A method for manufacturing an economic yield strength 800 MPa grade high plasticity steel

Abstract

The application discloses a method for manufacturing an economic yield strength 800MPa grade high plasticity steel, which comprises the following steps: 1) smelting; 2) thin slab continuous casting and rolling, the molten steel is produced by thin slab continuous casting and rolling, the casting blank heating temperature is 1220-1300 DEG C, the rolling is directly carried out in a multi-stand finishing mill, the finishing rolling opening rolling temperature is greater than or equal to 1100 DEG C, the first three pass reduction is greater than or equal to 50%, the pass temperature is greater than or equal to 1050 DEG C, and the pass reduction of each subsequent pass is greater than or equal to 15%; the constant speed rolling is adopted, the rolling speed is controlled to be 3.0-10.0 m / s, the finish rolling temperature is controlled to be 890-930 DEG C, and the finished steel plate thickness is 1.2-5.0 mm; 3) laminar cooling, two-stage controlled cooling is adopted, the first-stage water cooling speed is 100-180 DEG C / s, the cooling is carried out to the intermediate temperature 650-730 DEG C, then the water cooling speed is less than or equal to 15 DEG C / s, the cooling is carried out to 610-670 DEG C, and coiling is carried out. The yield strength of the high plasticity steel is greater than or equal to 800MPa, the tensile strength is greater than or equal to 850MPa, and the elongation after fracture is greater than or equal to 18%.

Description

Technical Field

[0001] This invention relates to the field of microalloyed steel manufacturing, specifically to a method for manufacturing an economical high-ductility steel with a yield strength of 800 MPa. Background Technology

[0002] Achieving structural lightweighting by increasing steel plate strength, thereby reducing energy consumption, alleviating labor intensity, and other benefits, has become an inevitable trend in the development of modern steel materials. For general structural steel, as material strength increases, its plasticity tends to decrease. Plasticity characterizes a material's deformation capacity; excessively low plasticity directly affects the forming of user-defined parts, easily leading to cracking and other problems, rendering the material unusable. Resolving this contradiction between strength and plasticity has become a key challenge in the research and development of steel materials.

[0003] High-strength steel with a yield strength of 800 MPa has been researched and manufactured by institutions and enterprises both domestically and internationally, such as:

[0004] Chinese patent CN101568659A discloses "Hot-rolled steel plate with excellent high strength and weather resistance and its manufacturing method", which introduces a high-strength hot-rolled steel plate with excellent weather resistance for containers, which is made of low carbon and high manganese (Mn preferably 2.0-2.5%), niobium-titanium composite microalloying, and Cu and Cr added.

[0005] Chinese patent CN101784688A, entitled "Manufacturing method of steel sheet with high tensile strength and ductility and sheet obtained therefrom", describes a hot-rolled high-strength steel sheet with a tensile strength greater than 800 MPa and an elongation at break greater than 10% by adding a high content of V and a certain content of Mo, resulting in a microstructure mainly composed of bainite.

[0006] Chinese patent CN101560629A discloses "A hot-rolled strip steel with a yield strength higher than 800MPa and its preparation method", which introduces a hot-rolled strip steel with a yield strength higher than 800MPa. Its chemical composition mainly adopts medium carbon high manganese (Mn 1.70~2.20%), with added high contents of niobium, vanadium and titanium (Nb 0.04~0.07%, V 0.00~0.07%, Ti 0.08~0.18%).

[0007] Chinese patent CN104513937A discloses "a high-strength steel with a yield strength of 800MPa and its production method". Its yield strength can reach 800MPa, but its elongation can only be guaranteed to be ≥12%. Its chemical composition adopts medium carbon with added Cr, Mo, B and a certain amount of Nb, V and Ti microalloying.

[0008] The aforementioned patents all primarily involve adding high levels of alloys and prioritize strength, without adequately considering the balance between strength and plasticity. Summary of the Invention

[0009] The purpose of this invention is to provide an economical method for manufacturing high-ductility steel with a yield strength of 800MPa. On the one hand, it ensures high strength (yield strength ≥ 800MPa, tensile strength ≥ 850MPa), and more importantly, it achieves high ductility (elongation after fracture ≥ 18%), that is, to obtain a high strength-ductility product. On this basis, it enables low-cost design and production.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] A method for manufacturing an economical high-ductility steel with a yield strength of 800 MPa, comprising the following steps:

[0012] 1) Smelting

[0013] The following components are smelted in a converter and refined in an LF ladle furnace, with the following chemical composition by weight percentage: C: 0.17~0.25%, Si: 0.1~0.20%, Mn: 1.2~2.0%, P≤0.010%, S≤0.002%, Ti: 0.10~0.40%, Cr: 0.3~1.0%, Als: 0.03~0.20%, Ca: 0.001~0.004%, As≤0.012%, N≤0.004%, with the balance including Fe and other unavoidable impurities;

[0014] 2) Thin slab continuous casting and rolling

[0015] Molten steel is continuously cast and rolled into thin slabs with a thickness of 60-110 mm. The slabs are heated in a soaking furnace at a temperature of 1220-1300℃ and rolled directly on a 7-stand finishing mill. The initial rolling temperature is ≥1100℃, the reduction rate for the first three passes is ≥50%, and the temperature for each pass is ≥1050℃. The reduction rate for each subsequent pass is ≥15%. Constant speed rolling is used, with the rolling speed controlled at 3.0-10.0 m / s and the final rolling temperature controlled at 890-930℃. The thickness of the finished steel plate is 1.2-5.0 mm.

[0016] 3) Laminar flow cooling

[0017] A two-stage controlled cooling system is adopted. The first stage water cooling rate is 100-180℃ / s, cooling to an intermediate temperature of 650-730℃. Then, the system is cooled to 610-670℃ by water cooling at a rate of ≤15℃ / s before winding.

[0018] In the manufacturing method of high-ductility steel with a yield strength of 800 MPa described in this invention:

[0019] This invention employs continuous casting and rolling of thin slabs, particularly using a reduction rate of ≥50% in the first three rolling passes. This is mainly achieved by utilizing the high reduction during the high-temperature stage to fully break down the short columnar crystals formed during the continuous casting process, promoting C and Mn diffusion. Simultaneously, the high-temperature conditions facilitate full recrystallization, further homogenizing and refining the austenite structure, improving center segregation, preventing and reducing the center C and Mn segregation problems that are prone to occur in conventional processes, and improving the performance and formability of the final product.

[0020] The reduction rate for each subsequent pass is ≥15%, which ensures that the billet microstructure can be continuously refined through recovery and dynamic recrystallization after each deformation pass.

[0021] Constant speed rolling can leverage the advantages of thin slab continuous casting and rolling production lines, achieving stable process parameters for steel strip during rolling, ensuring that all parts of the final steel strip are produced under the same conditions, and improving the uniformity of the final product's microstructure and properties.

[0022] The laminar flow cooling adopts a two-stage controlled cooling method. The first stage water cooling rate is 100-180℃ / s, cooling to an intermediate temperature of 650-730℃. Then, it is cooled to 610-670℃ at a water cooling rate of ≤15℃ / s before winding.

[0023] Existing conventional cooling processes employ a single-stage cooling method, either front-stage or rear-stage cooling, without considering the austenite-ferrite phase transformation and the control of ferrite morphology.

[0024] This invention employs a two-stage cooling control. The first stage, rapid cooling, quickly supercools the deformed austenite to the austenite-ferrite phase transformation temperature range, preserving the distortion energy in the supercooled austenite. This allows the supercooled austenite to generate more ferrite nucleation sites and rapidly complete the phase transformation, resulting in a finer and more abundant ferrite microstructure. The second stage, slow cooling, ensures the ferrite microstructure is equiaxed and homogeneous. Through this cooling control, a final, uniform and refined equiaxed ferrite + pearlite + small amount of bainite microstructure is obtained.

[0025] The high-ductility steel obtained by the manufacturing method described in this invention has the following chemical composition by weight percentage: C: 0.17-0.25%, Si: 0.1-0.20%, Mn: 1.2-2.0%, P≤0.010%, S≤0.002%, Ti: 0.10-0.40%, Cr: 0.3-1.0%, Als: 0.03-0.20%, Ca: 0.001-0.004%, As≤0.012%, N≤0.004%, with the balance including Fe and other unavoidable impurities.

[0026] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0027] The high-ductility steel described in this invention has a yield strength ≥800MPa, an elongation ≥18%, and a strength-ductility product ≥18GPa*.

[0028] In the composition design of the high-ductility steel described in this invention:

[0029] Carbon: Carbon is the most economical strengthening element. A carbon content of 0.07–0.16% falls within the peritectic region, where peritectic reactions easily occur in molten steel, directly affecting casting stability and slab quality. Furthermore, excessive carbon content can affect the weldability of the steel. Therefore, this invention limits the carbon content to 0.17–0.25%, preferably 0.175–0.215%.

[0030] Silicon: Silicon can purify ferrite and prevent the formation of coarse carbides during cooling. However, silicon also easily forms Fe2SiO4 and forms eutectoid products with FeO on the surface of the billet, which solidify into an anchor-like structure, making FeO difficult to remove and affecting the final surface quality. Therefore, the silicon content is limited to the range of 0.10-0.20%.

[0031] Manganese: Manganese is the most effective element for improving strength and toughness, and can effectively delay the pearlite transformation. However, adding too much manganese can also easily lead to peritectic reaction in molten steel, causing quality problems such as continuous casting cracks. Therefore, the present invention limits the manganese content to 1.20-2.00%, preferably 1.4-1.7%.

[0032] Chromium: Chromium is a carbide-forming element with a strong affinity for carbon, which hinders the diffusion of carbon atoms. Combined with the effects of manganese, this significantly delays the transformation of pearlite and bainite. Furthermore, chromium improves the hardenability of materials and also provides some corrosion resistance. In this invention, the chromium content is controlled at 0.30–1.00%, preferably 0.4–0.7%.

[0033] Titanium: Titanium has a prominent precipitation strengthening effect and is an economical and effective element to ensure the strength of steel. The titanium content of this invention is designed to be in the range of 0.10 to 0.40%, preferably 0.12 to 0.20%.

[0034] Phosphorus: Phosphorus is prone to center segregation, which affects molding performance. In this invention, the phosphorus content is controlled at ≤0.010%.

[0035] Aluminum: As an effective deoxidizer in the smelting process, it also has a certain effect on refining grains and improving the strength of steel, but it is also prone to forming Al2O3 inclusions. In this invention, the aluminum content is controlled at 0.03~0.20%.

[0036] Arsenic: It can significantly reduce the toughness and plasticity of materials. This invention strictly controls the arsenic content to ≤0.012%.

[0037] Sulfur and nitrogen: They easily combine with Ti in steel, affecting the strengthening effect of Ti and greatly affecting the plasticity of steel. In this invention, sulfur is controlled at ≤0.002% and nitrogen is controlled at ≤0.004%.

[0038] In addition to limiting the range of the above chemical components, from the point of view of improving the formability and economy of materials, this invention does not add expensive alloying elements such as Nb, Cu, Ni, and Mo.

[0039] Depending on the thickness of the finished product, the rolling speed is controlled at 3.0~10.0m / s for constant speed rolling in order to ensure the stability of the continuous coiling process and the overall performance stability.

[0040] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0041] This invention employs a low-cost medium-carbon-titanium-chromium composition design, with a focus on the formation and control of ferrite microstructure in the process. Fine primary austenite microstructure is obtained through strict pass-by-pass rolling reduction distribution. A fine and uniform ferrite matrix is ​​obtained through controlled cooling rate and temperature during the cooling process. The precipitation strengthening effect of Ti is fully considered. The resulting thin steel strip with a yield strength ≥800MPa has low process and alloy costs, and possesses higher elongation and plasticity. The product exhibits extremely high strength-ductility product, enabling its application in high-strength structural components with higher forming requirements. It can meet various forming processes such as rolling, stamping, and bending, as well as welding and other application requirements and environments. Attached Figure Description

[0042] Figure 1 This is a metallographic diagram of the steel in Example 2 of the present invention. Detailed Implementation

[0043] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0044] Table 1 shows the chemical composition of the steel in the embodiments of the present invention; Table 2 shows the main process parameters of the steel in the embodiments of the present invention; Table 3 shows the performance parameters of the steel in the embodiments of the present invention.

[0045] Comparative Example 1 (Chinese Patent CN101784688A), Comparative Example 2 (Chinese Patent CN106929759A), and Comparative Example 3 (Chinese Patent CN102978525A).

[0046] Compared with Comparative Examples 1-3, this invention does not add expensive alloys such as Nb and Mo, adopts a medium carbon design, selects and uses more economical alloying elements such as Ti and Cr, and makes full use of rolling process and cooling process control to obtain a uniform and fine equiaxed ferrite matrix to achieve high plasticity of the material. It also fully utilizes the comprehensive strengthening effects of solid solution strengthening of C, Mn and Cr, precipitation strengthening of Ti and fine grain strengthening effect to achieve high strength.

[0047] See Figure 1 The figure shown is a metallographic diagram of the steel in Embodiment 2 of the present invention.

[0048] As can be seen from the figure, the microstructure of the high-plasticity steel described in this invention is ferrite or ferrite + pearlite + a small amount of bainite.

[0049]

[0050]

[0051]

Claims

1. A method for manufacturing an economical high-ductility steel with a yield strength of 800 MPa, characterized in that, Includes the following steps: 1) Smelting The following components are smelted in a converter and refined in an LF ladle furnace, with the following chemical composition by weight percentage: C: 0.17~0.25%, Si: 0.1~0.20%, Mn: 1.2~2.0%, P≤0.010%, S≤0.002%, Ti: 0.10~0.40%, Cr: 0.3~1.0%, Als: 0.03~0.20%, Ca: 0.001~0.004%, As≤0.012%, N≤0.004%, with the balance including Fe and other unavoidable impurities; 2) Thin slab continuous casting and rolling Molten steel is continuously cast and rolled into thin slabs with a thickness of 60-110 mm. The slabs are heated in a soaking furnace at a temperature of 1220-1300℃ and rolled directly on a multi-stand finishing mill. The initial finishing rolling temperature is ≥1100℃, the reduction rate for the first three passes is ≥50%, and the temperature for each pass is ≥1050℃. The reduction rate for each subsequent pass is ≥15%. Constant speed rolling is used, with the rolling speed controlled at 3.0-10.0 m / s and the final rolling temperature controlled at 890-930℃. The thickness of the finished steel plate is 1.2-5.0 mm. 3) Laminar flow cooling A two-stage controlled cooling system is adopted. The first stage water cooling rate is 100-180℃ / s, cooling to an intermediate temperature of 650-730℃. Then, the system is cooled to 610-670℃ by water cooling at a rate of ≤15℃ / s before winding.

2. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, In step 2), a 7-stand finishing mill is used.

3. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, The high-ductility steel has a yield strength ≥800MPa, elongation ≥18%, and strength-ductility product ≥18GPa*.

4. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable impurities.

5. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, The C content of the high-plasticity steel is 0.175 to 0.215 wt%.

6. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, The Mn content of the high-plasticity steel is 1.4 to 1.7 wt%.

7. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, The Cr content of the high-plasticity steel is 0.4 to 0.7 wt%.

8. The method for manufacturing economical high-ductility steel with a yield strength of 800 MPa as described in claim 1, characterized in that, The high-plasticity steel has a Ti content of 0.12 to 0.20 wt%.

9. The high-ductility steel obtained by the manufacturing method as described in claim 1 has the following chemical composition by weight percentage: C: 0.17-0.25%, Si: 0.1-0.20%, Mn: 1.2-2.0%, P≤0.010%, S≤0.002%, Ti: 0.10-0.40%, Cr: 0.3-1.0%, Als: 0.03-0.20%, Ca: 0.001-0.004%, As≤0.012%, N≤0.004%, with the balance being Fe and other unavoidable impurities.

10. The high-ductility steel as described in claim 9, characterized in that, The balance consists of Fe and other unavoidable impurities.

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