Induction heating process for improving low-temperature toughness of flat-bulged steel

Abstract

This invention discloses an induction heating process for improving the low-temperature toughness of bulb flat steel, comprising: using a composite induction heating method of medium frequency and high frequency, with the heating rate controlled at 15-25℃ / s; heating the bulb head to 900-905℃ and the web to 870-880℃, and holding at this temperature for 2-3 minutes; using a water-polymer solution or a combination of water cooling and air cooling during quenching, with the cooling rate controlled at 15-25℃ / s; and controlling the tempering temperature at 640-670℃, holding at this temperature for 2-3 hours to allow for the complete decomposition of martensite and bainite, resulting in a composite microstructure of ferrite + tempered martensite + fine bainite. The bulb flat steel treated by this process exhibits the following mechanical properties: tensile strength ≥ 620 MPa, yield strength ≥ 520 MPa, elongation ≥ 30%, impact absorption energy at -60℃ ≥ 320 J, and ductile-brittle transition temperature ≤ -80℃.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for steel materials, and particularly relates to an induction heating process for improving the low-temperature toughness of bulb flat steel. Background Technology

[0002] As a key component in shipbuilding, bulb flat steel requires high strength, good low-temperature toughness, and excellent weldability. However, with the increasing size of ships, bulb flat steel specifications are constantly expanding, resulting in uneven cross-sectional dimensions. The impact toughness of the bulb's core decreases significantly at low temperatures, easily forming a brittle austenitic (MA) phase, leading to an increased risk of fracture. Currently used processes such as "hot rolling + tempering" or "integral furnace heating and quenching + tempering" for large-size bulb flat steel have the following problems: slow heating rate, uneven cross-sectional temperature, and insufficient austenitization in the core; coarse quenched microstructure, making it difficult for the MA phase to decompose, resulting in insufficient toughness; and insufficient traditional tempering strengthening, with coarse carbides that cannot adequately improve impact performance. Therefore, a new induction heating process is urgently needed to significantly improve the low-temperature toughness of large-size bulb flat steel while ensuring forming quality. Summary of the Invention

[0003] The purpose of this invention is to provide an induction heating process for improving the low-temperature toughness of bulb flat steel. By precisely controlling the heating frequency, power, heating rate, segmented heat preservation and cooling method, the microstructure is refined and the second phase is optimized for precipitation, so that the bulb flat steel can maintain excellent toughness even below -60℃.

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

[0005] This invention discloses an induction heating process for improving the low-temperature toughness of bulb flat steel, comprising:

[0006] A combination of medium-frequency and high-frequency induction heating method is used to heat the ball head and web of the bulb flat steel in sections, with the heating rate controlled at 15-25℃ / s.

[0007] Heat the ball head to 900-905℃ and the web to 870-880℃, and hold for 2-3 minutes to ensure uniform cross-sectional temperature and complete austenitization of the core.

[0008] During the quenching process, a water-polymer solution or a combination of water cooling and air cooling is used, and the cooling rate is controlled at 15-25℃ / s.

[0009] The tempering temperature is controlled at 640-670℃ and held for 2-3 hours to allow martensite and bainite to fully decompose and obtain a composite structure of ferrite + tempered martensite + fine bainite.

[0010] The chemical composition of the bulb flat steel is as follows (by mass percentage): C: 0.10–0.14%, Si: 0.20–0.40%, Mn: 1.20–1.50%, Cr: 0.20–0.40%, Ni: 0.20–0.40%, Cu: 0.20–0.35%, V: 0.05–0.12%, Nb: 0.015–0.045%, N: 0.003–0.008%, P, S: ≤0.015%, with the balance being Fe and unavoidable impurities;

[0011] The mechanical properties of the bulb flat steel after the above process meet the following requirements: tensile strength ≥620 MPa, yield strength ≥520 MPa, elongation ≥30%, impact absorption energy at -60℃ ≥320J, and ductile-brittle transition temperature ≤-80℃.

[0012] The intermediate frequency is further defined as 8–15 kHz, and the high frequency as 40–60 kHz.

[0013] Further cooling is achieved using an aqueous solution containing 8–12% polyallyl alcohol (PAG) to realize a controllable cooling rate.

[0014] The carbide particles precipitated during further tempering have a diameter of ≤80 nm and a grain size of ≤4 μm.

[0015] Furthermore, this process is applicable to large-size bulb flats with a cross-sectional height ≥ 300 mm.

[0016] More specifically:

[0017] Induction heating method: medium frequency (10 kHz + high frequency 50 kHz composite);

[0018] Heating rate: 20 ℃ / s;

[0019] Austenitizing temperatures: 900℃ for the ball head and 870℃ for the web;

[0020] Keep warm: 2 min;

[0021] Quenching: Water + 10% PAG solution, cooling rate 20 ℃ / s;

[0022] Fire: 650℃ × 2.5 h.

[0023] More specifically:

[0024] Induction heating: dual-frequency heating in zones, with a heating rate of 18 ℃ / s;

[0025] Austenitizing temperature: 880℃, increased to 905℃ for the ball head portion;

[0026] Keep warm: 3 min;

[0027] Cooling: Forced water spray + air cooling combination, average cooling rate 18 ℃ / s;

[0028] Tempering: 660℃×3h.

[0029] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0030] (1) Significantly improves low-temperature toughness: the impact absorption energy at -60℃ is increased to ≥320J, which is about 40% higher than the existing process; (2) the ductile-brittle transition temperature is reduced to below -80℃, which is far superior to the existing induction heating scheme; (3) the strength and toughness are well matched, with tensile strength ≥620 MPa, yield strength ≥520 MPa, and elongation ≥30%; (4) the process is highly efficient, energy-saving and consumption-reducing, and the production cost is reduced by more than 10% compared with the overall heating process. Detailed Implementation

[0031] The following is a more detailed description of an induction heating process for improving the low-temperature toughness of bulb flat steel according to the present invention.

[0032] The present invention provides an induction heating process for improving the low-temperature toughness of bulb flat steel, which mainly includes the following steps:

[0033] Chemical composition design: The chemical composition (mass fraction, %) of the bulb flat steel is as follows: C 0.10~0.14, Si 0.20~0.40, Mn 1.20~1.50, Cr 0.20~0.40, Ni 0.20~0.40, Cu 0.20~0.35, V 0.05~0.12, Nb 0.015~0.045, N 0.003~0.008, P≤0.015, S≤0.010, with the balance being Fe and unavoidable impurities.

[0034] Induction heating: A combination of medium-frequency (8-15 kHz) and high-frequency (40-60 kHz) heating method is used to heat the ball head and web in sections, with the heating rate controlled at 15-25 ℃ / s; the heating temperature of the ball head is 900-905 ℃, and that of the web is 870-880 ℃, and the temperature is held for 2-3 min to ensure uniform cross-sectional temperature and complete austenitization of the core.

[0035] Quenching and cooling: Quenching is carried out using a water-polymer solution or a combination of water cooling and air cooling. The cooling rate is controlled at 15-25 ℃ / s to obtain a fine mixed structure of martensite and bainite and avoid the formation of coarse MA phase.

[0036] Tempering treatment: Temper at 640-670 ℃ for 2-3 h to fully decompose martensite and precipitate dispersed carbides, ultimately forming a composite structure of ferrite + tempered martensite + fine bainite.

[0037] Performance indicators: After the above process, the bulb flat steel has a grain size ≤4 μm, a carbide grain size ≤80 nm, a tensile strength ≥620 MPa, a yield strength ≥520 MPa, an elongation ≥30%, an impact absorption energy of -60 ℃ ≥320J, and a ductile-brittle transition temperature (FATT50) ≤-80 ℃.

[0038] Example 1:

[0039] Induction heating method: medium frequency (10 kHz) + high frequency (50 kHz) composite;

[0040] Heating rate: 20 ℃ / s;

[0041] Austenitizing temperatures: 900℃ for the ball head and 870℃ for the web;

[0042] Keep warm: 2 min;

[0043] Quenching: Water + 10% PAG solution, cooling rate 20 ℃ / s;

[0044] Tempering: 650℃ × 2.5 h;

[0045] Performance results: Rm=625 MPa, ReL=523 MPa, A=30.8%, Z=81%, impact energy at -60℃=325J, FATT50=-82℃.

[0046] Example 2:

[0047] Induction heating: dual-frequency heating in zones, with a heating rate of 18 ℃ / s;

[0048] Austenitizing temperature: 880℃, increased to 905℃ for the ball head portion;

[0049] Keep warm: 3 min;

[0050] Cooling: Forced water spray + air cooling combination, average cooling rate 18 ℃ / s;

[0051] Tempering: 660℃×3h;

[0052] Test results (center of the ball head): Rm=635 MPa, ReL=528 MPa, A=31.2%, Z=82%, impact energy at -60℃=338J, FATT50=-85℃.

[0053] Comparative Example 1 (Traditional hot rolling + tempering):

[0054] Process: Hot-rolled state, tempered at 660℃ for 2 hours;

[0055] Performance: Rm=568 MPa, ReL=470 MPa, A=30%, -40℃ impact energy=165J, FATT50=+10℃.

[0056] Comparative Example 2 (Single Induction Heating):

[0057] Process: High-frequency heating, austenitization at 880℃, water cooling + tempering (650℃×2 h);

[0058] Performance: Rm=605 MPa, ReL=510 MPa, A=29%, -60℃ impact energy=260J, FATT50=-60℃.

[0059] Comparative Example 3 (Integral Furnace Heating + Quenching and Tempering):

[0060] Process: Furnace heating to 880℃, oil quenching + tempering;

[0061] Performance: Rm=610 MPa, ReL=515 MPa, A=28%, -60℃ impact energy=240J, FATT50=-55℃.

[0062] Table 1. Induction heating process parameters for different embodiments and comparative examples

[0063] serial number Heating method Austenitizing temperature (°C) Heating rate (°C / s) Cooling method tempering system Example 1 Mid-frequency + high-frequency composite Ball head 900 / Web plate 870 20 Water + 10% PAG solution 650℃×2.5 h Example 2 Zoned dual-frequency heating Ball head 905 / Web plate 880 18 Water cooling + air cooling 660℃×3 h Comparative Example 1 Hot rolling + tempering — — — 660℃×2 h Comparative Example 2 Single high frequency 880 — Water cooling 650℃×2 h Comparative Example 3 Furnace heating 880 — Oil quenching Tempering (650℃ × 2 h)

[0064] Table 2 Mechanical properties and low-temperature toughness of different embodiments and comparative examples

[0065] serial number Rm (MPa) ReL (MPa) A (%) Z (%) Impact energy (J) at -60℃ FATT50 (°C) Example 1 625 523 30.8 81 325 -82 Example 2 635 528 31.2 82 338 -85 Comparative Example 1 568 470 30 — 165 +10 Comparative Example 2 605 510 29 — 260 -60 Comparative Example 3 610 515 28 — 240 -55

[0066] As can be seen from the embodiments and comparative examples, the present invention has the following advantages:

[0067] (1) Significantly improved low-temperature toughness: The impact absorption energy at -60 ℃ reaches 320~338J, while the comparative ratio is only 165~260J, with an improvement of 30%~90%, meeting the requirements for extreme low-temperature service.

[0068] (2) The ductile-brittle transition temperature is significantly reduced: The process of this invention can reduce FATT50 to below -80 ℃, while the comparative examples are mostly at -60 ℃ or even above 0 ℃, which significantly improves low-temperature safety.

[0069] (3) The structure is more refined and uniform: the grain size in the example is ≤4μm and the carbide particle size is ≤80 nm, while the comparative example often shows coarse MA phase or incomplete decomposition of martensite, resulting in a decrease in toughness.

[0070] (4) Excellent balance between strength and toughness: While maintaining Rm≥620 MPa and ReL≥520 MPa, the elongation of this invention can reach more than 30%, achieving a balance between high strength and high toughness.

[0071] (5) Better process efficiency and economy: The use of induction heating + zoned cooling method results in uniform heating, short cycle, and reduced energy consumption by about 10% to 15%. Compared with the overall furnace heating, the cost is lower and the production efficiency is higher.

[0072] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from its spirit should fall within the protection defined by the claims.

[0073] Within the range.

Claims

1. An induction heating process for improving the low-temperature toughness of bulb flat steel, characterized in that, include: A combination of medium-frequency and high-frequency induction heating method is used to heat the ball head and web of the bulb flat steel in sections, with the heating rate controlled at 15-25℃ / s. Heat the ball head to 900-905℃ and the web to 870-880℃, and hold for 2-3 minutes to ensure uniform cross-sectional temperature and complete austenitization of the core. During the quenching process, a water-polymer solution or a combination of water cooling and air cooling is used, and the cooling rate is controlled at 15-25℃ / s. The tempering temperature is controlled at 640-670℃ and held for 2-3 hours to allow martensite and bainite to fully decompose and obtain a composite structure of ferrite + tempered martensite + fine bainite. The chemical composition of the bulb flat steel is as follows (by mass percentage): C: 0.10–0.14%, Si: 0.20–0.40%, Mn: 1.20–1.50%, Cr: 0.20–0.40%, Ni: 0.20–0.40%, Cu: 0.20–0.35%, V: 0.05–0.12%, Nb: 0.015–0.045%, N: 0.003–0.008%, P, S: ≤0.015%, with the balance being Fe and unavoidable impurities; The mechanical properties of the bulb flat steel after the above process meet the following requirements: tensile strength ≥620 MPa, yield strength ≥520 MPa, elongation ≥30%, impact absorption energy at -60℃ ≥320J, and ductile-brittle transition temperature ≤-80℃.

2. The induction heating process for improving the low-temperature toughness of bulb flat steel according to claim 1, characterized in that, The intermediate frequency is 8–15 kHz; the high frequency is 40–60 kHz.

3. The induction heating process for improving the low-temperature toughness of bulb flat steel according to claim 1, characterized in that, Cooling is achieved using an aqueous solution containing 8–12% polyallyl alcohol (PAG) to realize a controllable cooling rate.

4. The induction heating process for improving the low-temperature toughness of bulb flat steel according to claim 1, characterized in that, The carbide particles precipitated during tempering have a diameter ≤80 nm and a grain size ≤4 μm.

5. The induction heating process for improving the low-temperature toughness of bulb flat steel according to claim 1, characterized in that, This process is applicable to large-size bulb flats with a cross-sectional height ≥ 300 mm.

6. The induction heating process for improving the low-temperature toughness of bulb flat steel according to claim 1, characterized in that, Specifically: Induction heating method: medium frequency (10 kHz + high frequency 50 kHz composite); Heating rate: 20 ℃ / s; Austenitizing temperatures: 900℃ for the ball head and 870℃ for the web; Keep warm: 2 min; Quenching: Water + 10% PAG solution, cooling rate 20 ℃ / s; Fire: 650℃ × 2.5 h.

7. The induction heating process for improving the low-temperature toughness of bulb flat steel according to claim 1, characterized in that, Specifically: Induction heating: dual-frequency heating in zones, with a heating rate of 18 ℃ / s; Austenitizing temperature: 880℃, increased to 905℃ for the ball head portion; Keep warm: 3 min; Cooling: Forced water spray + air cooling combination, average cooling rate 18 ℃ / s; Tempering: 660℃×3h.