A heat treatment method to improve the microstructure and properties of bulb flat steel

Abstract

This invention discloses a heat treatment method for improving the microstructure and properties of bulb flat steel, comprising: induction heating quenching; rapid cooling; and high-temperature tempering. The bulb flat steel is a copper-containing high-strength low-alloy steel with the following chemical composition by mass percentage: C ≤ 0.06%, Si 0.30–0.50%, Mn 0.50–0.80%, Ni 1.5–2.0%, Cr 0.8–1.0%, Cu 1.2–1.5%, Mo 0.15–0.25%, Ti+Nb ≤ 0.05%, P ≤ 0.015%, S ≤ 0.010%, with the balance being Fe and unavoidable impurities. After the above process, the bainite lath width in the bulb flat steel microstructure is ≤ 0.6 μm, the average grain size is ≤ 3.5 μm, the precipitate particle size is controlled within the range of 30–40 nm, the impact energy at –40℃ is not less than 260 J, and the yield strength is not less than 640 MPa.

Description

Technical Field

[0001] This invention belongs to the field of steel heat treatment technology, and particularly relates to a heat treatment method for improving the microstructure and properties of bulb flat steel. It is especially suitable for optimizing the microstructure and improving the properties of bulb flat steel used in structural components such as ships, bridges, and offshore platforms. Background Technology

[0002] Due to its unique cross-sectional shape and structural properties, bulb flat steel is widely used in critical components such as ship skeletons, transverse bulkheads, and bridge webs. With the increasing demand for high-performance steel in marine engineering, bulb flat steel is required to possess not only high strength and toughness but also good weldability and processing stability. Currently, most bulb flat steel undergoes traditional resistance furnace heating followed by water quenching and high-temperature tempering, but this process suffers from problems such as coarse grains, uneven microstructure, and large performance fluctuations. In particular, the significant difference in mechanical properties between the bulb head and the web makes it highly susceptible to becoming a weak point in the structure's service life. Existing research indicates that induction heating can achieve faster and more uniform heating, shorten holding time, reduce austenite grain growth, and improve microstructure refinement. However, current methods still lack a synergistic optimization mechanism combining induction heating, rapid cooling, and efficient tempering, leaving room for further improvement. Summary of the Invention

[0003] The purpose of this invention is to provide a heat treatment method for improving the microstructure and properties of bulb flat steel. By optimizing the quenching heating method, temperature control path and tempering regime, a synergistic improvement of fine grain strengthening, dislocation strengthening and precipitation strengthening can be achieved.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a heat treatment method for improving the microstructure and properties of bulb flat steel, comprising:

[0006] Induction heating quenching: The bulb flat steel is heated to 940-980℃ using a medium-frequency induction heating device at a heating rate of 50-100℃ / s and a holding time of 2-4 minutes.

[0007] Rapid cooling: After heating and heat preservation, immediately perform water cooling or mist cooling to cool to room temperature, with a cooling rate of not less than 30℃ / s;

[0008] High-temperature tempering: Heat the quenched flat steel to 680-700℃ and hold for 2-3 hours, then perform high-temperature tempering treatment using a box furnace or resistance furnace.

[0009] The bulb flat steel is a copper-containing high-strength low-alloy steel with the following chemical composition by mass percentage: C≤0.06%, Si 0.30~0.50%, Mn 0.50~0.80%, Ni 1.5~2.0%, Cr 0.8~1.0%, Cu 1.2~1.5%, Mo 0.15~0.25%, Ti+Nb≤0.05%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities;

[0010] After the above process, the bainite lath width in the microstructure of the bulb flat steel is ≤0.6 μm, the average grain size is ≤3.5 μm, the precipitate particle size is controlled in the range of 30 to 40 nm, the impact energy at -40℃ is not less than 260 J, and the yield strength is not less than 640 MPa.

[0011] Furthermore, the induction heating device is a medium-frequency induction heating device with an induction frequency of 2 to 8 kHz and an induction coil spacing of 30 to 60 mm.

[0012] Furthermore, the rapid cooling method is water cooling tank cooling or atomized spray cooling, preferably a mist cooling system, with the droplet diameter controlled between 0.1 and 0.5 mm.

[0013] Furthermore, the tempering furnace is a multi-zone temperature-controlled box-type resistance furnace with a temperature control accuracy of ±3℃ and a heating rate of 3~8℃ / min.

[0014] Furthermore, the precipitated strengthening phase formed during the tempering process consists of fine Cu-enriched nanoparticles and some (Mo,Cr) carbides, with an average particle size between 30 and 40 nm.

[0015] Furthermore, after treatment, the bainite distribution in the cross-section of the bulb flat steel is uniform, and the difference in impact absorption energy between the bulb head and the web area is no greater than 15 J.

[0016] Furthermore, the heat treatment method is applicable to bulb flat steel profiles with a width of 160–320 mm and a thickness of 6–20 mm.

[0017] Furthermore, the heat-treated bulb flat steel, as analyzed by EBSD, has an average grain equivalent diameter ≤3.2 μm and a low-angle grain boundary ratio ≥50%.

[0018] Furthermore, specifically:

[0019] Quenching: The sample was heated to 970℃ using a medium-frequency induction heating device at a heating rate of 60℃ / s and held for 3 minutes to ensure a uniform temperature across the sample cross section.

[0020] Rapid cooling: After heating, the sample is immediately immersed in a water cooling bath for rapid cooling to room temperature at a rate of 35℃ / s;

[0021] High-temperature tempering: The water-quenched sample was placed in a box-type resistance furnace and tempered at 680℃ for 2.5 hours, followed by air cooling.

[0022] Furthermore, specifically:

[0023] Quenching: Medium-frequency induction heating is used to rapidly heat to 960℃ at a heating rate of 55℃ / s and hold for 2.5 minutes;

[0024] Rapid cooling: It adopts mist cooling to room temperature, with a cooling rate of 30℃ / s, and the cooling uniformity is better than water cooling;

[0025] Tempering process: tempering temperature 690℃, holding time 2 hours, air cooling to room temperature.

[0026] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0027] (1) The invention adopts a novel heat treatment path of induction heating quenching + high-efficiency tempering, which can significantly refine austenite grains, form a more uniform and fine bainite structure, and improve comprehensive mechanical properties; (2) Compared with the traditional heating process, the invention can shorten the heating and holding time from 1 hour to less than 3 minutes, reduce energy consumption by more than 70%, and at the same time reduce the tendency of grain growth and improve the grain size grade; (3) The yield strength of the ball head region in the embodiment is increased by 45-50 MPa, and the impact energy at -40℃ is increased by 25-40 J, which has a better strength and toughness match; (4) The process parameter window is wide and the adaptability is strong. It can be realized on existing medium frequency induction heating and box tempering furnace equipment without increasing equipment investment, and is suitable for large-scale continuous heat treatment; (5) It is applicable to the service application of Cu-containing high-strength low alloy ball flat steel in high-requirement environments such as ship skeletons and marine engineering structures, and improves material reliability. Detailed Implementation

[0028] The following is a detailed description of a heat treatment method for improving the microstructure and properties of bulb flat steel according to the present invention.

[0029] Example: This example is a preferred embodiment of the various embodiments of the present invention.

[0030] Example 1:

[0031] Chemical composition of steel (mass percentage): C 0.05, Si 0.38, Mn 0.65, Ni 1.65, Cr 0.85, Cu 1.32, Mo 0.22, Ti+Nb 0.035, P 0.011, S 0.008, balance Fe.

[0032] Initial state: The sample is a hot-rolled bulb flat steel with a specification of 220 mm × 14 mm and a sample length of 600 mm, which has been pickled and descaled.

[0033] Quenching heating process: Use medium frequency induction heating equipment to heat to 970℃, heating rate is about 60℃ / s, holding time is 3 minutes to ensure that the temperature of the sample cross section is consistent.

[0034] Rapid cooling: After the sample is heated, it is immediately immersed in a water cooling bath to rapidly cool to room temperature, with a cooling rate of approximately 35°C / s.

[0035] High-temperature tempering: The water-quenched sample was placed in a box-type resistance furnace and tempered at 680℃ for 2.5 hours, followed by air cooling.

[0036] Test results (center of the ball head): Yield strength: 641 MPa; Tensile strength: 715 MPa; Elongation: 21.7%; Impact energy absorbed at –40℃: 265 J; Average grain size: 3.2 μm; Bainite lath width: 0.57 μm; Precipitated particle size: 36.7 ± 12 nm.

[0037] Example 2:

[0038] Chemical composition of steel (mass percentage): C 0.06, Si 0.34, Mn 0.70, Ni 1.78, Cr 0.92, Cu 1.48, Mo 0.19, Ti+Nb 0.030, P 0.012, S 0.009, balance Fe.

[0039] Sample specifications: 210 mm wide, 12 mm thick, approximately 650 mm long, delivered in hot-rolled condition.

[0040] Quenching heating process: Medium frequency induction heating is used to quickly heat to 960℃, with a heating rate of about 55℃ / s, and hold for 2.5 minutes.

[0041] Cooling method: A mist cooling device is used to cool to room temperature, with a cooling rate of about 30℃ / s, and the cooling uniformity is better than that of water cooling.

[0042] Tempering process: tempering temperature 690℃, holding time 2 hours, air cooling to room temperature.

[0043] Test results (center of the ball head): Yield strength: 648 MPa; Tensile strength: 725 MPa; Elongation: 22.4%; Impact energy absorbed at –40℃: 272 J; Average grain size: 3.0 μm; Bainite lath width: 0.55 μm; Precipitated particle size: 38.9 ± 9 nm.

[0044] Comparative Example 1 (Conventional box furnace quenching):

[0045] The chemical composition of the steel is the same as that in Example 1.

[0046] Quenching heating process: Use a box-type resistance furnace to slowly heat to 980℃, with a heating rate of about 10℃ / min and a holding time of 60 minutes, so that the overall temperature of the sample is uniform.

[0047] Cooling method: Water quenching to room temperature, cooling rate approximately 5℃ / s.

[0048] Tempering process: 680℃, tempering for 2.5 hours.

[0049] Test results (center of the ball head): Yield strength: 596 MPa; Tensile strength: 682 MPa; Elongation: 21.0%; Impact energy at –40℃: 240 J; Grain size: 4.3 μm; Lath width: 0.86 μm; Precipitated particle size: 25.7 ± 7 nm.

[0050] Comparative Example 2 (Traditional Box Furnace + Fog Cooling):

[0051] The chemical composition of the steel is the same as in Example 2.

[0052] Quenching heating process: Heat to 950℃ in a resistance furnace and hold for 60 minutes.

[0053] Cooling method: A mist cooling device is used to cool to room temperature, with a cooling rate of approximately 6°C / s.

[0054] Tempering process: Temper at 680℃ for 2 hours.

[0055] Test results (center of the ball head): Yield strength: 595 MPa; Tensile strength: 680 MPa; Elongation: 20.1%; Impact energy at –40℃: 230 J; Grain size: 4.4 μm; Lath width: 0.84 μm; Precipitated particle size: 26.5 ± 8 nm.

[0056] Table 1 Performance Comparison of Examples and Comparative Examples

[0057] project Example 1 Example 2 Comparative Example 1 Comparative Example 2 Heating method Induction heating Induction heating Box furnace Box furnace Heating temperature (°C) 970 960 980 950 Insulation time (min) 3 2.5 60 60 Tempering temperature (°C) 680 690 680 680 Tempering time (h) 2.5 2.0 2.5 2.0 Cooling method Water cooling foggy cold Water cooling Water cooling Cooling rate (°C / s) ≥30 ≥25 ≈5 ≈5 Grain size (μm) 3.2 3.0 4.3 4.4 Slat width (μm) 0.57 0.55 0.86 0.84 Yield strength (MPa) 641 648 596 595 Impact energy at -40℃ (J) 265 272 240 230

[0058] As can be seen from the embodiments and comparative examples, the present invention has the following advantages:

[0059] (1) The grains are significantly refined and the structure is more uniform: Examples 1 and 2 use medium frequency induction heating, which significantly shortens the heating time and inhibits the growth of austenite grains, so that the average grain size is controlled between 3.0 and 3.2 μm, which is significantly refined compared to the 4.3 to 4.4 μm of the comparative example; the width of bainite laths is also reduced from more than 0.85 μm to 0.55 to 0.57 μm, and the structure is more dense and uniform.

[0060] (2) Significantly improved mechanical properties: The yield strength of the bulb flat steel sample using the heat treatment process of the present invention is increased to 641-648 MPa, and the impact energy at -40℃ reaches 265-272 J, which is significantly better than the comparative sample (yield strength less than 600 MPa, impact energy less than 240 J). This shows that the heat treatment path of the present invention can simultaneously improve strength and low-temperature toughness, achieving a balance between strength and toughness.

[0061] (3) The precipitated phase is more uniform and the strengthening effect is significant: In the example, the particle size of the precipitated nano-sized Cu enriched particles and Mo / Cr composite carbides is controlled in the range of 30 to 40 nm, and the distribution is uniform, which effectively enhances the precipitation strengthening effect; while the particle size of the precipitated phase in the comparative example is smaller (~25 nm), the quantity is uneven, and the strengthening effect is poor.

[0062] (4) Reduced energy consumption and higher efficiency: Compared with the traditional box furnace heating for 1 hour, the present invention uses induction heating for only 2 to 4 minutes, which increases energy efficiency by more than 70% and significantly shortens the production cycle, meeting the needs of modern steel plants for efficient and continuous heat treatment.

[0063] (5) The cooling path is well matched and the control capability is strong: the embodiment adopts rapid cooling (water cooling or mist cooling) with a cooling rate of not less than 30℃ / s, which effectively avoids bainite coarsening and the lag of supercooled austenite transformation, and helps to improve the consistency of the structure and the stability of the performance.

[0064] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method of improving the microstructure and properties of a flat-rolled steel by heat treatment, characterized in that, include: Induction heating quenching: The bulb flat steel is heated to 940-980℃ using a medium-frequency induction heating device at a heating rate of 50-100℃ / s and a holding time of 2-4 minutes. Rapid cooling: After heating and heat preservation, immediately perform water cooling or mist cooling to cool to room temperature, with a cooling rate of not less than 30℃ / s; High-temperature tempering: Heat the quenched flat steel to 680-700℃ and hold for 2-3 hours, then perform high-temperature tempering treatment using a box furnace or resistance furnace. The bulb flat steel is a copper-containing high-strength low-alloy steel with the following chemical composition by mass percentage: C≤0.06%, Si 0.30~0.50%, Mn 0.50~0.80%, Ni 1.5~2.0%, Cr 0.8~1.0%, Cu 1.2~1.5%, Mo 0.15~0.25%, Ti+Nb≤0.05%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities; After the above process, the bainite lath width in the microstructure of the bulb flat steel is ≤0.6 μm, the average grain size is ≤3.5 μm, the precipitate particle size is controlled in the range of 30 to 40 nm, the impact energy at -40℃ is not less than 260 J, and the yield strength is not less than 640 MPa.

2. The method of claim 1, wherein the method is characterized by: The induction heating device is a medium-frequency induction heating device with an induction frequency of 2 to 8 kHz and an induction coil spacing of 30 to 60 mm.

3. The method of claim 1, wherein the method is characterized by: The rapid cooling method is water cooling tank cooling or atomized spray cooling, preferably a mist cooling system, with the droplet diameter controlled between 0.1 and 0.5 mm.

4. The method of claim 1, wherein the method is characterized by: The tempering furnace is a multi-zone temperature-controlled box-type resistance furnace with a temperature control accuracy of ±3℃ and a heating rate of 3~8℃ / min.

5. The method of claim 1, wherein the method is characterized by: The precipitated strengthening phase formed during tempering consists of fine Cu-enriched nanoparticles and some (Mo,Cr) carbides, with an average particle size between 30 and 40 nm.

6. The method of improving the microstructure and properties of a bullet-nosed steel of claim 1, wherein, After treatment, the bainite in the cross-section of the bulb flat steel is uniformly distributed, and the difference in impact absorption energy between the bulb head and the web area is no more than 15 J.

7. The method of improving the microstructure and properties of a bullet-nosed steel of claim 1, wherein The heat treatment method is applicable to bulb flat steel profiles with a width of 160–320 mm and a thickness of 6–20 mm.

8. The method of claim 1, wherein the method is characterized by: The heat-treated bulb flat steel was analyzed by EBSD and its average grain equivalent diameter was ≤3.2 μm, and the proportion of low-angle grain boundaries was ≥50%.

9. The heat treatment method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, Specifically: Quenching: The sample was heated to 970℃ using a medium-frequency induction heating device at a heating rate of 60℃ / s and held for 3 minutes to ensure a uniform temperature across the sample cross section. Rapid cooling: After heating, the sample is immediately immersed in a water cooling bath for rapid cooling to room temperature at a rate of 35℃ / s; High-temperature tempering: The water-quenched sample was placed in a box-type resistance furnace and tempered at 680℃ for 2.5 hours, followed by air cooling.

10. The heat treatment method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, Specifically: Quenching: Medium-frequency induction heating is used to rapidly heat to 960℃ at a heating rate of 55℃ / s and hold for 2.5 minutes; Rapid cooling: It adopts mist cooling to room temperature, with a cooling rate of 30℃ / s, and the cooling uniformity is better than water cooling; Tempering process: tempering temperature 690℃, holding time 2 hours, air cooling to room temperature.