A 700mpa high tensile strength girder steel with high fracture toughness and a method of making the same

Abstract

A 700MPa girder steel with high fracture toughness and its preparation method. The 700MPa girder steel comprises the following components in percentage by mass: 0.08% of C, 0.10% of Si, 1.50% of Mn, 0.015% of Nb, 0.012% of Ti, 0.0006% of B, 0.015% of Ce, and the rest of Fe and inevitable impurities. The final obtained strip steel product is a low-carbon bainite steel, which not only has conventional high-strength performance, but also has a significantly improved fracture toughness at low temperature, with a fracture toughness value of 150MPa·m 1 / 2 at-20℃ and a fracture toughness value of 145MPa·m 1 / 2 at-40℃, which is about twice the fracture toughness value of the existing products of the same specification.

Description

Technical Field

[0001] This invention relates to the field of plate and strip rolling technology, and in particular to a high fracture toughness beam steel with a tensile strength of 700 MPa and its preparation method. Background Technology

[0002] To meet the requirements of safety, long service life, high range, high load capacity, and lightweight manufacturing in heavy-duty and commercial vehicles, the application of high-strength beam steel is becoming increasingly widespread. However, due to the complex operating conditions, in addition to basic mechanical properties, specific requirements are placed on the fracture toughness of these products. For the currently widely used high-strength 700L beam steel, the addition of multiple alloying elements achieves a yield strength of 600MPa and a tensile strength of 700MPa. However, with the increase in the types and contents of alloying elements, while strength is improved, the toughness of the product decreases, especially the fracture toughness at low temperatures, thus affecting its overall service performance and failing to meet the requirements for large-scale application in cold northern regions. Therefore, it is necessary to redesign the chemical composition and preparation process of 700MPa beam steel to obtain high-strength automotive beam steel products with high fracture toughness.

[0003] Chinese patent document CN 107604256 A discloses a method for preparing 700MPa grade automotive beam steel strip, which adds 0.05% to 0.07% Ti and 0.03% to 0.07% Nb to the composition, and finally obtains a 2.5 to 3.0 mm thick product with a tensile strength of 700 to 800 MPa. The method does not involve the low-temperature fracture toughness of the product.

[0004] Chinese patent document CN 115627417 A discloses an economical high-strength and high-toughness 700MPa grade beam steel and its production method. 0.08% to 0.15% Ti and 0.025% to 0.055% Nb are added to the composition, resulting in a product with a tensile strength of 720 to 840MPa and a full-size impact energy of ≥160J at -20℃. The low-temperature fracture toughness of the product is not discussed.

[0005] Chinese patent document CN 111809110 A discloses a rare earth-treated thick 700MPa grade automotive beam steel strip and its manufacturing method. The strip contains 0.07% to 0.10% Ti, 0.04% to 0.06% Nb, and 10ppm to 30ppm Ce. The product thickness is 10.0 to 16.0 mm, and its impact value at -40℃ is ≥70J. However, the low-temperature fracture toughness of the product is not discussed.

[0006] All of the above patents achieve the tensile strength requirement of 700MPa by adding alloying elements Ti and Nb, which is relatively expensive, and none of them address the fracture toughness of 700MPa beam steel at low temperatures.

[0007] Chinese patent document CN 103849812 A discloses a low-brittleness 700MPa grade steel for automobile beams and its manufacturing method. 0.08% to 0.10% Ti and 0.035% to 0.050% Nb are added to the composition, ultimately obtaining a 700MPa grade product with good low-temperature toughness, and its impact energy at -60℃ is 80J.

[0008] Chinese patent document CN 112030075 A discloses a 700MPa grade automotive beam steel with stable impact toughness and its production method. The steel contains 0.07% to 0.12% Ti and 0.02% to 0.08% Nb in its composition, resulting in a 700MPa product with stable impact toughness of 12.0 to 16.0 mm. The impact energy of its full-size sample at -20℃ is 150 to 180 J.

[0009] Both of the above patents relate to the impact performance of 700MPa automotive beam steel at low temperatures, but do not address the product's low-temperature fracture toughness.

[0010] Chinese patent document CN 111172467 B discloses a medium-high carbon steel with high fracture toughness. By controlling the Ca content in the steel, it reduces type D inclusions. Simultaneously, by adjusting the size and shape of soft phase inclusions, it improves the crack arrest effect, thereby increasing the room temperature fracture toughness (K0) of the steel. Q The value is 78.2 MPa·m 1 / 2 Increased to 109.3 MPa·m 1 / 2 The method involved in this patent is only applicable to medium and high carbon steel, and the product effect does not involve low-temperature fracture toughness.

[0011] Chinese patent document CN 111363896 A discloses a method for improving the fracture toughness of micro-niobium carbon steel through tempering. After the grinding, quenching, and tempering treatment, the fracture toughness K at room temperature is improved. Q The value is 78.7 MPa·m 1 / 2 Increased to 86.8 MPa·m 1 / 2 The process described in this patent is only applicable to high carbon steel, and the product effect does not involve low-temperature fracture toughness.

[0012] Chinese patent document CN 113166904 A discloses a high-strength steel plate with excellent low-temperature fracture toughness and its manufacturing method. It requires the addition of 0.3% Mo to the composition, which is costly. Moreover, the strength of the resulting product does not reach 700 MPa. Only the DWTT performance of the thick plate product was tested, which is different from the evaluation of the low-temperature fracture toughness of the beam steel product.

[0013] The above three patents all involve low-temperature fracture toughness, but they cannot be directly applied to improving the low-temperature fracture toughness of the 700MPa automotive beam steel product described in this invention.

[0014] In summary, currently, there are no patents addressing the control of the final product's microstructure through compositional design and process routes to significantly improve the low-temperature fracture toughness of 700MPa beam steel. Given the demands of heavy-duty trucks and commercial vehicles, beam steel products urgently need to possess both high strength and high low-temperature fracture toughness. Therefore, this invention provides a method for preparing 700MPa tensile strength beam steel with high fracture toughness. Summary of the Invention

[0015] The purpose of this invention is to provide a method for preparing high fracture toughness beam steel with a tensile strength of 700MPa. This method mainly involves designing the composition and hot rolling process to control the microstructure of the product. Without affecting its high strength, the product can be made to have high low-temperature fracture toughness, thereby improving its overall performance and expanding its application in low-temperature environments.

[0016] The technical solution of the present invention is as follows: a high fracture toughness tensile strength 700MPa beam steel, wherein the mass percentage of each element is as follows: C element mass percentage is 0.07%~0.09%, Si element mass percentage is 0.08%~0.12%, Mn element mass percentage is 1.48%~1.52%, Nb element mass percentage is 0.013%~0.017%, Ti element mass percentage is 0.011%~0.013%, B element mass percentage is 0.0005%~0.0007%, Ce element mass percentage is 0.014%~0.016%, and the remainder is Fe element and unavoidable impurities.

[0017] The mass percentages of each element in the high fracture toughness tensile strength 700MPa beam steel are as follows: C 0.08%, Si 0.10%, Mn 1.50%, Nb 0.015%, Ti 0.012%, B 0.0006%, Ce 0.015%, with the remainder being Fe and unavoidable impurities.

[0018] A method for preparing a high fracture toughness beam steel with a tensile strength of 700MPa specifically includes: slab heating, rolling, cooling, and coiling; an air-cooling relaxation process is added between the final rolling stage and the initial cooling stage of the rolling process.

[0019] The temperature drop during the air-cooled relaxation process is 50°C.

[0020] The heating temperature of the slab is 1200℃.

[0021] The slab thickness is 220mm, the heating temperature is 1200℃, and the heating and heat soaking time is 60min. After heating, the slab is rolled to obtain an intermediate slab with a thickness of 58mm. The finishing rolling start temperature is 1070℃, and the finishing rolling finish temperature is 850℃. The cooling start temperature is 800℃. The slab is cooled to 350℃ at a cooling rate of 50℃ / s and then coiled to obtain a steel strip with a thickness of 8mm.

[0022] The basis for this composition design and control method is as follows: Based on the requirement of low-cost control, 0.015% Nb (by mass) is used to regulate the high-temperature austenite grain size. Experimental verification shows that at a final rolling temperature of 850℃, the original austenite grain size can be effectively controlled at 45μm±5μm, which is beneficial for subsequent bainitic phase transformation and improves the strength and toughness of the steel. To effectively improve its low-temperature fracture toughness, a composite addition of 0.012% Ti (by mass), 0.0006% B (by mass), and 0.015% rare earth Ce (by mass) is used to regulate the N, O, and S elements in the steel. Ultimately, this achieves a suitable improvement in the material's hardenability while concentrating the inclusion size to 0.5μm~1μm, with a spherical shape, minimizing damage to its low-temperature fracture toughness. Combined with the hot rolling process, it has been verified that at a soaking temperature of 1200℃, the microalloying elements are completely solidified, which is beneficial for inhibiting austenite grain growth through high-temperature solid solution dragging and for subsequently controlling nano-precipitations in bainite. After the final rolling temperature and before the initial cooling temperature, there is an air-cooling relaxation process with a temperature drop of 50℃, which effectively allows for some dislocation recovery, beneficial for the subsequent high-toughness bainite phase transformation. The coiling temperature is 350℃, primarily to obtain high-toughness lower bainite; this temperature was obtained from the bainite isothermal phase transformation curve obtained by testing this composition. During the slow cooling process of coiling, the completely dissolved microalloying element Nb in the high-temperature stage will produce certain nano-precipitations. Measurements show that the average size of these precipitates is 3nm, which is beneficial for improving strength and ensuring good fracture toughness.

[0023] The beneficial effects of this invention are as follows: By designing the composition and preparation process of 700MPa tensile strength beam steel, this invention controls the final strip steel product to be a low-carbon bainitic steel. It not only possesses conventional high strength properties, but also significantly improves its fracture toughness at low temperatures, achieving a fracture toughness of 150MPa·m at -20℃. 1 / 2 The fracture toughness at -40℃ is 145 MPa·m. 1 / 2It is approximately twice the fracture toughness value of existing products of the same specification. Attached Figure Description

[0024] Figure 1 Microstructure of the high fracture toughness, 700MPa tensile strength beam steel prepared in this embodiment. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0026] This embodiment describes a high-fracture-toughness, 700MPa tensile strength main beam steel with a thickness of 8mm. The mass percentages of its elements are as follows: C 0.08%, Si 0.10%, Mn 1.50%, Nb 0.015%, Ti 0.012%, B 0.0006%, Ce 0.015%, with the remainder being Fe and unavoidable impurities. The production method includes slab heating, rolling, cooling, and coiling processes. Specific steps are as follows:

[0027] The slab thickness is 220mm, the heating temperature is 1200℃, the heating and holding time is 60min, the slab is rolled after heating, the rough rolling produces the intermediate slab with a thickness of 58mm, the finishing rolling start temperature is 1070℃, the finishing rolling finish temperature is 850℃, the cooling start temperature is 800℃, and then it is cooled to 350℃ at a cooling rate of 50℃ / s before being coiled.

[0028] The mechanical properties of a high-fracture-toughness beam steel with a tensile strength of 700 MPa in this embodiment are as follows: yield strength 675 MPa, tensile strength 750 MPa, and fracture toughness value of 150 MPa·m at -20℃. 1 / 2 The fracture toughness at -40℃ is 145 MPa·m. 1 / 2 The microstructure of the steel strip under a transmission electron microscope is shown Figure 1 ,Depend on Figure 1 It can be determined that its metallographic structure is lower bainite.

Claims

1. A high-fracture-toughness beam steel with a tensile strength of 700 MPa, characterized in that, The mass percentages of each element in the high fracture toughness tensile strength 700MPa beam steel are as follows: C 0.07%~0.09%, Si 0.08%~0.12%, Mn 1.48%~1.52%, Nb 0.013%~0.017%, Ti 0.011%~0.013%, B 0.0005%~0.0007%, Ce 0.014%~0.016%, with the remainder being Fe and unavoidable impurities. The preparation method of the high fracture toughness tensile strength 700MPa beam steel specifically includes: slab heating, rolling, cooling, and coiling; an air-cooling relaxation process is added between the final rolling stage and the initial cooling stage of the rolling process. The slab thickness is 220mm, the heating temperature is 1200℃, and the heating and heat soaking time is 60min. After heating, the slab is rolled to obtain an intermediate slab with a thickness of 58mm. The finishing rolling start temperature is 1070℃, and the finishing rolling finish temperature is 850℃. The cooling start temperature is 800℃. The slab is cooled to 350℃ at a cooling rate of 50℃ / s and then coiled to obtain a steel strip with a thickness of 8mm.

2. The high fracture toughness tensile strength 700MPa beam steel according to claim 1, characterized in that, The mass percentages of each element in the high fracture toughness, 700MPa tensile strength beam steel are as follows: C 0.08%, Si 0.10%, Mn 1.50%, Nb 0.015%, Ti 0.012%, B 0.0006%, Ce 0.015%, with the remainder being Fe and unavoidable impurities.

3. A method for preparing a high-fracture-toughness beam steel with a tensile strength of 700 MPa, characterized in that, Specifically, it includes: The slab is heated, rolled, cooled, and coiled; an air-cooling relaxation process is added between the final rolling stage and the initial cooling stage of the rolling process.

4. The method for preparing high fracture toughness tensile strength 700MPa beam steel according to claim 3, characterized in that, The heating temperature of the slab is 1200℃.

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