Control method for improving strength of 510 MPa girder steel

Abstract

The invention discloses a control method for improving the strength of 510 MPa grade girder steel. The control method comprises the following steps: strictly controlling the content of N in the 510 MPa grade girder steel to be 25 ppm to 40 ppm; the 510 MPa grade girder steel further comprises the following chemical components in percentage by weight: 0.06%-0.08% of C, less than or equal to 0.10% of Si, 0.80%-1.00% of Mn, less than or equal to 0.017% of P, less than or equal to 0.010% of S, 0.020%-0.050% of Alt, 0.025%-0.045% of Nb and the balance of Fe and inevitable impurities. By means of the method, the yield strength and the tensile strength of the girder steel can be stably improved by 15-30 MPa, and the ductility and other plasticity indexes of the girder steel are not affected.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical plate production technology, specifically relating to a control method for improving the strength of 510MPa beam steel. Background Technology

[0002] 510MPa grade beam steel is a crucial material for manufacturing key structural components such as automotive longitudinal beams and cross beams, requiring high strength, high fatigue performance, and good cold formability. Currently, methods to improve steel strength mainly include adding microalloying elements (such as Nb, V, and Ti), controlled rolling, and controlled cooling processes. Nitrogen (N), as a common residual element in steel, has a dual role. On the one hand, it can form nitrides with elements such as V and Ti, playing a precipitation strengthening role; on the other hand, excessively high free nitrogen content can deteriorate the toughness and aging properties of steel. In the production of 510MPa grade beam steel, nitrogen content is usually considered an impurity element with a controlled upper limit, but the quantitative relationship between its precise control within a specific range and strength is still insufficiently studied. Existing technologies typically control nitrogen content within 50ppm or a wider range, failing to fully utilize its positive contribution to strength. Therefore, developing a method to effectively improve the strength of 510MPa grade beam steel through simple and low-cost composition adjustment has significant industrial application value.

[0003] Patent publication number CN 112030071 A discloses a 510MPa grade high-toughness automotive beam steel and its preparation method. This patent proposes a 510MPa grade high-toughness automotive beam steel and its preparation method, wherein the chemical composition contains N ≤ 0.060% (i.e., 60ppm), but does not explicitly mention the specific impact of lower nitrogen content on strength improvement. In its preparation method, the converter process requires N ≤ 30ppm, but the nitrogen increment in subsequent refining and continuous casting processes may increase the final nitrogen content. This invention precisely controls the nitrogen content below 40ppm. Compared to the nitrogen content range (≤ 60ppm) and nitrogen increment control in the preparation process of the aforementioned literature, the control of nitrogen content is more stringent and precise, demonstrating technological progress and novelty in nitrogen content control.

[0004] Patent publication number CN 114214481 A discloses a method for reducing nitrogen content in steel. This patent provides a method to reduce nitrogen content in steel by smelting, deoxidation after tapping, RH refining, and fully protective casting, achieving a nitrogen content in slabs as low as 26-32 ppm. This method primarily focuses on reducing nitrogen content but does not explicitly mention the specific effect of reducing nitrogen content on the strength improvement of specific steel types (such as 510MPa beam steel). This invention reduces nitrogen content from 50 ppm to below 40 ppm. Although it does not reach the lowest nitrogen content (26-32 ppm) disclosed in the original patent, the focus of this invention is on improving the strength of 510MPa beam steel through precise control of nitrogen content, rather than simply reducing nitrogen content. Furthermore, the nitrogen content control range of this invention (25 ppm to 40 ppm) is closer to actual production needs and has greater practicality.

[0005] Patent publication number CN 119332049 A discloses a method for controlling nitrogen content in electric arc furnace (EAF) steel production. This method controls nitrogen content in molten steel to below 40 ppm by reducing nitrogen absorption and increasing nitrogen removal. While focusing primarily on nitrogen content control during EAF smelting, it doesn't explicitly address the specific effect of reducing nitrogen content on strength improvement in particular steel types (such as 510 MPa beam steel). This invention also controls nitrogen content below 40 ppm, but it places greater emphasis on strength improvement for the specific 510 MPa beam steel. By precisely controlling nitrogen content, this invention achieves increased strength while maintaining steel toughness, demonstrating greater specificity and practicality. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a control method for improving the strength of 510MPa grade beam steel. By precisely controlling the nitrogen content, the strength of beam steel can be significantly improved without changing other components and processes, thus solving the problems of high cost and complex processes in the prior art.

[0007] This invention is mainly achieved through the following technical solutions.

[0008] In one aspect, the present invention provides a method for controlling the strength of 510MPa grade beam steel, which includes strictly controlling the N content in the 510MPa grade beam steel to 25ppm to 40ppm (i.e. 0.0025% to 0.0040%).

[0009] The chemical composition of the 510MPa grade beam steel, by weight percentage, also includes: C: 0.06%~0.08%, Si≤0.10%, Mn: 0.80%~1.00%, P≤0.017%, S≤0.010%, Alt: 0.020%~0.050%, Nb: 0.025%~0.045%, with the balance being Fe and unavoidable impurities.

[0010] In some embodiments, the method includes strictly controlling the nitrogen content in the 510MPa grade beam steel to between 30ppm and 40ppm.

[0011] In some embodiments, the method includes the following process steps:

[0012] (1) Raw material pretreatment: Low nitrogen scrap steel and pig iron with nitrogen content ≤30 ppm are selected as smelting raw materials to avoid the introduction of excessive nitrogen elements into the raw materials;

[0013] (2) Converter blowing: control the oxygen content at the end of the blowing process to ≤500 ppm. The bottom blowing argon gas is used for stirring during the blowing process to reduce the contact time between the molten steel and the air and suppress the absorption of nitrogen.

[0014] (3) Refining treatment: Refining and vacuum degassing are carried out under argon protection to make the nitrogen content in the molten steel reach 25-40 ppm;

[0015] (4) Continuous casting: The continuous casting process is carried out by full protection casting process to obtain a continuous casting slab with nitrogen content that meets the requirements;

[0016] (5) Rolling and cooling: The continuously cast slab is heated, rolled under controlled conditions, cooled under controlled conditions and coiled to obtain the beam steel.

[0017] In some embodiments, the flow rate of the bottom-blown argon gas in step (2) is 10 to 15 L / min.

[0018] In some embodiments, the refining time in step (3) is 30 to 40 minutes, and the purity of the argon gas under argon protection is ≥99.99%.

[0019] In some implementations, the continuous casting speed in step (4) is 1.2 to 1.6 m / min, and a low-nitrogen mold flux is used.

[0020] In some embodiments, in step (5), the slab heating temperature is controlled at 1230-1250℃, the heating time is 180-220 minutes, the homogenization temperature is 1220-1260℃, the homogenization time is 30-40 minutes, and the furnace exit temperature is 1240-1250℃; the finishing rolling start temperature is controlled at 1000-1020℃, and the final rolling temperature is controlled at 880-920℃; the cooling rate is controlled at 12-16℃ / s, and the coiling temperature is controlled at 580-620℃.

[0021] On the other hand, the present invention provides a 510MPa grade main beam steel, the chemical composition of which, by mass percentage, includes: C: 0.06% to 0.08%, Si: ≤0.10%, Mn: 0.80% to 1.00%, P: ≤0.017%, S: ≤0.010%, Alt: 0.020% to 0.050%, Nb: 0.025% to 0.045%, with the balance being Fe and unavoidable impurities; characterized in that the nitrogen (N) content in the main beam steel is 25 ppm to 40 ppm.

[0022] In some embodiments, the nitrogen (N) content is 30 ppm to 40 ppm.

[0023] In some embodiments, the 510MPa grade beam steel has a yield strength ≥560 MPa, a tensile strength ≥610MPa, and an elongation after fracture ≥25%.

[0024] This invention optimizes the nitrogen content from 50 ppm in traditional 510MPa grade beam steel production methods to below 40 ppm (25 ppm to 40 ppm), reducing the number and size of nitrided inclusions in the steel and improving the density and continuity of the steel matrix, thereby significantly enhancing strength performance. Simultaneously, this method does not alter the original chemical composition and production process of the beam steel, requires no additional equipment, effectively controls production costs, and ensures that other key properties such as toughness and weldability are not affected, fully meeting the application requirements of 510MPa grade beam steel.

[0025] Compared with the prior art, the present invention has the following significant advantages:

[0026] Significant results: By simply adjusting the nitrogen content from 50 ppm to 25 ppm to 40 ppm, the yield strength and tensile strength of the main beam steel can be steadily increased by 15~30 MPa without affecting its elongation and other plasticity indicators.

[0027] Low cost: This invention does not require the addition of new alloying elements or changes to major processes such as rolling and cooling. It can be achieved simply by optimizing the control of degassing and protection during the steelmaking process, which increases production costs by almost nothing.

[0028] The process is simple and easy to implement: This method is applicable to existing converter or electric furnace smelting processes. By controlling the steelmaking raw materials, strengthening vacuum degassing operations, or optimizing protective casting, the nitrogen content can be precisely controlled, making it easy to promote on a large scale in steel enterprises.

[0029] Stable performance: By controlling the nitrogen content, performance fluctuations caused by nitrogen fluctuations are reduced, and the consistency of product performance is improved. Detailed Implementation

[0030] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0031] Examples 1-4 and Comparative Example 1

[0032] 510MPa grade beam steel was prepared by smelting, continuous casting, heating, controlled rolling, and controlled cooling according to the chemical composition (weight percentage, nitrogen content in ppm) shown in Table 1 below. The production process parameters remained consistent and included the following processes:

[0033] Raw material pretreatment: Low-nitrogen scrap steel and pig iron are selected as smelting raw materials, and the nitrogen content in the raw materials is ≤30ppm to avoid the introduction of excessive nitrogen elements.

[0034] Converter blowing: Control the oxygen content at the end of converter blowing to ≤500ppm. Use bottom blowing argon gas for stirring during the blowing process. The argon gas flow rate is 12 L / min to reduce the contact time between molten steel and air and suppress nitrogen absorption.

[0035] Refining process: Argon gas is used for protection throughout the refining process, with argon purity ≥99.99% and refining time controlled within 35 minutes. Nitrogen elements in the molten steel are further removed through vacuum degassing to reduce the nitrogen content to the required level in each embodiment or comparative example.

[0036] Continuous casting: The continuous casting process adopts a sealed casting process. Low-nitrogen protective slag is selected for the mold to prevent air from entering the molten steel during the casting process and causing the nitrogen content to rise. The continuous casting speed is controlled at 1.4 m / min to ensure uniform billet quality.

[0037] Rolling and Cooling: The continuously cast slab is heated, rolled under controlled conditions, cooled under controlled conditions, and coiled to obtain the beam steel. The controlled conditions are as follows: slab heating temperature 1240℃, heating time 200 minutes, soaking temperature 1240℃, soaking time 35 minutes, furnace exit temperature 1245℃, finishing rolling start temperature 1010℃, finishing rolling temperature 900℃, cooling rate 15℃ / s, and coiling temperature 600℃.

[0038] Table 1: Chemical composition (wt%) and properties of each embodiment and comparative example

[0039]

[0040] As shown in Table 1, with identical basic chemical composition and process, the yield strength and tensile strength of the steel significantly increased as the nitrogen content decreased from 50 ppm in Comparative Example 1 to 40 ppm, 35 ppm, 30 ppm, and 25 ppm in Examples 1-4 of this invention. The yield strength and tensile strength of Examples 1-4 increased by 15-30 MPa and 15-30 MPa respectively compared to Comparative Example 1, while the elongation remained essentially unchanged, indicating that this invention effectively improved strength without compromising the material's plasticity. However, in actual production, it is difficult to control the N content below 25 ppm, and the cost is high. Therefore, this invention preferably controls the N content in 510 MPa grade beam steel within the range of 30 ppm-40 ppm, which can save costs. Moreover, compared to the traditional method of controlling the N content to around 50 ppm, it can significantly improve the strength of 510 MPa grade beam steel without compromising the material's plasticity.

[0041] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. 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 method for controlling the strength of 510MPa grade beam steel, characterized in that, The method includes strictly controlling the N content in the 510MPa grade beam steel to between 25ppm and 40ppm. The chemical composition of the 510MPa grade beam steel, by weight percentage, also includes: C: 0.06%~0.08%, Si≤0.10%, Mn: 0.80%~1.00%, P≤0.017%, S≤0.010%, Alt: 0.020%~0.050%, Nb: 0.025%~0.045%, with the balance being Fe and unavoidable impurities.

2. The control method according to claim 1, characterized in that, The method includes strictly controlling the N content in the 510MPa grade beam steel to between 30ppm and 40ppm.

3. The control method according to claim 1 or 2, characterized in that, The method includes the following process steps: (1) Raw material pretreatment: Low nitrogen scrap steel and pig iron with nitrogen content ≤30 ppm are selected as smelting raw materials; (2) Converter blowing: control the oxygen content at the end of the blowing process to ≤500 ppm, and use bottom blowing argon gas for stirring during the blowing process; (3) Refining treatment: Refining and vacuum degassing are carried out under argon protection to make the nitrogen content in the molten steel reach 25-40 ppm; (4) Continuous casting: The continuous casting process is carried out by full protection casting process to obtain a continuous casting slab with nitrogen content that meets the requirements; (5) Rolling and cooling: The continuously cast slab is heated, rolled under controlled conditions, cooled under controlled conditions and coiled to obtain the beam steel.

4. The control method according to claim 3, characterized in that, The flow rate of the bottom-blown argon gas in step (2) is 10-15 L / min.

5. The control method according to claim 3, characterized in that, The refining time in step (3) is 30 to 40 minutes, and the purity of the argon gas under argon protection is ≥99.99%.

6. The control method according to claim 3, characterized in that, In step (4), the continuous casting speed is 1.2 to 1.6 m / min, and a low-nitrogen mold protective slag is used.

7. The control method according to claim 3, characterized in that, In step (5), the slab heating temperature is controlled at 1230-1250℃, the heating time is 180-220 minutes, the heat soaking temperature is 1220-1260℃, the heat soaking time is 30-40 minutes, and the furnace exit temperature is 1240-1250℃; the finishing rolling start temperature is controlled at 1000-1020℃, the finishing rolling temperature is 880-920℃; the cooling rate is controlled at 12-16℃ / s, and the coiling temperature is controlled at 580-620℃.

8. A 510MPa grade beam steel, the chemical composition of which, by mass percentage, includes: C： The main beam steel contains 0.06% to 0.08% nitrogen, Si: ≤0.10%, Mn: 0.80% to 1.00%, P: ≤0.017%, S: ≤0.010%, Alt: 0.020% to 0.050%, Nb: 0.025% to 0.045%, with the balance being Fe and unavoidable impurities; characterized in that the nitrogen (N) content in the main beam steel is 25 ppm to 40 ppm.

9. The 510MPa grade beam steel according to claim 8, characterized in that, The nitrogen (N) content is 30 ppm to 40 ppm.

10. The 510MPa grade beam steel according to claim 8 or 9, characterized in that, Its yield strength is ≥560 MPa, tensile strength is ≥610 MPa, and elongation after fracture is ≥25%.

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