Low-cost Q690D steel for coal mine machinery and production method thereof

Abstract

The invention relates to low-cost Q690D steel for coal mine machinery and a production method of the low-cost Q690D steel. The production method comprises the following steps of primary smelting, LF refining, VD treatment, continuous casting, casting blank stacking, cleaning, heating controlled rolling, online quenching, stacking slow cooling and tempering. The Q690D steel comprises the following chemical components in percentage by mass: 0.16%-0.18% of C, 0.2%-0.4% of Si, 1.45%-1.55% of Mn, less than or equal to 0.015% of P, less than or equal to 0.02% of S, 0.25%-0.3% of Cr, 0.2%-0.3% of Nb, 0.01%-0.018% of Ti, 0.02%-0.043% of Al, 0.001%-0.0018% of B and the balance of Fe and inevitable impurities. According to the design of the production method disclosed by the invention, the addition of precious metals Mo and Ni is avoided while the obdurability of the steel is ensured, the cost is reduced, the total heating time is reduced on the basis of traditional online quenching and tempering smelting of a steel plate, and the efficient and stable production of the Q690D steel is realized on the premise that the mechanical property and low-temperature impact toughness meet the requirements.

Description

Technical Field

[0001] This invention relates to the field of steel material production technology, and in particular to a low-cost Q690D steel for coal mining machinery and its production method. Background Technology

[0002] With the continuous development of the coal mining machinery industry, the steel materials sector is constantly advancing technological research and development towards lower costs and lighter weights. Currently, the main plate material used in the domestic coal mining machinery industry is Q690D steel, which has two mainstream production processes: one is online quenching followed by tempering, and the other is offline quenching followed by tempering, i.e., offline quenching and tempering. Offline quenching processes suffer from long production cycles and high costs; therefore, many advanced steel companies in China are actively committed to developing online quenching and tempering processes.

[0003] Under this process route, the conventional composition system design usually adopts a low carbon content and adds expensive alloying elements such as Mo and Ni. However, this will lead to an increase in production costs and weaken the product's competitiveness in the market. Therefore, it is necessary to propose a production process that reduces the amount of expensive alloying elements such as Mo and Ni while ensuring the strength and toughness of Q690D steel, so as to achieve stable production of Q690D steel under low-cost conditions. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a low-cost Q690D steel for coal mining machinery and its production method, so as to solve one or more problems in the prior art.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for producing low-cost Q690D steel for coal mining machinery, the method comprising the following steps:

[0007] Primary refining involves smelting molten iron to initially remove impurities and form molten steel.

[0008] LF refining includes intermediate ladle baking, refining ladle deoxidation, and fine-tuning alloy processing.

[0009] VD treatment includes pre-vacuum Ca treatment and vacuum boron treatment;

[0010] Continuous casting, controlling the superheat of continuous casting to 15-30℃, wherein electromagnetic stirring is used for 300mm and 330mm continuous casting billets, and the VD-treated molten steel is poured into the billet.

[0011] For continuous casting billet stacking, the stacking time for billet stacking is ≥48 hours.

[0012] Cleaning is performed to address surface defects in the cast billet.

[0013] Heating-controlled rolling is used to improve the microstructure and properties of steel.

[0014] Online quenching, controlling the water immersion temperature to ≥810℃ and the red-hot temperature to <200℃, to improve the hardness and strength of the steel;

[0015] Stacking and slow cooling, controlling the stacking time to ≥24h, to eliminate quenching stress;

[0016] Tempering is performed at a temperature of 530℃~550℃ and a tempering time of 28.8min / mm~72min / mm to adjust the toughness of the steel and ensure that the internal structure is fully transformed to obtain Q690D steel plate.

[0017] Furthermore, the intermediate batch baking process is protected by introducing argon gas to control the nitrogen content.

[0018] Furthermore, in the LF refining process, the refining time for white residue is controlled to be ≥25 min, and the total refining time is ≥45 min.

[0019] Furthermore, 10 minutes before the end of the LF refining process, the Al content is controlled to be ≥0.025%, and Ti iron is added.

[0020] Furthermore, during the fine-tuning alloy processing, argon gas is kept in the inlet state, and the Al content is controlled to be 0.12% by adding an Al line.

[0021] Furthermore, the vacuum boron treatment involves adding iron (B) and controlling the timing of the addition of iron and the continuity of the vacuum pumping.

[0022] Furthermore, during the VD treatment, the vacuum degree is controlled to be ≤66pa, the continuous casting furnace cycle time is ≥15min, and the soft blowing time after vacuuming is ≥8min.

[0023] Furthermore, during the heating control rolling process, the maximum heating temperature of the billet is controlled to be ≤1260℃, the heating time is 9min / cm~10min / cm, the holding temperature is 1220℃~1240℃, and the holding time in the soaking zone is ≥40min.

[0024] Furthermore, during the heating controlled rolling process, a two-stage controlled rolling process is adopted. In the first stage, the initial rolling temperature is controlled at 1050℃~1150℃, the reduction in three passes is controlled at ≥30mm, and the thickness of the steel plate is controlled at 1.5~2.0 times the thickness of the steel plate. In the second stage, the initial rolling temperature is controlled at 880℃~900℃, and the final rolling temperature is controlled at 840℃~850℃.

[0025] Q690D steel is produced using the aforementioned low-cost coal mining machinery Q690D steel production method. The Q690D steel comprises the following chemical composition and mass percentage: C: 0.16%–0.18%, Si: 0.2%–0.4%, Mn: 1.45%–1.55%, P: ≤0.015%, S: ≤0.02%, Cr: 0.25%–0.3%, Nb: 0.2%–0.3%, Ti: 0.01%–0.018%, Al: 0.02%–0.043%, B: 0.001%–0.0018%, with the balance being Fe and unavoidable impurities.

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

[0027] The design of the production method of this invention eliminates the addition of precious metals Mo and Ni while ensuring the strength and toughness of the steel, thus reducing costs. It also reduces the overall heating time compared to traditional online quenching and tempering of steel plates, and achieves efficient and stable production of Q690D steel while meeting the requirements for mechanical properties and low-temperature impact toughness. Attached Figure Description

[0028] Figure 1 The diagram shows a flow chart of a low-cost Q690D steel for coal mining machinery and its production method according to an embodiment of the present invention.

[0029] Figure 2 The microstructure of a low-cost Q690D steel for coal mining machinery and its production method according to an embodiment of the present invention is shown in Comparative Example 2 after tempering.

[0030] Figure 3 The microstructure of a low-cost Q690D steel for coal mining machinery and its production method according to an embodiment of the present invention is shown in Comparative Example 3 after tempering. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description, in conjunction with the accompanying drawings and specific embodiments, provides a low-cost Q690D steel for coal mine machinery and its production method. The advantages and features of this invention will become clearer from the following description. It should be noted that the drawings are in a very simplified form and use non-precise proportions, used only to facilitate and clearly illustrate the purpose of the embodiments of this invention. Please refer to the accompanying drawings to make the objectives, features, and advantages of this invention more apparent and understandable. It should be understood that the structures, proportions, sizes, etc., depicted in the accompanying drawings are only for illustrative purposes to aid those skilled in the art and are not intended to limit the implementation conditions of this invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effects and objectives achieved by this invention, should still fall within the scope of the technical content disclosed in this invention.

[0032] Please see Figure 1 The production method of low-cost Q690D steel for coal mining machinery in this embodiment includes the following steps:

[0033] Step 1: Initial refining, which involves smelting molten iron to initially remove impurities and form molten steel.

[0034] In the primary smelting process, oxidation reactions are used to remove harmful impurities such as phosphorus and sulfur from the molten iron. Strict control of smelting temperature and oxygen potential is crucial in the primary smelting stage to ensure efficient impurity removal while avoiding excessive oxidation. Simultaneously, controlling the smelting time ensures the full dissolution and uniform distribution of alloying elements, laying a solid foundation for subsequent refining processes. The final molten steel must meet the requirements of uniform composition and preliminary impurity content, providing qualified raw materials for LF refining. In practice, online monitoring equipment is preferable for real-time monitoring of the composition and temperature of the molten steel after primary smelting. Smelting parameters can be dynamically adjusted based on the monitoring results to ensure stable steel quality after primary smelting.

[0035] Step 2: LF refining, including intermediate ladle baking, refining ladle deoxidation, and fine-tuning alloy treatment.

[0036] The tundish, as a crucial piece of equipment in the continuous casting process, directly affects the temperature stability and purity of the molten steel during its baking treatment. In this embodiment, the tundish baking treatment utilizes argon gas for protection to control the nitrogen content, thereby preventing nitrogen from entering the molten steel and avoiding the formation of inclusions such as aluminum nitride that could negatively impact steel properties, thus ensuring the purity of the molten steel. The refining ladle, as the final treatment in the refining process, undergoes deoxidation treatment in this embodiment using an aluminum deoxidizer. Utilizing the strong reducing properties of aluminum, residual oxygen in the molten steel is further removed, improving its purity. During the LF refining process, the white slag time must be strictly controlled to ≥25 min, and the total refining time to ≥45 min. The white slag is the ideal slag with high alkalinity and strong reducing properties produced through deep deoxidation in the LF refining process. The white slag effectively adsorbs inclusions and harmful gases in the molten steel, significantly improving its purity and laying the foundation for producing high-quality steel. Extending the refining time allows inclusions to float to the surface, ensuring uniform steel composition.

[0037] Furthermore, in the 10 minutes before the end of LF refining, the Al content is controlled to be ≥0.025%, and Ti ferroalloy (titanium ferroalloy) is added. Precise control of the Al content creates favorable conditions for the subsequent addition of Ti. Simultaneously, Nb ferroalloy (niobium ferroalloy) and Cr ferroalloy (chromium ferroalloy) are added to achieve microalloying, laying the compositional foundation for improving the strength and toughness of the steel. During the fine-tuning alloying stage, argon gas is continuously introduced to ensure uniform composition of the molten steel through stirring. The Al content is precisely controlled to 0.12% by adding an Al line, utilizing deoxidation and nitrogen fixation reactions to deeply purify the molten steel. This creates a highly pure and stable environment for the addition of ferroboron alloy in the subsequent VD treatment. Excess aluminum and its reaction products are fully removed during vacuum degassing and soft blowing in the subsequent VD treatment, ensuring that the aluminum content in the final product remains stable within the target range.

[0038] Step 3: VD treatment, including pre-vacuum Ca treatment and vacuum boron treatment.

[0039] Pre-vacuum Ca treatment effectively controls the morphology of inclusions in molten steel. Calcium reacts with oxygen, sulfur, and other elements in the molten steel to form low-melting-point calcium aluminates and calcium sulfide. These inclusions are distributed in spherical or dot-like patterns, significantly reducing their adverse effects on the mechanical properties of the steel, especially reducing damage to toughness. During the vacuum boron treatment, ferroborone (B-Fe) alloy is added, and the timing of its addition and the continuity of vacuum extraction are strictly controlled to ensure that boron is uniformly dissolved in the molten steel, fully utilizing its role in improving hardenability while avoiding boron loss or segregation due to improper addition timing. Furthermore, during VD treatment, the vacuum degree must be controlled to ≤66 Pa, the continuous casting cycle time ≥15 min, and the soft blowing time after vacuum ≥8 min. High vacuum helps remove gases such as hydrogen and nitrogen from the molten steel, reducing porosity defects. Sufficient vacuum holding time and soft blowing time promote further flotation of inclusions and homogenization of the molten steel composition, providing high-quality molten steel for subsequent continuous casting processes.

[0040] Step 4: Continuous casting, controlling the superheat of continuous casting to 15-30℃. For 300mm and 330mm continuous casting billets, electromagnetic stirring is used to cast the VD-treated molten steel into billets.

[0041] Proper control of superheat in continuous casting is crucial for ensuring billet quality. Maintaining superheat within the range of 15–30°C avoids problems such as poor steel fluidity and nozzle blockage caused by excessively low superheat, while preventing excessively high superheat from leading to well-developed columnar crystals, central porosity, and severe segregation. For large-diameter continuous casting billets such as 300mm and 330mm, electromagnetic stirring technology utilizes electromagnetic force to drive the movement of molten steel. This breaks the tendency of columnar crystal growth, refines the grain structure, reduces central segregation and shrinkage cavities, and improves the density and uniformity of the billet, providing high-quality billets for subsequent rolling processes.

[0042] Step 5: Stacking of cast billets, with a stacking time of ≥48 hours for continuous casting billets.

[0043] By extending the stacking time, the billet cools slowly, reducing internal stress caused by rapid cooling and preventing cracks. Simultaneously, maintaining a uniform temperature drop during stacking helps ensure a uniform transformation of the internal microstructure, laying the foundation for grain refinement in subsequent hot rolling processes. Appropriate gaps must be maintained between billets during stacking to guarantee uniform heat dissipation and prevent localized overheating or underheating from negatively impacting the slow cooling effect.

[0044] Step 6: Cleaning to treat surface defects of the billet.

[0045] A combination of mechanical and flame cleaning is preferred for a comprehensive inspection of the billet surface, focusing on removing defects such as surface cracks, scale, and folds. For shallow defects, grinding with a grinding wheel is used until a metallic luster is revealed. For deeper cracks or large-area defects, the defective area is first removed by flame cutting, followed by grinding to ensure a smooth, undamaged billet surface and prevent the defects from expanding during subsequent rolling, thus affecting the quality of the finished steel plate. After cleaning, the billet undergoes a surface quality inspection; only those that pass the inspection can proceed to the next process.

[0046] Step 7: Heating controlled rolling to improve the microstructure and properties of the steel.

[0047] During controlled heating rolling, the maximum heating temperature of the billet is controlled to ≤1260℃, the heating time is 9 min / cm~10 min / cm, the holding temperature is 1220℃~1240℃, and the soaking temperature is ≥40 min. Reasonable control of heating temperature and time can prevent overheating or uneven heating of the billet, ensuring that microalloying elements are fully dissolved and evenly distributed. Specifically, the maximum heating temperature ≤1260℃ prevents excessive austenite grain growth, and the combination of a heating time of 9 min / cm~10 min / cm and a holding temperature of 1220℃~1240℃ ensures that the core temperature of the billet is consistent with the surface temperature, achieving uniform heating. The soaking temperature is ≥40 min, which allows for sufficient austenitization of the billet's internal structure, creating conditions for grain refinement in subsequent rolling processes. In the rolling process, a two-stage controlled rolling method is adopted. The first stage, rough rolling, is controlled at a temperature of 1050℃~1150℃, which is in the austenite recrystallization region. By controlling the reduction in three passes to ≥30mm, the austenite grains are broken up by the reduction. At the same time, the thickness of the steel plate is controlled to be 1.5~2.0 times the thickness of the steel plate to reserve sufficient deformation for the second stage of rolling. The second stage, finish rolling, is controlled at a temperature of 880℃~900℃, which is in the non-recrystallization region of austenite. At this time, the metal has greater resistance to deformation. Through cumulative deformation, a large number of deformation bands are generated in the austenite grains. The final rolling temperature is controlled at 840℃~850℃ to ensure that rolling is completed before the austenite transforms into ferrite. By utilizing deformation-induced precipitation and grain refinement, the strength and toughness of the steel are further improved.

[0048] Step 8: Online quenching, controlling the water immersion temperature to ≥810℃ and the red-hot temperature to <200℃, in order to improve the hardness and strength of the steel.

[0049] Precise control of the immersion water temperature is crucial for ensuring effective quenching. The steel plate should be rapidly immersed in water when its final rolling temperature drops to 810℃ or higher. At this temperature, the austenite structure is in a stable state and can transform into martensite and bainite to the maximum extent during rapid cooling, resulting in a significant increase in the steel's hardness and strength. The reheat temperature must be strictly controlled below 200℃. Excessive reheat temperature can cause tempering transformation of the martensite structure, reducing the quenching strengthening effect. A lower reheat temperature ensures a uniform and stable martensitic matrix after quenching, laying the foundation for adjusting toughness in subsequent tempering processes. During the quenching process, the cooling water volume, cooling rate, and cooling time must be adjusted appropriately according to the steel plate thickness and specifications to ensure that both the core and surface of the steel plate achieve the ideal quenching effect, avoiding defects such as incomplete quenching or quenching cracks.

[0050] Step 9: Stack and slow cool, control the stacking time to ≥24h, in order to eliminate quenching stress.

[0051] The quenched steel plates are quickly stacked, ensuring proper spacing between them to avoid direct contact that could lead to localized overheating or uneven cooling. A slow cooling process of at least 24 hours allows for the full release of internal stress, reducing stress concentration caused by quenching and preventing deformation or cracking during subsequent processing or use. After slow cooling, the temperature of the steel plates is monitored to ensure a uniform temperature drop to room temperature, providing a stable foundation for subsequent performance testing and finished product processing.

[0052] Step 10: Tempering. Control the tempering temperature to 530℃~550℃ and the tempering time to 28.8min / mm~72min / mm to adjust the toughness of the steel and ensure sufficient transformation of the internal structure. The tempering temperature range of 530℃~550℃ allows the quenched martensite to undergo tempering transformation, generating tempered sorbite. This significantly improves the toughness of the steel while maintaining high strength, avoiding the risk of brittle fracture. The tempering time of 28.8min / mm~72min / mm ensures that the core of the steel plate fully reaches the tempering temperature and completes the structural transformation. For thicker steel plates, extending the tempering time allows for the full precipitation and uniform distribution of carbides, further optimizing the strength-toughness balance. A stepped heating method is preferred during tempering, slowly heating to the target temperature to prevent thermal stress caused by excessive temperature differences between the inside and outside of the steel plate. In addition, air cooling is performed after tempering to avoid excessively rapid cooling that could lead to secondary hardening or performance fluctuations. Ultimately, this allows the steel to obtain the mechanical properties required for use in coal mining machinery, resulting in Q690D steel plates.

[0053] Q690D steel is produced using the aforementioned low-cost production method for Q690D steel used in coal mining machinery. The Q690D steel comprises the following chemical composition and mass percentages: C: 0.16%–0.18%, Si: 0.2%–0.4%, Mn: 1.45%–1.55%, P: ≤0.015%, S: ≤0.02%, Cr: 0.25%–0.3%, Nb: 0.2%–0.3%, Ti: 0.01%–0.018%, Al: 0.02%–0.043%, B: 0.001%–0.0018%, with the balance being Fe and unavoidable impurities. By controlling the C content to exceed 0.15%, unlike traditional low-carbon compositions, the amount of subsequent alloying elements can be reduced while ensuring the steel's strength and toughness. Meanwhile, the content of harmful impurities such as P and S is strictly limited, with P ≤ 0.015% and S ≤ 0.02%, which can effectively reduce the cold and hot brittleness tendency of steel and improve the weldability and safety of the material. Si, as a deoxidizing element, is controlled within the range of 0.2% to 0.4%, allowing it to react fully with oxygen, reducing the oxygen content in the steel, and without causing a decrease in toughness due to excessive content. Mn is controlled within 1.45% to 1.55%, improving the matrix strength through solid solution strengthening, and forming a multi-element strengthening system with microalloying elements such as Cr, Nb, and Ti. A Cr content of 0.25% to 0.3% can improve the hardenability and corrosion resistance of steel, while Nb and Ti, by forming carbonitridium compounds, refine the grains during rolling, improving the strength-toughness balance of the steel. Al, as the final deoxidizer, is controlled within 0.02% to 0.043%, which can further reduce the oxygen content in the steel and inhibit austenite grain growth. Adding 0.001% to 0.0018% of element B can significantly improve the hardenability of steel, ensuring a uniform martensitic structure during online quenching. This allows the mechanical properties of Q690D steel to be met even with reduced use of precious alloys such as Mo and Ni.

[0054] The following provides a verification analysis of the production method in this embodiment:

[0055] Comparative Examples 1-3 are provided, in which the steel plates are all obtained by rolling slabs. The steel plate production process is controlled through steps 1-10 above to ensure that the mode allocation of roughing and finishing rolling processes is the same and the cooling mode is consistent. The chemical composition and mass percentage of the steel plates are shown in Table 1, and the temperature parameters for controlling rolling and slow cooling during stacking are shown in Table 2.

[0056] Table 1

[0057] chemical composition C（%） Si (%) Mn (%) P（%） S（%） Cr（%） Nb (%) B（%） Comparative Example 1 0.165 0.25 1.46 0.012 0.005 0.29 0.025 0.0012 Comparative Example 2 0.178 0.33 1.53 0.009 0.006 0.28 0.028 0.0018 Comparative Example 3 0.171 0.28 1.51 0.011 0.007 0.25 0.027 0.016

[0058] Table 2

[0059] project Roughing rolling temperature (°C) Roughing and finishing rolling temperatures (°C) Finishing rolling start temperature (°C) Finishing rolling temperature (°C) Final cooling temperature (°C) Comparative Example 1 1150 994 880 840 150 Comparative Example 2 1133 960 888 845 180 Comparative Example 3 1109 950 895 850 120

[0060] In addition, relevant settings and measurement parameters for the production process of steel plates for comparison examples 1 to 3 are provided:

[0061] Comparative Example 1: The slab size was set at 300mm, the soaking temperature was 1232℃, the total furnace time was 288 minutes, the tempering temperature was 545℃, and the tempering time was 28.8 minutes. After the rough rolling process, the intermediate slab had a thickness of 32mm, and the finished steel plate had a thickness of 16mm.

[0062] Comparative Example 2: The slab size was set at 330mm, the soaking temperature was 1230℃, the total furnace time was 310 minutes, the tempering temperature was 550℃, and the tempering time was 72 minutes. After the rough rolling process, the intermediate slab size was 80mm thick, and the finished steel plate size after rolling was 40mm thick.

[0063] Comparative Example 3: The slab size was 300mm, the soaking temperature was 1228℃, the total furnace time was 292 minutes, the tempering temperature was 535℃, and the tempering time was 63 minutes. After the rough rolling process, the intermediate slab size was 70mm thick, and the finished steel plate size after rolling was 35mm thick. Observation of the microstructure after tempering of Comparative Examples 1 and 3 yielded the following results: Figure 2 and Figure 3 The metallographic structures shown are all combinations of martensite, bainite, and ferrite.

[0064] Based on this, after tempering, the impact toughness and longitudinal mechanical properties of Comparative Examples 1-3 were tested at the 1 / 4 mark of the steel plate. Standard tensile specimens were also prepared for tensile testing. The test results are shown in Table 3. All samples exhibited good yield strength, tensile strength, and elongation, meeting the mechanical property requirements of Q690D steel. In the impact toughness test, Comparative Examples 1-3 all demonstrated good low-temperature toughness, capable of meeting the needs of coal mining machinery under complex working conditions. This further verifies that this production method can stably control the comprehensive performance of the steel, achieving the goal of low-cost production of high-quality Q690D steel for coal mining machinery.

[0065] Table 3

[0066] project Yield strength (MPa) Tensile strength (MPa) Elongation (%) Longitudinal impact energy Akv (J) at -20℃ Comparative Example 1 740 856 17 374 Comparative Example 2 733 843 17 215 Comparative Example 3 756 866 16 263

[0067] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0068] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for producing low-cost Q690D steel for coal mining machinery, characterized in that: The production method includes the following steps: Primary refining involves smelting molten iron to initially remove impurities and form molten steel. LF refining includes intermediate ladle baking, refining ladle deoxidation, and fine-tuning alloy processing. VD treatment includes pre-vacuum Ca treatment and vacuum boron treatment; Continuous casting, controlling the superheat of continuous casting to 15-30℃, wherein electromagnetic stirring is used for 300mm and 330mm continuous casting billets, and the VD-treated molten steel is poured into the billet. For continuous casting billet stacking, the stacking time for billet stacking is ≥48 hours. Cleaning is performed to address surface defects in the cast billet. Heating-controlled rolling is used to improve the microstructure and properties of steel. Online quenching, controlling the water immersion temperature to ≥810℃ and the red-hot temperature to <200℃, to improve the hardness and strength of the steel; Stacking and slow cooling, controlling the stacking time to ≥24h, to eliminate quenching stress; Tempering is performed at a temperature of 530℃~550℃ and a tempering time of 28.8min / mm~72min / mm to adjust the toughness of the steel and ensure that the internal structure is fully transformed to obtain Q690D steel plate.

2. The method for producing low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: The intermediate batch baking process is protected by introducing argon gas to control the nitrogen content.

3. The production method of low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: In the LF refining process, the refining time for white residue is controlled to be ≥25 min, and the total refining time is ≥45 min.

4. The production method of low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: Ten minutes before the end of the LF refining process, the Al content is controlled to be ≥0.025%, and Ti iron is added.

5. The method for producing low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: During the fine-tuning alloy processing, argon gas is kept in the air supply, and the Al content is controlled to be 0.12% by adding Al lines.

6. The method for producing low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: The vacuum boron treatment involves adding iron (B) and controlling the timing of the iron addition with the continuity of the vacuum pumping.

7. The method for producing low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: During the VD treatment, the vacuum degree is controlled to be ≤66pa, the continuous casting furnace cycle time is ≥15min, and the soft blowing time after vacuuming is ≥8min.

8. The method for producing low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: During the controlled heating rolling process, the maximum heating temperature of the billet is controlled to be ≤1260℃, the heating time is 9min / cm~10min / cm, the holding temperature is 1220℃~1240℃, and the holding time in the soaking zone is ≥40min.

9. A method for producing low-cost Q690D steel for coal mining machinery as described in claim 1, characterized in that: During the controlled heating rolling process, a two-stage controlled rolling process is adopted. In the first stage, the initial rolling temperature is controlled at 1050℃~1150℃, the reduction in three passes is controlled at ≥30mm, and the thickness of the steel plate is controlled at 1.5~2.0 times the thickness of the steel plate. In the second stage, the initial rolling temperature is controlled at 880℃~900℃, and the final rolling temperature is controlled at 840℃~850℃.

10. Q690D steel, produced using a low-cost Q690D steel production method for coal mining machinery as described in any one of claims 1 to 9, characterized in that: The Q690D steel comprises the following chemical composition and its mass percentage: C: 0.16%–0.18%, Si: 0.2%–0.4%, Mn: 1.45%–1.55%, P: ≤0.015%, S: ≤0.02%, Cr: 0.25%–0.3%, Nb: 0.2%–0.3%, Ti: 0.01%–0.018%, Al: 0.02%–0.043%, B: 0.001%–0.0018%, with the balance being Fe and unavoidable impurities.

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