Hot-rolled pickling complex-phase steel with tensile strength of 800 MPa and manufacturing method of hot-rolled pickling complex-phase steel

Abstract

The invention discloses hot-rolled and pickled complex phase steel with tensile strength of 800MPa and a manufacturing method thereof. The complex phase steel comprises the following chemical components: 0.030 to 0.050 percent of C, 0.15 to 0.20 percent of Si, 1.30 to 1.50 percent of Mn, 1.30 to 1.50 percent of Cr, 0.15 to 0.20 percent of Al, less than or equal to 0.005 percent of S, less than or equal to 0.012 percent of P, 0.020 to 0.030 percent of Ti, 0.0015 to 0.0025 percent of B, less than or equal to 0.0040 percent of N and the balance of Fe and inevitable impurities. The manufacturing method comprises the steps of converter smelting, LF refining, RH refining, slab continuous casting, hot rolling, laminar cooling, coiling, slow cooling after coiling and acid pickling. The yield strength of the complex-phase steel is larger than or equal to 680 MPa, the tensile strength is larger than or equal to 800 MPa, the elongation is larger than or equal to 15%, the hole expansion rate is larger than or equal to 70%, and the complex-phase steel has low crack sensitivity.

Description

Technical Field

[0001] The invention belongs to the technical field of steel strip production, and relates to a hot-rolled pickled complex phase steel with a tensile strength of 800 MPa and a manufacturing method thereof. Background Art

[0002] Due to the requirements of low energy consumption, high environmental protection and high safety for automobiles, lightweight and high strength of automobile steel has become the development trend of the industry. Advanced high-strength steel represented by complex phase steel plays a pivotal role in the manufacturing of automobile parts. Complex phase steel is often used to manufacture left / right swing arm upper plates in automobile chassis parts. The processing of such parts often involves processes such as hole expansion, flanging, and drawing. Traditional complex phase steel has low elongation and no requirements for hole expansion performance, and can no longer meet production requirements.

[0003] Steel enterprises have conducted relevant research on complex phase steel. CN117344203A discloses a hot-rolled complex phase steel with a tensile strength of 800MPa and a manufacturing method thereof. The strength is improved by higher manganese and titanium and a certain amount of chromium. However, when the thickness of the ingot is thicker, it is more difficult to control the segregation of manganese, which increases the risk of cracking during the flanging of parts. Secondly, large-sized TiN precipitation phases are commonly found in high-titanium content steels, which increases the risk of cracking at the edges of stamped parts. The patent application with publication number CN103602895A discloses a high-expansion steel plate with a tensile strength of 780MPa and a manufacturing method thereof, and the patent application with publication number CN105483545A discloses a hot-rolled high-expansion steel plate with a tensile strength of 800MPa and a manufacturing method thereof. Both of them are achieved by adding a higher content of silicon to prevent the precipitation of carbides and reduce the appearance of pearlite phase, thereby improving the forming; but with the increase of silicon content, the surface quality of hot-rolled strip is difficult to guarantee. Patent application with publication number CN116043119A discloses an easy-to-weld ultra-high hole expansion performance 800MPa grade hot-dip galvanized complex phase steel and a preparation method. Molybdenum is added to inhibit the precipitation of pearlite structure and large-sized cementite, but the production cost increases sharply. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a hot-rolled pickled complex phase steel with a tensile strength of 800MPa and a manufacturing method thereof. The hot-rolled pickled complex phase steel has high flanging and hole expansion performance, and has high drawing performance compared with traditional high hole expansion steel. No cracking occurs on the edge during the processing after punching, and has low crack sensitivity.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The invention discloses a hot-rolled pickled complex phase steel with a tensile strength of 800 MPa. The chemical composition and mass percentage of the steel are as follows: C: 0.030-0.050%, Si: 0.15-0.20%, Mn: 1.30-1.50%, Cr: 1.30-1.50%, Al: 0.15-0.20%, S≤0.005%, P≤0.012%, Ti: 0.020-0.030%, B: 0.0015-0.0025%, N≤0.0040%, and the balance is Fe and unavoidable impurities.

[0007] Furthermore, in the microstructure of the complex phase steel of the present invention, the bainite content is ≥55%, the ferrite content is ≤20%, the amount of martensite, residual austenite and grain boundaries is ≥25%, and the amount of low-angle grain boundaries is ≥45%.

[0008] Furthermore, the yield strength of the complex phase steel of the present invention is ≥680MPa, the tensile strength is ≥800MPa, the elongation is ≥15%, and the hole expansion rate is ≥70%.

[0009] The functions of the alloying elements in the complex phase steel of the present invention and their design principles are as follows:

[0010] C: Since the complex phase steel of the present invention is required to have high formability and low crack sensitivity, and the microstructure of the strip is mainly bainite, the carbon content should not be too high; generally, when the carbon content is ≤0.05%, bainite structure can be obtained at a lower cooling rate, which is easy to achieve during the hot rolling laminar cooling process, and the low cooling rate can effectively improve the quality of the strip shape. The higher strength of the strip requires the steel grade to have a certain carbon content. When the carbon content is lower than 0.03%, it is not conducive to the formation of small-sized carbides, so that the strength of the strip cannot meet the requirements, so the carbon content is controlled between 0.030-0.050%.

[0011] Si, Al: In the present invention, silicon has two main functions. One is the solid solution strengthening effect to improve the strength of the ferrite matrix, reduce the hardness difference between different phases, and improve the flanging and hole expansion performance of the strip. The second is to work together with aluminum to prevent the formation of coarse carbides and pearlite structure in the microstructure after medium-temperature coiling. By adding a certain amount of aluminum to replace part of the silicon element, the surface quality of the hot-rolled strip can also be improved. When the aluminum content is ≥0.2% and completely replaces silicon, it will increase the difficulty of casting the ingot. When the silicon content is >0.2%, the surface quality of the strip is reduced. Therefore, considering comprehensively, the silicon and aluminum contents are controlled between 0.15-0.20%.

[0012] Mn: In the present invention, the effect of manganese on the strength of the strip is similar to that of carbon, but in low carbon steel, there is a peak range for the improvement of impact toughness as the manganese content increases. When the manganese content exceeds 1.5%, the impact toughness decreases rapidly. Therefore, the manganese content is controlled between 1.30-1.50% to take both strength and impact toughness into consideration.

[0013] Cr: In the present invention, chromium is the main alloying element. In common complex phase steels, molybdenum is usually added to inhibit the formation of pearlite. However, molybdenum is a precious metal, and a large amount of addition makes the cost too high, which is not conducive to industrial production. Secondly, the high hardenability of chromium is used to improve the strength of steel and replace part of manganese. When the manganese element is increased to more than 1.5%, segregation is likely to occur. The addition of chromium can improve the segregation of the ingot and improve the uniformity of the hot-rolled strip structure. Therefore, the chromium content is controlled between 1.30-1.50%.

[0014] B, Ti: In the present invention, boron and titanium are added in combination. The addition of titanium fixes nitrogen in the steel to form titanium nitride, thereby preventing nitrogen in the steel from forming boron nitride with boron, increasing the effective boron solid solution content in austenite, achieving the purpose of improving hardenability and thus improving product strength; since the optimal solid solution amount of boron in steel is between 0.001-0.002%, a certain amount of boron carbide or oxide will be formed during the smelting process to reduce the effective boron content, so the boron element is controlled at 0.0015-0.0025%.

[0015] The manufacturing method of the hot-rolled pickled complex phase steel with a tensile strength of 800 MPa comprises converter smelting, LF refining, RH refining, slab continuous casting, hot rolling, laminar cooling, coiling, slow cooling after coiling and pickling.

[0016] Furthermore, in the hot rolling process, the heating temperature range is 1180-1250°C; the finishing rolling inlet temperature is 1000-1050°C, and the final rolling temperature is 860-880°C.

[0017] Furthermore, the laminar cooling process adopts front-stage cooling with a cooling rate of 20-30°C / s.

[0018] Furthermore, in the coiling process, the coiling temperature is 450-510°C.

[0019] Furthermore, in the post-coiling slow cooling process, the steel coil enters a slow cooling pit for slow cooling after coiling, and the slow cooling time is more than 24 hours.

[0020] Furthermore, in the pickling process, the complex phase steel is pickled in 5 consecutive pickling tanks, the pickling speed is 50-80 m / min, and the temperature of the pickling tanks is 70-85°C.

[0021] Furthermore, in the pickling process, the free acid concentration of the No. 1 pickling tank is 30-60 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 110-130 g / L.

[0022] Furthermore, in the pickling process, the free acid concentration of the No. 2 pickling tank is 40-65 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 100-120 g / L.

[0023] Furthermore, in the pickling process, the free acid concentration of the No. 3 pickling tank is 60-120 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 50-90 g / L.

[0024] Furthermore, in the pickling process, the free acid concentration of the No. 4 pickling tank is 100-160 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 10-70 g / L.

[0025] Furthermore, in the pickling process, the free acid concentration of the No. 5 pickling tank is 145-180 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 5-45 g / L.

[0026] The design principle of the hot-rolled pickled complex phase steel manufacturing method of the present invention is as follows:

[0027] In the present invention, the main alloying elements of the steel are manganese, chromium and trace amounts of titanium and boron. Compared with high-titanium or high-niobium microalloys, the precipitated phase in the ingot is less, so the heating temperature of the ingot is not easy to be too high to prevent abnormal growth of austenite grains and affect the performance of the final product. The suitable heating temperature range is between 1180-1250°C. In the finishing rolling process, low temperature into the rolling mill and high temperature out of the rolling mill can effectively increase the rolling speed, improve the surface quality of the strip, and provide support for the surface quality of high pickling strip.

[0028] The laminar cooling of the steel strip determines various performance indicators. In the present invention, the high hardenability elements chromium and boron are contained, so the cooling rate needs to be properly controlled. If the laminar cooling rate is too fast, the surface hardness of the steel strip will be too high, which will affect the subsequent stamping process; if the cooling rate is low, the strength cannot meet the requirements, so the cooling is carried out at a cooling rate of 20-30°C / s. The coiling temperature determines the composition of the phases in the microstructure and the proportion of each phase. When the coiling temperature is higher than 510°C, the steel strip cannot effectively enter the bainite region and the effect of boron is weakened, so that the strength of the steel strip cannot meet the requirements. When the coiling temperature is lower than 450°C, the proportion of martensite and M / A in the microstructure increases, which affects the elongation of the steel strip and causes cracking during the drawing process. Slow cooling treatment after coiling can eliminate the residual stress generated during the laminar cooling of the steel strip and improve the quality of the steel strip. On the other hand, it can improve the uniformity of the transverse and longitudinal mechanical properties of the steel strip.

[0029] The beneficial effects of adopting the above-mentioned components and technical solutions are: achieving the production of hot-rolled pickled complex phase steel with a tensile strength of 800MPa, the yield strength of the strip steel ≥680MPa, the tensile strength ≥800MPa, the elongation ≥15%, the hole expansion rate ≥70%, and having the characteristics of high strength, high formability and no cracking during edge drawing processing, and can be widely used in the manufacture of automobile swing arms, Yuanbao beams, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a typical SEM photo of the steel in Example 1.

[0031] Figure 2 This is the KAM diagram of the steel in Example 1. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described in detail below through embodiments.

[0033] Examples 1-6

[0034] The invention discloses a hot-rolled pickled complex phase steel with a tensile strength of 800 MPa. The chemical composition and mass percentage of the steel are as follows: C: 0.030-0.050%, Si: 0.15-0.20%, Mn: 1.30-1.50%, Cr: 1.30-1.50%, Al: 0.15-0.20%, S≤0.005%, P≤0.012%, Ti: 0.020-0.030%, B: 0.0015-0.0025%, N≤0.0040%, and the balance is Fe and unavoidable impurities.

[0035] The chemical composition and content of the complex phase steel in each embodiment are shown in Table 1.

[0036] Table 1 Chemical composition and mass percentage of the complex phase steel in each embodiment (%)

[0037] C Mn Cr Si Al S P B Ti N Example 1 0.033 1.38 1.30 0.19 0.15 0.002 0.011 0.0020 0.025 0.0035 Example 2 0.042 1.50 1.48 0.17 0.18 0.003 0.012 0.0025 0.028 0.0034 Example 3 0.046 1.40 1.45 0.18 0.19 0.004 0.009 0.0015 0.020 0.0028 Example 4 0.030 1.48 1.42 0.20 0.20 0.003 0.010 0.0021 0.022 0.0036 Example 5 0.050 1.30 1.50 0.15 0.16 0.005 0.008 0.0023 0.025 0.0033 Example 6 0.038 1.45 1.37 0.18 0.17 0.003 0.011 0.0017 0.030 0.0038

[0038] The balance in Table 1 is Fe and unavoidable impurities.

[0039] The manufacturing method of the hot-rolled pickled complex phase steel with a tensile strength of 800 MPa comprises converter smelting, LF refining, RH refining, slab continuous casting, hot rolling, laminar cooling, coiling, slow cooling after coiling and pickling; wherein the key process parameters are controlled as follows:

[0040] (1) Hot rolling process: heating temperature range is 1180-1250℃; finishing rolling inlet temperature is 1000-1050℃, and final rolling temperature is 860-880℃.

[0041] (2) Laminar cooling process: adopt front-stage cooling, cooling rate 20-30℃ / s.

[0042] (3) Coiling process: Coiling temperature is 450-510°C;

[0043] (4) Slow cooling process after coiling: After coiling, the steel coil enters the slow cooling pit for slow cooling, and the slow cooling time is more than 24 hours;

[0044] (5) Pickling process: The complex phase steel is pickled in 5 consecutive pickling tanks at a pickling speed of 50-80 m / min and a temperature of 70-85°C. The free acid concentration of the No. 1 pickling tank is 30-60 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 110-130 g / L. The free acid concentration of the No. 2 pickling tank is 40-65 g / L, the total acid concentration is 180-220 g / L, and the iron ion concentration is 100-120 g / L. L; the free acid concentration of the No. 3 pickling tank is 60-120g / L, the total acid concentration is 180-220g / L, and the iron ion concentration is 50-90g / L; the free acid concentration of the No. 4 pickling tank is 100-160g / L, the total acid concentration is 180-220g / L, and the iron ion concentration is 10-70g / L; the free acid concentration of the No. 5 pickling tank is 145-180g / L, the total acid concentration is 180-220g / L, and the iron ion concentration is 5-45g / L.

[0045] The key process parameter controls in the complex phase steel production process of each embodiment are shown in Tables 2-7 respectively.

[0046] Table 2 Key production process parameters of complex phase steel in various embodiments

[0047]

[0048] Table 3 Pickling parameters of the complex phase steel in each embodiment in No. 1 pickling tank

[0049]

[0050] Table 4 Pickling parameters of the complex phase steel in the No. 2 pickling tank in each embodiment

[0051]

[0052] Table 5 Pickling parameters of the complex phase steel in each embodiment in No. 3 pickling tank

[0053]

[0054] Table 6 Pickling parameters of the complex phase steel in No. 4 pickling tank in each embodiment

[0055]

[0056] Table 7 Pickling parameters of the complex phase steel in each embodiment in No. 5 pickling tank

[0057]

[0058] The physical properties of the complex phase steels of various embodiments are shown in Table 8; the composition of each phase and the proportion of low-angle grain boundaries in the microstructure of the product are shown in Table 9.

[0059] Table 8 Physical properties of the complex phase steels of various embodiments

[0060] Tensile strength, MPa Yield strength, MPa Elongation after break, % Hole expansion rate, % Example 1 835 740 16 75 Example 2 821 746 18 82 Example 3 805 720 20 85 Example 4 811 718 19 80 Example 5 832 735 17 77 Example 6 810 720 19 85

[0061] Table 9 Microstructure composition of the complex phase steel of each embodiment (%)

[0062] Bainite ratio Ferrite ratio Martensite + retained austenite + grain boundaries Low-angle grain boundary ratio Example 1 55 18 27 47.7 Example 2 57 17 26 46.9 Example 3 56 19 25 50.1 Example 4 58 15 27 48.2 Example 5 57 17 26 47.2 Example 6 56 18 26 48.1

[0063] Example 1 The SEM photo of the complex phase steel is as follows Figure 1 As shown, it can be seen that its microstructure composition includes 55% bainite + 18% ferrite + 27% martensite, retained austenite and grain boundaries.

[0064] Example 1 The KAM diagram of the complex phase steel is as follows Figure 2 As shown, the proportion of small-angle grain boundaries is 47.7%.

[0065] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A hot-rolled pickled complex phase steel with a tensile strength of 800 MPa, characterized in that: The chemical composition and mass percentage of the complex phase steel are C: 0.030-0.050%, Si: 0.15-0.20%, Mn: 1.30-1.50%, Cr: 1.30-1.50%, Al: 0.15-0.20%, S≤0.005%, P≤0.012%, Ti: 0.020-0.030%, B: 0.0015-0.0025%, N≤0.0040%, and the balance is Fe and unavoidable impurities.

2. The hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 1, characterized in that: In the microstructure of the complex phase steel, the bainite content is ≥55%, the ferrite content is ≤20%, the amount of martensite, residual austenite and grain boundaries is ≥25%, and the amount of small-angle grain boundaries is ≥45%.

3. The hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 1, characterized in that: The yield strength of the complex phase steel is ≥680MPa, the tensile strength is ≥800MPa, the elongation is ≥15%, and the hole expansion rate is ≥70%.

4. A method for manufacturing a hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to any one of claims 1 to 3, characterized in that: It includes converter smelting, LF refining, RH refining, slab continuous casting, hot rolling, laminar cooling, coiling, slow cooling after coiling and pickling processes.

5. The method for manufacturing hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 4, characterized in that: In the hot rolling process, the heating temperature is 1180-1250°C; the finishing rolling inlet temperature is 1000-1050°C, and the final rolling temperature is 860-880°C.

6. The method for manufacturing hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 4, characterized in that: The laminar cooling process adopts front-stage cooling with a cooling rate of 20-30°C / s.

7. The method for manufacturing hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 4, characterized in that: In the coiling process, the coiling temperature is 450-510°C.

8. The method for manufacturing hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 4, characterized in that: In the post-coiling slow cooling process, the steel coil enters a slow cooling pit for slow cooling after coiling, and the slow cooling time is more than 24 hours.

9. The method for manufacturing hot-rolled pickled complex phase steel with a tensile strength of 800 MPa according to claim 4, characterized in that: In the pickling process, the complex phase steel is pickled in 5 continuous pickling tanks, the pickling speed is 50-80m / min, and the temperature of the pickling tanks is 70-85°C.

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