TMCP-state high-strength steel plate and production process thereof

Abstract

The invention discloses a TMCP-state high-strength steel plate and a production process thereof, and belongs to the technical field of high-strength steel manufacturing. The steel plate comprises the following chemical components in percentage by weight: 0.06 to 0.10 percent of C, 0.15 to 0.3 percent of Si, 1.2 to 2.0 percent of Mn, 0.15 to 0.25 percent of Cr, 0.15 to 0.25 percent of Ni, 0.1 to 0.2 percent of Mo, less than or equal to 0.1 percent of Nb + Ti + B, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S and the balance of Fe and impurities. Through two-stage large reduction rolling (the rough rolling reduction rate is larger than or equal to 65%, and the finish rolling reduction rate is larger than or equal to 70%) and dynamic cooling control (the water entering temperature is 780-820 DEG C, the cooling speed is larger than or equal to 20 DEG C / s, and the self-tempering temperature is 450-480 DEG C), a structure mainly comprising fine grain bainite (the grain size is smaller than or equal to 5 micrometers) is formed, and high-strength and toughness matching with the yield strength larger than or equal to 850 MPa, the tensile strength larger than or equal to 950 MPa, the percentage elongation after fracture A50 larger than or equal to 22% and the longitudinal impact energy larger than or equal to 200 J at the temperature of-40 DEG C is achieved; the alloy cost is reduced by more than 25%, and the technological process is shortened by 30%.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel manufacturing technology, specifically to a TMCP state high-strength steel plate and its production process that combines short process, low cost and ultra-high strength, toughness and plasticity matching, and is applicable to fields such as engineering machinery and heavy equipment with strict requirements for the strength, toughness and weldability of steel. Background Technology

[0002] With the development of modern industrial technology, high-strength structural steel, possessing excellent strength, toughness, plasticity, and weldability, is increasingly widely used in the field of engineering machinery. The application of high-strength structural steel has improved manufacturing efficiency, reduced the weight of engineering machinery, and effectively lowered energy consumption. This widespread application has led to a significant increase in demand; however, traditional manufacturing methods have consistently failed to balance short-process, low-cost operations with high performance.

[0003] By designing a reasonable alloy composition and adopting the TMCP process, the production process is shortened and costs are reduced. While ensuring high strength, the steel plate also exhibits good matching of plasticity and toughness, thus enabling the production of high-grade mechanical steel.

[0004] Publication No. CN 113106333A discloses a low-cost high-strength steel with a yield strength of 800 MPa and its production method. By controlling cooling and offline tempering, a high-strength steel with a yield strength > 800 MPa, tensile strength > 840 MPa, elongation > 13%, and impact energy > 150 J at -20℃ is obtained. It has a good strength and toughness match, but its process is relatively long, the cost is relatively high, and the plasticity is poor.

[0005] Publication No. CN 111647813B discloses a high-strength steel with a yield strength >800 MPa produced by TMCP process and its production method. By refining the grains through TMCP and a three-stage cooling process, a high-strength steel with a yield strength >800 MPa, tensile strength >850 MPa, elongation >20%, and impact energy >47 J at -20℃ is obtained. While achieving high strength, good elongation is obtained. The offline heat treatment after rolling is eliminated, but its process flow is still relatively long and the low-temperature impact toughness is relatively poor.

[0006] Publication No. CN 114959515A discloses a high-strength steel with a low cold cracking sensitivity coefficient of 890 MPa and a production method. The method does not require the addition of expensive alloys during production and reduces the process flow by controlling the cooling process. It yields a high-strength steel with a yield strength ≥890 MPa, tensile strength ≥940 MPa, elongation ≥11%, and longitudinal impact energy ≥100 J at -20℃. The alloy cost is relatively low, the process flow is short, and the cost is relatively low. However, while it has high strength, its plasticity is relatively poor.

[0007] Therefore, developing a TMCP-type high-strength steel with a short process flow, low alloy cost, and high strength, high toughness, high plasticity, and excellent weldability has become an urgent technical problem to be solved in this field. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a TMCP high-strength steel plate and its manufacturing method in light of the above-mentioned prior art. The method has lower alloy cost, shorter production process, and lower overall production cost. Moreover, the TMCP process yields ultra-high-strength steel with tensile strength >850 MPa and excellent low-temperature impact toughness.

[0009] The technical solution adopted by this invention to solve the above problems is as follows: a TMCP state high-strength steel plate, with the following chemical composition by weight percentage: C: 0.06-0.10%, Si: 0.15-0.3%, Mn: 1.2-2.0%, Cr: 0.15-0.25%, Ni: 0.15-0.25%, Mo: 0.1-0.2%, Nb+Ti+B≤0.1%, P≤0.025%, S≤0.015%, and the balance being Fe and unavoidable impurities; wherein, the carbon equivalent CEV≤0.41%.

[0010] The design basis and main functions of each chemical element are as follows: C: Controlled at 0.06-0.10%, it can both improve the strength of steel through solid solution strengthening and ensure that the carbon equivalent CEV ≤ 0.41%, avoiding the risk of cold cracking during welding due to excessive carbon content, thus balancing strength and weldability; Si: 0.15-0.3%, can inhibit austenite grain coarsening, improve the high-temperature stability of steel, and improve the deoxidation effect of steel; Mn: 1.2-2.0%, which can enhance the strength of steel through solid solution strengthening, reduce the phase transformation temperature, promote the formation of fine grain structure, and optimize the strength and toughness balance; Cr: 0.15-0.25%, Ni: 0.15-0.25%, Mo: 0.1-0.2%: The addition of these three compounds synergistically improves the hardenability of steel, refines the width of bainite laths to ≤0.2μm, and significantly improves toughness; among them, Ni can also improve the low-temperature toughness of steel, and Mo can improve the tempering stability of steel. Nb+Ti+B≤0.1%: Nb can inhibit austenite grain growth, Ti can form carbonitrides to refine grains, and B can improve hardenability. The three work together to ensure that the initial austenite grains of the steel are ≤50μm, thus optimizing the microstructure. P≤0.025%, S≤0.015%: Strictly control the content of these two harmful impurity elements to avoid the formation of brittle phases and reduce their negative impact on the toughness of steel.

[0011] The mechanical properties of the high-strength steel plate are as follows: yield strength Re≥850MPa, tensile strength Rm≥950MPa, elongation after fracture A50≥22%, longitudinal impact energy KV2≥200J at -40℃, and transverse and longitudinal cold bending performance meets d=2a (180° qualified); the microstructure is mainly composed of fine-grained bainite (grain size 3-5μm), accounting for 80-90%, with M / A islands of size ≤1μm distributed between the laths, forming a two-phase structure of fine-grained bainite and M / A islands.

[0012] The production method of the above-mentioned TMCP high-strength steel plate includes four processes: continuous casting billet heating, controlled rolling, controlled cooling, and post-rolling air cooling. The specific steps are as follows: Step 1: Heating the continuously cast billet A 150mm thick continuous casting billet is selected for producing finished steel plates with a thickness of 20~50mm. The continuous casting billet is fed into a heating furnace and heated for ≥240min. The average temperature of the soaking zone is ≥1200℃, ensuring that the surface temperature of the billet is ≥1150℃ at the end of heating. This ensures that the austenite in the continuous casting billet is fully homogenized, laying the foundation for subsequent grain refinement during rolling.

[0013] Step 2: Controlled rolling Controlled rolling is divided into two stages: roughing and finishing. The specific parameters are as follows: Rough rolling: The initial rolling temperature is ≥1000℃, and the rolling is completed in 8 passes. The single pass reduction rate is ≥5%, and the cumulative reduction rate is ≥65%. After rough rolling, the thickness of the steel plate to be heated is 3 to 4 times the thickness of the finished steel plate. The original austenite grains are broken by high temperature and high pressure, which provides conditions for subsequent fine rolling to refine the grains. Finishing rolling: initial rolling temperature 960-980℃, final rolling temperature ≥850℃, cumulative reduction rate ≥70%; through low temperature and high reduction cumulative strain to induce fine grains, further refine the austenite grains, and ensure the formation of fine bainite structure after subsequent phase transformation.

[0014] Step 3: Controlling Cooling The ACC (Accelerated Cooling) controlled cooling process is adopted, with the following parameters: water inlet temperature 780-820℃, cooling rate ≥20℃ / s, and red-hot temperature 450-480℃; wherein the difference between the red-hot temperature and the water inlet temperature ΔT=300-350℃, ensures complete bainitic phase transformation, forming a two-phase structure of fine-grained bainite and M / A islands, thereby improving the strength, toughness, and plasticity of the steel. Step 4: Post-rolling air cooling After controlled cooling is completed, the steel plate is naturally air-cooled to room temperature, eliminating the need for additional offline heat treatment processes, thus shortening the process flow and reducing production costs.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The high-strength steel produced by this invention has a yield strength ≥850MPa, tensile strength ≥950MPa, elongation after fracture A50 ≥22%, longitudinal impact energy ≥200J at -40℃, and transverse and longitudinal cold bending performance meets d=2a (180° qualified). Its strength, toughness and plasticity matching meets the requirements of high-end equipment use, and it is especially suitable for extreme environments such as low temperature and heavy load.

[0016] 2. This invention achieves a carbon equivalent (CEV) of ≤0.41% through precise control of C content and composite micro-alloying design, effectively avoiding welding cold cracks and meeting the welding and assembly requirements of fields such as engineering machinery.

[0017] 3. This invention eliminates the offline heat treatment process and uses continuous casting billets for direct production, reducing alloy costs by more than 25%, shortening the process flow by 30%, significantly improving production efficiency, and reducing overall production costs.

[0018] 4. This invention forms a two-phase structure of fine-grained bainite (3-5μm) and M / A islands (≤1μm) through two-stage high-reduction rolling and dynamic cooling control. The high-density dislocations and retained austenite work together to simultaneously improve the strength and toughness of the steel, providing microstructure protection for the performance breakthrough of high-strength steel. Attached Figure Description

[0019] Figure 1 This is the metallographic structure (500X) of the steel in Embodiment 1 of the present invention.

[0020] Figure 2 This is the metallographic structure (500X) of the steel in Example 2 of the present invention.

[0021] Figure 3 This is the metallographic structure (500X) of the steel in Example 3 of the present invention.

[0022] Figure 4 This is the metallographic structure (500X) of the steel in Example 4 of the present invention. Detailed Implementation

[0023] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0024] The TMCP-condition high-strength steel plate in this embodiment has the following chemical composition and mass percentage: C: 0.06-0.10%, Si: 0.15-0.3%, Mn: 1.2-2.0%, P: ≤0.025%, S: ≤0.015%, Cr: 0.15-0.25%, Ni: 0.15-0.25%, Mo: 0.1-0.2%, Nb+Ti+B≤0.1%, with the balance being Fe and unavoidable impurities, ensuring a CEV of 0.41%. This invention discloses a method for producing high-strength steel in the TMCP state, comprising continuous casting billet heating, controlled rolling, controlled cooling, and post-rolling air cooling.

[0025] In the continuous casting billet heating process, the continuous casting billet heating time is ≥240min, the average temperature of the soaking zone is ≥1200℃, and the surface temperature of the billet is guaranteed to be ≥1150℃ at the end of heating.

[0026] Controlled rolling is divided into two stages: roughing and finishing. The roughing temperature is ≥1000℃, the total reduction rate is ≥65%, and the thickness at which it is heated is 3~3.5 times the thickness of the finished steel plate. The finishing temperature is 960~980℃, the finishing temperature is ≥850℃, and the total reduction rate is ≥70%.

[0027] The cooling process is controlled by ACC, with an inlet water temperature of 780~820℃, a cooling rate of ≥20℃ / s, and a return temperature of 450~480℃.

[0028] The present invention will be further described below through specific embodiments 1 to 4: The alloy element contents of Examples 1-4 are shown in Table 1, and the rolling and cooling processes are shown in Table 2.

[0029] Table 1:

[0030] Table 2:

[0031] The steel plates obtained in Examples 1-4 were used as test samples. The chemical composition and mechanical properties of the steel plates were tested according to the standard GB / T2975. The low temperature impact toughness test was tested according to GB / T229, the tensile property test was tested according to GB / T228, and the bending property test was tested according to GB / T232. The test results are shown in Table 3 below.

[0032] Table 3:

[0033] As can be seen from Table 3, the TMCP state alloy structural steel provided in this application example has high strength and excellent low-temperature impact toughness.

[0034] The metallographic structure of each embodiment was observed (as shown in Figures 1-4). The results showed that the microstructure of each embodiment was mainly composed of fine lath bainite (accounting for 80-90%), with a grain size of 3-5 μm, a bainite lath width of ≤0.2 μm, and M / A islands with a size of ≤1 μm evenly distributed between the laths, forming a two-phase structure of fine-grained bainite and M / A islands. The high density of dislocations and the retained austenite work together to ensure the excellent strength, toughness and plasticity of the steel.

[0035] As shown in Table 3, the TMCP high-strength steel plates produced in the four sets of examples all meet the performance requirements of yield strength ≥850MPa, tensile strength ≥950MPa, elongation after fracture A50 ≥22%, and longitudinal impact energy ≥200J at -40℃. They also have qualified bending performance and excellent weldability (CEV≤0.41%). At the same time, no offline heat treatment was used in the production process, which reduced the alloy cost by more than 25% and shortened the process flow by 30%. This achieved an organic unity of short process, low cost and high performance, verifying the feasibility and superiority of the technical solution of the present invention.

[0036] By comparing with existing technologies:

[0037] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A high-strength steel plate in TMCP condition, characterized in that, The chemical composition, by weight percentage, includes: C: 0.06-0.10%, Si: 0.15-0.3%, Mn: 1.2-2.0%, Cr: 0.15-0.25%, Ni: 0.15-0.25%, Mo: 0.1-0.2%, Nb+Ti+B≤0.1%, P≤0.025%, S≤0.015%, with the balance being Fe and unavoidable impurities; the carbon equivalent (CEV) of the high-strength steel plate is ≤0.41%.

2. The TMCP high-strength steel plate according to claim 1, characterized in that, The mechanical properties are as follows: yield strength Re≥850MPa, tensile strength Rm≥950MPa, elongation after fracture A50≥22%, longitudinal impact energy KV2≥200J at -40℃, and transverse and longitudinal cold bending performance meets d=2a (180° qualified).

3. The TMCP-state high-strength steel plate according to claim 1, characterized in that, The microstructure is dominated by fine-grained bainite, with a grain size of 3-5 μm, accounting for 80-90%. M / A islands with a size ≤1 μm are distributed between the bainite laths, forming a two-phase structure of fine-grained bainite and M / A islands; the width of the bainite laths is ≤0.2 μm.

4. A production process for TMCP-state high-strength steel, used to prepare TMCP-state high-strength steel plates according to any one of claims 1-3, characterized in that, The process includes four steps: continuous casting billet heating, controlled rolling, controlled cooling, and post-rolling air cooling. The specific steps are as follows: Step 1: Heating of continuous casting billet: Select a continuous casting billet with a thickness of 150mm, heating time ≥240min, average temperature of soaking zone ≥1200℃, and billet surface temperature ≥1150℃ at the end of heating; Step 2: Controlled rolling: Divided into two stages: roughing and finishing rolling; Roughing starts at a temperature ≥1000℃, with 8 passes, a single-pass reduction rate ≥5%, and a cumulative reduction rate ≥65%, with the thickness after roughing being 3-4 times the thickness of the finished steel plate; Finishing starts at a temperature of 960-980℃, finishes at a temperature ≥850℃, and has a cumulative reduction rate ≥70%. Step 3: Controlled cooling: ACC controlled cooling process is adopted, with an inlet water temperature of 780-820℃, a cooling rate of ≥20℃ / s, and a reddening temperature of 450-480℃; the difference between the reddening temperature and the inlet water temperature is ΔT=300-350℃; Step 4: Post-rolling air cooling: After controlled cooling is completed, allow the material to air cool naturally to room temperature.

5. The production process of TMCP high-strength steel according to claim 4, characterized in that, The thickness of the finished steel plate is 20~50mm.

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