1200mpa grade hot-rolled high-strength steel and method for manufacturing the same

Abstract

The application discloses a 1200MPa-grade hot-rolled high-strength steel and a preparation method thereof, and belongs to the technical field of hot-rolled high-strength steel. The chemical components of the hot-rolled high-strength steel comprise, in terms of weight ratio, C 0.14-0.18%, Mn 1.5-1.8%, P≤0.015%, S≤0.005%, N≤0.005%, and further comprise Cr 0.3-0.6%, Mo 0.1-0.2%, B 0.0015-0.0030%, any two of the above, Si 0.5-1.1%, Al 0.3-0.5%, any one of the above, and the rest is Fe and impurities. The product steel obtained by the method has a yield strength≥800MPa, a tensile strength≥1200MPa, a yield strength ratio≤0.8 and an elongation≥16%, and can effectively solve the problems of high preparation cost and great preparation difficulty of the existing 1000MPa and above grade hot-rolled high-strength steel.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled high-strength steel technology, and relates to a production method of high-strength hot-rolled steel, specifically to a 1200MPa grade hot-rolled high-strength steel and its preparation method. Background Technology

[0002] Currently, the hot-rolled high-strength steels used in China in batches are mainly of 800MPa and below. Their chemical composition mainly adopts microalloying methods such as Ti, Ti-Nb, Ti-Cr, and Ti-Mo. The microstructure is ferrite or ferrite plus pearlite. The strengthening mechanism is mainly fine grain strengthening and precipitation strengthening. However, it is very difficult to further reduce the size of Ti precipitates and further increase the number density of precipitates. Therefore, it is quite difficult to improve precipitation strengthening and develop hot-rolled high-strength steels (non-heat-treated) of 1000MPa and above. In recent years, the research and application of the TRIP effect have provided new ideas for the development of hot-rolled high-strength steel with a strength of 1000 MPa and above. By adding hardenability elements such as Si, Cr, Mo, and B to the steel, a composite structure of bainite, martensite, and retained austenite is formed after hot rolling and laminar cooling. This not only improves the strength of the steel through structural strengthening, but also improves its toughness and plasticity by increasing the dislocation density. Furthermore, a certain proportion of the retained austenite in the steel can be transformed into martensite during subsequent forming processes. By improving the formability of the steel through the TRIP effect, it can be used to manufacture automotive parts with high formability and high hole expansion requirements.

[0003] A search revealed CN107557692B, which discloses a 1000MPa grade hot-rolled TRIP steel and its manufacturing method based on a CSP process. The steel's chemical composition is: 0.16-0.20% C, 1.50-1.60% Mn, 1.6-1.8% Si, 0.20-0.24% V, 0.015-0.060% Al, 0.015-0.025% N, P≤0.008%, S≤0.005%, with the remainder being Fe and unavoidable impurities. The steel's chemical composition process includes converter smelting, refining, thin slab continuous casting, continuous casting billet homogenization, high-pressure water descaling, controlled rolling, controlled cooling, and coiling. Through VN microalloying design, the ferrite and bainite phases are strengthened without reducing the carbon concentration, thus maintaining the stability of the retained austenite and exhibiting a good balance of strength and plasticity.

[0004] CN106119700B discloses a precipitation-strengthened high-strength and high-ductility steel of grade 1180MPa and its manufacturing method. The chemical composition of the steel is: 0.15-0.20%C, 0.8-2.0%Si, 1.5-2.0%Mn, 0.4-1.0%Als, 0.03-0.06%Nb, 0.1-0.2%Ti, V≤0.40%, P≤0.015%, S≤0.005%, O≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities. The microstructure of the steel consists of ferrite, bainite, and retained austenite. Nanoscale carbides are distributed within the ferrite grains. The average ferrite grain size is ≤5μm, the nanoscale carbide size is ≤10nm, the bainite lath width is ≤5μm, the yield strength is ≥1000MPa, the tensile strength is ≥1180MPa, and the elongation is ≥15%, exhibiting an excellent balance between high strength and high plasticity.

[0005] As can be seen from the above, most existing technologies add microalloying elements such as Nb, V, and Ti, which results in higher alloy costs. Furthermore, the production process employs a two-stage cooling method, making it difficult to control the cooling process. Summary of the Invention

[0006] The technical problem to be solved by the present invention is that the preparation cost of existing hot-rolled high-strength steel of 1000MPa and above is relatively high and the preparation is difficult.

[0007] The technical solution adopted by this invention to solve its technical problem is: 1200MPa grade hot-rolled high-strength steel, whose chemical composition by weight percentage includes: C 0.14-0.18%, Mn 1.5-1.8%, P≤0.015%, S≤0.005%, N≤0.005%, and also includes any two of Cr 0.3-0.6%, Mo 0.1-0.2%, and B 0.0015-0.0030%, and any one of Si 0.5-1.1% and Al 0.3-0.5%, with the remainder being Fe and unavoidable impurity elements.

[0008] Furthermore, the aforementioned 1200MPa grade hot-rolled high-strength steel has a yield strength ≥800MPa, tensile strength ≥1200MPa, yield strength ratio ≤0.8, and elongation ≥16%.

[0009] Furthermore, the microstructure of the aforementioned 1200MPa grade hot-rolled high-strength steel consists of ferrite, bainite, martensite, and retained austenite, with ferrite accounting for 40-50% and an average ferrite grain size ≤5μm; bainite accounting for 20-25% and a bainite lath width ≤5μm; martensite accounting for 20-30% and a martensite lath width ≤3μm; and retained austenite accounting for 5-10% and a retained austenite lamellar width ≤1μm.

[0010] The preparation method of the above-mentioned 1200MPa grade hot-rolled high-strength steel is as follows: a billet or ingot is obtained by a process of smelting in a converter or electric furnace → vacuum refining, and then the finished steel is obtained by a process of billet / ingot heating → rough rolling → finish rolling → laminar flow cooling → air cooling; wherein, after obtaining the billet or ingot, the billet / ingot is hot-charged for heating, and the furnace charging temperature is controlled at 400-700℃.

[0011] Furthermore, in the above-mentioned billet / ingot heating process, the billet / ingot is heated to 1000-1100°C at a heating rate of 4-8°C / min, and then heated to 1200-1260°C at a cooling rate of 10-15°C / min, with the total furnace time controlled to be 140-180 min.

[0012] Furthermore, in the above-mentioned roughing process, 5 to 7 rolling passes are used, with a cumulative deformation of 75 to 85%, and the roughing exit temperature is 1050 to 1100℃.

[0013] Furthermore, in the aforementioned finishing rolling process, 6 to 7 rolling passes are used, with a cumulative deformation of 85 to 95%, a finishing rolling inlet temperature of 1000 to 1050°C, and a finishing rolling outlet temperature of 880 to 920°C.

[0014] Furthermore, in the above-mentioned laminar flow cooling process, the cooling rate is 15-40℃ / s, and the winding temperature is 530-570℃.

[0015] The beneficial effects of this invention are as follows: This invention provides a 1200MPa grade hot-rolled high-strength steel and its preparation method. The steel provided by this invention does not contain large amounts of alloying elements such as Nb, V, and Ti, resulting in lower alloy costs. By adding hardenability elements such as Si, Cr, and B, the steel provided by this invention can achieve medium-low temperature transformation structures such as bainite, martensite, and retained austenite at moderate coiling temperatures. This avoids the difficulty in controlling the steel plate shape at lower coiling temperatures.

[0016] The steel provided by this invention possesses both high strength and good plasticity, for the following reasons: First, the presence of high-density dislocations in the bainitic structure enhances the steel's ability to suppress crack propagation; Second, the presence of retained austenite not only reduces the carbon content in martensite and decreases the hardness difference between phases, improving the coordinated strain capacity of each phase during material deformation, but also transforms into martensite during material deformation, absorbing a large amount of deformation energy and reducing stress concentration, thereby achieving a balance between high strength and high plasticity. Attached Figure Description

[0017] Figure 1 Metallographic image of the microstructure of the steel plate in Example 2 of this invention;

[0018] Figure 2 This is a TEM image of the microstructure of the steel plate in Example 2 of the present invention. Detailed Implementation

[0019] The technical solution of the present invention can be implemented in the following manner.

[0020] The chemical composition of 1200MPa grade hot-rolled high-strength steel, by weight percentage, includes: C 0.14-0.18%, Mn 1.5-1.8%, P≤0.015%, S≤0.005%, N≤0.005%, and any two of Cr 0.3-0.6%, Mo 0.1-0.2%, and B 0.0015-0.0030%, and any one of Si 0.5-1.1% and Al 0.3-0.5%, with the remainder being Fe and unavoidable impurity elements.

[0021] The aforementioned 1200MPa grade hot-rolled high-strength steel has a yield strength ≥800MPa, tensile strength ≥1200MPa, yield-to-tensile ratio ≤0.8, and elongation ≥16%. Its microstructure consists of ferrite, bainite, martensite, and retained austenite, with ferrite comprising 40–50% and an average ferrite grain size ≤5μm; bainite comprising 20–25% and a bainite lath width ≤5μm; martensite comprising 20–30% and a martensite lath width ≤3μm; and retained austenite comprising 5–10% and a retained austenite lamellar width ≤1μm.

[0022] The reasons for the restrictions on the main alloying elements in the steel described in this invention are explained below.

[0023] Carbon (C) is an important strengthening element in steel. The strength of bainite, martensite, and retained austenite structures in steel is related to their carbon content. Furthermore, increasing the C content shifts the CCT curve to the right, improves the stability of retained austenite, delays pearlite transformation, and increases the proportion of bainite and martensite. Therefore, the C content is controlled at 0.14–0.18%.

[0024] Mn has a strong effect on stabilizing austenite; a 1% Mn content can lower the Ms point by about 30°C. Adding Mn to steel helps retain more residual austenite in the final microstructure. However, excessively high Mn content can easily cause segregation in the cast billet, affecting the uniformity of the microstructure. Therefore, the Mn content is controlled at 1.5% to 1.8%.

[0025] During the phase transformation process, Si can increase the activity of C and promote the diffusion of C from ferrite to the retained austenite, thereby purifying ferrite and enriching austenite with carbon, thus improving the stability of the retained austenite. At the same time, Si inhibits the nucleation and precipitation of carbides, causing the pearlite transformation "C" curve to shift to the right, thus slowing down the formation of pearlite.

[0026] Al has a similar effect to Si in inhibiting cementite formation and has no adverse effect on the surface quality of steel. It can replace or partially replace Si to achieve the TRIP effect. Simultaneously, Al can reduce the austenite phase region in steel, increase the Ac3 and Ms points, reduce the phase transformation driving force and shear resistance, thereby increasing the Bs point. Therefore, this invention requires the addition of any one of 0.5–1.1% Si and 0.3–0.5% Al.

[0027] Cr, Mo, and B can all significantly improve the hardenability of steel, promote the transformation of bainite and martensite, and improve the stability of austenite. Therefore, this invention requires the addition of any two of the following: 0.3–0.6% Cr, 0.1–0.2% Mo, and 0.0015–0.0030% B.

[0028] The preparation method of the above-mentioned 1200MPa grade hot-rolled high-strength steel is as follows: a billet or ingot is obtained by a process of smelting in a converter or electric furnace → vacuum refining, and then the finished steel is obtained by a process of billet / ingot heating → rough rolling → finish rolling → laminar flow cooling → air cooling; wherein, after obtaining the billet or ingot, the billet / ingot is hot-charged for heating, and the furnace charging temperature is controlled at 400-700℃.

[0029] Preferably, in the above-mentioned billet / ingot heating process, the billet / ingot is heated to 1000-1100°C at a heating rate of 4-8°C / min, and then heated to 1200-1260°C at a cooling rate of 10-15°C / min, with the total furnace time controlled at 140-180 min. In the above-mentioned roughing process, 5-7 passes are used, with a cumulative deformation of 75-85%, and the roughing mill exit temperature is 1050-1100°C. In the above-mentioned finishing process, 6-7 passes are used, with a cumulative deformation of 85-95%, the finishing mill inlet temperature is 1000-1050°C, and the finishing mill exit temperature is 880-920°C. In the above-mentioned laminar flow cooling process, the cooling rate is 15-40°C / s, and the coiling temperature is 530-570°C.

[0030] The reasons for the limitations in the production process of the steel described in this invention will be explained below.

[0031] Because the steel contains a significant amount of hardenability-enhancing elements, such as Si or Al, Cr, Mo or B, its hardenability is good. Therefore, when the steel billet is air-cooled after smelting, a martensitic as-cast structure may form. This structure is coarse and brittle, potentially causing cracking or even fracture of the billet. Therefore, this invention requires hot charging, i.e., a charging temperature of 400–700°C. Simultaneously, to avoid stress concentration leading to cracking or fracture of the billet due to excessively rapid heating during the heating process, a two-stage heating method is required. First, the billet is heated to above the complete austenitization temperature (1000–1100°C) at a lower heating rate of 4–8°C / min. Then, it is heated to the tapping temperature (1200–1260°C) at a higher heating rate of 10–15°C / min. The tapping temperature is set to ensure sufficient solid solution of the microalloying elements.

[0032] Steel rolling is divided into two stages: roughing and finishing, which involve rolling in the austenite recrystallization zone and rolling in the non-recrystallization zone, respectively. Rolling in the austenite recrystallization zone requires a rolling temperature higher than the austenite recrystallization termination temperature. Therefore, this invention requires a roughing exit temperature of 1050–1100℃. Furthermore, since a higher roughing reduction results in higher rolling deformation energy, it is more conducive to austenite recrystallization, thereby refining the grains. Therefore, this invention requires a cumulative deformation of 75–85% in the roughing stage. Finishing requires rolling in the non-recrystallization zone of austenite. Flattening the austenite increases nucleation points and promotes the formation of fine and uniform grains during phase transformation. Therefore, this invention requires a finishing start temperature lower than the austenite recrystallization termination temperature, controlled at 1000–1050℃, with a cumulative deformation of 85–95%. In addition, this invention uses a relatively high finishing finishing temperature of 880–920℃, mainly because increasing the finishing temperature improves austenite stability and promotes subsequent microstructure transformation.

[0033] Since the addition of microalloying elements such as Si / Al, Cr / Mo / B to steel can improve hardenability and increase the phase transformation points Bs and Ms, the present invention uses medium-temperature coiling to obtain microstructures such as bainite, martensite, and retained austenite. In the present invention, the coiling temperature is controlled at 530-570℃ and the cooling rate is controlled at a relatively high 15-40℃ / s. At the same time, through the stable control of the cooling rate and the coiling temperature, the proportion of different phases in the microstructure is stably controlled.

[0034] The technical solution and effects of the present invention will be further explained below through practical examples.

[0035] Example

[0036] The 1200MPa grade hot-rolled high-strength steel of this invention comprises, by weight percentage: C 0.14-0.18%, Mn 1.5-1.8%, P ≤0.015%, S ≤0.005%, N ≤0.005%, and also contains any two of Cr 0.3-0.6%, Mo 0.1-0.2%, and B 0.0015-0.0030%, and any one of Si 0.5-1.1% and Al 0.3-0.5%, with the remainder being Fe and unavoidable impurity elements. Table 1 shows the specific chemical composition of Examples 1-4 of this invention; wherein, Al in Examples 1-3 and Si in Example 4 are elements that are inevitably introduced during the deoxidation process in the continuous casting process for preparing hot-rolled steel, and are impurity elements of this application, not intentionally added in the examples.

[0037] Table 1 Chemical composition / %

[0038]

[0039] The process flow of this invention embodiment is as follows: converter or electric furnace smelting → vacuum refining → billet or ingot casting → billet (ingot) heating → rough rolling + finish rolling → laminar flow cooling → air cooling → finished steel, and the key process parameters are shown in Table 2.

[0040] Table 2 Process Parameters

[0041] Process Example 1 Example 2 Example 3 Example 4 Slab heating temperature / °C 1220 1241 1255 1236 Cumulative reduction in rough rolling / % 76 80 82 78 Temperature at rough rolling exit / °C 1081 1092 1076 1058 Cumulative reduction in finish rolling / % 90 93 94 88 Opening temperature in finish rolling / °C 1038 1042 1025 1010 Final temperature in finish rolling / °C 913 895 889 911 Layer cooling speed / °C / s 21 18 19 22 Coiling temperature / °C 546 565 552 535

[0042] Appendix Figure 1 The image shows the metallographic structure of the steel plate in Example 2 of this invention. It can be seen that the microstructure mainly consists of ferrite, bainite, and martensite. (See attached image.) Figure 2 The TEM image of the steel plate in Example 2 of this invention shows the presence of bainite and martensite structures with high dislocation density, as well as lamellar retained austenite structures, with the width of the retained austenite lamellars being approximately 0.2 μm. This high dislocation density and retained austenite together improve the plasticity of the steel plate.

[0043] The mechanical properties of embodiments 1-4 of this invention are shown in Table 3. As can be seen from Table 3, the steel prepared using the composition and process provided by this invention has a yield strength ≥800MPa, tensile strength ≥1200MPa, yield-to-tensile ratio ≤0.80, and elongation ≥16%, exhibiting good strength and plasticity matching. This is due to the combined effect of fine ferrite structure, high dislocation density bainite and martensite structure, and a certain proportion of retained austenite structure (5-10%) in the steel.

[0044] Table 3 Mechanical Properties

[0045] Performance index Steel plate thickness / mm Yield strength / MPa Tensile strength / MPa Elongation / % Yield ratio Example 1 5.5 896 1221 20.0 0.73 Example 2 3.0 934 1219 19.0 0.77 Example 3 2.5 927 1234 19.5 0.75 Example 4 6.0 882 1206 19.5 0.73

Claims

1. 1200MPa grade hot-rolled high-strength steel, characterized in that, Its chemical composition by weight percentage includes: C 0.14~0.18%, Mn 1.5~1.8%, P≤0.015%, S≤0.005%, N≤0.005%, and also includes any two of Cr 0.3~0.6%, Mo 0.1~0.2%, and B 0.0015~0.0030%, and any one of Si 0.5~1.1% and Al 0.3~0.5%, with the remainder being Fe and unavoidable impurity elements; Its microstructure consists of ferrite, bainite, martensite, and retained austenite; among which, the proportion of ferrite is 40-50%, and the average grain size of ferrite is ≤5μm; the proportion of bainite is 20-25%, and the width of bainite laths is ≤5μm; the proportion of martensite is 20-30%, and the width of martensite laths is ≤3μm; the proportion of retained austenite is 5-10%, and the width of retained austenite lamellars is ≤1μm.

2. The 1200MPa grade hot-rolled high-strength steel according to claim 1, characterized in that: Its yield strength is ≥800MPa, tensile strength is ≥1200MPa, yield strength ratio is ≤0.8, and elongation is ≥16%.

3. The method for preparing 1200MPa grade hot-rolled high-strength steel according to claim 1 or 2, characterized in that: The billet or ingot is obtained through a process of smelting in a converter or electric furnace → vacuum refining, and then the finished steel is obtained through a process of heating the billet / ingot → rough rolling → finish rolling → laminar flow cooling → air cooling. In the billet / ingot heating process, the billet / ingot is heated to 1000-1100℃ at a heating rate of 4-8℃ / min, and then heated to 1200-1260℃ at a heating rate of 10-15℃ / min, with the total furnace time controlled to be 140-180min. In the roughing process, 5 to 7 passes of rolling are used, with a cumulative deformation of 75 to 85% and a roughing exit temperature of 1050 to 1100℃. In the finishing rolling process, 6 to 7 passes are used, with a cumulative deformation of 85 to 95%. The finishing rolling inlet temperature is 1000 to 1050℃, and the finishing rolling outlet temperature is 880 to 920℃. In the laminar flow cooling process, the cooling rate is 15~40℃ / s, and the winding temperature is 530~570℃.

4. The method for preparing 1200MPa grade hot-rolled high-strength steel according to claim 3, characterized in that: After obtaining the billet or ingot, it is hot-charged for heating the billet / ingot, and the charging temperature is controlled at 400~700℃.

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