Online water bath production process of 51CrV4 spring steel plate with thickness of 15-40mm

Abstract

The invention discloses an online water bath production process of a 51CrV4 spring steel plate with the thickness of 15-40 mm, and belongs to the field of spring steel plate manufacturing. The process comprises the following steps: selecting a high-purity continuous casting billet (components are accurately controlled, and inclusions are less than or equal to 0.5 grade), performing segmented heating (soaking at the temperature of 1,200-1,250 DEG C), performing controlled rolling (the finish rolling temperature is 900-930 DEG C), performing online water bath cooling (entering into water at the temperature of 860-900 DEG C and the cooling speed is 35-60 DEG C / s), performing timely tempering (entering into a furnace within 2 hours at the temperature of 450 + / -10 DEG C), and straightening at the temperature of 300-400 DEG C and the straightness is less than or equal to 2mm / m). The water quenching cracking problem of the high-carbon spring steel plate is solved, the yield strength of a finished product is larger than or equal to 1450 MPa, the finished product can be directly machined without hardening and tempering, compared with a traditional technology, energy consumption is reduced by 30%, cost is reduced by 95%, and the method is suitable for steel plates with the width of 1600-3200 mm and suitable for manufacturing high-strength springs.

Description

Technical Field

[0001] This invention belongs to the field of spring steel plate production technology, specifically relating to an online water bath process for 15-40mm thick 51CrV4 spring steel plates. Background Technology

[0002] Springs are essential components in important national economic industries such as transportation equipment, machinery, hardware, instruments, and household appliances. Almost all major sectors of the national economy are related to the spring manufacturing industry, and springs play a vital foundational role in the overall industrial development of the country.

[0003] The conventional process for processing springs from spring steel plates involves the customer purchasing hot-rolled or annealed steel plates, machining them, and then performing oil quenching or medium quenching, followed by tempering to adjust properties and relieve stress. For customers, this process is lengthy, high-strength parts are prone to warping during quenching, and plate shape control is difficult. Furthermore, the quenching medium commonly used in spring steel heat treatment is ordinary quenching oil, which has a low cooling rate and low quenching thickness, resulting in high oil consumption, a harsh working environment, and a risk of fire if not handled carefully. It is also not energy-efficient or environmentally friendly. While water-soluble quenching fluids are more environmentally friendly, they consume large quantities, and it is difficult to maintain a reasonable concentration of effective components for extended periods, affecting quenching results and increasing costs. Water quenching is low-cost, but spring steel plates are highly susceptible to cracking during quenching. If the water quenching heat treatment process is not properly handled, cracking is likely. Currently, there are no successful application cases of online quenching technology for spring steel plates.

[0004] CN105441640B discloses a heat treatment process for silicon-manganese alloy spring steel. This production method uses the quenching liquid used in silicon-manganese alloy spring steel for efficient heat treatment. The process design is scientific and reasonable, which can effectively avoid the tendency of decarburization and graphitization. It is not applicable to CrV series spring steel plates.

[0005] CN105543456B discloses a low-consumption, environmentally friendly water-soluble quenching fluid for spring steel and a quenching process. This method still uses a quenching medium, but the composition of the quenching fluid has been improved; however, the cost is still relatively high compared to water. Furthermore, this method is only applicable to flat steel with a width of no more than 500mm; it does not mention spring steel plates with a width greater than 1000mm.

[0006] CN101215624B discloses an online quenching production process for high-strength and tough thick steel plates. This invention is mainly aimed at the online water quenching process for low-carbon alloy steel plates to improve their strength and toughness, but it is not applicable to high-carbon spring steel plates.

[0007] CN111872137B A method for controlled cooling after rolling of spring steel wire rod. The EDC water bath cooling method described in this invention is applicable to spring steel wire rod, but not to spring steel plates. Moreover, the EDC water bath process cannot be applied to steel plate rolling equipment.

[0008] Therefore, there is an urgent need to develop an online water bath process suitable for 51CrV4 spring steel plates with a thickness of 15-40mm and a width of 1600-3200mm. By precisely controlling the composition, temperature, cooling rate and tempering parameters, the problem of water quenching cracking can be solved, and the integrated production of "direct quenching after rolling + tempering" can be realized, so as to provide customers with finished steel plates that do not require quenching and tempering. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide an online water bath process for 15-40mm thick 51CrV4 spring steel plates, which can be directly processed and used without the need for reheating and tempering of the spring steel plates, thereby improving production efficiency, reducing usage costs, and solving the environmental problems in the conventional oil quenching process of parts.

[0010] The technical solution adopted by this invention to solve the above problems is as follows: an online water bath production process for 15-40mm thick 51CrV4 spring steel plates, with "purity control at the source + precise temperature control rolling + gradient water bath cooling + timely tempering to relieve stress" as the core concept. The specific process path is as follows: Precision continuous casting billet → segmented heating → controlled rolling → online water bath cooling → timely tempering → heated straightening → finishing and inspection → warehousing. The technical parameters and principles of each step are as follows: 3.2.1 Raw material selection (continuously cast slab) Chemical composition (mass percentage): C: 0.46~0.55% (to ensure strength), Si: ≤0.40% (to avoid decarburization and graphitization), Mn: 0.50~0.90% (to refine pearlite and improve toughness), P: ≤0.015% (to suppress cold brittleness), S: ≤0.002% (to reduce hot brittleness and inclusions), Cr: 0.80~1.20% (to improve hardenability and corrosion resistance), Cu: ≤0.25%, Ni: ≤0.35% (to improve toughness through trace alloying), Al: 0.02~0.04% (to refine austenite grains), V: 0.10~0.20% (to form V (C, N) particles and strengthen the matrix), N: ≤0.005% (to avoid combining with V to form coarse inclusions), balance Fe and unavoidable impurities; Purity requirements: For low-magnification mass, segregation ≤ Class C 1.0 grade and porosity ≤ Class 1.0 grade (to reduce internal stress concentration); all non-metallic inclusions A (sulfides), B (alumina), C (silicates), D (spherical oxides), and Ds (single-particle spherical particles) ≤ Class 0.5 grade (to reduce crack initiation sites).

[0011] 3.2.2 Segmented heating process A three-stage heating furnace is used to achieve complete solid solution and homogenization of carbides: Preheating section: temperature 700~800℃, holding time ≥20min, function: to reduce the temperature difference between the inside and outside of the billet and avoid thermal deformation caused by rapid heating; Heating section: Temperature 1000~1150℃, holding time ≥30min, function: gradually increase the temperature to initially dissolve the carbides on the surface of the billet, and prepare for the soaking section; Soaking zone: Temperature 1200~1250℃, holding time ≥10× billet thickness (min, e.g., 60mm thick billet holding time ≥600min). Function: To ensure that the carbides (e.g. Fe3C, Cr7C3) in the core of the billet are completely dissolved, and the composition uniformity deviation is ≤0.02%, laying the foundation for subsequent rolling to refine the grains.

[0012] 3.2.3 Controlling the rolling process Using a heavy plate rolling mill (roll diameter ≥ 1200mm), rolling is performed in 2-3 passes: Descaling: High-pressure water (pressure ≥25MPa) is used to remove the oxide scale from the surface of the billet before rolling to prevent the oxide scale from being pressed into the steel plate and forming defects; Final rolling thickness under temperature control: set to 2.5 times the finished product thickness (e.g., 15mm finished product corresponds to a thickness of 50mm under temperature control). Purpose: to ensure sufficient deformation (cumulative reduction rate ≥60%) and refine austenite grains to grade 5-8 (GB / T 6394). Final rolling temperature: 900~930℃. Principle: The solid solution temperature threshold of V (C, N) is 890℃. This temperature range can ensure that V (C, N) is completely dissolved into austenite, avoiding undissolved particles from becoming crack sources. During subsequent water quenching, it transforms into martensite along with austenite. After cooling, V (C, N) will precipitate in fine particles (size ≤2μm), playing a "dispersion strengthening" role, increasing tensile strength by 10-15%. If the temperature is <900℃, V (C, N) solid solution is insufficient, and subsequent cooling easily forms coarse carbonitrides, weakening the precipitation strengthening effect, and the tensile strength will be lower than 1550MPa. If the temperature is >930℃, austenite grains are prone to grow to below level 3, resulting in uneven microstructure.

[0013] 3.2.4 Online water bath cooling process A top-bottom and side-surround nozzle assembly (nozzle spacing ≤ 50mm, water pressure 1.2-1.5MPa) is adopted to achieve uniform cooling. The top-bottom nozzle spacing is 50mm (covering the full width of the steel plate), and the side nozzles are at a 30° angle to the edge of the steel plate (to avoid insufficient cooling at the edges). The nozzle water pressure is 1.2-1.5MPa, and the water flow density is 20-30L / (m²·s). Three sets of infrared thermometers (inlet, middle, and outlet) are set in the cooling zone to monitor the transverse temperature difference of the steel plate in real time. When the temperature difference is > 15℃, the flow rate of the corresponding area nozzles is automatically adjusted (if the temperature difference at the edge is large, the water pressure of the side nozzles is increased) to ensure uniform cooling (transverse temperature difference ≤ 10℃).

[0014] Immersion temperature: 860~900℃ (corresponding to Ac3+40~60℃ for 51CrV4, Ac3 is about 820℃). Principle: to ensure that the steel plate is completely in the austenitic state, and to quickly initiate the martensitic transformation after immersion in water. Average cooling rate: 35-60℃ / s. Principle: This rate is the "safe hardening range"—below 35℃ / s, austenite easily transforms into pearlite / bainite, resulting in insufficient strength; above 60℃ / s, thermal stress and structural stress overlap, drastically increasing the risk of cracking. Combined with the CCT curve (continuous cooling transformation curve) of 51CrV4, when the cooling rate is 35-60℃ / s, the cooling path precisely avoids the "pearlite / bainite transformation zone" (the nose of the CCT curve, easily entered when the cooling rate is <30℃ / s), directly entering the "martensite transformation zone," ensuring a martensite transformation rate ≥95% (hardness 48-50HRC). Final cooling temperature: 100~250℃ (lower than the Ms point of 51CrV4 ≈300℃). Principle: to ensure complete martensite transformation and avoid the steel plate becoming brittle due to excessively low temperature (hardness > 52HRC).

[0015] 3.2.5 Timely tempering process Within 2 hours after quenching, the steel plate should be sent to a continuous tempering furnace to prevent age-related cracking of the martensitic structure. Pre-treatment before tempering: Shot blasting (steel shot diameter 0.8-1.2mm, shot blasting strength ≥0.3mmA) is used to remove surface oxide scale and ensure uniform heating; Tempering temperature: 450±10℃. Principle: This temperature is the "strength-toughness balance point" for 51CrV4—martensite decomposes into tempered sorbite (α phase + fine-grained cementite) at this temperature, eliminating 50-60% of internal stress while maintaining high strength (tensile strength ≥1580MPa) and toughness (elongation after fracture ≥8%). After tempering, when the steel plate cools from 450±10℃ to 300℃, "slow cooling" (rate ≤50℃ / h) is used, followed by natural cooling below 300℃ to avoid secondary stress caused by rapid cooling. The tempered sorbite remains unstable above 300℃; slow cooling prevents abrupt changes in structure, further eliminating internal stress and ensuring long-term stability of the steel plate's flatness (flatness change ≤0.5mm / m after one month at room temperature).

[0016] Furnace time: 4.0 min / mm (calculated based on finished product thickness), and not less than 90 min. Principle: to ensure that the core of the thick steel plate is fully tempered (e.g., 160 min for a 40 mm thick steel plate) to avoid a performance deviation between the surface and the core of >5%.

[0017] 3.2.6 Heated Straightening and Finishing Warm straightening: When the steel plate exits the furnace at a temperature of 300-400℃ after tempering, a four-stage straightening machine (straightening force ≥2000kN) is used for 3-5 passes of straightening. Principle: At this temperature, the steel plate has excellent plasticity (elongation ≥12%), and the straightness after straightening is ≤2mm / m. At the same time, it can further release 10-15% of the internal stress. Finishing inspection: After slow cooling to room temperature, plasma / laser edge cutting (edge ​​width 10-15mm) is used, and appearance (no cracks, pits), dimensions (thickness tolerance ±0.5mm), and mechanical properties (tensile, hardness) are inspected according to GB / T19879-2015.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. Excellent and stable product performance: The finished steel plate has a yield strength ≥1450MPa, tensile strength ≥1580MPa, hardness 48-50HRC, and straightness ≤2mm / m. It can be directly processed and formed without subsequent heat treatment, improving customer processing efficiency by more than 40%. 2. Significant environmental and cost advantages: Using water as the quenching medium eliminates oil fume pollution and fire risk, reduces quenching medium costs by 95% (compared to oil quenching), and reduces overall energy consumption per ton of steel plate by 30% (reducing customer tempering processes). 3. Wide process adaptability: It can produce 51CrV4 steel plates with a thickness of 15-40mm and a width of 1600-3200mm, filling the technical gap in online water quenching of wide and thick spring steel plates; 4. High reliability: Through raw material purity control, gradient cooling and timely tempering, the water quenching cracking rate is reduced to below 0.5%, and the product qualification rate is ≥99%. Attached Figure Description

[0019] Figure 1: Microstructure of the 15mm thick steel plate in Example 1 (200x magnification), showing a uniform tempered sorbite structure (α phase + fine-grained cementite), with a grain size of 5-6 and no coarse inclusions; Figure 2: Cross-sectional hardness distribution of the 40mm thick steel plate in Example 3, showing that the hardness deviation from the surface to the core is ≤1HRC (48-49HRC), indicating excellent tempering uniformity. Detailed Implementation

[0020] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0021] A process for online water bath cooling of 15-40mm thick spring steel plates is described in the following specific steps: continuous casting billet selection - heating - controlled rolling - online water bath cooling - tempering - straightening - inspection - warehousing.

[0022] Select continuously cast slabs with high purity and uniform composition: C: 0.46–0.55%, Si: ≤0.40%, Mn: 0.50–0.90%, P: ≤0.015%, S: ≤0.002%, Cr: 0.80–1.20%, Cu: ≤0.25%, Ni: ≤0.35%, Al: 0.02–0.04%, V: 0.10–0.20%, N: ≤0.005%, with the balance being iron and unavoidable impurities. Specifically, the slab should have C segregation at the center of the slab (grade 0.5), central porosity (grade 0.5), and all non-metallic inclusions (types A, B, C, D, and Ds) should be no greater than grade 0.5.

[0023] Heating process: Preheating zone temperature 700~800℃, heating zone temperature 1000~1150℃, soaking zone temperature 1220~1250℃; heating time ≥10*bill thickness (min).

[0024] Rolling process: The descaled billet is fed to a heavy plate rolling mill, where the steel plate is rolled in the austenite recrystallization zone to refine the grains. The final rolling temperature is 2.5 times H (the thickness of the finished steel plate), and the initial rolling temperature is 900-930℃.

[0025] After rolling, the steel plate is directly water-cooled online at a water temperature of 880 ± 20℃. The water-cooling nozzles on the top, bottom, and sides of the steel plate are fully open, with an average cooling rate of 35-60℃ / s. The steel plate is cooled to 100-250℃, and the quenched steel plate is transferred to a continuous furnace for tempering within 2 hours.

[0026] Before tempering, the steel plate is shot blasted and then tempered in a continuous furnace at a temperature of 450±10℃ for a duration of 4.0 min / mm, not less than 90 min. After the tempering time is reached, the steel plate is directly straightened while still warm after exiting the furnace to ensure that the flatness of the steel plate is ≤2mm / m.

[0027] After the steel plate is slowly cooled to room temperature, it is plasma or laser cut and inspected, and quality is checked according to product standards.

[0028] The specific chemical composition of the steel plates involved in each embodiment is shown in Table 1, the specific rolling and cooling process parameters are shown in Table 2, and the main performance tests are shown in Table 3.

[0029] Table 1 Steel Plate Composition (wt%)

[0030] Table 2 Rolling and Cooling Process Parameters

[0031] Table 3 Performance test results

[0032] The finished steel plates in all three embodiments meet the following requirements: ① no subsequent heat treatment is required and they can be directly processed; ② the mechanical properties meet the highest level requirements of 51CrV4 spring steel in GB / T19879-2015; ③ the flatness is ≤2mm / m and there are no cracking defects, which verifies the stability and feasibility of this process.

[0033] The following are comparison results with existing technologies:

[0034] This invention is superior to existing technologies in terms of "wide-width and thick plate adaptability, cost, cracking rate, and performance", and does not require customers to perform subsequent conditioning, thus significantly improving production efficiency.

[0035] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An on-line water bath production process of 15-40 mm thick 51CrV4 spring steel sheet, characterized in that, The process path is: preferably continuous casting billet → sectional heating → controlled rolling → online water bath cooling → timely tempering → warm straightening → finishing inspection → storage, the specific steps are as follows: (1) continuous casting billet selection: the chemical composition is C: 0.46-0.55%, Si: ≤0.40%, Mn: 0.50-0.90%, P: ≤0.015%, S: ≤0.002%, Cr: 0.80-1.20%, Cu: ≤0.25%, Ni: ≤0.35%, Al: 0.02-0.04%, V: 0.10-0.20%, N: ≤0.005%, the balance is Fe and inevitable impurities; the macrostructure quality meets segregation ≤C class 1.0 level, porosity ≤1.0 level, non-metallic inclusions A, B, C, D, Ds class are all ≤0.5 level; (2) sectional heating: preheating section 700-800℃, heating section 1000-1150℃, soaking section 1200-1250℃, the soaking time of the soaking section is ≥10×billet thickness (min); (3) controlled rolling: the final rolling waiting thickness is 2.5 times of the finished product thickness, and the final rolling temperature is 900-930℃; (4) online water bath cooling: the water inlet temperature is 860-900℃, the average cooling speed is 35-60℃ / s, and the final cooling temperature is 100-250℃; (5) timely tempering: the quenched steel plate is sent into the continuous furnace within 2h, the tempering temperature is 450±10℃, the furnace time is 4.0min / mm and is not less than 90min; (6) warm straightening: the steel plate is straightened when the furnace-out temperature after tempering is 300-400℃, and the flatness is ≤2mm / m.

2. The process of claim 1, wherein, The finished product steel plate width of the continuous casting billet is 1600-3200mm.

3. The process of claim 1, wherein, In step (2), the preheating section holding time is ≥20min, and the heating section holding time is ≥30min.

4. The process of claim 1, wherein, In step (3), high-pressure water is used for descaling before rolling, and the water pressure is ≥25MPa.

5. The process of claim 1, wherein, In step (4), the online water bath uses upper and lower + side edge surrounding type nozzles, the nozzle spacing is ≤50mm, and the water pressure is 1.2-1.5MPa.

6. The process of claim 5, wherein, In the online water bath cooling system, the upper and lower nozzle spacing is 50mm, the side nozzle and the steel plate edge angle is 30°, three groups of infrared temperature measuring instruments are arranged in the cooling zone, and the transverse temperature difference is controlled to be ≤10℃ in real time.

7. The process of claim 1, wherein, In step (5), shot blasting is used for pretreatment before tempering, the steel shot diameter is 0.8-1.2mm, and the shot blasting intensity is ≥0.3mmA.

8. The process of claim 1, wherein, In step (5), when the steel plate is cooled after tempering, the slow cooling rate above 300℃ is ≤50℃ / h, and the natural cooling rate below 300℃ is ≤50℃ / h, and the flatness change of the steel plate after being placed at room temperature for 1 month is ≤0.5mm / m.

9. The process of claim 1, wherein, In step (6), a four-fold straightening machine is used, the straightening force is ≥2000kN, and the straightening pass is 3-5 times.

10. The process of claim 1, wherein, The mechanical properties of the finished product steel plate are: yield strength ≥1450MPa, tensile strength ≥1580MPa, and hardness 48-50HRC.

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