750MPa-grade low-stress hot-rolled high-strength beam steel and manufacturing method thereof

Abstract

The invention discloses 750MPa-grade low-stress hot-rolled high-strength beam steel and a manufacturing method thereof, the method comprises the steps of smelting and continuous casting, casting blank slow cooling and soaking, rolling and segmented cooling, in the casting blank slow cooling and soaking step, the casting blank is cooled in air after red delivery, the furnace entering temperature is 300 DEG C or above, the soaking temperature is 1250-1290 DEG C, and the furnace staying time is 1-1.2 mm / min; in the rolling step, the initial rolling temperature of rough rolling ranges from 1100 DEG C to 1150 DEG C, the accumulated deformation of rough rolling is larger than or equal to 70%, the initial rolling temperature of finish rolling ranges from 950 DEG C to 1080 DEG C, and the final rolling temperature of finish rolling ranges from 810 DEG C to 880 DEG C; in the subsection cooling step, the first section is subjected to water cooling to 710-780 DEG C at the cooling speed of 10-25 DEG C / s and then is subjected to air cooling for 4-8 s, and then the second section is subjected to water cooling to the coiling temperature of 550-620 DEG C at the cooling speed of 30-50 DEG C / s. According to the low-stress hot-rolled high-strength large beam steel, the yield strength is larger than or equal to 700 MPa, the tensile strength is larger than or equal to 750 MPa, the ductility is larger than or equal to 20%, and the low-stress hot-rolled high-strength large beam steel not only has high strength, but also has good toughness and formability, and completely meets the requirements for use and performance of large beam steel in the industries of automobile manufacturing, engineering machinery and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials and their preparation, and particularly relates to a 750MPa-grade low-stress hot-rolled high-strength beam steel and a manufacturing method thereof. Background Art

[0002] In the era of the "dual carbon" goals, with the development of lightweight automobiles, safety and energy conservation and emission reduction have become the development trends of the automotive industry. For this reason, the selection of steel for automobiles has gradually developed towards high-strength steel. As a key structural component material in the automotive manufacturing field, beam steel has strict requirements on strength, toughness and formability. At present, there are many problems with the traditional hot-rolled high-strength beam steel production process on the market. For example, the parameter settings in the rolling process are not fine enough, and the internal structure of the steel plate is uneven; the synergistic effect of elements such as Nb and Ti is not fully considered, and the precipitation effect of (Ti, Nb) C is not good, making it difficult to achieve effective grain refinement and strengthening; the cooling process mostly uses a single cooling rate, which cannot meet the beam steel's requirements for low stress, high strength and high toughness, making the produced beam steel prone to deformation, cracking and other problems during use, limiting its application in the field of high-end equipment manufacturing.

[0003] Chinese patent application CN105316578A, entitled "Low-carbon equivalent, easily weldable, automobile beam steel with a yield strength of 750 MPa or more and a manufacturing method thereof," discloses a high-strength beam steel with a carbon equivalent of 0.354 to 0.394, a yield strength ≥750 MPa, a tensile strength of 780 to 950 MPa, and an elongation of 15%. This automobile beam steel has good welding performance but does not specify the material stress state and processing performance. In addition, its Mo alloy content is high and the cost is relatively high.

[0004] Chinese patent application CN101844162A, titled "A Method for Eliminating Residual Stress in Hot-Rolled High-Strength Steel," discloses a method for eliminating residual stress in hot-rolled high-strength steel. Steel coils thicker than 8mm are placed in a slow cooling pit for slow cooling, while coils less than 8mm are air-cooled. After cooling to room temperature, they are temper rolled. After cross-cutting, the flattened steel plates are subjected to stress relief annealing in a heat treatment furnace. After exiting the furnace, they are pile-cooled for slow cooling, and finally straightened in a straightening unit. This technology suffers from the requirement for a slow cooling pit, straightening, and heat treatment, resulting in a lengthy process and high costs.

[0005] Chinese patent application CN101670372A, entitled "A method for eliminating shape defects of hot-rolled high-strength steel plates," discloses a method for eliminating shape defects of hot-rolled high-strength steel plates. The method combines flattening, hood annealing, and roller straightening processes to eliminate internal stresses in high-strength steel. However, its disadvantage is that flattening and hood annealing are required, which complicates the process and increases production costs. Summary of the Invention

[0006] In order to solve the above-mentioned technical problems existing in the prior art, the present invention provides a 750MPa-grade low-stress hot-rolled high-strength beam steel and a manufacturing method thereof. By optimizing the chemical composition of the steel and combining the original ingot processing, rolling and segmented cooling process, the grain structure is effectively refined, the internal residual stress of the steel plate is reduced, and the strength, toughness and formability of the steel are improved. The yield strength of the produced beam steel is ≥700MPa, the tensile strength is ≥750MPa, and the elongation is ≥20%, meeting the demand for high-performance beam steel in industries such as automobile manufacturing and engineering machinery.

[0007] In one aspect of the present invention, a method for manufacturing 750 MPa grade low stress hot rolled high strength beam steel is provided, comprising the following steps:

[0008] (1) Smelting and continuous casting

[0009] The chemical composition of the steel is optimized and controlled by smelting in terms of mass percentage to be: C: 0.04-0.08%, Si≤0.15%, Mn: 1.3-1.8%, P≤0.015%, S≤0.004%, all-aluminum Alt≥0.02%, N≤0.008%, Nb: 0.025-0.055%, Ti: 0.10-0.15%, and the remainder is Fe and unavoidable impurity elements. The molten steel obtained by smelting and meeting the above composition requirements is then continuously cast into ingots.

[0010] (2) Slow cooling and uniform heating of the ingot

[0011] The billet is air-cooled after hot delivery, the billet temperature into the furnace is controlled above 300℃, the billet soaking temperature is controlled at 1250~1290℃, and the furnace dwelling time is controlled at 1~1.2mm / min according to the billet thickness;

[0012] (3) Rolling

[0013] The starting temperature of rough rolling is controlled at 1100-1150°C, the cumulative deformation of rough rolling is controlled at ≥70%, and the ingot is rough-rolled into an intermediate billet; the starting temperature of finishing rolling is controlled at 950-1080°C, the finishing temperature of finishing rolling is controlled at 810-880°C, and the intermediate billet is finish-rolled into a finished steel plate of target thickness;

[0014] (4) Segmented cooling

[0015] The finished steel plate is cooled by a staged cooling process, wherein the first stage of cooling is to water cool the steel plate to 710-780°C at a cooling rate of 10-25°C / s, followed by air cooling for 4-8s, and then the second stage of cooling is to water cool the steel plate to the coiling temperature of 550-620°C at a cooling rate of 30-50°C / s.

[0016] Furthermore, in the above-mentioned method for manufacturing 750 MPa grade low stress hot-rolled high strength beam steel, in the slow cooling and soaking steps of the ingot, the air cooling placement time is controlled to be ≥6 hours.

[0017] Furthermore, in the above-mentioned method for manufacturing 750 MPa grade low stress hot-rolled high strength beam steel, in the rolling step, the number of rough rolling passes is controlled to be 5 to 7 passes, and the number of finishing rolling passes is controlled to be 7 passes.

[0018] Furthermore, in the above-mentioned method for manufacturing 750 MPa grade low stress hot rolled high strength beam steel, in the smelting and continuous casting steps, the thickness of the ingot is controlled to be 180 to 230 mm.

[0019] In another aspect of the present invention, a 750MPa grade low stress hot-rolled high strength beam steel is provided. The 750MPa grade low stress hot-rolled high strength beam steel is produced using the above-mentioned 750MPa grade low stress hot-rolled high strength beam steel manufacturing method. The chemical composition of the 750MPa grade low stress hot-rolled high strength beam steel is as follows by mass percentage: C: 0.04-0.08%, Si≤0.15%, Mn: 1.3-1.8%, P≤0.015%, S≤0.004%, all-aluminum Alt≥0.02%, N≤0.008%, Nb: 0.025-0.055%, Ti: 0.10-0.15%, and the rest is Fe and unavoidable impurity elements.

[0020] Furthermore, the yield strength of the 750 MPa grade low stress hot rolled high strength beam steel is ≥700 MPa, the tensile strength is ≥750 MPa, and the elongation is ≥20%.

[0021] The 750MPa grade low stress hot rolled high strength beam steel and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0022] In terms of chemical composition design, the use of Mo alloys is avoided to reduce production costs, and the material strength is improved through (Ti, Nb)C composite precipitation;

[0023] In terms of process control, slow cooling of the ingot before entering the furnace achieves pre-refinement of the microstructure. Precise setting of the soaking temperature and dwell time ensures sufficient dissolution of Ti compounds, laying the foundation for subsequent precipitation behavior. Rational arrangement of the temperature, pass number, and deformation amount for roughing and finishing rolling further optimizes the microstructure. An innovative staged cooling process, through the control of cooling rate and air cooling time at different stages, effectively promotes the precipitation of (Ti, Nb) C in a fine and dispersed form, refines the grains, and significantly reduces the residual stress within the steel plate.

[0024] The low-stress hot-rolled high-strength beam steel of the present invention has a yield strength of ≥700MPa, a tensile strength of ≥750MPa, and an elongation of ≥20%. It not only has high strength, but also has good toughness and formability, and fully meets the use and performance requirements of high-performance beam steel in industries such as automobile manufacturing and engineering machinery. It can effectively reduce deformation and cracking problems in the manufacturing process of automobile beams, engineering machinery frames, etc., improve product quality and production efficiency, reduce production costs, and has good economic and social benefits. DETAILED DESCRIPTION

[0025] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0026] The chemical composition of the 750MPa grade low stress hot rolled high strength beam steel of the present invention is optimized and controlled in terms of mass percentage to be: C: 0.04-0.08%, Si≤0.15%, Mn: 1.3-1.8%, P≤0.015%, S≤0.004%, all-aluminum Alt≥0.02%, N≤0.008%, Nb: 0.025-0.055%, Ti: 0.10-0.15%, and the remainder is Fe and unavoidable impurity elements.

[0027] In the 750 MPa grade low stress hot rolled high strength beam steel of the present invention, the use of Mo alloy is avoided to reduce production costs, and the synergistic effect of Nb and Ti alloy elements is utilized by optimizing their contents to enhance the composite precipitation effect of (Ti, Nb) C, thereby achieving effective grain refinement and strengthening, thereby improving material strength.

[0028] The method for manufacturing 750MPa grade low stress hot rolled high strength beam steel of the present invention comprises the following steps:

[0029] (1) Smelting and continuous casting

[0030] Through smelting, the chemical composition of the steel is optimized and controlled in terms of mass percentage to be: C: 0.04-0.08%, Si≤0.15%, Mn: 1.3-1.8%, P≤0.015%, S≤0.004%, all-aluminum Alt≥0.02%, N≤0.008%, Nb: 0.025-0.055%, Ti: 0.10-0.15%, and the rest are Fe and unavoidable impurity elements. The molten steel obtained by smelting and meeting the above composition requirements is then continuously cast into ingots with a thickness of 180-230 mm.

[0031] (2) Slow cooling and uniform heating of the ingot

[0032] After hot delivery, the ingot is air-cooled for a period of ≥6 hours. The ingot temperature is lowered to below the Arl temperature through air cooling, which causes microstructural transformation and microstructure refinement, thus laying a good microstructural foundation for the subsequent rolling process.

[0033] The ingot entering the furnace is controlled at above 300°C, the ingot soaking temperature is controlled at 1250-1290°C, and the dwelling time is controlled at 1-1.2 mm / min according to the ingot thickness. By precisely controlling the soaking temperature and dwelling time, the Ti compounds in the steel are fully dissolved, creating favorable conditions for the uniform and refined precipitation of (Ti, Nb)C after subsequent rolling.

[0034] (3) Rolling

[0035] The starting temperature of rough rolling is controlled at 1100-1150°C, the number of rough rolling passes is controlled at 5-7 passes, and the cumulative deformation of rough rolling is controlled at ≥70%. Thus, the ingot is rough rolled into an intermediate ingot. The large deformation of rough rolling is used to break up the as-cast structure and further refine the grains.

[0036] The starting temperature of finishing rolling is controlled at 950-1080℃, the number of finishing passes is controlled at 7 passes, and the final temperature of finishing rolling is controlled at 810-880℃. In this way, the intermediate billet is finish-rolled into the finished steel plate of the target thickness. By reasonably controlling the finishing temperature and passes, the structure and performance of the steel can be further improved.

[0037] (4) Segmented cooling

[0038] The finished steel plates are cooled using a staged cooling process. In the first stage, the steel plates are water-cooled to a temperature of 710-780°C at a cooling rate of 10-25°C / s, followed by air cooling for 4-8 seconds to relieve some internal stress. The second stage, water-cooling the steel plates at a cooling rate of 30-50°C / s, brings them down to the coiling temperature of 550-620°C. By controlling the cooling rate, temperature range, and air cooling time at different stages, the precipitation of (Ti,Nb)C in a fine, dispersed form is effectively promoted, refining the grains and significantly reducing residual stress within the steel plates.

[0039] The 750 MPa grade low stress hot rolled high strength beam steel and its manufacturing method of the present invention are further described below with reference to specific embodiments.

[0040] Example 1

[0041] The method for manufacturing 750 MPa-grade low-stress hot-rolled high-strength beam steel according to Example 1 of the present invention specifically includes:

[0042] (1) Smelting and continuous casting

[0043] Through smelting, the chemical composition of the steel is optimized and controlled in mass percentage to: C: 0.070%, Si: 0.1%, Mn: 1.7%, P: 0.01%, S: 0.003%, all-aluminum Alt: 0.03%, N: 0.006%, Nb: 0.050%, Ti: 0.12%, and the rest are Fe and unavoidable impurity elements. The molten steel obtained by smelting and meeting the above composition requirements is then continuously cast into ingots with a thickness of 180 mm.

[0044] (2) Slow cooling and uniform heating of the ingot

[0045] The ingots are air-cooled after being red-delivered, the ingots are heated to a temperature of 1290°C, and the residence time is 3 hours.

[0046] (3) Rolling

[0047] After the ingot leaves the heating furnace, it is descaled once with high-pressure water, and then subjected to rough rolling and finish rolling. Rough rolling is performed on a 1-stand rolling mill, and after 5 rough rolling passes, the ingot is rough-rolled into an intermediate billet with a thickness of 50 mm, and the rough rolling outlet temperature is 1070°C. Finish rolling is performed on a 7-stand rolling mill, and after 7 finish rolling passes, the intermediate billet is finish-rolled into a finished steel plate with a target thickness of 10.0 mm. The start rolling temperature of the finishing rolling is 1000°C, the finishing rolling outlet speed is controlled at 4 m / s, and the finishing rolling final temperature is 820°C.

[0048] (4) Segmented cooling

[0049] After final rolling, the finished steel plate is cooled by a staged cooling process. In the first stage, the steel plate is water-cooled to 760°C at a cooling rate of 15°C / s. After air cooling for 7 seconds, the second stage is water-cooled to the coiling temperature of 560°C at a cooling rate of 40°C / s.

[0050] After actual production testing, the beam steel manufactured using Example 1 of the present invention has a yield strength of 722 MPa, a tensile strength of 770 MPa, and an elongation of 21%, which meets the performance requirements of the automotive manufacturing, engineering machinery and other industries for beam steel.

[0051] Example 2

[0052] The method for manufacturing 750 MPa-grade low-stress hot-rolled high-strength beam steel according to Example 2 of the present invention specifically includes:

[0053] (1) Smelting and continuous casting

[0054] Through smelting, the chemical composition of the steel is optimized and controlled in mass percentage to: C: 0.060%, Si: 0.12%, Mn: 1.4%, P: 0.012%, S: 0.002%, all-aluminum Alt: 0.025%, N: 0.007%, Nb: 0.035%, Ti: 0.11%, and the rest are Fe and unavoidable impurity elements. The molten steel obtained by smelting and meeting the above composition requirements is then continuously cast into ingots with a thickness of 230 mm.

[0055] (2) Slow cooling and uniform heating of the ingot

[0056] The ingots are air-cooled after being red-delivered, the ingots are heated to a temperature of 1250°C, and the furnace dwelling time is 3.5 hours.

[0057] (3) Rolling

[0058] After the ingot leaves the heating furnace, it is descaled once with high-pressure water, and then subjected to rough rolling and finish rolling. Rough rolling is performed on a 1-stand rolling mill, and after 7 rough rolling passes, the ingot is rough-rolled into an intermediate billet with a thickness of 40 mm, and the rough rolling outlet temperature is 1050°C. Finish rolling is performed on a 7-stand rolling mill, and after 7 finish rolling passes, the intermediate billet is finish-rolled into a finished steel plate with a target thickness of 2.0 mm. The finish rolling outlet speed is controlled at 9 m / s, and the finish rolling temperature is 850°C.

[0059] (4) Segmented cooling

[0060] After final rolling, the finished steel plate is cooled by a staged cooling process. In the first stage, the steel plate is water-cooled to 710°C at a cooling rate of 25°C / s. After air cooling for 4 seconds, the steel plate is water-cooled to the coiling temperature of 600°C at a cooling rate of 50°C / s in the second stage.

[0061] After actual production testing, the beam steel manufactured using Example 2 of the present invention has a yield strength of 745 MPa, a tensile strength of 810 MPa, and an elongation of 20%, which meets the performance requirements of the automotive manufacturing, engineering machinery and other industries for beam steel.

[0062] In summary, the 750MPa grade low stress hot rolled high strength beam steel and its manufacturing method of the present invention show significant advantages in many aspects compared with the existing technology: in terms of chemical composition design, the use of Mo alloy is avoided to reduce production costs, and the material strength is improved by composite precipitation of (Ti, Nb) C; in terms of process control, slow cooling before the ingot is put into the furnace to achieve pre-refinement of the structure, precise setting of the soaking temperature and the dwelling time in the furnace, ensuring that the Ti compound is fully dissolved, laying the foundation for subsequent precipitation behavior, reasonable arrangement of the temperature, pass number and deformation amount of rough rolling and finishing rolling, further optimizing the structure, and innovatively adopting a segmented cooling process. By controlling the cooling rate and air cooling time at different stages, the precipitation of (Ti, Nb) C in a fine and dispersed form is effectively promoted, the grains are refined, and the residual stress inside the steel plate is greatly reduced. The low-stress hot-rolled high-strength beam steel of the present invention has a yield strength of ≥700MPa, a tensile strength of ≥750MPa, and an elongation of ≥20%. It not only has high strength, but also has good toughness and formability, and fully meets the use and performance requirements of high-performance beam steel in industries such as automobile manufacturing and engineering machinery. It can effectively reduce deformation and cracking problems in the manufacturing process of automobile beams, engineering machinery frames, etc., improve product quality and production efficiency, reduce production costs, and has good economic and social benefits.

[0063] It should be noted that, unless otherwise specified, the noun terms herein have the meanings commonly understood by those skilled in the art. Moreover, when a numerical range is disclosed herein, the above range is deemed to be continuous and includes the minimum and maximum values ​​of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values ​​of the range is included. In addition, when multiple range description features are provided, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0064] It should also be noted that, in this article, the term "comprise" or any other variation thereof is intended to cover non-exclusive inclusion, so that an article or device that includes a series of elements includes not only those elements, but also includes other elements that are not explicitly listed, or also includes elements that are inherent to such article or device.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A method for manufacturing 750MPa grade low stress hot rolled high strength beam steel, characterized in that: The following steps are involved: (1) Smelting and continuous casting The chemical composition of the steel is optimized and controlled by smelting in terms of mass percentage to be: C: 0.04-0.08%, Si≤0.15%, Mn: 1.3-1.8%, P≤0.015%, S≤0.004%, all-aluminum Alt≥0.02%, N≤0.008%, Nb: 0.025-0.055%, Ti: 0.10-0.15%, and the remainder is Fe and unavoidable impurity elements. The molten steel obtained by smelting and meeting the above composition requirements is then continuously cast into ingots. (2) Slow cooling and uniform heating of the ingot The billet is air-cooled after hot delivery, the billet temperature into the furnace is controlled above 300℃, the billet soaking temperature is controlled at 1250~1290℃, and the furnace dwelling time is controlled at 1~1.2mm / min according to the billet thickness; (3) Rolling The starting temperature of rough rolling is controlled at 1100-1150°C, the cumulative deformation of rough rolling is controlled at ≥70%, and the ingot is rough-rolled into an intermediate billet; the starting temperature of finishing rolling is controlled at 950-1080°C, the finishing temperature of finishing rolling is controlled at 810-880°C, and the intermediate billet is finish-rolled into a finished steel plate of target thickness; (4) Segmented cooling The finished steel plate is cooled by a staged cooling process, wherein the first stage of cooling is to water cool the steel plate to 710-780°C at a cooling rate of 10-25°C / s, followed by air cooling for 4-8s, and then the second stage of cooling is to water cool the steel plate to the coiling temperature of 550-620°C at a cooling rate of 30-50°C / s.

2. The method for manufacturing 750MPa grade low stress hot rolled high strength beam steel according to claim 1, characterized in that: During the slow cooling and soaking steps of the ingot, the air cooling time is controlled to be ≥6h.

3. The method for manufacturing 750MPa grade low stress hot rolled high strength beam steel according to claim 1, characterized in that: In the rolling step, the rough rolling passes are controlled to be 5 to 7 passes, and the finishing rolling passes are controlled to be 7 passes.

4. The method for manufacturing 750MPa grade low stress hot rolled high strength beam steel according to claim 1, characterized in that: During the smelting and continuous casting steps, the thickness of the ingot is controlled to be 180 to 230 mm.

5. A 750MPa grade low stress hot rolled high strength beam steel, characterized in that: The 750MPa grade low stress hot rolled high strength beam steel is produced by the 750MPa grade low stress hot rolled high strength beam steel manufacturing method according to any one of claims 1 to 4, and the chemical composition of the 750MPa grade low stress hot rolled high strength beam steel is as follows by mass percentage: C: 0.04-0.08%, Si≤0.15%, Mn: 1.3-1.8%, P≤0.015%, S≤0.004%, all aluminum Alt≥0.02%, N≤0.008%, Nb: 0.025-0.055%, Ti: 0.10-0.15%, and the rest is Fe and unavoidable impurity elements.

6. The 750MPa grade low stress hot rolled high strength beam steel according to claim 5, characterized in that: The 750MPa-grade low-stress hot-rolled high-strength beam steel has a yield strength of ≥700MPa, a tensile strength of ≥750MPa, and an elongation of ≥20%.