Fine-grain reinforced ultralow-carbon cold forging steel and preparation process thereof

Abstract

The invention relates to the technical field of cold forging steel, and provides fine-grain reinforced ultralow-carbon cold forging steel and a preparation process thereof. The invention relates to fine grain strengthened ultralow carbon cold forging steel, which is prepared from the following raw materials in percentage by weight: less than or equal to 0.02% of C, 0.45 to 0.66% of Mn, 0.08 to 0.12% of Si, less than or equal to 0.02% of P, less than or equal to 0.008% of S, 0.025 to 0.045% of Als, 0.018 to 0.020% of Nb, 0.015 to 0.020% of V, 0.012 to 0.015% of Ti and the balance of Fe and inevitable impurities, and the ratio of Mn to (Nb + V + Ti) is 10 to 12. According to the technical scheme, the problem that the strength of the ultra-low carbon cold forging steel is insufficient in the related technology is solved.

Description

Technical Field

[0001] This invention relates to the field of cold heading steel technology, specifically to a fine-grained reinforced ultra-low carbon cold heading steel and its preparation process. Background Technology

[0002] Ultra-low carbon cold heading steel is a type of structural steel specifically designed for cold heading processes. With its excellent plastic deformation capacity, cold formability, and low cost, it is widely used in automotive manufacturing, machinery equipment, construction engineering, aerospace, and other fields, primarily for producing various fasteners such as bolts, nuts, rivets, and pins. As core components of mechanical connections, these fasteners must simultaneously meet the high plasticity requirements of cold heading and sufficient strength, fatigue resistance, and structural stability during actual service. Their performance directly determines the connection reliability and service life of the final product.

[0003] However, in the existing technology, in order to ensure the cold heading formability, the carbon content of ultra-low carbon cold heading steel is usually controlled below 0.02%. As carbon is an important strengthening element in steel, the reduction of its content will lead to a significant weakening of the solid solution strengthening effect of the matrix, resulting in insufficient strength of the steel.

[0004] Insufficient strength of cold-heading steel can lead to several hazards. In the connection scenarios of core power and load-bearing components such as automobile chassis, engine blocks, and transmissions, fasteners with insufficient strength are prone to premature plastic deformation, fatigue wear, or even fracture under long-term alternating loads, vibrations, and temperature fluctuations. This can directly cause component connection failure, power transmission interruption, and accidents such as vehicle breakdowns and brake failures, endangering the safety of drivers and passengers. In the fields of large-scale engineering equipment such as building steel structures, bridge supports, and tower cranes, low-strength fasteners cannot withstand the structural weight, external loads, and environmental corrosion, and are prone to loosening and slippage, posing safety hazards.

[0005] Therefore, it is very necessary to develop a high-strength, ultra-low carbon cold heading steel. Summary of the Invention

[0006] This invention proposes a fine-grained reinforced ultra-low carbon cold heading steel and its preparation process, which solves the problem of insufficient strength of ultra-low carbon cold heading steel in related technologies.

[0007] The technical solution of the present invention is as follows: The present invention proposes a fine-grained strengthened ultra-low carbon cold heading steel, which is composed of the following raw materials by weight percentage: C≤0.02%, Mn 0.45%~0.66%, Si 0.08%~0.12%, P≤0.02%, S≤0.008%, Als0.025%~0.045%, Nb 0.018%~0.020%, V 0.015%~0.020%, Ti 0.012%~0.015%, with the balance being Fe and unavoidable impurities, wherein Mn / (Nb+V+Ti)=10~12.

[0008] As a further technical solution, the Nb:V ratio is 1~1.2:1.

[0009] In the fine-grained strengthened ultra-low carbon cold heading steel of this invention, the Nb:V ratio is 1~1.2:1. Nb, as the core element for fine-grained strengthening, forms precipitates with strong stability, effectively pinning austenite grain boundaries and inhibiting austenite grain coarsening during rolling and cooling, laying the foundation for subsequent phase transformation to form fine ferrite grains. The V precipitates can be dispersed in the ferrite matrix, hindering dislocation slip and supplementing the precipitation strengthening effect. When the Nb:V ratio is 1~1.2:1, the Nb content is slightly higher than the V content, ensuring sufficient grain boundary pinning at high temperatures and avoiding incomplete grain refinement due to insufficient Nb, thus guaranteeing the dominant role of fine-grained strengthening. The V content avoids excessive V leading to precipitate agglomeration and coarsening while ensuring a sufficient quantity of precipitates; this balance effectively improves the strength of the ultra-low carbon cold heading steel.

[0010] This invention also proposes a method for preparing fine-grained reinforced ultra-low carbon cold heading steel, which includes the following steps: S1. According to the stated ingredients, the mixture is smelted, refined, and continuously cast to obtain a billet. S2. The billet is heated to 1180~1250℃, held at that temperature, and then subjected to rough rolling, finish rolling, and cooling to room temperature. The billet is then coiled to obtain the fine-grained reinforced ultra-low carbon cold heading steel.

[0011] As a further technical solution, the initial rolling temperature of the roughing mill is 1120~1160℃, and the final rolling temperature of the roughing mill is 1020~1050℃.

[0012] As a further technical solution, the initial rolling temperature of the finishing mill is 950~990℃, and the final rolling temperature of the finishing mill is 860~890℃.

[0013] As a further technical solution, the cooling includes the following steps: first, a first stage of cooling is performed, cooling to 600~650℃, for example, 600℃, 610℃, 620℃, 630℃, 640℃, or 650℃, preferably 620℃; then, a second stage of cooling is performed, cooling to 400~450℃, for example, 400℃, 410℃, 420℃, 430℃, 440℃, or 450℃, preferably 420℃; then, a third stage of cooling is performed, cooling to 200~220℃, for example, 200℃, 210℃, or 220℃, preferably 200℃; finally, cooling to room temperature. The cooling rates of the first, second, and third stages of cooling are different.

[0014] As a further technical solution, the cooling rate of the first cooling stage > the cooling rate of the second cooling stage > the cooling rate of the third cooling stage.

[0015] As a further technical solution, the cooling rate of the first stage of cooling is 12~15℃ / s, for example, it can be 12℃ / s, 13℃ / s, 14℃ / s, or 15℃ / s, preferably 12℃ / s; the cooling rate of the second stage of cooling is 5~8℃ / s, for example, it can be 5℃ / s, 6℃ / s, 7℃ / s, or 8℃ / s, preferably 6℃ / s; and the cooling rate of the third stage of cooling is 1~3℃ / s, for example, it can be 1℃ / s, 2℃ / s, or 3℃ / s, preferably 2℃ / s.

[0016] In the preparation of fine-grained strengthened ultra-low carbon cold heading steel according to this invention, a three-stage cooling process is employed, which, in conjunction with elemental composition, achieves both cooling and microalloying strengthening, thereby improving the strength of the ultra-low carbon cold heading steel. The first stage employs high-speed cooling to solidify the fine grain and precipitation foundation for strength enhancement, effectively inhibiting austenite grain growth. Driven by high-speed cooling, a sufficient quantity and uniform size of dispersed precipitates are formed. The second stage uses a medium-speed cooling rate, balancing strength and microstructure stability. This avoids excessive temperature differences and stress concentration issues caused by high-speed cooling, ensuring a uniform and controllable phase transformation process. It forms a fine, dense pearlite structure, supplementing the strengthening effect and preventing residual precipitates and uneven microstructure due to excessively rapid phase transformation, ensuring consistent strength across all parts of the steel. The third stage involves low-speed cooling, effectively releasing the internal stress generated during the phase transformation, preventing microcracks caused by stress concentration, and preventing microcracks from weakening the strength. The combination of the three-stage cooling process and the elemental composition strengthening system effectively improves the strength of the ultra-low carbon cold heading steel.

[0017] As a further technical solution, the billet is a square billet with a cross-sectional dimension of 150mm × 150mm.

[0018] As a further technical solution, the heat preservation time is 100~120 minutes.

[0019] The working principle and beneficial effects of this invention are as follows: In the fine-grained strengthened ultra-low carbon cold heading steel of this invention, C ≤ 0.02%, avoiding the damage to plasticity caused by excessive carbide precipitation and providing a fundamental guarantee for cold heading. The elemental composition satisfies Mn / (Nb+V+Ti) = 10~12, achieving a balance between Mn solid solution strengthening and ternary microalloying fine-grained strengthening. Mn, as a solid solution strengthening element, dissolves into the ferrite lattice, causing moderate lattice distortion and hindering dislocation movement, providing basic strength support for the steel matrix. The total amount of Nb, V, and Ti ternary microalloying elements forms a ratio of 10~12 with the Mn content, promoting the formation of precipitates, efficiently pinning austenite grain boundaries, and inhibiting grain coarsening during rolling and cooling. When the amounts of Mn, Nb, V, and Ti satisfy the formula, it avoids both insufficient solid solution strengthening due to insufficient Mn content and grain refinement saturation caused by relative excess of microalloying elements, effectively improving the strength of the ultra-low carbon cold heading steel. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1 A fine-grained reinforced ultra-low carbon cold heading steel is composed of the following raw materials by weight percentage: C 0.016%, Mn 0.45%, Si 0.08%, P 0.015%, S 0.006%, Als 0.025%, Nb 0.018%, V 0.015%, Ti 0.012%, with the balance being Fe and unavoidable impurities; A method for preparing fine-grained reinforced ultra-low carbon cold heading steel includes the following steps: S1. According to the ingredients, the raw materials are smelted, refined, and continuously cast to obtain the billet; S2. The billet is heated to 1180℃, held for 120 minutes, and then rough rolled, finish rolled, cooled to room temperature, and coiled to obtain fine-grained reinforced ultra-low carbon cold heading steel. The initial rolling temperature of the roughing mill is 1120℃, the final rolling temperature of the roughing mill is 1020℃, the initial rolling temperature of the finishing mill is 950℃, and the final rolling temperature of the finishing mill is 860℃; the cooling rate is 6℃ / s.

[0022] Example 2 A fine-grained reinforced ultra-low carbon cold heading steel is composed of the following raw materials by weight percentage: C 0.012%, Mn 0.50%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.019%, V 0.018%, Ti 0.013%, with the balance being Fe and unavoidable impurities; A method for preparing fine-grained reinforced ultra-low carbon cold heading steel includes the following steps: S1. According to the ingredients, the raw materials are smelted, refined, and continuously cast to obtain the billet; S2. The billet is heated to 1220℃, held for 110 minutes, and then rough rolled, finish rolled, cooled to room temperature, and coiled to obtain fine-grained reinforced ultra-low carbon cold heading steel. The initial rolling temperature of the roughing mill is 1150℃, the final rolling temperature of the roughing mill is 1030℃, the initial rolling temperature of the finishing mill is 980℃, and the final rolling temperature of the finishing mill is 880℃; the cooling rate is 6℃ / s.

[0023] Example 3 A fine-grained reinforced ultra-low carbon cold heading steel is composed of the following raw materials by weight percentage: C 0.02%, Mn 0.66%, Si 0.12%, P 0.02%, S 0.008%, Als 0.045%, Nb 0.020%, V 0.020%, Ti 0.015%, with the balance being Fe and unavoidable impurities; A method for preparing fine-grained reinforced ultra-low carbon cold heading steel includes the following steps: S1. According to the ingredients, the raw materials are smelted, refined, and continuously cast to obtain the billet; S2. The billet is heated to 1250℃, held for 100 minutes, and then rough rolled, finish rolled, cooled to room temperature, and coiled to obtain fine-grained reinforced ultra-low carbon cold heading steel. The initial rolling temperature of the roughing mill is 1160℃, the final rolling temperature of the roughing mill is 1050℃, the initial rolling temperature of the finishing mill is 990℃, and the final rolling temperature of the finishing mill is 890℃; the cooling rate is 6℃ / s.

[0024] Example 4 Compared with Example 2, Example 4 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.60%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.019%, V 0.018%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0025] Example 5 Compared with Example 4, Example 5 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.60%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.018%, V 0.018%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0026] Example 6 Compared with Example 4, Example 6 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.60%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.018%, V 0.015%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0027] Example 7 Compared with Example 4, Example 7 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.60%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.018%, V 0.019%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0028] Example 8 Compared with Example 4, Example 8 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.60%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.020%, V 0.015%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0029] Example 9 The difference between Example 4 and Example 9 is that, in step S2, the cooling rate is 12°C / s.

[0030] Example 10 Compared with Example 4, Example 10 differs in that, in step S2, the cooling includes the following steps: first cooling to 620°C at a cooling rate of 12°C / s, then cooling to 200°C at a cooling rate of 6°C / s, and finally cooling to room temperature.

[0031] Example 11 Compared with Example 4, Example 11 differs in that, in step S2, the cooling includes the following steps: first cooling to 420°C at a cooling rate of 6°C / s, then cooling to 200°C at a cooling rate of 2°C / s, and finally cooling to room temperature.

[0032] Example 12 Compared with Example 4, Example 12 differs in that, in step S2, the cooling includes the following steps: first cooling to 620°C at a cooling rate of 12°C / s, then cooling to 420°C at a cooling rate of 6°C / s, then cooling to 200°C at a cooling rate of 2°C / s, and finally cooling to room temperature.

[0033] Comparative Example 1 Compared with Example 2, Comparative Example 1 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.70%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.019%, V 0.018%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0034] Comparative Example 2 Compared with Example 2, Comparative Example 2 differs in that it is a fine-grained reinforced ultra-low carbon cold heading steel composed of the following raw materials by weight percentage: C 0.012%, Mn 0.40%, Si 0.10%, P 0.014%, S 0.005%, Als 0.030%, Nb 0.019%, V 0.018%, Ti 0.013%, with the balance being Fe and unavoidable impurities.

[0035] Experimental Example 1 The tensile strength of the fine-grained reinforced ultra-low carbon cold heading steels prepared in Examples 1-12 and Comparative Examples 1-2 was tested according to the test methods specified in GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature".

[0036] The test results are shown in Table 1: Table 1 Performance test results of Examples 1-12 and Comparative Examples 1-2

[0037] Table 1 shows that when the elemental composition of fine-grained reinforced ultra-low carbon cold heading steel satisfies Mn / (Nb+V+Ti) = 10~12, the strength of the ultra-low carbon cold heading steel can be improved. When Nb:V = 1~1.2:1, the strength of the obtained ultra-low carbon cold heading steel is even better. When a three-stage cooling process is combined with the elemental composition, the strength of the ultra-low carbon cold heading steel can be further improved.

[0038] Experiment Example 2 The fine-grained reinforced ultra-low carbon cold heading steel prepared in Example 2 was tested according to the test method specified in YB / T 5293-2014 "Metallic Materials Upsetting Test Method" with a forging ratio of 1 / 3. The results showed that there were no visible cracks in the sample, and the product was qualified.

[0039] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A fine-grained reinforced ultra-low carbon cold heading steel, characterized in that, It is composed of the following raw materials by weight percentage: C≤0.02%, Mn 0.45%~0.66%, Si 0.08%~0.12%, P≤0.02%, S≤0.008%, Als 0.025%~0.045%, Nb 0.018%~0.020%, V 0.015%~0.020%, Ti 0.012%~0.015%, with the balance being Fe and unavoidable impurities, wherein Mn / (Nb+V+Ti) = 10~12.

2. The fine-grained reinforced ultra-low carbon cold heading steel according to claim 1, characterized in that, The Nb:V ratio is 1 to 1.2:

1.

3. A method for preparing fine-grained reinforced ultra-low carbon cold heading steel, used to prepare the fine-grained reinforced ultra-low carbon cold heading steel according to any one of claims 1 to 2, characterized in that, Includes the following steps: S1. According to the stated ingredients, the mixture is smelted, refined, and continuously cast to obtain a billet. S2. The billet is heated to 1180~1250℃, held at that temperature, and then subjected to rough rolling, finish rolling, and cooling to room temperature. The billet is then coiled to obtain the fine-grained reinforced ultra-low carbon cold heading steel.

4. The method for preparing fine-grained reinforced ultra-low carbon cold heading steel according to claim 3, characterized in that, The initial rolling temperature of the roughing mill is 1120~1160℃, and the final rolling temperature of the roughing mill is 1020~1050℃.

5. The method for preparing a fine-grained reinforced ultra-low carbon cold heading steel according to claim 3, characterized in that, The initial rolling temperature of the finishing mill is 950~990℃, and the final rolling temperature of the finishing mill is 860~890℃.

6. The method for preparing a fine-grained reinforced ultra-low carbon cold heading steel according to claim 3, characterized in that, The cooling process includes the following steps: first, a first stage of cooling is performed, cooling to 600~650℃; then, a second stage of cooling is performed, cooling to 400~450℃; then, a third stage of cooling is performed, cooling to 200~220℃; and finally, cooling to room temperature. The cooling rates of the first, second, and third stages of cooling are different.

7. The method for preparing a fine-grained reinforced ultra-low carbon cold heading steel according to claim 6, characterized in that, The cooling rate of the first cooling stage > the cooling rate of the second cooling stage > the cooling rate of the third cooling stage.

8. The method for preparing fine-grained reinforced ultra-low carbon cold heading steel according to claim 6, characterized in that, The cooling rate of the first stage of cooling is 12~15℃ / s, the cooling rate of the second stage of cooling is 5~8℃ / s, and the cooling rate of the third stage of cooling is 1~3℃ / s.

9. The method for preparing a fine-grained reinforced ultra-low carbon cold heading steel according to claim 3, characterized in that, The billet is a square billet with a cross-sectional dimension of 150mm × 150mm.

10. The method for preparing a fine-grained reinforced ultra-low carbon cold heading steel according to claim 3, characterized in that, The heat preservation time is 100~120 minutes.

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