Hot-rolled beam steel with stable performance and preparation method thereof

Abstract

The present invention belongs to the field of steel material technology, specifically relating to a hot-rolled beam steel with stable performance and a preparation method thereof. The chemical composition and weight percentage of the beam steel are as follows: C 0.05-0.07%, Si 0.10-0.20%, Mn 1.10-1.40%, Al 0.020-0.050%, Nb 0.030-0.045%, V 0.040-0.055%, Ti 0.015-0.030%, P ≤ 0.015%, S ≤ 0.003%, N 0.0030-0.0060%, O 0.0010-0.0030%, with the balance being Fe and unavoidable inclusions. The present invention significantly improves the product's formability and performance stability by combining Nb and V with a low-Ti alloy component. The strength fluctuation range of products from the same furnace or batch does not exceed 50 MPa, and the impact energy fluctuation range does not exceed 30 J.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel materials, and in particular relates to a hot-rolled beam steel with stable performance and a preparation method thereof. Background Art

[0002] With the rapid development of the automotive industry, automotive beam steel is gradually moving towards higher strength and safety. Research on lightweight automotive technology has become a major trend in modern automotive design and manufacturing. Currently, the most widely used steel in the market has been upgraded to 600-700MPa grade hot-rolled beam steel, with a composition design primarily based on Nb-Ti composites, successfully replacing beam steels made of materials such as Q355B and 510L. However, with large-scale production, forming cracking, bending cracking, and large performance fluctuations are inevitable. Strength within the same furnace or batch can vary by up to 120MPa, and impact energy can vary by up to 60J, seriously impacting production efficiency and yield rates. This also creates significant difficulties for downstream OEMs during production and use, such as warping, bending, and excessive roll springback angle tolerances, and frequent adjustments to mold parameters. Chinese patent document CN110331344A discloses an automobile beam steel with stable strength performance Rm≥600MPa. The invention describes Nb-Ti composite strengthening, and the strength fluctuation range does not exceed 100MPa. The strength fluctuation of the embodiment is about 50MPa.

[0003] In summary, the market urgently needs a high-quality hot-rolled automobile beam steel with excellent forming performance, good strength and toughness matching, and stable performance. Summary of the Invention

[0004] The present invention provides a 650MPa-grade high-quality automobile beam steel with a thickness of 4.0mm-12.0mm and stable and uniform performance.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] The present invention provides a hot-rolled beam steel with stable performance. The chemical composition of the beam steel and the weight percentage thereof are as follows: C 0.05-0.07%, Si 0.10-0.20%, Mn 1.10-1.40%, Al 0.020-0.050%, Nb 0.030-0.045%, V 0.040-0.055%, Ti 0.015-0.030%, P≤0.015%, S≤0.003%, N 0.0030-0.0060%, O 0.0010-0.0030%, and the balance is Fe and unavoidable inclusions.

[0007] In the above technical solution, further, the chemical composition of the beam steel and its weight percentage are: C 0.05-0.07%, Si 0.10-0.20%, Mn 1.20-1.40%, Al 0.025-0.045%, Nb 0.033-0.043%, V 0.040-0.055%, Ti 0.018-0.030%, P 0.010-0.015%, S≤0.003%, N 0.0030-0.0055%, O 0.0015-0.0025%, and the balance is Fe and unavoidable inclusions.

[0008] In the above technical solution, further, the specification of the beam steel is 4.0mm~12.0mm.

[0009] In the above technical solution, further, the strength difference of the beam steel is ≤30MPa.

[0010] In the above technical solution, further, the method includes steelmaking, hot rolling, and coiling, and the steelmaking includes a raw material pretreatment process, a converter smelting process, a refining process, and a continuous casting process; the hot rolling process includes a heating furnace heating process, a rough rolling process, and a finishing rolling process; deoxidation and alloying are performed during the steel tapping process of the converter smelting process, and the order of adding alloys is: Si-Mn-Al-Nb-V; titanium iron is added in the refining process, and the mass percentage of Ti element in the molten steel is 0.018-0.030%, and other alloying elements are added and fine-tuned to meet the target range; in the finishing rolling process, when the thickness of the hot-rolled product is <6.0mm, the final rolling temperature is 870-900℃; when the thickness of the hot-rolled product is ≥6.0mm, the final rolling temperature is 840-870℃.

[0011] In the above technical solution, further, in the refining process, calcium silicon wire is used for treatment, and 500 meters of CaSi wire is fed.

[0012] In the above technical solution, further, in the hot furnace heating process, the heating temperature is 1220-1260°C, the soaking period in the furnace time is ≥90 minutes, and the total time in the furnace is ≥170 minutes.

[0013] In the above technical solution, further, in the rough rolling process, the rough rolling passes are in 3+3 mode, R1 is rolled back and forth 3 times, and R2 is rolled back and forth 3 times; the R1 starting rolling temperature is ≥1130℃, and water is sprayed for descaling at 1130℃ at high temperature; the intermediate billet thickness is 30~45mm, and the cumulative reduction rate in the rough rolling stage is greater than 75%.

[0014] In the above technical solution, further, in the finishing rolling process, the F1 inlet temperature is ≤1100°C, the F1 reduction ratio is ≥45%, and the F2 reduction ratio is ≥25%.

[0015] In the above technical solution, further, in the coiling process, the coiling temperature is 580-630°C, and the cooling rate is 25-35°C / s.

[0016] The main elements have the following functions:

[0017] C: 0.05-0.07wt%. Carbon is used to form sufficient carbide strengthening phase. The higher the carbon content, the greater the strength and hardness, but the lower the toughness. For every 0.1% increase in carbon content in steel, the cold-brittle transition temperature increases by about 13.9°C.

[0018] Si: 0.10-0.20wt%, Si has a strong affinity with O and is a strong deoxidizing element. It exists in the steel in the form of solid solution. Si can improve the strength, fatigue limit, corrosion resistance and wear resistance of steel. However, if the Si content is too high, oxides are easily generated during hot rolling, which reduces the surface quality of the steel.

[0019] Mn: 1.10-1.40wt%, Mn exists in the steel in solid solution state and is a solid solution strengthening element. It mainly exists in the steel in the form of MnS. Manganese is usually added as a desulfurizer and deoxidizer during the steel smelting process. Manganese and sulfur can prevent hot brittleness.

[0020] Nb: 0.030-0.045%. It has extremely strong bonds with carbon, nitrogen, and oxygen, forming extremely stable compounds with them, thereby refining grains. It also increases the austenite recrystallization temperature and recrystallization time. However, when the Nb content exceeds 0.06%, the strength-enhancing effect is greatly weakened and the cost is high. In the present invention, adding Nb together with V can further improve strength and toughness.

[0021] V: 0.040-0.055%. It has a strong binding force with carbon and nitrogen, precipitating in ferrite at the austenite grain boundaries. During rolling, it prevents austenite recrystallization and inhibits grain growth, thereby refining the ferrite grains and improving strength and toughness. It is well known that V (C, N) precipitates in small amounts in austenite, primarily in ferrite. Compared with Nb and Ti, it has a stronger precipitation strengthening effect. Increasing the N content and applying a certain amount of deformation can promote the precipitation of some V (C, N) in austenite, resulting in grain refinement and improved toughness. The remaining V element precipitates in ferrite, producing a precipitation strengthening effect.

[0022] Ti: 0.015-0.030%. Ti has both grain refinement and precipitation strengthening properties. At high temperatures, it dissolves into austenite, retarding the (γ→α) phase transformation. TiN and TiC precipitated in the austenite prevent grain growth, thereby refining the grains. However, Ti is relatively active, making precipitation difficult to control, which can lead to significant fluctuations in performance. High levels can also worsen low-temperature toughness.

[0023] P: not more than 0.015wt%. Generally speaking, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates welding performance, reduces plasticity, and deteriorates cold bending performance.

[0024] S: not more than 0.003 wt %. As a harmful element, it makes steel hot brittle, reduces the ductility and toughness of steel, easily generates cracks during rolling, and is also detrimental to weldability.

[0025] N: 0.0030-0.0060%. Originally considered a harmful impurity, it has become a useful and inexpensive element. The VN precipitated in the steel plays a strengthening role and at the same time liberates the strict control of gas in the steelmaking process.

[0026] Compared with the prior art, the present invention has the following beneficial effects: the present invention combines Nb and V with low Ti alloy components to significantly improve the product forming performance and performance stability, that is, the strength fluctuation range of products in the same furnace or the same batch does not exceed 50 MPa, and the impact energy fluctuation range does not exceed 30 J; at the same time, the V alloying design is adopted to reduce the restrictions on the steelmaking process and the hot rolling process, and the requirements for the N content in steelmaking, the controlled rolling and cooling process of hot rolling, and the rolling mill load are minimized; it solves the problems of warping, bending, excessive rolling rebound angle, frequent adjustment of mold parameters, etc. during the use of downstream customers, thereby improving the production efficiency of downstream customers. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Metallographic structure diagram of the beam steel of Example 3;

[0028] Figure 2 The metallographic structure diagram of the beam steel in Reference Document 1;

[0029] Figure 3 Electron microscope image of inclusions in the beam steel of Example 3;

[0030] Figure 4 Electron microscope image of inclusions in the beam steel of comparative document 1. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0032] Example

[0033] The preparation method of the beam steel of the present invention comprises the following steps:

[0034] 1. Steelmaking

[0035] 1. Raw material pretreatment process: Pre-treatment of the furnace S ≤ 0.0030%, remove the slag; use fine scrap steel.

[0036] 2. Converter smelting process: The converter carbon is pulled once to avoid spot blowing; the ladle is purged with argon before tapping, and the slag is blocked in the early stage: a slag blocking mud plug is used; according to the size of the tapping port after the previous heat of tapping, a suitable slag blocking mud plug is selected. The slag blocking mark is set in the later stage of tapping to ensure that the ladle slag thickness is less than 120mm. The tapping time is guaranteed to be 4 to 7 minutes, and the shape of the tapping port is controlled to avoid tapping dispersion. Deoxidation and alloying are started when 1 / 5 of the steel is tapped, and the alloy must be added when 4 / 5 is tapped. Deoxidation is carried out by adding ferrosilicon and ferroaluminum. First, a fixed amount of 650kg of ferrosilicon is added, and then high manganese is added. The ferroaluminum must be added after more than 1 minute (taking 170 tons per furnace as an example).

[0037] 3. Refining process: Single-path LF furnace is used for refining. LF uses active lime and fluorite to make reducing slag with good fluidity. The argon blowing intensity is strictly controlled to avoid exposure of molten steel as much as possible. Silicon calcium wire is used for calcium treatment. 500 meters of CaSi wire is fed to fully spheroidize inclusions and improve product performance.

[0038] 4. Continuous Casting Process: Before pouring, the tundish is purged with argon gas to ensure that no molten steel is exposed during the pouring process. The shroud is cleaned after each pour and maintained vertically during the pouring process. The shroud depth of the large ladle is maintained at 200-250mm. Slag detection is used to control the amount of slag released to prevent slag from forming in the large ladle. The secondary cooling stage utilizes a weak cooling mode with a light reduction mode, a 3-5mm casting depth, and a constant casting speed of 1.0-1.5m / min. The continuous casting superheat control target is ≤25°C.

[0039] 2. Hot rolling

[0040] 1. Heating process in the heating furnace: To ensure the slab shape, the preheating, first and second heating stages of the heating furnace are fully utilized to maximize the steel burning capacity. The high-temperature stage is moved forward to eliminate the temperature difference between the inside and outside of the slab and the watermark, ensuring sufficient dissolution of alloying elements and a good soaking effect. The heating temperature is 1200-1250°C, the soaking stage is in the furnace for ≥90 minutes, and the total time in the furnace is ≥170 minutes.

[0041] 2. Roughing: The roughing process uses a 3+3 pattern, with R1 rolling performed three times and R2 rolling performed three times. The R1 start temperature is controlled at ≥1130°C. Water spraying at high temperatures can effectively remove surface oxide scale. The intermediate bar thickness is 30-60 mm, and the cumulative reduction during the roughing stage is greater than 75%.

[0042] 3. Finishing Rolling: Hydraulic bending rolls (F1-F7) and CVC rolls (F2-F4) effectively control strip shape. V microalloyed steel is suitable for production on conventional rolling mills and requires no special production processes. This breaks away from the traditional controlled rolling process for Nb microalloyed steel, which requires controlled rolling in the uncrystallized zone. The finishing rolling F1 inlet temperature is ≤1100°C, the F1 reduction is ≥45%, and the F2 reduction is ≥25%. For hot-rolled finished product thicknesses <6.0mm, the final rolling temperature is 870-900°C; for hot-rolled finished product thicknesses ≥6.0mm, the final rolling temperature is 840-870°C.

[0043] 4. Coiling: Coil at 580-630°C to obtain a ferrite + small amount of pearlite structure. The coiling temperature is set at around 600°C. Considering the thicker beam steel used in this technology, the coiling temperature is slightly lower to balance cooling rate, microstructure refinement, and impact toughness.

[0044] According to the above preparation method, beam steel was prepared according to the components in Table 1 and the process in Table 2 to obtain Examples 1 to 6 and Comparative Examples 1 to 3.

[0045] The metallographic structures of Example 3 and Comparative Document 1 are shown in Figure 1 and Figure 2 As can be seen from the figure, the beam steel grains of the present invention are finer and less banded.

[0046] The electron microscope images of inclusions in Example 3 and Comparative Document 1 are shown in Figure 3 and Figure 4 It can be seen from the figure that under the strict control of steelmaking and hot rolling processes, the banding grade, inclusion grade, and grain size grade of the beam steel of the present invention are all better than those of the comparative example, which is also an important guarantee for the stable performance control of the product.

[0047] Table 1 Chemical composition / %

[0048]

[0049] Table 2 Process parameters

[0050]

[0051] Performance tests were conducted on Examples 1 to 6 and Comparative Examples 1 to 3, and the results are shown in Table 3. As can be seen from the results in Table 3, the differences in tensile strength and impact energy between the beam steels of the present invention, produced in different batches or in different furnaces, are small, not exceeding 30 MPa and 25 J, respectively. However, the differences in tensile strength and impact energy of the beam steels of the comparative examples are above 100 MPa and 45 J, respectively. The beam steels of the present invention have stable performance.

[0052] Table 3 Performance requirements

[0053]

[0054] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A hot-rolled beam steel with stable performance, characterized in that: The chemical composition of the beam steel and its weight percentage are: C 0.05-0.07%, Si 0.10-0.20%, Mn 1.20-1.40%, Al 0.025-0.045%, Nb 0.033-0.043%, V 0.040-0.055%, Ti 0.018-0.030%, P 0.010-0.015%, S≤0.003%, N 0.0030-0.0055%, O 0.0015-0.0025%, and the balance is Fe and unavoidable inclusions; The preparation method of the beam steel includes steelmaking, hot rolling, and coiling, wherein the steelmaking includes a raw material pretreatment process, a converter smelting process, a refining process, and a continuous casting process; Converter smelting process: Carbon pulling in the converter is done in one go to avoid spot blowing; the ladle is purged with argon before tapping, and slag blocking in the early stage is achieved by using a slag blocking plug; the appropriate slag blocking plug is selected based on the size of the tapping hole after the previous heat of tapping, and the slag blocking standard is set in the later stage of tapping to ensure that the ladle slag thickness is less than 120mm, the tapping time is guaranteed to be 4-7 minutes, and the tapping hole shape is controlled to avoid tapping dispersion; deoxidation and alloying begin when 1 / 5 of the steel is tapped, and alloying must be completed when 4 / 5 of the steel is tapped. Deoxidation is carried out by adding ferrosilicon and ferroaluminum, first adding a fixed amount of 650kg ferrosilicon, followed by high manganese, and the ferroaluminum must be added after more than 1 minute. Continuous casting process: Argon is used to purge the tundish before pouring. No molten steel is exposed during the pouring process. The shroud must be cleaned after each casting. The shroud must remain vertical during the casting process to ensure that the immersion depth of the shroud is 200-250mm. Slag detection is used to control the slag amount of the tundish to avoid slag in the tundish. The secondary cooling section adopts weak cooling mode and light pressure mode, with an input of 3-5mm and a constant casting speed of 1.0-1.5m / min. The continuous casting superheat control target is ≤25℃. The hot rolling process includes a heating furnace heating process, a rough rolling process and a finishing rolling process; deoxidation and alloying are performed during the steel tapping process in the converter smelting process, and the order of adding alloys is: Si-Mn-Al-Nb-V; ferrotitanium is added in the refining process, and the mass percentage of Ti element in the molten steel is 0.018-0.030%, and other alloying elements are added and fine-tuned to meet the target range; in the finishing rolling process, when the thickness of the hot-rolled product is less than 6.0 mm, the final rolling temperature is 870-900°C; when the thickness of the hot-rolled product is ≥6.0 mm, the final rolling temperature is 840-870°C.

2. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: The specification of the beam steel is 4.0mm to 12.0mm.

3. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: The strength difference of the beam steel is ≤30MPa.

4. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: In the refining process, calcium silicon wire is used for treatment, and 500 meters of CaSi wire is fed.

5. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: In the hot furnace heating process, the heating temperature is 1220-1260° C., the soaking period in the furnace is ≥90 min, and the total time in the furnace is ≥170 min.

6. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: In the rough rolling process, the rough rolling passes are in a 3+3 mode, with R1 rolling back and forth 3 times and R2 rolling back and forth 3 times; the R1 starting rolling temperature is ≥1130°C, and water spraying is performed at a high temperature of 1130°C for descaling; the intermediate billet thickness is 30-45 mm, and the cumulative reduction rate in the rough rolling stage is greater than 75%.

7. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: In the finishing rolling process, the F1 inlet temperature is ≤1100°C, the F1 reduction ratio is ≥45%, and the F2 reduction ratio is ≥25%.

8. The hot-rolled beam steel with stable performance according to claim 1, characterized in that: In the coiling process, the coiling temperature is 580-630° C., and the cooling rate is 25-35° C. / s.

Images