength, and at the same time improve the stability of austenite during cooling after hot rolling, prevent the precipitation of carbides, and improve the hardness of martensite. If the Si content is too low, the above effects cannot be achieved, but too high Si content will deteriorate the surface quality of the steel and increase the brittleness of the material. Therefore, in the present invention, the Si content is optimized and controlled to be 0.8 - 1.5%, and more preferably controlled to be 1.0 - 1.3%.

[0033] Mn is a commonly used economical solid solution strengthening element, which can improve the yield strength and tensile strength of steel. However, too high Mn content will reduce the plasticity of steel and lead to an increase in carbon equivalent and poor welding performance. Therefore, in the present invention, the Mn content is optimized and controlled to be 0.8 - 1.4%, and more preferably controlled to be 1.0 - 1.2%.

[0034] Ti is mainly used to play the role of fine grain strengthening and precipitation strengthening to improve the material strength. In the present invention, the Ti content is optimized and controlled to be 0.01 - 0.04%, and more preferably controlled to be 0.01 - 0.025%.

[0035] The main function of Al is to remove oxygen in the molten steel. When the Al content is too large, it will reduce the fluidity of the molten steel, and when the Al content is too small, the deoxidation in the steel is insufficient, and subcutaneous bubbles are likely to form in the billet. Therefore, in the present invention, the Al content is optimized and controlled to be 0.02 - 0.06%, and more preferably controlled to be 0.02 - 0.04%.

[0036] As harmful elements in steel, the lower the contents of P and S, the better. In the present invention, the P content and S content are respectively optimized and controlled to be P ≤ 0.02% and S ≤ 0.025%.

[0037] Thus, through the above-mentioned composition optimization design, C element with good hardenability is added to obtain martensite structure strengthening, and more Si element is added to prevent the precipitation of carbides in austenite, which is beneficial to improving the strength of high martensite. On this basis, the solid solution strengthening of Si and Mn elements and the fine grain strengthening of Ti element are exerted to further improve the strength to reach the level of 1100 MPa.

[0038] (2) The molten steel meeting the above-mentioned composition optimization design is obtained through converter smelting and secondary refining LF+RH, and the molten steel is continuously cast into slabs. Roll-type electromagnetic stirring is input during the continuous casting process to eliminate central segregation.

[0039] (3) The continuously cast slabs are heated to 1280 - 1320 °C, and a single-stand reciprocating roughing mill is used to rough the continuously cast slabs into intermediate billets with a thickness ≤ 35 mm. By setting the heating temperature of the continuously cast slabs to a relatively high 1280 - 1320 °C, not only can the micro-alloying elements be fully dissolved, but also the rolling load can be reduced; by controlling the thickness of the intermediate billets below 35 mm, the subsequent finishing rolling load can be reduced, the rolling stability can be ensured, which is beneficial to controlling and improving the finished product quality.

[0040] (4) A multi-stand hot tandem rolling mill is used to finish roll the intermediate billets into steel strips with the target thickness. The finishing rolling temperature is controlled at 850 - 900 °C. When the finishing rolling temperature is too low, the rolling load is large, which is not conducive to the shape quality of the plate; when the finishing rolling temperature is too high, the grains are coarse, which is not conducive to the strength improvement. Therefore, in the present invention, the finishing rolling temperature of the hot tandem rolling is controlled at 850 - 900 °C, which not only ensures good plate shape but also avoids coarse grains. The target thickness of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes is ≤ 2.0 mm, such as 1.5 - 2.0 mm.

[0041] (5) The post-rolling laminar cooling of the steel strip is carried out by a strong cooling process. The water ratio of the upper and lower water groups of the laminar cooling is controlled at 10:12, and the steel strip is cooled to below 150 °C at a cooling rate ≥ 60 °C / s to complete the martensite transformation of the steel strip material, and then coiled into a steel coil after cooling.

[0042] (6) The steel coil is subjected to tempering in a bell-type furnace to improve the cold bending performance. The tempering temperature is controlled at 300 - 400 °C, and the holding time is controlled at ≥ 15 h. If the tempering temperature is too low and the holding time is short, the cold bending performance cannot be improved, and cracking is likely to occur during subsequent pipe making; if the tempering temperature is too high, the strength will be significantly reduced. Therefore, in the present invention, the tempering temperature of the bell-type furnace is controlled at 300 - 400 °C, and the holding time is controlled at ≥ 15 h. Preferably, a nitrogen protection atmosphere is adopted during the tempering process in the bell-type furnace to reduce the oxidation of the steel coil in the furnace.

[0043] (7) Level the steel coils after tempering in a bell-type furnace. Control the leveling pressure at 600 - 800 tons and the leveling elongation at ≤2% to balance good flatness and plasticity. If the leveling pressure is too low, the flatness improvement effect is poor; if the leveling pressure is too high, it will lead to too high a leveling elongation, reducing the plasticity and elongation of the material and causing cracking during the subsequent pipe manufacturing process.

[0044] Through the implementation of the manufacturing method of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square pipes of the present invention above, a hot-rolled steel strip for 1100MPa grade thin-specification lightweight square pipes is obtained. The thickness of the steel strip product is 1.5 - 2.0mm. Through performance inspection, it can be known that the yield strength ReL≥900MPa, the tensile strength Rm≥1100MPa, the elongation A50≥10%, the cold bending performance (D = 4a, 180°) is qualified, the carbon equivalent CEV≤0.4%, and the metallographic structure of the steel strip product is mainly martensite, as Figure 1 shown.

[0045] Among them, as those skilled in the art know, D = 4a in the cold bending performance characterization parameters means that the bending die diameter D = 4× the steel strip thickness a during the steel strip bending test, and 180° means that the two sides of the steel strip are bent to be parallel during the steel strip bending test.

[0046] The following details the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square pipes of the present invention and its manufacturing method in conjunction with specific embodiments.

[0047] Example 1

[0048] The thickness of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square pipes involved in Example 1 of the present invention is 2mm, and its chemical composition is optimized and designed by mass percentage as follows: C: 0.13%, Si: 1.20%, Mn: 1.10%, P: 0.013%, S: 0.015%, Ti: 0.015%, Al 0.035%, and the rest is Fe and inevitable impurities.

[0049] The manufacturing method of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square pipes in Example 1 includes the following steps:

[0050] (1) Obtain molten steel that meets the above composition optimization design through converter smelting and secondary refining LF + RH, and continuously cast the molten steel into a slab with a thickness of 230mm.

[0051] (2) Heat the continuously cast slab to 1290°C, use a single-stand reciprocating roughing mill to rough the continuously cast slab with a thickness of 230mm into an intermediate slab with a thickness of 35mm, and then use a multi-stand hot continuous rolling mill to finish rolling the intermediate slab into a steel strip with a target thickness of 2mm. Control the final rolling temperature at 870°C.

[0052] (3) The laminar cooling after rolling is carried out on the steel strip by using the intensive cooling process. The water ratio of the upper and lower water groups in the laminar cooling is controlled at 10:12. The steel strip is cooled to below 150°C at a cooling rate of ≥60°C / s and coiled into a steel coil.

[0053] (4) The steel coil is subjected to tempering in a bell-type furnace. The tempering temperature is controlled at 350°C and the holding time is controlled at 15 h. Nitrogen is used as the protective atmosphere.

[0054] (5) The steel coil after tempering in the bell-type furnace is leveled. The leveling pressure is controlled at 700 tons and the leveling elongation is controlled at 1.5%.

[0055] The performance inspection is carried out on the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes prepared by using the embodiment 1. The results are as follows: the yield strength ReL is 932 MPa, the tensile strength Rm is 1203 MPa, the elongation A50 is 10.5%, the cold bending (D = 4a, 180°) is qualified, the carbon equivalent CEV is 0.31%, and the metallographic structure of the steel strip is mainly martensite.

[0056] Figure 2 The product actual drawing of the square tube made of the hot-rolled steel strip for 1100 MPa grade lightweight square tubes of the embodiment 1 of the present invention is shown.

[0057] Embodiment 2

[0058] The thickness of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes involved in the embodiment 2 of the present invention is 1.5 mm. Its chemical composition is optimized and designed by mass percentage as follows: C: 0.13%, Si: 1.15%, Mn: 1.10%, P: 0.010%, S: 0.010%, Ti: 0.017%, Al 0.03%, and the rest are Fe and inevitable impurities.

[0059] The manufacturing method of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes of the embodiment 2 includes the following steps:

[0060] (1) The molten steel meeting the above composition optimization design is obtained through converter smelting and secondary refining LF+RH. The molten steel is continuously cast into a slab with a thickness of 180 mm.

[0061] (2) The continuously cast slab is heated to 1280°C. A single-stand reciprocating roughing mill is used to rough the continuously cast slab with a thickness of 180 mm into an intermediate billet with a thickness of 27 mm, and then a multi-stand hot continuous rolling mill is used to finish rolling the intermediate billet into a steel strip with a target thickness of 1.5 mm. The finishing rolling temperature is controlled at 860°C.

[0062] (3) After rolling, laminar cooling is adopted for the steel strip. The water ratio of the upper and lower water groups in laminar cooling is controlled at 10:12. The steel strip is cooled to below 150°C at a cooling rate of ≥60°C / s and coiled into a steel coil.

[0063] (4) The steel coil is subjected to tempering in a bell-type furnace. The tempering temperature is controlled at 320°C and the holding time is controlled at 15 h. Nitrogen is used as the protective atmosphere.

[0064] (5) The steel coil after tempering in the bell-type furnace is leveled. The leveling pressure is controlled at 600 tons and the leveling elongation is controlled at 1.5%.

[0065] Performance inspection is carried out on the hot-rolled steel strip for 1100 MPa grade thin-spec lightweight square tubes prepared by Example 2. The results are as follows: the yield strength ReL is 952 MPa, the tensile strength Rm is 1219 MPa, the elongation A50 is 11%, the cold bending (D = 4a, 180°) is qualified, the carbon equivalent CEV is 0.32%, and the metallographic structure of the steel strip is mainly martensite.

[0066] Example 3

[0067] The thickness of the hot-rolled steel strip for 1100 MPa grade thin-spec lightweight square tubes involved in Example 3 of the present invention is 1.8 mm. Its chemical composition is optimized and designed by mass percentage as follows: C: 0.14%, Si: 1.22%, Mn: 1.27%, P: 0.007%, S: 0.015%, Ti: 0.016%, Al 0.03%, and the rest is Fe and unavoidable impurities.

[0068] The manufacturing method of the hot-rolled steel strip for 1100 MPa grade thin-spec lightweight square tubes in Example 3 includes the following steps:

[0069] (1) Molten steel meeting the above composition optimization design is obtained through converter smelting and secondary refining LF+RH, and the molten steel is continuously cast into a slab with a thickness of 210 mm.

[0070] (2) The continuously cast slab is heated to 1280°C. A single-stand reciprocating roughing mill is used to rough the continuously cast slab with a thickness of 210 mm into an intermediate billet with a thickness of 32 mm, and then a multi-stand hot tandem rolling mill is used to finish rolling the intermediate billet into a steel strip with a target thickness of 1.8 mm. The finish rolling temperature is controlled at 870°C.

[0071] (3) After rolling, laminar cooling is adopted for the steel strip. The water ratio of the upper and lower water groups in laminar cooling is controlled at 10:12. The steel strip is cooled to below 150°C at a cooling rate of ≥60°C / s and coiled into a steel coil.

[0072] (4) The steel coil is subjected to tempering in a bell-type furnace. The tempering temperature is controlled at 330°C and the holding time is controlled at 15 h. Nitrogen is used as the protective atmosphere.

[0073] (5) Level the steel coil after tempering in a bell-type furnace. Control the leveling pressure at 700 tons and the leveling elongation at 1.6%.

[0074] Perform performance tests on the hot-rolled steel strip for the 1100 MPa grade thin-gauge lightweight square tube prepared in Example 3. The results are as follows: the yield strength ReL is 963 MPa, the tensile strength Rm is 1205 MPa, the elongation A50 is 10.5%, the cold bending (D = 4a, 180°) is qualified, the carbon equivalent CEV is 0.34%, and the metallographic structure of the steel strip is mainly martensite.

[0075] Example 4

[0076] The thickness of the hot-rolled steel strip for the 1100 MPa grade thin-gauge lightweight square tube involved in Example 4 of the present invention is 1.6 mm. Its chemical composition is optimized and designed by mass percentage as follows: C: 0.14%, Si: 1.15%, Mn: 1.15%, P: 0.008%, S: 0.012%, Ti: 0.017%, Al 0.03%, and the rest is Fe and unavoidable impurities.

[0077] The manufacturing method of the hot-rolled steel strip for the 1100 MPa grade thin-gauge lightweight square tube in Example 4 includes the following steps:

[0078] (1) Obtain molten steel that meets the above composition optimization design through converter smelting and secondary refining LF + RH, and continuously cast the molten steel into a slab with a thickness of 190 mm.

[0079] (2) Heat the continuously cast slab to 1290 °C. Use a single-stand reciprocating roughing mill to rough the continuously cast slab with a thickness of 190 mm into an intermediate slab with a thickness of 29 mm, and then use a multi-stand hot tandem rolling mill to finish rolling the intermediate slab into a steel strip with a target thickness of 1.6 mm. Control the finishing rolling temperature at 880 °C.

[0080] (3) Adopt a strong cooling process to perform laminar flow cooling on the steel strip. Control the water ratio of the upper and lower water groups of the laminar flow cooling at 10:12, and cool the steel strip to below 150 °C at a cooling rate of ≥60 °C / s, and coil it into a steel coil.

[0081] (4) Perform tempering on the steel coil in a bell-type furnace. Control the tempering temperature at 350 °C and the holding time at 20 h, and use nitrogen as the protective atmosphere.

[0082] (5) Level the steel coil after tempering in a bell-type furnace. Control the leveling pressure at 600 tons and the leveling elongation at 1.4%.

[0083] The performance of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes prepared in Example 4 was tested, and the results are as follows: the yield strength ReL was 938 MPa, the tensile strength Rm was 1178 MPa, the elongation A50 was 10%, the cold bending (D = 4a, 180°) was qualified, the carbon equivalent CEV was 0.33%, and the metallographic structure of the steel strip was mainly martensite.

[0084] Example 5

[0085] The thickness of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes involved in Example 5 of the present invention is 1.7 mm, and its chemical composition is optimized by mass percentage as follows: C: 0.15%, Si: 1.06%, Mn: 1.25%, P: 0.008%, S: 0.012%, Ti: 0.018%, Al 0.04%, and the rest is Fe and inevitable impurities.

[0086] The manufacturing method of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes in Example 5 includes the following steps:

[0087] (1) Molten steel meeting the above composition optimization design was obtained through converter smelting and secondary refining LF + RH, and the molten steel was continuously cast into a slab with a thickness of 200 mm.

[0088] (2) The continuously cast slab was heated to 1290 °C, and a single-stand reciprocating roughing mill was used to rough the continuously cast slab with a thickness of 200 mm into an intermediate billet with a thickness of 30 mm, and then a multi-stand hot continuous rolling mill was used to finish rolling the intermediate billet into a steel strip with a target thickness of 1.7 mm, and the finish rolling temperature was controlled at 880 °C.

[0089] (3) The strip was subjected to laminar cooling after rolling using a strong cooling process. The water ratio of the upper and lower water groups of the laminar cooling was controlled at 10:12, and the strip was cooled to below 150 °C at a cooling rate of ≥60 °C / s and coiled into a steel coil.

[0090] (4) The steel coil was subjected to tempering in a bell-type furnace. The tempering temperature was controlled at 350 °C, the holding time was controlled at 20 h, and nitrogen was used as the protective atmosphere.

[0091] (5) The steel coil after tempering in the bell-type furnace was leveled. The leveling pressure was controlled at 600 tons, and the leveling elongation was controlled at 1.5%.

[0092] The performance of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes prepared in Example 5 was tested, and the results are as follows: the yield strength ReL was 955 MPa, the tensile strength Rm was 1195 MPa, the elongation A50 was 10%, the cold bending (D = 4a, 180°) was qualified, the carbon equivalent CEV was 0.33%, and the metallographic structure of the steel strip was mainly martensite.

[0093] Example 6

[0094] The thickness of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes involved in Example 6 of the present invention is 2.0mm, and its chemical composition is optimized by mass percentage as follows: C: 0.15%, Si: 1.18%, Mn: 1.23%, P: 0.008%, S: 0.015%, Ti: 0.017%, Al 0.04%, and the rest is Fe and inevitable impurities.

[0095] The manufacturing method of the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes in Example 6 includes the following steps:

[0096] (1) Molten steel meeting the above composition optimization design is obtained through converter smelting and secondary refining LF+RH, and the molten steel is continuously cast into a slab with a thickness of 230mm.

[0097] (2) The continuously cast slab is heated to 1290°C, and a single-stand reciprocating roughing mill is used to rough the continuously cast slab with a thickness of 230mm into an intermediate slab with a thickness of 35mm, and then a multi-stand hot tandem rolling mill is used to finish rolling the intermediate slab into a steel strip with a target thickness of 2.0mm, and the finishing rolling temperature is controlled at 880°C.

[0098] (3) The steel strip is subjected to laminar flow cooling after rolling by a strong cooling process, and the water ratio of the upper and lower water groups of the laminar flow cooling is controlled at 10:12, and the steel strip is cooled to below 150°C at a cooling rate of ≥60°C / s and coiled into a steel coil.

[0099] (4) The steel coil is subjected to tempering in a bell-type furnace, the tempering temperature is controlled at 360°C, the holding time is controlled at 20h, and nitrogen is used as the protective atmosphere.

[0100] (5) The steel coil after tempering in the bell-type furnace is leveled, the leveling pressure is controlled at 700 tons, and the leveling elongation is controlled at 1.6%.

[0101] Performance inspection is carried out on the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes prepared by using Example 6, and the results are as follows: the yield strength ReL is 939MPa, the tensile strength Rm is 1163MPa, the elongation A50 is 10%, the cold bending (D = 4a, 180°) is qualified, the carbon equivalent CEV is 0.35%, and the metallographic structure of the steel strip is mainly martensite.

[0102] In summary, the manufacturing method of hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes of the present invention uses molten steel mainly composed of C, Si, Mn, and Ti, and through processes such as smelting, continuous casting, hot continuous rolling, laminar cooling after rolling, box annealing, and tempering, a hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes is obtained. The thickness of the steel strip is 1.5 - 2.0mm, the yield strength ReL ≥ 900MPa, the tensile strength Rm ≥ 1100MPa, the elongation A50 ≥ 10%, the cold bending performance (D = 4a, 180°) is qualified, the carbon equivalent CEV ≤ 0.4%, and the metallographic structure of the steel strip product is mainly martensite.

[0103] Compared with the prior art, the hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes of the present invention and its manufacturing method have the following advantages and beneficial effects:

[0104] The present invention improves the strength of the steel for square tubes from the conventional 700MPa grade to 1100MPa grade, with high strength, good formability, low cost, and easy welding, meeting the requirements for making thin-specification and ultra-high-strength steel for lightweight square tubes;

[0105] While the present invention highly strengthens the material, it also takes into account good bending performance, and the cold bending (D = 4a, 180°) is qualified, meeting the requirements of the pipe-making process;

[0106] The thickness of the steel strip for square tubes of the present invention is reduced from more than 3mm in the conventional case to less than 2mm, with a weight reduction of more than 30%, which is beneficial to the lightweight development of the automotive industry. At the same time, it has a significant cost advantage compared with the existing method of using aluminum alloy materials to make square tubes.

[0107] It should be noted that the noun terms in this article have the meanings commonly understood by those skilled in the art unless otherwise specified. Moreover, when a numerical range is disclosed in this article, the above range is considered continuous and includes the minimum and maximum values of the range, as well as each value between such minimum and maximum values. Further, when the range refers to an integer, it includes each integer between the minimum and maximum values of the range. In addition, when multiple ranges are provided to describe features, these ranges can be combined. In other words, unless otherwise specified, all ranges disclosed in this article should be understood to include any and all sub-ranges subsumed therein.

[0108] It should also be noted that in this article, the term "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, such that an article or device comprising a series of elements not only includes those elements but also includes other elements not explicitly listed, or further includes elements inherent to such article or device.

[0109] In addition, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A manufacturing method for hot-rolled steel strips for thin-specification lightweight square tubes of 1100 MPa grade, characterized in that, It includes the following steps: (1) Component optimization design The chemical composition of the hot-rolled steel for 1100 MPa grade lightweight square tubes is optimized and controlled by mass percentage as follows: C: 0.08 - 0.18%, Si: 0.8 - 1.5%, Mn: 0.8 - 1.4%, P ≤ 0.02%, S ≤ 0.025%, Ti: 0.01 - 0.04%, Al: 0.02 - 0.06%, and the rest is Fe and inevitable impurities; (2) Molten steel meeting the component optimization design is obtained through converter smelting and secondary refining LF + RH, and the molten steel is continuously cast into slabs; (3) The continuously cast slabs are heated to 1280 - 1320 °C, and with a single-stand reciprocating roughing mill, the continuously cast slabs are roughly rolled into intermediate slabs with a thickness ≤ 35 mm; (4) The intermediate slabs are finish-rolled into steel strips with the target thickness by a multi-stand hot tandem rolling mill, and the finish rolling temperature is controlled at 850 - 900 °C; (5) The steel strips are subjected to laminar flow cooling after rolling by a strong cooling process, and the water ratio of the upper and lower water groups in the laminar flow cooling is controlled at 10:12, and the steel strips are cooled to below 150 °C at a cooling rate ≥ 60 °C / s, and then coiled into steel coils after cooling; (6) The steel coils are subjected to tempering in a bell-type furnace, the tempering temperature is controlled at 300 - 400 °C, and the holding time is controlled at ≥ 15 h; (7) The steel coils after tempering in the bell-type furnace are leveled, the leveling pressure is controlled at 600 - 800 tons, and the leveling elongation is controlled at ≤ 2%.

2. The manufacturing method of hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes according to claim 1, characterized in that, In the component optimization design step, the C content is optimized and controlled at 0.10 - 0.15%, the Si content is optimized and controlled at 1.0 - 1.3%, the Mn content is optimized and controlled at 1.0 - 1.2%, the Ti content is optimized and controlled at 0.01 - 0.025%, and the Al content is optimized and controlled at 0.02 - 0.04%.

3. The manufacturing method of hot-rolled steel strip for 1100MPa grade thin-specification lightweight square tubes according to claim 1, characterized in that, In the tempering step in the bell-type furnace, a nitrogen protection atmosphere is adopted during the tempering process.

4. A hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes, which is prepared by the manufacturing method of the hot-rolled steel strip for 1100 MPa grade thin-specification lightweight square tubes as described in claim 1. The thickness of the steel strip is 1.5 - 2.0 mm, the yield strength ReL ≥ 700 MPa, the tensile strength Rm ≥ 1100 MPa, the elongation A50 ≥ 8%, the cold bending is qualified, the carbon equivalent CEV ≤ 0.4%, and the metallographic structure of the steel strip is mainly martensite.

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